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1. What is refrigeration oil? The oil used to lubricate the various moving parts within a refrigeration compressor is called refrigeration oil, or lubricating oil. According to the standards set by the Petrochemical Industry Department, the refrigeration oils produced in China currently come in five grades: No. 13, No. 18, No. 25, No. 30, and Grade 40 based on enterprise standards. Among them, the commonly used lubricants for refrigeration compressors are types 13, 18, and 25. For R12 compressors, grade 18 is generally used, while for R22 compressors, grade 25 is generally used. 2. What are the functions of refrigeration oil? In compressors, refrigerant oil serves four main functions: lubrication, sealing, cooling, and energy regulation. (1) Lubrication: Refrigerant oil serves a lubricating function during the operation of the compressor, reducing friction and wear as the compressor runs, thereby extending its service life. (2) Sealing: Refrigeration oil serves a sealing function in the compressor, ensuring a seal between the piston and the cylinder wall inside the compressor, as well as between various rotating bearings, thereby preventing the leakage of refrigerant. (3) Cooling: When lubricating the various moving parts of the compressor, the refrigeration oil can carry away the heat generated during operation, keeping these parts at a lower temperature and thereby improving the efficiency of the compressor as well as its reliability in use. (4) Energy regulation: For refrigeration compressors equipped with an energy regulation mechanism, the oil pressure of the refrigerant oil can be used as power for the energy regulation mechanism. 3. What are POE and PAG refrigeration oils? To protect the ozone layer, international regulations have been imposed on the refrigerants used in air conditioning equipment, leading to the emergence of various alternative refrigerants; accordingly, the refrigeration oils have also changed. For air conditioners, the refrigerants replacing R134a and R410a/R407c are R134a and R410a/R407c respectively, with PAG and POE being used for their replacement. POE is short for Polyol Ester, also known as polyester oil; it is a type of synthetic polyol ester oil. PAG is an abbreviation for Polyalkylene Glycol, and it is a synthetic poly(ethylene) glycol-based lubricant. Among them, POE oil can be effectively used not only in HFC-based refrigeration systems but also in hydrocarbon refrigeration. PAG oil can be used as a lubricating oil in refrigeration systems that use HFCs, hydrocarbons, and ammonia as refrigerants. 4. What are the requirements for refrigeration oil in refrigeration equipment? Due to differences in application scenarios and refrigerants, refrigeration equipment requires different choices of refrigeration oils. The requirements for refrigeration oil include the following aspects: (1) Viscosity. Viscosity is an important parameter among the characteristics of refrigeration oil; different refrigerants require the use of corresponding refrigeration oils. If the viscosity of the refrigeration oil is too high, it will increase mechanical friction power, friction heat, and starting torque. Conversely, if the viscosity is too low, a proper oil film cannot be formed between the moving parts, thereby preventing the desired lubrication and cooling effects from being achieved. (2) Cloud point: The cloud point of refrigeration oil refers to the temperature at which, as the temperature drops to a certain value, paraffin begins to precipitate out of the oil, causing the lubricant to become cloudy. The cloud point of the refrigerant oil used in refrigeration equipment should be lower than the evaporation temperature of the refrigerant; otherwise, it may cause blockage of the throttle valve or affect heat transfer performance. (3) Freezing point: The temperature at which the refrigeration oil stops flowing when cooled under experimental conditions is called the freezing point. The freezing point of the refrigerant oil used in cooling equipment should be as low as possible (for example, in R22 compressors, the refrigerant oil should have a freezing point below -55°C); otherwise, it will affect the flow of the refrigerant, increase flow resistance, and as a result lead to poor heat transfer. (4) Flash point: The flash point of refrigeration oil refers to the highest temperature at which the lubricant, when heated, causes ignition when its vapor comes into contact with a flame. The flash point of the refrigerant oil used in cooling equipment must be 15–30°C or more higher than the exhaust temperature, to prevent the lubricating oil from burning and coking. (5) Others such as chemical stability and oxidation resistance, moisture and mechanical impurities, as well as insulation properties.