Why does water suppress fire? In most other chemical reactions, water acts as a catalyst, doesn’t it? (Water makes life on earth possible.) I would think, in paper for example, the water would make the fibers separate and be easier to burn.
Well:)?
It’s the fire triangle - sustained fire requires fuel, heat, oxygen.
Water primarily prevents sufficient available oxygen being in intimate contact with the fuel. (it may also quench the heat some)
Yes, mostly it’s just a convenient way of smothering the fire and keeping oxygen out, but also water does have a particulary high specific heat capacity (it takes a lot of energy to heat up a given volume of water by a given amount). That means that when you dump cold water on a fire, it will absorb a lot of heat before it boils away, which effectively cools down the blaze.
The previous two posters are right, it’s a convenient way of smothering many fires and removing heat. It’s not necessarily as effective as CO2 or foam, but water has the advantage of being plentiful, cheap and generally accessible.
I don’t believe water does usually act as a catalyst. Chemically, it often acts as a solvent.
I guess it’ll also absorb yet more heat as it boils away - latent heat of evaporation.
But the smothering is the main effect, as evidenced by the ineffectiveness of adding water to a reaction that is providing its own oxygen, such as thermite.
And in hyper-oxidising situations (burning metals, say) water can act as a source of oxygen, sustaining the fire and liberating H[sub]2[/sub] capable of exploding. Even CO[sub]2[/sub] won’t suppress burning metal fires.
Si
Water puts out fires by two mechanisms: direct contact cooling, and steam expansion cutting off oxygen.
Water flashing into steam expands about 250:1, and it happens fast, basically an explosion. This acts to displace atmospheric air, and cuts off the oxygen to the fire. Any liquid that makes contact with the burn interface will cool below the temperature of combustion.
I’ve fought a few attic/roof fires, and provided the space is still enclosed, a small spurt on a fog setting will do the trick.
ETA: Common construction and everyday materials. Burning metal, all bets are off. Magnesium is cool to watch burn, but don’t get any water near it. You put burning metal out by burying it in sand.
It is possable to put out a burning metal fire with water. But enough has to be dumped on the fire quickly enough to cool the fire before it explodes.
It’s even better than that - I think it is about 7000:1.
I don’t believe you guys. Water’s main effectiveness is the cooling due to its high specific heat. A secondary effect is that it smothers the fire. Water doesn’t help with metal fires because it can’t cool the fire enough.
Straight stream and smoothbore nozzles are used where steam expansion will have no effect, namely open air fires. There, the mechanism is what you claim above.
In an enclosed space, fog nozzles are used, and the primary mechanism is the steam expansion directly driving out the oxygen, and secondarily robbing heat by flashing the water.
If you want to play pedantic, I’ll dig out my *Essentials of Firefighting *when I get home tonight.
ETA: ‘Asphyxia’ in your cited source comes from steam expansion.
in an enclosed space misting the airspace can prevent the spread of fire by cooling hot combustible gases so they will not ignite. cooling the airspace also disrupts the convection which is bringing oxygen into the fire.
ETA: i had this sitting unrefreshed and VunderBob posted a better response during that time.