Trane TRG-TRC014-EN User Manual

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Air Conditioning Clinic

VAV Systems

One of the Systems Series

TRG-TRC014-EN

VAV Systems

One of the Systems Series

A publication of The Trane Company

Preface

VAV Systems

A Trane Air Conditioning Clinic

Figure 1

The Trane Company believes that it is incumbent on manufacturers to serve the industry by regularly disseminating information gathered through laboratory research, testing programs, and field experience.

The Trane Air Conditioning Clinic series is one means of knowledge sharing. It is intended to acquaint a nontechnical audience with various fundamental aspects of heating, ventilating, and air conditioning. We have taken special care to make the clinic as uncommercial and straightforward as possible. Illustrations of Trane products only appear in cases where they help convey the message contained in the accompanying text.

This particular clinic introduces the concept of variable air volume or “VAV” systems.

TRG-TRC014-EN

Contents

period one What Is Variable Air Volume? .......................... 1

Why VAV? ............................................................... 7

period two Components of a VAV System ...................... 12

period three System Configurations ..................................... 40

Perimeter Spaces .................................................. 41

Interior Spaces ...................................................... 46

Changeover/Bypass VAV System ........................... 47

System-Level Control Modes ................................ 48

period four Fan Modulation ................................................... 52

Fan Performance Curve ....................................... 52

System Resistance Curve .................................... 53

Fan Modulation Curve ......................................... 56

Fan Modulation Methods .................................... 58

System Static-Pressure Control .......................... 64

period five Application Considerations ............................. 68

period six Review ................................................................... 76

Quiz ......................................................................... 81

Answers ................................................................ 84

Glossary ................................................................ 85

TRG-TRC014-EN iii

iv TRG-TRC014-EN

notes

period one

What Is Variable Air Volume?

VAV Systems

period one

What Is Variable Air Volume?

Figure 2

A variable-air-volume (VAV) air-conditioning system varies the volume of constant-temperature air that is supplied to meet the changing load conditions of the space.

Constant-Volume, Variable-Temperature System

cooling

supply

fan

thermostat

For the purpose of comparison, we will look at a traditional constant-volume (CV), variable-air-temperature system.

This system delivers a constant volume of air to the space and, to maintain the required space temperature at all load conditions, varies the temperature of this air. In this example, the temperature of the air is varied by controlling the capacity of the central cooling coil.

cooling

coil

space

Figure 3

TRG-TRC014-EN 1

notes

period one

What Is Variable Air Volume?

Constant Volume–Full Load

Supply Airflow

This equation describes the performance of a constant-volume, variabletemperature system at various load conditions.

Supply Airflow (cfm)

With these design conditions:

space sensible heat gain = 40,000 Btu/hr [11,724 W]

space dry-bulb (DB) temperature = 75°F [23.9°C]

supply air dry-bulb (DB) temperature = 55°F [12.8°C]

constant = 1.085 [1,210]

the system balances the load with 1,840 cfm [0.87 m supply air.

Constant × (Space DB – Supply DB)

1.085 × (75°F – 55°F)

1,210 × (23.9°C – 12.8°C)

Sensible Heat Gain

40,000 Btu/hr

11,724 W

----------------------------------------------------- ------------- ------------------------------------=

Constant (Space DB Supply DB)–×

1,840 cfm

0.87 m3/s

Sensible Heat Gain

Figure 4

/s] of 55°F [12.8°C]

40,000 Btu/hr

Supply Airflow

2 TRG-TRC014-EN

---------------------------------------------------- ------

1.085 75° F55°F–()×

11,724 W

----------------------------------------------------- -----------

1,210 23.9°C12.8°C–×

1,840 cfm==

0.87m

/S==

notes

period one

What Is Variable Air Volume?

Constant Volume–Part Load

Supply

Rearranging the equation and using the constant supply airflow of 1,840 cfm [0.87 m

Btu/hr [5,862 W] space sensible load is:

Therefore, as the space sensible load drops from 40,000 Btu/hr to 20,000 Btu/hr [11,724 W to 5,862 W], this system modulates the temperature of the constant 1,840 cfm [0.87 m

Removing less energy from the air takes less energy from the central system. Therefore, with this particular system, refrigeration energy savings are realized at part load conditions, although control of space humidity levels suffers due to the warmer supply air condition.

/s], the supply air temperature required to balance a lesser 20,000

Supply DB Space DB

Supply DB 75° F

Supply DB 23.9°C

Space

75°F –

23.9°C –

1.085 × 1,840 cfm

/s] supply air from 55°F to 65°F [12.8°C to 18.3°C].

Sensible Heat Gain

–

Constant × Supply Airflow

20,000 Btu/hr

5,862 W

1,210 × 0.87 m

Sensible Heat Gain

---------------------------------------------------- ------------- ----------–=

Constant Supply Airflow×

20,000 Btu/hr

------------------------------------------------

1.085 1,840 cfm×

5,862W

-------------------------------------------------–=

210 0.87m

65°F=

18.3°C

65°F=–=

Figure 5

TRG-TRC014-EN 3

notes

period one

What Is Variable Air Volume?

Constant Volume–Multiple Spaces

single thermostat

It’s stuffy

in here!

I’m

hot!

However, because this type of system can respond to the demands of only one thermostat, it can serve only those building spaces with similar cooling requirements. If a building has many spaces with diverse cooling needs, each must be served by its own system.

I’m

fine.

I’m

freezing!

Figure 6

Constant Volume–Multiple Spaces

80°F

[26.7°C]

55°F

[12.8°C]

air mixing

air mixingair mixing

65°F

[18.3°C]

65°F

55°F

[12.8°C]

reheat coil

reheat coilreheat coil

Other constant-volume system designs can serve the cooling requirements of more than one space with a central fan and cooling coil. However, to do so, the cool primary air must be either reheated or mixed with warm air to produce the supply temperatures needed to balance the various space cooling loads.

4 TRG-TRC014-EN

65°F

[18.3°C]

Figure 7

notes

period one

What Is Variable Air Volume?

Terminal Reheat System

supply

cooling

coil

thermostat

supply

fan

The terminal reheat system uses a central air handler and cooling coil to deliver cool primary air to all the spaces. Each space has its own heating coil to temper the air to satisfy the space load. Of course, any heat added to meet the part-load requirements of a space becomes a cooling load that the refrigeration system must overcome. This can result in a nearly constant refrigeration load, even when the building is at part-load conditions.

Therefore, reheating cooled air to achieve part-load supply air temperature control is not very energy efficient and is used only in special constant-volume applications, or when there is a “free” source of heat (i.e., heat recovery).

Primary air (PA) is the air delivered by a central supply fan to a terminal unit. Supply air (SA) is the air delivered to a space.

reheat

coil

Figure 8

TRG-TRC014-EN 5

notes

period one

What Is Variable Air Volume?

Variable-Air-Volume (VAV) System

supply

cooling

coil

supply

fan

variable--

variable

speed drive

thermostat

In contrast, a variable-air-volume (VAV) system delivers the primary air at a constant temperature and varies the airflow to maintain the required space temperature at all load conditions.

VAV

box

Figure 9

VAV–Full Load

Supply Airflow

40,000 Btu/hr

1.085 × (75°F – 55°F)

1,840 cfm

Supply Airflow

Similar to the constant-volume system, at design conditions the VAV system will balance the same 40,000 Btu/hr [11,724 W] space sensible load with the same 1,840 cfm [0.87 m

6 TRG-TRC014-EN

1,210 × (23.9°C – 12.8°C)

11,724 W

/s] of 55°F [12.8°C] supply air.

0.87 m3/s

Figure 10

notes

period one

What Is Variable Air Volume?

VAV–Part Load

Supply

Airflow

Supply

Airflow

However, at part load, the VAV system produces a balance by modulating the volume of constant 55°F [12.8°C] supply air.

In this example, the air volume required to balance the lesser 20,000 Btu/hr [5,862 W] space sensible load is:

Supply Airflow

That is, 920 cfm [0.43 m3/s] of 55°F [12.8°C] supply air.

20,000 Btu/hr

1.085 × (75°F – 55°F)

1,210 × (23.9°C – 12.8°C)

5,862 W

20,000 Btu/hr

---------------------------------------------------- ------

1.085 75° F55°F–()×

----------------------------------------------------- -----------

1,210 23.9°C12.8°C–×

5,862 W

920 cfm

0.43 m3/s

Air Conditioning Clinic TRG-TRC014 -EN© American Stan dard Inc. 2 001

Air Conditioning Clinic TRG-TRC014 -EN

920 cfm==

0.43m

Figure 11

S⁄==

Why VAV? Energy Savings

▲ Reduced fan energy

▲ Reduced refrigeration energy

Figure 12

Why VAV?

One reason to use a VAV system is the potential for part-load energy savings. The part-load energy savings inherent with the VAV system are twofold. First,

TRG-TRC014-EN 7

period one

What Is Variable Air Volume?

notes

the air volume reduction creates an opportunity to reduce the fan energy required to move this air. The amount of energy saved depends on the method used to modulate the capacity of the fan (to be discussed in Period Four).

Second, the reduced airflow across the cooling coil causes the refrigeration system to throttle back in order to stabilize the primary air temperature. In turn, this results in a reduction in refrigeration energy compared to full load.

Why VAV? Comfort

▲ Dedicated terminal units

I’m

▲ Dedicated thermostats

It’s nice

in here!

I’m

fine.

Another reason for VAV is to cost-effectively provide improved comfort. A VAV system is capable of controlling space temperature in many spaces with dissimilar cooling and heating requirements, while using only one central air handling unit (AHU). This is accomplished by providing one VAV terminal unit and thermostat for each independently controlled space. When the sun is beating against the west side of the building in the late afternoon, a VAV system can provide an increased amount of cool supply air to keep the spaces on the west exposure comfortable, while throttling back the airflow to the spaces on the east exposure so as not to overcool them.

I’m

quite

cozy.

Ahhh!

Figure 13

8 TRG-TRC014-EN

notes

period one

What Is Variable Air Volume?

System Comparison

constant--

constant

single zone

◆

volume,

Constant fan

energy

Refrigeration

energy

savings

Delivers comfort

to only one

thermal zone

constant--

constant

terminal reheat

◆

volume,

Constant fan

energy

Nearly constant

refrigeration

energy

Delivers comfort

to many spaces

inefficiently

Reheat energy

increases at part

load

VAV

Fan energy

◆

Fan energy

savings

Refrigeration

◆

Refrigeration

energy

savings

Delivers comfort

◆

Delivers comfort

to many spaces

efficiently

Figure 14

Contrasting the three systems discussed in this example, the simple, constantvolume, variable-temperature system consumes constant fan energy while providing refrigeration energy savings at part load. However, it can only deliver comfort to spaces with similar loads.

The terminal reheat system can serve many spaces with dissimilar load requirements, but consumes constant fan energy and nearly constant refrigeration energy at part load. It also uses an increasing amount of reheat energy at part load.

On the other hand, the variable-air-volume system is capable of providing both fan and refrigeration energy savings at part load, making it the most energy efficient of these three systems. In addition, the VAV system can efficiently serve many spaces with dissimilar cooling and heating load requirements.

TRG-TRC014-EN 9

notes

period one

What Is Variable Air Volume?

VAV Building Characteristics

▲ Variable thermal load profiles

▲ Multiple, independently-controlled spaces

▲ Common return air path

Figure 15

There are some basic building characteristics that favor the application of VAV systems. We will discuss three:

n Variable thermal load profiles in the spaces.

n The need for multiple, independently-controlled spaces.

n The existence of a common return air path.

Variable Thermal Load Profiles

east--

facing

east

100

percent space loadpercent space load

A key feature of a VAV system is its ability to realize both fan and refrigeration energy savings at part-load conditions. A building with construction and utilization characteristics that cause the thermal load profiles of the spaces to vary throughout the day and year is an excellent application for a VAV system.

facing

space

6126

a.m.

noon

west--

west

space

p.m.

facing

Figure 16

The fact that the west-facing space in this example has a very different load profile than the east-facing space allows the VAV system to take advantage of this energy savings at all hours of the day.

10 TRG-TRC014-EN

notes

period one

What Is Variable Air Volume?

Independent Space Control

▲ Dedicated terminal units

▲ Dedicated thermostats

Figure 17

Another feature of the VAV system is the ability to efficiently satisfy the comfort requirements of many different spaces within the building. As we saw from the comparison of the VAV and terminal reheat systems, the VAV system is the most efficient at performing this task.

Common Return Air Path

Space 1

Because the VAV system uses one central cooling coil and fan to serve many spaces, the need for a common path to allow the air to return from these spaces is significant. A return air plenum is commonly used. If return ductwork is required to connect each conditioned space to the central air handling unit, the system becomes more difficult to balance and control.

Space 2

Space 3

Figure 18

TRG-TRC014-EN 11

notes

period two

Components of a VAV System

VAV Systems

period two

Components of a VAV System

Figure 19

Next, we will examine the individual components that compose a VAV system.

Components of a VAV System

return air path

relief

fan

supply

VAV

box

supply

ductwork

Figure 20

supply

fan

central

air handler

thermostat

A simple VAV system includes the following:

n Central air handler with a variable-volume supply fan, a cooling coil,

possibly a heating coil, controls, filters, a mixing box, and a return or relief fan

n Supply duct

n VAV terminal unit, or “box,” with a thermostat and supply diffusers for each

independently controlled space

n Thermostat and unit controller for each terminal unit

n Return plenum or duct

The VAV terminal units modulate the airflow supplied to each space.

12 TRG-TRC014-EN

notes

period two

Components of a VAV System

VAV Terminal Units

primary

air

controller

VAV Terminal Units

A VAV terminal unit is a sheet-metal assembly installed upstream of its respective space diffusers. The unit consists of an air-modulation device, control hardware and, depending on the system application, possibly a heating coil, a filter, and a small terminal mixing fan. Modulating the airflow to each individual space is accomplished using a temperature-controlled mechanical device that varies the airflow resistance in the supply duct to that space. The rotating blade damper changes airflow resistance by rotating the damper into the air stream, restricting the size of the air passage to the space. It is very cost-effective and flexible. Typically, either a pneumatic or electric controller can be used to adjust the damper. An understanding of the common VAV terminal unit types is important to understanding VAV systems.

▲ Heating coil

▲ Filter

▲ Mixing fan

airflow

modulation

device

supply

air

Figure 21

TRG-TRC014-EN 13

notes

period two

Components of a VAV System

Single Duct, Cooling Only

primary

air

supply

air

The simplest of all VAV terminal units is the single-duct, cooling-only terminal unit. It consists of an airflow modulation device with controls packaged in a sheet-metal enclosure. The unit can only modulate the primary airflow to the space. The primary air is supplied by a single, central air handler.

This VAV terminal unit is typically used for those zones that require year-round cooling, like the interior zones of a building. It is the most common and basic type of single-duct VAV terminal unit.

Figure 22

Single Duct, Cooling Only

remote source of heat

100%

% airflow to space

heating load

remote source of heat

design

heating tempering

heating

space load

tempering

primary air

This graph indicates how the air supplied to the space by the terminal unit varies as the space loads change. The vertical axis indicates the total airflow supplied to the space. The horizontal axis indicates the space load.

The supply airflow to the space is reduced as the cooling load in the space decreases. Responding to the space thermostat, the primary airflow is modulated between maximum and minimum settings. The maximum setting is

14 TRG-TRC014-EN

maximum primary airflow

minimum primary airflow

design

cooling load

Figure 23

period two

Components of a VAV System

notes

determined by the design cooling load of the space and the minimum setting is normally determined by the space ventilation requirement or minimum airflow for proper diffuser selection.

Most cooling-only units are applied to spaces that have no need for heat. These units would operate in the region on the right-hand portion of this chart, modulating between design and minimum primary airflow. When cooling-only units are applied to spaces that do have heating requirements, the heat is provided by a remote source such as finned radiation along the wall. In these spaces, when the cooling load drops below the minimum airflow setting for the unit, overcooling the space, the remote heat source activates. When space heating is required, the remote heat source satisfies the space heating load. Most terminal unit controllers provide an output signal to control this remote source of heat.

Space Heating with a VAV System

terminal

mixing fan

plenum air

remote

heating

coil

In addition to controlling this remote source of heat (perimeter baseboard radiation, in this example), single-duct VAV terminal units can directly provide heat to a space. This can be accomplished by adding a heating coil to each cooling-only unit or by mixing the primary air with warm plenum air before it is delivered to the space.

TRG-TRC014-EN 15

remote

heat source

Figure 24

notes

period two

Components of a VAV System

VAV Reheat

primary

air

terminal

heating coil

supply

air

VAV r e heat terminal units provide supply-air tempering or space heating by reheating the cool primary air. This is accomplished by adding an electric, or hot-water, heating coil to the discharge of a cooling-only unit.

Figure 25

The VAV reheat terminal unit is typically used for those zones that require seasonal cooling and heating, such as exterior zones of a building.

VAV Reheat

heating coil active

design

heating coil active

heating tempering

heating

space load

tempering

primary air

100%

% airflow to space

heating load

In the cooling mode, the unit is controlled in the same manner as the coolingonly unit. The supply airflow is reduced as the cooling load in the space decreases. When the space cooling load drops below the minimum primary airflow setting for the unit, overcooling the space, the heating coil warms (tempers) the primary air to balance the low space cooling load. When space heating is required, the heating coil further warms the primary air to satisfy the space heating load.

maximum primary airflow

minimum heating airflow

minimum cooling airflow

design

cooling load

Figure 26

16 TRG-TRC014-EN

period two

Components of a VAV System

notes

During the heating mode, the primary airflow must often be greater than the minimum setting of the unit for the cooling mode. When warm, buoyant air is supplied from the ceiling, a certain velocity is required to effectively deliver it down to the occupied portion of the space. Increased airflow may also be needed to meet the minimum requirement for proper operation of the heating coil.

VAV reheat is more efficient than the constant-volume reheat system discussed earlier because heat is provided at reduced airflow and only when required.

Fan-Powered Terminal Units

warm

plenum air

cool

primary air

parallel

series

Figure 27

Another method that a single-duct VAV terminal unit can employ to heat a space is to mix warm plenum air with cool primary air. Because this method uses a small fan to draw warm air from the plenum and mix it with the primary air, they are called fan-powered terminal units.

They come in two configurations, parallel and series.

TRG-TRC014-EN 17

notes

period two

Components of a VAV System

Parallel, Fan-Powered

plenum

air

primary

air

A parallel, fan-powered terminal unit consists of a primary airflow modulation device and a small, integral, constant-volume fan packaged to provide parallel airflow paths.

supply

air

terminal

mixing fan

Figure 28

Parallel, Fan-Powered

design

cooling load

maximum primary airflow

minimum primary airflow

Figure 29

100%

% airflow to space

design

heating load

heating

plenum air

space load

tempering

primary air

In the cooling mode, the unit is controlled in the same manner as the coolingonly unit. Primary airflow is reduced as the cooling load in the space decreases. When the space cooling load drops below the minimum primary airflow setting for the unit, overcooling the space, the small fan activates to mix warm plenum air with the cool primary air. This increases the total airflow to the space and creates a warmer supply air condition. If additional heating is required, with the terminal fan on, a heating coil can be used to further warm the supply air.

Operating the fan is a form of energy recovery. It tempers the supply air with heat from the building and lights carried by the return air, rather than with “new” energy, thus delaying the start of the heating coil.

18 TRG-TRC014-EN

period two

Components of a VAV System

notes

When in cooling mode, the supply airflow to the space equals the primary airflow. When in tempering and heating modes, the space receives a constant supply airflow equal to the minimum primary airflow plus the plenum airflow delivered by the terminal fan. Usually the primary airflow is not allowed to shut off completely because of the space requirement for ventilation air.

Series, Fan-Powered

primary

air

plenum

air

A series, fan-powered terminal unit consists of an airflow modulation device and a small, constant-volume fan, packaged so that their airflow paths are in series. The terminal unit fan operates continuously whenever the space is occupied. The fan draws air from either the primary air stream or the plenum, based on the thermostat in the space. This results in a constant volume of supply air delivered to the space at all times.

terminal

mixing fan

supply

air

Figure 30

TRG-TRC014-EN 19

notes

period two

Components of a VAV System

Series, Fan-Powered

design

cooling load

maximum primary airflow

minimum primary airflow

Figure 31

100%

tempering

primary air

% airflow to space

design

heating load

heating

plenum air

space load

In the cooling mode, the primary airflow is reduced as the cooling load in the space decreases. The total supply airflow to the space remains constant, a combination of cool primary air and warm plenum air. If the space cooling load drops below the minimum primary airflow setting for the unit, or if space heating is required, a heating coil can be used to further warm the supply air to the space.

The maximum airflow for the airflow modulation device typically equals the terminal fan airflow. This means that at the design cooling load, primary airflow equals supply airflow.

Parallel Versus Series Fan-Powered

▲ Constant airflow to the space

▲ Acoustics

▲ Fan energy consumption

Figure 32

Series, fan-powered units are generally considered the premium VAV system because while the central system operates as a variable-volume system, the spaces receive constant supply airflow. Increased air motion in the space improves comfort at all load conditions and the constant airflow simplifies diffuser selection.

20 TRG-TRC014-EN

period two

Components of a VAV System

notes

Most designers also believe that series units offer improved acoustical conditions in the space. The constant sound of the series unit, with the fan operating whenever the space is occupied, is generally preferred to the on-off sound generated by the cycling fan in the parallel unit.

However, because the fan runs continuously whenever the space is occupied, a series fan-powered unit consumes more energy than a parallel fan-powered unit. The development of high-efficiency motors has lessoned the energy consumption difference. The fan in a series fan-powered unit also costs more than an equivalent parallel fan-powered unit because it generally requires a larger terminal fan.

Fan-powered terminal units without a heating coil are typically used to provide tempering for those zones that require year-round cooling and have relatively high minimum airflow settings, such as the densely occupied interior zones of a building (i.e., a conference room). A fan-powered terminal unit with a heating coil is typically used for spaces that require seasonal cooling and heating, such as the exterior zones of a building. Units with heating coils are the most common of the fan-powered terminal units.

Single-Duct VAV Systems

central air handler

55°F

90°F

[32.2°C]

55°F

[12.8°C]

VSD

single--

80°F

[26.7°C]

duct

single

duct

VAV terminal units

Figure 33

The types of terminal units discussed so far are used in single-duct VAV systems. Single-duct VAV systems use a central return-air path that allows the air from the spaces to come back to the air handler. At the air handler, a portion of this return air is recirculated and mixed with outdoor air (introduced for space ventilation purposes). This mixture of outdoor and recirculated return air, or primary air, is then conditioned and delivered to the VAV terminal units through the supply duct system.

The remainder of the return air is exhausted from the building.

TRG-TRC014-EN 21

notes

period two

Components of a VAV System

Dual-Duct

warm

primary air

cool

primary air

supply

air

A dual-duct terminal unit consists of two airflow modulation devices with controls packaged in a sheet-metal enclosure. One controls the cool primary air and the other controls the warm primary air. Depending on the method of control, these two air streams may mix in the dual-duct unit before proceeding downstream to the space.

Figure 34

This VAV terminal unit is intended for zones that require seasonal cooling and heating, typically the exterior zones of a building. It is seldom used because of the high first cost incurred to provide two duct systems.

A dual-duct terminal unit can be controlled to provide either a variable volume or a constant volume of supply air to the space.

22 TRG-TRC014-EN

notes

period two

Components of a VAV System

Variable Air Volume to the Space

100%

% airflow to space

design

heating load

As before, this graph indicates how the air supplied to the space by the terminal unit varies as the space loads change. The vertical axis indicates the total airflow (cool primary air plus warm primary air) supplied to the space. The horizontal axis indicates the space load.

To deliver a variable volume of supply air to the space in a two-fan, dual-duct system in the cooling mode, the dual-duct unit is controlled in the same manner as the cooling-only unit. The cool primary airflow is reduced as the cooling load in the space decreases. When the space cooling load drops below the minimum cool primary airflow setting for the unit, overcooling the space, the second modulating device begins to open. This allows the warm primary air to mix with the cool primary air and provide warmer supply air to the space.

heating

warm

primary air

cool

space load

tempering

primary air

maximum primary airflow

minimum primary airflow

design

cooling load

Figure 35

As the cooling load decreases further and the space requires heating, more warm primary air is mixed with the minimum amount of cool primary air.

TRG-TRC014-EN 23

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