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Thursday, 3 October 2019

WORKING PRINCIPLE OF CENTRIFUGAL COMPRESSOR

We were discussing the basic principle of operations, functions of various parts and performance characteristics of different types of fluid machines such as hydraulic turbines, centrifugal pumps and reciprocating pumps in our previous posts. We have seen there that working fluid for above mentioned fluid machines was water. 

Now it’s time to discuss few other types of fluid machines such as centrifugal compressors, axial flow compressors, fans and blowers where working fluid will be steam, air or gases. 

Today we will be interested here to discuss the centrifugal compressor working principle and we will also find out here the various parts of centrifugal compressor and their function in the operation of a centrifugal compressor with the help of this post. Further we will see the energy transfer in the centrifugal compressor in our upcoming post. 

Centrifugal compressor: working principle, parts and their function 

Centrifugal compressor is also termed as turbo compressors. In centrifugal compressors, energy will be supplied from outside and again energy will be imparted to the fluid by the machine and by virtue of which fluid will gain its internal energy and this internal energy will be gained by the fluid in the terms of rise in static pressure and in the terms of kinetic energy i.e. high flow velocity. 

In centrifugal compressors, required rise in pressure will take place due to the continuous conversion of angular momentum imparted to the working fluid by a high speed impeller in to static pressure.
Centrifugal compressors are steady flow devices and hence they are subjected to less vibration and noise. 

In centrifugal compressors, working fluid i.e. air will enter to the compressor at the impeller eye axially and will be released radially outward or we can say that compressed air will leave the compressor in radial direction. 

Centrifugal compressor components 

There will be basically three important components in a centrifugal compressor as mentioned below.
  1. Stationary casing
  2. Impeller or rotor of centrifugal compressor
  3. Diffuser 

Casing 

The casing of a centrifugal compressor will be approximately similar to the casing of a centrifugal pump. Casing is basically an air tight passage surrounding the impeller and will be designed in such a way that the kinetic energy of working fluid (let us say air) discharged at the outlet of the impeller will be converted in to pressure energy before the fluid leaves the casing and enters to delivery pipe. 

Impeller or rotor of centrifugal compressor 

Impeller, which is also termed as rotor of centrifugal compressor, is displayed here in following figure.  

There will be one shaft of centrifugal compressor which will be coupled with the shaft of electric motor. 

Impeller or rotor of centrifugal compressor is basically a circular disc mounted over the compressor shaft with key assembly. Curved blades will be mounted on this circular disc. Number of these curved vanes will differ and it will be approximately 15 to 20 vanes in a impeller of a centrifugal compressor.  These curved vanes, fixed over the impeller of a centrifugal compressor, will provide the diverging passage to the working fluid or air. 

The center of the impeller will be termed as impeller eye and air will be sucked at this impeller eye when impeller will be rotated and further air will move along the curved vanes of impeller in radial outward direction.  

When impeller will be rotated, the pressure energy of the air will be increasing as air will move in radial outward direction along the curved vanes of the impeller. 

As impeller will be rotating, therefore a tangential velocity will be imparted on the fluid and hence fluid will secure a centripetal acceleration and it will be manifested in terms of the rise in pressure in radial outward direction. 

Working fluid i.e. air will have high velocity and high pressure at the outlet of the impeller or at the tip of the impeller of the centrifugal compressor. 

Diffuser

Diffuser is provided inside the casing. Impeller will be surrounded by a series of stationary guide vanes mounted on the diffuser ring as displayed here in following figure. 

The purpose of diffuser is to convert the kinetic energy in to static pressure energy and we must note it here that a diffuser is very important part of a centrifugal compressor in order to secure the high productivity or performance. 

Diffuser is basically provided in order to increase the pressure of working fluid i.e. air. Working fluid i.e. air will have high velocity and high pressure at the outlet of the impeller or at the tip of the impeller of the centrifugal compressor. 

Centrifugal compressor operation tends to develop more pressure and hence air from the outlet of the impeller will enter and pass through some stationary curved vanes. These stationary curved vanes will be fixed over the diffuser. 

These stationary curved guide vanes will provide a passage of diverging in nature and hence it increases the cross sectional area in the direction of flow which will convert the kinetic energy in to pressure energy. 

At the outlet of diffuser, air will be with very high static pressure and with relatively very lower velocity of flow. Therefore air with high static pressure will flow to delivery pipe of centrifugal compressor from the outlet of diffuser of compressor. 


Important points regarding centrifugal compressor

  1. Centrifugal compressor is basically a continuous flow machine suitable for large flow rate at moderate pressure.  
  2. A pressure ratio of 4:1 could be achieved with single stage centrifugal compressor and higher pressure ratio of 12:1 is also possible to achieve with multi stage centrifugal compressor. 
  3. Air will enter to the compressor at the impeller eye axially and compressed air will leave the compressor in radial direction


Losses in a centrifugal compressor 

There are basically following types of losses in a centrifugal compressor as mentioned below 
  1. Frictional losses 
  2. Incidence losses 
  3. Clearance and leakage losses 
  4. Inlet losses 

Hence, we have seen here the basics of a centrifugal compressor and the various important parts and their function in the operation of a centrifugal compressor. We have discussed above that working fluid such as air will be sucked at the eye of impeller of centrifugal compressor and further pressure of working fluid i.e. air  will be increased and finally air with high static pressure will be delivered at the outlet of compressor. 

High pressurized air will flow further through delivery pipe and will be utilized as per need of applications. 

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Further we will find out, in our next post, velocity diagram of centrifugal compressor.     

Reference: 

Fluid mechanics, By R. K. Bansal 
Image courtesy: Google    

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