Experimental Comparative energy, exergy flow and second ...

V.P. Venkataramanamurthy et al. / International Journal of Engineering Science and Technology Vol. 2(5), 2010, 1399-1412

Experimental Comparative energy, exergy flow and second law efficiency analysis of

R22, R436b vapour compression refrigeration cycles

V.P.VENKATARAMANAMURTHY*

Research Scholar, AU – CBE / Professor,

Dept. of Mechanical Engineering, K.S.Rangasamy College of Technology,

Tiruchengode 637 215 [email protected]

Dr.P.SENTHIL KUMAR

Prof., & Head, Dept. of Mechanical Engineering,

K.S.R College of Engineering, Tiruchengode 637 215

[email protected]

ABSTRACT

This study presents an experimental comparison of energy, exergy flow and second law efficiency of R22 and its substitute R436b (hydrocarbon mixture of 52% of propane (R290) 48% of isobutene (R600a)), vapour compression refrigeration cycles. The vapour compression refrigeration cycle was initially designed to operate with R22. The primary objective of this paper is to analyze the system processes separately and to identify and quantify the sites having largest energy and exergy flow losses .In addition the effect of various reference environment state on this analysis also presented .The exergy efficiency of the cycle is defined as the exergy flow rate in the evaporator to the isentropic compressor work. And second law efficiency is defined as the ratio of actual COP to the Carnot COP. Change in exergy flow rate, the exergy efficiency of components and piping of actual VCRC’s have been carried out under different evaporator temperature for both the cycles. The exergy flow of various points on vapour compression refrigeration cycle i.e. from 1-2, 2-3, 3-4, 4-5, and 5-1, exergy efficiency and second law efficiency for both R 22 and R436b refrigeration cycles are compared. Results have been presented graphically that shows the location of in efficiencies.

KEYWORDS: R22; R436b; Reference environment state; VCRC; Exergy flow rate; Exergy efficiency; Second law efficiency

1 INTRODUCTION AND LITERATURE REVIEW

The Montreal protocol 1. UNEP (1999) stipulate the phasing out of CFC s and HCFC s as refrigerants that deplete the ozone layer (ODP), while the Kyoto protocol 2. UNFCC (1998) encouraged promotion of policies for sustainable development and reduction of Global Warming Potential (GWP) including the regulations of HFCs .In the search for alternatives which have low Global Warming Potential (GWP) and reduced likelihood of other environmental impacts; natural refrigerants are gaining increased interest. Many researchers have reported that hydrocarbon mixed refrigerants is found to be an energy efficient and environment friendly alternative option to drop in replacement of R22 in vapour compression refrigeration system 3. Devotta, A. V. Waghmare, N. N. Sawant and B. M. Domkundwar (2001), 4. Mark W. Spatz and Samuel F. Yana Motta, (2004) 5. Devotta, A.S. Padalkar and N.K. Sane (2005) 6. Ki-Jung Park, Yun-Bo Shim, Dongsoo Jung (2008) 7. Ki-

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Jung Park and Dongsoo Jung. (2008) 8. Ki-Jung Park and Dongsoo Jung, (2009) 9. Ki-Jung Park, Yun-Bo Shim, Dongsoo Jung (2009). 10. Ki-Jung Park, Yun-Bo Shim and Dongsoo JungA. Now a days the R22 is replaced by hydrocarbon mixtures because the ozone depletion potential and global warming potential of R22 0.050 and 1810 11. Calm and Hourahan (2007) and the hydro carbons having zero ODP and low GWP. But they did only energy analysis but exergy analysis is first and foremost a technical tool for guiding efficiency improvement efforts in engineering and related fields 12. Marc A. Rosen (2002). Refrigerators are identified as major energy consuming domestic appliance in household environment 13. Radermacher and Kim, (1996). Energy is already conserved. What is not conserved is exergy, which is the useful work potential of the energy. Once exergy is wasted, it can never be recovered. When we use energy, we are not destroying any energy; we are merely converting it to a less useful form, a form of less exergy. On other side the energy savings in these appliances are very important for achieving a sustainable energy future. Thermal system unlike traditional energy analyses, the concept of exergy analysis combines both the first and second law of thermodynamics. Thus, exergy recognizes that energy not only has quantity but also quality. Consequently, different forms of energy can be compared directly using their exergetic values. The key to exergy analysis though is that, unlike energy, exergy can not only be transferred but also destroyed. Therefore, the total exergy of a system changes when it undergoes a real process due to exergy transfer and destruction. The parameter that quantifies the destruction of exergy is irreversibility. Thus, through a combination of evaluated exergy transfers and irreversibilities, the performance of thermal systems may be assessed and optimized. This degradation of energy, represented by an increase in entropy, is equivalent to the irretrievable loss of exergy due to all real processes being irreversible.

Thermodynamic processes in refrigeration systems release large amounts of heat to the environment. Heat transfer between the system and the surrounding environment takes place at a finite temperature difference, which is a major source of irreversibility for the cycle. Irreversibilities cause the system performance to degrade. The losses in the cycle need to be evaluated considering individual thermodynamic processes that make up the cycle. Energy (first law) 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. Exergy analysis is a powerful tool in the design, optimization, and performance evaluation of energy systems. The principles and methodologies of exergy analysis are well-established 14, 15. Bejan (1982) Bejan (1988) 16. Moran (1982) 17. Wark (1995)18. Cengel, and. Bole (2006) 19. Moran (1989) 20. Kotas (1995) 21.Ibrahim dincer(2003) 22. Ibrahim Dincer and Rosen (2004) 23. Rosen, Ibrahim Dincer, (2001) 24. Ibrahim Dincer and Cengel (2001.) The energy performance of vapour compression refrigeration systems is usually evaluated based on the first law of thermodynamics. However, compared to energy analysis, the exergy analysis can better and accurately show the location of inefficiencies. The results from exergy analysis can also be used to assess and optimize the performance of vapour compression refrigeration systems. In addition, integration of energy, entropy and exergy analysis can present a whole picture of the system performance. A number of applications of exergy analysis in vapour compression refrigeration system have shown its effectiveness. Akhilesh Arora, and Kaushik (2008) (25) computational model has been developed for computing Coefficient Of Performance (COP), exergy destruction, exergetic efficiency and efficiency defects for R502, R404A and R507A and derived a general expression for the optimum COP of refrigerator at minimum power input and given cooling load conditions. (26) Accadia (1998) and (27) wall (1991) used the exergoeconomic method blending second law and economic analysis for optimization of refrigeration cycle. Shilliday et.al (2009) (28). Explained detailed energy and exergy analysis of the low Global Warming Potential refrigerants R744 and R290 was preformed and compared against the commercial refrigerant R404A in a single-stage vapour compression cycle and R744 in a two-stage vapour compression cycle with an internal heat exchanger Recep Yumruta¸ Mehmet Kunduz , and Mehmet Kano glu(2002).(29) devolped A computational model based on the exergy analysis is for the investigation of the effects of the evaporating and condensing temperatures on the pressure losses, the exergy losses, the second law efficiency, and the Coefficient Of Performance (COP) of a vapor compression refrigeration. Mustapha Karkri et.al. (2009) (30) employed the first and second laws of thermodynamics to analyze the performance of an experimental Vapour-Compression Refrigerating Machine using R410A.

Ahmed Ouadha et.al(2005).(31) found the results of the exergy analysis of a two-stage refrigeration system operating between a constant evaporating temperature of -30°C and condensation temperatures of 30, 40, 50 and 60°C with two natural substitutes of HCFC22, namely, propane (R290) and ammonia (R717) as working fluids, are presented. It is found that the most significant losses occur in the compressors, expansion valves and condenser. Akhilesh Arora, and Sachdev (2009) (32) developed a computational model, in engineering equation solver software, it is employed for comparing the performance of these refrigerants in vapour compression refrigeration cycle. The parameters computed are volumetric cooling capacity, compressor discharge temperature, Coefficient of Performance (COP), exergetic efficiency and efficiency defects in system

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components. The results indicate that VCC, COP and exergetic efficiency for HCFC22 are higher in comparison with R422A, R422B, R422C and R422D. (33) Z. Oktay and I. Dincer (2009) explains the Problems with energy supply and use are related not only to global warming, but also to such environmental concerns as air pollution, ozone depletion, forest destruction, and emission of radioactive substances.. Aydın Dikici and Abdullah Akbulut (2008) (34) found the first law efficiency and exergetic efficiency of the whole system to be 65.6%, 30.8%, respectively. Consequently the energy and exergy loss analysis results show that the COP increase when the exergy loss of evaporator decrease. (35)Tahar KHIR, Rahim, JASSIM Galal M. ZAKI (2007) developed an optimization for the geometrical parameters of continuous fins on an array of tubes of a refrigeration evaporator by using exergy method. Jan Szargut (2001) (36) has been discussed the problem of calculation of partial exergy losses in cogeneration processes. Bilgen and Takahashi (2002) (37) carried out the Exergy analysis of heat pump air conditioner systems. The irreversibilities due to heat transfer and friction have been considered.

Mafi, Mousavi Naeynian and Amidpour (2009) (38) developed the equations of exergy destruction and exergetic efficiency for the main system components such as heat exchangers, compressors and expansion valves .The relations for total exergy destruction in the system and the system overall exergetic efficiency are obtained. Also, an expression for minimum work requirement for the refrigeration systems of olefin plants is developed. It shows that the minimum work depends only on the properties of incoming and outgoing process streams cooled or heated with refrigeration system and the ambient temperature Srinivasan, et.al,(2003 )(39) presents exergy charts for carbon dioxide (CO2) based on the new fundamental equation of state and the results of a thermodynamic analysis of conventional and trans-critical vapour compression refrigeration cycles using the data thereof Sarkar, Souvik Bhattacharyya and M. Ram Gopal (2005) (40) presents the exergetic analysis and optimization of a transcritical carbon dioxide based heat pump cycle for simultaneous heating and cooling applications. A computer model has been developed first to simulate the system at steady state for different operating conditions and then to evaluate the system performance based on COP as well as exergetic efficiency, including component wise irreversibility Reşat Selbaş Önder Kızılkan and Arzu Şencan (2006) (41) applied An exergy-based thermoeconomic optimization application to a subcooled and superheated vapor compression refrigeration system.. The application consists of determining the optimum heat exchanger areas with the corresponding optimum subcooling and superheating temperatures. A cost function is specified for the optimum conditions. All calculations are made for three refrigerants: R22, R134a, and R407c. Thermodynamic properties of refrigerants are formulated using the Artificial Neural Network methodology Morosuk and Tsatsaronis (2009 )(42) Splits the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts has many advantages for the detailed analysis of energy conversion systems. The paper presents the combined application of both concepts to vapor-compression refrigeration machines using different one-component working fluids (R125, R134a, R22 and R717) as well as azeotropic (R500) and zeotropic (R407C) mixtures. Aprea , De Rossi

, Greco , Renno( 2003) (43) presents an exergetic analysis of a vapour compressor refrigeration plant when the refrigeration capacity is controlled by varying the compressor speed. The aim is performance evaluation of both the whole plant and its individual components. The analysis of the exergy flow destroyed in each device of the plant varying the compressor speed has been carried out in order to determine the relative irreversibility of the plant components. The vapour compression plant is subjected to a commercially available cold store. The compressor working with R22, R407C and R507 and designed for a revolution speed corresponding to 50 Hz supply current frequency has been used varying the frequency in the range 30-50 Hz. In this range, the most suitable working fluids proposed as substitutes of R22, as R407C (R32/R125/R134a 23/25/52% in mass), R507 (R125/R143A 50/50% in mass) and R417A (R125/R134a/R600 46.6/50/3.4% in mass), have been tested.

In this paper, exergy flow analysis is applied to the vapor compression refrigeration cycle. The expressions for the exergy flow for the individual processes that make up the cycle as well as the coefficient of performance (COP) and exergy efficiency for the entire cycle are obtained. Effects of evaporating temperatures on the exergy losses, pressure losses, exergy efficiency and COP are investigated.

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Figure 1 Shows the saturated vapour pressure versus temperature for R22 and its substitute R436b hydrocarbon mixture of 58% of R290 42% of R600a

It was observed from Fig. 1 that the saturated vapour pressure for propane and isobutane mixture very near to the vapour pressure curves of the refrigerant R22 and can be used as a potential retrofit refrigerant [REFPROP] Lemmon, EW, Huber, ML, McLinden, (2007 ) (44)

Figure 2 Shows the actual vapour compression refrigeration cycle

2 MATERIALS AND METHODS

2.1EXERGY FLOW ANALYSIS.

The actual refrigeration capacity and COP, exergy flow rates of each component and piping of the refrigeration system were calculated as per the procedure followed by Ibrahim dincer(2003) (21 ) In exergy analysis we write exergy balance equation for each component of the refrigeration system and for the piping as well. In piping 1-2 the refrigerant generally experience a pressure drop and heat gain from the environment .2-3, mechanical work is supplied and converted into thermal energy. At the compressor discharge the substance is at high pressure and its flow availability is decreased .Then the substance passes through the condenser and leaves as saturated or sub cooled liquid 3-4 .in the condensing process, heat is transferred away from the refrigeration system .associated with heat transfer, exergy also is destructed in the system. Also, there is usually is a pressure drop across the condenser because of friction. When the substance goes through the expansion valve 4-5, it experiences a substantial pressure reduction while its enthalpy remains the same. in the final processes which takes place in the evaporator 5-1, the refrigerant receives heat from an external source and is changed back into vapour at either a saturated or slightly superheated state .Note that the term İ accounts for the time rate of exergy

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destruction due to irreversibilities within the system and is related to the rate of net entropy or entropy production After finding out the evaporator load, the mass of the refrigerant can be determined from

Qe=VXI (1) mr=Qe/h1-h5 (2) Condenser heat rejection rate Qcond=mr(h3-h4) (3) Assuming that the compressor is adiabaticWcomp=mr (h3-h2) (4)

Refrigerating effect = m cp dT / time Compressor work =10 *3600 / t * Emc

The energetic performance of the system is found by evaluating its coefficient of performance, between the

cooling capacity and compressor power

COP= Qe / Wcomp (5)

For piping 1-2

ΔĖf,1-2= -İ1-2 (6)

Where ΔĖf,1-2= ṁ (ef,2-ef,1) and İ1-2 =ToSgen =

- ṁTo [(s2-s1)-(h2-h1)/ To ]

Here ΔĖf,1-2 is the change in flow exergy rate ,kw.i is the irreversibility rate ,kw. Sgen is entropy generation rate

,kw/k.ef is specific flow exergy at the inlet and exit under consideration .kj/kg : t0 is the surrounding or reference

temperature ,k,and s is the specific entropy ,kj/kg k

The flow exergy rate difference for the compressor (2-3)

ΔĖf,2-3=-İ2-3=ṁ[(h3-h2)-To(s3-s2)] (7)

For condenser (3-4)

ΔĖf, 3-4=-İ3-4 = ṁ (ef,4-ef,3) and İ3-4 =ToSgen=-ṁTo[(s4-s3)+(h3-h4)/TH] (8)

For expansion valve (4-5)

ΔĖf,4-5=-I4-5 (9) Where ΔĖf, 4-5= ṁ(ef,5-

ef,4) and-İ4-5=-ToSgen= -ṁTo [ (s5-s4) ]

For evaporator (5-1)

ΔĖf, 5-1=-İ5-1 (10)

where ΔĖf, 5-1= ṁ(ef,1-ef,5)and -İ5-1=-ToSgen = -ṁTo [ (s1-s5)-(h1-h5)/ TL ]consequently, the exergy efficiency of the

cycle can be defined as follows

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η ex = ΔĖf, 5-1//w (11)

where the compressor work is the actual work ,not the isentropic one. Note that the value of ΔĖf, 5-1 is less than zero since its flow exergy decrease at the exit .in order to have efficiency positive, we drop the minus sign.

Second law efficiency is the ratio of actual cop to the Carnot cop.

ηII = COP actual / COP Carnot (12)

2.2 DESCRIPTION OF EXPERIMENTAL SETUP AND INSTRUMENTATION

An experimental setup of vapour compression refrigeration cycle was built up according to the National Standard of India to investigate the energy ,exergy flow rate and exergy efficiency of R22 and its substitute hydrocarbon mixture R436b (58% of R290 42% of R600a refrigerants) refrigeration cycles. Fig. 3 shows the schematic of the experimental setup. It consists of compressor, condenser, a filter-drier, expansion valve and evaporator. The compressor was hermitically sealed rotary compressor. In the condenser, the refrigerant flows through the inner tube while the air draw through the annular space between the inner and outer tubes. The evaporator cools the water. Puf insulation of 35 mm thickness for higher retention of cooling inside the freezer .A heater is provided at a bottom of the evaporator, whose output can be varied by a dimmer stat. Five pressure gauges are provided to measure pressures points after the evaporator ,before compressor ,after compressor ,after condenser and before evaporator . Eight suitable washer type RTD thermocouples are provided to measure temperatures after the evaporator ,middle of the evaporator,before compressor ,after compressor ,after condenser ,middle of the condenser ,before evaporator and ambient temperature . One energy meter with selector switch is provided to measure energy supplied to compressor and heater. Suitable electronic thermostat, voltmeter and ammeter are provided in the unit. The pressures at suction and discharge sides of the compressor were measured using a compound gauge manifold. This manifold is also used for charging the refrigeration system with the required refrigerant. The pressure gauges with resolution of 0.01 bar were used to indicate the pressure at points 1-2-3-4-5 in refrigerant circuit as shown in fig 2. The power consumed by the compressor, W, was measured using a power meter that has an accuracy of 0.01 kWh. These pressure gauges were calibrated using a dead weight tester. A digital watt meter which has 0.01 W resolution and a watt-hour meter were used to record the instantaneous power input to the refrigerator and the integrated energy consumption, respectively. A programmable charging meter of 1 g resolution was used to determine the amount of refrigerant charge input to the system during the charging process. The equipment was placed on a platform in a constant temperature room built with new heat insulation. Three heaters and an air-conditioner were used to control the room temperature. The fluctuations of the ambient temperature were ±0.5 0 C and the airflow velocity in the room was less than 0.25 m/s.

.

Figure 4 A schematic of the vapour-compression cycle with instrumentation

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Figure 5 Photograph view of experimental setup

Table 1 Specifications of main components

Evaporator Type - plate type Material copper, ms plate Length of evaporator panel 900 ± 1 mm Thickness of evaporator panel 35 ± 1 mm Passage way surface of panel 0.537 m2 Volume capacity of panel 0.0245m3 Outlet tube diameter 6.24 mm Maximum working pressure 10 bar Condenser Type Wire-on-tube Tube copper Tube diameter (inner/outer) 6.15/9.22 mm Outer surface area 0.549 m2 Maximum working pressure 30 bar Refrigerant R22 1010 g and R436b 360 g Compressor Number of cylinders One Type rotary Stroke volume 6.64 cm3 Power supply 1 phase, 220–240 V Current 6 A (starting), 1.2 (running) Frequency 50 Hz Speed 2900 rpm Capillary tube Material Copper Inner diameter 1.05 ± 0.02 mm Outer diameter 2.0 ± 0.05 mm Length 2.0 ± 0.03 m Initial charge 1010 g Parameters Measuring Instruments Measuring Space Accuracy Temperature washer Type RTD Thermocouple -50/100 °C 0.3 °C Pressure Bourdon Type Manometer -1/10, 0/34.33, 0/34.33, 0/34.33, 0/34.33 0.981/24.03 bar, 0.1/0.5 bar. Voltage Analog Voltmeter 0/400 V %1 Current Analog Ampermeter 0/20 A %1

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Table 2 Properties of refrigerants

Refrigerant Physical data Environmetal data Number

Chemical formula or blend composition, comman name.

Molecular mass

NBP (0C)

Tc (0C)

Pc Mpa

Safety group

Atm. life time

ODP

GWP 100 year

R22 Chclf2 86.47 -40.8 96.1 4.99 A1 12.0 .050

1810

R436b

52%of R290 48% of R600a 49.87 -33.4

114.0

4.053

A3 - 0 3

2.3 EXPERIMENTAL PROCEDURE

Initially, the system was flushed with nitrogen gas to 150 kgf/cm2 to check leakage ,to eliminate impurities, moisture and other foreign materials inside the system, which may affect the accuracy of the experimental results. The experiments were conducted according to ISO 8187 (45). To conduct no load pull down test, the evaporator door was kept open until temperature inside the freezer has reached the steady state condition with ambient. As per manufacturer’s recommendation, 1010 g of R22 was charged in the refrigerator for conducting baseline tests. During experimentation with R22, 2 m capillary tube length was used. The pull down characteristics and cycling running tests were carried out. The actual refrigeration capacity and COP ,exergy flow rates of each component and piping of the refrigeration system were calculated as per the procedure followed by Ibrahim dincer (21 ). The freezing capacity is determined by the mass of measurement packages placed in the frozen food storage compartment. These tests used 5 kg freezing load.The heater load was adjusted by a dimmerstat to maintain a temperature of −20 ± 0.5 ◦C inside the evaporator. The energy consumption of the compressor and heater were measured by energy meter. All the experimental observations were made after attaining the steady state conditions (4 h).The total system is kept in adiabatic room which is maintained at 35 0 CThen The cycling running tests were carried with different evaporator temperatures -18,-16,-12,-8−4,0,4,8 ,12,16 and 20 ◦C, the refrigerator was charged with R436b and these tests were repeated. Since the mixture is zeotrope, the refrigerant is charged in liquid state and the charge quantity was ensured with the help of electronic balance having an accuracy of ±0.01 g. In order to reduce the experimental uncertainties, experiments were repeated for five times and average values were considered. The variation in experimental values from the average value is within ±5%. Temperatures and Pressure at different locations were recorded at different evaporator temperatures. The instantaneous power consumption of the refrigerator during continuous running tests was measured after attaining the steady state condition. The energy consumption per day during cycling running tests was measured after 24 h by using a digital energy meter.

3 RESULTS AND DISCUSSIONS

comparative exergy flow analysis is presented for the investigation of the effects of the evaporating temperatures on the exergy flow losses, the second law efficiency, and the COP of a vapor compression refrigeration cycle. It is found that the evaporating temperatures have strong effects on the exergy flow losses is high from from 4-5 ,2-3,5-1,1-2 and 3-4.

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Figure 5 Comparison of freezer air temperature with time (no load) Pull down time in seconds

Figure 6 Comparison of discharge temperature with different evaporator temperatures

0

10

20

30

40

50

60

70

20 16 12 8 4 0 -4 -8 -12 -16 -18

Dis

ch

arg

e t

em

per

atu

r in

0C

Evaporator trmperature in 0 C

R22 R436b

0

500

1000

1500

2000

2500

20 16 12 8 4 0 -4 -8 -12 -16 -20 -24

Pu

ll d

ow

n t

ime

in s

eco

nd

s

Freezer air temperature

R22 R436b

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Figure 7 Comparison of COP at different evaporator temperature with fixed reference temperature (350C)

Figure 8 Comparison of refrigeration temperature vs time (with load )

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

20 16 12 8 4 0 -4 -8 -12 -16 -18

CO

P

Evaporator temperature 0C

R22 R436b

0

500

1000

1500

2000

2500

Pu

ll d

ow

n t

ime

in s

eco

nd

s

Evaporator temperature in 0C

R22 R436b

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Figure 9 Comparison of exergy flow rate of 1-2,2-3,3-4,4-5 and 5-1 at evaporator temperature 20 0C .

Figure 10 Comparison of exergy efficiency at different evaporator temperature with fixed reference temperature

0

0.5

1

1.5

2

2.5

3

3.5

4

20 16 12 8 4 0 -4 -8 -12 -16 -18

Ex

erg

y e

ffic

ien

cy in

%

Evaporator temperature in 0 C

R22 R436b

0

5

10

15

20

25

1-2, 2-3. 3-4, 4-5, 5-1.

Points on VCRC

Exe

rgy

flow

in k

wR22 R436b

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Figure 11 Comparison of seconlaw efficiency at different evaporator temperature with fixed reference temperature

and on the second law efficiency and COP of the cycle but little effects on the other components of the exergy losses. The second law efficiency and the COP increases, and the total exergy loss decreases with decreasing temperature difference between the evaporator and refrigerated space and between the condenser and outside air. The procedures given in this paper for the exergy analysis of vapor compression refrigeration cycles can be applied to actual cycles. To obtain good second law efficiency and minimum exergy the system has to be operated with 4 °C evaporator temperature, 35 to 40 °C con denser temperature, within 52 °C of compressor discharge temperature, and 14 °C of compressor suction temperature The exergy analysis along with the energy analysis of the vapour compression refrigeration cycle is presented. The analysis indicates that the second law efficiency is very low, although the first law efficiency is within a normal range .The reasons for such low exergy efficiency are due to large exergy destructions in the compressor and the condenser. As can be seen from Figure 9, the exergy flow from 4-5 is the largest contributor to the total exergy flow. The 2-3 is the next-largest contributor to the total exergy flow,5-1, 1-2 and 2-3 are the smallest.

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0

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20 16 12 8 4 0 -4 -8 -12 -16 -18

Sec

on

d la

w e

ffic

ien

gy

in %

Evaporator temperature in 0 C

R22 R436b

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NOMENCLATURE COP coefficient of performance GWP global warming potential ODP ozone depression potential HFC hydro fluorocarbon CFC chlorofluoro carbon HCFC hydro chlorofluorocarbon HC hydrocarbon VCRC vapour compression refrigeration cycle UNEP United National Environment Programme UNFCC United Nations Framework Convention on Climate Change P pressure (kPa) T temperature (0C) V voltage I current W compressor power consumption (kJ/ kg) X exergy

ISSN: 0975-5462 1411


I irreversibility h specific enthalpy S entropy T0 reference temperature Tl lowest temperature in the cycle Th higest temperature in the cycle ṁ Mass flow rate, (kg/s) ΔĖ exergy flow rate Subscripts 0 dead state H higher L lower

ISSN: 0975-5462 1412

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