Enhancement of COP in Vapour Compression Refrigeration System

Enhancement of COP in Vapour Compression Refrigeration System

M. Krishna Prasanna#1

M.E student, Heat Transfer studies in Energy Systems Department of Mechanical Engg., Andhra University

Visakhapatnam-530003, A.P, India.

P. S. Kishore*2

Ph.D, Professor, Department of Mechanical Engg., Andhra University

Visakhapatnam-530003, A.P, India

Abstract: Experimental analysis on vapour compression refrigeration (VCR) system with R-12 refrigerant was done and their results were recorded. The effects of the main parameters of performance analysis such as mass flow of refrigerant, degree of sub cooling and super heating on the refrigerating effect, coefficient of performance (COP) and power required to run the compressor for various evaporating temperatures, percentage increase in COP and percentage reduction of power required to run the compressor for VCR are dealt. Further the investigations are carried out by introducing shell and coil heat exchanger at the end of compressor.

Keywords: Vapour compression refrigeration, Heat exchanger, COP, power.

1. INTRODUCTION

   Vapour compression refrigeration system is based on vapour compression cycle. Vapour compression refrigeration system is used in domestic refrigeration, food processing and cold storage, industrial refrigeration system, transport refrigeration and electronic cooling etc. So improvement of performance of system is too important for higher refrigerating effect or reduced power consumption for same refrigerating effect. By sub-cooling using heat exchanger at condenser inlet refrigerating effect increases and power consumption or work input decreases. Thus performance of cycle is improved. Along with this waste heat also recovered. The essential quantity of heat recovered is not the amount but it is value.

   Lokapure and Joshi [1] In their article dealt energy conservation by using technique of utilizing waste heat from air-conditioning system and increasing COP. They said that the refrigeration heat recovery device is indirect type of system in which a refrigerant to water heat exchanger is installed between the host refrigeration system compressor and condenser. In this case they achieved their goal by recovering energy and improving COP up to 13%. Khurmi and Gupta [2] in their book gave evidence that the process of under cooling is also brought about by employing a heat exchanger. This increases refrigerating effect and finally improved coefficient of performance in vapour compression refrigeration system. Domanski [3] investigated the effect of LLSL-HX (Liquid line/ Suction line heat exchanger) on system performance by taking liquid refrigerant from condenser to exchange heat with vapor refrigerant from evaporator. They reported that coefficient of performance was increased after installing LLSL-HX. Jain et al. [4] analyzed a complex system in

   order to utilize waste heat rejected by condenser to the atmosphere by installing additional water cooled condenser. Baskaran and Mathews [5] described systems including various refrigerants improved by analyzing the effect of the super heating / sub cooling case. Better performance coefficient values (COP) than those of non- super heating /sub cooling case are obtained. Rajput [6] in his book concluded that sub-cooling results in increase of

   C.O.P and said that no further energy has to be spent to obtain the extra cold coolant. Thirumaleshwar [7] proposed correlations for overall heat transfer coefficient for parallel and counter flow heat exchangers. Coronel and Sandeep [8] determined convective heat transfer coefficient in both helical and straight tubular heat exchangers under turbulent flow conditions. The experiments were conducted in helical heat exchangers and their study shows that the heat transfer coefficient in coiled tubes is higher than that in straight tubes.

2. SYSTEM DESCRIPTION AND DESIGN

   Heat flows naturally from hot to colder body. But, in refrigeration system there is opposite phenomena i.e. heat flows from a cold to a hotter body. This is achieved by using a substance called a refrigerant. The refrigerant (R-

   12) absorbs heat and hence evaporates at a low pressure to form a gas. This gas is then compressed to a higher pressure, such that it transfers the heat it has gained to ambient air or water and turns back (condenses) into a liquid. Thus, heat is absorbed, or removed, from a low temperature source and transferred to a higher temperature source. The refrigeration cycle can be broken down into the following stages as in Fig 1.

   Fig.1: Schematic diagram of a proposed VCR System

   Considering Fig.2,

   1-2, the saturated vapour enters the compressor where its pressure is raised. There will also be a big increase in temperature, because a proportion of the energy input into the compression process is transferred to the refrigerant.

   2-3, the high pressure superheated vapour passes from the compressor into the condenser. There will be decrease in temperature due to condensation process. The cooling for this process is usually achieved by using air. After condensation, refrigerant enters the expansion device.

   3-3, shell and coil heat exchanger is installed between the host refrigeration system compressor and condenser. Water is circulated through one side of heat exchanger and hot refrigerant gas from the compressor is routed through the other side. Heat is transferred from the hot refrigerant gas to the water thus refrigerating effect increases and power consumption or work input decreases. Thus performance of cycle is improved. Along with this waste heat also recovered.

   3-4, the high-pressure liquid refrigerant passes through the expansion device, which both reduces its pressure and controls the flow into the evaporator.

   4-1, Low pressure liquid refrigerant in the evaporator absorbs heat from its surroundings. During this process it changes its state from a liquid to a gas, and at the evaporator exit is slightly superheated.

   Fig.2: p-h diagram of a basic VCR and VCR with heat exchanger

   1. Shell and Coil Heat Exchanger:

      Heat transfer in curved and helical circular tubes has been the subject of several studies due to the relatively high heat transfer coefficients associated with them. Flow in curved tubes is different from flow in straight tubes because of the presence of centrifugal forces. The centrifugal forces generate a secondary flow, normal to the primary direction of flow, with circulatory effects, that increases both the friction factor and the rate of heat

      Fig.3: HEAT EXCHANGER COIL (modeled in Pro-E)

   2. Heat Exchanger Unit:

   1. Overall heat transfer coefficient.

      1/U = 1/hi + 1/ho + dx/k + Fr + Fw (1)

   2. Outlet temperature of water

      t2 = (Q/ ( m Cp))+ t1 (2)

   3. Log mean temperature difference. =Tm

      Tm= T1-T2 / ln (T1/T2) (3)

   4. Area of Heat Exchanger

      A = Q/ U x Tm (4)

   5. Length of the Tube,

   L = A / x Do (5)

3. ANALYSIS OF THE PROBLEM

   Based on T-s diagram of basic VCR as shown in Fig.4

   1. Refrigerating effect =p-hf3 (6)

   2. Degree of superheat = T2 – T3 (7)

   3. p sup=p + Cpv (T2-T2) (8)

   where psup=enthalpy of vapour at superheated state Cpv (specific heat at constant pressure for the superheated vapour)

   1. Compressor work (Wcomp) = p sup-p (9)

      Fig.4: T-s diagram of basic VCR

      transfer. The helically coiled heat exchangers generate a lot of turbulence thus a higher heat transfer coefficient obtained. If te unit is vertically installed, this reduces

      space requirements. Fig.3 shows heat exchanger coil which is modeled in Pro-E.

   2. COP1 =

      =

      COP1 =

      Refrigeration Effect

      Work done by compressor

      p hf3

      p sup p

      (10)

      coefficient of performance of vapour compression

      refrigeration system without heat exchanger

   3. Mass flow of refrigerant, The results are extended to refrigerating capacity of 2 TR

      mR =

      Refrigerating capacity 60 x Wcomp x COP1

      (11)

4. RESULTS AND DISCUSSION

   Where Wcomp= Compressor work

   1. Heat available for desuperheating

      (Q) = mR (p sup-p) (12)

   2. Outlet temperature of water in heat exchanger

      (t2 °C) = (Q/ ( m Cp))+ t1 (13)

      Where Cp is specific heat of water t1= inlet temperature of the water t2= outlet temperature

   3. Degree of under cooling(DUC) = t2-t1 (14)

   4. hf3 = hf3 – Cpl x DUC (15) Where hf3=Enthalpy of liquid refrigerant (VCR with heat exchanger)

      Cpl =Specific heat at constant pressure for the superheated liquid

      150.00

      Refrigerating effect (kJ/kg)

      140.00

      130.00

      120.00

      110.00

      100.00

      -7 -4 0 2 5

      Evaporation temperature (°C)

      RE without HE REwith HE for 1TR RE with HE for 2TR

   5. COP2 =

      =

      Refrigeration Effect Work done by compressor

      p hf3

      p sup p

      (16)

      Fig.5: Evaporation temperature (°C) Vs Refrigerating effect (kJ/kg)

      The results in above Fig.5 reveal that refrigerating effect increases when the heat exchanger is attached to

      Where COP2 = coefficient of performance of vapour compression refrigeration system with heat exchanger

      VCR. This refrigerating effect further increases as the refrigerating capacity is increased from 1 TR to 2 TR.

      7

   6. % increase in COP =

      COP2 COP1

      COP1

      x100 (17)

      6

      C O P

      5

   7. mR1 = 14. (18)

   Refrigerating capacity

   60 x Wcomp x COP2 4

   mR1=Mass flow of refrigerant when VCR with heat

   exchanger 3

   15. P1 =

   Refrigerating capacity 60 x COP1

   (19)

   2

   -7 -4 0 2 5 -7 -4 0 2 5

   Evaporation temperature (°C)

   Where P1= Power required to run the compressor

   when VCR without heat exchanger

   R C is 1 TR R C is 2 TR

   COP without HE

   16. P2 =

   Refrigerating capacity 60 x COP2

   (20)

   COP with HE

   Fig.6: Evaporation temperature (°C) Vs C O P

   Where P2= Power required to run the compressor

   when VCR with heat exchanger

   17. % reduction of power required to run the compressor =

   x 100 (21)

   P1 P2

   P1

   The results in above Fig.6 reveal that the coefficient of performance increases when the VCR is connected to heat exchanger. This is further increases when the refrigerating capacity (RC) increased from 1 TR to 2TR.

   mR(kg/s)

   with HE for 2TR

   0.02

   -7 -4 0 2 5

   Evaporation temperature (°C)

   0.04

   mR(kg/s)

   without HE

   0.03 for 2TR

   mR(kg/s)

   with HE for 1TR

   0.05

   mR (kg/s)

   without HE for 1TR

   0.07

   0.06

   mass flow of refrigerant (mR) (kg/s)

   Fig.7: Evaporation temperature (°C) Vs Mass flow of refrigerant (kg/s)

   The results in above Fig.7 reveal that Mass flow of refrigerant is decreases when the VCR is connected to heat exchanger. This graph is plotted by taking VCR without heat exchanger, with heat exchanger for refrigerating capacities of 1 TR and 2 TR.

   Outlet temperature of water (°C)

   45

   40

   35

   30

   393.67

   438.76

   470.61

   606.21

   629.65

   787.34

   877.53

   941.22

   1212.41

   1259.3

   25

   For 2 TR

   For 1 TR

   Degree of under cooling (°C)

   18

   16

   14

   12

   10

   8

   6

   4

   2

   0

   % increase in C O P

   5.64

   6.287

   6.744

   8.68

   9.023

   11.28

   12.57

   13.48

   17.37

   18.045

   Fig.9: Degree of under cooling (°C) Vs percentage increase in C O P

   The results in above Fig.9 reveal that as the degree of under cooling increases the % increase in coefficient of performance also increases up to 16%. This is plotted by taking refrigerating capacities of 1 TR and 2TR.

   Power required to run the compressor (W)

   1400

   1200

   1000

   800

   600

   400

   200

   0

   Heat available for desuperheating (W)

   RC 1TR RC 2 TR

   Fig.8: Heat available for desuperheating (W) Vs Outlet temperature of water (°C)

   The results in above Fig.8 reveal that as the heat available for desuperheating increases, the outlet temperature of water in heat exchanger also increases.

   C O P

   2.95

   3.27

   4.26

   4.84

   5.69

   3.19

   3.52

   4.51

   5.09

   5.95

   COP without HE COP with HE

   Fig.10: C O P Vs Power required to run the compressor (W)

   The power required to run the compressor is reduced up to 7.5% when the VCR (having refrigerating capacity of 1 TR) is attached to heat exchanger. The power required to run the compressor for a VCR without heat exchanger and VCR with heat exchanger both were plotted and shown in Fig.10

   2500

   2000

   1500

   1000

   500

   0

   COP without HE

   COP with HE

   C O P

   Power required to run the compressor (W)

   2.95

   3.27

   4.26

   4.84

   5.69

   3.44

   3.77

   4.75

   5.347

   6.213

   Fig.11: C O P Vs Power required to run the compressor (W)

   The power required to run the compressor is reduced up to 14% when the VCR (having refrigerating capacity of 2 TR) is attached to heat exchanger. The power required to run the compressor for a VCR with and without heat exchanger were plotted and shown in Fig.11

5. CONCLUSIONS

The following conclusions are arrived from the VCR system connected with heat exchanger when the evaporation temperature decreases to -7°C:

1. The refrigeration effect of the system is increased up to 16% using the heat exchanger with vapour compression refrigeration system.

2. The C O P (coefficient of performance) of the system is increased up to 16% using the heat exchanger with vapour compression refrigeration system.

3. Mass flow of refrigerant (mR) is reduced up to 14% using the heat exchanger with vapour compression refrigeration system.

4. Heat available for desuperheating (Q) increases as the evaporation temperature decreases. So by attaching heat exchanger to the vapour compression refrigeration system and regulating water into heat exchanger, outlet temperature of the water (t2) in heat exchanger increases. That hot water can be used for useful purpose.

5. Power required to run the compressor is reduced up to 14% by using the heat exchanger with vapour compression refrigeration system.

ACKNOWLEDGMENT

We, authors express gratitude to all the anonymous reviewers for their affirmative annotations among our paper. Thanks to every reviewer for reviewing our paper and give valuable suggestions.

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