One of the major benefits in doing your own estimates, rather than turning everything over to a manufacturer, is that you develop a better understanding of the application. You are then in a better position to discuss it with the manufacturers, evaluate alternate selections, and even catch errors in manufacturer’s estimates.
Figure 200-25 is a plot of polytropic efficiency vs. inlet volume flow. This chart may be used for estimating polytropic efficiencies.
As discussed in Section100, manufacturers use a computer to calculate compressor performance on a stage-by-stage basis. Performance is based on each preceding stage, new impeller inlet conditions, including compressibility (Z) and k values to determine the individual performance for each successive stage.
If specific stage data is unavailable, overall calculations using average compressibility and a k value based on the average flange-to-flange temperature, will provide reasonably accurate results. (Refer to Section 100 for compressibility equations.)
Estimate overall efficiency from Figure200-25, using average CFM from:
where discharge ACFM is determined using Equation200-14 and an efficiency of 75%.
Determine n-1/n from:
Recalculate head, discharge temperature, and gas horsepower (GHP) from:
w = weight flow in lbs./min.
Estimate brake horsepower using:
BHP = GHP + bearing loss + oil seal loss
where bearing loss is determined from Figure200-26, and oil seal loss is determined from Figure200-27. The casing size in the figures is selected by comparing the cfmavg with the flow range in Figure 200-24.