Weight flow, w, may be calculated from any conditions of interest using the following equation (derived from Equation 100-3):
Refer to the nomograph in Appendix D for quick conversions between weight flow and inlet cubic feet per minute.
Weight flow can also be determined from SCFM:
Zo is often taken as 1.0 regardless of its actual value. It is important to use the same value for Zo in all calculations. Although the discrepancy would generally be no more than one or two percent in a single calculation, it could be compounded after conversions are made back and forth by several parties involved with the compressor project. Be consistent.
When specifying compressors, it is best to use weight flow and MMSCFD or SCFM, and to clarify the standard conditions to every one involved.