Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

ENGINEERING MANUAL of

AUTOMATIC CONTROL

COMMERCIAL BUILDINGS

SI Edition

for

in any form without permission of Honeywell Inc.

Library of Congress Catalog Card Number: 97-77856

Home and Building Control

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

Honeywell Latin American Region

480 Sawgrass Corporate Parkway Suite 200 Sunrise FL 33325

Home and Building Control

Honeywell Limited-Honeywell Limitée 155 Gordon Baker Road North York, Ontario M2H 3N7

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Printed in USA

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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 ha ve enjoyed ev en greater success in colleges, univ ersities, 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 outgro wth of that dedication. Our end user customers, the building owners and operators, will ultimately benef it 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 and use the International System of Units (SI). A ne w 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 third SI edition of the Engineering Manual of Automatic Control is our contribution to ensure that we continue to satisfy our customer’s requirements. The contr ib utions and encouragement receiv ed from pre vious users are gratefully acknowledged. Further suggestions will be most welcome.

Minneapolis, Minnesota December, 1997

KEVIN GILLIGAN President, H&BC Solutions and Services

ENGINEERING MANUAL OF AUTOMATIC CONTROL

iii

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 commer cial 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. Eng ineering 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.

Building management systems have ev olved 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

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

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

ASHRAE Psychrometric Charts .......................................................... 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 Centralization .................................................................... 89

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

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

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

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

How Electric Control Circuits are 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

Electronic 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 Priniples .................................................................................. 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............................................................... 196

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 ............................................................. 266

Airflow Control Applications................................................................. 280

References .......................................................................................... 290

Chiller, Boiler, and Distribution System Control Applications....................................................................... 291

Introduction.......................................................................................... 295

Abbreviations....................................................................................... 295

Definitions............................................................................................ 295

Symbols............................................................................................... 296

Chiller System Control......................................................................... 297

Boiler System Control.......................................................................... 327

Hot and Chilled Water Distribution Systems Control ........................... 335

High Temperature Water Heating System Control............................... 374

District Heating Applications................................................................ 380

Individual Room Control Applications ............................................................................................................ 395

Introduction.......................................................................................... 397

Unitary Equipment Control .................................................................. 408

Hot Water Plant Considerations .......................................................... 424

ENGINEERING MANUAL OF AUTOMATIC CONTROL

Engineering Information .......................................................................................... 425

Valve Selection and Sizing ............................................................................................................ 427

Introduction.......................................................................................... 428

Definitions............................................................................................ 428

Valve Selection .................................................................................... 432

Valve Sizing ......................................................................................... 437

Damper Selection and Sizing ............................................................................................................ 445

Introduction.......................................................................................... 447

Definitions............................................................................................ 447

Damper Selection................................................................................ 448

Damper Sizing..................................................................................... 457

Damper Pressure Drop ....................................................................... 462

Damper Applications ........................................................................... 463

General Engineering Data ............................................................................................................ 465

Introduction.......................................................................................... 466

Conversion Formulas and Tables ........................................................ 466

Electrical Data ..................................................................................... 473

Properties of Saturated Steam Data ................................................... 476

Airflow Data ......................................................................................... 477

Moisture Content of Air Data ............................................................... 479

Index .......................................................................................... 483

ENGINEERING MANUAL OF AUTOMATIC CONTROL

SMOKE MANAGEMENT FUNDAMENTALS

CONTROL

SYSTEM

FUNDAMENTALS

ENGINEERING MANUAL OF AUTOMATIC CONTROL

SMOKE MANAGEMENT FUNDAMENTALS

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

Control Fundamentals

ENGINEERING MANUAL OF AUTOMATIC CONTROL

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 V ariables ............................................................................ 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

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

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

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

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

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

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

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

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

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

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

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

Temperature 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

INTRODUCTION

CONTROL FUNDAMENTALS

This section describes heating, ventilating, and air conditioning (HV A C) 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 HVA C 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

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 low-limit 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 integra ted 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.

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: Continuousl y variable (e.g., a faucet controlling water

from off to full flow).

Automatic contr ol 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.

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).

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 V ariable: The quantity or condition that is measured

and controlled.

Controller: A device that senses changes in the controlled

variable (or receiv es input from a r emote sensor) and derives the proper correction output.

Corrective action: Control action that results in a change of

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

Cycle: One complete e xecution 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 v ariable from one

value to another. Out-of-control analog cycling is called “hunting”. T oo frequent on-off c ycling 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 c ycle.

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”.

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) contr ol:

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 perf orm control functions, such as actuating a switch or positioning a potentiometer. T he controller signal usually operates or positions an electric actuator or may switch an electrical load directly or through a relay.

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.

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.

Process: A gener al 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 operationg 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).

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

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

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.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

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.

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. 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 activ ated 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., Kelvins or degrees Celsius, percent relative humidity, kilopascals). 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. 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 requirements for the section.

MEASURED

VARIABLE

OUTDOOR

AIR

-2

OUTDOOR

AIR

CONTROLLED

MEASURED

VARIABLE

HOT WATER

RETURN

TEMPERATURE

MEDIUM

RESET SCHEDULE

-15

CONTROLLED

VARIABLE

HOT WATER

SUPPLY

AUTO

55 90

SETPOINT

CONTROL

POINT

HOT WATER

SUPPLY

TEMPERATURE

Fig. 1. Typical Control Loop.

SETPOINT

INPUT

PERCENT

OPEN

VALVE

SETPOINT

OUTPUT

FLOW

ALGORITHM IN

CONTROLLER

FINAL CONTROL ELEMENT

STEAM

CONTROL AGENT

MANIPULATED VARIABLE

M15127

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 lev el. 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 dif fusers. Larger buildings often hav e 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

CONTROL FUNDAMENTALS

Table 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

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

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

Cooling tower control

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

(DX) system control

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

Better Adjusts fan volumes to maintain proper duct and space pressures. Reduces system

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.

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

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

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

wet bulb temperature conditions.

Cycles compressor or DX coil solenoid valves to maintain temperature. If compressor is unloading type, cylinders are unloaded as required to maintain temperature.

required during unoccupied periods.

operating cost and improves performance (essential for variable air volume systems).

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

20°C

-7°C

WINDOW

PREVAILING WINDS

INFILTRATION

C3971

Transmission is the process by which energ y 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

OUT

) x HTC

Where:

TIN= indoor temperature

= outdoor temperature

OUT

HTC = heat transfer coefficient

HTC

Unit Time x Unit Area x Unit Temperature

joule

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 f an 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 inf iltration 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 SUPPL Y

FAN

COIL

UNIT HEATER

STEAM TRAP (IF STEAM SUPPLY)

CONDENSA TE OR HOT WATER RETURN

Fig. 5. Typical Unit Heater.

GRID PANEL

SERPENTINE PANEL

C2703

HOT WATER SUPPL Y

HOT WATER RETURN

HOT WATER SUPPL Y

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-c ycle 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 eff icient, 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

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 oil-fired 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 valv e to pro vide 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 heatrecovery systems are av ailable 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 r otates, 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 gi v es up the heat of v aporization 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 rev ersed 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. T erminal unit controls are typically factory installed. Boilers, heat pumps, and rooftop units are usually sold with a factoryinstalled 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 af fect 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 surf aces 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 117 watts. Incandescent lighting produces more heat than fluorescent lighting. Copiers, computers, and other office machines also contribute significantly to internal heat gain.

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 VALVE

CHILLED WATER COIL

SENSOR

CHILLED WATER RETURN

COOL AIR

C2707-2

Fig. 10. System Using Cooling Coil.

Compressors for chilled water systems are usually centrifugal, reciprocating, or screw type. The capacities of centr ifugal 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 Applications. — Air Handling System Control Applications. — Individual Room Control Applications.

TEMPERA TURE CONTROLLER

EVAPORATOR COIL

SOLENOID VALVE

COOL AIR

SENSOR

REFRIGERANT LIQUID

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 ar e always water cooled.

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.

For dehumidification, a cooling coil must have a capacity and surface temperature sufficient to cool the air belo w 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 gi v es up moisture to a stream of “scavenger” air , which is then exhausted. Scavenger air is often exhaust air or could be outdoor air.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

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 eff iciency and to provide yearround humidity control (winter humidification also).

MOISTURE

COOLING COIL

ELIMINATORS

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.

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 ventila tion 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 f or 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 f ilters, absorption chemicals (e.g., activ ated 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 v entilation, refer to ASHRAE Standard 62.

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

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 relati ve 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 w ater 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.

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

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 (HEP A) 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.

BAG FILTER

Fig. 17. Mechanical Filters.

Other types of mechanical filters include strainers, viscous coated filters, and diffusion f ilters. 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. Dif fusion 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 lar ge 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.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

–

PATH OF IONS

WIRES AT HIGH POSITIVE POTENTIAL

SOURCE: 1996 ASHRAE SYSTEMS AND EQUIPMENT HANDBOOK

AIRFLOW

POSITIVELY CHARGED PARTICLES

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.

ALTERNATE PLATES

–

GROUNDED

–

INTERMEDIATE PLATES

CHARGED TO HIGH POSITIVE

–

POTENTIAL

THEORETICAL PATHS OF

–

CHARGES DUST PARTICLES

C2714

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 v ariable, 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 sho ws 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.

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.

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.

Various 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 de vices have performed HVA C 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 dig ital 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 pr ovide even and uninterrupted control performance.

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.

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 giv en 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 electronicpneumatic transducer conver ts the electric output to a v ariable pressure output for pneumatic actuation of the final control element.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

ANALOG CONTROL SIGNAL

OPEN

FINAL CONTROL ELEMENT POSITION

CLOSED

OPEN

FINAL CONTROL ELEMENT POSITION

CLOSED

DIGITAL CONTROL SIGNAL

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 26°C, and to close the valve when the temperature drops to 25°C. The difference between the two temperatures (1°C or 1 Kelvin) 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 Tw o-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 v alues (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 dif ferential is the minimum possible swing of the controlled variable. Figure 21 shows a typical heating system cycling pattern.

TEMPERATURE (°C)

23.5 OVERSHOOT

CONDTION

DIAL SETTING

DIFFERENTIAL

OFF

22.5

21.5

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. T wo-position control is used in simple HV A C 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 variab le (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 mo v es to the other position and remains there until the controlled variable returns to the other limit.

20.5

TIME

UNDERSHOOT CONDTION

C3972

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., 22°C), 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 f inal control element lags) and where close control is not required.

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.

THERMOST AT

FURNACE

SOLENOID GAS VALVE

C2715

Fig. 22. Basic Two-Position Control Loop.

Timed Tw o-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.

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 v ariable 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).

BASIC TWO-POSITION CONTROL

OVERSHOOT CONDITION

DIAL SETTING

UNDERSHOOT CONDITION

TIME

TIMED TWO-POSITION CONTROL

TIME

DIFFERENTIAL

CONTROL POINT

C3973

TEMPERATURE (°C)

OFF

TEMPERATURE (°C)

OFF

23.5

22.5

21.5

20.5

23.5

22.5

21.5

20.5

Fig. 23. Comparison of Basic Two-Position

and Timed Two-Position Control.

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 whenev er 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.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

22.5

21.5

0.1

DROOP (°C)

CONTROL POINT (°C)

DESIGN

TEMPERATURE

OUTDOOR AIR

TEMPERATURE

100%

LOAD

C3974

NO LOAD

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.

TIME PROPORTIONING

Time proportioning control provides more effective twoposition control than heat anticipation control and is available with some electromechanical thermostats and in electronic and

Fig. 25. Relationship between Control Point,

Droop, and Load (Heating Control).

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.

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

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

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.

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.,

STEP CONTROL

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.

OFF

C2090

SELECTED

CYCLE TIME

(MINUTES)

7.5

2.5

100 75 50 25 0

LOAD (%)

Fig. 24. Time Proportioning Control.

Because the controller responds to average temperature or humidity , it does not w ait for a cyc lic change in the controlled variable before signaling correcti ve 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

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.

THROTTLING RANGE

DIFFERENTIAL

ONOFF

STAGES

ONOFF

SPACE TEMPERATURE (°C)

ONOFF

LOAD

ONOFF

100%0%

C3981

Fig. 26. Electric Heat Stages.

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.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

THROTTLING RANGE

STAGES

DIFFERENTIAL

OFF

SPACE TEMPERATURE (°C)

SETPOINT

LOAD

ONOFF

100%

C3982

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

DIRECT EXPANSION

COILS

RETURN AIR

THERMOSTAT

MULTISTAGE

ELECTRIC HEAT

DISCHARGE

AIR

C2716

full on, the modulating stage returns to 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 av ailable 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 two-position stage comes

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.

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Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

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