Polytropic Head of the compressor
Polytropic Head of the compressor
(OP)
Hi,
Many compressor design books suggest that the polytropic head is governed by the impeller gemorty and speed. To which I agree because the vector velocity diagram on the impeller is what determines the head the system along with the angular speed.
However, I have been recently involved in trending some compressor performace through our online PI systems where I am observing something quite contrary. I see a variation in the polytropic head (green line on attached sheet) being directly proportional to the variation in the p2/p1 pressure ratios (yellow trend)...The speed for the system during this time is fairly constant.....
If we go by the first princples the polytropic head (green line) should be fairly stable with changing p2/p1. I also understand that if the gas weight mw changes it should be reflected in the output pressure p2 and not the poly head which should only be dependent on the geometry and speed at the impeller. But this does not seem to be the case in the trends.
Could someone please shed some light on this. Would be much appreciated.
Thanks
Many compressor design books suggest that the polytropic head is governed by the impeller gemorty and speed. To which I agree because the vector velocity diagram on the impeller is what determines the head the system along with the angular speed.
However, I have been recently involved in trending some compressor performace through our online PI systems where I am observing something quite contrary. I see a variation in the polytropic head (green line on attached sheet) being directly proportional to the variation in the p2/p1 pressure ratios (yellow trend)...The speed for the system during this time is fairly constant.....
If we go by the first princples the polytropic head (green line) should be fairly stable with changing p2/p1. I also understand that if the gas weight mw changes it should be reflected in the output pressure p2 and not the poly head which should only be dependent on the geometry and speed at the impeller. But this does not seem to be the case in the trends.
Could someone please shed some light on this. Would be much appreciated.
Thanks





RE: Polytropic Head of the compressor
David
RE: Polytropic Head of the compressor
However, I have been recently involved in trending some compressor performace through our online PI systems where I am observing something quite contrary. I see a variation in the polytropic head (green line on attached sheet) being directly proportional to the variation in the p2/p1 pressure ratios (yellow trend)...The speed for the system during this time is fairly constant.....
the poly head value will change with varying pressure ratio, even if speed is relatively constant. examine the compressor curves.
If we go by the first princples the polytropic head (green line) should be fairly stable with changing p2/p1.
not true. examine the compressor curves and be sure the mfg is basing head on poly eqn and not otherwise. can have a constant speed with varying head and Rc.
I also understand that if the gas weight mw changes it should be reflected in the output pressure p2 and not the poly head which should only be dependent on the geometry and speed at the impeller. But this does not seem to be the case in the trends.
if MW changes from original design, for a given set of inlet conditions, the outlet conditions (namely P, Z & T) will change at a given speed; yet poly head will remain the same. examine the poly head equation and the compressor curves.
hope this helps.
-pmover
RE: Polytropic Head of the compressor