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

[Diver0001]

This is sort of a geeky physics/gas law question, but I thought someone here might be able to answer it for me or at least point me in the right direction. I've been diving for over 40 years, but I've never really understood why tanks get hot when you fill them from other tanks. I signed up here just to ask.

Charles law.

R..

 

[redacted]

Charles law.

R..

I believe Charles Law relates to a diabatic process. That is, there is an external thermal source involved: Charles's law - Wikipedia, the free encyclopedia

 

[Diver0001]

Really... they don't teach this in the states as related to scuba?

R..

 

[JohnN]

Well, we could have made it 100,000 cc's of gas. It can't be the volume of the gas that's important. Each 1cc should act the same. If one cc cools, then 2 should cool, and 100,000 should, too. I'm certain that the 1cc in the fill tank does cool and the 1 cc in the scuba tank does heat up.

Ignore the source of the gas. The gas in your scuba bottle goes from low pressure to high pressure. If you were to fill it with Helium it would cool, if you fill it with air it will get warm, if you fill it with 100% O2, it will get warmer.

Sic stat

 

[randini]

Charles law.

R..

Guy-Lussac's law is closer : P1/T1 = P2/T2

@OP: The physics doesn't care if you are filling the tank from a compressor or from higher pressure tank. In your original post you were on the right track when you talked about how the energy put into a tank from a compressor could be recovered by opening the tank's valve. That's what you're doing to the donor tank to fill the recipient tank: you open it's valve! Moreover you should notice that that the donor tank cools as the air leaves it (pressure decreases).

From your ideal gas law discussions, the volume does not significantly change because the tank's walls are rigid (you'd need a hydrostatic test station to measure the tank's expansion). What you are adding to the tank is molecules of gas (i.e. 'n' in the IGL), thereby increasing the density of the air inside the tank (hence why it gets heavier and negatively buoyant as you fill it) by compressing it (adding pressure).

 

[rjack321]

The pressure in the tank being filled is rising, there is adiabatic heating - even if the gas is cool to start with.

 

[jimmyw]

Charles law.

I know this isn't the answer either. Charles law essentially says that at a constant pressure as absolute temperature increases, the volume of the gas also increases in proportion. We aren't looking at constant pressure and I'm trying to understand why some of the gas gets hot as it expands while other parts of the gas get colder. I'm pretty sure it has to do with the fact that this is an irreversible process in which entropy increases and the fact that the scuba tank is not thermally connected to the fill tank.
It's not Charles's Law, Dalton's Law, Boyle's Law, Henry's law or even the Ideal Gas Law. I'm becoming more confident that it has to do with the path the second 1cc parcel of air follows through P, V and T as it moves from the fill tank to the scuba tank.

---------- Post added February 21st, 2013 at 02:56 PM ----------

Ignore the source of the gas. The gas in your scuba bottle goes from low pressure to high pressure.

Well that's the whole point of my question. The gas in the scuba tank doesn't go from low pressure to high pressure. It starts at high pressure in the fill tank and room temperature. It ends up in a larger volume, at a lower pressure and a higher temperature. Just focus on the gas. It's at a high pressure in the fill tank and ends up at a lower pressure in the scuba tank. There's no way to avoid that and any explanation has to take that into account.

 

[Diver0001]

When temperature changes, pressure changes

When pressure changes, temperature changes

expanding gasses cool. compressing gasses warm up.

That's pretty much the gist of it. It's not that hard.

R..

 

[jimmyw]

@OP: The physics doesn't care if you are filling the tank from a compressor or from higher pressure tank.

This can't be right. The situations are completely opposite. In the first case, you have a large volume of air (in the room) at room temp/low pressure and you compress it with the compressor. In the second case, you have a small volume of air (in the fill tank) at room temp/high pressure and you expand it. I don't mean to be stubborn, but one is expansion and one is compression.

In your original post you were on the right track when you talked about how the energy put into a tank from a compressor could be recovered by opening the tank's valve.

I agree. That's why I put it in there. That energy has got to be the source of the heat.

That's what you're doing to the donor tank to fill the recipient tank: you open it's valve! Moreover you should notice that that the donor tank cools as the air leaves it (pressure decreases).

I'm comfortable with understanding why the donor/fill tank is cooling off. But something is missing from the story when it comes to the gas in the scuba tank. That gas is at room temperature and it's at high pressure and a smaller volume. I've been working on this since I posted, and I'm now convinced that if I somehow slowly carried the gas from the donor/fill tank over to the scuba tank and slowly allowed it to expand to fill the tank, that it would cool off, just like the gas in the donor tank.

From your ideal gas law discussions, the volume does not significantly change because the tank's walls are rigid (you'd need a hydrostatic test station to measure the tank's expansion). What you are adding to the tank is molecules of gas (i.e. 'n' in the IGL), thereby increasing the density of the air inside the tank (hence why it gets heavier and negatively buoyant as you fill it) by compressing it (adding pressure).

I'm not sure where you are going with this. The "volume" I'm thinking about is the volume the gas in the donor/fill tank would take up if an imaginary box was drawn around it. That gas has an initial volume and pressure and temp. We know what they are. The same gas has a final pressure temp and volume. We know the volume - it's the volume of the scuba tank, and we want to know the pressure and temp of that expanded gas. I know from experience it's going to get hot, but I want to understand why.

Thanks for taking the time to help me work through it.

 

[redacted]

Well that's the whole point of my question. The gas in the scuba tank doesn't go from low pressure to high pressure. It starts at high pressure in the fill tank and room temperature. It ends up in a larger volume, at a lower pressure and a higher temperature. Just focus on the gas. It's at a high pressure in the fill tank and ends up at a lower pressure in the scuba tank. There's no way to avoid that and any explanation has to take that into account.

Follow the route a bit more closely in terms of pressure. The bank tank is high pressure. But there is quite a large pressure drop at the controlling valve. The drop from that point to the receiving tank may be all the way down to the pressure in the receiving tank depending on whether the receiving tank valve is being used to control fill rate. (When I use an equalizer hose, I control the fill rate with the donner tank so that is where the greatest pressure drop occurs. And that valve gets pretty cold due to adiabatic cooling.) Downstream at the receiving tank, pressure is increasing and so is temperature due to adiabatic heating. Go real slow and the temperature changes are relatively modest. Go fast and they are much more apparent.