Backdraft
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A backdraft is a situation which can occur when a fire is starved of oxygen; consequently combustion ceases but the fuel gases and smoke remain at high temperature. If oxygen is re-introduced to the fire, eg. by opening a door to a closed room, combustion can restart often resulting in an explosive effect as the gases heat and expand (see also flashover).
Characteristic signs of a backdraft situation include yellow or brown smoke, smoke which exits small holes in puffs (a sort of breathing effect) and is often found around the edges of doors and windows, and windows which appear brown or black when viewed from the exterior. These darker colors are caused by incomplete combustion. If the room contains a lot of soot, it indicates that the room lacks enough oxygen to permit combustion. Firefighters often look to see if there is soot on the inside of windows and in cracks around in the room. The window might have cracked because of the heat. The windows of the structure may also have a slight vibration due to the pressure differentials. The surrounding environment (e.g. the hallway outside the suspected backdraft room) will be extremely hot.
If firefighters discover a room pulling air into itself, for example through a crack, they generally evacuate immediately, because this is a strong indication that a backdraft is imminent. Due to pressure differentials, these puffs of smoke are sometimes "sucked" back into the enclosed space from which they emanate, which is where the term "backdraft" originates.
Backdrafts are very dangerous situations, often surprising firefighters, regardless of their level of experience. The most common tactic used by firefighters in defusing a potential backdraft is to ventilate from the highest point, allowing the heat and smoke to escape without igniting explosively.
[edit] In popular culture
Back drafts entered popular consciousness through the film Backdraft, where a serial arsonist was using them as a means of assassinating people.
An interesting variation on the phenomenon occurred after the 1906 San Francisco earthquake and fire. In the days after the blaze several business people opened warm safes to see if their papers were safe. The sudden inrush of oxygen into the still-hot metal interiors led to immediate and explosive combustion of the intact contents.
[edit] See also
Quotes on Backdraft:
"What is a backdraft?" "Well the technical explanation would be a fire, that has burned out all the oxygen in a room leaving only superheated gases." [Ron Howard]
[edit] Resources
- Hall, Richard & Adams, Barbara (1998). Essentials of firefighting (4th ed.). International Fire Service Training Association. ISBN 0-87939-149-9.
- White Smoke Warning
- Ron, Howard (1991 Film Backdraft) Explanation of what is a Backdraft.
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