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

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

Filling the empty tank by letting the high pressure air in the donor tank expand into the empty receiving tank. (As before, I continue to agree with the other part of your post).

And the high pressure air in the compressor cylinder "expands" into the empty receiving tank in exactly the same way. The compressor is generating a pressure higher than the pressure in the recieving tank just like hooking up a high pressure bank bottle and transfilling. How do you think compressors work, anyway? magic? What do you think a psi gauge reads out the outlet of a compressor? 0 psi just like room pressure?

If you're filling a tank, the other end of the hose has a higher pressure and the delta-p is driving the gas into the tank. Without a pressure difference (and a higher pressure at the source of the gas) there would be no force on the gas and it would not flow down the hose...

The difference that you're so fixated on (high pressure bottles vs. 0 psi atmospheric pressure) explains why bank bottles cool down when you transfill while compressors are hot.

And adiabatic processes happen to ideal gases, they're not caused by friction, you can get them from the kinetic theory of gases with entirely frictionless ideal gases, and helium, o2 and nitrogen are going to be basically the same modulo the heat capacity of gases (o2 and n2 probably heat up less since energy goes into making the molecules spin and vibrate). Its all little point particles elastically bouncing off the walls of a container and basic freshman physics...
 
I have done a little research on jimmyw - well maybe. Would you believe he is a world champion fly fisherman? He has set records for getting the most trout to rise to the same fly and for getting the same trout to rise to the fly repeatedly. Congrats jimmyw - I think you have done it again in another pool

Well, he caught me....sort of. I've read this whole damn thread and seen the answer to his question explained more than once. I continued to follow hoping to see the moment where the light bulb came on.

Time to unsubscribe.
 
I know it's not because the remaining air in the scuba tank being filled gets compressed. The tank gets hot even if it was at a a vacuum when filled.

Hey JimmyW,

This is a quote from your original post. Here is another perspective for you to consider Re the quote above. It is because of this reason exactly.

You must consider the air leaving the fill tank, and the air inside the scuba tank separately.

The air leaving the fill tank is expanding. No doubt about it. That occurs immediately after the restriction that limits the flow of air between the two tanks.

The air already in the scuba tank is compressing as more gas enters from the fill tank. Even from a vacuum, after the first molecules rush in, those that follow crowd the initial molecules, in effect compressing the group.

From your hypothetical example where two tanks are welded together and a hole drilled between them, both expansion and compression are occurring in the second cylinder!
The expansion immediately after the hole (restriction to air flow), and the compression everywhere else within the tank where the gas is not expanding. (Note the compression only occurs while gas is flowing into the second tank)

Also seeming as though you like formula’s I looked into the changes in stored energy from my first example where a full and empty tank are connected and allowed to equalize.

Compressed air energy storage - Wikipedia, the free encyclopedia

Check out the formula’s for isothermal storage. In particular the example provided. If you plug some numbers into these, you will find that a full tank has more energy stored than two half filled tanks! Which makes sense given that energy must be added to restore the system to its original state ie. One full tank (using a compressor for example). In fact the difference in energy that you have observed is exactly the energy required to restore to the initial state! (with no conversion losses).

Also note that this link refers to an isothermal process, which I referred to previously also. Well actually I referred to two isothermal processes coupled together. The decrease in pressure that would result from cooling the gas, and an increase in volume that would result from heating the gas. In my example, these two processes cancel each outer out to create an iso-thermal (constant-temperature) process, that I can only describe as a thing of physical beauty! The mathematics aint too bad either but I prefer to stick with the physics side of things :)

I hope that adds to your understanding of the problem.
 
Oh please lord stay, I am aiming to stretch this out to 50 pages :D

Ok - I think I know how to make that happen. Hold my beer, and watch this...

"Why does a SpareAir get hot when you fill it from a higher pressure tank?"

"Why does a SOLO DIVER's tank get hot when you fill them from higher pressure tanks?"

"If I use an Air2 will it get hot if I fill my tanks from higher pressure tanks?"

"If I dive with Split Fins and fill my tanks from higher pressure tanks will I be able to do a frog kick?"

"I just filled my tanks from higher pressure tanks and now they are hot and I'm afraid my jacket-style BCD will melt. Should I get a BP/W?"

"I just filled my tanks from higher pressure tanks - where can I get a fully evacuated 7-foot hose?"

"I'm thinking of getting certified. Should I go with PADI or NAUI ? I heard that PADI teaches that tanks don't get hot when you fill them from higher pressure tanks."

"If I fill my tanks from higher pressure tanks will they get hot enough to keep me warm so I can be my own boss and make $100,000 a year (or more!) diving for golf balls?"

"I just heard that GARY GENTILE is writing a new book entitled "Adiabatic Warming Exposed - The Real Story of Transfilling Tanks" wherein he conclusively hypothesizes that John Chatterton could not have entered the engine room of the U-869 by removing and sliding a single tank in front of him because that tank was transfilled from a higher pressure tank and everyone knows that means that Chatterton's tank would have expanded to twice it's original size and never would have fit through the restriction!"

:d
 
And the high pressure air in the compressor cylinder "expands" into the empty receiving tank in exactly the same way. The compressor is generating a pressure higher than the pressure in the recieving tank just like hooking up a high pressure bank bottle and transfilling. How do you think compressors work, anyway? magic? What do you think a psi gauge reads out the outlet of a compressor? 0 psi just like room pressure?

A compressor is provided with energy by plugging it into the wall, or by using muscle energy to press down on the plunger. The added energy appears as heat and increased pressure. The heated air flows into the tank and remains hot. That entire process involves adding energy to the gas. When you allow gas to expand from one tank into two tanks, the gas is expanding and no energy is added. The air leaving the donor tank is room temp at start (not hot like the air leaving the compressor) and gets colder as the process continues (it cools by expansion as energy is extracted from the air remaining in the original tank. The room temp air or colder than room temp air must heat up as it expands into the receiving tank. In the second process (transfilling), no energy is added to the gas. Unlike the first process (filling from a compressor), you don't have to plug in anything or push any plungers down to get the gas to expand out of the first tank into the second tank. You have to explain why the gas expanding in the first tank cools while the gas expanding but leaving the first tank heats up. The total energy stored by all the gas in the system (gas in both tanks) remains constant in the second process, but not the first.

If you're filling a tank, the other end of the hose has a higher pressure and the delta-p is driving the gas into the tank. Without a pressure difference (and a higher pressure at the source of the gas) there would be no force on the gas and it would not flow down the hose...
I completely agree. It's true for both processes.

The difference that you're so fixated on (high pressure bottles vs. 0 psi atmospheric pressure) explains why bank bottles cool down when you transfill while compressors are hot.
Yes. The bank bottle cools down because energy was extracted from the gas there. Where did that energy go? Answer my four questions or read
Doppler's answer and you will know:
quote_icon.png
Originally Posted by jimmyw

I'm game - let's do it
icosm14.gif

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?

{Doppler's answers:}

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.
The energy from the donor tank goes into accelerating the gas leaving the first as it expands into the the second tank. That energy appears as heat when the accelerating gas slows down in the second tank. This process is exactly like the process called "free expansion" or Joule expansion" described here. with one exception - the two gases aren't allowed to mix. Free expansion is a process in which no heat and no energy is added to the gas. That's what's happening in the transfill process. When the total gas is considered, the energy in all the gas is unchanged. If we let the gas in the two tanks freely mix, then the donor tank wouldn't get cold and the receiving tank wouldn't get hot as described in that link. Because we don't mix the gas and because we know the donor tank gets cold, the receiving tank must get hot. That's the answer.

And adiabatic processes happen to ideal gases, they're not caused by friction, you can get them from the kinetic theory of gases with entirely frictionless ideal gases, and helium, o2 and nitrogen are going to be basically the same modulo the heat capacity of gases (o2 and n2 probably heat up less since energy goes into making the molecules spin and vibrate). Its all little point particles elastically bouncing off the walls of a container and basic freshman physics...
Take a look at the free expansion process described in the Wikipedia article link above (also called "Joule expansion")
It explains why expanding gas does not always cool. It only cools when we extract energy from the potential energy stored in the compressed gas (the "internal energy" of the gas). If we just let it expand without extracting energy, it stays at room temp.

The transfill process extracts energy from the gas in the donor bottle, so it cools. It releases the same amount of energy into the receiving bottle, and it's the released energy that heats the receiving bottle above room temp.

Almost no one here will answer the four questions posted above - with the exception of Doppler. His answers are right. Those answers lead you directly to the conclusion that the gas leaving the donor bottle carries extra energy with it that was extracted from the expanding gas remaining in the donor bottle.

I'll ignore the personal attacks. They aren't fair. Those who made them should be ashamed of themselves. They just make this thread annoying to read. I've done nothing but try to understand the underlying physics of the gas. Perhaps I've been stubborn, but when I've asked before, I always got hammered down and gave up, instead of being stubborn, so here I refused to give up until I understood it.

I learned the gas laws in the 1960's, but could never reconcile why expanding gas gets hot in one tank and cold in the other. Anyone who thinks clearly about this knows that when you let high pressure gas out of a tank so it fills both the first tank and a second tank, the gas is now taking up more volume and that's called "expansion." If you read post #2 and those who think the answer is there, you will see how widespread is the belief that I started with - expanding gas always cools. That's what I thought, but if that's true and you believe as I do that gas that goes to larger volume and lower pressure is indeed "expanding" then you have to wonder, as I did for many years, why the expanding gas in the receiving tank gets hot when the expanding gas in the donor tank gets cold. The answer lies in the fact that expanding gas does not get cold just because it's expanding. It only gets cold when energy is extracted. In the transfill process, energy is extracted from the first tank, so it cools and that energy is sent to the receiving tank. The net energy added or extracted to/from the total gas is zero, so the heat/energy delivered to the second tank must balance out the cooling/energy extracted from the first tank.
 
I learned the gas laws in the 1960's, but could never reconcile why expanding gas gets hot in one tank and cold in the other.

I learned the gas laws in prep school and then as an undergraduate in the mid 1980's. I guess by then someone had figured out that the gas in the receiving tank is NOT EXPANDING.

You started with two tanks, each containing 10 liters of gas albeit at two different pressures. You ended the process with two tanks, each containing 10 liters of gas but at the same pressure.

The receiving tank ended the process with more molecules of gas in it than when it started. The only way to do that is for the gas in the receiving tank to have been COMPRESSED.
 
Jimmy...
We have established and you've so far agreed:
All fill sources are at a higher pressure than the receiving tank.
Gas flows from high pressure to low.
A tank starting at 500psi and finishing at 3000 psi is being filled.
Tanks being filled get hot.
The reason tanks being filled get hot is adiabatic heat.
Adiabatic heat is also known as heat of compression.


Yet you persist in defying physics by saying that a tank being filled is being filled from 500 to 3000psi is filling with "expanding" gas. You have to change your prespective.

From the perspective of the donor tank, yes the gas is "expanding", that's why the donor valve gets cold. From the perspective of the receiving tank, its being filled. The heat you feel on the donor tank is adiabatic heat of compression.

Midway along that transfill whip, where the hose is neither hot nor cold, that is where the perspective changes from "expanding" to "compressing".
 
Compressed air energy storage - Wikipedia, the free encyclopedia

Check out the formula’s for isothermal storage. In particular the example provided. If you plug some numbers into these, you will find that a full tank has more energy stored than two half filled tanks!
Thank you for the link. It does add to my understanding. :)

The equations cover an "isothermal" process as you say - i.e constant temperature. The "Joule expansion" or "free expansion" process described in my link above is also an isothermal process. No energy is added or extracted during free expansion. If you let the gas from the first tank expand into both tanks and freely mix, the temperature is unchanged, so it's isothermal, too. Since no energy was added or lost, one might ask how come the two tanks at half pressure have less "energy" than the original tank at higher pressure. It's a great question. Shouldn't the energy stored be the same?

The answer is that the page you link to is about "compressed air energy storage." It' all about how much energy can be extracted from the compressed air. During free expansion, the entropy of the gas increases. Some of the original energy went into increasing that entropy and can't be recovered. We could have recovered it (by using the pneumatic motor/generator discussed in earlier posts), but since we didn't. Once the gas expands and that available energy is dissipated as heat and the entropy is increased, the energy lost can't be recovered. Thus the available "compressed air energy stored" in the two tanks and available to be extracted is less than the amount of energy that could have been extracted from the original higher pressure tank at the same temperature.

---------- Post added February 26th, 2013 at 12:39 PM ----------

Jimmy...
We have established and you've so far agreed:
All fill sources are at a higher pressure than the receiving tank.
Gas flows from high pressure to low.
A tank starting at 500psi and finishing at 3000 psi is being filled.
Tanks being filled get hot.
I don't agree that tanks being filled always get hotter. "Free expansion" of gas involves letting gas expand from one tank into two tanks, and I'd characterize the second tank as being "filled" even though it's not getting hot.

Look. No one here seems to dispute the Wikipedia article on "free expansion." For an ideal gas, if you let it freely expand from one container into another, the temperature is unchanged. For a real gas, it's pretty close to unchanged. When I came here, I thought that expanding gas always cools. Others here seem to think that, too if you look at post #2. It doesn't. The transfill operation is more like the "free expansion" case where no energy is added or subtracted than it is like using a compressor to fill, where energy is added. The difference between direct compressor fill and transfill lies in the difference between free expansion (no energy added to the total gas) and not-free expansion (energy is extracted).

We don't have to come up with some imaginary compression to explain the heating when we all know the gas ends up in twice the original volume and that's called expansion.
 
Last edited:

Back
Top Bottom