The type of machine commonly installed at a colliery today where a large volume of compressed air is required. A single unit can deliver 10,000 ft ₃/min (283 m₃/min) or more of free air, and the floor space occupied is a minimum for these capacities. It is also ideally suited for direct drive by a steam turbine, and this combination is commonly found at collieries. The compressor consists essentially of a number of impellers keyed to a shaft and running in a fixed casing with specially shaped passages. Each impeller is in the form of a hollow wheel, the two sides being united by curved vanes.