Centrifugal compressor

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Centrifugal compressor, (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.

Contents 1 Advantages 2 Applications 3 Operating limits 4 See also 5 References 6 Further reading 7 External links

[edit] 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). One example of an application of centrifugal compressors is their use in 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. Tip speeds of centrifugal compressors can often reach Mach-1.3. In turbines, the high pressure rise per stage allows these modern compressors to obtain overall compression ratios of 15:1.

The speed limit of the centrifugal compressor of Mach 1.3 is due to the diffuser becoming inefficient at supersonic speeds. Centrifugal compressors also have tip-clearance losses, but for a slow radial flow and an inlet close to the axis they get very small. The stator in a compressor with a bearing after each stage has virtually zero tip-clearance losses, this is true for radial and axial compressors. Like in an axial design the rotor is fitted with wings. The leading edge lies so close to the axis that rotational speed due to the rotor is low, but not so close to the axis that the axial air flow is constricted and as such is high. The axis is as thin as possible to avoid further constriction. The trailing edge is sharp as with any other wing. The wings, called blades, usually do not start all at once, but are axially displaced to reduce the constriction for the first blades, and supply the later blades with already rotating fluid. Especially the first blades start with round edges to not avoid stall for a wide range of rations between rotational speed and axial flow. To the outside the rotor ideally consists of tapered blades and no disk as a disk just leads to friction with the walls, while spiraling blades can use some of the friction between the fluid and the housing for an increase in pressure. Often the diffuser does not feature straight leading edge blades, but is a cylindrical space around the rotor with spiraling grooves growing to the outside. This also avoids stall in a wide range of ratios between rotational speed and fluid flow. The supersonic flow in the middle of the cylindrical space has also the purpose to drag the slower air on the outside until it gets subsonic and can be converted into pressure by a diverging diffuser. Due to conservation of angular momentum supersonic flow can also be diffused isentropically. The supersonic part of the diffuser has to sport a significant ratio of radii.

[edit] 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

[edit] 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.

[edit] 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

[edit] References

[edit] 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. 

[edit] External links

• Compressor Surging Under Control
• MIT Gas Turbine Laboratory
• First Marine Gas Turbine 1947
• A history of Chrysler turbine cars
• SoftInWay engineering company
• Concepts ETI
• To find API codes, standards & publications
• To find ASME codes, standards & publications
• To find ASHRAE codes, standards & publications