The working fluid in the refrigerator to complete the thermal cycle. It absorbs the heat of the object to be cooled at low temperature, and then transfers it to cooling water or air at a higher temperature. In vapor compression refrigerators, refrigerants that can be liquefied at room temperature or lower temperature are used, such as Freon (fluorine, chlorine, and bromine derivatives of saturated hydrocarbons), and azeotropic mixture of refrigerants (composed of two Azeotropic solution of freon mixed in a certain proportion), hydrocarbons (propane, ethylene, etc.), ammonia, etc.; in gas compression refrigerators, gas refrigerants such as air, hydrogen, helium, etc. are used. The gas is always gaseous in the refrigeration cycle; in the absorption refrigerator, a binary solution composed of absorbent and refrigerant is used as a working fluid, such as ammonia and water, lithium bromide (molecular formula: LiBr. White cubic crystal or granular Powder, easily soluble in water) and water; steam jet refrigerators use water as the refrigerant. The main technical indicators of refrigerants include saturated vapor pressure, specific heat, viscosity, thermal conductivity, surface tension, etc. After 1960, people have conducted a large number of experimental studies on the application of non-azeotropic mixtures, and they have been used in the liquefaction and separation of natural gas. The application of single-stage compression of non-azeotropic mixed working fluid can obtain very low evaporation temperature, and can increase refrigeration capacity and reduce power consumption. Its nature is directly related to the refrigeration effect, economy, safety and operation management of the refrigeration device, so the understanding of the nature of the refrigerant cannot be ignored.