When the automobile air conditioning and refrigeration system is working, the refrigerant circulates in this closed system in different states. Each cycle has four basic processes:

1. Compression process: The compressor sucks in the low-temperature and low-pressure refrigerant gas at the evaporator outlet, compresses it into high-temperature and high-pressure (about 70°C, 1471kPa) gas and discharges it from the compressor.
2. Heat release process: The superheated refrigerant gas with high temperature and high pressure enters the condenser. Due to the decrease in pressure and temperature, the refrigerant gas condenses into a liquid and releases a large amount of heat.
3. Throttling process: The refrigerant liquid with higher temperature and pressure becomes larger in volume after passing through the expansion device, the pressure and temperature drop sharply, and is discharged from the expansion device in the form of mist (fine droplets).
4. Endothermic process: The mist-like refrigerant liquid enters the evaporator, so the boiling point of the refrigerant is much lower than the temperature inside the evaporator, so the refrigerant liquid evaporates into gas. During the evaporation process, a large amount of surrounding heat is absorbed, and then the low-temperature and low-pressure refrigerant vapor enters the compressor.