If a liquid is introduced into a vessel where there is initially a degree of vacuum and whose walls are kept at a constant temperature, it will immediately evaporate. In the process, the latent heat of vaporization, that is, the heat required for the change from liquid to vapor state is provided by the vessel walls. The resulting cooling effect is the starting point of the cooling cycle, which will be examined in this chapter.
As the liquid evaporates, the pressure within the vessel increases to eventually reach saturation pressure for the temperature considered. After this no amount of liquid will evaporate and of course the cooling effect will cease. Any additional amount of liquid introduced will remain in this state, that is, as liquid at the bottom of the vessel. If part of the vapor from the container is removed by connecting it to the suction side of a pump, the pressure will tend to drop, this will cause further evaporation of the liquid. In this regard, the cooling process can be considered continuous. And for that you need: a suitable fluid, the refrigerant; a container where vaporization and cooling are performed, called an evaporator; and a vapor removal element, called a compressor.
The system presented so far is not practical as it involves a continuous consumption of refrigerant. To avoid this problem it is necessary to convert the process into a loop. To return the vapor to liquid, it must be cooled and condensed. Usually, water or air is used as a cooling medium which is at a temperature substantially higher than the prevailing evaporator temperature. The vapor pressure corresponding to the condensing temperature must therefore be much higher than the evaporator pressure. The desired pressure increase is promoted by the compressor.
Refrigerant liquefaction is performed in a condenser which is essentially a container cooled externally by air or water. Hot (overheated) high pressure refrigerant gas is fed from the compressor to the condenser, where it is condensed. It now remains to complete the cycle, which can be done by adding a valve or other regulating device that will be used to inject liquid into the evaporator. This is an essential component of a refrigeration installation and is called an expansion valve.
