> How did you get that 4 cfm @ 90 PSI tool requirement equates to 28 scfm
> compressor capability? A 7X factor.
Ok, the confusion is "scfm" vs "cfm". "scfm" stands for "Standard Cubic
Feet per Minute", and as you noted, a "scf" is one cubic foot measured at a
specified pressure, temperature and RH. Ignoring the fine details, that
translates to free air conditions, ie 1 atmosphere and 70F (or maybe it's
60F, I forget offhand). (RH is a relatively small effect, we'll ignore it
here.) So, when someone talks about "scfm @ 90 psi", they actually mean the
number of cubic feet after the air has been allowed to expand back to one
atmosphere (and warmed to room temperature). This is the way almost all
compressor ratings are given, even when they only call it "cfm". Some
manufacturers call it "inlet cfm", which amounts to pretty much the same
thing except measured at the existing conditions at the compressor inlet
(which is still room temperature and pressure). Almost no one gives actual
outlet cfm, because it's such a small number.
Air does not follow the "ideal gas law" perfectly, but for our purposes it's
close enough. The ideal gas law is usually given as "PV = nRT", which means
that, for a given amount of air, pressure times volume divided by
temperature is a constant. IOW, to double the pressure (at the same
temperature), you have to halve the volume. And, if you double the volume,
you halve the pressure. Note that these units are all absolute, the
important part for our discussion is that the pressure is referenced to a
vacuum, not to atmospheric ("gauge") pressure. To translate gauge pressure
to absolute pressure, add one atmosphere. And again as you noted,
atmospheric pressure varies with altitude and barometric pressure; but for
the sake of simplicity I'm assuming it's 15 psi.
So, 90 psi (gauge) is 105 psi absolute, which is 7 atmospheres. If you
start with a standard cubic foot of air, and raise the pressure by a factor
of 7 (from 1 atm to 7 atm), the volume will decrease by a factor of 7 (once
it's cooled back to the starting temperature, and ignoring effects like
condensed water and whatnot). Thus, 1 cfm @ 90 psi is the same as 7 scfm @
90 psi.
You can find a more rigorous treatment at
http://www.cleandryair.com/scfm_vs__icfm_vs__acfm.htm
> Does that mean that I need a compressor that will produce 49 to 140 cfm?
> TP tools second largest compressor the Quincy QT-10 (dual stage 10hp)
> $2699 produces 39 cfm @ 100 psi. So the second largest compressor they
> sell is not adequate for their smallest blast cabinet/gun? Something
> doesn't seem right about this.
Well, that's why almost all compressors come with sizeable air tanks. The
deal is you very rarely use an air tool for very long at a stretch, so the
tank allows you to use a larger tool than the compressor alone will support,
for a limited amount of time. Kind of like the duty cycle on a welder, you
get so many minutes of use and then you have to wait for the tank to fill
again.
Of course, I _could_ be wrong about all this, and the TiP site certainly
implies so. But, if you take a 4 cfm die grinder and connect it to a 20 cfm
compressor; and run the grinder continuously, the compressor will not only
never catch up and shut off, but the pressure will continue to drop until
the compressor can keep up; at about 60-70 psi.
Randall
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