Refrigeration is the withdrawl of heat from
a substance or space so that temperature lower than that of the natural
surroundings is achieved.
Refrigeration may be produced by
- thermoelectric means
- vapor compression systems
- expansion of compressed gases
- throttling or unrestrained expansion
Vapor compression systems are employed in most refrigeration systems.
Here, cooling is accomplished by evaporation of a liquid refrigerant
under reduced pressure and temperature. The fluid enters the compressors
at state 1 where the temperature is elevated by mechanical compression
(state 2). The vapor condenses at this pressure, and the resultant heat
is dissipated to the surrounding. The high pressure liquid (state 3)
then passes through an expansion valve through which the fluid pressure
is lowered. The low-pressure fluid enters the evaporator at state 4
where it evaporates by absorbing heat from the refrigerated space, and
reenters the compressor. The whole cycle is repeated.