VARIATION IN REFRIGERATION COEFFICIENT OF PERFORMANCE AT VARIOUS PROCESS TEMPERATURES

1.0 INTRODUCTION

Refrigeration is used widely in various applications from industrial to domestic situations mainly for the storage and transport of perishable foodstuffs and chemical substances .it has the prime function to remove heat from a low temperature region and it can also be applied as heat pump for supplying heat to a region of high temperature.

2.0 OBJECTIVE

The objective of this experiment is to ascertain the variation in coefficient of performance(COP) of a vapor compression refrigeration system.

3.0 APPARATUS

The equipment used for this experiment is as follows:

3.1 Refrigeration Coefficient Of Performance from P.A Hilton Ltd

3.2 Computer

4.0 THEORY

A refrigeration cycle works to lower and maintain the temperature of a controlled space by heat transfer from a low to a high temperature region.

Refrigeration duty is another term for the cooling effect of the refrigeration system, which is the rate of heat being removed from the low temperature region with specified evaporation and condensation temperatures. The unit for “duty “measurements is in watts (for 1 ton of refrigeration=3517W)

4.1 The Vapor Compression Cycle

Ideal refrigeration system follows the theoretical reversed Carnot Cycle process .In practical refrigerators, compression and expansion of a gas and vapor mixture presents practical problems in the compressor and expander. Therefore , in practical refrigeration ,compression usually takes place in the superheated field and a throttling process is substituted for the isentropic expansion.

The cycle:

1-2 Isentropic compression of vapor, from the evaporating to the condensing pressures.

2-3 Condensation of the high pressure vapor during heat is transferred to the high temperature region.

3-4 Adiabatic throttling of the condensed vapor from the condensing to the evaporating pressure

4-1 Evaporating of the low pressure liquid during which heat is absorbed from the low temperature source.

4.2 Energy Transfers Analysis

4.2.1 Compressor

q 1-2 = h2 - h1+w

if compression is adiabatic, q 1-2 = 0, and w =h1-h2

Power requirement, P =m(h1-h2), where m is flow rate of working fluid per unit time.

4.2.2 Condenser

q 2-3 =h3-h2+w

w=0, therefore q 2-3 =h3-h2 and rate of heat rejection Q 2-3 =m(h3-h2)

4.2.3 Expansion Valve

q 3-4 =h4-h3+w

w=0, therefore q3-4 =h4-h3 and process is adiabatic,

therefore h4=h3

4.2.4 Evaporator

q 4-1 =h1-h4+w

w=0, therefore q 4-1 =h1-h4 and rate of heat absorbed Q 4-1 =m(h1-h4)

Coefficient of Performance (COP)

COPref = q 4-1 =h1-h4

w h2-h1

5. 0 PROCEDURES

The procedures of this experiment are as follow:

5.1 Experiment started at the condenser saturation temperature of 20˚C.

5.2 The evaporator load increased to approximately 10% in program 1.

5.3 Return to main menu and enter program 2.Select “no print out” and display the.

5.5 Condensing temperature, 2. Refrigerant flow rate and 14. Cooling water flow rate.

5.6 Maintained the condensing temperature of 20˚C by a small adjustments of cooling water flow rate.The system is stable when all three parameters show generally horizontal lines (approximately 1 minute).

5.7 Return to the main menu when the system is stabilized. Select program 1 with print out option(raw and calculated data).

5.8 Then increase evaporator load (by 10%) and print out the results. Repeated until evaporator is at 60%.

6.0 DATA & RESULTS

The Results Summary Table for this experiment are as follow:

Load

Evaporator temp

(˚C)

Condenser exit temp

(˚C)

Refrigerant flow rate

(kg/s)

Cooling water flow rate

(kg/s)

Q4-1

W1-2

Q2-3

COPref

0

11.23

16.72

75.54

11.87

191.11

-23.72

214.83

8.057

15

29.21

14.13

80.23

11.67

209.14

-13.89

223.03

15.056

30

30.15

14.06

81.39

13.04

210.12

-17.61

227.73

11.932

45

37.48

14.00

80.50

13.23

216.76

-14.31

231.07

15.147

60

39.76

15.27

81.21

12.84

217.73

-15.55

233.28

14.002

75

42.10

16.32

81.35

12.84

219.13

-16.42

235.55

13.345

7.0 DISCUSSION

7.1 The objectives of this experiment are to analyzed and apply the First Law of Thermodynamics and to ascertain the variation in coefficient of performance (COP) of a vapor compression refrigeration system has been achieved.

7.2 However the results of this experiment are not as accurate as theoretical. These discrepancies has been predicted and caused by several factors and as follows:

7.2.1 Parallax error. A common error that occurs during any experiment. This error normally happened during taking measurement from equipments or tool. If the eyes of the reader were not properly aligned with the scales, the result will differ and not accurate. This error can be reduced by taking a couple of reading or person and take the average. However this technique may time consuming for the whole process.

7.2.2 Zero Parallax. An error of a measurement from equipment’s or tool’s scale where when it supposed to be zero, but it still giving a reading. This error may occur if the equipments or tools used in the experiment are not calibrated. To reduce this error, before any experiment can be conducted ensure all the precisions equipment had been calibrated as per schedule.

7.2.3 Sensitivity of Equipment. The equipment used in this experiment is very sensitive from its surroundings. The readings will not consistent and will differ if any vibrations occurs around the equipment. To reduce this discrepancy, movement of a person must be limited and controlled.

7.2.4 Responsibilities. Every person must understand the whole procedures and processes. All processes have to be divided to each members involved in the experiment in order to reduce any errors to occur.

8.0 CONCLUSIONS

8.1 Overall it is a successful experiment, from the experiment we can conclude that in the vapor compression refrigerant system the Coefficient of Performance is varies the Torque was found indirectly proportional to the rotational speed.

8.2 We can apply the first and second law Thermodynamic while carried out this experiment 1. The torque and power required also determine from Experiment 2.

REFFERENCES

Thermodynamics an Engineering Approach – Fifth Edition, Yunos A. Cengel and Michael A. Boles