From Wikipedia, the free encyclopedia
showing the centrifugal compressor and other parts
Centrifugal flow compressors, sometimes
referred to as radial compressors, are a special
class of radial-flow work-absorbing turbomachinery that include pumps, fans,
blowers and compressors.
The earliest forms of these dynamic-turbo machines were
pumps, fans and blowers. What differentiates these early turbo
machines 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 0.3-0.5
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.
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 reciprocating compressors often achieve discharge
pressures of 8,000 to 10,000 psi (55 to 69 MPa). One
example of an application of centrifugal compressors is their use
in re-injecting natural gas back into oil fields to increase oil
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 a combination of rotor and variable
stator 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.
Additionally for aircraft gas-turbines; centrifugal flow
compressors offer the advantages of simplicity of manufacture and
relatively low cost. This is due to requiring fewer stages to
achieve the same pressure rise. The fundamental reason for this
stems from a centrifugal compressor's large change in radius
(relative to a multi-stage axial compressor); it is the change in
radius that allows the centrifugal compressor to generate large
increases in fluid energy over a short axial distance.
A partial list of centrifugal compressor applications
Many centrifugal compressors have one or more of the following
- 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 lower 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. 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 turbo
machines are designed to easily withstand occasional surging.
However, if the turbo machine is forced to surge repeatedly for a
long period of time or if the turbo machine is poorly designed,
repeated surges can result in a catastrophic failure. Of particular
interest, is that while turbo machines may be very durable, the
cycles/processes that they are used within can be far less
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