Knowledge of synthetic compressor oils and refrigeration oils*

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Synthetic lubricants are currently widely used in the compressor industry. Oil-fed and oil-immersed rotary as well as screw air compressors, along with large reciprocating compressors, represent some of the largest markets for such lubricants. For example, in Western Europe, the sales volume of synthetic compressor oils was 6,000 tons at the end of the 1980s, rising to 10,000 tons in the early 1990s. Sales in the United States are even higher, having exceeded 15,000 tons, and they are increasing at a rate of over 10% per year. 　　Currently, synthetic compressor oils are being used or increasingly utilized in the following types of compressors: II. Types of synthetic compressor oils and their characteristics: The synthetic oils currently used as compressor oils include esters (diesters, neopentyl polyol esters), synthetic hydrocarbons, polyethers, and silicone oils. The type of synthetic oil can be selected based on the compressor’s structure, compression medium, technical parameters, and operating conditions. In addition to sharing common characteristics, the various synthetic oils mentioned above also have their own distinct features. There are differences in the temperature limits for using these synthetic oils, as well as in their lubrication performance for different types of compressors. A comparison is also available regarding the biodegradability of synthetic oils versus mineral oils (using the CEC-L33-T-82 method). 1. Ester oils: The main esters used as synthetic compressor oils are neopentyl polyol esters and diesters. Ester oils meet the lubrication requirements of most screw and reciprocating air compressors thanks to their unique and excellent overall properties. The notable advantages of ester oils are their excellent performance at high and low temperatures, thermal oxidation stability, with a flash point and auto-ignition temperature that are about 40°C higher than those of mineral oils; moreover, their usable temperature range is 50°C higher than that of mineral oils. Extremely low volatility, resulting in low fuel consumption – only 1/8 of that of mineral fuels. It has a low residue content and good fluidity, which helps maintain high cleanliness in the lubrication system. Additionally, its high flash point and auto-ignition temperature **improve the safety of equipment operation and help prevent accidents. Ester oils are polar, which results in a strong oil film formed on metal surfaces; as a result, their friction coefficient is low, giving them excellent lubricating properties. Ester oils have a high thermal conductivity; their thermal conductivity is 15% higher than that of mineral oils. They also possess a higher specific heat, with a heat capacity that is 5–10% greater than that of mineral oils. As a result, it has good heat dissipation capabilities, which can effectively reduce the temperature of the fuel tank and the lubrication system. 　　Ester oils have extremely low toxicity and are biodegradable; in particular, diesters and polyesters used as compressor oils can achieve a biodegradation rate of over 90%, thereby reducing pollution to the ecological environment. 　　Many lubrication problems addressed by synthetic oils are related to temperature; they help prevent water and other refrigerants, which have higher heat resistance than conventional mineral oils, from condensing within the compressor system. Therefore, it is advantageous for the compressor to operate at high temperatures. Due to the gelling effect of waxes in mineral oil, as well as its poor performance at low temperatures and low viscosity index, it is very difficult to start compressors in field conditions and at low temperatures. The excellent high and low temperature performance, viscosity-temperature behavior, and wide operating temperature range of synthetic oils solve the aforementioned problems. Synthetic oil has a high viscosity at high temperatures, which facilitates the startup of the compressor at low temperatures. Its high viscosity during operation ensures that the screw compressor achieves a high volumetric efficiency. Synthetic ester-based compressor oils are more suitable for multi-stage high-pressure screw air compressors that operate at temperatures above 93°C under continuous heavy load conditions, and their service life can be doubled compared to PAO-based oils. Another key advantage of synthetic ester oils is their low residue content; they prevent the formation of sludge, sediment, and carbon deposits during use, thereby improving the cleanliness of the crankcase and the oil itself. This leads to better heat transfer, reduced friction, and less resistance in moving parts, which in turn reduces maintenance needs and downtime. As a result, energy consumption can be reduced by 7.2%, while power output can be increased by 3%. Synthetic esters are used in reciprocating air compressors, and their high flash point and good oxidation stability give them significant advantages. In the case of single-stage cylinder lubrication in large reciprocating air compressors, a common problem is the formation of carbon deposits and sludge in the cylinders and exhaust valves, which leads to frequent repairs and downtime. Even more serious is the fact that when mineral oils are used, excessive carbon deposits, rust, and high pressure can cause the compression system to catch fire at temperatures below 149°C, or even result in explosions. Significant economic benefits have been achieved due to the use of diester-based compressor oils. For example, a foreign company spent three years testing 100 rotary vane and screw compressors, carrying out continuous monitoring of the diester-based compressor oils as well as the compressors themselves; all the compressors used in these tests exceeded the expected time for oil replacement. The oil change interval for rotary vane compressors has been extended from 500 hours with mineral oil to over 4,000 hours; in some tests, this period even reached more than 10,000 hours. The service life of rotary screw compressors has been extended from 1,000 hours to over 8,000 hours, with several units having been tested for more than 13,000 hours. Tests were also conducted using diester oil in reciprocating compressors, which improved the cleanliness of the cylinders and crankcase and extended the cleaning cycle for valves by seven times. 　　At present, major oil companies around the world such as CPI, Mobil, Shell, BP, Esso, Castrol, Caltex, Ethel, Tanneco, Amoco, and others have developed their own series of ester-based synthetic compressor oils. Among them, CPI Engineering Company of the United States is the world’s largest manufacturer of compressor oils. 　　Typical data for diester-based compressor oils 2. Synthetic hydrocarbons (PAO) Synthetic hydrocarbons refer here mainly to polyalphaolefins; since synthetic hydrocarbon oils exhibit superior performance in many aspects compared to mineral oils, they are being used more and more widely in industrial applications, including those involving compressor oils. 　　Synthetic hydrocarbons feature a low pour point, good viscosity-temperature behavior and high viscosity index, a high flash point, low evaporation losses, low residue, excellent stability at high temperatures and under thermal oxidation, good compatibility with both non-metallic and metallic materials, and are non-toxic. 　　As can be seen from the above data, the viscosity index of the synthetic hydrocarbon oil is greater than 135; it can be used as a viscosity index improver in the formulation of various lubricants, and it possesses good shear resistance. Good high and low temperature performance enables use over a wide temperature range, ensuring that the oil provides excellent low-temperature starting performance and pumping capabilities for mechanical equipment used in outdoor operations in cold areas. Synthetic hydrocarbon oils possess the following chemical properties: good thermal stability. Comparison of the thermal stability of various oils (aluminum plate temperature: 310°C, oil temperature: 121°C). Note: The scoring scale assigns 10 for cleanliness and 0 for severe carbon deposition. 1. Good antioxidant stability: Polyα-olefins have better antioxidant stability than mineral oils; under identical test conditions, the time it takes for the pressure to drop to 172 Kpa is 100 minutes for mineral oil 100SN and 180 minutes for mineral oil 250SN. PAO 4 and 6 have times of 380 and 400 minutes respectively, which is twice as long as that of mineral oil.