What is the exact process by which the presence of water halts the oxidation reaction, does the water physically smother the fire, does it draw energy out of the reaction, or is there a more complicated chemical reaction ?. Would any of this be affected by the temperature of the water.
I would assume that in a fire fighting situation the temperature of the water might effect how much of it gets to the fire without being evaporated by the heat.
Would this make much of a difference in practice ? (I’d assume the water used to fight fire is pretty cold anyway)
I’m not sure how much difference it makes in practice, but yes - water acts to put out a fire by conducting heat away from the burning material, and by smothering it and preventing oxygen from reaching the flame.
The difference between using water at 2 degrees celsius and 26 degrees celsius might not be that noticeable, though… we as human beings react to those temperatures very differently, but they’re both very cold in comparison to a raging fire. Also, the amount of heat energy that it takes to turn water at 100 C to steam at 100C is quite high… maybe almost as much as it takes to bring that mass of water from 0C to 100C, or more??
Also, I think a lot of the smothering effect is not due to liquid water, but from the rapid expansion of steam as the water evaporates near the flame.
Maybe there is a marginal difference as [c]chrisk** said. Water cools things down to below the ignition temperature for the fuel surrounding the fire and prevents the fire from spreading. It also helps to cut off the oxygen needed to sustain the fire.
As for cooling down. It takes one calorie to raise the temperature of 1 gm of water 1 C degree. So if the water starts at 10 C vs. 20 C that’s an additional 10 calories/gm of heat removed from the area of the fire. On the other hand it takes 539 calories to produce vapor from one gram of water. So a gram of water at 10 C will remove 90 + 536 = 626 calories while water at 20 C will remove 80 + 536 = 616 calories. Hardly worth the trouble.
Is that 539 or 536 calories? You aren’t being quite consistent. In any event, that’s more than 5 times the heat necessary to raise the water from 0C to 100C, so I guess my ‘or more’ was far too timid… Oh well. Found it hard to believe it could be that much… though I admit I don’t often leave a pot of water boiling on the stove long enough for it to all vaporize away LOL.
As far as your comparison… you seem to be assuming either that the fire itself is just above 100C, or that the steam will not absorb any further heat from it?? How hot does fire tend to be, and hot hot does it have to be to still burn and recover? (I know this will vary enormously depending on the circumstances.)
FWIW in many petrochemical plants, a steam hose is used for fire control in small, local areas like a welding repair. The steam smothers the flame (quickly at that), but it is still water.
Not sure if this is along the lines of what you were asking, but it is still technically water on a fire.
Its 539 and 3 calories needs to be added to both sums. A typo crept in and I was sloppy in reviewing.
Flame temperatures are high, like 1200 F or more. But the temperatures around the place where the water is being heated will be held way down until all of the water is evaporated. And the fire hoses keep pumping in water to evaporate.
The water vapor takes about the same amount of heat to raise its temperature as does the liquid water, i.e. 1 calorie/C degree. The main thing that keeps the temperature down is the evaporation.
That’s the vast majority of it. For the fire to burn somthing, it first has to spend some of its own heat to get that new material up to its combustion point, and if that new material is soaked with water, it has to evaporate all the water first. Water is so good at dousing a fire because of its high latent heat at being converted to steam. Heating up a gram of water from room temp to 100C takes about 75 calories, but then turning that water at 100C to steam at 100C takes an additional 540 calories. That’s a huge amount of heat.
Heating up the steam after that takes some of the heat, but then, the fire would be heating up air anyway, right? Does humid air have that different of a specific heat than dry air?
As far as I can tell no. The amount of heat required to heat saturated air isn’t much greater than dry air.
The highest temperature data I could find without making a research paper out of it is 100 F.
At that temperature saturated air contains 2.86 x 10[sup]-3[/sup] lb/ft[sup]3[/sup] of water vapor which would take 2.86 x 10[sup]-3[/sup] Btu to go up 1 Fahrenheit degree. The cubic ft of air weighs 0.081 lb and has a specific heat of 0.242 at constant (atmospheric) pressure. So the dry air would take 19.6 x 10[sup]-3[/sup] to go up 1 Fahrenheit degree. If a particular quantity of such air took 6 Btu to raise one degree, saturated air would take about 6.15 Btu to do the same thing.
And if you consider steam, rather than just saturated air, the answers don’t get too different.
C[sub]p[/sub] for N[sub]2[/sub] is 29 JK[sup]-1[/sup]mol[sup]-1[/sup], and for O[sub]2[/sub] it’s also 29 JK[sup]-1[/sup]mol[sup]-1[/sup], so it’s also 29 for any arbitrary mix of N[sub]2[/sub] and O[sub]2[/sub], e.g. air.
C[sub]p[/sub] for steam is 35 JK[sup]-1[/sup]mol[sup]-1[/sup], so any given volume of steam will only only absorb 20% more heat than an equal volume of air, for the same temperature rise.
Just a quick note to add here, this is also part of why fires involving flammable liquids or gases are more difficult to fight, they are still flammable even near the freezing temperatures of water
Not exactly
If my old fire chem isn’t too rusty, someone else will probably catch me on something here
Flammable range is more the issue. Many solids and liquids don’t actually burn but release flamable gases or vapors when heated. You have to have a high enough ratio of that vapor in the air to create a sustained flame. Pump enough steam into the air around it and you can force the environment out of the flammable range even if you cannot directly apply water to the fire. There is still plenty of oxygen around and with a hot enough fire water is actually adding oxygen to the mix (magnesium or aluminum fires for example which are hot enough to disassociate water molecules).
Good point. It probably won’t arise to often, but if you’re ever around a magnesium or aluminum fire call the fire department first and only try to put it out if there is the proper fire extingusher handy. Water is a no no fer sure.
Actually its far from unknown to see a vehicle fire get a magnesium or aluminum rim or other component going. You can put them out with water, but reel lines are not going to cut it. You are just going to have to drown it with a bigger line.
Class D fires involve combustible metals, such as magnesium, titanium, potassium and sodium as well as pyrophoric organometallic reagents such as alkyllithiums, Grignards and diethylzinc. These materials burn at high temperatures and will react violently with water, air, and/or other chemicals.
There are also conditions under which water can disassociate in the presence of burning carbon forming hydrogen and carbon monoxide gases. As you can imagine, the results of this are not conducive to quenching a fire. Under most conditions it takes a metric buttload of carbon, so, unless you have a coalmine I wouldn’t worry about it. I’ve heard of it happening in a wood pulp warehouse as well, but you probably don’t have one of those either.