Comments by "" (@walterkronkitesleftshoe6684) on "The Slow Mo Guys"
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A backdraught is when a closed fire compartment (such as a room with a closed fire resisting door) traps the fire, heat and smoke within... the oxygen in the room is then consumed by the fire and the flames die down as no new oxygen is available due to the closed door/windows, but the room remains VERY hot and full of thick smoke together with a high concentration of partially combusted gases from the room's smouldering contents (fuel)... now only the oxygen is missing from the "triangle of combustion".... a door opens / window fails / or floor or ceiling burns through and fresh oxygen is now drawn into the room as the room's internal overpressure pushes out some of the previously trapped smoke/gases. The incoming oxygen mixes with the concentrated flammable gases accumulated within the room and brings the atmosphere into the "explosive range" not too lean and not too rich, and then all it needs is a smouldering ember to set of the explosive ignition. A tell tale sign of potential backdraught for firefighters attending an incident, is blackened windows with yellowish grey smoke pulsating from any gaps or cracks, also lots of heat and smoke, but little flame within a building is also a VERY dangerous situation.
A flashover is when the heat trapped at the top of a burning compartment eventually reaches such a high temperature that its radiance downwards is sufficient for every exposed combustible surface in the compartment to spontaneously combust within a second or two, turning a growing but defined fire source into a general firestorm in seconds, and it will even char nomex fire kit, and blister and crack the surface of a fire helmet. The way to remove this risk is to PULSE water spray into the searing hot upper area of the room to absorp and reduce the heat in the upper area, as a continous spray of water will completely fill the room with superheated steam in seconds, with resultant heavily "poached" & scalded firefighters.
A lesser known phenomena is known as a fire gas or smoke explosion, and is where the heat from a well developed fire in one room is transferred through the structure of a building usually into a room above the fire, this heat causes the floor covering to smoulder and thermally decompose though not necessarily in flames, filling the room again with heat and gases of thermal decomposition (pyrolysis) then if an ignition source is introduced (spark from an electrical fitting or a smouldering ember is exposed) and again an explosive conflagration can occur in a room that was not itself directly on fire. Such an incident killed two Welsh firefighters in a house fire in Blaina Gwent UK in 1996.
These are all reasons why properly managed & controlled ventilation of fire situations is now used far more often than in the past.
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