The properties of refrigerants will directly affect the type, structure, size and operation characteristics of refrigerators, as well as the form of refrigeration cycle, equipment structure and economic and technological performance. Therefore, the rational selection of refrigerants is a very important issue. The performance requirements of refrigerants are usually considered in terms of thermodynamics, physicochemistry, safety, environmental impact and economy.

I. Thermodynamic Requirements

1. Lower boiling point leads to lower evaporation temperature. At the same time, refrigerants with low boiling point have higher vapor pressure.
2. Higher critical temperature and lower solidification temperature. The critical temperature should be high and the solidification temperature should be low to ensure the safe operation of refrigerants in a wide temperature range.
3. Refrigerants should have suitable working pressure, requiring the evaporation pressure of refrigerants to be close to or slightly higher than the atmospheric pressure, so as to avoid vacuum in the low-pressure part of the refrigeration system and increase the chance of air mixing into the system. It is required that the condensation pressure should not be too high. Low condensation pressure can reduce the strength and construction requirements of refrigeration equipment and pipelines, reduce the construction investment of refrigeration system and the possibility of refrigerant leakage. The pressure ratio and pressure difference between condensation pressure and evaporation pressure are required to be small.
4. The latent heat of vaporization of refrigerants is larger. When the refrigeration system obtains the same cooling capacity, the large latent heat of vaporization can reduce the refrigerant circulation. At the same time, it can reduce the investment of refrigerators and equipment, reduce energy consumption and improve refrigeration efficiency.
5. For large refrigeration systems, the refrigerant capacity per unit volume is required to be as large as possible. In this way, when the cooling capacity is constant, the refrigerant circulation can be reduced, the size of the refrigerator and the diameter of the pipeline can be reduced. However, for small refrigeration systems, the refrigeration capacity per unit volume is required to be small, which can appropriately increase the channel cross section of refrigerant and reduce the flow resistance.
6. The adiabatic index of refrigerant should be small, which can reduce the compression power consumption, reduce the exhaust temperature, improve the operation performance and simplify the system design.
7. For centrifugal refrigeration compressors, refrigerants with moderate molecular weight should be used. Because the relative molecular mass can increase the boost ratio of each stage, the compression series can be reduced when the pressure ratio of the system is fixed. In addition, when most substances vaporize at boiling point, their molar entrainment increases are similar. Therefore, when the refrigerants with similar boiling point are difficult to be standardized, the latent heat of vaporization will be small when the relative molecular mass is large.
8. High thermal conductivity can increase the heat transfer coefficient of heat exchanger and reduce the heat transfer area of heat exchanger.

II. Physical and Chemical Requirements

1. Refrigerant viscosity should be small, refrigerant viscosity should be small, reduce the flow resistance of refrigerant in the system, reduce the diameter of refrigeration system pipeline belongs to consumption. Low viscosity can also increase the heat transfer performance of refrigerants.
2. Refrigerants should be of high purity. The selected refrigerants should be free of insoluble impurities, requiring refrigerants to have certain water absorption. When the refrigerants infiltrate very little water, ice jams will occur at low temperature and affect the normal operation of the refrigeration system.
3. Refrigerants have good thermochemical stability and are not easy to decompose at high temperature. When refrigerant is mixed with oil and water, it should not have obvious corrosion effect on metal materials, and the swelling effect on sealing materials of refrigerators should be as small as possible.
4. The oil solubility of refrigerants is characterized by complete dissolution, micro-dissolution and complete insolubility. When refrigerant and refrigerant oil are completely degraded, good conditions can be created for lubrication of engine parts. Oil film is not easy to form on the heat exchanger surface of condenser and other heat exchangers, and the heat transfer effect is excellent. However, the evaporating temperature of refrigerant will be raised, the refrigerating oil at low temperature will decrease, the foam will increase when refrigerant is boiling, the liquid level of evaporator is unstable, and the fuel consumption of refrigerating machine will increase when running, so it is not easy for the system to return oil. When refrigerant and frozen oil are completely unclear, the evaporation temperature of refrigeration system is less affected, but oil film is easily formed on the heat exchange surface of heat exchanger, which affects heat transfer. The advantages and disadvantages of oil-slightly soluble refrigerants lie between the two.
5. The effect of refrigerant on coil insulation material is as small as possible. In semi-enclosed and fully enclosed refrigerators, the direct contact between motor coil of compressor and refrigerant and refrigerant oil requires not only good electrical insulation of refrigeration penalty, but also the effect of refrigerant on coil insulation material as small as possible.

Ⅲ.Safety requirements

1. Refrigerants should not burn or explode in the working temperature range. When some flammable and explosive refrigerants must be used, fire and explosion-proof safety measures must be taken.
2. Refrigerants should be non-toxic or low toxic and have good relative safety. The toxicity, combustion and explosivity of refrigerants are all indicators to evaluate the safety of refrigerants, and all countries have set minimum safety standards.
3. Choose refrigerants that are easy to detect. Because some refrigerants are toxic and dangerous, it is required that the refrigerants chosen should have the characteristics of easy to detect leakage, so as to ensure the safety of operation.
4. Choose refrigerants that are harmless to human health and have no irritating odor. In case the leaked refrigerant comes into contact with the food, the food should not be discolored, tasted, polluted and damaged. Refrigerants for air conditioning should be harmless to human health and have no irritating odor.

Ⅳ.environmental impact requirements, the latest research found that some chemicals may have an impact on the global environment, has attracted the attention of the international community. The refrigerant selected should meet the following requirements:

(1) Life in the atmosphere is lower.
(2) The potential destructive effect on the ozone layer is small.
(3) Global greenhouse potential effect is lower.
(4) Non-light fog reaction has little effect on atmosphere, water source and soil.

V. Economic Requirements

(1) The production process of refrigerant should be simple and the production cost should be low.
(2) Refrigerants are cheap and readily available.
Because there are many kinds of refrigerants and their properties are very different, there is no ideal refrigerant that completely meets the requirements. When choosing, we should consider it comprehensively according to the actual situation.

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