On Mon, 29 Dec 2003 00:49:00 -0600, 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.

Assuming that there is no friction between the piston and cylinder and
assuming that there was _just enough_ over 50 psi from the regulator to start
the weight moving then:-

with the regulator, the weight would move up at a constant , infinitesimal,
speed until it got where it was going.

Without the regulator, the weight would accelerate until a little below the
speed of sound and carry on at that speed until decelerating to a standstill a
_long_ way above the other weight, and then come back down etc.


the rest is left as an exercise for the student :-)


Mark Rand
RTFM