project report
TRANSCRIPT
CERTIFICATE
This is to certify that thesis entitled “STUDY OF AUTOMOBILE AIR CONDITIONING
SYSTEM USING EXHAUST HEAT” being submitted by K.ADISESHAN (05331A302),
A.D.MANOHAR (05331A0309), A.PREETHAM (05331A0328) and SAI KRISHNA Y.N.M
(06335A0303) in partial fulfillment of the requirement for the award of the degree of Bachelor
of Technology in MECHANICAL ENGINEERING is a record of bonafied work done by him
under our supervision during the academic year “2008-2009”.
N.RAVI KUMAR Dr. R.Ramesh Associate Professor Professor, HOD Department of Mechanical Engineering Department of Mechanical EngineeringMVGR College of Engineering MVGR College of Engineering
INTERNAL EXAMINER EXTERNAL EXAMINER
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ACKNOWLEDGEMENT
We wish to express our deep sense of gratitude to Sri N.RAVI KUMAR, Associate Professor,
Mechanical Engineering for his wholehearted co-operation, unfailing inspiration and valuable guidance.
Throughout the project work, their useful suggestions, constant encouragement has gone a long way in
their valuable time at odd hours for their patience and understanding they showed.
We consider it our privilege to express our deepest gratitude to Dr.R.RAMESH, Professor, and
Head of the Department for his valuable suggestions and constant motivation that greatly helped the
project work to get successfully completed.
We also thank Dr.K.V.L.RAJU, Principal, for extending his utmost support and cooperation in
providing all the provisions for the successful completion of the project.
We sincerely thank all the members of the staff in the Mechanical Engineering Department for
their sustained help in our pursuits.
We thank all those who contributed directly or indirectly in successfully carrying out this work.
K.Adiseshan(05331A0302)
A.D.Manohar(05331A0309)
A.Preetham(05331A0328)
Y.N.M.Sai Krishna(06335A0303)
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ABSTRACT An automobile air conditioning system generally works on vapor compression cycle
comprising of compressor, condenser, expansion device and evaporator. The objective of project is to use vapor absorption system instead of vapor compression system having water as the refrigerant and lithium bromide as absorbent. The air conditioning system compresses of a generator, a segregator, a condenser, an evaporator, an absorber, a heat exchanger and a plurality of conducts intercommunicated thereto between to form a circulated cooled air production system. The improvement is characterized in utilizing the residual heat from the exhaust pipe of an engine of an engine by helically winding a coil tube around the main portion of the exhaust pipe so that the liquidized refrigerant water (H20) from the generator will flow through and be heated into a mixture of the vapor and lithium bromide and enter into the segregator for a process of separation. Then, the vapor enters into the condenser via a capillary tube and from there enters into the evaporator for a process of vaporization. Cooled air is therefore produced and vented into the interior of the automobile. The vapor from the evaporator will then go to the absorber and re-enter the generator after it is mixed with lithium bromide which is returned from the segregator after being processed there to. Whereby, a cooling circulation for this system is therefore completed.
Previously ammonia-water was used as refrigerant, due to the drawbacks such as depletion of ozone layer and hazardous to humans as it is a toxic gas. So water is used as a refrigerant. By implementing this absorption system, the compressor is eliminated thereby reducing the power consumption of the system. This results in higher overall efficiency and less fuel consumption of the engine. Further an analysis is made between vapor compression and vapor absorption system and various parameters are studied.
The main aim of the project is to study implementation of vapour absorption system for automobiles and make a working model.
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CONTENTS
CERTIFICATE i
ACKNOWLEDGEMENT ii
ABSTRACT iii
CONTENTS iv
LIST OF FIGURES ix
LIST OF TABLES x
CHAPTER 1 LITERATURE REVIEW 1
CHAPTER 2 INTRODUCTION 5
2.1 refrigeration methods 5
2.1.1 natural methods 5
2.1.1.1 ice making by nocturnal cooling 5
2.1.1.2 evaporative cooling 6
2.1.2 aerificial methods 6
2.2 air conditioning systems 6
2.3 vapor compression system 7
2.4 vapor absorption system 8
2.5 advantages ars over vcr 10
2.6 practical problems in libr system 11
2.7 crystallisation 11
2.8 capacity control 12
2.9 commercial systems 12
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CHAPTER 3 STUDY OF AUTMOBILE AIR CONDITIONING 15
3.1 description of absorption layout 16
3.2 waste heat recovery generator Alternatives 18
CHAPTER 4 THERMODYNAMIC ANALYSIS OF ABSORPTION SYSTEM 11
4.1 first law analysis 24
4.2 thermodynamic properties 24
4.3 performance calculations 25
4.4 model calculatios 29
4.5 conventional calculation 39
CHAPTER 5 RESULTS AND DISCUSSIONS 37
CHAPTER 6 FABRICATION OF ABSORPTION AIR CONDITIONING
SYSTEM 41
6.1 Absorber 42
6.2 Generator 43
6.3 Working model 44
6.4 water treatment 30
6.5 water level 30
6.6 high water temperature 32
CHAPTER 7 CONCLUSION 46
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LIST OF FIGURES
FIGURE NO. DESCRIPTION PAGE NO.
2.1 Vapor compression system 8
2.2 Vapor absorption system 9
2.3 Twin drum type libr system 13
3.1 Components of air cooled absorption
system for transport 17
3.2 Generator system with additional burner 19
3.3 Direct recovery generator system 19
3.4 Generator with air as intermediate fluid 20
3.5 Generator with intermediate fluid- closed 20
4.1 Layout of absorption system 23
4.2 Flow diagram of libr absorption system 26
4.3 Schematic representation of vapor absorption
with regenerator HE 32
4.4 Representation of absorption cycle on p-1/T diagram 33
5.1 Heat transfer in each component 38
5.2 Variation of COP at different generator temperatures 39
5.3 Variation of COP at different condeser temperatures 40
5.4 Variation of COP at different evaporator temperatures 40
6.1 Modified layout of absorption system 41
6.2 Absorber 42
6.3 Generator 43
6.4 Working model 44
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LIST OF TABLES
TABLE NO. DESCRIPTION PAGE NO.
4.1 Comparision between conventional and calculated results 36
5.1 Thermodynamic properties of each point 37
5.2 Heat transfer rate of components and performance parameters 3 8
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Chapter 1
LITERATURE SURVEY
Several eminent people have performed research studies on energy efficient source for
cooling and refrigeration. M.G. Rasul and A. Murphy [1] have evaluated the feasibility of an
absorption refrigeration unit on solar power. A prototype model that is capable of producing a
temperature change in the evaporator was designed, fabricated and tested. The performances and
effectiveness of the unit was studied by determining refrigeration effect (RE), coefficient of
performance (COP) and explaining operational issues of the unit. Johnny L. Kirby [2] studied the
working of absorption/adsorption air conditioning refrigeration system. He found that reclaiming
the waste energy is a great cost-effective advantage of the adsorption / absorption system, over a
conventional air-conditioning system. And finished by looking at various methods of powering
systems with minimal or better yet no, grid power assistance. Dr. Eng. Chiara Boccaletti [3] aimed
at individuating the main heat transfer and thermodynamic phenomena, at the basis of operation
of refrigerating machines. He particularly analyzed Absorption machines and suggested design
modifications and adaptations/improvements of schemes presently used. The final objective of
his study was to attain design of machines characterized by higher performance, lower specific
energy consumption and higher reliability.
V. Mittal, K. S. Kasana, N. S.Thakur [4] studied a detailed review on the past efforts in
the field of solar absorption cooling systems with the absorption pair of lithium-bromide and
water. They have investigated the influence of key parameters on the overall system
performance. They also performed the modeling and simulation of a solar absorption cooling
system of a solar-powered, single stage, absorption cooling system, using a flat plate collector
and water–lithium bromide solution. They developed a computer program for the absorption
system to simulate various cycle configurations with the help of various weather data for the
village Bahal, District Bhiwani, Haryana, India. They have also studied the effect of hot water
inlet temperatures on the coefficient of performance (COP) and the surface area of the absorption
cooling component. Jean Philippe Praene, Alain Bastide, Franck Lucas, François Garde, Harry
Boyer [5] Worked on solar-powered, single stage, absorption cooling system, using a water–
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lithium bromide solution. The first part of this work deals with the dynamic modeling of an
evacuated tube collector used for the simulation of heat production. In a second part, simulation
and optimization of the system has been investigated in order to determine the optimum of solar
collector plant surface, storage tank volume and nominal capacity of the absorption chiller. Hugo
Lima Moreira, Paulo Henrique Dias dos Santos, Celina Maria Cunha Ribeiro, Ednildo Andrade
Torres, Antonio Pralon Ferreira Leite, Carlos Antonio Cabral dos Santos [6] have worked on the
thermo economic or exergoeconomic analysis of a double-effect absorption refrigeration system
with the water-lithium bromide pair, operating with the direct combustion of natural gas. The
method combines exergetic and economic analysis and this study was done after the energetic
analysis of all system’s components. They performed exergoeconomic evaluation of the
thermodynamic flows, which go through this cycle, for operational conditions aimed at a
refrigerating capacity from 5 to 15 TR. And applied to the present system to reveal which
component in the cycle would be wasting energy. This method was also based on the incidence
matrix that represents the physical structure of the above-mentioned system.
Guozhen Xie Guogang Sheng Guang Li Shuyuan Pan[7] Have adopted an improved
cycle was adopted to raise the pressure inside the absorber of the machine in order to intensify
the absorption effect of thick Lithium bromide solution and enhance the COP of the absorption
refrigeration system. A mathematical model that is used for predicting the performance of the
system was developed, and the influence of pressure change on the overall performance of the
machine was studied. Omer Kaynakli and Recep Yamankaradeniz [8] have done the first and
second law thermodynamic analysis of a single-stage absorption refrigeration cycle with
water/lithium bromide as working fluid pair. Thermodynamic properties of each point in the
cycle were calculated using related equations of state. Heat transfer rate of each component in
the cycle and some performance parameters were calculated from the first law analysis. From the
second law analysis, the entropy generation of each component and the total entropy generation
of all the system components were obtained. They examined the Variation of the performance
and entropy generation of the system at various operating conditions. D.S. Kim, C.H.M.
Machielsen [9] have studied several air-cooled solar absorption cooling systems and compared
them in terms of cost and performance. They found that Compared to single effect system of the
same cooling capacity, half effect systems would require about 40% more heat exchange surface
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and -10~60% more collector area. Solar fraction of a single effect system would be lower than
half effect system, unless vacuum tube collectors or comparable types are used.
In the mean while another system gained importance which uses the waste energy
available to produce the cooling effect and air conditioning which is the vapor adsorption
system. R.Z. Wang and R.G. Oliveira [10] have presented the achievements gained in solid
sorption refrigeration prototypes the applications included are ice making and air conditioning.
The latter includes not only cooling and heating, but also dehumidification by desiccant systems.
The prototypes presented were designed to use waste heat or solar energy as the main heat
source. Miguel Ramos, Rafael L. Espinoza, Manfred J. Horn [11] have done experimental
evaluations of a prototype solar refrigerator, based on an Intermittent thermodynamic cycle of
adsorption, using water as refrigerant and the mineral zeolite as Absorber. This system uses a
mobile absorber, which is regenerated out of the refrigeration cycle and no Condenser is applied,
because the solar regeneration is made in the ambient air for the regeneration, aSK14 solar
cooker is considered.
By Li Yong and Ruzhu Z [12] have studied more than 100 patents filed mainly since year
2000 that propose technologies to improve adsorption system and make it become a realistic
alternative. And the patents surveyed were classified into four main groups: adsorption system
development, adsorbent bed innovation, adsorbent/adsorbate material development and novel
application of adsorption cooling system. The various technology options are discussed and
evaluated. “Hot spots” and key inventors/applicants are identified. An assessment is made about
current and future development of adsorption refrigeration technologies. Craig Christy and Reza
Toossi [13] have performed an investigation into the feasibility of meeting the cooling needs for
commercial tractor trailer refrigeration and transit bus air conditioning (A/C) by utilizing their
own exhaust heat to drive an adsorption refrigeration system. An experimental vapor
compression A/C system utilizing adsorption compression was refurbished and operated at
CSULB to verify previously reported coefficient of performance (COP) and specific cooling
power (SCP) values and to gain knowledge, experience, and insight into product design issues.
P. Seifert DBI Gas- und Umwelttechnik [14] have worked on a new and innovative
concept of the supply with power, heat and chilliness the Core of their demonstration project are
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a fuel cell, an adsorption Refrigeration machine as well as multi-solar collectors. First
experiences with this concept show, that an optimized co-operation of the components with an
adaptive control system based on the weather forecast as well as various storage’s for heat and
chilliness can be achieved. A continuously operation, high fuel utilization and reduced
environmental pollution can be demonstrated. J. R. Camargo [15] had worked on the basic
principles of the evaporative cooling process for human thermal comfort, the principles of
operation for the direct evaporative cooling system and the mathematical development of the
equations of thermal exchanges, allowing the determination of the effectiveness of saturation. He
also presented some results of experimental tests in a direct evaporative cooler that take place in
the Air Conditioning Laboratory at the University infatuate Mechanical Engineering Department,
and the experimental results are used to determinate the convective heat transfer coefficient and
to compare with the mathematical model.
P. K. Bansal [16] investigated the performance characteristics of three domestic
refrigerators, namely the vapor compression (VC), the thermoelectric (TE) and the absorption
refrigeration (AR). AR and TE refrigerators are the result of research and development in
refrigeration system in the quest to find a cooling system which does not use any refrigerant that
damages the ozone layer. Three refrigerators of similar capacity were compared for their usage
in the hotel industry in view of their energy efficiency, noise produced and cost.
From the above literature review it was found the air conditioning of automobiles can be
provided either by vapor absorption system or adsorption system. Work is being done on
absorption system using different refrigerants such as ammonia/water and LiBr/water presently
we have taken to study vapor absorption system with water as refrigerant and LiBr as absorbent
and make a working model.
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Chapter: 2
INTRODUCTION
The subject of refrigeration and air conditioning has evolved out of human need for food and
comfort, and its history dates back to centuries. Refrigeration may be defined as the process of
achieving and maintaining a temperature below that of the surroundings, the aim being to cool
some product or space to the required temperature. One of the most important applications of
refrigeration has been the preservation of perishable food products by storing them at low
temperatures. Refrigeration systems are also used extensively for providing thermal comfort to
human beings by means of air conditioning. Air Conditioning refers to the treatment of air so as
to simultaneously control its temperature, moisture content, cleanliness, odor and circulation, as
required by occupants, a process, or products in the space.
2.1 REFRIGERATION METHODS
Generally refrigeration methods are classified into two types
1. Natural methods
2. Artificial methods
2.1.1 NATURAL METHODS
In olden days refrigeration was achieved by natural means such as the use of ice or evaporative
cooling. In earlier times, ice was transported from colder regions, harvested in winter and
stored in ice houses for summer use or, made during night by cooling of water by radiation to
stratosphere.
2.1.1.1 Art of Ice making by Nocturnal Cooling
The art of making ice by nocturnal cooling was perfected in India. In this method ice was
made by keeping a thin layer of water in a shallow earthen tray, and then exposing the tray to the
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night sky. Compacted hay of about 0.3 m thickness was used as insulation. The water looses heat
by radiation to the stratosphere which is around -55oC and by early morning hours the water in
the tray freezes to ice. This method of ice production was very popular in India.
2.1.1.2 Evaporative cooling
As the name indicates, evaporative cooling is the process of reducing the temperature of
a system by evaporation of water. Human beings perspire and dissipate their metabolic heat by
evaporative cooling if the ambient temperature is more than skin temperature. Animals such as
the hippopotamus and buffalo coat themselves with mud for evaporative cooling. Evaporative
cooling has been used in India for centuries to obtain cold water in summer by storing the water
in earthen pots. The water permeates through the pores of earthen vessel to its outer surface
where it evaporates to the surrounding, absorbing its latent heat in part from the vessel, which
cools the water. It is said that Patliputra University situated on the bank of river Ganges used to
induce the evaporative-cooled air from the river. Suitably located chimneys in the rooms
augmented the upward flow of warm air, which was replaced by cool air. Evaporative cooling
by placing wet straw mats on the windows is also very common in India. The straw mat made
from “khus” adds its inherent perfume also to the air. Now-a-days desert coolers are being used
in hot and dry areas to provide cooling in summer.
2.1.2 ARTIFICIAL METHODS Refrigeration as it is known these days is produced by artificial means. Though it is very
difficult to make a clear demarcation between natural and artificial refrigeration, it is generally
agreed that the history of artificial refrigeration began in the year 1755, when the Scottish
professor William Cullen made the first refrigerating machine, which could produce a small
quantity of ice in the laboratory. Based on the working principle, refrigeration systems can be
classified as vapor compression systems, vapor absorption systems, gas cycle systems etc.
2.2 AIR CONDITIONING SYSTEMS Refrigeration systems are also used for providing cooling and dehumidification in
summer for personal comfort (air conditioning). The first air conditioning systems were used for
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industrial as well as comfort air conditioning. Eastman Kodak installed the first air conditioning
system in 1891 in Rochester, New York for the storage of photographic films. An air
conditioning system was installed in a printing press in 1902 and in a telephone exchange in
Hamburg in 1904. Many systems were installed in tobacco and textile factories around 1900. The
first domestic air conditioning system was installed in a house in Frankfurt in 1894. A private
library in St Louis, USA was air conditioned in 1895, and a casino was air conditioned in Monte
Carlo in 1901. Efforts have also been made to air condition passenger rail coaches using ice. The
widespread development of air conditioning is attributed to the American scientist and
industrialist Willis Carrier. Carrier studied the control of humidity in 1902 and designed a central
air conditioning plant using air washer in 1904. Due to the pioneering efforts of Carrier and also
due to simultaneous development of different components and controls, air conditioning quickly
became very popular, especially after 1923. At present comfort air conditioning is widely used in
residences, offices, commercial buildings, air ports, hospitals and in mobile applications such as
rail coaches, automobiles, aircrafts etc. Industrial air conditioning is largely responsible for the
growth of modern electronic, pharmaceutical, chemical industries etc. Most of the present day air
conditioning systems use either a vapor compression system or a vapor absorption system.
2.3 VAPOR COMPRESSION SYSTEMThe vapor compression refrigeration system consists of an evaporator, compressor,
condenser and an expansion valve. The refrigeration effect is obtained in the cold region as heat
is extracted by the vaporization of refrigerant in the evaporator. The refrigerant vapor from the
evaporator is compressed in the compressor to a high pressure at which its saturation temperature
is greater than the ambient or any other heat sink. Hence when the high pressure, high
temperature refrigerant flows through the condenser, condensation of the vapor into liquid takes
place by heat rejection to the heat sink. To complete the cycle, the high pressure liquid is made to
flow through an expansion valve. In the expansion valve the pressure and temperature of the
refrigerant decrease. This low pressure and low temperature refrigerant vapor evaporates in the
evaporator taking heat from the cold region. It should be observed that the system operates on a
closed cycle. The system requires input in the form of mechanical work. It extracts heat from a
cold space and rejects heat to a high temperature heat sink.
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Fig 2.1 Vapor compression system
2.4 VAPOR ABSORPTION REFRIGERATION SYSTEM
Vapor Absorption Refrigeration Systems (VARS) belong to the class of vapor cycles
similar to vapor compression refrigeration systems. However, unlike vapor compression
refrigeration systems, the required input to absorption systems is in the form of heat. Hence these
systems are also called as heat operated or thermal energy driven systems. Vapor absorption
refrigeration systems have also been commercialized and are widely used in various refrigeration
and air conditioning applications. Since these systems run on low-grade thermal energy, they are
preferred when low-grade energy such as waste heat or solar energy is available. Since
conventional absorption systems use natural refrigerants such as water or ammonia they are
environment friendly.
In the simplest absorption refrigeration system, refrigeration is obtained by connecting two
vessels, with one vessel containing pure solvent and the other containing a solution. Since the
pressure is almost equal in both the vessels at equilibrium, the temperature of the solution will be
higher than that of the pure solvent. This means that if the solution is at ambient temperature,
then the pure solvent will be at a temperature lower than the ambient. Hence refrigeration effect
is produced at the vessel containing pure solvent due to this temperature difference. The solvent
evaporates due to heat transfer from the surroundings, flows to the vessel containing solution and
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is absorbed by the solution. The solution used in absorption refrigeration systems may be
considered as a homogeneous binary mixture of refrigerant and absorbent.
Fig 2.2 Vapor absorption system
The most commonly used refrigerant-absorbent pairs in commercial systems are:
1. Water-Lithium Bromide (H2O-LiBr) system for above 0oC applications such as air
conditioning. Here water is the refrigerant and lithium bromide is the absorbent.
2. Ammonia-Water (NH3-H2O) system for refrigeration applications with ammonia as refrigerant
and water as absorbent.
Of late efforts are being made to develop other refrigerant-absorbent systems using both natural
and synthetic refrigerants to overcome some of the limitations of (H2O-LiBr) and (NH3-H2 O)
systems.
Currently, large water-lithium bromide (H2O-LiBr) systems are extensively used in air
conditioning applications, where as large ammonia-water (NH3-H2O) systems are used in
refrigeration applications, while small ammonia-water systems with a third inert gas are used in a
pump less form in small domestic refrigerators (triple fluid vapor absorption systems).
2.5 ADVANTAGES OF VAPOR ABSORPTION OVER VAPOUR
COMPRESSION 16
The function of the compressor in the vapor compression system is to continuously
withdraw the refrigerant vapor from the evaporator and to raise its pressure and hence
temperature, so that the heat absorbed in the evaporator along with the work of the compression
may be rejected in the condenser to the surroundings.
In the vapor absorption system the function of the compressor is accomplished in a three- step
process by the use of the absorber, pump and the generator as follows .
1. Absorber:
Absorption of the refrigerant vapor by its weak solution in a suitable absorbent forming a
strong or rich solution of the refrigerant in the absorbent.
2. Pump:
Pumping of the rich solution raising its pressure to the condenser pressure.
3. Generator :
Distillation of the vapor from the rich solution leaving the poor solution for recycling.
Compressor is connected to the crank shaft of the engine through a belt drive. If this unit is
removed, then the load engine decreases. Due to this the engine performance increases.
Vapor compression system traditionally uses halogenated hydrocarbon refrigerants,
which contribute to ozone depletion and greenhouse warming. Whereas refrigerant used
in vapor absorption system is ecofriendly.
Due to the absence of reciprocating parts, operation is noiseless.
2.6 PRACTICAL PROBLEMS IN WATER-LiBr SYSTEMSPractical problems typical to water-lithium bromide systems are:
1. Crystallization
2. Air leakage, and17
3. Pressure drops
As mentioned before to prevent crystallization the condenser pressure has to be
maintained at certain level, irrespective of cooling water temperature. This can be done by
regulating the flow rate of cooling water to the condenser. Additives are also added in practical
systems to inhibit crystallization. Since the entire system operates under vacuum, outside air
leaks into the system. Hence an air purging system is used in practical systems. Normally a two-
stage ejector type purging system is used to remove air from the system. Since the operating
pressures are very small and specific volume of vapor is very high, pressure drops due to friction
should be minimized. This is done by using twin and single drum arrangements in commercial
systems.
2.7 CRYSTALLIZATIONThe pressure-temperature-mass fraction and enthalpy-temperature-mass fraction charts
show lines marked as crystallization in the lower right section. The region to the right and below
these crystallization lines indicates solidification of LiBr salt. In the crystallization region a two-
phase mixture (slush) of water-lithium bromide solution and crystals of pure LiBr exist in
equilibrium. The water-lithium bromide system should operate away from the crystallization
region as the formation of solid crystals can block the pipes and valves. Crystallization can occur
when the hot solution rich in LiBr salt is cooled in the solution heat exchanger to low
temperatures. To avoid this the condenser pressure reduction below a certain value due to say,
low cooling water temperature in the condenser should be avoided. Hence in commercial
systems, the condenser pressure is artificially maintained high even though the temperature of
the available heat sink is low. This actually reduces the performance of the system, but is
necessary for proper operation of the system. It should be noted from the property charts that the
entire water-lithium bromide system operates under vacuum.
2.8 CAPACITY CONTROLCapacity control means capacity reduction depending upon load as the capacity will be
maximum without any control. Normally under both full as well as part loads the outlet
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temperature of chilled water is maintained at a near constant value. The refrigeration capacity is
then regulated by either:
Regulating the flow rate of weak solution pumped to the generator through the solution
pump
Reducing the generator temperature by throttling the supply steam, or by reducing the
flow rate of hot water
Increasing the condenser temperature by bypassing some of the cooling water supplied to
the condenser
Method 1 does not affect the COP significantly as the required heat input reduces with
reduction in weak solution flow rate, however, since this may lead to the problem of
crystallization, many a time a combination of the above three methods are used in commercial
systems to control the capacity.
2.9 COMMERCIAL SYSTEMSCommercial water-lithium bromide systems can be:
1. Single stage or single-effect systems, and
2. Multi stage or multi-effect systems
Single stage systems operate under two pressures, one corresponding to the condenser and
generator (high pressure side) and the other corresponding to evaporator and absorber.
Single stage systems can be either:
1. Twin drum type, or
2. Single drum type
Since evaporator and absorber operate at the same pressure they can be housed in a single vessel,
similarly generator and condenser can be placed in another vessel as these two components
operate under a single pressure. Thus a twin drum system consists of two vessels operating at
high and low pressures. Figure 2.3 shows a commercial, single stage, twin drum system.
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Fig 2.3 Twin drum type water- LiBr absorption system
As shown in the figure, the cooling water (which acts as heat sink) flows first to absorber,
extracts heat from absorber and then flows to the condenser for condenser heat extraction. This is
known as series arrangement. This arrangement is advantageous as the required cooling water
flow rate will be small and also by sending the cooling water first to the absorber, the condenser
can be operated at a higher pressure to prevent crystallization. It is also possible to have cooling
water flowing parallel to condenser and absorber, however, the cooling water requirement in this
case will be high. A refrigerant pump circulates liquid water in evaporator and the water is
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sprayed onto evaporator tubes for good heat and mass transfer. Heater tubes (steam or hot water
or hot oil) are immersed in the strong solution pool of generator for vapor generation. Pressure
drops between evaporator and absorber and between generator and condenser are minimized,
large sized vapor lines are eliminated and air leakages can also be reduced due to less number of
joints.
In multi-effect systems a series of generators operating at progressively reducing
pressures are used. Heat is supplied to the highest stage generator operating at the highest
pressure. The enthalpy of the steam generated from this generator is used to generate some more
refrigerant vapor in the lower stage generator and so on. In this manner the heat input to the
system is used efficiently by generating more refrigerant vapor leading to higher COPs.
However, these systems are more complex in construction and require a much higher heat source
temperatures in the highest stage generator.
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Chapter 3
STUDY OF AUTOMOBILE AIR CONDITIONING
In the future there will be a larger and larger diffusion of VCR systems because more
and more people will be ready to spend money for travelling comfort. At the same time, there is
the strong demand for improved vehicle performance and fuel consumption, reduced noise, easy
maintenance and high reliability.
A considerable portion of the total energy consumption of the western world is centered
in the transport sector. Automobiles and trucks alone account for approximately 80 percent of all
transportation energy expenditures. These internal combustion engines typically have a thermal
efficiency of 40 percent. The remaining energy is rejected to the atmosphere in the form of hot
exhaust gases or as energy convected from the radiator and the engine. Much work now in
progress is directed to the improvement of the thermal efficiency by achieving better
consumption of the fuel. Some effort has been devoted to the utilization of the vast amount of
waste energy dissipated in the exhaust gases. Unfortunately, few have focussed on using the
waste heat for air-conditioning and refrigeration. Automobiles and trucks alone account for
approximately 80 percent of all transportation energy expenditures. These internal combustion
engines typically have a thermal efficiency of 40 percent. The remaining energy is rejected to the
atmosphere in the form of hot exhaust gases or as energy convected from the radiator and the
engine. Much work now in progress is directed to the improvement of the thermal efficiency by
achieving better consumption of the fuel. Some effort has been devoted to the utilization of the
vast amount of waste energy dissipated in the exhaust gases. Unfortunately, few have focussed
on using the waste heat for air-conditioning and refrigeration.
Besides energy usage for transport refrigeration, another concern which has emerged in
the last five to ten years is the search for environmentally-benign refrigerants and refrigeration
techniques. Wide-spread efforts are currently underway to develop replacements for the
traditionally used halogenated hydrocarbon refrigerants, which contribute to ozone depletion and
greenhouse warming. One alternative to the vapor compression cycle which has been
increasingly discussed in recent years is the absorption refrigeration cycle.
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The adoption of air condition system based on absorption cycles may be seen as a good solution
with respect to vapor compression system. As a matter of fact the later causes reduction in
engine performance because of power consumption. Moreover, fuel can be saved by adopting
systems because of recovery of waste heat.
3.1 DESCRIPTION OF ABSORPTION SYSTEM LAYOUT
The principal difference between the absorption and the vapor-compression cycles is the
mechanism for circulating the refrigerant through the system and providing the necessary
pressure difference between the vaporizing and condensing processes. The vapor compressor
employed in the vapor-compression cycle is replaced in the absorption cycle by an absorber and
a generator or boiler, which compress the vapor as required. The energy input required by the
vapor-compression cycle is supplied to the compressor in the form of mechanical work. In the
absorption cycle, the energy input is mostly in the form of heat supplied to the generator. In the
present case the heat source is the exhaust heat of an internal combustion vehicle engine.
In the generator a mixture of lithium bromide and water is heated. The boiling point of
water is lower than that of lithium bromide, so it vaporizes, separating the refrigerant from the
absorbent. Since the vapor is not a pure water vapor, it must be purified as it flows through a
rectification column.
23
Fig 3.1 Components of the air-cooled absorption system for transportation
The almost pure water vapor flows from the top of the column to the condenser as a high-
temperature, high pressure mixture. As ambient air passes over the condenser, it removes heat
from the gas-mixture and the vapor condenses to a liquid. After the vapor is completely
condensed, the liquid leaves the condenser and passes to the liquid-suction heat exchanger
(LSHX). The LSHX is an aluminum plate-fin heat exchanger. It reduces the temperature of the
liquid before it reaches the evaporator. When the liquid-mixture leaves the LSHX, the pressure
drops as it passes through an expansion valve into the evaporator. Here it absorbs heat from the
air being cooled and vaporizes. After leaving the evaporator, the vapor is further heated as it
passes through the LSHX to the absorber mixing vessel.
The high-pressure weak absorbent solution leaving the generator enters a heat exchanger. Here,
its temperature is lowered by heat exchange with the strong solution coming from the absorber.
The weak solution temperature is lowered further in the air cooled absorber subcooler which is a 24
fin-and tube heat exchanger like the condenser and evaporator. Finally, the weak solution passes
through a throttling valve prior to entering the mixing vessel and mixing with the refrigerant
vapor coming from the evaporator and LSHX. The absorption of refrigerant into the absorbent
solution starts in this vessel, where the released heat of absorption significantly increases the
temperature of the two-phase binary mixture. The mixture is distributed into the three circuits of
the air cooled absorber heat exchanger. Removal of heat by ambient air is necessary to complete
absorption of water into the solution.
The absorption of refrigerant into the absorbent solution starts in this vessel, where the released
heat of absorption significantly increases the temperature of the mixture. The mixture is
distributed into the three circuits of the air cooled absorber heat exchanger. Removal of heat by
ambient air is necessary to complete absorption of water into the solution.
The solution leaving the absorber is strong because it has the water refrigerant absorbed into it.
After leaving the air cooled absorber, the strong solution pressure is raised by a pump. Pumping
power in the form of work must be input to the system, but this power requirement is relatively
small compared to the power input into the compressor of a vapor-compression cycle because
liquid is nearly incompressible. The solution passes on to the rectifier where it extracts the
rectifier cooling load, and to the solution heat exchanger, where more heat is absorbed from the
weak solution. The preheated solution passes through the stripping column section of the rectifier
in order to release some vapor before entering the generator.
3.2 WASTE HEAT RECOVERY GENERATOR ALTERNATIVESThe power can be transferred from the exhaust gas to the vapor generator either directly or by
means of an intermediate medium. Among the suitable secondary fluids, air seems a good
choice. When direct heat transfer to lithium bromide is adopted, the minimum required power is
the lowest possible. In the case of indirect heat transfer, the simple solution with air implies a
rather higher threshold. In the case that a burner is adopted when engine power is low, it will
provide the highest temperature necessary for the secondary fluid.
25
Fig 3.2 Generator system with additional burner
In the case of direct recovery, the temperature at the absorption system generator is some 1200C
and may be assumed to be constant.
Fig 3.3 Direct recovery generator system
When air is used as an intermediate fluid, various options are possible. The most suitable seems
to be discharging the exhaust at low temperatures, avoiding an air preheater. The temperature
26
difference between air and mixture is lower than the temperature difference between exhaust gas
and mixture in the case of direct recovery.
Fig 3.4 Generator with air as an intermediate fluid – open circuit
Another possibility is represented by a secondary fluid closed circuit. When air is used, there are
only small differences with respect to the previous open circuit. Among various media water
may be used and some advantages may come if phase change occurs in the exhaust pipe heat
exchanger as well as in the absorption system generator.
Fig 3.5 Generator with air as an intermediate fluid – closed circuit
If the heat recovery device is placed downstream of the catalyser, the temperature at the catalyser
inlet depends on the engine operating conditions .when the heat transfer occurs between exhaust
gas and a secondary fluid, the useful heat transfer surface could be insufficient to obtain the
necessary amount of recovered heat. Thus direct recovery vapor generator is preferred.
27
Chapter 4
THERMODYNAMIC ANALYSIS OF ABSORPTION
SYSTEM
The basis of thermodynamics is stated in the first and second laws. The first law of
thermodynamic analysis is still the most commonly used method in the analysis of thermal
systems. The first law is concerned only with the conservation of energy, and it gives no
information on how, where, and how much the system performance is degraded. The second law
of thermodynamic analysis is a powerful tool in the design, optimization, and performance
evaluation of energy systems.28
A basic vapor absorption cycle is a two pressure three temperatures level cycle which
makes use of a vaporizable liquid as the refrigerant and a second liquid or solid liquid as an
absorbent. It consists of a generator, a condenser, an absorber, an evaporator, a solution pump
and expansion valves.
The solution temperature in the generator and absorber are not uniform due to the
variation in solution concentration from inlet to outlet in these components. This causes heat
transfer irreversibility’s in addition to those due to internal mass exchange. The regenerated
absorbent leaves the generator at high temperature and is cooled to absorber temperature.
Similarly the solution leaving the absorber is heated to the level of the generator temperature. A
solution heat exchanger can be used to transfer the heat from the weak solution leaving the
generator to the strong solution leaving the absorber. This reduces the input heat required in the
generator. This internal heat recovery improves the COP.
Figure 4.1 Layout of absorption system
The main thermodynamics process in a absorption refrigeration cycle can be summarised as
follows:
- Pumping, pressurisation and sensible heat removal of the rich solution.
- Desorption process in the generator with external heat input, Qg at a temperature tg, which
represents the main input energy in this system.
- Desuperheating and condensation at constant pressure in the condenser, thus giving an energy
QC at a temperature tc.
- Cooling of the refrigerant liquid and its isenthalpic expansion through a throttling valve.29
- Evaporation of the refrigerant in the evaporator which produces the cooling load, Qe, at an
evaporator temperature te.
- Absorption of the low pressure refrigerant vapor within the weak solution in the absorber at the
absorber temperature ta with a heat output Qa.
In order to perform a thermodynamic analysis of this cycle, the following assumptions are made:
The generator, condenser, evaporator and absorber temperatures are supposed constant.
The huge and low pressures of the cycle correspond respectively to the saturation
temperature in the condenser and in the evaporator.
The pressures in the generator and in the absorber are supposed to be similar to the
pressures in the condenser and in the evaporator respectively.
The strong solution Mss is defined as the solution with a high concentration of the
refrigerant (water) leaving the absorber is supposed to be at a saturation state.
The weak solution, mws, is defined as the solution with a low concentration of the
refrigerant leaves the generator at a temperature tg.
The refrigerant (Water) leaves the condenser at a saturated liquid state at tc.
The refrigerant leaves the evaporator at a saturated vapor state at te.
The pumping work is negligible.
No heat losses to the surroundings
The system operates at a steady state regime.
4.1 FIRST LAW ANALYSISIn recent years, theoretical and experimental researches on the absorption refrigeration
system have increased, because these systems harness inexpensive energy sources in comparison
to vapor compression systems. The system is bounded by two concentration lines X1 and X2 for
absorber and generator concentrations, respectively, and two constant pressures pe and pc for
evaporator and condenser respectively. For an efficient air conditioning application, the
evaporator temperature te should be low enough to dehumidify the air. In practice it ranges from
4.5 to 100C according to the load conditions. The heat rejection temperatures ta and tc for the
absorber and the condenser respectively, vary according to the type of cooling medium. The
30
generator temperature tg depends on the source of heat available. However a minimum
temperature of 800C should be maintained to provide efficient operation. The operational
function of a liquid –liquid heat exchanger in the cycle will be the reduction of the weak solution
temperature t4, leaving the generator and increasing the strong solution temperature t1 leaving the
absorber.
4.2 THERMODYNAMIC PROPERTIES Enthalpies of the water (refrigerant) and LiBr (absorbent) solutions were calculated with
reference temperature at 250C. The following expressions were derived by F.L.Lansing [17] to
calculate the cycle performance.
1. The enthalpy of pure water liquid at temperature t0C = (t-25)*4.186 KJ/kg
2. The enthalpy of saturated water vapor at temperature t0C = (2397.74+1.745t) KJ/kg
3. The enthalpy or superheated steam at temperature t0C and pressure equal to saturation
pressure of steam at temperature t80C = (2397.74+1.925t-.179 t8) KJ/kg
4. The specific heat of lithium bromide/water solution of concentration X kg LiBr/kg
solution is given by Cx = 4.227-5.148X+2.01X2 KJ/kg
5. The enthalpy of LiBr/water solution of concentration X kg LiBr/kg solution at 250C is
Hx,25= 284.89-1911.62X+1744.18X2 KJ/kg solution
6. The enthalpy of LiBr/water solution of concentration X kg LiBr/kg solution at
temperature t0C = (179.2-1782.9X+1693.95X2)+(4.23-5.15X+2.01X2)(t)
7. In the range of concentration from 0.5-0.65 LiBr/kg solution, refrigerant temperature
tR0C =(205.28-563.64X)+(4.709-1.967X)(tm
0C). this can be written as X=(49.04+1.125tm-
tR)/(134.65+0.47tm)
8. The saturated vapour pressure P in Bar corresponding to saturation temperature T0K for
pure water is given by log10P bar = 0.01-(2.068/T)-(149.51/T2)
4.3 PERFORMANCE CALCULATIONThe determination of thermodynamic properties of each state in the cycle, the amount of
heat transfer in each component, and the flow rates at different lines depend on the following set
of input parameters:31
1. Generator temperature tg0C
2. Evaporator temperature te0C
3. Condenser temperature tc0C
4. Absorber temperature ta0C
5. Liquid-liquid heat exchanger effectiveness EL
6. Refrigeration load Qe
The above set can be determined from actual running measurements or assumed by a first
reasonable estimate to cycle performance.
Together with the assumptions of neglecting the pump work and neglecting the pressure
drop in components and lines and assigning saturation conditions after absorber, generator (weak
solution), condenser and evaporator, the properties are determined as follows:
4.3.1 ABSORBER CONCENTRATIONThis is determined from concentration equation using ta for the solution temperature and
te for the water temperature corresponding to the evaporator pressure Pe:
X1=X2=X3= X strong solution
X1= (49.04+1.125ta-te)/ (134.65+0.47ta) kg LiBr/kg solution………………………….. (eq. 4.1)
4.3.2 GENERATOR CONCENTRATIONThis is determined from concentration equation using tg for the solution temperature and
tc for the refrigerant temperature corresponding to the condenser pressure Pc:
X4=X5=X6= X weak solution
X4= (49.04+1.125tg-tc) / (134.65+0.47tg) kg LiBr/kg solution……………….……………(eq. 4.2)
32
Fig 4.2 flow diagram of LiBr – H2O absorption system
4.3.3 PRESSURE LIMITS IN THE CYCLEIt is possible to evaluate the pressure in the every line as follows:
Pevaporator,Pe=P1=P6=P9=P10 in bar :
log10Pe = 0.01-(2.068/te+273.15)-(149.51/(te+273.15)2)..........................................(eq. 4.3)
PcondenserPc=P2=P3=P4=P5=P7=P8 in bar:
log10Pc= 0.01-(2.068/tc+273.15)-(149.51/(tc+273.15)2)………………………….(eq. 4.4)
4.3.4 FLOW RATESEnthalpy of saturated liquid water after condenser (state 8), is given by the condenser
temperature tc.
h8= (tc -25) *4.186 kJ/kg………………………………………………...……… (eq. 4.5)
The throttling process from 8 to 9 give H8=H9
Enthalpy of saturated water vapour after evaporator (state 10) is given by the evaporative
temperature te as
h10= (2397.74+1.745te) kJ/kg…………………………………………………….(eq. 4.6)
According to the first law of thermodynamics to the evaporator will give
Qe=mR(h10-h9)……………………………...….….....….(eq.4. 7)
33
Where mR is the refrigerant flow rate, equals the difference between the strong and weak solution
rates. By using equation
mR = Qe / (h10- h8)…………………………..…………………….………(eq. 4.8)
The lithium bromide mass balance in absorber gives
mwX6+mRX10=msX1=(mw+mR)X1……………...………………….………(eq. 4.9)
By using the above equations,
mw= (Qe / (h10- h8)) (X1 / (X4-X1)) .....................................................................(eq. 4.10)
ms= (Qe / (h10- h8)) (X4 / (X4-X1))......................................................................(eq. 4.11)
since the concentrations x1 and X4 are restricted not to exceed certain limits to avoid
crystallization problems, and if the temperatures of the cycle are set to vary according the
ambient and load conditions, the mass flow rates in the different lines will be varies accordingly.
4.3.5 LIQUID-LIQUID HEAT EXCHANGER TEMPERATURESOnce the heat exchanger effectiveness EL , the mass flow rates and the concentrations are known
it is possible to determine the solution temperature t3 and t5 as follows
Based on the weak solution side, which has the minimum heat capacity the effectiveness EL is
given by
EL =(tg-t5)/(tg-ta)
or
Based on the strong solution side
EL=(mg*CX1)*(t3-ta)/((mw*CX4)*(tg-ta))
Where CX1 is the specific heat of the strong solution whose concentration is X1 and CX4 is the
specific heat of the weak solution whose concentration is X4 where
CX1= 4.227-5.148X1+2.01X12 KJ/kg.......................................................................(eq. 4.12)
CX4= 4.227-5.148X4+2.01X42 KJ/kg....................................................................(eq. 4.13)
From the above equations the values of the temperatures t3 and t5 are given as
t5 = tg- EL*(tg-ta)......................................................................................................(eq. 4.14)
34
t3= te + (EL (X1*CX4)(X4*CX1)(tg-ta)) ……………………………………..……..(eq. 4.15)
And the enthalpies h1 and h2 are the calculated using the following relations
h1=(179.2-1782.9X1+1693.95X12)+(4.23-5.15X1+2.01X1
2)(ta)...........................(eq. 4.16)
h5=(179.2-1782.9X4+1693.95X42)+(4.23-5.15X4+2.01X4
2)(t5).............................(eq.4.17)
4.3.6 HEAT TRANSFER IN CONDENSER, GENERATOR, ABSORBERThe enthalpy of water vapor leaving the generator and entering the condenser (state7) is given by
h7=2397.16+1.92tg-0.179tc……………………………………………………...(eq. 4.18)
The heat balance of the condenser gives
Qc=mr (h7-h8) …………………………………………………………….……..(eq. 4.19)
Heat balance for the combined generator and heat exchanger control volume gives
Qg=m10h5+mRh7-msh2…………………………………………………………..(eq. 4.20)
Heat balance for the absorber gives QA
Qa= mwh6+mRh10-msh1……………………………………...…………………..(eq.4.21)
The above equations are governed by the first law of thermodynamics in the form
Qc+Qa= Qg+Qe…………………………………………………...………….…..(eq. 4.22)
4.3.7 COEFFICIENT OF PERFORMANCE(COP)This is defined as COP= (refrigeration effect)/ (external heat input)
COP = Qe/Qg…………………………………………………..…..(eq. 4.23)
4.3.8 IDEAL COEFFICIENT OF PERFORMANCEThe maximum coefficient of performance of the absorption cycle is given by
COPmax =te (tg-ta)/ tg (tc-te) ……………………………………………………..…..(eq. 4.24)
The ratio COPactual/COPmax is called the ‘relative performance ratio’ to show the deviation from
reversible cycle operation.
4.4 MODEL CALCULATION35
X1(strong solution) = (49.04+1.125ta-te)/ (134.65+0.47ta) kg LiBr/kg solution
= (49.04+ (1.125*40)-10)/(134.65+0.47*40)
= 0.547
X4(weak solution) = (49.04+1.125tg-tc) / (134.65+0.47tg) kg LiBr/kg solution
= (49.04+(1.125*97)-40) / (134.65+0.47*97)
= 0.655
log10Pe = 0.01-(2.068/te+273.15)-(149.51/(te+273.15)2)
= 0.01-(2.068/283.15)-(149.51/283.152)
Pe =7.8 kPa = 0.078 bar
log10Pc = 0.01-(2.068/tc+273.15)-(149.51/(tc+273.15)2)
= 0.01-(2.068/313.15)-(149.51/313.152)
= 1.004 kPa = 0.01 bar
h8 = (t-25)*4.186 kJ/kg
= (40-25)*4.186
=62.79 kJ/kg
h10 = (2397.74+1.745te) kJ/kg
= (2.97.974+1.745*10)
= 2415.19 kJ/kg
mR = QE / (h10- h8).
= 3.5167/(2415.19-62.79)
= 0.00148 kg/s
mw= (QE / (h10- h8)) (X1 / (X4-X1))
= 0.00148(0.547/(0.655-0.547))
= 0.0075 kg/s
36
ms= (QE / (h10- h8)) (X4 / (X4-X1))
= 0.00148(0.655(0.655-0.547))
= 0.009 kg/s
CX1= 4.227-5.148X1+2.01X12 kJ/kg
= 4.227-(5.148*0.547)+(2.01*0.5472)
= 2.01 kJ/kg
CX4= 4.227-5.148X4+2.01X42 kJ/kg
= 4.227-(5.148*0.655)+(2.010.6552) = 1.71 kJ/kg
t5 = tg- EL*(tg-ta)
= 97-(0.6*(97-40)
= 62.80C
t3= te + ( EL (X1*CX4)(X4*CX1)(tg-ta))
=10+(0.6*(0.547*1.71)(0.655*2.01)(97-40)
=64.380C
h1 =(179.2-1782.9X1+1693.95X12)+(4.23-5.15X1+2.01X1
2)(ta)
=(179.2-(1782.9*0.547)+(1693.95*0.5472))+(4.23-(5.15*0.547)+(2.01*0.5472))*40
= (179.2-975.25+506.85)+(4.23-2.817+0.601)*40
= -208.64 Kj/kg
h5=(179.2-1782.9X4+1693.95X42)+(4.23-5.15X4+2.01X4
2)(t5)
=(179.2-(1782.9*0.655)+(1693.95*0.6552)) + (4.23-(5.15*0.655)+(2.01*0.6552))*62.8
= (179.2-1167.69+726.74)+(4.23-3.37+0.862)*62.8
= -153.6 KJ/kg
37
h7 = 2397.16+1.92tg-0.179tc = 2397.16 + (1.92*97)-(0.179*40) = 2576.24 KJ/kg
Qc= mr (h7-h8) = 0.00148*(2576.24-62.79) = 3.74 kW
Qg = mwh5+mRh7-msh1 = (0.0075*-173.24)+(0.00148*2576.24)-( 0.009*-206.98) =4.55 kW
Qa =mwh6+mRh10-msh1 = (0.007*-173.24)+(0.00148*2404.732)-(0.009*-206.98)= 4.31 kW
COP = QE/QG = 0.767
4.5 CONVENTIONAL CALCULATION The operting conditions for lithium bromide-water air conditioning system are given below
Generator temperature 97OC
Condenser temperature 40oC
Evaporator temperature 10oC
Absorber temperature 40 oC
38
Fig 4.3 schematic representation of of simple vapor absorption system with Liquid – liquid
regenerative heat exchanger
Form the fig 4.3 and form the table of the water vapour pressure in the Appendix we obtain the
condenser and evaporator pressures corresponding to their respective temperatures.
Codenser and generator pressure
Pc=7.357 kPa (at 40 oC) = 0.0735 bar
Absorber and evaporator pressure
Pe=1.224 kPa = 0.012 bar
39
Fig 4.4 Representation of Absorption cycle on the p-1/T Diagram
Now from fig 4.4 we get first the lithium bromide and then the refrigerant water concentration in
rich and poor solutions at states 4 and 2
State 4 Saturated cold solution form the absorber at
Pe=1.224 kPa and t=40oC
Xlibr=0.55 kg of LiBr per kg of solution
h4=93.5 kJ/kg(form h-x diagram)
Rich solution concentration of water
Xr= 1-0.55=0.45 kg of water per kg of solution
State 2 saturated hot solution from generator at
P=7.357 kPa and at tg=97 oC
Xlibr=0.65 kg of LiBr per kg of solution
h2=248 KJ/kg(from h-x diagram)
Poor solution concentration of water
Xr=0.35 kg of water per kg of solution40
State 1 saturated solution at condenser pressure and 0.55LiBr concentration
t1=74 oC (from fig 4.2)
h1=66 KJ/kg(h-x diagram)
State 3 saturated solution at evaporator pressure and 0.65LiBr concentration
T3=60 oC (from fig 4.2)
h3=180 KJ/kg(h-x diagram)
State point 3a has the same enthalpy, temperature and composition as state 3.but is at generator
pressure. Its represents a state sub cooled from 2 to 3 at55.32mm hg pressure
State 4a te=4 oC and Xlibr=0.55
h4a=h4=93.5 KJ/kg(neglecting pump work)
Specific solution circulation rates
f = (1-Xa)/(Xr - Xa)……………………………………………………….(eq. 4.25)
f = (1-0.35)/(0.45-0.35)
f = 6.5 kg/ kg of vapour
Heat available in the hot solution for transfer = (f-1)*(h2-h3)……………...............…….(eq. 4.26)
= 5.5*(248-180) = 374 kJ
Heat required by cold solution for heating = f*(h1-h4)……………………...……………(eq. 4.27)
= 6.58 (166-93.5) = 471 kJ >374 kJ
Hence, cold solution at 4a cannot be heated to 1.let it be heated to 1a.
State 1a. energy balance of the liquid – liquid heat exchanger gives
f (h1a – h4) = (f-1) (h2-h3)
where h1a = h4+((f-1)/f)*(h2-h3)……………………………………………..……………(eq. 4.28)
= 93.5+ (5.5/6.5)*(248-180) = 151 kJ/kg
41
State 5 It is that of water vapour at 7.35 kPa pressure and 97 0C temperature. At these conditions
it represents a superheated vapour state. The enthalpy of water vapour above the reference state
of saturated water at 00C had taken from empirical relation.
h=2051+1.88t………………………………………………………….…(eq. 4.29)
h5=2051+1.88(97)=2863 kJ/kg
State 6
Saturated water at 40 oC
h6=4.188(40) =167.5 kJ/kg
State 7 p=1.224 kPa and at t=10 oC (liquid+vapor)
h7=h6=167.5 kJ/kg
State 8 p=1.224 kPa at t=10 oC (staturated vapor)
h8=2501+1.88(10)=2520 kJ/kg
Refrigerating effect qe= h8-h7=2520-167.5=2532.5 kJ/kg
Heat added in the generator per unit mass of vapour distilled
qg=h5-h2 + f*(h2-h1a)……………………………………………………..(eq.4.30)
=2683-248+6.5*(248-151)
qg = 3066 kJ/ kg of vapour
Coefficient of Performance, COP= qe/ qg=0.77
Water vapour distilled per ton refrigeration
D=211/qe
=211/2532.5
=0.09kg/min
Mass flow rate of cold solution from the absorber
F=f*D=6.5(0.09)=0.495kg/min
42
Heat rejected in the condenser
Qc=D/60(h5-h6)…………………………………………………………… (eq 4.31)
=0.09/60(2683-167.5)
=3.77 kW
Heat rejected in the absorber
Qa=D*qa=D[(h8-h3)+f(h3-h4)]…………………………………………….(eq 4.32)
=0.009/60[(2520-180)+6.5(180-93.5)]
=4.35 kW
Heat supplied in the generator
Qg=D*qh=0.09/60(3066)
=4.6 kW
Properties Conventional calculations Calculations using
mathematical model
Qc 3.77 kW 3.74 kW
Qg 4.6 kW 4.56 kW
Qa 4.35 kW 4.32 kW
COP 0.77 0.767
Table 4.1 comparision between conventional and calculated results
As it is seen that the values obtained from the conventional calculations and that from the
mathematical calculation are approximately same, so the mathematical model is used for doing
first law analysis of vapor absorption system and by using this mathematical model a java
program has been developed for the ease of the calculations.
43
Chapter 5
RESULTS AND DISCUSIONS
The first law analysis is done on lithium bromide/water vapor absorption system. Table 5.1 and 5.2
shows the results for the thermodynamic properties and heat transfer rates of each component
respectively. In this analysis ,calculation were performed for 3.5kw cooling load and the parameters were
taken as te=4oC, tc=40 oC,,ta=40 oC, tg=97 oC effectiveness є=0.8. in table 5.1 chemical composition and
mass flow rate are provided along with temperature, concentration and enthalpy values of the working
fluids. As seen from the table 5.2 compared to other components the generator heat transfer rate is the
highest.
State pointChemical
composition
Temperature
TOC
Concentration
%
Enthalpy
kJ/kg
1 Water/LiBr 74 0.55 66
2 Water/LiBr 97 0.65 248
3 Water/LiBr 60 0.65 180
4 Water/LiBr 40 0.55 93.5
5 Vapour 97 0 2863
6 Water 40 0 167.5
7 Water 10 0 167.5
8 Vapour 10 0 2520
Table 5.1 thermodynamic properties of each point
44
Component Heat transfer rates (kW)
Absorber Qa 4.35
Condenser Qc 3.77
Generator Qg 4.6
Evaporator Qe 3.51
Performance parameters of ARS
Circulation ratio f 6.5 kg/kg vapour
Coefficient of performance COP 0.77
Table 5.2 heat transfer rate of the components and performance parameter of the system
Fig 5.1 heat transfer rate in each components
45
Fig 5.2 shows the relation between the generator temperature and coffecient of performance it shows that
as the generator temperature increases the COP increases keeping the evaporator temperature te at 6oC
condenser temperature Tc at 40oC and the absorber temperature ta at 40oC
Fig 5.2 variation of COP at different generator temperatures
As the generator temperature increases the net effect change in the enthalpies of vapour and weak
solution decreases thereby Qg decreases resulting in increase of COP.
Fig 5.3 shows the relation between the condenser temperature and coefficient of performance it shows
that as the condenser temperature increases the COP values decreases keeping tg=90oC te=6oC є=0.6
Qe=3.5 so the condensers should be maintained at low temperatures in order to attain high COP values at
low generator temperatures.
46
Fig 5.3 variation of COP at different condenser temperatures
With increase in the evaporator temperature the COP values increases keeping the generator temperature
Tg constant at 90oC. the evaporator temperature is varied in between 6 to 100C. The absorber and
condenser are maintained at 350C. The heat to be supplied for the generator is kept constant and the
variation in COP is observed by varying the evaporator temperature. As the evaporator temperature
increases the condensation required is less.
Fig 5.4 Variation of COP at different evaporator temperatures
Chapter 6
47
FABIRICATION OF ABSORPTION AIR
CONDITIONING SYSTEM
The vapor absorption system generally consists of a generator, a condenser, an absorber
an evaporator, a pump and a segregator. The condenser used is a conventional air cooled
condenser used in automobiles and coming to the evaporator instead of buying individual
components such as evaporator, fan, expansion device the whole evaporator unit had been
bought with fan and the expansion device connected to the evaporator placed inside a cabinet.
For the fan to run a 12v battery is provided. The selection of the condenser and the evaporator is
based on the calculations form the first law analysis of vapor absorption system as how much
heat needs to be transferred by them.
When the first working model was tested it was found that due to the pump the low pressures
which needs to be maintained for effective working is not being attained and more over the
separation of water vapor form LiBr/water solution is not happening. Hence the air conditioning
system was modified by removing the pump and combing the segregator and the generator into
one unit, and the circulation of the refrigerant will now be happening only due to its vapor
pressure.
Fig 6.1 Modified layout of absorption system
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6.1 ABSORBER
The absorber used is a cylinder made of mild steel, the main function of the absorber is it works
as a storage tank for LiBr/water solution and for the purpose of re-circulation.it consists of three
valves one for collecting the segregated LiBr solution that is the weak solution from the
segregator, the second for collecting water vapor coming from the evaporator and the third for
sending the LiBr/water solution to the generator for heating. Copper tube of 9mm diameter is
used for connecting the absorber and the generator to send LiBr/water solution to the generator
for heating, general rubber tubes used automobiles of diameter 15mm is used for connecting the
evaporator outlet and the absorber inlet through which water vapor flows. A flow indicating
valve is used for checking whether the strong solution is again being sent to the generator.
Fig 6.2 Absorber
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6.2 GENERATOR
The generator is also cylindrical in shape made of mild steel; here it works as both generator and
segregator. The main function of the generator is to heat the LiBr/water solution to saturation
temperature and the function of the segregator is to separate water vapor form the solution and
send the weak solution back to the absorber. The generator also has three valves one for
collecting of strong solution from the absorber, the second one for sending back the weak
solution back into the absorber and the third one for sending the water vapor into the condenser
for further process. The generator kept in a tilted position where the upper part when tilted acts
as a segregator which collects the water vapor and sends it to the condenser a tube is fitted inside
the generator and gas wielded through which the water vapor passes to go to the condenser.
Fig 6.3 Generator
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6.3 WORKING MODEL
Fig 6.4 Working model
Generator is provided with the electrical heater for which an ac current is provided. This
generator is packed with the insulating material by which the heat is not leaked to the
environment. By providing the insulation, time required to boil the water is reduced. The strong
solution would flow from the absorber to generator by gravity only. Thus the absorber is placed
above the generator. The insulated generator is placed in closed cabinet.
Due to the high temperature in the generator, the water in the solution will boil and vapor is
formed. Then the vapor is collected at the upper part of generator, this part will act as a rectifier.
The liquid molecules present in the water vapor are separated by providing baffles in the rectifier
(upper part of generator).
Air cooled condenser will receive hot vapor from the rectifier. A single flat tube of a condenser
will have 5 tubes of 5mm diameter. Vapor enters the condenser at the top opening and allowed to
flow. In this condensation process latent heat of vapor is removed and the vapor forms into
liquid.
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After condensation, the refrigerant enters the needle valve (expansion device). This expansion
device is provided at the inlet of evaporator. Condenser and expansion device is connected
through a 9 mm flexible pipe. Air conditioning of automobiles use these types of tubes only. This
throttling is an isenthalpic process. By expanding, the refrigerant pressure reduces and the low
pressure refrigerant enters the evaporator.
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Chapter 7
CONCLUSION
After studying and practical fabrication of vapor absorption system the following conclusion can
be made
To reduce the crystallization effect of lithium bromide, strong solutions should not be
maintained at low temperatures.
For better performance the condenser temperature should be maintained in between 35 to
400 C.
For small capacity absorption systems, pump should not be used in order to maintain
vacuum pressures.
Direct heat recovery is preferred due to the absence of intermediate working fluid.
FUTURE SCOPE
For better performance of the LiBr absorption system, double effect generation could be used.
For providing air conditioning at low generator temperatures vapor adsorption system can used
having R-134a as refrigerant and activated carbon as adsorbent.
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