First, high-rise buildings present the precise same, gravity bound predicament to emergency personnel that they do to the architect, or the lazy person. Going up is unduly hard. One important aspect of this is standpipe usage. It's hard labour to get the water upto the fire, and even more importantly, you lose pressure on every floor going up. You have to have all sorts of engineering safeguards so that the upper and the lower floors receive even adequate service in time of need. And in an emergency that manages to break the standpipe, all hell breaks loose. You have about zero pressure for the floor above the break, and considerably less even for the floors directly beneath, because then the pressure only comes from the fountain above to the breakpoint (plus friction, viscosity and upward momentum, which should mostly be neglected because in the "normal" situation any such thing would hinder the service to the upper floors, which are no less valuable).
So my first thought... One way to combat this would be to include low-pressure shutouts on every floor which would cut out the flow below the break. Difficult to do in a manner that doesn't rely unduly on fallible technology or impede pressure/flow under "normal" conditions, I know, but worthy of thought especially in very high rise buildings where the probability and the span of potentially inaccessible floors compound. The solution would probably take the form of mechanical devices designed to slowly and gradually activate upon the presence of significant flow and low pressure, and which furthermore draw power from velocity alone. That sort of thing is eminently constructible, and at least in time, it could be refined to a level where it also "works itself out", so that jamming becomes unlikely even in extended use. As for illicit use of such devices, they would obviously be in more or less plain sight, any control of them would be rapidly overridden by a firesquad, and so any tampering and anybody hanging about the switch would be detected pretty much the first or second time around over an operation.
Second, I wonder whether the water and the inherent power its rapid, pressured flow carries with it could be used to extra advantage, by varying how not just the water, but the power carried by it, is actually used. I mean, power is Power; why waste the huge power that is in the flowing water so that it dissipates in splashes and heat? Why not do something useful with it?
From an alternative viewpoint, this boils down to the basic thermochemical fundaments of sustained combustion or the lack of it: what it takes for a fire to sustain itself is something combustible, usually a mixture of an oxidizer (oxygen, anybody?), an otherwise energy rich material that is held back from spontaneous decomposition by a molecular energy threshold (quite often an organic material, but purified metals and the like serve more than well), and then enough temperature that the average molecular kinetic energy transcends the chemical reaction threshold, giving rise to an exothermic reaction at a wide enough scale, and then positive feedback (usually limited by the amount of the most common oxidizer, air, available).
Ergo, a fire. Which tends to spread. And tends to consume all of those valuable organic and/or otherwise energetically unstable, purified materials we hold as economically valuable. We don't like uncontrolled fire much, then, so we fight it.
From those basics we see that fighting spreading fire means taking one of the necessary elements away. Suffocate the fucker for the lack of an oxidizer. Atomize and isolate it to limit the positive feedback/"contagion". Cool it down to recontain the original combustible so that it is energetically favourable for it to stay in shape and not burn. And in the last part especially, make sure that it don't remain hot enough to restart the reaction spontaneously, no matter what temporary extinguishing measures you've done before that. (Unless, of course, those measures aren't that temporary, which is why we also have foam, powder, zero-ventilation upon Halon release, and so on. But that is another story.)
As a general rule, you must not only stop the reaction, but maintain the reacting system in a state that somehow stops it from reacting again before it's reached its "normal", "reaction- blockaded" state. I believe every firefighter knows this, both theoretically, and practically, and is exceedingly good at it. So what I'm talking about is not what firefighters do, can do or should do. It's about the technology at their disposal.
Here I'd like to take the example of the (very badly named and kitsch-sounding) IFEX 3000. Here the idea is to carry compressed air for power, and little water. You use the air to atomize a small amount of water, and to propel it right into the "heart" (i.e. the surface of the base combustible) of the fire very rapidly. The stated idea of the invention is to maximize the usage of water, by driving it into the fire by force, and maximizing both the proportion of it that is eventually vaporized completely (100%, drawing away the maximum amount of heat water can carry per volume, which is a lot, and which is why we also primarily fight fire with water), and also doing this rapidly (the rationale for this is not explained too well, but it's sound: it's about dividing the fire and bringing large surface areas of it back into chemical stability at once; the spatial backspread is then so slow that multiple blasts can bring even large fires back under control; every fire fighter does this when swinging the hose around over a centralized, expanding cold-zone, and is encouraged to do so in training, instead of just "spraying around"). This innovation does work, and is quite an idea as such...but only upto a point.
There are at least two problems. In the case of the fire I looked at, one principal source of nastiness was burning aluminium, along with updraft over the outer surface of a high-riser which enabled rapid vertical spread.
Aluminium, that stuff reacts with water when hot. So if you feed in small amounts of high surface area water mist, the fire will not go out; even if you were able to crowd out oxygen altogether, factually you would still be supplying enough oxidizer to the fire to keep it going. In that case, if water is all you have (and with aluminium you'd have to use something like pure nitrogen to actually suffocate it, which is not feasible in practical firefighting work, especially at that scale), your only hope is to cool it down enough. So that it stops reacting/burning.
And that's also going to be a challenge to the fine-mist-little-water-camp, because, although aluminium has a limited heat capacity by any metallurgical standard, it also has less than intuitive thermal conductivity for a metal, so that you can cool it down at the surface, but it would still have plenty of internal heat to reignite at the surface if it had burned long enough beforehand. So that sort of thing clearly calls out for conventional, quite heavy water cooling at the surface, well in excess of what you'd use with other sorts of fire, and also dogged persistence -- which is squarely what IFEX cannot give you with the limited amount of water it carries.
(By the way, I suspect that is why they always demonstrate the system with volatile gas/liquid fires. There it's highly efficient, because the total heat capacity of the coolant needed is limited. You only have to extinguish the gases in the flame to bring the fire under control. But were you to put a couple of coal bricks, or a red-hot cube of iron above the liquid surface, or heaven bid an active flare into the mix, it just wouldn't work.)
So, is there a middle ground? A practical one? I think there is. The main point of IFEX is a) rapid delivery and b) high surface area of c) water, which is really, really good at c1) carrying away heat and c2) once vaporized, usually extinguishing the source of oxidizer (atmospheric oxygen) as well.
You don't need compressed air to atomize your water. You only need energy. And every firefighter knows there's plenty of that right in the pressure hose; with enough pressure and men to hold the hose down, plaster walls and the like are gone in no time. Water it also has, quite evidently. The only thing missing is the capability of efficiently forming mist instead of spray or a run. So why not build long, turbulent (even cavitating) , extremely high inlet pressure, asymmetrical, vortex-inducing, sideflow-turbine -like nozzles for firefighting use? That sort of thing would:
- be powered only by the water pressure differential to ambient, making extra supplies unnecessary,
- under current standpipes, it might reduce flow, but instead it could be optimized towards optimum dissipation of heat, and would obviously be under the control of the firefighter as well, for expert effect,
- could yield many of the benefits of the IFEX design, without tripping any patents,
- would have no more moving parts, though perhaps some extra length and weight, compared to today's standard nozzle (perhaps they would also only be used when burning metals are involved, or something, so that my idea is an addition to, not a replacement of, the well-tested arsenal of today), so that they could be easy to use and also robust,
- unlike IFEX-type systems, would ensure continual replenishing, so that even high heat capacity targets could be fought continually until they truly give up,
- unlike conventional type systems, would minimize drainage, which is a hazard not only to firefighters in heavy action (water clutters up the scene, and any water spilled is also a step away from water security in tight spots, exposing firefighters to unnecessary personal risk), but also to the property and the lives that are being protected, and finally,
- they would be a logical step beyond the flashover/hot-ceiling spray techniques that are already commonplace, thus providing a physically well-founded step beyond simple spray, and perhaps even an impetus for going towards spray instead of splash in ordinary scenarios where high heat capacity of the combustible is not an issue.
Sure, this intuition comes from a lesson already learnt; namely the fire I'm now looking at. But it is also informed by the systemic perspective on things, much magnified by the failure of etc. the WTC towers -- which were also very, *very* well engineered, and in fact eventually stood much more than they were even designed to. So what I'm asking here is, did we forget some essential element in our social science, engineering science, economical science, chemical/combustion, the actuarial one, whatever, about the distinct character of high-rise? That is, that they're *high*; as in "gravitationally challenged" in so many ways?
I know that the intuitive aspect of this has been thoroughly grasped within the rescue community. The eventual, surprisingly fast extinguishment of it by the LAFD bears witness to this. But still, do we really, *really* have any hard science underlying our responses to high rise stuff? Something that explains what we do there, and at the same time suggests verifiably better ways to improve our response?
I don't really think so. And that distresses me. As someone who'd very much like to acquaint oneself with at least one of these marvellous products of the human mind and spirit.
