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This post was last edited by tiandiyi*an on 2017-12-1 at 13:35. Refrigerants, also known as cooling media, are commonly referred to as \"snow agents\" in some areas of the south. It is a working substance that circulates continuously within the refrigeration system and achieves cooling through changes in its own state. The refrigerant absorbs heat from the medium to be cooled (such as water or air) in the evaporator, thereby vaporizing; it then releases that heat to the surrounding air or water in the condenser, where it condenses. Today, the editor from Jiuqi Refrigeration will take you on a detailed exploration of the classification of refrigerants and the criteria for selecting them. I. Classification of Refrigerants Refrigerants can be divided into three categories based on their condensation pressure at normal temperatures and their evaporation temperature at atmospheric pressure: 1. Low-pressure, high-temperature refrigerants The evaporation temperature is higher than 0°C, and the condensation pressure is lower than 29.41995×104 Pa. 2. Medium-pressure, medium-temperature refrigerant: Evaporation temperature of -50~0°C, condensation pressure of (196.113~29.41995)×10^4 Pa. 3. High-pressure, low-temperature refrigerants: The evaporation temperature is below -50°C, and the condensation pressure is above 196.133×104 Pa. II. Requirements for selecting a refrigerant 1. The operating temperature and pressure of the refrigerant should be moderate. At atmospheric pressure, the evaporation temperature of the refrigerant must be low enough to meet the cooling requirements ； At room temperature, the refrigerant should have a relatively low condensing pressure; as the condensing pressure increases, higher requirements are placed on the sealing performance and structural strength of the refrigeration system. The required condensing pressure of the refrigerant is generally: 12×105~15×105 Pa ； At room temperature, the refrigerant must have a relatively high evaporation pressure; otherwise, if the pressure inside the evaporator is lower than atmospheric pressure, outside air can easily enter the refrigeration system through gaps, causing the pressure within the system to rise, reducing the cooling capacity and increasing power consumption. At the same time, moisture in the air can cause ice buildup in the refrigeration system and other adverse effects. 2. The refrigerant should have a high cooling capacity per unit volume. For refrigeration equipment of the same specification, choosing a refrigerant with a high cooling capacity per unit volume enables higher cooling performance. Under the same operating conditions, when the cooling capacity remains constant, a higher cooling capacity per unit volume of refrigerant allows for a reduction in the volume of refrigerant required in the system, which in turn enables the compressor to be smaller in size. 3. The refrigerant should have a high critical temperature and a low freezing point; a high critical temperature facilitates the condensation of the refrigerant into a liquid at ambient temperatures ； It has a low freezing point, allowing for the achievement of lower temperatures; this expands the range of operating temperatures for refrigerants, reduces throttling losses, and improves the coefficient of performance. 4. The viscosity and density of the refrigerant should be as low as possible. Lower viscosity and density result in less flow resistance in the refrigerant circulation within the system, thereby reducing the energy required for circulation. It is also possible to reduce the diameter of the pipes, and allowing for smaller bending radii in the pipelines (which is very important for reducing pressure losses in the evaporator). This helps to lessen the impact forces on the valve assemblies in the compressor, thus extending its service life. 5. The thermal conductivity and heat release coefficient of the refrigerant should be high. Higher thermal conductivity and heat release coefficients allow for a reduction in the size of the heat exchangers in the refrigeration system, thereby improving the efficiency of heat exchange in those exchangers. 6. Other requirements for refrigerants include being non-flammable, non-explosive, non-toxic, non-corrosive, having an affordable price, and being easily available for purchase. What are the requirements for selecting refrigerants for cold storage? The properties of the refrigerant directly affect the type, structure, size, and operating characteristics of the refrigerator. They also influence the design of the refrigeration cycle, the structure of the equipment, as well as its economic and technical performance. Therefore, selecting an appropriate refrigerant is a very important matter. The performance requirements for refrigerants are typically considered from aspects such as thermodynamics, physicochemistry, safety, environmental impact, and economics. I. Thermodynamic requirements 1. The boiling point should be low, so as to achieve a lower evaporation temperature; at the same time, refrigerants with a low boiling point have a higher vapor pressure. 2. The critical temperature should be high, and the solidification temperature should be low. The critical temperature should be high and the freezing temperature low, to ensure that the refrigerant can operate safely over a wide temperature range. 3. The refrigerant must have an appropriate operating pressure; its evaporation pressure should be close to or slightly higher than atmospheric pressure, in order to prevent a vacuum from forming in the low-pressure areas of the refrigeration system, which could increase the chances of air entering the system. The condensation pressure must not be too high. A low condensing pressure can reduce the strength requirements and construction needs for refrigeration equipment and pipelines, thereby lowering the investment required for building the refrigeration system and minimizing the risk of refrigerant leakage. It is required that the pressure ratio and pressure difference between the condensation pressure and the evaporation pressure be small. 4. The latent heat of vaporization of the refrigerant should be high; when a refrigeration system achieves the same cooling capacity, a higher latent heat of vaporization allows for a reduced circulation volume of the refrigerant. It can also reduce investment in refrigerators and equipment, lower operating energy consumption, and improve cooling efficiency. 5. For large refrigeration systems, it is necessary to maximize the cooling capacity per unit volume of the refrigerant; this allows, for a given amount of cooling required, to reduce the volume of refrigerant that needs to circulate, thereby minimizing the size of the refrigeration machine and the diameter of the pipes. However, for small refrigeration systems, a low cooling capacity per unit volume is required, which allows the cross-sectional area of the refrigerant channels to be increased appropriately, thereby reducing flow resistance. 6. The adiabatic index of the refrigerant should be low, as this reduces the work required for compression, lowers the exhaust temperature, improves operational performance, and simplifies system design. 7. For centrifugal refrigeration compressors, refrigerants with a moderate molecular weight should be used, as a high molecular weight can increase the pressure rise per stage; when the system’s pressure ratio remains constant, this allows for a reduction in the number of compression stages. Furthermore, most substances have similar molar enthalpies of vaporization at their boiling points. Therefore, for refrigerants with similar boiling points, a larger relative molecular mass results in a smaller latent heat of vaporization. 8. The thermal conductivity should be high, as this can increase the heat transfer coefficient of heat exchange equipment and reduce its heat exchange area. II. Requirements in physical chemistry 1. The viscosity of the refrigerant should be low; a low viscosity reduces the flow resistance of the refrigerant within the system, and smaller diameters for the pipes in the refrigeration system lead to reduced consumption. A low viscosity can also improve the heat transfer performance of the refrigerant. 2. The purity of the refrigerant must be high; the selected refrigerant should be free from any insoluble impurities. It should also have a certain degree of water absorption, as even trace amounts of water in the refrigerant can lead to ice formation at low temperatures, thereby disrupting the proper operation of the refrigeration system. 3. The refrigerant should have good thermochemical stability and should not decompose easily at high temperatures. When mixed with oil and water, it should not cause significant corrosion to metal materials, and its swelling effect on the sealing materials of the refrigeration unit should also be as minimal as possible. 4. The oil solubility of refrigerants is categorized as complete dissolution, slight dissolution, and no dissolution at all. When the refrigerant and refrigeration oil are completely dissolved, it creates favorable conditions for lubricating the components; an oil film does not easily form on the heat exchange surfaces of heat exchangers such as condensers, resulting in excellent heat transfer efficiency. However, it raises the evaporation temperature of the refrigerant; the viscosity of the refrigeration oil decreases at low temperatures. It also increases foam formation when the refrigerant boils, leads to instability in the liquid level within the evaporator, increases the fuel consumption of the compressor during operation, and makes it difficult for the oil to return to the system. When the refrigerant and the refrigeration oil are not completely separated, the impact on the evaporation temperature of the refrigeration system is minimal; however, an oil film tends to form on the heat exchange surfaces, thereby affecting heat transfer. The advantages and disadvantages of refrigerants that are slightly soluble in oil lie between those of the two mentioned above. 5. The effect of the refrigerant on the insulation material of the coils should be minimized. In semi-hermetic and hermetic refrigerators, since the motor coils of the compressor are in direct contact with the refrigerant and refrigeration oil, it is necessary not only for the refrigerant to have good electrical insulation properties but also for its effect on the coil insulation material to be as minimal as possible. III. Requirements regarding safety 1. The refrigerant must not be flammable or explosive within the operating temperature range. When using certain flammable or explosive refrigerants, fire and explosion prevention safety measures must be in place. 2. The refrigerant should be non-toxic or low in toxicity, offering good relative safety. The toxicity, flammability, and explosiveness of refrigerants are all indicators used to assess their safety level, and various countries have established standards for minimum safety requirements. 3. Choose a refrigerant that is easy to detect leaks. Since some refrigerants are toxic and hazardous, it is necessary to select a refrigerant that allows for easy detection of leaks, in order to ensure safe operation. 4. Choose a refrigerant that is harmless to human health and has no irritating odor. In the event that the refrigerant leaks and comes into contact with food, it is necessary to ensure that the food does not change color or taste, and that its tissues are not contaminated or damaged. Refrigerants for air conditioning should be harmless to human health and have no irritating odor. IV. Requirements regarding environmental impact Recent studies have shown that certain chemicals may have an impact on the global environment, which has drawn the attention of the international community. The selected refrigerant should meet the following requirements: (1) It should have a short lifespan in the atmosphere. (2) The potential destructive effect on the ozone layer should be low. (3) The potential global greenhouse effect should be low. (4) There is no haze formation, resulting in less impact on the atmosphere, water sources, and soil. V. Requirements regarding economy (1) The production process of the refrigerant should be simple, and the production costs should be low. (2) The refrigerant should be inexpensive and readily available. Due to the wide variety of refrigerants and the significant differences in their properties, there is no ideal refrigerant that meets all requirements. When making a choice, it is necessary to take all relevant factors into consideration.