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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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