Refrigeration and Air Conditioning

DEFINATION OF REFRIGERATION

Refrigeration is defined as “ the branch of science that deal with the process of reducing and maintaining the temperature of confined space or material below the temperature of surrounding” the system maintain at lower temperature is known as refrigerated system and equipment used to obtain this is known as refrigerator. The cooling effect produced by this equipment is the refrigerating effect.

PRINCIPLE OF REFRIGERATION

According to Clausius’s statement of second law of thermodynamics ,heat does not flow from low temperature region to high temperature region without help of external energy.

Thus ,in order to transfer heat from low temperature to high temperature ,work must be supplied to refrigerator, which produces cooling effect.such cooling may be obtain by one of the following principle. 1. By chemical mean ,in which chemical reaction is carried out to absorb the heat for its completion. The heat required for reaction taken from the substance to be cooled. 2. By bringing the substance to be directly or indirectly in contact with some cooling medium such as chilled water or ice.

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3. By using mechanical and heat energy to operate heat pump.

METHOD OF REFRIGERATION

Refrigerating effect can be produced by using following methods.—

1) Ice refrigeration.
2) Refrigeration by expansion of air.
3) Refrigeration by throttling of air
4) Steam jet refrigeration
5) Vapour compression cycle
6) Non convention method of refrigeration
a) Vortex tube refrigeration
b) Pulse tube refrigeration
c) Solar refrigeration

REFERIGERATION CYCLES

1) REVERSED CARNOT CYCLE
2) AIR REFRIGERATION CYCLE
a) BELL COLEMAN AIR REFRIGERATION CYCLE
3) VAPOUR COMPRESSION CYCLE

REVERSED CARNOT CYCLE

A machine work on reversed carnot cycle would act as refrigerator if driven by power an external source and would by theoretically most efficient .

no such machine has been used in pratical because adiabatic portion of piston stroke need extremely slow speed. This variation of speed is not possible in actual practice.

The reversed carnot cycle consist of two reversible isothermal isothermal and two reversible adiabatic processes. Process 1-2 : reversible isothermal heat absorbtion
Process 2-3: reversible adiabatic compression
Process 3-4: reversible isothermal heat rejection
Process 4-5: reversible adiabatic expansion

Thus in cornot cycle the heat absorbtion and heat rejection taken place at constant temperature and compression and expansion is adiabatic.

AIR REFERIGERATION CYCLES

Air refrigeration cycles are knows as gas refrigeration cycle, in which air is used as refrigerant. Air transfers only its sensible heat and does not undergo a change of phase. a) Bell coleman air refrigeration cycle

The earlist type of refrigerator works on Bell coleman cycle with air as working substance. the isothermal process from carnot cycle can be replaced by constant pressure processes. The schematic diagram of bell-colleman cycle can be shown in fig.

Schetimatic diagram of Bell-coleman cycle

Bell-coleman cycle consist of compressor ,expansion ,heat exchanger and evaporator. The air from evaporator is drawn into compressor and is comprssed to heat exchanger pressure isentropically. During the compression along with pressure rise,temperature if air also increases. This worm compressed air is force into heat exchanger, which cooled the air at constant pressure.it has an effect of reducing volume. Simultaneously its temp also reduce due to rejection of heat.this low temperature air is drawn to expander, where it is expanded isentropically. The isentropic expansion reduces temperature of air bellows that of evaporated temperature. This at least temperature is passes to evaporator, where it absorb the heat at constant pressure and producing refrigerating effect. P-V diagram of bel-coleman cycle is as shown in fig.

VAPOUR COMPRESSION CYCLE

PRINCIPLE OF VAPOUR COMPRESSION CYCLE

In vapour compression cycle liquid is used as refrigerant in place of air as in case of air refrigeration cycle. The liquid refrigerant boil in evaporator which is at low pressure ,by absorbing latent.the vapour form are condenced in condencer which is at high pressure,by rejecting latent heat.thus vapour compession cycle transfer heat in the form of sensible heat as well as latent heat, giving higher coefficient of performance.