The first screw compressors had a symetric profile and did not use liquid in the compression chamber, so-called oil-free or dry screw compressors. At the end of the 1960s a high speed, oil-free screw compressor was introduced with an asymetric screw profile. The new rotor profile resulted in significantly improved efficiency, due to reduced internal leakage.
An external gear is used in dry screw compressors to synchronise the counter rotating rotors. As the rotors neither come into contact with each other nor with the compressor housing, no particular lubrication is required in the compression chamber. Consequently the compressed air is com
pletely oil-free. The rotors and housing are manufactured with great precision to minimise leakage from the pressure side to the inlet. The integrated pressure ratio is limited by the temperature difference between the intake and the discharge. This is why oil-free screw compressors are frequently built with several stages.