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Centrifugal compressor

Centrifugal compressors, sometimes termed radial compressors, are a sub-class of dynamic axisymmetric work-absorbing turbomachinery.

The idealized compressive dynamic turbo-machine 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 potential energy/static pressure by slowing the flow through a diffuser.

Imagine a simple case where flow passes through a straight pipe to enter centrifugal compressor. The simple flow is straight, uniform and has no swirl. As the flow continues to pass into and through the centrifugal impeller, the impeller forces the flow to spin faster and faster. According to a form of Euler's fluid dynamics equation, known as "pump and turbine equation," the energy input to the fluid is proportional to the flow's local spinning velocity multiplied by the local impeller tangential velocity. In many cases the flow leaving centrifugal impeller is near or above 1000 ft./s or approximately 300 m/s. It is at this point, in the simple case according to Bernoulli's principle, where the flow passes into the stationary diffuser for the purpose of converting this velocity energy into pressure energy.

Historical contributions, the pioneers
Over this past 100 years, applied scientists like Stodola (1903, 1927–1945),[2] Pfleiderer (1952),[3] Hawthorne (1964),[4] Shepard (1956),[1] Lakshminarayana (1996),[5] and Japikse (many texts including, 1997),[6] have tried to educate young engineers in the fundamentals of turbomachinery. These understandings apply to all dynamic, continuous-flow, axisymmetric pumps, fans, blowers, and compressors in axial, mixed-flow and radial/centrifugal configurations.

This relationship is why advances in turbines and axial compressors often find their way into other turbomachinery including centrifugal compressors. Figures 1.1 and 1.2[7][8] illustrate the domain of turbomachinery with labels showing centrifugal compressors. Improvements in centrifugal compressors have not been achieved through large discoveries. Rather, improvements have been achieved through understanding and applying incremental pieces of knowledge discovered by many individuals.

Components of a simple centrifugal compressor
A simple centrifugal compressor has four components: inlet, impeller/rotor, diffuser, and collector.[1] Figure 3.1 shows each of the components of the flow path, with the flow (working gas) entering the centrifugal impeller axially from right to left. As a result of the impeller rotating clockwise when looking downstream into the compressor, the flow will pass through the volute's discharge cone moving away from the figure's viewer.

Centrifugal impeller
The key component that makes a compressor centrifugal is the centrifugal impeller, Figure 01. It is the impeller's rotating set of vanes (or blades) that gradually raises the energy of the working gas. This is identical to an axial compressor with the exception that the gases can reach higher velocities and energy levels through the impeller's increasing radius. In many modern high-efficiency centrifugal compressors the gas exiting the impeller is traveling near the speed of sound.

Applications
Below, is a partial list of centrifugal compressor applications each with a brief description of some of the general characteristics possessed by those compressors. To start this list two of the most well-known centrifugal compressor applications are listed; gas turbines and turbochargers.

Performance
While illustrating a gas turbine's Brayton cycle,[9] Figure 5.1 includes example plots of pressure-specific volume and temperature-entropy. These types of plots are fundamental to understanding centrifugal compressor performance at one operating point. Studying these two plots further we see that the pressure rises between the compressor inlet (station 1) and compressor exit (station 2). At the same time, it is easy to see that the specific volume decreases or similarly the density increases. Studying the temperature-entropy plot we see the temperature increase with increasing entropy (loss). If we assume dry air, and ideal gas equation of state and an isentropic process, we have enough information to define the pressure ratio and efficiency for this one point. Unfortunately, we are missing several other key pieces of information if we wish to apply the centrifugal compressor to another application.

References
1. a b c d e f g h i j k l m n Shepard, Dennis G. (1956). Principles of Turbomachinery. McMillan. ISBN 0 - 471 - 85546 - 4. LCCN 56002849.
2. Aurel Stodola (1945). Steam and Gas Turbines. New York: P. Smith. OL18625767M.
3. Pfleiderer, C. (1952). Turbomachines. New York: Springer-Verlag.
4. W. R. Hawthorne (1964). Aerodynamics Of Turbines and Compressors. Princeton New Jersey: Princeton University Press. ISBN LCCN 58-5029.
5. a b c d e Lakshminarayana, B. (1996). Fluid Dynamics and Heat Transfer of Turbomachinery. New York: John Wiley & Sons Inc.. ISBN 0-471-85546-4.
6. a b c d e f Japikse, David & Baines, Nicholas C. (1997). Introduction to Turbomachinery. Oxford: Oxford University press. ISBN 0-933283-10-5.
7. Peng, W. W. (2007). Fundamentals of Turbomachinery. New York: John Wiley & Sons Inc..
8. a b c d e f g h Wislicenus, George Friedrich (1965). Fluid Mechanics of Turbomachinery in two volumes. New York: Dover. ISBN 978-0-486-61345-1.
9. a b c d Wood, Bernard D. (1969). Applications of Thermodynamics. Reading, Massachusetts: Addison - Wesley Publishing Company. ISBN LCCN 75-79598.
10. a b c Streeter, Victor L. (1971). Fluid Mechanics fifth edition. New York: McGraw Hill Book Company. ISBN 07-062191-8.

This article uses material from the Wikipedia article "Centrifugal compressor", which is released under the Creative Commons Attribution-Share-Alike License 3.0.