CO2 12g cartridge pressure manipulation

[Jim Cliff]

Hi all,

I am wanting to reduce the 852.8 psi of pressure from a 12g CO2 cartridge. My application would ideally require 3 times less pressure at roughly 284 psi.

I know there are pressure regulators, relief valves available. But in theory if I was to screw a 12g cartridge into a vessel / canister with 3 times the volume, would the pressure in this larger canister be roughly 3 times less?

CO2 is rather unique as pressure is determined by temperature. But surely increasing vessel volume while keeping the same amount of gas will allow expansion and therefor lower pressure?

Thanks,
Jim

 

[zeusfaber]

I don't think that's going to work the way you expect it to.

The important thing here is that most of the CO2 in the cartridge is there as a liquid (with exactly enough gas above it to create enough pressure to force the boiling point up to room temperature). If you expand the gas in the headspace into a larger volume, all that will happen is that some of the liquid will boil off until the additional headspace is filled to the same pressure (this might take a little while as the boiling process chills everything down and you won't get full pressure back until the liquid has warmed back up to room temperature). This system is less like a cylinder of gas and more like a boiler.

To get 20 bar at room temperature, you'll need to use an expansion vessel that's big enough to boil off all of the liquid and then expand the resulting gas threefold after that. You can estimate how big that has to be by working out how many moles of CO2 (molecular mass 44) are in 12g (0.273), remembering that one mole of a gas fills 22.4 litres at STP and then correcting for your target pressure of 20 bar to give you a required volume of 0.273 * 22.4 / 20 = about 0.3 litres (a lot bigger than 22 ml of liquid).

One other thing to bear in mind: A useful feature of liquid CO2 is that it lets you draw off gas at a pressure that remains virtually constant until the last bit of liquid has been consumed - which is what makes it so good for things like paintball guns. Remember that when you store it in the gas phase, the supply pressure will drop more or less linearly as you consume the gas.

A.

 

[Jim Cliff]

Zeusfaber, many thanks for your reply! It was extremely helpful and I greatly appreciate the time you've taken to explain this to me.

0.273 * 22.4 / 20 = about 0.3 litres (a lot bigger than 22 ml of liquid).

One thing though. I am confused about where you have got the 22mL of liquid from?

But basically, from your calculation..... In order to reduce the pressure from a 12g Co2 cartridge down to 20 bar, I will need to have an expansion vessel with a volume of 300 mL?

Thanks,
Jim

 

[zeusfaber]

I reckon 300 ml will more or less do it.

Where did the 22ml come from? That's the answer you get if you use the wrong value for the density of liquid CO2. 14 ml is probably a more useful figure. Sorry about that: it was late.

A.