Differences between oil-free screw air compressors and oil-injected screw compressors, along with their operating principles and structural diagrams.
The first twin-screw air compressor employs a symmetrical rotor profile and operates without any cooling medium within the compression chamber.
Time:
2026-05-29
Oil-free screw air compressor
The first twin‑screw air compressor employed a symmetrical rotor profile and operated without any cooling medium inside the compression chamber. Such compressors are known as oil‑free or dry‑type screw air compressors. In contrast, modern high‑speed oil‑free screw air compressors utilize an asymmetrical rotor design, which significantly enhances energy efficiency by reducing internal leakage.
External gear engagement is the most common method for reversing the rotor rotation and achieving synchronization. Since there is no contact between the rotors or between the rotors and the housing, no lubrication is required in the compression chamber, ensuring that the compressed air remains completely oil-free. To minimize leakage from the compression side to the suction side, both the rotors and the housing must be manufactured with high precision. The built-in compression ratio is limited by the maximum pressure differential between the suction and discharge ports. This is why oil-free screw air compressors typically employ multi-stage compression and are equipped with internal cooling systems to achieve higher pressure ratings.
A host unit of an oil-free screw air compressor, wherein the rotor housing adopts a liquid-cooling structure, is equipped with both gas seals and oil seals at both ends, and is fitted with a set of synchronizing gears to ensure that the minimum clearance between the rotors is maintained.
In liquid-injected screw air compressors
The liquid enters the compression chamber and often flows to the air compressor’s bearings. Its functions include cooling and lubricating the compressor’s moving parts, cooling the internally compressed air, and reducing leakage back into the intake pipeline.
Currently, lubricating oil is the most commonly used injection fluid due to its excellent lubrication and sealing properties. However, other fluids such as water or polymers can also serve as injection media. Components of liquid-injection screw air compressors can achieve high compression ratios, and single-stage compression often suffices to reach pressures of 14 bar or even 17 bar, although this may reduce energy efficiency.
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