Centrifugal compressors use centrifugal force to displace and compress air. A rotating impeller is used to impart energy to air or a gas to compress it. After flowing through a fixed guide vanes at the compressor entrance, air enters the center of the compressor impeller hub and moves radially outward. At first the air flows axially, parallel to the shaft on which the impeller rotates, then it is channeled by the impeller with a 900 change in direction, and discharged into the diffuser. The mechanical energy of the rotating impeller is transferred to the air by increasing its velocity. The air flows radially outward along the vanes of the impeller and the air’s velocity and pressure increase as it moves outward. Some energy from the impeller increases the air’s temperature from friction between the air and the blades and due to the work done on the air. When the air leaves the impeller, it enters divergent nozzle vanes in the diffuser that convert the kinetic energy of the air into pressure energy. It is common to design the impeller so that half the pressure rise occurs in the impeller and half occurs in the diffuser.