Equation 300-11 can be used to obtain a quick and rough estimate (plus or minus 10%) of compressor horsepower. It was developed for large low-speed (300-450 RPM) units handling 0.65 specific gravity gases and having stage compression ratios above 2.5. The equation may produce a result that is as much as 20% low for high-speed compressors (900-1200 RPM) due to the higher valve losses in these machines.
Bhp = (22) (MMCFD) (f) (n) (r)
r = compression ratio per stage (should be between 2.5 and 4.0)
n = number of stages
f = 1.0 for single-stage compression
= 1.08 for two-stage
= 1.10 for three-stage
roa = overall compression ratio
Equation 300-11 will also provide a rough estimate of horsepower for gases with higher specific gravities or for lower stage compression ratios, with modification. For specific gravities between 0.8 and 1.0 use a multiplication factor of 20 instead of 22 in the equation. Likewise, use a factor of 16 to 18 for stage compression ratios between 1.5 and 2.0.