Refrigeration & Air Conditioning
Contents
• Definition
• Introduction
• Vapour Compression Refrigeration
• Vapour Absorption Refrigeration
• Domestic Refrigerator
Definition
 Refrigeration is the science of providing and
maintaining temperature below that of the
surrounding atmosphere.
 Air conditioning refers to maintaining a space for
the human comfort i.e. temperature, humidity and
ventilation are the parameters to be controlled.
 Equipment used for removing the heat continuously
for maintaining low temperature in a space is called
‘refrigerator’.
 Cycles on which it operates are called ‘refrigeration
cycles’.
 The working fluids used for carrying away heat are
called
‘refrigerants’
which
are
used
in
both
refrigeration and air conditioning equipment.
 Equipment used for air conditioning are called air
conditioners.
 Refrigeration is extensively used for increasing the
storage life of perishable items specially food products,
vegetables, fruits, milk, beverages, chilling of water, ice
formation etc.
 Industrial
applications
include
chemical
manufacturing, petroleum refinery, petrochemical
plants, paper and pulp industry etc.
Methods for Refrigeration
• Refrigeration by evaporation
• Refrigeration by ice
• Refrigeration by expansion of air
• Refrigeration by throttling process
• Refrigeration by dry ice
• Vapour Refrigeration system
Performance Parameter
Unit of Refrigeration
 Refrigeration effect is the amount of heat extracted
by refrigerator from the refrigerated space.
 Higher the capacity of refrigerator means higher
shall be the refrigeration effect.
 This refrigeration effect is defined by the unit of
refrigeration called ‘Ton’ of refrigeration.
 “One ‘Ton’ of refrigeration can be defined as
the refrigerating effect produced by melting 0f
1 tonne of ice from and at 0 degrees in 24 hrs.
 1 Tonne of refrigeration (TR) = (336x1000)/ 24

= 14000 KJ/hr
CARNOT REFRIGERATION CYCLE
 Different processes in reversed Carnot cycle are,
 1–2 Reversible adiabatic compression
 2–3 Reversible isothermal heat rejection, Qh at
temperature, Th
 3–4 Reversible adiabatic expansion
 4–1 Reversible isothermal heat absorption, Ql at
temperature, Tl
Bell-Coleman Cycle
 Refrigeration cycles using air as refrigerant are
termed as air refrigeration cycles
 Bell-Coleman cycle is one such cycle running on
air as refrigerant.
 Bell-Coleman cycle is modified form of reversed
Carnot cycle in order to make it feasible cycle.
 Here isothermal heat addition and release are
replaced by isobaric processes
VAPOUR COMPRESSION CYCLE
 Refrigerant is gas or vapour
 Refrigerant is compressed
compressor from state 1 to 2.
isentropically
in
 High pressure and high temperature refrigerant
enters the condenser at state 2 where its
condensation occurs and refrigerant is available in
liquid form at state 3.
 Refrigerant in the form of saturated liquid at high
pressure is passed through expansion valve where
isenthalpic expansion occurs
 Refrigerant leaving expansion valve at state 4 is in
the form of low pressure wet mixture of liquid and
vapour
 Low pressure liquid-vapour mixture is passed
through evaporator section in which refrigerant
picks up heat from surroundings thereby showing
refrigeration effect.
 As a result of this heat absorption the liquidvapour mixture refrigerant gets transformed into
dry gaseous refrigerant in case of dry compression.
 Wet
compression is also possible, that is
refrigerant is in wet state i.e. liquid-vapour mixture
at inlet of compressor.
Merits of Vapour compression system over Airrefrigeration system
 Thermodynamic cycle involved in vapour compression
system is very close to Carnot cycle hence its COP is
quite high of the order of 3–4 while for air refrigeration
COP is generally lesser.
 Refrigerant required is smaller in vapour compression
system because the heat is carried away by latent heat
of vapour and so the amount of liquid refrigerant
circulated per ton is less. Also size of evaporator is
small.
 Operating cost of vapour compression system is very
less as compared to air refrigeration system on ground.
 Refrigerated space temperature can be easily
regulated by controlling the throttle valve.
Demerits of Vapour Compression System Over Airrefrigeration System
 Initial investment is too large as compared to air
refrigeration system.
 There are chances of leakage of refrigerant as large
pressure differential exists throughout the cycle.
 Air leakage into refrigerant offers reduced
performance of vapour compression system and so it
should be prevented.
VAPOUR ABSORPTION CYCLE
 The Solubility of ammonia in water at low
temperature and pressure is higher than it is at
higher temperature and pressure.
 Specific volume of vapour/gas is more than liquid
 More work is required to increase the pressure for
gas
 Compressor is being replaced by pump, as the
refrigerant is in liquid form at the beginning of
compressor in vapour absorption system
 Ammonia works as refrigerant and water acts as
carrier for refrigerant in this cycle.
 Ammonia





vapour leaving evaporator enter the
absorber where ammonia vapours are released through
perforated pipe into water for getting absorbed in
water.
Dissolving of NH3 in water result in strong aquaammonia solution
Mixing of ammonia into water is more when temp of
water mixing is low
This solution is pumped using a pump to generator at
high pressure.
Generator is used for separating out NH3 vapour
For driving out ammonia from strong-aqua ammonia
solution heat is added to generator.
 heat addition is done using burners.
 Ammonia vapours leaving generator at high pressure





are sent to condenser at state 2.
Condensation of ammonia takes place yielding high
pressure ammonia in liquid form at state 3.
High pressure ammonia liquid is passed through
expansion valve from state 3 to 4.
The combination of absorber, pump and generator
helps in getting high pressure refrigerant (ammonia)
without using compressor.
Low pressure ammonia leaving expansion valve may be
pure liquid or liquid-vapour mixture for being sent to
evaporator at state 4.
Ammonia leaving evaporator after picking up heat
from refrigerating space gets transformed into vapour
at 1.
Merits Of Vapour Absorption System Over Vapour
Compression System
 Vapour absorption system has no moving component
throughout cycle, so it works quietly and less
maintenance is requirement.
 Lesser work input is required for pump as compared to
compression in vapour compression system.
 Vapour absorption system may be run with some
installation rejecting waste heat, which may be used to
run generator, thus energy conservation.
 Vapour absorption system is well suited for large
refrigeration capacity applications such as even above
1000 tons, which is difficult in vapour compression
systems.
Demerits Of Vapour Absorption System Over
Vapour Compression System
 Size of absorption system is large and is bulky to
handle.
 If heating source (such as electric heating) is costly
then the vapour absorption system becomes costly
than vapour compression system.
 At very high loads the COP of vapour compression
system is more than that of vapour absorption
system.
REFRIGERANTS
 Refrigerant
is the working fluid used in
refrigeration/air conditioning equipment
 Capable of carrying heat/rejecting heat in the form
of sensible heat or latent heat.
 Examples:
Ammonia, CO2, SO2, methyl chloride, ice, solid
CO2
Desirable Properties:
Thermodynamic properties:
• Low boiling point
• Low freezing point
• High latent heat of vaporisation
Chemical properties:
• Non toxic
• Non flammable and non explosive
• Non corrosive to metal
Physical properties:
• Low specific volume of vapour
• Low specific heat
• Low viscosity
Other properties:
• Ease of leakage location
• Availability and low cost
• High COP
Domestic Refrigeration
Air Conditioning
PSYCHROMETRY
 Psychrometry or Psychometrics refers to the study of
system involving dry air and water.
 Properties of mixture of air and water are called
psychometric properties.
 Dry air: Atmospheric air having 79% nitrogen and 21%
oxygen by volume is considered dry air.
 Moist air: Moist air is the mixture of dry air and water
vapour.
 Humidity ratio or Specific Humidity: This is
given by the ratio of the mass of water vapour to
the mass of dry air.
 Dry bulb temperature: Dry bulb temperature refers
to the temperature of air measured with ordinary
thermometer having its bulb open.
 Relative Humidity: It is the ratio of the partial
pressure of the water vapour in the mixture to the
saturated partial pressure at the dry bulb
temperature.
AIR CONDITIONING SYSTEMS
 Air conditioning systems require basic arrangement
for getting refrigeration effect through cooling coil
followed
by
subsequent
humidification/dehumidification and heating etc. in
order to provide air conditioned space with air at
desired temperature and humidity.
 Air
conditioning
systems
require
different
arrangements depending upon the atmospheric air
condition and comfort condition requirement.
 summer air conditioning systems and winter air
conditioning systems
 The comfort conditions in summer may require
delivery of air to air-conditioned space at about 25ºC
DBT and 60% relative humidity where the outdoor
conditions may be up to 40–44º C DBT and 20%
relative humidity in Indian conditions.
 air blower which blows air across the air filter between




(1) and (2).
Air coming out from filter passes over cooling coils and
is subsequently sent for humidification between states
(3) and (4).
Large size water particles carried by air are retained by
water eliminator.
Air finally coming out at state (5) is sent to air
conditioned space.
Winter air conditioning has slightly different
arrangement as compared to summer air conditioning.
 Let us assume air from atmosphere to be available at
10ºC and 80% relative humidity while comfort
conditions are 25ºC DBT and 60% relative humidity
 Here atmospheric air is blown into air filter and it
comes out at (2).
 Air is passed across heating coil for being heated. Hot
air at (3) passes through humidifier section where its
humidity increases.
 It is further passed through water eliminator to retain
large sized water particles.
 Air may again be passed through heating coil section
so as to compensate for temperature reduction in
humidifier section and attain desired temperature.