Tank volume calculations..what was the temp used?

[pescador775]

Rab, the temperature differential between interior air and metal tank is key. The only way to elevate this differential and maintain it is to lower the temp of the metal, and thereby the air inside, through convection and conduction in a water bath.

 

[Big-t-2538]

Rab, the temperature differential between interior air and metal tank is key. The only way to elevate this differential and maintain it is to lower the temp of the metal, and thereby the air inside, through convection and conduction in a water bath.

Can you please explain to me what happens to the water in the bath? I understand that the heat will dissipate from the walls of the tank into water better than it will in the air. But since your really only lowering the extertnal temp of the tank by at most 30 degrees (assuming room temp is 70 and the water bath is 40) how does this significantly increase the cooling rate of the tank? Also, as the water gets warm, without an exchange of new water how does the water stay cool in order to keep the tank cool?

 

[pescador775]

When the tank is immersed in air, dissipation of heat is by convection which is inefficient. In a water bath, heat is carried away by conduction which is very efficient given the physical characteristics of water. This is why a diver's body loses heat so rapidly, even in relatively warm water. Normally, it is very difficult to convect away heat from the air inside the tank. In order to speed this process the temp of the metal must be as low as possible and kept that way. During filling the metal can rise to 120F and the air inside even hotter. When the temp of the metal is lowered to 70F (in water) the rate of convection (inside) is improved because the metal is so much cooler than the interior air, I estimate as much as 70F differential in a hot fill. There are some interesting twists. There is a lag in the rate of cooling in a water bath due to the inefficiencies of transfer of heat from interior air to even cold metal. Hence, when a tank is removed from the water bath it initially feels cold to touch. However, placing a hand on the metal reveals that the tank is gradually getting hotter again. This is latent heat remaining in the interior air which is working its way out. That is relatively unimportant but interesting. There are also twists in the method of tank filling. I don't have much experience in decanting air. I conduct tests on compressor fill rates only. However, dive shops fill SCUBA bottles by decanting compressed air from large reservoirs called 'banks'. There are similarities but the method of filling introduces some different physics which I can speculate about. For example, even though the air in the tank is being compressed it is also true that it is expanding. The air released from a 3000 psi bank is expanding as it enters an empty SCUBA tank. This is a dynamic process so that at the same time the air is recompressing in the tank it is simultneously absorbing and emitting heat although at slightly different points in time. I would guess that a tank filled from a bank would show less overall heating than one filled straight from a compressor. Therefore, the slow fill method might work, I don't know. My comments have only spoke to air compressors. These machines also precompress air because the pump works against a back pressure valve. This air then expands and recompresses in the Scuba tank. However, some heat is added by the compression process in the PUMP, no matter how well the output is cooled by the mechanics of the system. This could make for some differences from the dive shop methods.

PS Water has a very high specific heat. This means that the heat transferred from large quantities of compressed air can be dissipated in water without raising the water temp by very much while simultaneously lowering the temp of the air dramatically.

Can you please explain to me what happens to the water in the bath? I understand that the heat will dissipate from the walls of the tank into water better than it will in the air. But since your really only lowering the extertnal temp of the tank by at most 30 degrees (assuming room temp is 70 and the water bath is 40) how does this significantly increase the cooling rate of the tank? Also, as the water gets warm, without an exchange of new water how does the water stay cool in order to keep the tank cool?

 

[jhelmuth]

yeah... what pescador775 said.

Actually - nice job on the explaination. I've never put that much thought into it - but it makes alota sense (to me).

 

[Big-t-2538]

When the tank is immersed in air, dissipation of heat is by convection which is inefficient. In a water bath, heat is carried away by conduction which is very efficient given the physical characteristics of water. This is why a diver's body loses heat so rapidly, even in relatively warm water.

Actually...no, they are both due to conduction, and yes, water is about 23 times more conductive than air, I agree with you there, but that's not the point I was getting at here.

I was getting at the point of forced convection, which seems to be used much more widely than any conductive cooling systems.

We both agree that heat is transferrred from the air inside the tank into the walls of the tank. And we both agree that is a slow proces, not an instantaneous one as you continuously fill the tank. Now that heat has to dissipate somewhere. You choose a water bath which is nice b/c water dissipates heat pretty well, but it has a drawback, is there's no flow. Without flow, you cannot have forced convective heat transfer, which seems to be the application of choice in many cooling systems. Why? Because the rate of heat transfer now becomes dependent on things such as the velocity of the surrounding fluid (in this case, it would be air).

So as I see it, there is 1 "pro" to using a water bath, and that is that it dissipates heat better than air, where as the risk of introducing moisture into your fill whip ot tank valve has now greatly increased. As we all know, water inside of a tank is bad.

On the flip side, you could just fill slowly (my brother and I use the term "slow like a turtle" when we fill tanks) and allow the heat to dissipate at the same rate it is generated and not have to worry about the mariginal decrease in pressure you'll get at the end of the cycle.

 

[pescador775]

Big T, the air inside a tank being filled is constantly circulating and constantly conveying or 'convecting' heat to the tank walls. The heat is transferred to metal and conducted through the metal and into the water where it is absorbed. Or, if the tank is exposed to air, circulating currents carry the heat away, abeit more slowly.

The stuff about getting water into a tank is a red herring. It doesn't hurt to watch the fill process however. DIN valves can act as a water trap so I would suggest that users of this valve format make sure any water is blown out before filling. The same caution applies to yoke valves but the chore is easier to verify. There is no reason to avoid the obviously advantageous water bath fill because of a theoretical concern. Diving is besieged by cautions and concerns. I would place that one on the list and let it go, not use it to push an agenda.

 

[Big-t-2538]

The stuff about getting water into a tank is a red herring.

So you do not agree that filling a tank in a water bath does not significantly increase the chance of getting water inside your tank or fill whip?

It doesn't hurt to watch the fill process however.

I make it a habit to watch who fills my tanks and how...and I recommend it to students who own their own tanks.

There is no reason to avoid the obviously advantageous water bath fill because of a theoretical concern.

I am not blaming you, don't take it personally, but I've seen places just sling tanks in and out of water baths with no regard or concern for where that splashing water is going. It is not theoretical, it is an unfortunate fact. Water will cause damage to the interior of a tank. Many places are convinced that a water bath is a wonderful idea, but seem not to care about the flip side of the coin of introducing moisture to a tank.

A water bath is a way to minimize pressure drops after filling, but it comes at a cost, it's not the only method available, and it's arguably not the best method either.

For those reasons, I don't see the benefit.

 

[TCDiver1]

http://www.psicylinders.com/library/wetfills.htm

No doubt that heat transfer is greater in the water bath. But it doesn't appear that a water bath is required at the recommended fill rates. So why risk the potential problems that go with it?

I gotta agree with Big-t on this one.

 

[MikeFerrara]

Let me re-phrase....

Tank Temp SHOULD NOT be a big factor. Fill slower.

So 10% of a 3000psi fill is what....300psi..

To be honest, I'm not buying the fact that your 3.5 CFM compressor heats a tank up that much and you need to fill it to 3300psi to ensure a good fill?

So what is 3.5cfm per psi in an AL80 as in psi/min. Well, Let's find out. 3.5 / 77.4 * 3000 = approximately 135 psi / min.

So if you're tank needs 2000 psi to be filled...or 2300 to overfill and it takes approximately 15 to 17 minutes to fill your tank....you're telling me that it will rise 55 degrees in that amount of time? (300 / 5.5 = approximately 55 degrees) So you're telling me that your tank raises it's temp to 125 deg F? I just don't buy it.

Don't know about a 3.5 cfm compressor but filling directly off my 5 cfm (actually a little less) will easily result in a couple hundred psi drop in an al 80 when it cools. The tank will be warm very warm to the touch and something like a HP 100 will feel pretty hot. Done it thousands of times...honest.

 

[Big-t-2538]

Mike -

I don't usually fill with a low volume compressor, so I was sort of calculating on the fly. When we fill for the school, we've got 4 banks (only one of which is open at a time) and 4 whips. We try to keep the fill rate between 100 and 200 psi per minute for the AL80s, and that seems to keep things pretty stable in the long run. YEs, the tanks do get warm, but nothing exorbent enough to noticably affect the fill.