Rotary screw air compressors can be oil-flooded or oil-free. An oil-flooded version uses oil to lubricate and seal the rotors on the air-end, enabling them to generate high pressure quickly and deliver compressed air in a single stage. The oil separates from the air before leaving the system. The air then recycles into the air-end for another use.

Oil-flooded air compressors use lubricating oil that enters the space between the two rotor screws, producing a hydraulic seal and transferring mechanical energy from the driving to the driven rotor. Air enters the air-end of the rotor from the suction side and moves through the vanes as the screws rotate. The rotors force oil and air through the air-end, causing the air to exit at the end of the screws. The air and oil then separate, and the air is cooled, filtered and routed back into the air-end.