Centrifugal compressor

From Academic Kids

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 reciprocating compressor. Their primary drawback is that they cannot achieve the high compression ratio of reciprocating compressors without multiple stages. Centrifugal compressors 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; when similar blowers are used in pipelines they are sometimes called jets.

Big centrifugal compressors are used for gas transportation in gas pipelines all around the world. They 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 increase rapidly, becoming dangerous for the equipment. Stonewall or Choke: this occurs when the velocity of the gas approaches its sonic speed somewhere in the compressor (it may occur at the impeller inlet or at the vaned diffuser inlet). It is 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 enough power to overcome the discharge pressure. This causes flow reversal (surge), high vibration, temperature increases, and rapid changes in axial thrust that can damage the labyrinth seals or even the driver.

Missing image
CompressorMap.jpg
This figure shows a centrifugal compressor map with the surge line to the left and choke to the right. The x axis is the actual inlet flow, and the y axis is the isentropic head generated by the compressor. The close-to-vertical curves are lines of constant efficiency, and the close-to-horizontal curves are lines of constant speed.
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