Transcript 5. centrifugal compressors and axial compressors

TURBOMACHINES
Chapter 5
CENTRIFUGAL compressors
& axial flow compressors
1
Introduction
• Compressor is a device used to produce large
pressure rise ranging from 2.5 to 10 bar or more.
• A single stage compressor generally produce a
pressure rise up to 4 bar.
• In general centrifugal compressors may be known
as fan, blower, supercharger etc, depending upon
the need to be served.
• Fans are low pressure compressors and blowers are
medium pressure compressors.
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Elements of a centrifugal compressor
• It mainly consists of (i) inlet casing with the
converging nozzle (ii) the impeller (iii) the diffuser
and (iv) the outlet casing.
3
Variation of pressure and velocity
• Fig below shows the variation of pressure and
velocity in a centrifugal compressor.
4
Velocity triangle at the Eye Hub and Tip
• In ideal condition the fluid enters the eye section radially
with no whirl component.
• The velocity remains constant from hub to tip of the eye.
• The tangential velocities of the impeller at the hub (root)
and the tip of the eye are calculated based on the
corresponding hub and tip diameters of the eye
respectively.
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Different vane shape
• The impellers may be classified depending on the
exit angle β2 into (i) Backward curved vanes, (ii)
Radial vanes and (iii) Forward curved blades.
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• The above equation is
known as H-Q characteristic
curve for the centrifugal fan,
blower and compressor.
• The value of K1 represents
the K.E of the fluid moving
in the tangential tip speed
of the impeller and the
constant K2 represents the
slope the H-Q curve which
may be positive, zero or
negative for fixed value of
β2 .
• The H-Q relationship can be
obtained as in fig.
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• Where Cd is the total drag coefficient.
• Total loss in the system,
• Actual head produced can therefore be obtained by
deducing these losses from ideal head developed by
the machine, i.e.,
• Fig shows the actual characteristic of C.F
compressor,
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Slip and Slip Co-efficient
• It is assumed that the velocities are constant over
the cross sectional area.
• But in actual practice this assumption is not correct
as shown if fig,
9
Surging
• The phenomenon of momentary fluctuations
in head and discharge due to unsteady flow,
flow reversal and vibration at low flow rate is
called surging.
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Pre-rotation or Pre-whirl
• We know that the
velocity at inlet have
more effect on Mach
number at inlet.
• The relative velocity at
the inlet should be
minimum,
which
reduces
the
Mach
number for a given eye
tip diameter.
• For a fixed eye tip
diameter the Mach
number can be reduced
by providing pre-whirl at
the inlet using guide
vanes.
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Diffuser
• Diffuser is used in the centrifugal compressor to
convert large kinetic energy of the fluid exiting
from the impeller to useful fluid or pressure
energy.
• Diffuser may be (i) Vaneless type (ii) Vaned type
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Axial flow compressor
• It is essentially an axial flow turbine driven in the
reverse direction except that in order to achieve a
sufficiently high efficiency, it is necessary to design
blades by taking extreme care.
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• The general velocity triangles at inlet and outlet are
as shown in fig.
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• If the reaction is not equal to 50% then the velocity
triangles are unsymmetric and then design is on
whether absolute velocity of air at rotor inlet or
outlet is axial.
• The velocity triangles for the two cases are as
shown in fig.
15
Work done & efficiencies of compressor
• The actual work is given
by
• The ideal work
• Then the static-to-static
efficiency is defined as
16
Work Done Factor
• Fig shows the axial velocity distributions in the first and last
stage of a multistage axial compressor.
• The degree of distortion in the axial velocity distributions
will depend on the number of the stages.
• On account of this, the axial velocity in the hub and tip
regions is much less than the mean value, whereas in the
central region its value is higher than the mean.
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• Consider the velocity triangle shown in fig.
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• Form Euler’s equation,
• In terms of blade angles, w.r.t tangential direction is given
by
• From geometry,
• Therefore,
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