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:
Originally Posted by
jimmyw 
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?
{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.
