Centrifugal Compressor Curves - variation due to MW, Ts, Ps

[centeng11]

I have a query regarding centrifugal compressor curves that is starting to drive me a bit crazy; make that very crazy. According to the polytropic head equation, at the suction of the compressor, an increase in gas molecular weight (MW) will cause a subsequent decrease in the polytropic head (H). Likewise, an increase in gas suction temperature will cause an increase in polytropic head, and so forth.

BUT while quite reputable compressor books say this, they then show compressor curves (H vs Q) whereby the head curve for a heavier gas is above that of a lighter gas, and it's the same case for the suction termpature, which is opposite to the polytropic head equation. Surely any variation in the curve follows the formula. I'm very confused.

Can someone very kindly explain to me why this is the case. Why does a heavier gas for example raise the head curve whereas the formula says it polytropic head will be reduced.

Thanks in advance.

 

[centeng11]

I am very suprised that I never got a response to my post above considering that it forms the fundamentals of compressor design any engineer involved in the field should know.

Anyway, I figured it out in the meantime.