chapter 03 theoretical concepts of vapour absorption

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CHAPTER 03

THEORETICAL CONCEPTS OF VAPOUR ABSORPTION

CHILLERS & EXHAUST GAS

This chapter describes the about the vapour absorption chillers. Further to that

describe the types and characteristics of the vapour absorption chillers. Vapour

absorption chiller is the most important part of the cogeneration system

3.1 Absorption Chillers

Absorption chiller technologies are one of a group of technologies classified as heat

pumps. Heat pumps may be either heat driven or work driven. Absorption

technologies are heat driven, transferring heat from a low temperature to a high

temperature using heat as the driving energy. Heat pumps operate on the principle of

the absorption refrigeration cycle, which is similar to the vapor-compression cycle.

Both the absorption refrigeration cycle and the vapor-compression cycle will be

examined to draw analogies between the two.

Vapor-compression refrigeration systems are the most common refrigeration

systems in use today. The vapor-compression cycle is a work- driven cycle that is

illustrated in Figure 3.1. In the vapor-compression cycle, work is input to compress

the refrigerant to a high pressure and temperature at State 2. At State 2, the

refrigerant condensation temperature is below the ambient temperature. As the high-

pressure and high-temperature refrigerant vapor passes through the condenser, heat

is rejected to the ambient air and the refrigerant vapor condenses to a liquid to

achieve State 3. The high-pressure liquid at State 3 passes through an expansion

valve. As the liquid passes through the expansion valve, the refrigerant experiences

a reduction in both temperature and pressure to reach State 4. At State 4, the boiling

temperature of the refrigerant is lower than that of the surroundings. The low-

pressure liquid refrigerant passes through the evaporator, absorbing heat from the

ambient environment when boiling occurs in the evaporator and creating a low-

pressure refrigerant vapor at State 1. The low-pressure refrigerant vapor at State 1

enters the compressor completing the cycle.

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Figure 3.1: Vapor-compression cycle schematic

Source: Mississippi state university, 2005

The absorption cycle has some features in common with the vapor compression

cycle. For example, the absorption cycle has a condenser, an evaporator, and an

expansion valve. However, the absorption cycle and the vapor-compression cycle

differ in two very important aspects. The absorption cycle uses a different

compression process and different refrigerants than the vapor-compression cycle.

The absorption cycle operates on the principle that some substances (absorbents)

have an affinity for other liquids or vapors and will absorb them under certain

conditions. Instead of compressing a vapor between the evaporator and condenser as

in Figure 3.1, the refrigerant of an absorption system is absorbed by an absorbent to

form a liquid solution. The liquid solution is then pumped to a higher pressure.

Because the average specific volume of a liquid is much smaller than that of the

refrigerant vapor, significantly less work is required to raise the pressure of the

refrigerant to the condenser pressure. This corresponds to less work input for an

absorption system as compared to a vapor- compression system.

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Because the absorbent used in the absorption cycle forms a liquid solution, some

means must also be introduced to retrieve the refrigerant vapor from the liquid

solution before the refrigerant enters the condenser. This process involves heat

transfer from a relatively high-temperature source. Because the thermal energy input

into the system is much higher than the work input through the pump, absorption

chillers are considered to be heat driven.

The components used to achieve the pressure increase in an absorption chiller are

viewed as a “thermal compressor” and replace the compressor in the vapor-

compression cycle shown in Figure 3.1. The components of the absorption cycle are

shown schematically in Figure 3.2. The components of the thermal compressor are a

pump, an absorber, and a (heat) generator and are shown to the right of the dashed

Z-Z line. The components to the left of the dashed Z-Z line are the same as the ones

used in the vapor-compression system

Figure 3.2: Basic absorption cycle schematic


The operation of the absorption cycle shown in Figure 3-2 is as follows: At State 1,

the low-pressure refrigerant vapor exits the evaporator and enters the absorber.

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In the absorber, the refrigerant vapor is dissolved in an absorbent and rejects the heat

of condensation and the heat of mixing to form a liquid solution. The

refrigerant/absorbent solution is then pumped to the condenser pressure and passed

to the generator. In the generator, heat is added to the refrigerant/absorbent solution

to vaporize the refrigerant, removing the refrigerant from the solution. The liquid

absorbent has a higher boiling temperature than the refrigerant and, therefore, stays

in the liquid form. There are two streams exiting the generator. The refrigerant exits

to the condenser at a high temperature and pressure (State 2) while the absorbent

passes through an expansion valve, decreasing the pressure of the absorbent to the

evaporator pressure before entering the absorber again.

The remainder of the operation is much the same as the vapor compression cycle.

The high-temperature, high-pressure refrigerant vapor at State 2 enters the

condenser with a pressure such that the ambient temperature is higher than the

condensation temperature of the refrigerant.

The refrigerant vapor condenses as it passes through the condenser, rejecting heat to

the ambient environment to achieve State 3. At State 3, the high-pressure, low-

temperature liquid refrigerant enters the expansion valve where the refrigerant

experiences a decrease in pressure to the evaporator pressure. The low- pressure,

low-temperature liquid refrigerant that results at State 4 is at a pressure such that the

boiling temperature of the refrigerant is lower than the ambient temperature of the

environment. As the liquid refrigerant passes through the evaporator, the refrigerant

boils, absorbing heat from the ambient air. The refrigerant exits the evaporator as a

high-temperature, low-pressure vapor to complete the cycle.

3.2 Refrigerant-Absorbent Selection

Though all absorption chillers operate on the basic cycle presented in Figure 3.2,

each chiller design is dependent on the refrigerant-absorbent selection. Current

refrigerant/ absorber media for absorption chillers are either water/lithium bromide

or ammonia/water. Water/lithium bromide absorption chillers utilize water as the

refrigerant and lithium bromide as the absorbent.

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Because water is used as the refrigerant, applications for the water/lithium bromide

absorption chillers are limited to refrigeration temperatures above 00C. This

combination of refrigerant and absorbent is advantageous in areas where toxicity is a

concern because lithium bromide is relatively non-volatile. Absorption machines

based on water/lithium bromide are typically configured as water chillers for air-

conditioning systems in large buildings. Water/lithium bromide chillers are available

in sizes ranging from 10 to 1500 tons. The coefficient of performance (COP) of

these machines typically falls in the range of 0.7 to 1.2 (Herold et al.1996).

3.2.1 Water lithium bromide vapour absorption system

Vapour absorption refrigeration systems using water-lithium bromide pair are

extensively used in large capacity air conditioning systems. In these systems water is

used as refrigerant and a solution of lithium bromide in water is used as absorbent.

Since water is used as refrigerant, using these systems it is not possible to provide

refrigeration at sub-zero temperatures.

Hence it is used only in applications requiring refrigeration at temperatures above

0oC. Hence these systems are used for air conditioning applications. The analysis of

this system is relatively easy as the vapour generated in the generator is almost pure

refrigerant (water), unlike ammonia-water systems where both ammonia and water

vapour are generated in the generator.

3.3. Properties of Water-Lithium Bromide Solutions

3.3.1. Composition

The composition of water-lithium bromide solutions can be expressed either in mass

fraction (ξ) or mole fraction (x). For water-lithium bromide solutions, the mass

mass of solution, i.e,

Where mL and mW are the mass of anhydrous lithium bromide and water in solution,

respectively.

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The composition can also be expressed in terms of mole fraction of lithium bromide

as:

Where, nL and nW are the number of moles of anhydrous lithium bromide and water

in solution, respectively. The number moles of lithium bromide and water can easily

be obtained from their respective masses in solution and molecular weights, thus;

: and

Where ML (= 86.8 kg/kmol) and MW (= 18.0 kg/kmol) are the molecular weights of

anhydrous lithium bromide and water respectively.

3.3.2. Vapour pressure of water-lithium bromide solutions

Applying Raoult’s law, the vapour pressure of water-lithium bromide solution with

the vapour pressure exerted by lithium bromide being negligibly small is given by:

Where, PW is the saturation pressure of pure water at the same temperature as that of

the solution and x is the mole fraction of lithium bromide in solution. It is observed

that Raoult’s law is only approximately correct for very dilute solutions of water

lithium bromide ( 0). Strong aqueous solutions of water-lithium bromide

are found to deviate strongly from Raoult’s law in a negative manner. For example,

at 50 percent mass fraction of lithium bromide and 25oC, Raoult’s law predicts a

vapour pressure of 26.2 mbar, whereas actual measurements show that it is only 8.5

mbar. The ratio of actual vapour pressure to that predicted by Raoult’s law is known

as activity coefficient. For the above example, the activity coefficient is 0.324

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The vapour pressure data of water-lithium bromide solutions can be very

conveniently represented in a Dühring plot. In a Dühring plot, the temperature of the

solution is plotted as abscissa on a linear scale, the saturation temperature of pure

water is plotted as ordinate on the right hand side (linear scale) and the pressure on a

logarithmic scale is plotted as ordinate on the left hand side. The plot shows the

pressure-temperature values for various constant concentration lines (isosters),

which are linear on Dühring plot. Figures 15.1 show the Dühring plot. The Dühring

plot can be used for finding the vapour pressure data and also for plotting the

operating cycle. Figure 15.2 shows the water-lithium bromide based absorption

refrigeration system on Dühring plot. Other types of charts showing vapour pressure

data for water-lithium bromide systems are also available in literature. Figure 15.3

shows another chart wherein the mass fraction of lithium bromide is plotted on

abscissa, while saturation temperature of pure water and vapour pressure are plotted

as ordinates. Also shown are lines of constant solution temperature on the chart.

Pressure-temperature composition data are also available in the form of empirical

equations

Fig.3.3: A Typical Duhring plot

Source: IIT Kharagpur, 2008

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Fig.3.4: H2O- LiBr System with a solution heat exchanger on Duhring plot


Fig.3.5: Pressure-temperature-concentration diagram for H2O-LiBr solution


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3.3.3. Enthalpy of water-lithium bromide solutions

Since strong water-lithium bromide solution deviates from ideal solution behavior, it

is observed that when water and anhydrous lithium bromide at same temperature are

mixed adiabatically, the temperature of the solution increases considerably. This

indicates that the mixing is an exothermic process with a negative heat of mixing.

Hence the specific enthalpy of the solution is given by:

Where hL and hW are the specific enthalpies of pure lithium bromide and water,

respectively at the same temperature. Figure 15.4 shows a chart giving the specific

enthalpy-temperature-mass fraction data for water-lithium bromide solutions. The

chart is drawn by taking reference enthalpy of 0 kJ/kg for liquid water at 0oC and

solid anhydrous lithium bromide salt at 25oC.

Fig.3.6: Enthalpy –temperature - concentration diagram for H2O-LiBr solution


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3.3.4. Enthalpy values for pure water (liquid and superheated vapour)

The enthalpy of pure water vapour and liquid at different temperatures and pressures

can be obtained from pure water property data. For all practical purposes, liquid

water enthalpy, hW, liquid at any temperature T can be obtained from the equation:

Where Tref is the reference temperature, 0oC

The water vapour generated in the generator of water-lithium bromide system is in

super heated condition as the generator temperature is much higher than the

saturation water temperature at that pressure. The enthalpy of superheated water

vapour, hW,sup at low pressures and temperature T can be obtained approximately by

the equation:

hW,sup = 2501 + 1.88 (T – T ref ) (3.8)

3.3.5. Crystallization

The pressure-temperature-mass fraction and enthalpy-temperature-mass fraction

charts (Figs. 3.5 and 3.6) show lines marked as crystallization in the lower right

section. The region to the right and below these crystallization lines indicates

solidification of LiBr salt. In the crystallization region a two-phase mixture (slush)

of water-lithium bromide solution and crystals of pure LiBr exist in equilibrium. The

water-lithium bromide system should operate away from the crystallization region as

the formation of solid crystals can block the pipes and valves. Crystallization can

occur when the hot solution rich in LiBr salt is cooled in the solution heat exchanger

to low temperatures. To avoid this condenser pressure reduction below a certain

value due to say, low cooling water temperature in the condenser should be avoided.

Hence in commercial systems, the condenser pressure is artificially maintained high

even though the temperature of the available heat sink is low. This actually reduces

the performance of the system, but is necessary for proper operation of the system. It

should be noted from the property charts that the entire water-lithium bromide

system operates under vacuum.

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3.4. Commercial systems

Commercial water-lithium bromide systems can be:

1. Single stage or single-effect systems, and

2. Multi stage or multi-effect systems

Single stage systems operate under two pressures – one corresponding to the

condenser-generator (high pressure side) and the other corresponding to evaporator-

absorber. Single stage systems can be either:

1. Twin drum type, or

2. Single drum type

Since evaporator and absorber operate at the same pressure they can be housed in a

single vessel, similarly generator and condenser can be placed in another vessel as

these two components operate under a single pressure. Thus a twin drum system

consists of two vessels operating at high and low pressures. Figure 3.8 shows a

commercial, single stage, twin drum system.

Fig.3.7: A commercial, twin-drum type, water-lithium bromide system


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As shown in the figure, the cooling water (which acts as heat sink) flows first to

absorber, extracts heat from absorber and then flows to the condenser for condenser

heat extraction. This is known as series arrangement. This arrangement is

advantageous as the required cooling water flow rate will be small and also by

sending the cooling water first to the absorber, the condenser can be operated at a

higher pressure to prevent crystallization. It is also possible to have cooling water

flowing parallel to condenser and absorber; however, the cooling water requirement

in this case will be high.

A refrigerant pump circulates liquid water in evaporator and the water is sprayed

onto evaporator tubes for good heat and mass transfer. Heater tubes (steam or hot

water or hot oil) are immersed in the strong solution pool of generator for vapour

generation.

Pressure drops between evaporator and absorber and between generator and

condenser are minimized, large sized vapour lines are eliminated and air leakages

can also be reduced due to less number of joints.

Figure 3.9 shows a single stage system of single drum type in which all the four

components are housed in the same vessel. The vessel is divided into high and low

pressure sides by using a diaphragm.

Fig.3.8: A commercial, single-drum type, water-lithium bromide system


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In multi-effect systems a series of generators operating at progressively reducing

pressures are used. Heat is supplied to the highest stage generator operating at the

highest pressure. The enthalpy of the steam generated from this generator is used to

generate some more refrigerant vapour in the lower stage generator and so on.

Double-effect absorption systems use a second generator, condenser, and heat

exchanger that operate at higher temperature. A double-effect water/lithium bromide

absorption system is shown schematically in Figure 3.10. Refrigerant vapor is

recovered from the first-stage generator in the high- temperature condenser. The

refrigerant/absorbent in the second-stage generator is at a lower temperature than the

solution in the first-stage generator. The refrigerant vapor from the first stage

generator flows through the second- stage generator where a portion of the

refrigerant condenses back into liquid while the remainder remains in the vapor

phase.

Additional refrigerant is vaporized in the second-stage generator by the high

temperature and the heat of vaporization supplied by the refrigerant from the first-

stage generator. The refrigerant vapor from both generator stages flows to the

condenser while the absorbent solution flows back to the absorber.

Figure 3.9: Double-effect water/lithium bromide absorption chiller schematic


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The purpose of the two stages that make up the double-effect absorption cycle is to

increase the COP of the cycle. This is made possible through the use of the

recuperative heat exchangers used in the system. Double-effect chillers yield higher

COPs than single-effect chillers. The COP for double-effect absorption chillers

varies from 1.0 to 1.2 for water/lithium bromide chillers (Herold et al. 1996).

Triple-effect absorption chillers have been in prototype development for several

years. These chillers will be direct-fired and are expected to provide a 50% thermal

efficiency improvement over double-effect absorption chillers. Triple-effect

absorption chillers do not feature a distinct third generator stage; rather they use

internally-recovered heat to achieve high efficiencies. Triple- effect water/lithium

bromide chillers can achieve COPs of 1.6 and greater (Peltchers, 2003).

In this manner the heat input to the system is used efficiently by generating more

refrigerant vapour leading to higher COPs. However, these systems are more

complex in construction and require a much higher heat source temperatures in the

highest stage generator. Figure 3.10 show commercial double-effect systems. Figure

3.11 shows the double effect cycle on Dühring plot.

Fig.3.10: A commercial, double-effect, water-lithium bromide system


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Fig.3.11: Double effect VARS on Dühring plot


3.5 Heat Sources for Water-Lithium Bromide Systems

Water-lithium bromide systems can be driven using a wide variety of heat sources.

Large capacity systems are usually driven by steam or hot water. Small capacity

systems are usually driven directly by oil or gas. A typical single effect system

requires a heat source at a temperature of about 120oC to produce chilled water at

7oC when the condenser operates at about 46oC and the absorber operates at about

40oC. The COPs obtained in the range of 0.6 to 0.8 for single effect systems while it

can be as high as 1.2 to 1.4 for multi-effect systems (IIT Kharagpur, 2008).

3.5.1 Minimum heat source temperatures for Libr-Water systems

Application data for a single-stage water-lithium bromide vapour absorption system

with an output chilled water temperature of 6.7oC (for air conditioning applications)

is shown in Table 3.1.

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Table 3.1 Application data for a single-stage water-lithium bromide system


The above values were validated on actual commercial systems with very efficient

heat and mass transfer design. If the heat and mass transfer is not very efficient, then

the actual required heat source temperatures will be higher than the reported values.

For given cooling water temperature, if the heat source temperature drops below the

minimum temperature given above, then the COP drops significantly. For given

cooling water temperature, if the heat source temperature drops below a certain

temperature (minimum generation temperature), then the system will not function.

Minimum generation temperature is typically 10 to 15oC lower than the minimum

heat source temperature.

If air cooled condensers and absorbers are used, then the required minimum heat

source temperatures will be much higher (≈ 150oC). The COP of the system can be

increased significantly by multi- effect (or multi-stage) systems. However, addition

of each stage increases the required heat source temperature by approximately 50oC.

3.6 Capacity Control

Capacity control means capacity reduction depending upon load as the capacity will

be maximum without any control. Normally under both full as well as part loads the

outlet temperature of chilled water is maintained at a near constant value.

The refrigeration capacity is then regulated by either:

1. Regulating the flow rate of weak solution pumped to the generator through

the solution pump

2. Reducing the generator temperature by throttling the supply steam, or by

reducing the flow rate of hot water

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3. Increasing the condenser temperature by bypassing some of the cooling

water supplied to the condenser

Method 1 does not affect the COP significantly as the required heat input reduces

with reduction in weak solution flow rate, however, since this may lead to the

problem of crystallization, many a time a combination of the above three methods

are used in commercial systems to control the capacity.

3.7 Application

Absorption chillers can be directly fired or indirectly fired. Direct-fired absorption

chillers utilize a natural gas burner and can supply waste heat for a desiccant

dehumidification device or hot water. Direct-fired chillers are often used in areas

where electric rates are high and gas utilities offer lower rates or rebate programs to

replace vapor-compression chillers.

Indirect-fired fired absorption chillers are utilized where there is an existing source

of heat that can be recovered. The supplied heat can be in the form of hot water,

steam, or exhaust gases.

All of the prime mover technologies that are applicable to CHP can produce waste

heat sufficient to drive an absorption chiller. This coupling ability makes absorption

cycle chiller systems very desirable for CHP applications. A typical Exhaust gas

fired absorption chiller is shown in following figure 3.13

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Fig.3.12: Schematic diagram for typical exhaust fired absorption chiller.

Source: www.thermaxindia.com

Absorption chillers offer many advantages over electric chillers, especially when

there is a source of waste heat available. As compared to electric chillers, absorption

chillers have lower operating costs, shorter payback periods, quiet operation, low

maintenance, and high reliability. Absorption systems also operate at lower

pressures and offer safer operation. However, absorption chillers have higher initial

costs and are not as widely available as electric chillers.

Vapor-compression systems are much more widely manufactured and more

available than absorption chillers. Still, the fact that absorption chillers do not have

mechanical compressors and have fewer moving parts gives absorption technologies

an advantage over vapor-compression systems in terms of lower maintenance,

higher reliability and quieter operation. A typical cogeneration system, with exhaust

fired absorption chiller as shown in following figure.

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Figure 3.13: Schematic diagram for cogeneration system with absorption chiller.

Source: www.thermaxindia.com

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3.8 Practical Problems in Water-Lithium Bromide Systems

Practical problems typical to water-lithium bromide systems are:

1. Crystallization

2. Air leakage, and

3. Pressure drops

As mentioned before to prevent crystallization the condenser pressure has to be

maintained at certain level, irrespective of cooling water temperature. This can be

done by regulating the flow rate of cooling water to the condenser. Additives are

also added in practical systems to inhibit crystallization. Crystallization occurs when

the machine operates too close to the saturation temperature of the lithium bromide

solution and the lithium bromide begins to precipitate out of the solution. While this

will not damage the machine, it is a nuisance and usually requires application of

external heat to get the lithium bromide back into the solution.

Newer machines have electronic controls which prevent the chiller from operating at

temperatures and concentrations which allow crystallization to occur. If the unit is

operated properly, and maintained, crystallization is nearly a thing of the past.

Since the entire system operates under vacuum, outside air leaks into the system.

Hence an air purging system is used in practical systems. Normally a two-stage

ejector type purging system is used to remove air from the system. Since the

operating pressures are very small and specific volume of vapour is very high,

pressure drops due to friction should be minimized. This is done by using twin- and

single-drum arrangements in commercial systems.

Vacuum leaks are a serious problem adversely affecting the efficiency of the

machine and causing corrosion in the unit for this reason, points of entry for air,

such as valves, must be maintained and repaired as necessary. Newer machines have

automatic purges which help to eliminate the effects of vacuum leaks, but the purge

tank has a manual valve which must be cycled weekly to remove non-condensable

gases. Since the operating pressures are very small and specific volume of vapour is

very high, pressure drops due to friction should be minimized.

This is done by using twin- and single-drum arrangements in commercial systems.

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Other than these problems (which are infrequent on newer machines), absorption

chillers are relatively trouble-free, partly due to the fact that there are only two

moving parts, the purge, and the pump.

3.9 Disadvantages of Absorption Chillers

Two of the primary disadvantages of absorption chillers are their size and

weight, and their requirement for larger cooling towers. Absorption chillers are

larger and heavier than electric chillers of the same capacity. If they are used to

replace both a boiler and a chiller, however, the size and weight of an absorption

chiller/heater is less than that of a combined electric chiller and a boiler.

Absorption chillers require cooling tower capacities approximately 1/3

greater than electric chillers of the same size. An absorption chiller of the

same size as an electric chiller can use the same cooling tower but its capacity

and efficiency will be reduced.

3.10 Summary and Conclusion

This chapter describes the technical overview, advantages, disadvantages and etc., of

the absorption chillers. Water/ lithium bromide absorption chiller is most effective

for air conditioning applications. Most efficient water/ lithium bromide absorption

chillers are triple effect chillers. This can achieve higher COP, up to 1.6. However in

the exhaust fired triple effect absorption chillers are not available in the market.

Since select the double effect water/ lithium bromide absorption chillers for the

cogeneration system for most effective operation.

chapter 03 theoretical concepts of vapour absorption

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