tank filling theory...e.g. getting the pressure right

[pescador775]

Fred, if we want to nitpick, a black tank will radiate more heat than a white one. However, we'll leave 'radiation' aside since the primary mode of heat transfer is not radiation but convection. That is why a water bath works so well. Water has a higher conduction and heat coefficient than air resulting in higher convection. Aluminum itself has a higher specific heat, about 2X as much, as steel. This means that an equivalent weight of aluminum can absorb 2X as much heat from the tank's air as does steel resulting in a greater drop in the aluminum tank air temperature while the aluminum itself appears not to 'heat up' much at all. Therefore, the 'thermal mass' of aluminum is related to the specific heat, not directly to the weight or mass of metal although there is certainly a larger volume of aluminum than in an equivalent steel tank. Similarly, aluminum has much a higher heat conduction number than steel which allows the aluminum tank of equal temperature to shed heat more rapidly through convection than steel whether bathed in air or water. However, this heat loss, whether through convection or radiation, is moderated by the fact that the temperature of aluminum will not rise as much as steel in the first place. Clear?
Pesky

 

[sheck33]

pescador775 once bubbled...
Fred, if we want to nitpick, a black tank will radiate more heat than a white one. However, we'll leave 'radiation' aside since the primary mode of heat transfer is not radiation but convection. That is why a water bath works so well. Water has a higher conduction and heat coefficient than air resulting in higher convection. Aluminum itself has a higher specific heat, about 2X as much, as steel. This means that an equivalent weight of aluminum can absorb 2X as much heat from the tank's air as does steel resulting in a greater drop in the aluminum tank air temperature while the aluminum itself appears not to 'heat up' much at all. Therefore, the 'thermal mass' of aluminum is related to the specific heat, not directly to the weight or mass of metal although there is certainly a larger volume of aluminum than in an equivalent steel tank. Similarly, aluminum has much a higher heat conduction number than steel which allows the aluminum tank of equal temperature to shed heat more rapidly through convection than steel whether bathed in air or water. However, this heat loss, whether through convection or radiation, is moderated by the fact that the temperature of aluminum will not rise as much as steel in the first place. Clear?
Pesky

Radiation plays an insignificant role here as temperatures are far to low for that. When we fill a tank it gets hot. Whether the tank is in air or water there are 3 pieces to the heattransfer path.
first the air inside the tank gets hot and heats up the tank by convection in the tank itself. The air or water the tank is in takes away heat from the tank by, again mainly, convection. However, since the tank walls are not infinitely thin heat has to be transferred from the inside of the tank wall to the outside of the tankwall. This of course takes place by means of conduction. Conduction in this whole process is just as important as convection is. If convection where the only means of heat transportation than the tank material would be irrelevant.
Since aluminium has a higher heat conductivity it makes more sense to put aluminium tanks in a waterbath than steel tanks, this aside from the corrosion problem that is more important in steel tanks. The key is to fill a tank as slowly as possible.
LP tanks have the advantage here. :doctor:

 

[pescador775]

Since aluminium has a higher heat conductivity it makes more sense to put aluminium
tanks in a waterbath than steel tanks, this aside from the corrosion problem that is
more important in steel tanks.

Sorry, that's wrong. Since steel has a lower coefficient of conductivity its transfer of heat must be assisted by increasing the temp gradient as much as possible. Therefore, it is most necessary to immerse steel tanks in water.

 

[sheck33]

pescador775 once bubbled...


Sorry, that's wrong. Since steel has a lower coefficient of conductivity its transfer of heat must be assisted by increasing the temp gradient as much as possible. Therefore, it is most necessary to immerse steel tanks in water.

You are right, i goofed. Steel having a lower coefficient of conductivity needs more help, greater temperature gradient, to facilitate transport of heat. Al has a higher conductivity than steel so benefits even more from being immersed in water. But my conclusion got screwed up from there. Thanks for the correction.

 

[Dyno Bill]

Thanks for the science lesson, I might have been ignorant about all that stuff for years if it hadn't been for the discussion about materials and their abilities to dissipate heat. As I might have mentioned about where I dive, there are between 20 and 30 divers on a boat so thats how many tanks that get filled between dives with the use of two wips. The time span is usually between 30 minutes to an hour while the boat is moved to another dive site so the initial tank fill is the only one that is done other than full blast and without the advantage of a cooling tank so again I have to say the most practical tank is a LP steel for this type of scenario. Forgive my spelling due to the lack of practice in recent years.
Dive Safe

 

[deepstops]

Is it possible to over-cool the tank and cause condensation inside the tank during the fill process??

 

[Stone]

If the separator and filters are doing their jobs, the gas in the tank should be dry.

 

[pescador775]

Stone is correct, condensation during filling is not a concern. As a practical matter it is necessary to 'crack' the valve prior to filling in case any moisture is lodged in the valve orifice. This should be a lifelong practice just as the diver has learned to blow moisture off a dustcap before replacing on the regulator orifice. High pressure air at the outlet of a good compressor has a dew point of -40 degrees F. However, as the air is compressed into the tank this dew point could go much higher, and did so, when years ago, compressors did not routinely come equipped with back pressure valves. When I was a teenager, there was a dive shop in Balt, Md. The owner, Delucca was his name, was killed when a storage bottle exploded. It was rusted out. Air from a modern compressor will raise the dew point level somewhat when compressed in a high pressure tank but not enough to cause visable condensation. However, common sense says it would not be a good idea to store tanks at extreme low temperatures unless most of the air has been drained. Draining the air and lowering the pressure also lowers the dew point. There is some controversy over the susceptibility to rusting caused by EAN. Needless to say, only modern equipment with tricked out filtration (large condenser, filter with chemical stack & backpressure valve) should be used for this.