Refrigerating oil use characteristics and performance requirements

Refrigerating oil use characteristics and performance requirements Refrigerating machine oil is mainly used to lubricate the parts that need to be lubricated in the freezing machine, and is closely related to the running performance and service life of the compressor, so its selection and application are very important. 1. Working characteristics of the freezer Refrigerators can be divided into three types according to their structure: piston type (or reciprocating type), rotary type (including screw type, rotor type and sliding plate type), and centrifugal type (or turbine type). The parts that need to be lubricated in piston refrigerators include pistons and cylinder walls, connecting rod bushes and piston pins or crank pins, main journals and front and rear bearing bushes, crankcases, shaft seals and valves. The parts of the screw refrigerator that need to be lubricated include the rotating meshing parts of the convex and concave screws, the sliding bearings before and after the screw, and the balancing piston of the active screw. The parts that need to be lubricated in the centrifugal refrigerator include speed-increasing gears, main bearings, shaft end sealing surfaces, etc. Due to the reciprocating movement of the piston, part of the lubricating oil may enter the cylinder and be brought into the system with high-speed gas. At the same time, due to the evaporation of the lubricating oil, it may also enter the system and mix with the refrigerant. It still enters the cooler and other equipment with the refrigerant. When the temperature decreases, the lubricating oil condenses, adhering to the cooler and the tube wall of the system to form an oil film, which reduces the heat transfer effect and increases the temperature of the refrigerator compartment. On the other hand, high-pressure refrigerant gas may also leak into the lubricating oil, diluting the lubricating oil. Second, the main function of refrigeration oil 1. Lubricate the friction surface so that the friction surface is completely separated by the oil film, thereby reducing friction work, friction heat and wear; 2. The flow of the refrigerating machine oil takes away the heat of friction and keeps the temperature of the friction parts within the allowable range; 3. The sealing part is filled with oil to ensure the sealing performance and prevent the leakage of refrigerant; 4. The movement of oil takes away the abrasive debris generated by metal friction, and plays the role of cleaning the friction surface; 5. Provide hydraulic power for the unloading mechanism. Third, the classification of refrigeration oil One is the traditional mineral oil; the other is the synthetic polyol ester oils such as POE (PolyolEster), often called polyester oil PAG (PolyalkyleneGlyco1) is also a synthetic poly (B) glycol lubricants, their Chinese name Not very uniform. POE oil can be used not only in HFC refrigerant systems, but also in hydrocarbon refrigerants. PAG oil can be used in HFC, hydrocarbon and ammonia as refrigerant systems. 4. Performance requirements of refrigerating machine oil 1. Appropriate viscosity In addition to ensuring that the friction surface of each moving part has good lubricity, the viscosity of the refrigerating machine oil must also take away some heat from the refrigerator and play a sealing role. If the refrigerant used in the refrigerator is relatively miscible with the refrigerating machine oil, it should be considered to use oil with higher viscosity to overcome the effect of the lubricant diluted by the refrigerant. 2. Low volatility and high flash point The greater the volatilization amount of the refrigerator oil, the more oil will circulate with the refrigerant. Therefore, the narrower the fraction range of the refrigerator oil, the better, and the flash point should be higher than the exhaust temperature of the refrigerator by 25-30 ℃. 3. Good chemical stability and thermal oxidation stability The final compression temperature in the operation of the refrigerator can reach 130 ℃ ~ 160 ℃, at this temperature, the refrigerator oil will be heated and continue to decompose and deteriorate, forming carbon deposits, which leads to the failure and wear of the refrigerator. On the other hand, the decomposition products of oil will react chemically with the refrigerant to deteriorate the refrigeration effect, and the generated acidic substances will strongly corrode the refrigerator parts. Therefore, the refrigerator oil is required to have good thermal oxidation stability, not coking and carbonization at the high temperature of the outlet valve, and the refrigerator oil is required to have good chemical stability to avoid interaction with refrigerants such as halogenated hydrocarbons (RCI, RF) This produces acidic corrosive substances. 4. No water and impurities Because water freezing in the evaporator will affect the heating efficiency, contact with the refrigerant will accelerate the decomposition of the refrigerant and corrode the equipment, so the refrigerator oil must not contain water and impurities. 5. Other The refrigerating machine oil should also have good anti-foam properties, and it should not dissolve or swell rubber, enameled wire and other materials. When used in a closed refrigerator, it should have good electrical insulation. V. Factors to be considered when choosing refrigeration oil 1. Viscosity The higher the speed of the compressor, the greater the viscosity of the refrigerating machine oil. In actual use, the low-speed vertical double-cylinder compressor can use L-DRAl5 refrigeration oil, and the medium-speed and high-speed multi-cylinder compressors should use L-DRA22. Or L-DRA32 refrigeration oil, some high-speed heavy-duty compressors have a large calorific value, high oil temperature and high temperature. 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 refers to the temperature at which the steam of the refrigerating machine oil flashes after a fire. The flash point of the refrigerating machine oil must be higher than the discharge temperature of the compressor. For example, the flash point of the refrigerating machine oil used in R717, R12, R22 compressors should be above 160 ℃. 3. Liquidity The refrigerating machine oil should have good fluidity at low temperature. In the evaporator, due to the low temperature and the increased viscosity of the oil, the fluidity becomes poor. When the temperature reaches a certain temperature, the refrigerating machine oil stops flowing. The temperature at this time is called the freezing point of the oil . The freezing oil of refrigerators is required to have a low freezing point, especially the freezing point of low-temperature refrigerators is very important. Otherwise, the fluidity is reduced, which affects both the heat transfer of the evaporator and the lubrication of the machine. The freezing point of various refrigeration oils is below -40 ℃, which can meet the needs of general-purpose refrigerators. When the evaporation temperature is lower, precision instrument oil can be used, and its freezing point is generally not higher than -60 ℃. 4. Solubility The miscibility of various refrigerants with refrigerating machine oil is roughly divided into three categories: one is insoluble in each other, the other is infinitely soluble in each other, and the other is in between. The pros and cons of solubility have already been discussed in the refrigerant section and will not be repeated here. 5. Cloud point The temperature at which the refrigerating machine oil begins to precipitate paraffin (the oil becomes turbid) is called the cloud point. When the refrigerant is present, the cloud point of the refrigerating machine oil will decrease. 6. In addition, fully enclosed and semi-enclosed refrigerators have certain requirements for the electrical breakdown voltage of the refrigerating machine oil, which is generally required to be above 25kV. Six, the main reason for the deterioration of refrigeration oil 1. Mixed with water Due to the infiltration of air into the refrigeration system, the moisture in the air comes into contact with the refrigerating machine oil and mixes in; when the refrigerant contains a large amount of water, the water will also be mixed into the refrigerating machine oil. When water is mixed into the refrigerating machine oil, the viscosity decreases, causing corrosion to the metal. In the Freon refrigeration system, it will also cause "ice plug" phenomenon; 2. Oxidation During the use of refrigerating machine oil, when the discharge temperature of the compressor is high, it may cause oxidative deterioration, especially the refrigerating machine oil with poor chemical stability, which is more prone to deterioration. The lubrication at other places is bad. Organic fillers, mechanical impurities, etc. mixed into the refrigerator oil will also accelerate its aging or oxidation; 3. Mixed use of refrigerating machine oil When several different brands of refrigeration oil are used in combination, the viscosity of the refrigeration oil will be reduced, and even the formation of the oil film will be damaged, and the bearing will be damaged; if the two refrigeration oils contain antioxidant additives of different properties, when mixed together , It is possible to produce chemical changes, the formation of precipitates, which will affect the lubrication of the compressor, so pay attention when using it. 4. There are impurities in the refrigerating machine oil. 7. Selection criteria of refrigerating machine oil There are many specifications of refrigeration oil. In order to ensure the normal operation of refrigeration compressors, the performance of refrigeration oil must be understood and selected correctly. The performance of refrigerating machine oil can be determined by many indicators. The main quality indicators are briefly introduced below. Viscosity, viscosity is one of the most important performance parameters of lubricating oils, which determines the bearing capacity, friction power consumption and sealing ability of the sealing surface of the sliding bearing. The size of viscosity is usually expressed in kinematic viscosity, the legal unit is m2 / s, and the commonly used units are s (st) and centimeters (cst). 1st = 10-4m2 / s = 25px2 / s, 1cst = 10-6m2 / s = 1mm2 / s. The lubricating oil viscosity of the refrigeration compressor must be appropriate, the viscosity is too large, the oil film carrying capacity is large, and it is easy to maintain liquid lubrication, but the flow resistance is large, the friction power consumption and starting resistance of the compressor will increase; The friction heat is small, but it is not easy to form a lubricating oil film, and it also affects the sealability. Since the discharge temperature of the refrigerant on the high-pressure side of the refrigeration compressor is high during operation, the viscosity of the lubricating oil should not be too small. Due to the low-temperature gas sucked by the low-pressure side, the viscosity of the lubricating oil should not be too large at this time. The viscosity of various lubricating oils decreases with varying degrees as the temperature increases. In the refrigeration compressor, lubricating oil whose viscosity changes as little as possible with temperature should be selected, that is, special refrigeration oil. The viscosity of lubricating oil is also related to the miscibility of the refrigerant in the lubricating oil. If ammonia and lubricating oil are incompatible, a lubricating oil of lower viscosity should be used. The refrigerating machine oil can be selected according to the following aspects: 1. Select lubricating oil according to compression type The compressor of the refrigerator has three types: piston type, screw type and centrifugal type. The first two types of lubricating oil are in direct contact with the compressed refrigerant, and the interaction between the lubricating oil and the refrigerant must be considered. The lubricating oil used in the centrifugal type is only used to lubricate the rotor bearings, and can also be selected according to the size of the load and speed. 2. Select lubricating oil according to the type of refrigerant Lubricating oils that are in direct contact with the refrigerant must consider the interaction between the two. Refrigerants such as Freon are soluble in mineral oil, so the viscosity grade of the lubricating oil used should be one higher than that of the insoluble refrigerant, to prevent the lubricating oil from being diluted and unable to guarantee lubrication. In addition, it should also be noted whether a small amount of lubricating oil mixed in the refrigerant will affect the operation of the refrigeration system. The flocculation point of the refrigerating machine oil is the quality index for checking whether the lubricating oil mixed in the refrigerant precipitates wax crystals and blocks the refrigeration system. 3. Select lubricating oil according to the evaporation temperature of the refrigerant In general, refrigerators with a low refrigerant evaporation temperature should use refrigerator oil with a low freezing point to prevent the lubricant carried by the refrigerant into the refrigeration system from condensing on the throttle valve and evaporator, which affects the refrigeration efficiency. For refrigerators using ammonia as the refrigerant, the freezing point of the lubricating oil used should be lower than the evaporation temperature. With Freon as the refrigerant, the freezing point of the lubricating oil can be slightly higher than the evaporation temperature. 4. Select lubricating oil according to the working conditions of the freezer

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