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Should compressor Np be adjusted for RPMs to compare to design?

Should compressor Np be adjusted for RPMs to compare to design?

Should compressor Np be adjusted for RPMs to compare to design?

(OP)
I have calculated a polytropic efficiency for a centrifugal compressor at the set of conditions when the survey was conducted.  I also have the design curve showing the predicted efficiency vs icfm.  

Latest survey:
RPM = 8610
icfm = 14600
Np = 75.6

From Design Curve:
RPM = 8800
icfm = 14600
Np = 79.3

Does this mean that the efficiency is 3.6% below design or do I need to adjust to the design RPM?  And if so, how do I do that?

I've used Fan Laws to adjust polytropic head for RPM but I can't find any info on the efficiency.

Thanks!

RE: Should compressor Np be adjusted for RPMs to compare to design?

(OP)
Hi folks, just wondering if perhaps I need to move this to a different forum (i.e. gas compression engineering)?  Or perhaps my question needs rephrased or more info supplied?  

I have several surveys of this compressor at different RPMs and I'm just trying to understand if I can compare the efficiencies directly or if I need to compensate for the speed of the compressor.  

Thank you!  

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