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International e-Journal For Technology And Research-2017

A Review on Performance Comparison of VCRs &VARs 1

Madhvendra Kumar Dwivedi, 2 Mr. Dheerandra Singh 1, 2

Mechanical Engineering Department Madan Mohan Malaviya University of technology Gorakhpur, India

ABSTRACT:

Vapour Absorption Refrigeration Systems (VARS) and vapour compression refrigeration system (VCRS) both are widely used for refrigeration purpose. The major differences in both the system are in vapour compression refrigeration systems the input supply is compressor work which is high grade of energy whereas in vapour absorption systems main input is heat supply in the generator which is low grade energy. Energy is an essential requirement for the economic development of any country, so our main aim is to save energy, unless VCRS have high performance future scope is VARS are primarily used in large commercial and industrial installations .In this paper discusses the various methods for increasing the performance of both systems so this paper focuses on the suitability of both systems.

Keywords: Refrigeration, Coefficient of Performance, Vapour Absorption Refrigeration System, Vapour Compression Refrigeration System.

range of temperature for high coefficient of performance. In this system the low pressure temperature of the vapour refrigerant is enters into the compressor through the inlet valve where it’s compressed to a high temperature and pressure and this refrigerant is discharge into the condenser where heat is removed. An automatic expansion valve is used for reducing the pressure and this low pressure refrigerant enters into the evaporator [4].

1. INTRODUCTION 1.1. Vapour compression refrigeration system (VCRS).

When the term “refrigeration” comes we make up the mind about refrigerator that works on the electrical supply and provides us cooling, ice and preserves our beverages and food items from getting exhausted. In these types of refrigerators the components that are used are compressor, condenser, and evaporator and expansion valves.

Basic principle taken from first and second law of thermodynamics .This system would have wide IDL - International Digital Library

Fig 1- A simple VCRS [4] 1.2. Vapour Absorption refrigeration system (VARS).

The vapour absorption refrigeration system (VARS) comprises all processes in the vapour compression refrigeration system like Condensation, Expansion and Evaporation are same only change is replacing the Compressor with 1|P a g e

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International e-Journal For Technology And Research-2017 Generator Absorber and Pump. In the vapour absorption system the refrigerant used is ammoniawater, lithium bromide etc. The refrigerant gets condensed in the condenser and it gets evaporated when enter in the evaporator. Low pressure ammonia vapour increases its pressure and enters into the generator by Pump. Weak solution reverses back the absorber whereas strong solution enters the condenser through the liquid separator. The refrigerant produces cooling effect in the evaporator and releases heat to the atmosphere via the condenser [3].VARS are primarily used in large commercial and industrial installations. Compared with vapour-compression systems, ARS have one major advantage. The mixture should be safe, chemically stable, noncorrosive, and inexpensive and should be available easily. The refrigerant should have high heat of vaporization [17].A liquid is compressed instead of a vapour and as a result the work input is very small (on the order of one percent of the heat supplied to the generator) The NH3-H2O mixture is environmental friendly which is the only useful working pair currently used for refrigeration purposes in absorption systems, and despite of the new mixtures under investigation, the ammonia- Water mixture is the only one with a clear future [14]

R. A. Saidur et.al The COP of VCRS is higher than VARS for same temperature limit There is no any requirement of superheating and sub cooling phenomenon in VARS but in VCRS some time refrigerant which was in liquid form is the serious problem for the compressor. In VARS liquid refrigerant in the evaporator is not a serious problem. The COP of VCRS decreases at part loads but VARS does not .Also the VCRS has the severity of noise and vibration along with requirement of maintenance more than that of a VARS. The VARS are the echo friendly so there are no any issue regarding environment for using. [10] P. L.Vikas et.al. The result shows that the hot vapour from the generator which enters the evaporator coil without disturbing its phase and thus reduced the cooling effect .Concentration of Ammonia was design for 50% but commercially 25% concentration is available in the market [14].

V. K Gupta et.al .The system therefore has less power consumption and higher capacity due to the compression of flash gas only from the intermediate pressure which is higher than evaporating pressure and a low quality of refrigerant which enters into evaporator. Its exergetic efficiencies were 26% and 24% higher than these of the conventional single-stage cycle for R12 and R134a, respectively although the addition of extra components such as flash tank, expansion valves etc causes extra irreversibility. Total overall irreversible losses are lower to the single stage cycle. [15] When the temperature increases, heat pumps with a single-stage cycle become increasingly inefficient. This is especially true for conventional air-source heat pumps operating in colder climates. The disadvantages may be overcome by operating the two-stage heat pump of compression/expansion. Figure 3 shows a two-stage cycle utilizing for flash tank economizer. The flash tank has three functions: separated the liquid and vapour phases, desuperheat the discharge gas from low pressure compressor and to cool the liquid with the condenser to the saturated temperature for the intermediate pressure.[15]

Figure: 2 A simple vapour absorption refrigeration systems [1]

2. Literature Review on VARS &VCRS IDL - International Digital Library

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International e-Journal For Technology And Research-2017 increase, decrease and remain same depends upon the refrigerants.

3.3 Effect of increase in condenser pressure:

Figure: 3 Two-stage heat Pump

Siddhesh S. Arote et.al .Electric based VARS and Evaluate the COP for mixture of lithium bromide water and ammonia-water. When exhaust gas as a source of energy the same COP obtained and observed that it depends on the working conditions of the condenser, absorber, evaporating temperature, and generator. The water-ammonia vapour concept for utilizing exhaust waste heat, because it is the important source of energy, only to operate the pump very small electric power used.

The input heat energy potential power is the source for VARS [16].

Khaled S.et.al. Worked on the topic for liquid absorption and solid absorption system for household, industrial and automobile applications .They show that liquid absorption is more common in industry applications, where as solid absorption systems were suitable for running automobiles, owing for their limitations of liquid handling, with the recycle of waste heat and also the utilization of clean solar energy [18].

When increasing the condenser pressure this causes Reduction in refrigerating effect (RE), increase in compressor Work because it is the function of initial temperature and pressure ratio.

Hence Reduction in COP, by increasing the pressure ratio volumetric efficiency decreases. In simple languages volumetric efficiency is the breathing capacity of the compressor [8].

Figure:4 Increase in condenser pressure:

3.4 Effect of decrease in evaporator pressure: When decrease the evaporator pressure or low pressure no change in refrigerating effect (RE), by increasing the pressure ratio increase in work input so COP decreases and due to increase in pressure ratio volumetric efficiency decreases.

3. Performance of V- C System.

3.1 Effect of sub cooling: When sub cooling perform in the condenser refrigerating Effect (RE) increases there are no change work input so by the sub cooling COP increases.

3.2 Effect of superheating (in evaporator): If superheating is done there is increase in refrigerating Effect (RE) for the same pressure ratio refrigerants work input increases so COP may

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International e-Journal For Technology And Research-2017 Figure: 5 decrease in evaporator pressure

3.5 Effect of flash chamber use in V-C system: Thermodynamically there is no change in refrigerating effect & cop of the system when using flash chamber only liquid refrigerant was absorbing heat from storage space and after using flash chamber it is the same liquid refrigerant which absorb heat from storage space. At the inlet of evaporator only liquid refrigerant absorbs heat from the storage so no need of vapour refrigerants in evaporator therefore by using flash chamber liquid refrigerants separate from vapour at the inlet of evaporator so this decreases the size of evaporator [6].

3.6 Decreasing the throttling losses

coil of heat exchanger by the heat it was absorbed and compressed in the second and pass through the cooling chamber [5]

4. Types of vapour absorption refrigeration system 4.1: Water-Lithium Bromide (LiBr-H2O) Systemfor moderate temperatures (0ºC and above) applications especially in large capacity air conditioning system. Here H2O is the refrigerant and Li Br is the absorbent. The Lithium BromideWater pair since water is use as refrigerant for these reasons Li-Br - H2O systems are becoming more popular. [4].

In VCRS throttling is one of the irreversible processes so this device is one of the main causes of irreversible loss in cycle efficiency. A portion of the refrigerant flashing to vapour during the expansion process is reduces this losses. Other way

4.2 Ammonia-Water (NH3- H2O) System-for low temperature less than 0℃ refrigeration applications with NH3 as refrigerant and as H2O as a absorbent [2].There is another substance used in combination with refrigerant and that is known as absorbent.

These pairs are frequently used in industrial purpose as cold storage; ice plant etc where minimum temp. is requirement.

To eliminate these throttling losses is to change the single-stage expansion to a multistage expansion where flash gas is removed after each throttling stage. For smaller expansions and compressions reduce the refrigerant pressure to the required pressure of evaporator. [7]

5 Modification of Vapour Absorption Refrigeration System [5]

3.7 Cascading of vapour compression system

There are some drawbacks which induced in VARS are as below:

In the cascade system consists there are two or three separate simple cycles operating in adding with each other at different temperature levels. The connecting point is a heat exchanger between the different stages. This interstate heat exchanger is the condenser for the first stage and the second stage is the evaporator .Starting with the low pressure cycle, the vapour from the evaporator was compressed into the different stage compressor and send to the interstate heat exchanger where it rejects up its heat to the second evaporator coil.

The condensed liquid was flows to the first stage expansion valve and the evaporator, complete the low pressure cycle. The vapour generated in the IDL - International Digital Library



The rich solution entering the generator are low temperature and the poor solution which was entering the absorber are at high temperature.



.Present of absorbent in the vapors leaving the generator and going to refrigerant circuit.

These two are discussed along with proposed modification.

5.1. Used liquid-liquid Heat exchanger in the solution channel.

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International e-Journal For Technology And Research-2017 The saturated rich solution must be heated from the absorber temp. (ta) to the bubble temp t1 where the saturated poor solution must be cooled to t 3 in the absorber. This was insufficient since useful heat must be added in generator and rejected in absorber. System can be improved by inducing regenerative heat exchanger .Thus reduction in the amount of heat added in generator and increase COP. Size would also reduce in this system .[7]

5.2. Using analyzers - the exhausting column and Dephlegmator In NH3-H2O system the vapour coming out from the generator containing considerable amount of absorbent vapour which reaches evaporator after condensation this cause evaporation would not be isothermal and required low temp. Would not be reach in the evaporator.fig 7

6. Components of VARS 6.1 Generator: Generator denotes as the power house of absorption refrigeration system. . Heat is then supplied to the mixture, due to refrigerants vaporizes and leaves a weak solution of ammonia where the weak solution would drain out through the pressure reducing valve from other side of absorber. Heat supply to Generator uses waste energy sources so it is useful in future or its higher scope.

6.2 Condenser:

Fig 6- Diagram shows cooling and heating of poor and rich solution.

The condenser is a finned tube heat exchanger where coolant separated in both the low� generator and the single� effect generator is directly condensed by the outdoor air or water. Refrigerant flows inside a cooper tube while air or water circulates through the aluminum fins in a cross flow configuration. As well know, the heat transfer coefficients of the air are quite low but both have the purpose of cooling.

6.3 Evaporator absorber assembly:

Fig 7- Diagram shows the effect of absorbent in the evaporator.

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The evaporator and the absorber are assembled together in the same chamber for the purpose of water vapor generated in the evaporator is directly absorbed by the solution in the absorber. This minimizes the piping between both components it’s also reduces the pressure drop and consequently improves the absorption process when the chilled water circulates inside a cooper coil and the refrigerant drops on its outer surface. With the purpose of improving the heat transfer from the chilled water to the refrigerant, both inner and outer 5|P a g e

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International e-Journal For Technology And Research-2017 surfaces of the tube coil tube are enhanced surfaces fins.

6.4 Expansion devices: Two expansion valves where installed in the first at the outlet of the condenser and the second at the exit of the sub-cooler. They consist of two flow restrictor devices generating a pressure drop in the refrigerant entering the evaporator chamber. These are hand controlled valves [2].

6.5 Pump: Pump helps to pumping of the rich solution from evaporator pressure to condenser pressure. Specific volume of liquid is small which was present at the entry of pump (W= − đ?‘‰đ?‘‘đ?‘? ) so pump work is negligible in VARS. [8]

7 Coefficient of performance of systems: COPVCRS =

đ??‘đ??žđ??&#x;đ??Ťđ??˘đ?? đ??žđ??Ťđ??šđ??­đ??˘đ??§đ?? đ??„đ??&#x;đ??&#x;đ??žđ??œđ??­ đ??–đ??¨đ??Ťđ??¤ đ???đ??¨đ??§đ??ž

In case of VCRS performance should be increased either by increasing refrigerating effect or decrease work input to the system. COP of VACRS is usually greater than 1. [12] COPVARS=

đ??‡đ??žđ??šđ??­ đ??šđ??›đ??Źđ??¨đ??Ťđ??›đ??žđ??? đ??˘đ??§ đ??žđ??Żđ??šđ??Šđ??¨đ??Ťđ??šđ??­đ??¨đ??Ť đ??‡đ??žđ??šđ??­ đ??Źđ??Žđ??Šđ??Šđ??Ľđ??˛ đ??­đ??¨ đ?? đ??žđ??§đ??žđ??Ťđ??šđ??­đ??¨đ??Ť

In vase of VARS performance should be increased by decreasing the heat supply to the generator.COP of VARS is usually less than 1.

8 CONCLUSIONS: In VARS liquid refrigerant are not a serious problem while is in VCRS it have very large demerits. Unless VARS have low COP, this system is more preferable because of scarcity of electricity and also waste heat is available and waste heat which was the heat input of this system.

When increasing the concentration of ammonia at the absorber exit COP continuously increases IDL - International Digital Library

because energy supplied to the generator decreases which is the only input for the system. Also, the absorber heat rejection decreases, so the requirement of cooling media (water) decreases.

Apart from lower COP of vapour absorption refrigeration system it is better to use because of its eco-friendly in nature or behaviour.

9 REFERENCES: [1] S.C. Kaushik, Akhilesh Arora, (2009), Energy and exergy analysis of single effect and series flow double effect water–lithium bromide absorption refrigeration systems. 1247 – 1258 [2]Demirocak, Dervis (2008), Emerging thermodynamic and economic analysis of a solar thermal powered adsorption cooling system, Oct2008.

[3] Gomri, R (2009)., Second law comparison of single effect and double effect vapour absorption refrigeration systems, Energy Conversion and Management, Vol. 50, pp. (1279-1287 ).

[4] Nwanya Stephen Chijioke, (2010), use of energy method to simulate the performance of LiBr-H2O absorption system, Nigerian journal of technology, vol. 29 no.1.

[5] Uppal AH, Norton B, Probert SD (1986), A low-cost solar-energy stimulated absorption refrigerator for vaccine storage Applied Energy 1986; 25: 167–74.

[6] Nguyen Q. Minh, Neil James Hewitt, Philip Charles Eames (2006) Improved Vapour Compression Refrigeration Cycles� International Refrigeration and Air Conditioning Conference.

[7] Sohail Bux, A.C.Tiwari (2014), Natural Refrigerants Based Automobile Air Conditioning System, International Journal of Emerging Science and Engineering (IJESE) ISSN: 2319–6378, Volume-2.

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International e-Journal For Technology And Research-2017 [8] V.D.Patela, A.J.Chaudhari, R.D.Jilte (2012), Theoretical and Experimental Evaluation of Vapour Absorption Refrigeration System, International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 National Conference on Emerging Trends in Engineering & Technology [9] Arora .P.C., Domkundwar.S. S.(2000), A Course in Refrigeration and Air-conditioning.

[10] Kyoto Protocol to the United Nations Framework Convention on Climate Change, 1997 [11] Arora. C.P. edition (2002),A Course in Refrigeration and Air-Conditioning,Tata McGraw Hill, New Delhi.

[12] Kumar, .D. S. (2017), Refrigeration & Air-Conditioning First edition, S.K. Kataria & Sons New Delhi.

[13] R. A. Saidur, M.Rezaei A, W.K.Muzammil A, M.H.Hassan A, S.Paria A, M. Hasan uzzaman (2007),Technologies to recover exhaust heat from internal combustion engines Elsevier .

[14] Sreeshankar K. K, Vikas P L, Sooraj (2013), Vapour Absorption Refrigeration System For Cold

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Storage & Power Generation in Automobiles Using Exhaust Gas circulation [15] Gupta, V. K., Prasad, M.(1984), Graphic Estimation of Design Parameters for Two-Stage Ammonia Refrigerating Systems Parametrically Optimized, Mech. Engg. Bull., vol.15,ppno-100104 [16] Prof. Nilesh B. Totla, Siddhesh S. Arote, Sujaya V. Gaikwad, Shubham Kattimani (2016) , Comparison of the Performances of NH3-H20 and Libr-H2O Vapour Absorption Refrigeration Cycles.

Prof. Nilesh B. Totla.et al. Int. Journal of Engineering Research and Application www.ijera.com ISSN: 2248-9622, Vol. 6, Issue 4, (Part - 7), pp.08-13 [17] Subhash Kumar, Dr.R.RArakerimath, (2015) Comparative Study On Performance Analysis Of Vapour Absorption Refrigeration System Using Various Refrigerants IPASJ International Journal Of Mechanical Engineering (IIJME) Volume 3, Issue 1, Pg-07-11 [18] Khaled S.(2011), Evaluation And Performance of water- Ammonia Auto Air Conditioner System Using Exhaust Waste, Science-Direct Energy Procedia6,pp-467-476

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