Domestic and industrial refrigeration works on a principle called the vapour compression cycle. The basic refrigeration system consists of a compressor feeding into a condensor. The output from the condensor, called the liquid line, then feeds into an evaporator. The other end of the evaporator, termed the suction line, leeds back into the compressor. The system is charged with a refrigerant gas which boils at a low temperature.
In addition to the components of a refrigeration system two terms have to be understood. These are latent heat and sensible heat. Latent heat is heat energy gained or lost without a change in temperature. Sensible heat is heat energy gained or lost that results in change in temperature which can be sensed. A change in latent heat is much more efficient than a change in sensible heat and this is the principle on which the vapour compression cycle works.
Starting from the compressor the vapour compression cycle flows as follows.
1) The compressor takes low pressure refrigerant gas and compresses it into a high pressure gas. Since pressure is linked to temperature the gas gains sensible heat during compression.
2) The now high pressure gas is fed into a condensor where it is cooled and also changes state into a liquid. A change of state is a latent process and so the refrigerant must lose latent heat to change from gas to liquid.
3) The liquid refrigerant is now piped through the liquid line to a metering device at the start of the evaporator, usually an expansion valve. Since this valve is much smaller in diameter than the pipe the liquid enters it at high pressure but leaves at low pressure.
4) Now at a low pressure the liquid enters the evaporator where it tries to change state to a gas. This is a latent heat process so the refrigerant draws heat from the surrounding area to fuel its change of state.
5) Once it reaches the end of the evaporator the refrigerant is once again a low pressure gas which is drawn into the compressor and the cycle starts again.
In domestic refrigerators and freezers the compressor and condensor are located on top or in the rear of the case whilst the evaporator is placed inside the case.
In industrial refrigeration systems the compressors are usually located in a plant near the shopfloor and the liquid refrigerant is piped out to the cases. The returning hot gas is piped to a condensor, usually on an outside wall or roof, where it is cooled.