leichter@lrw.com (Jerry Leichter) (05/20/91)
Several recent TELECOM Digest messages have described Halon as "a deadly poison" or as "displacing the oxygen in a room". All this is nonsense. Halons are not poisonous, and in the amounts used in fire supression systems do not have a significant effect on the amount of oxygen in a room. (I don't remember the exact numbers, but I think a typical Halon system tries to get a 5-10% concentration of Halon in the room. That's no worse an effect on the availability of oxygen than dropping the air pressure by 10% -- which is almost within the range of normal barometric pressure variations!) Halon fire supression is a very interesting effect. For a fire to burn, you need three basic components: Fuel, oxygen, and heat. Traditional techniques aim at one or more of these components. Water mainly removes heat. Carbon dioxide fire extinguishers mainly remove oxygen: Since CO2 is heavier than air, it will settle over the fire and smother it. Sand will do the same. Halons don't attack ANY of these three components: Their mechanism of action is more subtle. If you look at a fire more closely, you find that there's an intermediate stage between fuel and oxygen: The heat first causes the fuel and the oxygen to break up into active free radicals; it is these free radicals that then combine, producting more heat. The flame region itself is full of these radicals. Halons act on the radicals: They grab onto them strongly and carry them away from the flame. I believe this is a catalytic process: Once the Halon/radical combination gets away from the heated area, it breaks up, re-constituting the Halon to grab more radicals. (The released free radicals are spread out and away from the hot zone, and so are not a problem.) Because this mechanism strikes right at the heart of the reactions that keep the fire going, it can act very quickly and very effectively, with relatively little Halon. Halons are being phased out because they persist in the atmosphere and destroy ozone. I'd guess it's pretty much the same reactions that allows them to put out fires that also makes them so effective at destroying ozone. Chloroflorocarbons like Halon are essentially non-reactive with biological materials -- it takes a fair amount of energy to split them up. (That's also why they persist in the atmosphere until they make it up to high enough for ultraviolet light to provide the necessary energy.) That's one of the things that has made them so useful: They have been widely used as non-toxic refrigerants, foam blowing materials, and so on. I watched a test of a Halon fire supression system installed at Yale a number of years ago. The testers remained inside the room as the system was set off; they seemed quite unconcerned. A decent-sized Halon system is pretty impressive to watch when fired: It has to get a sufficient concentration of Halon throughout a room FAST. This requires that the Halon be forced into the room at high pressure, creating quite a wind -- all sorts of things go flying. Also, as the Halon expands rapidly, it undergoes adiabadic cooling, rapidly dropping the temperature in the room -- so the room instantly fills with fog. In all, a startling experience if you aren't prepared for it -- but not in and of itself dangerous. Jerry
"Louis J. Judice 21-May-1991 0916" <judice@foamer.enet.dec.com> (05/21/91)
One more quick word on Halon systems. There's a big difference in the destructive velocity that Halon is discharged with, depending up the design and implementation of the system. Generally, the cost of a system is directly proportional to the number of nozzles. Low bid installations with 2-3 nozzles in a 5000 sq ft room will release with destructive force. Better systems with many nozzles with almost instantly fill the room with no ill-effects. I've seen $15,000 microscopes smashed and nearly a hundred ceiling tiles dislodged in the test of a low-cost halon system at my former employer. The system I installed in a DEC facility several years ago, though, was much more expensive, and caused no damage. I hope this is some help to those with halon systems. I certainly understand the environmental issues, but I sure hope CO2 is not re-established for this purpose, since it is definitely dangerous. ljj