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Halon; A chemist's perspective
In the current spate of letters on halon and fire extingushers there are a
couple of statements made that I simply cannot let go unchallenged.
I was the chairman of the safety committee of the chemistry department
of a major researsch university, and in that position I learned quite a
bit about fire safety and fire extingushers.
First let me emphasize that there is simply no completely safe and
completely nontoxic way to extingush an electrical fire in an enclosed
space as small as a Velocity cabin. That being said, by far the best way
is to use a halon fire extingusher. In fact, I have already purchased one
because I am afraid that they might not be available when I complete the
Most elementary fire safety lectures (and believe me, I have sat through a
lot of them) start with the fire triangle. To sustain any fire you need
fuel, heat, and oxygen. Remove any one of the three and the fire goes out.
the three common types of fire extingushers work in the following way, the
powder extingushers work simply by coating the fuel with a sticky layer of
powder which prevents contact between the fuel and the oxygen while both
the halon and carbon dioxide extingushers both cool the area and displace
the oxygen. They do this in different ways. The carbon dioxide
extingushers do this in a pretty obvious way. David, you are about right in
what you said about this except that the correct way to use a carbon
dioxide extingusher on a large fire you first knock down the flame from a
distance (smother it) and then move in to cool it before the CO2
dissipates and it reignites. The halon works somewhat differently however,
contrary to the impression that some of you seem to have the principal
action is NOT chemical. Halon is a very dense, inert (i.e. slow to react),
gas with a very high heat capacity. It is not really drawn into the flame
but it is expelled from the hot zone much more slowly than the oxygen and
so the effect is the same in that it tends to collect in the hot zone and
effectively excludes the oxygen and at the same time it cools the area by
soaking up the heat because of its high heat capacity.
David, your understanding the chemistry is incorrect because the halogens
in the halon are already fully oxidized and so there is not way that they
can take up more electrons. The only reasonable bulk reaction between
halon and oxygen would result in CO2(or CO) and free halogen and this
reaction would be so slow under any reasonable fire extingushing scenio
that it would not be a major concern. In any event breathing a small
amount of CO or free bromine, while not exactly good for you beats the
hell out breathing fire or the fumes from an extensive fire in a cabin made
of modern synthetic materials. The fumes from any extensive cabin fire in
a Velocity are going to be toxic as hell. In that sense the suggestion of
getting a smoke hood is a good one that I intend to follow.
Martin, I don't know where you get the idea that the decomposition
products of halon are so toxic. The fumes from overheated teflon are much
worst. Yes teflon does not support combustion, but when heated to the
decomposition point, it gives off extemely toxic fumes. In a sense the
comments by Martin and Simom directly contradict each other. If the halon
is inert (slow to react not necessarily unreactive) enough to get to the
stratosphere to destroy the ozone, will be too inert to decompose much in
a typical fire.
The halon itself is odorless, tasteless and as nontoxic as anything can
be (ever water will kill you). In the bad old days when DuPont was making
multiton lots of halon they used demostrate it's safety by filling a room
completely full of halon and having the prospective customers walk
through it. They never lost a customer.
The halon extingusher that I bought from Spruce says to not use it in an
enclosed space smaller than 150 cubic feet . That is 3 X 5 X 10 feet that
is close enough to the size of a Velocity cabin that I am not going to
worry about it because such warnings are always very conservative.
In short I am with Bill Wade all the way.
My personal opinion is use a large CO2 fire extingusher for ground engine
fires or hangar fires, use halon for a cabin fire, and and don't even
think of using a powder extingusher if any thing else is available.