Centrifugal compressor


Centrifugal compressors, sometimes called impeller compressors or radial compressors, are a sub-class of dynamic, axisymmetric, work-absorbing turbomachinery.
They achieve pressure rise by adding energy to the continuous flow of fluid through the rotor/impeller. The equation in the next section shows this specific energy input. A substantial portion of this energy is kinetic, which is converted to increased potential energy/static pressure by slowing the flow through a diffuser. The static pressure rise in the impeller may roughly equal the rise in the diffuser.

Components of a simple centrifugal compressor

A simple centrifugal compressor stage has four components : inlet, impeller/rotor, diffuser, and collector. Figure 1.1 shows each of the components of the flow path, with the flow entering the centrifugal impeller axially from left to right. This turboshaft impeller is rotating counter-clockwise when looking downstream into the compressor. The flow will pass through the compressors from left to right.

Inlet

The simplest inlet to a centrifugal compressor is typically a simple pipe. Depending upon its use/application, inlets can be very complex. They may include other components such as an inlet throttle valve, a shrouded port, an annular duct, a bifurcated duct, stationary guide vanes/airfoils used to straight or swirl flow, movable guide vanes. Compressor inlets often include instrumentation to measure pressure and temperature in order to control compressor performance.
Bernoulli's principle plays an important role in understanding vaneless stationary components like an inlet. In engineering situations assuming adiabatic flow, this equation can be written in the form:
where:
  • Subscript is the inlet of the compressor, station 0
  • Subscript is the inlet of the impeller, station 1
  • is the pressure
  • is the density and indicates that it is a function of pressure
  • is the flow speed
  • is the ratio of the specific heats of the fluid

    Centrifugal impeller

The identifying component of a centrifugal compressor stage is the centrifugal impeller rotor. Impellers are designed in many configurations including "open", "covered or shrouded", "with splitters", and "without splitters". Figures 1.1, 1.2.1, and 1.3 show three different open, full-inducer rotors with alternating full blades/vanes and shorter-length splitter blades/vanes. Generally, the accepted mathematical nomenclature refers to the leading edge of the impeller with subscript 1. Correspondingly, the trailing edge of the impeller is referred to as subscript 2.
As the working-gas/flow passes through the impeller from stations 1 to 2, the kinetic and potential energy increase. This is identical to an axial compressor with the exception that the gases can reach higher 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.
Most modern high-efficiency impellers use "backsweep" in the blade shape.
Derived from the general Euler equations is Euler's pump and turbine equation, which plays an important role in understanding impeller performance. This equation can be written in the form:
  • Subscript 1 is the impeller leading edge, station 1
  • Subscript 2 is the impeller trailing edge, station 2
  • is the energy added to the fluid
  • is the acceleration due to gravity
  • is the impeller's circumferential velocity, units velocity
  • is the velocity of flow relative to the impeller, units velocity
  • is the absolute velocity of flow relative to stationary, units velocity

    Diffuser

The next component downstream of the impeller within a simple centrifugal compressor may be the diffuser. The diffuser converts the flow's kinetic energy into increased potential energy by gradually slowing the gas velocity. Diffusers can be vaneless, vaned, or an alternating combination. High-efficiency vaned diffusers are also designed over a wide range of solidities from less than 1 to over 4. Hybrid versions of vaned diffusers include wedge, channel, and pipe diffusers. Some turbochargers have no diffuser. Generally accepted nomenclature might refer to the diffuser's leading edge as station 3 and the trailing edge as station 4.
Bernoulli's principle plays an important role in understanding diffuser performance. In engineering situations assuming adiabatic flow, this equation can be written in the form:
where:
  • Subscript is the inlet of the diffuser, station 2
  • Subscript is the discharge of the diffuser, station 4

  • Collector

The collector of a centrifugal compressor can take many shapes and forms. When the diffuser discharges into a large, empty, circumferentially constant-area chamber, the collector may be termed a Plenum. When the diffuser discharges into a device that looks somewhat like a snail shell, bull's horn, or a French horn, the collector is likely to be termed a volute or scroll.
When the diffuser discharges into an annular bend, the collector may be referred to as a combustor inlet or a return channel. As the name implies, a collector's purpose is to gather the flow from the diffuser discharge annulus and deliver this flow downstream into whatever component the application requires. The collector or discharge pipe may also contain valves and instrumentation to control the compressor. In some applications, collectors will diffuse flow far less efficiently than a diffuser.
Bernoulli's principle is important in understanding how diffusers perform. In engineering situations assuming adiabatic flow, this equation can be written in the form:
where:
  • Subscript is the inlet of the diffuser, station 4
  • Subscript is the discharge of the diffuser, station 5

  • Historical contributions, the pioneers

Over the past 100 years, applied scientists including Stodola, Pfleiderer, Hawthorne, Shepherd, Lakshminarayana, and Japikse, have educated 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 the reason advances in turbines and axial compressors often find their way into other turbomachinery, including centrifugal compressors. Figures 2.1 and 2.2 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.

Aerodynamic-thermodynamic domain

Figure 2.1 represents the aero-thermo domain of turbomachinery. The horizontal axis represents the energy equation derivable from the first law of thermodynamics. The vertical axis, which can be characterized by Mach number, represents the range of fluid compressibility. The Z-axis, which can be characterized by Reynolds number, represents the range of fluid viscosities. Mathematicians and physicists who established the foundations of this aero-thermo domain include Isaac Newton, Daniel Bernoulli, Leonhard Euler, Claude-Louis Navier, George Stokes, Ernst Mach, Nikolay Yegorovich Zhukovsky, Martin Kutta, Ludwig Prandtl, Theodore von Kármán, Paul Richard Heinrich Blasius, and Henri Coandă.

Physical-mechanical domain

Figure 2.2 represents the physical or mechanical domain of turbomachinery. Again, the horizontal axis represents the energy equation with turbines generating power to the left and compressors absorbing power to the right. Within the physical domain the vertical axis differentiates between high speeds and low speeds depending upon the turbomachinery application. The Z-axis differentiates between axial-flow geometry and radial-flow geometry within the physical domain of turbomachinery. It is implied that mixed-flow turbomachinery lie between axial and radial. Key contributors of technical achievements that pushed the practical application of turbomachinery forward include Denis Papin, Kernelien Le Demour, Daniel Gabriel Fahrenheit, John Smeaton, Dr. A. C. E. Rateau, John Barber, Alexander Sablukov, Sir Charles Algernon Parsons, Ægidius Elling, Sanford Alexander Moss, Willis Carrier, Adolf Busemann, Hermann Schlichting, Frank Whittle, and Hans von Ohain.

Partial timeline of historical contributions

Turbomachinery similarities

Centrifugal compressors are similar in many ways to other turbomachinery and are compared and contrasted as follows:

Similarities to axial compressor

Centrifugal compressors are similar to axial compressors in that they are rotating airfoil-based compressors. Both are shown in the adjacent photograph of an engine with 5 stages of axial compressors and one stage of a centrifugal compressor. The first part of the centrifugal impeller looks very similar to an axial compressor. This first part of the centrifugal impeller is also termed an inducer. Centrifugal compressors differ from axials as they use a significant change in radius from inlet to exit of the impeller to produce a much greater pressure rise in a single stage than does an axial stage. The 1940s-era German Heinkel HeS 011 experimental engine was the first aviation turbojet to have a compressor stage with radial flow-turning part-way between none for an axial and 90 degrees for a centrifugal. It is known as a mixed/diagonal-flow compressor. A diagonal stage is used in the Pratt & Whitney Canada PW600 series of small turbofans.

Centrifugal fan

Centrifugal compressors are also similar to centrifugal fans of the style shown in the neighboring figure, as they both increase the energy of the flow through the increasing radius. In contrast to centrifugal fans, compressors operate at higher speeds to generate greater pressure rises. In many cases, the engineering methods used to design a centrifugal fan are the same as those to design a centrifugal compressor, so they can look very similar.
For purposes of generalization and definition, it can be said that centrifugal compressors often have density increases greater than 5 percent. Also, they often experience relative fluid velocities above Mach number 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 five percent and peak relative fluid velocities below Mach 0.3.