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Saturday, 26 October 2019

AXIAL FLOW COMPRESSOR CONSTRUCTION AND WORKING PRINCIPLE

We were discussing various important topics based on the basics of centrifugal compressor in our previous posts. 

Today we will be interested here to understand the axial flow compressor construction and working principle with the help of this post. We will see here how an axial flow compressor works and what will be the various important components of an axial flow compressor. 

After ready this post, we will have following information about an axial flow compressor. 
  1. Basics of axial flow compressor 
  2. Working principle of axial flow compressor 
  3. Difference between the axial flow compressor and centrifugal compressor 
  4. Advantage of axial flow compressor over centrifugal compressor 
  5. Construction of axial flow compressor 
  6. Pressure velocity diagram for axial flow compressor 

Axial flow compressor construction and working principle 

Axial flow compressor: Brief introduction 

Axial flow compressor also works on the same principle on which centrifugal compressor works i.e. energy will be supplied from outside to the rotor of the compressor and this energy will be further imparted to the working fluid i.e. air in terms of static pressure energy and kinetic energy. 

Further, working fluid i.e. air will flow finally through the diffuser where kinetic energy will also be converted in to static pressure energy. Hence, at the outlet of compressor, there will be air with relatively lower velocity and with high static pressure energy. 

Axial flow compressor: Important components 

Axial flow compressor will also have similar basic parts such as the rotor and the stator like centrifugal compressor. 

Rotor is the rotating component of the axial flow compressor which will have the number of blades attached over its surface. Mechanical energy will be supplied to the rotor of the compressor from outside in the terms of external torque. Working fluid i.e. air will gain the energy from the rotor of the compressor in terms of static pressure and kinetic energy. 

Stator of the axial flow compressor will be basically the diffuser which will be the responsible for the diffusion process where kinetic energy of the working fluid coming out from the rotor will be converted in to pressure energy and therefore working fluid coming out from the diffuser will have high static pressure with relatively lower velocity. 

Hence, the working principle of axial flow compressor is similar with the working principle of centrifugal compressor. 

So, what is the basic difference between the axial flow compressor and the centrifugal compressor? 


In case of axial flow compressor, flow will take place in axial direction and in case of centrifugal compressor flow will take place in radially outward direction. 

In case of axial flow compressor, there will be number of stages and each stage consist a rotor and a stator. 

Axial flow compressor can handle a large amount of air as compared to centrifugal compressor or we can say that flow rate in case of axial flow compressor will be higher as compared to the centrifugal compressor. 

Axial flow compressor will run more efficiently with higher flow rate as compared to centrifugal compressor. 

Weight of the axial flow compressor will be less as compared to the weight of the respective centrifugal compressor. 

Axial flow compressor: Construction 

Now we will see here the construction of axial flow compressor here. There are basically two types of axial flow compressors and these are as mentioned here in following figure. 
  1. Drum type axial flow compressor 
  2. Disc type axial flow compressor 


Disc type axial flow compressor will be used where lower weight will be quite important.
There will be a rotating drum in case of drum type axial flow compressor. Rotor blades will be fixed over the surface of this rotating drum along its circumference. 

Similarly, in case of disc type axial flow compressor, there will be one disc and rotor blades will be mounted over the surface of the disc. 

There will be stationary casing, as displayed in following figure, stator blades will be mounted with this stationary casing of axial flow compressor. 

For an axial flow compressor, a row of rotor blades followed by a row of stator blades will be termed as a stage. 

There will be one row of inlet guide vanes which will be upstream to the first row of rotor. Working fluid will be directed by these inlet guide vanes in the axial direction. Working fluid will flow through the number of rotor blades and stator blades in a direction parallel to the axis of rotation of the drum or disc of the axial flow compressor i.e. flow will take place in axial direction and that’s why this compressor is termed as axial flow compressor. 

There is very important point that we must know about the designing of the axial flow compressor and that is the flow annulus area. Annulus area will be decreasing along the direction of flow or from lower to higher pressure end of the axial flow compressor. 

You might have one question that why it is important to have decreasing annulus area along the direction of flow. 

Let us recall the continuity equation 

Mass flow rate (m) = Density (ρ) x Flow annulus area (A) x Velocity of flow (Vf

When fluid flow will take place, pressure will be increasing along the direction of flow and therefore density of fluid will be increased. Mass flow rate i.e. m will also be constant for steady state flow and therefore if we want to keep axial velocity of flow constant, we will have to decrease the flow annulus area. 

Pressure and velocity variation through a compressor stage 

Following figure displayed here indicates the pressure velocity variation through a compressor stage.
Working fluid will be directed by the inlet guide vanes in the axial direction. Working fluid will flow through the number of rotor blades and stator blades in a direction parallel to the axis of rotation of the drum or disc of the axial flow compressor. 

Pressure in the inlet guide vanes will be dropped slightly as displayed in figure due to the frictional losses. While velocity of flow will be increased slightly as the cross-sectional area of the inlet guide vanes will be slightly converging type. 

Once the working fluid will pass through the rotor blades, energy will be imparted to the working fluid in terms of static pressure energy and kinetic energy and therefore the velocity of flow and static pressure of working fluid both will be increased and it is displayed in figure. 

Further working fluid will pass through the stator blades i.e. diffuser of the axial flow compressor. There will be the diffusion action over there and therefore static pressure of the working fluid will be increased and velocity of flow will be decreased. 

Working fluid coming out from the stator blades will have high static pressure with relatively lower velocity of flow. 

Above diagram is the pressure velocity diagram for the axial flow compressor. 

Therefore, we have seen here the basics of axial flow compressor, working principle of axial flow compressor, difference between the axial flow compressor and centrifugal compressor, advantage of axial flow compressor over centrifugal compressor, construction of axial flow compressor and finally we have discussed that the pressure velocity diagram for axial flow compressor. 

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Further we will find out, in our next post, difference between fans and blowers.  

Reference:  

Fluid Machines, By Prof S.K. Som 
Image courtesy: Google   

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