There are three types of pollutants in compressed air, namely water, oil and dust. The sources of these three pollutants are different:-water: inherent in natural air;  -Oil: Most of it comes from air compressors (air in areas with serious air pollution will also contain trace oil stains);  -solid particles: partly from air and partly from inside the compressed air system.  Compressed air drying equipment (such as cold dryer, suction dryer, etc.) can remove water from compressed air, while oil and solid particles can be removed by compressed air filters.  The oil stain in compressed air is weakly acidic, so it will not only not lubricate the gas-using equipment, but also corrode it, which will lead to the failure of the gas-using equipment within a certain period of time.  In some places with strict requirements, such as pneumatic control system, a drop of oil can change the condition of air holes and paralyze the production line that normally operates automatically.  Sometimes, the oil will swell the sealing ring and the supporting body of the pneumatic valve, causing slow operation, serious blockage.  In the process completed by air, such as blow molding, oil will also cause product appearance defects or surface pollution.  Compressed air is produced by compressors. Most air compressors use lubricating oil to lubricate and cool their moving parts (such as screws, pistons, etc.).  Lubricating oil in the air compressor is in direct contact with high-temperature compressed air.  Oil stains in compressed air exist in three forms: "oil drops", "oil mist (aerosol)" and "oil vapor".  Fig. 1 shows the working flow of the oil screw compressor: when air is sucked into the air compressor, it first passes through the suction filter (1), then enters the compression chamber (2) to be compressed, and the compressed air reaching a certain pressure enters the oil-gas separator (5), (at this time, the temperature of the compressed air is about 100 deg c) the lubricating oil in the compressed air is recycled, the separated compressed air enters the rear cooler (3) to be cooled to about 40 deg c by cooling water or air, and the condensate is discharged through the automatic drainer (6).  At the same time of compressed air, lubricating oil is injected into the compression chamber to lubricate and cool the screw. The lubricating oil enters the oil-gas separator together with the compressed air, and the separated lubricating oil is discharged into the oil cooler (7), cooled and injected into the compression chamber again for recycling.  Although all air compressors with oil have "oil and gas separators", the temperature of compressed air entering the "oil and gas separators" is higher, i.e. the partial pressure of oil vapor is also higher, and oil vapor cannot be separated in the "oil and gas separators";  After the compressed air is cooled by the rear cooler, the partial pressure of oil vapor in the compressed air decreases, and some oil vapor condenses into oil mist.  Therefore, the compressed air discharged from the screw compressor with oil contains a certain amount of oil.  We can confirm this point from the side of the air compressor which needs to be supplemented with lubricating oil within a certain period of time.  The oil pollution content in compressed air is not high, so it is generally expressed in mg/m3 (i.e. oil pollution content per cubic meter of compressed air), and also expressed in ppmw (weight ratio) and ppmv (volume ratio).  Apart from the air compressor, a small part of the oil in the compressed air comes from the ambient air. Due to air pollution, the air contains about 0.05ppm to 0.25ppm of oil. Strictly speaking, the compressed air discharged by the oil-free air compressor contains oil.