ijaerv9n6spl_06.pdf

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International Journal of Applied Engineering Research

ISSN 0973-4562 Volume 9, Number 6 (2014) pp. 655-662

Research India Publications

http://www.ripublication.com

Theoretical Analysis of Modified Refrigeration Cycle of a

Single Effect Lithium Bromide-Water Vapour Absorption

System Using Exhaust Gases of IC Engine

Raghvendra Kumar Singh1and Trinath Mahala

2

1

M. Tech (Automobile Engg.), Galgotias University, Greater Noida, U. P2Galgotias University, Greater Noida, U. P

Abstract:

As the utilization of automobiles is increasing, companies are launching their

improved vehicles to fulfill the needs of passengers. Air-conditioning is one of

the essential needs of passengers in vehicles. In an internal combustion engine,

55-60% of total energy of burnt fuels is waste, only 35-39% heat energy isconverted into useful work.

In road transport vapour compression refrigeration system is commonlyused for air conditioning purpose. In this system compressor extracts the

energy from engine, so engine has to do some extra work which results in

increase of fuel consumption.

In this research, LiBr-H2O vapour absorption system driven by exhaust

gases of the engine is used in the automobiles for air-conditioning purpose.

Because of this there is improvement in the engine efficiency and reduction inexhaust emission. The exhaust gases of the engine are used as a heat sources

in the generator thus it avoids the extraction of power from the engine. Forbetter improvement of COP LiBr-H2O vapour absorption cycle is modified. In

this modification, another heat exchanger between evaporator and generator is

incorporated. From the evaporator, 20% of mass of refrigerant flow to thegenerator through this heat exchanger. After this modification it is found that

the maximum COP is increased to 0. 9451 from 0. 7961. Maximum COP is

achieved when generator temperature is 81. 20C, condenser and absorber

temperature are 31. 7C and evaporator temperature is 8. 6C. The COP of the

system is improved by 18. 72%.

Nomenclature

COP coefficient of performance

a absorber


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656 Raghvendra Kumar Singh and Trinath Mahala

Mws mass flow rate of weak solution

c condenser

X mass fraction of LiBr in solutiong generator

Cp1 specific heat of rich LiBr solutione evaporator

Cp2 specific heat of weak LiBr solution

i in

Q heat transfer rate

o out

Cpws specific heat of saturated steam

hn enthalpy at respective point from evaporator

Cpw specific heat of superheated steam represent sum of from generator

n represent state pointsm mass flow rate

Introduction:

Refrigeration and air-conditioning is the most important facility in human comfort in

the modern day. Due to some refrigerant deplete the ozone layer; the interest in

lithium bromide-H2O absorption refrigeration system has been growing. In his system

waste heat or sunlight can be used as energy source.

In automobiles the current air-conditioning system is based on vapourcompression refrigeration system. Due to extraction of the power from the engine bythe compressor, it affects the fuel economy. To improve fuel economy the vapour

absorption system can be used.

Eisa et al.[1]

did an experiment on LiBr-water absorption cooling system and

found that the COP of the system is increases with the increment in generator

temperature and COP is decreased as the condenser and absorber temperatures are

increased. Joudi and Lafta [2]conducted a study on LiBr-H2O cooling machine, for the

analysis of heat and mass transfer processes in the absorber using finite difference

analysis. They had shown that COP and cooling capacity is increased with sourcetemperature. Jason et al. [3]proposed a detailed solution procedure and validated and

shows that the performance of the system evaluated based on the circulation ratiowhich is measured of the system size and cost.

Vapour absorption system description:A systematic representation of simple single effect vapour absorption system is shown

in fig. 1. Vapour absorption consists 8 main components: a generator, a condenser, a

absorber, a evaporator, a pump, 2 throttle valves and a heat exchanger. The working

fluid is LiBr-H2O solution where LiBr used as absorbent and water is used asrefrigerant.


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Mass balance

mi = mo (1)

Material balance

moXi = meXo (2)

Energy balanceQ-W= moho-mihi (3)

Where are Q is the heat transfer rate and W is the work transfer rate.

Heat exchanger calculation

1= (T3-Ta) / (Tg-Ta) (4)

T5= Tg-Mws*Cp2* (T3-T2)/ (M4*Cp1) (5)2= (T12-T11)/ (T7-T11) (6)

T7p=T7-M11*Cpw* (T12-T11)/ (M7*Cpws) (7)

Where 1 and 2are the effectiveness of the heat exchanger-1 and heat exchanger-

2 respectively and specific heats of LiBr-H2O solutions are calculated[6]as:

Cp1=A0+A1. Xrich+ (B0+B1. Xrich). Tg (8)Cp2=A0+A1. Xweak+ (B0+B1. Xweak). Ta (9)

Where A0, A1, B0, B1 are constants and the value of these constants are

A0= 3. 462023A1=-2. 679895/100

B0= 1. 3499/1000

B1=-6. 55/10000000

The COP of the system is defined as

COP =

(Assumed pump work Wp= 0) (10)

Assumptions

The thermodynamic analysis presented here is based on these assumptions: The heat losses from the components of the system are negligible.

The refrigerant leaving the condenser and evaporator assumed to be saturated

condition.

Refrigerant leaving generator and absorber are in supersaturated in

equilibrium conditions at their respective temperature, pressure andconcentrations.

There is the negligible pump work.

There are no temperature losses during flow from one component to other.


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Effect of variation in absorber & condenser temperatureThe effect of the variation in the absorber & condenser temperatures on the COP is

illustrated in Fig. 3a & 3b. The increment in the absorber temperature, specificcirculation ratio is also increased which increases the generator heat duty and pump

work. The mass flow rate of refrigerant is assumed constant so evaporator load isconstant.

Fig. 3a

Fig. 3b

Fig. 3a Effect of absorber temperature on COP in single effect cycle and Fig. 3b effect

of absorber temperature on COP in modified cycle (Tg=80C, Ta=Tc, 1=0. 7 and

2=0. 1)

Effect of evaporator temperatureFig 4a and 4b show the effect of evaporator temperature on COP. The COP of both

systems increases with increase in evaporator temperature. Evaporator temperaturevaries 4C-10C at different generator and evaporator temperatures.


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Theoretical Analysis of Modified Refrigeration Cycle 661

Fig. 4a

Fig. 4b

Fig. 4a Effect of evaporator temperature on COP in simple cycle and Fig. 4b effect of

absorber temperature on COP in modified cycle (Tg=80C, Ta=Tc, 1=0. 7 and 2=0.

1)

ConclusionsA computer program is developed to compare the performance of simple cycle and

modified refrigeration cycle of single effect vapour absorption system. The various

effects are in variation in absorber, condenser evaporator and generator temperature

on COP. It is shown that an increase in the generator temperature increases the COP

up to an optimum generator temperature. The COP of the modified cycle of single

effect vapour absorption system is nearly 18. 72% greater than simple cycle of LiBr-H2O vapour absorption system. The optimum value of COP is achieved at generator

temperature is 81. 20C, condenser and absorber temperature are 31. 7C and

evaporator temperature is 8. 6C. It is also shown that increase in evaporator


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temperature increases the COP while increase in absorber and condenser temperature

decreases the COP. The analysis of both cycles proves that the performance of the

modified cycle is higher than simple cycle of LiBr-H2O vapour absorption system.

References

[1] Eisa MAR, Diggory PJ, Holland FA. Experimental studies to determine the

effect of difference in absorber and condenser temperatures on the

performance of a water-lithium bromide absorption cooler. Energy Convers

Manag 1987; 27 (2): 253e9.

[2] Joudi KA, Lafta AH. Simulation of a simple absorption refrigeration system.

Energy Convers Manag 2001; 42 (13): 1575e605.[3] Jason Wonchala, Maxwell Hazledine, Kiari Goni Boulama. Solution

procedure and performance evaluation for a water LiBr absorptionrefrigeration machine. Energy 65 (2014): 272e284

[5] Y. Kaita. Thermodynamic analysis of lithium bromide-water solution at high

temperature. International journal of refrigeration 24 (2001) 374-390

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ijaerv9n6spl_06.pdf