Grant Erwin wrote:
Think about it. Do you really think a pump pumping into twice the
pressure is going to flow double the amount of input air? That's
what you're claiming if the specs are CFM of pressurized air.

I did think about it. I really do think that, yes. This is a 2-stage
compressor, so the first stage always "sees" a fairly constant load.
However, I don't want this thread to degrade to "will too .. will not
.." or some such unhelpful discourse. Let me ask you - if they are
specifying input air then why would that change with output pressure?

I would expect it to change somewhat, the air is not going to flow as easily
into a fully pressurized tank as it would into 0 psi. As it turns out, it
doesn't change much. Why would it flow double the CFM at a higher pressure?


And why say an output pressure at all? Why not simply read "14.7 cfm
@ 170 psi" at face value? To me that reads "this compressor will
deliver 14.7 cfm of compressed air into a tank if that tank is at a
pressure of 170 psi". Why shouldn't I read this spec this way?

Grant Erwin

Well, that's what I always thought, too, but I think Richard Kinch has
enlightened us and the compressor manufacturer websites seem to support his
position. Measuring the inlet air makes sense from a practical point of view
as well, since it eliminates converting for pressure, which would make
comparisons more difficult. Ned Simmons has steered us in the right
direction regarding the potential energy of compressed air, which is
definitely lessened when you allow the volume to increase without recovering
any energy in the process. The transformer analogy is flawed, transformers
work both ways and with minimal loss. The same cannot be said for pressure
changes in compressed air.