Trane TRG-TRC014-EN User manual

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

TRG-TRC014-EN

ii

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.

A Trane Air Conditioning Clinic

VAV Systems

Figure 1

TRG-TRC014-EN iii

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

iv TRG-TRC014-EN

TRG-TRC014-EN 1

notes

period one

What Is Variable Air Volume?

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.

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.

VAV Systems

period one

What Is Variable Air Volume?

Figure 2

Constant-Volume,

Variable-Temperature System

space

OA

supply

fan

SA

cooling

coil

thermostat

RA

Figure 3

2 TRG-TRC014-EN

notes

period one

What Is Variable Air Volume?

This equation describes the performance of a constant-volume, variable-

temperature system at various load conditions.

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

3

/s] of 55°F [12.8°C]

supply air.

Sensible Heat Gain

Constant × (Space DB – Supply DB)

Supply

Airflow

=

40,000 Btu/hr

1.085 × (75°F – 55°F)

Supply

Airflow

=

1,840 cfm

=

Constant Volume–Full Load

11,724 W

1,210 × (23.9°C – 12.8°C)

Supply

Airflow

=

0.87 m

3

/s

=

Figure 4

Supply Airflow (cfm)

Sensible Heat Gain

Constant (Space DB Supply DB)–×

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

Supply Airflow

40,000 Btu/hr

1.085 75°F55°F–()×

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

1,840 cfm==

Supply Airflow

11,724 W

1,210 23.9°C12.8°C–×

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

0.87m

3

/S==

TRG-TRC014-EN 3

period one

What Is Variable Air Volume?

notes

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

[0.87 m

3

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

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

3

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

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.

75°F –

20,000 Btu/hr

1.085 × 1,840 cfm

Supply

DB

=

65°F=

Constant Volume–Part Load

Space

DB

Sensible Heat Gain

Constant × Supply Airflow

–

=

Supply

DB

23.9°C –

5,862 W

1,210 × 0.87 m

3

/s

Supply

DB

=

18.3°C

=

Figure 5

Supply DB Space DB

Sensible Heat Gain

Constant Supply Airflow×

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

Supply DB 75°F

20,000 Btu/hr

1.085 1,840 cfm×

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

65°F=–=

Supply DB 23.9°C

5,862W

1

,

210 0.87m

3

/

S

×

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

4 TRG-TRC014-EN

notes

period one

What Is Variable Air Volume?

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.

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.

I’m

freezing!

I’m

freezing!

I’m

fine.

I’m

fine.

I’m

hot!

I’m

hot!

It’s stuffy

in here!

It’s stuffy

in here!

Constant Volume–Multiple Spaces

single thermostat

Figure 6

Constant Volume–Multiple Spaces

reheat coilreheat coil

reheat coil

80°F

[26.7°C]

55°F

[12.8°C]

55°F

[12.8°C]

65°F

[18.3°C]

65°F

[18.3°C]

air mixingair mixing

air mixing

Figure 7

TRG-TRC014-EN 5

period one

What Is Variable Air Volume?

notes

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.

Terminal Reheat System

EA

PA

OA

RA

supply

fan

cooling

coil

reheat

coil

thermostat

SA

Figure 8

6 TRG-TRC014-EN

notes

period one

What Is Variable Air Volume?

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.

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

3

/s] of 55°F [12.8°C] supply air.

Variable-Air-Volume (VAV) System

EA

OA

RA

variable

-

speed drive

supply

fan

cooling

coil

thermostat

VAV

box

PA

SA

Figure 9

VAV–Full Load

40,000 Btu/hr

1.085 × (75°F – 55°F)

Supply

Airflow

=

1,840 cfm

=

11,724 W

1,210 × (23.9°C – 12.8°C)

Supply

Airflow

=

0.87 m

3

/s

=

Figure 10

TRG-TRC014-EN 7

period one

What Is Variable Air Volume?

notes

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:

That is, 920 cfm [0.43 m

3

/s] of 55°F [12.8°C] supply air.

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,

Air Conditio ning Clinic TRG-TRC0 14-EN

Air Conditio ning Clinic TRG-TRC0 14-EN© Amer ican Standard Inc. 2001

VAV–Part Load

20,000 Btu/hr

1.085 × (75°F – 55°F)

Supply

Airflow

=

920 cfm

=

5,862 W

1,210 × (23.9°C – 12.8°C)

Supply

Airflow

=

0.43 m

3

/s

=

Figure 11

Supply Airflow

20,000 Btu/hr

1.085 75°F55°F–()×

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

920 cfm==

Supply Airflow

5,862 W

1,210 23.9°C12.8°C–×

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

0.43m

3

S⁄==

▲ Reduced fan energy

▲ Reduced refrigeration energy

Why VAV? Energy Savings

Figure 12

8 TRG-TRC014-EN

notes

period one

What Is Variable Air Volume?

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.

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.

Why VAV? Comfort

Ahhh!

I’m

fine.

I’m

fine.

I’m

quite

cozy.

I’m

quite

cozy.

It’s nice

in here!

It’s nice

in here!

▲ Dedicated terminal units

▲ Dedicated thermostats

Figure 13

TRG-TRC014-EN 9

period one

What Is Variable Air Volume?

notes

Contrasting the three systems discussed in this example, the simple, constant-

volume, 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.

System Comparison

constant

-

volume,

single zone

constant

-

volume,

terminal reheat

◆

Fan energy

savings

◆

Refrigeration

energy

savings

◆

Delivers comfort

to many spaces

efficiently

◆

Constant fan

energy

◆

Refrigeration

energy

savings

◆

Delivers comfort

to only one

thermal zone

◆

Constant fan

energy

◆

Nearly constant

refrigeration

energy

◆

Delivers comfort

to many spaces

inefficiently

◆

Reheat energy

increases at part

load

VAV

Figure 14

10 TRG-TRC014-EN

notes

period one

What Is Variable Air Volume?

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.

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.

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.

▲ Variable thermal load profiles

▲ Multiple, independently-controlled spaces

▲ Common return air path

VAV Building Characteristics

Figure 15

Variable Thermal Load Profiles

100

75

50

25

0

100

75

50

25

0

percent space loadpercent space load

6126

noon

a.m.

p.m.

west

-

facing

space

east

-

facing

space

Figure 16

TRG-TRC014-EN 11

period one

What Is Variable Air Volume?

notes

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.

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.

Independent Space Control

▲ Dedicated terminal units

▲ Dedicated thermostats

Figure 17

Common Return Air Path

Space 1

Space 2

Space 3

Figure 18

12 TRG-TRC014-EN

notes

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

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.

VAV Systems

period two

Components of a VAV System

Figure 19

supply

ductwork

VAV

box

thermostat

return air path

Components of a VAV System

central

air handler

supply

fan

relief

fan

Figure 20

period two

Components of a VAV System

TRG-TRC014-EN 13

period two

Components of a VAV System

notes

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.

VAV Terminal Units

primary

air

supply

air

airflow

modulation

device

▲ Heating coil

▲ Filter

▲ Mixing fan

controller

Figure 21

14 TRG-TRC014-EN

notes

period two

Components of a VAV System

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.

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

Single Duct, Cooling Only

primary

air

supply

air

Figure 22

space load

design

cooling load

design

heating load

% airflow to space

100%

minimum

primary

airflow

0%

primary air

Single Duct, Cooling Only

maximum

primary

airflow

remote source of heat

heating

heating tempering

tempering

Figure 23

TRG-TRC014-EN 15

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.

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.

Space Heating with a VAV System

remote

heat source

heating

coil

plenum air

terminal

mixing fan

Figure 24

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Trane TRG-TRC014-EN User manual

Trane TRG-TRC014-EN User manual

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