Reciprocating compressors consist of one or more cylinders, each with an intake valve, a discharge valve, and a piston that is attached to a crankshaft. The intake valve opens on the downstroke of the piston and allows gas to enter the cylinder. On the upstroke the intake valve closes, and the piston compresses the gas and then forces it out through the discharge valve, which opens during the upstroke.
Rotary compressors trap a gas between rotating blades or vanes that compress the gas into a steadily decreasing volume as they sweep it along.
Centrifugal-flow, or radial-flow, compressors consist of two alternating sets of rotating blades enclosed in a casing. The rotation of one set of blades, called the impellers, forces the gas away from the rotating shaft toward the casing and at the same time increases the speed of the gas. The second set of blades, called the diffusers, tends to make the flowing gas slow down. In accordance with Bernoulli's principle, the reduction in speed increases the pressure of the gas.
Axial-flow compressors also consist of pairs of moving and stationary blades and operate much like radial-flow compressors. In axial-flow compressors, however, the gas flows parallel to the shaft on which the moving blades are fastened.
