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1. Types and characteristics of lubricants
Depending on the type of screw compressor and the application, the lubricant in the unit plays a different role.
In oil-free screw compressors, lubricating oil mainly plays the role of lubrication. In oil-injected screw compressors, the role of lubricating oil is extremely important, such as lubrication, sealing, cooling, noise reduction, etc., but also as a hydraulic oil to control and adjust the movement of components such as slide valves.
In addition, in addition to the above functions in the compressor, the lubricating oil must also have the characteristics of resistance to chemical decomposition, oxidation, and carbonization, and during use, it can maintain the original viscosity and saturated vapor pressure for a long time. , Ignition point and flow characteristics.
The lubricants used in screw compressors are mainly mineral oil, semi-synthetic and fully synthetic oil.
Mineral oil uses crude oil as a raw material. After crude oil undergoes distillation, solvent extraction and dewaxing and other refining processes, "semi-finished products" of various viscosity mineral oils can be obtained. After adding various additives to these semi-finished products to improve their characteristics, various specifications of mineral oil are obtained.
The main additives include anti-friction agents, anti-corrosion agents, water-resistant agents, dispersants, anti-foaming agents, antioxidants, rust inhibitors, etc. It is worth noting that additives should be used according to specific applications, for example, the additives in the oil for air compressors and the oil for refrigeration compressors are different. Some applications do not allow the use of certain additives, because these additives may react chemically with the compressed gas.
Semi-synthetic oil also uses crude oil as a raw material, but the refining process is different from mineral oil. In the refining process of semi-synthetic oil, the solvent extraction process is not used, and the "hydrocracking" process is used instead. After the crude oil undergoes refining processes such as distillation, hydrocracking, and dewaxing, it can be mixed with some of the aforementioned additives as needed to obtain various semi-synthetic oils.
Compared with mineral oil, semi-synthetic oil has more stable characteristics and can reach higher viscosity. In addition, semi-synthetic oils have a low saturated vapor pressure, making them less volatile. Can greatly reduce the oil content in the air supply of the oil-injected screw compressor.
The most essential difference between synthetic oil and mineral oil and semi-synthetic oil is that this lubricating oil is not refined on the basis of crude oil. The semi-finished product of synthetic oil is a pure chemical substance that does not contain any impurities. After the selected various semi-finished products are mixed, the synthetic oil with the required characteristics can be directly obtained.
If it has a suitable viscosity, it will not react with the compressed gas, it can protect the surface of the lubricated parts, and it will not harm the subsequent processes and equipment. Although the price of synthetic oil is higher, the service life is obviously increased, and in many cases, it has better lubrication performance.
In addition, in some specific applications, in order to prevent the chemical reaction between the lubricating oil and the compressed gas, only synthetic oil can be used.
2. Selection of lubricating oil
Obviously, different types of compressors and different applications have significantly different requirements for lubricants.
In oil-free screw compressors, lubricating oil is never in contact with the compressed gas, and its role is to lubricate and cool bearings, shaft seals, synchronous gears and speed-increasing gears. Therefore, the selection of lubricating oil in the oil-free screw compressor is relatively simple, as long as the trade-offs are made in terms of lubrication performance, life and price. Usually cheap mineral oil can be selected, but anti-foaming agent, anti-corrosion and anti-wear agent are required. In this application, although mineral oil has a long enough life, semi-synthetic oil can also be used to make the life of the lubricating oil longer and have better lubrication performance. In oil-free screw compressors, it is generally not necessary to use higher-priced synthetic oil.
In oil-injected screw compressors, the lubricating oil is directly mixed with the compressed gas. When selecting, the following should be considered: 1. The lubricating oil is not required to chemically react with the compressed gas; 2. The dissolving of the compressed gas in the lubricating oil and the dilution of the lubricating oil are considered; 3. Since the lubricating oil is subjected to frequent heating and The cooling process requires that the lubricating oil can maintain a suitable viscosity for a long time, and has excellent anti-oxidation, anti-rust, anti-corrosion and other properties.
Although some mineral oils can provide satisfactory lubrication, the application of mineral oil in oil-injected screw compressors is limited. Semi-synthetic oils and synthetic oils with better performance than mineral oils are increasingly used in oil-injected screw compressors. many.
In the oil-injected screw air compressor, since the air does not chemically react with the composition in the lubricating oil and the solubility in the lubricating oil is also very small, various lubricating oils can be used. Mineral oil has the advantage of low price, but has a short life and poor high temperature resistance. Semi-synthetic oil has good high temperature resistance. Compared with mineral oil, the life can be at least doubled, and the oil content in the exhaust gas can also be greatly reduced. Synthetic oil has a longer life and better resistance to high temperatures, but it may chemically react with the synthetic rubber components in the compressor, which is worth noting.
In oil-injected refrigeration screw machines, the refrigerants widely used in screw refrigeration compressors are ammonia and R22, and some units use mixed refrigerants such as R134A, R407C, and R410A. Mineral oil can be used in the refrigeration system of ammonia and R22, but the life is limited. When semi-synthetic oil or synthetic oil is used, not only can the life of the lubricating oil be extended, it can be easily recovered from the low temperature side below -45 ° C. For unit systems using R134A, R407C, R410A and other refrigerants, semi-synthetic oil or synthetic oil is mostly used.
The situation in oil-injected screw process compressors is more complicated. Although mineral oil can be used in applications such as methane, ammonia, carbon oxides, nitrogen, etc., the application of mineral oil is limited when the compressed gas is propane, butane, chloride, etc. Therefore, in some oil-injected screw process compressors, although the price is higher, only semi-synthetic or synthetic oil can be used.
