Why do tanks get hot when you fill them from higher pressure tanks?

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I'm game - let's do it :)


Great - yes 10 litre tanks.


If you insist on keeping air in the second tank, OK. If you can do it with a vacuum in there, the math is easier, but I'll do it your way.


OK


Remember, the "2000 litres in one cylinder" are at a totally different pressure from the receiving tank, and you said "one litre in the other" when I think you meant 10 litres in the other.

I'm going to work out the numbers this way -
We have 10 liters of air at 200 bar and temp 273 K (0° Celsius) .... I got to this point and then realized you didn't actually do the math. I have done the math. Repeatedly. Is it worth going through it? To do that, I have to state whether we are adding energy to the gas or not. That's why I asked these questions:

1) Did it take energy to compress the gas into the donor tank?
2) Could we get some of that energy back by putting a pneumatic motor/generator in the whip line between the tanks and driving the generator by the pressure differential during the fill?
3) Would the recipient tank and donor tank equalize pressure regardless of whether we put a pneumatic motor/generator in the whip line?
4) What happens to the energy we could have extracted from the gas with the pneumatic motor/generator if we don't put the pneumatic motor/generator in the whip line?

Are they that hard?

Let me give the answers I think are pretty clear. If my answers are wrong, tell me which ones are wrong. At least give me some hints on where you think this is wrong:

1) Did it take energy to compress the gas into the donor tank?
A: Yes. We have to do work on the gas to push it into the donor tank. The compressor does that work. As a side note, this is called a reversible adiabatc process of constant entropy. The internal energy of the gas is raised as it goes from 80cf at STP to a higher temperature pressure and smaller volume. Because this process is "reversible" (another way of saying the entropy is unchanged), we could extract the stored energy by letting the gas expand and do mechanical work on a turbine/generator. The gas would cool if we did that and return to STP. The cooling would exactly counteract the heating from the original compression. The extracted energy could be used to reverse the process again and push the gas back into the donor tank, which would heat it up. In theory, we can go back and forth - compress hot - expand cool. Below I will say we could let the expanding gas out into a tall vertical cylinder with a weight on a piston. We could let it push the cylinder of mass m upwards to a height h and store mgh potential energy. We could then push the cylinder and weight downward to compress the gas and make it hotter. We can go back and forth (neglecting friction).

2) Could we get some of that energy back by putting a pneumatic motor/generator in the whip line between the tanks and driving the generator by the pressure differential during the fill?
A: Yes. As described above. We don't have to use a pneumatic motor and generator. We could let the gas expand into a cylinder and push a weight on a piston upwards and use mgh where m is the mass of the rising piston and h is the height the gas pushes that weight up. It doesn't matter how we extract the energy or how we store it. What is important, however, is that if we *do* extract the energy, all the gas in the donor tank, including the gas we let out of the donor tank into the cylinder not only expands, it also cools. Again this is a reversible constant entropy process. I'm sorry I'm using technical terms, but they accurately describe what's happening.

3) Would the recipient tank and donor tank equalize pressure regardless of whether we put a pneumatic motor/generator in the whip line?
A: Yes. If we let the gas push a mass upwards, it's a reversible process, and we extract energy, but if we were to just let the gas into an empty cylinder, with the weight already at the top of the cylinder, it would be an irreversible, process. If I can be excused for using another technical term, this type of expansion is called constant enthalpy. The constant enthalpy process is not reversible, but has constant internal energy of the gas. Either way, the final pressure would be the same in both tanks, but in one case, the temperature of the gas would decrease and in the other (called "free expansion") the temperature would be unchanged in the "Joule expansion = free expansion" process - provided we let the gas freely mix. If we don't let it freely mix, some gas is hotter and some gas is colder.

4) What happens to the energy we could have extracted from the gas with the pneumatic motor/generator if we don't put the pneumatic motor/generator in the whip line?

A: The energy that could have been extracted is released as heat into the gas in the receiving tank. Instead of the expansion cooling the gas in the receiving tank, as happened in the reversible process, it is released as heat, and there's just enough energy to counteract the cooling of all the gas from the expansion that would have occurred if we'd let the expansion raise the weight in a reversible process.

I can do the math on all of this. I can start with 80cf of air at STP and calculate how hot it gets when we compress it reversibly. If preferred, I can let the excess heat escape and figure out the pressure (PV is constant since T is constant) I can then assume no energy (no work no heat) is added to the gas and determine the final temp during free expansion. It will be room temp if all the gas is allowed to mix and equalize temp. I can also calculate how hot the gas in the receiving tank gets if we assume that the gas in the donor tank expanded reversibly, and all the released energy is transferred to the gas in the receiving tank. I can even calculate what happens if there is residual gas in the receiving tank. And if necessary, I can even add in the final details of assuming that the gas is not ideal so that the J-T coefficient is non-zero.

It's long and boring, and I suspect that it won't convince anyone, but it convinced me.

If anyone wants to understand the physics, they have to figure out the difference between extracting energy from the compressed high pressure gas in the donor cylinder versus not extracting/storing that energy and letting it appear as heat in the receiving tank gas. All of this works for the ideal gas assumptions, and for real world gases.

Here's another example - we let the gas from the expanding donor tank push a bullet through the whip hose. The bullet has mass and picks up energy, then slams into the receiving tank and it's kinetic energy converts to heat. It's the same as letting the expanding gas of the donor tank push and accelerate the gas leaving the donor tank, but it might be easier to see it that way.

Ok. You've conviced us...

[video=youtube;vYabrQrXt4A]https://www.youtube.com/watch?v=vYabrQrXt4A[/video]

That being said, now that you've explained what happens if the tanks are on the moon or there's a magic bullet in the fill whip, can you explain just one more thing that I'm not clear on...

Why do tanks get hot when you fill them from higher pressure tanks?
 
Jimmy,
Nobody cares. All the questions of merit have been answered.
Tobin
I care. The thread is up to 44 "likes", has many posts and I've seen the question asked by others on the web, located when I was searching for the answer, so perhaps someone else will eventually ask the same question. You seemed to recognize that the net heating of all the gas in the system is zero since no energy is added or removed. That's an important point that I missed when I first asked the question. I thought that gas always cools when it expands. I had no knowledge of what "free expansion" of gas was all about and didn't know that gas could expand without changing temperature. I didn't know that the cooling was a result of extracting energy from the gas as it expanded.

If you had to steer someone to a single post that you think best answers the original question, which one would you point to? I'd really like to look it over.
 
If you had to steer someone to a single post that you think best answers the original question, which one would you point to? I'd really like to look it over.

The very first reply answered the original question to the satisfaction of anyone but the most ardent pedant.

If someone insisted in carrying on after reading that one, post 18 nails it pretty well with all the science needed.

(Note to self: call Mythbusters to find out if a full scuba tank will implode if it is shot.)
 
Ok. You've conviced us...
I don't want to convince you. I want to make sure I haven't made a mistake. You don't have to believe me, and you are free to attack me when you can't explain why I'm wrong, but I'm still going to be as polite as I can be. When I'm certain that I'm right, I'll leave (or if others ask me to leave). When I'm certain I'm wrong, I'll admit that and thank everyone for their help. Many here have been extremely polite about their answers, even when they disagree with me. Some of those answers were very helpful to me.

For many, many, years I didn't know the answer, but I was uncomfortable with the claim that it was "just the same as filling with a compressor." I knew that couldn't be right. Filling with a compressor adds energy, filling from a tank doesn't. Filling from a a compressor is compression, filling from another tank is expansion. They are very different operations. Now I've got an answer and at least one piece of the puzzle - "free expansion." Perhaps it's wrong or incomplete, but it doesn't leave me with the feeling I had before that the answers others gave me were clearly wrong.

---------- Post added February 25th, 2013 at 03:53 PM ----------

post 18 nails it pretty well with all the science needed.
Post 18 says:
When pressure changes, temperature changes
expanding gasses cool. compressing gasses warm up.
That's pretty much the gist of it.

During free expansion, gas temperature is unchanged and the gas arriving in a vacuum empty receiving tank from a donor tank gets hotter, even though it's expanding from the higher pressure donor tank. I don't see that post as telling anyone very much. It's where I started, and it's why I asked my question, since it's clearly wrong or at least incomplete. Most beginner scuba classes required us to learn Charles's law, the ideal gas law, Dalton's law, Henry's law and a few others, and they are a lot more explanatory than post 18. I expected more from people having advanced training.

Thanks for the reply, anyway.
 
Filling with a compressor adds energy, filling from a tank doesn't.

If you don't believe that a scuba tank with 3,000psi of gas in it has "energy" in it - a WHOLE LOT of energy - there is no help for you ever understanding this.

[video=youtube;NkhtTPU9KjM]https://www.youtube.com/watch?v=NkhtTPU9KjM[/video]
 
The very first reply answered the original question to the satisfaction of anyone but the most ardent pedant.

If someone insisted in carrying on after reading that one, post 18 nails it pretty well with all the science needed.

Jimmy you have to accept the fact that gas going INTO a tank, is compressing. The source whether a compressor or another tank will ALWAYS be at a higher pressure. There must be a pressure gradient for it to flow into the receiving tank in the first place. That does not matter.

The source gas is not "expanding" into the tank being filled. All of your math and "entropy" talk is distracting from the fact that you think the gas is "expanding" when its actually being compressed.

Your assumption is wrong and all subsequent arguments are wrong because of that. Re-read post #2, esp the middle paragraph. It answers your question.


You need a bit more physics/physical-chemistry.

If the gas were ideal (remember PV=nRT is the "Ideal Gas Law"), there would be no heating/cooling on compression/expansion. Almost all gases are not ideal. The deviation from this (how much they cool when they expand or heat when compressed) is called the Joule-Thompson coefficient (Joule?Thomson effect - Wikipedia, the free encyclopedia) and is the basis of how your refigerator/air-conditioner work.

Back to filling tanks, you should expect to see a cooling of the tanks doing the filling (their pressure is reduced) and an increase in temperature of the filled tank (its pressure is increasing).

FWIW, Helium has a negative Joule-Thompson coefficient at 1 atm (it warms when it expands and cools when it compresses)
 
If you don't believe that a scuba tank with 3,000psi of gas in it has "energy" in it - a WHOLE LOT of energy - there is no help for you ever understanding this.
I do believe it has energy, but the sum of the energy in the scuba tank plus the energy in the donor tank is unchanged. That's the point. We don't add any energy to the gas and some gets hot and some gets cold.
 
I do believe it has energy, but the sum of the energy in the scuba tank plus the energy in the donor tank is unchanged. That's the point. We don't add any energy to the gas and some gets hot and some gets cold.

Ah, so you DO understand!

You just don't seem to UNDERSTAND that you understand.
 
Filling with a compressor adds energy, filling from a tank doesn't.

Absolutely false. The full tank is losing energy and the empty tank is gaining energy.
 
Jimmy you have to accept the fact that gas going INTO a tank, is compressing.
This would be so easy if this were true. I weld a vacuum empty tank onto the side of a high pressure tank and drill a hole between them. The gas in the high pressure tank rushes into the vacuum empty tank and the pressure equalizes as the volume doubles. The gas in the high pressure tank started at high pressure and expands into double the volume and you want me to accept that the gas was compressed? I just can't. It isn't compressed. It now occupies double the volume and is at a lower pressure. That's expansion. There's no way to turn it into compression. I put a piston cylinder on the side and push teh piston donw to zero volume, now let the gas from the first tank into the cylinder and let that gas push the piston away - same thing. It's expansion.
 
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