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If a compressor is rated 10 CFM at 100 PSI, does that mean that 10 cubic feet of air at 100 PSI is generated (per minute) at the output? Or is the 10 cubic feet drawn in (at STP, say) on the input side?

I think it's the latter.

Moles in = moles out.

Here's my equation (assume T is constant for simplicity, and the gas constant is scaled to the units of PSI and cubic feet).

If moles out per minute is 10*(100+14.7)/(RT), and moles in per minute = (CFM_in)*14.7/(RT),

then CF_in per minute must be 10*114.7/14.7 = 78.027. That seems like a lot of aspiration on the input side! For instance, half of that (a 5 CFM machine) is available as portable equipment, and 40 CFM aspiration (intake) for a portable compressor seems like a lot to me...

If the 10 CFM is on the input side, then the CFM on the output side is calculated as

CFM_out = 147/114.7 = 1.22 cfm (@ 90 PSI)

I guess I am ignorant. Is the specified 10 CFM of the compressor measured at the intake or output? It seems like a simple question, but it is not clear who is right when perusing the internet. My modest 10 CFM example implies "a lot of aspiration". If that were changed to 1000 CFM (which does exist), the input "aspiration" would be 7800 CFM. That's wind-tunnel flow! That would empty a large room in 1 minute, an enclosed shop in 10-20 seconds. Of course, such a large machine would probably have access to the Earth's atmosphere...

Another hint is the use of SCFM specifications for Standard CFM. Also, a particular case on the Grainger site specifies a Dewalt 30-gallon compressor with "Free Air Flow Rate: 6 cfm @ 90 PSI". I think this string of characters gives an ambiguous meaning. "Free Air" and "90 PSI" are inconsistent, are they not?

Obviously, the answer is known. Based on the above heuristics, I think compressor CFM refers to the intake flow at STP (or near STP). Can someone confirm this, or disabuse me?

Many thanks!

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  • $\begingroup$ Generally they are at a minimum rated at a CFM with pretty much no load and some max pressure at no flow, all on the output side. If one is generous enough to include a rating with both flow and pressure, it should all be on the output side (the pressure is on the output side so input flow, which is at no change in pressure is less useful when combined with it). $\endgroup$ Commented Jul 4 at 17:37
  • $\begingroup$ Small air compressors (like the Makita MAC2400 in my garage) are rated by free air flow, standard CFM. There are calculators available for compressor power, mine said it needed about 1 hp to meet 4.2 scfm at 90 psi, versus the spec of 2.1 running hp. The same calc at 4.2 of 90 psi air would be 4 hp, which would blow my breaker. Note that air tool air requirments are at pressure because of course they are. checalc.com/calc/compress.html $\endgroup$ Commented Jul 6 at 8:41
  • $\begingroup$ the rating could easily be the volume of air at the intake if the manufacturer and/or seller is unscrupulous $\endgroup$ Commented Jul 6 at 18:18
  • $\begingroup$ @jsotola - thank you, typo corrected. $\endgroup$ Commented Jul 12 at 18:18

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Air compressor specifications include the pressure delivered and the FAD (free air delivered) this is the volume of air drawn in at the input that is delivered at rated pressure at the output.

Clause 3.4.1 of ISO1217 states:

“Actual volume flow rate of a compressor is the actual volume of gas, compressed and delivered at the standard discharge point, referred to conditions of total temperature, total pressure and composition prevailing at the standard inlet point.”

This is the actual volume of air delivered by the air compressor referred back to the conditions of the free air at the compressor inlet. So FAD is the amount of free air drawn into the compressor that is actually delivered by the air compressor at its compressed air outlet.

from bestairpractices.com

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  • $\begingroup$ Thank you so much for finding the definitive answer. The corollary issue is the tool air flow specification. My tool requires (consumes) 10 CFM operating at 90 psi. Here the specified psi is at the input to the tool. I surmise that my compressor, specified at >10 CFM at 150 psi, will be adequate for the tool's "consumption". The language seems ambiguous, which is often the case in technical discussions (hence the existence of standards like ISO1217). The key fact for me is that the tool does not require a flow of 10 CFM of 90-psi air parcels, requiring a much larger compressor. $\endgroup$ Commented Jul 17 at 16:22
  • $\begingroup$ I think that that 10cfm does refer to free air (SCFM) and not volume of compressed air , unless it is specifically marked as ACFM (actual air flow at compressed rating. See here - engineeringtoolbox.com/rating-air-compressors-d_848.html Also doesn't your compressor have an air tank? That should take care of any shortcomings in immediate supply needs. $\endgroup$ Commented Jul 17 at 16:51
  • $\begingroup$ Yes, the 20 gallon compressor tank is a big factor in the duty cycle of the tool (if it is not 100%, due to insufficient compressor capability). I am attempting to perfrom small abrasive blasting tasks. I expect that I may have to work in bursts because blasting operations are among the hungriest in terms of CFM. I'm not refinishing a ship-hull! Just cleaning up small objects, tools, BBQ grills, etc. I had a very small Makita compressor, but that wasn't going to suffice. OTOH, I don't have room for the 60 gallon Dewalt, unfortunately. Gary $\endgroup$ Commented Jul 24 at 15:21
  • $\begingroup$ With a tool operating at 10cfm and 90psi, and with a 20gal tank, each 10psi above 90 in the tank will give you 10 seconds of operation before tank pressure falls below 90. For example 120psi in the tank will give you about 30 seconds of tool operation. A 1HP compressor operating continually will just about double that time. If you have a 2HP comp that should provide enough air for continuous operation. $\endgroup$ Commented Jul 24 at 16:24

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