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When a gas expands, it doesn't gain or lose any energy. It's energy remains equal, but is distributed over a larger volume, thus reducing the temperature. This effect is demonstratable by holding a canister of dust remover upside-down and spraying. Observe the ice crystals that form.
If you had an
large extra-reinforced pressurized container filled with one of two things:
pure, near frozen CO2
or
liquid nitrogen
CO2, compressed and mounted on a truck, would be effective in that: a) the CO2 puts out the fire, and b) the lower temperature also helps put out the fire.
Of course, for the cold to be truly effective, we would need cryogenic temperatures, but if we had enough pressure in a CO2 tanker that was already cooled almost to the freezing point of CO2, it could be really effective. Which is where liquid nitrogen comes in.
You could instead fill the tank with liquid nitrogen, which would also be effective in that it absorbs heat to change from a liquid, plus the fact that it is freezing cold in the first place. Also, the nitrogen gas would smother the fire.
*NOTE* To avoid suffocating people, try to get the out first. Though I'd rather almost suffocate and have a fireman (with airpack) pull me out, rather than frying and sizzling in the fire.
(?) Fire suppression gasses
http://www.tsfire.com/supp.htm
[Worldgineer, Feb 24 2005]
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I seem to recall an essay by (I think ) Primo Levi where he describes using a CO2 extinguisher on an inflammable liquid fire, only to find out later that this is "A Bad Thing" because the dry CO2 crystals in the nozzle can become statically charged, and produce a spark, thereby providing a potential source of ignition. |
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And you might suffocate people trying to escape from the fire. |
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Of course, we should only use this in fires without people trapped inside. We don't want to kill them. |
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And Absinthe, CO2 can't ignite. It is the product of combustion. Besides, it isn't solid CO@. You can't spray that, except maye as a powder. Almost freezing the CO2 solid, leaving it a freezing gas. Though I suspect that the Liquid Nitrogen would be more effective. |
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[df] I didn't say the CO2 could ignite - I said it could provide a source of ignition, ie a spark .Observe the ice crystals that form - they are very, very dry and produce static electricity. |
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True. But wouldn't it not really matter as the place is already on fire? That's what I thought. |
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Thank you for the input. And my mistake about the igniting CO2. |
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I figured that there was some reason why. |
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[df] I don't get why you would want to chill the water. To raise 1 kilogram of water from 273K to 373K takes 420kJ, but to get that water at 373K to boil requires more than five times that energy - it hardly seems worth the effort of chilling it. |
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Because when you chill the water, it's energy is dissappated safely, and in the fire, it absorbs energy really well. Besides, that doesn't matter now. |
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CO2 is often used in fire suppression. Designed into a building it can be used safely, allowing for enough oxygen to hopefully not kill anyone off. Because spraying into buildings from fire trucks doesn't have the advantage of design, I'd recommend switching to something like Inergen, which contains enough oxygen to breathe. |
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Assuming that the notrogen is still very cold when it enters the compartment that is on fire, you are presented with two problems. One is the thermal shock to objects in the compartment that may result in them shattering with associated shrapnel. The other uis that unless the compartment is well ventilated there will be an increase in pressure as the liquid turns to a gas and this could reslt in all sorts of problems for the structure. |
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Additionally you would be unable to use this on any buring liquid fires and when the nitrogen hit the hot liquid it would expand massively, spraying burning liquid everywhere, like adding water to a chip-pan fire. |
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If I remember correctly: Although Halon is not preferred because of environmental concerns, it works by removing the heat from the flame. Only a little bit is required. It doesn't work by removing Oxygen. In that respect, in a closed room (switchgear or transformers) I would prefer to have a few percent of Halon, than be completely flooded with CO2 or N2. The latter need interlock systems to prevent entry when armed. |
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