Trane Trg Trc016 En Users Manual

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Air Conditioning

Clinic

Chilled-Water Systems

One of the Systems Series

TRG-TRC016-EN

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The Trane Company • Worldwide Applied Systems Group

3600 Pammel Creek Road • La Crosse, WI 54601-7599

www.trane.com

An American-Standard Company

Chilled-Water Systems

One of the Systems Series

A publication of

The Trane Company

Chilled-Water Systems

Preface

TRG-TRC016-EN

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 reader to chilled-water systems.

A Trane Air Conditioning Clinic

Chilled-Water Systems

Figure 1

TRG-TRC016-EN ii

Contents

period one Types of Water Chillers ...................................... 1

Vapor-Compression Water Chillers .......................... 3

Air-Cooled or Water-Cooled Condensing .................. 6

Packaged or Split Components .............................. 11

Absorption Water Chillers ...................................... 15

Equipment Rating Standards ................................. 18

period two Chilled-Water System Design ........................ 26

Load-Terminal Control ........................................... 28

Parallel Configuration ............................................. 35

Series Configuration .............................................. 38

Primary-Secondary System Operation .................... 53

period three System Variations ............................................. 59

Alternate Fuel Choice ............................................ 59

Low-Flow Systems ............................................... 61

Variable-Primary-Flow Systems .............................. 64

Preferential Loading .............................................. 66

Heat Recovery ...................................................... 68

Asymmetric Design ............................................... 72

“Free” Cooling ...................................................... 75

Application Outside the Operating Range

of the Chiller ......................................................... 78

period four Chiller-Plant Control .......................................... 79

Chiller Sequencing ................................................ 82

Failure Recovery and Contingency Planning ........... 90

System Tuning ...................................................... 92

System Optimization ............................................. 96

Operator Interface ............................................... 100

period five Review ................................................................. 103

Quiz ....................................................................... 108

Answers .............................................................. 110

Glossary .............................................................. 112

iii TRG-TRC016-EN

TRG-TRC016-EN 1

notes

period one

Types of Water Chillers

Water chillers are used in a variety of air conditioning and process cooling

applications. They cool water that is subsequently transported by pumps

and pipes. The water passes through the tubes of coils to cool air in an air

conditioning system, or it can provide cooling for a manufacturing or industrial

process. Systems that employ water chillers are commonly called chilled-

water systems.

When designing a chilled-water system, one of the first issues that must be

addressed is to determine which type of water chiller to use. This period

discusses the primary differences in chiller types.

The refrigeration cycle is a key differentiating characteristic between chiller

types. The vapor-compression and absorption refrigeration cycles are the two

most common cycles used in commercial air conditioning.

period one

Types of Water Chillers

Chilled-Water Systems

Figure 2

absorption

water chiller

centrifugal

water chiller

Figure 3

2TRG-TRC016-EN

notes

period one

Types of Water Chillers

Water chillers using the vapor-compression refrigeration cycle vary by the type

of compressor used. Reciprocating, scroll, helical-rotary, and centrifugal

compressors are common types of compressors used in vapor-compression

water chillers.

Absorption water chillers make use of the absorption refrigeration cycle.

Vapor-compression water chillers use a compressor to move refrigerant around

the system. The most common energy source to drive the compressor is an

electric motor.

Absorption water chillers use heat to drive the refrigeration cycle. They do not

have a mechanical compressor involved in the refrigeration cycle. Steam, hot

water, or the burning of oil or natural gas are the most common energy sources

for these types of chillers.

Driving Sources

heat-driven

compressor-driven

Figure 4

TRG-TRC016-EN 3

period one

Types of Water Chillers

notes

Vapor-Compression Water Chillers

In the vapor-compression refrigeration cycle, refrigerant enters the evaporator

in the form of a cool, low-pressure mixture of liquid and vapor (A). Heat is

transferred from the relatively-warm air or water to the refrigerant, causing

the liquid refrigerant to boil. The resulting vapor (B) is then drawn from the

evaporator by the compressor, which increases the pressure and temperature

of the refrigerant vapor.

The hot, high-pressure refrigerant vapor (C) leaving the compressor enters

the condenser, where heat is transferred to ambient air or water at a lower

temperature. Inside the condenser, the refrigerant vapor condenses into a

liquid. This liquid refrigerant (D) then flows to the expansion device, which

creates a pressure drop that reduces the pressure of the refrigerant to that of

the evaporator. At this low pressure, a small portion of the refrigerant boils

(or flashes), cooling the remaining liquid refrigerant to the desired evaporator

temperature. The cool mixture of liquid and vapor refrigerant (A) travels to the

evaporator to repeat the cycle.

The vapor-compression refrigeration cycle is reviewed in detail in the

Refrigeration Cycle Air Conditioning Clinic.

Vapor-Compression Cycle

compressor

expansion

device

energy in

absorb heat

reject heat

evaporator

condenser

Figure 5

4TRG-TRC016-EN

notes

period one

Types of Water Chillers

The type of compressor used generally has the greatest impact on the efficiency

and reliability of a vapor-compression water chiller. The improvement of

compressor designs and the development of new compressor technologies

have led to more-efficient and -reliable water chillers.

The reciprocating compressor was the workhorse of the small chiller market

for many years. It was typically available in capacities up to 100 tons [350 kW].

Multiple compressors were often installed in a single chiller to provide chiller

capacities of up to 200 tons [700 kW].

Scroll compressors have emerged as a popular alternative to reciprocating

compressors, and are generally available in hermetic configurations in

capacities up to 15 tons [53 kW] for use in water chillers. As with reciprocating

compressors, multiple scroll compressors are often used in a single chiller to

meet larger capacities. In general, scroll compressors are 10 to 15 percent more

efficient than reciprocating compressors and have proven to be very reliable,

primarily because they have approximately 60 percent fewer moving parts than

reciprocating compressors. Reciprocating and scroll compressors are typically

used in smaller water chillers, those less than 200 tons [700 kW].

Helical-rotary (or screw) compressors have been used for many years in

air compression and low-temperature-refrigeration applications. They are now

widely used in medium-sized water chillers, 50 to 500 tons [175 to 1,750 kW].

Like the scroll compressor, helical-rotary compressors have a reliability

advantage due to fewer moving parts, as well as better efficiency than

reciprocating compressors.

Centrifugal compressors have long been used in larger water chillers.

High efficiency, superior reliability, reduced sound levels, and relatively low

cost have contributed to the popularity of the centrifugal chiller. Centrifugal

compressors are generally available in prefabricated chillers from 100 to

3,000 tons [350 to 10,500 kW], and up to 8,500 tons [30,000 kW] as built-up

machines.

centrifugal

scroll

reciprocating

helical-rotary

Compressor Types

Figure 6

TRG-TRC016-EN 5

period one

Types of Water Chillers

notes These various types of compressors are discussed in detail in the Refrigeration

Compressors Air Conditioning Clinic.

The capacity of a centrifugal chiller can be modulated using inlet guide vanes

(IGV) or a combination of IGV and a variable-speed drive (adjustable-frequency

drive, AFD). Variable-speed drives are widely used with fans and pumps, and as

a result of the advancement of microprocessor-based controls for chillers, they

are now being applied to centrifugal water chillers.

Using an AFD with a centrifugal chiller will degrade the chiller’s full-load

efficiency. This can cause an increase in electricity demand or real-time pricing

charges. At the time of peak cooling, such charges can be ten (or more) times

the non-peak charges. Alternatively, an AFD can offer energy savings by

reducing motor speed at low-load conditions, when cooler condenser water

is available. An AFD also controls the inrush current at start-up.

Certain system characteristics favor the application of an adjustable-frequency

drive, including:

nA substantial number of part-load operating hours

nThe availability of cooler condenser water

nChilled-water reset control

Chiller savings using condenser- and chilled-water-temperature reset, however,

should be balanced against the increase in pumping and cooling-tower energy.

This is discussed in Period Four. Performing a comprehensive energy analysis

is the best method of determining whether an adjustable-frequency drive is

desirable. It is important to use actual utility costs, not a “combined” cost, for

demand and consumption charges.

Depending on the application, it may make sense to use the additional money

that would be needed to purchase an AFD to purchase a more efficient chiller

instead. This is especially true if demand charges are significant.

Variable-Speed Drives

variable

variable-

speed

drive

Figure 7

6TRG-TRC016-EN

notes

period one

Types of Water Chillers

Air-Cooled or Water-Cooled Condensing

The heat exchangers in the water chiller (the condenser and evaporator) have

the second greatest impact on chiller efficiency and cost. One of the most

distinctive differences in chiller heat exchangers continues to be the type of

condenser selected—air-cooled versus water-cooled.

When comparing air-cooled and water-cooled chillers, available capacity is the

first distinguishing characteristic. Air-cooled chillers are typically available in

packaged chillers ranging from 7.5 to 500 tons [25 to 1,580 kW]. Packaged

water-cooled chillers are typically available from 10 to 3,000 tons [35 to

10,500 kW].

Condenser Types

water-cooled

air-cooled

Figure 8

Air-Cooled or Water-Cooled

0 tons

[0 kW]

1,000 tons

[3,517 kW]

2,000 tons

[7,034 kW]

chiller capacity

water

water-

-cooled

cooled

air

air-

-cooled

cooled

1,500 tons

[5,276 kW]

2,500 tons

[8,793 kW]

500 tons

[1,759 kW]

3,000 tons

[10,551 kW]

Figure 9

TRG-TRC016-EN 7

period one

Types of Water Chillers

notes

A major advantage of using an air-cooled chiller is the elimination of the cooling

tower. This eliminates the concerns and maintenance requirements associated

with water treatment, chiller condenser-tube cleaning, tower mechanical

maintenance, freeze protection, and the availability and quality of makeup

water. This reduced maintenance requirement is particularly attractive to

building owners because it can substantially reduce operating costs.

Systems that use an open cooling tower must have a water treatment program.

Lack of tower-water treatment results in contaminants such as bacteria and

algae. Fouled or corroded tubes can reduce chiller efficiency and lead to

premature equipment failure.

air-cooled or water-cooled

Maintenance

▲Water treatment

▲Condenser tube brushing

▲Tower maintenance

▲Freeze protection

▲Makeup water

cooling tower

cooling tower Figure 10

8TRG-TRC016-EN

notes

period one

Types of Water Chillers

Air-cooled chillers are often selected for use in systems that require year-round

cooling requirements that cannot be met with an airside economizer. Air-cooled

condensers have the ability to operate in below-freezing weather, and can do so

without the problems associated with operating the cooling tower in these

conditions. Cooling towers may require special control sequences, basin

heaters, or even an indoor sump for safe operation in freezing weather.

For process applications, such as computer centers that require cooling year-

round, this ability alone often dictates the use of air-cooled chillers.

air-cooled or water-cooled

Low Ambient Operation

air

air-

-cooled

cooled

chiller

Figure 11

TRG-TRC016-EN 9

period one

Types of Water Chillers

notes

Water-cooled chillers are typically more energy efficient than air-cooled chillers.

The refrigerant condensing temperature in an air-cooled chiller is dependent

on the ambient dry-bulb temperature. The condensing temperature in a

water-cooled chiller is dependent on the condenser-water temperature, which

is dependent on the ambient wet-bulb temperature. Since the wet-bulb

temperature is often significantly lower than the dry-bulb temperature, the

refrigerant condensing temperature (and pressure) in a water-cooled chiller

can be lower than in an air-cooled chiller. For example, at an outdoor design

condition of 95°F [35°C] dry-bulb temperature, 78°F [25.6°C] wet-bulb

temperature, a cooling tower delivers 85°F [29.4°C] water to the water-cooled

condenser. This results in a refrigerant condensing temperature of

approximately 100°F [37.8°C]. At these same outdoor conditions, the refrigerant

condensing temperature in an air-cooled condenser is approximately 125°F

[51.7°C]. A lower condensing temperature, and therefore a lower condensing

pressure, means that the compressor needs to do less work and consumes

less energy.

This efficiency advantage may lessen at part-load conditions because the

dry-bulb temperature tends to drop faster than the wet-bulb temperature

(see Figure 12). As a result, the air-cooled chiller may benefit from greater

condenser relief.

Additionally, the efficiency advantage of a water-cooled chiller is much less

when the additional cooling tower and condenser pump energy costs are

considered. Performing a comprehensive energy analysis is the best method

of estimating the operating-cost difference between air-cooled and water-cooled

systems.

air-cooled or water-cooled

Efficiency

outdoor temperature

midnight

noon

noon 12

midnight

dry bulb

wet bul b

Figure 12

10 TRG-TRC016-EN

notes

period one

Types of Water Chillers

Another advantage of an air-cooled chiller is its delivery as a “packaged

system.” Reduced design time, simplified installation, higher reliability, and

single-source responsibility are all factors that make the factory packaging of

the condenser, compressor, and evaporator a major benefit. A water-cooled

chiller has the additional requirements of condenser-water piping, pump,

cooling tower, and associated controls.

Water-cooled chillers typically last longer than air-cooled chillers. This

difference is due to the fact that the air-cooled chiller is installed outdoors,

whereas the water-cooled chiller is installed indoors. Also, using water as the

condensing fluid allows the water-cooled chiller to operate at lower pressures

than the air-cooled chiller. In general, air-cooled chillers last 15 to 20 years, while

water-cooled chillers last 20 to 30 years.

To summarize the comparison of air-cooled and water-cooled chillers, air-cooled

chiller advantages include lower maintenance costs, a prepackaged system for

easier design and installation, and better low-ambient operation. Water-cooled

chiller advantages include greater energy efficiency (at least at design

conditions) and longer equipment life.

water-cooled

▲Greater energy efficiency

▲Longer equipment life

air-cooled or water-cooled

Comparison

air-cooled

▲Lower maintenance

▲Packaged system

▲Better low-ambient

operation Figure 13

TRG-TRC016-EN 11

period one

Types of Water Chillers

notes

Packaged or Split Components

Water-cooled chillers are rarely installed with separable components. Air-cooled

chillers, however, offer the flexibility of separating the components in different

physical locations. This flexibility allows the system design engineer to place

the components where they best serve the available space, acoustic, and

maintenance requirements of the customer.

A packaged air-cooled chiller has all of the refrigeration components

(compressor, condenser, expansion device, and evaporator) located outdoors.

A major advantage of this configuration is factory assembly and testing of all

chiller components, including the wiring, refrigerant piping, and controls.

This eliminates field labor and often results in faster installation and improved

system reliability. Additionally, all noise-generating components (compressors

and condenser fans) are located outdoors, easing indoor noise concerns.

Finally, indoor equipment-room space requirements are minimized.

Packaged Air-Cooled Chiller

air-cooled chiller

Figure 14

12 TRG-TRC016-EN

notes

period one

Types of Water Chillers

An alternative to the packaged air-cooled chiller is to use a packaged

condensing unit (condenser and compressor) located outdoors, with a remote

evaporator barrel located in the indoor equipment room. The two components

are connected with field-installed refrigerant piping. This configuration locates

the part of the system that is susceptible to freezing (evaporator) indoors and

the noise-generating components (compressors and condenser fans) outdoors.

This usually eliminates any requirement to protect the chilled-water loop from

freezing during cold weather.

This configuration is particularly popular in schools and other institutional

applications, primarily due to reduced seasonal maintenance for freeze

protection. A drawback of splitting the components is the requirement for

field-installed refrigerant piping. The possibility of system contamination

and leaks increases when field-installed piping and brazing are required.

Additionally, longer design time is generally required for the proper selection,

sizing, and installation of this split system.

Remote Evaporator Barrel

condensing unit

remote

evaporator

refrigerant

piping

Figure 15

TRG-TRC016-EN 13

period one

Types of Water Chillers

notes

Another popular configuration is to use an outdoor air-cooled condenser

connected to a packaged compressor and evaporator unit (also called a

condenserless chiller) that is located in the indoor equipment room. Again,

the components are connected with field-installed refrigerant piping.

The primary advantage of this configuration is that the compressors are located

indoors, which makes maintenance easier during inclement weather and

virtually eliminates the concern of refrigerant migrating to the compressors

during cold weather.

Remote Air-Cooled Condenser

air

air-

-cooled

cooled

condenser

refrigerant

piping

condenserless

chiller

Figure 16

14 TRG-TRC016-EN

notes

period one

Types of Water Chillers

The final configuration includes a packaged compressor-and-evaporator unit

that is located in an indoor equipment room and connected to an indoor,

air-cooled condenser. The air used for condensing is ducted from outdoors,

through the condenser coil, and rejected either outdoors or inside the building

as a means for heat recovery. Indoor condensers typically use a centrifugal fan

to overcome the duct static-pressure losses, rather than the propeller fans used

in conventional outdoor air-cooled condensers. Again, the components are

connected with field-installed refrigerant piping.

This configuration is typically used where an outdoor condenser is

architecturally undesirable, where the system is located on a middle floor of a

multistory building, or where vandalism to exterior equipment is a problem.

A disadvantage of this configuration is that it typically increases condenser fan

energy within compared to a conventional outdoor air-cooled condenser.

Similarly, a packaged cooling tower in a water-cooled system can also be

located indoors. This configuration also requires outdoor air to be ducted to

and from the cooling tower, and again, typically requires the use of a centrifugal

fan. Centrifugal fans use about twice as much energy as a propeller fan, but can

overcome the static-pressure losses due to the ductwork. Alternatively, the

tower sump can be located indoors, making freeze protection easier.

Indoor Air-Cooled Condenser

condenserless

chiller

indoor

air

air-

-cooled

cooled

condenser

refrigerant

piping

Figure 17

TRG-TRC016-EN 15

period one

Types of Water Chillers

notes

Absorption Water Chillers

So far, we have discussed water chillers that use the vapor-compression

refrigeration cycle. Absorption water chillers are a proven alternative to vapor-

compression chillers. The absorption refrigeration cycle uses heat energy as the

primary driving force. The heat may be supplied either in the form of steam or

hot water (indirect-fired), or by burning oil or natural gas (direct-fired).

There are two fundamental differences between the absorption refrigeration

cycle and the vapor-compression refrigeration cycle. The first is that the

compressor is replaced by an absorber, pump, and generator. The second is

that, in addition to the refrigerant, the absorption refrigeration cycle uses a

secondary fluid called the absorbent. The condenser, expansion device, and

evaporator sections, however, are similar.

Warm, high-pressure liquid refrigerant (D) passes through the expansion

device and enters the evaporator in the form of a cool, low-pressure mixture of

liquid and vapor (A). Heat is transferred from the relatively-warm system water

to the refrigerant, causing the liquid refrigerant to boil. Using an analogy of the

vapor-compression cycle, the absorber acts like the suction side of the

compressor—it draws in the refrigerant vapor (B) to mix with the absorbent.

The pump acts like the compression process itself—it pushes the mixture of

refrigerant and absorbent up to the high-pressure side of the system. The

generator acts like the discharge of the compressor—it delivers the refrigerant

vapor (C) to the rest of the system.

The refrigerant vapor (C) leaving the generator enters the condenser, where

heat is transferred to cooling-tower water at a lower temperature, causing the

refrigerant vapor to condense into a liquid. This high-pressure liquid refrigerant

(D) then flows to the expansion device, which creates a pressure drop that

reduces the pressure of the refrigerant to that of the evaporator, repeating

the cycle.

The absorption refrigeration cycle is discussed in more detail in the Absorption

Water Chillers Air Conditioning Clinic.

Absorption Refrigeration Cycle

pump

expansion

device

absorb heat

reject heat

heat energy in

reject heat

evaporator

condenser

absorber

generator

Figure 18

16 TRG-TRC016-EN

notes

period one

Types of Water Chillers

Absorption water chillers generally have a higher first cost than vapor-

compression chillers. The cost difference is due to the additional heat-transfer

tubes required in the absorber and generator(s), the solution heat exchangers,

and the cost of the absorbent. This initial cost premium is often justified when

electric demand charges or real-time electricity prices are a significant portion

of the electric utility bill. Because electric demand charges are often highest at

the same time as peak cooling requirements, absorption chillers are often

selected as peaking or demand-limiting chillers.

Because the absorption chiller uses only a small amount of electricity, backup-

generator capacity requirements may be significantly lower with absorption

chillers than with electrically-driven chillers. This makes absorption chillers

attractive in applications requiring emergency cooling, assuming the alternate

energy source is available.

Some facilities, such as hospitals or factories, may have excess steam or hot

water as a result of normal operations. Other processes, such as a gas turbine,

generate waste steam or some other waste gas that can be burned. In such

applications, this otherwise wasted energy can be used to fuel an

absorption chiller.

Finally, cogeneration systems often use absorption chillers as a part of their

total energy approach to supplying electricity in addition to comfort cooling

and heating.

Absorption Chillers Offer Choice

▲Avoid high electric

demand charges

▲Minimal electricity

needed during

emergency situations

▲Waste heat recovery

▲Cogeneration

Figure 19

TRG-TRC016-EN 17

period one

Types of Water Chillers

notes

There are three basic types of absorption chillers. They are typically available

in capacities ranging from 100 to 1,600 tons [350 to 5,600 kW].

Indirect-fired, single-effect absorption chillers operate on low-pressure

steam (approximately 15 psig [205 kPa]) or medium-temperature liquids

(approximately 270°F [132°C]), and have a coefficient of performance (COP) of

0.6 to 0.8. In many applications, waste heat from process loads, cogeneration

plants, or excess boiler capacity provides the steam to drive a single-effect

chiller. In these applications, absorption chillers become conservation devices

and are typically base-loaded. This means that they run as the lead chiller to

make use of the “free” energy that might otherwise be wasted.

Indirect-fired, double-effect absorption chillers require medium-pressure

steam (approximately 115 psig [894 kPa]) or high-temperature liquids

(approximately 370°F [188°C]) to operate and, therefore, typically require

dedicated boilers. Typical COPs for these chillers are 0.9 to 1.2.

The direct-fired absorption chiller includes an integral burner, rather than

relying on an external heat source. Common fuels used to fire the burner are

natural gas, fuel oil, or liquid petroleum. Additionally, combination burners are

available that can switch from one fuel to another. Typical COPs for direct-fired,

double-effect chillers are 0.9 to 1.1 (based on the higher heating value of the

fuel). Higher energy efficiency and elimination of the boiler are largely

responsible for the increasing interest in direct-fired absorption chillers. These

types of absorption chillers have the added capability to produce hot water for

heating. Thus, these “chiller–heaters” can be configured to produce both chilled

water and hot water simultaneously. In certain applications this flexibility

eliminates, or significantly down-sizes, the boilers.

Absorption Chiller Types

single-effect double-effect

direct-fired

Figure 20

18 TRG-TRC016-EN

notes

period one

Types of Water Chillers

Equipment Rating Standards

The Air-Conditioning & Refrigeration Institute (ARI) establishes rating standards

for packaged HVAC equipment. ARI also certifies and labels equipment through

programs that involve random testing of a manufacturer’s equipment to verify

published performance. These equipment rating standards have been

developed to aid engineers in comparing similar equipment from different

manufacturers. Chiller full-load efficiency is described in terms of kW/ton and

coefficient of performance (COP). Additionally, two efficiency values developed

by ARI that are receiving increased attention are the Integrated Part-Load

Value (IPLV) and Non-Standard Part-Load Value (NPLV).

ARI’s part-load efficiency rating system establishes a single number to estimate

both the full- and part-load performance of a stand-alone chiller. As part of ARI

Standard 550/590–1998, Water-Chilling Packages Using the Vapor-Compression

Refrigeration Cycle, and ARI Standard 560–1992, Absorption Water Chilling-

Heating Packages, chiller manufacturers may now certify their chiller part-load

performance using the IPLV and NPLV methods. This gives the engineering

community an easy and certified method to evaluate individual chillers.

Understanding the scope and application limits of IPLV and NPLV is, however,

crucial to their validity as system performance indicators.

Equipment Rating Standards

▲Air-Conditioning &

Refrigeration Institute (ARI)

◆Standard 550/590–1998:

centrifugal and helical-rotary

water chillers

◆Standard 560–1992:

absorption water chillers

Figure 21

TRG-TRC016-EN 19

period one

Types of Water Chillers

notes

The IPLV predicts chiller efficiency at the ARI standard rating conditions, using

weighted-average load curves that represent a broad range of geographic

locations, building types, and operating-hour scenarios, both with and without

an airside economizer. The NPLV uses the same methods to predict chiller

efficiency at non-standard rating conditions. Although these weighted-average

load curves place greater emphasis on the part-load operation of an average,

single-chiller installation, they will not—by definition—represent any

particular installation.

Additionally, ARI notes that more than 80 percent of all chillers are installed in

multiple-chiller systems. Chillers in these systems exhibit different unloading

characteristics than the IPLV weighted formula indicates. Appendix D of

Standard 550/590–1998 explains this further:

The IPLV equations and procedure are intended to provide a single-

number, part-load performance number for water-chilling products.

The equation was derived to provide a representation of the average

part-load efficiency for a single chiller only. However, it is best to use a

comprehensive analysis that reflects the actual weather data, building

load characteristics, operational hours, economizer capabilities, and

energy drawn by auxiliaries, such as pumps and cooling towers, when

calculating the chiller and system efficiency.

Here is the important part:

This becomes increasingly important with multiple-chiller systems

because individual chillers operating within multiple-chiller systems

are more heavily loaded than single chillers within single-chiller

systems.

Part-Load Efficiency Rating

▲Integrated Part-Load Value (IPLV)

◆Weighted-average load curves

◆Based on an “average” single-chiller installation

◆Standard operating conditions

▲Non-Standard Part-Load Value (NPLV)

◆Weighted-average load curves

◆Based on an “average” single-chiller installation

◆Non-standard operating conditions

Figure 22

20 TRG-TRC016-EN

notes

period one

Types of Water Chillers

The standard rating conditions used for ARI certification represent a particular

set of design temperatures and flow rates for which water-cooled and air-cooled

systems may be designed. They are not suggestions for good design practice

for a given system—they simply define a common rating point to aid

comparisons.

In fact, concerns toward improved humidity control and energy efficiency have

changed some of the design trends for specific applications. More commonly,

chilled-water systems are being designed with lower chilled-water

temperatures and lower flow rates. The water flow rate required through the

system is decreased by allowing a larger temperature difference through the

chiller.

chiller type

evaporator

flow rate

vapor-compression

• reciprocating

•scroll

• helical-rotary

• centrifugal

2.4 gpm/ton

[0.043 L/s/kW]

absorption

• single-effect

• double-effect,

indirect-fired

• double-effect,

direct-fired

Standard Rating Conditions

condenser

flow rate

3.0 gpm/ton

[0.054 L/s/kW]

2.4 gpm/ton

[0.043 L/s/kW]

3.6 gpm/ton

[0.065 L/s/kW]

2.4 gpm/ton

[0.043 L/s/kW]

4.0 gpm/ton

[0.072 L/s/kW]

rating

standard

ARI

550/590–1998

ARI

560–1992

water leaving evaporator = 44°F [6.7°C]

water entering condenser = 85°F [29.4°C]

4.5 gpm/ton

[0.081 L/s/kW]

Figure 23

TRG-TRC016-EN 21

period one

Types of Water Chillers

notes

The temperature difference (∆T) through the chiller and the water flow rate are

related. For a given load, as the flow rate is reduced, the ∆T increases, and vice

versa.

where,

nQ = load, Btu/hr [W]

nflow rate = water flow rate through the chiller, gpm [L/s]

n∆T = temperature difference (leaving minus entering) through the chiller,

ºF [°C]

Realize that 500 [4,184] is not a constant! It is the product of density, specific

heat, and a conversion factor for time. The properties of water at conditions

typically found in an HVAC system result in this value. Other fluids, such as

mixtures of water and antifreeze, will cause this factor to change.

Density of water = 8.33 lb/gal [1.0 kg/L]

Specific heat of water = 1.0 Btu/lb°F [4,184 J/kg°K]

Flow Rates and Temperatures

equation for water only

QBtu/hr = 500 ×flow rate ×∆

∆∆

∆T

[QW= 4,184 ×flow rate ×∆T ]

Figure 24

Q 500 flow rate ∆T××=

Q 4,184 flow rate ∆T××=[]

8.33 lb/gal 1.0 Btu/lb°F 60 min/hr 500=××

1.0 kg/L 4,184 J/kg°K4,184=×[]

22 TRG-TRC016-EN

notes

period one

Types of Water Chillers

In the example system shown in Figure 25, the chilled water is cooled from 57°F

[13.9°C] to 41°F [5°C] for a 16°F [8.9°C] ∆T. This reduces the water flow rate

required from 2.4 gpm/ton [0.043 L/s/kW] to 1.5 gpm/ton [0.027 L/s/kW].

Reducing water flow rates either: 1) lowers system installed costs by reducing

pipe, pump, valve, and cooling tower sizes, or 2) lowers system operating costs

by using smaller pumps and smaller cooling tower fans. In some cases, both

installed and operating costs can be saved. Low-flow systems will be discussed

in more detail in Period Three.

The two ARI rating standards mentioned previously, as well as ASHRAE/IESNA

Standard 90.1–1999 (the energy standard), allow reduced chilled-water

temperatures and flow rates. System design engineers should examine the use

of reduced flow rates to offer value to building owners.

Flow Rates and Temperatures

2.4 gpm/ton

[0.043 L/s/kW]

44°F

[6.7°C]

54°F

[12.2°C]

85°F

[29.4°C]

95°F

[35°C]

3.0 gpm/ton

[0.054 L/s/kW]

ARI conditions

1.5 gpm/ton

[0.027 L/s/kW]

41°F

[5°C]

57°F

[13.9°C]

85°F

[29.4°C]

100°F

[37.8°C]

2.0 gpm/ton

[0.036 L/s/kW]

low-flow conditions

evaporator

flow rate

condenser

flow rate

evaporator

flow rate

condenser

flow rate

Figure 25

TRG-TRC016-EN 23

period one

Types of Water Chillers

notes

ASHRAE/IESNA Standard 90.1–1999, Energy Standard for Buildings, Except

Low-Rise Residential Buildings, went into effect in October 1999. ASHRAE

is the American Society of Heating, Refrigerating and Air-Conditioning

Engineers and IESNA is the Illuminating Engineering Society of North America.

This standard addresses all aspects of buildings except low-rise residential

buildings. It contains specific requirements for both water chillers and

chilled-water systems.

Standard 90.1 contains minimum full- and part-load efficiency requirements

for packaged water chillers. The table in Figure 27 is an excerpt from Table

6.2.1C of Addendum J to the standard. It includes the minimum efficiency

requirements for electric vapor-compression chillers operating at standard ARI

conditions. The standard also contains tables of minimum efficiency

requirements for these chillers operating at nonstandard conditions. The test

procedure for these chillers is ARI Standard 550/590–1999. Notice that these

requirements go into effect on October 29, 2001.

ASHRAE/IESNA Standard 90.1–1999

▲Energy Standard

◆Building design and

materials

◆Minimum equipment

efficiencies

◆HVAC system design

Figure 26

standard 90.1-1999 efficiency requirements

Electric Vapor-Compression Chillers

chiller type

air cooled

water-cooled

reciprocating

helical-rotary, scroll

centrifugal

chiller type

air cooled

water-cooled

reciprocating

helical-rotary, scroll

centrifugal

capacity

all capacities

< 150 tons [528 kW]

150 to 300 tons [528 to 1,056 kW]

> 300 tons [1,056 kW]

< 150 tons [528 kW]

150 to 300 tons [528 to 1,056 kW]

> 300 tons [1,056 kW]

capacity

all capacities

< 150 tons [528 kW]

150 to 300 tons [528 to 1,056 kW]

> 300 tons [1,056 kW]

< 150 tons [528 kW]

150 to 300 tons [528 to 1,056 kW]

> 300 tons [1,056 kW]

minimum efficiency*

2.8 COP 3.05 IPLV

4.2 COP 5.05 IPLV

4.45 COP 5.2 IPLV

4.9 COP 5.6 IPLV

5.5 COP 6.15 IPLV

5.0 COP 5.25 IPLV

5.55 COP 5.9 IPLV

6.1 COP 6.4 IPLV

minimum efficiency*

2.8 COP 3.05 IPLV

4.2 COP 5.05 IPLV

4.45 COP 5.2 IPLV

4.9 COP 5.6 IPLV

5.5 COP 6.15 IPLV

5.0 COP 5.25 IPLV

5.55 COP 5.9 IPLV

6.1 COP 6.4 IPLV

* as of October 29, 2001

* as of October 29, 2001 Figure 27

24 TRG-TRC016-EN

notes

period one

Types of Water Chillers

Note: Addendum J updates the minimum IPLV efficiency requirements in the

standard to match the methods included in the most current version of the ARI

rating standard. At the time this booklet was printed, Addendum J had not been

formally adopted as part of the standard. However, its adoption was deemed

sure enough to include in this publication.

The standard requires that both full- and part-load conditions be met. For

example, the efficiency of a water-cooled centrifugal chiller with a capacity

greater than 300 tons [1,056 kW] must be 6.1 COP, or better, at ARI standard

conditions. This is equivalent to 0.576 kW/ton. The part-load (IPLV) efficiency

must also be 6.4 (based on the efficiency units of COP) or better. This is

equivalent to an IPLV of 0.549 kW/ton.

Coefficient of performance (COP) is a unitless expression of efficiency, defined

as useful energy out divided by energy input. A higher COP designates a higher

efficiency.

The table in Figure 28 includes the minimum efficiency requirements for

absorption water chillers. For an absorption chiller, COP is defined as

evaporator cooling capacity divided by the heat energy required by the

generator, excluding the electrical energy needed to operate the pumps, purge,

and controls.

Again, these minimum efficiency requirements take effect on October 29, 2001.

The test procedure for these chillers is ARI Standard 560–1992. Note that the

efficiency requirement for single-effect absorption chillers is higher than many

manufacturers have offered in the past.

Section 6.3.4 of Standard 90.1 includes additional requirements for the design

and operation of chilled-water systems. These requirements will be mentioned

in later periods.

kW/ton 3.516

COP

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

standard 90.1-1999 efficiency requirements

Water-Cooled Absorption Chillers

chiller type

single-effect

double-effect

indirect-fired

direct-fired

chiller type

single-effect

double-effect

indirect-fired

direct-fired

capacity

all capacities

capacity

all capacities

minimum efficiency*

0.7 COP

1.0 COP 1.05 IPLV

1.0 COP 1.0 IPLV

minimum efficiency*

0.7 COP

1.0 COP 1.05 IPLV

1.0 COP 1.0 IPLV

* as of October 29, 2001

Figure 28

TRG-TRC016-EN 25

period one

Types of Water Chillers

notes

Another standard that is related to chilled-water systems, ASHRAE Standard

15–1994, Safety Code for Mechanical Refrigeration, is intended to specify

requirements for safe design, construction, installation, and operation of

refrigerating systems. This standard covers mechanical refrigeration systems

of all sizes that use all types of refrigerants. Because absorption chillers use

water as the refrigerant, however, they are exempt from this standard.

For many chilled-water systems in which the chillers are located indoors, the

standard requires the refrigeration equipment to be installed in a mechanical-

equipment room. The requirements for this mechanical-equipment room

include refrigerant monitors and alarms, mechanical ventilation, pressure-relief

piping, and so forth.

ASHRAE Standard 15–1994

▲Safety standard for

refrigerating systems

▲Mechanical equipment

room

◆Refrigerant monitors

◆Alarms

◆Mechanical ventilation

◆Pressure-relief piping

Figure 29

26 TRG-TRC016-EN

notes

Proper design of a chilled-water system can greatly impact the first cost,

operating costs, and flexibility of the HVAC system. The purpose of this period

is to discuss the design of reliable chilled-water systems.

The conventional chilled-water system consists of combinations of the

following primary components:

nWater chillers

nLoad terminals (chilled-water cooling coils in comfort-cooling applications)

nCooling towers in water-cooled systems

nChilled- and condenser-water pumps

nChilled- and condenser-water distribution systems that include piping,

an expansion tank, control valves, check valves, strainers, and so forth.

period two

Chilled-Water System Design

Chilled-Water Systems

Figure 30

Chilled-Water System Components

chiller

pumps

cooling coil

cooling tower

Figure 31

period two

Chilled-Water System Design

TRG-TRC016-EN 27

period two

Chilled-Water System Design

notes

This period focuses on the chilled-water side of the system; that is, the water

that flows through the chiller evaporator and out through the load terminals.

Specifically, we will review methods of load-terminal control and various

multiple-chiller system configurations. These topics apply to systems using

both air-cooled and water-cooled chillers.

Fundamentally, the function of the chilled-water system is to transport the

cooling fluid from the chillers to the load terminals and back to the chillers.

Assuming that the distribution system is adequately sized, we will concentrate

on the hydraulic interaction between the load terminals and the chillers.

Chilled-Water System

pump

pump coil

coil

control

valve

air

air-

-cooled

cooled

chiller

Figure 32

28 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

Load-Terminal Control

The purpose of load-terminal control is to modulate the flow of air or water

through the coil to maintain building space comfort. This is accomplished by

measuring the temperature of the supply air or space. The temperature then

converted to an electronic signal that modulates the capacity of the cooling

coil to match the changing load in the space.

Three methods of load-terminal control are commonly used in

chilled-water systems.

nThree-way modulating valve control

nTwo-way modulating valve control

nFace-and-bypass damper control

Each of these methods has a different effect on the operation of the system.

Load-Terminal Control Options

▲Three-way

modulating valve

▲Two-way

modulating valve

▲Face-and-bypass

dampers

Figure 33

TRG-TRC016-EN 29

period two

Chilled-Water System Design

notes

A three-way control valve is one method used to regulate the flow of chilled

water through a cooling coil. As the space cooling load decreases, the

modulating valve directs less water through the coil, decreasing its capacity.

The excess water bypasses the coil and mixes downstream with the water that

flows through the coil. As a result, the temperature of the water returning from

the system decreases as the space cooling load decreases.

Systems that use three-way valves have the following characteristics:

nThe temperature of the water returning from the system varies as the

cooling load varies.

nThe water flow through each load terminal (water through the coil plus

water bypassing the coil) is relatively constant at all load conditions.

nThe pump energy is constant at all loads because the use of three-way

valves results in constant water flow throughout the system.

nWater-flow balance is very critical to proper operation because the flow

is constant.

Three-Way Valve Control

three

three-

-way

way

modulating

valve

airflow

bypass

pipe

Figure 34

30 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

A two-way modulating valve is similar to a three-way valve in that the water

flow through the coil is modulated proportionately to the load. The primary

difference is that the two-way valve does not bypass any unused water, it

simply throttles the amount of water passing through the coil.

The coil and the air being conditioned experience no difference in the cooling

effect of using a two-way versus a three-way valve. The chilled-water system,

however, sees a great difference. Recall that with a three-way valve, the

terminal water flow (water through the coil plus the water bypassing the coil)

is constant at all loads. With a two-way valve, the terminal water flow varies

proportionately with the load. Because there is no mixing of coil and bypassed

water, the temperature of the water leaving the load terminal remains relatively

constant at all conditions. In fact, this return-water temperature may actually

rise slightly as the load decreases, due to coil heat-transfer characteristics.

Systems that use two-way valves have the following characteristics:

nThe temperature of the water returning from the system is constant

(or increases) as the cooling load decreases. This increases the effectiveness

of options such as heat recovery, free cooling, and base-loading, which will

be discussed further in Period Three.

nThe water flow through each load terminal varies proportionately to the

load, resulting in pump energy savings at part load.

nA variable-flow system is less sensitive to water balance than most

constant-flow systems.

A variable-flow, chilled-water distribution system, however, may require

another method to provide constant water flow though the chillers, or else

the chillers must be equipped to handle variable water flow.

Two-Way Valve Control

two

two-

-way

way

modulating valve

airflow

Figure 35

TRG-TRC016-EN 31

period two

Chilled-Water System Design

notes

The final method of modulating the coil capacity to match the cooling load is

through the use of face-and-bypass dampers. A linked set of dampers varies

the amount of air flowing through the coil by diverting the excess air around

the coil. As the cooling load decreases, the face damper closes, reducing the

airflow through the coil and reducing its capacity. At the same time, the linked

bypass damper opens, allowing more air to bypass around the coil. A unique

characteristic of this method of load-terminal control is that the coil is allowed

to “run wild,” meaning that the water flow through the coil is constant.

Similar to the three-way valve, systems that use face-and-bypass dampers have

the following characteristics:

nThe temperature of the water returning from the system varies as the

cooling load varies.

nThe water flow through each load terminal and, therefore, pump energy are

constant at all load conditions.

An advantage of face-and-bypass control with a “wild” cooling coil is that it can

better dehumidifiy of the conditioned air when compared to varying the water

flow through the coil. As the airflow through the coil decreases at part-load

conditions, assuming that the temperature of the water entering the coil is

constant, the temperature of the air leaving the coil also decreases. That is,

the air is cooled further and more moisture is removed.

Face-and-Bypass Damper Control

bypass

damper

face

damper

airflow

Figure 36

32 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

Properly designed, operated, and maintained, any of these three methods can

result in good space comfort control. However, they have different effects on

the chilled-water system.

The use of three-way valves or face-and-bypass dampers results in variable

return-water temperature and relatively constant chilled-water flow through

the entire system. The use of two-way valves results in constant return-water

temperature and variable water flow through the entire system. Before

choosing one of these control methods, it is necessary to determine the

effect that it will have on the other parts of the chilled-water system.

In the past, the water flow rate through the chiller evaporator was to remain

as constant as possible. The vast majority of chilled-water systems employ

pumping schemes that maintain a constant flow rate of water through each

chiller evaporator. Even in the most-carefully-designed chilled-water systems,

however, the flow through the chillers will still vary slightly due to system

Load-Terminal Control Options

▲Three-way modulating valve

◆Constant water flow

◆Variable system return-water temperature

▲Two-way modulating valve

◆Variable water flow (pump energy savings)

◆Constant system return-water temperature

▲Face-and-bypass dampers

◆Constant water flow

◆Variable system return-water temperature

◆Enhanced dehumidification capability with “wild” coils Figure 37

Chiller Evaporator Flow

▲Constant flow is most

common

▲Variable flow is

possible

◆Can reduce energy

consumption

◆Use only with

advanced chiller and

system controls

evaporator

Figure 38

TRG-TRC016-EN 33

period two

Chilled-Water System Design

notes effects. System effects include pump–system curve interaction, dynamic head

variations, and variation in distribution system flow.

There are benefits to maintaining a constant water flow rate through the chiller

evaporator. Constant flow provides more-stable and-simple chiller and system

operation. However, there is potential for energy savings by varying the water

flow in the distribution system. Applying these two seemingly-conflicting

principles to chilled-water systems requires careful planning and a thorough

understanding of hydraulic system operation.

Due to advances in technology, however, many of today’s chillers can operate

with variable evaporator water flow. Chilled-water systems that are specifically

designed to vary evaporator water flow are discussed in Period Three. This

period focuses on systems that employ constant water flow through the chiller

and either constant or variable water flow through the rest of the distribution

system.

Another factor that influences chilled-water system design is the number of

chillers used. Single chillers are sometimes used in small systems (less than

100 tons [35 kW]), while larger or critical systems typically use multiple chillers.

Many single-chiller systems resemble the one shown in Figure 39. This system

uses a single pump to move water through the chiller and load terminals. The

load terminals are controlled using three-way modulating valves. The pump

delivers a constant flow of water throughout the entire system, and flow

balance is relatively easy.

Single-Chiller System

pump

pump coil

coil

three

three-

-way valve

way valve

air

air-

-cooled

cooled

chiller

Figure 39

34 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

Multiple-chiller systems are more common than single-chiller systems for the

same reason that most commercial airplanes have more than one engine—

redundancy provides reliability. Additionally, because cooling loads typically

vary widely, multiple-chiller systems can often operate with less than the full

number of chillers. During these part-load periods, the system saves the energy

required to operate the additional chillers, pumps, and, in water-cooled

systems, cooling tower fans.

There are several configurations used to connect multiple chillers in these

systems. Some of these configurations work well, others do not. Next, we will

look at the most-commonly-used system configurations, including their

advantages and drawbacks.

Multiple-Chiller Systems

▲Redundancy

▲Part-load efficiency

Figure 40

TRG-TRC016-EN 35

period two

Chilled-Water System Design

notes

Parallel Configuration

Parallel piping is one common configuration of multiple-chiller systems.

Figure 41 shows a system that uses a single pump to deliver chilled water both

to chillers and to the system load terminals. This configuration can be used in

systems that use constant-flow methods of terminal control (three-way valves

or face-and-bypass dampers), or in systems that use variable-flow methods of

terminal control (two-way valves). Varying the flow through the load terminals

using two-way valves in this type of system results in variable water flow

through the chiller evaporators. Chilled-water systems that are specifically

designed to vary evaporator water flow will be discussed in Period Three.

This section will focus on systems that use constant-flow methods of

terminal control.

Water is pumped through both chillers continuously, regardless of whether only

one chiller or both chillers are operating. This example system is at 50 percent

load, with one chiller operating and the second chiller off. Return water from the

system at 54°F [12.2°C] continues to flow through the non-operating chiller and

mixes with the chilled 42°F [5.6°C] water produced by the operating chiller. The

resulting mixed-water temperature leaving the plant is 48°F [8.9°C]. This rise in

supply-water temperature may result in problems with building comfort or

humidity control. A chiller-plant controller may be used to reset the set point of

the operating chiller downward, in an attempt to compensate for this condition,

and more-closely maintain the desired supply-water temperature. Reducing the

set point of the operating chiller has its limits, however, depending on the

operating characteristics and evaporator freeze limits of the specific chiller. The

more chillers in the system, the worse the problem becomes. For this reason,

this configuration is seldom used in systems with more than two chillers.

Additionally, ASHRAE/IESNA Standard 90.1–1999 (Section 6.3.4.2) prohibits this

type of system when the pump is larger than 10 hp [7.5 kW]. The standard

requires that, in systems that contain more than one chiller piped in parallel,

system water flow must be reduced when a chiller is not operating.

54° F

[12.2° C]

42°F

[5.6°C]

54°F

[12.2°C]

off

48° F

48°F

[8.9°C]

chillers piped in parallel

Single Pump

Figure 41

36 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

If separate, dedicated pumps are used with each chiller, a pump-and-chiller pair

can be turned on and off together as the cooling load varies. This solves the

temperature mixing problem that occurred in the previous, single-pump

configuration, but it presents a new problem in a system that uses a constant-

flow method of terminal control.

Below 50-percent load, only one chiller and one pump are operating. The total

water flow in the system decreases significantly, typically 60 to 70 percent of

full system flow. Ideally, at this part-load flow rate, all of the coils will receive

less water, regardless of their actual need. Typically, however, some coils

receive full water flow and others receive little or no water. In either case,

heavily-loaded coils will usually be “starved” for flow. Examples of spaces

with constant heavy loads that may suffer include computer rooms, conference

rooms, photocopy rooms, and rooms with high solar loads.

54°F

[12.2°C]

42° F

[5.6°C]

off

60% to 70%

of system flow

coil starved for flow

chillers piped in parallel

Dedicated Pumps

Figure 42

TRG-TRC016-EN 37

period two

Chilled-Water System Design

notes

Figure 43 shows an example of the pump–system curve relationship. When

both pumps are operating, the system receives 100 percent of design flow.

When only one pump is operating, the intersection of the pump’s performance

curve with the system curve results in about 65 percent of design flow.

This configuration also presents problems to chiller operation. The starting or

stopping of a pump for one chiller affects the flow through the other chiller.

Using this same example, if one chiller is operating and a second chiller and

pump are started, the total water flow in the system does not double. The

system and pump performance curves will “rebalance,” resulting in an increase

in system flow of only 35 percent of total flow. The new total flow rate,

however, is now divided equally between the two chillers. This results in a rapid

reduction in water flow through the original operating chiller, from 65 percent of

total system flow to 50 percent. This rapid decrease in flow often results in a

loss of temperature control and may cause the chiller to shut off on a safety.

In order to overcome this problem, the chiller-plant control system should

anticipate the starting of additional pumps and unload operating chillers prior

to the start of an additional chiller. Again, this configuration is sometimes

acceptable for two-chiller systems, but is not often used in larger systems

because the part-load system flow problems are further multiplied.

head pressure

percent flow

2 pumps

1 pump

system

curve

100%

65%

chillers piped in parallel

Dedicated Pumps

Figure 43

38 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

Series Configuration

Another way to connect multiple chillers is to configure the chiller evaporators

in series. Series chilled-water systems typically use three-way valves at the coils

to ensure constant system flow. With two chillers in series, both the

temperature mixing and the flow problems associated with the parallel

configurations shown previously disappear. All of the chilled water passes

through both chillers, and there is full system-water flow at all loads.

However, the flow rate through each individual chiller is equal to the entire

system flow rate. When compared to chillers piped in parallel at the same

system ∆T, this is twice as much water flowing through each chiller. This means

that the chiller-tube pass arrangement must accommodate double the water

quantity within acceptable velocity and pressure drop limits. This typically

requires a reduced number of passes in the evaporator and may impact chiller

efficiency. This efficiency impact, however, is often offset by the gain in system

efficiency due to thermodynamic staging.

System pressure drop also increases because the pressure drops through the

chillers are additive. This can result in increased pump size and energy costs.

This increase in pumping energy can be substantially reduced by designing the

system for a higher system ∆T and, therefore, a reduced water flow rate.

Because of the pressure drop limitations, it is difficult to apply more than two

chillers in series. Systems involving three or more chillers typically use either

the primary-secondary configuration or parallel sets of two chillers in series.

Chillers Piped in Series

three

three-

-way valve

way valve

absorption

chiller

electric

chiller

Figure 44

TRG-TRC016-EN 39

period two

Chilled-Water System Design

notes

Temperature control in a series system can be accomplished in several ways,

depending on the desired operating sequence. The first method, shown in

Figure 45, has both set points adjusted to the desired system supply-water

temperature. Assuming equally-sized chillers, either chiller can meet the load

below 50 percent. Above 50-percent load, both chillers operate and the

upstream chiller is preferentially loaded. This means that the upstream chiller is

operated at full capacity and any portion of the load that remains is handled by

the downstream chiller.

This strategy may be desirable in systems that benefit from preferentially

loading the upstream chiller. Examples include:

nUsing a heat-recovery chiller in the upstream position. Because the chiller is

at full capacity whenever the system load exceeds 50 percent, the amount of

heat available for recovery is maximized.

nUsing an absorption chiller in the upstream position. An absorption chiller

works more efficiently, and has a higher cooling capacity, with higher

leaving-chilled-water temperatures. The absorption chiller in the upstream

position provides a warmer leaving-chilled-water temperature at design

conditions, 48°F [8.9°C] in this example. This arrangement preferentially

loads the gas-burning absorption chiller, allowing the system to maximize

the use of a lower-cost fuel during periods of high electrical-energy cost.

Alternatively, equal loading of the two chillers in series can be accomplished

using a chiller-plant control system to monitor system load and balance chiller

loading. The set point for the downstream chiller is set equal to the desired

system supply-water temperature, and the set point for the upstream chiller is

then dynamically reset to maintain equal loading on both chillers. The control

system must be stable enough to prevent control hunting or chiller cycling

during periods of changing load.

chillers piped in series

Equal Set Points

54° F

54°F

[12.2°C]

42°F

42° F

[5.6°C]

set point =

set point = 42° F

42° F [5.6°C]

[5.6°C] set point =

set point = 42° F

42°F [5.6°C]

[5.6°C]

48°F

[8.9°C]

Figure 45

40 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

An alternative method of controlling chillers in series involves staggering the

set points of the two chillers. This results in the downstream chiller operating

first and being preferentially loaded. Any portion of the load that the

downstream chiller cannot meet is handled by the upstream chiller.

The example in Figure 46 shows the system operating at about 80 percent of

design cooling load. As we mentioned earlier, with three-way valves at the

coils, the temperature of the water returning to the chillers decreases at part

load. At 80-percent load, the return-water temperature is 52°F [11.1°C], instead

of the 54°F [12.2°C] at 100-percent load. The upstream chiller is partially loaded,

cooling the water to the 48°F [8.9°C] set point, while the downstream chiller

remains fully loaded, cooling the water the rest of the way to 42°F [5.6°C].

chillers piped in series

Staggered Set Points

52° F

52°F

[11.1°C]

42°F

42° F

[5.6°C]

set point =

set point = 48° F

48° F [8.9°C]

[8.9°C] set point =

set point = 42° F

42°F [5.6°C]

[5.6°C]

48°F

[8.9°C]

Figure 46

TRG-TRC016-EN 41

period two

Chilled-Water System Design

notes

Primary-Secondary (Decoupled) Configuration

If the water flow through the chillers (production) could be hydraulically

isolated from the water flow through the coils (distribution), many of the

problems encountered in parallel and series configurations could be

eliminated.

Figure 47 shows a configuration that separates, or “decouples,” the production

capacity from the distribution load. This scheme is known as a primary-

secondary system, also referred to as a decoupled system. This

configuration is unique because it dedicates separate pumps to the

“production” and “distribution” loops. A bypass pipe that connects the supply

and return pipes is the key component in decoupling the system.

The chillers in the production loop receive a constant flow of water, while the

coils in the distribution loop, controlled by two-way modulating valves, receive

a variable flow of water.

Primary-Secondary Configuration

production

pumps

two

two-

-way valve

way valve

distribution

pump

distribution

loop

production

loop

bypass pipe

Figure 47

42 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

The bypass pipe is common to both production and distribution loops.

The purpose of the bypass pipe is to hydraulically decouple the production

(primary) and distribution (secondary) pumps. Because water can flow freely

between the supply and return pipes for both loops, a change in flow in one

loop does not affect the flow in the other loop.

The actual extent of hydraulic decoupling depends on the pressure drop due to

the bypass pipe. Total decoupling is accomplished only if the bypass pipe is free

from restrictions and large enough to produce no pressure loss at all flow rates.

Because zero pressure loss is not practical, some insignificant pump coupling

will exist. Bypass pipes are typically sized so that the water velocity in the pipe

will be 10 to 15 ft/s [3 to 4.5 m/s], based on the water flowing through the

bypass pipe at the design flow rate of the largest chiller in the system.

Additionally,

to further minimize pressure drop, the bypass pipe is usually relatively short in

length. To prevent random mixing of the supply and return water streams,

however, the minimum length of the bypass pipe is typically 5 to 10 pipe

diameters.

When designing the bypass pipe, an important issue to keep in mind is that the

bypass pipe must be kept free of unnecessary restrictions. For example, a check

valve must not be installed in the pipe. Restrictions cause hydraulic coupling

that can result in unacceptable chiller flow variations or unstable, and

potentially harmful, system pressure variations due to the resulting series-

pumping effects. If a manual isolation valve is required for service, it should

be large enough to ensure that it does not add significant pressure drop to the

bypass pipe.

Primary-Secondary System Rules

▲The bypass pipe should be free of restrictions

◆Sized for minimal pressure drop

◆Avoid random mixing of supply- and return-water

streams

◆No check valve

Figure 48

TRG-TRC016-EN 43

period two

Chilled-Water System Design

notes

The production pumps circulate water only from the return tee, through a

chiller, to the supply tee, and through the bypass pipe. This represents a

relatively-small pressure loss and, therefore, relatively-low pump energy. In

addition, each pump only operates when its respective chiller is operating.

A primary-secondary system provides a high degree of flexibility in the

production loop. Not only are the individual chiller “loops” decoupled from

the distribution loop, they are also decoupled from each another. In this

configuration, the production loop consists of independent pairs of chillers and

pumps. Each pump is turned on and off with its respective chiller. Supply water

temperature is maintained by the controls supplied with the chiller. Because the

bypass pipe prevents flow interaction between chillers, there is little worry of

flow disturbances. In addition, the chillers can be of any type, size, or age, or

even from different manufacturers. Because each chiller has a dedicated pump,

the chillers can have different evaporator pressure drops.

Production Loop

return

tee

supply

tee

production

pumps

bypass pipe

Figure 49

44 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

Alternatively, the production loop can be configured with manifolded pumps

and automatic, two-position isolation valves at each chiller. When turning on a

chiller, a pump is turned on and the isolation valve is opened.

This manifolded-pump configuration provides greater redundancy because the

system can change the pump-and-chiller combinations. This redundancy is at

the cost of system complexity, and somewhat limits the flexibility of selecting

chillers of different capacities and types. If the production pumps are

manifolded, the chillers must be selected with the same evaporator-water

pressure drop, or else some method of flow balancing must be employed.

Pump sizing also becomes an issue if chillers are of different capacities and

flow rates, because the proper pump needs to be turned on to match the chiller

flow rate.

The drawback of manifolding production pumps is that the chiller flows become

hydraulically coupled again. If an isolation valve is opened before a pump is

started, flow through the operating chillers will drop suddenly, causing

potential control instability. If a pump is started before a valve is open, the

operating chillers will see a momentary flow increase, causing control

instability or water hammer.

Manifolded Production Pumps

production

pumps

bypass pipe

isolation

valves

Figure 50

TRG-TRC016-EN 45

period two

Chilled-Water System Design

notes

The distribution pump circulates water from the supply tee, through the load

terminals, and back to the return tee. Although the same water is pumped twice

by different pumps, there is no duplication of pumping energy. The production

pumps overcome only the pressure drop through the production loop, and the

distribution pumps overcome the pressure drop through the distribution loop.

The distribution pump(s) should be capable of varying the flow through the

distribution loop. Typically this is accomplished by using a pump with a

variable-speed drive to modulate the flow of water through the pump.

In a properly-designed and -operating system, distribution-pump energy

consumption will decrease significantly at part load. The pump power reduction

approaches the theoretical cubic relationship to flow. That is, when the load is

50 percent of design, requiring 50 percent of design water flow, the energy

consumed by the variable-flow distribution pump is 12.5 percent of full load:

The total installed pump capacity required in a primary-secondary system is

typically less than in a system not designed for primary-secondary pumping.

This is because the total system head (production plus distribution) is divided

between pumps. Each pump is more efficient because it works against a lower

head. Furthermore, the distribution pump is sized to meet the diversified (block)

system load, not the sum-of-peaks coil loads. This can represent a 20- to

25- percent reduction in the size of the distribution pump.

Distribution Loop

two

two-

-way valve

way valve

distribution

pump

bypass pipe

return

tee

supply

tee

Figure 51

0.50()

30.125=

46 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

Obviously, in order to achieve variable flow at the distribution pump, the load

terminals must be configured to vary the system flow. This requires the use of a

modulating two-way control valve for each coil. Typically, no three-way valves

or wild coils need to be used in a primary-secondary system. Their use

decreases the energy savings potential from the variable-flow distribution

pump. In fact, ASHRAE/IESNA Standard 90.1–1999 (Section 6.3.4.1) requires

the use of modulating two-way control valves, and thus variable water flow,

in most systems. Primary-secondary systems comply with this requirement.

Some systems, however, will use one three-way valve at the load terminal

furthest from the distribution pump, to ensure that cold water is immediately

availability to all terminals in the system.

Primary-Secondary System Rules

▲The bypass pipe should be free of restrictions

▲Load terminals should use two-way

modulating control valves

Figure 52

TRG-TRC016-EN 47

period two

Chilled-Water System Design

notes

The distribution pump is typically equipped with a variable-speed drive that is

controlled to maintain a certain pressure difference between the supply- and

return-water piping. In response to a reduced cooling load, the two-way valve

modulates closed, restricting the flow of water through the coil. This causes an

increase in system differential pressure, which can be measured and used to

signal a reduction in the speed of the distribution pump.

An alternative is to allow the pump to “ride its pump curve.” As the two-way

valves modulate closed, the increase in system pressure causes the pump to

“ride up” its performance curve (A to B), resulting in a reduction to 50 percent

of design flow in this example. This method, however, generally results in less

energy savings than a pump with a variable-speed drive. Also, proper pump

selection is important and part-load operating conditions must be considered.

In variable-flow systems, ASHRAE/IESNA Standard 90.1–1999 (Section 6.3.4.1)

requires the use of a modulation device, such as a variable-speed drive, on

pump motors larger than 50 hp [37 kW] that have a pump head greater than

100 ft H2O [300 kPa].

Varying Distribution Flow

pressure

difference

riding the pump curve

variable-speed control

head pressure

percent flow

100

10050

pump

curve

Figure 53

48 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

Another advantage of the primary-secondary system is that the production loop

is not affected by the distribution pumping arrangement.

For example, multiple distribution pumps can be used to vary the flow within

the distribution loop. Providing variable flow through the application of

multiple pumps, or through variable-speed drives on one or more pumps, is

more energy efficient than simply riding the pump curve. It also provides

greater system redundancy.

Multiple Distribution Pumps

distribution

pumps

bypass pipe

supply

to loads

return

from loads

Figure 54

TRG-TRC016-EN 49

period two

Chilled-Water System Design

notes

A variation of the multiple-pump configuration is to use separate pumps to

deliver water to specific, dedicated loads. An example is a chilled-water system

serving a college campus. Separate distribution pumps supply water to the east

(A), west (B), and central (C) portions of the campus. A primary advantage of

this configuration is flexibility. Expanding the system can be achieved by simply

adding another distribution pump to the existing plant and connecting it to the

piping that runs to the new building.

A variation of this multiple, dedicated pump configuration is often called

distributed pumping. It is sometimes used in very large systems that serve

multiple buildings. In a distributed pumping system, a dedicated distribution

pump is located out in the system at each building, instead of all the pumps

being housed in the chiller plant. This configuration offers the potential for

additional pump energy savings, because each pump only needs to pump the

water required for the building it serves.

Multiple Distribution Pumps

distribution

pumps

bypass pipe

supply

to loads

return

from loads

from loads A

ABC

Figure 55

50 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

In very large systems, a primary-secondary-tertiary pumping configuration is

sometimes used. The primary pumps circulate water through the chillers. The

secondary distribution pumps circulate the water around the distribution loop.

The individual load terminals are decoupled from the distribution loop and each

load terminal has a dedicated tertiary pump.

A “load terminal” in this case may be an individual cooling coil or an entire

building. In systems that use tertiary pumping, the load terminal must be

controlled so that only the quantity of water required is drawn from the

distribution loop. Water must not be allowed to flow into the return piping until

it has experienced the proper temperature rise. The two-way valve modulates

to maintain the design return-water temperature. A constant-volume tertiary

pump circulates water through the load terminal.

Some tertiary pumping systems use a small bleed line to ensure that water will

be immediately available when the tertiary pump is started, and to provide an

accurate control signal when the two-way valve is closed. It also keeps the

distribution pump from “dead-heading,” or trying to pump when all of the

two-way valves are closed. If a bleed line is used, it should be of a much smaller

diameter than the rest of the piping.

Tertiary Pumping

bypass pipe

distribution

pump

tertiary

pump

two

two-

-way

way

valve

valve bleed line

bleed line

Figure 56

TRG-TRC016-EN 51

period two

Chilled-Water System Design

notes

Let’s summarize: When designed and operated correctly, the distribution loop

of the primary-secondary system has the following characteristics:

nVariable water flow. Only the amount of water that is actually used at the

load terminals is pumped throughout the distribution loop. Under most

operating conditions, this flow rate is less than the design flow rate,

resulting in reduced pumping energy.

nLoad diversity. Not all of the load terminals peak at the same time.

Therefore, the quantity of water that flows at any given time is less than

the constant water flow required in a system using three-way valves.

This allows for reduced distribution pump and pipe sizes.

nHigher return-water temperature at all loads. Properly-operating two-

way valves do not allow unused chilled water to bypass the load terminals.

Water is only allowed to enter the return pipe after it has accomplished

vague useful cooling. If the system is operating properly, the temperature

of the water returning from the load terminals will be at least as high as it

is at design load conditions, and may actually rise at part-load conditions.

This warm return water is especially advantageous in systems using heat

recovery, free cooling, or preferential loading of chillers. These options will

be discussed in Period Three.

Distribution Loop Characteristics

▲Reduced pump

energy use

▲Distribution loop

sized for system

diversity

▲Higher return-water

temperatures

Figure 57

52 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

For simplicity of system control, all of the chillers in a primary-secondary

system should be selected to operate with the same leaving-water temperature

and with the same temperature difference (∆T). This allows all operating chillers

to be loaded to equal percentages.

Control of supply-water temperature is fairly simple. The set points of the

individual chillers are all equal to the desired system supply-water temperature.

Because water flows only through operating chillers, there is no water mixing in

the production loop, and the production loop supplies the water temperature

corresponding to the individual chiller set points.

Primary-Secondary System Rules

▲The bypass pipe should be free of restrictions

▲Load terminals should use two-way

modulating control valves

▲All chillers should be selected for the same

leaving chilled-water temperature and ∆

∆∆

∆T

Figure 58

TRG-TRC016-EN 53

period two

Chilled-Water System Design

notes

Primary-Secondary System Operation

We have seen that the production and distribution loops of the primary-

secondary system act independently. The next consideration is to match the

capacity of the production loop to the load of the distribution loop. The

operation of a primary-secondary system focuses on the direction and

amount of flow in the bypass line.

At the supply tee, which connects the supply and bypass pipes, a supply-and-

demand relationship exists. The total water flow from all operating production

(chiller) pumps is the “supply” flow. The “demand” flow is the total water flow

required to meet the loads on the cooling coils. Whenever the supply and

demand flows are unequal, water will either flow into, or out of, the bypass

pipe at the supply tee.

System Operation

distribution

loop

production

loop

bypass pipe

return

tee

supply

tee

supply

flow

dem

and

dem

and

flow

Figure 59

54 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

If production supply is inadequate to meet the load demand, a “deficit” of

supply water exists. To make up for this deficit, the distribution pump will pull

water from the return pipe of the distribution loop through the bypass pipe.

This is called deficit flow.

In this example, the pumps operating in the production loop are supplying

1,000 gpm [63 L/s] of water while the distribution pump is pumping 1,200 gpm

[76 L/s] to meet the demand of the cooling coils. The result is that 200 gpm

[13 L/s] of system return water flows through the bypass pipe to be mixed with

the supply water from the production loop. The temperature of the mixed water

supplied to the distribution loop is 44.3°F [6.8°C].

Control of the water temperature supplied by the distribution loop is

compromised due to this mixing. When this deficit flow condition exists,

starting an additional chiller and pump increases the supply water flow from the

production loop. It also changes the supply-and-demand relationship in order to

restore the temperature of the chilled water supplied to the distribution loop.

Deficit Flow

1,200

1,200 gpm

gpm at 56°F

at 56°F

[76 L/s at 13.3°C]

1,000

1,000 gpm

gpm at 56° F

at 56° F

[63 L/s at 13.3°C]

1,000

1,000 gpm

gpm at 42°F

at 42°F

[63 L/s at 5.6° C]

1,200

1,200 gpm

gpm at 44.3° F

at 44.3° F

[76 L/s at 6.8° C]

200

200 gpm

gpm

at 56°F

[13 L/s at 13.3°C]

[13 L/s at 13.3°C

]

Figure 60

TRG-TRC016-EN 55

period two

Chilled-Water System Design

notes

When the flow of chilled water from the production loop exceeds the demand of

the distribution loop, the direction of flow in the bypass pipe reverses. Chilled

water flows from the supply side of the production loop, through the bypass

pipe, and mixes with warm water returning from the distribution loop. This is

called excess flow.

In this example, the pumps operating in the production loop are supplying

2,000 gpm [126 L/s] of water, while the distribution pump is pumping

1,800 gpm [114 L/s] to meet the demand of the cooling coils. The result is that

200 gpm [13 L/s] of supply water flows through the bypass pipe to be mixed

with the water returning from the production loop. The temperature of the

water returning to the chillers decreases to 54.6°F [12.6°C], reducing the load

on the operating chillers.

Some excess flow is normal in the operation of a primary-secondary system.

The amount of excess flow is almost always less than the flow of one

production pump. The energy consumed by pumping this excess water through

the production loop is typically very low, because the production pump only

needs to produce enough head to push the water through the chiller evaporator

and the bypass pipe.

If a pump-and-chiller pair is turned off as soon as this excess flow condition

occurs, deficit flow will result and the pump and chiller will be turned on again.

To prevent this from happening, a production pump and its respective chiller

are not turned off until the excess bypass flow exceeds the capacity of the next

production pump that is to be turned off.

Some systems are designed with variable flow also in the production loop.

Although this minimizes excess flow in the bypass pipe and further reduces

production-energy consumption, it results in a significantly-more-complex

control system. This type of system will be discussed in Period Three.

Excess Flow

1,800

1,800 gpm

gpm at 56°F

at 56°F

[114 L/s at 13.3°C]

2,000

2,000 gpm

gpm at 54.6°F

at 54.6° F

[126 L/s at 12.6°C]

2,000

2,000 gpm

gpm at 42°F

at 42°F

[126 L/s at 5.6° C]

1,800

1,800 gpm

gpm at 42°F

at 42°F

[114 L/s at 5.6° C]

200

200 gpm

gpm

at 42°F

[13 L/s at 5.6°C

]

[13 L/s at 5.6°C

]

Figure 61

56 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

Starting and stopping of pump-and-chiller pairs in a primary-secondary system

depends on the direction and quantity of water flow in the bypass pipe.

nWhenever there is deficit flow through the bypass pipe for a specified period

of time (typically 15 to 30 minutes in a comfort-cooling system), another

pump-and-chiller pair started.

nWhenever there is excess flow through the bypass pipe that is greater than

the flow being produced by the next pump-and-chiller pair to be turned off,

that pump and chiller are turned off. To prevent short cycling as the result of

a slight increase in load, the chiller-plant control system will typically allow

excess flow of from 110 to 115 percent of the flow produced by the next

production pump to be turned off.

nIf neither of the above conditions exist, no action is taken.

Control of Primary-Secondary System

condition

deficit flow for

specified period of

time

excess flow greater

than 110% to 115% of

next pump to turn off

neither

condition

deficit flow for

specified period of

time

excess flow greater

than 110% to 115% of

next pump to turn off

neither

response

start another

chiller and pump

turn off next chiller

and pump

do nothing

response

start another

chiller and pump

turn off next chiller

and pump

do nothing

Figure 62

TRG-TRC016-EN 57

period two

Chilled-Water System Design

notes

The direction and quantity of flow in the bypass pipe may be determined either

directly by using a flow meter or indirectly by sensing temperatures.

Direct flow measurement can be accomplished using a variety of flow-meter

technologies. These include pressure-based flow meters (pitot tubes, venturi

meters, orifice plates, and differential pressure sensors), turbine and impeller

meters, vortex meters, magnetic flow meters, and ultrasonic transit-time

meters. The accuracy, ease of installation, required maintenance, and cost of

these meter technologies vary widely. The accuracy and reliability of the flow

meter will directly impact the efficiency and reliability of the chilled-water

system. High-quality flow meters are critical to proper system operation.

When using a flow meter, it is important to understand the range of flows and

velocities that the specific device can accurately measure. The accuracy of

some flow meters is dependent on the velocity of the flow and the development

of a smooth flow profile in the stream being measured. To obtain accurate

measurements, several diameters of straight pipe may be required, both

upstream and downstream of the meter. Finally, in order to give accurate

results, many types of flow meters require periodic calibration. This is often

overlooked in the maintenance of chilled-water systems.

Types of Fluid Flow Meters

▲Pressure-based

◆Pitot tube

◆Venturi

◆Orifice plate

◆Differential pressure

▲Turbine and impeller

▲Vortex

▲Magnetic

▲Ultrasonic

Figure 63

58 TRG-TRC016-EN

notes

period two

Chilled-Water System Design

The advent of microprocessor-based controls has led to another method for

determining flow in the bypass pipe. Temperature sensors are placed in the

supply and return pipes of the production and distribution loops, and in the

bypass pipe. With these temperatures, a chiller-plant control system,

programmed with fluid mixing equations, can determine the quantity of excess

or deficit flow that exists at any time. Because a small change in temperature

may indicate a relatively large change in load or flow in the bypass pipe,

it is important to use accurate, calibrated sensors to ensure acceptable system

operation.

The primary advantage of this method is that it does not depend on flow

velocity in the pipe. Also, it is reliable and cost effective because the

temperature sensors are relatively-low-cost devices.

Integrating the chiller controls with a chiller-plant control system, however, is

imperative for efficient system operation. The control of chilled-water systems

will be discussed further in Period Four.

Temperature-Based Calculations

bypass pipe

return

tee

supply

tee

system

system-

-level

level

controller

Figure 64

TRG-TRC016-EN 59

notes

period three

System Variations

Period Two discussed several standard configurations of chilled-water systems.

In addition, there are many variations available to reduce installed costs,

enhance the efficiency of the system, improve reliability, or increase operational

flexibility.

These variations are worth examining, because improving the reliability or

efficiency of the system by even a small percentage can result in a large

payback over the life of a building.

Alternate Fuel Choice

Many building owners and operators are investigating the use of fuels other

than electricity. There are several reasons for this renewed interest.

Today, the utility industry in many regions is going through some degree of

deregulation. One of the first United States locations to experience this was

San Diego, California. Due to a number of factors, the price of electricity

period three

System Variations

Chilled-Water Systems

Figure 65

Electric Utility Deregulation

price of electricity, $/kWh

0.20

0.10

0.30

May

May June

June July

July August

August September

September

San Diego, CA

2000

2000 Figure 66

60 TRG-TRC016-EN

notes

period three

System Variations

doubled, and even tripled, during periods of high demand in the first summer

of deregulation. Utility deregulation will occur differently in various locations,

but the possibility of high electricity costs during peak periods has building

owners and operators looking for ways to use different fuels during those

periods.

Another reason for using different fuel types is that it provides the building

owner with leverage to negotiate for reduced utility prices from competing

utilities. If a building uses both electricity and natural gas for cooling, and

can switch between the two, the owner can often negotiate better rates for

both cooling and heating.

A chilled-water system that uses more than one type of fuel is referred to as a

hybrid system. The most obvious option for using an alternate fuel is an

absorption chiller. This type of chiller can be powered by natural gas, fuel oil,

or even waste heat in the form of steam or hot water.

Another option is to use natural gas to operate an engine-and-generator set that

produces electricity, and then use that electricity to run a standard electric

chiller. This indirect coupling of the gas engine to the chiller allows the flexibility

of operating the chiller using the gas engine during times of high electricity

costs, and operating the chiller on utility (line) electricity during times of low

electricity costs. A second benefit of indirect coupling is that the engine can

be sized to provide enough power for the chiller, the pumps, and, in a water-

cooled system, the cooling tower. If the engine is also to be used for emergency

backup, the pumps and cooling tower would not need a second generator to

provide them with power.

An alternative to this approach is to directly couple the engine and chiller.

A significant drawback of this approach is that the building owner does not

have the flexibility to switch between natural gas and electricity—the chiller

must always operate on natural gas. Also, only the chiller is connected to the

engine. If emergency backup is necessary, a second generator is required to

operate the pumps and cooling tower.

Fuel Choice Options

absorption thermal storage

indirectly-coupled,

gas-engine

chillers

control interface

power

Figure 67

TRG-TRC016-EN 61

period three

System Variations

notes A third method of using an “alternate” fuel is actually to use the same fuel

(electricity) but to use it at a different time. The highest electricity costs occur at

the time of highest demand. For example, a real-time-pricing rate for electricity

may be $0.50/kWh at times of peak demand during the day but only $0.03/kWh

at night. By using either ice or chilled water to store cooling capacity at night

when the cost of electricity is low, and then using that stored energy to help

cool the building during the day when the cost of electricity is high, total electric

costs can be reduced substantially. Although thermal storage does not use a

different fuel, it is certainly an option for avoiding high electricity costs during

peak periods.

Low-Flow Systems

Building owners are becoming more conscious about how improved efficiency

reduces system operating costs and overall environmental impact. Typically,

the largest piece of equipment in the chilled-water system is the water chiller.

However, it is also the most efficient piece of equipment in the system.

Figure 68 shows the dramatic improvements in chiller efficiency, at standard

ARI conditions, since 1970. High-efficiency compressors and motors,

economizers on multiple-stage centrifugal compressors, more heat-transfer

tubes, and tubes with special geometry to enhance heat transfer in both the

evaporator and condenser, have all contributed to these efficiency

improvements. Manufacturers continue to strive to improve chiller efficiency

by redesigning chiller components.

Chiller Efficiency Improvements

chiller efficiency (kW/ton)

1970

year

1980

1980 2000

2000

COP

kW/ton

1990

chiller efficiency (COP)

0.9

0.8

0.7

0.6

0.5

8.0

7.0

6.0

5.0

4.0

Figure 68

62 TRG-TRC016-EN

notes

period three

System Variations

Realize, however, that the chiller is only one component of the chilled-water

system. Although chiller efficiency is important, overall system efficiency is

more important because the building owner pays to operate the entire system,

not just the chiller. Said another way, “The meter is on the building!”

With this in mind, many system design engineers are looking for ways to

optimize the efficiency of the entire system, not just the chiller.

One approach to increase overall system efficiency has been to reduce pump

and cooling-tower energy by reducing the amount of water being pumped

through the system. In the past, the conditions shown in the center column of

the table in Figure 70 were often used when designing a water-cooled, chilled-

water system. These flow rates result in a 10°F [5.6°C] temperature difference

(∆T) through both the evaporator and the condenser. In fact, they are the

standard conditions at which electric, vapor-compression chillers are rated by

Greater Focus on System Efficiency

Figure 69

Trend Toward Lower Flow Rates

yesterday

evaporator

flow rate

leaving

chilled-water

temperature

condenser

flow rate

entering

condenser-water

temperature

2.4 gpm/ton

[0.043 L/s/kW]

44°F

[6.7°C]

3.0 gpm/ton

[0.054 L/s/kW]

85°F

[29.4°C]

today

1.5 gpm/ton

[0.027 L/s/kW]

41°F

[5°C]

2.0 gpm/ton

[0.036 L/s/kW]

85°F

[29.4°C]

electric-driven chiller

Figure 70

TRG-TRC016-EN 63

period three

System Variations

notes ARI. They are not, however, suggestions for good design practice for any given

system—they simply define a common rating point to aid comparisons.

Trends toward improved humidity control and system-level energy efficiency

have led many design engineers to reduce the flow rates on both the chilled-

and condenser-water sides of the system. This results in smaller motors in the

pumps and cooling-tower fans, as well as smaller pipes and control valves in

the distribution system. The right column of this table shows one possible set

of conditions for a low-flow system. For comparison, 1.5 gpm/ton [0.027 L/s/

kW] through the evaporator results in a 16°F [8.9°C] ∆T, and 2.0 gpm/ton

[0.036 L/s/kW] through the condenser results in a 15°F [8.3°C] ∆T.

Figure 71 shows the combined annual energy consumption of the chiller,

chilled- and condenser-water pumps, and cooling-tower fans for these two

system designs. In fact, a growing number of design engineers and utilities

have published papers or manuals that recommend that system flow rates be

reduced. A number of them have found that using lower flow rates can reduce

both installed and operating costs.

“…there are times you can ‘have your cake and eat it too.’ In most

cases, larger ∆T’s and the associated lower flow rates will not only save

installation cost but will usually save energy over the course of the

year. This is especially true if a portion of the first-cost savings is

reinvested in more efficient chillers. With the same cost chillers, at

worst, the annual operating cost with the lower flows will be about

equal to ‘standard’ flows but still at a lower first cost.”

(Source: Kelly, David W. and Chan, Tumin, “Optimizing Chilled Water Plants,”

Heating/Piping/Air Conditioning, January 1999)

Low-Flow Systems

annual energy consumption, kWh

base casebase case

base case

chiller

cooling tower fans

low flowlow flow

low flow

750,000

600,000

450,000

300,000

150,000

150,000 pumps

pumps

Figure 71

64 TRG-TRC016-EN

notes

period three

System Variations

Variable-Primary-Flow Systems

One of the reasons that many chilled-water systems are installed using the

primary-secondary configuration is that, in the past, chillers could not respond

well to varying water flow through the evaporator. Therefore, the production

loop was designed for a constant flow through the chillers, and the distribution

loop was designed for variable flow to take advantage of the pump energy

savings. The system was hydraulically decoupled to meet these two goals.

Alternatively, in a variable-primary-flow (VPF) system, the flow of water

varies throughout the entire system—through the evaporator of each operating

chiller as well as through the cooling coils. The VPF system differs from the

primary-secondary system in that it no longer hydraulically decouples the two

loops. The variable-flow pumps move the water through the entire system.

The primary benefit of this system is the elimination of the separate distribution

pump(s) and the associated electrical and piping connections. There is also a

small reduction in operating cost because there is seldom excess water flowing

through the bypass pipe.

VPF systems, however, require chillers that can operate properly when the

water flow through the evaporator varies. Many of today’s chillers can tolerate

variable water flow through the evaporator, within limits. These limits include

minimum and maximum flow rates and a limitation on how quickly the flow can

vary. Exceeding these operating limits may cause control instability or even

catastrophic failure. The VPF system therefore requires a method of monitoring

the flow rate through each chiller and a control system to ensure that the flow

through the evaporator stays within the limits for the specific chiller. Do not

attempt to use a VPF system with chillers that have older, analog electric,

or pneumatic controls that cannot handle variable evaporator flow.

Variable-Primary-Flow Systems

bypass

two

two-

-way

way

valve

variable

variable-

-flow

flow

pumps

control

valve

check

valves

optional bypass

with three

with three-

-way valve

way valve Figure 72

TRG-TRC016-EN 65

period three

System Variations

notes Notice also that the VPF system must continue to include a bypass. Although

a control valve prevents flow in the bypass for most system operating

conditions, the modulating valve and bypass are required to ensure that the

water flow through the system remains above the minimum flow limit of the

operating chillers. This bypass may be in the same location as in the primary-

secondary system, or it may be a three-way valve on a few of the cooling coils.

Although VPF systems have been successfully installed and operated, they

are more complex both to design and to operate when compared to a primary-

secondary system. The sequencing of chillers and pumps requires a thorough

understanding of system dynamics because flow rates will vary through every

operating chiller. The control system needs to avoid cycling (restarting the

chiller too soon) and maintain the rate-of-flow variation through the chiller

evaporators within the allowable limits. This becomes very complicated as

the number of chillers increases.

Another important consideration when investigating VPF systems is the fact

that they take more time and planning to design and commission properly than

other systems. The system design engineer must thoroughly define the control

sequence early in the design process, and clearly communicate it to the

controls provider. Also, the system operators must understand how the VPF

system works; therefore, training is mandatory. The success of a system design

is directly related to the ability of the operator to carry out the design intent.

Critical VPF System Requirements

▲Chillers must handle variable evaporator flow

▲System must include a bypass

▲System-level controls must limit the rate-of-

flow change

▲Adequate time to design and commission

controls

▲Operator must understand the system

Figure 73

66 TRG-TRC016-EN

notes

period three

System Variations

Preferential Loading

To take full advantage of a high-efficiency, heat-recovery or alternate-fuel chiller,

the system may need a method to preferentially load these chillers. The

following two system configurations are variations of the primary-secondary

system.

In the basic primary-secondary system, all operating chillers are loaded to equal

percentages. In this first preferential-loading configuration, the preferentially-

loaded chiller is moved to the distribution side of the bypass pipe. This chiller

is preferentially (or most fully) loaded when it is turned on, because it always

receives the warmest system return water. In this example, an absorption chiller

is located on the distribution side of the bypass pipe so that it can be

preferentially loaded during periods of high electricity costs.

Preferential Chiller Loading

bypass pipe

distribution

pump

absorption

chiller

preferentially

loaded

equally

loaded

Figure 74

TRG-TRC016-EN 67

period three

System Variations

notes

The second preferential-loading configuration, shown in Figure 75, ensures

that the chiller in the sidestream position receives the warmest entering-water

temperature, and that it can be fully loaded whenever the system load is high

enough. This arrangement is unique because it not only allows preferential

loading, but it also permits the chiller (or other cooling device) in the sidestream

position to operate at any temperature difference. In other words, it does not

need to supply water at the same temperature that the other operating chillers

do. The chiller in this position precools the system return water, reducing the

load on the downstream chillers. In this example, a heat-recovery chiller is

located in the sidestream position so that it can be preferentially loaded to

maximize the amount of heat recovered, thus reducing the overall building

energy consumption. Because a heat-recovery chiller is typically less efficient

than a standard cooling-only chiller, the heat-recovery chiller only has to

provide as much cooling as is required to meet the heat-recovery load, letting

the more efficient cooling-only chillers meet the rest of the cooling load.

One drawback of the sidestream arrangement is that it does not add water flow

capability to the system, it simply reduces the load on other chillers. Therefore,

the other system pumps must ensure that the system flow requirements are

met. For this reason, the capacity of the sidestream chiller is often smaller than

the other chillers in the plant.

Either of these arrangements allows a chiller to be preferentially loaded.

Preferential loading is typically most beneficial in the following applications:

nIn a system that has a high-efficiency chiller along with several standard-

efficiency chillers, the high-efficiency chiller can be preferentially loaded to

reduce system energy consumption.

nIn a system with a heat-recovery chiller, preferentially loading the heat-

recovery chiller maximizes the amount of heat recovered, thus reducing

the overall building energy consumption.

Sidestream Configuration

bypass pipe

distribution

pump

heat

heat-

-recovery

recovery

chiller

Figure 75

68 TRG-TRC016-EN

notes

period three

System Variations

nIn a system with an alternative-fuel chiller, such as an absorption chiller,

preferentially loading the alternative-fuel chiller during times of high

electricity costs minimizes system energy cost.

Heat Recovery

Heat recovery is the process of capturing the heat that is normally rejected from

the chiller condenser and using it for space heating, domestic water heating,

or another process requirement. Heat recovery has been successfully applied in

virtually all types of buildings, including hotels, schools, manufacturing plants,

and office buildings. It typically provides an attractive return on investment for

building owners. The use of heat recovery should be considered in any building

with simultaneous heating and cooling requirements, or in facilities where the

heat can be stored and used at a later time. Buildings with high year-round

internal cooling loads are excellent opportunities for heat recovery.

Additionally, ASHRAE/IESNA Standard 90.1–1999 (Section 6.3.2) includes

restrictions on the amount of reheat that can be performed in an HVAC system

unless it is recovered heat. It is therefore likely that heat recovery will be used

more in the future.

Heat recovery can be applied to practically any type of water chiller. It can be

accomplished either by operating at higher condensing temperatures and

recovering heat from the water leaving the standard condenser, or by using a

separate condenser, as shown in Figure 76 for a centrifugal chiller. In smaller

chillers, heat recovery is sometimes accomplished using a device called a

desuperheater. A desuperheater is a device that is connected to the

refrigeration circuit between the compressor and condenser to recover heat

from the hot refrigerant vapor.

Heat-Recovery Chiller

heat

heat-

-recovery

recovery

condenser

standard

condenser

evaporator

Figure 76

TRG-TRC016-EN 69

period three

System Variations

notes

For water-cooled centrifugal chillers, there are generally three methods of

implementing heat recovery.

The dual-condenser, or double-bundle, heat-recovery chiller contains a

second, full-size condenser that is connected to a separate hot-water loop.

It is capable of more heat rejection and higher leaving-hot-water temperatures

than an auxiliary condenser. The amount of heat rejected is controlled by

varying the temperature or flow of water through the standard condenser.

Chiller efficiency is degraded slightly in order to reach the higher condensing

temperatures.

An auxiliary-condenser, heat-recovery chiller makes use of a second, but

smaller, condenser bundle. It is not capable of rejecting as much heat as the

dual-condenser chiller. Leaving hot-water temperatures are also lower, so it is

often used to preheat water upstream of the primary heating equipment or

water heater. It requires no additional controls, and actually improves chiller

efficiency because of the extra heat-transfer surface for condensing.

A heat-pump chiller is a standard chiller (no extra shells are required) used

and controlled primarily for the heat it can produce in the condenser. The

evaporator is connected to the chilled-water loop, typically in the sidestream

position discussed earlier, but it only removes enough heat from the chilled-

water loop to handle the heating load served by its condenser. This application

is useful in a multiple-chiller system where there is a base or year-round heating

or process load, or where the quantity of heat required is significantly less than

the cooling load. The heating efficiency of a heat-pump chiller is the highest of

any heat-producing device.

Heat-Recovery Chiller Options

heat-recovery

(dual) condenser

auxiliary

condenser heat pump

◆No extra

condenser

◆Large base-heating

loads or continuous

operation

◆High hot-water

temperatures

◆Controlled

◆Good heating

efficiency

◆Second, full-

size condenser

◆Large heating

loads

◆High hot-water

temperatures

◆Controlled

◆Degrades

chiller efficiency

◆Second, smaller-

size condenser

◆Preheating loads

◆Moderate

hot-water

temperatures

◆Uncontrolled

◆Improves chiller

efficiency

Figure 77

70 TRG-TRC016-EN

notes

period three

System Variations

There is usually an efficiency penalty associated with the use of heat recovery

with a chiller. The cost of this efficiency penalty, however, is typically much less

than the energy saved by recovering the “free” heat.

The energy consumption of a heat-recovery chiller will be higher than that of

a cooling-only chiller, because of the higher pressure differential at which the

compressor must operate. In this example, the energy consumption of a

centrifugal chiller operating in heat-recovery mode (producing 105°F [40.6°C]

condenser water) is 0.69 kW/ton [5.1 COP]. The efficiency of the same chiller

operating in the cooling-only mode (no heat being recovered and producing

95°F [35°C] condenser water) is 0.60 kW/ton [5.9 COP]. A comparable cooling-

only chiller of the same capacity and operating at the same cooling-only

conditions consumes 0.57 kW/ton [6.2 COP]. In this example, the heat-recovery

chiller uses four percent more energy in the cooling-only mode than the chiller

designed and optimized for cooling-only operation. It is therefore important

to perform a life-cycle cost analysis to determine when heat recovery is a

viable option.

It should also be noted that the chiller can only recover the amount of heat

transferred into the evaporator plus the energy input to the compressor.

Therefore, the more load on the chiller evaporator, the more heat can be

recovered. To maximize the heat available for recovery, a heat-recovery chiller is

often piped in one of the preferential-loading configurations described earlier.

An advantage of the sidestream position is that the heat-recovery chiller is not

required to maintain a chilled-water temperature set point. It can be loaded just

enough to satisfy the heating load while the more efficient cooling-only chillers

provide the rest of the cooling. The heat removed from the chilled-water loop

benefits the system by precooling the return water.

Heat-Recovery Chiller Efficiency

chiller type

cooling

mode

cooling-only

centrifugal chiller

heat-recovery

centrifugal chiller

0.57 kW/ton

[6.2 COP]

heat-recovery

mode

0.60 kW/ton

[5.9 COP]

0.69 kW/ton

[5.1 COP]

cooling mode conditions:

• evaporator ∆T = 44°F to 54°F [6.7°C to 12.2°C]

• condenser ∆T = 85°F to 95°F [29.4°C to 35.0°C]

heat-recovery mode conditions:

• evaporator ∆T = 44°F to 54°F [6.7°C to 12.2°C]

• condenser ∆T = 85°F to 105°F [29.4°C to 40.6°C]

not

applicable

Figure 78

TRG-TRC016-EN 71

period three

System Variations

notes

The control of a heat-recovery centrifugal chiller, although seemingly simple,

is critical to reliable chiller operation. Typically, either the temperature or the

flow of the water entering the standard condenser is modulated to meet the

capacity required by the heat-recovery condenser.

Controlling heat-recovery capacity based on the temperature of the hot water

leaving the heat-recovery condenser can cause operational problems for a

centrifugal chiller. This is explained best by using a map of centrifugal-

compressor operation (see Figure 79). Control based on the temperature of the

water leaving the heat-recovery condenser causes the condenser-to-evaporator

pressure differential to remain relatively high at all loads (line A to B). High

pressure differentials at low cooling loads increases the risk of a centrifugal

compressor operating in its unstable region, commonly known as surge.

The preferred method is to control heat-recovery capacity based on the

temperature of the hot water entering the heat-recovery condenser. This allows

the condenser-to-evaporator pressure differential to decrease as the chiller

unloads (line A to C), thereby keeping the centrifugal chiller from surging and

resulting in more stable operation. If high leaving-hot-water temperatures are

required at low-cooling-load conditions, another method to prevent surge is

to use hot gas bypass on the centrifugal chiller.

For other types of chillers that are not prone to surge, operating at these high

pressure differentials at low cooling loads may cause the chiller to consume

more energy than it recovers in the form of heat.

Control of a Heat-Recovery Chiller

percent maximum pressure differential

percent load

unstable

operation

(surge)

unloading with

constant

constant entering

entering hot

hot-

water temperature

100

120

100

unloading with

constant

constant leaving

leaving hot

hot-

water temperature

Figure 79

72 TRG-TRC016-EN

notes

period three

System Variations

Asymmetric Design

Many system design engineers seem to default to using chillers of equal

capacity in a multiple-chiller system. There are benefits to using chillers of

varying capacities to more favorably match the system loads. Remember that

when a chiller is started, so is the associated ancillary equipment (pumps and

cooling tower). In general, the smaller the chiller, the smaller the ancillary

equipment. Operating the least number of chillers, and the smallest chiller

possible, to meet the system load minimizes system energy consumption.

Figure 80 examines the use of a “60/40 split,” that is, one chiller (the “lead”

chiller) is sized for 40 percent of the total system capacity and the second chiller

(the “lag” chiller) for 60 percent. Notice that the number of hours of chiller (and

ancillary equipment) operation is reduced by 15 percent, by changing from two

chillers of equal capacity to one chiller at 40 percent capacity and the second

chiller at 60 percent. This is because up to 60 percent load, only one chiller is

operating. Below 40 percent load, only the lead chiller is operating; between

40 and 60 percent load, only the lag chiller is operating. In the same system with

chillers of equal capacity, both chillers and their ancillary equipment are

operating when the load is 50 percent or greater.

Asymmetric Design

annual operating hours

50% / 50%

lag

chiller

lag

chiller

60% / 40%

10,000

8,000

6,000

4,000

2,000

lead

chiller

lead

chiller

▲Different chiller

capacities

▲Different chiller

efficiencies

chiller split

Figure 80

TRG-TRC016-EN 73

period three

System Variations

notes

Another benefit of unequal chiller capacities is that the system load can be

more closely matched with the operating chiller (and ancillary equipment)

capacity, increasing overall system efficiency.

Figure 81 shows a system that includes two large chillers of equal capacity,

along with one smaller capacity chiller. The smaller capacity chiller, called

a swing chiller, in this combination presents an opportunity for significant,

overall system-energy savings.

Swing Chiller

equal

equal-

-capacity

capacity

large chillers

small

small-

-capacity

capacity

“swing” chiller

Figure 81

74 TRG-TRC016-EN

notes

period three

System Variations

The smaller swing chiller is turned on to handle the low cooling loads either

during the night or during unoccupied periods of time. When the building load

exceeds the capacity of the swing chiller, it is turned off and a larger chiller is

turned on. The larger chiller handles the building cooling load alone until it

becomes fully loaded. Then the swing chiller is turned on again. The swing

chiller is alternated on and off between the larger chillers' operation to serve

as a smaller incremental step of loading. This sequence more favorably

matches the capacity of the chiller plant to the system load. It keeps the large

chillers loaded in their peak efficiency range and operates with the fewest,

and smallest, pieces of ancillary equipment (pumps and cooling towers) at any

system load.

A common concern is to prevent the swing chiller from cycling too frequently,

which could shorten the life of the equipment. In large chilled-water systems,

however, the changes in building load typically occur slowly enough that this

is not a problem.

A final asymmetric design option is to use one high-efficiency chiller and one or

more standard-efficiency chillers. In this type of system, the high-efficiency

chiller should be preferentially loaded to minimize system energy consumption.

Swing Chiller

percent cooling load

chiller 1

100

swing chiller

chiller sequence

chiller 2

swing chiller

Figure 82

TRG-TRC016-EN 75

period three

System Variations

notes

“Free” Cooling

There are a number of methods that use cool outdoor conditions to reduce

cooling energy costs. They are often referred to as “free cooling” because they

reduce or eliminate the energy consumed by the compressor. They are not truly

free, but really reduced-cost cooling options.

The most prevalent method is the use of an airside economizer. When the

temperature, or enthalpy, of the outdoor air is low enough, the outdoor-air and

return-air dampers in the air handler are modulated and the cooler outdoor air

is used to reduce the temperature of air entering the cooling coil. This can

reduce or totally eliminate the requirement for mechanical cooling for much

of the year in many climates.

In water-cooled systems, there are also several types of waterside

economizers. The most direct method, but typically the least desirable, is to

use a strainer cycle. In this system, the condenser- and chilled-water systems

are connected. When the outdoor wet-bulb temperature is low enough, cold

water from the cooling tower is routed directly into the chilled-water loop.

Although the strainer cycle is the most efficient waterside economizer option, it

greatly increases the risk of fouling in the chilled-water system and cooling coils

with the same type of contamination that is common in open-cooling-tower

systems. A strainer or filter can be used to minimize this contamination, but the

potential for fouling prevents widespread use of the strainer-cycle system.

“Free” Cooling

▲Airside economizer

▲Waterside economizer

◆Strainer cycle

◆Plate-and-frame heat exchanger

◆Refrigerant migration

Figure 83

76 TRG-TRC016-EN

notes

period three

System Variations

A second type of waterside economizer is the plate-and-frame heat

exchanger. In this case, water from the cooling tower is kept separate from

the chilled-water loop by a plate-and-frame heat exchanger. This is a popular

configuration because it can achieve a high heat-transfer efficiency without the

potential for cross-contamination. With the addition of a second condenser-

water pump and proper piping modifications, this heat exchanger can operate

simultaneously with the chiller. As much heat as possible is rejected through

the heat exchanger, while the chiller handles any excess cooling load.

If the plate-and-frame heat exchanger is piped in the sidestream position, it can

be used for more hours in the year because it does not need to maintain a

leaving-chilled-water temperature set point. It can provide some useful cooling

at any time that it can precool the system return water.

If simultaneous free cooling and mechanical cooling are performed, care must

be taken to control the evaporator-to-condenser pressure differential inside the

chiller. When very cold condenser water flows through the chiller condenser for

an extended period of time, operational problems may result due to a low

pressure differential between the evaporator and the condenser. Using a

three-way, modulating bypass valve to mix the warm water leaving the chiller

condenser with the cold water entering the condenser, or a two-way

modulating valve and a variable-speed condenser water pump, can eliminate

this problem. Consult with the chiller manufacturer to determine the limits for

the specific chiller being used. This issue is discussed further in Period Four.

waterside economizer

Plate-and-Frame Heat Exchanger

bypass pipe

distribution

pump

plate

plate-

-and

and-

-frame

frame

heat exchanger

condenser

water loop

Figure 84

TRG-TRC016-EN 77

period three

System Variations

notes

The final method of “free” cooling is to transfer heat between the cooling tower

water and the chilled water inside a centrifugal chiller through the use of

refrigerant migration. When the temperature of the water from the cooling

tower is colder than the desired chilled-water temperature, the compressor is

turned off and automatic shutoff valves inside the chiller refrigerant circuit are

opened. This allows refrigerant to circulate between the evaporator and

condenser without the need to operate the compressor. Because refrigerant

vapor migrates to the area with the lowest temperature, refrigerant boils in the

evaporator and the vapor migrates to the cooler condenser. After the refrigerant

condenses into a liquid, it flows by gravity back into the evaporator.

There are no additional fouling concerns because the cooling-tower water flows

through the chiller condenser and is separated from the chilled-water loop.

Although not as effective as a plate-and-frame heat exchanger, it is possible for

refrigerant migration in a centrifugal chiller to satisfy many cooling load

requirements (up to 40 percent of the chiller’s design capacity) without

operating the compressor. This can increase further if the system can

accommodate warmer chilled-water temperatures at part-load conditions.

waterside economizer

Refrigerant Migration

from

compressor

evaporator

condenser

shutoff

valve

shutoff

valve

vapor

migration

liquid

flow

Figure 85

78 TRG-TRC016-EN

notes

period three

System Variations

Application Outside the Operating Range of the Chiller

Some process-load applications involve either temperatures or flow rates that

are outside the capabilities of any chiller. This may include high return-water

temperatures, high or low fluid flow rates, or high or low system ∆Ts. By using

special piping arrangements, a standard chiller can still be used to satisfy the

requirements of the process load.

Figure 86 shows a system in which the water flow requirement of the process

load is below the minimum flow rate for a chiller with the required capacity.

The system is designed like a primary-secondary system, but the production

loop has a higher design flow rate than the distribution loop. This allows the

water flow rate and ∆T through the chiller to be within the acceptable limits,

while the water flow rate and ∆T through the process meet its requirements.

Alternatively, in a smaller system with a single chiller, a single pump on the

chiller side of the bypass and a diverting valve to maintain the proper flow

through the process load can achieve the same result.

Application Outside Range of Chiller

air

air-

-cooled

cooled

chiller

240

240 gpm

gpm at 45° F

at 45° F

[15 L/s at 7.2°C]

80 gpm

gpm at 45° F

at 45°F

[5 L/s at 7.2°C]

160

160 gpm

gpm

at 45°F

[10 L/s at 7.2°C]

process load

240

240 gpm

gpm at 56.7°F

at 56.7° F

[15 L/s at 13.7°C]

80 gpm

gpm at 80°F

at 80°F

[5 L/s at 26.7°C]

Figure 86

TRG-TRC016-EN 79

notes

period four

Chiller-Plant Control

It is important to understand that no matter how good the system design is,

adequate controls are necessary for all the components to operate properly as

a system. It is equally important to understand that you cannot “control your

way out of a bad system design.”

The chiller plant consists of chillers, pumps, pipes, coils, cooling towers,

temperature sensors, control valves, and many other devices. It is similar to

an orchestra with many instruments. The existence of these pieces does not

guarantee that the system will work properly. There needs to be an orchestra

conductor. In the case of a chilled-water system, that conductor is a chiller-plant

control system. How well the plant works depends on how well the control

system gets all the pieces to work together.

period four

Chiller-Plant Control

Chilled-Water Systems

Figure 87

80 TRG-TRC016-EN

notes

period four

Chiller-Plant Control

The largest change to chillers in the last decade has undoubtedly been in the

area of controls. In the past, chillers were pneumatically controlled, and they

were protected by turning them off if the flow rates or temperatures changed

too quickly. Today’s microprocessor-based controls provide accurate chilled-

water temperature control, as well as monitoring, protection, and adaptive

limit functions.

These controls monitor chiller operation and prevent the chiller from operating

outside its acceptable limits. They can also adapt to unusual operating

conditions, keeping the chiller operating by modulating its components and

sending a warning message, rather than doing nothing more than shutting it

down when a safety setting is violated. Improved control accuracy allows

chillers to be applied in systems and applications that were previously avoided.

When problems occur, diagnostic messages aid troubleshooting. Modern

chiller controls also interface with a chiller-plant control system for integrated

system operation.

Chiller Controls

▲Start–stop

▲Chilled-water

temperature

control

▲Monitor and

protect

▲Adapt to

unusual

conditions

Figure 88

TRG-TRC016-EN 81

period four

Chiller-Plant Control

notes

There are primarily five issues to address in a chiller-plant control system.

1When should a chiller be turned on or off?

2After we know that a chiller must be turned on or off, which one should

it be?

3If we attempt to turn on a chiller, pump, or cooling tower, and there is a

malfunction, what do we do next?

4How can we minimize the energy cost of operating the system?

5How can the chiller-plant control system effectively communicate with the

operator?

What Is Important?

▲When to turn a chiller on or off

▲Which chiller to turn on or off

▲How to recover from an equipment failure

▲How to optimize system efficiency

▲How to communicate with the operator

Figure 89

82 TRG-TRC016-EN

notes

period four

Chiller-Plant Control

Chiller Sequencing

Chiller sequencing refers to making decisions about when to turn chillers on

and off, and in what order. Typically, turning chillers on and off is performed

with the goal of matching the capacity of the chiller plant to the system cooling

load. In order to do this successfully, the design of the chilled-water system

must provide the control system with variables that are good indicators of

system load.

The hydraulic design and size of the chilled-water system will determine the

possible method(s) for effectively monitoring system load. Typical methods

for load monitoring include:

nIn series- or parallel-piped systems, the supply- and return-water

temperatures, and sometimes chiller current draw, are monitored.

nIn a primary-secondary system, the system supply and chiller return-water

temperatures and/or the direction and quantity of flow in the bypass pipe

are typically measured.

nIn a variable-primary-flow system, the system supply-water temperature

and the system flow rate may be monitored.

nDirect measurement of the system load (in tons, kW, or amperes) has also

been used in some systems.

Other methods are also possible. It is imperative that the chilled-water system

be designed with the control variables in mind; otherwise, the result may be a

system that is impossible to efficiently control.

Chiller Sequencing

▲Turning on an additional chiller

▲Turning off a chiller

▲Which chiller to turn on or off?

Figure 90

TRG-TRC016-EN 83

period four

Chiller-Plant Control

notes

Today’s chiller controls can very accurately control the chiller’s leaving-water

temperatures over a wide range of loads. This is especially true of centrifugal

and helical-rotary chillers. This fact allows constant-flow chilled-water systems,

similar to the system shown in Figure 91, to use the system supply- and return-

water temperatures to determine system load.

By sensing a rise in the temperature of the water leaving the chiller plant,

the control system can determine when the operating chillers can no longer

maintain the desired temperature. Often, the supply-water temperature is

allowed to drift a predetermined amount before an additional chiller is turned

on, to ensure that there is enough load to keep an additional chiller operating.

Deciding when it is appropriate to turn a chiller off is more complex. The control

system may monitor the system ∆T, that is, return-water temperature minus

supply-water temperature. This information, along with the capacities of the

operating chillers, allows the control system to determine when a chiller can be

turned off. To help stabilize system operation, the control system should use

logic to prevent load transients from causing unwarranted chiller cycling.

In constant-flow systems that are suffering from “low ∆T syndrome” (airside

systems that return water to the plant at lower temperatures than desired),

some of the load terminals may starve for flow before the capacity of the

operating chiller is exceeded. To preserve system efficiency, this situation is

best dealt with by solving the airside problem. Typical causes of low ∆T

syndrome include: a poorly-balanced flow system, dirty filters or coils,

poorly performing air-handler controls, incorrect coil control valves, or

undersized air handlers.

load indicators

Temperature

supply

supply-

-water

water

temperature

return

return-

-water

water

temperature

chiller

chiller-

-plant

plant

controller

Figure 91

84 TRG-TRC016-EN

notes

period four

Chiller-Plant Control

In a primary-secondary system, the direction and quantity of flow in the bypass

pipe is an excellent indicator of when to turn a chiller on or off . As discussed in

Period Two, the water flow in the bypass pipe can be measured directly using a

flow meter, or indirectly by measuring system water temperatures and applying

flow-mixing equations. The rules applied to the bypass flow to determine when

to turn a chiller on and off are:

nWhen there is a deficit flow, a chiller may be added.

nWhen there is excess flow greater than that of the next chiller to be turned

off, plus a 10 to 15 percent safety factor, that chiller may be turned off.

nIf neither of the above conditions exists, do nothing.

As an alternative to measuring flow in a primary-secondary system with four or

less chillers, system supply and chiller-plant return-water temperatures may be

used to decide when to turn a chiller on or off. This is similar to the logic applied

to constant-flow systems. It is simple and has a low installed cost, but it is less

accurate than flow determination, especially as the number of chillers

increases.

“Low ∆T syndrome” can also affect the operation of primary-secondary

systems. Unlike constant-flow systems, the primary-secondary system will

maintain the required system flow and supply-water temperature, and therefore

maintain occupant comfort. However, it accomplishes this by turning on

additional chillers before all operating chillers are fully loaded. This may reduce

overall system efficiency.

Although some have proposed solutions such as putting a valve in the bypass

line, lowering the supply-water temperature, or controlling the system

differently, these are only band-aids that mask the actual problem and often

cause other operational difficulties. Fixing the root cause of low ∆T syndrome in

the distribution system is the best course of action for proper and efficient

system operation.

bypass pipe

load indicators

Flow

flow

meter

chiller

chiller-

-plant

plant

controller

Figure 92

TRG-TRC016-EN 85

period four

Chiller-Plant Control

notes

Another method of monitoring system cooling load is to measure the system

water flow rate and temperatures directly, and then calculate the load. Although

it would appear that direct measurement of the actual system load would be an

excellent way to determine when to turn chillers on and off, this method has

several drawbacks. It requires the use of flow meters with high accuracy and

high turndown capacities. Although flow meters have become more accurate

and less expensive, they require special installation conditions for reliable

accuracy—conditions seldom achievable in real installations. Also, the

equipment typically requires regular calibration. For these reasons, direct

measurement of load has not been used as much as the simple and reliable

methods discussed previously.

An alternate way to monitor chiller load is by measuring the current draw of the

chiller motor. By itself, this does not provide an adequate control indicator, but

when used in conjunction with other information, such as system supply-water

temperature, it can be effective. System supply-water temperature is used to

determine when to turn an additional chiller on, and operating chiller

compressor-motor current draw is used to determine when a chiller can be

turned off.

The most effective load indicator for any chilled-water system is dependent on

the design of that system. Creative designers have used the control strategies

as described here and in various combinations to effectively control a wide

variety of chiller plants. It is highly recommended that one of the first tasks

undertaken in the design process is to create a simplified flow diagram and a

load model of the system that allows for the evaluation of various control

strategies and sensor placements. This will help to ensure that effective

chiller-plant control can be implemented.

load indicators

Capacity

flow

meter

chiller

chiller-

-plant

plant

controller

supply

supply-

-water

water

temperature

return

return-

-water

water

temperature

Figure 93

86 TRG-TRC016-EN

notes

period four

Chiller-Plant Control

When the system has determined that a chiller needs to be turned on or off,

the next issue is to determine the sequence in which to turn chillers on and off.

It is assumed that the first chiller in the sequence will always be turned on

whenever cooling is required.

When the system consists of identical chillers, the choice of which chiller is

turned on or off next has little impact on system efficiency. Some design

engineers and operators prefer to equalize the run time and the number of

starts for all chillers in the system. This is typically done by rotating the

sequence of chillers on a periodic basis, often every few days or weeks. This

method generally keeps the run time equalized reasonably well, and the

operator knows exactly when to expect the rotation to occur. An alternative

approach is to total the actual run hours on each chiller, in an attempt to rotate

the chillers when a significant imbalance in the run time or the number of starts

occurs. Rotation that is based on actual run time has the disadvantage of the

operator not knowing when rotation will occur. In some installations, operating

personnel prefer to manually initiate rotation.

On the other hand, some design engineers and operators believe that

equalizing run times will result in all of the chillers needing to be overhauled or

replaced at the same time. They tend to operate the most-efficient chiller first,

followed by the next-most-efficient, and so on. With this approach, all chillers

are turned on at least once a month to ensure that they will be able to start

when required.

Chiller Rotation

equally

equally-

-capacity chillers

capacity chillers

Figure 94

TRG-TRC016-EN 87

period four

Chiller-Plant Control

notes

When the system consists of chillers with different capacities, efficiencies, or

fuel types, the question of which chiller to turn on or off next becomes more

complex. Although each system requires a complete analysis, there are some

general principles that apply to most systems.

In systems with chillers of different capacities, such as the “swing” chiller

concept introduced in Period Three, the goal is to operate the least number

of chillers and the smallest chiller possible. This typically minimizes overall

system energy consumption by closely matching the capacity of the plant to

the system load, thus reducing the energy used by ancillary equipment.

In systems with chillers of different efficiencies, it makes sense to operate the

most efficient chillers first and the least efficient chillers last. If different fuel

types are involved, the control system may receive data on the costs of natural

gas and electricity and calculate the real-time cost of operating the electric-

versus gas-driven chillers.

large electric

chiller

chiller absorption

absorption

chiller

small electric

chiller

Chiller Rotation

Figure 95

88 TRG-TRC016-EN

notes

period four

Chiller-Plant Control

The system might also benefit from having a heat-recovery chiller fully loaded.

As discussed in Period Three, to maximize the amount of heat recovered, it is

often desirable to preferentially load that chiller, sequencing it as a base

chiller—“first on” and “last off.” Other chillers can then be turned on when the

heat-recovery chiller cannot handle the cooling load alone.

A variation on this idea is an absorption chiller fueled by waste heat. It is

preferentially loaded to handle as much of the cooling load as possible before

turning on other chillers. The absorption chiller would be sequenced as a base

chiller to make use of the free energy operating this chiller.

Heat Recovery

preferentially

preferentially-

-loaded

loaded

heat

heat-

-recovery

recovery

chiller

standard

electric chillers

Figure 96

TRG-TRC016-EN 89

period four

Chiller-Plant Control

notes

The variable-primary-flow system, introduced in Period Three, is designed to

operate with variable flow through the chiller evaporators. Sequencing chillers

in this type of system cannot be based solely on temperature, because in a

properly-operating system the supply- and return-water temperatures will be

nearly constant. Determining when to turn chillers on or off is not a simple task.

For control stability and chiller reliability, the flow rates through the chillers,

and the rate of flow change, must be kept within allowable ranges.

Therefore, control of a variable-primary-flow system must:

nInclude a method to determine system load. Many systems measure flow

rates and temperatures.

nEnsure that flow rates through the chillers are within the allowable

minimum and maximum limits. Modulation of a control valve in the bypass

pipe is commonly used to ensure minimum flow rates through the chillers.

nControl the rate at which the system flow rate changes, to ensure that it does

not change more rapidly than the chillers can adapt. This is especially critical

when turning on additional chillers.

Adequate time must be spent designing the control sequence and

commissioning the system after installation, to ensure proper operation of a

variable-primary-flow system.

Variable-Primary-Flow Systems

bypass

two

two-

-way

way

valve

variable

variable-

-flow

flow

pumps

control

valve

check

valves

optional bypass

with three

with three-

-way valve

way valve Figure 97

90 TRG-TRC016-EN

notes

period four

Chiller-Plant Control

Failure Recovery and Contingency Planning

In addition to normal chiller sequencing, the chiller-plant control system must

react when a chiller or another piece of associated equipment fails. Failure

recovery, or ensuring the reliable supply of chilled water, is a very important

part of the chiller-plant control system, and is an area where many systems

have fallen short. This is especially true in field-programmed systems because

of the difficulty of thorough debugging.

During periods of equipment malfunction, it is important to focus on the

primary goal of the system, which is to provide the required flow of chilled

water to the system at the proper temperature. It seems reasonable that the

simplest and most reliable failure-recovery sequence is to simply turn on the

next chiller in the sequence, and not try to turn several chillers on and off in

an attempt to re-optimize the system.

During an equipment failure, it is especially important to notify the operator

of the status, as well as to help the operator understand where the problem is

and what might be the cause. The control system must also allow the operator

to easily analyze the situation and to intervene if the failure condition will exist

for an extended period of time. A system that provides this information will

ensure that the system itself will be maintained and operated in proper

condition.

System Failure Recovery

▲Maintain flow of chilled water

▲Keep it simple

◆Lock out failed equipment

◆Turn on the next chiller in the sequence

▲Notify the operator

▲Allow the operator to intervene

Figure 98

TRG-TRC016-EN 91

period four

Chiller-Plant Control

notes

In addition to failure recovery, it is wise for the system design engineer to

work with the building owner to develop a contingency plan for chilled water

in the case of an emergency shutdown or an extended breakdown. Many

organizations have contingency plans for critical areas of their business.

Some deal with natural disasters and others with the loss of power in critical

areas. However, few have taken the time to think about what a loss of cooling

would mean to their facility. This is often especially critical for process-cooling

applications.

Cooling contingency planning is intended to minimize the losses a facility

may incur as a result of a total or partial loss of cooling capacity. It allows a

building operator to act more quickly by having a plan in place and by

proactively preparing the facility. Such a plan often includes working with

suppliers to temporarily lease cooling equipment. During initial construction,

it is easy and cost-effective to provide piping stubs, which are built into the

chilled-water system for quick connection, and easily accessible electrical

connections. When equipment leasing is combined with these simple additions

to the system, a contingency plan can be put into action quickly and the system

can produce chilled water again in a short period of time.

It is important to first identify the minimum, or critical, cooling capacity

required. With multiple chillers in a facility, it may be acceptable to have less

than full capacity in an emergency situation. For example, the chiller plant

may consist of 1,800 tons [6,330 kW], but the minimum capacity required in

an emergency situation may only be 1,200 tons [4,220 kW]. Therefore, it is also

important to identify a contingency plan if Chiller 1 fails, if Chiller 2 fails, if

Chillers 2 and 3 fail, and so on.

Contingency Planning

electrical

connections

piping

stubouts

Figure 99

92 TRG-TRC016-EN

notes

period four

Chiller-Plant Control

System Tuning

In addition to turning chillers on and off, there are other functions of the chiller-

plant control system that help prevent system flow instability from disrupting

chiller operation. Flow instability can often be caused by normal valve and

pump operation. The first is time delays.

Excessive cycling can be detrimental to the life of a motor. For this reason,

turning a large motor (such as those used in large chillers) on and off should

be minimized. Chilled-water systems typically have a large thermal mass

(water in the system) and benefit from the diversity and slow rate of change

of the system cooling load. Fast reactions, therefore, are typically not required.

In fact, a response that is too fast will often cause system instability, waste

energy, and cause unnecessary wear on mechanical equipment. To achieve

stable and accurate control, many chiller-plant control systems provide time

delays that can be adjusted by the operator to help minimize chiller cycling.

The first time delay is the load-confirmation timer. Its purpose is to delay

turning on an additional chiller for a period of time following the initial

indication that an additional chiller is required. This confirms that the indicated

load is not a transient condition that would cause the chiller to be turned on and

then quickly turned off.

The second time delay, which works in conjunction with the first, is a

staging-interval timer. Its purpose is to allow the system time to respond

after a chiller has been turned on. This prevents more chillers from turning on

than are actually required, particularly during periods of pull-down or rapid

load variation.

The third time delay is a minimum-cycle timer. This timer should have the

highest priority. It requires a fixed period of time between turning an individual

chiller on and turning it back off. This ensures that the chiller is not cycled too

frequently.

It is important to understand that these timers are lower priority than the

safeties built into the individual chiller controls. At all times, the individual

System Timers

▲Load-confirmation timer

◆Avoids transient conditions

▲Staging-interval timer

◆Allows time for the system to

respond to turning a chiller on

▲Minimum-cycle timer

◆Prevents excessive cycling

Figure 100

TRG-TRC016-EN 93

period four

Chiller-Plant Control

notes chiller safeties must be capable of shutting the chiller down to avoid equipment

damage.

The next control function is to partially unload the operating chillers before

an additional chiller and pump are turned on. Depending on the system

configuration, there can be very rapid variations in water flow through the

chiller evaporator when a pump is turned on or off, or when a control valve is

opened or closed. Partially unloading the chiller prior to such variations allows

the chiller to continue to operate without interruption.

This can be explained by looking at a flow diagram for a chilled-water system

with multiple pumps (see Figure 36). This diagram shows that, with one pump

and chiller operating, the flow rate through the chiller is 610 gpm [38.5 L/s].

When the second, same-size pump and chiller are turned on, the flow rate

through the system increases to 870 gpm [54.9 L/s], but the flow through each

chiller drops to 435 gpm [27.4 L/s]. This is an instantaneous reduction of

175 gpm [11 L/s], or 30 percent, through the first chiller.

The temperature of the water leaving the chiller and the temperature of the

refrigerant in the evaporator drop as a result of this drastic flow reduction.

New, advanced chiller controls may allow the refrigerant temperature to drop

below the fluid’s freezing point for a brief period of time while the compressor

unloads. The evaporator low-temperature safety may, however, turn off the

chiller if the controls and compressor cannot react quickly enough.

The “unload-before-start” function partially unloads the operating chillers,

raising the refrigerant temperature in the evaporator, before the flow reduction

occurs. The chillers are allowed to reload as soon as the additional chiller is

turned on.

Unload Before Start

head pressure

system flow rate

2 pumps

1 pump

system

curve

870

870 gpm

gpm

[54.9 L/s]

610

610 gpm

gpm

[38.5 L/s]

[38.5 L/s] Figure 101

94 TRG-TRC016-EN

notes

period four

Chiller-Plant Control

Another control function that is desirable is called soft loading. It is typically

used when the system has been off for an extended period of time and the

chilled-water temperature is the same as the ambient temperature inside the

building.

Soft loading either delays turning on additional chillers or varies the chilled-

water set point, allowing the operating chillers to gradually catch up to the

building pull-down load. This results in a very smooth pull-down, prevents

overshooting the set point, and operates only the equipment required to satisfy

the actual system load.

Soft Loading

supply

supply-

-water temperature

water temperature

operating time, minutes

two chillers

soft loading

(one chiller)

set point

80°F

[26.7°C]

60°F

[15.6°C]

40°F

[4.4°C]

Figure 102

TRG-TRC016-EN 95

period four

Chiller-Plant Control

notes

Constant-flow chilled-water systems frequently require individual chiller set-

point control. Its purpose is to help maintain the system supply-water

temperature by compensating for the bypass of return water through

non-operating chillers.

The chiller-plant control system adjusts the individual set points for the

operating chiller to “overcool” the water before it mixes with the higher-

temperature water that bypasses through the non-operating chiller. The result

is that the chilled water supplied to the system is as close as possible to the

desired temperature. There are limits to the amount of overcooling. Depending

on the design of the chilled-water system, one of two situations may exist.

Either the chiller may not have been selected to produce cold-enough water,

or the temperature required may be below the freezing point of the water being

cooled. In either case, the control system must be intelligent enough to limit

overcooling in order to prevent damage to the chiller.

Additionally, the control system must know when to turn another chiller on to

meet the system chilled-water-temperature set point. Turning an additional

chiller on may be required to meet the system demand for flow, even though

the operating chiller may not be fully loaded.

constant-volume pumping system

Chilled-Water Set Point Control

54° F

[12.2° C]

37° F

[2.8°C]

54°F

[12.2°C]

off

45.5°F

[7.5°C]

lowering operating

chiller set point

helps regain control

helps regain control Figure 103

96 TRG-TRC016-EN

notes

period four

Chiller-Plant Control

System Optimization

The chiller-plant control system can also be used for system optimization.

For the purposes of this discussion, we will define optimization as minimizing

the energy used by the chiller plant (including chillers, chilled-water pumps,

condenser-water pumps, and cooling tower) while still maintaining comfort

or satisfying process loads.

The first step is to examine the energy use of the major components of the

chiller plant, to see what can be done to minimize each component individually.

The chiller energy usage can be reduced by lowering the condenser-water

temperature or by raising the chilled-water temperature.

In a variable-flow system, chilled-water pumping energy can be reduced by

lowering the chilled-water temperature while increasing the system ∆T. With

the lower water temperature and increased ∆T, the coil requires less water

flow to handle the same load.

Cooling-tower energy can be reduced by increasing the condenser-water

temperature. This allows the tower fans to cycle or slow down. Condenser-

water pumping energy can be reduced by increasing the ∆T through the

condenser side of the system, thereby pumping less water. This is achieved

by reducing the water flow through the condenser.

Obviously, looking at only a single component presents a conflicting picture for

energy reduction, and a change in one component has an impact on other

components. To truly optimize the chiller plant, all components must be

analyzed together.

System Optimization

▲Chiller

◆Decrease condenser-water temperature

◆Increase chilled-water temperature

▲Chilled-water pump (variable-flow system)

◆Increase chilled-water ∆T

▲Cooling tower

◆Increase condenser-water temperature

▲Condenser-water pump (variable-flow system)

◆Increase condenser-water ∆T

Figure 104

TRG-TRC016-EN 97

period four

Chiller-Plant Control

notes

As previously stated, as the chilled-water temperature set point is reset

upwards, the chiller will use less energy. In constant-flow systems, this

chilled-water reset strategy is fairly simple to implement and can be

controlled based on the drop in return-water temperature.

In a variable-flow system, however, as the chilled-water temperature increases,

the pumping energy also increases. While the COP of the chiller is

approximately 6.5, the COP of the pump is about 0.65. Often the increase in

pump energy will be more than the amount of chiller energy saved, especially

because the chiller will often operate at part-load conditions. Another potential

problem with resetting the chilled-water temperature upward is that space

humidity control can be compromised if the water gets too warm. Finally, the

chiller-plant control system must account for the changing supply-water

temperature.

ASHRAE/IESNA Standard 90.1–1999 (Section 6.3.4.3) requires the use of

chilled-water temperature reset in systems larger than 25 tons [88 kW]. It does,

however, exclude variable-flow systems and systems where space humidity

control will be compromised.

In Period Three, the concept of designing for reduced chilled-water temperature

and flow rates was briefly discussed. Some engineers feel that designing the

system for low flow rates and a lower supply-water temperature, thus

minimizing pump energy use, might be a better answer than attempting to

reset the temperature upward.

▲Cons

◆Increases pump energy

in variable-flow systems

◆Can cause loss of

space humidity control

◆Complicates chiller

sequencing control

Chilled Water Reset

▲Pros

◆Reduces chiller energy

◆Can work in constant-

flow systems

Figure 105

98 TRG-TRC016-EN

notes

period four

Chiller-Plant Control

Lowering the temperature of the condenser water can also reduce the energy

consumption of the chiller. Depending on the system load and outdoor

conditions, cooling towers typically have the ability to supply colder condenser

water than at design conditions. This, however, increases the energy

consumption of the cooling tower fans. The key to maximizing energy savings

is knowing the relationship of cooling-tower energy consumption to chiller

energy consumption.

At design conditions, a chiller typically uses five to ten times more energy

than a cooling tower. This would suggest that it might be beneficial to use

more cooling-tower energy to save chiller energy. However, there is a point of

diminishing return where the chiller energy savings is less than the additional

energy used by the cooling tower. Figure 106 shows the combined annual

energy consumption of a chiller and cooling tower in a system that is controlled

to various condenser-water-temperature set points. The third column shows a

system that attempts to supply 55°F [12.8°C] water from the cooling tower at

all times. Of course, at design conditions, the cooling tower may not be able to

supply this temperature, but it will supply the water at the coldest temperature

possible.

The fourth column shows a system that uses a control system to dynamically

determine the optimal condenser-water temperature that minimizes the

combined energy use of the chiller plus cooling tower. It is obvious that this

method of optimal control minimizes overall system energy consumption.

Condenser-Water Temperature

annual energy consumption, kWh

cooling

tower

cooling

tower

300,000

chiller

condenser-water temperature set pointcondenser

condenser-

-water temperature set point

water temperature set point

85°F

[29.4°C]

85°F

[29.4°C]

70°F

[21.1°C]

70°F

[21.1°C]

55°F

[12.8°C]

55°F

[12.8°C]

optimal

control

optimal

control

200,000

100,000

Figure 106

TRG-TRC016-EN 99

period four

Chiller-Plant Control

notes

Related to the issue of condenser-water-temperature control is the control of

condensing pressure. Every chiller requires a minimum refrigerant-pressure

difference between the evaporator and the condenser, in order to ensure that

refrigerant and oil circulate properly inside the chiller. This pressure difference

varies based on the chiller design and operating conditions. The chiller must

develop the required pressure difference within a certain amount of time, as

specified by the manufacturer, or the chiller controls will turn it off due to a

safety limit. During some start-up conditions, this pressure difference may be

difficult to achieve within the time required.

An example of such a condition is an office building that has been unoccupied

during a cool autumn weekend. The temperature of the water in the sump of

the cooling tower is 40°F [4.4°C]. Monday is sunny and warm, and the building

cooling load requires a chiller to be started. Because the chiller is operating

at part load and the tower sump is relatively large, the minimum pressure

difference may not be reached before the chiller is turned off on a safety.

If, however, the flow of water through the condenser is reduced, the minimum

pressure difference can be obtained. The lower flow rate increases the

temperature of the water leaving the condenser, which results in a higher

refrigerant pressure inside the condenser. After the minimum pressure

difference is reached, the flow may again be increased.

Either the refrigerant pressure in the condenser or the condenser-evaporator

refrigerant-pressure differential can be monitored and used to control the

temperature or flow rate of the condenser water, to prevent this pressure

differential from dropping below the limit.

Control of Condensing Pressure

condenser

control

panel

evaporator

Figure 107

100 TRG-TRC016-EN

notes

period four

Chiller-Plant Control

Operator Interface

System-level communication and control is very important. Today, the amount

of communication between the components (chillers, cooling towers, pumps,

control valves, and so forth) has increased immensely, allowing many chilled-

water systems to be fully automated.

In some facilities, however, the largest energy user in the HVAC system (the

chiller plant) has not progressed beyond manual control. In some cases it was

reduced to manual control shortly after the building was commissioned.

Why does this occur? Chillers are large, with very expensive pieces of

equipment which, if damaged by incorrect operation, can cost the owner a

substantial amount of money to repair or replace. Operators are, therefore, very

sensitive to chiller plant operation. If the operator does not understand how the

system is designed and controlled, it is likely that the system will be put into a

manual control mode. Therefore, initial and ongoing operator training and

support is critical.

Operator Training and Support

Figure 108

TRG-TRC016-EN 101

period four

Chiller-Plant Control

notes

There is an amazing amount of information available within a chilled-water

system. Often the problem is not a lack of information, but how to interpret

that information. Therefore, a clear and concise interface between the control

system and the system operator is extremely important.

Information that should be communicated to the operator includes:

nChiller-water system temperatures

nChiller status (on or off)

nInformation specified by ASHRAE Guideline 3

nAny pending control actions (chiller about to turn on or off)

nStatus of system time delays

nStatus of ancillary equipment (pumps, cooling towers, and so forth)

In addition, the chiller-plant control system should notify the operator of

problems that are occurring, or are about to occur, in the system. These

warning or diagnostic messages may point to a single piece of equipment

malfunctioning, or be indicative of system changes that may cause problems.

Diagnostics that occur at the chiller control panel should be communicated

to the chiller-plant control system.

Operator Interface

Figure 109

102 TRG-TRC016-EN

notes

period four

Chiller-Plant Control

ASHRAE Guideline 3, Reducing Emission of Halogenated Refrigerants in

Refrigeration and Air Conditioning Equipment and Systems, includes a list of

recommended data points to be logged daily for each chiller. Much of this data

may be available from the display on the chiller control panel. It is also helpful

to the operator if this information is available at the chiller-plant control system

and presented in a clear format.

In addition to current status, historical operating information is valuable for

keeping the equipment operating at peak efficiency and for identifying

operating trends that signal either impending problems or a drop in system

performance. For example, the condenser approach temperature is the

temperature difference between the water leaving the condenser and the

refrigerant inside the condenser. If there has been a problem with water

treatment in the cooling tower, fouling may build up inside the tubes in the

chiller condenser. This will cause the difference between the condenser water

and refrigerant temperatures to increase, reducing chiller efficiency. By noting

an increase in this approach temperature, the operator can schedule cleaning of

the condenser tubes. By monitoring system and equipment trends, the operator

has a chance to fix minor issues before they cause operational problems.

chiller operating log

ASHRAE Guideline 3

▲Chilled-water inlet and outlet

temperatures and pressures

▲Chilled water flow

▲Evaporator-refrigerant

temperature and pressures

▲Evaporator approach

temperature

▲Condenser-water inlet and

outlet temperatures and

pressures

▲Condenser water flow

▲Condenser-refrigerant

temperature and pressures

▲Condenser approach

temperature

▲Compressor-refrigerant suction

and discharge temperatures

▲Oil pressures, temperature, and

levels

▲Refrigerant level

▲Vibration levels

▲Addition of refrigerant or oil

Figure 110

TRG-TRC016-EN 103

notes

We will now review the main concepts that were covered in this clinic on

chilled-water systems.

In Period One, we learned about the different types of vapor-compression and

absorption water chillers. Vapor-compression chillers are differentiated

primarily by the type of compressor used and whether they use an air-cooled

or water-cooled condenser. Absorption chillers are primarily differentiated by

whether they are indirectly or directly fired.

We compared the use of air-cooled versus water-cooled chillers. Air-cooled-

chiller advantages include lower maintenance costs, a prepackaged system for

easier design and installation, and better low-ambient operation. Water-cooled-

chiller advantages include greater energy efficiency (at least at design

conditions) and longer equipment life.

period five

Review

Chilled-Water Systems

Figure 111

Review—Period One

▲Vapor-compression

water chillers

◆Air-cooled versus water-

cooled

▲Absorption water

chillers

▲Equipment rating

standards

▲ASHRAE/IESNA

Standard 90.1-1999

Figure 112

period five

Review

104 TRG-TRC016-EN

notes

period five

Review

ARI Standards 550/590 and 560 are common industry standards used for rating

water chiller performance. ASHRAE/IESNA Standard 90.1–1999, the energy

standard, contains minimum full-load and part-load chiller efficiency

requirements, as well as requirements for the design and operation of chilled-

water systems.

Period Two examined the methods of load-terminal control, including using

three-way modulating valves, two-way modulating valves, and face-and-bypass

dampers. We then examined several common system configurations, including

chillers piped in parallel, in series, and in a primary-secondary arrangement.

The majority of the time was spent discussing the design and operation of the

primary-secondary (or decoupled) chilled-water system. The primary-secondary

system eliminates many of the hydraulic problems associated with multiple-

chiller systems. It provides a reliable and energy-efficient supply of chilled

water, and its simplicity and flexibility make it easy to design, expand, and

operate.

Review—Period Two

primary

primary-

-secondary system

secondary system

▲Load-terminal control

◆Three-way valve

◆Two-way valve

◆Face-and-bypass

dampers

▲Parallel configuration

▲Series configuration

▲Primary-secondary

configuration

Figure 113

TRG-TRC016-EN 105

period five

Review

notes

Period Three reviewed several variations in the design of chilled-water systems.

These variations may allow the system design engineer to provide added value

to the building owner and operator in the areas of improved reliability, greater

flexibility, reduced installed costs, and lower operating costs.

Topics included:

nHybrid chilled-water systems using chillers that operate on different fuels

nLow-flow systems that use lower chilled-water temperatures and lower flow

rates

nVariable-primary-flow systems that are designed to vary the water flow

through the chiller evaporator

nConfigurations that allow a chiller to be preferentially loaded, specifically

in the case of a high-efficiency, heat-recovery or alternate-fuel chiller

nHeat-recovery chillers that are capable of providing heat to another part of

the system

nAsymmetric system designs using chillers of different capacities or

efficiencies, such as the “swing” chiller concept

nSeveral “free” cooling options that reduce cooling energy costs

nApplications in which the required conditions are outside of the normal

operating range of the chiller

Review—Period Three

▲Hybrid systems

▲Low-flow systems

▲Variable-primary-flow systems

▲Preferential loading

▲Heat recovery

▲Asymmetric design

▲“Free” (reduced-energy) cooling

▲Application of a chiller outside its normal

operating range

Figure 114

106 TRG-TRC016-EN

notes

period five

Review

Period Four discussed the issues related to chiller-plant control, including chiller

sequencing, failure recovery, contingency planning, system tuning, and system-

level optimization.

Remember that you cannot “control your way out of” a poor system design.

Although the control system can attempt to minimize the impact of a poor

design, it cannot eliminate the cause of the poor design.

Second, even a properly installed system with good components requires

system-level control to make those components work together effectively.

Third, even if the components are working together, the system, not the

individual components, needs to be optimized. Remember: the meter is on

the building.

Finally, a very important issue that is related to chiller-plant control is the issue

of interfacing with the person who is operating the system. Simplicity is

important, and it gives the system a much better chance of working without

intervention by the operator.

Review—Period Four

▲Chiller sequencing

▲Failure recovery

▲Contingency

planning

▲System tuning

▲System optimization

▲Operator interface

Figure 115

TRG-TRC016-EN 107

period five

Review

notes

For more information, refer to the following references:

nMultiple-Chiller-System Design and Control Applications Engineering Manual

(Trane literature order number SYS-APM001-EN)

nRefrigeration Cycle Air Conditioning Clinic (Trane literature order number

TRG-TRC003-EN)

nRefrigeration Compressors Air Conditioning Clinic (Trane literature order

number TRG-TRC004-EN)

nCentrifugal Water Chillers Air Conditioning Clinic (Trane literature order

number TRG-TRC010-EN)

nAbsorption Water Chillers Air Conditioning Clinic (Trane literature order

number TRG-TRC011-EN)

nHelical-Rotary Water Chillers Air Conditioning Clinic (Trane literature order

number TRG-TRC012-EN)

nASHRAE Handbook – Refrigeration

nASHRAE Handbook – Systems and Equipment

Visit the ASHRAE Bookstore at www.ashrae.org.

For information on additional educational materials available from Trane,

contact your local Trane sales office (request a copy of the Educational

Materials price sheet—Trane order number EM-ADV1) or visit our online

bookstore at www.trane.com/bookstore/.

Figure 116

108 TRG-TRC016-EN

Questions for Period 1

1What are the four types of compressors commonly used in vapor-

compression water chillers?

2List one advantage of an air-cooled chiller and one advantage of a

water-cooled chiller.

3True or False: The IPLV equation included in the ARI standards for rating

chillers was derived to provide a representation of the average part-load

efficiency for a system with multiple chillers.

Questions for Period 2

4What are the three most common methods of load-terminal (coil) control?

5The system shown in Figure 117 contains two chillers piped in parallel with

a single, constant-volume pump. What is the drawback of this system

configuration when only one chiller is operating?

6In a conventional primary-secondary chilled-water system, each production

pump delivers a __________ (constant or variable) flow of water and the

distribution pump delivers a __________ (constant or variable) flow of water.

7What method of load-terminal control should be used in the distribution

loop of a primary-secondary chilled-water system?

8Deficit flow in the bypass pipe of a primary-secondary system is an

indication to turn an additional chiller __________ (on or off).

Figure 117

Quiz

TRG-TRC016-EN 109

Quiz

Questions for Period 3

9Why does a variable-primary-flow (VPF) system require a bypass in

the system?

10 Identify one scenario where preferential loading is beneficial to a

chilled-water system.

11 What is the benefit of using a smaller-capacity “swing” chiller in a

multiple-chiller system?

Questions for Period 4

12 Making decisions about when to turn chillers on and off is commonly

referred to as chiller __________.

13 Lowering the temperature of the water leaving the cooling tower __________

(increases or decreases) the energy consumption of the chiller and

__________ (increases or decreases) the energy consumption of the cooling

tower fans.

14 An increase in the condenser approach temperature (that is, the

temperature difference between the water and the refrigerant inside the

condenser) may be a sign of what?

110 TRG-TRC016-EN

1Centrifugal, helical-rotary, reciprocating, and scroll

2Air-cooled chiller advantages include lower maintenance costs, a pre-

packaged system for easier design and installation, and better low-ambient

operation. Water-cooled chiller advantages include greater energy efficiency

(at least at design conditions) and longer equipment life.

3False. The IPLV equation was derived to provide a representation of the

average part-load efficiency for a single-chiller system only.

4Three-way modulating control valve, two-way modulating control valve,

and face-and-bypass dampers

5When only one chiller is operating, warm return water continues to flow

through the non-operating chiller and mixes with the chilled water leaving

the operating chiller. The temperature of the mixture of these two streams

is higher than the desired supply-water temperature, possibly resulting in

building comfort or space humidity control problems.

6Constant; variable

7Two-way modulating control valves

8Tur n on

9To ensure that the water flow through the system remains above the

minimum flow limit of the operating chiller(s)

10 Preferential loading is typically most beneficial in the following scenarios:

nIn a system that has a high-efficiency chiller along with several

standard-efficiency chillers, the high-efficiency chiller can be

preferentially loaded to reduce system energy consumption.

nIn a system with a heat-recovery chiller, preferentially loading the

heat-recovery chiller maximizes the amount of heat recovered, thus

reducing the overall system energy consumption.

nIn a system with an alternate-fuel chiller, such as an absorption chiller,

preferentially loading the alternate-fuel chiller during times of high

electricity costs minimizes system energy cost.

11 The swing chiller is alternated on and off between the larger chillers'

operation, in order to serve as a smaller incremental step of loading.

This sequence more favorably matches the capacity of the chiller plant to

the system load and operates the fewest, and smallest, pieces of ancillary

equipment (pumps and cooling towers) at any system load.

Answers

TRG-TRC016-EN 111

Answers

12 Sequencing

13 Decreases; increases

14 Fouling inside the tubes of the condenser, possibly indicating a problem

with water treatment in the cooling tower

112 TRG-TRC016-EN

absorbent A substance used to absorb refrigerant and transport it from the

low-pressure to the high-pressure side of the absorption refrigeration cycle.

In absorption water chillers, the absorbent is commonly lithium bromide.

absorber A component of the absorption refrigeration cycle in which

refrigerant vapor is absorbed by the absorbent solution and rejects heat to

cooling water.

air-cooled condenser A type of condenser in which refrigerant flows through

the tubes and rejects heat to air that is drawn across the tubes.

airside economizer A method of free cooling that involves using cooler

outdoor air for cooling instead of recirculating warmer indoor air.

ARI Air-Conditioning and Refrigeration Institute

ARI Standard 550/590 A publication titled Standard for Water Chilling

Packages Using the Vapor-Compression Cycle that promotes consistent rating

and testing methods for all types and sizes of water chillers. It covers factory-

designed, prefabricated water chillers, both air-cooled and water-cooled, using

the vapor-compression refrigeration cycle.

ARI Standard 560 A publication titled Absorption Water Chilling and Water

Heating Packages that promotes consistent rating methods for many types and

sizes of absorption water chillers in which water is the refrigerant and lithium

bromide is the absorbent. It covers single-effect chillers operating on steam or a

hot fluid; indirect-fired double-effect chillers operating on steam or a hot fluid;

and direct-fired double-effect chillers operating on natural gas, oil, or liquid

petroleum (LP).

ASHRAE American Society of Heating, Refrigerating and Air-Conditioning

Engineers

ASHRAE Guideline 3 A publication titled Reducing Emission of Halogenated

Refrigerants in Refrigeration and Air Conditioning Equipment and Systems that

includes a recommended list of data points to be logged daily for each water

chiller.

ASHRAE Standard 15 A publication titled Safety Code for Mechanical

Refrigeration that specifies safe design, construction, installation, and operation

of refrigerating systems.

ASHRAE/IESNA Standard 90.1 A publication titled Energy Standard for

Buildings, Except Low-Rise Residential Buildings that provides minimum

requirements for the energy-efficient design of buildings (except low-rise

residential buildings), including the HVAC system.

centrifugal compressor A type of compressor that uses centrifugal force,

generated by a rotating impeller, to compress the refrigerant vapor.

Glossary

TRG-TRC016-EN 113

Glossary

chilled-water system Uses water as the cooling media. The refrigerant inside

the evaporator absorbs heat from the water, and this water is pumped to coils

in order to absorb heat from the air used for space conditioning.

coefficient of performance (COP) A dimensionless ratio used to express the

efficiency of a refrigeration machine. A higher COP designates a higher

efficiency. For an electric chiller, it is defined as evaporator cooling capacity

divided by the electrical energy input. For an absorption water chiller, it is

defined as evaporator cooling capacity divided by the heat energy required by

the generator, excluding the electrical energy needed to operate the pumps,

purge, and controls.

compressor A mechanical device used in the vapor-compression refrigeration

cycle to increase the pressure and temperature of the refrigerant vapor.

condenser A component of the refrigeration cycle in which refrigerant vapor is

converted to liquid as it rejects heat to air, water, or some other fluid.

cooling tower A device used to reject the heat from a water-cooled condenser

by spraying the condensing water over the fill while drawing outdoor air

upward through the fill.

decoupled system See primary-secondary system.

deficit flow A condition in a primary-secondary chilled-water system in which

the production loop provides less flow than is required by the distribution loop.

To make up for this deficit, water travels from the return side of the distribution

loop, through the bypass pipe, and mixes with the water supplied by the

production loop.

direct-fired A type of absorption chiller that uses the combustion of a fossil

fuel (such as natural gas or oil) directly to provide heat to the high-temperature

generator.

double-effect A type of absorption chiller that uses two generators,

a high-temperature generator and a low-temperature generator.

evaporator A component of the refrigeration cycle where cool, liquid

refrigerant absorbs heat from air, water, or some other fluid, causing the

refrigerant to boil.

excess flow A condition in a primary-secondary chilled-water system in which

the production loop is providing more flow than is required by the distribution

loop. This excess water travels from the supply side of the production loop,

through the bypass pipe, and mixes with the water returning from the

distribution loop.

expansion device A component of the refrigeration cycle used to reduce

the pressure and temperature of the refrigerant to the evaporator conditions.

114 TRG-TRC016-EN

Glossary

expansion tank A component of a closed piping system that accommodates

the expansion and contraction of the water as temperature and, therefore,

density, changes.

fouling Minerals in the water that form scaling on the internal surfaces of the

heat exchanger tubes.

generator A component of the absorption refrigeration cycle in which

refrigerant vapor boils and is separated from the absorbent solution as it

absorbs heat from the primary heat source.

heat recovery The process of capturing the heat that is normally rejected from

the chiller condenser and using it for space heating, domestic water heating,

or another process seems unnecessary.

helical-rotary (screw) compressor A type of compressor that uses two mated

rotors to trap the refrigerant vapor and compress it by gradually shrinking the

volume of the refrigerant.

hybrid system A chilled-water system that can use more than one type of fuel.

indirect-fired A type of absorption chiller that uses steam or a hot fluid

(such as water) from an external source to provide heat to the generator.

Integrated Part-Load Value (IPLV) An equation that predicts chiller efficiency

at the ARI standard rating conditions, using weighted-average load curves

that represent a broad range of geographic locations, building types, and

operating-hour scenarios, both with and without an air side economizer.

Nonstandard Part-Load Value (NPLV) An equation that predicts chiller

efficiency at nonstandard rating conditions, using weighted-average load

curves that represent a broad range of geographic locations, building types,

and operating-hour scenarios, both with and without an air side economizer.

primary-secondary (decoupled) system A configuration of a multiple-chiller

system that uses separate production and distribution pumps to hydraulically

decouple the production capacity of the chillers from the load of the distribution

system.

reciprocating compressor A type of compressor that uses a piston that travels

up and down inside a cylinder to compress the refrigerant vapor.

refrigerant migration A method of free cooling that allows the chiller to be

used as a heat exchanger without operation of the compressor. It is possible,

when the condensing temperature of the refrigerant is low enough, for

refrigerant to migrate from the evaporator to the condenser.

scroll compressor A type of compressor that uses two opposing scrolls to trap

the refrigerant vapor and compress it by gradually shrinking the volume of the

refrigerant.

single-effect A type of absorption chiller that uses a single generator.

TRG-TRC016-EN 115

Glossary

notes swing chiller A smaller-capacity chiller used in a multiple-chiller system.

It is alternated on and off between the larger chillers' operation to serve as

a smaller, incremental step of loading.

variable-primary-flow (VPF) system A type of chilled-water system that is

designed to vary the flow of water throughout the entire system—through

the evaporator of each operating chiller as well as through the cooling coils.

variable-speed drive A device used to vary the capacity of a centrifugal pump

by varying the speed of the motor that rotates the pump impeller.

water-cooled condenser A type of condenser in which water flows through the

tubes and absorbs heat from the refrigerant that fills the surrounding shell.

The Trane Company

An American Standard Company

www.trane.com

For more information contact

your local distict office or

e-mail us at comfort@trane.com

Literature Order Number TRG-TRC016-EN

File Number E/AV-FND-TRG-TRC016-0201-EN

Supersedes CWS-CLC-1, CWS-CLC-2, CWS-CLC-3, and CWS-CLC-4

Stocking Location La Crosse

Since The Trane Company has a policy of continuous product and product data improvement, it reserves the right

to change design and specifications without notice.

Trane Trg Trc016 En Users Manual

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