Refrigerator and AC conditioning

1. ELEMENTS OF MECHANICAL
ENGINEERING
(22ME210-22ME110)
UNIT - 5
Refrigeration and Air Conditioning
By
Tejas M R
Assistant Professor
Mechanical engineering department
JSSSTU (SJCE) Mysuru

2. What is Refrigeration?
 Refrigeration is any of various types of cooling of a space, substance, or system to lower and/or maintain
its temperature below the ambient one (while the removed heat is ejected to a place of higher
temperature). Refrigeration is an artificial, or human-made, cooling method.

3. What are Refrigerants?
 A refrigerant is a working fluid used to carry the heat in a refrigeration
system.
 The refrigerant dose not undergo any chemical change but carries the
heat by evaporating at a low temperature and pressure and gives up this
heat by condensing at a high temperature and pressure.
OR
 A refrigerant is a working fluid used in the refrigeration cycle of air
conditioning systems and heat pumps where in most cases they
undergo a repeated phase transition from a liquid to a gas and back
again.

4. Types of Refrigerants
 Ammonia (NH3)
It is the most widely used refrigerant in vapour absorption system. It has a normal boiling
point temperature of -33.3°C. It is highly toxic, inflammable, irritating and corrosive. Hence,
these food-destroying properties make it unsuitable for domestic refrigerators. But this
refrigerant is used where its toxic nature is not considered seriously. It is used in large
industries for cold storage, ice manufacturing plants, packaging plants etc.
 Carbon dioxide (CO2)
It is non-toxic and non-flammable. Its normal boiling point is -77.6°C. It is nearly 1.53 times
heavier than air and hence requires high operating pressure. It is used in dry ice making Due
to its low specific volume, the plant size is compact. It is used in ships where space
consideration is more important.

5. Types of Refrigerants
 Sulphur dioxide (SO2)
- It has a good thermodynamic properties, low refrigerating effect. It is colorless,
suffocating and possesses irritating odor. It has a high boiling point of-10°C. This refrigerant
was used in household refrigerator in olden days.
 Freon 12
It is non-flammable, non-explosive, non-corrosive and odorless. Hence it is a widely
accepted refrigerant for various applications. It has a boiling point of -29.8°C.It is used in
small capacity equipment such as domestic refrigerators, water coolers, air-conditioners etc.
 Freon 22
It has a normal boiling point of -40.8°C that is about 10° less than that of Freon-12. It is
therefore, a comparatively high-pressure refrigerant. Freon-22 is employed for air-
conditioners in large capacity plants, food freezing, freeze drying, etc.

6. Properties of Refrigerant
 Thermodynamic Property
a) Low boiling point
b) Low freezing point
c) High latent heat of evaporation
d) High critical temperature
e) Condenser and evaporator pressure should be slightly above the atmospheric
pressure
 Physical properties
a) Low viscosity
b) Low specific heat
c) High thermal conductivity
d) High electrical insulation

7. Properties of Refrigerant
 Chemical properties
a) Non-toxic
b) Non-flammable and non-explosive
c) Non-corrosive
d) Good chemical stability
 Other properties
a) Availability
b) Low cost
c) Ease of handling

8. Definition Of Terms Used In Refrigeration
 Refrigeration
It is defined as the process of reducing the temperature of a substance below that of the
surrounding atmosphere and maintaining this lower temperature within the boundary of a
given space.
 Refrigerating Effect
It is the amount of cooling produced by a refrigeration system. It is defined as the rate at
which the heat is removed from the space to be cooled in a cycle. It is also called 'capacity
of refrigerator'. It is expressed in kW or kJ/s.
 Ton of Refrigeration
The unit of refrigeration is expressed in terms of ton of refrigeration. It is defined as the
amount of heat absorbed in order to produce one ton of ice in 24 hours from water, whose
initial temperature is 0°C. In S.I. units/the value of 1 Ton of refrigeration = 210 kJ/min or
3.5 kW OR Generally, ton of refrigeration is defined as the amount of heat transferred to
freeze or melt 1 short ton of ice at 0 deg. C in 24 hours.
(Note One ton of refrigerating machine will not produce one ton of ice. Due to the fact that
in earlier days, refrigeration was produced by ice, the refrigeration effect was expressed in
terms of ton of refrigeration)

9. Definition Of Terms Used In Refrigeration
 Ice Making Capacity
Ice making capacity is the ability of a refrigerating system to make ice. In other words, it is the
capacity of a refrigerating system to remove heat from water to make ice.
 Co-efficient of Performance
The performance of a refrigerator is measured by a factor known as Co-efficient Of Performance
(COP). It is defined as the ratio of the amount of heat removed from a given space to the work
supplied to achieve the heat removal.
COP =
amount of heat removed from a refrigerator
Work supplied
=
𝐐
𝐖
COP is used as a measure of the steady state performance or energy efficiency of heating/cooling and
refrigeration appliances. The higher the COP, the more efficient is the device
 Relative COP
It is defined as the ratio of actual COP to the theoretical COP of a refrigerator.
Relative COP=
Actual COP
Theoretical COP
where Q = heat removed in kJ/s and
W = work supplied or work done in kJ/s

10. Working principle of Vapour Compression
Refrigeration

11. Working principle of Vapour Compression
Refrigeration
 Vapour compression system of refrigeration is widely used in modern
refrigerating plants. A liquid refrigerant (heat carrying substance) used in this
system alternatively undergoes a change of phase from vapour to liquid
(condensation) and from liquid to vapour phase (evaporation) during the working
cycle.
 Figure shows the line diagram of a vapour compression refrigeration system. The
system consists of evaporator, compressor, condenser and an expansion valve.
 The liquid refrigerant in the evaporator, absorbs the heat from the medium
(cabinet/refrigerated space) which is to be cooled and undergoes a change of
phase form liquid to vapour. The vapour at low temperature and pressure is
drawn into the compressor where it is compressed to a high pressure and
temperature. The compressed vapour then enters the condenser.

12. Working principle of Vapour Compression
Refrigeration
 In the condenser, the vapour refrigerant is cooled and condensed by giving its
latent heat to the circulating cooling medium (air or water). The high-pressure
liquid refrigerant leaves the condenser and passes through the expansion valve
where it is expanded to low pressure and temperature. The temperature of the
refrigerant falls to a value less than that of the refrigerated space.
 The low pressure-low temperature refrigerant again enters the evaporator where
it absorbs the heat from the medium (cools the medium) and evaporates. The
low pressure-low temperature vapour is drawn into the compressor and the
cycle repeats.
 Thus, heat is continuously extracted from the medium, thereby keeping the
contents at the required lower temperature

13. Working principle of Vapour Absorption
Refrigeration

14. Working principle of Vapour Absorption
Refrigeration
 Vapour absorption refrigerator is a heat operated system. It differs from the vapour
compression system only in the manner by which the circulation of the refrigerant is.
achieved. In the vapour absorption system, the compressor is replaced by an absorber, a
generator and a pump. The refrigerant used in this system must be highly soluble in the
solution known as 'absorbent'. The system uses ammonia as the refrigerant and water as
absorbent.
 Figure shows the line diagram of a vapour absorption system.
 The liquid refrigerant (ammonia) in the evaporator, absorbs the heat from the medium that is
to be cooled and it undergoes a change of phase form liquid to vapour. The low pressure
vapour is then passed to the absorber.

15. Working principle of Vapour Absorption
Refrigeration
 In the absorber, the low pressure ammonia vapour is dissolved in the weak ammonia
solution producing strong ammonia solution at low pressure. The strong ammonia
solution is then pumped to a generator through the heat exchanger at high pressure.
While passing through the heat exchanger, the strong ammonia solution is warmed
up by the hot weak ammonia solution flowing from the generator to the absorber.
 The warm strong ammonia solution is heated by an external source in the generator.
Due to heating, the vapour gets separated from the solution. The vapour which is at
high pressure and high temperature is condensed to low temperature in a condenser
by cold water circulation.
 The high pressure liquid ammonia then passes through the expansion valve where it
is expanded to low pressure and temperature. The low pressure- low temperature
ammonia liquid again enters the evaporator where it absorbs the heat from the
medium (cools the medium) and the cycle repeats.

16. Comparison between ‘Vapour Compression’ and
‘Vapour Absorption System’
Sl.No. Vapour Absorption System Vapour Compression System
1 Works on heat energy. Works on mechanical energy (compressor).
2 Absorption system dose not depend on
electric power.
Depends on electric power for running
compressor.
3 Since a pump is required only to circulate
the refrigerant, energy required to run the
pump is less.
Mechanical energy required is more because
refrigerant vapours are compressed to high
pressure.
4 Due to absence of moving parts,
maintenance cost is less.
Maintenance cost is more due to compressor.
5 Absorption unit can be built in capacity
well above 1000 tons.
Maximum capacity for vapour compression
system is 1000 tons.
6 At reduced loads, the absorption system is
almost as efficient as at full load.
The COP of vapour compression system
decreases with increase in load.
7 There is no chance of refrigerant leakage in
the system.
Chances of leakage is more due to wear of the
reciprocating parts in the compressor.

17. Air Conditioning

18. Air Conditioning
 Air conditioning is defined as the process of simultaneous control of temperature,
humidity, cleanliness and air motion of the confined space.

19. Room Air Conditioning

20. Room Air Conditioning
 Figure shows the schematic diagram of a window type room air conditioner. The unit
consists of a vapour compression refrigeration system, an air filter and a double shaft motor
that drives a fan at one end and a blower at the other end. The room side and the outdoor
side of the unit are separated by an insulated partition wall within the casing.
 The blower sucks the warm air from the room through the air filter and the evaporator. The
refrigerant inside the evaporator coil gets vaporized by absorbing the heat from the warm
air. The motor runs a blower to deliver the cooled air into the room. This air mixes with the
air present in the room, thereby bringing down the temperature and humidity level so as to
maintain comfortable conditions.

21. Room Air Conditioning
 The refrigerant vapour from the evaporator is compressed in the compressor to high
pressure and temperature. The high-pressure vapour enters the condenser where it is
cooled by the outside atmospheric air circulated by a fan. The high-pressure
refrigerant is reduced to low pressure by passing it through an expansion valve.
 The low pressure and temperature refrigerant again enters the evaporator where it
absorbs the heat from the room (cools the room) and the cycle repeats.
 Desired temperature in the room can be maintained by controlling the operation of
the compressor with the help of a thermostat