Design for Performance : Centrifugal Compressors

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Design for Performance Centrifugal Compressors

• P M V Subbarao
• Professor
• Mechanical Engineering Department

An Efficient Compressor may better than compact
Compressor ????
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Tip Speed Materials

• The precise stress limits of a given impeller
  material will depend upon factors such as the
  required cyclic duty (number of startstop cycles
  per unit time).
• Cast aluminium to be used up to a tip speed of
  around 200300 m/s,
• forged machined aluminium up to where a maximum
  of about 500 m/s.
• titanium up to around 650700 m/s.
• Titanium aluminides and titanium metal matrix
  composites are currently being researched for the
  higher tip speeds.

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The balance of factors affecting stage stability,
derived from the work factor is to limit
impeller stress levels usually results in a
backsweep angle of at least 30
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Efficiency of A Centrifugal Compressor

• Broadly speaking, two approaches are used for to
  determining stage efficiency at the preliminary
  design stage.
• An approach that at first sight appears to be
  less dependent upon empiricism is to formulate a
  general 1D compressor model that includes some
  system of loss estimation for the principal flow
  elements of the stage.
• The most comprehensive method includes, models
  for IGV, impeller, vaneless space and vaned
  diffuser.

The loss models are tuned so that the method
obtains reasonable agreement with a
representative range of test cases.
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Development of Loss Model

• A best empirical approach is to correlate
  efficiency with parameters such as specific speed
  or the flow coefficient.
• In aero applications, the specific speed is
  defined as

and the flow coefficient as
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Rodgers efficiency versus specific speed
h
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Impeller Exit Geometry Vs Tip Losses
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Losses in Stator Passages
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Irreversible Diffuser
T
s
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Gas Dynamics of A Real Impeller
Va2
Vr2
Vf2
Vw2 lt U
Vr1
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Slip Factor, Power Input Factor Efficiency

• Power input factor and slip factor are neither
  independent of one another nor of efficiency.
• The power input factor represents an increase in
  the work input.
• The whole of this increment is absorbed in
  overcoming frictional loss and therefore degraded
  into thermal energy.
• Power input factor should be as close as possible
  to unity.
• Low values of Power input factor imply that the
  impeller is very efficient.
• However, the value of compressor efficiency also
  depends on friction losses in the diffuser which
  does not affect power input factor.
• The slip factor limits the capacity of the
  compressor and this should be as high as
  possible.
• A high value of slip factor requires higher
  number of vanes.
• Higher number vanes will increase frictional
  losses and hence increase the value of power
  input factor and decrease the value of
  efficiency.
• A suitable compromise must be found, and present
  day practice is
• 19 21 vanes to get a slip factor value of 0.9.
• There is high demand for Compact design of a
  centrifugal.

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Optimum design of a centrifugal compressor inlet

• To obtain high efficiencies from high pressure
  ratio compressors it is necessary to limit the
  relative Mach number at the eye.
• The flow area at the eye can be written as

Optimum ratio
Tip velocity of eye
Eye tip
Eye hub
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With uniform flow velocity the continuity
equation is
Ut1
bt1
at1