What is backdraft, and under what conditions does it occur? Or, is it an ever-present danger? Tell me more about it…
- Jinx
Not a fireman, but I figured I’d give it a shot until one comes along. I think this is it.
Basically, you need three ingredients for a fire – heat, fuel, and oxygen. Now, lets say a fire breaks out in a windowless room w/ the door shut. The fire will keep burning until all of the fuel or oxygen in the room have been used up. In a backdraft situation it’s the oxygen that runs out first. So, you have a room full of fuel and heat. When some poor unsuspecting person opens the door a rush of oxygen is sucked into the room and it all ignites again. I don’t know if it’s as explosive a situation as in the movie, though.
I could be wrong, but this is how I understand it.
Yeah, I found this via Google, but was does it mean, in layman’s terms?
3.3 Backdraught:
Limited ventilation can lead to a fire in a compartment producing fire gases containing significant proportions of partial combustion products and un-burnt pyrolysis products. If these accumulate then the admission of air when an opening is made to the compartment can lead to a sudden deflagration. This deflagration moving through the compartment and out of the opening is a backdraught.
For a perfectly adequate translation of that text in simple English, please refer to the second post in this thread.
Backdraft can occur when the fire has plenty of heat, and lots of VOC (Volatile Organic Compounds) present in the air inside the room. The only thing missing is oxygen. No oxygen, no fire. Someone opens door and lots of nice cool oxygen enters room. The technical term for what happens next is WHOOMP!
Here is a very cool site that has a very good explanation of backdraft as it applies to backyard bbqs. We call it a flashback. There are some good pictures, and at the bottom of the page an excellent video clip of a flashback.
Jinx,
Ryanbobo gave a good simple answer, but it seems you’re still wanting a bit more. I never experienced one during the years I worked as a firefighter, but the way they explained it in training was like this.
When there’s a fire in a tightly closed room, it’s possible for all the oxygen to be used before all the fuel is gone. The fire does not go completely out; instead it changes to smoldering rather than open flames. By this point the room is completely filled with dense smoke and heat, both from the previous burning and the current smoldering. Smoke particles are very small bits of fuel that have not completely combusted. Given oxygen and flame, smoke particles can burn some more. This is where you get your backdraft; also known as flashover or smoke explosion. It’s that last term that may help you understand just what’s burning when the backdraft occurs. In the room in question, the air is filled with dense smoke (lots of particles) and high heat; the air itself is capable of rapid combustion.
Think of a toy balloon filled with hydrogen, then touch a match to it; what happens? The very air explodes (seemingly) as all that airborne fuel combines with oxygen and flame.
If someone opens the door to this room, when oxygen rushes in, the smoldering fire will once again burst into flame. That flame in turn ignites the heated smoke, which burns very rapidly. Backdraft!
Hope this helps.
Are you all sure?
I thought a backdraft is what happens when you have to change into nothing but one of those flimsy paper gowns at the proctologist’s office.
I thought it was the result of too many eggs and beer.
Backdrafts and flashovers/smoke explosions are two totally different phenomenon.
Backdrafts (as said above) are from sufficent heat and fuel remaining in a compartment, but not enough oxygen to allow combustion. By the opening of a horizontal vent of some type (door or window, generally), oxygen is allowed in and the fire comes back with a vengance (the deflagration talked about by Mr Grimwood over in Britain). You prevent a backdraft by opening up above the fire - this lets the heat out, but not enough oxygen in to let the fire come back.
Backdrafts are very rare, I only know of two of them off the top of my head in my area in the past 20 years (I wasn’t at either of them). One was a single room in a 3 decker in Fall River, the other was an entire floor of a 5 story department store in Providence back in 1986 or '87 (the Outlet, for those keeping track).
Flashover and smoke explosions, on the other hand, happen with most fires.
When a fire starts in a compartment, it is usually in what is called the incipient phase. Think of a waste paper basket on fire, thats an incipient fire. Something you can still be in the room with and not suffer too much discomfort. After the incipient phase, you get the growth phase. This continues as the fire spreads throughout the compartment. As the fire is burning away, it is releasing smoke (filling the room from top-down) and heat (making the room a bit warm). This smoke layer filling the room is also releasing heat, so the smoke and heat are doubly heating other objects in the room. Once the contents of the room are heated enough (it generally happens when the smoke is down to doorknob height), they release enough vapors to all ignite at once. This is flashover - the whole room lights up at the same time. Any unprotected people in the room will die, and firefighters wearing their full personal protective equipment correctly will have about 6 seconds to leave the room without injury. Usually flashover is the point that a fire leaves its original compartment and begins to move around the structure.
Smoke explosions, on the other hand, are when the carbon monoxide in the smoke reaches its lower explosive limit (about 2.5%, I think, but I very well may be wrong on that), and the smoke catches fire. This also happens around the same time as flashover does (when the smoke layer reaches doorknob height), but doesn’t ignite everything in the room.