Selection requirements for cold storage refrigerants

Selection requirements for cold storage refrigerants

The nature of the refrigerant will directly affect the type, structure, size and operating characteristics of the refrigerator, as well as the form, equipment structure, and economic and technical performance of the refrigeration cycle. Therefore, a reasonable choice of refrigerant is a very important issue. Generally, the performance requirements of refrigerants are considered from the aspects of thermodynamics, physical chemistry, safety, environmental impact and economy.

　　 1. Thermodynamic requirements

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

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

　　 (3) The refrigerant must have an appropriate working pressure. The evaporation pressure of the refrigerant is required to be close to or slightly higher than the atmospheric pressure to avoid vacuum in the low pressure part of the refrigeration system and increase the chance of air being mixed into the system. The condensing pressure must 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. The pressure ratio and pressure difference between the condensing pressure and the evaporation pressure are required to be small.

　　(4) The latent heat of vaporization of the refrigerant is larger. When the refrigeration system gets the same cold output, the latent heat of vaporization can reduce the circulation of refrigerant. At the same time, it can also reduce the investment of refrigerators and equipment, reduce operating energy consumption, and improve cooling efficiency.

　　(5) For large 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 amount of refrigerant can be reduced, and the size of the refrigerator and the diameter of the pipe can be reduced. However, for small refrigeration systems, the cooling capacity per unit volume is required to be small, so that the channel cross-section of the refrigerant can be appropriately increased to reduce the flow resistance.

　　(6) The adiabatic index of the refrigerant should be small. This can reduce compression power consumption, lower exhaust temperature, improve operating performance and simplify system design.

　　(7) For centrifugal refrigeration compressors, refrigerants with moderate molecular mass should be used. Because the relative molecular mass is large, the boost ratio of each stage can be increased. When the pressure ratio of the system is constant, the number of compression stages can be reduced. In addition, when most substances vaporize at the boiling point, their molar increase is similar. Therefore, when the relative molecular mass of refrigerants with similar difficult boiling points is large, the latent heat of vaporization is small.

　　(8) The thermal conductivity should be high. This can increase 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, reducing the flow resistance of the refrigerant in the system, and reducing the pipe diameter of the refrigeration system is a 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 be free of insoluble impurities. The refrigerant must have a certain degree of water absorption. When very little water penetrates into the refrigerant, it will cause ice plugs at low temperatures and affect the refrigeration system. Works normally.

　　(3) The thermochemical stability of 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 dissolved, slightly dissolved and completely insoluble. When the refrigerant and the refrigerating machine oil are completely disintegrated, it can create good conditions for the lubrication of the mechanical parts. It is not easy to form an oil film on the heat exchange surface of the heat exchanger such as the condenser, and the heat transfer effect is excellent. However, the evaporation temperature of the refrigerant will increase, the refrigerating machine oil at low temperatures will decrease, and the foam will increase when the refrigerant is boiling, the liquid level in the evaporator will be unstable, and the oil consumption of the refrigerator will increase during operation. The oil return of the system is not easy. When the refrigerant and the refrigerating machine oil are completely unclear, the effect on the evaporation temperature of the refrigeration system is small, but an oil film is easy to form on the heat exchange surface of the heat exchanger, which affects the heat exchange. The advantages and disadvantages of refrigerants that are slightly soluble in oil are between the above two.

(5) In semi-enclosed and fully enclosed refrigerators, the direct contact of the motor coil of the compressor with the refrigerant and the refrigerating machine oil not only requires good electrical insulation for the refrigeration punishment, but also requires the role of the refrigerant on the coil insulation material As small as possible.

　　 Three, safety requirements

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

　　(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) Because some refrigerants are toxic and dangerous, it is required that the selected refrigerant should be easy to pick up and leak 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, or be contaminated or damage tissues. Refrigerants for air conditioners should not harm human health and have no irritating odor.

　　 Fourth, environmental impact requirements

　　 The latest research has found that certain 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 global greenhouse effect is low.

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

　　 Five, economic requirements

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

　　 (2) Refrigerants are cheap and easily available.

　　 Due to the wide variety of refrigerants and their properties vary greatly, there is no ideal refrigerant that fully meets the requirements. The selection should be considered comprehensively according to the actual situation.