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17
CHAPTER 2
Literature Review
2.1 Introduction
A refrigeration system utilizes work supplied by an electric motor to transfer
heat from a space to be cooled to a high temperature sink (place to be heated). Low
temperature boiling fluids called refrigerants absorb thermal energy to get vaporized in
the evaporator causing a cooling effect in the region being cooled. While comparing
the advantages and disadvantages of various cooling systems, two most important
parameters i.e the operating temperature and the coefficient of performance are of
vital importance in these systems. These systems can be evaluated using energy and
exergy analyses which are based on first and second law of thermodynamics,
respectively and have been described in the previous chapter in detail.
An extensive review of the literature has been done on different refrigeration and
heat pump systems in present chapter. The main idea was to have possible future
direction of research. The literature review has been classified as under:
1. Vapor Absorption Refrigeration Systems.
2. Vapor Compression Refrigeration Systems.
3. Vapor Compression-Absorption Refrigeration Systems.
2.2 Vapor Absorption System
Vapor Absorption system is an attractive method for utilizing low grade energy directly
for cooling. This is an important advantage as against the conventional vapor
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compression system which operates on high grade energy. Another important feature
of these systems is that it does not use any moving component except a very small
liquid pump. Vapor absorption system consists of four basic components viz. an
evaporator, an absorber (located on low pressure side), a generator and a compressor
(located on high pressure side). A refrigerant flows from the condenser to the
evaporator, then via absorber to the generator and back to condenser, while the
absorbent passes from absorber to the generator and back to absorber. For maximum
efficiency, the pressure difference between the low pressure side and high pressure
side is maintained as small as possible. Although, the initial cost of these systems is at
present higher but their operating expenses are often appreciably lower, which can
further be reduced if efficient absorption and distillation can be achieved. Since, the
efficiency of these processes is determined largely by thermodynamic properties of the
refrigerant –absorbent combination, an extensive study of these properties is of utmost
importance in the development of an efficient absorption refrigeration cycle.
A large number of researchers have carried out research in the field of vapor
absorption refrigeration using different working pairs and the most common working
pairs are LiBr-H2O and NH3-H2O. Alizadeh et al [1] carried out theoretical study on
design and optimization of water – lithium bromide refrigeration cycle. They concluded
that for a given refrigerating capacity higher generator temperature causes high
cooling ratio with smaller heat exchange surface and low cost. There is a limiting factor
for water lithium bromide cycles because of the problem of crystallization. Anand and
Kumar [2] carried out availability analysis and calculation of irreversibility in system
components of single and double effect series flow water lithium bromide absorption
19
systems. The assumed parameters for computation of results were condenser and
absorber temperature equal to 87.8oC and 140.6oC for single effect and double effect
systems respectively.
Tyagi [3] carried out the detailed study on aqua-ammonia VAR system and
plotted the coefficient of performance, mass flow rates as a function of operating
parameters i.e. absorber, evaporator and generator temperatures. He showed that
COP and work done are the function of evaporator, absorber, and condenser and
generator temperature and also depends on the properties of binary solution. Ercan
and Gogus [4] showed the irreversibility’s in components of aqua-ammonia absorption
refrigeration system by second law analysis. They calculated the dimensionless
exergy loss of each component, exergetic coefficient of performance, coefficient of
performance and circulation ratio for different generator, absorber evaporator and
condenser temperature. They concluded that aqua-ammonia system needs a rectifier
for high ammonia concentrations but it will lead to additional exergy loss in the system.
They observed the highest exergy loss in evaporator followed by absorber. I was also
concluded that the dimensionalless total exergy loss depends on generator
temperature.
Oh et al [5] investigated a gas fired, air cooled LiBr/H20 double effect parallel
flow type absorption heat pump of 2TR being used as an air conditioner. They
investigated the performance of the absorption heat pump in the cooling mode through
cycle simulation. They obtained the system characteristics depending on the inlet
temperature of air to the absorber, the working solution concentration, the solution
distribution ratio of the mass of the solution into the first generator to he total mass of
20
the solution from the absorber, and the leaving temperature differences of the heat
exchanging components. They concluded that there exists a critical value of the
solution distribution ratio that maximizes the cooling performance of the system.
Aphornratana and Eames [6] investigated single effect water lithium bromide system
using exergy analysis approach. It was shown that the irreversibility in generator was
highest followed by absorber and evaporator.
Bell et al [7] developed a LiBr-H20 experimental absorption cooling system
driven by heat generated by solar energy. The components of the system are housed
in evacuated glass cylinders to observe all the processes. They determined the
thermodynamic performance of the system by applying mass and energy balance for
all the components. Their work was based on the assumption that the working fluids
are in equilibrium and the temperature of the working fluid leaving the generator and
absorber is equal to the temperature of generator and absorber respectively. They
concluded that the COP of the system depends on generator temperature and there is
optimum value of generator temperature at which COP is maximum. They also
concluded that by operating the system at low condenser and absorber temperatures
a satisfactory COP is obtained at a generator temp. as low as 68oC. Horuz [8]
explained the fundamental vapour absorption refrigeration system and carried out
comparative study of such system based on ammonia-water and water lithium bromide
working pairs. The comparison of two systems is presented in respect of COP, cooling
capacity and maximum and minimum pressures. He concluded that VAR system
based on water-lithium bromide is better than ammonia-water. However, problem of
crystallization lies with water-lithium bromide system.
21
Kumar et al [9] studied in detail the exergy variation in solar assisted absorption
system. They found that rise in first generator heat transfer, decreases the heat
transfer in second stage generator. The increase in generator-II temperature
decreases the exergy and energy transfer rates at the condenser. They concluded that
the availability at the devices varies with respect to quality of the device. Talbi and
Agnew [10] carried our exergy analysis on single effect absorption refrigeration cycle
with lithium bromide water as the working fluid pairs. They developed a computer
simulation model based on heat and mass balance, heat transfer equations and
thermodynamic properties. The cycle collects free energy from the exhaust of diesel
engine. They calculated the dimensionless total exergy loss and exergy loss of each
component. They found that the absorber has the highest exergy loss of 59.06%
followed by generator. They concluded that the absorption refrigeration cycle is
effective in demonstrating the advantages of exergy process which are other wise not
accounted in the heat balance method.
Lee and sheriff [11] carried out the second law analysis of a single effect water
lithium bromide absorption refrigeration system. The effect of heat source temperature
on COP and exergetic efficiency was evaluated. However, they did not analyzed effect
of variation in absorber and condenser temperatures and also the effectiveness of
solution heat exchanger was also not specified. Lee and sheriff [12] carried out the
second law analysis of single effect and various double effect lithium bromide water
absorption chillers for chilled water temperature of 7.22oC and cooling water
temperatures 29.4oC and 35oC and computed COP and exergetic efficiency. The
effect of heat source temperature on COP and exergetic efficiency was investigated. In
22
this study, the effectiveness values of solution heat exchangers considered for
analysis has not been specified and their results are only valid for water cooled
systems.
Sozen [13] studied the effect of heat exchangers on the system performance in
an ammonia water absorption refrigeration system. Thermodynamic performance of
the system is analyzed and the irreversibility’s in the system components have been
determined for three different cases. The COP, ECOP, circulation ratio, and non
dimensional exergy loss of each component of the system is calculated. They
concluded that the evaporator, absorber, generator, mixture heat exchanger and
condenser show high non-dimensional exergy losses. They also concluded that using
refrigerant exchanger in addition to mixture heat exchanger does not increase the
system performance. Fernandez-Seara and Vazquez [14] studied the optimal
generator temperature in single stage ammonia – water absorption refrigeration
system. They studied the behavior of this temperature on thermal operating conditions
and system design parameters. They carried out study based on parametric analysis
by developing a computer program and based on the results designed a control
system. The control system developed maintains a constant temperature for the space
to be refrigerated and also control the optimal temperature in the system generator.
De Francisco et al [15] developed and tested the prototype of a 2kW capacity
water ammonia absorption system operating on solar energy for rural applications.
The system also suffered from leakages in different components and need further
improvements. They concluded that the efficiency of the system is very low. The new
and improved prototype has to be developed. Horuz and Callander [16] described the
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experimental investigation of the performance of a commercially available absorption
refrigeration system. The system is natural gas fired with a capacity of 10 kW. They
studied the response of the refrigeration system to variations in chilled water
temperature, chilled water level in evaporator drum; chilled water level flow rate and
variable heat input are presented. They concluded that lower the energy input, lower
will be the cooling effect.
De Lucas et al [17] studied the use of alternative absorbent used in absorption
refrigeration cycles to replace the existing absorbent. New absorbent used is a mixture
of lithium bromide and potassium formate in a 2:1 w/w. The performance of the system
is compared by developing a program. They concluded that less energy is required in
the generator and due to this; waste heat with a temperature of 328.15K is required.
The efficiency of the system is increased and the new absorbent is less corrosive and
less expensive to manufacture. Sencan et al [18] carried out the exergy analysis of a
single effect water lithium bromide absorption refrigeration system and calculated the
exergy losses in the system components. The effect of heat source temperature on
COP and exergetic efficiency was computed. They did not analyze the effect of
variation in absorber and condenser temperature. They concluded that the cooling and
heating COP of the system increases slightly when increasing the heat source
temperature but the exergetic efficiency of the system decreases when increasing the
heat source temperature for both cooling and heating applications.
Kilic and Kaynakli [19] investigated single effect and series flow double effect
vapour absorption systems using energy analysis approach. The effect of different
parameters such as generator temperature, absorber temperature, condenser
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temperature, solution circulation ratio and solution concentration etc had been
investigated by these researchers on COP. Their results revealed that COP of double
effect absorption refrigeration system is higher in comparison to single effect system.
Kilic and Kaynakli [20] used the first and second law of thermodynamics to analyze the
performance of a single stage water lithium bromide absorption refrigeration system by
varying some working parameters. They introduced a mathematical model based on
exergy method. They found that the performance of the ARS increases with increasing
generator and evaporator temperatures but decreases with increasing condenser and
absorber temperatures. They concluded that the highest exergy loss occurs in
generator regardless of operating conditions and therefore it is most important
component of the system.
Gong et al [21] presented the method of product exergy cost for scheme
selection optimization of cooling and heating source system of air conditioning system.
They developed the optimization algorithm which adopts an integrative; multiple
objective decision method with the analysis of the product exergy cost and concluded
that the method is scientific and reliable. Kaynakli and Yamankaradeniz [22] studied
the single effect VA system on the basis of entropy generation method. Kaynakli and
Yamankaradeniz [22] performed calculations for a 10kW cooling load system. The
evaporator and condenser temperature was taken as 4oC and 38oC respectively. The
generator temperature was taken as 90oC. Effectiveness of solution heat exchanger
was assumed as 0.5 and efficiency of pump was assumed equal to 0.9. They
concluded that entropy generation of the generator is an important fraction of the total
entropy generation in the system basically due to the temperature difference between
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the heat source and the working fluid and in order to decrease the total entropy
generation of the system, the generator should be developed
Morosuk and Tsatsaronis [23] used an absorption refrigeration machine to
represent splitting the exergy destruction into endogenous/exogenous and
unavoidable/avoidable parts and this is new development in the exergy analysis of
energy conversion systems. They concluded that advanced exergetic evaluation of an
ARM supplies useful additional information which is not provided by exergy analysis.
The avoidable exergy destruction identifies the potential for improving each system
component. Gomri and Hakimi [24] carried out exergy analysis of double effect lithium
bromide/water absorption refrigeration system. They showed that the performance of
the system increases with increasing LP generator temperature, but decreases with
increasing HP generator temperature. They concluded that the highest exergy loss
occurs in the absorber and in the HP generator and therefore the absorber and HP
generator is the most important component of the double effect refrigeration system.
Gomri [25] carried out the comparative study between single effect and double
effect absorption refrigeration systems. They developed the computer program based
on energy balances, thermodynamic properties to carry out thermodynamic analysis.
They concluded that for each condenser and evaporator temperature, there is an
optimum generator temperature where change in exergy of single effect and double
effect absorption refrigeration system is minimum. Their study showed that the COP of
double effect system is approximately twice the COP of single effect system but there
is marginal difference between the exergetic efficiency of the system. Kaushik and
Arora [26] presented the energy and exergy analysis of single effect and series flow
26
double effect water–lithium bromide absorption system. They developed the
computational model for parametric investigation. Their analysis involves the effect of
generator, absorber and evaporator temperatures on the energetic and exergetic
performance. They concluded that the irreversibility is highest in the absorber in both
systems as compared to other systems. Zhu and Gu [27] used the first and second law
of thermodynamic to analyze the performance of ammonia–sodium thiocyanate
absorption system for cooling and heating applications. A mathematical model based
on exergy analysis was developed. The performance of the system is analyzed using
different operating conditions. They concluded that the cooling and heating COP
increases with increasing generator and evaporator temperatures but it decreases with
increasing condenser and absorber temperatures. Garousi Farshi et al [28] developed
a computational model to study and compare the effects of operating parameters on
crystallization phenomena in three classes of double effect lithium bromide–water
absorption refrigeration systems (series, parallel and reverse parallel) with identical
refrigeration capacities. They concluded that the range of operating conditions without
crystallization risks in the parallel and the reverse parallel configurations is wider than
those of the series flow system. Behrooz and Ziapour [29] carried out thermodynamic
analysis of a diffusion absorption refrigeration heat pipe (DARHP) cycle. A computer
code was developed for an ammonia–water DARHP cycle with helium as the auxiliary
inert gas using EES software. The second law efficiency was examined parametrically
by the computer simulation. They validated the model by comparison with previously
published experimental data for DARHP system. The cycle performance results under
different conditions indicated that the best performance was obtained for the
27
concentration rich solution of 0.35 ammonia mass fraction and the concentration of
weak solution about 0.1. They concluded that the exergy losses in the evaporator,
condenser and dephlegmator were small. Also the second law efficiency increases
with increasing evaporator temperature; and decreases with increasing thermosyphon
temperature.
Khaliq et al [30] carried out first and second law investigation of waste heat
based combined power and ejector-absorption refrigeration cycle using R141b as a
working fluid. Estimates for irreversibilities of individual components of the cycle lead
to possible measures for performance improvement. Results show that around 53.6%
of the total input exergy is destroyed due to irreversibilities in the components, 22.7%
is available as a useful exergy output, and 23.7% is exhaust exergy lost to the
environment, whereas energy distribution shows 44% is exhaust energy and 19.7% is
useful energy output. They concluded that proposed cogeneration cycle yields much
better thermal and exergy efficiencies than the previously investigated cycles and the
current investigation clearly show that the second law analysis is quantitatively
visualizes losses within a cycle and gives clear trends for optimization.
2.3 Vapour Compression System
In vapor compression system there are four major components: evaporator,
compressor, condenser and expansion device. Power is supplied to the compressor
and heat is added to the system in the evaporator, whereas in the condenser heat
rejection occurs. Heat rejection and heat addition are dissimilar to different
refrigerants. A standard vapor compression cycle consists of four processes viz. a
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reversible adiabatic compression from the saturated vapor to the compressor pressure
followed by a reversible heat rejection at constant pressure causing de-superheating
and condensation. This is further extended to an irreversible expansion at constant
enthalpy from saturated liquid to evaporator pressure and there after a reversible heat
addition at constant pressure causing evaporation to saturated vapor.
Keshwani and Rastogi [31] determined the optimum interstage pressure in a
two stage VCR system for refrigerant CFC12. They concentrated their research on
minimization of overall compressor work. Arora and Dhar [32] used the discrete
maximum principle, discusses by Katz (1962), to solve the problem of optimum
interstage pressure allocation in multistage compression systems for R-12, with and
without intercooling between the stages. They concluded that the optimum interstage
pressure approximately equals the geometric means of the condensation and
evaporation pressure but when the flash inter cooler was incorporated, they found a
considerable difference between the geometric means and the optimal pressure
values. Prasad [33] determined the optimum interstage pressure in a two stage vapour
compression refrigeration system for the refrigerant R-12 with a view to maximize the
COP. They concluded that the inter-stage temperature of a two-stage refrigeration
cycle is given by the geometric mean of the condensation and evaporation
temperatures.
Kumar et al [34] explained a method of carrying out exergy analysis on a
vapour compression refrigeration system using R-11 and R-12 as refrigerants. They
presented the exergy-enthalpy diagrams to facilitate the analysis. They explained the
procedure to calculate various losses in different components of the system.
29
McGovern and Harte [35] presented an exergy method for compressor analysis. This
is used to find and quantify defects in the use of compressor shaft power and will lead
to the improvement in design of the compressor. The exergy destruction and its
location are identified. They analyzed the refrigerant compressor using R-12 as
refrigerant. They presented the graphs of the instantaneous rates of exergy
destruction. They concluded that it is particularly suitable for applications in computer
simulation of compressors and provides a sound basis for design optimization.
Zubair and Khan [36] showed that the optimum interstage pressure for a two
stage refrigeration system can be approximated by the saturation pressure
corresponding to the arithmetic mean of the condensation and evaporation
temperatures. Zubair et al [37] found that optimum interstage pressure for refrigerant
R-134a for maximum COP of the system was close to the saturation pressure
corresponding to the arithmetic mean of the refrigerant condensation and evaporation
temperatures. They showed that the system irreversible losses are lowest at an
intermediate saturation temperature near to arithmetic mean of the condensation and
evaporation temperatures. Aprea et al [38] reported that vapour compression
refrigeration systems are widely used for cold storage and super market refrigeration.
The suitable working fluid for these applications is the refrigerant R-502 which is an
azeotropic mixture of refrigerants R-22 and R-115.
Aprea et al [39] experimentally evaluated the general characteristics and
system performances of substitutes for R-502 in a refrigeration plant. They examined
different refrigerants such as R-402A, R-402B, R-403B, R-408A, R-404A, R-407A and
FX-40. All the refrigerants showed performances very close to those of R-502 except
30
R-403B whose COP was found to be about 8% lower than that of R-502. They
concluded that the above refrigerants can be used as substitutes to R-502. Doring et
al [40] carried out an experimental study of R-507 to measure thermodynamic data.
They presented the data in the form of mathematical correlations. Their theoretical
results show that the compressor discharge temperature for R-507 was approximately
8K below in comparison to R-402. Camporese et al [41] experimentally investigated
the mixtures such as HC290/HFC134a, HFC125/HC290/HFC134a,
FC125/HFC143a/HC290, HFC125/HFC143a/HCC270 and HFC32/HFC125/HFC143a
for their influence on the solubility of various lubricant oils by measuring critical
solubility temperatures. The experiments were conducted to compare refrigerating
capacity, COP, discharge temperature and mass flow rates. The mixtures selected for
new units were the mixture of HFC143a/HC290/R-22 showed the best performance
and its COP and cooling capacity were found to be higher in comparison to R-502
Nikolaidis and Probert [42] used exergy analysis to investigate the behaviour of
two stage compound compression cycle with flash intercooling using R-22 as
refrigerant by varying the condenser saturation temperature and evaporator saturation
temperature from 298 to 308 K and 238 to 228 K respectively. They determined the
effect of temperature change in condenser and evaporator on plants irreversibility rate.
They concluded that the changes in the temperatures of condenser and evaporator
significantly effect the plants overall irreversibility and therefore the system needs
optimization. Sami and Desjardins [43] carried out performance evaluation of R-407B,
R-507, R-408A and R-404A as substitutes to R-502. Their results revealed that R-
408A blend has a superior performance than R-502 but it is characterized by high
31
discharge pressure. Ratts and Brown [44] used entropy generation minimization
method to compute the optimum reduced intermediate temperature for R-134a in a
two stage VCR system. They concluded that this method gives better results than
geometric means method for evaluation of interstage temperature charge pressure
compared to R-502. Rakehesh et al [45] carried out experimental study on a heat
pump with different refrigerant R-22, R-407C and R-407A. They concluded that R-
407C heat pump chiller systems offered higher exergy efficiency than those operating
with R-22 and in the case of R-407A systems; the exergy efficiency was higher than
that of HCFC at condensing temperatures less than 50oC.
Aprea et al [46] studied the performance of a VCR experimental plant both as
water chiller and as a heat pump using R-22 and its substitute R-417A. The use of
R-417A does not require lubricant change and equipment redesign. The results
showed that the COP and exergetic efficiency of the plant is higher for R-22 than
R-417A. Aprea and Renno [47] experimentally investigated the energetic and
exergetic performance of a VCR plant for cold storage application using both R-22 and
its substitute R-417A. The results showed that COP was 15% greater than R-417A
where as exergy destruction for R-417 was greater than R-22. Areaklioglu et al [48]
numerically calculated the rational efficiency and components based irreversibility
ratios of a cooling system based on the second law of thermodynamics using HFC and
HC based pure refrigerants such as HFC32, HFC125. HFC134a, HFC143a, HFC152a,
HC290, HC600a and there binary and turnery mixtures, along with CFC12, R-22 and
R-502. The effect of temperature glide, occurring at the condenser and evaporator, on
the rational efficiency of the cooling system was evaluated. The irreversibility in
32
condenser was found to varying between 40-55% of the total irreversibility. The results
also suggest that for both binary and ternary mixtures the rational efficiency increases
against temperature glide.
Xuan and Chen [49] carried out an experimental study of a ternary near
azeotropic mixture of HFC161 as an alternative to R-502. Without any modification to
system components, experimental tests were performed on a vapour compression
refrigeration plant with a reciprocating compressor which was originally designed to
use R-404A, a major substitute for R-502. The experimental results under two different
rated working conditions indicated that the pressure ratios of this new refrigerant were
nearly equal to those of R-404A. Under lower evaporative temperature, its COP was
almost equal to that of R-404A and its discharge temperature was found to be slightly
higher than that of R-404A, while under higher evaporative temperature, its COP was
found was found to greater than that of R-404A and its discharge temperature was
lower than that of the latter. This new refrigerant achieved a high level of COP and
hence was considered as a promising retrofit refrigerant to R-502.
Park and Jung [50] studied two pure hydrocarbon refrigerants, R-1270
(Propylene) and R-290(Propane) and three binary mixturescompared to R-1270, R-
290 anf R-152a were tested in a refrigerating bench tester with a scroll compressor I
an attempt to substitute R-502. The results showed that all refrigerants tested had 9.6
to 18.7% higher capacity and 17.1 to 27.3% higher COP than R-502. There was
problem with mineral oil. They concluded that these alternative refrigerants offer better
system performance and reliability than R-502. They studied the thermodynamic
performance of two pure hydrocarbons and seven mixtures composed of propylene
33
(R-1270), Propane (R-290), HFC152a and dimethyl ether (RE170,DME) in an attempt
to substitute R-22 in residential air conditioners. The mixtures are all near azeotropic
having the gliding temperature difference of less than 0.6oC. Test results revealed that
the COP of these mixtures was up to 5.7% higher than that of R-22. The compressor
discharge temperatures were reduced by 11-17oC with these fluids. There was no
problem found with mineral oil since the mixtures were mainly composed of
hydrocarbons. These fluids provide good thermodynamic performance with reasonable
energy saving.
Arora et al [51] carried out parametric investigation of actual vapour
compression refrigeration cycle in terms of COP, exergy destruction and exergy
efficiency for R-22, R-407C and R-410A by developing a computational model. The
results showed that COP and exergy efficiency for R-22 are higher in comparison to
R-407C and R-410A. It was concluded that R-410A is better alternate as compared to
R-407C with high coefficient of performance and low exergy destruction ratio when
considering for refrigeration applications. For air conditioning application R-407A is
better alternative than R-410A. Park et al [52] experimentally tested the
thermodynamic performance of R-433A, R-432A for possible replacement to R-22 in a
heat pump bench tester under air conditioning and heat pumping conditions. The test
results showed that the COP of R-433A was 4.9-7.6% higher than that of R-22 while
the capacity of R-433A was found to be 1.0-5.5% lower than that of R-22 under both
test conditions. The COP of R-432A was found to be 8.5-8.7% higher than HCFC and
its refrigerating capacity was 1.9-6.4% higher than that of R-22 under both test
conditions. The compressor discharge temperatures of R-432A and R-433A were
34
lower than that of R-22. The amount of charge required for both of these refrigerants
were 50-57% lower than that of R-22 due to their low density. They concluded that
these refrigerants are good long term environmental friendly alternatives to replace
R-22 in residential air conditioners and heat pumps due to their excellent
thermodynamic and environmental properties with minor adjustments. However, they
did not comment on the compatibility of these refrigerants with lubricating oil.
Palm [53] reported tat vapor pressure curves of the propane and propene are
quite similar to those of R-22 and ammonia, indicating that the application areas would
be same. Recently, air conditioning provided by ammonia refrigeration systems has
found application on college campuses and office parks, small scale building such as
convenience stores, and larger office buildings. These applications have been
achieved by using water chillers, ice thermal storage units and district cooling systems.
Pearson [54] specified that ammonia is widely used in industrial systems for food
refrigeration, cold storage, distribution ware housing and process cooling. It has more
recently been proposed for use in applications such as water chilling for air
conditioning systems. Bhattacharyya et al [55] carried out analysis of an
endoreversible two-stage cascade cycle and obtained an optimum intermediate
temperature for maximum exergy and refrigeration effect. They developed a
comprehensive numerical model of a transcritical CO2-C3H8 cascade system. The
cycle was optimized with the operating temperatures and the results obtained were in
line with the simulation results.
Arora and Kaushik [56] presented a detailed exergy analysis of an actual
vapour compression refrigeration cycle. They developed a computational model for
35
computing coefficient of performance, exergy destruction, exergetic efficiency and
efficiency defect for different refrigerants. They concluded that R-507A is a better
substitute to R-502 than R-404A and the efficiency defect in condenser is highest and
lowest in liquid vapour heat exchanger for the refrigerant considered. Mafi et al [57]
carried out exergy analysis for multistage cascade low temperature refrigeration
systems used in olefin plants. They developed the equations of exergy destruction and
exergetic efficiency for heat exchanger, compressors and expansion valves. The
relations for total exergy destruction in the system and the system overall exergetic
efficiency are obtained. They also developed the expression for minimum work
requirement for cascade low temperature refrigeration used in olefins plants. They
determined the overall exergetic efficiency to be 30.88%
Dopazo et al [58] reported the analysis of the parameters in design and
operation of a CO2/NH3 cascade cooling system and their effect on system COP and
exergetic efficiency. They carried out the analysis based on general mathematical
model which was validated using experimental results. They concluded that for
specific installation, the isentropic efficiency for each compressor in cascade system
should be determined as accurately as possible from the manufacturer or experimental
data in order to obtain reliable values for the optimum CO2 condensing temperature
and maximum COP. Miguel Padilla et al [59] did exergy analysis of the impact of direct
replacement of R-12 with zeotropic mixture R-413A on the performance of a domestic
vapour compression refrigeration system originally designed to work with R-12 using a
simulated analysis model. They concluded that the overall energy and exergy
performance of system working with R-413A is better than R-12.
36
Qureshi and Zubair [60] investigated the performance degradation due to fouling in a
vapor compression cycle for various applications. The investigation was carried out
using refrigerants R-134a, R-410A and R-407C. The first law analysis indicates that
R-134a always performs better unless only the evaporator is being fouled. However,
the second law shows that R-134a performs the best in all cases. While considering
the second set of refrigerants i.e. R-717, R-404A and R-290. The first law shows that
R-717 always performs better unless only the evaporator is being fouled. In contrast to
this, from a second-law standpoint, the second-law efficiency indicates that R-717
performs the best in all cases. Volumetric efficiency of R-410A and R-717 remained
the highest under the respective conditions studied. Furthermore, performance
degradation of the evaporator often has a larger effect on compressor power
requirement while that of the condenser has an overall larger effect on the COP.
Sayyaadi and Nejatolahi [61] considered a cooling tower assisted vapor
compression refrigeration machine has for optimization with multiple criteria. Two
objective functions, the total exergy destruction of the system and the total product
cost of the system are considered as thermodynamic and economic criterion
respectively, have been considered simultaneously. They developed the
thermodynamic model based on energy and exergy analyses and an economic model
according to the Total Revenue Requirement (TRR) method. The exergetic and
economic results obtained for three optimized systems have been compared and
discussed. They concluded that the multi-objective design more acceptably satisfies
generalized engineering criteria than other two single-objective optimized designs.
37
Zhu and Jiang [62] developed a refrigeration cycle which combines a basic
vapor compression refrigeration cycle with an ejector cooling cycle. The ejector cooling
cycle is driven by the waste heat from the condenser in the vapor compression
refrigeration cycle. The additional cooling capacity from the ejector cycle is directly
input into the evaporator of the vapor compression refrigeration cycle. The governing
equations are derived based on energy and mass conservation in each component
including the compressor, ejector, generator, booster and heat exchangers. They
concluded that the COP is improved by 9.1% for R-22 system.
Qureshi and Zubair [63] investigated the performance characteristics due to use
of different refrigerant combinations in vapor compression cycles with dedicated
mechanical sub-cooling. For basic designs, R-134a used in both cycles produced the
best results in terms of COP, COP gain and relative compressor sizing. In retrofit
cases, considering the high sensitivity of COP to the relative size of heat exchangers
in the sub-cooler cycle and the low gain in COP obtained due to installation of a
dedicated sub-cooling cycle when R-717 is the main cycle refrigerant, it seems that
dedicated mechanical sub-cooling may be more suited to cycles using R-134a as the
main cycle refrigerant rather than R-717. With R-134a as the main cycle refrigerant, no
major difference was noted, by changing the sub-cooler cycle refrigerant, in the
degradation of the performance parameters such as COP and cooling capacity, due to
equal fouling of the heat exchangers.
A cascade system consists of two independently operated single-stage refrigeration
systems: a lower system that maintains a lower evaporating temperature and
produces a refrigeration effect and a higher system that operates at a higher
38
evaporating temperature. These two separate systems are connected by a cascade
condenser in which the heat released by the condenser in the lower system is
extracted by the evaporator in the higher system.
Wang et al. [64] examined the potential of a double-stage coupled heat pumps heating
system, whereby an air source heat pump was coupled to a water source heat pump.
Comparatively, they found that such a coupling process improved energy efficiency
ratio by 20% compared to a purely air source heat pump.
Satoru Okamoto [65] carried out the analysis of a heat pump system with a latent heat
storage utilizing sea water installed in an aquarium. In this study a heat pump installed
in an aquarium with latent heat storage potential using sea water is analyzed. This
installation is quite helpful in maintaining the indoor conditions at constant temperature
and humidity. In this study the comparison of the actual operating characteristics and
efficiency of sea water source heat pump is carried out with two assumed conventional
systems that are, an air source heat pump without ice storage and an oil-fired
absorption refrigeration system. The results indicate that cost of generation of heat
energy with sea water heat pump is significantly lower than that of air-source heat
pump and the oil-fired heat pump and. The actual operating costs of sea water heat
pump is 42% lesser than the air fired and oil fired heat pumps and the energy
consumption for the generation of heat is also 19% lesser for the sea water heat
pump. Also the emission of the harmful gases like CO2 is also lesser for sea water
heat pump when compared to air fired and oil fired heat pumps.
Zhen et al. [66] carried out a study on the use of the ocean energy or sea water as
heat source as well as sink for district cooling and heating in Dalin city of China. They
39
suggested that coastal areas are the best suitable location for the use of sea water
source heat pump technology for both cooling as well as heating. The government
helped in the commissioning of the plants both for heating as well as cooling with the
capacity of 68 MW and 76 MW respectively for this study. In this study the economic,
energy and environment impacts of the sea water source heat pump technology are
analyzed. In this study, the system is compared with coal fired heating system and the
conventional air-conditioning system in terms of the economic, energy and
environment impacts. The economic impacts include the analysis of money through
series of sensitivity cases which needs to be invested in different forms like annual
cost, net present value used for the calculation of coal price, electricity price and
interest rate of loans. The energy effects include the analysis of change in the sea
water temperature which has been done with the simulation study of sea water
temperature field with a two dimensional convection-diffusion equation. The results of
the study indicates that the sea water source heat pump can be used for both heating
as well cooling applications and has a great potential if used in other locations
depending on the geographical conditions and local environment.
Shu Haiwen et al. [67] carried out the study of the energy saving judgment of electric-
driven sea water source heat pump district heating system over boiler house district
heating system. They concluded that for renewable energy utilization system, the
electricity driven heat pumps are gaining popularity, but the energy saving condition is
still not clear. In this study, an expression of the critical COP of the heat pump system
for energy saving is derived through the comparison of the system and conventional
boiler house district heating system in the energy consumption aspects. Also, the
40
actual COP values of the heat pump unit are calculated by the experimental data
regression model based on the details from the supplier of the heat pump. The
comparison of the values of both COP’s critical as well as actual brings out a judgment
on the energy saving aspect of an electric driven sea water heat source pump for
district heating. The results also indicate that both the heating radius and the natural
conditions of sea water are the most important factors to determine the energy
efficiency of the system. The results indicate that selection of sea water area should
be done in a way that it should have the largest sea water temperature difference that
could be utilized thereby decreasing the water head of the sea water pump. The type
of fuel used in the boiler greatly influences the critical COP value of the heat pump.
2.4 Vapor Compression-Absorption System
Vapor Compression-absorption heat pump/refrigeration cycle represents a
cycle in which vapor is mechanically compressed, absorbed and then desorbed using
a liquid solution cycle. These systems may be considered as hybrid systems between
conventional vapor compression and vapor absorption systems. The hybrid vapor
compression/absorption heat pump cycle combines two well known heat pump
concepts, the compression heat pump and the absorption heat pump. It uses a mixture
of refrigerants as the working fluid, one as the absorbent and the other as the
desorbent. A key advantage of the hybrid heat pump is the extended range of
temperatures available for a mixture compared to pure refrigerants. This is the effect of
the reduced vapor pressure obtained for a refrigerant in a mixture with less volatile
component. Another advantage is the gliding temperature obtained in the absorber
41
and desorber. It reduces irreversibility during heat exchange process between working
fluids and results in improved system performance.
Pourreza-Djourshari and Radermacher [68] presented the performance
calculation of two vapour compression heat pump cycles, one with single stage
solution circuit and the other with two stage solution circuit. The working fluid chosen
was R-22-DEGDME. They found that both cycles show a significant increase in COP
as compared to R-22. The results indicate that there is potential of control capacity by
a ratio of 7:1, energy saving up to 50% and significant reduction in pressure ratio
compared to conventional R-22 cycle. Radermacher [69] examined the performance of
vapour compression heat pump cycle with desorber/absorber heat exchange working
on R-22-R-113 mixture using successive substitution method. The results showed an
improvement in the cooling COP by 57% and a reduction in pressure ratio by 69%
compared to a conventional R-22 cycle. Stroker and Trepp [70] presented the first
simulation model which includes the calculation of the overall heat transfer resistance.
The heat transfer resistance has been calculated as a function of the mass flow rate
for the working pair NH3-H2O from the experimental data. They presented design and
experimental results of a compression heat pump with solution circuit. The test plant
heats water from 40 to 70oC and cools water from 40 to 15oC. A COP of 4.3 was
measured and an energy saving of 23% was achieved.
George et al [71] studied the performance of compression-absorption heat
pump working on R-22-Dimethyl formamide (DMF) through thermodynamic analysis.
The heating COP, concentration difference and the circulation ratio are calculated by
varying compression ratio and operating temperatures at the absorber and desorber.
42
The assumptions taken are that the absorbent does not evaporate in the considered
temperature range to necessitate rectification; equilibrium conditions exist at the exit of
absorber and desorber; effectiveness of heat exchanger is 100%; isentropic
compression in compressor; isenthalpic expansion in pressure reducing valve and no
heat losses and pressure drops in various components etc. They concluded that at
certain operating conditions COP as high as 6 and temperature lift as high as 60oC
can be achieved. Amrane et al [72] developed two simulation models, one for vapour
compression cycle with single stage solution circuit and another for vapour
compression cycle with two stage solution circuit utilizing NH3-H2O mixture. The
analysis of heat exchangers has been carried out by using UA values as input to
program.
Herold et al [73] analyzed a hybrid refrigeration cycle which combines a
mechanical compressor and an absorption cycle using single evaporator. The analysis
involves using the output from internal combustion engine efficiently. LiBr-H2O is used
as working fluid and the cycle has been analyzed assuming oil free compressor.
Although, these types of compressors are available, they are rarely used due to their
high capital cost and low isentropic efficiency. High initial cost and low performance
results in poor economics for the hybrid cycles. Some other assumptions taken in the
analysis are no pressure drops in heat exchangers and pipes; all phases are in
thermodynamic equilibrium. In this analysis also, only internal behavior of the cycle
has been studied and external temperatures are not taken into account.
Ahlby et al [74] carried out optimization study on the compression-absorption
cycle operating on NH3-H2O mixture. The improvement in cycle performance which is
43
gained by optimizing the temperature gradient in the absorber is considerable
particularly for situations with small external temperature gradient. The assumptions
taken in the analysis are: saturated conditions at the desorber and absorber outlets;
adiabatic absorption and desorption in the first part of the absorber and desorber,
respectively, until equilibrium is reached; constant UA values for heat exchanger; no
pressure drops and heat losses. The optimum point of operation is found by studying
the changes in the compressor and pump and the heat loss obtained in the solution
heat exchanger with the working conditions. They concluded that for each external
situation, an optimum working condition can be found. The improvement in cycle
performance gained by optimizing the temperature gradient in the absorber is
considerable. A comparison of performance with the vapour compression cycle shows
that compression-absorption cycle is better or equally good.
Ahlby et al [75] studied the performance of compression-absorption heat pump
with the ternary working fluid NH3-H2O-LiBr. At 60% by mass salt concentration,
ternary mixture showed 10% better cycle performance than binary fluid NH3-H2O. The
calculations are uncertain since the properties of such mixtures are estimated from
properties for NH3-H2O and NH3-H2O-60%LiBr and have not been experimentally
validated. Results indicate that the best mixture would be a solution with a salt content
of about 40-50% by mass. Riffat and Shankland [76] described the integration of
different types of absorption systems and vapour compression system. They analyzed
the performance of such systems using various refrigerant/absorber pairs. Their study
is concerned with the intermittent absorption system, intermittent absorption/vapour
compression system and combined intermittent absorption/vapour compression
44
system. They concluded that integrated compression absorption systems could
provide higher COP than individual systems
Rane et al [77] compared the performances of four versions of two stage
vapour compression heat pump with solution circuits. This represents a cascade
system. They developed a computer simulation model based on heat and mass
balances of each component. For heat exchangers, calculations, UA values have been
taken as input parameter. Performance of the cycles has been compared and it has
been found that the cycle with bleed line and desuperheater has 40-50% higher COP
than the cycle with rectifier. Various parameters, viz., cooling COP, solution heat
exchanger effectiveness, pressure ratio, temperature glides in the absorber and
desorber, and low temperature desorber load have been studied as a function of weak
solution concentration. The results indicate that the above system can work at
temperature above 100oC and achieve a temperature lift of more than 100K.
Groll and Radernacher [78] presented the simulation model for the vapour
compression cycle with single stage solution circuit and cycle with desorber/absorber
heat exchange. The working pair used was CO2-acetone and R-23-DEGDME. It has
been found that the mixtures CO2-acetone and R-23-DEGDME are not suitable for
higher absorber temperatures in heat pump applications due to low COP and high
absolute pressures compared to that for NH3-H2O mixture. Contrary to this, high COP
and low absolute pressures were obtained with CO2-acetone and R-23-DEGDME
compared to NH3-H2O mixture for low desorber temperature in refrigeration
applications. It has also been found that for temperature lifts below 70K, the vapour
compression cycle with single solution circuit is better compared to the cycle with
45
desorber/absorber heat exchange because the former gives more COP and capacity
at lower pressure ratio but for temperature lifts above 70K, the cycle with
desorber/absorber heat exchange was found better than the cycle with single stage
solution circuit.
Itard and Machielsen [79] surveyed the problems encountered when modeling
compression/resorption heat pumps. Their design showed that LMTD method cannot
be used for modeling of heat exchanger and for COP calculations when working with
large temperature glides. They concluded that a mixture can be more advantageous
than a pure refrigerant. They concluded that for certain external conditions, an
optimum overall concentration exists which is the determining factor for the COP of the
system. Ayala et al [80] carried out simulation study of ammonia/lithium nitrate
absorption/compression refrigeration cycle. They modeled a cycle over a range of
proportions from 0 to 100% mechanical vapour compression. They considered
different power generation and distribution efficiencies in deriving the primary energy
ratio. The main consideration for the hybrid model was the assumption that the heat
losses were zero, except in the generator, the pump work was not considered. They
concluded that it is possible to achieve up to a 10% increase in overall efficiency using
combined absorption/compression refrigeration systems.
Groll [81] presented the simulation results of vapour compression cycles with
solution circuit for the working pair carbon dioxide and acetone. The two cycles
investigated are vapour compression cycles with single stage solution circuit and
vapour compression cycles with desorber/absorber heat exchange and parameters
studied are circulation ratio, mass concentration, temperature level in desorber and
46
temperature lift between heat source and sink. Itard [82] carried out experimental and
simulation work on wet compression cycle and found that wet compression cycle is
better cycle because it gives better COP that solution recirculation cycle. Tarique and
Siddiqui [83] compared the performances and economic analysis of the combined
absorption/compression cycle using NH3-NaSCN solution and pure ammonia in the
compression cycle under various operating conditions. They concluded that the capital
and running costs are highly reduced while working with NH3-NaSCN as compared to
the cycle with pure ammonia.
Arun et al [84] carried out analysis of a single stage compression-absorption
heat pump using R-134a-dimethyl acetamide. Effect of variation in suction and
discharge pressures and generator and absorber temperatures on circulation ratio,
discharge temperature and heating COP have been studied. Results of the analysis
show that at low pressure ratios and high temperature lifts, compression-absorption
heat pump exhibits better performance than compression heat pump. For compression
absorption system the discharge temperature remains constant with the increase in
heat delivery temperature for a given pressure ratio and solution concentration
whereas for vapour compression system discharge temperature varies almost linearly
and increases sharply as the great delivery temperature reaches the critical
temperature of the refrigerant.
Swinney et al [85] investigated away of manipulating the composition change of
a refrigerant mixture using two components of similar volatility in order to reduce the
compression ratio. They examined the use of composition change with a mixed
refrigerant to achieve a temperature lift. They also examined the possibility of
47
integrating the column into a closed cycle along with implication of this in energy
requirements. They concluded that the performance is shown to be comparable to
conventional absorption refrigeration units. The cycle is able to use heat sources
below 100oC as input to the distillation column. Satapathy et al [86] carried out
thermodynamic analysis of a compression-absorption heat pump working on R-22-
DMETEG for simultaneous heating and cooling. It is found that by operating the
system at slightly higher discharge pressures than normal, excellent performance is
achieved despite the required large temperature lift.
Fernandez-Seara et al [87] investigated a compression absorption cascade
refrigeration system. The results were computed for refrigerants carbon-dioxide and
ammonia in the compression stage and ammonia water in absorption stage. It is
shown that the intermediate temperature level is an important design parameter that
causes an opposite effect on the COP of the compression and absorption systems.
Infante Ferreira et al [88] investigated the use of twin screw oil free compressor
operating under wet compression conditions in an ammonia–water compression
absorption heat pump cycle. The compressor performance is assessed with respect to
the influence of the location of liquid intake, injection angle and mass flow rate of the
injected liquid on compression performance. Labyrinth seals are used to separate the
oil free side from the lubricated side. They concluded that there is significant impact of
the liquid injection location and due to this the isentropic efficiency increases from 5 to
50% in the model and 10 to 35% in the experiment. The labyrinth leakage flow is
substantial and has a very large impact on the compression performance.
48
Kairouani and Nahdi [89] developed a novel combined refrigeration system and
discuss the thermodynamic analysis of the cycle. The possibility of using geothermal
energy for hybrid system is studied. They selected three working fluids R-717, R-22
and R-134a for the conventional and ammonia–water Pair for the absorption system.
The geothermal temperature source is in the range of 343-349K and the results show
that the COP of a combined system is significantly higher than that of a single stage
refrigeration system. The system presents an opportunity to reduce the continuously
increasing electrical energy consumption. Satapathy et al [90] carried out a
comparative thermodynamic investigation on R-22-E181 and R-134a-E181 working
pairs for vapour compression-absorption system for cooling and heating applications.
Results show that R-134a-181 working pair gives better performance at lower solution
concentration and lower system capacity. At higher solution concentration and higher
system capacity R-22-E181 is slightly better than R-134a-E181. They concluded that
since R-22 can be used for some more years, it may be considered as it gives higher
volumetric capacities. They also concluded that the actual performance is poor due to
use of non-optimized components.
Garimella et al [91] analyzed a novel cascaded absorption/vapor-compression cycle
with a high temperature lift for a naval ship application. A single-effect LiBr–H2O
absorption cycle and a subcritical CO2 vapor-compression cycle were coupled together
to provide low-temperature refrigerant −40°C for high heat flux electronics applications,
medium-temperature refrigerant 5°C for space conditioning and other low heat flux
applications, and as an auxiliary benefit, provide medium-temperature heat rejection
∼48°C for water heating applications. They developed a thermodynamic model to
49
analyze the performance of the cascaded system, and parametric analyses were
conducted to estimate the performance of the system over a range of operating
conditions. The performance of the cascaded system was also compared with an
equivalent two-stage vapor-compression cycle. This cycle was found to exhibit very
high COPs over a wide range of operating conditions and when compared to an
equivalent vapor-compression system, was found to avoid up to 31% electricity
demand. Yari et al [92] studied and compared the GAX and GAX hybrid absorption
refrigeration cycles from the viewpoint of both first and second law of thermodynamics.
They performed the exergy analyses in order to calculate the total exergy destruction
rate within the cycles and also reveal the contribution of different components to the
destructions. They concluded that in both cycles the generator temperature (Tgen) has
more influence on the second law efficiency whereas, the coefficient of performance
(COP) of the cycles are comparatively less affected by this temperature. An increase
of about 75% in the second law efficiency of the GAX cycle was found as the
generator temperature was varied from 400 to 440 K. With this variation of the
generator temperature, the increase in the corresponding COP was around 5%. In
addition, compared to that in the GAX cycle, the maximum value of exergetic efficiency
in the GAX hybrid cycle occurs at a slightly higher value of Tgen.
Zheng and Meng [93] studied the thermodynamic mechanism of the hybrid
refrigeration cycle. They proposed the two fundamental concepts, which are the
ultimate refrigerating temperature (or the ultimate temperature lift) and the behavior
turning. They investigated the impact of compressor pressure increasing on the cycle
performance. The key-parameters include the concentration difference, the circulation
50
ratio of working fluid, etc. They showed that the refrigeration cycle performance varies
with the change of compressor outlet pressure and depends on which one achieves
dominance in the hybrid refrigeration cycle, the absorption sub-system or the
compression sub-system.
2.5 Conclusions of Literature Review
A comprehensive review of the literature on Vapour Absorption Systems,
Compression-Absorption System and Vapour Compression System has been carried
out on various aspects of energy analysis, the type of cycles analyzed, working pairs
used and exergy analysis. With regards to vapour absorption cycles, it is found that
mostly the studies are carried out on large capacity systems and the investigation had
been carried out with in a limited range of system design parameters. The literature on
small vapour absorption systems is scant and very few studies have been done on
smaller systems. The above studies are simulation studies.
Regarding compression-absorption systems studies have been carried out by
many researchers mostly analytically and experimentally. The investigations have
been done on wet compression cycles which eliminated the need of solution pump.
The literature provides details with regard to the applications of this cycle. However the
literature on exergy analysis of such systems is scant. In CA systems, refrigerant –
absorbent mixtures are used as working fluids which provide temperature gradient
profiles in the absorber and desorber. Literature reveals that NH3-H2O is the most
suitable working fluid due to its high latent heat and excellent heat and mass transfer
properties.
51
Literature review revealed that thermodynamic optimization on compressor-
absorption system was carried out to find optimum working condition for a given
external condition. The temperature gradient in the absorber is optimized. The
literature reveals that cost optimization of the system is essential to minimize the cost
as this system is more capital intensive than the conventional VC and VA system. With
regard to vapour compression systems, literature review revealed that natural
refrigerants such as ammonia, propane, propylene are halogen free and are safe for
the environment. Many researchers have carried out theoretical and experimental
investigations on alternative refrigerants. Much is talked about the replacement of
R-22 but proper substitutes are still to be found out. The literature on exergy analysis
of vapour compression refrigeration system is available but the exergy analysis of
such system with variable refrigerant charge is not reported.
In view of the increase in the cost of our existing resources, the advantage of
minimizing losses in the use of this energy is very important and essential. Exergy
analysis is a prime area for effective improvement of the systems. In the present work
energy and exergy analysis of the refrigeration and heat pump systems is done in
order to improve the system thermodynamically. The main aim of the study is to locate
for components in the system for maximum irreversibility and to find ways to improve
the system. The overall objective is to accomplish the thermodynamic analysis of the
refrigeration and heat pump systems and study their thermodynamic viability.
52
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