You are looking to purchase the DeWalt air compressor. Assuming the compressor o

Question

You are looking to purchase the DeWalt air compressor. Assuming the compressor operates (outputs) air at 90psi and at 110F. If the atmospheric pressure is 0 psig and the atmospheric temperature is 65F, how much free air will the compressor consume in cubic feet (ft3 )/Minute to handle the 5 standard cubic feet per minute (SCFM)? • DeWalt Specifications -- 1.6 HP electric engine gets the job done. High-pressure motor with consistent draw requirements can be run with a 12-Gauge or higher extension cord that is under 50 ft. 200 psi maximum pressure enables long air-tool run time Delivers 5 SCFM at 90 psi for quick recovery time

Explanation / Answer

When you speak of PD devices, either mass flowrate or volume flowrate at standard conditions (i.e scfm) remains constant irrespective of discharge pressure, keeping suction pressure constant, but actual volumetric flowrate (i.e acfm) changes. The calculation from scfm to acfm and vice-a-versa is ACFM = SCFM*[(Pstd - (Pvpstd*RHstd))/(P1-(P1*RH1))]*(T1/Tstd) Standard conditions are Pstd = 14.696 psia, Tstd = 60deg.F and RHstd = 0% P1, T1 and RH1 are pressurised air parameters. Generally, for small flowrates, the term P*RH can be neglected and the simplified formula will be ACFM = SCFM*(Pstd/P1)*(T1/Tstd) If the air temperature is same before and after compression (you can consider the temperature after aftercooler) then the equation is much simplified to ACFM = SCFM*(Pstd/P1) Likewise, CFM1/CFM2 = (P2/P1)*(T1/T2) All these terms are in absolute units. If your examples says the flowrate at 175psig is 25 acfm then at 90psig, it is 25*(175+14.696/90+14.696) = 45.3 cfm (at actual conditions) This method can be applied where you are reducing pressure by a pressure reducing valve after the air is compressed to 175psig at the compressor. If the compressor setting is changed to 90psig, rather, then adiabatic conditions are to be used for volumetric calculations.

Reference https://www.physicsforums.com/threads/calculating-cubic-ft-min-and-psi.155966/

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