Firefigthting is what we usually associate with the fire service, in the meaning ”putting out fire”.
But – and statistics is very clear about this – putting out fires is a small part of the work by the fire service. In Sweden, the number of killed in fires has been more or less constant for a number of years (appr. 100 per year). A large part of the turnouts are to traffic accidents, false alarms or a countless number of other more or less unthinkable situations that occurs. But still, fires causes large costs for society.
In the majority of cases water is used for putting out fires.
Water is a very good extinguishing agent, not least because of its fantastic properties. The heat absorption capacity is 4.18 kJ/kgC°, the heat of vaporization is 2260 kJ/kg and the heat absorption capacity of water vapor is 2 kJ/kgC°, so even here water is more or less unbeatable as an extinguishing agent. For every liter of water applied per second, it is therefore possible (in theory) to extinguish a fire of 3 MW in a flash. And in our part of the world, it also occurs in abundance.
Water for fire extinguishing can be said to be linked to five types of extinguishing mechanisms:
- Gas phase cooling (including both flames and hot fire gases)
- Lowering the oxygen content and diluting flammable gases (mainly at the molecular level)
- Wetting and cooling of fuel surfaces
- Radiation attenuation (mainly at the molecular level), and
- Kinetic effects.
The ability of water to cool hot gases (and flames) is related, among other things, to the size of the droplets. Small droplets have a larger surface area in relation to their volume than large droplets. Small droplets therefore heat up faster and evaporate faster than large droplets. However, small droplets have a shorter throw than large ones, partly because they have less mass. The ability to reach fuel surfaces is therefore worse (in addition, they evaporate completely or partially on the way).
When water evaporates, the water vapor will take up a certain amount of space. The oxygen content will then drop and the fire gases will be diluted and become less flammable. The effect is generally small, but cannot be ruled out.
Water that hits a fuel surface will cool the surface and stop or prevent the pyrolysis that is required for the fire to continue. Here, large droplets are better than small ones, partly because large droplets have a better throw distance. Partly, a large drop wets more surface than a small drop. The size and speed of the drop will also affect what happens when it hits a hot surface.
Water vapor also has the property of preventing or at least dampening radiation. And radiation is an important mechanism for the spread of fire.
Finally, the water also has a so-called kinetic effect.
The droplets simply move around in the fire gases, which partly draws in cold (colder) air that cools the hot gas and the flames. Partly, the droplets can bring air from outside with them, which can cause the fire to increase in intensity. But since we add water, the cooling effect of the droplets will take over, provided the flow is large enough.
For the sake of simplicity, we can say that small droplets are smaller than about 0.5 mm.
Large droplets are thus larger than that, but the equipment we have at our disposal delivers a whole spectrum, a distribution, of droplet sizes. In practice, we cannot deliver just one droplet size. Except for a closed water jet, which can be considered as a single long continuous drop. But even this will break up into smaller droplets on the way to the target. Or if they hit an object or a surface. It is also not possible to produce droplets that are just as small, because small droplets collide with other small droplets and then become larger droplets.
We also often forget that a certain minimum amount of water is normally required to extinguish a fire of a certain size. A little simplified: a large fire requires more water than a small fire. But to extinguish a fire, it is normally required that we reach the fuel with water so that the pyrolysis is stopped. Working only with fire gas cooling will therefore not normally extinguish a fire. Regardless of which equipment is used and how we apply the water. Also note that both the flow and nozzle pressure will affect the manageability of the equipment.
Depending on how the water is applied, these extinguishing mechanisms will have a greater or lesser effect on the extinguishing. It is therefore not an either or and we cannot choose to eliminate any of these mechanisms.
In a slightly simplified way, we can say that water can be delivered with a closed jet, with a spread jet or with a mist jet (or water mist). A mist jet/water mist then becomes a special case of a spread jet: an even more spread jet than a spread jet, but above all smaller drops (with all that this entails). Depending on how the delivery takes place, the properties of water are utilized in slightly different ways. There is therefore no one right way, but you have to adapt the delivery to what you want to achieve with the properties of the water. If I want to throw water far, a closed jet is undeniably the best. In addition, the closed jet is better at hitting surfaces, which means that the temperature of the surfaces drops and that pyrolysis decreases. On the other hand, I have to move the jet around to reach all surfaces. Also note that a closed jet can be considered as a single large continuous drop. If I want to cover a larger surface or a larger volume, a spread jet is better. The gas phase effect is better than for a closed jet, but the surface effect may be slightly worse than with a closed jet, since I may not reach the fuel. On the other hand, I can reach a larger surface (and a larger volume) at the same time, without having to crank the jet around.
Fire fighting (=putting out fires) is a craft that has to be done manually. From time to time we come up with ingenious gadgets that are said to put out fires even better than before. But to actually put out a fire, someone has to get inside and do the job, eventually. In many cases, it is even better to use a shovel and a bucket of water than a nozzle. And no matter how you get the wet stuff on the red stuff, you need to know how large the flow rate should be, how high the pressure in a hose/nozzle, how long do I have to put water on the fire, and, of course, where do I put the water?
In addition, one needs to pay attention to a large number of other parameters. What are the conditions in the building? What does the building look like? What does the fire safety in the building look like?
After a fire, ask yourself if it was the fire service that put out the fire or did the fire go out of fuel?