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

[jimmyw]

Get your nose out of the hypothetical world of textbooks and formulas and go to your local LDS. Ask them to transfill a tank at 3,000psi into a tank at 0psi. During the process, put your hand on the valve of the donor tank. Then put your hand on the receiving tank.

You are the one arguing that expanding gas always cools off. I'm the one arguing it doesn't.
The expanding gas in the receiving tank heats up. We're discussing why. If you don't think expanding gas always cools, then your original argument only makes sense if you still believe that gas going from one tank at 3000 psi to two tanks at half that pressure is compressing, not expanding.

 

[rjack321]

You are the one arguing that expanding gas always cools off. I'm the one arguing it doesn't.

LMFAO!

The expanding gas in the receiving tank heats up. We're discussing why. If you don't think expanding gas always cools, then your original argument only makes sense if you still believe that gas going from one tank at 3000 psi to two tanks at half that pressure is compressing, not expanding.

Has it ever dawned on you that you're the only one arguing that the gas in a tank being filled is experiencing "expansion"?

There is no "heat of expansion"

 

[lowviz]

brings me back to the Monty Python "argument" skit..... ...//...

That was nothing short of inspired. Thanks for a good laugh.

Maybe Marg or Q would be willing to put on a trenchcoat and put us out of our misery...

Best!
lowviz

 

[jimmyw]

Has it ever dawned on you that you're the only one arguing that the gas in a tank being filled is experiencing "expansion"?

Yes, I've thought of it. Except that Wikipedia calls it expansion. Joule called it expansion - that type of expansion was named for the guy - Joule expansion - and his name was given to the fundamental units of energy, so he must have known something about it. Doppler called it expansion, at least as I interpret his comments and the non-Wikipedia link to "free expansion" calls it expansion.

There is no "heat of expansion"

We agree. That's why I asked the question. How can expanding gas heat up? I thought expanding gas cools down. It doesn't. If you add or remove zero energy, it just expands at constant temperature. I didn't know that when I started. Now I do. You still have to explain how the expanding gas gets hot, but it's a lot easier if you know that all the gas taken together shouldn't change temperature.

It's easy to see why we come down on opposite sides. It explains why this bothered me for years and it didn't bother you. I believe that gas starting out in one high pressure tank is expanding when it's released to fill two tanks at lower pressure. I believed that expanding gas always cools, so I saw an inconsistency. You believe that the gas entering the second tank is being compressed and it's being heated by compression, while the gas leaving the first tank is expanding/cooling and that fits naturally with your belief that expansion is cooling and compression is heating.

I respect your position. You, and others, believe it in good faith, but you are wrong. Gas that starts in one tank and ends up in two tanks and twice the volume is expanding. There's no way to call that compression. Joule called it expansion and it was named for him - Joule expansion.

If I accepted that gas expanding from one tank into two tanks is actually undergoing "compression" there could be no such thing as expansion. As soon as you make the container even slightly bigger, a first one or two gas molecules would enter that new volume, then all the rest of the gas would rush in and "compress" the gas in the new volume. It just doesn't work that way. You look at the starting volume and the final volume of all the gas and if the total volume has increased, it's called expansion. It is expansion.

 

[RJP]

Yes, I've thought of it. Except that Wikipedia calls it expansion. Joule called it expansion - that type of expansion was named for the guy - Joule expansion - and his name was given to the fundamental units of energy, so he must have known something about it. Doppler called it expansion, at least as I interpret his comments and the non-Wikipedia link to "free expansion" calls it expansion.

We agree. That's why I asked the question. How can expanding gas heat up? I thought expanding gas cools down. It doesn't. If you add or remove zero energy, it just expands at constant temperature. I didn't know that when I started. Now I do. You still have to explain how the expanding gas gets hot, but it's alot easier if you know that all the gas taken together shouldn't change temperature.

Joule expansion that you keep citing is about "expansion into a vaccuum" . That's not the situation here. That's why no one here needs to explain it... it's irrelevant.

In the real world, the gas in the receiving tank has not expanded into that tank. It was compressed into that tank.

 

[rjack321]

Yes, I've thought of it. Except that Wikipedia calls it expansion. Joule called it expansion - that type of expansion was named for the guy - Joule expansion - and his name was given to the fundamental units of energy, so he must have known something about it. Doppler called it expansion, at least as I interpret his comments and the non-Wikipedia link to "free expansion" calls it expansion.

Jimmy there is no other explanation. Its not expansion. Halfway down that transfill whip the gas changes from expansion to compression. The gas cools on expansion and the heats up on compression, exactly as adiabatic cooling and compression would describe.

Gas in the donor going from 3000 to 2900 psi is expanding.
That same gas on the other end of the whip in the receiver going from 100 to 200psi is compressing.

All of your wiki articles are describing (various) different circumstances, not the filling of scuba tanks.

 

[iluvtheocean]

i apologize if this hijacks the thread -- but it goes along with the original post --

If a tank is hot filled to above the max pressure and allowed to cool (no water tank avail). How long might it take to cool sufficiently to top it off to max pressure? (assume steel HP100,3440psi,75-80F room temp) best guess is fine for me. i'm just curious since many of the shops i've seen don't have water tubs for cool down.

 

[RJP]

i apologize if this hijacks the thread -- but it goes along with the original post --

If a tank is hot filled to above the max pressure and allowed to cool (no water tank avail). How long might it take to cool sufficiently to top it off to max pressure? (assume steel HP100,3440psi,75-80F room temp) best guess is fine for me.

"How long is a piece of string?"

The answer to your question depends on:

- How overfilled is it overfilled? If I fill it to 3800psi it might well cool to 3500psi
- How hot is it? I can fill your tank to 3440psi very slowly and not get it too hot. If I slam 3440 into it, it will get very hot. Total amout of time it takes to give you a "good fill" might be the same, but the latter approach gets hotter, so more of the total time will be allocated to letting the tank cool.

The answer to your question depends on:

- How overfilled is it overfilled? If I fill it to 3800psi it might well cool to 3500psi
- How hot is it? I can fill your tank to 3440psi very slowly and not get it too hot. If I slam 3440 into it, it will get very hot. Total amout of time it takes to give you a "good fill" might be the same, but the latter approach gets hotter, so more of the total time will be allocated to letting the tank cool.

However, if I set up one of the new Catalina/Joule 80cf "vaccuum expansion tanks" and put it next to your tank the cooling of the mythical expanding gas scuba tank might cool your tank... and enable faster fills!

 

[jimmyw]

Joule expansion that you keep citing is about "expansion into a vaccuum" . That's not the situation here. That's why no one here needs to explain it... it's irrelevant.

If I close the fill valve and stop the process of expansion into the "vacuum empty" tank after 10 seconds, the gas will be hotter in that tank. We now have gas in the originally vacuum empty tank, just like the gas in a not-vacuum empty scuba tank. Now I reopen the valve and let the process continue. More heat is released. The tank being filled continues to heat up. You think all this heating is irrelevant to the heat produced when we fill a scuba tank? I don't. The same process is at work in Joule expansion and scuba tank filling. In both we are letting high pressure air out of one tank so it ends up filling two tanks. This has nothing to do with compression. You can't just conveniently ignore the facts.

If I accepted that gas expanding from one tank into two tanks is actually undergoing "compression" there could be no such thing as expansion. As soon as you make the container even slightly bigger, a first one or two gas molecules would enter that new volume, then all the rest of the gas would rush in and "compress" the gas in the new volume. It just doesn't work that way. You look at the starting volume and the final volume of all the gas and if the total volume has increased, it's called expansion. It is expansion.

 

[rjack321]

If I close the fill valve and stop the process of expansion into the "vacuum empty" tank after 10 seconds, the gas will be hotter in that tank. We now have gas in the originally vacuum empty tank, just like the gas in a not-vacuum empty scuba tank. Now I reopen the valve and let the process continue. More heat is released. The tank being filled continues to heat up. You think all this heating is irrelevant to the heat produced when we fill a scuba tank? I don't. The same process is at work in Joule expansion and scuba tank filling. In both we are letting high pressure air out of one tank so it ends up filling two tanks. This has nothing to do with compression. You can't just conveniently ignore the facts.

If I accepted that gas expanding from one tank into two tanks is actually undergoing "compression" there could be no such thing as expansion. As soon as you make the container even slightly bigger, a first one or two gas molecules would enter that new volume, then all the rest of the gas would rush in and "compress" the gas in the new volume. It just doesn't work that way. You look at the starting volume and the final volume of all the gas and if the total volume has increased, it's called expansion. It is expansion.

You are a scientific juggernaut. Please work this into your next post.
Metric expansion of space - Wikipedia, the free encyclopedia

---------- Post added February 27th, 2013 at 12:34 PM ----------

i apologize if this hijacks the thread -- but it goes along with the original post --

If a tank is hot filled to above the max pressure and allowed to cool (no water tank avail). How long might it take to cool sufficiently to top it off to max pressure? (assume steel HP100,3440psi,75-80F room temp) best guess is fine for me. i'm just curious since many of the shops i've seen don't have water tubs for cool down.

Fills at about 300psi per min don't get all that hot and typically cool to room temp in about 5 hours. The first hour cools the fastest, but the rate of cooling is dependent on how hot the compressor outlet gas was, the cylinder material, if the cylinder is in the sun etc. Overfilling about 2-300psi is usually enough to compensate for the shrinkage post cooling on my (slow) compressor. It party depends on how empty the cylinder was and how much new gas is needed to fill it. Rapidly filling 1000psi/min from a bank, that gas is wicked hot. Overfills of 600+ psi might be required to have the hot filled cylinder cool to rated pressure. That is a much faster rate of fill than cylinder manufacturers recommend so most places don't fill that fast.