Refrigerant classification and selection requirements

Refrigerant classification and selection requirements

Refrigerants are also called refrigeration working fluids, commonly known as refrigerants in some areas in the south. It is a working substance that continuously circulates in the refrigeration system and realizes refrigeration through its own state changes. The refrigerant absorbs the heat of the cooling medium (water or air, etc.) in the evaporator to vaporize, and transfers the heat to the surrounding air or water in the condenser to condense. Today, okmarts.com editor takes you to learn more about the classification and selection requirements of refrigerants.

　　 1. Classification of refrigerant

　　 Refrigerants can be divided into three categories according to the condensation pressure at normal temperature and the evaporation temperature at atmospheric pressure:

　　1, low pressure and high temperature refrigerant

　　 The evaporation temperature is higher than 0℃, and the condensing pressure is lower than 29.41995×104Pa.

　　2, medium pressure and medium temperature refrigerant

　　 Evaporation temperature -50~0℃, condensation pressure (196.113~29.41995)×104Pa.

　　3, high pressure low temperature refrigerant

　　 The evaporation temperature is lower than -50°C, and the condensation pressure is higher than 196.133×104Pa.

　　 2. Requirements for the selection of refrigerant

　　 1. The working temperature and working pressure of the refrigerant should be moderate.

　　Under atmospheric pressure, the evaporation temperature of the refrigerant should be low enough to meet the cooling temperature requirements;

　　 At room temperature, the refrigerant must have a relatively low condensing pressure. When the condensing pressure is too high, the structural strength of the sealing performance of the refrigeration system is high. Generally, the condensing pressure of the refrigerant is required to be: 12×105~15×105Pa;

At normal temperature, the refrigerant must have a relatively high evaporation pressure, because if the pressure in the evaporator is lower than the atmospheric pressure, the outside air can easily enter the refrigeration system through the gap, increasing the pressure in the system and reducing the cooling capacity. Increase power consumption. At the same time, the moisture in the air will cause ice blockage and other adverse effects in the refrigeration system.

　　2, the refrigerant must have a relatively large unit volume of cooling capacity

　　 The refrigeration equipment of the same specification can obtain a larger refrigeration capacity when the selected refrigerant has a large refrigeration capacity per unit volume.

　　Under the same working conditions, when the cooling capacity is constant, the unit volume of the refrigerant has a large cooling capacity, which can reduce the system's refrigerant volume and reduce the size of the compressor accordingly.

　　3. The critical temperature of the refrigerant should be high and the freezing point should be low

　　 The high critical temperature makes it easy for the refrigerant to condense at ambient temperature as a liquid; the low freezing point can make a lower temperature, expand the use temperature range of the refrigerant, reduce throttling loss, and improve the coefficient of refrigeration.

　　4. The viscosity and density of the refrigerant should be as small as possible

The low viscosity and density can make the flow resistance of the refrigerant cycle in the system small, reduce the cycle power consumption, appropriately reduce the pipe diameter, and allow the pipe to have a smaller bending radius (and this is useful for reducing the pressure loss of the evaporator Is very important), it can also reduce the impact of the refrigerator on the valve group in the compressor and extend the service life of the compressor.

　　5. The thermal conductivity and heat release coefficient of the refrigerant should be high

　　 high thermal conductivity and heat release coefficient, can appropriately reduce the structure of the heat exchanger in the refrigeration system, and can improve the heat exchange efficiency of the heat exchanger.

　　 6. Other requirements for refrigerant

　　 non-flammable, non-explosive, non-toxic, non-corrosive, affordable, easy to buy, etc.

　　What are the requirements for the selection of 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, the rational 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 solidification temperature should be low.

　　 The critical temperature should be high and the freezing temperature should be low to ensure the safe operation of the refrigerant 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 production, the large 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-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 circulation 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 weight 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,

　　 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 the consumption. The low viscosity can also increase the heat transfer performance of the refrigerant.

　　2, the purity of the refrigerant should be high

　　The selected refrigerant should be free of insoluble impurities. The refrigerant is required to 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 normal operation of the refrigeration system.

　　3. The thermochemical stability of the refrigerant is better, and it is not easy to decompose under high temperature

　　 When the refrigerant is mixed with oil and water, there should be no 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 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 condenser and other heat exchangers, 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. The effect of refrigerant on the coil insulation material is as small as possible

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 the refrigeration to have good electrical insulation, but also requires the refrigerant to have as little effect on the coil insulation material 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 z* low safety standards.

　　 3. Choose a refrigerant that is easy to detect leaks

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

　　4. Choose a refrigerant that does not harm human health and has no pungent odor

　　 In case the leaked refrigerant comes into contact with food, it is required that the food will not change color, taste, or be contaminated and 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 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 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, their properties vary greatly, and there is no ideal refrigerant that fully meets the requirements. The selection should be considered comprehensively according to the actual situation.