A roots blower is a valve-less displacement compressor without internal compression. When the compression chamber comes into contact with the outlet port, compressed air flows back into the housing from the pressure side. Subsequently, further compression takes place when the volume of the compression chamber further decreases with continued rotation. Accordingly, compression takes place against full counter-pressure, which results in low efficiency and a high noise level. Two identical, usually symmetrical, counter-rotating rotors work in a housing, synchronized by means of a set of gear wheels. Blowers are usually air-cooled and oil-free. Their low efficiency limits these blowers to very low pressure applications and compression in a single stage, even if two- and three-stage versions are available. Roots blowers are frequently used as vacuum pumps and for pneumatic conveyance.