Honeywell AUTOMATIC CONTROL Engineering Manual

HONEYWELL

ENGINEERING MANUAL of

AUTOMATIC
CONTROL

for

COMMERCIAL BUILDINGS

ENGINEERING MANUAL OF AUTOMATIC CONTROL

i

Copyright 1934, 1940, 1953, 1988, 1991 and 1997 by Honeywell Inc.

All rights reserved. This manual or portions thereof may not be reporduced

in any form without permission of Honeywell Inc.

Library of Congress Catalog Card Number: 97-72971

Home and Building Control

Honeywell Inc.
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P.O. Box 524
Minneapolis MN 55408-0524

Honeywell Latin American Region

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Home and Building Control

Honeywell Limited-Honeywell Limitée
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North York, Ontario
M2H 3N7

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1140 Brussels
Belgium

Printed in USA

ii

Honeywell Asia Pacific Inc.

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ENGINEERING MANUAL OF AUTOMATIC CONTROL

FOREWORD

The Minneapolis Honeywell Regulator Company published the first edition of the Engineering Manual of
Automatic Control in l934. The manual quickly became the standard textbook for the commercial building
controls industry. Subsequent editions have enjoyed even greater success in colleges, universities, and contractor
and consulting engineering offices throughout the world.

Since the original 1934 edition, the building control industry has experienced dramatic change and made
tremendous advances in equipment, system design, and application. In this edition, microprocessor controls are
shown in most of the control applications rather than pneumatic, electric, or electronic to reflect the trends in
industry today. Consideration of configuration, functionality, and integration plays a significant role in the
design of building control systems.

Through the years Honeywell has been dedicated to assisting consulting engineers and architects in the
application of automatic controls to heating, ventilating, and air conditioning systems. This manual is an outgrowth
of that dedication. Our end user customers, the building owners and operators, will ultimately benefit from the
efficiently designed systems resulting from the contents of this manual.

All of this manual’s original sections have been updated and enhanced to include the latest developments in
control technology. A new section has been added on indoor air quality and information on district heating has
been added to the Chiller, Boiler, and Distribution System Control Applications Section.

This twenty-first edition of the Engineering Manual of Automatic Control is our contribution to ensure that
we continue to satisfy our customer’s requirements. The contributions and encouragement received from previous
users are gratefully acknowledged. Further suggestions will be most welcome.

Minneapolis, Minnesota
October, 1997

KEVIN GILLIGAN
President, H&BC Solutions and Services

ENGINEERING MANUAL OF AUTOMATIC CONTROL

iii

iv

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PREFACE

The purpose of this manual is to provide the reader with a fundamental understanding of controls and how
they are applied to the many parts of heating, ventilating, and air conditioning systems in commercial buildings.

Many aspects of control are presented including air handling units, terminal units, chillers, boilers, building
airflow, water and steam distribution systems, smoke management, and indoor air quality. Control fundamentals,
theory, and types of controls provide background for application of controls to heating, ventilating, and air
conditioning systems. Discussions of pneumatic, electric, electronic, and digital controls illustrate that applications
may use one or more of several different control methods. Engineering data such as equipment sizing, use of
psychrometric charts, and conversion formulas supplement and support the control information. To enhance
understanding, definitions of terms are provided within individual sections. For maximum usability, each section
of this manual is available as a separate, self-contained document.

Building management systems have evolved into a major consideration for the control engineer when evaluating
a total heating, ventilating, and air conditioning system design. In response to this consideration, the basics of
building management systems configuration are presented.

The control recommendations in this manual are general in nature and are not the basis for any specific job or
installation. Control systems are furnished according to the plans and specifications prepared by the control
engineer. In many instances there is more than one control solution. Professional expertise and judgment are
required for the design of a control system. This manual is not a substitute for such expertise and judgment.
Always consult a licensed engineer for advice on designing control systems.

It is hoped that the scope of information in this manual will provide the readers with the tools to expand their
knowledge base and help develop sound approaches to automatic control.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

v

vi

ENGINEERING MANUAL OF AUTOMATIC CONTROL

ENGINEERING MANUAL of

AUTOMATIC
CONTROL

CONTENTS

Foreward ............................................................................................................................................................. iii

Preface ................................................................................................................................................................ v

Control System Fundamentals ............................................................................................ 1

Control Fundamentals ....................................................................................................................................... 3

Introduction.......................................................................................... 5

Definitions............................................................................................ 5

HVAC System Characteristics ............................................................. 8

Control System Characteristics ........................................................... 15

Control System Components .............................................................. 30

Characteristics And Attributes Of Control Methods............................. 35

Psychrometric Chart Fundamentals ................................................................................................................ 37

Introduction.......................................................................................... 38

Definitions............................................................................................ 38

Description of the Psychrometric Chart............................................... 39

The Abridged Psychrometric Chart ..................................................... 40

Examples of Air Mixing Process .......................................................... 42

Air Conditioning Processes ................................................................. 43

Humidifying Process............................................................................ 44

ASHRAE Psychrometric Chart ............................................................ 53

Pneumatic Control Fundamentals .................................................................................................................... 57

Introduction.......................................................................................... 59

Definitions............................................................................................ 59

Abbreviations ....................................................................................... 60

Symbols............................................................................................... 61

Basic Pneumatic Control System ........................................................ 61

Air Supply Equipment .......................................................................... 65

Thermostats ........................................................................................ 69

Controllers ........................................................................................... 70

Sensor-Controller Systems ................................................................. 72

Actuators and Final Control Elements ................................................. 74

Relays and Switches ........................................................................... 77

Pneumatic Control Combinations ........................................................ 84

Pneumatic Centeralization .................................................................. 89

Pneumatic Control System Example ................................................... 90

Electric Control Fundamentals ......................................................................................................................... 95

Introduction.......................................................................................... 97

Definitions............................................................................................ 97

How Electric Control Circuits Classified .............................................. 99

Series 40 Control Circuits.................................................................... 100

Series 80 Control Circuits.................................................................... 102

Series 60 Two-Position Control Circuits............................................... 103

Series 60 Floating Control Circuits ...................................................... 106

Series 90 Control Circuits.................................................................... 107

Motor Control Circuits.......................................................................... 114

ENGINEERING MANUAL OF AUTOMATIC CONTROL

vii

Electronic Control Fundamentals..................................................................................................................... 119

Introduction.......................................................................................... 120

Definitions............................................................................................ 120

Typical System .................................................................................... 122

Components ........................................................................................ 122

Electtonic Controller Fundamentals .................................................... 129

Typical System Application .................................................................. 130

Microprocessor-Based/DDC Fundamentals .................................................................................................... 131

Introduction.......................................................................................... 133

Definitions............................................................................................ 133

Background ......................................................................................... 134

Advantages ......................................................................................... 134

Controller Configuration ...................................................................... 135

Types of Controllers............................................................................. 136

Controller Software.............................................................................. 137

Controller Programming ...................................................................... 142

Typical Applications ............................................................................. 145

Indoor Air Quality Fundamentals ..................................................................................................................... 149

Introduction.......................................................................................... 151

Definitions............................................................................................ 151

Abbreviations ....................................................................................... 153

Indoor Air Quality Concerns ................................................................ 154

Indoor Air Quality Control Applications................................................ 164

Bibliography ......................................................................................... 170

Smoke Management Fundamentals ................................................................................................................. 171

Introduction.......................................................................................... 172

Definitions............................................................................................ 172

Objectives............................................................................................ 173

Design Considerations ........................................................................ 173

Design Principles ................................................................................ 175

Control Applications ............................................................................ 178

Acceptance Testing ............................................................................. 181

Leakage Rated Dampers .................................................................... 181

Bibliography ......................................................................................... 182

Building Management System Fundamentals................................................................................................. 183

Introduction.......................................................................................... 184

Definitions............................................................................................ 184

Background ......................................................................................... 185

System Configurations ........................................................................ 186

System Functions ................................................................................ 189

Integration of Other Systems............................................................... 197

viii

ENGINEERING MANUAL OF AUTOMATIC CONTROL

Control System Applications ............................................................................................... 199

Air Handling System Control Applications...................................................................................................... 201

Introduction.......................................................................................... 203

Abbreviations ....................................................................................... 203

Requirements For Effective Control .................................................... 204

Applications-General ........................................................................... 206

Valve and Damper Selection ............................................................... 207

Symbols............................................................................................... 208

Ventilation Control Processes ............................................................. 209

Fixed Quantity of Outdoor Air Control ................................................. 211

Heating Control Processes.................................................................. 223

Preheat Control Processes ................................................................. 228

Humidification Control Process ........................................................... 235

Cooling Control Processes .................................................................. 236

Dehumidification Control Processes ................................................... 243

Heating System Control process ......................................................... 246

Year-Round System Control processes .............................................. 248

ASHRAE Psychrometric Charts .......................................................... 261

Building Airflow System Control Applications ............................................................................................... 263

Introduction.......................................................................................... 265

Definitions............................................................................................ 265

Airflow Control Fundamentals ............................................................. 267

Airflow Control Applications................................................................. 281

References .......................................................................................... 292

Chiller, Boiler, and Distribution System Control Applications ....................................................................... 293

Introduction.......................................................................................... 297

Abbreviations....................................................................................... 297

Definitions............................................................................................ 297

Symbols............................................................................................... 298

Chiller System Control......................................................................... 299

Boiler System Control.......................................................................... 329

Hot And Chilled Water Distribution Systems Control........................... 337

High Temperature Water Heating System Control .............................. 376

District Heating Applications................................................................ 382

Individual Room Control Applications ............................................................................................................ 399

Introduction.......................................................................................... 401

Unitary Equipment Control .................................................................. 412

Hot Water Plant Considerations .......................................................... 428

ENGINEERING MANUAL OF AUTOMATIC CONTROL

ix

Engineering Information....................................................................................................... 429

Valve Selection and Sizing ................................................................................................................................ 431

Introduction.......................................................................................... 432

Definitions............................................................................................ 432

Valve Selection .................................................................................... 436

Valve Sizing ......................................................................................... 441

Damper Selection and Sizing ............................................................................................................................ 451

Introduction.......................................................................................... 453

Definitions............................................................................................ 453

Damper Selection ................................................................................ 454

Damper Sizing ..................................................................................... 463

Damper Pressure Drop ....................................................................... 468

Damper Applications ........................................................................... 469

General Engineering Data ................................................................................................................................. 471

Introduction.......................................................................................... 472

Weather Data ...................................................................................... 472

Conversion Formulas And Tables........................................................ 475

Electrical Data ..................................................................................... 482

Properties Of Saturated Steam Data................................................... 488

Airflow Data ......................................................................................... 489

Moisture Content Of Air Data .............................................................. 491

Index ....................................................................................................................................... 494

x

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL

CONTROL FUNDAMENTALS

SYSTEMS

FUNDMENTALS

1

ENGINEERING MANUAL OF AUTOMATIC CONTROL

Control

Fundamentals

Contents

Introduction ............................................................................................................ 5

Definitions ............................................................................................................ 5

HVAC System Characteristics ............................................................................................................ 8

General................................................................................................ 8

Heating ................................................................................................ 9

General................................................................................................ 9

Heating Equipment .............................................................................. 10

Cooling ................................................................................................ 11

General................................................................................................ 11

Cooling Equipment .............................................................................. 12

Dehumidification .................................................................................. 12

Humidification ...................................................................................... 13

Ventilation............................................................................................ 13

Filtration............................................................................................... 14

Control System Characteristics ............................................................................................................ 15

Controlled Variables ............................................................................ 15

Control Loop ........................................................................................ 15

Control Methods .................................................................................. 16

General ........................................................................................... 16

Analog And Digital Control .............................................................. 16

Control Modes ..................................................................................... 17

Two-Position Control....................................................................... 17

General ....................................................................................... 17

Basic Two-Position Control......................................................... 17

Timed Two-Position Control ........................................................ 18

Step Control .................................................................................... 19

Floating Control............................................................................... 20

Proportional Control ........................................................................ 21

General ....................................................................................... 21

Compensation Control ................................................................ 22

Proportional-Integral (Pi) Control .................................................... 23

Proportional-Integral-Derivative (Pid) Control ................................. 25

Enhanced Proportional-Integral-Derivative (epid) Control .............. 25

Adaptive Control ............................................................................. 26

Process Characteristics....................................................................... 26

CONTROL FUNDAMENTALS

Load ................................................................................................ 26

Lag .................................................................................................. 27

General ........................................................................................... 27

Measurement Lag ........................................................................... 27

Capacitance .................................................................................... 28

Resistance ...................................................................................... 29

Dead Time....................................................................................... 29

Control Application Guidelines ............................................................ 29

Control System Components ............................................................................................................ 30

Sensing Elements ............................................................................... 30

Te mperature Sensing Elements...................................................... 30

Pressure Sensing Elements............................................................ 31

Moisture Sensing Elements ............................................................ 32

Flow Sensors .................................................................................. 32

Proof-Of-Operation Sensors ........................................................... 33

Transducers ........................................................................................ 33

Controllers ........................................................................................... 33

Actuators ............................................................................................. 33

Auxiliary Equipment............................................................................. 34

Characteristics And Attributes Of Control Methods .............................................................................................. 35

ENGINEERING MANUAL OF AUTOMATIC CONTROL

4

INTRODUCTION

CONTROL FUNDAMENTALS

This section describes heating, ventilating, and air
conditioning (HVAC) systems and discusses characteristics and
components of automatic control systems. Cross-references
are made to sections that provide more detailed information.

A correctly designed HVAC control system can provide a
comfortable environment for occupants, optimize energy cost
and consumption, improve employee productivity, facilitate
efficient manufacturing, control smoke in the event of a fire,
and support the operation of computer and telecommunications
equipment. Controls are essential to the proper operation of
the system and should be considered as early in the design
process as possible.

Properly applied automatic controls ensure that a correctly
designed HVAC system will maintain a comfortable
environment and perform economically under a wide range of
operating conditions. Automatic controls regulate HVAC
system output in response to varying indoor and outdoor
conditions to maintain general comfort conditions in office
areas and provide narrow temperature and humidity limits
where required in production areas for product quality.

DEFINITIONS

The following terms are used in this manual. Figure 1 at the
end of this list illustrates a typical control loop with the
components identified using terms from this list.

Analog: Continuously variable (e.g., a faucet controlling water

from off to full flow).

Automatic control system: A system that reacts to a change

or imbalance in the variable it controls by adjusting
other variables to restore the system to the desired
balance.

Automatic controls can optimize HVAC system operation.
They can adjust temperatures and pressures automatically to
reduce demand when spaces are unoccupied and regulate
heating and cooling to provide comfort conditions while
limiting energy usage. Limit controls ensure safe operation of
HVAC system equipment and prevent injury to personnel and
damage to the system. Examples of limit controls are low­limit temperature controllers which help prevent water coils
or heat exchangers from freezing and flow sensors for safe
operation of some equipment (e.g., chillers). In the event of a
fire, controlled air distribution can provide smoke-free
evacuation passages, and smoke detection in ducts can close
dampers to prevent the spread of smoke and toxic gases.

HVAC control systems can also be integrated with security
access control systems, fire alarm systems, lighting control
systems, and building and facility management systems to
further optimize building comfort, safety, and efficiency.

Controlled medium: The medium in which the controlled

variable exists. In a space temperature control system,
the controlled variable is the space temperature and
the controlled medium is the air within the space.

Controlled Variable: The quantity or condition that is

measured and controlled.

Controller: A device that senses changes in the controlled

variable (or receives input from a remote sensor) and
derives the proper correction output.

Algorithm: A calculation method that produces a control

output by operating on an error signal or a time series
of error signals.

Compensation control: A process of automatically adjusting

the setpoint of a given controller to compensate for
changes in a second measured variable (e.g., outdoor
air temperature). For example, the hot deck setpoint
is normally reset upward as the outdoor air
temperature decreases. Also called “reset control”.

Control agent: The medium in which the manipulated variable

exists. In a steam heating system, the control agent is
the steam and the manipulated variable is the flow of
the steam.

Control point: The actual value of the controlled variable

(setpoint plus or minus offset).

Corrective action: Control action that results in a change of

the manipulated variable. Initiated when the
controlled variable deviates from setpoint.

Cycle: One complete execution of a repeatable process. In

basic heating operation, a cycle comprises one on
period and one off period in a two-position control
system.

Cycling: A periodic change in the controlled variable from

one value to another. Out-of-control analog cycling
is called “hunting”. Too frequent on-off cycling is
called “short cycling”. Short cycling can harm electric
motors, fans, and compressors.

Cycling rate: The number of cycles completed per time unit,

typically cycles per hour for a heating or cooling
system. The inverse of the length of the period of the
cycle.

5

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

Deadband: A range of the controlled variable in which no

corrective action is taken by the controlled system
and no energy is used. See also “zero energy band”.

related. Also, the delay in response of the sensing
element of a control due to the time required for the
sensing element to sense a change in the sensed
variable.

Deviation: The difference between the setpoint and the value

of the controlled variable at any moment. Also called
“offset”.

DDC: Direct Digital Control. See also Digital and Digital

control.

Digital: A series of on and off pulses arranged to convey

information. Morse code is an early example.
Processors (computers) operate using digital
language.

Digital control: A control loop in which a microprocessor-

based controller directly controls equipment based
on sensor inputs and setpoint parameters. The
programmed control sequence determines the output
to the equipment.

Droop: A sustained deviation between the control point and

the setpoint in a two-position control system caused
by a change in the heating or cooling load.

Enhanced proportional-integral-derivative (EPID) control:

A control algorithm that enhances the standard PID
algorithm by allowing the designer to enter a startup
output value and error ramp duration in addition to
the gains and setpoints. These additional parameters
are configured so that at startup the PID output varies
smoothly to the control point with negligible
overshoot or undershoot.

Electric control: A control circuit that operates on line or low

voltage and uses a mechanical means, such as a
temperature-sensitive bimetal or bellows, to perform
control functions, such as actuating a switch or
positioning a potentiometer. The controller signal
usually operates or positions an electric actuator or
may switch an electrical load directly or through a
relay.

Load: In a heating or cooling system, the heat transfer that

the system will be called upon to provide. Also, the
work that the system must perform.

Manipulated variable: The quantity or condition regulated

by the automatic control system to cause the desired
change in the controlled variable.

Measured variable: A variable that is measured and may be

controlled (e.g., discharge air is measured and
controlled, outdoor air is only measured).

Microprocessor-based control: A control circuit that operates

on low voltage and uses a microprocessor to perform
logic and control functions, such as operating a relay
or providing an output signal to position an actuator.
Electronic devices are primarily used as sensors. The
controller often furnishes flexible DDC and energy
management control routines.

Modulating: An action that adjusts by minute increments and

decrements.

Offset: A sustained deviation between the control point and

the setpoint of a proportional control system under
stable operating conditions.

On/off control: A simple two-position control system in which

the device being controlled is either full on or full off
with no intermediate operating positions available.
Also called “two-position control”.

Pneumatic control: A control circuit that operates on air

pressure and uses a mechanical means, such as a
temperature-sensitive bimetal or bellows, to perform
control functions, such as actuating a nozzle and
flapper or a switching relay. The controller output
usually operates or positions a pneumatic actuator,
although relays and switches are often in the circuit.

Electronic control: A control circuit that operates on low

voltage and uses solid-state components to amplify
input signals and perform control functions, such as
operating a relay or providing an output signal to
position an actuator. The controller usually furnishes
fixed control routines based on the logic of the solid­state components.

Final control element: A device such as a valve or damper

that acts to change the value of the manipulated
variable. Positioned by an actuator.

Hunting: See Cycling.

Lag: A delay in the effect of a changed condition at one point

in the system, or some other condition to which it is

ENGINEERING MANUAL OF AUTOMATIC CONTROL

Process: A general term that describes a change in a measurable

variable (e.g., the mixing of return and outdoor air
streams in a mixed-air control loop and heat transfer
between cold water and hot air in a cooling coil).
Usually considered separately from the sensing
element, control element, and controller.

Proportional band: In a proportional controller, the control

point range through which the controlled variable
must pass to move the final control element through
its full operating range. Expressed in percent of
primary sensor span. Commonly used equivalents are
“throttling range” and “modulating range”, usually
expressed in a quantity of engineering units (degrees
of temperature).

6

Proportional control: A control algorithm or method in which

the final control element moves to a position
proportional to the deviation of the value of the
controlled variable from the setpoint.

Proportional-Integral (PI) control: A control algorithm that

combines the proportional (proportional response)
and integral (reset response) control algorithms. Reset
response tends to correct the offset resulting from
proportional control. Also called “proportional-plus­reset” or “two-mode” control.

Proportional-Integral-Derivative (PID) control: A control

algorithm that enhances the PI control algorithm by
adding a component that is proportional to the rate of
change (derivative) of the deviation of the controlled
variable. Compensates for system dynamics and
allows faster control response. Also called “three­mode” or “rate-reset” control.

Reset Control: See Compensation control.

Sensing element: A device or component that measures the

value of a variable.

CONTROL FUNDAMENTALS

Step control: Control method in which a multiple-switch

assembly sequentially switches equipment (e.g.,
electric heat, multiple chillers) as the controller input
varies through the proportional band. Step controllers
may be actuator driven, electronic, or directly
activated by the sensed medium (e.g., pressure,
temperature).

Throttling range: In a proportional controller, the control point

range through which the controlled variable must pass
to move the final control element through its full
operating range. Expressed in values of the controlled
variable (e.g., degrees Fahrenheit, percent relative
humidity, pounds per square inch). Also called
“proportional band”. In a proportional room
thermostat, the temperature change required to drive
the manipulated variable from full off to full on.

Time constant: The time required for a dynamic component,

such as a sensor, or a control system to reach 63.2
percent of the total response to an instantaneous (or
“step”) change to its input. Typically used to judge
the responsiveness of the component or system.

Two-position control: See on/off control.

Setpoint: The value at which the controller is set (e.g., the

desired room temperature set on a thermostat). The
desired control point.

Short cycling: See Cycling.

MEASURED

VARIABLE

OUTDOOR

AIR

30

OUTDOOR

AIR

CONTROLLED

MEASURED

VARIABLE

MEDIUM

RESET SCHEDULE

60

0

OA

TEMPERATURE

CONTROLLED

VARIABLE

HOT WATER

SUPPLY

148

130

190

HW

SETPOINT

CONTROL

POINT

HOT WATER

SUPPLY

TEMPERATURE

Zero energy band: An energy conservation technique that

allows temperatures to float between selected settings,
thereby preventing the consumption of heating or
cooling energy while the temperature is in this range.

Zoning: The practice of dividing a building into sections for

heating and cooling control so that one controller is
sufficient to determine the heating and cooling

ALGORITHM IN

CONTROLLER

SETPOINT

160

SETPOINT

INPUT

OUTPUT

PERCENT

159

OPEN

VALVE

41

FLOW

FINAL CONTROL
ELEMENT

STEAM

MANIPULATED
VARIABLE

CONTROL
AGENT

HOT WATER

RETURN

requirements for the section.

AUTO

Fig. 1. Typical Control Loop.

7

M10510

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

HVAC SYSTEM CHARACTERISTICS

GENERAL

An HVAC system is designed according to capacity
requirements, an acceptable combination of first cost and
operating costs, system reliability, and available equipment
space.

DAMPER

AIR
FILTER

COOLING
COIL

FAN

COOLING

TOWER

Figure 2 shows how an HVAC system may be distributed in
a small commercial building. The system control panel, boilers,
motors, pumps, and chillers are often located on the lower level.
The cooling tower is typically located on the roof. Throughout
the building are ductwork, fans, dampers, coils, air filters,
heating units, and variable air volume (VAV) units and
diffusers. Larger buildings often have separate systems for
groups of floors or areas of the building.

DUCTWORK

HEATING

UNIT

VAV BOX

DIFFUSER

CHILLER

PUMP

Fig. 2. Typical HVAC System in a Small Building.

The control system for a commercial building comprises
many control loops and can be divided into central system and
local- or zone-control loops. For maximum comfort and
efficiency, all control loops should be tied together to share
information and system commands using a building
management system. Refer to the Building Management
System Fundamentals section of this manual.

BOILER

CONTROL
PANEL

M10506

The basic control loops in a central air handling system can

be classified as shown in Table 1.

Depending on the system, other controls may be required
for optimum performance. Local or zone controls depend on
the type of terminal units used.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

8

CONTROL FUNDAMENTALS

r

Ta ble 1. Functions of Central HVAC Control Loops.

Control

Loop Classification Description

Ventilation Basic Coordinates operation of the outdoor, return, and exhaust air dampers to maintain

the proper amount of ventilation air. Low-temperature protection is often required.

Better Measures and controls the volume of outdoor air to provide the proper mix of

outdoor and return air under varying indoor conditions (essential in variable air
volume systems). Low-temperature protection may be required.

Cooling Chiller control Maintains chiller discharge water at preset temperature or resets temperature

according to demand.

Cooling tower
control

Controls cooling tower fans to provide the coolest water practical under existing

wet bulb temperature conditions.
Water coil control Adjusts chilled water flow to maintain temperature.
Direct expansion

(DX) system
control

Cycles compressor or DX coil solenoid valves to maintain temperature. If

compressor is unloading type, cylinders are unloaded as required to maintain

temperature.

Fan Basic Turns on supply and return fans during occupied periods and cycles them as

required during unoccupied periods.
Better Adjusts fan volumes to maintain proper duct and space pressures. Reduces system

operating cost and improves performance (essential for variable air volume

systems).

Heating Coil control Adjusts water or steam flow or electric heat to maintain temperature.

Boiler control Operates burner to maintain proper discharge steam pressure or water temperature.

For maximum efficiency in a hot water system, water temperature should be reset

as a function of demand or outdoor temperature.

HEATING

GENERAL

Building heat loss occurs mainly through transmission,

infiltration/exfiltration, and ventilation (Fig. 3).

TRANSMISSION

VENTILATION DUCT

EXFILTRATION

DOOR

Fig. 3. Heat Loss from a Building.

The heating capacity required for a building depends on the
design temperature, the quantity of outdoor air used, and the
physical activity of the occupants. Prevailing winds affect the
rate of heat loss and the degree of infiltration. The heating
system must be sized to heat the building at the coldest outdoor
temperature the building is likely to experience (outdoor design
temperature).

ROOF

70°F

20°F

WINDOW

PREVAILING
WINDS

INFILTRATION

C2701

Transmission is the process by which energy enters or leaves
a space through exterior surfaces. The rate of energy
transmission is calculated by subtracting the outdoor
temperature from the indoor temperature and multiplying the
result by the heat transfer coefficient of the surface materials.
The rate of transmission varies with the thickness and
construction of the exterior surfaces but is calculated the same
way for all exterior surfaces:

Energy Transmission per
Unit Area and Unit Time = (TIN - T

) x HTC

OUT

Where:

TIN=indoor temperature

T

= outdoor temperature

OUT

HTC = heat transfer coefficient

HTC

=

Unit Time x Unit Area x Unit Temperatu

Btu

9

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

Infiltration is the process by which outdoor air enters a
building through walls, cracks around doors and windows, and
open doors due to the difference between indoor and outdoor
air pressures. The pressure differential is the result of
temperature difference and air intake or exhaust caused by fan
operation. Heat loss due to infiltration is a function of
temperature difference and volume of air moved. Exfiltration
is the process by which air leaves a building (e.g., through
walls and cracks around doors and windows) and carries heat
with it. Infiltration and exfiltration can occur at the same time.

Ventilation brings in fresh outdoor air that may require
heating. As with heat loss from infiltration and exfiltration,
heat loss from ventilation is a function of the temperature
difference and the volume of air brought into the building or
exhausted.

HEATING EQUIPMENT

Selecting the proper heating equipment depends on many
factors, including cost and availability of fuels, building size
and use, climate, and initial and operating cost trade-offs.
Primary sources of heat include gas, oil, wood, coal, electrical,
and solar energy. Sometimes a combination of sources is most
economical. Boilers are typically fueled by gas and may have
the option of switching to oil during periods of high demand.
Solar heat can be used as an alternate or supplementary source
with any type of fuel.

STEAM OR
HOT WATER
SUPPLY

FAN

COIL

UNIT HEATER

STEAM TRAP
(IF STEAM SUPPLY)

CONDENSATE
OR HOT WATER
RETURN

Fig. 5. Typical Unit Heater.

GRID PANEL

SERPENTINE PANEL

C2703

HOT WATER
SUPPLY

HOT WATER
RETURN

HOT WATER
SUPPLY

HOT WATER
RETURN

C2704

Figure 4 shows an air handling system with a hot water coil.
A similar control scheme would apply to a steam coil. If steam
or hot water is chosen to distribute the heat energy, high­efficiency boilers may be used to reduce life-cycle cost. Water
generally is used more often than steam to transmit heat energy
from the boiler to the coils or terminal units, because water
requires fewer safety measures and is typically more efficient,
especially in mild climates.

THERMOSTAT

HOT WATER
SUPPLY

FAN

VALVE

HOT WATER
RETURN

DISCHARGE
AIR

C2702

Fig. 4. System Using Heating Coil.

An air handling system provides heat by moving an air
stream across a coil containing a heating medium, across an
electric heating coil, or through a furnace. Unit heaters (Fig.

5) are typically used in shops, storage areas, stairwells, and
docks. Panel heaters (Fig. 6) are typically used for heating
floors and are usually installed in a slab or floor structure, but
may be installed in a wall or ceiling.

Fig. 6. Panel Heaters.

Unit ventilators (Fig. 7) are used in classrooms and may
include both a heating and a cooling coil. Convection heaters
(Fig. 8) are used for perimeter heating and in entries and
corridors. Infrared heaters (Fig. 9) are typically used for spot
heating in large areas (e.g., aircraft hangers, stadiums).

FAN

DISCHARGE
AIR

COOLING
COIL

RETURN
AIR

C3035

WALL

OUTDOOR
AIR

HEATING
COIL

DRAIN PAN

MIXING

DAMPERS

Fig. 7. Unit Ventilator.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

10

CONTROL FUNDAMENTALS

FINNED TUBE

WARM AIR

RETURN AIR

SUPPLY

RETURN

FLOOR

TO OTHER
HEATING UNITS

FROM OTHER
HEATING UNITS

C2705

Fig. 8. Convection Heater.

REFLECTOR

INFRARED
SOURCE

RADIANT HEAT

C2706

Fig. 9. Infrared Heater.

In mild climates, heat can be provided by a coil in the central
air handling system or by a heat pump. Heat pumps have the
advantage of switching between heating and cooling modes
as required. Rooftop units provide packaged heating and
cooling. Heating in a rooftop unit is usually by a gas- or oil­fired furnace or an electric heat coil. Steam and hot water coils
are available as well. Perimeter heat is often required in colder
climates, particularly under large windows.

A heat pump uses standard refrigeration components and a
reversing valve to provide both heating and cooling within the
same unit. In the heating mode, the flow of refrigerant through
the coils is reversed to deliver heat from a heat source to the
conditioned space. When a heat pump is used to exchange heat
from the interior of a building to the perimeter, no additional
heat source is needed.

A heat-recovery system is often used in buildings where a
significant quantity of outdoor air is used. Several types of
heat-recovery systems are available including heat pumps,
runaround systems, rotary heat exchangers, and heat pipes.

In a runaround system, coils are installed in the outdoor air
supply duct and the exhaust air duct. A pump circulates the
medium (water or glycol) between the coils so that medium
heated by the exhaust air preheats the outdoor air entering the
system.

A rotary heat exchanger is a large wheel filled with metal
mesh. One half of the wheel is in the outdoor air intake and
the other half, in the exhaust air duct. As the wheel rotates, the
metal mesh absorbs heat from the exhaust air and dissipates it
in the intake air.

A heat pipe is a long, sealed, finned tube charged with a
refrigerant. The tube is tilted slightly with one end in the
outdoor air intake and the other end in the exhaust air. In a
heating application, the refrigerant vaporizes at the lower end

in the warm exhaust air, and the vapor rises toward the higher
end in the cool outdoor air, where it gives up the heat of
vaporization and condenses. A wick carries the liquid
refrigerant back to the warm end, where the cycle repeats. A
heat pipe requires no energy input. For cooling, the process is
reversed by tilting the pipe the other way.

Controls may be pneumatic, electric, electronic, digital, or
a combination. Satisfactory control can be achieved using
independent control loops on each system. Maximum operating
efficiency and comfort levels can be achieved with a control
system which adjusts the central system operation to the
demands of the zones. Such a system can save enough in
operating costs to pay for itself in a short time.

Controls for the air handling system and zones are
specifically designed for a building by the architect, engineer,
or team who designs the building. The controls are usually
installed at the job site. Terminal unit controls are typically
factory installed. Boilers, heat pumps, and rooftop units are
usually sold with a factory-installed control package
specifically designed for that unit.

COOLING

GENERAL

Both sensible and latent heat contribute to the cooling load
of a building. Heat gain is sensible when heat is added to the
conditioned space. Heat gain is latent when moisture is added
to the space (e.g., by vapor emitted by occupants and other
sources). To maintain a constant humidity ratio in the space,
water vapor must be removed at a rate equal to its rate of
addition into the space.

Conduction is the process by which heat moves between
adjoining spaces with unequal space temperatures. Heat may
move through exterior walls and the roof, or through floors,
walls, or ceilings. Solar radiation heats surfaces which then
transfer the heat to the surrounding air. Internal heat gain is
generated by occupants, lighting, and equipment. Warm air
entering a building by infiltration and through ventilation also
contributes to heat gain.

Building orientation, interior and exterior shading, the angle
of the sun, and prevailing winds affect the amount of solar
heat gain, which can be a major source of heat. Solar heat
received through windows causes immediate heat gain. Areas
with large windows may experience more solar gain in winter
than in summer. Building surfaces absorb solar energy, become
heated, and transfer the heat to interior air. The amount of
change in temperature through each layer of a composite
surface depends on the resistance to heat flow and thickness
of each material.

Occupants, lighting, equipment, and outdoor air ventilation
and infiltration requirements contribute to internal heat gain.
For example, an adult sitting at a desk produces about 400 Btu
per hour. Incandescent lighting produces more heat than
fluorescent lighting. Copiers, computers, and other office
machines also contribute significantly to internal heat gain.

11

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

COOLING EQUIPMENT

An air handling system cools by moving air across a coil
containing a cooling medium (e.g., chilled water or a
refrigerant). Figures 10 and 11 show air handling systems that
use a chilled water coil and a refrigeration evaporator (direct
expansion) coil, respectively. Chilled water control is usually
proportional, whereas control of an evaporator coil is two­position. In direct expansion systems having more than one
coil, a thermostat controls a solenoid valve for each coil and
the compressor is cycled by a refrigerant pressure control. This
type of system is called a “pump down” system. Pump down
may be used for systems having only one coil, but more often
the compressor is controlled directly by the thermostat.

CHILLED
WATER
SUPPLY

TEMPERATURE
CONTROLLER

CONTROL
VALV E

CHILLED
WATER
COIL

SENSOR

CHILLED
WATER
RETURN

COOL AIR

C2707-2

Fig. 10. System Using Cooling Coil.

TEMPERATURE
CONTROLLER

EVAPORATOR
COIL

SOLENOID
VALV E

D

X

COOL AIR

SENSOR

REFRIGERANT
LIQUID

Compressors for chilled water systems are usually
centrifugal, reciprocating, or screw type. The capacities of
centrifugal and screw-type compressors can be controlled by
varying the volume of refrigerant or controlling the compressor
speed. DX system compressors are usually reciprocating and,
in some systems, capacity can be controlled by unloading
cylinders. Absorption refrigeration systems, which use heat
energy directly to produce chilled water, are sometimes used
for large chilled water systems.

While heat pumps are usually direct expansion, a large heat
pump may be in the form of a chiller. Air is typically the heat
source and heat sink unless a large water reservoir (e.g., ground
water) is available.

Initial and operating costs are prime factors in selecting
cooling equipment. DX systems can be less expensive than
chillers. However, because a DX system is inherently two­position (on/off), it cannot control temperature with the
accuracy of a chilled water system. Low-temperature control
is essential in a DX system used with a variable air volume
system.

For more information control of various system equipment,
refer to the following sections of this manual:

— Chiller, Boiler, and Distribution System

Control Application.
—Air Handling System Control Applications.
—Individual Room Control Applications.

DEHUMIDIFICATION

Air that is too humid can cause problems such as
condensation and physical discomfort. Dehumidification
methods circulate moist air through cooling coils or sorption
units. Dehumidification is required only during the cooling
season. In those applications, the cooling system can be
designed to provide dehumidification as well as cooling.

REFRIGERANT
GAS

C2708-1

Fig. 11. System Using Evaporator

(Direct Expansion) Coil.

Two basic types of cooling systems are available: chillers,
typically used in larger systems, and direct expansion (DX)
coils, typically used in smaller systems. In a chiller, the
refrigeration system cools water which is then pumped to coils
in the central air handling system or to the coils of fan coil
units, a zone system, or other type of cooling system. In a DX
system, the DX coil of the refrigeration system is located in
the duct of the air handling system. Condenser cooling for
chillers may be air or water (using a cooling tower), while DX
systems are typically air cooled. Because water cooling is more
efficient than air cooling, large chillers are always water cooled.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

For dehumidification, a cooling coil must have a capacity
and surface temperature sufficient to cool the air below its dew
point. Cooling the air condenses water, which is then collected
and drained away. When humidity is critical and the cooling
system is used for dehumidification, the dehumidified air may
be reheated to maintain the desired space temperature.

When cooling coils cannot reduce moisture content
sufficiently, sorption units are installed. A sorption unit uses
either a rotating granular bed of silica gel, activated alumina
or hygroscopic salts (Fig. 12), or a spray of lithium chloride
brine or glycol solution. In both types, the sorbent material
absorbs moisture from the air and then the saturated sorbent
material passes through a separate section of the unit that
applies heat to remove moisture. The sorbent material gives
up moisture to a stream of “scavenger” air, which is then
exhausted. Scavenger air is often exhaust air or could be
outdoor air.

12

CONTROL FUNDAMENTALS

HUMID

ROTATING
GRANULAR
BED

SORPTION
UNIT

AIR

DRY AIR

HUMID AIR
EXHAUST

HEATING
COIL

SCAVENGER
AIR

C2709

Fig. 12. Granular Bed Sorption Unit.

Sprayed cooling coils (Fig. 13) are often used for space
humidity control to increase the dehumidifier efficiency and
to provide year-round humidity control (winter humidification
also).

MOISTURE

COOLING
COIL

ELIMINATORS

VENTILATION

Ventilation introduces outdoor air to replenish the oxygen
supply and rid building spaces of odors and toxic gases.
Ventilation can also be used to pressurize a building to reduce
infiltration. While ventilation is required in nearly all buildings,
the design of a ventilation system must consider the cost of
heating and cooling the ventilation air. Ventilation air must be
kept at the minimum required level except when used for free
cooling (refer to ASHRAE Standard 62, Ventilation for
Acceptable Indoor Air Quality).

To ensure high-quality ventilation air and minimize the
amount required, the outdoor air intakes must be located to
avoid building exhausts, vehicle emissions, and other sources
of pollutants. Indoor exhaust systems should collect odors or
contaminants at their source. The amount of ventilation a
building requires may be reduced with air washers, high
efficiency filters, absorption chemicals (e.g., activated
charcoal), or odor modification systems.

Ventilation requirements vary according to the number of
occupants and the intended use of the space. For a breakdown
of types of spaces, occupancy levels, and required ventilation,
refer to ASHRAE Standard 62.

SPRAY
PUMP

M10511

Fig. 13. Sprayed Coil Dehumidifier.

For more information on dehumidification, refer to the

following sections of this manual:

— Psychrometric Chart Fundamentals.
—Air Handling System Control Applications.

HUMIDIFICATION

Low humidity can cause problems such as respiratory
discomfort and static electricity. Humidifiers can humidify a
space either directly or through an air handling system. For
satisfactory environmental conditions, the relative humidity
of the air should be 30 to 60 percent. In critical areas where
explosive gases are present, 50 percent minimum is
recommended. Humidification is usually required only during
the heating season except in extremely dry climates.

Humidifiers in air handling systems typically inject steam
directly into the air stream (steam injection), spray atomized
water into the air stream (atomizing), or evaporate heated water
from a pan in the duct into the air stream passing through the
duct (pan humidification). Other types of humidifiers are a
water spray and sprayed coil. In spray systems, the water can
be heated for better vaporization or cooled for
dehumidification.

For more information on humidification, refer to the
following sections of this manual:

— Psychrometric Chart Fundamentals.
—Air Handling System Control Applications.

Figure 14 shows a ventilation system that supplies 100
percent outdoor air. This type of ventilation system is typically
used where odors or contaminants originate in the conditioned
space (e.g., a laboratory where exhaust hoods and fans remove
fumes). Such applications require make-up air that is
conditioned to provide an acceptable environment.

EXHAUST

SUPPLY
FAN

RETURN
AIR

MAKE-UP
AIR

SPACE

C2711

TO
OUTDOORS

OUTDOOR
AIR

SUPPLY

FILTER

EXHAUST
FAN

COIL

Fig. 14. Ventilation System Using 100 Percent

Outdoor Air.

In many applications, energy costs make 100 percent outdoor
air constant volume systems uneconomical. For that reason,
other means of controlling internal contaminants are available,
such as variable volume fume hood controls, space
pressurization controls, and air cleaning systems.

A ventilation system that uses return air (Fig. 15) is more
common than the 100 percent outdoor air system. The return­air ventilation system recirculates most of the return air from
the system and adds outdoor air for ventilation. The return-air
system may have a separate fan to overcome duct pressure

13

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

losses. The exhaust-air system may be incorporated into the
air conditioning unit, or it may be a separate remote exhaust.
Supply air is heated or cooled, humidified or dehumidified,
and discharged into the space.

DAMPER RETURN FAN

EXHAUST
AIR

RETURN
AIR

FILTER COIL SUPPLY FAN

MIXED
AIR

SUPPLY
AIR

C2712

OUTDOOR
AIR

DAMPERS

Fig. 15. Ventilation System Using Return Air.

Ventilation systems as shown in Figures 14 and 15 should
provide an acceptable indoor air quality, utilize outdoor air
for cooling (or to supplement cooling) when possible, and
maintain proper building pressurization.

For more information on ventilation, refer to the following
sections of this manual:

—Indoor Air Quality Fundamentals.
—Air Handling System Control Applications.
— Building Airflow System Control Applications.

FILTRATION

Air filtration is an important part of the central air handling
system and is usually considered part of the ventilation system.
Two basic types of filters are available: mechanical filters and
electrostatic precipitation filters (also called electronic air
cleaners). Mechanical filters are subdivided into standard and
high efficiency.

PLEATED FILTER

Filters are selected according to the degree of cleanliness
required, the amount and size of particles to be removed, and
acceptable maintenance requirements. High-efficiency
particulate air (HEPA) mechanical filters (Fig. 16) do not
release the collected particles and therefore can be used for
clean rooms and areas where toxic particles are released. HEPA
filters significantly increase system pressure drop, which must
be considered when selecting the fan. Figure 17 shows other
mechanical filters.

CELL

AIR FLOW

PLEATED PAPER

C2713

Fig. 16. HEPA Filter.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

BAG FILTER

Fig. 17. Mechanical Filters.

Other types of mechanical filters include strainers, viscous
coated filters, and diffusion filters. Straining removes particles
that are larger than the spaces in the mesh of a metal filter and
are often used as prefilters for electrostatic filters. In viscous
coated filters, the particles passing through the filter fibers
collide with the fibers and are held on the fiber surface.
Diffusion removes fine particles by using the turbulence present
in the air stream to drive particles to the fibers of the filter
surface.

An electrostatic filter (Fig. 18) provides a low pressure drop
but often requires a mechanical prefilter to collect large
particles and a mechanical after-filter to collect agglomerated
particles that may be blown off the electrostatic filter. An
electrostatic filter electrically charges particles passing through
an ionizing field and collects the charged particles on plates
with an opposite electrical charge. The plates may be coated
with an adhesive.

14

CONTROL FUNDAMENTALS

–

PATH
OF
IONS

WIRES
AT HIGH
POSITIVE
POTENTIAL

SOURCE: 1996 ASHRAE SYSTEMS AND EQUIPMENT HANDBOOK

AIRFLOW

AIRFLOW

POSITIVELY CHARGED
PA RTICLES

+

ALTERNATE
PLATES

–

GROUNDED

+

–

INTERMEDIATE
PLATES

+

CHARGED
TO HIGH
POSITIVE

–

POTENTIAL

+

THEORETICAL
PATHS OF

–

CHARGES DUST
PARTICLES

C2714

Fig. 18. Electrostatic Filter.

CONTROL SYSTEM CHARACTERISTICS

Automatic controls are used wherever a variable condition
must be controlled. In HVAC systems, the most commonly
controlled conditions are pressure, temperature, humidity, and
rate of flow. Applications of automatic control systems range
from simple residential temperature regulation to precision
control of industrial processes.

CONTROLLED VARIABLES

The sensor can be separate from or part of the controller
and is located in the controlled medium. The sensor measures
the value of the controlled variable and sends the resulting
signal to the controller. The controller receives the sensor
signal, compares it to the desired value, or setpoint, and
generates a correction signal to direct the operation of the
controlled device. The controlled device varies the control
agent to regulate the output of the control equipment that
produces the desired condition.

Automatic control requires a system in which a controllable
variable exists. An automatic control system controls the
variable by manipulating a second variable. The second
variable, called the manipulated variable, causes the necessary
changes in the controlled variable.

In a room heated by air moving through a hot water coil, for
example, the thermostat measures the temperature (controlled
variable) of the room air (controlled medium) at a specified
location. As the room cools, the thermostat operates a valve
that regulates the flow (manipulated variable) of hot water
(control agent) through the coil. In this way, the coil furnishes
heat to warm the room air.

CONTROL LOOP

In an air conditioning system, the controlled variable is
maintained by varying the output of the mechanical equipment
by means of an automatic control loop. A control loop consists
of an input sensing element, such as a temperature sensor; a
controller that processes the input signal and produces an output
signal; and a final control element, such as a valve, that operates
according to the output signal.

HVAC applications use two types of control loops: open
and closed. An open-loop system assumes a fixed relationship
between a controlled condition and an external condition. An
example of open-loop control would be the control of perimeter
radiation heating based on an input from an outdoor air
temperature sensor. A circulating pump and boiler are energized
when an outdoor air temperature drops to a specified setting,
and the water temperature or flow is proportionally controlled
as a function of the outdoor temperature. An open-loop system
does not take into account changing space conditions from
internal heat gains, infiltration/exfiltration, solar gain, or other
changing variables in the building. Open-loop control alone
does not provide close control and may result in underheating
or overheating. For this reason, open-loop systems are not
common in residential or commercial applications.

A closed-loop system relies on measurement of the
controlled variable to vary the controller output. Figure 19
shows a block diagram of a closed-loop system. An example
of closed-loop control would be the temperature of discharge
air in a duct determining the flow of hot water to the heating
coils to maintain the discharge temperature at a controller
setpoint.

15

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

SETPOINT

FEEDBACK

SENSING

ELEMENT

CONTROLLER

CORRECTIVE
SIGNAL

FINAL CONTROL

ELEMENT

MANIPULATED
VARIABLE

PROCESS

CONTROLLED
VARIABLE

SECONDARY

INPUT

DISTURBANCES

C2072

Fig. 19. Feedback in a Closed-Loop System.

In this example, the sensing element measures the discharge
air temperature and sends a feedback signal to the controller.
The controller compares the feedback signal to the setpoint.
Based on the difference, or deviation, the controller issues a
corrective signal to a valve, which regulates the flow of hot
water to meet the process demand. Changes in the controlled
variable thus reflect the demand. The sensing element continues
to measure changes in the discharge air temperature and feeds
the new condition back into the controller for continuous
comparison and correction.

Automatic control systems use feedback to reduce the
magnitude of the deviation and produce system stability as
described above. A secondary input, such as the input from an
outdoor air compensation sensor, can provide information
about disturbances that affect the controlled variable. Using
an input in addition to the controlled variable enables the
controller to anticipate the effect of the disturbance and
compensate for it, thus reducing the impact of disturbances on
the controlled variable.

CONTROL METHODS

GENERAL

An automatic control system is classified by the type of
energy transmission and the type of control signal (analog or
digital) it uses to perform its functions.

The most common forms of energy for automatic control
systems are electricity and compressed air. Systems may
comprise one or both forms of energy.

Systems that use electrical energy are electromechanical,
electronic, or microprocessor controlled. Pneumatic control
systems use varying air pressure from the sensor as input to a
controller, which in turn produces a pneumatic output signal
to a final control element. Pneumatic, electromechanical, and
electronic systems perform limited, predetermined control
functions and sequences. Microprocessor-based controllers use
digital control for a wide variety of control sequences.

Self-powered systems are a comparatively minor but still
important type of control. These systems use the power of the
measured variable to induce the necessary corrective action.
For example, temperature changes at a sensor cause pressure
or volume changes that are applied directly to the diaphragm
or bellows in the valve or damper actuator.

Many complete control systems use a combination of the
above categories. An example of a combined system is the
control system for an air handler that includes electric on/off
control of the fan and pneumatic control for the heating and
cooling coils.

Va rious control methods are described in the following
sections of this manual:

— Pneumatic Control Fundamentals.
—Electric Control Fundamentals.
—Electronic Control Fundamentals.
—Microprocessor-Based/DDC Fundamental.

See CHARACTERISTICS AND ATTRIBUTES OF
CONTROL METHODS.

ANALOG AND DIGITAL CONTROL

Traditionally, analog devices have performed HVAC control.
A typical analog HVAC controller is the pneumatic type which
receives and acts upon data continuously. In a pneumatic
controller, the sensor sends the controller a continuous
pneumatic signal, the pressure of which is proportional to the
value of the variable being measured. The controller compares
the air pressure sent by the sensor to the desired value of air
pressure as determined by the setpoint and sends out a control
signal based on the comparison.

The digital controller receives electronic signals from
sensors, converts the electronic signals to digital pulses
(values), and performs mathematical operations on these
values. The controller reconverts the output value to a signal
to operate an actuator. The controller samples digital data at
set time intervals, rather than reading it continually. The
sampling method is called discrete control signaling. If the
sampling interval for the digital controller is chosen properly,
discrete output changes provide even and uninterrupted control
performance.

Figure 20 compares analog and digital control signals. The
digital controller periodically updates the process as a function
of a set of measured control variables and a given set of control
algorithms. The controller works out the entire computation,
including the control algorithm, and sends a signal to an
actuator. In many of the larger commercial control systems,
an electronic-pneumatic transducer converts the electric output
to a variable pressure output for pneumatic actuation of the
final control element.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

16

CONTROL FUNDAMENTALS

ANALOG CONTROL SIGNAL

OPEN

FINAL

CONTROL

ELEMENT
POSITION

CLOSED

OPEN

FINAL

CONTROL

ELEMENT
POSITION

CLOSED

DIGITAL CONTROL SIGNAL

TIME

TIME

C2080

Fig. 20. Comparison of Analog and Digital Control Signals.

CONTROL MODES

Control systems use different control modes to accomplish
their purposes. Control modes in commercial applications
include two-position, step, and floating control; proportional,
proportional-integral, and proportional-integral-derivative
control; and adaptive control.

An example of differential gap would be in a cooling system
in which the controller is set to open a cooling valve when the
space temperature reaches 78F, and to close the valve when
the temperature drops to 76F. The difference between the two
temperatures (2 degrees F) is the differential gap. The
controlled variable fluctuates between the two temperatures.

Basic two-position control works well for many applications.
For close temperature control, however, the cycling must be
accelerated or timed.

BASIC TWO-POSITION CONTROL

In basic two-position control, the controller and the final
control element interact without modification from a
mechanical or thermal source. The result is cyclical operation
of the controlled equipment and a condition in which the
controlled variable cycles back and forth between two values
(the on and off points) and is influenced by the lag in the
system. The controller cannot change the position of the final
control element until the controlled variable reaches one or
the other of the two limits of the differential. For that reason,
the differential is the minimum possible swing of the controlled
variable. Figure 21 shows a typical heating system cycling
pattern.

TEMPERATURE
(°F)

75

OVERSHOOT
CONDTION

DIAL SETTING

DIFFERENTIAL

OFF

ON

74

73

72

71

70

TWO-POSITION CONTROL

GENERAL

In two-position control, the final control element occupies
one of two possible positions except for the brief period when
it is passing from one position to the other. Two-position control
is used in simple HVAC systems to start and stop electric
motors on unit heaters, fan coil units, and refrigeration
machines, to open water sprays for humidification, and to
energize and deenergize electric strip heaters.

In two-position control, two values of the controlled variable
(usually equated with on and off) determine the position of
the final control element. Between these values is a zone called
the “differential gap” or “differential” in which the controller
cannot initiate an action of the final control element. As the
controlled variable reaches one of the two values, the final
control element assumes the position that corresponds to the
demands of the controller, and remains there until the controlled
variable changes to the other value. The final control element
moves to the other position and remains there until the
controlled variable returns to the other limit.

69

68

TIME

UNDERSHOOT
CONDTION

C2088

Fig. 21. Typical Operation of Basic Two-Position Control.

The overshoot and undershoot conditions shown in Figure
21 are caused by the lag in the system. When the heating system
is energized, it builds up heat which moves into the space to
warm the air, the contents of the space, and the thermostat. By
the time the thermostat temperature reaches the off point (e.g.,
72F), the room air is already warmer than that temperature.
When the thermostat shuts off the heat, the heating system
dissipates its stored heat to heat the space even more, causing
overshoot. Undershoot is the same process in reverse.

In basic two-position control, the presence of lag causes the
controller to correct a condition that has already passed rather
than one that is taking place or is about to take place.
Consequently, basic two-position control is best used in
systems with minimal total system lag (including transfer,
measuring, and final control element lags) and where close
control is not required.

17

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

Figure 22 shows a sample control loop for basic two-position
control: a thermostat turning a furnace burner on or off in
response to space temperature. Because the thermostat cannot
catch up with fluctuations in temperature, overshoot and
undershoot enable the temperature to vary, sometimes
considerably. Certain industrial processes and auxiliary
processes in air conditioning have small system lags and can
use two-position control satisfactorily.

THERMOSTAT

FURNACE

SOLENOID
GAS VALVE

C2715

Fig. 22. Basic Two-Position Control Loop.

TIMED TWO-POSITION CONTROL

GENERAL

The ideal method of controlling the temperature in a space

is to replace lost heat or displace gained heat in exactly the
amount needed. With basic two-position control, such exact
operation is impossible because the heating or cooling system
is either full on or full off and the delivery at any specific instant
is either too much or too little. Timed two-position control,
however, anticipates requirements and delivers measured
quantities of heating or cooling on a percentage on-time basis
to reduce control point fluctuations. The timing is accomplished
by a heat anticipator in electric controls and by a timer in
electronic and digital controls.

BASIC TWO-POSITION CONTROL

TEMPERATURE
(°F)

OFF

ON

TEMPERATURE
(°F)

OFF

ON

75

74

73

72

71

70

69

68

TIME

TIMED TWO-POSITION CONTROL

75

74

73

72

71

70

69

68

TIME

OVERSHOOT
CONDITION

DIAL SETTING

UNDERSHOOT
CONDITION

DIFFERENTIAL

CONTROL
POINT

C2089

Fig. 23. Comparison of Basic Two-Position and Timed

Two-Position Control.

In timed two-position control, the basic interaction between

the controller and the final control element is the same as for
basic two-position control. However, the controller responds
to gradual changes in the average value of the controlled
variable rather than to cyclical fluctuations.

Overshoot and undershoot are reduced or eliminated because

the heat anticipation or time proportioning feature results in a
faster cycling rate of the mechanical equipment. The result is
closer control of the variable than is possible in basic two­position control (Fig. 23).

HEAT ANTICIPATION

In electromechanical control, timed two-position control can
be achieved by adding a heat anticipator to a bimetal sensing
element. In a heating system, the heat anticipator is connected
so that it energizes whenever the bimetal element calls for heat.
On a drop in temperature, the sensing element acts to turn on
both the heating system and the heat anticipator. The heat
anticipator heats the bimetal element to its off point early and
deenergizes the heating system and the heat anticipator. As
the ambient temperature falls, the time required for the bimetal
element to heat to the off point increases, and the cooling time
decreases. Thus, the heat anticipator automatically changes
the ratio of on time to off time as a function of ambient
temperature.

Because the heat is supplied to the sensor only, the heat
anticipation feature lowers the control point as the heat
requirement increases. The lowered control point, called
“droop”, maintains a lower temperature at design conditions
and is discussed more thoroughly in the following paragraphs.
Energizing the heater during thermostat off periods
accomplishes anticipating action in cooling thermostats. In
either case, the percentage on-time varies in proportion to the
system load.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

18

TIME PROPORTIONING

73-1/4

72

70-3/4

0

1-1/4

2-1/2

DROOP (°F)

CONTROL POINT (°F)

DESIGN

TEMPERATURE

OUTDOOR AIR

TEMPERATURE

0%

100%

LOAD

C2091-1

NO LOAD

TEMPERATURE

Time proportioning control provides more effective two­position control than heat anticipation control and is available
with some electromechanical thermostats and in electronic and
microprocessor-based controllers. Heat is introduced into the
space using on/off cycles based on the actual heat load on the
building and programmable time cycle settings. This method
reduces large temperature swings caused by a large total lag
and achieves a more even flow of heat.

CONTROL FUNDAMENTALS

In electromechanical thermostats, the cycle rate is adjustable
by adjusting the heater. In electronic and digital systems, the
total cycle time and the minimum on and off times of the
controller are programmable. The total cycle time setting is
determined primarily by the lag of the system under control.
If the total cycle time setting is changed (e.g., from 10 minutes
to 20 minutes), the resulting on/off times change accordingly
(e.g., from 7.5 minutes on/2.5 minutes off to 15 minutes on/5
minutes off), but their ratio stays the same for a given load.

The cycle time in Figure 24 is set at ten minutes. At a 50
percent load condition, the controller, operating at setpoint,
produces a 5 minute on/5 minute off cycle. At a 75 percent
load condition, the on time increases to 7.5 minutes, the off
time decreases to 2.5 minutes, and the opposite cycle ratio
occurs at 25 percent load. All load conditions maintain the
preset 10-minute total cycle.

SELECTED

CYCLE TIME

(MINUTES)

10

7.5

5

2.5

0

100 75 50 25 0

LOAD (%)

ON

OFF

C2090

Fig. 25. Relationship between Control Point, Droop,

and Load (Heating Control).

Time proportioning control of two-position loads is
recommended for applications such as single-zone systems that
require two-position control of heating and/or cooling (e.g., a
gas-fired rooftop unit with direct-expansion cooling). Time
proportioning control is also recommended for electric heat
control, particularly for baseboard electric heat. With time
proportioning control, care must be used to avoid cycling the
controlled equipment more frequently than recommended by
the equipment manufacturer.

STEP CONTROL

Step controllers operate switches or relays in sequence to
enable or disable multiple outputs, or stages, of two-position
devices such as electric heaters or reciprocating refrigeration
compressors. Step control uses an analog signal to attempt to
obtain an analog output from equipment that is typically either
on or off. Figures 26 and 27 show that the stages may be
arranged to operate with or without overlap of the operating
(on/off) differentials. In either case, the typical two-position
differentials still exist but the total output is proportioned.

Fig. 24. Time Proportioning Control.

Because the controller responds to average temperature or
humidity, it does not wait for a cyclic change in the controlled
variable before signaling corrective action. Thus control system
lags have no significant effect.

Droop in heating control is a lowering of the control point
as the load on the system increases. In cooling control, droop
is a raising of the control point. In digital control systems,
droop is adjustable and can be set as low as one degree or even
less. Figure 25 shows the relationship of droop to load.

19

STAGES

THROTTLING RANGE

DIFFERENTIAL

5

4

3

2

1

ONOFF

74

ONOFF

SPACE TEMPERATURE (°F)

ONOFF

LOAD

ONOFF

Fig. 26. Electric Heat Stages.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

ONOFF

72

100%0%

C2092-1

CONTROL FUNDAMENTALS

THROTTLING RANGE

STAGES

DIFFERENTIAL

4

3

2

OFF

1

72

SPACE TEMPERATURE (°F)

0%

74

SETPOINT

LOAD

ON

ONOFF

ONOFF

ONOFF

76

100%

C2093

Fig. 27. Staged Reciprocating Chiller Control.

Figure 28 shows step control of sequenced DX coils and
electric heat. On a rise in temperature through the throttling
range at the thermostat, the heating stages sequence off. On a
further rise after a deadband, the cooling stages turn on in
sequence.

SPACE OR

ACTUATOR

6

5

4

3

2

1

STEP

CONTROLLER

FAN

STAGE NUMBERS

SOLENOID

VALVES

D

X

DIRECT EXPANSION

COILS

RETURN AIR

THERMOSTAT

D

X

MULTISTAGE

ELECTRIC HEAT

DISCHARGE

AIR

C2716

zero, and the sequence repeats until all stages required to meet
the load condition are on. On a decrease in load, the process
reverses.

With microprocessor controls, step control is usually done
with multiple, digital, on-off outputs since software allows
easily adjustable on-to-off per stage and interstage differentials
as well as no-load and time delayed startup and minimum on
and off adjustments.

FLOATING CONTROL

Floating control is a variation of two-position control and is
often called “three-position control”. Floating control is not a
common control mode, but is available in most microprocessor­based control systems.

Floating control requires a slow-moving actuator and a fast­responding sensor selected according to the rate of response
in the controlled system. If the actuator should move too slowly,
the controlled system would not be able to keep pace with
sudden changes; if the actuator should move too quickly, two­position control would result.

Floating control keeps the control point near the setpoint at
any load level, and can only be used on systems with minimal
lag between the controlled medium and the control sensor.
Floating control is used primarily for discharge control systems
where the sensor is immediately downstream from the coil,
damper, or device that it controls. An example of floating
control is the regulation of static pressure in a duct (Fig. 29).

FLOATING

STATIC

PRESSURE

CONTROLLER

ACTUATOR

REFERENCE

PRESSURE

PICK-UP

AIRFLOW

STATIC

PRESSURE

PICK-UP

Fig. 28. Step Control with Sequenced DX Coils and

Electric Heat.

A variation of step control used to control electric heat is
step-plus-proportional control, which provides a smooth
transition between stages. This control mode requires one of
the stages to be a proportional modulating output and the others,
two-position. For most efficient operation, the proportional
modulating stage should have at least the same capacity as
one two-position stage.

Starting from no load, as the load on the equipment increases,
the modulating stage proportions its load until it reaches full
output. Then, the first two-position stage comes full on and
the modulating stage drops to zero output and begins to
proportion its output again to match the increasing load. When
the modulating stage again reaches full output, the second two­position stage comes full on, the modulating stage returns to

ENGINEERING MANUAL OF AUTOMATIC CONTROL

DAMPER

C2717

Fig. 29. Floating Static Pressure Control.

In a typical application, the control point moves in and out
of the deadband, crossing the switch differential (Fig. 30). A
drop in static pressure below the controller setpoint causes the
actuator to drive the damper toward open. The narrow
differential of the controller stops the actuator after it has moved
a short distance. The damper remains in this position until the
static pressure further decreases, causing the actuator to drive
the damper further open. On a rise in static pressure above the
setpoint, the reverse occurs. Thus, the control point can float
between open and closed limits and the actuator does not move.
When the control point moves out of the deadband, the
controller moves the actuator toward open or closed until the
control point moves into the deadband again.

20

CONTROL FUNDAMENTALS

ON

“CLOSE”
SWITCH

DIFFERENTIAL

SETPOINT

CONTROLLER

LOAD

DAMPER

POSITION

T1 T2 T3 T4 T5 T6

TIME

Fig. 30. Floating Control.

PROPORTIONAL CONTROL

GENERAL

Proportional control proportions the output capacity of the
equipment (e.g., the percent a valve is open or closed) to match
the heating or cooling load on the building, unlike two-position
control in which the mechanical equipment is either full on or
full off. In this way, proportional control achieves the desired
heat replacement or displacement rate.

In a chilled water cooling system, for example (Fig. 31), the
sensor is placed in the discharge air. The sensor measures the
air temperature and sends a signal to the controller. If a
correction is required, the controller calculates the change and
sends a new signal to the valve actuator. The actuator
repositions the valve to change the water flow in the coil, and
thus the discharge temperature.

CONTROLLER

CHILLED

WATER

RETURN

AIR

VALVE

COIL

SENSOR

DISCHARGE

AIR

C2718

Fig. 31. Proportional Control Loop.

OFF

DEADBAND

OFF

“OPEN”

CONTROL POINT

FULL LOAD

NO LOAD

OPEN

CLOSED

T7

DIFFERENTIAL

ON

SWITCH

C2094

In proportional control, the final control element moves to a
position proportional to the deviation of the value of the
controlled variable from the setpoint. The position of the final
control element is a linear function of the value of the controlled
variable (Fig. 32).

POSITION OF FINAL
CONTROL ELEMENT

73 74 75 76 77

CONTROL POINT (°F)

THROTTLING RANGE

ACTUATOR

POSITION

100%

OPEN

50%

OPEN

CLOSED

Fig. 32. Final Control Element Position as a Function of

the Control Point (Cooling System).

The final control element is seldom in the middle of its range
because of the linear relationship between the position of the
final control element and the value of the controlled variable.
In proportional control systems, the setpoint is typically the
middle of the throttling range, so there is usually an offset
between control point and setpoint.

C2095

21

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

An example of offset would be the proportional control of a
chilled water coil used to cool a space. When the cooling load
is 50 percent, the controller is in the middle of its throttling
range, the properly sized coil valve is half-open, and there is
no offset. As the outdoor temperature increases, the room
temperature rises and more cooling is required to maintain the
space temperature. The coil valve must open wider to deliver
the required cooling and remain in that position as long as the
increased requirement exists. Because the position of the final
control element is proportional to the amount of deviation, the
temperature must deviate from the setpoint and sustain that
deviation to open the coil valve as far as required.

Figure 33 shows that when proportional control is used in a
heating application, as the load condition increases from 50
percent, offset increases toward cooler. As the load condition
decreases, offset increases toward warmer. The opposite occurs
in a cooling application.

WARMER

CONTROL POINT

SETPOINT

0%
LOAD

COOLER

OFFSET

50%

LOAD

OFFSET

C2096

100%
LOAD

Fig. 33. Relationship of Offset to Load

(Heating Application).

The throttling range is the amount of change in the controlled
variable required for the controller to move the controlled
device through its full operating range. The amount of change
is expressed in degrees Fahrenheit for temperature, in
percentages for relative humidity, and in pounds per square
inch or inches of water for pressure. For some controllers,
throttling range is referred to as “proportional band”.
Proportional band is throttling range expressed as a percentage
of the controller sensor span:

Proportional Band =

Throttling Range

Sensor Span

x 100

Where:

V=output signal
K=proportionality constant (gain)

E=deviation (control point - setpoint)

M=value of the output when the deviation is

zero (Usually the output value at 50 percent
or the middle of the output range. The
generated control signal correction is added
to or subtracted from this value. Also called
“bias” or “manual reset”.)

Although the control point in a proportional control system
is rarely at setpoint, the offset may be acceptable.
Compensation, which is the resetting of the setpoint to
compensate for varying load conditions, may also reduce the
effect of proportional offset for more accurate control. An
example of compensation is resetting boiler water temperature
based on outdoor air temperature. Compensation is also called
“reset control” or “cascade control”.

COMPENSATION CONTROL

GENERAL

Compensation is a control technique available in proportional
control in which a secondary, or compensation, sensor resets
the setpoint of the primary sensor. An example of compensation
would be the outdoor temperature resetting the discharge
temperature of a fan system so that the discharge temperature
increases as the outdoor temperature decreases. The sample
reset schedule in Table 2 is shown graphically in Figure 34.
Figure 35 shows a control diagram for the sample reset system.

Table 2. Sample Reset Schedule.

Fig. 34. Typical Reset Schedule for Discharge Air

Outdoor Air Discharge Air

Temperature Temperature

Condition (F) (F)

Outdoor design

0 100

temperature
Light load 70 70

“Gain” is a term often used in industrial control systems for
the change in the controlled variable. Gain is the reciprocal of
proportional band:

Gain =

100

Proportional Band

The output of the controller is proportional to the deviation
of the control point from setpoint. A proportional controller
can be mathematically described by:

V=KE + M

ENGINEERING MANUAL OF AUTOMATIC CONTROL

22

100

(FULL RESET)

DISCHARGE AIR

70

TEMPERATURE SETPOINT (°F)

0

(FULL

RESET)

OUTDOOR AIR TEMPERATURE (°F)

Control.

70
(RESET
START)

C2719

CONTROL FUNDAMENTALS

OUTDOOR AIR

TEMPERATURE

SENSOR

RETURN

FAN

SENSOR

SUPPLY

TEMPERATURE
CONTROLLER

DISCHARGE

AIR

C2720

Fig. 35. Discharge Air Control Loop with Reset.

Compensation can either increase or decrease the setpoint
as the compensation input increases. Increasing the setpoint
by adding compensation on an increase in the compensation
variable is often referred to as positive or summer
compensation. Increasing the setpoint by adding compensation
on a decrease in the compensation variable is often referred to
as negative or winter compensation. Compensation is most
commonly used for temperature control, but can also be used
with a humidity or other control system.

Some controllers provide compensation start point capability.
Compensation start point is the value of the compensation
sensor at which it starts resetting the controller primary sensor
setpoint.

COMPENSATION AUTHORITY

Compensation authority is the ratio of the effect of the
compensation sensor relative to the effect of the primary sensor.
Authority is stated in percent.

The basic equation for compensation authority is:

Authority =

Change in setpoint

Change in compensation input

x 100

In an application requiring negative reset, a change in outdoor
air temperature at the reset sensor from 0 to 60F resets the hot
water supply temperature (primary sensor) setpoint from 200
to 100F. Assuming a throttling range of 15 degrees F, the
required authority is calculated as follows:

Authority =

Change in setpoint + TR

Change in compensation input

200 – 100 + 15

=

60 – 0

x 100

x 100

Authority = 192%

The previous example assumes that the spans of the two
sensors are equal. If sensors with unequal spans are used, a
correction factor is added to the formula:

Authority =

Compensation sensor span

Primary sensor span

Change in setpoint ± TR

x

Change in compensation input

x 100

Correction Factor

Assuming the same conditions as in the previous example,
a supply water temperature sensor range of 40 to 240F (span
of 200 degrees F), an outdoor air temperature (compensation)
sensor range of -20 to 80F (span of 100 degrees F), and a
throttling range of 10 degrees F, the calculation for negative
reset would be as follows:

100

Authority =

200 – 100 + 10

x

200

60 – 0

x 100

Authority = 92%

For proportional controllers, the throttling range (TR) is
included in the equation. Two equations are required when the
throttling range is included. For direct-acting or positive
compensation, in which the setpoint increases as the
compensation input increases, the equation is:

Authority =

Change in setpoint – TR

Change in compensation input

x 100

Direct-acting compensation is commonly used to prevent
condensation on windows by resetting the relative humidity
setpoint downward as the outdoor temperature decreases.

For reverse-acting or negative compensation, in which the
setpoint decreases as the compensation input increases, the
equation is:

Authority =

Change in setpoint + TR

Change in compensation input

x 100

The effects of throttling range may be disregarded with PI reset
controls.

PROPORTIONAL-INTEGRAL (PI) CONTROL

In the proportional-integral (PI) control mode, reset of the
control point is automatic. PI control, also called “proportional­plus-reset” control, virtually eliminates offset and makes the
proportional band nearly invisible. As soon as the controlled
variable deviates above or below the setpoint and offset
develops, the proportional band gradually and automatically
shifts, and the variable is brought back to the setpoint. The
major difference between proportional and PI control is that
proportional control is limited to a single final control element
position for each value of the controlled variable. PI control
changes the final control element position to accommodate
load changes while keeping the control point at or very near
the setpoint.

23

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

The reset action of the integral component shifts the
proportional band as necessary around the setpoint as the load
on the system changes. The graph in Figure 36 shows the shift
of the proportional band of a PI controller controlling a
normally open heating valve. The shifting of the proportional
band keeps the control point at setpoint by making further
corrections in the control signal. Because offset is eliminated,
the proportional band is usually set fairly wide to ensure system
stability under all operating conditions.

HEATING

VALVE

POSITION

CLOSED

50% OPEN

100% OPEN

0%

LOAD

90 95

= CONTROL POINT
THROTTLING RANGE = 10 DEGREES F

50%

LOAD

PROPORTIONAL BAND

FOR SEPARATE LOAD

100

SETPOINT (°F)

CONDITIONS

100%

LOAD

105 110

C2097-1

Fig. 36. Proportional Band Shift Due to Offset.

Reset of the control point is not instantaneous. Whenever
the load changes, the controlled variable changes, producing
an offset. The proportional control makes an immediate
correction, which usually still leaves an offset. The integral
function of the controller then makes control corrections over
time to bring the control point back to setpoint (Fig. 37). In
addition to a proportional band adjustment, the PI controller
also has a reset time adjustment that determines the rate at
which the proportional band shifts when the controlled variable
deviates any given amount from the setpoint.

SETPOINT

Reset error correction time is proportional to the deviation
of the controlled variable. For example, a four-percent deviation
from the setpoint causes a continuous shift of the proportional
band at twice the rate of shift for a two-percent deviation. Reset
is also proportional to the duration of the deviation. Reset
accumulates as long as there is offset, but ceases as soon as
the controlled variable returns to the setpoint.

With the PI controller, therefore, the position of the final
control element depends not only upon the location of the
controlled variable within the proportional band (proportional
band adjustment) but also upon the duration and magnitude of
the deviation of the controlled variable from the setpoint (reset
time adjustment). Under steady state conditions, the control
point and setpoint are the same for any load conditions, as
shown in Figure 37.

PI control adds a component to the proportional control
algorithm and is described mathematically by:

V = KE + ∫ Edt + M

K

T

1

Integral

Where:

V=output signal
K=proportionality constant (gain)

E=deviation (control point - setpoint)

T1=reset time

K/T1=reset gain

dt = differential of time (increment in time)

M=value of the output when the deviation

is zero

Integral windup, or an excessive overshoot condition, can
occur in PI control. Integral windup is caused by the integral
function making a continued correction while waiting for
feedback on the effects of its correction. While integral action
keeps the control point at setpoint during steady state
conditions, large overshoots are possible at start-up or during
system upsets (e.g., setpoint changes or large load changes).
On many systems, short reset times also cause overshoot.

DEVIATION

FROM

SETPOINT

VALVE

POSITION

CONTROL POINT (LOAD CHANGES)

INTEGRAL ACTION

PROPORTIONAL CORRECTION

T1 T2 T3 T4

TIME

Fig. 37. Proportional-Integral Control Response to

Load Changes.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

OPEN

CLOSED

C2098

Integral windup may occur with one of the following:

—When the system is off.
—When the heating or cooling medium fails or is not

available.

—When one control loop overrides or limits another.

Integral windup can be avoided and its effects diminished.
At start-up, some systems disable integral action until measured
variables are within their respective proportional bands.
Systems often provide integral limits to reduce windup due to
load changes. The integral limits define the extent to which
integral action can adjust a device (the percent of full travel).
The limit is typically set at 50 percent.

24

CONTROL FUNDAMENTALS

PROPORTIONAL-INTEGRAL-DERIVATIVE (PID)
CONTROL

Proportional-integral-derivative (PID) control adds the
derivative function to PI control. The derivative function
opposes any change and is proportional to the rate of change.
The more quickly the control point changes, the more corrective
action the derivative function provides.

If the control point moves away from the setpoint, the
derivative function outputs a corrective action to bring the
control point back more quickly than through integral action
alone. If the control point moves toward the setpoint, the
derivative function reduces the corrective action to slow down
the approach to setpoint, which reduces the possibility of
overshoot.

The rate time setting determines the effect of derivative
action. The proper setting depends on the time constants of
the system being controlled.

The derivative portion of PID control is expressed in the
following formula. Note that only a change in the magnitude
of the deviation can affect the output signal.

V = KT

dE

D

dt

Where:

V=output signal
K=proportionality constant (gain)

TD=rate time (time interval by which the

derivative advances the effect of
proportional action)

KTD=rate gain constant

dE/dt = derivative of the deviation with respect to

time (error signal rate of change)

The complete mathematical expression for PID control
becomes:

V = KE + ∫Edt + KTD + M

T

1

dEK

dt

The graphs in Figures 38, 39, and 40 show the effects of all
three modes on the controlled variable at system start-up. With
proportional control (Fig. 38), the output is a function of the
deviation of the controlled variable from the setpoint. As the
control point stabilizes, offset occurs. With the addition of
integral control (Fig. 39), the control point returns to setpoint
over a period of time with some degree of overshoot. The
significant difference is the elimination of offset after the
system has stabilized. Figure 40 shows that adding the
derivative element reduces overshoot and decreases response
time.

SETPOINT

CONTROL

POINT

T1 T2 T3 T4 T5 T6

TIME

OFFSET

C2099

Fig. 38. Proportional Control.

SETPOINT

CONTROL

T1 T2 T3 T4 T5 T6

TIME

POINT

OFFSET

C2100

Fig. 39. Proportional-Integral Control.

OFFSET

SETPOINT

Where:

Proportional Integral Derivative

V=output signal
K=proportionality constant (gain)

E=deviation (control point - setpoint)

T1=reset time

K/T1=reset gain

dt = differential of time (increment in time)

TD=rate time (time interval by which the

derivative advances the effect of
proportional action)

KTD=rate gain constant

dE/dt = derivative of the deviation with respect to

time (error signal rate of change)

M=value of the output when the deviation

is zero

T1 T2 T3 T4 T5 T6

TIME

C2501

Fig. 40. Proportional-Integral-Derivative Control.

ENHANCED PROPORTIONAL-INTEGRAL­DERIVATIVE (EPID) CONTROL

The startup overshoot, or undershoot in some applications,
noted in Figures 38, 39, and 40 is attributable to the very
large error often present at system startup. Microprocessor­based PID startup performance may be greatly enhanced by
exterior error management appendages available with
enhanced proportional-integral-derivative (EPID) control.
Two basic EPID functions are start value and error ramp time.

25

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

The start value EPID setpoint sets the output to a fixed value
at startup. For a VAV air handling system supply fan, a suitable
value might be twenty percent, a value high enough to get the
fan moving to prove operation to any monitoring system and
to allow the motor to self cool. For a heating, cooling, and
ventilating air handling unit sequence, a suitable start value
would be thirty-three percent, the point at which the heating,
ventilating (economizer), and mechanical cooling demands are
all zero. Additional information is available in the Air Handling
System Control Applications section.

The error ramp time determines the time duration during
which the PID error (setpoint minus input) is slowly ramped,
linear to the ramp time, into the PID controller. The controller
thus arrives at setpoint in a tangential manner without
overshoot, undershoot, or cycling. See Figure 41.

100

SETPOINT

START

PERCENT OPEN

VALUE

ACTUATOR POSITION

0

ERROR

RAMP

TIME

T1 T2 T3 T4 T5 T6

ELAPSED TIME

OFFSET

CONTROL

POINT

T7 T8

M13038

Fig. 41. Enhanced Proportional-Integral-Derivative

(EPID) Control.

ADAPTIVE CONTROL

Adaptive control is available in some microprocessor-based
controllers. Adaptive control algorithms enable a controller to
adjust its response for optimum control under all load
conditions. A controller that has been tuned to control
accurately under one set of conditions cannot always respond
well when the conditions change, such as a significant load
change or changeover from heating to cooling or a change in
the velocity of a controlled medium.

An adaptive control algorithm monitors the performance of
a system and attempts to improve the performance by adjusting
controller gains or parameters. One measurement of
performance is the amount of time the system requires to react
to a disturbance: usually the shorter the time, the better the
performance. The methods used to modify the gains or
parameters are determined by the type of adaptive algorithm.
Neural networks are used in some adaptive algorithms.

An example of a good application of adaptive control is
discharge temperature control of the central system cooling
coil for a VAV system. The time constant of a sensor varies as
a function of the velocity of the air (or other fluid). Thus the
time constant of the discharge air sensor in a VAV system is
constantly changing. The change in sensor response affects
the system control so the adaptive control algorithm adjusts
system parameters such as the reset and rate settings to maintain
optimum system performance.

Adaptive control is also used in energy management
programs such as optimum start. The optimum start program
enables an HVAC system to start as late as possible in the
morning and still reach the comfort range by the time the
building is occupied for the lease energy cost. To determine
the amount of time required to heat or cool the building, the
optimum start program uses factors based on previous building
response, HVAC system characteristics, and current weather
conditions. The algorithm monitors controller performance by
comparing the actual and calculated time required to bring the
building into the comfort range and tries to improve this
performance by calculating new factors.

PROCESS CHARACTERISTICS

As pumps and fans distribute the control agent throughout
the building, an HVAC system exhibits several characteristics
that must be understood in order to apply the proper control
mode to a particular building system.

LOAD

Process load is the condition that determines the amount of
control agent the process requires to maintain the controlled
variable at the desired level. Any change in load requires a
change in the amount of control agent to maintain the same
level of the controlled variable.

Load changes or disturbances are changes to the controlled
variable caused by altered conditions in the process or its
surroundings. The size, rate, frequency, and duration of
disturbances change the balance between input and output.

Four major types of disturbances can affect the quality of
control:

— Supply disturbances
— Demand disturbances
— Setpoint changes
— Ambient (environmental) variable changes

Supply disturbances are changes in the manipulated variable
input into the process to control the controlled variable. An
example of a supply disturbance would be a decrease in the
temperature of hot water being supplied to a heating coil. More
flow is required to maintain the temperature of the air leaving
the coil.

Demand disturbances are changes in the controlled medium
that require changes in the demand for the control agent. In
the case of a steam-to-water converter, the hot water supply
temperature is the controlled variable and the water is the
controlled medium (Fig. 42). Changes in the flow or
temperature of the water returning to the converter indicate a
demand load change. An increased flow of water requires an
increase in the flow of the control agent (steam) to maintain
the water temperature. An increase in the returning water
temperature, however, requires a decrease in steam to maintain
the supply water temperature.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

26

HEAT LOSS

COLD

AIR

VALVE

THERMOSTAT

C2074

SPACE

VALV E

STEAM

(CONTROL AGENT)

FLOW (MANIPULATED

VARIABLE)

HOT WATER SUPPLY

(CONTROLLED

HOT WATER

RETURN

CONTROLLER

TEMPERATURE

(CONTROLLED

VAR IABLE)

MEDIUM)

CONTROL FUNDAMENTALS

WATER

LOAD

CONVERTER

STEAM TRAP

CONDENSATE RETURN

C2073

Fig. 42. Steam-to-Water Converter.

A setpoint change can be disruptive because it is a sudden
change in the system and causes a disturbance to the hot water
supply. The resulting change passes through the entire process
before being measured and corrected.

Ambient (environmental) variables are the conditions
surrounding a process, such as temperature, pressure, and
humidity. As these conditions change, they appear to the control
system as changes in load.

LAG

GENERAL

Time delays, or lag, can prevent a control system from
providing an immediate and complete response to a change in
the controlled variable. Process lag is the time delay between
the introduction of a disturbance and the point at which the
controlled variable begins to respond. Capacitance, resistance,
and/or dead time of the process contribute to process lag and
are discussed later in this section.

One reason for lag in a temperature control system is that a
change in the controlled variable (e.g., space temperature) does
not transfer instantly. Figure 43 shows a thermostat controlling
the temperature of a space. As the air in the space loses heat,
the space temperature drops. The thermostat sensing element
cannot measure the temperature drop immediately because
there is a lag before the air around the thermostat loses heat.
The sensing element also requires a measurable time to cool.
The result is a lag between the time the space begins to lose
heat and the time corrective action is initiated.

Fig. 43. Heat Loss in a Space Controlled by a

Thermostat.

Lag also occurs between the release of heat into the space,
the space warming, and the thermostat sensing the increased
temperature. In addition, the final control element requires time
to react, the heat needs time to transfer to the controlled
medium, and the added energy needs time to move into the
space. Total process lag is the sum of the individual lags
encountered in the control process.

MEASUREMENT LAG

Dynamic error, static error, reproducibility, and dead zone
all contribute to measurement lag. Because a sensing element
cannot measure changes in the controlled variable instantly,
dynamic error occurs and is an important factor in control.
Dynamic error is the difference between the true and the
measured value of a variable and is always present when the
controlled variable changes. The variable usually fluctuates
around the control point because system operating conditions
are rarely static. The difference is caused by the mass of the
sensing element and is most pronounced in temperature and
humidity control systems. The greater the mass, the greater
the difference when conditions are changing. Pressure sensing
involves little dynamic error.

Static error is the deviation between a measured value and
the true value of the static variable. Static error can be caused
by sensor calibration error. Static error is undesirable but not
always detrimental to control.

Repeatability is the ability of a sensor or controller to output
the same signal when it measures the same value of a variable
or load at different times. Precise control requires a high degree
of reproducibility.

27

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

The difference between repeatability and static error is that
repeatability is the ability to return to a specific condition,
whereas static error is a constant deviation from that condition.
Static error (e.g., sensor error) does not interfere with the ability
to control, but requires that the control point be shifted to
compensate and maintain a desired value.

The dead zone is a range through which the controlled
variable changes without the controller initiating a correction.
The dead zone effect creates an offset or a delay in providing
the initial signal to the controller. The more slowly the variable
changes, the more critical the dead zone becomes.

CAPACITANCE

Capacitance differs from capacity. Capacity is determined
by the energy output the system is capable of producing;
capacitance relates to the mass of the system. For example,
for a given heat input, it takes longer to raise the temperature
of a cubic foot of water one degree than a cubic foot of air.
When the heat source is removed, the air cools off more
quickly than the water. Thus the capacitance of the water is
much greater than the capacitance of air.

A capacitance that is large relative to the control agent tends
to keep the controlled variable constant despite load changes.
However, the large capacitance makes changing the variable
to a new value more difficult. Although a large capacitance
generally improves control, it introduces lag between the time
a change is made in the control agent and the time the controlled
variable reflects the change.

Figure 44 shows heat applied to a storage tank containing a
large volume of liquid. The process in Figure 44 has a large
thermal capacitance. The mass of the liquid in the tank exerts
a stabilizing effect and does not immediately react to changes
such as variations in the rate of the flow of steam or liquid,
minor variations in the heat input, and sudden changes in the
ambient temperature.

LIQUID

IN

STEAM

IN

Figure 45 shows a high-velocity heat exchanger, which
represents a process with a small thermal capacitance. The
rate of flow for the liquid in Figure 45 is the same as for the
liquid in Figure 44. However, in Figure 45 the volume and
mass of the liquid in the tube at any one time is small compared
to the tank shown in Figure 44. In addition, the total volume
of liquid in the exchanger at any time is small compared to the
rate of flow, the heat transfer area, and the heat supply. Slight
variations in the rate of feed or rate of heat supply show up
immediately as fluctuations in the temperature of the liquid
leaving the exchanger. Consequently, the process in Figure 45
does not have a stabilizing influence but can respond quickly
to load changes.

HEATING

LIQUID IN

LIQUID OUT

HEATING

MEDIUM OUT

MEDIUM IN

C2076

Fig. 45. Typical Process with Small Thermal

Capacitance.

Figure 46 shows supply capacitance in a steam-to-water
converter. When the load on the system (in Figure 44, cold
air) increases, air leaving the heating coil is cooler. The
controller senses the drop in temperature and calls for more
steam to the converter. If the water side of the converter is
large, it takes longer for the temperature of the supply water to
rise than if the converter is small because a load change in a
process with a large supply capacitance requires more time to
change the variable to a new value.

VALVE

STEAM

CONVERTER

CONTROLLER

HOT WATER SUPPLY

(CONSTANT FLOW,

VARYING

TEMPERATURE)

CONDENSATE

TANK

Fig. 44. Typical Process with Large Thermal

Capacitance.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

LIQUID

OUT

RETURN

C2075

PUMP

HOT WATER

RETURN

CONDENSATE

RETURN

STEAM

TRAP

COLD AIR

(LOAD)

HEATING

COIL

Fig. 46. Supply Capacitance (Heating Application).

28

HOT AIR

(CONTROLLED

VARIABLE)

C2077

CONTROL FUNDAMENTALS

VALVE

HWS

PROCESS

CONTROLLED

MEDIUM IN

HWR

24 FT

2 FT

CONTROLLED

MEDIUM OUT

SENSOR AT

LOCATION 1

SENSOR AT

LOCATION 2

CONTROLLER

VELOCITY OF CONTROLLED MEDIUM: 12 FT/S

DEAD TIME FOR SENSOR AT LOCATION 1: = 0.166 SEC

DEAD TIME FOR SENSOR AT LOCATION 2: = 2.0 SEC

2 FT

12 FT/S

24 FT

12 FT/S

C2079

In terms of heating and air conditioning, a large office area
containing desks, file cabinets, and office machinery has more
capacitance than the same area without furnishings. When the
temperature is lowered in an office area over a weekend, the
furniture loses heat. It takes longer to heat the space to the
comfort level on Monday morning than it does on other
mornings when the furniture has not had time to lose as much
heat. If the area had no furnishings, it would heat up much
more quickly.

The time effect of capacitance determines the process
reaction rate, which influences the corrective action that the
controller takes to maintain process balance.

RESISTANCE

Resistance applies to the parts of the process that resist the
energy (or material) transfer. Many processes, especially those
involving temperature control, have more than one capacitance.
The flow of energy (heat) passing from one capacitance through
a resistance to another capacitance causes a transfer lag
(Fig. 47).

COLD WATER

IN

HOT

STEAM

IN

HEAT CAPACITY

OF STEAM

IN COILS

(E.G., PIPES, TANK WALLS)

RESISTANCE TO

HEAT FLOW

HEAT CAPACITY

OF WATER

IN TANK

WATER

OUT

C2078

Fig. 47. Schematic of Heat Flow Resistance.

A transfer lag delays the initial reaction of the process. In
temperature control, transfer lag limits the rate at which the
heat input affects the controlled temperature. The controller
tends to overshoot the setpoint because the effect of the added
heat is not felt immediately and the controller calls for still
more heat.

DEAD TIME

Dead time, which is also called “transportation lag”, is the
delay between two related actions in a continuous process
where flow over a distance at a certain velocity is associated
with energy transfer. Dead time occurs when the control valve
or sensor is installed at a distance from the process (Fig. 48).

Fig. 48. Effect of Location on Dead Time.

Dead time does not change the process reaction
characteristics, but instead delays the process reaction. The
delay affects the system dynamic behavior and controllability,
because the controller cannot initiate corrective action until it
sees a deviation. Figure 48 shows that if a sensor is 24 feet
away from a process, the controller that changes the position
of the valve requires two seconds to see the effect of that
change, even assuming negligible capacitance, transfer, and
measurement lag. Because dead time has a significant effect
on system control, careful selection and placement of sensors
and valves is required to maintain system equilibrium.

The office described in the previous example is comfortable
by Monday afternoon and appears to be at control point.
However, the paper in the middle of a full file drawer would
still be cold because paper has a high thermal resistance. As a
result, if the heat is turned down 14 hours a day and is at comfort
level 10 hours a day, the paper in the file drawer will never
reach room temperature.

An increase in thermal resistance increases the temperature
difference and/or flow required to maintain heat transfer. If
the fins on a coil become dirty or corroded, the resistance to
the transfer of heat from one medium to the other medium
increases.

CONTROL APPLICATION GUIDELINES

The following are considerations when determining control

requirements:

—The degree of accuracy required and the amount of offset,

if any, that is acceptable.

—The type of load changes expected, including their size,

rate, frequency, and duration.

— The system process characteristics, such as time

constants, number of time lag elements, and
reaction rate.

29

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

Each control mode is applicable to processes having certain
combinations of the basic characteristics. The simplest mode
of control that meets application requirements is the best mode
to use, both for economy and for best results. Using a control

mode that is too complicated for the application may result in
poor rather than good control. Conversely, using a control
mode that is too basic for requirements can make adequate
control impossible. Table 3 lists typical control applications
and recommended control modes.

Ta ble 3. Control Applications and Recommended Control Modes.

Control Application Recommended Control Mode

a

Space Temperature P, PID
Mixed Air Temperature PI, EPID
Coil Discharge Temperature PI, EPID
Chiller Discharge Temperature PI, EPID
Hot Water Converter Discharge Temperature PI, EPID
Airflow PI Use a wide proportional band and a fast reset rate. For some

applications, PID may be required.
Fan Static Pressure PI , EPID
Humidity P, or if very tight control is required, PI
Dewpoint Temperature P, or if very tight control is required, PI

a

PID, EPID control is used in digital systems.

CONTROL SYSTEM COMPONENTS

Control system components consist of sensing elements,

controllers, actuators, and auxiliary equipment.

SENSING ELEMENTS

A sensing element measures the value of the controlled
variable. Controlled variables most often sensed in HVAC
systems are temperature, pressure, relative humidity, and flow.

TEMPERATURE SENSING ELEMENTS

The sensing element in a temperature sensor can be a bimetal
strip, a rod-and-tube element, a sealed bellows, a sealed bellows
attached to a capillary or bulb, a resistive wire, or a thermistor.
Refer to the Electronic Control Fundamentals section of this
manual for Electronic Sensors for Microprocessor Based
Systems.

A bimetal element is a thin metallic strip composed of two
layers of different kinds of metal. Because the two metals have
different rates of heat expansion, the curvature of the bimetal
changes with changes in temperature. The resulting movement
of the bimetal can be used to open or close circuits in electric
control systems or regulate airflow through nozzles in
pneumatic control systems. Winding the bimetal in a coil
(Fig. 49) enables a greater length of the bimetal to be used in
a limited space.

M10518

Fig. 49. Coiled Bimetal Element.

The rod-and-tube element (Fig. 50) also uses the principle

of expansion of metals. It is used primarily for insertion directly
into a controlled medium, such as water or air. In a typical
pneumatic device, a brass tube contains an Invar rod which is
fastened at one end to the tube and at the other end to a spring
and flapper. Brass has the higher expansion coefficient and is
placed outside to be in direct contact with the measured
medium. Invar does not expand noticeably with temperature
changes. As the brass tube expands lengthwise, it pulls the
Invar rod with it and changes the force on the flapper. The
flapper is used to generate a pneumatic signal. When the flapper
position changes, the signal changes correspondingly.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

30

FLAPPER

SPRING

SIGNAL PORT

BRASS TUBE

INVAR ROD

EXTENSION SPRING

SENSOR BODY

C2081

Fig. 50. Rod-and-Tube Element.

In a remote-bulb controller (Fig. 51), a remote capsule, or
bulb, is attached to a bellows housing by a capillary. The remote
bulb is placed in the controlled medium where changes in
temperature cause changes in pressure of the fill. The capillary
transmits changes in fill pressure to the bellows housing and
the bellows expands or contracts to operate the mechanical
output to the controller. The bellows and capillary also sense
temperature, but because of their small volume compared to
the bulb, the bulb provides the control.

MECHANICAL OUTPUT

TO CONTROLLER

CONTROL FUNDAMENTALS

The temperature sensor for an electronic controller may be
a length of wire or a thin metallic film (called a resistance
temperature device or RTD) or a thermistor. Both types of
resistance elements change electrical resistance as temperature
changes. The wire increases resistance as its temperature
increases. The thermistor is a semiconductor that decreases in
resistance as the temperature increases.

Because electronic sensors use extremely low mass, they
respond to temperature changes more rapidly than bimetal or
sealed-fluid sensors. The resistance change is detected by a
bridge circuit. Nickel “A”, BALCO, and platinum are typical
materials used for this type of sensor.

In thermocouple temperature-sensing elements, two
dissimilar metals (e.g., iron and nickel, copper and constantan,
iron and constantan) are welded together. The junction of the
two metals produces a small voltage when exposed to heat.
Connecting two such junctions in series doubles the generated
voltage. Thermocouples are used primarily for high­temperature applications.

Many special application sensors are available, including
carbon dioxide sensors and photoelectric sensors used in
security, lighting control, and boiler flame safeguard
controllers.

BELLOWS

LIQUID

FILL

CAPILLARY

CONTROLLED

MEDIUM

(E.G., WATER)

BULB

C2083

Fig. 51. Typical Remote-Bulb Element.

Two specialized versions of the remote bulb controller are
available. They both have no bulb and use a long capillary (15
to 28 feet) as the sensor. One uses an averaging sensor that is
liquid filled and averages the temperature over the full length
of the capillary. The other uses a cold spot or low temperature
sensor and is vapor filled and senses the coldest spot (12 inches
or more) along its length.

Electronic temperature controllers use low-mass sensing
elements that respond quickly to changes in the controlled
condition. A signal sent by the sensor is relatively weak, but is
amplified to a usable strength by an electronic circuit.

PRESSURE SENSING ELEMENTS

Pressure sensing elements respond to pressure relative to a
perfect vacuum (absolute pressure sensors), atmospheric
pressure (gage pressure sensors), or a second system pressure
(differential pressure sensors), such as across a coil or filter.
Pressure sensors measure pressure in a gas or liquid in pounds
per square inch (psi). Low pressures are typically measured in
inches of water. Pressure can be generated by a fan, a pump or
compressor, a boiler, or other means.

Pressure controllers use bellows, diaphragms, and a number
of other electronic pressure sensitive devices. The medium
under pressure is transmitted directly to the device, and the
movement of the pressure sensitive device operates the
mechanism of a pneumatic or electric switching controller.
Va riations of the pressure control sensors measure rate of flow,
quantity of flow, liquid level, and static pressure. Solid state
sensors may use the piezoresistive effect in which increased
pressure on silicon crystals causes resistive changes in the
crystals.

31

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

MOISTURE SENSING ELEMENTS

Elements that sense relative humidity fall generally into two
classes: mechanical and electronic. Mechanical elements
expand and contract as the moisture level changes and are
called “hygroscopic” elements. Several hygroscopic elements
can be used to produce mechanical output, but nylon is the
most commonly used element (Fig. 52). As the moisture
content of the surrounding air changes, the nylon element
absorbs or releases moisture, expanding or contracting,
respectively. The movement of the element operates the
controller mechanism.

NYLON ELEMENT

LOW HIGH

RELATIVE HUMIDITY SCALE

C2084

FLOW SENSORS

Flow sensors sense the rate of liquid and gas flow in volume
per unit of time. Flow is difficult to sense accurately under all
conditions. Selecting the best flow-sensing technique for an
application requires considering many aspects, especially the
level of accuracy required, the medium being measured, and
the degree of variation in the measured flow.

A simple flow sensor is a vane or paddle inserted into the
medium (Fig. 53) and generally called a flow switch. The
paddle is deflected as the medium flows and indicates that the
medium is in motion and is flowing in a certain direction. Vane
or paddle flow sensors are used for flow indication and
interlock purposes (e.g., a system requires an indication that
water is flowing before the system starts the chiller).

ON/OFF SIGNAL

TO CONTROLLER

SENSOR

PIVOT

FLOW

Fig. 52. Typical Nylon Humidity Sensing Element.

Electronic sensing of relative humidity is fast and accurate.
An electronic relative humidity sensor responds to a change
in humidity by a change in either the resistance or capacitance
of the element.

If the moisture content of the air remains constant, the
relative humidity of the air increases as temperature decreases
and decreases as temperature increases. Humidity sensors also
respond to changes in temperature. If the relative humidity is
held constant, the sensor reading can be affected by temperature
changes. Because of this characteristic, humidity sensors
should not be used in atmospheres that experience wide
temperature variations unless temperature compensation is
provided. Temperature compensation is usually provided with
nylon elements and can be factored into electronic sensor
values, if required.

Dew point is the temperature at which vapor condenses. A
dew point sensor senses dew point directly. A typical sensor
uses a heated, permeable membrane to establish an equilibrium
condition in which the dry-bulb temperature of a cavity in the
sensor is proportional to the dew point temperature of the
ambient air. Another type of sensor senses condensation on a
cooled surface. If the ambient dry-bulb and dew point
temperature are known, the relative humidity, total heat, and
specific humidity can be calculated. Refer to the Psychrometric
Chart Fundamentals section of this manual.

PADDLE (PERPENDICULAR TO FLOW)

C2085

Fig. 53. Paddle Flow Sensor.

Flow meters measure the rate of fluid flow. Principle types
of flow meters use orifice plates or vortex nozzles which
generate pressure drops proportional to the square of fluid
velocity. Other types of flow meters sense both total and static
pressure, the difference of which is velocity pressure, thus
providing a differential pressure measurement. Paddle wheels
and turbines respond directly to fluid velocity and are useful
over wide ranges of velocity.

In a commercial building or industrial process, flow meters
can measure the flow of steam, water, air, or fuel to enable
calculation of energy usage needs.

Airflow pickups, such as a pitot tube or flow measuring
station (an array of pitot tubes), measure static and total
pressures in a duct. Subtracting static pressure from total
pressure yields velocity pressure, from which velocity can be
calculated. Multiplying the velocity by the duct area yields
flow. For additional information, refer to the Building Airflow
System Control Applications section of this manual.

Applying the fluid jet principle allows the measurement of
very small changes in air velocity that a differential pressure
sensor cannot detect. A jet of air is emitted from a small tube
perpendicular to the flow of the air stream to be measured.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

32

The impact of the jet on a collector tube a short distance away
causes a positive pressure in the collector. An increase in
velocity of the air stream perpendicular to the jet deflects the
jet and decreases pressure in the collector. The change in
pressure is linearly proportional to the change in air stream
velocity.

Another form of air velocity sensor uses a microelectronic
circuit with a heated resistance element on a microchip as the
primary velocity sensing element. Comparing the resistance
of this element to the resistance of an unheated element
indicates the velocity of the air flowing across it.

PROOF-OF-OPERATION SENSORS

Proof-of-operation sensors are often required for equipment
safety interlocks, to verify command execution, or to monitor
fan and pump operation status when a central monitoring and
management system is provided. Current-sensing relays,
provided with current transformers around the power lines to
the fan or pump motor, are frequently used for proof-of­operation inputs. The contact closure threshold should be set
high enough for the relay to drop out if the load is lost (broken
belt or coupling) but not so low that it drops out on a low
operational load.

Current-sensing relays are reliable, require less maintenance,
and cost less to install than mechanical duct and pipe devices.

CONTROL FUNDAMENTALS

Controllers may be electric/electronic, microprocessor, or
pneumatic. An electric/electronic controller provides two­position, floating, or modulating control and may use a
mechanical sensor input such as a bimetal or an electric input
such as a resistance element or thermocouple. A
microprocessor controller uses digital logic to compare input
signals with the desired result and computes an output signal
using equations or algorithms programmed into the controller.
Microprocessor controller inputs can be analog or on/off
signals representing sensed variables. Output signals may be
on/off, analog, or pulsed. A pneumatic controller receives input
signals from a pneumatic sensor and outputs a modulating
pneumatic signal.

ACTUATORS

An actuator is a device that converts electric or pneumatic
energy into a rotary or linear action. An actuator creates a
change in the controlled variable by operating a variety of final
control devices such as valves and dampers.

In general, pneumatic actuators provide proportioning or
modulating action, which means they can hold any position in
their stroke as a function of the pressure of the air delivered to
them. Two-position or on/off action requires relays to switch
from zero air pressure to full air pressure to the actuator.

TRANSDUCERS

Transducers convert (change) sensor inputs and controller
outputs from one analog form to another, more usable, analog
form. A voltage-to-pneumatic transducer, for example, converts
a controller variable voltage input, such as 2 to 10 volts, to a
linear variable pneumatic output, such as 3 to 15 psi. The
pneumatic output can be used to position devices such as a
pneumatic valve or damper actuator. A pressure-to-voltage
transducer converts a pneumatic sensor value, such as 2 to 15
psi, to a voltage value, such as 2 to 10 volts, that is acceptable
to an electronic or digital controller.

CONTROLLERS

Controllers receive inputs from sensors. The controller
compares the input signal with the desired condition, or
setpoint, and generates an output signal to operate a controlled
device. A sensor may be integral to the controller (e.g., a
thermostat) or some distance from the controller.

Electric control actuators are two-position, floating, or
proportional (refer to CONTROL MODES). Electronic
actuators are proportional electric control actuators that require
an electronic input. Electric actuators are bidirectional, which
means they rotate one way to open the valve or damper, and
the other way to close the valve or damper. Some electric
actuators require power for each direction of travel. Pneumatic
and some electric actuators are powered in one direction and
store energy in a spring for return travel.

Figure 54 shows a pneumatic actuator controlling a valve.
As air pressure in the actuator chamber increases, the
downward force (F1) increases, overcoming the spring
compression force (F2), and forcing the diaphragm downward.
The downward movement of the diaphragm starts to close the
valve. The valve thus reduces the flow in some proportion to
the air pressure applied by the actuator. The valve in Figure 54
is fully open with zero air pressure and the assembly is therefore
normally open.

33

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

ACTUATOR

CHAMBER

F1

VALVE

AIR

PRESSURE

SPRING

FLOW

DIAPHRAGM

F2

Fig. 54. Typical Pneumatic Valve Actuator.

C2086

Electric actuators are inherently positive positioning. Some
pneumatic control applications require accurate positioning
of the valve or damper. For pneumatic actuators, a positive
positioning relay is connected to the actuator and ensures that
the actuator position is proportional to the control signal. The
positive positioning relay receives the controller output signal,
reads the actuator position, and repositions the actuator
according to the controller signal, regardless of external loads
on the actuator.

Electric actuators can provide proportional or two-position
control action. Figure 56 shows a typical electric damper
actuator. Spring-return actuators return the damper to either
the closed or the open position, depending on the linkage, on a
power interruption.

ACTUATOR

DAMPER

CRANK ARM

PUSH ROD

A pneumatic actuator similarly controls a damper. Figure
55 shows pneumatic actuators controlling normally open and
normally closed dampers.

NORMALLY
OPEN DAMPER

AIR
PRESSURE

ACTUATOR ACTUATOR

SPRING

PISTON

ROLLING
DIAPHRAGM

NORMALLY
CLOSED DAMPER

AIR
PRESSURE

C2087

Fig. 55. Typical Pneumatic Damper Actuator.

C2721

Fig. 56. Typical Electric Damper Actuator.

AUXILIARY EQUIPMENT

Many control systems can be designed using only a sensor,
controller, and actuator. In practice, however, one or more
auxiliary devices are often necessary.

Auxiliary equipment includes transducers to convert signals
from one type to another (e.g., from pneumatic to electric),
relays and switches to manipulate signals, electric power and
compressed air supplies to power the control system, and
indicating devices to facilitate monitoring of control system
activity.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

34

CONTROL FUNDAMENTALS

CHARACTERISTICS AND ATTRIBUTES OF CONTROL METHODS

Review the columns of Table 4 to determine the characteristics and attributes of pneumatic, electric, electronic, and

microprocessor control methods.

Table 4. Characteristics and Attributes of Control Methods.

Pneumatic Electric Electronic Microprocessor

Naturally
proportional

Requires clean
dry air

Air lines may
cause trouble
below freezing

Explosion proof

Simple, powerful,
low cost, and
reliable actuators
for large valves
and dampers

Simplest
modulating
control

Most common for
simple on-off
control

Integral sensor/
controller

Simple sequence
of control

Broad
environmental
limits

Complex
modulating
actuators,
especially when
spring-return

Precise control

Solid state
repeatability and
reliability

Sensor may be
up to 300 feet
from controller

Simple, remote,
rotary knob
setpoint

High per-loop
cost

Complex
actuators and
controllers

Precise control

Inherent energy management

Inherent high order (proportional plus integral)
control, no undesirable offset

Compatible with building management system.
Inherent database for remote monitoring,
adjusting, and alarming.

Easily performs a complex sequence of control

Global (inter-loop), hierarchial control via
communications bus (e.g., optimize chillers based
upon demand of connected systems)

Simple remote setpoint and display (absolute
number, e.g., 74.4)

Can use pneumatic actuators

35

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

ENGINEERING MANUAL OF AUTOMATIC CONTROL

36

PSYCHROMETRIC CHART FUNDAMENTALS

Psychrometric Chart

Fundamentals

Contents

Introduction ............................................................................................................ 38

Definitions ............................................................................................................ 38

Description of the Psychrometric Chart ............................................................................................................ 39

The Abridged Psychrometric Chart ............................................................................................................ 40

Examples of Air Mixing Process ............................................................................................................ 42

Air Conditioning Processes ............................................................................................................ 43

Heating Process .................................................................................. 43

Cooling Process .................................................................................. 44

Humidifying Process ............................................................................................................ 44

Basic Process...................................................................................... 44

Steam Jet Humidifier....................................................................... 46

Air Washes...................................................................................... 49

Vaporizing Humidifier ...................................................................... 50

Cooling and Dehumidification.............................................................. 51

Basic Process ................................................................................. 51

Air Washes...................................................................................... 51

Dehumidification and Reheat .............................................................. 52

Process Summary ............................................................................... 53

ASHRAE Psychrometric Charts ............................................................................................................ 53

37

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PSYCHROMETRIC CHART FUNDAMENTALS

INTRODUCTION

This section provides information on use of the psychrometric
chart as applied to air conditioning processes. The chart provides
a graphic representation of the properties of moist air including
wet- and dry-bulb temperature, relative humidity, dew point,
moisture content, enthalpy, and air density. The chart is used to
plot the changes that occur in the air as it passes through an air
handling system and is particularly useful in understanding these

DEFINITIONS

To use these charts effectively, terms describing the
thermodynamic properties of moist air must be understood.
Definition of these terms follow as they relate to the
psychrometric chart. Additional terms are included for devices
commonly used to measure the properties of air.

Adiabatic process: A process in which there is neither loss

nor gain of total heat. The heat merely changes from
sensible to latent or latent to sensible.

British thermal unit (Btu): The amount of heat required to

raise one pound of water one degree Fahrenheit.

Density: The mass of air per unit volume. Density can be

expressed in pounds per cubic foot of dry air. This is
the reciprocal of specific volume.

Dew point temperature: The temperature at which water

vapor from the air begins to form droplets and settles
or condenses on surfaces that are colder than the dew
point of the air. The more moisture the air contains,
the higher its dew point temperature. When dry-bulb
and wet-bulb temperatures of the air are known, the
dew point temperature can be plotted on the
psychrometric chart (Fig. 4).

Dry-bulb temperature: The temperature read directly on an

ordinary thermometer.

Isothermal process: A process in which there is no change of

dry-bulb temperature.

Latent heat: Heat that changes liquid to vapor or vapor to

liquid without a change in temperature or pressure of
the moisture. Latent heat is also called the heat of
vaporization or condensation. When water is
vaporized, it absorbs heat which becomes latent heat.
When the vapor condenses, latent heat is released,
usually becoming sensible heat.

changes in relation to the performance of automatic HVAC
control systems. The chart is also useful in troubleshooting a
system.

For additional information about control of the basic
processes in air handling systems, refer to the Air Handling
System Control Applications section.

Moisture content (humidity ratio): The amount of water

contained in a unit mass of dry air. Most humidifiers
are rated in grains of moisture per pound of dry air
rather than pounds of moisture. To convert pounds to
grains, multiply pounds by 7000 (7000 grains equals
one pound).

Relative humidity: The ratio of the measured amount of

moisture in the air to the maximum amount of moisture
the air can hold at the same temperature and pressure.
Relative humidity is expressed in percent of saturation.
Air with a relative humidity of 35, for example, is
holding 35 percent of the moisture that it is capable of
holding at that temperature and pressure.

Saturation: A condition at which the air is unable to hold any

more moisture at a given temperature.

Sensible heat: Heat that changes the temperature of the air

without changing its moisture content. Heat added to
air by a heating coil is an example of sensible heat.

Sling psychrometer: A device (Fig. 1) commonly used to

measure the wet-bulb temperature. It consists of two
identical thermometers mounted on a common base.
The base is pivoted on a handle so it can be whirled
through the air. One thermometer measures dry-bulb
temperature. The bulb of the other thermometer is
encased in a water-soaked wick. This thermometer
measures wet-bulb temperature. Some models provide
slide rule construction which allows converting the
dry-bulb and wet-bulb readings to relative humidity.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

38

PSYCHROMETRIC CHART FUNDAMENTALS

WATER-SOAKED WICK

DRY-BULB THERMOMETER

WET-BULB THERMOMETER

PIVOT

Fig. 1. Sling Psychrometer.

Although commonly used, sling psychrometers can
cause inaccurate readings, especially at low relative
humidities, because of factors such as inadequate air
flow past the wet-bulb wick, too much wick wetting
from a continuous water feed, thermometer calibration
error, and human error. To take more accurate readings,
especially in low relative humidity conditions,
motorized psychrometers or hand held electronic
humidity sensors are recommended.

Specific volume: The volume of air per unit of mass. Specific

volume can be expressed in cubic feet per pound of
dry air. The reciprocal of density.

Total heat (also termed enthalpy): The sum of sensible and

latent heat expressed in Btu or calories per unit of mass
of the air. Total heat, or enthalpy, is usually measured
from zero degrees Fahrenheit for air. These values are
shown on the ASHRAE Psychrometric Charts in
Figures 33 and 34.

HANDLE

RELATIVE HUMIDITY SCALE

C1828

Wet-bulb temperature: The temperature read on a thermom-

eter with the sensing element encased in a wet wick
(stocking or sock) and with an air flow of 900 feet
per minute across the wick. Water evaporation causes
the temperature reading to be lower than the ambient
dry-bulb temperature by an amount proportional to
the moisture content of the air. The temperature re­duction is sometimes called the evaporative effect.
When the reading stops falling, the value read is the
wet-bulb temperature.

The wet-bulb and dry-bulb temperatures are the easiest
air properties to measure. When they are known, they
can be used to determine other air properties on a
psychrometric chart.

DESCRIPTION OF THE PSYCHROMETRIC CHART

The ASHRAE Psychrometric Chart is a graphical represen­tation of the thermodynamic properties of air. There are five
different psychrometric charts available and in use today:

Chart No. 1 — Normal temperatures, 32 to 100F
Chart No. 2 — Low temperatures, –40 to 50F
Chart No. 3 — High temperatures, 50 to 250F
Chart No. 4 — Normal temperature at 5,000 feet above

sea level, 32 to 120F

Chart No. 5 — Normal temperature at 7,500 feet above

sea level, 32 to 120F

Chart No. 1 can be used alone when no freezing temperatures
are encountered. Chart No. 2 is very useful, especially in
locations with colder temperatures. To apply the lower range
chart to an HVAC system, part of the values are plotted on
Chart No. 2 and the resulting information transferred to Chart
No. 1. This is discussed in the EXAMPLES OF AIR MIXING
PROCESS section. These two charts allow working within the
comfort range of most systems. Copies are provided in the
ASHRAE PSYCHROMETRIC CHARTS section.

39

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PSYCHROMETRIC CHART FUNDAMENTALS

THE ABRIDGED PSYCHROMETRIC CHART

Figure 2 is an abridged form of Chart No. 1. Some of the
scale lines have been removed to simplify illustrations of the
psychrometric processes. Smaller charts are used in most of
the subsequent examples. Data in the examples is taken from
full-scale charts

The major lines and scales on the abridged psychrometric
chart identified in bold letters are:

—Dry-bulb temperature lines
—Wet-bulb temperature lines
— Enthalpy or total heat lines
— Relative humidity lines
— Humidity ratio or moisture content lines
—Saturation temperature or dew point scale
—Volume lines in cubic feet per pound of dry air

5000

3000

1.0

2000

0.8

0.6

0.5

0.4

1500

S

E

N

S

T

I

O

0.3

B

L

T

E

A

H

E

L

H

E

A

T

0.2

0.1

E

N

T

H

A

H

U

M

I

D

I

T

Y

∆H

A

T

=

-0.3

-0.1

1000

L

P

Y

R

A

I

O

T

ENTHALPY—

20

15

35

12.5

35

45

40

35

40

45

10

1.0

2.0

4.0

-1000

∞

0

-4.0

S

∆H

-0.5

∆ h

∆ W

BTU PER POUND OF DRY AIR

25

-2.0

T

-1.0

500

30

50

13.0

40

505055

=

35

SATURATIO

55

45

N TEM

60

60

40

70

PERATURE -

65

13.5

50

65

70

15

°F

Fig. 2. Abridged Chart No. 1.

The chart also contains a protractor nomograph with the

following scales:

— Enthalpy/humidity ratio scale
— Sensible heat/total heat ratio scale

When lines are drawn on the chart indicating changes in

psychrometric conditions, they are called process lines.

With the exception of relative humidity, all lines are straight.
Wet-bulb lines and enthalpy (total heat) lines are not exactly
the same so care must be taken to follow the correct line. The
dry-bulb lines are not necessarily parallel to each other and
incline slightly from the vertical position. The purpose of the
two enthalpy scales (one on the protractor and one on the chart)
is to provide reference points when drawing an enthalpy (total

50

85

45

80

75

80%

14.5

VOLUME-CU FT PER POUND OF DRY AIR

REALTIVE HUMIDITY - %

80

60%

40%

70

14.0

65

60

55

75

WET BULB -

°F

20%

DRY BULB - °F

75

80

85

90

95

20

100

105

15.0

85

110

25

115

.030

.028

.026

.024

.022

.020

.018

.016

.014

.012

.010

.008

.006

.004

HUMIDITY RATIO(W) - POUNDS OF MOISTURE PER POUND OF DRY AIR

.002

M10306

120

60

55

45

40

35

30

ENGINEERING MANUAL OF AUTOMATIC CONTROL

40

heat) line. The protractor nomograph, in the upper left corner,

C

D

A

E

B

LATENT HEAT

18.7 BTU/LB

SENSIBLE
HEAT

60% R.H.

C1831

77°F DB

31.6 BTU/LB

is used to establish the slope of a process line. The mechanics
of constructing this line are discussed in more detail in the
STEAM JET HUMIDIFIERS section.

PSYCHROMETRIC CHART FUNDAMENTALS

31.6 BTU/LB
D

60% RH

The various properties of air can be determined from the chart
whenever the lines of any two values cross even though all
properties may not be of interest. For example, from the point
where the 70F dry-bulb and 60F wet-bulb lines cross (Fig. 3,
Point A), the following additional values can be determined:

D

26.3
BTU/LB

B

54°F DP

A

70°F DB

56% RH

13.505 CF/LB

60°F WB

C

0.0088 LB/LB

C1829

Fig. 3.

— Relative humidity is 56 percent (Point A)
—Volume is 13.505 cubic feet per pound of dry air

(Point A)
—Dew point is 54F (Point B)
—Moisture content is 0.0088 pounds of moisture per pound

of dry air (Point C)
— Enthalpy (total heat) is 26.3 Btu per pound of dry air

(Point D)
— Density is 0.074 pounds per cubic foot (reciprocal of

volume)

C

62.5°F DP

B

E

77°F DB

A

67.5°F WB

13.8 CF/LB

0.012 LB/LB

C1830

Fig. 4.

Figure 5 is the same as Figure 4 but is used to obtain latent
heat and sensible heat values. Figures 4 and 5 indicate that the
enthalpy (total heat) of the air is 31.6 Btu per pound of dry air
(Point D). Enthalpy is the sum of sensible and latent heat (Line
A to E + Line E to D, Fig. 5). The following process determines
how much is sensible heat and how much is latent heat. The
bottom horizontal line of the chart represents zero moisture
content. Project a constant enthalpy line to the enthalpy scale
(from Point C to Point E). Point E enthalpy represents sensible
heat of 18.7 Btu per pound of dry air. The difference between
this enthalpy reading and the original enthalpy reading is latent
heat. In this example 31.6 minus 18.7 equals 12.9 Btu per pound
of dry air of latent heat. When the moisture content of the air
changes but the dry-bulb temperature remains constant, latent
heat is added or subtracted.

Figure 4 is another plotting example. This time the dry-bulb
temperature line and relative humidity line are used to establish
the point. With the relative humidity equal to 60 percent and
the dry-bulb temperature at 77F (Fig. 4, Point A), the following
values can be read:

—Wet-bulb temperature is 67.5F (Point A)
—Volume is 13.8 cubic feet per pound of dry air (Point A)
— Dew point is 62.5F (Point B)
— Moisture content is 0.012 pounds of moisture per pound

of dry air (Point C)
— Enthalpy is 31.6 Btu per pound of dry air (Point D)
— Density is 0.0725 pounds per cubic foot (reciprocal of

volume)

41

Fig. 5.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PSYCHROMETRIC CHART FUNDAMENTALS

EXAMPLES OF AIR MIXING PROCESS

The following examples illustrate use of the psychrometric
chart to plot values and determine conditions in a ventilating
system. The examples also show how to obtain the same results
by calculation. Example A requires only Chart No. 1. Example
B requires both Charts No. 1 and 2 since the outdoor air
temperature is in the range of Chart No. 2.

EXAMPLE A:

Plotting values where only Chart No. 1 (Fig. 6) is required.

RA

A

75°F DB

MA
62°F DB

62.5°F WB

C1834

B

OA
36°F DB
40% RH

C

Fig. 6. Example A, Chart No. 1.

In this example:

1. A fixed quantity of two-thirds return air and one-third
outdoor air is used.

2. The return air condition is 75F dry bulb and 62.5F wet
bulb.

3. Outdoor air condition is 36F dry bulb and 40 percent rh.

covers the –40 to 50F temperature range. This is the temperature
range immediately below that of Chart No. 1. Note that there is
an overlap of temperatures between 35F and 50F. The overlap
is important when transferring values from one chart to another.

RA

DA

C2055

SUPPLY
FAN

OA

N.C.

Fig. 7. Example B, Ventilating System.

This example illustrates mixing two different air conditions
with no change in total heat (enthalpy). Any changes in the
total heat required to satisfy space conditions are made by
heating, cooling, humidification, or dehumidification after the
air is mixed.

In this example:

1. A fixed quantity of two-thirds return air and one-third
outdoor air is used.

2. The return air condition is 75F dry bulb and 62.5F wet
bulb.

3. Outdoor air condition is 10F dry bulb and 50 percent rh.

To find the mixed air condition:

1. Plot the outdoor air (OA) condition on Chart No. 2,
Fig. 8

To find the mixed air conditions at design:

1. Plot the return air (RA) condition (Point A) and outdoor
air (OA) condition (Point B).

2. Connect the two points with a straight line.

3. Calculate the mixed air dry-bulb temperature:

(2/3 x 75) + (1/3 x 36) = 62F dry bulb

4. The mixed air conditions are read from the point at which
the line, drawn in Step 2, intersects the 62F dry-bulb line
(Point C).

EXAMPLE B:

Plotting values when both Chart No. 1 and Chart No. 2 are

required.

In this example, a ventilating system (Fig. 7) is used to
illustrate how to plot data on Chart No. 2 and transfer values to
Chart No. 1. Chart No. 2 is similar to Chart No. 1 except that it

ENGINEERING MANUAL OF AUTOMATIC CONTROL

3.1 BTU/LB

OA
10°F DB
50% RH

.

B

0.00065 LB/LB

C1833

Fig. 8. Example B, Chart No. 2.

2. Plot the return air (RA) condition on Chart No. 1, Fig. 9.

42

28.2

B

A

55°F DB
40% RH

85°F DB
12% RH

24.4
BTU/LB

17.1
BTU/LB

0.0035 LB/LB

C1835

BTU/LB

RA
75°F DB

19.8
BTU/LB

FROM CHART 2

C

MA

53.3°F DB
49°F WB

A

62.5°F WB

Fig. 9. Example B, Chart No. 1

3. Calculate the mixed air enthalpy as follows:
a. For the return air, project a line parallel to the

enthalpy line from Point A to the enthalpy scale
on Figure 9. The value is 28.2 Btu per pound of
dry air.

b. For the outdoor air, project a line parallel to the

enthalpy line from Point B to the enthalpy scale on
Figure 8. The value is 3.1 Btu per pound of dry air.

c. Using the determined values, calculate the mixed

air enthalpy:
(2/3 x 28.2) + (1/3 x 3.1) = 19.8 Btu per

pound of dry air

0.0094 LB/LB

0.00648 LB/LB

0.00065 LB/LB

C1832

PSYCHROMETRIC CHART FUNDAMENTALS

4. Calculate the mixed air moisture content as follows:
a. For the return air, project a line from Point A hori-

zontally to the moisture content scale on Figure 9.
The value is 0.0094 pounds of moisture per pound
of dry air.

b. For the outdoor air, project a line from Point B

horizontally to the moisture content scale on
Figure 8. The value is 0.00065 pounds of moisture
per pound of dry air. Also, project this value on to
Chart No. 1 as shown in Figure 9.

c. Using the determined values, calculate the mixed

air moisture content:
(2/3 x 0.0094) + (1/3 x 0.00065) = 0.00648 pounds

of moisture per pound of dry air

5. Using the enthalpy value of 19.8 and the moisture content

value of 0.00648, plot the mixed air conditions, Point C,
on Chart No. 1, Figure 9, by drawing a horizontal line
across the chart at the 0.00648 moisture content level
and a diagonal line parallel to the enthalpy lines starting
at the 19.8 Btu per pound of dry air enthalpy point. Point
C yields 53.3F dry-bulb and 49F wet-bulb temperature.

6. Read other conditions for the mixed air (MA) from Chart

No. 1 as needed.

AIR CONDITIONING PROCESSES

HEATING PROCESS

The heating process adds sensible heat to the system and
follows a constant, horizontal moisture line. When air is heated
by a steam or hot water coil, electric heat, or furnace, no
moisture is added. Figure 10 illustrates a fan system with a
heating coil. Figure 11 illustrates a psychrometric chart for this
system. Air is heated from 55F dry bulb to 85F dry bulb
represented by Line A-B. This is the process line for heating.
The relative humidity drops from 40 percent to 12 percent and
the moisture content remains 0.0035 pounds of moisture per
pound of air. Determine the total heat added as follows:

HEATING COIL

55°F DB
40% RH

85°F DB
12% RH

Fig. 10. Fan System with Heating Coil.

SUPPLY FAN

AIR FLOW

C2056

Fig. 11.

43

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PSYCHROMETRIC CHART FUNDAMENTALS

1. Draw diagonal lines parallel to the constant enthalpy lines
from Points A and B to the enthalpy scale.

2. Read the enthalpy on the enthalpy scale.

3. Calculate the enthalpy added as follows:

Total heat at Point B – total heat at Point A = total
heat added.

24.4 – 17.1 = 7.3 Btu per pound of dry air

Since there is no change in moisture content, the total heat added
is all sensible. Whenever the process moves along a constant
moisture line, only sensible heat is changed.

COOLING PROCESS

The cooling process removes sensible heat and, often, latent
heat from the air. Consider a condition where only sensible
heat is removed. Figure 12 illustrates a cooling process where
air is cooled from 90F to 70F but no moisture is removed. Line
A-B represents the process line for cooling. The relative
humidity in this example increases from 50 percent (Point A)
to 95 percent (Point B) because air at 70F cannot hold as much
moisture as air at 90F. Consequently, the same amount of
moisture results in a higher percentage relative humidity at 70F
than at 90F. Calculate the total heat removed as follows:

COOLING COIL

90°F DB
50% RH

37.9 BTU/LB

33.3 BTU/LB

70°F DB
95% RH

B

70°F DB
95% RH

SUPPLY FAN

A

90°F DB
50% RH

AIRFLOW

C1836

Fig. 12.

Total heat at Point A - total heat at Point B =

total heat removed.

37.9 – 33.3 = 4.6 Btu per pound of dry air

This is all sensible heat since there is no change in moisture
content.

HUMIDIFYING PROCESS

BASIC PROCESS

The humidifying process adds moisture to the air and crosses
constant moisture lines. If the dry bulb remains constant, the
process involves the addition of latent heat only.

Relative humidity is the ratio of the amount of moisture in
the air to the maximum amount of moisture the air can hold at
the same temperature and pressure. If the dry-bulb temperature
increases without adding moisture, the relative humidity
decreases. The psychrometric charts in Figures 13 and 14
illustrate what happens. Referring to Chart No. 2 (Fig. 13),
outdoor air at 0F dry bulb and 75 percent rh (Point A) contains
about 0.0006 pounds of moisture per pound of dry air. The

0.0006 pounds of moisture per pound of dry air is carried over
to Chart No. 1 (Fig. 14) and a horizontal line (constant moisture
line) is drawn.

A

0°F DB
75% RH

Fig. 13. Chart No. 2.

0.0006 LB/LB

C1837

ENGINEERING MANUAL OF AUTOMATIC CONTROL

44

PSYCHROMETRIC CHART FUNDAMENTALS

SUPPLY FAN
10,000 CFM

70°F DB
35% RH

DA

0°F DB
75% RH

OA

HEATING COIL

70°F DB

4.5% RH

FROM CHART 2

70°F DB

4.5% RH

A

0.0006 LB/LB

C1838

Fig. 14. Chart No. 1.

The outdoor air (0F at 75 percent rh) must be heated to a
comfortable indoor air level. If the air is heated to 70F, for
example, draw a vertical line at that dry-bulb temperature. The
intersection of the dry-bulb line and the moisture line determines
the new condition. The moisture content is still 0.0006 pounds
of moisture per pound of dry air, but the relative humidity drops
to about 4.5 percent (Point A, Fig. 14). This indicates a need to
add moisture to the air. Two examples of the humidifying
process follow.

EXAMPLE 1:

Determine the amount of moisture required to raise the
relative humidity from 4.5 percent to 35 percent when the air
temperature is raised from 0 to 70F and then maintained at a
constant 70F.

Figure 15 provides an example of raising the relative humidity
by adding moisture to the air. Assume this example represents
a room that is 30 by 40 feet with an 8-foot ceiling and two air
changes per hour. Determine how much moisture must be added
to raise the relative humidity to 35 percent (Point B).

35% RH

FROM CHART 2

B

A

70°F DB

4.5% RH

0.0056 LB/LB

0.0006 LB/LB

C1839

Fig. 15.

The space contains the following volume:

30 x 40 x 8 = 9600 cubic feet

Two air changes per hour is as follows:

2 x 9600 = 19,200 cubic feet per hour

This amount of air is brought into the room, heated to 70F, and
humidified. Chart No. 2 (Fig. 13) illustrates that outdoor air at
0F has a volume of 11.5 cubic feet per pound. The reciprocal
of this provides the density or 0.087 pounds per cubic foot.
Converting the cubic feet per hour of air to pounds per hour
provides:

19,200 x 0.087 = 1670 pounds of air per hour

For the space in the example, the following moisture must

be added:

1670 x 0.005 = 8.5 pounds of water per hour

To raise the relative humidity from 4.5 percent (Point A) to
35 percent (Point B) at 70F, the moisture to be added can be
determined as follows:

1. The moisture content required for 70F air at 35 percent
rh is 0.0056 pounds of moisture per pound of dry air.

2. The moisture content of the heated air at 70F and

4.5 percent rh is 0.0006 pounds of moisture per pound
of dry air.

3. The moisture required is:

0.0056 – 0.0006 = 0.005 pounds of moisture per
pound of dry air

Line A-B, Figure 15, represents this humidifying process on
the psychrometric chart.

Since a gallon of water weighs 8.34 pounds, it takes about one
gallon of water per hour to raise the space humidity to 35 percent
at 70F.

EXAMPLE 2:

Determine the moisture required to provide 75F air at

50 percent rh using 50F air at 52 percent rh.

In this example, assume that 10,000 cubic feet of air per
minute must be humidified. First, plot the supply air Point A,
Figure 16, at 50F and 52 percent rh. Then, establish the condition
after the air is heated to 75F dry bulb. Since the moisture content
has not changed, this is found at the intersection of the
horizontal, constant moisture line (from Point A) and the
vertical 75F dry-bulb temperature line (Point B).

45

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PSYCHROMETRIC CHART FUNDAMENTALS

The air at Points A and B has 0.004 pounds of moisture per
pound of air. While the moisture content remains the same after
the air is heated to 75F (Point B), the relative humidity drops
from 52 percent to 21 percent. To raise the relative humidity to
50 percent at 75F, find the new point on the chart (the
intersection of the 75F dry-bulb line and the 50 percent rh curve
or Point C). The moisture content at this point is 0.009 pounds
of moisture per pound of dry air. Calculate the moisture to be
added as follows:

0.009 – 0.004 = 0.005 pounds of moisture per
pound of dry air

Line B-C in Figure 16 represents this humidifying process on
the psychrometric chart.

SUPPLY FAN

50°F DB
52% RH

MA

HEATING COIL

A

50°F DB
52% RH

75°F DB
21% RH

75°F DB

10,000 CFM

C

B

21% RH

13.56 CF/LB

75°F DB
50% RH

DA

50% RH

0.009 LB/LB

0.004 LB/LB

C1840

Fig. 16.

If each pound of dry air requires 0.005 pounds of moisture,

then the following moisture must be added:

736 x 0.005 = 3.68 pounds of moisture per minute

This converts to:

3.68 x 60 minutes = 220.8 pounds per hour

Since one gallon of water weighs 8.34 pounds, the moisture to
be added is as follows:

220.8 ÷ 8.34 = 26.5 gallons per hour

Thus, a humidifier must provide 26.5 gallons of water per hour
to raise the space humidity to 50 percent at 75F.

STEAM JET HUMIDIFIER

The most popular humidifier is the steam-jet type. It consists
of a pipe with nozzles partially surrounded by a steam jacket.
The jacket is filled with steam; then the steam is fed through
nozzles and sprayed into the air stream. The jacket minimizes
condensation when the steam enters the pipe with the nozzles
and ensures dry steam for humidification. The steam is sprayed
into the air at a temperature of 212F or higher. The enthalpy
includes the heat needed to raise the water temperature from
32 to 212F, or 180 Btu plus 970 Btu to change the water into
steam. This is a total of 1150 Btu per hour per pound of water
at 0 psig as it enters the air stream. (See Properties of Saturated
Steam table in General Engineering Data section). The
additional heat added to the air can be plotted on Chart No. 1
(Figure 17) to show the complete process. In this example, air
enters the heating coil at 55F dry-bulb temperature (Point A)
and is heated to 90F dry-bulb temperature (Point B) along a
constant moisture line. It then enters the humidifier where the
steam adds moisture and heats the air to Point C.

At 75F and 21 percent relative humidity, the psychromet-

ric chart shows that the volume of one pound of air is about

13.58 cubic feet. There are two ways to find the weight of the
air. One way is to use the volume to find the weight. Assuming
10,000 cubic feet of air:

10,000 ÷ 13.58 = 736 pounds of air

The other way is to use the density to find the weight. The
reciprocal of the volume provides the density as follows:

1 ÷ 13.58 = 0.0736 pounds per cubic foot

The weight is then:
10,000 x 0.0736 = 736 pounds of air per minute

ENGINEERING MANUAL OF AUTOMATIC CONTROL

Figure 17 also shows use of the protractor nomograph.
Assume the relative humidity of the air entering the
humidifier at Point B is to be raised to 50 percent. A
process line can be constructed using the protractor
nomograph. The total heat of the entering steam in Btu
per pound is located on the enthalpy/humidity ratio scale
(∆h / ∆W) of the nomograph. This value, 1150 Btu per
pound, is connected to the reference point of the
nomograph to establish the slope of the process line on
the psychrometric chart. A parallel line is drawn on the
chart from Point B up to the 50 percent relative humidity
line (Point C). The Line B-C is the process line. The Line
X-Y (bottom of the chart) is simply a perpendicular
construction line for drawing the Line B-C parallel to the
line determined on the nomograph. Note that the dry-bulb
temperature increased from 90 to 92F.

46

REFERENCE POINT

PSYCHROMETRIC CHART FUNDAMENTALS

5000

1.0

0.8

3000

0.6

0.5

2000

1500

THIS LINE IS
PARALLEL
TO THE SOLID
LINE C-B ON
THE PSYCH
CHART

0.4

0.3

1.0

∆ h

S

∆H

-0.5

∆ W

2.0

4.0

-1000

∞

0

-4.0

-2.0

T

-1.0

500

50% RH

C

0.0164 LB/LB

B

0.0065 LB/LB

S

E

N

S

T

I

O

B

L

T

E

A

H

E

L

H

E

A

T

0.2

0.1

E

N

T

H

A

H

U

M

I

D

I

T

Y

1150

∆H

A

T

=

-0.3

-0.1

1000

L

P

Y

=

R

A

I

O

T

A

55°F DB

X

CONSTRUCTION LINE

Y

92°F DB

90°F DB

C1841

Fig. 17.

47

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PSYCHROMETRIC CHART FUNDAMENTALS

Figure 18 is the same as the chart shown in Figure 17 except
that it graphically displays the amount of heat added by the
process. Enthalpy (total heat) added is determined by
subtracting the enthalpy of the dry, heated air at Point B from
the enthalpy of the humidified air at Point C as follows:

40.3 – 28.7 = 11.6 Btu per pound of dry air

The steam raised the temperature of the air from 90F dry bulb
to 92F dry bulb. To find the latent heat added by the steam
humidifier to the air, determine the enthalpy at Point D (the
enthalpy of the heated air without added moisture) and subtract
it from the enthalpy of the humidified air at Point C. This is
as follows:

40.3 – 29.6 = 10.7 Btu per pound of dry air

REFERENCE POINT

5000

3000

1.0

2000

0.8

0.6

0.5

0.4

0

1500

STEAM
ENTHALPY
1150

S

E

N

S

T

I

O

B

L

T

E

A

H

E

L

H

E

A

T

.3

0.2

0.1

E

N

T

H

A

H

U

M

I

D

I

T

Y

∆H

A

T

=

-0.3

-0.1

1000

L

P

Y

=

R

A

I

O

T

∆ h

S

∆H

-0.5

∆ W

T

-1.0

BTU/LB

-4.0

-2.0

∞

40.3

4.0

1.0

2.0

500

-1000

0

The remaining 0.9 Btu is sensible heat. The actual moisture
added per pound of dry air is 0.0099 pounds. The specific
volume of the entering air at Point B is 14 cubic feet per pound.

For a 10,000 cubic feet per minute system, the weight of the

air passing through is:

10,000 ÷ 14 = 714.3 pounds per minute

The weight of the moisture added is:

714.3 x 0.0099 = 7.07 pounds per minute of moisture

Since one gallon of water weighs 8.34 pounds, the moisture to
be added is as follows:

7.07 ÷ 8.34 = 0.848 gallons per minute

TOTAL
ENTHALPY

SENSIBLE

(0.9 BTU/LB)

28.7

BTU/LB

29.6

BTU/LB

LATENT

AB

55°F DB 90°F DB 92°F DB

C

D

50% RH

Fig. 18.

0.0164 LB/LB

0.0065 LB/LB

C1842

ENGINEERING MANUAL OF AUTOMATIC CONTROL

48

This converts to:

0.848 x 60 minutes = 50.9 gallons per hour

Recalling that the steam added 11.6 Btu per pound of dry air,

the total heat added is:

714.3 x 11.6 = 8286 Btu per minute

PSYCHROMETRIC CHART FUNDAMENTALS

This converts to:

8286 x 60 minutes = 497,160 Btu per hour

Summarized, a steam humidifier always adds a little sensible
heat to the air, and the Process Line B–C angles to the right of
the 90F starting dry-bulb line because of the added sensible
heat. When the process line crosses the moisture content lines
along a constant dry-bulb line, only latent heat is added. When
it parallels a constant, horizontal moisture line, only sensible
heat is added.

AIR WASHERS

Air washers are also used as humidifiers particularly for
applications requiring added moisture and not much heat as in
warm southwestern climates. A washer can be recirculating as
shown in Figure 19 or heated as shown in Figure 20. In
recirculating washers, the heat necessary to vaporize the water
is sensible heat changed to latent heat which causes the dry­bulb temperature to drop. The process line tracks the constant
enthalpy line because no total heat is added or subtracted. This
process is called “adiabatic” and is illustrated by Figure 21.
Point A is the entering condition of the air, Point B is the final
condition, and Point C is the temperature of the water. Since
the water is recirculating, the water temperature becomes the
same as the wet-bulb temperature of the air.

SUPPLY FAN

CONSTANT

C

B

ENTHALPY
LINE

A

C1843

Fig. 21.

The next two psychrometric charts (Fig. 22 and 23) illustrate
the humidifying process using a heated air washer. The
temperature to which the water is heated is determined by the
amount of moisture required for the process. Figure 22 shows
what happens when the washer water is heated above the air
dry-bulb temperature shown at Point A. The temperature of
the water located at Point B on the saturation curve causes the
system air temperature to settle out at Point D. The actual
location of Point D depends upon the construction and
characteristics of the washer.

As the humidity demand reduces, the water temperature
moves down the saturation curve as it surrenders heat to the
air. This causes the water temperature to settle out at a point
such as Point C. The final air temperature is at Point E. Note
that the final air temperature is above the initial dry-bulb
temperature so both sensible and latent heat have been added
to the air.

HWS

HWR

PUMP

Fig. 19. Recirculating Air Washer.

SUPPLY FAN

HEAT EXCHANGER

PUMP

Fig. 20. Heated Air Washer.

C2598

C2599

49

SATURATION

CURVE

B

C

E

D

A

C1844

Fig. 22.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PSYCHROMETRIC CHART FUNDAMENTALS

B

C

D

E

SATURATION CURVE

washer is always located on the saturation curve. Note that the
dry-bulb temperature of the air is reduced as it passes through
the washer. This happens because some of its heat is used to
evaporate the water; however, the humidity of the air rises
considerably. In this case, some of the sensible heat of the air
becomes latent heat in the water vapor, but the enthalpy of the
air is increased because of the heat in the water.

A

C1845

Fig. 23.

Figure 23 illustrates a heated washer where the water
temperature is between the dry-bulb and wet-bulb temperatures
of the air. The air is humidified but also cooled a little. Point B
represents the initial and Point C the final temperature of the
water with reduced humidity demand. Point A represents the
initial and Point E the final temperature of the air. The location
of Points D and E depends on the construction and
characteristics of the washer. The temperature of the water in a

32°F WATER = 0 BTU/LB

1.0

2.0

4.0

∞

-4.0

-2.0

-1.0

500

-1000

0

5000

3000

1.0

2000

0.8

0.6

0.5

0.4

1500

0.3

C

S

E

N

S

T

I

O

B

L

T

E

A

H

E

L

A

T

H

E

A

T

0.2

0.1

E

N

T

H

A

L

H

U

P

M

I

D

I

T

Y

R

A

S

∆H

T

∆H

=

-0.5

-0.3

-0.1

1000

∆ h

Y

=

∆ W

I

O

T

VAPORIZING HUMIDIFIER

Va porizing and water spray humidifiers operate on the
principal of breaking water up into small particulates so they
are evaporated directly into the air. This process is essentially
adiabatic since the enthalpy lines of the water vapor for 32 and
212F are so close. The enthalpy of water at 32F is zero and at
212F it is 180 Btu per pound. If air at Point A (Fig. 24) is
humidified by 212F water, the process follows a line parallel
to line C-D and the 80F WB line and ends at a point such as
Point B. The actual water temperature of a vaporizing or water
spray humidifier will be between 32 and 212F and will usually
be around room temperature so using the zero enthalpy line
(C-E) as reference will not introduce a significant error into
the process.

80°F WB LINE

212°F WATER = 180 BTU/LB

Fig. 24. Psychrometric Chart Showing Line A–B Parallel to Line C–D.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

E

B

A

CONSTRUCTION LINE, FOR LINE A -B,
PERPENDICULAR TO LINES C-D AND A-B

D

C1846

50

PSYCHROMETRIC CHART FUNDAMENTALS

COOLING AND DEHUMIDIFICATION

BASIC PROCESS

Cooling and dehumidification can be accomplished in a single
process. The process line moves in a decreasing direction across
both the dry-bulb temperature lines and the constant moisture
lines. This involves both sensible and latent cooling.

Figure 12 illustrates cooling air by removing sensible heat
only. In that illustration, the resulting cooled air was 95 percent
relative humidity, a condition which often calls for reheat (see
DEHUMIDIFICATION AND REHEAT). Figure 25 illustrates
a combination of sensible and latent cooling. Whenever the
surface temperature of the cooling device (Point B), such as a
chilled water coil, is colder than the dew point temperature of
the entering air (Point A), moisture is removed from the air
contacting the cold surface. If the coil is 100 percent efficient,
all entering air contacts the coil and leaving air is the same
temperature as the surface of the coil.

85°F DB
63% RH

OA

COOLING COIL

50°F DB

SUPPLY FAN

60°F DB
93% RH

DA

To remove moisture, some air must be cooled below its
dew point. By determining the wet-bulb and the dry-bulb
temperatures of the leaving air, the total moisture removed per
pound of dry air can be read on the humidity ratio scale and is
determined as follows:

1. The entering air condition is 85F dry bulb and 63 percent
rh (Point A). The moisture content is 0.0166 pounds of
moisture per pound of dry air.

2. The leaving air condition is 60F dry bulb and 93 percent
rh (Point C). The moisture content is 0.0100 pounds of
moisture per pound of dry air.

3. The moisture removed is:

0.0166 – 0.0100 = 0.0066 pounds of moisture
per pound of dry air

The volume of air per pound at 85F dry bulb and 75F wet bulb
(Point A) is 14.1 cubic feet per pound of dry air. If 5000 cubic
feet of air per minute passes through the coil, the weight of
the air is as follows:

5000 ÷ 14.1 = 355 pounds per minute

The pounds of water removed is as follows:

355 x 0.0066 = 2.34 pounds per minute
or

2.34 x 60 minutes = 140.4 pounds per hour

B

50°F DB

C

60°F DB
93% RH

A

85°F DB
63% RH

14.1 CF/LB

58°F WB

0.0166 LB/LB

0.0100 LB/LB
75°F WB

C1847

Fig. 25.

All coils, however, are not 100 percent efficient and all air
does not come in contact with the coil surface or fins. As a
result, the temperature of the air leaving the coil (Point C) is
somewhere between the coolest fin temperature (Point B) and
the entering outdoor air temperature (Point A). To determine
this exact point requires measuring the dry-bulb and wet-bulb
temperatures of the leaving air.

Since one gallon of water weighs 8.34 pounds, the moisture to
be removed is as follows:

140.4 ÷ 8.34 = 16.8 gallons per hour

AIR WASHERS

Air washers are devices that spray water into the air within a
duct. They are used for cooling and dehumidification or for
humidification only as discussed in the HUMIDIFYING
PROCESS—AIR WASHERS section. Figure 26 illustrates an
air washer system used for cooling and dehumidification. The
chiller maintains the washer water to be sprayed at a constant
50F. This allows the chilled water from the washer to condense
water vapor from the warmer entering air as it falls into the
pan. As a result, more water returns from the washer than has
been delivered because the temperature of the chilled water is
lower than the dew point (saturation temperature) of the air.
The efficiency of the washer is determined by the number and
effectiveness of the spray nozzles used and the speed at which
the air flows through the system. The longer the air is in contact
with the water spray, the more moisture the spray condenses
from the air.

51

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PSYCHROMETRIC CHART FUNDAMENTALS

SUPPLY FAN

90°F DB
52% RH

CWS

CWR

PUMP

58°F DB
85% RH

C2597

Fig. 26. Air Washer Used for Cooling and

Dehumidification.

Figure 27 is a chart of the air washer process. If a washer is
100 percent efficient, the air leaving the washer is at Point B.
The result as determined by the wet-bulb and dry-bulb
temperatures is Point C and is determined as follows:

90°F DB
52% RH

A

0.0153 LB/LB

the wet-bulb temperature of the air as the process line extends.
Note that whenever the washer water temperature is between
the dew point (Point B) and the dry-bulb (Point D) temperature
of the air, moisture is added and the dry-bulb temperature of
the air falls. If the water temperature is above the dry-bulb
temperature of the air (to the right of Point D), both the air
moisture and the dry-bulb temperature increase. Whenever the
water temperature is below the dew point temperature (Point
B), dehumidification occurs as well as dry-bulb cooling. This
process always falls on a curved line between the initial
temperature of the air and the point on the saturation curve
representing the water temperature. The exact leaving air
temperature depends upon the construction and characteristics
of the washer.

D

C

B

A

B

C

58°F DB
85% RH

55°F WB50°F DB

0.0085 LB/LB

75°F WB

C1848

Fig. 27.

1. The entering condition air is 90F dry bulb and 52 percent
rh (Point A). The moisture content is 0.0153 pounds of
moisture per pound of dry air.

2. Air that contacts the spray droplets follows the saturation
curve to the spray temperature, 50F dry bulb (Point B),
and then mixes with air that did not come in contact with
the spray droplets resulting in the average condition at
Point C.

3. The leaving air is at 58F dry bulb and 85 percent rh
(Point C). The moisture content is 0.0085 pounds of
moisture per pound of dry air.

4. The moisture removed is:

0.0153 – 0.0085 = 0.068 pounds of moisture per
pound of dry air

C1849

Fig. 28.

DEHUMIDIFICATION AND REHEAT

Dehumidification lowers the dry-bulb temperature, which
often requires the use of reheat to provide comfortable
conditions. Dehumidification and reheat are actually two
processes on the psychrometric chart. Applications, such as
computer rooms, textile mills, and furniture manufacturing
plants require that a constant relative humidity be maintained
at close tolerances. To accomplish this, the air is cooled below
a comfortable level to remove moisture, and is then reheated
(sensible heat only) to provide comfort. Figure 29 is an air
conditioning system with both a cooling coil and reheat coil.

Figure 28 summarizes the process lines for applications using
washers for humidification or dehumidification. When the
water recirculates, the process is adiabatic and the process line
follows the Constant Enthalpy Line A-C. The water assumes

ENGINEERING MANUAL OF AUTOMATIC CONTROL

52

PSYCHROMETRIC CHART FUNDAMENTALS

HEATING

COIL

60°F DB

51.2°F WB
56% RH

V

T

C2600

90°F DB

71.3°F WB
40% RH

COOLING

COIL

48°F DB
46°F WB
85% RH

V

SUPPLY FAN

H

Fig. 29. Fan System with Dehumidification and Reheat.

Figure 30 illustrates cooling and dehumidification with reheat
for maintaining constant relative humidity. Air enters the coils
at Point A, is cooled and dehumidified to Point B, is reheated
to Point C, and is then delivered to the controlled space. A space
humidistat controls the cooling coil valve to maintain the space
relative humidity. A space thermostat controls the reheat coil
to maintain the proper dry-bulb temperature.

— Enthalpy and humidity ratio, or moisture content, are

based on a pound of dry air. Zero moisture is the bottom
line of the chart.

—To find the sensible heat content of any air in Btu, follow

the dry-bulb line to the bottom of the chart and read the
enthalpy there, or project along the enthalpy line, and
read the Btu per pound of dry air on the enthalpy scale.

LATENT HEAT CHANGE

SENSIBLE HEAT CHANGE

C1851

Fig. 31.

A

B

48°F DB
46°F WB
85% RH

C

60°F DB

51.2°F WB
56% RH

90°F DB

71.3°F WB
40% RH

C1850

Fig. 30.

PROCESS SUMMARY

Figures 31 and 32 summarize some principles of the air
conditioning process as illustrated by psychrometric charts.

— Sensible heating or cooling is always along a constant

moisture line.

—When latent heat is added or removed, a process line

always crosses the constant moisture lines.

ASHRAE PSYCHROMETRIC CHARTS

C

D

E

F

SUMMARY OF ALL PROCESSES CHARTABLE.
PROCESS MOVEMENT IN THE DIRECTION OF:
— A, HEATING ONLY - STEAM, HOT WATER OR ELECTRIC HEAT COIL
— B, HEATING AND HUMIDIFYING - STEAM HUMIDIFIER
OR RECIRCULATED HOT WATER SPRAY
— C, HUMIDIFYING ONLY - AIR WASHER WITH HEATED WATER
— D, COOLING AND HUMIDIFYING - WASHER
— E, COOLING ONLY - COOLING COIL OR WASHER AT

DEWPOINT TEMPERATURE
— F, COOLING AND DEHUMIDIFYING - CHILLED WATER WASHER
— G, DEHUMIDIFYING ONLY - NOT PRACTICAL
— H, DEHUMIDIFYING AND HEATING - CHEMICAL DEHUMIDIFIER

B

A

H

G

Fig. 32.

C1852

The following two pages illustrate ASHRAE Psychrometric Charts No. 1 and No. 2.

53

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PSYCHROMETRIC CHART FUNDAMENTALS

Fig. 33. ASHRAE Psychrometric Chart No. 1.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

54

PSYCHROMETRIC CHART FUNDAMENTALS

Fig. 34. ASHRAE Psychrometric Chart No. 2.

55

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PSYCHROMETRIC CHART FUNDAMENTALS

ENGINEERING MANUAL OF AUTOMATIC CONTROL

56

PNEUMATIC CONTROL FUNDAMENTALS

Pneumatic Control

Fundamentals

Contents

Introduction ............................................................................................................ 59

Definitions ............................................................................................................ 59

Abbreviations ............................................................................................................ 60

Symbols ............................................................................................................ 61

Basic Pneumatic Control System ............................................................................................................ 61

General................................................................................................ 61

Air Supply and Operation .................................................................... 61

Restrictor ............................................................................................. 62

Nozzle-Flapper Assembly.................................................................... 62

Pilot Bleed System .............................................................................. 62

Signal Amplifier.................................................................................... 63

Feed and Bleed System ...................................................................... 63

Sensing Elements ............................................................................... 63

Bimetal ............................................................................................ 63

Rod and Tube ................................................................................. 64

Remote Bulb ................................................................................... 64

Averaging Element.......................................................................... 64

Throttling Range Adjustment ............................................................... 64

Relays and Switches ........................................................................... 65

Air Supply Equipment ............................................................................................................65

General................................................................................................ 65

Air Compressor ................................................................................... 65

Air Drying Techniques ......................................................................... 66

General ........................................................................................... 66

Dry Air Requirement ........................................................................ 66

Condensing Drying ......................................................................... 67

High-Pressure Drying ................................................................. 67

Refrigerant Drying....................................................................... 67

Desiccant Drying ............................................................................. 67

Pressure Reducing Valve Station ........................................................ 68

Air Filter........................................................................................... 68

Pressure Reducing Valves .............................................................. 69

Single-Pressure Reducing Valve ................................................ 69

Two-Pressure Reducing Valve.................................................... 69

Thermostats ............................................................................................................ 69

57

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PNEUMATIC CONTROL FUNDAMENTALS

Controllers ............................................................................................................ 70

General................................................................................................ 70

Te mperature Controllers...................................................................... 71

Humidity Controllers ............................................................................ 71

Pressure Controllers............................................................................ 71

Sensor-Controller Systems ................................................................. 72

Pneumatic Controllers ......................................................................... 72

Proportional-Integral (PI) Controllers .............................................. 72

Controller Adjustments .................................................................... 72

Pneumatic Sensors ............................................................................. 73

Velocity Sensor-Controller................................................................... 73

Actuators and Final Control Elements ............................................................................................................ 74

Actuators ............................................................................................. 74

General ........................................................................................... 74

Spring Ranges ................................................................................ 74

Control Valves ..................................................................................... 75

Dampers .............................................................................................. 76

Relays and Switches ............................................................................................................ 77

Switching Relay ................................................................................... 77

Snap Acting Relay ............................................................................... 78

Lockout Relay...................................................................................... 78

High-Pressure Selector Relay ............................................................. 79

Low-Pressure Selector Relay .............................................................. 79

Load Analyzer Relay ........................................................................... 79

Capacity Relay .................................................................................... 80

Reversing Relay .................................................................................. 80

Averaging Relay .................................................................................. 80

Positive-Positioning Relay ................................................................... 80

Ratio Relay .......................................................................................... 81

Pneumatic Potentiometer .................................................................... 81

Hesitation Relay .................................................................................. 82

Electrical Interlocking Relays .............................................................. 82

Electric-Pneumatic Relay ................................................................ 82

Pneumatic-Electric Relay ................................................................ 82

Electronic-Pneumatic Transducer ....................................................... 83

Pneumatic Switch ................................................................................ 83

Manual Positioning Switch .................................................................. 84

Pneumatic Control Combinations ............................................................................................................ 84

General................................................................................................ 84

Sequence Control................................................................................ 85

Limit Control ........................................................................................ 85

Manual Switch Control ........................................................................ 86

Changeover Control for Two-Pressure Supply System ....................... 87

Compensated Control System ............................................................ 87

Electric-Pneumatic Relay Control........................................................ 87

Pneumatic-Electric Relay Control........................................................ 88

Pneumatic Recycling Control .............................................................. 88

Pneumatic Centralization ............................................................................................................ 89

Pneumatic Control System Example ............................................................................................................ 90

Start-Stop Control Sequence .............................................................. 90

Supply Fan Control Sequence ............................................................ 92

Return Fan Control Sequence............................................................. 92

Warm-up/Heating Coil Control Sequence ........................................... 92

Mixing Damper Control Sequence ...................................................... 93

Discharge Air Temperature Control Sequence .................................... 94

Off/Failure Mode Control Sequence .................................................... 94

ENGINEERING MANUAL OF AUTOMATIC CONTROL

58

INTRODUCTION

PNEUMATIC CONTROL FUNDAMENTALS

This section provides basic information on pneumatic control
systems and components commonly used to control equipment
in commercial heating and air conditioning applications. The
information in this section is of a general nature in order to
explain the fundamentals of pneumatic control. Some terms
and references may vary between manufacturers (e.g., switch
port numbers).

Pneumatic control systems use compressed air to operate
actuators, sensors, relays, and other control equipment.
Pneumatic controls differ from other control systems in several
ways with some distinct advantages:

— Pneumatic equipment is inherently proportional but can

provide two-position control when required.

— Many control sequences and combinations are possible

DEFINITIONS

Actuator: A mechanical device that operates a final control

element (e.g., valve, damper).

Authority (Reset Authority or Compensation Authority):

A setting that indicates the relative effect a
compensation sensor input has on the main setpoint
(expressed in percent).

Branch line: The air line from a controller to the controlled

device.

with relatively simple equipment.

— Pneumatic equipment is suitable where explosion

hazards exist.

— The installed cost of pneumatic controls and materials

may be lower, especially where codes require that low­voltage electrical wiring for similar electric controls be
run in conduit.

—Quality, properly installed pneumatic equipment is

reliable. However, if a pneumatic control system requires
troubleshooting or service, most building-maintenance
people have the necessary mechanical knowledge.

Controlled variable: The quantity or condition that is measured

and controlled (e.g., temperature, relative humidity,
pressure).

Controller: A device that senses the controlled variable or

receives an input signal from a remote sensing
element, compares the signal with the setpoint, and
outputs a control signal (branchline pressure) to an
actuator.

Branchline pressure (BLP): A varying air pressure signal

from a controller to an actuator carried by the branch
line. Can go from atmospheric to full main line
pressure.

Compensation changeover: The point at which the

compensation effect is reversed in action and changes
from summer to winter or vice versa. The percent of
compensation effect (authority) may also be changed
at the same time.

Compensation control: A process of automatically adjusting

the control point of a given controller to compensate
for changes in a second measured variable such as
outdoor air temperature. For example, the hot deck
control point is reset upward as the outdoor air
temperature decreases. Also know as “reset control”.

Compensation sensor: The system element which senses a

variable other than the controlled variable and resets
the main sensor control point. The amount of this
effect is established by the authority setting.

Control point: The actual value of the controlled variable

(setpoint plus or minus offset).

Differential: A term that applies to two-position devices. The

range through which the controlled variable must pass
in order to move the final control element from one to
the other of its two possible positions. The difference
between cut-in and cut-out temperatures, pressures, etc.

Direct acting (DA): A direct-acting thermostat or controller

increases the branchline pressure on an increase in
the measured variable and decreases the branchline
pressure on a decrease in the variable. A direct-acting
actuator extends the shaft on an increase in branchline
pressure and retracts the shaft on a decrease in
pressure.

Discharge air: Conditioned air that has passed through a coil.

Also, air discharged from a supply duct outlet into a
space. See Supply air.

Final control element: A device such as a valve or damper

that acts to change the value of the manipulated
variable. Positioned by an actuator.

59

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PNEUMATIC CONTROL FUNDAMENTALS

Main line: The air line from the air supply system to controllers

and other devices. Usually plastic or copper tubing.

Manipulated variable: Media or energy controlled to achieve

a desired controlled variable condition.

Measuring element: Same as sensing element.

Mixed air: Typically a mixture of outdoor air and return air

from the space.

Modulating: Varying or adjusting by small increments. Also

called “proportional”.

Offset: A sustained deviation between the actual system

control point and its controller setpoint under stable
operating conditions. Usually applies to proportional
(modulating) control.

Proportional band: As applied to pneumatic control systems,

the change in the controlled variable required to
change the controller output pressure from 3 to 13
psi. Usually expressed as a percentage of sensor span.

Reset control: See compensation control.

Restrictor: A device in an air line that limits the flow of air.

Reverse acting (RA): A reverse-acting thermostat or controller

decreases the branchline pressure on an increase in
the measured variable and increases the branchline
pressure on a decrease in the variable. A reverse-acting
valve actuator retracts the shaft on an increase in
branchline pressure and extends the shaft on a
decrease in pressure.

Sensing element: A device that detects and measures the

controlled variable (e.g., temperature, humidity).

Sensor: A device placed in a medium to be measured or

controlled that has a change in output signal related
to a change in the sensed medium.

Sensor Span: The variation in the sensed media that causes

the sensor output to vary between 3 and 15 psi.

Setpoint: The value on the controller scale at which the

controller is set (e.g., the desired room temperature
set on a thermostat). The desired control point.

Supply air: Air leaving an air handling system.

Thermostat: A device that responds to changes in temperature

and outputs a control signal (branchline pressure).
Usually mounted on a wall in the controlled space.

Return air: Air entering an air handling system from the

occupied space.

ABBREVIATIONS

The following port abbreviations are used in drawings of

relays and controllers:

B — Branch

C —Common

E — Exhaust

M — Main

O — Normally connected*
X — Normally disconnected*

P —Pilot (P1 and P2 for dual-pilot relays)

S — Sensor (S1 and S2 for dual-input controllers)
N.C. — Normally closed
N.O. — Normally open

Throttling range: Related to proportional band, and expressed

in values of the controlled variable (e.g., degrees,
percent relative humidity, pounds per square inch)
rather than in percent.

* The normally connected and common ports are connected

on a fall in pilot pressure below the relay setpoint, and the
normally disconnected port is blocked. On a rise in pilot
pressure above the relay setpoint, the normally disconnected
and common ports are connected and the normally
connected port is blocked. Refer to Figure 37 in RELAYS
AND SWITCHES.

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60

SYMBOLS

PNEUMATIC CONTROL FUNDAMENTALS

MAIN AIR SUPPLY

RESTRICTOR

NOZZLE

M

OR

OR

M

BASIC PNEUMATIC CONTROL SYSTEM

GENERAL

A pneumatic control system is made up of the following

elements:

— Compressed air supply system
— Main line distribution system
—Branch lines
— Sensors
— Controllers
—Actuators
—Final control elements (e.g., valves, dampers)

In a typical control system, the final control element (a valve
or a damper) is selected first because it must produce the desired
control results. For example, a system designed to control the
flow of water through a coil requires a control valve. The type
of valve, however, depends on whether the water is intended
for heating or cooling, the water pressure, and the control and
flow characteristics required. An actuator is then selected to
operate the final control element. A controller and relays
complete the system. When all control systems for a building
are designed, the air supply system can be sized and designed.

FIXED POINT

FULCRUM

PIVOT POINT

C1082

A basic pneumatic control system consists of an air supply,
a controller such as a thermostat, and an actuator positioning
a valve or damper (Fig. 1).

TO OTHER
CONTROLLERS

THERMOSTAT

M

COMPRESSED
AIR SUPPLY
SYSTEM

MAIN BRANCH

B

ACTUATOR

VALV E

C2353

Fig. 1. Basic Pneumatic Control System.

The controller receives air from the main line and regulates
its output pressure (branchline pressure) as a function of the
temperature, pressure, humidity, or other variable. The
branchline pressure from the controller can vary from zero to
full mainline pressure. The regulated branchline pressure
energizes the actuator, which then assumes a position
proportional to the branchline pressure applied. The actuator
usually goes through its full stroke as the branchline pressure
changes from 3 psi to 13 psi. Other pressure ranges are
available.

AIR SUPPLY AND OPERATION

The main line air supply is provided by an electrically driven
compressor pumping air into a storage tank at high pressure
(Fig. 2). A pressure switch turns the compressor on and off to
maintain the storage tank pressure between fixed limits. The
tank stores the air until it is needed by control equipment. The
air dryer removes moisture from the air, and the filter removes
oil and other impurities. The pressure reducing valve (PRV)
typically reduces the pressure to 18 to 22 psi. For two-pressure
(day/night) systems and for systems designed to change from
direct to reverse acting (heating/cooling), the PRV switches
between two pressures, such as 13 and 18 psi. The maximum
safe air pressure for most pneumatic controls is 25 psi.

AIR

SUPPLY

IN

AIR
COMPRESSOR

FILTER

Fig. 2. Compressed Air Supply System.

STORAGE
TANK

PRESSURE
GAGES

PRESSURE
REDUCING
VALV E

AIR
DRYER

MAIN AIR TO
PNEUMATIC
CONTROL
SYSTEM

C2616-1

61

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PNEUMATIC CONTROL FUNDAMENTALS

From the PRV, the air flows through the main line to the
controller (in Figure 1, a thermostat) and to other controllers
or relays in other parts of the system. The controller positions
the actuator. The controller receives air from the main line at a
constant pressure and modulates that pressure to provide
branchline air at a pressure that varies according to changes in
the controlled variable, as measured by the sensing element.
The controller signal (branchline pressure) is transmitted via
the branch line to the controlled device (in Figure 1, a valve
actuator). The actuator drives the final control element (valve)
to a position proportional to the pressure supplied by the
controller.

When the proportional controller changes the air pressure
to the actuator, the actuator moves in a direction and distance
proportional to the direction and magnitude of the change at
the sensing element.

RESTRICTOR

The restrictor is a basic component of a pneumatic control
system and is used in all controllers. A restrictor is usually a
disc with a small hole inserted into an air line to restrict the
amount of airflow. The size of the restrictor varies with the
application, but can have a hole as small as 0.003 inches.

NOZZLE-FLAPPER ASSEMBLY

To create a branchline pressure, a restrictor (Fig. 3) is
required. The restrictor and nozzle are sized so that the nozzle
can exhaust more air than can be supplied through the restrictor
when the flapper is off the nozzle. In that situation, the
branchline pressure is near zero. As the spring tension increases
to hold the flapper tighter against the nozzle, reducing the air
escaping, the branchline pressure increases proportionally.
When the spring tension prevents all airflow from the nozzle,
the branchline pressure becomes the same as the mainline
pressure (assuming no air is flowing in the branch line). This
type of control is called a “bleed” control because air “bleeds”
continuously from the nozzle.

With this basic mechanism, all that is necessary to create a
controller is to add a sensing element to move the flapper as
the measured variable (e.g., temperature, humidity, pressure)
changes. Sensing elements are discussed later.

PILOT BLEED SYSTEM

The pilot bleed system is a means of increasing air capacity
as well as reducing system air consumption. The restrictor and
nozzle are smaller in a pilot bleed system than in a nozzle­flapper system because in a pilot bleed system they supply air
only to a capacity amplifier that produces the branchline
pressure (Fig. 4). The capacity amplifier is a pilot bleed
component that maintains the branchline pressure in proportion
to the pilot pressure but provides greater airflow capacity.

The nozzle-flapper assembly (Fig. 3) is the basic mechanism
for controlling air pressure to the branch line. Air supplied to
the nozzle escapes between the nozzle opening and the flapper.
At a given air supply pressure, the amount of air escaping is
determined by how tightly the flapper is held against the nozzle
by a sensing element, such as a bimetal. Thus, controlling the
tension on the spring also controls the amount of air escaping.
Ve ry little air can escape when the flapper is held tightly against
the nozzle.

SENSOR

NOZZLE

RESTRICTOR

M

FORCE

AIR SUPPLY

FLAPPER

SPRING

BRANCH

C1084

Fig. 3. Nozzle-Flapper Assembly with Restrictor.

FLAPPER

NOZZLE

PILOT
CHAMBER

BRANCH
CHAMBER

M

FEED
VALV E
DISC

CAPACITY
AMPLIFIER

VENT

BLEED
VALV E

SPRING

BRANCH

C1085

Fig. 4. Pilot Bleed System with Amplifier Relay.

The pilot pressure from the nozzle enters the pilot chamber
of the capacity amplifier. In the state shown in Figure 4, no air
enters or leaves the branch chamber. If the pilot pressure from
the nozzle is greater than the spring force, the pilot chamber
diaphragm is forced down, which opens the feed valve and
allows main air into the branch chamber. When the pilot
pressure decreases, the pilot chamber diaphragm rises, closing

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62

PNEUMATIC CONTROL FUNDAMENTALS

the feed valve. If the pilot chamber diaphragm rises enough, it
lifts the bleed valve off the feed valve disc, allowing air to
escape from the branch chamber through the vent, thus
decreasing the branchline pressure. Main air is used only when
branchline pressure must be increased and to supply the very
small amount exhausted through the nozzle.

SIGNAL AMPLIFIER

In addition to the capacity amplifier, pneumatic systems also
use a signal amplifier. Generally, modern amplifiers use
diaphragms for control logic instead of levers, bellows, and
linkages.

A signal amplifier increases the level of the input signal and
provides increased flow. This amplifier is used primarily in
sensor-controller systems where a small signal change from a
sensor must be amplified to provide a proportional branchline
pressure. The signal amplifier must be very sensitive and
accurate, because the input signal from the sensor may change
as little as 0.06 psi per degree Fahrenheit.

Another use for a signal amplifier is to multiply a signal by
two to four times so a signal from one controller can operate
several actuators in sequence.

SETPOINT
ADJUSTMENT

M

SENSING
FORCE

EXH

FEED VALVE

PRESSURE
CHAMBER

BLEED VALVE

BRANCHLINE
PRESSURE

C2382-1

Fig. 5. Feed and Bleed System.

A force applied by the sensing element at the sensor input
point is opposed by the setpoint adjustment spring and lever.
When the sensing element pushes down on the lever, the lever
pivots on the bleed ball and allows the feed ball to rise, which
allows main air into the chamber. If the sensing element reduces
its force, the other end of the lever rises and pivots on the feed
ball, and the bleed ball rises to exhaust air from the system.
The sensor can be any sensing element having enough force
to operate the system.

FEED AND BLEED SYSTEM

The “feed and bleed” (sometimes called “non bleed”) system
of controlling branchline pressure is more complicated than
the nozzle-flapper assembly but theoretically uses less air. The
nozzle-flapper system exhausts some air through the nozzle
continually, whereas the feed and bleed system exhausts air
only when the branchline pressure is being reduced. Since
modern nozzle-flapper devices consume little air, feed and
bleed systems are no longer popular.

The feed and bleed system consists of a feed valve that
supplies main air to the branch line and a bleed valve that
exhausts air from the branch line (Fig. 5). Each valve consists
of a ball nested on top of a tube. Some pneumatic controllers
use pressure balance diaphragm devices in lieu of springs and
valves. A spring in the tube continually tries to force the ball
up. The lever holds the ball down to form a tight seal at the
end of the tube. The feed and bleed valves cannot be open at
the same time.

SENSING ELEMENTS

BIMETAL

A bimetal sensing element is often used in a temperature
controller to move the flapper. A bimetal consists of two strips
of different metals welded together as shown in Figure 6A. As
the bimetal is heated, the metal with the higher coefficient of
expansion expands more than the other metal, and the bimetal
warps toward the lower-coefficient metal (Fig. 6B). As the
temperature falls, the bimetal warps in the other direction
(Fig. 6C).

A. CALIBRATION TEMPERATURE

B. INCREASED TEMPERATURE

C. DECREASED TEMPERATURE

63

METALS:

HIGH COEFFICIENT OF EXPANSION
LOW COEFFICIENT OF EXPANSION

C1087

Fig. 6. Bimetal Sensing Element.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PNEUMATIC CONTROL FUNDAMENTALS

A temperature controller consists of a bimetal element linked
to a flapper so that a change in temperature changes the position
of the flapper. Figure 7 shows a direct-acting thermostat
(branchline pressure increases as temperature increases) in
which the branchline pressure change is proportional to the
temperature change. An adjustment screw on the spring adjusts
the temperature at which the controller operates. If the tension
is increased, the temperature must be higher for the bimetal to
develop the force necessary to oppose the spring, lift the
flapper, and reduce the branch pressure.

CONTACT POINT FOR
THROTTING RANGE

BIMETAL

SETPOINT
SCREW

M

ADJUSTMENT

FLAPPER

NOZZLE

BRANCH

C1088

Fig. 7. Temperature Controller with

Bimetal Sensing Element.

ROD AND TUBE

The rod-and-tube sensing element consists of a brass tube
and an Invar rod, as shown in Figure 8. The tube expands and
contracts in response to temperature changes more than the
rod. The construction of the sensor causes the tube to move
the rod as the tube responds to temperature changes. One end
of the rod connects to the tube and the other end connects to
the flapper spring to change the force on the flapper.

diaphragm chamber. The expansion causes the diaphragm pad
to push the pin toward the lever, which moves the flapper to
change the branchline pressure.

DIAPHRAGM PAD

PIN

CONTROLLER

DIAPHRAGM CHAMBER

Fig. 9. Remote-Bulb Temperature Sensor.

LIQUID FILL

CAPILLARY

BULB

C1090-1

Remote-bulb temperature sensors are used in bleed-type
controllers. Capillary length of up to 2.5 meters are normally
used for inserting the bulb in duct, tank, or pipe.

AVERAGING ELEMENT

The averaging-element sensor is similar to the remote-bulb
sensor except that it has no bulb and the whole capillary is the
measuring element. The long, flexible capillary has a slightly
wider bore to accommodate the equivalent liquid fill that is
found in a remote-bulb sensor. The averaging-element sensor
averages temperatures along its entire length and is typically
used to measure temperatures across the cross section of a
duct in which two air streams may not mix completely.
Averaging element sensors are used to provide an input signal
to a controller.

TUBE

CONNECTION
TO FLAPPER
SPRING

ROD

C1089

Fig. 8. Rod-and-Tube Insertion Sensor.

On a rise in temperature, the brass tube expands and draws the
rod with it. The rod pulls on the flapper spring which pulls the
flapper closed to the nozzle. The flapper movement decreases
the air-bleed rate, which increases branchline pressure.

REMOTE BULB

The remote-bulb sensing element has as measuring element
made up of a capillary and bulb filled with a liquid or vapor
(Fig. 9). On and increase in temperature at the bulb, the liquid
or vapor expands through the capillary tubing into the

THROTTLING RANGE ADJUSTMENT

A controller must always have some means to adjust the
throttling range (proportional band). In a pneumatic controller,
the throttling range is the change at the sensor required to
change the branchline pressure 10 psi. The setpoint is usually
at the center of the throttling range. For example, if the
throttling range of a temperature controller is 4F and the
setpoint is 72F, the branchline pressure is 3 psi at 70F, 8 psi at
72F, and 13 psi at 74F for a direct acting controller.

In all pneumatic systems except the sensor-controller system,
the throttling range is adjusted by changing the effective length
of a lever arm. In Figure 7, the throttling range is changed by
moving the contact point between the bimetal and the flapper.
(For information on adjusting the throttling range in a sensor­controller system, see SENSOR-CONTROLLER SYSTEMS.)

RELAYS AND SWITCHES

Relays are used in control circuits between controllers and
controlled devices to perform a function beyond the capacity
of the controllers. Relays typically have diaphragm logic

ENGINEERING MANUAL OF AUTOMATIC CONTROL

64

PNEUMATIC CONTROL FUNDAMENTALS

construction (Fig. 10) and are used to amplify, reverse, average,
select, and switch controller outputs before being sent to valve
and damper actuators.

SPRING

O

NORMALLY

COMMON

PORT

PILOT

PORT

P

CONNECTED PORT

X

NORMALLY
DISCONNECTED PORT

CONTROL
CHAMBER

C2608

Fig. 10. Typical Switching Relay.

AIR SUPPLY EQUIPMENT

GENERAL

A pneumatic control system requires a supply of clean, dry,
compressed air. The air source must be continuous because
many pneumatic sensors, controllers, relays, and other devices
bleed air. A typical air supply system includes a compressor,
an air dryer, an air filter, a pressure reducing valve, and air
tubing to the control system (Fig. 11).

The following paragraphs describe the compressor, filter,
pressure reducing valves, and air drying techniques. For
information on determining the moisture content of compressed
air, refer to the General Engineering Data section.

The controlling pressure is connected at the pilot port (P),
and pressures to be switched are connected at the normally
connected port (O) or the normally disconnected port (X).
The operating point of the relay is set by adjusting the spring
pressure at the top of the relay.

When the pressure at the pilot port reaches the relay
operating point, it pushes up on the diaphragm in the control
chamber and connects pressure on the normally
disconnected port (X) to the common port as shown. If the
pilot pressure falls below the relay setpoint, the diaphragm
moves down, blocks the normally disconnected (X) port,
and connects the normally connected port (O) to the common
port.

AIR COMPRESSOR

The air compressor provides the power needed to operate
all control devices in the system. The compressor maintains
pressure in the storage tank well above the maximum required
in the control system. When the tank pressure goes below a
minimum setting (usually 70 to 90 psi), a pressure switch starts
the compressor motor. When the tank pressure reaches a high­limit setting, the pressure switch stops the motor. A standard
tank is typically large enough so that the motor and compressor
operate no more than 50 percent of the time, with up to twelve
motor starts per hour.

Some applications require two compressors or a dual
compressor. In a dual compressor, two compressors operate

INTAKE FILTER

COMPRESSOR

STORAGE

TANK

NORMALLY OPEN
SERVICE/TEST VALVE

NORMALLY CLOSED
SERVICE/TEST VALVE

PRESSURE SWITCH

HIGH PRESSURE SAFETY RELIEF VALVE

DRIVE BELT

MOTOR

AIR DRYER

TEST COCK

Fig. 11. Typical Air Supply.

AUTO TRAP

SERVICE
BYPASS
VALVE

TEST COCK

HIGH-PRESSURE

GAGE

AUTO
SEPARATOR
FILTER/TRAP

DRAIN
COCK

PIPED TO DRAIN

PRESSURE
REDUCING
VALVE

LOW-PRESSURE GAGE

SUBMICRON
FILTER

SAFETY REFIEF VALVE

MAIN AIR

TO SYSTEM

C2617-2

65

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PNEUMATIC CONTROL FUNDAMENTALS

alternately, so wear is spread over both machines, each capable
of supplying the average requirements of the system without
operating more than half the time. In the event of failure of
one compressor, the other assumes the full load.

Contamination in the atmosphere requires a compressor
intake filter to remove particles that would damage the
compressor pump. The filter is essential on oil-less compressors
because a contaminated inlet air can cause excessive wear on
piston rings. The intake filter is usually located in the equipment
room with the compressor, but it may be located outdoors if
clean outdoor air is available. After the air is compressed,
cooling and settling actions in the tank condense some of the
excess moisture and allow fallout of the larger oil droplets
generated by the compressor pump.

A high pressure safety relief valve which opens on
excessively high tank pressures is also required. A hand valve
or automatic trap periodically blows off any accumulated
moisture, oil residue, or other impurities that collect in the
bottom of the tank.

AIR DRYING TECHNIQUES

GENERAL

Air should be dry enough to prevent condensation. Con­densation causes corrosion that can block orifices and valve
mechanisms. In addition, dry air improves the ability of filters
to remove oil and dirt.

Moisture in compressed air is removed by increasing
pressure, decreasing temperature, or both. When air is
compressed and cooled below its saturation point, moisture
condenses. Draining the condensate from the storage tank
causes some drying of the air supply, but an air dryer is often
required.

An air dryer is selected according to the amount of moisture
in the air and the lowest temperature to which an air line will
be exposed. For a chart showing temperature and moisture
content relationships at various air pressures, refer to the
General Engineering Data section.

DRY AIR REQUIREMENT

is the temperature at which moisture starts to condense out
of the air.

The coldest winter exposure is normally a function of
outdoor air temperature. Summer exposure is normally a
function of temperature in cold air ducts or air conditioned
space. The typical coldest winter application is an air line and
control device (e.g., damper actuator) mounted on a rooftop
air handling unit and exposed to outdoor air temperatures (Fig.

12). The second coldest winter exposure is an air line run in a
furred ceiling or outside wall.

70

60

50

40

30
24

20

10

REQUIRED MAXIMUM

0

DEWPOINT OF MAIN AIR (°F)

-10

-10 0 10 20 30 405060 7080

TUBING IN
FURRED
CEILING

TUBING AT
OUTDOOR AIR
TEMPERATURE

OUTDOOR AIR TEMPERATURE (°F)

C1098

Fig. 12. Winter Dew Point Requirement.

A typical summer minimum dew point application is a cold
air plenum. Figure 13 shows a 50F plenum application along
with winter requirements for a year-round composite.

SUMMER REQUIREMENT

50

REQUIREMENT

40

AT OUTDOOR AIR

TEMPERATURE

30

20

10

REQUIRED MAXIMUM

0

DEWPOINT OF MAIN AIR (°F)

-10

-10 0 10 20 30405060

OUTDOOR AIR TEMPERATURE (°F)

COLD AIR PLENUM

WINTER

C1099

The coldest ambient temperature to which tubing is exposed

is the criterion for required dryness, or dew point. Dew point

ENGINEERING MANUAL OF AUTOMATIC CONTROL

Fig. 13. Twelve-Month Composite Dew Point

Requirement.

66

PNEUMATIC CONTROL FUNDAMENTALS

CONDENSING DRYING

The two methods of condensing drying are high-pressure

drying and refrigerant drying.

High-Pressure Drying

High-pressure drying may be used when main air piping is
kept away from outside walls and chilling equipment. During
compression and cooling to ambient temperatures, air gives
up moisture which then collects in the bottom of the storage
tank. The higher the tank pressure, the greater the amount of
moisture that condenses. Maintaining a high pressure removes
the maximum amount of moisture. The compressor should have
a higher operating pressure than is required for air supply
purposes only. However, higher air pressure requires more
energy to run the compressor. The tank must include a manual
drain valve or an automatic trap to continually drain off
accumulated moisture. With tank pressures of 70 to 90 psi, a
dew point of approximately 70F at 20 psi can be obtained.

Refrigerant Drying

Lowering air temperature reduces the ability of air to hold
water. The refrigerated dryer (Fig. 14) is the most common
means of obtaining dry, compressed air and is available in
several capacities. It provides the greatest system reliability
and requires minimal maintenance.

HOT GAS
BYPASS

HEAT
EXCHANGER

AIR IN

CONTROL

REFRIGERANT
LINES

The heat exchanger reduces the temperature of the com­pressed air passing through it. A separator/filter condenses
both water and oil from the air and ejects the condensate
through a drain. A temperature-sensing element controls the
operation of the refrigeration system to maintain the tempera­ture in the exchanger.

With a dew point of 35F and an average compressor tank
pressure of 80 psi, air is dried to a dew point of 12F at 20 psi.
Under severe winter conditions and where piping and devices
are exposed to outside temperatures, the 12F dew point may
not be low enough.

DESICCANT DRYING

A desiccant is a chemical that removes moisture from air. A
desiccant dryer is installed between the compressor and the
PRV. Dew points below –100F are possible with a desiccant
dryer. The desiccant requires about one-third of the process
air to regenerate itself, or it may be heated. To regenerate,
desiccant dryers may require a larger compressor to produce
the needed airflow to supply the control system and the dryer.

It may be necessary to install a desiccant dryer after the
refrigerant dryer in applications where the 12F dew point at
20 psi mainline pressure does not prevent condensation in air
lines (e.g., a roof-top unit exposed to severe winters).

The desiccant dryer most applicable to control systems uses
the adsorbent principle of operation in which porous materials
attract water vapor. The water vapor is condensed and held as
a liquid in the pores of the material. The drying action continues
until the desiccant is saturated. The desiccant is regenerated
by removing the moisture from the pores of the desiccant
material. The most common adsorbent desiccant material is
silica gel, which adsorbs over 40 percent of its own weight in
water and is totally inert. Another type of adsorbent desiccant
is the molecular sieve.

AIR OUT

REFRIGERATION
UNIT

REFRIGERANT DRYER

CONDENSOR

C1888

Fig. 14. Typical Refrigerant Dryer Airflow Diagram.

The refrigerant dryer uses a non cycling operation with a
hot gas bypass control on the refrigerant flow to provide a
constant dew point of approximately 35F at the tank pressure.
The refrigeration circuit is hermetically sealed to prevent loss
of refrigerant and lubricant and to protect against dirt.

A desiccant is regenerated either by heating the desiccant
material and removing the resulting water vapor from the
desiccant chamber or by flushing the desiccant chamber with
air at a lower vapor pressure for heatless regeneration. To
provide a continuous supply of dry air, a desiccant dryer has
two desiccant chambers (Fig. 15). While one chamber is being
regenerated, the other supplies dry air to the system. The
cycling is accomplished by two solenoid valves and an electric
timer. During one cycle, air passes from the compressor into
the left desiccant chamber (A). The air is dried, passes through
the check valve (B), and flows out to the PRV in the control
system.

67

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PNEUMATIC CONTROL FUNDAMENTALS

DESICCANT
CHAMBERS

A

CHECK
VALVE

C

B

ORIFICE

F

E

CHECK
VALVE

G

PRESSURE REDUCING VALVE STATION

The pressure reducing valve station is typically furnished
with an air filter. The filter, high-pressure gage, high pressure
relief valve, pressure reducing valve (PRV), and low-pressure
gage are usually located together at one point in the system
and may be mounted directly on the compressor. The most
important elements are the air filter and the PRV.

ORIFICE

AIR FILTER

DRY AIR OUT

SOLENOID

D

AIR FROM COMPRESSOR

SOLENOID

H

C1889

Fig. 15. Typical Heatless Desiccant

Dryer Airflow Diagram.

Simultaneously, some of the dried air passes through the
orifice (G) to the right desiccant chamber (E). The air is dry
and the desiccant chamber is open to the atmosphere, which
reduces the chamber pressure to near atmospheric pressure.
Reducing the air pressure lowers the vapor pressure of the air
below that of the desiccant, which allows the moisture to
transfer from the desiccant to the air. The timer controls the
cycle, which lasts approximately 30 minutes.

During the cycle, the desiccant in the left chamber (A)
becomes saturated, and the desiccant in the right chamber (E)
becomes dry. The timer then reverses the flow by switching
both of the solenoid valves (D and H). The desiccant in the
right chamber (E) then becomes the drying agent connected to
the compressor while the desiccant in the left chamber (A) is
dried.

The process provides dry air to the control system continually
and requires no heat to drive moisture from the desiccant. A
fine filter should be used after the desiccant dryer to filter out
any desiccant discharged into the air supply.

The air filter (Fig. 16) removes solid particulate matter and

oil aerosols or mist from the control air.

AIR IN

LIQUID
DRAIN

AIR OUT

INNER FOAM
SLEEVE

FILTERING
MEDIUM

OUTER FOAM
SLEEVE

PERFORATED
METAL
CYLINDER

C2601

Fig. 16. Typical Air Filter.

Oil contamination in compressed air appears as a gas or an
aerosol. Gaseous oil usually remains in a vapor state throughout
the system and does not interfere with operation of the
controls. Aerosols, however, can coalesce while flowing
through the system, and turbulence can cause particles to
collect in device filters, orifices, and small passages.

Many filters are available to remove solids from the air.
However, only an oil-coalescing filter can remove oil aerosols
from control air. An oil coalescing filter uses a bonded fibrous
material to combine the small particles of oil mist into larger
droplets. The coalesced liquids and solids gravitate to the
bottom of the outer surface of the filter material, drop off into
a sump, and are automatically discharged or manually drained.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

The oil coalescing filter continues to coalesce and drain off
accumulated oil until solid particles plug the filter. An increase

68

in pressure drop across the filter (to approximately 10 psi)
indicates that the filter element needs replacement. For very
dirty air, a 5-micron prefilter filters out large particles and
increases the life of the final filter element.

PRESSURE REDUCING VALVES

A pressure reducing valve station can have a single-pressure
reducing valve or a two-pressure reducing valve, depending
on the requirements of the system it is supplying.

PNEUMATIC CONTROL FUNDAMENTALS

Two-Pressure Reducing Valve

A two-pressure reducing valve is typically set to pass 13 or
18 psi to the control system, as switched by a pilot pressure.
The two-pressure reducing valve is the same as the single­pressure reducing valve with the addition of a switchover
diaphragm and switchover inlet to accept the switchover
pressure signal. Switchover to the higher setting occurs when
the inlet admits main air into the switchover chamber.
Exhausting the switchover chamber returns the valve to the
lower setting.

Single-Pressure Reducing Valve

After it passes though the filter, air enters the PRV (Fig.

11). Inlet pressure ranges from 60 to 150 psi, depending on
tank pressures maintained by the compressor. Outlet pressure
is adjustable from 0 to 25 psi, depending on the control air
requirements. The normal setting is 20 psi.

A safety relief valve is built into some PRV assemblies to
protect control system devices if the PRV malfunctions. The
valve is typically set to relieve downstream pressures above
24 psi.

THERMOSTATS

Thermostats are of four basic types:
—A low-capacity, single-temperature thermostat is the

basic nozzle-flapper bleed-type control described earlier.
It is a bleed, one-pipe, proportional thermostat that is
either direct or reverse acting.

—A high-capacity, single-temperature thermostat is a low-

capacity thermostat with a capacity amplifier added. It
is a pilot-bleed, two-pipe, proportioning thermostat that
is either direct or reverse acting.

—A dual-temperature thermostat typically provides

occupied/unoccupied control. It is essentially two
thermostats in one housing, each having its own bimetal
sensing element and setpoint adjustment. A valve unit
controlled by mainline pressure switches between the
occupied and unoccupied mode. A manual override lever
allows an occupant to change the thermostat operation
from unoccupied operation to occupied operation.

The switchover signal is typically provided by an E/P relay
or a two-position diverting switch. An automatic time clock
can operate an E/P relay to switch the main pressure for a
day/night control system. A diverting switch is often used to
manually switch a heating/cooling system.

In many applications requiring two-pressure reducing
valves, a single-pressure reducing valve is also required to
supply single-pressure controllers which do not perform well
at low pressures. Higher dual pressure systems operating at
20 and 25 psi are sometimes used to eliminate the need and
expense of the second PRV.

—A dual-acting (heating/cooling) thermostat is another

two-pipe, proportioning thermostat that has two bimetal
sensing elements. One element is direct acting for heating
control, and the other, reverse acting for cooling control.
Switchover is the same as for the dual-temperature
thermostat but without manual override.

Other thermostats are available for specific uses. Energy con­servation thermostats limit setpoint adjustments to reasonable
minimums and maximums. Zero energy band thermostats
provide an adjustable deadband between heating and cooling
operations.

The thermostat provides a branchline air pressure that is a
function of the ambient temperature of the controlled space
and the setpoint and throttling range settings. The throttling
range setting and the setpoint determine the span and operating
range of the thermostat. The nozzle-flapper-bimetal assembly
maintains a fixed branchline pressure for each temperature
within the throttling range (Fig. 17). The forces within the
nozzle-flapper-bimetal assembly always seek a balanced
condition against the nozzle pressure. If the setpoint is changed,
the forces in the lever system are unbalanced and the room
ambient temperature must change in a direction to cause the
bimetal to rebalance the lever system.

69

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PNEUMATIC CONTROL FUNDAMENTALS

13

8

open. Heat enters the space until the temperature at the
thermostat increases and the force of the bimetal is again in
equilibrium with the opposing force of the pressure at the
nozzle. Decreasing the setpoint causes the reverse to occur.

3

0

SETPOINT

BRANCHLINE PRESSURE (PSI)

THROTTLING RANGE

NOTE: SETPOINT IS AT MIDDLE OF

THROTTING RANGE

C1091

Fig. 17. Relationship between Setpoint, Branchline

Pressure, and Throttling Range.

For example, if the setpoint of a direct acting thermostat is
increased, the bimetal reduces the force applied to the flapper
and raises the flapper off the nozzle. This movement causes
the branchline pressure to bleed down and a heating valve to

CONTROLLERS

GENERAL

A controller is the same as a thermostat except that it may
have a remote sensing element. A controller typically measures
and controls temperature, humidity, airflow, or pressure.
Controllers can be reverse or direct acting, proportional or two­position, single or two pressure, and bleed, feed and bleed, or
pilot bleed.

A two-position controller changes branchline pressure
rapidly from minimum to maximum (or from maximum to
minimum) in response to changes in the measured condition,
thus providing ON/OFF operation of the controlled device.

A proportional controller changes branchline pressure
incrementally in response to a change in the measured
condition, thus providing modulating operation of the
controlled device.

A proportional-integral (PI) controller adds to the
proportional controller a component that takes offset into
account. The integral component eliminates the control point
offset from the setpoint.

The throttling range adjustment provides the means for
changing the effective length of the cantilever bimetal in the
lever system. When the throttling range adjustment is
positioned directly over the nozzle, the force of the bimetal
increases and a narrow throttling range or very high sensitivity
results. For example, a change in temperature of 1 degree F
could result in a branchline pressure change of 5 psi.

When the throttling range adjustment is moved toward the
end of the bimetal and away from the nozzle, the force of the
bimetal is reduced. This reduction requires a greater
temperature change at the bimetal to throttle the flapper over
the nozzle. The result is a wider throttling range or very low
sensitivity. For example, a temperature change of 1 degree F
could result in a branchline pressure change of only 1 psi.

main air goes into the controller, through an internal restrictor
in the controller, and out of the controller through a branch
line to the actuator. All pilot-bleed and feed-and-bleed
controllers are two pipe.

CONTROLLER

MAIN BRANCH

M

VALV E

C2342

Fig. 18. One-Pipe Controller System.

CONTROLLER

M

B

MAIN BRANCH

M

Bleed-type controllers can be used in one-pipe or two-pipe
configurations. In a one-pipe system (Fig. 18), the main air
goes through a restrictor to the controller and actuator in the
most expeditious routing. In a two-pipe system (Fig. 19), the

ENGINEERING MANUAL OF AUTOMATIC CONTROL

70

C2343

VALV E

Fig. 19. Two-Pipe Controller System.

PNEUMATIC CONTROL FUNDAMENTALS

Controllers may also be classified as single-pressure or two­pressure controllers. Single-pressure controllers use a constant
main air pressure. Two-pressure controllers use a main air
pressure that is alternately switched between two pressures,
such as 13 and 18 psi. For example, occupied/unoccupied
controllers automatically change setpoint from a occupied
setting at a mainline pressure of 13 psi to a lowered unoccupied
setting at 18 psi. Heating/cooling controllers change from
reverse acting at mainline air pressure of 13 psi for cooling to
direct acting at 18 psi for heating.

TEMPERATURE CONTROLLERS

Temperature controllers can be one- or two-pipe. The sensing
element is typically bimetal, liquid filled remote bulb, or liquid
filled averaging capillary tube. Dimensional change of the
element with temperature change results in flapper position
change and therefore, pilot and branch pressure change.

HUMIDITY CONTROLLERS

Principles that apply to temperature controllers also apply
to humidity controllers. The primary difference between
temperature and humidity controllers is in the type of sensing
element. The sensing element in a humidistat is usually a band
of moisture-sensitive nylon. The nylon expands and contracts
with changes in the relative humidity of the air.

in respect to atmospheric pressure or another pressure source.
The low-pressure controller is available in both bleed-type and
pilot-bleed designs.

Figure 20 shows a schematic of a bleed-type, low-pressure
controller. The direct-acting pressure sensor measures static
pressure from a pressure pickup located in a duct. A reference
pressure, from a pickup located outside the duct, is applied to
the other side of the diaphragm.

SETPOINT
SPRING

MAIN LEVER

M

FEEDBACK
BELLOWS

STATIC
PRESSURE

REFERENCE
PRESSURE

BRANCH

PRESSURE SENSOR

DIAPHRAGM

C2609

Fig. 20. Bleed-Type Static Pressure Controller.

The humidistat can be used in a one-pipe or two-pipe
configuration and is available as either a bleed-type humidistat
or a two-pipe capacity humidistat using a capacity amplifier.
The humidistat may be direct or reverse acting. The high­capacity humidistat has a capacity amplifier.

PRESSURE CONTROLLERS

Pressure controllers can be divided into two classes
according to the pressure range of the measured variable. High­pressure controllers measure and control high pressures or
vacuums measured in pounds per square inch or in inches of
mercury (e.g., steam or water pressures in an air conditioning
system). Low-pressure controllers measure and control low
pressures and vacuums measured in inches of water (e.g.,
pressure in an air duct).

High- and low-pressure controllers have different size
diaphragms. In both types, one side of the diaphragm is
connected to the pressure to be controlled, and the other side
is connected to a reference pressure. Pressures can be measured

On an increase in static pressure, the increased force on the
diaphragm exceeds the force of the setpoint spring, pulling
the main lever downward. A setpoint adjustment screw
determines the tension of the setpoint spring. As the main lever
is pulled downward, it moves closer to the nozzle, restricts the
airflow through the nozzle, and increases the pressure in the
branch. The action continues until the pressure on the feedback
bellows balances the static pressure on the diaphragm.

On a decrease in static pressure, or if the static pressure
sensor is piped for reverse action (high- and low-pressure
pickups reversed), the diaphragm moves upward to move the
main lever away from the nozzle and reduce the pressure in
the branch.

For differential pressure sensing, the two pressure pickup
lines connect to opposite sides of the pressure sensor
diaphragm.

71

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PNEUMATIC CONTROL FUNDAMENTALS

SENSOR-CONTROLLER SYSTEMS

A sensor-controller system is made up of a pneumatic
controller, remote pneumatic sensors, and a final control
element. The controller provides proportional or proportional­integral control of temperature, humidity, dew point, or pressure
in HVAC systems. Sensors do not have a setpoint adjustment
and provide a linear 3 to 15 psi signal to the controller over a
fixed sensor range. The controller compares the sensor input
signal with the setpoint signal. The difference is the pilot input
to a signal amplifier, which provides a branchline pressure to
the controlled device. Thus the controller acts as a general­purpose pneumatic amplifier.

PNEUMATIC CONTROLLERS

Controllers generally use diaphragm logic, which allows
flexible system application, provides more accurate control,
and simplifies setup and adjustment for the needs of each
system. Controllers may be proportional only or proportional­integral (PI). The integral function is also called “automatic
reset”. Proportional and PI controllers are available with single­sensor input or dual-sensor input for resetting the primary
sensor setpoint from a second sensor. They are also available
with integral or remote setpoint adjustment.

The single-input controller consists of a signal amplifier
feeding a capacity amplifier. The capacity amplifier is discussed
under PILOT BLEED SYSTEM. A dual-input controller has
inputs from a primary temperature sensor and a reset
temperature sensor. The reset sensor resets controller setpoint.
Reset can be negative or positive.

Figure 21 depicts a single-input controller as it would appear
in a simple application. Figure 22 depicts a dual-input
controller with manual remote setpoint control. In Figures 21
and 22 the sensors are fed restricted main air from the
controllers. Where sensors are located extremely remote from
the controller, a remote restrictor may be required.

MAIN AIR
(18 PSI)

M

TEMPERATURE
SENSOR

HOT WATER
VALV E

Fig. 21. Single-Input Controller.

SINGLE INPUT
CONTROLLER

M10293

PRIMARY
SENSOR

RESET SENSOR

M

MANUAL REMOTE
SETPOINT CONTROL

M10294

HOT WATER
VALV E

MAIN AIR
(18 PSI)

M

Fig. 22. Dual-Input Controller with

Manual Remote Setpoint.

PROPORTIONAL-INTEGRAL (PI) CONTROLLERS

Va riations of single-input and dual-input controllers can
provide proportional-integral (PI) control. PI controllers are
used in critical applications that require closer control than a
proportional controller. A PI controller provides close control
by eliminating the deviation from setpoint (offset) that occurs
in a proportional controller system. PI controllers are similar
to the controllers in Figures 21 and 22 and have an additional
knob for adjusting the integral reset time.

CONTROLLER ADJUSTMENTS

Controller operation is adjusted in the following ways:

— Adjusting the setpoint

— Changing between direct and reverse control action

— Adjusting the proportional band (throttling range)

— Adjusting the reset authority

— Adjusting the integral control reset time

The setpoint can be manually adjusted with a dial on the
controller. Remote setpoint adjustment is available for all con­trollers. Control action may be direct or reverse, and is field
adjustable. The proportional band setting is typically adjust­able from 2.5 to 50 percent of the primary sensor span and is
usually set for the minimum value that results in stable con­trol. In a sensor with a span of 200 degrees F, for example,
the minimum setting of 2.5 percent results in a throttling
range of 5 degrees F (0.025 x 200 = 5 degrees F). A change of
5 degrees F is then required at the sensor to proportionally
vary the controller branchline pressure from 3 to 13 psi. A maxi­mum setting of 50 percent provides a throttling range of 100
degrees F (0.50 x 200 = 100 degrees F).

ENGINEERING MANUAL OF AUTOMATIC CONTROL

72

PNEUMATIC CONTROL FUNDAMENTALS

Reset authority, also called “reset ratio”, is the ratio of the
effect of the reset sensor compared to the primary sensor.
Figure 23 shows the effect of authority on a typical reset
schedule. The authority can be set from 10 to 300 percent.

130

DA TEMPERATURE

CONTROL POINT (°F)

30

0

OUTDOOR AIR TEMPERATURE (°F)

60

COMPENSATION

START POINT

C1094

Fig. 23. Typical Reset Schedule for

Discharge Air Control.

The integral control reset time determines how quickly the
PI controller responds to a change in the controlled variable.
Proportional correction occurs as soon as the controlled
variable changes. The integral function is timed with the reset
time adjustment. The reset time adjustment is calibrated from
30 seconds to 20 minutes. The proper setting depends on
system response time characteristics.

VELOCITY SENSOR-CONTROLLER

The velocity sensor-controller combines a highly sensitive
air velocity sensor with a pneumatic controller to detect and
control airflow regardless of system static pressure. It is used
in air terminal units and other air handling systems. Reverse­and direct-acting models are available for normally closed and
normally open dampers.

The velocity sensor measures actual velocity and does not
require the conversion of velocity pressure to velocity.
Although the sensor is typically used in duct air velocity
applications, it can accurately sense velocities as low as 100
feet per minute. Flow-limiting orifices inserted into the sensor
sampling tube can measure velocity ranges up to 3,500 feet
per minute.

Figure 24 shows the operation of a velocity sensor. A
restrictor supplies compressed air to the emitter tube located
in the air stream to be measured. When no air is flowing in the
duct, the jet of air from the emitter tube impinges directly on
the collector tube and maximum pressure is sensed. Air flowing
in the duct blows the air jet downstream and reduces the
pressure on the collector tube. As the duct air velocity increases,
less and less of the jet enters the collector tube. The collector
tube is connected to a pressure amplifier to produce a usable
output pressure and provide direct or reverse action.

PNEUMATIC SENSORS

Pneumatic sensors typically provide a direct acting 3 to 15 psi
pneumatic output signal that is proportional to the measured
variable. Any change in the measured variable is reflected as a
change in the sensor output. Commonly sensed variables are
temperature, humidity, and differential pressure. The sensors
use the same sensing elements and principles as the sensors in
the controllers described earlier, but do not include setpoint
and throttling range adjustments. Their throttling range is the
same as their span.

A gage connected to the sensor output can be used to indicate
the temperature, humidity, or pressure being sensed. The gage
scale is calibrated to the sensor span.

Temperature sensors may be vapor-filled, liquid-filled,
averaging capillary, or rod-and-tube. The controller usually
provides restricted air to the sensor.

Humidity sensors measure the relative humidity of the air
in a room (wall-mounted element) or a duct (insertion element).
Nylon is typically used as the sensing element. Humidity
sensors include temperature compensation and operate on a
force-balance principle similar to a wall thermostat.

M

AIR FLOW

TO PRESSURE
AMPLIFIER

EMITTER TUBE

GAP

COLLECTOR
TUBE

C2610

Fig. 24. Velocity Sensor Operation.

A controller connected to the pressure amplifier includes
setpoints for maximum and minimum dual air velocity limits.
This allows the air volume to be controlled between the limits
by a thermostat or another controller.

Two models of the controller are available. One model
operates with a one-pipe, bleed-type thermostat, and the other
with a two-pipe thermostat. The two-pipe model also allows
sequencing for reheat applications.

The low-pressure sensor measures duct static pressure and
differential pressure. When the duct static pressure or the
pressure differential increases, branchline pressure increases.

73

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PNEUMATIC CONTROL FUNDAMENTALS

Figure 25 shows a typical application of a thermostat and

M

velocity controller on a Variable Air Volume (VAV) terminal
unit with hot water reheat. The thermostat senses a change in
room temperature and resets the velocity setpoint of the
velocity controller. The controller repositions the VAV damper
to increase or decrease airflow accordingly. If a change in duct
static pressure modifies the flow, the controller repositions the
actuator to maintain the correct flow. The reheat valve operates
only when the thermostat has reset the velocity setpoint down
to minimum airflow and the thermostat calls for heating.

VAV BOX
REHEAT VALVE

ROOM
THERMOSTAT

Fig. 25. VAV Box Velocity Controller Control System.

ACTUATORS AND FINAL CONTROL ELEMENTS

A pneumatic actuator and final control element such as a
valve (Fig. 26) or damper (Fig. 27) work together to vary the
flow of the medium passing through the valve or damper. In
the actuator, a diaphragm and return spring move the damper
push rod or valve stem in response to changes in branchline
pressure.

DIAPHRAGM

BRANCH
LINE

VALV E

SPRING

VALV E
STEM

INLET
FLOW

OUTLET
FLOW

ACTUATOR

VALV E

ACTUATORS

GENERAL

Pneumatic actuators position damper blades and valve
stems. A damper actuator typically mounts on ductwork or on
the damper frame and uses a push rod and crank arm to position
the damper blades (rotary action). A valve actuator mounts on
the valve body and positions the valve stem directly (linear
action) for a globe valve or rotary action via linkage for a
butterfly valve. Valve actuator strokes typically are between
one-quarter and one and one-half inch. Damper actuator
strokes range from one to four inches (longer in special
applications). In commercial pneumatic actuators, air pressure
positions the actuator in one direction and a spring returns it
the other direction.

VELOCITY
CONTROLLER

DAMPER
ACTUATOR

VAV BOX
DAMPER

VELOCITY
SENSOR IN
DUCTWORK

M10296

Fig. 26. Pneumatic Actuator and Valve.

M10361

DAMPER

ROLLING
DIAPHRAGM

AIRFLOW

PISTON

PUSH
ROD

SPRING

DAMPER
ACTUATOR

C2611

BRANCH
LINE

Fig. 27. Pneumatic Actuator and Damper.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

Valve actuators are direct or reverse acting. Damper actuators
are direct acting only. A direct-acting actuator extends on an
increase in branchline pressure and retracts on a decrease in
pressure. A reverse-acting actuator retracts on an increase in
branchline pressure and extends on a decrease in pressure.

Pneumatic valve and damper actuator assemblies are termed
“normally open” or “normally closed.” The normal position is
the one assumed upon zero actuator air pressure. Three-way
valves have both normally open (N.O.) and normally closed
(N.C.) ports.

SPRING RANGES

Springs used in valve and damper actuators determine the
start pressure and pressure change required for full movement
of the actuator from open to closed, or from closed to open.
Actuators designed for special applications can move through
the full range, open to closed or closed to open, on a limited
change in pressure from the controller. Such actuators can

74

PNEUMATIC CONTROL FUNDAMENTALS

provide a simple form of sequence control (e.g., operating
heating and cooling valves from a single thermostat). Typical
spring pressure ranges are 2-7 psi, 8-12 psi, and 3-13 psi.

CONTROL VALVES

Single-seated globe valves (Fig. 28) are used where tight
close-off is required. The valve body can be either direct acting
or reverse acting. A direct-acting valve body allows flow with
the stem up, while a reverse-acting valve body shuts off flow
with the stem up. The combination of valve body and actuator
(called the valve assembly) determines the normal valve stem
position.

BRANCH
LINE

FLOW

NORMALLY OPEN VALVE ASSEMBLY (DIRECT-ACTING

A.

VALVE BODY AND DIRECT-ACTING ACTUATOR)

BRANCH
LINE

FLOW

B.

NORMALLY CLOSED VALVE ASSEMBLY (DIRECT-ACTING
VALVE BODY AND REVERSE-ACTING ACTUATOR)

BRANCH
LINE

(F2

F1

F2

F3

F1

)

F3

F1

F2

The position maintained by the valve stem depends on the

balance of forces acting on it:

—Force F1 from the air pressure on the diaphragm
— Opposing force F2 from the actuator spring
— Controlled-medium force F3 acting on the valve disc and

plug due to the difference between inlet and outlet
pressures

An increase in controller branchline pressure increases force
F1, (Fig. 28A), moving the diaphragm down and positions
the valve stem toward closed until it has moved far enough
that the sum of the spring force F2 and the controlled-medium
force F3 increases balance the increased force F1 on the
diaphragm. Conversely, a decrease in controller branchline air
pressure in the diaphragm chamber of a direct-acting actuator
decreases force F1, allowing forces F2 and F3 to push the
diaphragm upward and move the valve stem toward the open
position.

In Figure 28B, branchline pressure is applied on the bottom
surface of the diaphragm. An increase in air pressure in the
diaphragm chamber increases force F1 causing the actuator
diaphragm to move upward and open the valve. Motion
continues until the increase in pressure on the diaphragm plus
the controlled-medium force F3 is balanced by the increase in
spring compression (force F2). On a decrease in air pressure
in the diaphragm chamber, the compressed spring moves the
diaphragm down toward its normal position and the valve stem
toward closed. A normally closed valve assembly usually has
a lower close-off rating against the pressure of the controlled
medium than a normally open valve because the spring force
F2 is the only force available to close the valve.

In Figure 28C, an increase in branchline pressure in the
actuator increases force F1 causing the diaphragm to move
downward and open the valve. Motion continues until the
increase in pressure on the diaphragm (force F1) plus the
controlled-medium force F3 is balanced by the increase in
spring compression (force F2). On a decrease in air pressure
in the diaphragm chamber, the compressed-spring pressure
moves the diaphragm up and the valve stem moves toward
the closed position.

F3

FLOW

C.

NORMALLY CLOSED VALVE ASSEMBLY (REVERSE-ACTING
VALVE BODY AND DIRECT-ACTING ACTUATOR)

Fig. 28. Single-Seated Valves.

C2613

In a double-seated valve (Fig. 29), the controlled agent flows
between the two seats. This placement balances the inlet
pressures between the two discs of the plug assembly and
reduces the actuator force needed to position the plug assembly.
Double-seated valves generally do not provide tight close-off
because one disc may seat before the other and prevent the
other disc from seating tightly.

75

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PNEUMATIC CONTROL FUNDAMENTALS

BRANCH
LINE

INLET
FLOW

F1

F2

F3

NORMALLY OPEN VALVE

OUTLET
FLOW

C2612

Fig. 29. Double-Seated Valve.

Figure 30 shows three-way globe valve assemblies. The
mixing valve has two inlets and a common outlet. The diverting
valve has a common inlet and two outlets.

BRANCH
LINE

F1

F2

Two- and three-way butterfly valves can be operated by long

stroke pneumatic actuators and appropriate linkage (Fig. 31).

One or two low pressure actuators powered directly by
branchline pressure can operate butterfly valves up to about
12 inches, depending on the differential close-off rating of the
valve. For other applications high pressure pneumatic cylin­ders can be used to provide the force required by the valve. A
pneumatic positioner provides an appropriate high pressure
signal to the cylinder based on a 3 to 15 psi input signal.

INLET
FLOW

MIXING VALVE, NORMALLY CLOSED TO
STRAIGHT-THROUGH FLOW

BRANCH
LINE

INLET
FLOW

DIVERTING VALVE, NORMALLY OPEN TO
STRAIGHT-THROUGH FLOW

INLET FLOW

F1

F2

OUTLET FLOW

OUTLET
FLOW

OUTLET
FLOW

C2615

Fig. 30. Three-Way Valve Assemblies.

Three-way valves may be piped to be normally open or
normally closed to the heating or cooling load. If a three-way
valve has linear characteristics and the pressure differentials
are equal, constant total flow is maintained through the
common inlet or outlet port.

M10403

Fig. 31. Butterfly Valve Assembly.

For a more detailed discussion of valves, see the Valve

Selection And Sizing section.

DAMPERS

Dampers control the flow of air in air-handling systems. The
most common type of damper, a multiblade louver damper,
can have parallel or opposed blades (Fig. 32).

PARALLEL
BLADES

Fig. 32. Parallel- and Opposed-Blade Dampers.

OPPOSED
BLADES

C2604

ENGINEERING MANUAL OF AUTOMATIC CONTROL

76

Figure 33 shows normally open and normally closed paral-

C2605

NORMALLY OPEN
DAMPER ASSEMBLY

BRANCH LINE

BRANCH LINE

ACTUATOR

NORMALLY CLOSED
DAMPER ASSEMBLY

ACTUATOR

lel-blade dampers. A normally open damper returns to the open
position with low air pressure in the actuator diaphragm
chamber. An increase in branchline pressure forces the rolling
diaphragm piston to move against the spring, and a decrease
allows the compressed spring to force the piston and diaphragm
back to the normal position. As with valve actuators, interme­diate positions depend on a balance between the force of the
control air pressure on the diaphragm and the opposing force
of the actuator spring.

PNEUMATIC CONTROL FUNDAMENTALS

A normally closed damper returns to the closed position with
low air pressure in the actuator diaphragm chamber. The way
the damper blades, crank arm, and push rod are oriented during
installation determines the normal (open or closed) position
of the damper blades.

For a more detailed discussion of dampers, see the Damper
Selection and Sizing section.

RELAYS AND SWITCHES

In the following illustrations, common (C) and the normally
connected port (O) are connected on a fall in pilot pressure (P)
below the relay setpoint, and the normally disconnected port
(X) is blocked (Fig. 34). On a rise in pilot pressure above the
relay setpoint, C and X are connected and O is blocked.

PO

CX

PILOT SIGNAL BELOW
RELAY SETPOINT

PORTS: P= PILOT

C= COMMON
O= NORMALLY CONNECTED
X= NORMALLY DISCONNECTED

Fig. 34. Relay Port Connections.

PO

CX

PILOT SIGNAL ABOVE
RELAY SETPOINT

C2344

Fig. 33. Normally Open and Normally Closed Dampers.

Figure 35 shows a typical spdt switching relay application for
heating/cooling operation in which the thermostat controls the
heating/cooling coil valve. Seasonal mainline pressure changes
cause the action of the thermostat to be reversed. A discharge
low-limit control is switched into the control circuit for heat­ing and out of the circuit for cooling. The switching is done
from mainline pressure connected to the pilot port (P).

During the heating cycle, the 18 psi mainline pressure is
above the preset switching pressure. The common port (C)
connects to the normally disconnected port (X), connecting
the low-limit controller to the thermostat branchline to prevent
discharge temperatures below the controller setting. The
normally connected port (O) is blocked.

SWITCHING RELAY

available with either single-pole, double-throw (spdt) or
double-pole, double-throw (dpdt) switching action. Pneumatic
heating and cooling control systems use relays to switch a valve
or damper actuator from one circuit to another or to positively
open or close a device. Both spdt and dpdt switching relays
are available with a variety of switching pressures.

A switching relay requires a two-position pilot signal and is

77

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PNEUMATIC CONTROL FUNDAMENTALS

ROOM
THERMOSTAT

DA WINTER
RA SUMMER

MB

RESTRICTOR

M

VALVE

COIL

M

SWITCHING
RELAY

OC

XP

CAP

LIMIT
CONTROLLER

DISCHARGE
AIR

C2379

Fig. 35. Typical Switching Relay for Application.

During the cooling cycle, the 13 psi mainline pressure at the
pilot port (P) is below the minimum switching pressure of the
preset limits. The common port (C) connects to the normally
connected port (O), which is capped. The normally
disconnected port (X) is closed and removes the low-limit
controller from the system.

In a dpdt model, the common, normally connected, and
normally disconnected ports are duplicated in the second
switch section.

SNAP ACTING RELAY

The snap acting relay is a spdt switch that provides two­position switching action from a modulating signal and has an
adjustable switching point. The switching differential is less
than 1.0 psi. The switching pressure is manually adjustable
for 3 to 15 psi operation.

Figure 36 shows a snap acting relay application. Operation
is similar to the switching relay. When the branchline pressure
from the outdoor air thermostat equals or exceeds the preset
switchover pressure, the relay connects the normally
disconnected port (X) and blocks the normally connected port
(O) to deliver main air to the normally open heating valve
and provide positive close off. When the outdoor air thermostat
pressure drops below the relay setpoint, the normally
disconnected port (X) is blocked and the normally connected
port (O) connects to the common port (C) to connect the valve
actuator to the room thermostat.

DA OUTDOOR AIR
THERMOSTAT

M

B

N.O. HEATING
VALV E

PCX

SNAP ACTING
RELAY

O

DA ROOM
THERMOSTAT

MB

MM

VAV TERMINAL UNIT
DAMPER ACTUATOR

C2360

Fig. 36. Typical Application for Snap Acting Relay.

LOCKOUT RELAY

The lockout relay is a three-port relay that closes off one
pressure signal when a second signal is higher. Figure 37 shows
a typical application in which mixed air control becomes
disabled when outdoor air temperature is higher than return
air temperature. To prevent air from being trapped in the line
between the lockout relay and the snap acting relay, a small
bleed must be present either in the pilot chamber of the snap
acting relay or in the line.

MIXED AIR
TEMPERATURE
CONTROLLER

BLEED

LOCKOUT
RELAY

P

1

P

2

B

THERMOSTAT

P

2

OUTDOOR AIR
DAMPER ACTUATOR

OUTDOOR AIR
THERMOSTAT

M

B

RESTRICTOR

TO
ACTUATOR

C2355

M

C2362

M

SNAP ACTING

M

B

RELAY

M

EXH

RETURN AIR
THERMOSTAT

M

XOC

P

M

B

Fig. 37. Lockout Relay in Economizer Cycle.

Figure 38 shows the lockout relay used as a repeater. This
application provides circuit isolation by repeating the pilot
signal with a second air source.

OUTPUT

P

1

B

LOCKOUT
RELAY

M

RESTRICTOR

EXH

ENGINEERING MANUAL OF AUTOMATIC CONTROL

Fig. 38. Lockout Relay as Repeater.

78

PNEUMATIC CONTROL FUNDAMENTALS

LOWEST

M

HIGHEST

12 3

N

LOAD ANALYZER RELAY

DA ZONE THERMOSTATS

TO
ZONE
DAMPERS

C2369

MBMMB

M

MB

M

M

M

MB

8-12 PSI
N.C. COOLING
VALVE

2-7 PSI
N.O. HEATING
VALVE

HIGH-PRESSURE SELECTOR RELAY

The high-pressure selector relay is a three-port relay that
transmits the higher of two input signals to the output branch.
The high sensitivity of the relay allows it to be used in sensor
lines with an accuracy of 2 to 3 degrees F.

The application shown in Figure 39 uses pressures from two
zones and a high-pressure selector relay to determine control.
A separate thermostat controls each zone damper. The
thermostat that calls for the most cooling (highest branchline
pressure) controls the cooling valve through the high-pressure
selector relay.

DA ZONE
THERMOSTAT

MB

M

P

1

P

2

B

HIGH-PRESSURE
SELECTOR RELAY

N.C. ZONE
DAMPER

2-7 PSI
DAMPER
ACTUATOR

C2363

N.C. ZONE
DAMPER

2-7 PSI
DAMPER
ACTUATOR

DA ZONE
THERMOSTAT

MB

M

8-12 PSI N.C.
COOLING COIL
VALVE

DA ZONE

N.O. ZONE
DAMPER

8-12 PSI DAMPER
ACTUATOR

RESTRICTOR

2-7 PSI N.O.
HEATING COILVALVE

THERMOSTAT

MB

M

EXH

DA ZONE
THERMOSTAT

MB

M

P

1

P

2

B

LOW-PRESSURE
SELECTOR RELAY

N.O. ZONE
DAMPER

8-12 PSI
DAMPER
ACTUATOR

C2364

Fig. 40. Typical Application for

Low-Pressure Selector Relay.

LOAD ANALYZER RELAY

The load analyzer relay is a bleed-type, diaphragm-logic
pressure selector. The relay selects the highest and lowest
branch pressure from multiple inputs to operate final control
elements (Fig. 41). The relay contains multiple diaphragms
and control nozzles. Each input pressure connects to two
diaphragms.

Fig. 39. Typical Application for

High-Pressure Selector Relay.

LOW-PRESSURE SELECTOR RELAY

The low-pressure selector relay is a three-port relay that
selects the lower of two input pressure signals and acts as a
repeater for the lower of the two inputs. The relay requires an
external restrictor on the input to the branch port. Figure 40
shows a low-pressure selector relay controlling the heating coil
valve from the thermostat that calls for the most heat.

Fig. 41. Load Analyzer Relay in
Multizone Air Unit Application.

In Figure 41, the load analyzer relay selects the lowest
pressure signal from the thermostat in the coldest zone and
transmits that signal to a normally open heating valve. The
relay transmits the highest pressure signal from the thermostat
in the warmest zone to a normally closed cooling valve.

79

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PNEUMATIC CONTROL FUNDAMENTALS

CAPACITY RELAY

The capacity relay is a direct-acting relay that isolates an
input and repeats the input pressure with a higher capacity
output. Figure 42 shows a capacity relay enabling a single
bleed-type thermostat to operate multiple damper actuators
quickly by increasing the output capacity of the thermostat.

BLEED-TYPE
THERMOSTAT

DAMPER
ACTUATOR

EXH

E

M

M

P

B

M

CAPACITY
RELAY

DAMPER
ACTUATOR

DAMPER
ACTUATOR

C2365

Fig. 42. Typical Capacity Relay Application.

REVERSING RELAY

The reversing relay is a modulating relay with an output
that decreases at a one-to-one ratio as the input signal increases.
Figure 43 shows a reversing relay application. A falling
temperature at the direct-acting thermostat causes the
branchline pressure to decrease. The reversing relay branch
pressure increases and opens the normally closed heating valve.

DA ROOM
THERMOSTAT

M B

M

Fig. 43. Reversing Relay Application.

M

B

M

E

P

REVERSING
RELAY

EXH

N.C. HEATING
VALVE

C2354

POSITIVE-POSITIONING RELAY

The positive-positioning relay (Fig. 44) mounts directly on
a valve or damper actuator. The relay positions the valve or
damper precisely according to the branchline pressure from a
thermostat or other controller, regardless of the load variations
affecting the valve stem or damper shaft. The relay is typically
used for large actuators for sequencing, or in applications
requiring precise control.

ROOM
THERMOSTAT

M

DAMPER
ACTUATOR

M B

P

M

B

FEEDBACK
SPRING

DAMPER

POSITIVE­POSITIONING
RELAY

M

VALVE
ACTUATOR

ROOM
THERMOSTAT

M B

P

M

B

VALVE

POSITIVE­POSITIONING
RELAY

INTERNAL
FEEDBACK
SPRING

C2366

Fig. 44. Positive-Positioning Relay

with Damper and Valve Actuators.

When the relay is connected to an actuator, the feedback
spring produces a force proportional to the actual valve or
damper position. The relay positions the actuator in proportion
to the branchline input. If the connected load attempts to
unbalance the required valve stem position, the relay either
exhausts or applies main pressure to the actuator to correct the
condition. If the valve or damper sticks or the load prevents
proper positioning, the relay can apply the pressure required
(up to full main pressure) or down to zero to correct the
condition.

The positive-positioning relay also permits sequenced
operation of multiple control valves or dampers from a single
thermostat or controller. For example, a normally open heating
valve and a normally closed outdoor air damper could be
controlled from a single thermostat piloting relays on two
actuators. Relays typically have a 3, 5, or 10 psi input pressure
span and an adjustable start pressure. As the space temperature
rises into the low end of the thermostat throttling range, the
heating valve positioner starts to close the valve.

AVERAGING RELAY

The averaging relay is a direct-acting, three-port relay used
in applications that require the average of two input pressures
to supply a controller input or to operate a controlled device
directly.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

80

PNEUMATIC CONTROL FUNDAMENTALS

Figure 45 shows an averaging relay in a typical application
with two thermostat signals as inputs. The average of the
thermostat signals controls a valve or damper actuator.

THERMOSTAT 1

M B

M

THERMOSTAT 2

M B

P

2

B

P

1

AVERAGING
RELAY

TO
ACTUATOR

C2345

Fig. 45. Averaging Relay Application.

RATIO RELAY

The ratio relay is a four-port, non bleed relay that produces
a modulating pressure output proportional to the thermostat
or controller branchline output. Ratio relays can be used to
control two or three pneumatic valves or damper actuators in
sequence from a single thermostat. The ratio relay has a fixed
input pressure range of either 3 or 5 psi for a 10 psi output
range and an adjustable start point. For example, in a ratio
relay with a 5 psi range set for a 7 psi start, as the input pressure
varies from 7 to 12 psi (start point plus range), the output
pressure will vary from 3 to 13 psi.

In Figure 46, three 3 psi span ratio relays are set for 3 to 6,
6 to 9, and 9 to 12 psi inputs, respectively. The thermostat
signal through the relays proportions in sequence the three
valves or actuators that have identical 3 to 13 psi springs.

PNEUMATIC POTENTIOMETER

The pneumatic potentiometer is a three-port, adjustable
linear restrictor used in control systems to sum two input signal
values, average two input pressures, or as an adjustable flow
restriction. The potentiometer is a linear, restricted air passage
between two input ports. The pressure at the adjustable output
port is a value based on the inputs at the two end connections
and the location of the wiper between them.

Figure 47 shows a pneumatic potentiometer providing an
average of two input signals. The wiper is set at mid-scale for
averaging or off-center for a weighted average. It can be used
this way to average two air velocity transmitter signals from
ducts with different areas by positioning the wiper according
to the ratio of the duct areas. This outputs a signal proportional
to the airflow.

PNEUMATIC POTENTIOMETER

INPUT 1

OUTPUT

INPUT 2

SIGNALS
FROM
SENSORS

Fig. 47. Pneumatic Potentiometer as Averaging Relay.

WEIGHTED
AVERAGE
SIGNAL

AVERAGE
SIGNAL

C2374

DA ZONE
THERMOSTAT

M

M B

M

EXH

M

EXH

M

EXH

RATIO RELAY 1
3-6 PSI

M

EBP

RATIO RELAY 2
6-9 PSI

M

EBP

RATIO RELAY 3
9-12 PSI

M

EBP

HEATING
VALVE
3–13 PSI

MIXING
DAMPERS
3–13 PSI

COOLING
VALVE
3–13 PSI

C2370

Fig. 46. Ratio Relays in Sequencing Control Application.

Figure 48 shows a pneumatic potentiometer as an adjustable
airflow restrictor.

Fig. 48. Pneumatic Potentiometer as

PNEUMATIC POTENTIOMETER

INPUT 1

OUTPUT

INPUT 2

M

CAP

TO
CONTROLLED
DEVICE

C2372

Adjustable Airflow Restrictor.

81

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PNEUMATIC CONTROL FUNDAMENTALS

)

HESITATION RELAY

The hesitation relay is used with a pneumatic actuator in
unit ventilator applications. The output pressure goes to
minimum whenever the input pressure is below the minimum
setting. Figure 49 shows a graph of the output of a hesitation
relay as controlled by the relay knob settings (piloted from the
thermostat).

18

16

14

12

10

8
7

6

RELAY OUTPUT (PSI)

4

2

0

0 246810121416 18

RELAY INPUT (PSI

Fig. 49. Hesitation Relay Output Pressure

as a Function of Knob Setting.

The hesitation relay has an internal restrictor. Figure 50
shows a typical application of a hesitation relay and a
pneumatic damper actuator. When the thermostat branchline
pressure reaches 1.5 psi, the relay output goes to its preset
minimum pressure. When the branchline pressure of the
thermostat reaches the setting of the hesitation relay, the
thermostat controls the damper actuator. When the thermostat
branchline pressure drops below the hesitation relay setting,
the relay holds the damper actuator at the minimum position
until the thermostat branchline pressure drops below 1.5 psi.
At that point, the hesitation relay output falls to zero.

DA ROOM
THERMOSTAT

HESITATION

MB

M

RELAY

P

B

M

M

DAMPER
ACTUATOR

C2346

Fig. 50. Typical Hesitation Relay Application.

100

80
60
40
20

0

KNOB SETTING (%)

C1097

ELECTRICAL INTERLOCKING RELAYS

Electrical interlocking relays bridge electric and pneumatic
circuits. The electric-pneumatic relay uses electric power to
actuate an air valve in an associated pneumatic circuit. The
pneumatic-electric relay uses control air pressure to make or
break an associated electrical circuit.

ELECTRIC-PNEUMATIC RELAY

The electric-pneumatic (E/P) relay is a two-position, three­way air valve. Depending on the piping connections to the
ports, the relay performs the same functions as a simple
diverting relay. A common application for the E/P relay is to
exhaust and close an outdoor air damper in a fan system when
the fan motor is turned off, as shown in Figure 51.

E/P RELAY

SOLENOID

FAN
INTERLOCK
VOLTAGE

OXC

EXH

M

Fig. 51. E/P Relay Application.

When the relay coil is de-energized, the solenoid spring seats
the plunger. The normally disconnected port (X) is blocked
and the normally connected port (O) connects to the common
port (C). The connection exhausts the damper actuator which
closes the damper. When the relay coil is energized, the plunger
lifts against the tension of the spring and blocks the normally
connected port (O). Main air at the normally disconnected port
(X) connects to the common port (C) and opens the damper.

PNEUMATIC-ELECTRIC RELAY

Figure 52 shows a simplified pneumatic-electric (P/E) relay
with a spdt switch. The P/E relay makes the normally closed
contact on a fall in pilot pressure below the setpoint, and makes
the normally open contact on a rise above a value equal to the
setpoint plus the differential. For example, with a setpoint
adjustment of 3 psi and a differential of 2 psi, the pump is
energized at pilot pressures below 3 psi and turns off at pilot
pressures above 5 psi.

SPRING

RELAY COIL

PLUNGER

OUTDOOR
AIR DAMPER
ACTUATOR

C2602

ENGINEERING MANUAL OF AUTOMATIC CONTROL

82

PNEUMATIC CONTROL FUNDAMENTALS

TEMPERATURE
CONTROLLER

MB

M

P/E RELAY

BELLOWS

SPRING

SETPOINT
ADJUSTMENT

N.O.

N.C.

C

PUMP

PUMP
VOLTAGE

C2384

Fig. 52. P/E Relay Application.

ELECTRONIC-PNEUMATIC TRANSDUCER

The electronic-pneumatic transducer is a proportional relay
that varies the branch air pressure linearly 3 to 15 psi in
response to changes in an electrical input of 2 to 10 volts or 4
to 20 ma. Electronic-pneumatic transducers are used as the
interface between electronic, digital, or computer-based control
systems and pneumatic output devices (e.g., actuators).

A resistance-type temperature sensor in the discharge air
duct is the input to the controller, which provides all of the
system adjustments and logic requirements for control. The
controller output of 2 to 10 volts dc is input to the electronic­pneumatic transducer, which converts the signal to a 3 to 15
psi output to position the heating valve.

PNEUMATIC SWITCH

The pneumatic switch is available in two- or three-position
models (Fig. 54). Rotating the switch knob causes the ports to
align in one of two ways in a two-position switch, and in one
of three ways in a three-position switch. The two-position
switch is used for circuit interchange. The three-position switch
sequentially switches the common port (Port 2) to the other
ports and blocks the disconnected ports.

1

2

1

3

4

TWO-POSITION SWITCH

3

1

2

1

3

4

3

Figure 53 shows discharge air temperature control of a
heating coil using digital control for sensing and control. The
output of the transducer positions the valve on a heating coil.

DDC
CONTROLLER

ELECTRONIC­PNEUMATIC

M B

TRANSDUCER

M

HEATING
VALVE

HEATING COIL

ELECTRONIC
TEMPERATURE
SENSOR

DISCHARGE

AIR

C2378

Fig. 53. Typical Electronic-Pneumatic

Transducer Application.

2

4

1

2

THREE-POSITION SWITCH

2

3

4

4

C1887

Fig. 54. Pneumatic Switches.

Figure 55 shows a typical application for sequential
switching. In the OPEN position, the valve actuator exhausts
through Port 4 and the valve opens. In the AUTO position, the
actuator connects to the thermostat and the valve is in the
automatic mode. In the CLOSED position, the actuator
connects to main air and the valve closes.

83

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PNEUMATIC CONTROL FUNDAMENTALS

M

MB

M

B
P

M

OA DAMPER
ACTUATOR

MINIMUM
POSITION
SWITCH

DA MIXED AIR
TEMPERATURE
CONTROLLER

VALVE

C2348

DA
THERMOSTAT

MB

M

123

M

4

EXH

THREE-POSITION
SWITCH

N.O. VALVE

NOTE: POSITION 1: OPEN—PORTS 2 AND 4 CONNECTED
POSITION 2: AUTO—PORTS 2 AND 3 CONNECTED

POSITION 3: CLOSED—PORTS 2 AND 1 CONNECTED

M10295

Fig. 55. Typical OPEN/AUTO/CLOSED Application.

MANUAL POSITIONING SWITCH

A manual positioning switch is used to position a remote
valve or damper or change the setpoint of a controller. The
switch takes input air from a controller and passes a preset,
constant, minimum air pressure to the branch regardless of
the controller output (e.g., to provide an adjustable minimum
position of an outdoor air damper). Branchline pressure from
the controller to other devices connected to the controller is
not affected.

Figure 56 shows the switch functioning as a minimum
positioning switch. The damper will not close beyond the
minimum setting of the positioning switch. As the controller
signal increases above the switch setting, the switch positions
the damper according to the controller signal.

Fig. 56. Typical Three-Port Minimum

Position Switch Application.

Manual switches are generally panel mounted with a dial
plate or nameplate on the front of the panel which shows the
switch position. Gages are sometimes furnished to indicate
the main and branch pressures to the switch.

PNEUMATIC CONTROL COMBINATIONS

GENERAL

A complete control system requires combinations of several
controls. Figure 57 shows a basic control combination of a
thermostat and one or more control valves. A normally open
control valve assembly is selected when the valve must open
if the air supply fails. A normally open control valve requires
a direct-acting thermostat in the heating application shown in
Figure 56. Cooling applications may use normally closed
valves and a direct-acting thermostat. The thermostat in Figure
56 has a 5 degree throttling range (output varies from 3 to 13
psi of the 5 degree range) and the valves have an 8 to 12 psi
spring range, then the valve will modulate from open to closed
on a 2 degree rise in temperature at the thermostat.

X 5F° = 2F°

4 psi

10 psi

ENGINEERING MANUAL OF AUTOMATIC CONTROL

DA

THERMOSTAT

MB

M

N.O. HEATING COIL VALVES

Fig. 57. Thermostat and One or

TO OTHER
VALV ES

C2349

More Normally Open Valves.

A normally open or a normally closed valve may be
combined with a direct-acting or a reverse-acting thermostat,
depending on the requirements and the conditions in the
controlled space. Applications that require several valves
controlled in unison (e.g., multiple hot water radiation units
in a large open area) have two constraints:

—All valves that perform the same function must be of

the same normal position (all normally open or all
normally closed).

84

PNEUMATIC CONTROL FUNDAMENTALS

— The controller must be located where the condition it

measures is uniformly affected by changes in position
of the multiple valves. If not, the application requires
more than one controller.

A direct- or reverse-acting signal to a three-way mixing or
diverting valve must be selected carefully. Figure 58 shows
that the piping configuration determines the signal required.

HOT WATER
SUPPLY

DIRECT­ACTING
SIGNAL

HOT WATER COIL

THREE-WAY
MIXING VALVE

HOT WATER
RETURN

REVERSE­ACTING
SIGNAL

HOT WATER COIL

THREE-WAY
MIXING VALVE

HOT WATER
RETURN

HOT WATER
SUPPLY

C2377

Fig. 58. Three-Way Mixing Valve Piping

with Direct Actuators.

SEQUENCE CONTROL

In pneumatic control systems, one controller can operate
several dampers or valves or several groups of dampers or valves.
For example, year-round air conditioning systems sometimes
require heating in the morning and evening and cooling in the
afternoon. Figure 59 shows a system in which a single controller
controls a normally open heating valve and normally closed
cooling valve. The cooling valve is set for an 8 to 13 psi range
and the heating valve, for a 2 to 7 psi range. The controller operates
the two valves in sequence to hold the temperature at the desired
level continually.

valve is closed. As the temperature rises, the branchline
pressure increases and the heating valve starts to close. At
7 psi branchline pressure, the heating valve is fully closed. If
the temperature continues to rise, the branchline pressure
increases until the cooling valve starts to open at 8 psi. The
temperature must rise enough to increase the branchline
pressure to 13 psi before the cooling valve will be full open.
On a drop in temperature, the sequence is reversed.

Valves with positive positioners ensure tight close-off of the
heating valve at 7 psi branchline pressure, and delay opening
of the cooling valve until 8 psi branchline pressure is reached.
Positive positioners prevent overlapping caused by a variation
in medium pressure, a binding valve or damper, or a variation
in spring tension when using spring ranges for sequencing.

A greater deadband can be set on the positioners to provide
a larger span when no energy is consumed. For example, if the
positioners are set for 2 to 7 psi on heating and 13 to 18 psi on
cooling, no energy is used when the controller branchline
pressure is between 7 and 13 psi. The positioners can also be
set to overlap (e.g., 4 to 9 and 7 to 12 psi) if required.

Valve and damper actuators without positioners have various
spring ranges. To perform the sequencing application in Figure
59 without positioners, select a heating valve actuator that has
a 2 to 7 psi spring range and a cooling valve actuator that has
an 8 to 13 psi spring range. Although this method lessens
precise positioning, it is usually acceptable in systems with
lower pressure differentials across the valve or damper and on
smaller valves and dampers .

LIMIT CONTROL

Figure 60 shows a sensor-controller combination for space
temperature control with discharge low limit. The discharge
low limit controller on a heating system prevents the discharge
air temperature from dropping below a desired minimum.

SENSOR

C2357

DA

CONTROLLER

S M B

M

M

POSITIVE
POSITIONING

M P M P

ACTUATORS

N.C. COOLING
VALVE
8-13 PSI

M

N.O. HEATING
VALVE
2-7 PSI

Fig. 59. Pneumatic Sequencing of Two Valves with

Positive Positioning Actuators.

When the temperature is so low that the controller calls for
full heat, the branchline pressure is less than 3 psi. The normally
open heating valve is open and the normally closed cooling

RETURN AIR
SENSOR

PRIMARY
CONTROLLER (DA)

B S M

M

N.O.
VALVE

LOW-PRESSURE

P

SELECTOR
RELAY

BP

EXH

HEATING COIL

LOW-LIMIT
CONTROLLER (DA)

B S M

SENSOR

Fig. 60. Low-Limit Control (Heating Application).

85

ENGINEERING MANUAL OF AUTOMATIC CONTROL

M

DISCHARGE

AIR

C2380

PNEUMATIC CONTROL FUNDAMENTALS

Low-limit control applications typically use a direct-acting
primary controller and a normally open control valve. The
direct-acting, low-limit controller can lower the branchline
pressure regardless of the demands of the room controller, thus
opening the valve to prevent the discharge air temperature from
dropping below the limit controller setpoint. Whenever the low­limit discharge air sensor takes control, however, the return
air sensor will not control. When the low-limit discharge air
sensor takes control, the space temperature increases and the
return air sensor will be unable to control it.

A similar combination can be used for a high-limit heating
control system without the selector relay in Figure 61. The
limit controller output is piped into the exhaust port of the
primary controller, which allows the limit controller to limit
the bleed-down of the primary controller branch line.

PRIMARY
CONTROLLER (DA)

B S M

PRIMARY
SENSOR

N.O.
VALVE

M

LOW-LIMIT
CONTROLLER (DA)

B S M

B S M

M

LIMIT
SENSOR

DISCHARGE

AIR

MANUAL SWITCH CONTROL

Common applications for a diverting switch include on/off/
automatic control for a heating or a cooling valve, open/closed
control for a damper, and changeover control for a two-pressure
air supply system. Typical applications for a proportional
switch include manual positioning, remote control point
adjustment, and minimum damper positioning.

Figure 63 shows an application for the two-position manual
switch. In Position 1, the switch places the thermostat in
control of Valve 1 and opens Valve 2 by bleeding Valve 2 to
zero through Port 1. When turned to Position 2, the switch
places the thermostat in control of Valve 2 and Valve 1 opens.

THERMOSTAT

B

M

M

EXH

NOTE:
POSITION 1: PORTS 3 AND 2, 1 AND 4 CONNECTED
POSITION 2: PORTS 3 AND 4, 1 AND 2 CONNECTED

Fig. 63. Application for Two-Position Manual Switch.

314

2

TWO-POSITION

SWITCH

N.O. VALVE 2

N.O. VALVE 1

C2351

HEATING COIL

C2381

Fig. 61. High-Limit Control (Heating Application).

Bleed-type, low-limit controllers can be used with pilot­bleed thermostats (Fig. 62). A restrictor installed between
the thermostat and the low-limit controller, allows the low
limit controller to bleed the branch line and open the valve.
The restrictor allows the limit controller to bleed air from
the valve actuator faster than the thermostat can supply it,
thus overriding the thermostat.

N.O. VALVE

DA
LOW-LIMIT
CONTROLLER

C2350

THERMOSTAT

M

DA

B

M

Figure 64 shows an application of the three-position switch

and a proportioning manual positioning switch.

DAMPER
ACTUATOR

MANUAL

EXH

POSITIONING
SWITCH

B

M

E

EXH

C2352

M

DA

THERMOSTAT

NOTE: POSITION 1: AUTO—PORTS 2 AND 4 CONNECTED
POSITION 2: CLOSED—PORTS 2 AND 3 CONNECTED

M

M

POSITION 3: MANUAL—PORTS 2 AND 1 CONNECTED

B

THREE-POSITION
SWITCH

241

3

Fig. 64. Application for Three-Position Switch and

Manual Positioning Switch.

In Position 1, the three-position switch places the thermostat
in control of the damper. Position 2 closes the damper by
bleeding air pressure to zero through Port 3. Position 3 allows
the manual positioning switch to control the damper.

Fig. 62. Bleed-Type, Low-Limit Control System.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

86

PNEUMATIC CONTROL FUNDAMENTALS

EXH

M

XOC

MIXED AIR
SENSOR

BMS

FAN
VOLTAGE

E/P
RELAY

N.C. DAMPER
ACTUATOR

N.C. DAMPER

C2361

DA CONTROLLER

CHANGEOVER CONTROL FOR TWO­PRESSURE SUPPLY SYSTEM

Figure 65 shows a manual switch used for changeover from
13 to 18 psi in the mains. Either heating/cooling or day/night
control systems can use this arrangement. In Position 1, the
switch supplies main pressure to the pilot chamber in the PRV.
The PRV then provides 18 psi (night or heating) main air
pressure to the control system.

MANUAL

SWITCH

CAP

13

EXH

MAIN PRESSURE
GAUGE

FROM
COMPRESSOR

HIGH PRESSURE

GAGE

FILTER

TWO-PRESSURE

REDUCING VALVE

Fig. 65. Two-Pressure Main Supply System

with Manual Changeover.

In Position 2, the manual switch exhausts the pilot chamber
in the PRV. The PRV then provides 13 psi (day or cooling) to
the system.

Figure 66 shows a two-pressure system with automatic
changeover commonly used in day/night control. A switch in
a seven-day time clock and an E/P relay provide the
changeover. When the E/P relay energizes (day cycle), the pilot
chamber in the PRV exhausts and controls at 13 psi. When the
electric-pneumatic relay de-energizes, the pilot chamber
receives full main pressure and the PRV provides 18 psi air.

24

MAIN
AIR

C2375

according to a preset schedule. The system then provides the
scheduled water temperature to the convectors, fan-coil units,
or other heat exchangers in the system.

HOT WATER SUPPLY
TEMPERATURE
SENSOR

OUTDOOR AIR
TEMPERATURE
SENSOR

C2356

VALVE

M

B M S

1

S

CONTROLLER

2

Fig. 67. Compensated Supply Water System

Using Dual-Input Controller.

ELECTRIC-PNEUMATIC RELAY CONTROL

Figure 68 shows one use of an E/P relay in a pneumatic
control circuit. The E/P relay connects to a fan circuit and
energizes when the fan is running and de-energizes when the
fan turns off, allowing the outdoor air damper to close
automatically when the fan turns off. The relay closes off the
controller branch line, exhausts the branch line going to the
damper actuator, and allows the damper to go to its normal
(closed) position. Figure 69 shows an E/P relay application
that shuts down an entire control system.

THERMOSTAT
OR
TIME CLOCK

MAIN
AIR

ELECTRIC
POWER

C2376

FROM
COMPRESSOR

TWO-PRESSURE
REDUCING VALVE

E/P RELAY

C
X

EXH

GAUGE

O

X = NORMALLY DISCONNECTED
O = NORMALLY CONNECTED

Fig. 66. Two-Pressure Main Supply System with

Automatic Changeover.

COMPENSATED CONTROL SYSTEM

In a typical compensated control system (Fig. 67), a dual­input controller increases or decreases the temperature of the
supply water as the outdoor temperature varies. In this
application, the dual-input controller resets the water
temperature setpoint as a function of the outdoor temperature

Fig. 68. Simple E/P Relay Combination.

SYSTEM
INTERLOCK
VOLTAGE

E/P RELAY

M

XO

EXH

C

THERMOSTAT

M

B

Fig. 69. E/P Relay Combination for System Shutdown.

87

ENGINEERING MANUAL OF AUTOMATIC CONTROL

N.C.
VALV E

C2358

PNEUMATIC CONTROL FUNDAMENTALS

PNEUMATIC-ELECTRIC RELAY CONTROL

A P/E relay provides the interlock when a pneumatic
controller actuates electric equipment. The relays can be set
for any desired pressure. Figure 70 shows two P/E relays
sequenced to start two fans, one at a time, as the fans are needed.

WIRED TO
START FAN 1

THERMOSTAT

M

M

B

C N.C. N.O.

SET
5-7 PSI

P/E RELAYS

Fig. 70. P/E Relays Controlling Fans in Sequence.

On a rise in temperature, Relay 1 puts Fan 1 in operation as
the thermostat branchline pressure reaches 7 psi. Relay 2 starts
Fan 2 when the controller branchline pressure reaches 12 psi.
On a decrease in branchline pressure, Relay 2 stops Fan 2 at
10 psi branchline pressure, and Relay 1 stops Fan 1 at 5 psi
branchline pressure.

Figure 71 shows two spdt P/E relays starting and stopping a
two-speed fan to control condenser water temperature.

WIRED TO
START FAN 2

C N.C. N.O.

SET
10-12 PSI

C2359

branchline pressure to 14 psi, Relay 2 breaks the normally
closed circuit and makes the normally open circuit, removing
voltage from Relay 1, shutting down the low speed, and
energizing the high speed. On a decrease in temperature, the
sequence reverses and the changes occur at 12 and 7 psi
respectively.

PNEUMATIC RECYCLING CONTROL

E/P and P/E relays can combine to perform a variety of logic
functions. On a circuit with multiple electrically operated
devices, recycling control can start the devices in sequence to
prevent the circuit from being overloaded. If power fails,
recycling the system from its starting point prevents the circuit
overload that could occur if all electric equipment restarts
simultaneously when power resumes.

Figure 72 shows a pneumatic-electric system that recycles
equipment when power fails.

TO LOAD SIDE

THERMOSTAT

OF POWER

SUPPLY SWITCH

HG

WIRED TO START

ELECTRICAL EQUIPMENT

COOLING TOWER
FAN STARTER
CONTROL VOLTAGE

OVERLOAD

LOW SPEED

HIGH AUXILIARY

C N.C.

P/E RELAY 1
SET 7-9 PSI

N.O.

C N.C.

HIGH SPEED

LOW AUXILIARY

FAN STARTER

N.O.
P/E RELAY 2

SET 12-14 PSI

CONTROLLER

DA

B M S

M

SENSOR IN
CONDENSER
WATER

C2367

Fig. 71. Two-Speed Fan Operated by P/E Relays.

Vo ltage is applied to the common contact of Relay 1 from
the normally closed contact of Relay 2. When the controller
branchline pressure rises to 9 psi, the cooling tower fan is
started on low speed by Relay 1 which makes common to
normally open. As a further rise in temperature increases the

M

B

M

CHECK
VALV E

ADJUSTABLE

RESTRICTOR

E/P
RELAY

XOC

EXH

P/E
RELAYS

C2368

Fig. 72. Recycling System for Power Failure.

When power is applied, the E/P relay operates to close the
exhaust and connect the thermostat through an adjustable
restrictor to the P/E relays. The electrical equipment starts in
sequence determined by the P/E relay settings, the adjustable
restrictor, and the branchline pressure from the thermostat. The
adjustable restrictor provides a gradual buildup of branchline
pressure to the P/E relays for an adjustable delay between
startups. On power failure, the E/P relay cuts off the thermostat
branch line to the two P/E relays and bleeds them off through
its exhaust port, shutting down the electrical equipment. The
check valve allows the thermostat to shed the controlled loads
as rapidly as needed (without the delay imposed by the
restrictor).

ENGINEERING MANUAL OF AUTOMATIC CONTROL

88

PNEUMATIC CONTROL FUNDAMENTALS

PNEUMATIC CENTRALIZATION

Building environmental systems may be pneumatically
automated to any degree desired. Figure 73 provides an
example of the front of a pneumatic automation panel. This
panel contains pneumatic controls and may be local to the
controlled HVAC system, or it may be located centrally in a
more convenient location.

In this example, the on-off toggle switch starts and stops the
fan. The toggle switch may be electric, or pneumatic with a
Pneumatic-Electric (P/E) relay.

Two pneumatic “target” gauges are shown for the outside
air damper and the supply fan. The ON/OFF Supply Fan Gauge
is fed from a fan proof-of-flow relay, and the OPEN/CLOSED
Damper Gauge is fed from the damper control line.

PSI

10

0

RETURN

AIR

The Discharge Air Temperature Indicator is fed from the
pneumatic discharge air temperature sensor and the Three-Way
Va lve Gauge is fed from the valve control line.

When pneumatic automation panels are located local to the
HVAC system, they are usually connected with 1/4 inch plastic
tubing. When there are many lines at extended lengths, smaller
diameter plastic tubing may be preferable to save space and
maintain responsiveness. When the panel devices are remote,
the air supply should be sourced remotely to avoid pressure
losses due to long flow lines. The switching air may be from
the automation panel or it may be fed via a remote restrictor
and piped in an exhaust configuration.

DISCHARGE AIR

20

30

4-11 PSI
NORMALLY
OPEN

SUPPLY

FAN

ON OFF

TEMPERATURE

60

50

70

40

30

20

10

0

80

90

100

OUTSIDE

AIR

COOLING

E

N

P

O

COIL

N

O

AHU 6

M10297

Fig. 73. Pneumatic Centralization

89

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PNEUMATIC CONTROL FUNDAMENTALS

PNEUMATIC CONTROL SYSTEM EXAMPLE

The following is an example of a typical air handling system
(Fig. 74) with a pneumatic control system. The control system
is presented in the following seven control sequences (Fig. 75
through 79):

—Start-Stop Control Sequence.
— Supply Fan Control Sequence.
— Return Fan Control Sequence.
—Warm-Up/Heating Coil Control Sequence.
—Mixing Damper Control Sequence.
—Discharge Air Temperature Control Sequence.
—Off/Failure Mode Control Sequence.

Controls are based upon the following system information
and control requirements:

System Information:

—VAV air handling system.
— Return fan.
— 35,000 cfm.
—4,000 cfm outside air.
—3,000 cfm exhaust air.
—Variable speed drives.
—Hot water coil for morning warm-up and to prevent

discharge air from getting too cold in winter .
— Chilled water coil.
—Fan powered perimeter VAV boxes with hot water reheat.
—Interior VAV boxes.
—Water-side economizer.
—8:00 A.M to 5:00 P.M. normal occupancy.
— Some after-hour operation.

Control Requirements:

— Maintain design outside air airflow during all levels of

supply fan loading during occupied periods.

— Use normally open two-way valves so system can heat

or cool upon compressed air failure by manually running
pumps and adjusting water temperatures.

—Provide exhaust/ventilation during after-hour occupied

periods.

— Return fan sized for 35,000 cfm.

START-STOP CONTROL SEQUENCE

Fans 1M through 3M (Fig. 75) operate automatically

subject to starter-mounted Hand-Off-Automatic Switches.

The Supply Fan 1M is started and controls are energized
by Electric-Pneumatic Relay 2EP at 0645 by one of the
following:

— An Early Start Time Clock 1TC
—A drop in perimeter space temperature to 65F at Night

Thermostat TN

— An after-hour occupant setting the Spring-Wound

Interval Timer for 0 to 60 minutes.

The Supply Fan 1M operation is subject to manually reset
safety devices including Supply and Return Air Smoke
Detectors; a heating coil, leaving air, Low Temperature
Thermostat; and a supply fan discharge, duct High Static
Pressure Cut-Out.

GRAVITY
RELIEF

OUTSIDE
AIR

MIXED
AIR

Fig. 74. Typical Air Handling System.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

RETURN FAN

90

RETURN
AIR

SUPPLY FAN

EXHAUST

DISCHARGE
AIR

EAST
ZONE

WEST
ZONE

M10298