Developed in the 1930s, screw technology is a type of rotating positive displacement compressor or rotary compressor, with a male and female rotor driven either by the male rotor or timing gear. The male and female rotors spin in opposite directions. This draws air into the pockets of the screws. These pockets of air are forced towards the outlet. This is a positive displacement operation. There are no oscillating parts and the reciprocating compressor is a very efficient way to compress air. The variable speed drive units, adjust the speed of the rotors, to only deliver the volume of air consumed. This minimizes the electrical power consumed.