UNIVERSITI TEKNOLOGI MARAFAKULTI KEJURUTERAAN MEKANIKAL

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Programme : Bachelor of Engineering (Hons) Mechanical Course : Thermofluids Laboratory IICourse Code : KJM 470Lecturer : Assoc. Prof. Shif Bin Ismail______________________________________________________________________

Laboratory Report Assignment

Title of Experiment

Variation in refrigeration coefficient of performance at various process temperatures

No. Name Student ID Number Signature

1. Khairunnisa Binti Mohammad Yunus 2004346361

2. Muhammad Mustaqim Bin Mad Saad 2004346116

3. Umar Bin Muhamad Suffian 2004104465

4. Azlan Bin Rahmat 2004104618

5. Mohd Alieff Bin Zainal 2004346210

6. Wan Azliza Binti Wan Aziz 2004104657

Practical Session : 6th February 2006 Staff certification :________________

(Signature)

Report Submission : 20th February 2006 Staff certification :________________

(Signature)

Contents

Page No:

1. Title 3

2. Introduction 3

3. Objective 3

4. Theory 3- 5

5. Energy transfers analysis 5

6. Equipment 6

7. Energy Balance Formula’s 6 - 7

8. Results 6

9. Discussion and Conclusion 9 - 16

10. References 16

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TITLE

Variation in refrigeration coefficient of performance at various process temperatures.

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 the low temperature region, and it can also be applied as a heat pump for supplying heat to a region of high temperature.

OBJECTIVE

To investigate the variation in Coefficient of Performance (COPR) of a vapor compression refrigeration system.

THEORY

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

Refrigeration duty is another term for the cooling effect for the refrigeration system, which is rate of heat being removed from the low temperature region which specified evaporation and condensation temperatures. The unit for “duty’ measurements is in Watts (for 1 ton of refrigeration = 3157W).

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3.1 The vapor compression cycle

Ideal refrigeration systems follow 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.

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The cycle:

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

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

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

4-5 Evaporation of the low pressure liquid during which hat is absorbed from the low temperature source.

Energy transfers analysis

Compressor

If compression is adiabatic, , and .

Power requirement, , where m is the flow rate of working fluid per unit time.

Condenser

, therefore , and rate of heat rejection .

Expansion valve

at the expansion valve, and process is adiabatic.Therefore .

Evaporator

, therefore , and rate of heat rejection .

Coefficient of Performance (COP)

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EQUIPMENT

RC713 Computer Linked Refrigeration Unit (P.A.HILTON)

EXPERIMENTAL PROCEDURES

(i) The experiment is start at a condenser saturation temperature of 20°C.(ii) Programme 1 is enter the evaporator load is increase to approximately 10%.(iii) The main menu is return and Programme 2 is enter. “No print-out” is select

and these three parameters, 5. Condensing Temperature; 2. Refrigerant Flow Rate; and 14. Cooling Water Flow Rate is display.

(iv) By small adjustments of the cooling water flow rate the condensing temperature of 20°C is maintained. All three parameters shows generally horizontal lines (approximately 1 minute) which mean the system is stable.

(v) The main menu is return and Programme 1 with print-out option (raw and calculate data) is select.

(vi) Then evaporator load (by 10%) is increase, and the result is print-out. Step (i) to (v) is repeat until evaporator load is 60%.

ENERGY BALANCE FORMULA’S

Evaporator

Evaporator heat input (W)

Refrigerant Enthalpy Change Rate

Condenser

Heat transfer to cooling water (W)

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Refrigerant Enthalpy Rate

Compressor

Shaft Power (W)

Friction Power

Indicated Power (W)

Enthalpy Change Rate

Electrical Motor

Electrical Power Input

Coefficient of Performance (COPR)

COPR (based on electrical input)

SYMBOLS AND UNITS

Symbol Quantity Unit

Cp Specific Heat J kg-1 K-1

F Force Nh Specific enthalpy J kg-1

I Current AMass flow rate kg/s

N Rotational speed Rev/minq Heat Transfer per unit Mass J kg-1

Heat Transfer Rate WT Temperature KV Potential Difference Voltsw Work per unit Mass J kg-1

ω Angular velocity Rad s-1

RESULT AND DISCUSSION

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7.1 By using data obtained from the experiments, for 1 set of data, plot the data on the Pressure-enthalpy (P-h) diagram, and show the calculations and parameters below using the energy equations based on enthalpy:

(a) Refrigeration duty,(b) Compressor work, (c) Heat rejected from condenser, (d) Coefficient of Performance, COPref

Result summary table

Load Evaporator Condenser exit Refrigerant Cooling Water

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temperature(°C)

temperature(°C)

flow rate(kg/s)

Flow rate(kg/s)

0 14.08 22.29 76.47 14.9820 21.08 19.72 77.78 14.7940 34.19 17.65 76.83 14.2060 26.01 16.84 77.30 13.6280 27.88 17.49 76.92 13.42

Load h1

(kJ/kg)h2

(kJ/kg)h3=h4

(kJ/kg) (kW) (kW) (kW)COPref

0 312.5 337.5 127.1 14.1775 -1.9118 -3.1518 7.41620 329.2 343.8 125.0 15.8827 -1.1317 -3.2361 13.98640 329.2 350.0 122.9 15.8500 -1.5981 -3.2248 9.91860 335.4 354.2 122.9 16.4263 -1.4532 -3.1503 11.30380 339.6 354.2 125.0 16.5070 -1.1230 -3.0759 14.699

Sample of calculation

From experimental data load = 20

Based from the graph , h1=392.2kJ/kg, h4=125kJ/kg.

Based from the graph , h1=392.2kJ/kg, h2=343.8kJ/kg.

Based from the graph , h2=343.8kJ/kg, h3=125kJ/kg.

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Based from the graph h1=392.2kJ/kg, h2=343.8kJ/kg, h4=125kJ/kg.

Coefficient of Performance (COP)

7.2 Discussion

1) Plot the graph of COPref against evaporator load at constant condenser saturation temperature. From the graph, discuss the effect on the COPref as the evaporation load is increase at a constant condenser temperature.

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COPref Vs Evaporator load

02468

10121416

0 20 40 60 80 100

Load

CO

Pref

2) Fill in the parameters from a set of experimental data into the refrigeration system diagram (Figure 5).

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Figure 5

From experimental data at load = 20

Evaporator heat input

Shaft Power

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;

Heat transfer to cooling water

;

3) Explain the term COPref and its effect in rating refrigeration systems against economic considerations.

The efficiency of a refrigerator or refrigerator performances are defined by means of the coefficient of performance, COP denoted by COPref which is

given by

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where COP is sometimes called the performance ratio. The best COP will be given by a circle which is a Carnot cycle operation between the

given temperature conditions. The objective of a refrigerator is to remove heat

QL from the refrigerated space. To accomplish this objective, it requires a work input of Wnet,in.

For a refrigerator the important quantity is the heat supply to the system from the surrounding, Q1. The power input, W is important

because it is the quantity which has to be paid for and constitutes the main item of the running cost.

4) Give example with appropriate diagrams and explanations of actual loads in refrigeration practice in a factory.

An example of actual loads is storage of specific food. A refrigerator is design to maintain the freezer section at -18°C and the refrigerator section

Wnet,in

QH

QL

Cold refrigerated at TL

Warm environment at TH>TL

Reversed heat engine

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at 3°C. Lower freezer temperatures increase energy consumption without improving the storage life of frozen food significantly. Different temperatures for the storage of specific foods can be maintained in the refrigerator section by using special-purpose compartment.

Generally, all full size refrigerators have the a large air-tight drawer for leafty vegetable and fresh foods to seal moisture and protect from drying effect. Some have a temperature controlled meat compartment maintained at -0.5C, which keeps meat at lowest safe temperature without freezing it.

For specified external dimensions, a refrigerator is desired to have maximum food storage volume, minimum energy consumption, and the lowest possible cost to the consumer.

The size of compressor and another components of a refrigeration system are determined on the basis of the anticipated heat load (or refrigeration

load), which is heat flow into the refrigerator. The heat load consist of the predictable part, fan motor, defrost heaters and the unpredictable part.

The cross section of a refrigerator showing the relative magnitudes of various effects that constitutes the predictable heat load.

5) Outline at least 3 measures to increase the COPref of a refrigeration system.

i. Work input, Wnet,in.

ii. Temperature differenceiii. Secondary working fluid

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6) Conclusion of the experiment.

As a conclusion from the experiment the value of COPref for each load 0, 20, 40, 60 and 80 are 7.416, 13.986, 9.918, 11.303 and 14.699 respectively while the value of the work input, are -1.9118, -1.1317, -1.5981, -1.4532 and -1.1230 respectively. From the result it shows that all value of COPref are more than unity. Noticed that the value of COPref can be greater than unity. That is, the amount of heat remove from the refrigerated space can be greater than the amount of the work input. This is in contrast to the thermal efficiency, which can never be greater than 1. While the only reason for expressing the efficiency of a refrigerator by another term the coefficient of performance is desire to avoid the oddity of having efficiencies greater than unity.

REFERENCES

1. Yunus A. Cengel, Michael A. Boles, Thermodynamics: An Engineering Approach 5th Edition, Mc Graw Hill, 2006.

2. Eastop & McConkey, Applied Thermodynamics for Engineering Technologists 5th Edition, Prentice Hall, 1993.

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