Refrigerator is a machine that transfers the heat of the low-temperature cooled object to the environmental medium to obtain refrigeration capacity. The heat transmitted from a lower temperature object is customarily called cooling capacity. The working medium that participates in the change of thermodynamic process (energy conversion and heat transfer) in refrigerator is called refrigerant. The temperature range of refrigeration is usually above 120K, and 120K belongs to cryogenic technology. Refrigerators are widely used in industrial and agricultural production and daily life. The main performance indexes of the refrigerator include working temperature (evaporation temperature and condensation temperature of vapor compression refrigerator, temperature of the cooled object of gas compression refrigerator and semiconductor refrigerator and temperature of cooling medium), refrigerating capacity (heat taken away from the cooled object by the refrigerator in unit time), power or heat consumption, refrigeration coefficient (index to measure the economy of compression refrigerator, which refers to the refrigerating capacity obtained by consuming unit work) and thermodynamic coefficient (index to measure the economy of absorption refrigerator and steam jet refrigerator). Steam compression refrigerator is the most widely used one in modern refrigerators.

Air conditioner, namely room air conditioner, is a device for providing processing air to a room (or enclosed space or area). Its function is to adjust the temperature, humidity, cleanliness and air flow of indoor (or enclosed space or area) air to meet the requirements of human comfort or technological process. The air conditioning refrigeration system consists of evaporator, compressor, condenser and capillary. According to the working sequence of refrigeration cycle, they are connected into a whole by pipes in turn. When the system works, the refrigerant in the evaporator absorbs the heat of indoor air and evaporates into low-pressure and low-temperature steam. After the refrigerant is sucked and compressed by the compressor, the pressure and temperature rise, and then it is discharged into the condenser. By releasing heat to outdoor air in the condenser, refrigerant vapor is condensed into liquid with higher pressure. The refrigerant liquid is throttled by capillary tube, the pressure and temperature are reduced, and then it enters the evaporator to evaporate, so it works circularly, thus reducing the indoor temperature.