absorption refrigeration systems

7/21/2019 Absorption Refrigeration Systems

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Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramcos

employees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,

or disclosed to third parties, or otherwise used in whole, or in part,

without the written permission of the Vice President, Engineering

Services, Saudi Aramco.

Chapter : Process For additional information on this subject, contact

File Reference: CHE21003 R.A. Al-Husseini on 874-2692

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Absorption Refrigeration Systems


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Engineering Encyclopedia Process


Saudi Aramco DeskTop Standards

CONTENTS PAGES

OPERATION OF ABSORPTION REFRIGERATION SYSTEMS ..............................................................1

General Operation of Absorption Refrigeration Systems ................................................................1Operation of Aqua-Ammonia Systems............................................................................................3

Operation of Lithium Bromide Systems .................................................................................6

SELECTING APPROPRIATE ABSORPTION SYSTEMS, GIVEN PROCESS

REQUIREMENTS ..........................................................................................................................8

Advantages, Features, and Capacities..............................................................................................8

Aqua-Ammonia .................................................................................................................8

Lithium Bromide ...............................................................................................................9

Sizing Calculations..........................................................................................................................10

Aqua-Ammonia .................................................................................................................10

Sample Problem.................................................................................................................11

Lithium Bromide ...............................................................................................................18

WORK AID.....................................................................................................................................27

GLOSSARY....................................................................................................................................28

ADDENDUM..................................................................................................................................29


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Saudi Aramco DeskTop Standards

LIST OF FIGURES

Figure 1. Absorption Refrigeration System.......................................................................................1

Figure 2. Ammonia-Water Single-Stage Absorption Refrigeration System ......................................4

Figure 3. Lithium Bromide-Water Single-Stage Absorption Refrigeration System ..........................6

Figure 4. Enthalpy-Concentration Diagram for Ammonia-Water Solution.......................................12

Figure 5. Enthalpy Values.................................................................................................................15

Figure 6. Enthalpy-Concentration Diagram for Lithium Bromide-Water Solutions .........................22

Figure 7. Enthalpy Values Summary For This Particular Lithium Bromide System.........................23

Figure 8. Typical Values for Li Br Systems......................................................................................25

Figure 9. Refrigerant Temperature Enthalpy of Lithium Bromide Solutions ....................................26


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Saudi Aramco DeskTop Standards 1

Operation of Absorption Refrigeration Systems

This section discusses the operation of absorption refrigeration systems in general and the operation of the

following absorption refrigeration systems, specifically:

Aqua-ammonia systems

Water-lithium bromide systems

General Operation of Absorption Refrigeration Systems

Absorption refrigeration systems convert heat into cooling power. Figure 1 is a diagram of a basic absorption

refrigeration system.

Figure 1. Absorption Refrigeration System


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In Step 1, hot liquid refrigerant is expanded into a low-pressure, low-temperature liquid and vapor in the

evaporator. The expansion of the refrigerant cools the refrigerant due to the heat of vaporization. The cold

refrigerant provides the cooling power.

In Step 2, the cold, low-pressure refrigerant vapor is converted to a liquid in solution while the low-pressure ismaintained. This conversion occurs when the refrigerant is miscible with the solution base (absorbent) and

when there is a high affinity between the refrigerant molecules and the absorbent molecules. Thermal energy

due to the heat of condensation, sensible heats, and heat of dilution is generated in this process. This thermal

energy must be removed with minimal heating of the refrigerant-absorbent mixture.

In Step 3, the refrigerant-absorbent mixture is pressurized.

In Step 4, the refrigerant-absorbent mixture passes through a heat exchanger where the mixture is heated.

In Step 5, the refrigerant-absorbent mixture undergoes distillation and separates the refrigerant from the

absorbent. If the pure absorbent material is nonvolatile, a simple still is adequate. If the pure absorbent

material is volatile, fractional distillation is required. It is critical that the refrigerant that is obtained in this step

be free of absorbent. The refrigerant from this step is a hot, high-pressure vapor. The absorbent obtained inthis step may contain significant amounts of refrigerant.

In Step 6, the hot, high-pressure refrigerant vapor passes into a condenser. The condenser cools the vapor

sufficiently to convert the vapor to a liquid.

The hot absorbent from Step 5 is transferred through the heat exchanger in Step 4 to heat the refrigerant-

absorbent mixture, and at the same time, to cool the regenerated absorbent. The regenerated absorbent is then

supplied as the absorbent for Step 2.

Critical characteristics of the refrigerant-absorbent pair are as follows:

The refrigerant and the absorbent should not form a solid over the range of composition and

temperature to which they may be subjected.

The refrigerant should be much more volatile than the absorbent.


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The absorbent should have a strong affinity for the refrigerant under the conditions in which the

absorption takes place.

The operating pressures should be moderate. The operating pressures are largely determined by

the refrigerant.

The refrigerant, the absorbent, and the mixture should all be chemically stable. They are subjected

to severe conditions over years of service.

The refrigerant, the absorbent, and the mixture should be non- corrosive. Corrosion inhibitors

should be used as necessary.

The fluids should be nontoxic and nonflammable.

The refrigerant's latent heat should be high. High latent heats allows the circulation rate of the

refrigerant and absorbent to be kept to a minimum.

The fluids should have good transport properties (viscosity, surface tension, thermal diffusivity,

and mass diffusivity).

No known refrigerant-absorbent pair has all of the critical characteristics that are listed. Two commonly used

pairs are ammonia-water and water-lithium bromide. In the following section, the operations of these

refrigerant-absorbent pairs are discussed. Critical characteristics are discussed in a later section.

Operation of Aqua-Ammonia Systems

Figure 2 illustrates the flow of an ammonia-water absorption refrigeration system. In this type of system, water

is the absorbent, ammonia is the refrigerant, and ammonia-water is the refrigerant-absorbent mixture. The

following description refers to the circled numbers in Figure 2 to identify specific points in the process. The

discussion references these points as the steps in the process are described.


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Figure 2. Ammonia-Water Single-Stage Absorption

Refrigeration System


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Point 1 to 2

Hot water leaves the generator. This water is cooled in the heat exchanger by the incoming ammonia-water

(4A). The cold water is then sent to the absorber.

Point 2 to 3

In the absorber, the cold water absorbs low-pressure ammonia that comes from the evaporator. The ammonia-

water then enters the strong aqua (SA) tank.

Point 3 to 4

The ammonia-water from the (SA) is pumped to the generator system through the heat exchanger. In the heat

exchanger, the ammonia-water is heated by the water that was produced at point 1.

Point 4 to 5

The hot ammonia-water from the heat exchanger enters the distillation tower and, through the tower, the

generator system. In the tower and generator system, the ammonia is separated from the water. The hot water

then leaves the generator at point 1.

Point 5 to 6

The gaseous ammonia leaves the tower and enters the condenser where the gas is converted to liquid ammonia.

Point 6 to 7

The liquid ammonia is expanded in the evaporator, where the liquid is converted into cold liquid and gaseous

ammonia. The cold liquid vaporizes and cools the evaporator. The cold, gaseous ammonia is then transferred

to the absorber. In the absorber, the cold, gaseous ammonia is absorbed by the water.

Point 8

Upsets in the tower or changes in evaporator operation could cause the concentration of water in the liquid

ammonia to increase. The periodic spillover connection allows the return of accumulated water to the system.


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Operation of Lithium Bromide Systems

Figure 3 illustrates the flow of a lithium bromide-water absorption refrigeration system. In this type of system,

lithium bromide is the absorbent, water is the refrigerant, and lithium bromide-water is the refrigerant-absorbent

mixture. The following description refers to the circled numbers in Figure 3 to identify specific points in the

process. The discussion references these points as the steps in the process are described.

Figure 3. Lithium Bromide-Water Single-Stage Absorption Refrigeration System


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Point 1 to 2

Hot lithium bromide leaves the generator. This lithium bromide is cooled in the heat exchanger by the

incoming lithium bromide-water. The cold lithium bromide is then sent to the absorber.

Point 2 to 4

In the absorber, cold lithium bromide absorbs the low-pressure water vapor that comes from the evaporator.

Point 4 to 5

The cool, lithium bromide-water is pumped from the absorber to the generator through the heat exchanger. In

the heat exchanger, the lithium bromide-water is heated by the lithium bromide that was produced at point 1.

Point 5 to 1

Hot lithium bromide-water from the heat exchanger enters the generator. In the generator, heat is added todistill the water. Hot lithium bromide leaves the generator at point 1.

Path 6 to 7

Hot, high-pressure water vapor (6) condenses (7).

Point 7 to 8

The hot water expands in the evaporator. The expansion vaporizes the water, which becomes the evaporator.

The cold, low-pressure water vapor is absorbed by the solution in the absorber .


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Selecting Appropriate Absorption Systems, Given Process Requirements

This section compares and contrasts aqua-ammonia and lithium bromide-absorption systems through a

discussion of the following:

Advantages, features, and capacities

Sizing calculations

Advantages, Features, and Capacities

Absorption refrigeration equipment has the following advantages:

The system can be powered with waste, low-level heat

There are no expensive compressors

They are quiet

Absorption refrigeration equipment is classified by how it is fired and whether it has a single-stage or two-stagegenerator. Units that use steam or hot fluid heat sources are indirect fired. Units that use a flame heat source

are direct fired. Units that use hot waste gases as a heat source are indirect fired, but are often referred to as

heat-recovery units. Units with two-stage generators are called dual-effect or double-effect units.

Aqua-Ammonia

The ammonia-water pair (aqua-ammonia absorption system) meets most of the ideal requirements for an

absorption refrigeration system. However, the volatility ratio is too low; high operating pressures are required;

and ammonia is an ASHRAE 15-1978 Safety Code Group 2 fluid, restricting its indoor use.

The low volatility ratio means that it is difficult to completely separate the ammonia from the water and

requires a compromise in the refrigeration system. There are two possible solutions. In the first solution, the

refrigerant will have water vapor present as it passes through the condenser and evaporator, which restricts therefrigerant pathway which can freeze at 0C (32F). In the second solution, the water that serves as an

absorbent will have a significant amount of ammonia already absorbed and will consequently perform as a less

efficient absorber. Because water is present throughout the system, ammonia-water absorption should not be

used for applications near or below 0C (32F).


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The high operating pressures increase the cost of the refrigeration equipment and also increase the inherent

dangers associated with the equipment. The condenser pressures are around 2068 kPa (300 psia) and the

evaporator pressures are around 483 kPa (70 psia).

Ammonia is an ASHRAE 15-1978 Safety Code Group 2 fluid because it can cause serious injury to people atconcentrations of 0.5 to 1.0 percent for durations of exposure over one half hour. Therefore, the use of an aqua-

ammonia refrigeration cycle in an enclosed space, especially in a space routinely inhabited, is discouraged.

Direct-fired, air-cooled ammonia-water liquid chillers are available in 3- to 5-ton (10- to 18-kW) capacities.

Lithium Bromide

The water-lithium bromide pair (lithium bromide-absorption system) meets most of the ideal requirements for

an absorption refrigeration system. However, the pair has the following disadvantages:

The refrigerant (water) freezes at 0C (32F)

The water-lithium bromide mixture tends to form solids

Lithium bromide solutions have a high viscosity

The fact that water freezes at 0C (32F) means that a lithium bromide-absorption system cannot be used for

applications requiring refrigeration near or below 0C (32F).

The tendency of the water-lithium bromide mixture to form solids means that the equipment must be designed

to allow a partially crystallized solution to flow, especially from the generator, through the heat exchanger and

into the absorber. It may be necessary to provide a mechanism for removing crystals from the solution, re-

dissolving them, and returning the reconstituted solution to the system. Because the crystallization of the

solution occurs most readily when the solution is air-cooled, the absorber should be water cooled.

Small lithium bromide units of 3- to 30-ton (10- to 105-kW) capacity are available as indirect- or direct-fired,liquid chiller, chiller-heater, or air-conditioning equipment. Indirect-fired liquid chillers are available in

capacities of 50 to 1500 tons (175 to 5275 kW).


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Sizing Calculations

The following material demonstrates sizing calculations for aqua-ammonia and lithium bromide refrigeration

systems. The objective of this module is to provide only a basic understanding of sizing calculations to help

determine the correct absorption refrigeration process to use for a given application.

For absorption refrigeration systems, the following equation determines the refrigerant flow rate:

RE

=Q

E

hv

hl( ) (EQN A)

where: R E = Mass (weight) flow of refrigerant from the evaporator

QE = Heat load at the evaporator

hv = Enthalpy of refrigerant vapor from the evaporator

hl = Enthalpy of refrigerant liquid from the condenser

Aqua-Ammonia

The following material, including the sample calculation, is extracted from the 1989 ASHRAE Fundamentals

Handbook. In many cases, calculations include values with greater precision than that obtainable from charts

included in this module.

For aqua-ammonia absorption refrigeration systems, the following equation provides the solution flow rate per

unit refrigerant rate (see Figure 2):

WFSA(X) WFSG(X 1) = 1 (EQN B)

where: WFSA = Mass fraction of ammonia in solution coming from the absorber

WFSG = Mass fraction of ammonia in solution coming from the generator

X = Mass of solution from the absorber per unit mass of refrigerant flow

X-1 = Mass of solution from the generator per unit mass of refrigerant

flow


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Sample Problem

The sample calculation is performed for a large aqua-ammonia absorption refrigeration plant with the following

conditions:

Refrigeration load, 1758.5 kW (500 tons)

Evaporator temperature, 5.0C (41.1F)

Evaporator pressure, 517 kPa (75 psia)

Absorber pressure, 507 kPa (73.5 psia)

Strong aqua solution (absorbent-refrigerant mixture) temperature, 41C (105F)

Condenser temperature, 38C (100F)

Condenser and tower pressure, 1461 kPa (211.9 psia)

Concentrate split (WFSG- WFSA), 6% by weight Cooling tower water temperature, 29.4C (85F)

Assume a 3% increase in refrigerant flow due to heat gains in equipment

Figure 4 provides the enthalpy-concentration diagram for ammonia-water solution.


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Source: ASHRAE 1989 Fundamentals Handbook (IP Edition) , Atlanta, GA, American Society of Heating,

Refrigerating, and Air Conditioning Engineers, Inc., 1989, p. 17.69, figure 33.

Figure 4. Enthalpy-Concentration Diagram for Ammonia-

Water Solution

From Figure 4 and the given conditions of 507 kPa (73.5 psia) and 41C (105F) at the absorber, the strong

aqua (SA) solution has an ammonia concentration of 49% by mass.


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A 6% to 8% concentration increase across the absorber allows sufficient flow and adequate liquid wetting of

plain horizontal tubes % up to 25 mm (1 in) in diameter. The difference between the concentration of ammonia

in solution that enters the absorber and the concentration of ammonia in solution that exits the absorber is

referred to as the concentration split. Concentration splits of 6% to 8% ensure reasonable maximum liquid

flows for cost effective exchangers and towers, and a practical minimum temperature of heat source for thegenerator. Large splits reduce the flow rate, efficiency, and cost effectiveness of absorbers and exchangers and

raise the required temperature of the heat source.

For comparison purposes, the calculations will be shown for two splits: 6% and 30%.

With a 6% concentration split, there is a 43% ammonia concentration by mass in the absorbent returned from

the generator to the absorber via the heat exchanger.

Using Equation B, the solution flow rates for a 6% split are the following:

0.49X 0.43(X1) = 1

0.06X + 0.43 = 1

0.06X = 0.57X = 9.5 lb SA/lb refrigerant

X1 = 8.5 lb WA/lb refrigerant

Given the condenser and tower pressure of 1461 kPa (211.9 psia) Figure 4 can be used to determine the

equilibrium to minimal temperature for the 49% ammonia solution that will enter the heat exchanger. Figure 4

can also be used to determine the equilibrium temperature for the 43% ammonia solution that will leave the

generator as absorbent for the 6% split.

With a 30% concentration split, there is a 19% ammonia concentration by mass in the absorbent returned from

the generator to the absorber via the heat exchanger.


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Using Equation B the solution flow rates for a 30% split are the following:

0.49X-0.19(X-1) = 1

0.30X+0,19 = 1

0.30X = 0.81 X = 2.7 lb SA/lb refrigerant

X-1 = 1.7 lb WA/lb refrigerant

As with the 6% split, Figure 4 can be used to determine the equilibrium temperature of the 19% ammonia

solution that leaves the generator as absorbent for the 30% split.

For the 49% ammonia solution, the equilibrium temperature for the tower and generator is 80.5C (176.9F).

For the 6% split, the 43% concentration ammonia-water leaves the tower and generator at a temperature of

90.5C (194.9F). For the 30% split, the 19% concentration ammonia-water leaves the tower and generator at a

temperature of 144.4C (291.9F).


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Enthalpy values for the ammonia-water from Figure 4 are as presented in Figure 5.

6% SPLIT 30% SPLIT

ENTHALPY kJ/kg Btu/lb kJ/kg Btu/lb

hl of (1)* WA 166.0 71.4 508.4 218.7

hlof (4)* 112.3 48.3 112.3 48.3

hl of (3)* SA -65.8 -28.3 -65.8 -28.3

*Numbers in parentheses refer to the circled numbers and corresponding point in the

refrigeration system shown in Figure 2.

Figure 5. Enthalpy Values

The enthalpy at point (2) in Figure 2 in the refrigeration system is determined by mass enthalpy flow rate

balance. This calculation is as follows:

For 6% split (43% ammonia-water):

71.4 Btu/lb 9. 5 48.3 Btu / lb 28.3 Btu/ lb( )

8.5= 14. 21 Btu/ l b

For 30% split (19% ammonia-water):

218.7 Btu/ lb 2.7 48.3 Btu/lb 28.3 Btu/lb( )1.7

= 97.04 Btu/lb


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At these concentrations, Figure 4 indicates the following temperatures for solution entering the absorber (Point

2):

For 6% split: 47.8C (118.0F)

For 30% split: 83.1C (181.6F)

For ammonia-water mixtures at 49% SA, feed conditions of 174F, tower pressure of 211.9 psia, and a vapor

product at 99.95% ammonia, the internal reflux rate is .167 lb/lb refrigerant (1.15 times the minimum reflux).

These values, while specific for the conditions stated, should be adequate for checking a proposal since the

vapor product should always be about the same. For exact calculations, the tower should be calculated through

use of computer programs such as "Process."

Use Figure 4 to calculate the refrigerant flow rate and condenser heat level as follows:

Tower Top

(Point 5) saturated vapor at 211.9 psia at 99.9% purity, hv=

(Figure 2) read T = 110F hv= 569 Btu/lb

Condenser Outlet

(Point 6) saturated liquid at 100F and 210 psia

(Figure 2) read hl= 60 Btu/lb

Evaporator Outlet

(Point 7) saturated vapor at 41.1F and 75 psia

(Figure 2) read hv= 548 Btu/lb

H across evaporator = 548 60 = 488 Btu/lb

Refrigerant flow rate (assuming 3% for heat gain through insulation):

1.03 x 500 Tons x12000 Btu/hr

Ton x

lb

488 Btu = 12664 lb/hr


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Generator Heat Load:

Heat Out 6% Split (Btu/hr) 30% Split (Btu/hr)

(Point 6) Ql 12,664 x 60 = 759,840

(Condenser) Qc 12,664 x (569 60) = 6,445,976

(Reflux) Qr .167 x 12,664 x (569 60) = 1,076,478

Sub Total Heat Out:8,282,294 8,282,294

(Point 1) QWA 8.5 x 12,664 x 71.4 = 7,685,782 1.7 x 12,664 x 218.7 = 4,708,349

Total Heat Out: 15,968,076 12,990,643

Heat In

(Feed) Qf 9.5 x 12,664 x 48.3 = 5,810,876 2.7 x 12,664 x 48.3 = 1,651,512

Generator Heat

by Difference: 10,157,200 11,339,131

Absorber Load:

Heat In 6% Split (Btu/hr) 30% Split (Btu/hr)

(Point 7) 12,664 x 548 = 6,939,872 12,664 x 548 = 6,939,872

(Point 2) 8.5 x 12,664 x (-14.21) = -1,529,621 1.7 x 12,664 x 97.04 = 2,089,155

Sub Total Heat In:5,410,251 9,029,027

Heat Out

(Point 3) 9.5 x 12,664 x (-28.3) = -3,404,716 2.7 x 12,664 x (-28.3) = 967,656

Qa Absorber Load

by Difference: 8,814,967 9,996,683


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Heat Balance:

Heat In 6% Split (Btu/hr) 30% Split (Btu/hr)

Evaporator 12,664 x (548 60) = 6,180,032 12,664 x (548 60) = 6,180,032

Generator 10,157,200 11,339,131

16,337,232 17,519,163

Heat Out

Condenser 6,445,976 6,445,976

Reflux Coil 1,076,478 1,076,478

Absorber 8,814,967 9,996,683

16,337,421 17,519,137

Coefficient of Performance (COP)

6,180,032 (1.03 x 10,157,200)

=.591

6,180,032 (1.03 x 11,339,131) =.529

The Coefficient of Performance (COP) for absorption refrigeration is the amount of cooling derived from the

system divided by the amount of heat required to separate the refrigerant from the absorbent.

Lithium Bromide

The following material, including the sample calculation, is extracted from the 1989 ASHRAE Fundamentals

Handbook. In many cases, greater precision is obtained through use of calculations than through use of the

charts in this module.


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For lithium bromide-absorption refrigeration systems, the following equation provides the solution flow rate per

unit refrigerant rate:

1 WFSA

( ) X( ) 1 WFSG( ) X 1( ) = 1 (EQN C)

where: WFSA = Mass fraction of lithium bromide in solution coming from the absorber

WFSG = Mass fraction of lithium bromide in solution coming from the generator


X-1 = Mass of solution from the generator per unit mass of refrigerant flow

The sample calculation is performed for a large lithium bromide-absorption refrigeration plant with the

following conditions (single-digit numbers in parentheses refer to circled numbers in Figure 3):

Refrigeration load, 1758 kW (500 tons)

Evaporator temperature (8), 5.0C (41.1F)

Absorber equilibrium temperature (3), 42C (107.2F)

Actual solution temperature (4), 38C (100.9F)

Solution temperature (5), 76.8C (170.3F)



Refrigerant vapor temperature (6), 93.3C (200F)

Refrigerant temperature (7), 43.3C (110F)

Refrigerant spillover rate (9), 2.5% of (8)

Concentration of solutions: WFSAis 0.595 and WFSGis 0.646

Chilled water temperature, 12.2-6.7C (54-44F)

Cooling water temperature entering, 29.4C (85F)

Assume no inerts present

Cooling water tower flow rate, 408 m3/hr (1800 gpm)


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For the given values for WFSAand WFSG, X can be calculated using Eqn. C as follows:

(1 0.595)X (1 0.646)(X 1)= 1

0.405X (0.354)(X 1)= 1

0.051X + 0.354 = 1

0.051X = 0.646

X = 12.67 lb solution from

absorber/ lb refrigerant

To calculate the amount of coolant flow required to provide the 500 tons of refrigeration, the enthalpy change

of the coolant as it passes through the evaporator is needed. The refrigerant enters the evaporator as a liquid at

110F and leaves the evaporator as a gas at 41.1F.

Enthalpy Evaporator Vapor (8) = 1,079.80 Btu/lb

Enthalpy of Liquid From Condenser (7) = -77.94

Difference H = 1,001.86 Btu/lb

Use standard steam tables for saturated vapor at 41.1F

Refrigerant Flow (including 2.5% spillover) =

1.025 x500 tons

1,001.86x

12,000 Btu/ hr

tonx

hr

60 min= 102.3 lb/min

Solution Flow Rate From Absorber =

12.67 lb solution/lb refrigerant x 102.3 lb/min = 1,296.14 lb/min dilute solution

Concentrated Solution From Generator = 1,296.14

-102.30 lb/min refrigerant

1,193.84 lb/min conc. solution


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Enthalpy of Liquid Spillover (9) =9.15 Btu/lb

Use standard steam tables for saturated vapor at 41.1F

Enthalpy of Generator Bottoms From Exchanger (2) = 71.70 Btu/lbUse Figure 6 at concentrations of 64.6% and temperature of 121.1F

Enthalpy of Absorber Bottoms (4) = 47.20 Btu/lb

Use Figure 6 at concentrations of 59.5% and temperature of 100.9F

Enthalpy of Feed to Generator (5) = 79.00 Btu/lb


Enthalpy of Liquid From Generator (1) = 107.00 Btu/lb


Enthalpy of Generator Overhead (6) = 1,150.30 Btu/lb

The approximate pressure of the generator overhead is equal to 65.9 mm

(1.27 psia) (the saturation pressure of water at the condenser outlet with a temperature of 110F). The enthalpy

of the generator overhead is the vapor enthalpy of steam at 200F and 1.27 psia.


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Source: ASHRAE 1989 Fundamentals Handbook (IP Edition) , Atlanta, GA, American Society of Heating,Refrigerating, and Air Conditioning Engineers, Inc., 1989, p. 17.71, Figure 34.

Figure 6. Enthalpy-Concentration Diagram for Lithium Bromide-

Water Solutions


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Enthalpy values are summarized in Figure 7.

ENTHALPY kJ/kg Btu/lb

hvof (8)* 2,509.9 1,079.8

hlof (7)* 181.2 77.94hlof (9)* 21.3 9.15

hlof (2)* 166.7 71.7

hlof (4)* 109.7 47.2

hlof (5)* 183.6 79.0

hlof (1)* 248.7 107.0

hlof (6)* 2,673.8 1,150.3

*Numbers in parenthesis refer to circled points in flow diagram, Figure 2.

Figure 7. Enthalpy Values Summary For This Particular Lithium

Bromide System

Material and heat balance the system:

Absorber:

Heat In Calculation

Solution

(Btu/min)

(Point 2) 1,193.84 lb/min conc. solution x 71.7 85,598.3

(Point 8) 102.3 1.025 lb/min refrig. vapor x 1,079.8 Btu/lb 107,769.3

(Point 9) (102.3 1.025) x (.025) refig. liq. x 9.15 Btu/lb 22.8

193,390.4

Heat Out

(Point 4) 1,296.14 dilute solution lb/hr x 47.2 61,177.8

Absorber Load by Difference: 132,212.6


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Generator:

HeatOut

Calculation

Solution

(Btu/min)

(Point 1) 1,193.84 conc. solution lb/hr x 107 127,741

(Point 6) 102.3 refrigerant lb/hr x 1,150.3 117,676

245,417

Heat In

(Point 5) 1,296.14 x 79 102,395

Generator Load by Difference: 143,022

Condenser:

Heat In Calculation

Solution

(Btu/min)

(Point 6) 102.3 x 1,150.3 117,676

Heat Out

(Point 7) 102.3 x 77.94 7,973

Condenser Load by Difference: 109,703

Evaporator:

500 tons x12,000 Btu/ hr

tonx

hr60 min

= 100,000 Btu min

Overall Heat Balance:

Heat In Btu/min

Evaporator Load 100,000

Generator Load 143,022

243,022

Heat Out

Absorber Load 132,213

Condenser Load 109,703

241,916


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The two totals should be the same. However, there are small differences in reading enthalpy values. The

overall balance is within .46% and is considered complete. The coefficient of performance (COP) is calculated

as follows:

COP = Evaporator LoadGenerator Load

= 100,000143,022

= .699

Consider a 2% loss to surroundings: the effective COP is .699 x .98 = .685.

The steam load to the generator can be calculated knowing the steam/ condensate properties and the heat load

of 143,022 Btu/min.

The cooling water rate can be calculated given the water inlet and allowable temperature use with the duty of

109,703 Btu/min.

Warmer cooling water will lower the COP since the generator pressure/ temperature will change, which will

change the concentration of Li Br that can be achieved. Similarly, cooler steam temperatures will lower COPfor the same reason. Cooling water temperatures and steam temperatures can be traded within limits to

maintain the COP.

Typical Values for Li Br Systems

Single Stage Double Effect

Leaving chilled water temp. 6.7C (44F) 6.7C (44F)

Chilled water differential 5.5C (10F) 5.5C (10F)

Entering condenser water temp. 29.4C (85F) 29.4C (85F)

Steam pressure at control valve inlet

(gage pressure, dry and saturated)

62-83 kPa

(9-12 psig)

296-896 kPa

(43-130 psig)

Exchanger fouling factor 90 mm2k/w

(.0005 hr ft2F/Btu)

90 mm2k/w

(.0005 hr ft2F/Btu)

Cooling water rate .065 L/s per kw

(3.6 gpm/ton)

.054 L/s per kw

(3.6 gpm/ton)

Cooling water temp. rise 9C (16F) 8C (15F)

Steam rate 1.5 kw/kw

(18.5 lb/hr per ton)

.43 g/s per kw

(12 lb/hr per ton)

Source: ASHRAE 1989 Fundamentals Handbook (IP Edition), Atlanta, GA, American Society of Heating, Refrigerating,

and Air Conditioning Engineers, Inc., 1989, p. 13.7, Table 1 & 2.

Figure 8. Typical Values for Li Br Systems


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Refrigerant Temperature (t ' = F) and Enthalpy (h = Btu/lb) of Lithium Bromide Solutions

Percent Li Br

Temp.(t= F)

0 10 20 30 40 45 50 55 60 65 70

80t'

h

80.0

48.0

78.2

39.2

75.6

31.8

70.5

25.6

60.9

21.6

53.5

21.2

42.1

23.0

28.6

28.7

13.8

38.9

#-0.2

#52.7

#-11.6

#67.1

100t'

h

100.0

68.0

98.1

56.6

95.3

47.0

89.9

38.7

79.6

33.2

71.8

32.1

60.0

33.2

46.1

38.2

30.9

47.8

#16.2

#61.1

#3.8

#75.1

120t'

h

120.0

87.9

117.9

73.6

114.9

61.7

109.2

51.7

98.3

44.7

90.1

43.0

77.9

43.6

63.6

48.0

48.1

56.9

32.7

69.4

#19.1

#83.0

140t'

h

140.0

107.9

137.8

91.0

134.6

77.0

128.5

65.1

117.1

56.5

108.5

54.1

95.8

54.1

81.2

57.9

65.2

66.1

49.1

78.0

#34.4

#91.1

160t'

h

160.0

127.9

157.7

108.2

154.3

92.0

147.9

78.2

135.8

68.1

126.8

65.1

113.8

64.7

98.7

67.9

82.3

75.4

65.6

86.6

#49.7

#99.2

180t'

h

180.0

147.9

177.5

125.4

173.9

107.9

167.2

91.9

154.5

80.4

145.1

76.6

131.7

75.3

116.2

77.7

99.5

84.6

82.0

95.1

#65.1

#107.2

200t'

h

200.0

168.0

197.4

143.4

193.6

123.3

186.5

105.3

173.3

92.1

163.5

87.4

149.6

85.9

133.7

87.8

116.6

94.1

98.5

104.0

#80.4

#115.6

220t'

h

220.0

188.1

217.2

160.7

213.3

138.2

205.8

119.0

192.0

104.1

181.8

99.0

167.5

96.5

151.3

97.8

133.7

103.3

114.9

112.5

95.7

123.6

240t'

h

*240.0

*208.3

*237.1

*178.4

232.9

154.0

225.2

132.6

210.7

116.0

200.2

110.3

185.4

107.1

168.8

107.7

150.9

112.5

131.4

121.1

111.0

131.6

260t'

h

*260.0

*228.6

*256.9

*195.7

*252.6

*169.1

*244.5

*146.2

229.4

128.1

218.5

121.6

203.3

117.6

186.3

117.6

168.0

121.6

147.9

129.5

126.4

139.5

280t'

h

*280.0

*249.1

*276.8

*213.8

*272.3

*185.1

*263.8

*159.7

*248.2

*140.0

*236.8

*132.8

221.2

128.1

203.9

127.5

185.1

130.6

164.3

137.9

141.7

147.6

300t'

h

*300.0

*269.6

*396.7

*231.6

*291.9

*200.7

*283.1

*173.5

*266.9

*152.1

*255.2

*144.1

*239.2

*138.9

221.4

137.3

202.3

139.8

180.8

146.5

157.0

155.5

320t'

h

*320.0

*290.3

*316.5

*249.7

*311.6

*216.3

*302.5

*187.2

*285.6

*164.2

*273.5

*155.3

*257.1

*149.5

*238.9

*147.1

219.4

148.8

197.2

154.9

172.4

163.4

340t'

h

*340.0

*311.1

*336.4

*267.9

*331.3

*232.1

*321.8

*201.0

*304.4

*176.1

*291.9

*166.6

*275.0

*160.1

*256.4

*157.0

*236.5

*158.0

213.7

163.5

187.7

171.0

360t'

h

*360.0

*332.2

*356.2

*286.1

*350.9

*248.0

*341.1

*214.9

*323.1

*188.2

*310.2

*178.0

*292.9

*170.6

*274.0

*166.8

*253.7

*167.0

230.1

171.9

203.0

178.3

*Extensions of data above 235F are well above the original data and should be used with care.

Source: ASHRAE 1989 Fundamentals Handbook (IP Edition) , Atlanta, GA, American Society of Heating, Refrigerating, and

Air Conditioning Engineers, Inc., 1989, p. 17.70.

Figure 9. Refrigerant Temperature Enthalpy of Lithium Bromide

Solutions


30/33




Work Aid 1: Procedure for Sele cting Appropriate Absorption systems,

Given Process Requirements

1. Decide if the refrigeration system must provide cooling to a temperature at or below the freezing point of

water (0C or 32F). Do not use an ammonia-water or lithium bromide-water system.

2. If the refrigeration system is to be installed inside an enclosed structure, and especially if it is to be

installed inside an inhabited structure, select a lithium bromide system.

3. If Step 1 or Step 2 does not force a choice, select the system with the best economy. Obtain the

investment and utility requirements from the vendor. Use utility costs specific for the location, and

calculate the operating cost for each proposal. Check the vendor's utility requirements using the methods

given in the sample problems. Choose the most economical system.


31/33




GLOSSARY

absorbent A material which, in contact with a liquid or gas, extracts one or more

components for which it has an affinity.

absorption refrigeration Refrigeration in which cooling is effected by expansion of a liquid

refrigerant into a gas and absorption of the gas by absorbent; the

refrigerant is reused after separation from the absorbent.

affinity The ability of an absorbent to easily suck in (absorb) the refrigerant.

aqua-ammonia system A system that uses a combination of ammonia and water. The ammonia

functions as the refrigerant, and the water as the absorbent.

COP Coefficient of Performance. The amount of cooling derived from the

absorption refrigeration system, divided by the amount of heat required

to separate the refrigerant from the absorbent.

direct-fired Refrigeration units that use a flame as their heat source.

double-effect

(dual-effect)

Absorption refrigeration units with two-stage generators.

heat-recovery Refrigeration units that are indirect-fired and use hot waste gases.

indirect-fired Refrigeration units that use steam or hot fluid heat as their heat source.

miscible The refrigerant is capable of being completely mixed with the absorbent

and forms a single phase.

lithium bromide system A system that uses a combination of lithium bromide and water. Thewater functions as the refrigerant, and the lithium bromide as the

absorbent.

refrigerant A substance that releases or absorbs a large latent heat when it undergoes

a change in phase (liquid-to-gas or gas-to-liquid).

spillover Movement of high water concentration liquid ammonia to the absorber

from the bottom of the evaporator.


32/33




ADDENDUM

ADDENDUM A: EQUATIONS USED IN ChE 210.03

RE =QE

hv hl( ) (EQN A)

where: R E = Mass (weight) flow of refrigerant from the evaporator


hv = Enthalpy of refrigerant vapor from the evaporator

hl = Enthalpy of refrigerant liquid from the condenser

WFSA(X) WFSG(X 1) = 1 (EQN B)

where: WFSA = Mass fraction of ammonia in solution coming from the absorber

WFSG = Mass fraction of ammonia in solution coming from the generator



1 WFSA( ) X( ) 1 WFSG( ) X 1( ) = 1 (EQN C)

where: WFSA = Mass fraction of lithium bromide in solution coming from the absorber

WFSG = Mass fraction of lithium bromide in solution coming from the generator




33/33



ADDENDUM B: SYMBOLS USED IN ChE 210.03

hv = Enthalpy of refrigerant vapor

hl = Enthalpy of refrigerant liquid


RE = Mass (weight) flow of refrigerant from the evaporator

WFSA = Mass fraction in solution coming from the absorber

WFSG = Mass fraction in solution coming from the generator


X1 = Mass of solution from the generator per unit mass of refrigerant flow

absorption refrigeration systems

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