Refrigeration

MEM554 – Thermalfluids LabComputer Linked Refrigeration

TABLE OF CONTENTS

Page

Front Page

Table of Contents i

Title 1

1. Objective 1

2. Introduction 1

3. Theory 1

4. Equipment 1

5. Experimental Procedure 1

6. Result 1

7. Sample Calculation 1

8. Analysis Result 1

9. Discussion 1

10. Conclusion 1

11. References 1

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TITLE:VARIATION IN REFRIGERATION COEFFICIENT OF PERFORMANCE AT VARIOUS PROCESS TEMPERATURES

1. OBJECTIVE

Investigate the variation in Coefficient of Performance (COPR) of a vapour compression refrigeration system.

2. INTRODUCTION

A major application area of thermodynamics is refrigeration, which is the transfer of heat from a lower temperature medium to a higher temperature medium. The devices that produce refrigeration are caller refrigerators, and the cycles on that operate is called refrigeration cycles. It is known that the purpose of refrigration is to lowered the temperature of a region and also maintaining the region.

The first use of refrigeration system is in 1756 by William Cullen at the University of Glasgow in Scotland, and when i widespread use in commercial from 1870 till present. 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 low temperature region and it can also be applied as a heat pump for supplying heat to a region of high temperature.

Probably the most widely used current applications of refrigeration are for the air-conditioning of private homes and public buildings, and the refrigeration of foodstuffs in homes, restaurants and large storage warehouses. The use of refrigerators in kitchens for the storage of fruits and vegetables has permitted the addition of fresh salads to the modern diet year round, and to store fish and meats safely for long periods.

In commerce and manufacturing, there are many uses for refrigeration. Refrigeration is used to liquify gases like oxygen, nitrogen, propane and methane for example. In compressed air purification, it is used to condense water vapor from compressed air to reduce its moisture content. In oil refineries, chemical plants, and petrochemical plants, refrigeration is used to maintain certain processes at their required low temperatures (for example, in the alkylation of butenes and butane to produce a high octane gasoline component). Metal workers use refrigeration to temper steel and cutlery. In transporting temperature-sensitive foodstuffs and other materials by trucks, trains, airplanes and sea-going vessels, refrigeration is a necessity.

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3. THEORY:

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

Figure 1: The objective of a refrigerator is to remove heat (QL) from the cold medium

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

The Vapor Compression Cycle (known as reversed Carnot cycle)

Ideal refrigeration systems follow the theoretical Reversed Carnot Cycle process. In practical problems in the compression and expansion of a gas and vapor mixture present practical problems in the compressor and expander. Therefore in practical refrigeration compression usually take place in the superheated field and a throttling process is substituted for the isentropic expansion.

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Figure 2: Schematic and T-s diagram for the ideal vapor-compression refrigeration cycle or reversed Carnot cycle (1)

Figure 3: Schematic and T-s diagram for the ideal vapor-compression refrigeration cycle or reversed Carnot cycle (2)

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h1-h4 h2-h1

h2-h3


Figure 4: T-s diagram of the ideal vapor-compression refrigeration cycle

The cycle:1 - 2 Isentropic compression of the vapor, from the evaporating to 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 condensing to the evaporating

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

temperature source.

Energy Transfer Analysis for reversed Carnot cycle

Compressor

q1−2=h 1−h2+w1-2

If compression is adiabatic, q1-2 = 0, and w1-2 = h2 − h1 = wamp.

Power requirement; P=m¿

(h 2−h 1) =w¿

, where m¿

is the flow rate of working fluid per unit time.

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Condenser

q2-3 = h2 − h3 + w

w = 0, therefore q2-3 = h2 − h3, and the rate of heat rejection Q2−3=m¿

(h 2−h 3 )

Expansion valve

q3-4 = h4 − h3 + ww = 0 at the expansion valve, and the process is assumed adiabatic (q=0)Therefore h4 = h3

Evaporator

q4-1 = h1 − h4 +w

w = 0, therefore q4-1 = h1 − h4, and rate of heat absorbed Q4−1=m¿

( h1−h4 )

Coefficient of Performance (COP)

COPref =

q4−1

w¿ =

h1−h4

h2−h1

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4. EQUIPMENTS

RC 713 Computer Linked Refrigeration Unit (P.A Hilton)

Figure 5: RC 713 Computer Linked Refrigeration Unit (P.A Hilton)

.

Figure 6: Computer and Printer

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Evaporator Pressure Transducer

Condenser Pressure Transducer Evaporator Evaporator Load

Main Switch

Dynamometer

Compressor

Water Flow meterCooling Water Control Valve

Condenser

Refrigerant Flowmeter

Interface Status

System Sampling


5. EXPERIMENTAL PROCEDURE

a. The experiment is started at a condenser saturation temperature of 20oC.b. Program 1 is entered and the evaporator load is increased approximately 10%.c. Return to the main menu and Program 2 is entered. “no print out” is selected and

the three parameters; 5. Condensing Temperature; 2. Refrigerant Flow Rate; and 14. Cooling Water Flow Rate is displayed.

d. The condensing temperature of 20oC may be maintained by small adjustments of the cooling water flow rate. The system is stable when all three parameters show generally horizontal lines (approximately 1 minute).

e. Return to the main menu and Program 1 is selected with print-out option (raw and calculated data) when the system is stabilized.

f. Evaporator load is increased (by 10%) and the results are printed out. Repeat until evaporator load is 60%.

6. RESULT

Load(%)

h1

(kJ/kg)h2

(kJ/kg)h3

(kJ/kg)h4

(kJ/kg)

0 316.67 338.67 116.67 116.67

15 329.17 345.83 116.67 116.67

30 335.42 350.00 116.67 116.67

45 336.46 352.08 116.67 116.67

60 340.63 356.25 116.67 116.67

7. SAMPLE OF CALCULATION

At Load = 30 % h1 = 335.42 kJ/kgh2 = 350.00 kJ/kgh3 = 116.67 kJ/kgh4 = 116.67 kJ/kgmref = 0.08104kg/smcw = 0.02938kg/s

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Evaporator

Q4 – 1 = mref (h1 – h4) = 0.08104 kg/s (335.42kJ/kg – 116.67 kJ/kg) = 17.73 kW

Compressor

W1 – 2 = mref (h1 – h2) = 0.08104 kg/s (335.42kJ/kg – 350.00 kJ/kg) = 1.18 kW

Condenser

Q2 – 3 = mref (h2 – h3) = 0.08104 kg/s (350.00 kJ/kg – 116.67 kJ/kg) = 18.91 kW

Coefficient of Performance (COP)

COPref = (q4 – 1) / (w) = (h1 - h4) / (h2 - h1)

= 335.42kJ/kg - 116.67 kJ/kg 350.00 kJ/kg - 335.42kJ/kg

= 15.00

QCW = mcw (h1 – h4) = 0.02938 kg/s (335.42kJ/kg – 116.67 kJ/kg) = 6.43 kW

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8. ANALYSIS RESULT

Load(%)

Evaporator Temp(OC)

Condenser Exit Temp(OC)

Refrigerant flow rate (kg/s)

Cooling Water flow rate (kg/s)

0 12.03 15.90 0.05562 0.02938

15 24.95 12.52 0.08087 0.02918

30 26.10 12.94 0.08104 0.02938

45 28.02 13.13 0.08128 0.02938

60 34.56 13.05 0.08090 0.02918

Load(%)

Q4-1 (kW) W1-2 (kW) Q2-3 (kW) COPref

QCW

(kW)

0 11.12 1.22 12.35 9.09 5.88

15 17.18 1.35 18.53 12.76 6.20

30 17.73 1.18 18.91 15.00 6.43

45 17.86 1.27 19.13 14.07 6.46

60 18.12 1.26 19.38 14.34 6.54

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9. DISCUSSION

Syed Mohd. Hasif Wafa b. Syed Mohd. Hassan2009643852EMD5M9A

1. Experiment data parameters for load = 0

Figure 5

2. What do you understand by the term load? Give examples of actual loads in refrigeration practice in a domestic fridge, in a room and in a factory.The term load is the amount of heat which must be transferred per unit time from the cold chamber and is known as the refrigeration capacity. Heats, both sensible and latent, enter an enclosure through door openings whenever the air surrounding the enclosure is warmer than the box temperature. Knowing the location, size and number of the door openings and the temperature to which they are exposed will greatly aid in determining the heat load of the infiltrating air.

Regardless of its size or complexity, a refrigeration system has one basic function. That function is to remove heat from a place or substance where it is not wanted, and transport it to a location where it can be diffused into air or water. In order to select the proper equipment for a given application, we must carefully estimate the amount of heat to be moved.

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There are many examples of actual loads in refrigeration practice in a domestic fridge, in a room and in a factory. These examples are shown below:

DOMESTIC FRIDGE ROOM FACTORY The product load is heat from warm food or other solids or liquids that are being refrigerated.

The heat leakage load is heat that infiltrates the refrigerated area through walls, ceilings, roofs, and floors. Entering temperature People Lights Motors including fan motors

The miscellaneous heat load is heat introduced by lights, motors, and other heat producing devices located in the refrigerated area. Manufacturing processes and human occupancy also may contribute to the miscellaneous heat load. forklifts, conveyers People

3. What is the effect on the COPref as the load is increased? Why?The coefficient of performance is the ratio of the heat removed at the colder temperature to the work required to do it. So, as the load increased, the COP should increase. From the experiment that we have done the value of COP that we get is increase until it reached the maximum COP of 15.00 for load 30, but then the COP decrease to 14.07 for load 45 but then increase again to 14.34 as it reached load 60. This is because by looking at the relation of COPref = (q4 – 1) / (w), if the heat remove (q4 – 1) increase and the work required (w) is small, the COP will be higher but if the heat remove decrease and the work required is bigger, the COP will be lower.

4. What is the effect on the condenser temperature as the load is increased? Why?Base on our experiment, the condenser temperature shows an unsteady value, as the load increase, the condenser temperature decrease and increase. This may be due to the environment. In theory, as the load increase, the condenser temperatures also increase. More heat is required to be removed from the cold space by the refrigerant. This means that the refrigeration duty will increase. As more heat to be removed, the temperature of refrigerant exit from evaporator will increase. And as it been compress in the compressor, the temperature of the refrigerant become hotter. Before the refrigerant flow into the expansion valve and evaporator, it must be cool in the condenser. The high-pressure refrigerant gas, coming from the compressor, flows through the condenser and becomes a

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liquid. As this occurs, the refrigerant gives off heat. The process is more likely as the heat exchanger. In our case, the refrigerant gives of heat to the cooling water. There are many factors that determine the rate of heat transfer. By assuming the condenser have constant hot and cold fluid flow rate, the amount of heat transfer will be the same as the load increase. This means that as the inlet temperature of hot fluid (in our case R134a) increase, the outlet temperature should also increase. That is why the condenser temperature increases as the load increase. But in our experiment, the rate of heat transfer of the refrigerant to the cooling water is not constant due to the changing of the flow rate of the refrigerant and the flow rate of the cooling water and that’s why we get uncertain value for condenser temperature and some other value.

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Musaddiq bin Mohsin2007284454EMD5M9A

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CONDENSER

EVAPORATOR

C

14

32

eQ

QC = 12.35 kW

CW


Muhamad Nazrul bin Sulaiman2007126715EMD5M9A

1. Fill the parameter from one set of experimental data in to the refrigeration System diagram.

At percentage load = 0

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CONDENSER

EVAPORATOR

C

14

32

eQ

QC = 18.53 kW

CW

CONDENSER

EVAPORATOR

C

14

32

eQ

QC = 18.91 kW

CW




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CONDENSER

EVAPORATOR

C

14

32

eQ

QC = 19.13 kW

CW

CONDENSER

EVAPORATOR

C

14

32

eQ

QC = 19.38 kW

CW




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2. What do you understand by the term load? Give example of actual loads in refrigeration practice in a domestic fridge, in a room, and in a factory.

Domestic FridgeThe present invention concerns domestic refrigerators of the type having a refrigerator body defining a first internal refrigerated cavity, and, in this first internal cavity, at least one compartment in which at least a part is manually movable.

Known domestic refrigerators generally have two compartments of the type mentioned above, namely the compartment for making ice, commonly called "freezer", containing a refrigeration coil and closed by means of a pivoting door, and a compartment in the form of a vat, generally disposed opposite the "freezer" and serving as a removable vegetable tray, in an environment at a low temperature between 2° and 5° C.

There is a need for the consumers to be able to rely on refrigerators which are provided with means enabling to substantially extend the period of preservation of fruits and vegetables, experience showing that in known vegetable trays, the vegetables have a tendency to fade or rot relatively rapidly.

Processes for extending the length of preservations of plant food products, in particular fruits and vegetables, are known, and these processes consist in placing these products under an oxygen starved controlled atmosphere and to keep them at low temperature, typically between 0° and 15° C. These processes are found either in storage silos of substantial size, where the composition of the controlled atmosphere is permanently supervised and adjusted, namely for their conditioning in wrappings intended for sale and having selective properties of gas diffusion.

It is an object of the present invention to propose a domestic refrigerator enabling to establish and maintain in a portion of its internal cavity an atmosphere adapted for the extended preservation of fruits and vegetables, in an autonomous arrangement, at low cost, with reliable operation and which does not modify the overall size of the refrigerator.For this purpose, according to a characteristic of the invention, the compartment of the refrigerator defines a second internal cavity which is substantially water tight with respect to the first internal cavity and communicates with a gas source containing less oxygen than air.

According to a more particular characteristic of the invention, the gas source comprises a separation module periodically supplied with air under pressure, advantageously by means of a moto-compressor unit controlled by a control module comprising a timer, and preferably, coupled to a detector which is responsive to the movement of the movable part of the compartment.

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It is another object of the present invention to propose a process for feeding an oxygen starved gas into a compartment of a refrigerator of the above type, enabling, at lower cost, to keep, in the compartment, an atmosphere which is adapted for the preservation of fruits and vegetables.

RoomIn a room, the specification that needs to know is the chilling or freezing times now worked out, the size of the room can be determined. To achieve this, the operation of the whole abattoir may have to be changed and also the flow of carcasses to and from chiller or freezer, the position of doors and so on.

If it size and position of the room has been rigidly fixed before this stage, the cooling times determined above will not be met. When loading a chiller the doors are invariably left open for long periods allowing a fully established air flow to take place to and from the room either from gravity through a single door or by a through flow of air if more than one door is open.

Another point to notice is that the load on the room, when used as a store, even when the outside temperatures are very high, is very small compared to both the peak and average load and is for the most part due to the evaporator fans running continuously. The load then increase when the doors are opened and the room is washed out or possibly unloaded. Warm carcasses are then loaded into the room and the load rapidly reaches the peak product load that occurs at the end of the loading period. Thereafter, the doors are closed and the load rapidly declines. At the end of the chilling cycle, the doors are again opened to remove the carcasses and the infiltration load so caused increase.

FactoryAn 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 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.

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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 consists 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.

3. What is the effect on the COPref as the load is increased? Why?

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

COPref=Q1

∑W

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.

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Wnet,in

QH

QL

Cold refrigerated at TL

Warm environment at TH>TL

Reversed heat engine


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. What is the effect on the condenser temperature as the load is increased? Why?

The effect of the condenser temperature is when the load is increase, the temperature also increased. When the more load is added, the more heat is rejected from the system. When more heat is rejected, the temperature is more cooled do to the increases of the load. Cooling effect of the refrigeration system, which is the rate of heating being removed from low temperature region with specified evaporator and condensation temperatures. The unit “duty” measurement is in the watts (for 1 ton of refrigeration = 351 W)

The function of the refrigeration system is to transport the heat from the lower temperature in the conditioned space to the higher temperature outside. It is thus a heat pump. The required energy input of a heat pump is a function not only of the amount of cooling it has to do but also of the temperature elevation against which it is pumping. The reduction of this temperature difference is the goal of optimization.

The isothermal processes in this cycle are also isobaric (constant pressure). The efficiency of a refrigerator is defining as the ratio between the heat removed from the process and the work required to achieve this heat removal.

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9. CONCLUSION

Syed Mohd. Hasif Wafa b. Syed Mohd. Hassan2009643852EMD5M9A

From the experiment that we had done, we can conclude that we have achieved our main objective to investigate the variation in Coefficient of Performance (COPref) of a vapour

compression refrigeration system. This is achieved by looking at the effect of the COPref as the load increased. The effect of COPref that we get from this experiment is that the higher the value of load, the COPref increased but due to some factor during the experiment, the COPref

decrease at some point and increase back. Apart from this experiment, we are able to find out that the higher the value of COPref, the better the refrigeration cycle is. Also for this experiment, it is recommended that, we carefully done the experiment and follows the entire steps to avoid getting errors and it is highly recommended that the refrigerant flow rate and cooling water flow rate can be fixed so that we can clearly see the effect of COPref of the refrigeration cycle.

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Musaddiq bin Mohsin2007284454EMD5M9A

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Muhamad Nazrul bin Sulaiman2007126715EMD5M9A

This experiment achieved its objective to find the COPR value of a vapour compression refrigeration system. For the experiment the value of COPR as stated below:

1. The value of COPR when the load is 0% is 9.092. The value of COPR when the load is 15% is 12.763. The value of COPR when the load is 30% is 15.004. The value of COPR when the load is 45% is 14.075. The value of COPR when the load is 60% is 14.34

It is recommended that future versions of this experiment take steps to make sure while taken the value of evaporator load to be in steady reading. Before print the result after percentage load is inserted wait for 5 minutes in order to make the process become steady. This will ensure that the experiment will get less error.

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10. 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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Refrigeration