Backdraught The most dangerous fire phenomenon is known as a ‘backdraught’ . For backdraught to occur a fire must start in a closed room environment, for example, where windows and doors are shut and with little airflow into the room. As the initial fire burns it will use up all the oxygen within the room. The fire can then die down but often does not go out. The smouldering fire that remains will then fill the room with large quantities of high- temperature smoke. As smoke consists of flammable gases mixed with combustible fuel particles, it would normally burn off. However, due to the lack of oxygen in the room, these flammable products will not burn, even though they are above their spontaneous ignition temperatures. When someone opens the door to the room and lets oxygen into this environment the flammable gases or fuel can instantly ignite. Should this happen, it does so with an explosive force that will drive the flames back out of the opening that the air came from. The flames that are produced will come out of the opening at 10 metres per second and 1100°C (30 feet per second and 2000°F). Backdraught is the most dangerous fire phenomenon, and the risk associated with opening doors onto a possible fire situation should be included in all employee fire training. E2.1 Summary In this section we have covered: The principles of combustion, fire growth and fire spread • The fire triangle. • Combustion. • Methods of heat transfer; conduction, convection, radiation, and direct burning and how they contribute to fire and smoke spread through buildings and to neighbouring properties. • The stages of fire. • Factors that influence fire growth rates and smoke movement. • Building design (such as cavities, ducts, shafts). • The conditions required for, mechanisms of, and impacts of flashover and backdraft.
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