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

[halocline]

I really think the OP is getting confused because he's feeling the warmth of the tank being pressurized, but not feeling the cooling of the air being released from the original tank. So he thinks that heat energy is being produced without an apparent source. He also doesn't seem to get that the air from the donor tank first gets depressurized to near ambient (assuming the recipient tank is empty) before being pressurized to the final pressure.

He's feeling metal, not air. The air getting pressurized in the recipient tank remains in that tank, in contact with the metal of the tank. So it has more of a radiant effect on the metal of the tank than does the air getting cooled as it exits the first tank. That tank's valve will get very cold, but overall the tank is not going to exhibit the same level of radiant temperature change.

As I mentioned before and many others have explained ad nauseum, if you could measure the true amount of heat loss through expansion and heat gain through pressurization, in the air itself, not the containers, you'd have a net zero or minus.

But it is pretty fun in a banging-your-head sort of way to follow this thread!

 

[redacted]

During the expansion, the stored energy in the compressed gas was released as heat. The amount of heat released exactly counteracted the cooling that would have occurred if the gas had expanded reversibly (if we'd captured the stored energy from the compressed air.)

Again, I thought about it quite a bit last night. The physics all makes sense.

And therefore adiabatic cooling is obviously a myth.!!!!!!

 

[jimmyw]

Jimmy, at least a half dozen of us have told you the same thing in various words. You have been watching too much Big Bang Theory or Nature. This is not quantum physics. Its strictly adiabatic cooling and heating. There is absolutely no energy being added or subtracted by the transfill whip.

The half dozen answers are basically wrong. Ghanguss understands what's going on. To say that it's "strictly adiabatic heating and cooling" misses the effect of the reversible versus irreversible aspect of the process. The donor tank expands more or less reversibly while the scuba tank expands irreversibly.

When we compress gas to get adiabatic heating, we add energy with the compressor. When we do the reverse, we get energy back. The gas in the scuba tank is gas that started out under high pressure in the donor tank and ended up at lower pressure in the scuba tank. That's expansion. The cooling from that expansion is overcome by the energy released by the expanding gas in the donor tank (plus energy released from its own expansion) and all of that energy is released in the form of heat. That's the source of the heat in the scuba tank.

The whole point of the Wikipedia article on Joule expansion is that they do exactly what we are talking about, but they use a large diameter opening so the hot gas from the scuba tank and the cold gas from the donor tank freely mix - leaving the temperature of the gas unchanged, but the entropy increased.

The gas in the donor/fill tank is adiabatically and reversibly expanding/cooling, but the gas in the scuba filling tank is .

 

[cool_hardware52]

I haven't read all the posts, so this may have been covered. I would suggest googling the "refrigeration cycle" If you understand tank filling / gas transfer will hold few mysteries.

Tobin

 

[jimmyw]

And therefore adiabatic cooling is obviously a myth.!!!!!!

No, it's not a myth, but "adiabatic cooling" should really be called adiabatic reversible expansion cooling. If the expansion is not a reversible expansion, there is no cooling. That's exactly what Joule expansion is. "Adiabatic" just means that no heat is added. If you expand "reversibly", you must extract energy in the form of work from the compressed gas and the gas gets cold. If you leave that energy in the gas (in the form of heat), then it expands adiabatically, but you get no cooling. The gas remains at constant temperature during the expansion.

I'm sure you understand that "adiabatic heating" always requires that energy be added to the gas by doing work on it with a compressor. You have to account for that energy when the gas expands. It's got to go somewhere. It can go into heat, as in Joule expansion, or it can be captured and stored (and later used to run a compressor to return the gas to its original compressed state).

---------- Post added February 23rd, 2013 at 12:04 PM ----------

I haven't read all the posts, so this may have been covered. I would suggest googling the "refrigeration cycle" If you understand tank filling / gas transfer will hold few mysteries.

Can you explain further? Why does the gas that started in the donor/fill tank at high pressure and room temperature end up hotter after it has expanded into the scuba tank while the gas that remained in the donor fill tank and expanded got colder?

---------- Post added February 23rd, 2013 at 12:28 PM ----------

I really think the OP is getting confused because he's feeling the warmth of the tank being pressurized, but not feeling the cooling of the air being released from the original tank.

No, I've accounted for both. The scuba tank is filled with gas that is at a lower pressure than it started at. There's no way to get away from that. That's where I started. The air expands when it first enters the scuba tank, but it later gets compressed and the expansion is always greater than teh subsequent compression.

So he thinks that heat energy is being produced without an apparent source.

No. There's no energy being added. That's the important fact that others are missing. We added energy to compress the donor tank and now when that gas expands the energy being released is being ignored, as thought it was never added.

He also doesn't seem to get that the air from the donor tank first gets depressurized to near ambient (assuming the recipient tank is empty) before being pressurized to the final pressure.

See above. I even did the math on this.

He's feeling metal, not air. The air getting pressurized in the recipient tank remains in that tank, in contact with the metal of the tank.

The air in the scuba tank came from the donor tank. It started at some temp/pressure volume and ends up at another temp pressure volume. I was just trying to figure out why it did what it did - some got hot and some got cold even though both expand.

As I mentioned before and many others have explained ad nauseum, if you could measure the true amount of heat loss through expansion and heat gain through pressurization, in the air itself, not the containers, you'd have a net zero or minus.

I've been trying repeatedly to focus only on the gas, not on the containers. I agree 100% that no energy is added to the entire system (donor plus scuba tanks) and no heat is added. However, because of the arrangement of a whip connecting the tanks, the donor tank does work (pushes on the escaping gas, so it cools, while the scuba tank receives that energy and heats. I lso agree that for teh enetire system "you'd have a net zero" of heat gain or loss, but because the donor tank is doing work, it loses energy and the scuba tank gains energy. If we let them mix, then all the gas would be at room temp, but at a lower pressure.

But it is pretty fun in a banging-your-head sort of way to follow this thread!

You should be me I've been asking myself and others the same question for years and I always get responses like this. The first few times I figured I must be missing something. But, it kept bugging me. I know the gas in both tanks expands, but the gas in the scuba tank ended up hotter while the gas in the donor tank ends up colder. I kept getting told that it was "just the same as when filled with a compressor," but a compressor has to be plugged into the wall and you have to add energy to make the gas hotter. Filling from another tank doesn't involve adding energy. Nothing gets plugged in.

 

[RJP]

Can you explain further? Why does the gas that started in the donor/fill tank at high pressure and room temperature end up hotter after it has expanded into the scuba tank while the gas that remained in the donor fill tank and expanded got colder?

Another fundamental error. The gas didn't "expand into the scuba tank." The gas from the donor tank expanded into the donor tank's valve and then the fill whip. That gas was then COMPRESSED into the receiving tank. The pressure of the gas remaining in the donor tank, compresses the gas that expanded into the valve and the whip into the receiving tank. If that wasn't the case, you couldn't get the gas INTO the receiving tank.

Let's try a "duckies and horsies" explanation.

One more time, from the top...

Let's follow two gas molecules - we'll call them Brian and Carl. (You won't be able to follow "a single cc of gas" without getting even more confused, since it will become more cc's as it leaves the tank and then those cc's will again become fewer cc's as they are pressurized into the second tank, with the net effect being that, on average, all the cc's that leave the donor tank... will become TWO cc's in the receiving tank.)

So, donor tank valve is closed and receiving tank valve is open. The tanks are connected by a standard 6ft transfill whip.

Brian and Carl (the two molecules of gas) in the donor tank are at 3,000psi at ambient temperature. Let's say 70F. Since there's lots of molecules packed in there, Brian and Carl (and all the other molecules) are crashing into each other pretty often. The molecules of gas in the empty tank (yes, there is gas in the "empty" tank, roughly 14psi at sea level) are also at ambient temperature, but they are also at ambient pressure. There's plenty of room in the receiving tank, so the molecules in there are crashing into each at a much lower rate compared to the molecules in the donor tank.

[FONT=&amp]When the donor valve is opened, our two gas molecules (Brian and Carl) move into the valve where the pressure on those molecules drops to ambient pressure of 14psi/1atm. With less pressure, these two gas molecules can move further apart and now crash into each other less frequently, so there is a net cooling effect. So the temperature of those two gas molecules drops from room temperature to [FONT=&amp]a few[/FONT] degrees cooler than when they were in the tank. The temp of the gas leaving the tank doesn't drop to freezing, it just drops a bit.

As Brian and Carl enter the fill whip they immediately begin to warm back to ambient temperature. Gas doesn't hold temp very well, it changes temp pretty quickly. (Think about when you take a breath off a reg. The air you breathe is somewhat cooler than ambient temp, but it's not really COLD.)

Our two heros - the first two out of the donor tank - now enter the receiving tank at roughly ambient pressure and temperature. Here they meet some other gas molecules that are at 1atm (14psi) and since the pressure in the tank is low, there's plenty of room for Brian and Carl, so they mingle with their new friends and are not crashing into each other all that often, so all the molecules in the receiving tank remain at about ambient temperature and pressure.

However, those Brian and Carl are not the only two molecules coming into the receiving tank. They are being followed by billions more! And as those other molecules enter the receiving tank, the pressure increases. Dramatically. As the two tanks equalize, the pressure in the receiving tank increases from 14psi to 1500psi. So the pressure in the receiving tank has increased by 10,000%! The receiving tank is starting to get crowded. Brian and Carl and their new friends - all of whom were at ambient temperature and pressure - are now crashing into each other >1,000x more often. Because of this, their temperature increases. As does the temperature of all the other molecules in the receiving tank.

Meanwhile, in the donor tank, the pressure is decreasing as the gas molecules move to the receiving tank. As the pressure decreases, the molecules of gas remaining in the donor tank crash into each other less. So their temperature drops. (At the risk of blowing your mind further, consider this: while the pressure in the receiving tank increases 10,000%... the pressure in the donor tank only drops by 50%! How is that possible?)

So, as the tanks equalize, you end up with one tank cooler than when it started... and the other warmer.

However, the total amount of energy and matter remains the same. Because energy/matter can't be created or destroyed. (Or Brian and Carl, for that matter.)

[/FONT]Perhaps you are further confused by the fact that you are not actually feeling the temperature of the gas, but rather the temperature of the tanks/valves? The names of the tanks and valves are not important, but keep in mind that the temperature of the tanks/valves is a factor of the CUMULATIVE effect of the temperature change of all the molecules passing through the valves and into the tanks. For instance, why does a wide open scuba valve form frost? Is it because the air coming out has dropped to <32F? No, it's because as each molecule of gas passing through the expands it cools and essentially "sucks heat" out of the valve. (At tobin suggested, google "refrigeration cycle" or merely compare the temperature of the inside of your refrigerator to the temperature of the coils on the back of it.)

http://www.rdac.com/sites/default/files/How HVAC Works.pdf

 

[halocline]

Can you repeat the question? hehe....

 

[redacted]

When we compress gas to get adiabatic heating, we add energy with the compressor. When we do the reverse, we get energy back. The gas in the scuba tank is gas that started out under high pressure in the donor tank and ended up at lower pressure in the scuba tank. That's expansion. The cooling from that expansion is overcome by the energy released by the expanding gas in the donor tank (plus energy released from its own expansion) and all of that energy is released in the form of heat. That's the source of the heat in the scuba tank.

I don't have a compressor so I fill my small pony tanks from an S80 and an HP100 using an equalizer hose. When I fill the pony, the doner tank gets cool and the pony gets warm. The temperature of the equalizer hose may get cooler or warmer depending on which tank valve I use to control the rate of transfer.

 

[JohnN]

When we compress gas to get adiabatic heating, we add energy with the compressor. When we do the reverse, we get energy back

That's your problem right there. The fact is that some gasses do this. SOME do the opposite (expansion would warm, compression would cool).

An Ideal Gas would neither.

I'm back to my original point, the JT Coefficient predicts how much cooling or heating takes place (and how much the gas deviates from being Ideal). None of your 'arguments' attempt to explain why compressing O2 generates more heat than compressing N2. In fact if a tri-mix diver were foolish enough to get involved in this discussion, they would confirm that filling a bottle with He causes the bottle to cool.

So forget the Ideal Gas law. It is a predictor of general behavor and is useful to a limited extent, just not for this scenario.

 

[DivemasterDennis]

Strangely, I enjoyed reading this thread. I am not sophisticated about th physics of tank filling, but I have filled thousands of tanks from a bank. Here is some observational data for the scients to incoprate in their continued explanation:
1. The speed at which you fill a tank impacts the temperature of the receiving tank ( and the air in it). The faster you fill, the hotter it gets.
2. As the "hot" tank cools, the pressure reduces due to cooling of the contents, resulting in what was a 3000 PSI fill ending up being a 2400 psi fill ( more or less). The full cooling of the filled tank will take an hour or more on a hot summer day, even if in the shade.
3. If you fill a tank at about 500 psi per minute or less, the heating is minimal, but it takes a long time to fill 50 tanks after the day's dive boat trip is done.
4. If you place the receiving tanks in cold water as you fill them, you can fill faster, as the heat dissipates quicker in the cold water ( just like the heat from a submerged diver's body).

So, if anyone cares to assay this question, please explain the relationship between rte of fill and temperature of air in the recipient tank.
DivemasterDennis