On Tue, 30 Dec 2003 02:55:01 GMT, "ATP"
wrote:

Don Foreman wrote:
On Sun, 28 Dec 2003 23:56:33 -0500, Ned Simmons
wrote


It's easy to demonstrate, without resorting to
thermodynamics, that air expanding thru a regulator loses
some of its capacity to do mechanical work, and that energy
does *not* remain in the air behind the regulator.

If "some" means more than a few percent, please say more. Consider
a reservoir of 1 cu ft volume containing 200 PSIG air, and a piston
& cylinder of 1 sq inch area lifting a 50 lb weight. First exhaust
air directly from the reservoir into the cylinder: how far does it
lift the weight? Then consider a regulator set to 50 PSI between
reservoir and cylinder and repeat the experiment. Let's assume
adiabatic expansion (no heat exhange with the environment) to keep
things simple enough for me to understand.

I think the results would be about the same but I'm definitely open
to learning why not.

That's a good example. It exposes the fallacy in considering the regulated
air to be escaping into an open system. They would be close to being the
same in such a closed system, which is a somewhat similar system to some
practical compressed air uses. The air is going to flow until an equilibrium
is established at 50 psi. The inefficiency here is in compressing the air to
200 psi rather than 50 in the first place. In this case very little is
wasted. But in the extreme case of compressing air to 175 psi only to let it
flow through a spray gun at 5-10 psi, there is a great deal of waste, and
that's the way this thread started.

Yes, that makes sense. Real compressors aren't isothermal so the
compressed air is hot. Some of the work done by the compressor goes
into heating the air, and that heat is lost to the environment so the
process is inefficient. Thanks for the "get back on track" nudge!