Refrigerant oil

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Refrigerant oil is primarily used to lubricate the parts in a refrigerator that require lubrication. It has a close relationship with the operating performance and service life of the compressor; therefore, its selection and application are extremely important. 1. Main functions of refrigeration oil: (1) Lubricate the friction surfaces, keeping them completely separated by an oil film, thereby reducing frictional work, frictional heat, and wear ； (2) The flow of the refrigerant oil carries away the friction heat, keeping the temperature of the friction parts within the allowable range ； (3) Fill the sealing area with oil to ensure good sealing performance and prevent refrigerant leakage ； (4) The movement of oil carries away the wear debris generated by metal friction, thus cleaning the friction surfaces ； (5) Provide hydraulic power for the unloading mechanism. 2. The classifications of refrigeration oils include traditional mineral oils on one hand, and synthetic polyol ester oils such as POE (Polyol Ester), which are often referred to as polyester oils. PAG (Polyalkylene Glycol) is also a synthetic poly(ethylene) glycol-based lubricant; the Chinese names for these substances are not always consistent. POE oil can be effectively used not only in HFC-based refrigerant systems but also in hydrocarbon refrigerants. PAG oil can be used in systems where HFCs, hydrocarbons, and ammonia are used as refrigerants. 3. Factors to consider when selecting refrigeration oil: (1) Viscosity. The higher the speed of the compressor, the greater the viscosity of the refrigeration oil required. In practice, low-speed vertical two-cylinder compressors can use L-DRAl5 refrigeration oil, while medium-speed and high-speed multi-cylinder compressors should use L-DRA22 or L-DRA32 refrigeration oil. Some high-speed, high-load compressors generate a lot of heat, resulting in high oil temperatures and high ambient temperatures; in such cases, it is best to use L-DRA46 or L-DJRA68 refrigeration oil. (2) Thermal stability: Thermal stability is generally measured by the flash point of refrigeration oil. The flash point is the temperature at which the vapor of refrigerant oil catches fire when exposed to heat. The flash point of the refrigerant oil must be higher than the exhaust temperature of the compressor; for example, the flash point of the refrigerant oil used in R717, R12, and R22 compressors should be above 160°C. (3) Liquid refrigeration oil should have good fluidity at low temperatures. In the evaporator, as the temperature drops and the viscosity of the oil increases, its fluidity declines; when a certain temperature is reached, the refrigeration oil stops flowing, and this temperature is known as the oil’s freezing point. Refrigerator refrigeration oils are required to have a low freezing point; this is particularly important for low-temperature refrigerators. Otherwise, liquidity decreases, which affects both the heat transfer in the evaporator and the lubrication of the machine. The freezing points of various refrigeration oils are all below -40°C, meeting the requirements of refrigerators for general purposes. At even lower evaporation temperatures, precision instrument oils can be used, whose freezing point is generally not higher than -60°C. (4) The solubility of various refrigerants in refrigeration oils varies; they can be roughly divided into three categories: those that are immiscible with each other, those that are infinitely soluble in one another, and those that fall somewhere in between. The advantages and disadvantages regarding solubility have been discussed in the section on refrigerants, so they will not be repeated here. (5) The temperature at which paraffin begins to precipitate out of the sludge-point refrigerant oil (causing the oil to become cloudy) is called the sludge point; the sludge point of the refrigerant oil decreases in the presence of a refrigerant. (6) In addition, fully enclosed and semi-enclosed refrigerators have certain requirements regarding the electrical breakdown voltage of the refrigeration oil, which is generally required to be above 25 kV. 4. The main reasons for the deterioration of refrigeration oil are (1) the presence of water; as air gets into the refrigeration system, the water contained in that air mixes with the refrigeration oil ； When there is a high amount of water in the refrigerant, it can also cause water to mix into the refrigeration oil. When moisture enters the refrigeration oil, its viscosity decreases, causing corrosion to metals. In Freon refrigeration systems, it can also cause the \"ice plug\" phenomenon ； (2) Oxidation: During use, refrigerant oils can undergo oxidation and deterioration when the exhaust temperature of the compressor is high. This is especially true for refrigerant oils with poor chemical stability, which are more prone to degradation. Over time, residues form within the oil, which impairs the lubrication in areas such as bearings. The inclusion of organic fillers, mechanical impurities, etc. in refrigeration oil can also accelerate its aging or oxidation ； (3) Mixing of refrigeration oils: When several different grades of refrigeration oils are used together, it can lead to a decrease in the viscosity of the refrigeration oil; in some cases, it may even prevent the formation of an oil film, thereby damaging the bearings ； If the two types of refrigeration oil contain antioxidant additives with different properties, mixing them together may lead to chemical reactions that result in the formation of precipitates, thereby affecting the lubrication of the compressor; therefore, care must be taken when using them. (4) There are impurities in the refrigeration oil. 5. Selection criteria for refrigeration oil. There are many different specifications and types of refrigeration oil. To ensure the proper operation of refrigeration compressors, it is necessary to understand the properties of these oils and be able to select them appropriately. The performance of refrigerant oil can be determined by many indicators; the following briefly introduces its main quality parameters. Viscosity is one of the most important performance parameters of lubricating oils; it determines the load-carrying capacity of the oil film in sliding bearings, the frictional power loss, and the sealing ability of the sealing surfaces. The value of viscosity is usually expressed in terms of dynamic viscosity, with the legal unit being m2/s; common units are st and cst. 1st = 10^-4 m2/s = 1 cm2/s, 1 cst = 10^-6 m2/s = 1 mm2/s. The viscosity of the lubricating oil in a refrigeration compressor must be appropriate. If the viscosity is too high, the oil film has a greater load-bearing capacity and liquid lubrication is easier to maintain, but the flow resistance increases, resulting in higher frictional power consumption and starting resistance in the compressor ； Too low viscosity results in low flow resistance and less frictional heat, but it makes it difficult to form a lubricating film, which also affects sealing performance. Due to the high exhaust temperature of the refrigerant on the high-pressure side during operation of the refrigeration compressor, it is desirable that the viscosity of the lubricating oil not be too low. Also, due to the low-temperature gas drawn in on the low-pressure side, the viscosity of the lubricating oil should not be too high at this time. The viscosity of various lubricants decreases to varying degrees as the temperature rises. In refrigeration compressors, it is necessary to use lubricating oil whose viscosity changes as little as possible with temperature, that is, specialized refrigeration oil. The viscosity of the lubricating oil is also related to the miscibility of the refrigerant in the lubricating oil. If ammonia and lubricant are immiscible, a lubricant with lower viscosity should be selected.