The working principle of refrigeration is that the compressor compresses steam with lower pressure into steam with higher pressure, so that the steam volume decreases and the pressure increases. The compressor sucks the working medium vapor with lower pressure from the evaporator, sends it to the condenser after the pressure rises, condenses it into liquid with higher pressure in the condenser, becomes liquid with lower pressure after throttling by the throttle valve, then sends it to the evaporator to absorb heat and evaporate it into vapor with lower pressure, and then sends it to the inlet of the compressor to complete the refrigeration cycle.

Evaporator is a very important part of the four refrigeration components. The low-temperature condensed gas passes through the evaporator, exchanges heat with the outside air, liquefies and absorbs heat, and achieves the refrigeration effect.

The evaporator consists of a heating chamber and an evaporation chamber. The heating chamber provides the heat needed for evaporation to the liquid, and promotes the liquid to boil and evaporate; The evaporation chamber completely separates the gas phase from the liquid phase. The steam generated in the heating chamber carries a large amount of liquid foam, which can be separated from the steam by self-condensation or demister after reaching the evaporation chamber in a larger space. Usually, the demister is located at the top of the evaporation chamber.

Evaporators are divided into normal pressure, pressurization and decompression according to working pressure. According to the movement of the solution in the evaporator, there are: ① circulation type. Boiling solution passes through the heating surface many times in the heating chamber, such as central circulation pipe, hanging basket, external heating, Levin and forced circulation. ② One-way type. The boiling solution passes through the heating surface once in the heating chamber without circulating flow, that is, the concentrated solution is discharged, such as rising film type, falling film type, stirring film type and centrifugal film type. ③ Direct contact type. The heating medium is in direct contact with the solution to transfer heat, such as submerged combustion evaporator. During operation, the evaporation device consumes a lot of heating steam. In order to save heating steam, multi-effect evaporator and steam recompression evaporator can be used. Five-ring refrigeration shell-and-tube evaporator is widely used in chemical industry, light industry, petroleum and other departments.