300-500 psi per minute is the fill rate I was taught in the PSI VCI class, which would mean 6-10 minutes from empty to full. Reduce that by whatever gas remains from the previous fill, of course.
As for your two shops' fill times, that sounds a lot like the quick fill is from a bank and the slow fill is from a compressor. I know at our shop, I can fill much faster (still 300-500 psi/minute or less) from the bank than from our compressor. I prefer to get my fills from the compressor, of course, as a slower fill has more time to dissipate the heat, meaning more gas in the cylinder.
(I also live and work about a mile from the shop, so it's not like I need to hurry.)
Of course, you're not *losing* any air from a hot fill. It's just the gas laws, specifically Amonton's Law:
P1/T1 = P2/T2
Let's say the hot fill is a hot but not scalding 120°F (580°R) and your summer temperature is 80°F (540°R). If the hot fill pressure is 3000 psi, once the cylinder cools to ambient temperature, the pressure inside will only read 2800 psi (i.e. 3000 psi * 540°R / 580°R). If the hot fill is a painful 140°F (600°R), the cooled cylinder would only show 2700 psi.
Note: You may notice that I made sure to use absolute temperatures (in this case, Rankine for ease of calculation) and wonder whether I also needed to be *absolute* pressure. The answer to that is that you must *absolutely* (hehe) use absolute pressures as well, but in the case of an air fill, you don't have precise enough instruments to tell the difference between 3000 psi and 3015 psi, so I rounded the absolute pressure to 3000 psi, even. If you were talking about a few atmospheres of pressure (such as a salvage lift, perhaps), the difference could be significant, but it's lost in rounding error for cylinder fills.
It's not pointless, I clearly notice a difference in the temperature of my tanks between a water bath and just air cooled, why is it soo hard for people to understand that water dissapates heat far better than air, ie. why people wear drysuits surrounded by air are warm vs. people that wear wetsuits and are wet are cold.
Ah, rox, you're falling victim to one of the classic blunders (slightly below land-war-in-asia and sicilians-and-death). What you're clearly noticing is a difference in the temperature of the outside of your cylinders, and you're assuming that that correlates to a significant difference in the temperature of the gas inside that cylinder. It all comes back to heat transfer.
The metal cylinder is in direct contact with the water bath (ignoring coatings for this post), and water conducts heat away from the metal at a vastly higher rate than air. On the other hand, the gas inside the cylinder is still a gas, and the rate at which it transfers heat to the cylinder is the limiting factor. A water bath greatly facilitates conduction of heat from the outside of the cylinder, but the only impact it can have on heat transfer on the inside of the cylinder is by increasing the delta-T across the gas-cylinder interface.
It is plainly obvious that the increased heat transfer to a water bath does mean that more heat is transferred from the gas in the cylinder as it is filled, but it should not be assumed that the difference is significant. The experimental design to investigate the difference, however, is quite trivial:
- Drain two identical cylinders and allow them to reach thermal equilibrium (an hour should be more than ample).
- Place one cylinder in a water bath, the other alongside.
- Attach both cylinders to fill whips branching from the same manifold.
- Record the starting time.
- Fill the cylinders to the desired pressure, then close the valves.
- Record the ending time. (We really just need the elapsed time.)
- Measure and record the pressure in each cylinder (they should certainly match).
- Allow both cylinders to reach thermal equilibrium.
- Measure and record the pressure in each cylinder and the ambient temperature.
The pressures once the cylinders have reached thermal equilibrium will be a direct indicator of the temperature of the gas inside the cylinder at the point the cylinder was "full". At the end of filling, the pressures are identical, and at the end of the cooling period, the temperatures are identical. The volumes, of course, are constant (within the precision of this experiment).
If anyone wants to speak up, I'd be more than willing to collect and analyze the results of this experiment. (I should be able to perform a few iterations of the experiment myself, but I don't have fast-filling capability at my disposal.) It would be very interesting to see just how large or small the difference of water-bath versus ambient-air fills *actually* is. Once we have that knowledge, the "is it worth it" conversation can take place in a far more informed manner.
