Don Foreman wrote:
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!

Wasn't trying to get you back on track, I like some of the twists and turns
this thread has taken, and your post corrected my thinking which had gone a
bit off track. I was initially thinking that if we allow a gas to expand, we
have lost potential energy since the larger volume of gas at a lower
pressure is capable of doing less work. This is not the case if the gas does
work in the process of that expansion, as in your example. The additional
turbulence/restriction created by the regulator must be accounted for, but
it may not have practical consequences in every application.

In any case, as a practical matter, we should strive to limit pressures,
allowing just enough headroom for variations in the volume needed, and use
turbines or vortex blowers for high volume/low pressure application. Venturi
jets are appropriate for some hi vol applications, more efficient than just
allowing the compressed air to expand.