Industrial Lubrication｜Refrigeration Oil Performance and Testing
February 23, 2022
Refrigeration oil is a special lubricating oil for refrigeration compressors. Like general lubricating oil, it plays the role of lubrication, sealing, rust prevention and heat removal in refrigeration compressors. important component. High-quality refrigeration oil must not only have excellent thermochemical stability and compatibility with refrigerants, excellent low-temperature fluidity, lubricity, and anti-foaming properties, but also must be easy to produce, reliable source of raw materials, and no pollution to the environment. . Because the refrigeration oil works in the special environment of the refrigerant, it has the following characteristics: 1. Refrigerator oil and refrigerant are in direct contact in the refrigeration compression system 2. A small amount of refrigeration oil is carried into the refrigerant pipeline to participate in the refrigeration cycle 3. In a hermetic compressor, the refrigerating machine oil is in close contact with organic materials such as the coils and seals of the motor 4. Refrigeration oil is under extreme temperature conditions such as high temperature of compressor discharge valve and low temperature of expansion valve and evaporator Each of these characteristics will be described below.
01. Viscosity and viscosity temperature characteristics Viscosity refers to the viscosity of oil, which is one of the important characteristics of refrigeration oil. It is also one of the basis for the classification of refrigeration oil. The commonly used viscosity of refrigeration oil is kinematic viscosity. Appropriate refrigeration oil is an important factor to ensure that refrigeration compressors are in a state of hydrodynamic lubrication, reduce wear and reduce energy consumption. If the viscosity is too low, a suitable oil film thickness cannot be formed, which will lead to mechanical wear of the refrigeration system components and mechanical failures such as cylinder pulling and shaft holding, and will also affect the sealing performance of the system. If the viscosity is too high, the power consumption of the compressor will be too large. Therefore, different refrigeration equipments use different viscosity grades of oil. Under normal circumstances, refrigeration tends to use low-viscosity refrigeration oil, and air conditioners tend to use high-viscosity refrigeration oil. In addition, due to the wide use temperature of refrigeration oil in the refrigeration cycle system, the cylinder of some compressors can sometimes be exposed to a high temperature of 150 ° C, while in the evaporation system, it may be exposed to a temperature of -40 ° C or lower, so It is required that the viscosity of the oil varies little with temperature to ensure that the refrigeration oil has good lubricity and fluidity at various temperatures. 02. Thermochemical stability The thermochemical stability of refrigerating machine oil and refrigerant in the coexistence determines its service life. Under the catalysis of high temperature and metal, the refrigerating machine oil can chemically react with the refrigerant (Freon, methyl chloride, etc.) to generate corrosive acid, sludge and other products, which will cause the blockage of the refrigeration system, affect the refrigeration efficiency, corrode the metal, and destroy the insulation. The material caused the motor to burn out.
03. Thermal stability Refrigeration oil will encounter high temperature in the refrigeration compressor, especially near the discharge port of the compressor valve plate, and the temperature is sometimes as high as 160 ℃. The thermally stable oil will decompose here to produce carbon deposits and other decomposition products, thereby blocking the movement of the valve plate and reducing the cooling effect. The thermal stability of refrigeration oil is related to factors such as the type of oil and the structure of the device. Generally speaking, paraffin-based and alkylbenzene refrigeration oils have high thermal stability, while naphthenic-based refrigeration oils have poor thermal stability. The generation of carbon deposits is also related to the flash point and volatility of the oil. Oils with high volatility and low flash point tend to volatilize and become viscous at the high temperature valve plate to form carbon deposits. Generally speaking, the flash point of the refrigeration oil should be about 20-30℃ higher than the maximum discharge temperature of the refrigeration compressor.
04. Low temperature performance Different refrigerants have different solubility characteristics in refrigeration oil. Due to the different solubility of different refrigerants and oils, the conditions that occur in the low temperature region of the refrigeration cycle system are different, so the requirements for the low temperature performance of the refrigerating machine oil will vary with the change of the refrigerant used in the refrigerating machine. It is described as follows: 1. Low temperature fluidity - refrigerants that are insoluble in oil From the outlet of the evaporator of the refrigeration cycle system to the inlet of the compressor, a series of problems easily occur due to the poor low-temperature fluidity of the oil. Especially for refrigeration systems containing R502, R503, R13 and ammonia that are difficult to dissolve in refrigeration oil, if the freezing point of the oil is too high, when it enters the refrigeration system with the refrigerant, it will be in the evaporator coil, etc. The low-temperature parts stay or solidify, and in severe cases, the pipeline may even be blocked, making the equipment unable to operate normally. Therefore, high-quality refrigeration oil should have a lower freezing point and good low-temperature fluidity. Generally, freezing point, pour point and U-tube fluidity are used to evaluate and measure the low temperature fluidity of refrigeration oil. 2. Solubility - refrigerant partially dissolved with oil Freon refrigerant, which is only partially soluble in oil, will separate from the oil as the temperature of the system decreases. Due to the low density of the oil, the oil floats on top of the refrigerant. If the viscosity of the refrigerant layer is too low, it will cause sintering of sliding parts or abnormal vibration of the body. In addition, due to the separation of oil and refrigerant, it will cause problems such as difficulty in reflux and affect heat transfer efficiency in low temperature areas such as evaporators and condensers. Therefore, the oil and refrigerant must have good compatibility, and the temperature of oil-refrigerant phase separation should be as low as possible to ensure the normal operation of the refrigeration compressor. The compatibility of oil and refrigerant is affected by the type of refrigerant and the composition, structure and viscosity of refrigeration oil. Generally speaking, alkylbenzene type synthetic oil has the best compatibility with refrigerants, followed by naphthenic oil, and paraffin-based refrigeration oil has the worst compatibility.
05. Lubricity Relatively speaking, the requirement of refrigeration compressors on the lubricating performance of refrigeration oil is relatively moderate, because refrigerants, especially Freon refrigerants, play the role of extreme pressure additives under boundary lubrication conditions. However, with the miniaturization, light weight, high speed and rotation of compressors this year, higher and higher requirements have been placed on the lubricating performance of refrigeration oil products. Falex EP load has almost doubled from 1335-1880N (300-400 lb) to 3115N (700 lb). Only by adding lubricating additives can the lubricating performance of refrigeration oil be significantly improved and meet the above stringent requirements. In addition, since refrigerating machine oil is used in a refrigerant environment, not only the lubricity of the oil itself, but also the lubrication problem caused by the viscosity reduction after the oil is mixed with the refrigerant should be considered.
06. Moisture Refrigeration oil has very strict requirements on moisture, especially for fully-closed and semi-closed refrigeration compressor oil using Freon as refrigerant, its moisture content should generally not exceed 35×10-6. If the water content exceeds 100×10-6, it should be scrapped. Even a small amount of water in the refrigeration cycle can cause ice plugs in the throttling device and cause premature flocculation of the refrigeration oil. At high temperatures, moisture also reduces the stability of the oil and promotes the decomposition of Freon under the catalysis of the metal. On the other hand, the water in the oil also reduces the insulating properties of the refrigeration oil Ammonia refrigeration systems also have strict moisture requirements. The dissolved ammonia can emulsify the oil and create a lot of foam in the tank. This not only reduces the lubricity of the oil, but also prevents the lubrication system from supplying oil properly, resulting in wear and failure of the compression system. In addition, when water dissolves in ammonia, it produces ammonium hydroxide with strong chemical activity. This basic compound not only corrodes metal parts, but also promotes the deterioration of oil products.
