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Sunday, 13 October 2019

DIFFERENT TYPES OF LOSSES IN CENTRIFUGAL COMPRESSOR

We were discussing the working principle of centrifugal compressorvelocity diagram of centrifugal compressorslip 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.  

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. 

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 
mentioned here.
  1. Frictional losses
  2. Incidence losses
  3. Clearance losses

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

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. 

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. 

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. 

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.  

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. 

Reference:  

Fluid Machines, By Prof S.K. Som 

Image courtesy: Google   

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