We were discussing the working
principle of centrifugal compressor, velocity
diagram of centrifugal compressor, slip
phenomenon and slip factor for centrifugal compressor, Mach
number for centrifugal compressor and function of diffuser in centrifugal compressor in our recent posts, where we were also discussing the
various important parts of centrifugal compressor and their functions and we
have also secured there the work done on air in centrifugal compressor.

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mentioned here.

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Reference:

Today we will be interested here to discuss another
very important topic i.e. losses in centrifugal compressor with the help of
this post i.e. different types of losses in centrifugal compressor.

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**Different
types of losses in centrifugal compressor **

When fluid flows through the centrifugal compressor,
there will be some losses in the mechanical energy or we can also say that
there will be drop in static pressure energy at the outlet of compressor. This losses or drop in static pressure energy at the outlet of centrifugal compressor will be termed as losses in centrifugal compressor.

There will be basically three types of losses in the
centrifugal compressor and these losses are as

- Frictional losses
- Incidence losses
- Clearance losses

Now let us discuss each type of loss in centrifugal
compressor in detail.

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**Frictional
loss**

Frictional loss is the major portion of losses in
centrifugal compressor and it is basically due to the friction. Working fluid
i.e. air will flow through the compressor blade passages and will come in
contact with solid surfaces and hence fluid will face resistance i.e. friction
while flowing through the compressor blade passages. Due to viscosity, there
will be friction between fluid layers also. Therefore, there will be skin
friction.

There will also be boundary layer separation losses
and loss due to shock.

Frictional losses, hence, will be the combined
effect of losses due to skin friction and boundary layer separation.

Frictional losses basically depend over the friction
factor, square of the fluid flow velocity and length of the flow passage. We can
see here the variation of frictional losses with respect to the mass flow with
the help of following curve as displayed here in following figure.

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**Incidence
losses **

Incidence losses occur when the compressor will not
work at its design condition. When flow velocity of working fluid does not follow
the vane angle while flowing through the vane, incidence losses will be there.

When compressor will work on off-design condition, the
direction of relative velocity of flowing fluid at inlet will not match with
the inlet blade angle and hence working fluid i.e. air will not enter the blade
passage smoothly by gliding along the blade surface.

We can see here the variation of incidence losses
with respect to the mass flow with the help of following curve as displayed
here in following figure.

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**Clearance
losses**

There must be minimum clearance between impeller
shaft and casing of the centrifugal compressor and between the outlet periphery
of the impeller eye and the casing.

Clearance losses could be reduced by providing
proper sealing arrangement.

We must note it here that the contribution of
clearance losses in the total losses in the centrifugal compressor will be very
less. Hence, in order to avoid the losses in the centrifugal compressor, we
must have to reduce the losses due to friction and incidence losses. Frictional losses and incidence losses are the main
parts of total losses in the centrifugal; compressor.

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**Losses
in the centrifugal compressor Vs Mass flow **

Let us draw here the curve between the losses and
mass flow as displayed here in following figure.

If we see the curve, we can easily conclude that
frictional losses will be increasing with respect to mass flow.

Incidence loss is also displayed here in following
figure and we can conclude that incidence losses will be minimum at design
condition only.

Curve which is showing the total loss is basically the combined effect of frictional losses and incidence losses. Therefore this curve indicates the total losses of energy or static pressure head in the centrifugal compressor.

Curve which is showing the total loss is basically the combined effect of frictional losses and incidence losses. Therefore this curve indicates the total losses of energy or static pressure head in the centrifugal compressor.

Therefore, we have studied here the various types of
losses in the centrifugal compressor and we have also seen here a curve between
mass flow and losses.

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Further we will find out, in our next post, performance characteristics of centrifugal compressor.

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Reference:

Fluid Machines, By Prof S.K. Som

Image courtesy: Google

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