[Shop-talk] single vs. dual stage compressors

[Randall]

> I thought that the point of 2-stage was higher PSI, but that 
> doesn't seem to
> be the case here.

I think you are confusing maximum pressure with the pressure for a rated
flow.  "19.1 cfm @ 125 psi" does not mean it won't go higher, only that 125
is where they measured the 19.1 cfm.

Also note that compressor cfm figures are always kind of misleading; the
cubic feet are actually measured at atmospheric pressure (scfm,
approximately) rather than the quoted pressure.

> Also, what is 19.1 cfm@ 125 PSI at 90 PSI?  I bet it's a ton.

Not really.  Because the ratings are really scfm, and 2-stage compressors
keep their efficiency to higher pressures than single stage ones do; a
2-stage rated for 19.1 @ 125 will still produce about 19.1 @ 90.  

IOW, the (s)cfm stays pretty much the same, until the pressure gets high
enough that the compressor's efficiency drops off.  In theory you could
change that by using a variable speed motor (or transmission), but I've
never heard of an air compressor that does so.  It's much easier to just
design everything for the normal operating pressure, and let the motor loaf
at lower pressures.

Randall