SCFM and CFM??

[Wade Lippman]

I have a PC compressor that is 2.6scfm at 90psi. I am considering replacing it with CAT that does 2.2cfm. How do they compare? I have googled it, but only find gibberish. Thanks.

[Grant Wilkinson]

What do you use it for? How big is the tank in the CAT? Clearly, you are losing a bit of cfm, but that will only be an issue if the tools you are using need more air that the CAT can produce. Even if you are borderline, you may find the CAT runs too much.

[Wade Lippman]

If a cfm equaled a scfm it would be fine; 15% less capacity would be adequate. But I can't find a converter between the two.

[Ken Combs]

CFM is simply the displacement of the pump x the speed. So it's a calculated theoretical number that a pump will never actually deliver.

SCFM is also calculated but it starts with the measured air delivery of a specific pump/motor combo. That measurement is then adjusted to reflect the conditions (temperature/humidity/atmospheric pressure) at the time of the measurement. Those conditions are never the same as the standard set so the results are corrected to the standard to allow comparison of different units.


There is not direct accurate correlation between the two since the pump efficiency isn't known. Pumping efficiency changes with air pressure. at low pressures they move close to their displacement. As output pressure rises delivery drops. The CAT pump's 2.2 has no pressure rating so it's likely zero. So, I'd just assume 80 percent or so and go from there. I'd guess the 2.2CFM would produce somewhere around 1.8 SCFM, so a significant difference from the other, 2.6SCFM PC unit.

Whether you need that air flow is unknown to me.

[Mike Gresham]

The problem is you don't have the information to make the conversion. You would need to know the temp and barometric pressure the CFM measurement was made at. CAT could have done that, but didn't. They could also have done the test at 68F and sea level BP which produces the SCFM reading, but apparently didn't. I don't know why they chose that route.

[Ken Combs]

The problem is you don't have the information to make the conversion. You would need to know the temp and barometric pressure the CFM measurement was made at. CAT could have done that, but didn't. They could also have done the test at 68F and sea level BP which produces the SCFM reading, but apparently didn't. I don't know why they chose that route.

Probably due to cost. Small, low priced unit, costly testing and not likely to influence sales. They probably did no tests at all. Just calculated the displacement.

[Doug Garson]

Is this the CAT compressor you are considering? http://www.californiaairtools.com/ultra-quiet-oil-free-air-compressors/2-0-hp-air-compressors/cat-10020c/
If so it s also rated at 90 psig. I'm a little rusty on this but my guess is that your PC compressor capacity is misstated and should be CFM not SCFM. SCFM is standard cubic feet per minute which is at 14.696 psia (psia is absolute pressure 14.696 psia = 0 psig where psig is "gauge pressure" or the pressure you would read on a pressure gauge or manometer. You cannot state the capacity as SCFM at 90 psig since SCFM means at 0 psig. So the CAT compressor capacity is about 15% less than the PC compressor.
If the PC compressor capacity was actually 2.6 SCFM then at 90 psig it would only be 0.5 CFM.

[andrew whicker]

CFM is simply the displacement of the pump x the speed. So it's a calculated theoretical number that a pump will never actually deliver.

SCFM is also calculated but it starts with the measured air delivery of a specific pump/motor combo. That measurement is then adjusted to reflect the conditions (temperature/humidity/atmospheric pressure) at the time of the measurement. Those conditions are never the same as the standard set so the results are corrected to the standard to allow comparison of different units.


There is not direct accurate correlation between the two since the pump efficiency isn't known. Pumping efficiency changes with air pressure. at low pressures they move close to their displacement. As output pressure rises delivery drops. The CAT pump's 2.2 has no pressure rating so it's likely zero. So, I'd just assume 80 percent or so and go from there. I'd guess the 2.2CFM would produce somewhere around 1.8 SCFM, so a significant difference from the other, 2.6SCFM PC unit.

Whether you need that air flow is unknown to me.


SCFM is just CFM at some standard conditions as far as I know. I work in a company that designs large centrifugal compressors for oil and gas. The volume flow the customer needs is given in CFM at their process conditions or as SCFM. If it is SCFM, they just have to give us the conditions (pressure and temperature) that were used to calculate the SCFM. Then we convert it to CFM at the process conditions. No efficiencies needed. It's just a conversion.

I'd imagine that in this case, the PC compressor company provided you with a typo. You can't have SCFM at 90 PSI, unless they are trying to say 90 PSI at 68 F and X% of Relative Humidty. Maybe that's what they mean. I'd ignore it. Probably true that the air compressor industry uses the same temperature and relative humdity standards.

Regardless, the link given above gives us:

PC Compressor: P = 90 PSIG / V = 2.6 CFM

CAT: P = 90 PSIG / V = 5.3 CFM

edit: used PSIA when I meant to use PSIG. oops

[Doug Garson]

Looks like the link I provided is not the CAT compressor the OP was considering, not sure what happened because when I looked at it it was for a compressor with 2.2 CFM at 90 psig. There is a 1 HP compressor linked to on that page on the right hand side that is rated at 2.2 cfm at 90 psig. Note psig not psia. psig is what you read on a pressure gage, psia is 14.7 psi higher and is not generally used except in some volumetric calculations. Perhaps the OP could confirm if this is the one he was looking at. Try this link http://www.californiaairtools.com/ultra-quiet-oil-free-air-compressors/1-0-hp-air-compressors/cat-4610ac/