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Kavindra Pant Page 1

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CONTENTS

What is refrigerant? History of refrigerants. Classification of refrigerants. Primary refrigerants. Secondary refrigerants.

Properties of different refrigerants. Designation of refrigerants. Desirable properties of refrigerants.

Thermal properties. Chemical properties. Physical properties.

Ozone depletion and global warming. Applications. Air refrigeration system. Bootstrap refrigeration.

Conclusion.

References.


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REFRIGERANTS

WHAT IS REFRIGERANT?

Any substance capable of absorbing heat from another required substance can be used as

refrigerant, i.e., ice, water or brine. A mechanical refrigerant is a refrigerant which will

absorb the heat from the source (which is at lower temperature) and dissipate the same to

the sink (which is at high temperature than source) either in form of sensible heat (as in the

case of air-refrigeration) or in form of latent heat (as in case of vapour refrigeration).

The refrigerants which carry heat in form of latent heat and also dissipate heat in

form of latent heat are more efficient than the refrigerants which carry the heat in form ofsensible heat.

HISTORY OF REFRIGERANT:

The first refrigerant used was ether, employed by Perkins in his had operated vapour -

compression machine. In the earlier days, ethyl chloride (C2H5Cl) was used as ref referent

which soon gave way to ammonia as early in 1875.At about the same time, sulphur dioxide

(SO2) in 1874, methyl chloride (CH3Cl) in 1878 and carbon dioxide (CO2) in 1881, found

application as refrigerants, During 1910-30 many new refrigerants, such as N2O3, CH4,

C2H6, C2H4, C3H8, were employed for medium and low-temperature refrigeration.

Hydrocarbons were, however, found extremely inflammable. Dichloromethane (CH2Cl2),

dichloroehylene (C2H2Cl2) and monobromomethane (CH3Br) were also used as refrigerants

for centrifugal machines.

CLASSIFICATION OF REFRIFERANT:

Refrigerants are classified into two groups:

1: Primary refrigerants.

A: Halocarbon compounds.

B: Azeotropes.

C: Hydro-carbons.

D: Inorganic compounds.

E: Unsaturated organic compounds.

2: Secondary refrigerants.


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A: Water.

B: Brines.

PRIMARY REFRIGERANTS:

Halocarbon:

This is group of refrigerants was invented and developed by Charles Kettering and

Dr. Thomas Migley in 1928.these refrigerants are sold in the market under trade name as

Freon, Genetron, Isotron, and Arcton. This group includes refrigerants which contain one or

more of three halogens, chlorine, fluorine, and bromine.

Eg:

Azetropes:

The refrigerants under this group consist of mixtures of different refrigerants

which do not separate into their components with the changes in pressure or temperature

or both. They have fixed thermo-dynamic properties.

Eg:

Refrigerant 500 which contains 73.8% F-12 and 26.2% F-152.

Hydro-carbons:

Most of the organic compounds are considered as refrigerant under this group.Many hydrocarbons are successfully used as refrigerants in industrial and commercial

installations. Most of them possess satisfactory thermodynamic properties but are highly

flammable.

Inorganic compounds:

The refrigerants under this group were universally used for all purpose before the

introduction of halocarbon group. They are still used for different purposes due to theirinherent thermodynamic and physical properties.


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SECONDARY REFRIGERANTS:

Primary refrigerants directly take part in refrigeration system where secondary

refrigerants are first cooled with the help of primary refrigerants and are further used for

cooling purpose.

Water:

When the required temperature to be maintained is above the freezing point of

water, then water is universally used as secondary refrigerant mostly in air conditioning

plant and industrial cooling installations. In air conditioning plants, chilled water is used for

cooling.

Brine:

When the temperature required to be maintained are below the freezing point of

water than the water cannot be used as secondary refrigerant. In such case brine solutions

can be used.


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PROPERTIES OF DIFFERENT REFRIGEERANTS:

REFRIGERANT CHEMI

CAL

FORM

ULA

DESIGNA

TION

M

(MOLEC

ULAR

WEIGHT)

Ts

(CRITICAL

TEMPERA

TURE)

Pc

(CRITIC

AL

PRESS

URE)

bar

Vc

(CRITICA

L

VOLUME

)L/kg

Tf

(FREEZ

ING

POINT)

Ts

(N.B.

P.)

INORGANIC

REFRIGERANT

Water H2O R 718 18.016 314.15 221.3 3.26 0.0 100

Ammonia NH3 R 717 17.031 133.0 112.97 4.13 -77.7 -33.3Carbon Dioxide CO2 R 744 44.01 31.1 73.72 2.135 -56.6 -78.4

Sulphur

Dioxide

SO2 R 764 64.06 157.2 78.7 1.92 -73.2 -10.0

Nitrous Oxide N2O - 44.02 36.5 72.7 2.188 -80.2 -88.4

Sulphur

hexafluoride

SF6 - 146.0 45.56 37.4 1.35 -50.8 -63.8

ORGANIE

REFRIGERANT

TrichlorofluoroM

ethane

CCl3F R 11 137.39 197.78 43.7 1.805 -111 23.7

Dichlorodifluoro

Methane

CCl2F2 R 12 120.92 112.04 41.15 1.793 -136 -29.8

ChlorodifuoromE

thane

CHClF2 R 22 86.48 96.02 49.88 1.949 -160 -40.8

Difluoromethane CH2F2 R 32 52.024 78.41 58.3 2.326 - -51

Pentafluoroetha

ne

CHF2CF

3

R 125 120.02 66.25 36.31 1.748 - -48.5

Trifluoro

Methane

CHF3 R23 70.01 25.83 48.2 1.748 -155 -82.2

HYDROCARBO

N

Ethane C2H4 R 170 30.06 32.1 49.3 4.7 -183.2 -88.6

Propane C3H8 R 290 44.1 96.8 42.56 4.545 -187.1 -42.1

n-Butane C4H10 R 600 58.1 153.0 35.3 4.29 -135 -0.5

UNSATURATED

HYDROCARBO

N

Propylene C2H4 - 42.08 94.4 46.0 4.2 -185 -47.7


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Dichloroethyle

ne

C2H2C

l2

- 96.9 243 54.9 - -56.6 90.9

DESIGNATION OF REFRIGERANTS:

The American Society of Refrigerating Engineers (ASRE) has developed certainconventions for use in naming different types of refrigerants. These naming conventions

differ according to the type of refrigerant. Each refrigerant type is denoted by a different

series. Thus, we have separate series for halogenated refrigerants and other types. The

naming conventions are simple and easy to follow. These conventions are now accepted

worldwide and help to name the large variety of refrigerants available commercially

nowadays.

Halocarbon Compounds:

These are represented by a three digit nomenclature. Here, the first digit represents

the number of carbon atoms in the compound minus one, the second digit stands for the

number of hydrogen atoms plus one while the third digit stands for the number of fluorine

atoms. The remaining atoms are chlorine.

As an example, let us consider the refrigerant having R22 as its three digit nomenclature.

According to the above mentioned convention,

No. of C atoms in R22: C 1 = 0 => C = 1

No. of H atoms in R22: H + 1 = 2 => H = 1

No. of F atoms in R22: F = 2

Since there is only one carbon atom in the compound, this compound has originated from

the methane series (CH4). From the calculation, we can see there is one hydrogen atom and

two fluorine atoms. The remaining valence bond of carbon will be balanced by chlorine.

Thus, the substance is

Graphical Representation of Monochloro-Difluoro-Methane

Therefore, chemical formula of R22 is CHClF2 and has the name Monochloro-difluoro-

methane.

Taking again the example of R134, we can calculate its chemical formula as above which

gives us


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No. of C atoms: C 1 = 1 => C = 2

No. of H atoms: H + 1 = 3 => H = 2

No. of F atoms: F = 4

Therefore, no. of Cl atoms: Cl = 0

Graphical Representation of Tetrafluoroethane

The compound is C2H2F2 and its name is Tetrafluoroethane.

The non-halogenated refrigerants follow a different naming convention which is dependant

upon the series of the refrigerant.

DESIRABLE PROPERTIES OF REFRIGERANTS:

The vast number of refrigerants available in the market today allows us to choose a

refrigerant depending upon the operating conditions of the refrigeration system. As such,

there is no refrigerant that can be advantageously used under all operating conditions and

in all types of refrigeration systems. In spite of that, we can state certain desirable

properties that a refrigerant should possess. These properties can be divided into

favourable thermodynamic, chemical and physical properties:

THEERMODYNAMIC PROPERTIES:

Critical Temperature and Pressure

The critical temperature of the refrigerant should be as high as possible above the

condensing temperature in order to have a greater heat transfer at a constant temperature.

If this is not taken care of, then we will have excessive power consumption by the

refrigeration system.

The critical pressure should be moderate and positive. A very high pressure will make the

system heavy and bulky whereas in case of very low pressures, there is a possibility of airleaking into the refrigerating system.


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Specific Heat:

The specific heat of the liquid should be as small as possible. This ensures that the

irreversibilitys associated with throttling are small and there is greater sub cooling of the

liquid. On the other hand, the specific heat of vapour should be high to have lesssuperheating of the vapour.

Enthalpy of Vaporization:

This should be as large as possible to minimize the area under superheat and the

area reduction due to throttling. Also, the higher value of enthalpy of vaporization lowers

the required flow rate per ton of refrigeration.

Taking these three factors into account, the T-s and p-h diagrams of an ideal refrigerant

would be as shown in Figures.

These properties are

practically not found in any refrigerant. So, a trade-off has to be done in order to achieve as

high a COP as possible.

Conductivity:

The conductivity of the refrigerant should be as high as possible so that the size of

the evaporator and condenser is manageable. From this viewpoint, ammonia has a better

conductivity than that of R12 or R22 and is more suitable than the latter. But, ammonia is

toxic and this does not allow its use in home refrigeration systems.

T-S Plot of an Ideal

Refrigerant

p-h Plot of an Ideal

Refrigerant


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Evaporator and Condenser Pressure:

Both the evaporator and condenser pressures need to be above atmospheric

pressure otherwise there is a possibility of air leaking into the system. Presence of air

drastically reduces the capacity of the refrigeration system. Also, due to presence of

moisture in air, acids or other corrosive compounds may form and this may affect the tubingof the refrigeration system.

Compression Ratio:

The compression ratio needs to be as small as possible otherwise the leakage of refrigerant

occurs across the piston. Also, the volumetric efficiency is affected.

Freezing Point:

It should be as low as possible or else there will be a possibility of blockage of

passages during flow of fluid through evaporator.

Volume of Refrigerant Handled Per Ton of Refrigeration:

This should be as small as possible in order to have a small size of the compressor.

The type of compressor is decided by this value. For refrigerants like R12, R500, R22 etc., a

reciprocating compressor is suitable. For others like R11 and water, a centrifugal

compressor is required to handle the large volume.

Coefficient of Performance:The Coefficient of performance or COP has a direct bearing on the running cost of

the refrigeration system. Higher the magnitude of COP, lower will be the running cost. Since,

the COP of any refrigeration system is limited by the Carnot COP, for large operating

pressures a multi-stage refrigeration system should be employed. CO2 has a very low COP.

Hence, it is not suitable for use as a refrigerant.

Density:

The density of the refrigerant should be as large as possible. In reciprocating

compressors, the pressure rise is accomplished by squeezing the entrapped fluid inside the

piston-cylinder assembly. Hence, density decides the size of the cylinder. Again in

centrifugal compressors pressure rise is related to the density of the vapour. A high value of

density results in high pressure rise.

Compression Temperature:

Whenever a refrigerant gets compressed, there is a rise in the temperature of the

refrigerant resulting in the heating of the cylinder walls of the compressor. This necessitates

external cooling of the cylinder walls to prevent volumetric and material losses. Refrigerantshaving lowest compression temperatures are thus better than others.


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CHEMICAL PROPERTIES:

Chemical Stability and Inertness:

It should be chemically stable for the operating ranges of temperature. Also, it

should not react with the materials of the refrigeration system or with which it comes into

contact. Further, it should be chemically inert and must not undergo polymerization

reactions at either the lower or higher ranges of temperatures.

Action on Rubber or Plastics:

Rubber and plastics are used extensively in the refrigeration system. These materials

are mostly used in the seals and gaskets of the refrigeration system. They help to prevent

the leakage of the refrigerant and ensure the smooth functioning of the compressor. The

refrigerant should not react with them or else there might be leakage of refrigerant from

the system or loss of functioning of the compressor.

Flammability:

The refrigerant should be inert and not catch fire when subjected to high

temperatures. From this viewpoint CO2 is the most suitable as it is not only non-flammable,

but also acts as a fire-extinguisher. Ethane, butane, isobutene are highly undesirable as they

catch fire quickly.

Effect on Oil:

The refrigerant should not react with the lubricating oil else, there is a possibility of

loss of lubricating action due to either thickening or thinning of the oil. It should not be

soluble in the oil else there will be reduction in the viscosity of the lubricating oil.

Effect on Commodity:

If the refrigerant is directly used for chilling, then it should not affect the commodity

kept in the conditioned space. Also, in case where direct cooling is not employed, the

refrigerant should still not affect the commodity if there is any leakage.

Toxicity:

The refrigerant used in air conditioning, food preservation etc. should not be toxic as

they will come into contact with human beings.


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PHYSICAL PROPERTIES:

Leakage and Detection:

Since pressures higher than atmospheric are usually employed in refrigeration

systems, there is a possibility of leakage of refrigerants after long period of operation. It is

desirable to detect this leak early else the system would operate under reduced capacity or

stop functioning altogether. Hence, it is desirable that the refrigerant has a pungent smell so

that its leakage can be detected immediately.

Miscibility with Oil:

The refrigerant should not be miscible with the oil else the lubricating strength will

be reduced.

Viscosity:

It should be as small as possible to ensure that the pressure drop in the system is as

small as possible. A low viscosity refrigerant will require less energy for its circulation

through the refrigeration system.

Safety Criteria:

Under safety criteria, we consider the toxicity, flammability, action on perishable

food and formation of explosive compound on exposure to air. An ideal refrigerant should

be non-toxic, non-flammable, have no effect on food products and should not react withatmospheric air. No refrigerant satisfy these criteria fully. We can therefore, group

refrigerants into different sub-groups based on their flammability and toxicity levels.

Economic Criteria:

Apart from the thermodynamic, chemical, physical and safety criteria, there is

another criterion by which we judge an ideal refrigerant. The economic criterion takes into

account the cost of the refrigerant, the availability and supply levels of the refrigerant, cost

of storage and handling. We discuss each of these in detail below.

Cost of Refrigerant:

The cost of the refrigerant has a big impact on the overall cost of the refrigeration

system. Hence, its cost should be as low as possible. From this viewpoint, ammonia and

water are ideally suited, but their low thermodynamic and chemical properties restrict their

use in all types of refrigeration systems. Particularly, for flooded type evaporator or

condenser, the refrigerant amount required is high and their cost needs to be factored in

while making the initial investments.

Availability and Supply:


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The refrigerant should be easily available in the market and in abundant quantity.

This ensures that the cost of the refrigerant is not prohibitive. An abundant and free supply

of the refrigerant ensures that refrigeration systems will be designed specifically for use

with them.

Storage and Handling:

The refrigerant should be such that it can be conveniently stored and handled during

transportation and charging. It should be stored in as small a pressure vessel as possible.

Also, if we have to handle a toxic or flammable refrigerant, then the cost involved will be

higher compared to handling and storage cost of non-toxic and non-flammable refrigerant.

From the above discussions of the ideal properties of refrigerants, we can come to the

conclusion that none of the refrigerants in current use and available satisfy these conditions

fully. As such, we have to make a detailed analysis of the different factors like cost,

performance of the refrigeration system and safety issues before deciding on using a

particular refrigerant.

OZONE DEPLETION AND GLOBAL WARMING POTENTIAL:

An issue of growing concern for the present day environment is the impact of the

various refrigerants on the ozone depletion and global warming of the environment. The

main culprits in this case are the chlorine containing halogenated hydrocarbons, commonlyknown as chlorofluorocarbons or CFC which are being used as refrigerants.

The Earths atmosphere is made up of various layers. The layer just above the Earths

surface is known as the troposphere. The troposphere extends up to 10 km from the

surface. The ozone layer is just above the troposphere and located in the stratosphere. The

stratospheric ozone is Earths natural protection to harmful ultraviolet (UV) radiation from

the sun. UV radiation is harmful to human, plant and animal life. The ozone layer gets

depleted by the action of these refrigerants.

CFCs, when they are released from the surface of the Earth, rise slowly into the

stratosphere. Here they are bombarded by the incoming UV light from the Sun, whichreleases the chlorine atoms from the parent compound. It is this chlorine atom which reacts

with the ozone molecules. The detailed reactions are given below:

Cl + O3 ClO + O2

ClO + O Cl + O2

The free chlorine atom can again take part in the reaction with another ozone atom. A single

chlorine atom, released by the action of UV radiation on CFCs can catalytically destroy tens


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of thousands of ozone molecules during its residence in the stratosphere

Graphical Representation of the Reactions Involved in Ozone Depletion

Ozone depletion will permit UV rays to reach earth which can result in several harmful

effects on living creatures. The UV radiation can cause skin cancer, cataracts and destruction

of the bodys immune system.

Along with ozone depletion, CFC refrigerants also contribute to a large extent in the global

warming of the planet. These gases create a greenhouse effect which traps the heat in the

lower atmosphere. This makes the Earth warmer because the greenhouse gases do not

allow infrared radiation to pass through tem. The earth emits IR rays during its cooling when

sun is not there. CO2 is the most important greenhouse gas but one molecule of CFC has

warming potential which is more than 1000 times the warming potential of one molecule of

CO2. Suns rays are allowed into the lower atmosphere, but the heat from these rays is not

allowed to escape.

The Montreal Protocol on Substances that Deplete the Ozone Layer signed in 1987 by

several countries stipulates the gradual phase-out of CFC refrigerants. Use of HCFC

refrigerant is advocated as an interim measure, but even these are to be eventually phased

out. This therefore necessitates the need for new refrigerants which can at least perform as

well as the refrigerants they replace without harming the atmosphere. Based on this

requirement, HFC 134a emerges as the refrigerant of the future.


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APPLICATION:

Aircraft cooling system:

In an aircraft, cooling systems are required to keep the cabin temperatures at a

comfortable level. Even though the outside temperatures are very low at high altitudes, still

cooling of cabin is required due to:

i. Large internal heat generation due to occupants, equipment etc.

ii. Heat generation due to skin friction caused by the fast moving aircraft

iii. At high altitudes, the outside pressure will be sub-atmospheric. When air at this low

pressure is compressed and supplied to the cabin at pressures close to

atmospheric, the temperature increases significantly. For example, when outside

air at a pressure of 0.2 bar and temperature of 223 K (at 10000 m altitude) is

compressed to 1 bar, its temperature increases to about 353 K. If the cabin is

maintained at 0.8 bar, the temperature will be about 332 K. This effect is called

as ram effect. This effect adds heat to the cabin, which needs to be taken out by

the cooling system.

iv. Solar radiation

For low speed aircraft flying at low altitudes, cooling system may not be required,

however, for high speed aircraft flying at high altitudes, a cooling system is a must.

Even though the COP of air cycle refrigeration is very low compared to vapour

compression refrigeration systems, it is still found to be most suitable for aircraftrefrigeration systems as:

i. Air is cheap, safe, non-toxic and non-flammable. Leakage of air is not a problem

ii. Cold air can directly be used for cooling thus eliminating the low temperature heat

exchanger (open systems) leading to lower weight

iii. The aircraft engine already consists of a high speed turbo-compressor, hence

separate compressor for cooling system is not required. This reduces the weight

per kW cooling considerably. Typically, less than 50% of an equivalent vapour

compression system

iv. Design of the complete system is much simpler due to low pressures. Maintenancerequired is also less.


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Schematic of a simple aircraft refrigeration cycle

Figure 9.5 shows the schematic of a simple aircraft refrigeration system and the operating

cycle on T-s diagram. This is an open system. As shown in the T-s diagram, the outside low

pressure and low temperature air (state 1) is compressed due to ram effect to ram pressure

(state 2). During this process its temperature increases from 1 to 2. This air is compressed in

the main compressor to state 3, and is cooled to state 4 in the air cooler. Its pressure is

reduced to cabin pressure in the turbine (state 5), as a result its temperature drops from 4

to 5. The cold air at state 5 is supplied to the cabin. It picks up heat as it flows through the

cabin providing useful cooling effect. The power output of the turbine is used to drive the

fan, which maintains the required air flow over the air cooler. This simple system is good for

ground cooling (when the aircraft is not moving) as fan can continue to maintain airflow

over the air cooler.

By applying steady flow energy equation to the ramming process, the temperature rise at

the end of the ram effect can be shown to be:

where M is the Mach number, which is the ratio of velocity of the aircraft (C) to the sonic

velocity a:


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Other modifications over the simple system are: regenerative system and reduced ambient

system. In a regenerative system, a part of the cold air from the cooling turbine is used for

precooling the air entering the turbine. As a result much lower temperatures are obtained

at the exit of the cooling turbine, however, this is at the expense of additional weight and

design complexity. The cooling turbine drives a fan similar to the simple system. Theregenerative system is good for both ground cooling as well as high speed aircrafts. The

reduced ambient system is well-suited for supersonic aircrafts and rockets.

Schematic of a bootstrap system


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CONCLUSION:

The field of refrigeration and air-conditioning has undergone tremendous changes in the

last century. More and more new refrigerants having improved properties are being

produced globally. Research in this field is now directed towards producing betterenvironment friendly refrigerants and in replacing old refrigeration systems using

halogenated refrigerants with the newer ones. We can be sure that in the future,

refrigerants will be produced which will not only match the performance characteristics of

the present day refrigerants, but also surpass them. And all this will be done without

causing any destructive effect upon the environment.


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REFRENCES:

1. Arora Domkundwar, Refrigeration & Air Conditioning.2. Arora , Refrigeration & Air conditioning.3. www.wikipedia.com4. www.google.com/images
http://www.wikipedia.com/http://www.wikipedia.com/http://www.google.com/imageshttp://www.google.com/imageshttp://www.google.com/imageshttp://www.wikipedia.com/

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