Principles & Uses of Refrigeration

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Unit P-14-01

Refrigeration &

Cryogenics

Principles & Uses

of Refrigeration

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Principles & Uses of Refrigeration - Unit P-14-01

UNIT P-14-01

PRINCIPLES AND USES OF REFRIGERATION

TABLE OF CONTENTS

1.0. OBJECTIVES/INTRODUCTION..........................................................................4

2.0 USES OF REFRIGERATION.............................................................5

2.1 To Reduce the Rate of Reaction....................................................5

2.2 To Store Flammable Materials......................................................5

2.3 To Condense and Store the Vapour of Low Boiling Point Liquids.....6

2.4 Air Conditioning...........................................................................7

2.5 Freeze Drying..............................................................................7

3.0 BASIC REFRIGERATION PRINCIPLES...............................................8

3.1 Adiabatic Change.........................................................................8

3.2 Isothermal Change.......................................................................9

3.2.1 Adiabatic and Isothermal Compression....................................................10

3.3 The Joule-Thomson Effect...........................................................10

3.4 Summary...................................................................................11

4.0 REFRIGERATION CYCLE...............................................................13

4.1 Compression..............................................................................15

4.2 Condensation............................................................................16

4.3 Expansion..................................................................................17

4.4 Evaporation...............................................................................17

4.5 Re-Circulation............................................................................18

4.6 Summary...................................................................................19

5.0 PRIMARY REFRIGERANTS............................................................20

5.0.1 Low Boiling Point......................................................................................20

5.0.2 Low Freezing Point....................................................................................20

5.0.3 High Latent Heat.......................................................................................20

5.0.4 High Critical Temperature.........................................................................21

5.0.5 Low Critical Pressure................................................................................21

5.0.6 Non - Corrosive.........................................................................................21

5.0.7 Non - Toxic, Non - Flammable...................................................................21

5.0.8 Non - Reactive with and Easily Separated from Oil.............................................21

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5.1 Common Primary Refrigerants....................................................22

5.1.1 Ammonia..................................................................................................22

5.1.2 Chloro-methane (methyl chloride)............................................................22

5.1.3 Sulphur Dioxide........................................................................................22

5.1.4 Propane....................................................................................................22

5.1.5 Halocarbon Refrigerants (often called CFCs)............................................23

5.2 Alternative Refrigerants.............................................................24

6.0 SECONDARY REFRIGERANTS.......................................................25

6.0.1 Very Low Freezing Point...........................................................................25

6.0.2 Low Viscosity............................................................................................25

6.0.3 Cheapness................................................................................................25

6.1 Common Secondary Refrigerants................................................26

6.2 Summary.....................................................................................................27


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1.0. OBJECTIVES/INTRODUCTION

Objectives

At the end of this section, the trainee must be able to:

State the applications of refrigeration in industry,

Define ‘critical temperature’ and ‘critical pressure’,

Define the term ‘liquefaction’,

Distinguish between ‘adiabatic’ and ‘isothermal’ expansion,

Explain the Joule-Thomson effect and state its relevance to refrigeration.

Explain the industrial vapour compression system of refrigeration,

Describe the primary and secondary refrigerants,

Describe the components of a refrigerant system.

Define the terms 'primary refrigerant' and 'secondary refrigerant'.

State the desirable properties of each of the above groups of refrigerants.

List typical refrigerants and their uses.

Introduction

Refrigeration is the process of lowering the temperature of a substance to below that of its surroundings. The oil and chemical industry are major user of refrigeration facilities. Often, refrigeration is carried out at a central point and the low temperature produced is then transferred to different areas of the plant via coolants within a ring main. It is often classified as one of the services available to the plant.

This first lesson looks at some of the more common uses and also investigates the basic principles used to produce the low temperatures required.

Although the size of refrigeration plant may vary for different applications, the basic principle of operation remains unaltered. The principle used in most refrigeration systems is the Joule-Thomson effect. This lesson considers how this principle is used industrially in the vapour compression refrigeration cycle to achieve low temperatures and cooling.

In the vapour compression refrigeration cycle, the refrigerant is the fluid medium that transfers heat from one part of the circuit to another. The choice of refrigerant is based on the required duty. In this lesson, refrigerants are classified, the 'desirable properties' of refrigerants are described and the characteristics of several common refrigerants are listed.


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2.0 USES OF REFRIGERATION

Refrigeration is the process of lowering the temperature of a substance to below that of the ambient surroundings. Refrigeration of materials is carried out for the following reasons, which are in effect, its main uses:

to reduce the rate of reaction,

to store flammable material,

to condense and store the vapour of low boiling point liquids,

air conditioning,

freeze drying.

These main uses are now considered in more detail.

2.1 To Reduce the Rate of Reaction

Some chemical reactions take place so rapidly that they can only be kept under control if the reactants are cooled to a low temperature before and during the reaction process. This is because the reaction rate is slowed by reducing the temperature.

An example is the decomposition of food. In this case, the rate of decomposition (reaction with air caused by bacteria) is usually slow. However, the rate can be slowed even further, by reducing the temperature.

For example, consider meat. If it is left in the kitchen at 20°C, it will probably go bad in a day or two. If kept in a domestic refrigerator, which lowers the temperature to about 5°C, meat will probably be useable for about a week. If placed in a deep freeze at -25°C, it will be useable for at least six months.

There are many similar examples to be found in the chemical industry e.g. storage of drugs. Refrigeration can also be used for materials whose reaction rate is very much faster. For example, if strong acid is to be added to strong alkali, the heat produced by the fast reaction may cause boiling and splashing of the liquids, giving rise to a safety hazard. If, however, both are cooled to a low temperature and kept cool during the reaction, the reaction is slower and less likely to create a hazard.

2.2 To Store Flammable Materials

Some liquids are flammable. A measure of their flammability (ease of catching fire) is known as the flash point. This is the lowest temperature at which the material gives off a vapour that will ignite when exposed to a flame.

There is a second higher temperature known as the auto-ignition temperature. This is the lowest temperature at which the material will spontaneously ignite without any external source of ignition.


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Liquids that have flash points below the normal ambient temperature are likely to explode if a spark is present. This may mean they need to be cooled to a temperature below ambient to store them safely below their flash point.


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2.3 To Condense and Store the Vapour of Low Boiling Point Liquids

Normal butane has a boiling point of -0·5°C and is a gas at normal atmospheric temperatures. As a gas, it occupies a volume over 1,000 times more than that of the same mass of liquid butane. Storage space for butane will be smaller if it is converted to a liquid by lowering its temperature to below -0·5°C using refrigeration.

An illustration of this (in reverse) is that in cold countries, on cold winter days, butane cylinders cannot be used if left outside because the low temperatures prevent the liquid butane from turning into gas. In winter, propane cylinders should be used as the liquid propane boils at -42°C under atmospheric conditions and will boil even on very cold winter days!

Another way of condensing the butane is to increase the pressure, which in turn increases its boiling point. A pressure of 5 bar (500 kPa) gives butane a boiling point of 42°C. This method is utilised to produce the liquid butane used to fill the storage cylinders. However, some gases at normal atmospheric temperatures will not condense, no matter what pressure is applied to them. They will condense, however, if cooled to a specific temperature. This temperature is known as the critical temperature for the gas concerned.

Critical temperature is defined as: the temperature above which a gas will not liquefy, irrespective of the pressure applied. Even at the critical temperature, large amounts of pressure may be needed to liquefy the gas.

Associated with the critical temperature is critical pressure, which is defined as: the pressure required to change a gas to a liquid at the critical temperature.

Some typical critical temperatures and pressures are given in the table below.

Note: a unit conversion table is located at the end of this section of the manual

To condense the first three gases in the above table, they must be cooled to very low temperatures as well as being pressurised (5 mPa is approximately 700 p.s.i.). This process of cooling below critical temperature and compressing to condense gases is known as liquefaction and the process is used to produce liquid air. This,


GAS CRITICAL PRESSURE CRITICAL TEMPERATURE

Nitrogen 3·4 mPa -147°C

Oxygen 5·0 mPa -118°C

Methane 4·6 mPa -83°C

Ethane 4·9 mPa 32°C

Propane 4·25 mPa 96°C

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in turn, is used to obtain liquid oxygen and liquid nitrogen. The process is also used to produce Liquified Natural Gas (LNG), which is discussed in Part 4.


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2.4 Air Conditioning

In very hot weather we feel uncomfortable because of the heat and the amount of water in the atmosphere (humidity). To prevent this in a working environment where uncomfortable conditions may increase the risk of accidents and reduce the efficiency of the workers, air conditioning is used.

Basically, this consists of a refrigeration system that cools the air and removes some of the water vapour by condensation. This reduces the temperature and lowers the humidity of the air, making conditions more comfortable.

2.5 Freeze Drying

The normal method used to dry materials is to heat them to evaporate the moisture. This cannot be used if the material is sensitive to heat, as the process of drying may harm it. To overcome this difficulty, freeze drying is used.

The material to be dried is pre-frozen to around -25°C and is then placed inside a chamber. The temperature is slowly increased and the ice sublimes (turns directly from a solid to a gas) causing drying of the material.


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3.0 BASIC REFRIGERATION PRINCIPLES

When petrol or other volatile (low boiling point) material is accidentally splashed over the hands, there is a cooling effect as the material evaporates. This is because the change of state occurring (from liquid to gas) requires heat (latent heat) which the liquid takes from its surroundings (in this case the hands). Water has the same effect, which is less noticeable because the rate of evaporation is slower.

3.1 Adiabatic Change

If a volatile liquid is placed in a highly insulated vessel so that heat can neither enter nor leave the system, evaporation of the liquid causes cooling of the liquid itself. The sensible heat removed from the liquid is now latent heat in the gas. This change of state is an example of an adiabatic change - known as adiabatic evaporation.

An adiabatic change is a process that occurs without heat entering or leaving a system. In general, an adiabatic change involves a fall or rise in temperature of the system.


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3.2 Isothermal Change

A true adiabatic change is very difficult to achieve in practice. When heat is added to the evaporating liquid to keep the temperature constant, the process is called isothermal.

An isothermal change is a change that occurs at constant temperature by the process taking heat in from, or giving heat out to, the surroundings.

Once again, a true isothermal change is difficult to achieve in practice.

Boiling pure water at exactly 100°C at atmospheric pressure by adding heat is an example of an isothermal process. Adiabatic and isothermal processes are shown in diagrammatic form in Figure 3-1.

Figure 3-1

In theory, low temperatures can be achieved by using the adiabatic evaporation principle. As the liquid cools however, the rate of evaporation will slow and less cooling will take place. Eventually a low temperature limit is reached where no evaporation occurs.

The lower the boiling point of the material, the lower is the temperature produced by its evaporation. Remember, petrol (which starts to boil at 40°C) has a greater cooling effect on the hands than water (boiling point 100°C). This is because, in general, the lower the boiling point, the more evaporation will occur at a given temperature.

A lowering of the pressure above the liquid will cause higher rates of evaporation at a given temperature due to the lower resistance present above the liquid.

The evaporation of a volatile liquid is one of the principles used in refrigeration. The sensible heat removed from the liquid is converted to latent heat in the gas.


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1.0.1.Adiabatic and Isothermal Compression

If a gas is compressed within a highly insulated vessel so that heat cannot enter or leave the vessel, the temperature of the gas rises. This is called adiabatic compression.

If the heat of compression is removed to produce a pressurised gas at the same temperature as the original un-pressurised gas, this is called isothermal compression.

If the pressure on the gas is now reduced without any addition of heat from the surroundings, the temperature will fall. This is called adiabatic expansion. This is because the energy required to expand the gas must come from within the gas itself since energy can neither be created nor destroyed. Therefore the temperature of the gas is reduced as energy is provided for expansion.

If heat is supplied from outside the system in order to keep the temperature constant, this is called isothermal expansion.

3.3 The Joule-Thomson Effect

Consider what may happen if a gas, which has a low condensing point at atmospheric pressure, is compressed to a higher pressure but kept at the same temperature, i.e. is subjected to isothermal compression.

Pressure affects the boiling point of a liquid; the higher the pressure, the higher is the boiling point. This is, in effect, the same as saying that an increase in pressure on a gas will raise the temperature at which the gas will condense and form a liquid. So, if a gas is subjected to isothermal compression, it may condense and form a pressurised liquid.

However, this will be the case only if the constant temperature chosen is below the critical temperature of the gas. The cooling system must remove the latent heat present in the gas as well as any heat of compression to condense the gas. Thus large amounts of heat are removed from the system to keep the temperature constant.

If the pressure on the liquid is now reduced, the liquid will boil. For this to happen, latent heat has to be supplied. If the process is carried out so that heat cannot be taken from the surroundings (adiabatic change), then the latent heat can only be supplied by the liquid itself losing sensible heat.

The liquid left will therefore cool down until a temperature is reached at which the liquid is below its boiling point for the new pressure. This cooling caused by the adiabatic expansion of a gas or liquid is called the Joule-Thomson effect.

This process of compressing a gas, cooling to its original temperature to form a liquid and then allowing the pressure on the liquid to drop causing the liquid to vaporise and cool forms the basis of the main type of refrigeration system used by industry - the Vapour Compression System. This is the subject of the next lesson.


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3.4 Summary

Refrigeration is a service supplied to many chemical plants. The principle uses of refrigeration are:

to reduce the rate of reaction

to store flammable liquids

to dry-heat sensitive materials

to condense and store vapour from low boiling point liquids

for air conditioning.

A gas can only be turned into a liquid by the application of pressure if it is at a temperature below its critical temperature. The process of cooling a gas to below its critical temperature and compressing to form a liquid is called liquefaction.

An adiabatic change is one where heat is neither added to, nor removed from the surroundings by the change.

An isothermal change is one where heat is added or removed by the surroundings to keep the temperature constant.

The Joule-Thomson effect is the cooling caused by the adiabatic expansion of a gas or liquid. This principle is used in creating the low temperatures used in refrigeration.


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CONVERSION TABLE

Pressur

e Area

bar mbar Pa kPa in.H2O

mm.H2

O lb/in2 m2 cm2 ft2 yds2

1 1000 100000 100 401 10200 14.5 1 10000 10.76 1.196

0.001 1 100 0.1 0.401 10.2 0.00145 0.0001 1

0.00107

6

0.00011

96

0.00001 0.01 1 0.001 0.00401 0.102

0.00014

5 0.0929 929 1

0.11111

11

0.01 10 0.001 1 4.01 102 0.145 0.8361 8361 9 1

0.0024 2.49 249 0.249 1 25.4 0.0361

0.00009

81 0.0981 9.81 0.0981 0.0394 1 0.00142

0.0689 68.9 0.00689 6.89 27.7 703 1 Volume

Temperature Velocity yds3 ft3 m3 in3

1 27 0.7646 46656

Deg. C. Deg. F. m/s ft/min

0.03703

7 1 0.02832 1728

0 32 1 197 1.308 35.31 1 61023

-

17.7777

78 0 0.00508 1

2.143E-

05

0.00057

87

1.639E-

05 1

Volume Flow Power


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m3/s ft3 / min l/s

galls/

min galls/hr Watts Kew Btu/Hr kcal/Hr HP

1 2119 1000 13200 792000 1 0.001 3.412 0.86 0.00136

47190 1 0.4719 6.229 373.74 1000 1 3412.1 860 1.36

1000 2.119 1 13.2 792 0.2931

0.00029

31 1 0.2519

0.00039

85

757700 0.1605 0.07577 1 60 1.1628

0.00116

3 3.968 1

0.00158

13

4546200

0 9.63 4.5462

0.01666

67 1

735.294

11

0.73529

41

2508.89

7

632.352

9 1


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4.0 REFRIGERATION CYCLE

The purpose of a refrigeration system is to maintain a material at a temperature below that of the surroundings. If this is achieved, then heat will flow from the warmer surroundings to the colder material. If the material has to be kept at the low temperature, then the heat flowing from the surroundings must be minimised and continually removed.

The principles used in the vapour compression refrigeration system are:

As a gas is compressed, its temperature rises. If the gas is cooled to remove the heat of compression, then the gas may now be at its condensation point (an increase in pressure causes the condensing or boiling point to rise). If the latent heat is now removed, the gas will condense and form a liquid.

If a liquid is expanded adiabatically, it will boil. This causes the liquid to cool as it loses some of its sensible heat, which is used to provide the latent heat energy required for the change to gas.

The vapour compression refrigeration system combines these principles to produce a situation where heat can be both added and removed from the same temperature surroundings.

The vapour compression system requires:

a compression stage to compress the gas,

a cooler (condenser) to remove the heat of compression and latent heat (condensing the gas),

an expansion device to reduce the pressure causing boiling of the liquid,

a heat exchanger where the cold liquid produced is used to cool down a second material (the material to be cooled).


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The gas formed in the expansion system is then recycled to the compression stage where the process is repeated. The gas/liquid that is recycled in this system is known as the primary refrigerant.

The vapour compression refrigeration system is illustrated in Figure 4-1. Refer to this diagram as each stage is now studied in more detail.

Figure 4-1

The vapour compression refrigeration cycle can be broken down into the following five stages, which are studied in turn. The number of each stage is included on Figure 4-1.

Compression

Condensation

Expansion

Evaporation

Re-circulation.


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4.1 Compression

A compressor is used to pressurise the primary refrigerant gas to a pressure such that, on cooling the gas back down to atmospheric temperatures (compressing a gas raises its temperature), it will condense and form a liquid.

The pressure required will depend on how much the condensing point of the gas is affected by pressure (an increase in pressure raises the condensing point of a gas). The compressor used generally operates on one of two main principles:

Reciprocating motion

This consists of a piston inside a cylinder that is moved backwards and forwards by a driver, sucking gas in on the backstroke and forcing it out on the forward stroke.

The driver may be outside the main compressor housing (the open type, reciprocating compressor) or the motor and compressor are connected by a common shaft and totally enclosed in the same housing (the hermetic type, reciprocating compressor). The latter type eliminates leaks but is more difficult to service.

Both types will require lubrication of the piston to reduce friction and to help seal the piston within the cylinder. This can lead to oil contamination of the refrigerant, which is undesirable as it reduces the efficiency of the refrigerant. If oil contamination is likely to occur, then an oil separation stage is included. This consists of a device that uses centrifugal force to throw the heavier oil particles to the outside of a vessel from where they can be removed. Another method is by the impingement of oil droplets onto baffles so that the droplets form into large drops, which sink to the bottom of the vessel from where they are removed.

If a totally oil-free system is required (e.g. if the oil reacts with the refrigerant chosen) then a diaphragm pump is often used. This type may also be used if the refrigerant chosen is extremely corrosive.

Usually, reciprocating compressors are used on very low temperature duties because of their ability to produce high pressures on the delivery side and high vacuum on the suction side. However, they are less likely to be used on very large installations as they cannot handle large amounts of material unless they themselves are extremely large. This would make them very expensive due to the high cost of the precision engineering required.

Centrifugal action

These are multistage centrifugal compressors where the high speed created by the impeller is converted into pressure energy.

These have the advantages of low cost, they can handle large quantities of gas and do not require lubrication of parts in contact with the refrigerant. However, they have a tendency to leak refrigerant unless the shaft seal is well maintained. Also, they cannot produce the large suction pressures possible with a reciprocating compressor.


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Rotary compressors are rarely used in refrigeration systems. This is due to the need for lubrication between the moving parts (e.g. the gears) in contact with the refrigerant and other operating conditions beyond the scope of this course.


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4.2 Condensation

The hot compressed gas from the compressor needs to be cooled to remove the heat of compression. Also, the latent heat needs to be removed at the condensing point to enable the gas to turn into liquid at the higher pressure.

In small-scale refrigeration systems such as domestic refrigerators, the condenser consists of a pipe designed to give the maximum heat transfer surface exposed to the air surrounding it. Using air as the coolant is cheap as the air costs nothing and does not need to be supplied from an external source. The condenser on the back of domestic refrigerators can be easily identified.

The major problems with this system are that:

Air is a poor conductor of heat, has a low specific heat capacity and as it is a gas, has a relatively low density. It cannot therefore, remove very much heat per kilogram (1 kg of air is about 700 litres in volume).

The air in contact with the condenser will heat up and cease to cause cooling. Movement of the hot air away from the pipe relies on natural convection currents. These cause the hot air to rise pulling cold air in behind it. The convection currents can be hindered by placing the refrigerator underneath a work surface without proper venting at the top (to allow the hot air out) and the bottom and sides (to allow the cold air in).

In this situation, the efficiency of the condenser is reduced and the gas passing back to the compressor becomes hotter and hotter.

Then, the compressor has to develop higher pressures than normal, which can lead to premature failure of the refrigerator.

On larger industrial systems, where natural convection currents are not very effective, a fan may be used to force air over the condenser surfaces. The design of the condenser can now become more compact, i.e. the pipes and the fins are tightly packed (like a car radiator) as the pressure developed by the fan can push the air through smaller spaces.

On very large systems, air-cooling is replaced by water-cooling. Water is a better conductor and has a higher specific heat capacity than air and will cause larger amounts of cooling. This means that for a specific duty, the water-cooled exchanger is much smaller and less expensive. Also, the provision of water to a process becomes progressively cheaper the more that is supplied.

One water-cooled condenser design consists of water cascading over the outside of the pipes containing the hot gas (a cheap system of low efficiency). Another uses a counter-current shell and tube heat exchanger where the hot gas is passed into the shell and water passes through the tubes (a more expensive and effective system). See Heat Exchange, Unit 4 for revision of this type of equipment.

The cooled liquid at high pressure leaving the condenser then passes to a storage vessel (the receiver) ready for the next stage.


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4.3 Expansion

The cold pressurised liquid then passes through an expansion (or 'let-down') valve. The expansion valve is a pressure reducer that automatically controls the differential pressure at a set value. In addition, it can be thermostatically controlled so that it adjusts the differential pressure to maintain the temperature in the refrigerator system.

The expansion valve allows the liquid to pass from the high-pressure region to a low pressure region. The reduction in pressure causes the liquid refrigerant to boil. The latent heat required by the liquid to turn into a gas is provided by the liquid itself, which therefore cools until a temperature is reached where it stops boiling (the boiling point of the liquid at the controlled low pressure).

The cooled liquid then passes to an evaporator and the gas formed in the proess passes to the suction of the compressor.

4.4 Evaporation

The evaporator takes the cooled liquid from the expansion valve and adds heat to cause further boiling of the primary refrigerant. The design of this varies according to the size and duty of the refrigeration system. In the domestic refrigerator, the evaporator is the metal icebox at the top, which absorbs heat from the rest of the contents of the fridge, thereby cooling them. In a deep freeze, the evaporator forms one or more internal sides of the freezer.

Note that the icebox and some sides of fridges and freezers are hollow and contain the primary refrigerant. This is why sharp tools should not be used to remove ice built-up.

In industry the evaporator may be simply a pipe or coil containing a fluid to be cooled inside a vessel containing the primary refrigerant or may be a more complex shell and tube design of heat exchanger.

The heat required by the evaporator is provided either by the surroundings or by a process fluid known as the secondary refrigerant which, in turn, cools down.

The heat taken in by the evaporator causes the primary refrigerant liquid to boil by providing the latent heat required. The temperature remains at the low value since the liquid is at its boiling point and cannot be heated above this without the pressure increasing (the pressure is controlled by the expansion valve!) or until all the liquid has boiled away. Thus the evaporator gives a controlled low temperature. The net effect is that sensible heat is taken from the surroundings or secondary refrigerant and is given to the primary refrigerant as latent heat.


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4.5 Re-Circulation

The cold gas produced in the evaporator mixes with that from the expansion valve and passes to the inlet of the compressor where the whole process is repeated. Thus the primary refrigerant is recycled. Since the main duty of the refrigeration system is to create cooling in the expansion/evaporator section, the cold gas is kept as cold as possible as it passes to the compressor.

As it is at a temperature below that of its surroundings, the pipe-work is usually lagged to prevent loss of the cold by the addition of heat from a part of the surroundings that does not require cooling. It is also usual to lag all parts of the expansion/evaporator section that are exposed to higher temperature material that does not require cooling. For example, a domestic fridge has lagging in the sides, top and bottom to keep the whole of the fridge from gaining heat from the kitchen, but the icebox is not lagged allowing the 'cold' to spread from the box through to all internal parts of the fridge.

Note: the icebox in a domestic fridge is cold enough to freeze water but the main part of the fridge will not. This is because the icebox is the coldest part due to the evaporation taking place there, whilst this 'cold' has to spread through the rest of the fridge. Compare this to a combustion chamber in a heating system, this is the hottest part of the system. The greater the distance from the heat source, the lower the system temperature becomes.


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Figure 4-2

With the domestic refrigerator (refer to Figure 4-2), the circulating LP vapour (5) is compressed (1) and then condensed in an air-cooled finned condenser (2). The liquid formed passes to a receiver from where it is forced through the piping to an expansion valve (3) where the pressure is reduced. This causes evaporation of the liquid in the icebox (4), creating low temperatures (-6°C).

This low temperature spreads throughout the rest of the fridge but due to leaks of heat into the system, the temperature in the rest of the fridge is above that of the icebox. The temperature within the fridge is controlled by an adjustable thermostat, which switches the compressor on (if temperature above set point) and off (if temperature below set point).

In the industrial refrigerator, the hot compressed gas is cooled and condensed in a water condenser. The liquid then passes via a reservoir to the expansion valve, which reduces the pressure. The evaporating liquid creates cooling of a brine solution (the secondary refrigerant) surrounding the evaporator.

This cooled brine solution is then pumped through a system of pipes within the vessel to be cooled. This helps to spread the cold from a single point (the evaporator) throughout the whole of the vessel to be cooled. (This can be compared to a fire in one room trying to heat up a whole house compared to a central heating boiler in one room with pipes and radiators in other rooms). In this case the low temperature in the vessel is controlled by both the temperature in the evaporator and the rate of circulation of the brine.

4.6 Summary

The vapour compression refrigeration system consists of five stages:

a primary refrigerant gas is compressed by a compressor (usually of the reciprocating or centrifugal type). Any oil contamination of the primary refrigerant must be removed at this stage.

cooling the pressurised gas to ambient temperature and condensing: it into a liquid in a heat exchanger (either a finned air cooled type or a water cooled, shell and tube heat exchanger).

reducing the pressure on the liquid by passing it through an expansion valve (a pressure reducing valve which may be thermostatically controlled) causing cooling of the liquid.

evaporating the liquid to form a gas in an evaporator by adding heat from the atmosphere or from a secondary refrigerant.

re-cycling the gas back to the compressor using a system of lagged pipework.


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5.0 PRIMARY REFRIGERANTS

In the vapour compression cycle a gas is compressed, cooled and condensed to a liquid. This is then expanded to reduce the pressure (causing the liquid to boil and cool) producing a gas, which is recycled back to the compressor. The gas/liquid circulating in the vapour compression refrigeration cycle is known as the primary refrigerant.

The desirable properties of primary refrigerants are:

low boiling point

low freezing point

high latent heat

high critical temperature

low critical pressure

non - corrosive

non - toxic, non - flammable

non - reactive with and easily separated from oil.

The primary refrigerant should also be cheap.

5.0.1 Low Boiling Point

The primary refrigerant has to be a gas at normal atmospheric temperature and pressure. If not, the system will have to operate (on the recycle side) at a pressure below atmospheric to evaporate the liquid and form a gas. This could lead to air leaking into the system. Any moisture within the air will freeze at the expansion valve, causing blockage of the evaporator. To prevent this, in most refrigeration systems, the primary refrigerant usually has a boiling point within the range -100°C to + 10°C (typically -30°C to - 40°C).

5.0.2 Low Freezing Point

The freezing point of the material must be well below that of its boiling point so that any cooling that occurs as the liquid is evaporated does not cause the remaining liquid to freeze.

A frozen refrigerant will block the piping system, reduce the efficiency of the compression system and will eventually damage the compressor.


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5.0.3 High Latent Heat

Latent heat is the heat required to change the state of a material. The liquid refrigerant should require a large amount of heat to cause it to change its state from liquid to gas i.e. a high specific latent heat of vaporisation.

If it possesses a high latent heat, it will remove a lot of sensible heat from itself

or the surroundings, giving large amounts of cooling per kilogram of liquid evaporated. This will mean that the amount of refrigerant circulating in the compression cycle can be kept small, thus keeping costs down.


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5.0.4 High Critical Temperature

Critical temperature is the temperature above which a gas will not form a liquid on compression. In the vapour compression system, a gas is compressed to form a liquid. Its critical temperature must therefore be above normal operating temperatures within the system.

5.0.5 Low Critical Pressure

Critical pressure is the pressure required to turn a gas into a liquid at its critical temperature. This should be low so that the pressures required to convert the gas to a liquid in the vapour compression system are low. The compressor used will then only require a small amount of energy to produce the low compression pressure, saving on running costs. Also, because of lower pressures, the strength of the construction needed to contain the refrigerant can be lower.

5.0.6 Non - Corrosive

If the chosen refrigerant is corrosive, the materials of construction would need to be capable of withstanding the corrosion. Corrosion resistant materials (e.g. stainless steel) are usually of higher cost than 'normal' materials such as mild steel. Also, if corrosion did occur, refrigerant that could be hazardous to personnel and other equipment might leak.

5.0.7 Non - Toxic, Non - Flammable

The vapour compression system operates under pressure throughout most of the system, therefore any leaks will lead to refrigerant gas escaping into the surroundings. If the gas is toxic or flammable, then a loss of containment could be very hazardous.

5.0.8 Non - Reactive with and Easily Separated from Oil

The compressor used in the vapour compression system may need lubricating and the oil used may come into contact with the refrigerant. If it does, it should not react as this will affect the properties of the refrigerant and lead to lower efficiency. The oil should also be easily removed from the refrigerant for the same reasons.

So far, no single substance has been found which ideally fits all these properties and it is therefore necessary to compromise in order to select the best refrigerant for each particular duty required. It may be that after considering the above points, several refrigerants will be suitable.

The last point to consider is the price of the alternatives. Far too often, cost is put at the top of a list of priorities!


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5.1 Common Primary Refrigerants

5.1.1 Ammonia

Ammonia was the first refrigerant to be used on a large scale. It is efficient, but it is also very toxic and flammable. That is why safer substitutes have often been preferred and ammonia is now generally restricted in use.

Ammonia is a colourless, toxic gas with a characteristically pungent smell.

It is a good refrigerant in many ways, with fairly low boiling and freezing points and it is cheap. It also gives a large amount of refrigeration for a given compressor size, which keeps costs down. It is non - flammable, but explosions have occurred when the gas has come into contact with welding torches.

Against these advantages, however, must be set the fact that it is highly toxic and irritant, and absorbs water readily. When it has absorbed water it is severely corrosive to copper alloys.

It is generally used in large-scale industrial refrigeration plants, such as cold storage plants and breweries. Leak testing for ammonia gas is by means of hydrochloric acid, which forms a dense white smoke of ammonium chloride in the presence of ammonia.

5.1.2 Chloro-methane (methyl chloride)

Chloromethane is a colourless gas with a faintly sweetish odour. It has a relatively low operating pressure, high latent heat capacity and boils at -24°C.

If chloromethane absorbs water, it becomes very corrosive to aluminium, magnesium and zinc alloys. Better refrigerants are now available and chloro-methane is little used. Leak testing is done with a special alcohol burning torch whose flame exhibits a colour change when chloro-methane is present.

5.1.3 Sulphur Dioxide

Sulphur dioxide is a colourless gas with a pungent smell and is highly toxic. It is non - explosive and non - flammable, but when mixed with water, forms sulphurous acid which is highly corrosive to metals. It has good latent heat capacity and is capable of achieving low refrigeration temperatures.

The toxicity and corrosive properties of sulphur dioxide have led to its replacement by more modern refrigerants. Leak tracing can be done using an ammonia swab, which will give white fumes in the presence of sulphur dioxide.

5.1.4 Propane

Propane is a highly flammable gas whose boiling point is - 42°C. It is a good refrigerant in many ways, its main disadvantage being its high flammability. So, it should not be used if there is the possibility of escaped refrigerant coming into contact with a naked flame.


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5.1.5 Halocarbon Refrigerants (often called CFCs)

The defects of the refrigerants generally available led to much research in the late 1930s aimed at producing a more satisfactory material. Many substances were investigated and in the 1940s a range of refrigerants were developed based mainly on two hydrocarbon chemicals, ethane and methane. These are the principal refrigerants in use today, usually under trade names such as 'Arcton' and 'Freon'.

Typical refrigerants in this group are dichlorodifluoromethane and trichlorotrifluoroethane.

HFCs are pratically non-toxic and non- flammable. They can be used in a large number of applications. HFC present minimum risk, even in the case of an accidental leak. They also offer very good energy performance. However, whilst they have no effect on stratospheric or tropospheric ozone, they are greenhouse gases.

In industrial applications, refrigerants are not known by their actual chemical name but are given a numerical designation. Some of these numerical designations are given in the following text.,

R11 is a single chlorofluorocarbon or CFC compound. It has a high chlorine content and ozone depletion potential (ODP) and high global warming potential (GWP). The use and manufacture of R11 and similar CFC refrigerants is now banned within the European Union.

R22 is a single hydrochlorofluorocarbon or HCFC compound. It has low chlorine content and ozone depletion potential and only a modest global warming potential. R22 can still be used in small heat pump systems, but no more new systems can be manufactured for use in the EU after late 2003. From 2010 only recycled or saved stocks of R22 can be used, as it will no longer be manufactured.

Phase out dates for R22

From 1 July 2002 no more cooling or air conditioning equipment can be manufactured that uses refrigerant R22.

From 1 January 2004 no more heat pump equipment can be manufactured that uses refrigerant R22.

After 1 January 2010 no more virgin refrigerant R22 can be used in existing systems.

After 2015 no more recycled refrigerant R22 can be used in existing systems.If you have recently installed an R22 air conditioning system the phase out dates should not cause you concern. Your system will only require additional refrigerant should a leak or major repair is required and this can be effected within current legislation until 2015.

There is already a "drop in" replacement refrigerant for R22 with zero ODP - R417A - See below.

R134A is a single hydrofluorocarbon or HFC compound. It has no chlorine content, no ozone depletion potential, and only a modest global warming potential.


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R407C is a ternary blend of hydrofluorocarbon or HFC compounds, comprising 23% of R32, 25% of R125 and 52% of R134a. It has no chlorine content, no ozone depletion potential, and only a modest direct global warming potential.

R410A is a binary blend of hydrofluorocarbon or HFC compounds, comprising 50% of R32 and 50% of R125) it has no chlorine content, no ozone depletion potential, and only a modest global warming potential.

R417A is the zero ODP replacement for R22 suitable for new equipment and as a drop-in replacement for existing systems.

There are currently no restrictions on equipment or use of the following refrigerants: R134A, R407C, R410A, and R417A.

All these refrigerants are non-flammable, have low toxicity and are non -irritant at normal temperatures. They have good latent heat capacities and a wide range of physical and thermodynamic properties which enable them to be 'tailored' to meet the requirements of most refrigerator systems.

5.2 Alternative Refrigerants

R290 - Pure propane, a hydrocarbon (HC) an efficient naturally occurring refrigerant with similar properties to R22, but has no ozone depletion potential and an extremely low global warming potential. Whilst it is environmentally safe, it is also highly flammable and must only be used after careful consideration is given to safety.

Leak detection is usually carried out using an electronic leak detector. Although the halocarbon refrigerants are not health hazards at normal temperatures, at high temperatures, such as those encountered in welding or in a fire on the plant, they can break down chemically, forming very irritating and toxic compounds.

Modern research has shown that halocarbons (or CFCs) affect the ozone layer, which protects the earth from the harmful ultraviolet radiation emitted by the sun and contribute to global warming. ‘Environmentally friendly’ substances are gradually replacing halocarbons, which are used in air conditioning as well as refrigeration systems. Typically, these hydrocarbon-based substances have an atmospheric life-time of less than 1 year (compared to the atmospheric life-time of 130 years for CFCs).

In industrial applications, refrigerants are not known by their actual chemical name but are given a numerical designation. Some of these numerical designations together with the corresponding chemical name are given in the table in the Appendix (for information purposes only).

In the vapour compression system the evaporation of the primary refrigerant creates cooling. In the domestic refrigerator this occurs in the icebox which then cools down the rest of the fridge. However, the rest of the fridge is never as cold as the icebox.

Heat is transferred by conduction, convection and radiation. In a cool fridge, radiation will not occur. The fridge has mainly air inside, which is a poor conductor of heat, so conduction will be low. This will mean that the air will cool down very slowly. For convection there must be a temperature difference to cause hot air to


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rise and colder air to fall. Thus the 'cold' will not easily transfer from the icebox to the rest of the fridge.

In industrial systems, the 'fridge' is much larger so the cold needs to be forcibly spread throughout the space if it is to be made cold. To do this, a second refrigerant is used to take the cold from the evaporator and spread it through the fridge. This refrigerant is known as the secondary refrigerant.


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6.0 SECONDARY REFRIGERANTS

These are materials that are circulated through the evaporator, cooled and then passed to other parts of the plant to spread the cold. In the process of cooling the other parts, the secondary refrigerants are heated. The heated material is then recycled back to the evaporator to be re-cooled.

Note: primary and secondary refrigerants never come into direct contact with each other; they are always separated by a heat exchange surface.

Secondary refrigerants, like primary refrigerants, should possess certain properties:

very low freezing point

low viscosity, i.e. easily pumped

cheapness

non-toxic

non-flammable

non-corrosive.

6.0.1 Very Low Freezing Point

The secondary refrigerant passes through the evaporator, which is the coldest part of the vapour compression system. It must not freeze as it would block the piping system and not spread the cold. Boiling point is not so important except that it should be above normal working temperatures to maintain the refrigerant as a liquid. In other words, it should not easily boil or evaporate at working temperatures.

6.0.2 Low Viscosity

The refrigerant should have a low viscosity so that it is easily pumped through the piping system. As temperature decreases, viscosity increases. This means that the liquid will become more viscous during its passage through the evaporator. The change of viscosity with temperature must be taken into account when choosing the refrigerant.

6.0.3 Cheapness

Large amounts of the secondary refrigerant are required to spread the cold around the plant system. If the material is expensive, it would involve a high cost. However, as before, the cost should only be a factor when the properties mentioned earlier are met.

Water meets most of the above properties (especially cost) with one exception i.e. it is unsuitable because it freezes at low temperatures. Most secondary refrigerants however, are based on water with additives to reduce the possibility of freezing.

A secondary refrigerant must also be non – toxic, non – flammable and non – corrosive for the same reasons as primary refrigerants.


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6.1 Common Secondary Refrigerants

Secondary refrigerants can be formed:

By the addition of a solute to water

Common salt (sodium chloride) when dissolved in water will lower its freezing point. This is why roads are salted when the temperature is likely to fall below freezing point (0°C). The stronger the solution made by adding salt, the lower the temperature at which the water will freeze. A typical solution strength used is 25% which freezes at around -20°C. This solution is often called brine.

The major problem with salt solution is that it is very corrosive especially to steel (e.g. it causes cars to rust). Other problems are:

The stronger the solution, the more difficult it is to pump

The maximum strength possible (saturated solution) limits the lowest temperature to about -25°C.

Calcium chloride solution, which acts in a similar way to sodium chloride, can be used instead. Although less corrosive, it is more expensive.

By the addition of a liquid to water

Methanol and ethylene glycol have low freezing points (methanol -98°C and ethylene glycol -16°C). When added to water, the mixture produced has a lower freezing point than water alone. These are commonly used in car radiators in winter to stop the cooling system from freezing and are known as antifreeze. The strength of the mixture will once again determine the freezing point.

Methanol is relatively cheap, but at normal temperatures its rate of evaporation is high and the strength of the mixture will therefore change with time giving less protection from low temperatures. Glycol is more expensive and more corrosive but will not evaporate. Even though ethylene glycol has a higher freezing point than methanol, when mixed with water it has a greater effect on depressing the freezing point. A 50% mixture of ethylene glycol/water freezes at -49°C whilst a 50% methanol/water mixture freezes at a higher temperature.

In both these cases, a corrosion inhibitor is usually added to the mixtures to reduce the amount of corrosion. Also, it is important that any loss of liquid is replaced using the correct strength solution and not simply topped up with water. Regular checks of refrigerant strength should also be carried out to ensure that it gives the necessary protection.


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Other secondary refrigerants used in industry, together with some of their important properties, are:

air - cheap, does not freeze but has a low capacity for removing heat and is a poor conductor

carbon dioxide - can be converted into a solid (dry ice) and used to give portable 'cold' transfer

oils - these have low freezing points and are non-corrosive but tend to be very viscous

process fluid - sometimes it is the process fluid that is cooled by the refrigeration system so its properties cannot be listed.

6.2 Summary

Primary refrigerants are the materials circulating in the vapour compression refrigeration cycle, which produce the cooling effect as they change from liquid to gas.

They must possess certain properties, the most important of which is that they must easily change from a liquid to a gas.

The most commonly used primary refrigerants in modern industrial systems are the halocarbons (Freons and Arctons).

Secondary refrigerants are materials used to spread the cold to different parts of the refrigeration system.

These must possess certain properties, the most important of which is that they must not freeze.

Typical secondary refrigerants are water mixed with substances that depress its freezing point, eg. salt, antifreeze.


Principles & Uses of Refrigeration

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introduction refrigeration

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