Compare ROTARY SCREW COMPRESSORS. Oil injected rotary screw compressors are the most common air compressors.. The rotary screw compressor uses the rotary motion of two interlocking screws. The continuous flow in one direction draws air in on one side, slowly compresses it and ejects it at the other end as compressed air.

The continuous motion of the screws allows large amounts of air to be compressed, depending on the size of the compressor screws. The large contact areas of the screws allows a lot of air leakage. Therefore, the rotary compressor can not efficiently compress air much above 8 atm.

The compressed air is hot and high in oil and moisture. An after cooler or refrigerated air dryer usually comes with a rotary screw compressor. Air dyers and oil and water separators are required to bring the compressed air to useful quality for most applications. The Rotary screw compressor, because of its high air capacity at relatively low pressure ( 8 atm or 120 psig) is often used to precede a piston compressor to produce a high volume of high pressure compressed air ( 35 atm or 525 psig).

Capacity FAD is the measure of free air delivery which is the amount of air drawn into the compressor.