Compressors can be divided into volumetric compressors and velocity compressors according to their principles. Volumetric compressors are divided into reciprocating compressors and rotary compressors; Speed compressors are divided into axial compressors, centrifugal compressors, and mixed flow compressors.　　Nowadays, compressors for household refrigerators and air conditioners are of the positive displacement type, which can be further divided into reciprocating and rotary types. Reciprocating compressors use piston, crank, connecting rod mechanisms or piston, crank, sliding tube mechanisms, while rotary compressors mostly use rolling rotor compressors. In commercial air conditioning, there are mostly centrifugal, vortex, and screw types.　　According to the application scope, it can be divided into low back pressure type, medium back pressure type, and high back pressure type. Low back pressure (evaporation temperature -35~-15 ℃), generally used for household refrigerators, food freezers, etc. Medium back pressure type (evaporation temperature -20~0 ℃), generally used for cold drink cabinets, milk refrigerators, etc. High back pressure type (evaporation temperature -5~15 ℃), generally used for room air conditioners, dehumidifiers, heat pumps, etc.

　　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.

　　1. Adjustment of pressure level: Turn the pressure adjustment screw 1 clockwise to increase the applied pressure, and vice versa to decrease the applied pressure, but the pressure difference remains unchanged　　2. Adjustment of pressure level: Turn the pressure adjustment screw 2 clockwise to increase the applied pressure, and vice versa to decrease the applied pressure, but the pressure difference remains unchanged　　3. When the air compressor reaches the set pressure, the saturation and relief valves will discharge the gas inside the steel pipe, which is a normal phenomenon and not a leakage　　4. 1-2 adjustment screws are interrelated　　5. Adjust the pressure to ensure that it exceeds the discharge pressure of the compressor