In article ,
says...
On Sun, 28 Dec 2003 23:56:33 -0500, Ned Simmons wrote:
In article ,
says...
On Sat, 27 Dec 2003 00:50:14 -0600, Richard J Kinch wrote:


In general, regulation of *any power source* implies an inherent waste
versus the unregulated source: electrical power, compressed air power,
motive power, water behind a dam, etc.

So, if you draw 1 gallon of water a minute through a valve at the base
of a dam, the other 999,999,999,999,999 gallons in the reservoir are
wasted. Obviously not true at all. You use what you use, the rest stays
in the reservoir (with all of its potential energy intact) until you need it.


Another faulty analogy.

Not an analogy. The man said regulation of *any power source* and
specifically listed "water behind a dam". Not all regulators are shunt
regulators which waste any energy their downstream load does not
demand. Most regulation is obtained simply by not allowing any more
energy than the downstream load requires to be released from the
reservoir, be that water behind a dam, the energy stored in a flywheel,
or the energy stored in a capacitor, or a compressed air tank.

OK, let's look at the water behind a dam. If you remove a
gallon of water from the reservoir you lower the potential
energy of the remaining mass of water. The reduction in PE
is the same whether you:

A. remove the water under pressure at the bottom of the dam
and generate electricity,

B. remove the water under pressure at the bottom of the
dam, regulate the pressure down and generate less
electricity,

C. bypass the generator completely and spray the water into
the air, or

D. dip the water out of the top of the reservoir in a
bucket and spill it over the other side of the dam.

In each case the decrease in the mass of the water and the
change in water level (elevation in the gravitational
field) is the same, thus the delta PE is the same in all
four cases.



Since water is essentially
incompressible, the capacity of the water behind a dam to
do mechanical work is almost entirely due to its potential
energy -- its elevation in a gravitational field.

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.

It is easy to illustrate that if you're just venting air to atmosphere,
but it is wrong if you're requiring the air to do work beyond the
regulator. The latter is the normal case except when you're just
bleeding down the tank.

If that were true you'd be able to determine how much
mechanical work had been done by the air that has left the
tank by simply monitoring the gas in the tank. Do you
really think any data you gather *in the tank* will tell
whether the air that has *left the tank* was used to
perform mechanical work, or simply released to the
atmosphere?

The same aplies in the case of the water in the reservoir
above.


Example, assume an air cylinder with a piston surface area of
12 square inches and a stroke of 1 foot. If you feed that cylinder
50 PSI air regulated from a 60 gallon 175 PSI tank, you can do
600 ft-lbs of work by lifting a 600 pound weight the 1 foot stroke
of the cylinder. Now vent the cylinder and fill it again from the
regulated source. You do another 600 ft-lbs of work. Do it again,
and again. You can keep doing it until the pressure in the tank
falls below 50 PSI.

Now try it without the regulator. You can't do it as many times,
because energy stored in the tank has been *wasted* at each fill,
ie the pressure in the cylinder and tank equalize each time you
open the fill valve even though that's more energy than you *need*
to lift the weight one foot, and that excess energy is lost when you
vent the cylinder. This is a clear case where the *absence* of a
regulator wastes energy. It is in fact the *usual* case.

This is like saying there is no loss in a shunt regulator
because it can be used to supply the proper voltage to a
device that may self destruct or draw excessive current in
the absence of the regulator. (Just had to work in an
electrical analogy.)

Ned Simmons