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Centrifugal compressor

Also called a radial blower, squirrel cage, or squirrel wheel compressor, a centrifugal compressor consists of an axle to which is mounted a cylindrical assembly of compressor blades. The compressor operates by using the centrifugal force applied to an air mass to achieve compression. Centrifugal compressors are used throughout industry because they have few moving parts, are very energy efficient, and give higher airflow than a similarly sized reciprocal compressor. Their primary drawback is that they cannot achieve the high compression ratio of reciprocal compressors without multiple stages. Centrifugal compressors however are more suited to continuous duty applications such as ventilation fans, air movers, cooling units, and other uses that require high volume but fairly low pressures. While technically centrifugal blowers can operate in reverse, due to blade design and other factors their efficiency is greatly reduced. Centrifugal blowers are used in some small jet turbine engines.

Big Centrifugal Compressors are used for Gas transportation in gas pipelines all around the world, thay have the following operating limits: Minimum Operating Speed: is the minimum speed for sustentation, below this value the compressor stops or goes to the called "Idle Speed". Maximum Allowable Speed: is the maximum design speed for the compressor, beyond this value the vibrations increases rapidly being dangerous for the equipment. Stonewall or Choke: it occurs when the velocity of the gas approaches its sonic speed in the compressor,it may occur at the impeller inlet or at the vaned diffuser inlet, generally not detrimental to the compressor. Surge: normally occurs at about 50% of design inlet capacity at design speed, is the point at which the impeller cannot add any more power to overcome the discharge pressure, it causes reversal flow (surge), high vibration, temperature increases, and rapid changes in axial thrust that can damage the labyrinth seals or even the driver.

Figure shows a centrifugal compressor map with the surge line to the left and choke to the right, it also shows efficiency percent lines and rpm lines.