Fires 4
Fires 4 : When a material burns, Thermal Radiation is emitted. This form of heat transfer other than by conduction or convection can cause harm or damage to people and objects. The amount and rate of energy emission depends upon a number of factors, the first of which is the type of Fire. A Pool Fire may result from the ignition of flammable vapour from a spill of liquid which has collected on the ground or in a container. The linear rate of evaporation of liquid from a pool fire is usually termed the Buring Rate. This term is also used to describe the mass-burning rate of other types of fires. If the ignition takes place when material is emerging from the release point under pressure, a Jet Flame can be produced. If, however, the release produces a cloud of gas which is then ignited, the Flame Front usually moves through the cloud in a Flash Fire, ultimately consuming at least those portions of the cloud in which the concentration is above or below the Lower and Upper Flammable Limits respectively. Portions of the cloud which are at concentrations above the upper flammable limit will not burn until further diluted with air. The lower and upper flammable limits are not constant and depend upon a number of factors. On occasions, a cloud fire can induce sufficient buoyancy to rise in the air, burning as a Fireball. An adequate supply of oxygen is necessary to sustain combustion of the material and therefore a fire will draw air into the combustion zone. In the case of extremely large fires (over several kilometers square) caused by large scale wartime bombing this in-rush of air reached hurricane force. This phenomenon is known as a Fire Storm. However, the relevance of fire storms to chemical plant hazards is very doubtful. For each type of fire, the radiation emitted depends, among other things, on how fast the burning material is consumed. Calculations of the rate of energy received at a target are based on the behaviour of black body radiators. The extent to which the source approaches the emissive power of a black body at the same temperature is known as the Emissivity. This can be used to predict the Surface Flux of a flame. To estimate the proportion of this flux received at any specified "target", it is necessary to know the View Factor, which depends on the special configuration of source and target and the Transmissivity of the intervening medium which allow for the attenuation of radiation passing through that medium. The Absorptivity of the target material determines what fraction of the incident energy will go towards raising the target's temperature. Fire, in the sense discussed above, refers, to the combustion of materials in air. However, there are reactions in which air is not involved but which produce similar hazards; e.g. the burning of military propellants or the burning of iron in a chlorine atmosphere. No attempt has been made to produce definitions for such specialised areas. Safety measures associated with fire fall into two broad categories. Fire Prevention measures are those intended to reduce the likelihood of a fire occurring. Fire Protection measures are those which seek to minimise the extent of damage from fire should it occur. Fire protection systems may detect, extinguish, contain, or allow persons or property to tolerate a fire
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