The nature of the refrigerant will directly affect the type, structure, size and operating characteristics of the chiller, and will also affect the form of the refrigeration cycle, equipment structure and economic and technical performance. Therefore, reasonable selection of refrigerant is a very important issue. The performance requirements of refrigerants are usually considered in terms of thermodynamics, physical chemistry, safety, environmental impact and economics.

1. Thermodynamic requirements

(1) The boiling point should be low to obtain a lower evaporation temperature. At the same time, refrigerants with low boiling points have higher vapor pressures.

(2) The critical temperature should be high and the solidification temperature should be low to ensure that the refrigerant can work safely in a wide temperature range.

(3) The refrigerant should have an appropriate working pressure. The evaporation pressure of the refrigerant is required to be close to or slightly higher than the atmospheric pressure, so as to avoid a vacuum in the low-pressure part of the refrigeration system and increase the chance of air being mixed into the system. It is required that the condensing pressure should not be too high. Low condensing pressure can reduce the strength and construction requirements of refrigeration equipment and pipelines, reduce the construction investment of refrigeration systems and the possibility of refrigerant leakage.

(4) The latent heat of vaporization of the refrigerant should be large. When the refrigeration system obtains the same cooling capacity, the latent heat of vaporization can reduce the circulation of refrigerant. At the same time, it can also reduce the investment of chillers and equipment, reduce energy consumption and improve refrigeration efficiency.

(5) For large-scale refrigeration systems, the refrigeration capacity per unit volume of the refrigerant is required to be as large as possible. In this way, when the cooling capacity is constant, the circulating volume of the refrigerant can be reduced, and the size of the refrigerator and the diameter of the pipeline can be reduced. However, for small refrigeration systems, the cooling capacity per unit volume is required to be small, so that the channel section of the refrigerant can be appropriately increased to reduce flow resistance.

(6) The adiabatic index of the refrigerant should be small. This reduces compression power consumption, lowers exhaust temperature, improves operational performance and simplifies 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, when the pressure ratio of the system is constant, the number of compression stages can be reduced. In addition, most substances have similar molar increases when they vaporize at boiling points. Therefore, for refrigerants with similar boiling points, when the relative molecular mass is large, the latent heat of vaporization is small.

(8) The thermal conductivity should be high. This can improve the heat transfer coefficient of the heat exchange equipment and reduce the heat exchange area of the heat exchange equipment.

2. Physical and chemical requirements

(1) The viscosity of the refrigerant should be small to reduce the flow resistance of the refrigerant in the system and reduce the diameter of the refrigeration system pipes as consumption. The low viscosity can also increase the heat transfer performance of the refrigerant.

(2) The purity of the refrigerant should be high, and the selected refrigerant should have no insoluble impurities. It is required that the refrigerant has a certain degree of water absorption. When a small amount of water seeps into the refrigerant, it will cause ice plugs at low temperatures and affect the refrigeration system. normal work.

(3) The thermochemical stability of the refrigerant is better, and it is not easy to decompose at high temperature. When the refrigerant is mixed with oil and water, it should not have obvious corrosive effect on metal materials, and the swelling effect on the sealing material of the refrigerator should be as small as possible.

(4) The oil solubility of the refrigerant is completely soluble, slightly soluble and completely insoluble. When the refrigerant and the chiller oil are completely pan-dissolved, it can create good conditions for the lubrication of the machine parts, and it is not easy to form an oil film on the heat exchange surface of the condenser and other heat exchangers, and the heat transfer effect is good. But it will increase the evaporation temperature of the refrigerant, reduce the temperature of the cold water machine oil at low temperature, increase the foam when the refrigerant boils, cause the liquid level in the evaporator to be unstable, and increase the oil consumption of the refrigerator during operation. It is not easy to return oil to the system. When the refrigerant and the refrigerating machine oil are completely unclear, it has little effect on the evaporation temperature of the refrigeration system, but it is easy to form an oil film on the heat exchange surface of the heat exchanger and affect the heat exchange. The advantages and disadvantages of refrigerants that are slightly soluble in oil are in between the above two.

(5) In semi-hermetic and fully enclosed refrigerators, the direct contact between the motor coil of the compressor and the refrigerant and chiller oil not only requires good electrical insulation of the refrigerant, but also requires the effect of the refrigerant on the coil insulation material as small as possible.

3. Security requirements

(1) The refrigerant should not burn or explode within the working temperature range. When certain flammable and explosive refrigerants must be used, there must be fire and explosion-proof safety measures.

(2) The refrigerant should be non-toxic or low-toxic and relatively safe. The toxicity, flammability, and explosiveness of refrigerants are all indicators for evaluating the safety of refrigerants, and all countries have stipulated minimum safety standards.

(3) Due to the certain toxicity and danger of some refrigerants, it is required that the selected refrigerants should have the characteristics of easy leakage to ensure safe operation.

(4) In case the leaked refrigerant comes into contact with food, it is required that the food will not change color, taste, be polluted or damage tissue. Refrigerants for air conditioners should have no damage to human health and no irritating odor.

4. Requirements for environmental impact

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

(1) The lifetime in the atmosphere is low.

(2) The potential damage effect on the ozone layer is small.

(3) The potential effect of global warming should be low.

(4) There is no light and fog reaction, and the impact on the atmosphere, water source and soil is small.

5. Economic requirements

(1) The production process of the refrigerant should be simple and the production cost should be low.

(2) Refrigerant should be cheap and easy to get.

Due to the wide variety of refrigerants and their very different properties, there is no ideal refrigerant that fully meets the requirements. When choosing, it should be considered comprehensively according to the actual situation.