The equation is (P1*V1)/T1 = (P2*V2)/T2,
where P1 and P2 are pressures before and after compression/expansion,
measured in Absolute units, V1 and V2 are the volumes before and after
compression/expansion, and T1 and T2 are the temperatures before and after
compression/expansion, also measured in absolute units.
Absolute pressure = gauge pressure + 14.7 psi (units are psia)
Absolute temperature = degF + 459.9 deg (units are degrees Rankine or degR)
Thus, if you apply this relationship to the example Randall gave, 1 cu ft of
air at room temperature and pressure (0 psig or 14.7 psia, 60 degF or 519.9
degR) compressed to 15 psig (29.7 psia) will occupy just under .5 cu ft,
assuming the temperature stays constant.
The website at http://www.cleandryair.com/scfm_vs__icfm_vs__acfm.htm has a
pretty good explanation of the terms used in Air Compressor specifications.
Usually, manufacturers of industrial compressore define their capacity in
SCFM.
Michael Marr
1960 TR3A
Whitewater, WI
> -----Original Message-----
> From: owner-triumphs@autox.team.net
> [mailto:owner-triumphs@autox.team.net]On Behalf Of Randall Young
> Sent: Sunday, November 16, 2003 1:02 AM
> To: triumphs@autox.team.net
> Subject: RE: Air Compressor Redux
>
>
> > I'm not sure how to translate CFM at one PSI to CFM at another PSI
> > (maybe it's exponential by PSI or something), but it would seem that if
>
> Another slightly tricky part is that we are talking about "gauge"
> pressures
> here, meaning relative to atmospheric pressure rather than absolute.
>
> So, if you start with a cubic foot of room pressure air, and
> compress it to
> 15 psi (roughly twice room pressure), it's only 1/2 cubic foot.
> Carry that
> a little farther, and you'll hopefully see that "19.5 scfm @ 100
> psi" really
> means "2.5 cfm @ 100 psi" meaning that big, honkin air compressor
> won't keep
> up ...
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