The Differences Between Oil-Filled and Oil-Free Screw Air Compressors

Oil-Free Screw Compressor: The screw rotors do not come into direct contact with each other, maintaining a certain gap between them. The outer rotor drives the inner rotor through synchronized gears, ensuring a gap during power transmission. "Oil-Free" refers to the fact that the gas does not come into contact with oil during the compression process; however, the compressor's bearings and synchronized gears still require lubrication, but only between these lubricated parts and the compression chamber. An effective seal isolates the oil-injected screw compressor. A large amount of lubricating oil is injected into the compressed gas medium for lubrication, sealing, cooling, and noise reduction. The oil-injected screw compressor does not have synchronized gears. The anode rotor directly drives the rotation of the cathode rotor. In addition to lowering the exhaust temperature, the lubricating oil provides lubrication for the bearings, mechanical seals, and sliding valves.

Due to the non-contacting rotors of oil-free screw compressors, the medium gas and lubricating oil are separated by the shaft seal, thus preventing direct contact. As a result, oil-free screw compressors are widely used in waste gas recovery. Media gases such as coking gas recovery, coalbed methane pressurization, and pressure swing adsorption contain dust and liquids, with varying gas compositions. The oil-free screw compressor can meet the long-term continuous operation requirements of industrial production, with a continuous running time of up to 24,000 hours. The volumetric efficiency of oil-free screw compressors is lower than that of oil-lubricated screw compressors. Generally, the single-stage pressure ratio does not exceed 10.

The rotor of the screw air compressor directly engages in meshing transmission, resulting in high volumetric efficiency. The single-stage pressure ratio can reach over 10. However, due to the direct contact between lubricating oil and the process medium, the medium gas should be relatively clean and free of a large amount of impurities and water, otherwise, it will affect the lubricating oil. Additionally, it is necessary to use a lubricating oil that does not react with the medium gas during the selection process, typically a special synthetic oil. Therefore, oil-filled screw air compressors are usually used for medium-pressure gases in a single operating state, such as air compressors, refrigeration compressors, etc.