Trane TRG-TRC011-EN User Manual

Air Conditioning Clinic

Absorption Water Chillers

One of the Equipment Series

TRG-TRC011-EN

Absorption Water Chillers

One of the Equipment Series

A publication of The Trane Company— Worldwide Applied Systems Group

Preface

Figure 1

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

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

This particular clinic introduces the concept of absorption water chillers.

TRG-TRC011-EN

Introduction ........................................................... 1

period one Absorption Refrigeration Cycle ....................... 3

Absorption System Fluids ........................................ 6

Components of the Absorption Cycle ...................... 8

Equilibrium Chart ................................................... 15

period two Absorption Chiller Types ................................. 18

Single-Effect Chiller ............................................... 19

Double-Effect Chiller ............................................. 21

Direct-Fired Chiller ................................................. 27

Chiller/Heater ........................................................ 30

period three Capacity Control ................................................. 34

Crystallization ........................................................ 37

Purge System ....................................................... 44

period four Maintenance Considerations .......................... 46

period five Application Considerations ............................. 53

Cooling-Water Temperature Limitations ................. 54

Combination Chiller Plants ..................................... 55

Special Considerations for Direct-Fired Chillers ...... 57

Equipment Rating Standards ................................. 59

period six Review ................................................................... 60

Quiz ......................................................................... 65

Answers ................................................................ 68

Glossary ................................................................ 69

TRG-TRC011-EN iii

iv TRG-TRC004-EN

notes

Introduction

Figure 2

Water chillers are used in a variety of air conditioning and process cooling applications. They are used to make cold water that can be transported throughout a facility using pumps and pipes. This cold water can be passed through the tubes of coils to cool the air in an air conditioning application, or it can provide cooling for a manufacturing or industrial process.

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

Figure 3

Although water chillers come in many sizes and types, they all produce cooling using the same basic principles of heat transfer and change-of-phase of the refrigerant. This is accomplished by the chiller refrigeration cycle. They differ from each other based on the refrigeration cycle and the type of refrigerant fluid used.

TRG-TRC011-EN 1

Introduction

notes

Water chillers using the vapor-compression refrigeration cycle vary by the type of compressor used. The compressor works to draw in refrigerant vapor and increase its pressure and temperature to create the cooling effect. Reciprocating, scroll, helical-rotary (or screw), or centrifugal compressors are generally used in water chillers that employ the vapor-compression refrigeration cycle.

Absorption water chillers make use of the absorption refrigeration cycle and do not use a mechanical compressor. The absorption refrigeration cycle is used in both small and large air-conditioning equipment. This clinic, however, focuses on large water-chiller applications of the absorption cycle. The different types of absorption water chillers will be discussed in detail in Period Two.

2 TRG-TRC011-EN

notes

period one

Absorption Refrigeration Cycle

period one

Figure 4

This period describes the components of the absorption refrigeration cycle. Comparing the absorption refrigeration cycle with the more familiar vaporcompression refrigeration cycle is often an easy way to introduce it. Like the vapor-compression refrigeration cycle, the absorption refrigeration cycle uses the principles of heat transfer and change-of-phase of the refrigerant to produce the refrigeration effect.

Both the vapor-compression and absorption refrigeration cycles accomplish cooling by absorbing heat from one fluid (chilled water) and transferring it to another fluid (cooling water or ambient air). Both cycles circulate refrigerant inside the chiller to transfer this heat from one fluid to the other. Both cycles also include a device to increase the pressure of the refrigerant and an expansion device to maintain the internal pressure difference, which is critical to the overall heat transfer process.

TRG-TRC011-EN 3

notes

period one

Absorption Refrigeration Cycle

reject heat

condenser

expansion

device

evaporator

In the vapor-compression refrigeration cycle, refrigerant enters the evaporator in the form of a cool, low-pressure mixture of liquid and vapor ($). Heat is transferred from the relatively warm air or water to the refrigerant, causing the liquid refrigerant to boil. The resulting vapor (%) is then pumped from the evaporator by the compressor, which increases the pressure and temperature of the refrigerant vapor.

The hot, high-pressure refrigerant vapor (&) 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 (') 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 ($) travels to the evaporator to repeat the cycle.

evaporator

absorb heat

compressor

energy in

Figure 5

The vapor-compression refrigeration cycle is discussed in detail in the Refrigeration Cycle clinic.

4 TRG-TRC011-EN

notes

period one

Absorption Refrigeration Cycle

reject heat

condenser

expansion

device

evaporator

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 the same.

evaporator

absorb heat

generator

absorber

heat energy in

pump

reject heat

Figure 6

Refrigerant enters the evaporator in the form of a cool, low-pressure mixture of liquid and vapor ($). Heat is transferred from the relatively warm water to the refrigerant, causing the liquid refrigerant to boil. Using an analogy of the vaporcompression cycle, the absorber acts like the suction side of the compressor—it draws in the refrigerant vapor (%) 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 (&) to the rest of the system.

The refrigerant vapor (&) leaving the generator enters the condenser, where heat is transferred to water at a lower temperature, causing the refrigerant vapor to condense into a liquid. This liquid refrigerant (') then flows to the expansion device, which creates a pressure drop that reduces the pressure of the refrigerant to that of the evaporator. The resulting mixture of liquid and vapor refrigerant ($) travels to the evaporator to repeat the cycle.

The components of the absorption refrigeration cycle will be discussed in detail in a moment.

TRG-TRC011-EN 5

notes

period one

Absorption Refrigeration Cycle

▲

◆ Stable

◆ Nontoxic

◆ Low cost

◆ Readily available

◆ Environmentally friendly

◆ High latent heat of

vaporization

Figure 7

Absorption System Fluids

Probably the greater of these differences between the vapor-compression and absorption refrigeration cycles, however, is the types of fluids used. The vaporcompression refrigeration cycle generally uses a halocarbon (such as HCFC-123, HCFC-22, HFC-134a, etc.) as the refrigerant. The particular absorption refrigeration cycle discussed in this clinic uses distilled water as the refrigerant.

Distilled water is stable, nontoxic, low in cost, readily available, environmentally friendly, and has a relatively high heat of vaporization (1000 Btu/lb [2326 kJ/kg]). The heat of vaporization is the amount of heat required to fully transform (evaporate) liquid to a vapor at a given pressure. For the water to be used as a refrigerant, the cycle must operate in a vacuum, that is, at a pressure below atmospheric pressure. This will be discussed shortly. Finally, large quantities of water are easily absorbed by the absorbent and separated within the absorption cycle.

Throughout the remainder of this clinic, when the term refrigerant is used, it refers to distilled water.

6 TRG-TRC011-EN

notes

period one

Absorption Refrigeration Cycle

▲

◆ High affinity for water

(refrigerant)

◆ In solution, higher

boiling point than water

◆ Nontoxic

Figure 8

Additionally, the absorption refrigerant cycle uses a second fluid called an absorbent solution. The absorbent solution is confined to the absorber and generator sections of the cycle, and is used to carry the refrigerant from the low-pressure side (evaporator) to the high-pressure side (condenser) of the chiller. For this purpose, the absorbent should have a strong affinity (attraction) for the refrigerant and, when in solution with the refrigerant, a boiling point that is substantially higher than that of the refrigerant.

The absorbent commonly used with water (the refrigerant) is lithium bromide. Lithium bromide, a nontoxic salt, has a high affinity for water. Also, when in solution with water, the boiling point of lithium bromide is substantially higher than that of water. This makes it easy to separate the refrigerant from the absorbent at low pressures. A certain quantity of absorbent solution, therefore, is pumped from the absorber to the generator in order to transport the refrigerant.

Another common refrigerant–absorbent pair is ammonia as the refrigerant and water as the absorbent. These fluids are more common in small residential applications. There are other refrigerant–absorbent combinations; this clinic, however, will focus on water as the refrigerant and lithium bromide as the absorbent.

TRG-TRC011-EN 7

notes

period one

Absorption Refrigeration Cycle

dilute

solution

intermediate

solution

concentrated

solution

Figure 9

These two fluids, the refrigerant and the absorbent, are mixed inside the chiller in various concentrations. The term dilute solution refers to a mixture that has a relatively high refrigerant content and low absorbent content. A concentrated solution has a relatively low refrigerant content and high absorbent content. An intermediate solution is a mixture of dilute and concentrated solutions.

steam or

hot water

generator

evaporator

exchanger

absorber

heat

cooling

water

condenser

chilled

water

Figure 10

Components of the Absorption Cycle

The four basic components of the absorption refrigeration cycle are the generator and condenser on the high-pressure side, and the evaporator and absorber on the low-pressure side. The pressure on the high-pressure side of the system is approximately ten times greater than that on the low-pressure side.

8 TRG-TRC011-EN

period one

Absorption Refrigeration Cycle

notes

The operating conditions used in this section of the clinic are approximate, subject to variation with changing load and cooling-water temperature conditions.

115°F

[46.1°C]

temperature

45°F

[7.2°C]

0.15

[1.034

psia

kPa]]

kPa

pressure

1.5

[10.34

psia

kPa]]

kPa

Figure 11

At a given pressure, the temperature at which a liquid will boil into a vapor is the same temperature at which the vapor will condense back into a liquid. This curve illustrates the pressures and corresponding temperatures at which water (the refrigerant) boils and condenses.

At atmospheric pressure (14.7 psia [101.3 kPa]), water boils and evaporates at 212 °F [100 °C]. When the pressure is decreased, water boils at a lower temperature. At the lower pressure, there is less force pushing against the water molecules, allowing them to separate easier.

Just like in the vapor-compression refrigeration cycle, this change in pressure allows the evaporator temperature to be low enough for the refrigerant to absorb heat from the water being cooled. Likewise, it allows the condenser temperature to be high enough for the refrigerant to reject heat to water at normally available temperatures. Inside of the evaporator, the pressure is very low, 0.15 psia [1.034 kPa] in this example, so that the refrigerant boils at 45ºF [7.2ºC]. In the condenser, however, the pressure is much higher (1.5 psia [10.34 kPa]) so that the refrigerant condenses at 115ºF [46.1ºC].

TRG-TRC011-EN 9

notes

period one

Absorption Refrigeration Cycle

steam or

hot water

generator

concentrated

solution

dilute

solution

Starting on the high-pressure side of the cycle, the purpose of the generator is to deliver the refrigerant vapor to the rest of the system. It accomplishes this by separating the water (refrigerant) from the lithium bromide-and-water solution.

solution

refrigerant

vapor

Figure 12

In the generator, a high-temperature energy source, typically steam or hot water, flows through tubes that are immersed in a dilute solution of refrigerant and absorbent. The solution absorbs heat from the warmer steam or water, causing the refrigerant to boil (vaporize) and separate from the absorbent solution. As the refrigerant is boiled away, or “generated,” the absorbent solution becomes more concentrated.

The concentrated absorbent solution returns to the absorber and the refrigerant vapor migrates to the cooler condenser. Physically, the generator and condenser are contained inside of the same shell. The pressure in the condenser section is less than the pressure in the generator section. This is because the temperature of the cooling water flowing through the tubes of the condenser is less than the temperature of the steam or hot water flowing through the tubes of the generator.

10 TRG-TRC011-EN

notes

period one

Absorption Refrigeration Cycle

refrigerant

vapor

condenser

cooling

water

liquid

refrigerant

Figure 13

Inside the condenser, cooling water flows through tubes and the hot refrigerant vapor fills the surrounding space. As heat transfers from the refrigerant vapor to the water, refrigerant condenses on the tube surfaces. The condensed liquid refrigerant collects in the bottom of the condenser before traveling to the expansion device.

In absorption water chillers, the cooling water system is typically connected to a cooling tower.

liquid

evaporator

From the condenser, the liquid refrigerant flows through an expansion device into the evaporator. The expansion device is used to maintain the pressure difference between the high-pressure (condenser) and low-pressure (evaporator) sides of the refrigeration system. In this example, the expansion device is a throttling pipe, which is a long section of pipe with an orifice restriction in it. It creates a liquid seal that separates the high-pressure and lowpressure sides of the cycle.

liquid

refrigerant

expansion

device

Figure 14

TRG-TRC011-EN 11

period one

Absorption Refrigeration Cycle

notes

As the high-pressure liquid refrigerant flows through the expansion device, it causes a pressure drop that reduces the refrigerant pressure to that of the evaporator. This pressure reduction causes a small portion of the liquid refrigerant to boil off, or “flash,” cooling the remaining refrigerant to the desired evaporator temperature. The cooled mixture of liquid and vapor refrigerant then flows into the evaporator pan.

refrigerant

vapor

chilled

evaporator

absorber

evaporator

spray pump

Inside the evaporator, relatively warm return water from the chilled-water system flows through the tubes. An evaporator pump draws the liquid refrigerant from the bottom of the evaporator and continuously circulates it to be sprayed over the tube surfaces. This maximizes heat transfer.

chilled

water

liquid

refrigerant

Figure 15

As heat transfers from the water to the cooler liquid refrigerant, the refrigerant boils (vaporizes) and the resulting refrigerant vapor is drawn into the lowerpressure absorber. Physically, the evaporator and absorber are contained inside the same shell.

12 TRG-TRC011-EN

notes

period one

Absorption Refrigeration Cycle

refrigerant

vapor

absorber

intermediate

solution

concentrated

solution

Inside the absorber, the refrigerant vapor is absorbed by the lithium bromide solution. As the refrigerant vapor is absorbed, it condenses from a vapor to a liquid, releasing the heat it acquired in the evaporator. This heat, along with the heat generated during the process of being absorbed, is rejected to the cooling water that is circulated through the absorber tube bundle. Absorption of the refrigerant vapor creates a low pressure area within the absorber. This lower pressure, along with the absorbent’s affinity for water, induces a continuous flow of refrigerant vapor from the evaporator.

absorber

spray pump

cooling

water

dilute

solution

Figure 16

Maximum surface area is provided by spraying the solution over the tube bundle. This also provides maximum heat transfer to the cooling water. The absorber spray pump mixes concentrated absorbent solution (returning from the generator) with dilute solution (from the bottom of the absorber) and delivers this intermediate solution to the absorber sprays.

There are two reasons for using an intermediate solution rather than a concentrated solution in the absorber sprays. First, for effective tube wetting, a greater quantity of solution is required than is available from the generator. Therefore, dilute solution is mixed with the concentrated solution to increase the total quantity of solution being sprayed over the tube surfaces. Second, if concentrated solution were sprayed directly upon the absorber tube bundle, it would be subjected to temperatures that could cause it to crystallize—a solidification of the bromide salt. Therefore, the concentration is reduced by mixing it with dilute solution.

TRG-TRC011-EN 13

notes

period one

Absorption Refrigeration Cycle

concentrated

solution

heat

exchanger

dilute

solution

As the lithium bromide solution absorbs the refrigerant, it becomes diluted and has less ability to absorb water vapor. To complete the cycle and sustain operation, the absorbent solution must be reconcentrated. Consequently, the generator pump continuously returns the dilute solution to the generator to again separate the refrigerant vapor from the solution and reconcentrate the solution, thus repeating the cycle.

generator pump

Figure 17

This cool dilute solution that is pumped from the absorber to the generator, and the hot concentrated solution returning from the generator, pass through a heat exchanger. This transfer of heat preheats the dilute solution, reducing the heat energy required to boil the refrigerant within the generator, and also precools the concentrated solution, reducing the required flow rate of cooling water through the absorber.

Notice that in this example cycle, the cooling water passes through the condenser after passing through the absorber. Some absorption chiller designs split the cooling water and deliver it directly to both the absorber and the condenser.

14 TRG-TRC011-EN

notes

period one

Absorption Refrigeration Cycle

psia

0.1

[0.69

psia

kPa]]

kPa

[6.9

psia

1 1 psia

kPa]]

kPa

1515psia

[103.4

kPa]]

[103.4

psia

5 5 psia

[34.5

kPa]]

[34.5

kPa

concentration

50°F

[10°C]

100°F

[37.8°C]

solution temperature

150°F

[65.6°C]

200°F

[93.3°C]

LiBr

solution

Figure 18

Equilibrium Chart

The performance of the absorption refrigeration cycle can be analyzed using a special chart called an Equilibrium Chart for Aqueous Lithium Bromide Solutions. This chart plots the vapor pressure (vertical axis) versus the temperature (horizontal axis) and concentration (diagonal lines) of the lithium bromide (LiBr) solution.

The chart shows that an increase in concentration ($ to %), at a constant solution temperature, results in a decrease in vapor pressure. Conversely, a decrease in solution temperature ($ to &), at a constant concentration, results in a decrease in vapor pressure. Assuming that no air or other noncondensables are inside the chiller, the vapor pressure of the solution determines the temperature at which the refrigerant will vaporize. In other words, the combination of solution temperature and concentration determines the temperature at which the refrigerant will boil (vaporize).

TRG-TRC011-EN 15

notes

period one

Absorption Refrigeration Cycle

steam or

hot water

generator

evaporator

exchanger

absorber

heat

(

)

A diagram of a typical absorption refrigeration cycle can be superimposed on this equilibrium chart to demonstrate the function of each component in the system.

cooling

water

condenser

chilled

water

expansion

device

Figure 19

Realize that the equilibrium chart can only be used for those portions of the cycle where the lithium bromide solution is present. It cannot be used for the condenser or evaporator sections. The properties of the refrigerant as it passes through the condenser, expansion device, and evaporator can be analyzed using a pressure–enthalpy chart for the refrigerant (water, in this case).

16 TRG-TRC011-EN

notes

period one

Absorption Refrigeration Cycle

psia

1515psia

[103.4

kPa]]

[103.4

kPa

(

150°F

[65.6°C]

200°F

[93.3°C]

0.1

[0.69

psia

kPa]]

kPa

[6.9

psia

1 1 psia

kPa]]

kPa

50°F

[10°C]

psia

5 5 psia

[34.5

kPa]]

[34.5

kPa

1.5

psia

1.5

psia

[10.3

kPa]]

[10.3

kPa

)

100°F

[37.8°C]

solution temperature

Starting at the absorber, the dilute lithium bromide solution leaves the absorber ($) at 105°F [40.6ºC] and 59% concentration. This solution passes through the heat exchanger, where it is preheated to 175°F [79.4°C] (%). (Notice that there is no change in concentration as the solution passes through the heat exchanger.) In the generator, the solution absorbs heat from the steam or hot water flowing through the tubes. Initially, this only sensibly heats the solution to &, that is, the temperature of the solution increases while the concentration stays the same. At this point, the refrigerant begins to boil (vaporize) and separate from the solution. This increases the concentration of the lithium bromide solution as the temperature continues to increase (').

LiBr

concentration

solution

Figure 20

The concentrated solution ('), now at 215°F [101.7 ºC] and 64.5%, passes through the heat exchanger where it is cooled to 135°F [57.2ºC] ((). This cooled, concentrated solution (() is then mixed with dilute solution from the absorber ($), and this intermediate solution ()) (118°F [47.8ºC] and 62% concentration) is pumped to the absorber spray trees. In the absorber, refrigerant vapor is absorbed by the intermediate solution, decreasing its concentration to 59%, while heat is transferred to the cooling water. The resulting cooled, dilute solution ($) returns to the generator to repeat the cycle.

This chart also can be used to demonstrate the operating pressures of the cycle. In this example, the low-pressure sections of the cycle are operating at approximately 0.15 psia [1.034 kPa], and the high-pressure sections are operating at approximately 1.5 psia [10.34 kPa].

TRG-TRC011-EN 17

notes

period two

Absorption Chiller Types

period two

Figure 21

Lithium bromide-and-water absorption chillers are classified by the firing method—that is, how the primary generator is heated and whether it has a single- or a multiple-effect generator. Indirect-fired chillers are heated with steam or a hot liquid (such as water) that is typically supplied by an on-site boiler or a local utility. It can also be heated by waste energy that is recovered from the exhaust of a gas turbine or by some other heat recovery device. Directfired chillers are heated via the combustion of fossil fuels. An absorption chiller with a single generator is called a single-effect chiller. Multiple-effect chillers have multiple generators.

Like vapor-compression water chillers, absorption chillers can also be classified by the condensing method employed, either air-cooled or water-cooled. Physical size limitations typically constrain air-cooled condensing to ammoniaand-water absorption equipment that is applied in residential and small commercial applications (3 to 5 tons [10 to18 kW]). Most large commercial (20 to 1,500 tons [70 to 5,300 kW]) water-and-lithium bromide absorption chillers employ water-cooled condensing with cooling towers, because of the higher energy efficiency at design conditions.

18 TRG-TRC011-EN

notes

period two

Absorption Chiller Types

condenser

evaporator

absorber

Single-Effect Chiller

The single-effect absorption water chiller uses a cycle similar to the one presented in Period One. It includes a single generator, condenser, evaporator, absorber, heat exchanger, and pumps.

generator

Figure 22

These chillers are typically operated on low-pressure steam (approximately 15 psig [204.8 kPa]) or medium-temperature liquids (approximately 270°F [132.2°C]). Typical coefficients of performance for single-effect water chillers are

0.6 to 0.8. The coefficient of performance (COP) is a dimensionless ratio used to express the efficiency of a refrigeration machine. For an absorption water chiller, COP is defined as the ratio of evaporator cooling capacity divided by the heat energy required by the generator. A higher COP designates a higher efficiency.

Notice that the COP used to express the efficiency of absorption water chillers excludes the electrical energy needed to operate the pumps, purge, and controls.

TRG-TRC011-EN 19

+ 53 hidden pages

Trane TRG-TRC011-EN User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

Contents

Introduction

Absorption System Fluids

Components of the Absorption Cycle

Equilibrium Chart

Single-Effect Chiller