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

[Doppler]

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.

Well, I was thinking real world and the empty tank has not been vacuum-pumped, so it does indeed have air in it, and yes, it will be ten litres and not one as I mistakenly wrote. Apologies for any confusion.

We have 10 liters of air at 200 bar and temp 273 K (0° Celsius) ....

no we do not... We have a ten litre cylinder at 200 bar which translates as 2000 litres of gas. And actually, I have done the calculations required (not math)... A cursory glance at my profile will indicate that I've been teaching this stuff for a while and have a little background in applied physics... which is actually about as useful as an ashtray on a motorcycle with this stuff, but whatever... Point is, I did the arithmetic.

I'm game - let's do it

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?

1 and 2) Yes, this is then stored as the potential energy in the cylinder at 200 bar... lots of potential for that compressed gas to do work... we could run a haskel booster for example... but that would be energy inefficient... remember the three laws of thermodynamics: You can't win, you can't tie, and you can't get out of the game.

3) Yes, this process is called transfilling. Divers do it often. I had to do it last week. I used a transfill whip to transfer some diluent from a steel 16 litre cylinder to a steel 3 litre cylinder for my rebreather. (sidebar... a more useful and instructive topic and a little homework for you is this. The starting pressure in the 16 litre steel cylinder was 230 bar, while the three-litre was empty. When the pressure in the two had equalized, what was it, and how many litres of gas (in this case a 10/50 trimix) were left in the supply cylinder and how many in my diluent bottle? I am not interested in what the temperature shift was... but you are welcome to let us know.)

4) A portion of that potential energy is converted to heat. Some of that heat is dissipated through system components rather rapidly, some heats the gas that is compressed... and is then dissipated a little more slowly. Some remains in both tanks as potential energy. The net effect is that energy is neither created or destroyed... in keeping with the first law of thermodynamics and all that jazz... it is simply converted and spread around a little... wasted if you like.

This is all we need to know.

Your remaining "arguments" -- if I am reading them correctly -- seem to assume that energy can be created without any more work being added. It seems to me you are arguing a kind of perpetual motion machine.

Anyhow, I am done here.

 

[jimmyw]

Absolutely false. The full tank is losing energy and the empty tank is gaining energy.

We agree. The entire "system," meaning all the gas, gets no energy added, and the reason the empty tank gas gets hot is that energy is lost from the full tank and that energy is gained by the gas going into the empty tank. I agree 100%. If you let the two tanks freely mix, since no energy was added, the temperature would be unchanged and both tanks would remain at room temp.

If you look at all the gas in the case of the direct compressor fill, it's different. There, the gas starts at room temp/pressure and gets more energy as we compress it into the donor tank..

---------- Post added February 25th, 2013 at 04:17 PM ----------

1 and 2) Yes, this is then stored as the potential energy in the cylinder at 200 bar... lots of potential for that compressed gas to do work... we could run a haskel booster for example... but that would be energy inefficient... remember the three laws of thermodynamics: You can't win, you can't tie, and you can't get out of the game.

3) Yes, this process is called transfilling. Divers do it often. I had to do it last week. I used a transfill whip to transfer some diluent from a steel 16 litre cylinder to a steel 3 litre cylinder for my rebreather. (sidebar... a more useful and instructive topic and a little homework for you is this. The starting pressure in the 16 litre steel cylinder was 230 bar, while the three-litre was empty. When the pressure in the two had equalized, what was it, and how many litres of gas (in this case a 10/50 trimix) were left in the supply cylinder and how many in my diluent bottle? I am not interested in what the temperature shift was... but you are welcome to let us know.)

4) A portion of that potential energy is converted to heat. Some of that heat is dissipated through system components rather rapidly, some heats the gas that is compressed... and is then dissipated a little more slowly. Some remains in both tanks as potential energy. The net effect is that energy is neither created or destroyed... in keeping with the first law of thermodynamics and all that jazz... it is simply converted and spread around a little... wasted if you like.

This is all we need to know..

And with those answers, you seem to arrive at exactly the same conclusion I did. Energy stored as potential energy in the donor tank is released from the donor tank as the gas in it is allowed to expand and cool. That energy is is transferred to the gas being sent to the receiving tank and takes the form of heat. That's the answer. I don't know where you think the "perpetual motion" came from - nothing that I argued or meant to say. This answer is 100% right - at least as I see it. So far, it's the only one that answers my original question.

 

[rjack321]

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.

You keep constructing imaginary vessels which you don't have. You have 2 tanks which are partially thermally isolated from one another. Not a single exploding tank doubling in volume.

Do you have a transfill whip? Connect it up and let her rip. The starting valve gets cold. The receiving valve gets hot. Somewhere around 1/2 way along, the hose is neither hot nor cold. Amazing huh?

 

[cool_hardware52]

We agree

No, *We* no longer care.

Please, go turnover the cushions on your "bed", you know the couch in mom's basement. Maybe you and the rest of us will get lucky and you will find a couple quarters.

Use them as a down payment on a clue.

Tobin

 

[jimmyw]

Use them as a down payment on a clue.

I keep getting hammered on. Why not hammer on Doppler? He's got a clue. That's the post I would point to to explain what's happening.
@Doppler: Sorry if that tars you with what seems to be my bad boy/stubborn rep here. I've really tried to be polite and just figure out the answers.

---------- Post added February 25th, 2013 at 04:32 PM ----------

You keep constructing imaginary vessels which you don't have. You have 2 tanks which are partially thermally isolated from one another. Not a single exploding tank doubling in volume.

Thought experiments have a long and honorable reputation

Do you have a transfill whip? Connect it up and let her rip. The starting valve gets cold. The receiving valve gets hot. Somewhere around 1/2 way along, the hose is neither hot nor cold. Amazing huh?

Yes, I do, and it's the results that sent me here. The gas expands from one volume into two and the expanding gas in the donor gets cold, while the expanding gas in the receiving tank gets hot. I wanted to know why. What's wrong with that?

 

[lowviz]

...//...You can't win, you can't tie, and you can't get out of [-]the game[/-] this thread. ...//...

So now, where were we??? Something to do with entropy reduction, I believe...

 

[Rick Murchison]

Well now, let's see here...
This is actually real simple, once yew know what's goin' on.
Ye see, there's these little critters that live inside scuba cylinders called Adia Bats. Normally they just sit there kind of chillin', but if yew open the tank valve they'll escape, and the loss of their body heat inside the tank causes the tank to cool off.
Now if yew shove the Adia Bats into another cylinder using a fill whip, the trip really irritates the little rascals, and they buzz around inside the filling tank all hot & bothered & ticked off, which causes the tank being filled to warm up.
Once the pressures are equal and the Adia Bats ain't bein' aggravated anymore, they'll settle down to their usual tranquil state, and the tank that got cooler will warm up and the tank that got hotter will cool off, 'till all is once again right with the world.
Now that wasn't so hard, was it?

Rick

 

[cool_hardware52]

Now that wasn't so hard, was it?

Rick

Just wait, you'll soon see the folly of your ways.

Tobin

 

[rjack321]

Thought experiments have a long and honorable reputation


Yes, I do, and it's the results that sent me here. The gas expands from one volume into two and the expanding gas in the donor gets cold, while the expanding gas in the receiving tank gets hot. I wanted to know why. What's wrong with that?

IF you had one tank and you instantly doubled its volume then yes it would cool down.

You don't have 1 tank, you have 2 tanks and they are thermally isolated from one another. And the receiving tank is being compressed. It started out at zero and ended up at 1500psi. That's compression.

You're obviously very hung up on the "it started out at higher pressure it must be expanding" issue. But despite all your incorrect arguments, gas will only flow from high pressure to low. So all gas going "into" a tank is "expanding" by your flawed logic. Its not expanding - the receiving tank is being compressed. The midpoint of transfill hose is where the gas from the donor tank ceases to be expanding and starts to be compressed.

 

[jimmyw]

IF you had one tank and you instantly doubled its volume then yes it would cool down.

The equations don't care how quickly the process is done, only whether we add any energy. There are no "rate of change" terms in the gas laws. We are taking the gas in one tank and letting it expand into two.

You don't have 1 tank, you have 2 tanks and they are thermally isolated from one another. And the receiving tank is being compressed. It started out at zero and ended up at 1500psi. That's compression.

No, We have gas that starts at 3000 psi and let it drop pressure as it expands. That's expansion.

You're obviously very hung up on the "it started out at higher pressure it must be expanding" issue.

Yes, I am. It's the source of all my questions.

But despite all your incorrect arguments, gas will only flow from high pressure to low.

Agreed. It goes from a 3000 psi tank to a 0 psi tank.

So all gas going "into" a tank is "expanding" by your flawed logic. Its not expanding - the receiving tank is being compressed. The midpoint of transfill hose is where the gas from the donor tank ceases to be expanding and starts to be compressed.

Just look at a 1cc parcel of high pressure air that starts in the donor tank and flows through the whip to the receiving tank. It ends up at a lower pressure. Another 1cc parcel of air that doesn't flow through the whip stays in the donor tank. It ends up at the same lower pressure. Both expand due to the lower pressure. I know the starting temp pressure and volume of both 1 cc parcels. I know the final pressure of both - they are the same and are close to 1500 psi. I don't see how you can say that after they both end up at half their starting pressure one is compressed. it isn't. They have both expanded as a result of being allowed to drop to half their initial pressure.