Dinc Ibrahim. Refrigeration systems and applications 2th edition

Подождите немного. Документ загружается.

Refrigeration Cycles and Systems 191

in sizes from 10 to 1000 tons, leading to the lowest evaporation temperature of 4

◦

C (with a mini-

mum pressure of 0.8 kPa) because the water is used as the refrigerant. In practical applications, the

temperature is 5

◦

C. Low-pressure steam is the main energy source for these H

O–LiBr absorption

systems. Despite their COPs less than unity, cheap energy can make these systems economically

competitive with much higher COP values for vapor-compression systems. In practical H

O–LiBr

ARSs, the evaporator and absorber are combined in a shell at the lower-pressure side and the con-

denser and generator are combined in another shell at the higher-pressure level. A liquid–liquid heat

exchanger is arranged to increase system efﬁciency and hence to improve the COP. Its operating

principle is the same as that of other ARSs. In the H

O–LiBr ARS, crystallization (which is a

solidiﬁcation of the LiBr) appears to be a signiﬁcant problem. The crystallization lines are shown

on the pressure–temperature and enthalpy–concentration charts. Dropping into the crystallization

region causes the formation of slush, resulting in blockage of the ﬂow inside the pipe and interrup-

tion of the system operation. In order to prevent this problem, practical systems are designed with

control devices to keep the condensation pressure artiﬁcially high. Note that absorption chillers

and/or refrigeration systems are classiﬁed into three categories as follows:

• Single-effect ARS. Units using low pressure (135 kPa or less) as the driving force. These units

typically have a COP of 0.7.

• Double-effect ARS. Units are available as gas-ﬁred (either direct gas ﬁring, or hot exhaust gas

from a gas-turbine or engine) or steam-driven with high-pressure steam (270 to 950 kPa). These

units typically have a COP of 1.0 to 1.2. To achieve this improved performance they have a

second generator in the cycle and require a higher temperature energy source.

• Triple-effect ARS. Although the units are not fully available for commercial applications, the

concept is well-developed and experiments are conducted for applications through various patents

(e.g., Patent Storm, 2010) and some papers (e.g., Kaita, 2001). This triple-effect ARS can use

any heat source from waste heat to renewable energy sources, including solar and geothermal

heat. The pressure of steam further increases here due to the additional effect (stage) and may

easily go beyond the double-effect ARS pressures. The COPs of these three-effect units may

become 13 and higher for ammonia-water ARSs, and 1.6 and higher for water-LiBr ARSs. Such

COPs are really encouraging for practical applications. The operation of this kind of triple-effect

ARS may be described brieﬂy as follows:

An absorber provides strong solution to three generators, including a high-temperature generator,

an intermediate-temperature generator, and a low-temperature generators in which all may be

connected in parallel or inverse series. Each generator feeds refrigerant vapor to a corresponding

condenser, including a high-temperature condenser, an intermediate-temperature condenser, and

a low-temperature condenser. The higher-temperature condensers are essentially coupled with

the lower temperature generators, respectively. Hence, the system is referred to as a double-

coupled condenser triple effect absorption system. The three heat exchangers may be provided

in the parallel or inverse series ﬂowpath from the absorber. It is possible to conﬁgure this

system differently which requires further research and development to ﬁnd the best option for

applications.

4.7.5.1 Single-Effect ARS

As stated earlier, in ARS, an absorber, generator, pump, and recuperative heat exchanger replace

the compressor. Like mechanical refrigeration, as shown in Figure 4.25, the cycle begins when

high-pressure liquid refrigerant from the condenser passes through a metering device (1) into the

lower-pressure evaporator (2) and collects in the evaporator pan or sump. As before, the ﬂashing that

occurs at the entrance to the evaporator cools the remaining liquid refrigerant. Similarly, the transfer

of heat from the comparatively warm system water to the now-cool refrigerant causes the latter

to evaporate (2), and the resulting refrigerant vapor migrates to the lower-pressure absorber (3).

192 Refrigeration Systems and Applications

Heat

exchanger

Absorb

Evaporate

Expand

Generate

Condense

Cooling

water

Cooling

water

Chilled

water

Figure 4.25 Schematic of a single-effect ARS.

There, it is soaked up by an absorbent lithium bromide solution. This process not only creates a low-

pressure area that draws a continuous ﬂow of refrigerant vapor from the evaporator to the absorber,

but also causes the vapor to condense (3) as it releases the heat of vaporization picked up in the evap-

orator. This heat – along with the heat of dilution produced as the refrigerant condensate mixes with

the absorbent – is transferred to the cooling water and is released in the cooling tower. Of course,

assimilating refrigerant dilutes the lithium bromide solution and reduces its afﬁnity for refrigerant

vapor. To sustain the refrigeration cycle, the solution must be reconcentrated. This is accomplished

by constantly pumping (4) dilute solution from the absorber to the generator (5), where the addition

of heat boils the refrigerant from the absorbent. Once the refrigerant is removed, the reconcentrated

lithium bromide solution returns to the absorber, ready to resume the absorption process. Mean-

while, the refrigerant vapor liberated in the generator migrates to the cooler condenser (6). There,

the refrigerant returns to its liquid state as the cooling water picks up the heat of vaporization

carried by the vapor. The liquid refrigerant’s return to the metering device (1) completes the cycle.

4.7.5.2 Double-Effect ARS

The energy efﬁciency of absorption can be improved by recovering some of the heat normally

rejected to the cooling tower circuit. A two-stage or two-effect ARS accomplishes this by taking

vapors driven off by heating the ﬁrst-stage concentrator (or generator) to drive off more water in a

second stage. Many ARS manufacturers offer this higher efﬁciency alternative.

The double-effect ARS takes absorption to the next level. The easiest way to picture a double-

effect cycle is to think of two single-effect cycles stacked on top of each other (as shown in

Figure 4.26). Note that two separate shells are used. The smaller is the ﬁrst-stage concentrator.

The second shell is essentially the single-effect ARS from before, containing the concentrator,

condenser, evaporator, and ARS. The temperatures, pressures, and solution concentrations within

the larger shell are similar to the single-effect ARS as well. The cycle on top is driven either directly

by a natural gas or oil burner, or indirectly by steam. Heat is added to the generator of the topping

cycle (primary generator), which generates refrigerant vapor at a relatively higher temperature and

pressure. The vapor is then condensed at this higher temperature and pressure and the heat of

condensation is used to drive the generator of the bottoming cycle (secondary generator), which is

at a lower temperature. If the heat added to the generator is thought to be equivalent to the heat of

condensation of the refrigerant, it becomes clear where the efﬁciency improvement comes from.

Refrigeration Cycles and Systems 193

Heat

exchanger

Absorb

Evaporate

Expand

Generate

Condense

Cooling

water

Cooling

water

Chilled

water

Heat

exchanger

Generate

Condense

Figure 4.26 Schematic of a double-effect ARS.

For every unit of heat into the primary generator, two masses of refrigerant are boiled out of

solution, or generated: one in the primary generator and one in the secondary generator. In a single-

effect cycle only one mass is generated. Therefore, in a double-effect system, twice the mass ﬂow of

refrigerant is sent through the refrigerant loop per unit of heat input, so twice the cooling is delivered

per unit of heat input. Using this approach a double-effect system has a COP that is roughly twice

that of a single-effect cycle. However, this simplifying assumption does not account for cycle

inefﬁciencies and losses. In actuality, a single-effect system has a COP of about 0.65 and a double-

effect system has a COP of about 1.0. Note that the reuse of the vapors from the ﬁrst-stage generator

makes this machine more efﬁcient than single-stage absorption chillers, typically by about 30%.

4.7.5.3 Crystallization

Some absorption chillers are notorious for “freezing up” or crystallizing. The basic mechanism of

failure is simple enough – the lithium bromide solution becomes so concentrated that crystals of

lithium bromide form and plug the machine (usually the heat exchanger section). The most frequent

causes are as follows:

• air leakage into the machine,

• low-temperature condenser water, and

• electric power failures.

The ﬁrst two are actually very similar since they both drive the heat input up to the point that

crystallization can occur. Whether air leaks into the machine or the condenser water temperature is

194 Refrigeration Systems and Applications

too low, the water vapor pressure in the absorption chiller evaporator has to be lower than normal

to produce the required cooling. This forces the heat input to the machine to be higher to increase

the solution concentration. Air leakage into the machine can be controlled by designing the machine

with hermetic integrity and routinely purging the unit using a vacuum pump.

Excessively cold condenser water (coupled with a high load condition) can also cause crystalliza-

tion. While reducing condenser water temperature does improve performance, it could cause a low

enough temperature in the heat exchanger to crystallize the concentrate. Sudden drops in condenser

water temperature could cause crystallization. For this reason, some of the early absorption chillers

were designed to produce a constant condenser water temperature. Modern absorption chillers have

special controls that limit the heat input to the machine during these periods of lower condenser

water temperatures.

Power failures can cause crystallization as well. A normal absorption chiller shutdown uses a

dilution cycle that lowers the concentration throughout the machine. At this reduced concentration,

the machine may cool to ambient temperature without crystallization. However, if power is lost

when the machine is under full load and highly concentrated solution is passing through the heat

exchanger, crystallization can occur. The longer the power is out, the greater the probability of

crystallization.

Major absorption chiller manufacturers now incorporate devices that minimize the possibility

of crystallization. These devices sense impending crystallization and shut the machine down after

going through a dilution cycle. These devices also prevent crystallization in the event of power

failure. A typical anti-crystallization device consists of two primary components: (i) a sensor in

the concentrated solution line at a point between the concentrator and the heat exchanger and

(ii) a normally open, two-position valve located in a line connecting the concentrated solution line

and the line supplying refrigerant to the evaporator sprays.

4.7.6 The Steam Ejector Recompression ARS

The ejector recompression absorption cycle, which was developed by Eames and Wu (2000), is

similar to the conventional single-effect lithium bromide absorption cycle. The difference between

them is that there is a steam ejector in this novel cycle for enhancing the concentration process.

Because of the use of the steam ejector, the performance and the operating characteristics of the

novel cycle are different from the conventional cycle.

The steam ejector recompression absorption cycle is shown schematically in Figure 4.27a. In

this ﬁgure, the expansion of the high-pressure steam causes a low pressure at the exit of the

primary nozzle of the steam ejector; therefore, the vapor at point 8 in the concentrator is entrained

by the primary ﬂow. The two streams are mixed in the steam ejector and condensed in the heat

exchanger of the concentrator. The condensation heat is used to heat the solution in the concentrator.

Obviously, the heat of the entrained vapor is recovered by the steam ejector in this process. Water

at point 3 splits into two streams; one ﬂows back to the steam generator and the other ﬂows into

the condenser. In stable operation, the mass ﬂow rate of the ﬁrst stream equals that of primary

ﬂow, while the mass ﬂow rate of the second stream equals that of the entrained vapor. The rest

of the cycle is similar to that of the conventional single-effect lithium bromide absorption cycle.

Figure 4.27b shows the novel cycle on a P–T–C diagram. As shown in Figure 4.27b, the cycle

6–7–9–10–6 takes up water at the absorber (10−6) and releases it as vapor at the concentrator

(7–9). In the conventional absorption cycle, the vapor is condensed at 8



and the condensation heat

is rejected to the surroundings. In the novel cycle, this vapor undergoes a compression process

through the ejector to point 2. Since the vapor temperature is greater than the solution temperature

in the concentrator, this vapor is used to heat the solution by condensation to point 3. Therefore

the heat otherwise wasted is recovered and the energy efﬁciency is improved.

Eames and Wu (2000) investigated the energy efﬁciency and the performance characteristics of

the novel cycle and the theoretical results showed that the COP of the novel cycle is better than

Refrigeration Cycles and Systems 195

(

)(

)

Concentrated

Dilute

0 %

2,3

8,9

8′

con

Water

pump

Evaporator

Absorber

Restrictor

Solution

pump

Solution heat

exchanger

Condenser

Concentrator

8,T

Steam generator

Ejector

Figure 4.27 (a) The steam ejector recompression ARS and (b) its P –T –C diagram (Eames and Wu, 2000)

(Reprinted with permission from Elsevier Science).

that of the conventional single-effect absorption cycle. The characteristics of the cycle performance

show its promise in using high-temperature heat source at low cost.

In the past, Kang et al. (2000) undertook a study to propose and evaluate advanced

absorption cycles for the COP improvement and temperature lift enhancement applications. The

characteristics of each cycle are assessed from the viewpoints of the ideal cycle COP and its

applications. The advanced cycles for the COP improvement are categorized according to their

heat recovery method: condensation heat recovery, absorption heat recovery, and condensation/

absorption heat recovery. In H

O–LiBr systems, the number of effects and the number of stages

can be improved by adding a third or a fourth component to the solution pairs. The performance

of NH

–H

O systems can be improved by internal heat recovery because of their thermal

characteristics such as temperature gliding. NH

–H

O cycles can be combined with adsorption

cycles and power generation cycles for waste heat utilization, performance improvement, panel

heating, and low-temperature applications. The H

O–LiBr cycle is better from the high COP

viewpoint for evaporation temperature over 0

◦

C while the NH

–H

O cycle is better from the

viewpoint of low-temperature applications. This study suggests that the cycle performance would

be signiﬁcantly improved by combining the advanced H

O–LiBr and NH

–H

Ocycles.

4.7.7 The Electrochemical ARS

In another study, Newell (2000) proposed a new electrochemical ARS as shown in Figure 4.28,

which consists of four main components. An electrochemical cell is the heat absorber, equivalent

to an evaporator in a conventional vapor-compression refrigeration system. A fuel cell rejects heat

in a manner similar to a condenser in a common vapor-compression refrigeration cycle. The third

component is a heat exchanger between gas streams and water ﬂow stream. The fourth component

is a current pump for elevating the fuel cell’s voltage output to a level sufﬁcient for driving the

electrochemical cell. The voltage required is sufﬁciently low such that the cycle may be one that is

conveniently matched for solar photovoltaic cells or other direct current electric energy conversion

systems. In fact, the system shown in Figure 4.28 can be used as a thermally driven power cycle

by operating the fuel cell at a temperature lower than the electrochemical cell. The voltage supply

196 Refrigeration Systems and Applications

Heat exchanger

Electrochemical

cell

Fuel

cell

Current flow

Heat and

work IN

Heat and

work OU

∆ V electrochemical

cell

∆ V fuel cell

∆ V added

Figure 4.28 Schematic representation of the electrochemical ARS (Newell, 2000).

becomes a load driven by the electric circuit. Lowering component irreversibilities is essential

to reach a breakeven operating condition where the fuel cell is generating sufﬁcient power for

operation of the electrochemical cell. Newell’s system is based on a water/hydrogen/oxygen fuel

cell and electrochemical cell combination. Other combinations are also considered. Each one has its

own advantages or disadvantages. The conﬁguration envisioned for the system operates near atmo-

spheric pressure. The components could be operated at nearly uniform pressures with gravitation,

surface tension, or low head pumping used for transporting the working ﬂuids within and between

components. Water may be moved from the electrochemical cell and fuel cell to external heat

exchange surfaces, or the cells could be conﬁgured for direct heat exchange with their surroundings.

Engine

Absorption

system

Compressor

Engine coolant

Exhaust

Cooling water

Exhaust

Brine

LP evaporator

Precooler

LP evaporator

Evaporator

Brine

Fuel

Condenser

Condense

coolant

Figure 4.29 Absorption-augmented engine-driven refrigeration system (Turpin, 2000).

Refrigeration Cycles and Systems 197

4.7.8 The Absorption-Augmented Refrigeration System

Recently, a new absorption-augmented refrigeration system has been under development. The sys-

tem is based on another development called the generator absorber heat exchange (GAX) cycle.

These heat-activated absorption cycles excel at using low-temperature waste heat and turning it into

refrigeration or air conditioning. In the absorption-augmented refrigeration system, the prime mover

is a gas-ﬁred engine. Gas-ﬁred engines are quite efﬁcient at using high-temperature heat; however,

they leave a lot of their energy (approximately 65 or 70%) behind as low-temperature waste heat,

which is ideal for absorption (Turpin, 2000). The total system combines an internal combustion

engine with a mechanical compression refrigeration system powered by the engine shaft power and

the waste heat driven ARS (Figure 4.29).

Example 4.7

In this example, we present one of our earlier works (Dincer and Ture, 1993; Dincer, Edin and

Ture, 1996) on the design and construction of a solar powered ARS (Figure 4.30) using a mixture

of R-22 and DMETEG as the working ﬂuid. In this project, a combined water-heating and cooling

system based on absorption refrigeration was designed and constructed. The system consists of

four plate collectors, an evaporator, an absorber, a generator, a condenser, a solution pump, and

two heat exchangers. Each part was custom-designed to provide 4000 kcal/h cooling load, although

R-22 which has a less damaging effect on the ozone layer compared to other CFCs was employed

as a refrigerant. The energy analysis results of the experimental system were compared with the

theoretical calculations and a reasonably good agreement was found. The results show that the ARS

appeared to be efﬁcient and effective.

Solar radiation

Collector

Pump

Generator

Condenser

Tank

Heat exchanger

Pump

Expansion

valve

Water in

Water out

Expansion

valve

Evaporator

Absorber

Figure 4.30 An R-22 and DMETEG ARS (Dincer et al., 1996) (Reprinted with permission from Elsevier

Science).

198 Refrigeration Systems and Applications

Solution

In the energy analysis, we used the energy balance equations presented earlier in this section.

Experimental Apparatus and Procedure. The experimental system used a working ﬂuid com-

bining R-22 as refrigerant and DMETEG as absorbent. The cycle efﬁciency and the operational

characteristics of an ARS are dependent on the properties of the refrigerant, the absorbent, and

their relative mixtures. The combination of R-22 and DMETEG was suggested as one of the new

alternative combinations and was employed in the present system.

A schematic diagram of the solar powered ARS built in the Solar Energy Laboratory of the Energy

Systems Department at Marmara Research Centre in Gebze, Turkey, is depicted in Figure 4.30.

Generally, the system is considered as one which incorporates solar energy equipment with con-

ventional ARS. The basic elements of the system were four ﬂat-plate collectors, an evaporator,

an absorber, a generator, a condenser, a solution pump, and two heat exchangers. The system has

been designed speciﬁcally to provide 4000 kcal/h cooling load in the evaporator. Although the con-

struction of the components of the system followed closely that of Van Den Bulck, Trommelmans

and Berghmans 1982, some modiﬁcations were introduced into the individual components of the

system, not only to achieve the desired design parameters, but also to improve the operation of

the ARS. For instance, a 1.2 m long and 1.3 cm diameter copper pipe with 2 mm holes in every 3 cm

of length was employed in the absorber to provide homogeneous, fast, and effective absorption of

DMETEG and R-22 vapor.

The operation of the system is described as follows. Starting from the change of R-22 from liquid

to gas via the throttling effect of the expansion valve, the resulting R-22 vapor begins to absorb heat

from its immediate surroundings in a conventional natural convection type evaporator. Cool vapor

leaving the evaporator passes through the second heat exchanger into the absorber where it combines

with DMETEG which absorbs the gaseous R22. Absorption proceeds because of the chemical

afﬁnity between the absorbent DMETEG and refrigerant R-22 molecules. This absorption activity

lowers the pressure in the absorber to cause the vapor to ﬂow from the evaporator. When the vapor

goes into liquid solution it releases both its latent heat and a heat of dilution. This energy release

has to be continuously dissipated by the cooling water. When the effective cooling is achieved,

the process continues until the liquid solution reaches the equilibrium saturation condition which

exists for each absorber temperature and pressure. Because of the physical limitations, complete

equilibrium saturation may not be reached in the absorber and the strong liquid leaving the absorber

may not be as fully saturated with R-22 as its pressure and temperature would require. The resulting

liquid solution reaches the equilibrium saturation condition consistent with the temperature and

pressure of the absorbent. This strong solution, fully saturated with R-22, now passes through

a solution pump which raises the pressure, passing it through the ﬁrst heat exchanger and into

the generator. The generator meanwhile is being heated by circulating hot water in the higher-

pressure portion of the system, whose heat is derived from the solar collectors. The temperature of

the strong R-22/DMETEG solution increases, driving off R-22 and a small amount of DMETEG

vapor. The weak solution returns to the absorber down through the ﬁrst heat exchanger while

warming the upward ﬂowing strong solution. It is then throttled into the absorber by the expansion

valve, to be further cooled as it picks up a new charge of R-22 coming from the second heat

exchanger. Meanwhile, the hot R-22 vapor driven off in the generator passes to the condenser where

it loses energy and passes into the liquid phase. The liquid R-22 after passing down through the

second heat exchanger experiences a drop in pressure and enters the evaporator as the low portion

of the system to complete the cycle. The reduction in pressure through this valve 2 facilities the

vaporization of R-22 which ultimately effects the heat removal from the environment. The cycle

is completed when the desired cooling load is achieved in the evaporator. Consequently, it can be

Refrigeration Cycles and Systems 199

seen that there are essentially three circuits for the absorption cooling system; (a) the almost pure

R22 circuit – condenser, heat exchanger 2, evaporator, and heat exchanger 2 to the absorber, (b) the

strong solution circuit – absorber, pump, and heat exchanger 1 to the generator, and (c) the weak

solution circuit – from the generator through the heat exchanger 1 and into the absorber.

Results and Discussion. All the required parameters for the design of a solar powered absorption

cooling system were obtained by using the calculation techniques for the individual components.

In the theoretical calculations for the design of an ARS, an enthalpy–concentration diagram for the

R-22 and DMETEG pair (Figure 4.30) was used. Some obtained results are as follows: T

...T

(

◦

C) =39, 30, 30, 65, 90, 87, 53, 82, 40, 27, −5, −5, 20; T

= 20

◦

C; P

= 4.8bar;P

= 16 bar;

= 4.65 kW; Q

= 5.0kW;Q

= 7.5kW;Q

= 7.6kW;W

= 0.25 kW; COP = 0.6; m

290.6 kg/h; m

= 90.0 kg/h; m

= 200.6 kg/h. In addition, some results related to the collector

system are as follows: L = 40

◦



; S = 36

◦

; Q

= 4313 kcal/m

·day; R = 1.08; E

= 0.65; E

0.60; E

= 0.80; T

= 45

◦

C; T

= 18.7

◦

C; Q

= 1450 kcal/m

·day; m = 420 kg/day; C

= 1

kcal/kg·

◦

C; F

= 8m

; n = 4; V = 0.48 m

; G = 8 L/min; ε

= 0.06 m

; f

= 1 L/min·m

;

= 1,1046 kcal/day. Note that Figure 4.31, known as the enthalpy–concentration diagram of

the R-22/DMETEG pair, was used to ﬁnd the enthalpy and other relevant data. This graph was

originally developed by Jelinek, Yaron and Borde 1980.

Under a working regime, the measured experimental and theoretical values were used to deter-

mine the values of COP which were plotted against the variations in evaporator temperature as

shown as Figure 4.32a. In this graph, in order to eliminate the negative evaporator temperatures,

the term (T

− T

) was used where T

is the initial evaporator temperature whose average value

DMETEG–R22

Temperature (°C)

Pressure (kg/cm

)

165

160

155

150

145

140

135

130

125

120

115

110

105

100

Enthalpy, h (kcal/kg)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.

Weight fraction (x)

120

110

100

Figure 4.31 Enthalpy-weight fraction (concentration) diagram for the pair of R-22 and DMETEG (Dincer

et al., 1996) (Reprinted with permission from Elsevier Science).

200 Refrigeration Systems and Applications

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

COP

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

COP

20 22 24 26 28 30

−T

(°C) T

(°C)

Theoretical

Experimental

Theoretical

Experimental

80 82 84 86 88 90 92 94 96 98 100

(a) (b)

Figure 4.32 Variation of COP versus (a) evaporator temperature and (b) generator temperature (Dincer et al.,

1996) (Reprinted with permission from Elsevier Science).

was around 20

◦

C. The experimental evaporator temperature values which were found to be lower

than that of the theoretical ones clearly indicated some heat losses in the system. Similarly, when

the actual and theoretical values of COP were plotted against the variation of generator temperature

(Figure 4.21b) a slight increase in COP with increasing temperature was observed. This indicated

that the operating performance of the system can be considered stable in that range of temperatures.

Although the cost of the system was higher than that of compression refrigerators of equivalent

performance (almost double), it is believed that the basic design is amenable to low cost mass

production. This makes it attractive for widespread use especially in developing countries. In

addition, it must be remembered that this system was developed and built as a one-off research

prototype. Reﬁnement of the manufacturing process and economical selection of materials will

further reduce the cost per unit.

Example 4.8

In this example, we present one of our works (Dincer and Dost, 1996) on energy analysis of a lithium

bromide–water ARS to determine heat and work capacities of the system’s components varying

with the mass ﬂow rates of weak solution. The heat and work capacity expressions developed, based

on optimum operation conditions, are proposed as useful equations for practical design calculations

of lithium bromide–water ARSs.

In the energy analysis, we used the mass and energy balance equations for the system shown in

Figure 4.33, based on the methodology presented above.

The main goal was to ﬁnd simple expressions that can be utilized in design calculation of

lithium bromide–water ARS. We considered some optimum design parameters, such as evaporator

temperature T

= 4.5

◦

C, condenser and absorber temperatures T

= T

= 30

◦

C, heat exchanger

efﬁciency E = 0.9 for two cases of the generator temperatures T

= 90 and 100

◦

C. For comparison

purposes, two temperature values of the generator were investigated. In addition, the concentration

values of LiBr on a mass basis were taken as X

= 0.685 and 0.695 for weak solution, and X

= 0.5

for strong solution to avoid crystallization at the two generator temperatures given above, using

Figure 4.34. Using the optimum conditions, the following temperature and enthalpy values for two