10 seconds of transfilling is about 9.99 seconds longer than it would actually take to fill a "fully evacuated" scuba tank from 0psi to 14psi (atmospheric pressure) from a donor tank pressurized to 3000psi.
So make it 0.01 seconds, or open the valve a lot less
Think about this: Imagine you had a tank at 0 psi (vaccuum) with a stopper that you could simply pull out of the tank opening. If you pulled that stopper out, how long would it take that tank to equalize from 0psi to 14psi. A quarter of a second? Half a second? Now put 3000psi of pressure behind that equalization from 0psi to 14psi. How long would that take? Let's assume it's painfully slow at half a second. The next 9.5 seconds of air going into that tank is being compressed. Follow along with me
Yeah - good point. I can see you have a good feel for reality.
PS - while I do think you're dead wrong here, and can't figure out how someone can be so far off-base, I do appreciate your willingness to tangle on this issue. I'm busting your chops a bit... and you seem like a good sport. Hope you don't take my digs as more than "good clean fun" - Ray
Ray, thanks for this. I think you're wrong, too, but no more wrong than I was, and you seem like a decent guy, too. I do think I understand it now, but I also recognize that being out on the limb as I am I need to be sure of my footing. I'm not trying to convince others, nor stir up trouble. I really do think that gas leaving the donor tank expands and I really do think it gets hot. It's the "why" I needed to work out. If anyone is able to knock me off this branch, they are going to have to do it with some convincing arguments, and each time I go to the text books or the math or Wiki, I find that the branch gets just a bit wider. It would just be nice to find someone who understands the arguments and can refute them or explain with decent support why they are wrong.
---------- Post added February 27th, 2013 at 04:34 PM ----------
Aarggggghhhhhhhhhhhh!
There IS energy added when a tank is filled from another tank! That energy is stored in the donor tank in the form of 3,000psi of pressure. 1500psi of that energy is "lost" from the donor tank (which cools) and is "added" to the receiving tank... WHICH HEATS UP!
Yes, I agree. The gas in the receiving tank gets energy and that energy is the source of the heat.. But the total gas in the system - the original gas in the donor tank, gets no additional energy. The fill process separates out the expanding gas into two parts - the gas expanding in the donor tank, from which energy is extracted, and the gas expanding out of the donor tank into the receiving tank, which picks up that additional energy. The expanding gas in the donor tank is doing work and cooling down. The expanding gas going to the receiving tank is not doing work and stays at constant temperature by "free expansion = Joule expansion" and carries the energy released by the expanding gas in the donor, ultimately resulting in heated expanded gas above its original temperature. I think it's interesting (cool!
) that expanding gas going to the receiving tank gets hotter when no energy is added to the entire system of gas and the system is able to separate itself into two expanding parts - one getting hotter and one getting colder.
The difference is this. When direct filling from a compressor, you start with 40 cf of air at STP and compress it into a mere 11 liters of the tank to end up at about 1500 psi. You have to add energy (plug in the compressor) and the compression results in heat. In the case of transfilling, you start with 11 liters of air at room temp and 3000 psi and let it expand out into 22 liters of volume at about 1500 psi. No energy is needed - you just open the valve and release previously stored energy. One is compression heating and one expansion with a system that divides itself into hot expanded gas and cold expanded gas. In one you don't need to have ever heard of Joule expansion. To explain the other, you need to understand Joule expansion - it's key to how it's possible for expanding gas to heat up.