1. What is refrigerant? Refrigerant is the medium used to complete energy conversion in various heat engines, and it is also the working fluid that completes the thermodynamic cycle in refrigeration machines. When machines operate, they emit heat, and refrigerants can absorb heat from the cooled objects at low temperatures and then transfer it to cooling water or air at higher temperatures. Generally, refrigerants are required to have excellent thermodynamic properties, specifically including: critical temperature higher than condensation temperature, saturation pressure corresponding to condensation temperature not too high, standard boiling point relatively low, low specific heat capacity, low adiabatic index, large heat quantity per unit volume, high heat transfer coefficient, low viscosity, small density, and good chemical stability, etc.

　　2. How many types of refrigerants are there? There are many types of refrigerants, and different types have different codes, generally following the standards set by the American Society of Heating, Refrigerating and Air-Conditioning Engineers in 1967, using a naming method that links the refrigerant's code with its species and chemical composition, which can be divided into the following three major categories:

　　1. Inorganic compounds. The numbering method is R7XX (XX - molecular weight of inorganic compounds), commonly found are:

　　(1) Ammonia: R717, a widely used medium-pressure medium-temperature refrigerant, and one of the earliest refrigerants. As a refrigerant, it has advantages such as being readily available, low price, moderate pressure, large cooling capacity per unit, high heat release coefficient, and easy detection when leaking. However, it also has disadvantages such as irritating odor, toxicity, flammability and explosiveness, and corrosiveness.

　　(2) Carbon dioxide: R744, belongs to hydrocarbon refrigerants, is a natural working fluid, has no pollution to the atmosphere, no damage to the ozone layer, and almost zero greenhouse effect, with advantages of energy saving and environmental protection.

　　(3) Water: a very simple refrigerant.

　　2. Organic compounds. Mainly chlorofluorocarbons, which are derivatives of methane and ethane, replacing all or part of the hydrogen atoms in the original compounds with atoms of fluorine, chlorine, and bromine. There are many types, commonly used are:

　　(1) Chlorofluorocarbon-12: R12, a halogenated alkane, chemically named dichlorodifluoromethane, is a widely used medium-pressure medium-temperature refrigerant in medium and small refrigeration devices.

　　(2) Chlorofluorocarbon-22: R22, also a halogenated alkane, chemically named difluorochloromethane, its properties are similar to R12, but has a larger cooling capacity per unit volume, although its chemical stability is not as good as R12, and its toxicity is slightly higher than R12.

　　(3) R-134a: a widely used medium-low temperature environmentally friendly refrigerant, as an effective and safe alternative to R12 (dichlorodifluoromethane), can be applied in most fields using R12 refrigerant.

　　3. Mixed working fluids. Refrigerants formed by mixing two or more refrigerants in certain proportions, mainly include:

　　(1) Azeotropic refrigerants: such as R502, its properties are the same as those of a single refrigerant, having a constant evaporation temperature at a constant pressure, and the components of the gas phase and liquid phase are the same.

　　(2) Non-azeotropic refrigerants: solutions formed by mixing two or more single refrigerants that do not form azeotropic solutions with each other. When the solution is heated, at a certain evaporation pressure, the proportion of the more volatile components evaporating is large, while the proportion of the less volatile components evaporating is small, thus the compositions of the gas and liquid phases are different, and the temperature of the refrigerant changes during the evaporation process, with similar characteristics during the condensation process, including R400, R401, etc.

　　(3) Hydrocarbon refrigerants: belong to alkane refrigerants, including methane, ethane, propane, ethylene, and propylene, mainly in the petrochemical industry.