

Refer simple vapour compression cycle on Blog:
Vapour compression refrigeration is most important and practical form of refrigeration system for domestic & commercial utility. The working fluid (refrigerants) used are CO2, NH3 and Feron etc are liquid gas mediums, which at normal temperature may exit either as a liquid or a gas depending upon pressure.
The system comprises namely compressor, condenser, expansion valve or capillary tube and evaporator. The refrigerant is not allowed to leave the system, but is circulated again & again alternately condensing and evaporating.
The sequence of operation of the cycle is:
1. Reversible adiabatic compression (1-2): The vapour at low temperature T1 & pressure p1 preferably dry (state1) is drawn from evaporator during suction stroke of compressor. Vapour is compressed isentropically to a higher pressure p2 and temperature T2. At compressor exit, the vapour may be wet, dry or superheated. Compressor creates the pressure difference for maintaining continuous flow of refrigerant in system by raising the pressure.
2. Constant pressure condensation (2-3): The vapour refrigerant at high temperature and pressure (state2) coming from the compressor is delivered to the condenser, where the condensation is at constant pressure. The condenser first absorbs the heat of superheat and temperature of the vapour falls to saturation temperature. Subsequently the refrigerant loses its latent heat and changes to high pressure liquid (state3)
3. Throttling (3-4): Expansion of high pressure liquid refrigerant is done by throttling process. During expansion pressure drops, but enthalpy remains constant. Due to pressure drop, the liquid starts boiling and required heat is provided by refrigerant itself. Refrigerant is mixture of liquid and vapour. Throttling is an irreversible process( shown by dotted line)
4. Constant pressure evaporation(4-1): The wet vapour after throttling passes through evaporator coils placed in the medium (usually brine), which is to be cooled. The temperature of vapour is below that of brine. It absorbs latent heat from the brine and gets evaporated at constant pressure (state1) the refrigerant may remains wet, dry saturated or slightly superheated.
Notes:
Rotary type compressor…………… for domestic refrigerators.
Reciprocating type compressor……for large plants & machines.
Centrifugal compressor……………..for plants over 50 tons.
Cooling medium atmosphere………for domestic refrigerators.
Cooling medium steam of water …for large units refrigerations.
The components of the system are connected by pipes:
Suction line from evaporator to compressor, delivery line from compressor to condenser and liquid line from receiver tank to expansion/ throttling----(i.e. capillary tube, thermostatic expansionvalve or automatic expansion valve.
If the unit is a reach in, with the condensing unit in the bottom, and it's located in a dusty area, dust can build up on the condenser coil and filter in a surprisingly short time, so make it a point to establish a maintenance schedule, and stick to the schedule.
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