The compressor is regarded as the heart of the refrigeration system, and the specialized term that can represent the characteristics of the compressor is called a "vapor pump". The actual responsibility of the compressor is to increase the pressure and raise the suction pressure state to the discharge pressure state.

　　Compression ratio is a technical representation of pressure difference, which means the pressure on the high-pressure side divided by the pressure on the low-pressure side. The calculation of compression ratio must use pressure values. To avoid negative values in the compression ratio calculation, pressure must be used instead of gauge pressure when calculating the pressure ratio. Only by using pressure values can the calculated compression ratio be positive, which is meaningful.

　　There are five types of compressors used in the refrigeration and air conditioning industry: reciprocating, screw, rotary, vortex, and centrifugal. Among them, reciprocating is a commonly used compressor in small and medium-sized commercial refrigeration systems. Screw compressors are mainly used in large commercial and industrial systems. Rotary compressors and scroll compressors are mainly used in household and small capacity commercial air conditioning units, while centrifugal compressors are widely used in air conditioning systems for large buildings.

　　Various reciprocating compressors are generally classified according to the form of the compressor housing and the setting of the driving mechanism. According to the shell form, there are open and closed semi closed compressors. Closed type refers to the entire compressor being housed within a single casing.