[koljo45]

Hi,

I have a CO₂ extinguisher. The volume of the container is 6.7 dm³, the mass of the extinguisher casing is 8 kg, and the mass of the CO₂ inside is 4.7 kg. I contacted the distributor and asked for information, they stated that the internal pressure at 20°C is 55 bar and the CO₂ eject time is 20 seconds. I really don't get how this works. I tried calculating the internal pressure using mass, volume and temperature information but get results around 200 bar (3 times the actual pressure!!!). I'm missing something big and simply don't know what. As I understand, the CO² inside the tank is a gas because the temperature is not low enough as well as the pressure not being high enough for it to be a liquid. I realize CO₂ doesn't behave as an ideal gas, but I have taken that into consideration, it's compression factor at ≈ 50 bar and 300 K is ≈ 0.6. How can I calculate the thrust of the gas being expelled? I hope someone out there has some experience with fire extinguishers, or is this the wrong forum for this sort of question

P*V = 0.6*n*R*T
P = 0.6*(m/M)*R*T/V
P = 0.6*(4700 g/44.01 g mol^-1)*8.314 J mol^-1 K^-1*300 K/0.0067 m³
P = 23853615.3588 Pa

http://www.pastor.hr/tfpdf/vatrogasni_aparat.php?id=24
http://www.analytix.co.uk/wp-content/uploads/2015/02/TN-32-Compression-and-Condensation-of-CO2.pdf

[Arkcon]

There may be some engineering calculations you can use, once you have a better idea of the conditions.  But I want to get something settled right away.  The Ideal Gas Law -- pV-nRT only applies to ideal gasses, which are mathematical points that don't interact with each other.  Only low concentrations, of dilute gasses at low pressures approach ideal gas behavior.  Your CO2 extinguisher is likely at high enough pressure to contain liquid CO2 or dry ice snow.  So the force of that expansion has to be taken into account.

[AWK]

Below 304 K liquid carbon dioxide exists. Using ideal gas law makes no sense  (even with compression factor since most of CO₂ is liquified).

[koljo45]

Thank you for the info. The thread that I was reading had conflicting arguments about the state of CO₂ (liquid/gas) inside fire extinguishers, I had the wrong conclusion. Anyway, I found a website describing the pressure of CO₂ bottles used in paintball. It seems that the fire extinguisher has 35% of it's volume filled with liquid CO₂.

P*V = 0.6*m/M*R*T
m = P*V*M/(0.6*R*T)
m = 55*10⁵ Pa*0.0067 m³*44.017 g mol^-1/(0.6*8.314 J mol^-1 K^-1*300 K)
m = 1083.86553471 g

Δm = 4.7 kg - 1.083 kg
Δm = 3.617 kg

So when 3.617 kg of CO₂ flow out there is only gas left? I didn't take into account the cooling of the tank due to CO₂ evaporation. I guess the thrust is constant until that point, after that I would need integrals to calculate anything.

Hope I got this right.

Tnx for all the help.

http://www.warpig.com/paintball/technical/gasses/co2dynamics.shtml#2