Differences Between Dry Screws and Oil-Flooded Screws: Oil-Flooded Screw Compressors

In oil flooded screw compressors, sometimes referred to as oil-injected or wet screw compressors, oil is injected directly into the rotor chamber continuously during operation. The oil is discharged with the gas into an oil separator vessel and then must be separated from the gas on the discharge side before being injected back into the compressor again. The injection oil serves several purposes:

1. It provides a lubricating film between the male and female rotors. Oil flooded screw compressors do not have timing gears. Instead, one rotor drives the other through direct contact (with an oil film between the two rotors). The drive rotor refers to the rotor that is coupled to the motor, while the driven rotor refers to the rotor that is moved by the drive rotor. Most oil-flooded screw compressors are male-rotor drive, but many female rotor drive machines are available as well.

2. It carries away much of the heat of compression. This enables higher pressure ratios than are thermally possible in oil-free screw compressors (in which the compression power increases the gas temperature, and a small part of the heat of compression is absorbed by the casing and rotors).

3. It fills the internal clearances, increasing volumetric efficiency. The volumetric efficiency of an oil-flooded screw compressor can be 10%–20% higher than a similar oil-free screw compressor.

4. It continuously flushes away contamination that might enter the machine from the suction header

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