Author Topic: Tank sizes  (Read 574 times)

bnowlin

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Tank sizes
« on: February 18, 2018, 06:45:43 PM »
Bob,
I am sure you have answered this many many times, but how do you calculate what size tank, in cu ft , you have if you only have the ccs ie 105 cc 144 cc etc.  in 4500 psi SCBA tanks?
Thanks the other Bob



rsterne

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Re: Tank sizes
« Reply #1 on: February 18, 2018, 07:29:21 PM »
To convert from the inside "water volume", usually in cc or CI.... to the amount of air it will hold in CF.... you need to know the fill pressure (in bar).... If the tank is listed in cc, convert to CI by dividing by 16.4 (cc/CI).... If you know the pressure in psi, convert to bar by dividing by 14.5 (psi/bar)....

First multiply the water volume by the fill pressure, to give the number of CI or air inside the tank at that pressure.... Then divide by 1,728 to convert CI to CF....

For example if the inside volume is 144 cc, that is (144 / 16.4) = 8.78 CI.... If the fill pressure is 3,000 psi, that is (3,000 /14.5) = 207 bar.... When filled to 207 bar, that tank will hold (8.78 x 207) = 1,817 CI of air at 1 bar.... Converted to CF, that is (1,817 / 1,728) = 1.05 CF....

The above is according to Boyle's Law for ideal gasses, and works up to about 3,000 psi.... It gets a bit more complicated when the tank is filled to more than 3,000 psi because of the VanDerWaals effect.... If we use the above calculation for a standard 1 hour SCBA tank (which is 550 CI water volume) we get the following at 4500 psi (310 bar)....

(550 x 310) = 170,500 CI.... (170,500 / 1728) = 98.7 CF (according to Boyle's Law).... However, that tank only holds 88 CF of air at 4,500 psi.... about 11% less than you calculated.... Up to 3,000 psi, you can use the basic Boyle's calculation as I detailed it.... At 4,500 psi, you need to reduce that by 11% because of the VanDerWaals effect.... At 6,000 psi the effect is much greater, about 18% compared to at 3000 psi.... At 10,000 psi a tank only holds 64% as much air as the simplified calculations tell you.... Sorry, but there is no easy formula to use once the pressure goes over 3,000 psi....

Bob
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bnowlin

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Re: Tank sizes
« Reply #2 on: February 18, 2018, 08:22:56 PM »
Thanks Bob,
That means to me that some of the sellers are over advertising on some of their big tanks and smaller also just to sell them especially if they are the same size like 88 and 98.  The fire and diving, etc  people do it by minutes. for safety reasons and leave 15 minutes or so in them.
Appreciate ya. 
BoBn

rsterne

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Re: Tank sizes
« Reply #3 on: February 19, 2018, 08:37:54 AM »
Absolutely correct, Bob.... ALL the 1 hour size SCBA tanks hold 88 CF of air, not 98 (or 100) as advertised by some companies.... Likewise, the 45 min. tanks are 66 CF and the 30 min. are 44 CF.... The best way, if you are unsure, is to go by the internal volume (water capacity) of the tanks to compare them....

Incidently, this is why the pressure in your 4500 psi tank drops faster from 4500 to 3000 than from 3000 to 1500 psi.... and why a 6000 psi welding tank does not hold twice the air it would at 3000 psi.... Incidently, you can't put as many CF of Helium in a tank at the same pressure as you can with air (and the loss is linear).... Here is a graph showing how air, Nitrogen and Helium compare to an ideal gas....



Note that if you filled a 1 hour SCBA tank with Helium to 4500 psi it would only contain about 84 CF of Helium.... and Nitrogen would be about 86 CF....

Bob
« Last Edit: February 19, 2018, 07:40:28 PM by rsterne »
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bnowlin

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Re: Tank sizes
« Reply #4 on: February 19, 2018, 02:17:42 PM »
Thanks Bob,
> Carelton says they make a 100 cu ft carbon tank for the military etc but not for public.  I think he told me they make larger with like 6000 psi also but not for public.  Tote them around if you will.  Thanks for all the info I needed that.
> Bob

rsterne

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Re: Tank sizes
« Reply #5 on: February 19, 2018, 07:21:36 PM »
I made an error with the Helium, it is corrected in blue in the post above....

One other thing that the VanDerWaals effect causes that I mentioned briefly above.... For each 500 psi increase in pressure above about 2500 psi, you put less air in the tank.... This means that if you are using a regulator to tether to a high pressure tank, and shooting at, say, 2000 psi.... as the tank pressure drops by 500 psi, you will get fewer shots at high pressure than at low.... Let's say that you get 100 shots from 2500 psi down to 2000 on an SCBA tank....  If the same tank is filled to 4500 psi, from 4500 down to 4000 you will only get about 80 shots.... If you are tethered to a 6000 psi tank of the same internal volume, from 6000 psi down to 5500 you will only get about 60 shots... The higher the pressure, the fewer the shots you will get from a 500 psi drop.... This also means that you won't get as many fills from a 6000 psi welding tank as you thought, particularly when the tank is full.... the pressure will drop very fast at the beginning compared to when it is at only "half full" at 3000 psi....

Bob
« Last Edit: February 19, 2018, 07:59:30 PM by rsterne »
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