Co2 fire extinguishers

[Tom Sutton]

Is the CO2 gas in Co2 fire extinguishers at room temperature liquid or in gas form? We had a discussion in our company and had lots of different ideas (but no knowledge  ). Anybody can help?

I received the above question the other day and the more I researched it the more complicated it got. Any ideas?

[kurnal]

Its a supercritical liquid.

Dont know if this helps
http://en.wikipedia.org/wiki/Supercritical_carbon_dioxide

[Fishy]

According to the Wikipedia Phase Diagram...

If you assume CO2 fire extinguishers are at 200 - 300Bar, then at less than the temperature of its 'Critical Point' (31.03 °C) it's a liquid.  Above that temperature, at that pressure it'll be a supercritical fluid.

[nearlythere]

Opened one up this morning and it's definitely not liquid.

[AnthonyB]

It's both. The bulk is liquid with a vapour layer in the ullage space providing pressure. Liquid fill is to 2/3 of the capacity of the cylinder, unless using tropical fill.

[Tom Sutton]

Thanks Anthony it confirms what I thought however the 2/3 and tropical fill is new to me.

[AnthonyB]

It's all about fill ratios for different environments, the so called 'tropical fill' is for very hot countries or environments - I don't have the fill ratio to hand though, it's not often called for with our weather!

[fireftrm]

Anthony is absolutely spot on - it is liquid and the gas above also acts as an expellant. It is DEFINITELY NOT a supercritical liquid and please do NOT 'assume' it is at 200-300 bar! That level of pressure applies to BA cylinders, think 52-57 bar instead............As the liquid vapourises it is so endothermic that it can (and usually does) form a solid (frozen in simple language) form, which then sublimes. You will see this if you aim the extinguisher directly onto a surface, just don't touch that solid as it will be below -70'C, this does not mean the ice that forms on the horn/valve as this is water freezing from the atmosphere, but this will be at a similar temperature.

PS NEVER, NEVER, NEVER use wikipedia for research, remembering that you are relying on an individual's post of their interpretation, or possibly limited knowledge. The 200-300 bar bit proves the point and dramatically. If you do any higher education study you would not be allowed to include any wiki research and your use of it would be very badly viewed - quite rightly.

People who use CO2 extinguishers to feed MIG welders (so much cheaper than special CO2 cyliders they think) come across problems as there is liquid dipelled which 'blocks' the tubing due to solidification, or liquid getting to the jet and delivering a sporadic gas flow.

[Fishy]

I didn't get the 200 - 300 bar from Wikipedia - I asked a colleague who used to work for a company who manufactured fire extinguishers!  Oh, well...

Point taken about Wikipedia, though, but I did check the phase diagram on several scientific websites.

[kurnal]

Just to add more information / misinformation to the debate, heres the link to the last time we discussed this :

http://fire.org.uk/forum/index.php?topic=2955.0

[Tom Sutton]

As the result of some research and your help, especially AB my understanding is,

CO2 is a solid at absolute zero and as the temperature increases the pressure increases until -56.4C / 5.11 bar known as the triple point where it exists as a solid, liquid and gas. As the temperature increases to 31.1C / 73bar (critical point) it exists as a liquid and gas, above that, it becomes a super critical fluid. Check Phase Diagram.

CO2 bottles contain copper "burst disks" which will blow out above a certain pressure and release the contents of the bottle into the air in a relatively safe manner. The blowout pressure of burst discs vary with manufacturer but the minimum value is about 150bar and the maximum value is about 190bar.

CO2 cylinders are built to hold pressures about 109bar. The normal operating pressure for a CO2 cylinder at 21C is about 55bar and the pressure will vary depending on temperature and % rated fill. Temperature & percentage of rated fill greatly affects the pressure and in a cylinder the way to control the pressure is by adjusting temperature and/or percentage rated fill. At 38C the pressure in full bottle will have raised to 95bar. At 60% fill the pressure would be 81bar. Consider tropical fill for instance. Check P vs F & T.

As you can see the pressure at 38C (100F) is still in the working range but I personally would sooner have it in a lower temperature and if I was ever required to site a CO2 extinguisher I would certainly consider ambient temperature especially direct sunlight.

Check CO2 Phase Diagram http://i419.photobucket.com/albums/pp272/twsutton/phasediagramco2.jpg

Check Pressure vs fill and temperature graph http://i419.photobucket.com/albums/pp272/twsutton/CO2PV-1.jpg