basic mechanical engineering - refrigeration
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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.
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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.
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Major applications
• Food processing ,preservation & distribution.
• Chemical & process industries.
• Special applications such as cold treatment of metals , medical , construction etc.
• Comfort air conditioning
• 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=
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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
T1< T2
Sink T1
Source T2
Refrigerator Work W
Q 1
Q 2
• 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.
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. [email protected]
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compressor
Low pr gauge
Evaporator
Throttle valve
condenser
High pr gauge
Simple Vapour Compression
System
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High pressure Low temperature liquid
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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.
T-S & P-H Diagram
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Components of the Practical Cycle
Practical Vapour Compression Cycle.
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A home refrigerator with its door open
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
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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. [email protected]
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. [email protected]
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.
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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.
