Centrifugal compressor
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A centrifugal compressor, also called a radial blower, squirrel cage, or squirrel wheel compressor, consists of a cylindrical assembly of compressor blades mounted on an axle. 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. When centrifugal blowers are used in pipelines they are sometimes called jets.
Centrifugal compressors are often used in some small gas turbine engines, mainly because, at small size, all-axial units are less efficient than the equivalent CF compressor. Also because the rear axial stages become too small to be robust. Although some centrifugal compressors are capable of over 10:1 pressure ratio at a reasonable efficiency, temperature and stress considerations severely limit the pressure ratio that a CF unit can develop within the compression system of modern, high overall pressure ratio, gas turbine. Turboshaft engines often use an axial-CF or double centrifugal compressor unit to achieve a high overall pressure ratio.
[edit] Advantages
Centrifugal flow compressors offer several advantages including simplicity of manufacture, relatively low cost, low weight, low starting power requirements, and operating efficiency over a wide range of rotational speeds. In addition, a centrifugal flow compressor’s short length and spoke-like design allow it to accelerate air rapidly and immediately deliver it to the diffuser in a short distance. Top speeds of centrifugal compressors may reach Mach 1.3, but the pressure within the compressor casing prevents airflow separation and provides a high transfer of energy into the airflow. Although most centrifugal compressors are limited to two stages, the high pressure rise per stage allows modern centrifugal compressors to obtain compressor ratios of 15:1. [Source: A&P technician Powerplant Textbook]
[edit] Operating limits
Centrifugal compressors have the following operating limits:
- Minimum Operating Speed - the minimum speed for sustentation, below this value the compressor stops or goes to the called "Idle Speed".
- Maximum Allowable Speed - the maximum design speed for the compressor, beyond this value the vibrations increase rapidly, becoming dangerous for the equipment.
- Stonewall or Choke - 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.
Large centrifugal compressors are used for gas transportation in gas pipelines all around the world.
