about the rtg if the heated helium is released would it cause a refrigeration effect and if so would the psi change

Sir: You compress a gas ...say helium, .....it gets hot,(confined mass ,concentrated
heat), at which point you find a way to draw off the heat....( radiator...heat exchanger
etc) and then release the compressed...cooled gas through an expansion valve.....
= refrigeration effect , and the gas is at lower pressure ......return to compresor.

lets say u heat a steel tank externally (unsafe) but lets say u release the gas into a steam or air engine would that obsolete the heating if the pressure drops due to refigration
refrigration is not my goal

Odd terminology.

Due to the pressure drop of moving the piston, the temperature would drop acording to the Ideal gas law. This does not "obsolete the heating" because mechanical power has been produced. Heat energy was converted to mechanical energy. This is the purpose of the Stirling radioisotope generator.

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That's how a steam engine works. Superheated steam enters they cylinder and moves the piston causing the steam to expand, as it expands it cools. There is enough heat energy left in the steam to repeta the exercise several times. A Marine steam engine has 3 or 4 cylinders of increasing size and the steam exhusts from one to the other, expanding and cooling each time until it finaly exhausts into a heat exchanger (radiator if you like) where the last of the heat is extracted condesing the steam back into water for return to the boiler.

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What do you mean by "obsolete the heating"?

But thats exactly how a steam Boiler works. It's a big steel tank that you heat up and it's perfectly safe. How is what you are describingany different to any steam plant?

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There are, in fact, refrigeration systems that run on propane instead of compressors. Still does the job.

Dan

Yes, they are often used in Recreational Vehicles. I think a small pilot light provides the energy required.

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obsolete the heating i mean if it cooled when released into the engine it pretty much pointless to heat in the first place
so the gas would cool after it leaves the engine and enter a low pressure atmosphere yes or no

If you do not heat the working gas, the engine will grind to a halt and do no useful work. You cannot get something for nothing.

Technically you could run an engine on compressed gas drawn from a high pressure tank, but you would not get much range. The same tank can store much more usable energy in the form of fuel which provides heat for the engine while under way.

When the hot gas expands while pushing a piston or spinning a turbine, yes indeed it cools.

after it expands the piston or during

It cools while expanding. That is during the power stroke, and subsequently while escaping through the exhaust valve, if the pressure at bottom center is still higher than the ambient pressure.

A bit of a nitpick: The Ideal gas law does not predict a drop in temperature. In fact, I believe that an "ideal" gas would not change temperature, but instead would merely change its volume (The "V" in PV = nRT) when the pressure changes.

But since "real" gases are not ideal, they can experience a change in temperature when they expand. Usually the gas will cool, though in some cases (I know hydrogen gas is one) the gas will heat up.
</nitpick>

There is a very interesting side note in this: THINK....we waste....yes WASTE
a hell of a lot of heat from the exhaust manifold throughout the exhaust system ....and draw 15 horsepower for the air conditioning compressor.
Imagine for a moment that we employed the tremendous heat at the exhaust to the refrigerant( like the RV refrigerators) .....hmmm........ almost something for nothing. We need every mile per gallon we can get.

It could wor.....nah !!

Dan

Although it has been some 40 years since my thermodynamics course in college physics, I am completely confident that you are mistaken in saying that an ideal gas would not cool upon expansion.

A gas that is under pressure in a cylinder is applying a force to the piston. If we let the piston move under that pressure, thus letting the gas expand, the gas will be doing work, which means a transfer of energy. If the cylinder is insulated so no heat can pass one way or the other across the walls, that energy transfer will be realized as a loss of molecular kinetic energy. Presto! The gas becomes cooler. No non-ideal characteristic is needed for this.

Can you provide a link to a source that backs up your belief that hydrogen warms on expansion?

But what if there's no piston? Connect two cylinders - one with gas under pressure and one in a vacuum - with a valve. Open the valve. Isn't the gas now doing a lot less work?

I would describe it as working on itself. We still have force acting on each infinitesimal sample of the gas over a distance. That is work, and the energy expended in doing the work comes from the thermal kinetic energy of the molecules.

Try Googling "adiabatic expansion". There are plenty of references out there that can explain it better than I can, provided you have a taste for calculus.

Yeah, I agree. But I also agree with the nitpick because at least mathematically, the ideal gas formula doesn't indicate a temp change when volume is doubled and pressure is halved.

As to the OP: Useful cooling relying soley on the expasion of a gas from a higher pressure to a lower pressure? I don't think so simply because I don't know of any examples where that system is used comercially. It's change of state - from a liquid to a vapor - where small system size equates to large cooling capacities.

What was "ace holmes" referring to in the original post? I know that "RTG"
means "radioisotope thermoelectric generator" in the context of spacecraft
power sources such as on Voyager and Cassini, but it isn't entirely clear that
that is what he's talking about. The helium he mentions must be the helium
emitted by the decaying plutonium-238. I'm pretty sure that all heat sources
using plutonium-238 are vented to allow the helium to escape, so that there
is no pressure buildup, by design.

-- Jeff, in Minneapolis

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