Need help sizing N2 filling bottles for gas bottle precharging. Simple 2

[jomofro39]

I think I am just running my own mind in circles with this one, but I feel like I would ask you guys before committing to anything.

I need to find out how much 2,200 psi nitrogen is needed in order to precharge (3) 15 gallon gas bottles with 1,200 psi of pressure each. The place we can order the nitrogen sells them in 20, 40, 80, etc. cubic feet of nitrogen at 2,200 psi. My calculations yielded that for 45 gallons (45*231= 10,395 cu in) @ 1200 psi, I would need 12,474 cu in (or 7.22 cu. ft.) of 2,200 psi nitrogen to attain this precharge. It seems low to me. Keep in mind that the 2,200 psi bottles can only be depleted to 1,200 psi, so basically only 1,000 psi of that pressure is usable.

Thanks in advance!

Tell me if any more information is necessary.

 

[zeusfaber]

I get 7.22 Cu Ft too - but that's the Water Capacity of the cylinders (54 gallons - makes sense: you want to end up with a bit more than half your original pressure, so the bottles you're filling need to be a bit less than half of the combined volume).

Charged to 2200 psi, that's 1100 Cu Ft of free nitrogen.

A.

 

[zdas04]

My guess is that they're selling standard cubic feet (SCF), so let's assume that you are at 60F at sea level (P(atm)=14.7 psia), which is close enough to STP.

Density(stp)=0.076 lbm/ft^3
Density(1215 psia)=6.308 lbm/ft^3

deltaMass=(6.308-0.076) lbm/ft^3 * 6.016 ft^3=37.49 lbm
Vol(stp)=deltaMass/Density(stp)=493 SCF

The density of the supply bottles is immaterial.

David

 

[jomofro39]

I got the 37.5 lb answer as well, but my issue with that is that is assuming all of the gas from the supply bottles gets INTO the 3 15 gallon gas bottles I am trying to fill to 1200 psi. The supply bottles will all be depleted until they reach 1200 psi, not 0 psi. Their supply will not be exhausted when I fill the gas bottles. But thank you for your help because that at least answers how much SCF of nitrogen I am trying to get into the tanks. Appreciate it!

 

[zdas04]

You really have to quit focusing on the supply. Assume that the supply is infinite (i.e., that no matter how much you take out, the pressure remains 2,200 psig).

Now if you own the supply bottles, the trick is to use them wisely. Fill from 0 psig to 800 psig with nearly depleted bottles, then top them off with 2,200 psig bottles for example.

Regardles, it is going to take 37.5 lbm of nitrogen to get your 3 bottles to 1200 psig. If for some perverse reason you wanted ACF at 1200 psig then divide mass by actual density and get 5.94 ACF, but there really isn't much you can do with that number.

David