Centrifugal compressor

Centrifugal compressors, (sometimes referred to as radial compressors) are a special class of radial-flow work-absorbing turbomachinery that includes pumps, fans, blowers and compressors.^[1]

The earliest forms of these dynamic-turbomachines^[2] were pumps, fans and blowers. What differentiates these early turbomachines from compressors is that the working fluid can be considered incompressible thus permitting accurate analysis through Bernoulli's equation. In contrast, modern centrifugal compressors are higher in speed and analysis must deal with compressible flow.

For purposes of definition, centrifugal compressors often have density increases greater than 5 percent. Also, they often experience relative fluid velocities above Mach 0.3 when the working fluid is air or nitrogen. In contrast, fans or blowers are often considered to have density increases of less than 5 percent and peak relative fluid velocities below Mach 0.3

In an idealized sense, the dynamic compressor achieves a pressure rise by adding kinetic-energy/velocity to a continuous flow of fluid through the rotor or impeller. This kinetic energy is then converted to an increase in static pressure by slowing the flow through a diffuser.

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Advantages

Centrifugal compressors are used throughout industry because they have fewer rubbing parts, are relatively energy efficient, and give higher airflow than a similarly sized reciprocating compressor (i.e. positive-displacement). Their primary drawback is that they cannot achieve the high compression ratio of reciprocating compressors without multiple stages. Centrifugal fan/blowers are more suited to continuous-duty applications such as ventilation fans, air movers, cooling units, and other uses that require high volume with little or no pressure increase. In contrast, multi-stage centrifugal compressors often achieve discharge pressures of 8,000 to 10,000 psi (59 MPa to 69MPa) re-injecting natural gas back into oil fields to increase oil production.

Centrifugal compressors are often used in small gas turbine engines like APUs (auxiliary power units) and smaller aircraft gas turbines. A significant reason for this is that with current technology, the equivalent flow axial compressor will be less efficient due primarily to tip-clearance losses. There are few single stage centrifugal compressors capable of pressure-ratios over 10:1, due to stress considerations which severely limit the compressor's safety, durability and life expectancy.

For aircraft gas-turbines; 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 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. The most significant drawback is the relatively larger frontal area/unit flow. For these reasons and others, aircraft gas turbines that utilize centrifugal stages within the compressor tend to be smaller and are used in turboshaft or turboprop applications (ref List of aircraft engines). These smaller compressor configurations vary, but generally fall into one of two categories; the axi-centrifugal and the 2-stage centrifugal. Tip speeds of centrifugal compressors can often reach Mach-1.3. In current 2-stage gas-turbines, the high pressure rise per stage allows these modern compressors to obtain overall compression ratios of 15:1.

Applications

A partial list of centrifugal compressor applications include:

• in pipeline transport of natural gas to move the gas from the production site to the consumer.
• in oil refineries, natural gas processing plants, petrochemical and chemical plants
• in air separation plants to manufacture purified end product gases.
• in refrigeration and air conditioner equipment refrigerant cycles: see Vapor-compression refrigeration.
• in industry and manufacturing to supply compressed air for all types of pneumatic tools.
• in gas turbines and auxiliary power units
• in pressurized aircraft to provide atmospheric pressure at high altitudes.
• in automotive engine and diesel engine turbochargers
• in oil field re-injection of high pressure natural gas to improve oil recovery

Operating limits

Many centrifugal compressors have one or more of the following operating limits:

• Minimum Operating Speed - the minimum speed for acceptable operation, below this value the compressor may be controlled to stop or go into an "Idle" condition.
• Maximum Allowable Speed - the maximum operating speed for the compressor. Beyond this value stresses may rise above prescribed limits and rotor vibrations may increase rapidly. At speeds above this level the equipment will likely become very dangerous and be controlled to slower speeds.
• Stonewall or Choke - occurs under one of 2 conditions. Typically for high speed equipment, as flow increases the velocity of the gas/fluid can approach the gas/fluid's sonic speed somewhere within the compressor stage. This location may occur at the impeller inlet "throat" or at the vaned diffuser inlet "throat". In most cases, it is generally not detrimental to the compressor. For low speed equipment, as flows increase, losses increase such that the pressure ratio drops to 1:1.
• Surge - is the point at which the compressor cannot add enough energy to overcome the system resistance^[3]. This causes a rapid flow reversal (i.e. surge). As a result, high vibration, temperature increases, and rapid changes in axial thrust can occur. These occurrences can damage the rotor seals, rotor bearings, the compressor driver and cycle operation. Most turbomachines are designed to easily withstand occasional surging. However, if the turbomachine is forced to surge repeatedly for a long period of time or if the turbomachine is poorly designed, repeated surges can result in a catstrophic failure. Of particular interest, is that while turbomachines may be very durable, the cycles/processes that they are used within can be far less robust.

See also

• Daniel Bernoulli
• Ernst Mach
• Leonhard Euler
• Fluid dynamics
• Multiphase flow
• Navier-Stokes equations
• Computational Fluid Dynamics
• Reynolds-averaged Navier-Stokes equations
• Reynolds transport theorem
• Reynolds number
• Finite element analysis
• Gas compressor
• Centrifugal fan
• Choked flow
• HVAC
• Refineries
• List of mechanical engineering topics
• List of aerospace engineering topics
• List of engineering topics

References

Further reading

• Lakshminarayana, B. Fluid Dynamics and Heat Transfer of Turbomachinery. Wiley-Interscience. ISBN 0-471-85546-4. 
• Wilson, D.G. and Korakianitis, T. (1998). The Design of High-Efficiency Turbomachinery and Gas Turbines, 2nd Edition, Prentice Hall. ISBN 0-13-312000-7. 
• Cumpsty, N.A. (2004). Compressor Aerodynamics. Krieger Publishing. ISBN 1-57524-247-8. 
• Whitfield, A. and Baines, N.C. (1990). Design of Radial Turbomachines. Longman Scientific & Technical. ISBN 0-470-21667-0. 
• Saravanamuttoo, H.I.H., Rogers, G.F.C. and Cohen, H. (2001). Gas Turbine Theory, 5th Edition, Prentice Hall. ISBN 0-13-015847-X. 
• Japikse, David and Baines, N.C. (1994). Introduction to Turbomachinery. Oxford University Press. ISBN 0-933283-06-7. 
• Japikse, David (1996). Centrifugal Compressor Design and Performance. Concepts ETI. ISBN 0-933283-03-2. 
• Japikse, David and Baines, N.C. (1998). Diffuser Design Technology. Concepts ETI. ISBN 0-933283-08-3. 
• Wennerstrom, Arthur J. (2000). Design of Highly Loaded Axial-Flow Fans and Compressors. Concepts ETI. ISBN 0-933283-11-3. 
• Japiske, D., Marschner, W.D., and Furst, R.B. (1997). Centrifugal Pump Design and Performance. Concepts ETI. ISBN 0-933283-09-1. 
• Editor:David Japikse (1986). Advanced Experimental Techniques in Turbomachinery, 1st Edition, Concepts ETI. ISBN 0-933283-01-6. 
• Shepard, Dennis G. (1956). Principles of Turbomachinery. Mcmillan. LCCN 56002849. 
• Baines, Nicholas C. (2005). Fundamentals of Turbocharging. Concepts ETI. ISBN 0-933283-14-8.