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Creating a temperature differential on SF6 to evacuate a cylinder 
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#1
Aug1214, 05:21 PM

P: 8

I have a 50KG cylinder with 12LB of SF6 at 15PSI. I want to transfer it into another cylinder by temperature differential.
Keeping it simple. I try to model it with two cylinders that has a valve on each cylinder. Connecting the cylinder with a 1/4" hose. Both valves are closed. The cylinder with the SF6 is at ambient temperature (25C). I cool the other cylinder to 40C (~50C is solidifying point).. Open both valves. How would I go about calculating the flow rate? I try to find the pressure differential using ideal gas law PV=nRT; P1/T1 = P2/T2, 15 PSI / 298.15K = P2/233.15K; P2 = 11.73PSI. This is the wrong way since I know it is a compressible gas and the pressure should be much lower. Any help or direction would greatly be appreciated. Thanks! 


#2
Aug1214, 06:46 PM

Mentor
P: 12,113

Weird unit system :(.
PV=nRT takes compression into account, but you won't get a pressure difference if your valves are open. You just get a density difference, while the total amount of SF6 is constant (and known). Your calculation would require the same amount of SF6 in both cylinders and closed valves. 


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