The purpose of the evaporator is to receive low-pressure, low-temperature fluid from the expansion valve and to bring it in close thermal contact with the load.
The refrigerant takes up its latent heat from the load and leaves the evaporator as a dry gas. Evaporators are classified according to their refrigerant flow pattern and their function.
The flow pattern can be one of two types. Either the refrigerant flows continuously through the heat exchanger whilst it evaporates and becomes superheated, or alternatively it resides in a vessel at low pressure whilst it evaporates or from which it is taken to individual coolers, returning as liquid/vapour mixture. The most common type by far is the continuous flow type, referred to as a direct expansion evaporator.
The function of the evaporator is to cool air or liquid in almost all cases. The air or liquid then cools the load. For example in a refrigerated display cabinet, the air is cooled and circulated to keep the contents at the required temperature; in a water chiller system, the water is circulated to individual fan-coil units to provide air conditioning. In heat pumps, the function can be described as recovering heat from air or liquid, but the evaporator construction will be very similar.
Posted in Refrigeration and Air Conditioning