unit 1 introduction to refrigeration

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UNITS IN THIS COURSE

UNIT 1 INTRODUCTION TO REFRIGERATION

UNIT 2 THE REFRIGERATION TRAINER

TABLE OF CONTENTS

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1.0 OBJECTIVES 3

1.1 TERMS USED IN REFRIGERATION PROCESSES 4

1.2 THE BASIC PRINCIPLES OF REFRIGERATION 6

1.2.1 Heat Transfer Methods 7

1.2.2 Heat Transfer 9

1.2.3 Heat and Refrigeration 10

1.3 THE COMPONENTS OF A SINGLE STAGE REFRIGERATION CYCLE 10

1.4 THE BASIC REFRIGERATION CYCLE 12

1.0 OBJECTIVES

At the end of this unit the trainee will be able to:

Explain the terms used in refrigeration processes.

Explain the principle of refrigeration and its use of heat.

Identify and explain the functions of the components that make up a singlestage refrigeration cycle.

Explain the basic refrigeration cycle.

1.1 TERMS USED IN REFRIGERATION PROCESSES

To understand the process of refrigeration you need to know the special terms andwords which are used. Some of the more common terms are listed and definedbelow.

Evaporation takes place when a liquid changes state to a gas. In a refrigerationsystem evaporation takes place in an evaporator.

Condensation takes place when a gas changes state to a liquid. In a refrigeration

system condensation takes place in a condenser.

Vapour pressure. All liquids give off vapour, which consists of molecules of theModuleNo.

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liquid. If the liquid is in an enclosed space (for example, a tank) there is a pointwhere, for every molecule of liquid changing to vapour there is also one molecule ofvapour changing back to a liquid at exactly the same time. The pressure thishappens at is called the vapour pressure of the liquid.

The Joule-Thomson effect is named after the two scientists who discovered that

when a fluid is expanded through a small hole there is a change in temperature.The temperature of the fluid is lowered. The change in temperature is proportionalto the pressure difference across the hole.

A system is an organised group of items working together to perform a function.

The state of a system is a set of properties (see below) which characterise thesystem. For example, the state of a gas may be determined by its pressure,temperature and volume.

A property is a special quality of a material. Examples of properties are density,volume or specific heat.

A process is a series of steps through which a system changes from one state toanother.

Something is said to be in equilibrium when there is balance between opposingforces or effects. When a liquid is at its vapour pressure there is equilibriumbetween the liquid and its vapour. Equilibrium can also mean the initial state of asystem. It is the condition that exists before a system is changed by any kind ofmotion or action.

A cycle is a series of operations that end at the starting point.

A refrigerant is the fluid that actually does the heat transfer in a refrigerationsystem.

A closed circuit or closed system is one in which the fluids in the system remainin that system even though they may change state. This means that when a closedsystem is working there is no need to add more fluids to it because the fluids arere-circulated, not released from the system.

A thermostat is an instrument for keeping something at a constant temperature.The thermostat checks the temperature of the thing we want to keep at a constanttemperature and instructs another device to open or close, or to switch on or switch

off to keep the temperature constant.

Compression ratio describes how much a compressor compresses a gas. It is theratio of the outlet pressure from the compressor compared to the inlet (suction)pressure of that same compressor. A compressor that discharges gas at 80 psi andreceives it at 10 psi has a compression ratio of 8 to 1, which we write as 8:1 Thebest compression ratio that a single stage compressor can work at is about 10: 1 .If a larger compression ratio is needed the compressor will need to have at leastone more stage.

The British thermal unit (Btu) is a measure of the quantity of heat there is insomething (you will learn about heat later). The Btu is defined as the amount of

heat energy needed to raise the temperature of one pound of water by one degreeFahrenheit. Removing one Btu of heat from one pound of water will lower thetemperature of the water by one degree Fahrenheit. See figure 1 - 1 .

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The Btu is an old unit of heat measurement but it is still often used in the oil and gasindustries. The modern unit of heat measurement is the 'joule' (J). 1054 Btu equal 1J.

Figure 1-1. The British Thermal Unit (Btu)

1.2 THE BASIC PRINCIPLES OF REFRIGERATION

Before you can understand how a refrigeration system works you need to learnsomething about heat.

Heat is a form of energy. Energy cannot be made or destroyed, it can only bechanged from one form of energy to another.

As an example, think of a single cylinder compressor driven by an electric motor.

Electricity, one form of energy, makes the motor work, and work is another form ofenergy. The work energy is passed to the piston of the compressor. The pistoncompresses the gas in the cylinder and two things happen; the pressure of the gasis increased, and the temperature of the gas is increased. The work energy hasbeen changed mainly into pressure energy and, through the increase intemperature, into some heat energy.

Although heat and temperature are related, they are not the same. Somethingwhich feels hot to you may not contain a lot of heat. More difficult to understand isthat something which feels cold to you could contain a lot of heat.

Figure 1-2. Quantity of Heat

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Figure 1-2 shows two containers of water, one of 1 litre capacity and the other of 5litres. Both containers have the same amount of heat, but the temperature of the 1litre container is higher.

The reason for this is that the 1 litre of water contains more heat per unit of volume

than the 5 litres of water. The heat energy in the 1 litre of water is moreconcentrated -than in the 5 litres of water. This shows that the temperature ofsomething does not tell us how much heat it contains.

It is easy to understand that something which feels hot probably contains heat. It isnot so easy to understand how something at a very low temperature, say - 100C,can also contain heat.

The lowest temperature we know of is called 'absolute zero. Absolute zero

measures -273

on the Celsius scale (-273C) and -460

on the Fahrenheit scale

(-460F). At -273C an object contains no heat. If we raise the temperature of thatobject by just one degree there will be some heat in the object. The definition of theBtu shows this is true.

If we raise the temperature of the object by 173C it will be at a temperature of -100C. The object is still very cold to us, but now it contains quite a bit of heat.

This is heat that we can use. The actual amount of heat in the object will dependon its size. .

Heat always flows from a higher temperature to a lower temperature. As anexample, think of a drink you want to make cold. The drink is poured into a glass

and ice cubes are added. The heat in the drink flows to the colder ice cubes whichabsorb. it. The heat is not destroyed, it has been taken out of the drink to make itcolder. The increase in heat in the ice cubes makes them melt.

If you just want the drink to be cool, perhaps two ice cubes will be enough. If youwant the drink cold you may need four ice cubes. For an even colder drink youcould use six or eight ice cubes. The cooling effect depends on the volume as wellas the temperature of the ice cubes.

There are two forms of heat, latent heat and sensible heat.

Latent heat is the heat energy required to make something change its state without

any rise or fall in temperature. It doesn't matter if the change of state is from solidto liquid or from liquid to vapour, latent heat is still involved. Latent heat cannot besensed by a thermometer or felt by the hand.

Sensible heat raises the temperature of anything it is added to and lowers thetemperature of anything it is removed from. Sensible heat can be detected by achange in temperature of the substance it is being added to or removed from.

1.2.1 Heat Transfer Methods

Heat is transferred from one substance to another by three basic methods:

conduction, convection or radiation. Figure 1-3 shows how each method takesplace.

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Conduction

Conduction is heat transfer by molecular collisions (molecules crashing together).See figure 1-3(a).

For example, if we hold one end of an iron rod in a fire, the heat will eventually

reach our hand through the process of conduction as it travels along the rod. Whena substance is heated, the molecular activity of that substance is greatly increased.That increased activity at the heated end is passed on from molecule to moleculeuntil it reaches the hand. This process will continue as long as there is a differencein temperature along the rod.

Figure 1-3. Three Heat Transfer Methods

Convection

Convection is heat transfer by mass motion of fluid matter. A change of temperaturein a fluid causes a difference in density. This makes a current in the fluid callednatural convection. An example of natural convection is easily seen in the rising ofsmoke from a fire. When a fluid is moved by a fan or a pump, the action is calledforced convection. Hot air circulation (air is a fluid) carries the heat, transfers mostof it to cooler air, then returns to take more heat and repeat the cycle. See figure1-3(b).

Radiation

If you hold your hand near (but not on) something which is hot, the heat you feel isthrough radiation. See Figure 1-3(c). The heat is radiated through the space

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between your hand and the hot object by something called electromagnetic waves.A good example of radiated heat is the heat we feel from the sun or when sittinground a fire.

1.2.2 Heat Transfer

Heat can be transferred from one substance to another using a heat exchanger.The substances used in heat transfer are usually liquids and gases. Heat transfercan be between liquid and liquid, gas and gas or liquid and gas. The heatexchanger brings the two substances close together but keeps them separated bysomething, usually the walls of the pipe containing one of the substances.

Two examples of heat exchangers can be found in a car.

Figure 1-4 The Heat Exchangers in a Car

The heat from the engine (made by burning fuel) is transferred to the water in theengine block. The water circulates through the radiator where it gives up its heat tothe air passing through the radiator fins. The air then carries the heat in the wateraway.

1.2.3 Heat and Refrigeration

We already know that even something at a temperature of - 100C has heat. If ithelps, you can think of this as negative heat. We also know that this (negative)heat can be transferred and that heat flows from a higher temperature to a lowertemperature.

Imagine two substances which we will call A and B. We can lower the temperature

of substance A so that it has negative heat. If we bring substance A and substanceB together in a heat exchanger (so that they do not mix) the heat in substance Bwill be transferred to substance A. The temperature of substance B will be loweredbecause heat has been removed.

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That is how an ordinary household refrigerator works. Substance A is a fluid calledrefrigerant which flows through pipes surrounding and inside the food compartment.Substance B is the food-and drinks put into the food compartment. The food anddrinks are warm and the refrigerant is cold. Heat is transferred from the food anddrinks (making them colder) into the refrigerant (making it warmer). The refrigerant

then gives up the heat it has gained from the food and drinks to the air around therefrigerator.

1.3 THE COMPONENTS OF A SINGLE STAGE REFRIGERATION CYCLE

A refrigeration system contains the following equipment:

A compressor.

A condenser.

An expansion valve.

An evaporator.

A high pressure receiver.

The equipment in a household refrigerator, and the way it works, is the same as allrefrigeration systems, no matter how large or small, with only a few differences.See figure 1-5.

Figure 1-5. Household Refrigerator and Refrigeration Loop

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Figure 1-6. Single-Stage Refrigeration Cycle

The cycle of operations is as follows.

1. .In the evaporator the liquid refrigerant vaporises (becomes a gas) and

expands. The vaporisation is caused by picking up heat from the surroundingarea. This raises the temperature of the refrigerant to its boiling point. Thesurrounding area is the space we want to make cool. The area will containwarm (or hot) substances.

2. The evaporated refrigerant (a gas) enters the compressor, where it iscompressed, increasing its pressure and temperature.

3. The refrigerant leaving the compressor goes to the condenser, where itbecomes liquid and gives off much of its heat in the process.

4. The liquid refrigerant may be stored temporarily in the high pressure receiver.

5. The liquid refrigerant leaves the condenser and goes through the expansionvalve. When it passes through the valve, expansion occurs and the pressureand temperature are reduced. From the expansion valve the refrigerant again

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passes through the evaporator. This completes the cycle.

Let us look at this in more detail using figures 1 -7 to 1 - 10. On these figures thelarge, black arrow shows you where you are in the system. We start at thecompressor.

Figure 1-7. Compressor Increases Pressure and Temperature

The compressor receives cool, low pressure vapour from the evaporator andincreases the vapour pressure and temperature. When the refrigerant leaves the

compressor it is still a vapour, but at a much higher pressure and temperature. Thehot refrigerant vapour is then pumped to the condenser.

Compressor input Cool, low pressure vapour

Compressor output Hot, high pressure vapour

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Figure 1-8. Hot Vapour Goes to the Condenser

In the condenser the hot gas is cooled and condensed to a liquid. To cause-heattransfer, the condensing agent, either air or water, is at a. lower temperature thanthat at which the refrigerant condenses at the existing pressure.

Condenser input Hot, high pressure vapour.

Condenser output High pressure liquid.

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Figure 1 -9. High Pressure Liquid Goes to the Expansion Valve

High pressure liquid refrigerant flows from the condenser to the expansion valve.The expansion valve slows the flow of liquid refrigerant, so that a pressuredifference exists between the upstream and downstream sides of the valve.

When the liquid passes through the expansion valve, its pressure is suddenlyreduced to that of the evaporator. At this low pressure, a small portion of the liquidboils off immediately, chilling the remaining liquid to evaporator temperature.

Expansion valve input High pressure liquid.

Expansion valve output Lower pressure liquid with a small amount of vapour.

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Figure 1 -10. The Evaporator

The cold refrigerant, along with a small amount of vapour, passes into the

evaporator where it absorbs heat and evaporates. The refrigerant absorbs heatfrom the relatively warm air or water to be cooled and vaporises. This transfer ofheat causes refrigeration.

Evaporator input Liquid with a small amount of vapour

Evaporator output Low pressure refrigerant vapour.

The output from the evaporator passes to the inlet of the compressor and the basicrefrigeration cycle is completed.

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