The performance of the refrigerant will directly affect the type, structure, size and operating characteristics of the refrigerator, as well as the form of the refrigeration cycle, equipment structure and economic and technical performance. Therefore, reasonable selection of refrigerants is a very important issue. The performance requirements of refrigerants are usually considered from the aspects of thermodynamics, physical chemistry, safety, environmental impact and economics.
1. Thermodynamic requirements
1. Lower boiling point leads to lower evaporation temperature. At the same time, refrigerants with low boiling points have higher vapor pressures.
2. Higher critical temperature and lower solidification temperature. The critical temperature must be high and the solidification temperature must be low to ensure the safe operation of the refrigerant within a wide temperature range.
3. The refrigerant should have a suitable working pressure, and 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 mixing. 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, reducing the construction investment of the 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 the refrigerant is large. When the refrigeration system obtains the same cooling capacity, the latent heat of vaporization is larger, which can reduce the amount of refrigerant circulation. At the same time, it can reduce investment in 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 volume can be reduced, and the volume of the refrigerator and the diameter of the pipe can be reduced. However, for small refrigeration systems, which require a small cooling capacity per unit volume, the refrigerant channel cross-section can be appropriately increased to reduce flow resistance.
6. The adiabatic index of the refrigerant should be small, which can reduce compression power consumption, lower exhaust temperature, improve operating performance, and simplify system design.
2. Physical and chemical requirements
1. The viscosity of the refrigerant should be small. The viscosity of the refrigerant should be small to reduce the flow resistance of the refrigerant in the system and reduce the diameter consumption of the refrigeration system pipes. Low viscosity also increases the refrigerant's heat transfer properties.
2. The refrigerant should be of high purity. The refrigerant selected should not contain insoluble impurities, and the refrigerant is required to have a certain degree of water absorption. When very little water penetrates into the refrigerant, ice jams will occur at low temperatures, affecting the normal operation of the refrigeration system.
3. The refrigerant has good thermochemical stability and is not easy to decompose at high temperatures. When the refrigerant is mixed with oil and water, it should not cause obvious corrosive effects on metal materials, and the swelling effect on the refrigerator sealing materials should be as small as possible.
4. The oil solubility of refrigerants has the characteristics of completely soluble, slightly soluble and completely insoluble. When the refrigerant and refrigeration oil are completely degraded, good conditions can be created for the lubrication of engine parts. The heat exchange surface of heat exchangers such as condensers is not easy to form oil film and has excellent heat transfer effect. However, the evaporation temperature of the refrigerant increases, the low-temperature refrigeration oil decreases, the foam increases when the refrigerant boils, the evaporator liquid level is unstable, and the fuel consumption increases when the refrigerator is running. Therefore, it is difficult to return oil to the system. When the refrigerant and refrigeration oil are completely unclear, the effect on the evaporation temperature of the refrigeration system is small, but an oil film is easily formed on the heat exchange surface of the heat exchanger, which affects heat transfer. The advantages and disadvantages of slightly oil-soluble refrigerants lie somewhere in between.
3. Safety requirements
1. The refrigerant should not burn or explode within the operating temperature range. When certain flammable and explosive refrigerants must be used, fire and explosion-proof safety measures must be taken.
2. The refrigerant should be non-toxic or low-toxic and have good relative safety. The toxicity, flammability, and explosiveness of refrigerants are all indicators for evaluating the safety of refrigerants, and all countries have established minimum safety standards.
3. Choose a refrigerant that is easy to detect. Since some refrigerants are toxic and dangerous, it is required that the selected refrigerant should have the characteristics of easy detection of leaks to ensure safe operation.
4. Choose refrigerants that are harmless to human health and have no irritating odor. If the leaked refrigerant comes into contact with food, the food should not change color, taste, contamination or damage. The refrigerants used in air conditioners should be harmless to human health and have no irritating odor.
4.environmental impact requirements
The latest research has discovered that certain chemicals 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) Life in the atmosphere is low.
(2) The potential damage to the ozone layer is small.
(3) The potential global greenhouse effect is low.
(4) The non-light haze reaction has little impact on the atmosphere, water sources and soil.
5. Economic requirements
1. The production process of refrigerant should be simple and the production cost should be low.
2. Refrigerant is cheap and easy to obtain.
Due to the wide variety of refrigerants with widely varying performance, no ideal refrigerant can fully meet the requirements. When choosing, comprehensive considerations should be made based on the actual situation.