How to calculate air compressor capacity and power needed to drive it.

GoceKU

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I have an old truck air compressor pump with no specs for it. I took off the head and measured the cylinder bore and stroke and want to calculate how many CFM i can expect and how big of a electric motor i need to drive the compressor pump. The cylinder is 74mm bore X 28mm stroke that makes the cylinder 120cc, now how to calculate CFM and how much horsepower i need to turn it and how fast.
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As a rough estimation, the energy required to compress a gas is equal to the volume x the change in pressure and the power required will be equal to the volume x pressure increase divided by the time. The volume per rev. x rpm/60 will be the volume/sec.. Your displacement is 120cc or 1.2E-4 m^3. so 1.2E-4/60 x rpm will be the volume/time and that times the change in pressure will be power required. Assuming 100 rpm at the pump and a pressure increase of 100 psi (=6.895E5 nt/m^2) the power required will be 1.2E-4/60*100*7E5 or 140 joules/sec. = 140 watts. The volume delivered would be 2E-4 m^3/sec or 1.2E-2m^3/min which would be .424 scfm for this side of the pond.

If you wanted a larger scfm, You would need to increase the pump speed accordingly. 500 rpm would give you about 2scfm but the power requirement will go up proportionately to around 700 watts. This would be under ideal conditions. Factoring motor efficiencies frictional losses, etc. I would estimate the power required would probably be about twice as much.

(Hopefully, I did all the math correctly ;))
 
Thank you RJ, i thought this pump has a bigger capacity. I'll have to spin it at 1000 rpm to get something usable (4CFM). I have this electric motor 1,5Kw (2HP) with 1400rpm. The pump has a 8,8" pully and i'll need a 5.7" one on the motor to get 1000 rpm at the pump. The pump is a truck part so i think even if its been under driven diesel engines rev to 3000 rpm to 1000 rpm should be fine with some cooling.
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