Compressor Curves in Hysys - Off-design correction

[Heyerdahl]

Hi
when "Off-design" correction is applied in Hysys due to a change in MW and T with reference to what the original curves are based on, Hysys corrects the Head, Flow and Speed based on the new MW and T.
The question is; The corrected Head and Flow are ok, but what does the corrected speed mean? In the documentation a formula is given for a corrected Speed. I thought a compressor impeller (variable speed) gives a certain speed and that doesn`t change without installing a new impeller?

Best regards
Heyerdahl

 

[apetri]

for a dynamic compressor if the head increases the flowrate decreases,
head is related to density (and Molecular weight) so if you change composition (and Molecular Weight) the operating conditions change,
polytropic head is related to speed (see performance curves) and you can adjust speed for a new condition, the real accuracy of these estimates is difficult to predict being related to each compressor characteristics, you can find correlations in books discussing compressors.
for preliminary design I do these calc's in Excel with a process library (Prode Properties) and a suitable correlation, then I contact the manufacturer for a detailed analysis.