This is Module 5 notes in EME as per VTU syllabus

1. 1
Module-5
Refrigeration and air conditioning
1.1 Introduction to Refrigeration
Refrigeration is a science of keeping a body relatively at a lower temperature than
its surroundings by continuously extracting heat from the body. Refrigeration is an age old
known process followed by the Indians and Egyptians before the rest of the world actually
practiced. In the recent days we use refrigerating systems to preserve the cereals, greens,
vegetables, food and other perishable items for a longer time
Frequently changing extreme climatic conditions have necessitated human beings to
find Air conditioning system that can keep him in comfort condition. Air conditioning has
become a part and parcel of the human comforts in the modern world. It has been found
that human beings perform better under moderate temperatures than at extreme conditions
thus many offices install Air conditioning systems to increase work efficiency of its
employees.
Refrigeration works on the basis of second law of thermodynamics which states that
“External work has to be expended in transferring heat from a body at lower temperature to
a body at higher temperature in order to keep the body at a temperature relatively lesser
than that of its surroundings.”
The fluid medium which continuously extracts the heat from a cold body (sink) to
deliver the same to a body which is relatively at a higher temperature (source), to maintain
the cold body at a temperature lesser than that of its surrounding temperature is called as
refrigerant. This process of continuously keeping a body at a relatively lower temperature
is called a refrigeration.
In almost all refrigeration cycles the refrigerant undergoes a phase transition from
liquid to gas and vice versa. The commonly used refrigerants are chlorofluorocarbons
ammonia, freons, carbon di oxide, methyl chloride, sulphur dioxide, non-halogenated
hydrocarbons such as propane etc,. chlorofluorocarbons is phased out as refrigerant because
of its ozone depletion effects.
Introduction to refrigeration, basic definitions like TOR. Ice making capacity, COP, Unit
of refrigeration Refrigeration, properties of refrigerants, List of some commonly used
refrigerants Principle and working of vapour compression refrigeration system Principle
and working of vapour absorption refrigeration system Domestic Refrigerator, Principle
and application of air conditioning system. Window and split air conditioning system.

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1.2Different types of refrigerating system
a. Ice refrigeration system
b. Air refrigeration system
c. Vapour absorption refrigeration system
d. Vapour compression refrigeration system
e. Special refrigeration system.
1.3Properties of a good refrigerants
The properties of a good refrigerant can be broadly classified as
1.3.1 Physical property:
a. Specific volume: It should have low specific volume to reduce the size of the
compressor.
b. Specific heat: It should posses low specific heat, because high specific heat
decreases the refrigerating effect per kg of refrigerant
a. Viscosity: It should have a low viscosity at all stages of the refrigeration as it
reduces the pumping effect
1.3.2 Thermodynamic property
c. Boiling point: a good refrigerant should possess low boiling point.
d. Freezing point: a good refrigerant should possess low freezing point
e. Evaporator and condenser pressure The evaporator and condenser pressures
should be low enough to reduce the material cost and must be positive to avoid
leakage of air into the system
f. Latent heat of evaporation: It should possess high latent heat, because high latent
heat at low temperature increases the refrigerating effect per kg of refrigerant.
g. Critical pressure and temperature: It must have high critical pressure and
temperature to avoid large power requirements
h. Thermal conductivity: It must have high thermal conductivity to reduce the area of
heat transfer in evaporator and condenser
1.3.3 Safe working properties
a. Chemical stability: It should be chemically stable and should not break down
during all the processes of the refrigerating cycle.
b. Corrosiveness: it should be non corrosive so as to avoid corrosion of the container
material that contains the refrigerant
c. Flammability: It should be non-flammable so as to avoid fire accidents if there is
any leakage of the refrigerant from the system to the atmosphere.
d. Toxicity: It should not be toxic so as to avoid any bad effects on the stored material
or food, when any leak develops in the system.
1.3.4 Other properties

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a. COP: It should give high COP in the working temperature range. This is necessary
to reduce the running cost of the system.
b. Odour: It should not have a bad odour so as to prevent the spoiling of the
substances or food products stored in the refrigerator in case of any leakage
c. Leak: A suitable detector may be added to the refrigerant so as to identify the
leakages if any from the system
d. Action with lubricating oil: It must have high miscibility with lubricating oil and it
should not have reacting properly with lubricating oil in the temperature range of
the system
1.4Commonly used Refrigerants in refrigerating systems
Some of the commonly used refrigerants in the refrigerating systems are
a. Air: It is one of the primitively used refrigerants by man as it is freely available, non
toxic and non flammable.
b. Ammonia: This is one of the best refrigerants man has known, it has its boiling
point at -330
c, it is available at lower cost, has a high specific volume, high COP
and high refrigerating effect. It finds its application where large quantity of
refrigeration is required and toxicity is of secondary importance.
c. Carbon-di-oxide: It is a chemically stable, Non-flammable, non toxic, refrigerant.
It has a low specific volume and has a critical temperature at 310
C which makes it
unstable in some hot countries like India.
d. Sulphur-di-oxide: It is a colourless, suffocating, irritating gas it is twice as heavy
as air at atmospheric conditions. It is non-flammable but becomes poisonous when
comes in contact with food.
e. Hydrocarbons: These are non poisonous refrigerants and highly explosive when
exposed to air. The larger the number of hydrogen and carbon atoms the heavier is
the gas and higher is its boiling point.
f. Halocarbon refrigerants: One or two hydrogen atoms in the hydrocarbons are
replaced by one or two halogens, namely fluorine, chlorine, bromine.
1.5Definitions used in Refrigeration
a. Refrigerating effect: Refrigerating effect is defined as the rate of heat absorption by a
refrigerant, from the system operating in a cycle, to maintain the system at a temperature
lower than that of its surrounding temperature.
b. Ton of Refrigeration: It is the unit of refrigeration, which is defined as the
amount of heat absorbed from water at zero degree Celsius over a period of 24 hours
to make one ton of ice.
1Ton of refrigeration =
( ) ( )
=
( ) ( )
kJ/s
= 3.5kJ/s

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Note: The latent heat of fusion of ice is 336 kJ/s
c. Ice making capacity: It is the capacity of a refrigerating system to convert water
at room temperature to solid ice.
d. Co-efficient of Performance: In general the performance of a refrigerating
system is expressed in terms of co-efficient of performance, it may be defined as the
ratio of heat absorbed by a refrigerant in a refrigerating system to the work supplied.
It is mathematically expressed as
COP = =
Where „Q‟ is the amount of heat absorbed
„RE‟ is the refrigerating effect
„W‟ is the work supplied.
e. Relative Co-efficient of performance: It is defined as the ratio of actual COP
to the theoretical COP,
Relative COP =
1.6 Elements of a refrigerating system
The basic elements of a refrigerating system are as discussed below
1. A low temperature Sink into which heat will flow from the cooling space
2. A device for extracting energy from the sink, increasing the temperature level of the
energy and delivering it to a heat receiver.
3. A receiver to receive heat from the high temperature and high pressure refrigerant
4. A device to reduce the temperature and pressure of the refrigerant while it returns
from receiver to sink
1.7 Vapour compression refrigeration system
Principle of Vapour compression refrigeration:
A refrigerant having low boiling temperature is made to absorb the heat from
objects placed in the evaporator to attain the vapour state. This high temperature,
low pressure refrigerant vapour is compressed and then expanded in a throttle valve
to bring it back to liquid state and is again passed to the refrigerating chamber
(evaporator) where it absorbs the heat and thus the cycle repeats.
Vapour compression Refrigerator parts and their functions:
The vapour compression refrigeration system essentially comprises of the following
parts,
1. Evaporator.
2. Compressor.
3. Condenser.
4. Expansion valve

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Evaporator: the Evaporator region is surrounded by long coiled tubing in which the
refrigerant is circulated to provide the cooling effect; because of this, the tubing is often
called as the freezing coil. In this region liquid refrigerant entering at low pressure and low
temperature is converted into high temperature and low pressure refrigerant vapours, after
absorbing the heat from the objects stored in the Evaporator. Thus these objects are
maintained at a lower temperature than that of its surrounding temperature.
Compressor or pump: The sole purpose of a compressor in a refrigerator is to compress the
low pressure, high temperature refrigerant vapours from the evaporator and convert it to
high pressure, high temperature refrigerant, such that the saturation temperature
corresponding to these pressures is higher than that of the cooling medium flowing into the
condenser. Compressors are generally electrically driven and this is the external work that
is being done in a refrigerator.
Condenser: The condenser converts the high pressure high temperature vapours of the
refrigerant to high pressure and low temperature refrigerant; this is accomplished when the
refrigerant is made to pass in the long coiled condenser tube. Here the latent heat in the
vapour attained during compression and heat absorption from the objects placed in the
refrigerating unit is absorbed by the fins of the condenser due to increased area of contact.
Expansion device: The high pressure, low temperature refrigerant is converted into Low
temperature and low pressure liquid refrigerant during the expansion of the refrigerant in
the expansion device. When the HP, LT refrigerant is made to expand in the expansion
valve, the heat present in the refrigerant is given out and rendered liquid. Thus this liquid
refrigerant which is at low temperature and low pressure is once again admitted to the
evaporator and the refrigeration cycle continues.
Fig. 1 Schematic representation of Vapour Compression Refrigeration System
1.8Working of a vapour compression refrigeration system
Working of a vapour compression refrigeration system is illustrated in the fig.2 The
Liquid refrigerant at low temperature and low pressure is admitted into the evaporator, here
it absorbs the heat from the objects to be maintained at a lower temperature than that of its
Compressor
Condenser
Expansion valve
Evaporator

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surroundings and converts itself into the vapours of the refrigerant and leaves the
evaporator.
The compressor draws this vapour and compresses it to a higher pressure so that the
high temperature and high pressure refrigerant can reject heat in the condenser and be ready
to expand in the throttle valve (expansion valve) to lower the evaporator pressure again.
Thus the H.T and H.P refrigerant heads the condenser, where it gives away its latent heat of
vaporisation and becomes once again a liquid refrigerant. This liquid refrigerant is again
admitted to the evaporator and the refrigeration cycle continues, by continuously extracting
heat from contents of the evaporator and rejecting to the atmospheric air. Through the
condenser and evaporator.
1.9Vapour absorption refrigeration
Vapour absorption refrigeration system is one of the oldest refrigeration systems. In this system
ammonia is mainly used as refrigerant and water is used as absorbent. The boiling point
temperature difference between ammonia and water is not that high, both ammonia and water
are generated from the solution in the generator. presence of large Quantity of water in
refrigerant circuit is disadvantageous to the system performance, thus rectification of the
generated vapour is carried out using a rectification column and a dephlegmator. Since
ammonia is used as the refrigerant, these systems can be used for both refrigeration and air
conditioning applications.
Principle: The HT and LP vapours of the refrigerant leaving the evaporator are mixed in
the absorber with the cold and dilute refrigerant coming from the heat exchanger and
pumped to the heat exchanger. Here the LT, HP refrigerant is heated in the heater to make a
concentrated refrigerant solution. This HT, HP refrigerant in turn is passed through the
condenser and then expanded in the expansion valve. Finally the LT , LP and concentrated
liquid refrigerant is admitted to the evaporator.
1.10 Parts of Vapour absorption refrigeration system and their
functions.
The following are the essential parts of a Vapour absorption refrigeration system.
1. Evaporator
2. Absorber
3. Circulating pump
4. Heat exchanger
5. Heater
6. Condenser

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7. Expansion valve
Evaporator: It is the region where the low pressure, low temperature liquid refrigerant is
converted into high temperature, low pressure vapours of the refrigerant. This takes place
due to the absorption of heat from the objects kept in the refrigeration chamber which will
have to be kept in a temperature lesser than that of the surrounding temperature.
The evaporator consists of simple tubing which surrounds the regions to be cooled. The
refrigerant is circulated inside this tubing to provide the cooling effect; because of this the
tubing is often called as the freezing coil.
Absorber: The main function of the absorber is to mix the dry ammonia vapour leaving the
Evaporator in HT LP state with the cold water coming from the heat exchanger to produce
an aqueous solution of ammonia.
Circulating pump: The Circulating pump aids in extracting the LP aqueous solution from
the absorber and to convert it into HP aqueous solution of ammonia. It also feeds the same
to the heat exchanger.
Heat exchanger: The heat exchanger exchanges the heat between the weak hot solution
coming from the heater separator and the cold solution coming from the absorber. There are
separate channels for the flow of the solutions from the absorber and the heater and
separator. The hot weak solution of ammonia is passed in a tube which is immersed in a
bath of ammonia solution coming from the absorber. This strong solution coming from the
absorber after exchanging heat enters the heater separator.
Heater-Separator: The heater separator comprises of a heating coil which heats the strong
solution of aqueous ammonia coming from the heat exchanger and separates the vapours of
ammonia from water. Thus the separated ammonia vapours enters the condenser.
Condenser: The condenser condenses the high pressure high temperature vapours of the
refrigerant to high pressure and low temperature refrigerant; this is accomplished when the
refrigerant is made to pass in the long coiled condenser tube. Here the latent heat in the
Evaporator Expansion
device
Absorber
Circulating
Pump
Heat
exchanger

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vapour attained during compression and heat absorption from the objects placed in the
refrigerating unit is absorbed by the fins of the condenser due to increased area of contact.
Expansion valve: The high pressure, low temperature refrigerant is converted into Low
temperature and low pressure liquid refrigerant during the expansion of the refrigerant in
the expansion device. When the HP, LT refrigerant is made to expand in the expansion
valve, the heat present in the refrigerant is given out and rendered liquid. Thus this liquid
refrigerant which is at low temperature and low pressure is once again admitted to the
evaporator and the refrigeration cycle continues
1.11 Working of a vapour compression refrigeration system
Fig. 2 Working of a vapour compression refrigeration system
The working of a vapour compression refrigeration system is as shown in Fig. 3, the dry
refrigerant coming out of the evaporator is sent to the absorber were it mixes with the weak
ammonia solution coming from the heater separator, to form a strong solution of ammonia.
This solution is further drawn by the pump and supplied to the heat exchanger, the hot and
weak aqueous solution of ammonia coming from the heat exchanger exchanges its heat in
the tube which is immersed in a strong solution of ammonia coming from the absorber.
Thus the warm strong solution of ammonia from the absorber leaves the heat exchanger to
heater-separator unit.

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In the heater separator unit the heater heats the solution and separates the vapour of
ammonia from the aqueous ammonia solution. The vapours of ammonia are further sent to
the expansion valve where the vapours are expanded and brought to the liquid state.
This ammonia in its liquid state is again made to enter the evaporator and the cycle
continues.
1.12 Application of refrigeration system
a. Transportation of food items
b. Medical and surgical applications
c. In chemical industries
d. Ice making industries
e. Industrial Air conditioning
f. Food and beverages processing industries
g. Comfort air conditioning
h. Rubber manufacturing industries
i. Oil refineries
j. Metal treatment processes.
1.13 Differences between Vapour absorption refrigeration and vapour
compression refrigeration.
SL.
No.
Parameter Vapour absorption refrigeration Vapour compression refrigeration
1. Input Energy Takes in low grade energy such
as waste heat from furnace,
exhaust steam or solar heat for its
operations.
High grade energy such as
electrical or mechanical energy
for its operation of compressor
is used in the cycle.
2. Moving part a small pump run by a small
motor is used as a moving part,
A compressor driven by an
electric motor or engine is used.
3. Evaporator
pressure
It can be operated even at reduced
evaporator pressure, with little
decrease in refrigerant capacity.
The refrigerant capacity
decreases with lowered
evaporator pressure.
4. Load
variation
The performance of vapour
absorption system does not
change with load variation
The performance of vapour
compressing system is very poor
at partial load.
5. Evaporator
exit
In vapour absorption system, the
liquid refrigerant leaving the
evaporator does not put any bad
effect on the system except to
reduce the refrigerant effect.
In a vapour compression system,
it is desirable to superheat
vapour before leaving the
evaporator, so no liquid can
enter the compressor.
6. Lowest
temperature
Since water is used as refrigerant,
thus the lowest temperature
attained is above 0°C.
With cascading, the temperature
can be lowered upto -150°C or
even less temperature.
7. Coefficient of
Performance
The COP of the system is poor. The COP of the system is
excellent.

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8. Capacity It can be built in capacities well
above 1000 TR.
For a single compression
system, it is not possible to have
a system with more than 1000
TR capacity.
9. Refrigerant Water and ammonia is used as
refrigerant, lithium bromide .
lithium chloride
Chloroflourocarbon,
hydroflorocarbon and
hydrochlorofluorocarbon are
used in most of the systems.
10 Noise Almost quiet in operation as there is
no compressor
Noise is more due to the presence
of compressor
11 Maintenance Less maintenance The maintenance is high because of
the compressor
12 Operating cost The electrical energy required to run
the pump is relatively less and this
system works on other sources of
energy other than electrical energy,
hence the operating cost is less
As this system depends on
electrical energy, the operational
cost will be high
13 Space Requires large place Small space is sufficient
1.14 Air conditioning system.
Air conditioning may be defined as artificial cooling process where in the
temperature is maintained congenial at all times regardless of weather conditions either for
human comforts or industrial purpose. Air conditioning system involves main three steps
namely dehumidifying, cleaning and circulation air from surroundings.
An air conditioner basically draws the air from the indoor space to be cooled, cools
it and sends back the same to the region to be air conditioned. A typical air conditioning
system comprises of an evaporator, condenser and a compressor. The comfort air
conditioning unit is generally mounted on a window sill such that the evaporator unit is
inside the room and the condenser part projects outside the room.
1.15 Working of an Air conditioning system.
The high-pressure, low-temperature liquid refrigerant from the condenser is passed
to the evaporator coils through the capillary tube where it undergoes expansion. The low-
pressure, low temperature liquid refrigerant passes through the evaporator coils. The
evaporator-fan continuously draws the air from the interior space within the room through
an air filter by forcing it to pass over the evaporator coils. The air from the interior passing
over the evaporator coils is cooled by the refrigerant which consequently evaporates by
absorbing the heat from the air. The high-temperature evaporated refrigerant from the
evaporator is drawn by the suction of the compressor which compresses it and delivers it to
the condenser. The high-pressure, high-temperature refrigerant vapour now flows
through the condenser coils. The condenser-fan draws the atmospheric air from the exposed
side-portions of the air conditioner which is projecting outside the building into the space
behind it and discharges to pass through the centre section of the condenser unit over the

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condenser coils. The high-pressure, high-temperature refrigerant passing inside the
condenser coils condenses by giving off the heat to the atmospheric air. The cooled high-
pressure refrigerant from the condenser passes through the capillary tube where it
undergoes expansion and is again re-circulated to repeat the cycle continuously.
1.16 Application of Air conditioners.
Air conditioners find wide applications in almost all areas, starting from comforts to
industries, however the following are few applications of AC systems.
a. Houses
b. Hospitals.
c. Theatres.
d. Pharmaceuticals
e. Laboratories.
f. Data centres.
g. Textile industries.
h. Industrial environment.
i. Automobiles.
j. Cooking and processing units.
k. Cold storage systems
1.17 Summary
 Refrigerant: The fluid media used to absorb the heat from the objects stores in the
evaporator to maintain them at a temperature lesser than that of their surroundings.
 Refrigeration: This process of continuously keeping a body at a relatively lower
temperature is called as refrigeration
 Refrigeration system types: Ice refrigeration system , Air refrigeration system,
Vapour absorption refrigeration system, Vapour compression refrigeration system,
Special refrigeration system.
 Commonly used Refrigerants in refrigerating systems: Air, Ammonia, Carbon-
di-oxide, Sulphur-di-oxide, Hydrocarbons, Halocarbon refrigerants
 Refrigerating effect: the rate of heat absorption by a refrigerant, from the system
operating in a cycle, to maintain the system at a temperature lower than that of its
surrounding temperature
 Ton of refrigeration: the amount of heat absorbed from water at zero degree
Celsius over a period of 24 hours to make one ton of ice.
 Ice making capacity; It is the capacity of a refrigerating system to convert water at
room temperature to solid ice.
 Co-efficient of Performance: In general the performance of a refrigerating system
is expressed in terms of co-efficient of performance, it may be defined as the ratio
of heat absorbed by a refrigerant in a refrigerating system to the work supplied.

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 Relative Co-efficient of performance: It is defined as the ratio of actual COP to
the theoretical COP,