Basic Mechanical Engineering - Refrigeration

1

Refrigeration & Air Conditioning

M S Steve
Assistant professor
Dept of Mechanical Engineering
Amal Jyothi College of Engineering

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Refrigeration System
The mechanism used for lowering or
producing low temp. in a body or a space, whose
temp. is already below the temp. of its
surrounding, is called the refrigeration system.
Here the heat is being generally pumped
from low level to the higher one & is rejected at
high temp.
msstevesimon@gmail.com

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Refrigeration

The term refrigeration may be defined as the process
of removing heat from a substance under controlled
conditions.

It also includes the process of reducing heat &
maintaining the temp. of a body below the general temp. of
its surroundings.

In other words the refrigeration means a
continued extraction of heat from a body whose temp is
already below the temp. of its surroundings.

Major applications
• Food processing ,preservation & distribution.
• Chemical & process industries.

• Special applications such as cold treatment of
metals , medical , construction etc.
• Comfort air conditioning

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• Refrigerating system
• Refrigerated system
• Refrigerant – working fluid
The equipment used to maintain a lower
temperature inside the system than its surroundings
is known as refrigerating system & the working fluid
used inside the system is known as refrigerant.

• TYPES OF REFRIGERATORS
▫ Air refrigerator
▫ Vapour refrigerator

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Unit of refrigerating capacity
• The rate of heat absorbed from a body to be cooled
is called refrigerating effect.
• Refrigerating effect: It is the amount of heat energy
removed per unit time from the space to be cooled
by the refrigeration process. It is expressed in kw or
kJ/s. It is also called capacity of a refrigerator
• The main unit of refrigeration is tones of
refrigeration (TR).
• It is the rate at which the heat is removed from one
U.S tone of water at 0 d.c to ice at 0 d.c in 24 hrs.

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• The rate of heat absorbed by the system from the
body to be cooled, equivalent to the latent heat
of fusion on 1ton of ice from and at 0 d.c in 24
hours is called one ton refrigeration.
• One ton of refrigeration is the rate of heat
absorbed by the system from the body to be
cooled ,equivalent to the latent heat of fusion of
one ton of ice, from and at zero degree Celsius in
24 hours.
• In general 1 ton refrigeration is equivalent to
3.5167 kJ of heat removal per second.


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Coefficient of Performance (COP)
COP=

desired refrigerating effect
________________________________
work spent in producing the refrigerating effect

COP of a refrigerator will be greater than unity

• Coefficient of performance (COP)
It is the measure of the effectiveness of
the refrigeration system. It is the ratio of
refrigerating effect (Q) to the input work
required to produce the effect(w) .
COP =refrigerating effect/work input
= Q/W
• Relative COP: It is the ratio of actual COP to the
theoretical COP.


COP & EFFICIENCY
• COP indicates the effectiveness of a refrigerator
or heat pump.
• For refrigerator the desired result is extraction
of heat from space.
• Generally the value of COP of refrigerator is
more than unity but the value of efficiency is less
than unity.
• It is for this reason the term COP is used for
rating the refrigerator.

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Thermodynamics of a Refrigerator
T< T
1

2

Source T2
Q2

COP (coefficient of
Performance)= Q1/ W

Refrigerator

Work W

Q1
Sink T1

• For refrigerator maximum Q1 should be taken out with minimum
expense of W ,so performance of refrigerator is evaluated by COP
(coefficient of Performance)= Q1/ W.

Methods of producing refrigeration
1.Sensible cooling by cold medium.
eg :cooling of fish with ice, cooling with cold air /cold
water or brine.
2.Endothermic mixing of substances- refrigeration effect
obtained is small.
3.Phase changing process such as sublimation ,melting &
evaporation - eg: melting of ice produces a refrigeration
effect in the surroundings.
4.Expansion of liquids- it is a popular method of cooling.
(isentropic & isenthalpic process)

Methods of producing refrigeration
5.Expansion of gases.(isentropic & isenthalpic
process)

6.Magnetic refrigeration-adiabatic
demagnetisation
7.Thermoelectric refrigeration- it is based on the
reverse Seebeck effect

Air refrigeration system
• The air is taken in to the compressor from the
atmosphere & is compressed.
• The compressed air is cooled

to atmospheric

temperature in a heat exchanger.
• The cooled air is then expanded in an expander.
• The low temperature air coming out of the expander
enters the evaporator and absorbs heat from the
source.


Air refrigeration system
•.


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Simple Vapour Compression
System
Evaporator
Throttle
valve

Low pr gauge

condenser

High pr gauge

compressor


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High pressure
Low temperature
liquid

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Vapour compression systems


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Parts
•
•
•
•

Compressor
Condenser
Expansion valve
Evaporator


Vapour Compression Refrigeration System

Vapour Compression Refrigeration
System
• The function of the compressor is to
continuously draw the refrigerant vapour from
the evaporator and compress it to a high
pressure.
• Condensor is used to condense the refrigerant
from vapour to liquid.
• Throttle valve acts as an expansion device.
• In evaporator the lower temperature refrigerant
absorbs the heat from the space to be cooled and
the refrigerant gets evaporated.

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Practical Vapour Compression Cycle.

Components of the Practical Cycle

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A home refrigerator with its door open


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Vapour absorption system


Vapour Absorption Refrigeration
System
• It differs from the vapour compression system
only in the method of compressing the
refrigerant.
• The compressor is replaced by a combination of
absorber, generator & a pump.
• Ammonia is a very suitable refrigerant for this
system

Vapour Absorption Refrigeration System


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Vapour Absorption System
• No moving parts
• Low grade thermal energy like solar energy can
be the input energy.
• Load variation does not affect system
performance.
• Environmental friendly.


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Refrigerants
• Defined: Any substance capable of absorbing
heat from another required substance can be
used as refrigerant i.e. ice ,water, brine, air etc.
Primary Ref

Refrigerants:
Secondary Ref


They take part

in refrigerating systems.

eg: Freon Ammonia , Carbon dioxide, Sulphur
dioxide etc.

First they are
and then they are circulated .
eg: Chilled water, brine solutions etc.


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Properties of Refrigerants
•
•
•
•
•
•
•
•

Low Boiling Point
Low Freezing Point.
High Latent Heat.
Chemically Inert & stable .
Non Flammable
Non toxic
Should not react with lubrication oil of comp.
Should not be corrosive

Important refrigerants
• Refrigerant is the working fluid used in refrigerators . Commonly
used refrigerants are:
1.Ammonia : used in ice plants, large cold storages,
skating rings etc.
2.Carbon dioxide : used in marine refrigerators,
dry ice making etc.
3.Methyl chloride: used in domestic & industrial refrigerators
(now obsolate)
4.Fluorocarbons : Freon -11,12,13,22,113,114, etc.

These are used in domestic & industrial applications.These
chemicals damage the ozone layer and hence they are being
replaced.

PROPERTIES OF AN IDEAL REFRIGERANT
• An ideal refrigerant should satisfy the following thermodynamic ,
chemical & physical requirements:
a. Thermodynamic properties.
1. low boiling point
2. high critical temperature & low critical pressure.
3.Freezing point should be lower than the refrigeration
system temperature to avoid blocking of pipes.
b. Thermo physical properties.
1. low viscosity.
2. high thermal conductivity.
3.high latent heat of vapourization.
4.low specific volume.
5.low specific heat.

PROPERTIES OF AN IDEAL REFRIGERANT
c. Chemical requirements.
1. Should be non inflammable.
2. It should not be toxic.
3. Should be non corrosive to metals.
4.Should be capable of mixing well with
lubricating oil.
5.Electric resistance should be high.
6.Leakage should be detected easily.
7.Should be odourless.

Domestic refrigerator
• Vapour compression cycle is normally preferred
over absorption system in domestic refrigerator
because of its compactness and more efficient
use of electrical energy.
• Refrigerant used is R-12 or R – 22.


Lay out of an Ice plant


Ice Plant
• Ammonia is used as the refrigerant.

• The

high temperature, high pressure ammonia vapours are

condensed in a condenser .
• The condensed liquid ammonia is collected in a receiver and then

expanded through the expansion valve .
• As the low pressure ammonia passes through the evaporator coils,
surrounding the ice cans in the brine tank in which brine solution is
filled, it absorbs heat from the solution and gets vapourized .
• It is again fed to the compressor to complete the cycle.

Basic Mechanical Engineering - Refrigeration

Basic Mechanical Engineering - Refrigeration

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