Centrifugal separators utilize centrifugal action for the separation of materials of different densities and phases. They are built in (a) stationary and (b) rotary types. Various modifications of stationary units are used more than any other kind for separation problems. The cost is moderate; it is simple in construction and is reasonably flexible in service, being useful for gas–liquid or gas–solid systems. In addition to serving as finishing separators centrifugal units are also used to take a “rough cut” into a separation problem. They may be followed by some additional unit of special cyclone action or filtration through woven cloth pads, and so on, to completely remove last traces of entrained particles.