According to the molecular structure of the refrigerant, the refrigerant can be divided into inorganic compounds and organic compounds; according to the composition of the refrigerant, it can be divided into single refrigerant and mixed refrigerant; according to the physical properties of the refrigerant, the refrigerant can be divided into high temperature (low pressure) , Medium temperature (medium pressure), low temperature (high pressure) refrigerant. Usually according to chemical composition, refrigerants can be divided into five categories: inorganic compound refrigerants, freons, saturated hydrocarbon refrigerants, unsaturated hydrocarbon refrigerants and azeotropic refrigerants.
1. Inorganic compound refrigerant
This type of refrigerant was used earlier, such as ammonia (NH3), water (H2O), air, carbon dioxide (CO2) and sulfur dioxide (SO2). For inorganic compound refrigerants, the internationally prescribed code is R and the following three digits, where the first digit is "7" and the last two digits are molecular weight. Compound molecular weight (rounded part) plus 700 to get its refrigerant number. For example, the molecular weight of ammonia is 17, and its number is R717 . The numbers for carbon dioxide and water are R744 and R718, respectively. Ammonia is currently the most widely used medium pressure and medium temperature refrigerant. The advantages of ammonia as a refrigerant are: easy availability, low price, moderate pressure, large unit refrigeration capacity, high heat release coefficient, almost insoluble in oil, low flow resistance, and easy to find when leaking. Its disadvantages are: pungent odor, toxic, combustible and explosive, and corrosive to copper and copper alloys. Compared with CFCS, the advantage of CO2 is that the pressure is close to the best economic level; it can fully adapt to various lubricating oils and common machine parts; it has low kinematic viscosity, low compression ratio, and large refrigeration capacity per unit volume.
2. Halogenated hydrocarbons-Freon
Freon is a transparent, odorless, non-toxic, incombustible, explosive and chemically stable refrigerant. The thermodynamic properties of Freon refrigerants with different chemical compositions and structures are very different, and can be applied to high temperature, medium temperature and low temperature refrigerators to meet the requirements of different refrigeration temperatures. Freon has a low solubility to anyone. Acidic substances will be generated when moisture enters the refrigeration device, which will easily cause "ice plugs" in the low-temperature system and block the throttle or pipeline. In addition, to avoid the effect of Freon and natural rubber, the device should use nitrile rubber as gasket or sealing ring. The commonly used Freon refrigerants are R12, R22, R502 and R1341a. Freon 12 (CF2CL2, R12) is one of the most widely used Freon refrigerants. It is widely used in refrigeration devices such as medium and small food storage, household refrigerators, and water and road refrigeration transportation. R12 has better thermodynamic properties and lower storage pressure. The standard evaporating temperature of R12 is -29°C, which is a warm refrigerant. It is used in medium and small piston compressors to obtain a low temperature of -70°C. Freon 22 (CHF2CL, R22) is also one of the most widely used Freon refrigerants, mainly used in household air conditioners and low-temperature refrigerators. The thermodynamic properties of R22 are similar to ammonia. The standard vaporization temperature is -40.8°C, and the condensing pressure usually does not exceed 1.6MPa. R22 is non-flammable and non-explosive, safer and more reliable than ammonia in use. Freon and water are almost completely insoluble in each other, and the solubility to water is extremely small. Moisture can also hydrolyze Freon to produce acid, causing "copper plating" in the refrigeration system.
3. Saturated hydrocarbon refrigerants
Such refrigerants mainly include methane, ethane, propane, butane and cyclic organic compounds. The code name uses "R" like Freon. This type of refrigerant is flammable and explosive, and has poor safety.
4. Unsaturated hydrocarbon refrigerant
These refrigerants are mainly ethylene (C2H4), propylene (C3H6) and their halogen derivatives.
5. Azeotrope refrigerant
This type of refrigerant is an azeotropic mixture formed by mixing two or more different refrigerants in a certain ratio. This type of refrigerant can maintain a certain evaporation temperature under a certain pressure, and its gas or liquid phase always maintains the same composition ratio. However, their thermal properties are different from those before mixing. The use of azeotropic mixtures can improve the characteristics of refrigerants.