Overfill Differences for LDS - Question

[ams511]

where do you dive that you need 120cuft of gas for a shore dive multiple times a day?

The OP profile says Winter Springs which is outside of Orlando. So he is either diving springs or beach dives (Vero Beach would be the closest).

 

[ams511]

Many years ago when we were designing a filling station we instrumented a water bath to see what the heat transfer was to that water while filling. I hate to burst bubbles and crush life long beliefs, but there is very little heat transfer to the water during a 5 to 10 minute fill (at least with steel 72s).

A steel 72 is a LP tank, while the OP uses HP tanks. It would make sense that the higher tank pressure would generate more heat. So a water bath may be more beneficial for this type of tank than yours.

Also you would need to look at the conductivity of the material the tank is made of. All things equal a water bath would probably work better on an aluminium tank then on a steel one. Being aluminium conducts heat better than steel.

 

[captndale]

Water takes the heat away from the tank alot faster than the air. That's why you can get hypothermic in relatively warm water given enough time, even though you'd be fine in the same temperature air for allof your life. Water baths don't necessarliy make a safety difference, when your tank blows up it'll hurt things water tank or not.

What you say is true, as far as it goes. The problem, as you state, is that heat transfer between the steel (or aluminum) and air takes place slowly. It takes a long time for the air in the tank to transfer its heat to the metal tank. In fact heat transfer by convection is much more efficient outside the tank than inside the tank. Although you can speed up heat transfer away from the tank by immersing it in water, that does nothing to speed up the heat transfer from the hot, compressed air to the tank itself. That is really the limiting factor and the reasom that water baths do not appreciably speed up cooling the tank contents.

 

[Thalassamania]

We were considering putting in a chilled bath, and decided not to after testing, Rather we put some active intercoolers between the compressor stages.

 

[captain]

I made a water bath out of a length of 8" diameter 24" tall PVC pipe capped on one end. A continous flow of water enters the bottom end and overflows out the top. It takes my compressor 20 minutes to fill a steel 72. Doing it this way I never have to let them cool and top them off. I may lose 25 psi once they cool completely. Without the water I lose 200 psi and have to top off. If the water is not moving constantly pass the tank the effect is not as great.

 

[UB]

The OP profile says Winter Springs which is outside of Orlando. So he is either diving springs or beach dives (Vero Beach would be the closest).

I beach dive Venice (FL West Coast) and West Palm Area (FL East Coast). These are both ~3hr drives for me so I try and make a day of it with two dives.

Thanks for the feedback - I do know that the Shop with the larger loss in pressure uses banks and I am not sure of the other shop.

 

[pescador775]

UB, a the water bath is the difference. Theoretical arguments about thermometers in the bath and convection of interior tank air are pointless. It can be easily demonstrated that a water bath drops the temp and press of a tank and allows more air to be jammed. Yes, during a fast fill, the bath cools the metal faster than the internal air but the air catches up as it loses heat slowly to the cold metal. All you have to do is handle a steel tank and you will see that the metal continues to warm up AFTER the tank is removed from the bath. This delay ultimately causes the wet tank to lose some pressure, maybe 100 psi instead of 300 psi as after a dry fill. As far as aluminum vs steel. Aluminum has higher conductivity but steel temperature rises faster due to its lower heat capacity. This results in higher temperature differential with the water bath thus speeding cooling.

UB, since you are probably using an SPG to low ball the actual pressure, my guess is number 1 is filling spot on and number 2 is underfilling by a tad.

 

[Rick Murchison]

I hate to burst bubbles and crush life long beliefs, but there is very little heat transfer to the water during a 5 to 10 minute fill (at least with steel 72s).

Hmmm... my casual observation of fills on 72's and LP85's is that a water bath makes about a 200psi difference in final pressure in the 2500-3000 psi range at "normal" fill rates (about 125psi/min).
Now I'm gonna have to run a side-by-side comparison to see if I'm right or not.
Rick

 

[wedivebc]

with banked air you can fill a tank in less time this will result in greater heat production....

That is not actually true. The first law of thermodynamics is heat can not be created or destroyed. There is not heat generated when transferring gas from one container to another other than the heat picked up from the piping and valves between the source and the destination container. Ever notice as you tranfer from a bank it gets cooler? There is more heat from compressed air because usually ambient air has more heat energy than compressed air that has been allowed to cool off for a period of time.

 

[Thalassamania]

Hmmm... my casual observation of fills on 72's and LP85's is that a water bath makes about a 200psi difference in final pressure in the 2500-3000 psi range at "normal" fill rates (about 125psi/min).
Now I'm gonna have to run a side-by-side comparison to see if I'm right or not.
Rick

We all wait for youir results ... well, at least I do.

That is not actually true. The first law of thermodynamics is heat can not be created or destroyed. There is not heat generated when transferring gas from one container to another other than the heat picked up from the piping and valves between the source and the destination container. Ever notice as you tranfer from a bank it gets cooler? There is more heat from compressed air because usually ambient air has more heat energy than compressed air that has been allowed to cool off for a period of time.

Picky, picky picky.