How capacity control is achieved by IGVs in centrifugal compressor?

[realit]

Ok, it is clear that Inlet Guide Vanes effectively controls the flow rate. But, what I do not understand is the exact thermodynamic or physical phenomenon happening at IGVs. How they reduce the outlet flow for same inlet flow?

 

[LittleInch]

http://www.eng-tips.com/viewthread.cfm?qid=91398

Please do some searching on this site first then come back if you don't understand the answer.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[realit]

Littleinch, I already had read that thread. I got what it says. But, what I want to know is how the throttling (i.e., reduced pressure) ultimately results into reduced flow and constant pressure at compressor's outlet.

 

[rotw]

realit,

Please be realist. I agree with LittleInch as your OP requires to read first the basics. If you have a question it has to be specific then we will be glad to help.

You are also mixing flow, pressure etc concepts.

Varying IGVs angle means changing the prerotation angle at inlet of impeller. To understand why this happens you need to study a bit speed triangles and Euler equation. In practice, the compressor is capable to generate more or less head at different IGVs angles, the curve is more or less steep also depending on the prerotation angles.

Considering the same suction conditions, if the pressure at outlet needs to increase say because it is required by process, IGV angle can be varied so that a higher pressure ratio is developed by the compressor without having to reduce the inlet flow provided the surge limit is not reached. Conversely if the pressure at discharge is required to be lower, it normally means you need to increase the inlet flow (potentially applying recycle) to stay on the operating curve, however varying the IGV angle will here again permit to operate at the same flow capacity at reduced discharge pressure.

At higher outlet pressure, the actual volume flow decreases at the discharge - but does it really matter? typically we look at pressure at the discharge, eventually at temperature.



"If you want to acquire a knowledge or skill, read a book and practice the skill".

 

[realit]

Rotw
Thanks
...will take your advice positively.