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Fire Triangle/Tetrahedron & Combustion Cheat Sheet by

safety     fire     triangle     tetrahedron     combustion

The Fire Tetrah­edron

The concept of fire was symbolized by the Triangle of Combustion and repres­ented, fuel, heat, and oxygen. Further fire research determined that a fourth element, a chemical chain reaction, was a necessary component of fire. The fire triangle was changed to a fire tetrah­edron to reflect this fourth element. A tetrah­edron can be described as a pyramid which is a solid having four plane faces. Essent­ially all four elements must be present for fire to occur, fuel, heat, oxygen, and a chemical chain reaction. Removal of any one of these essential elements will result in the fire being exting­uished.

The Four Elements

The four elements are oxygen to sustain combus­tion, sufficient heat to raise the material to its ignition temper­ature, fuel or combus­tible material and subseq­uently an exothermic chemical chain reaction in the material. Each of the four sides of the fire tetrah­edron symbolise the Fuel, Heat, Oxygen and Chemical Chain Reacti­on. Theore­tic­ally, fire exting­uishers put out fire by taking away one or more elements of the fire tetrah­edron.

Exting­uishing a Fire

The symbol although simpli­stic, is a good analogy, how to theore­tically extinguish a fire, by creating a barrier using foam for instance and prevent oxygen getting to the fire. By applying water you can lower the temper­ature below the ignition temper­ature or in a flammable liquid fire by removing or diverting the fuel. Finally interf­ering with the chemical chain reaction by mopping up the free radicals in the chemical reaction using, BCF and other halon exting­uis­hers, it also creates an inert gas barrier. However this type of exting­uisher is being phased out and in the future other exting­uishing agents may be found using this principle.

Classes of Fire

Class A fires involve Organic solids like paper, wood, ...
Class B fires involve Flammable Liquids
Class C fires involve Flammable Gasses
Class D fires involve Metals
Class F fires involve Cooking oils.
 

The Fire Tetrah­edron

The Combustion Modes

Flaming Mode
Non-flaming Mode, smoldering or glowing embers.

Flaming mode: it is necessary for solid and liquid fuels to be vaporized. The solid fuel vapors are thermally driven off, or distilled and the liquid fuel vapors evapor­ated. It is this volatile vapor from the solid or liquid fuels that we see actually burning in the flaming mode. This gas or vapor produc­tion, emitted from the fuel is referred to as pyrolysis. Once a flame has been establ­ished, heat transfer from the flame to the fuel surface continues to drive off more volatile gases and perpet­uates the combustion process. For continued burning in the flaming mode requires a high burning rate, and the heat loss associated with transfer of heat from the flame area by conduc­tion, convec­tion, and radiation must be less than the energy output of the fire. If the heat loss is greater than the energy output of the fire the fire will exting­uish.

Both modes, flaming and non-fl­aming surface modes, can occur singly, or in combin­ation. Flammable liquids and gases only burn in the flaming mode. Wood, straw, and coal both modes may exist simult­ane­ously.

Flaming combustion forms

Premixed flames where the fuel and oxygen are mixed prior to ignition. For example the flame on a bunsen burner, gas stove, or propane torch.
Diffusion flames, more common, where the fuel and oxygen are initially separate but burn in the region where they mix, like a burning of a pool of flammable liquid or the burning of a log.

Three Stages of a Fire

There are three generally recognized stages to a fire. The incipient stage, smoldering stage, and flame stage.

Inci­pient stage: where prehea­ting, distil­lation and slow pyrolysis are in progress. Gas and sub-micron particles are generated and transp­orted away from the source by diffusion, air movement, and weak convection movement, produced by the buoyancy of the products of pyrolysis.
Smol­dering stage: Fully developed pyrolysis that begins with ignition and includes the initial stage of combus­tion. Invisible aerosol and visible smoke particles are generated and transp­orted away from the source by moderate convection patterns and background air movement.
Flaming stage: Rapid reaction that covers the period of initial occurrence of flame to a fully developed fire. Heat transfer from the fire occurs predom­inantly from radiation and convection from the flame.

Explosions

Generally, an explosion is defined as a very rapid release of high-p­ressure gas into the enviro­nment. The energy from this very rapid release of the high-p­ressure gas is dissipated in the form of a shock wave.

Explosions can be classified as physical, a balloon bursting, as physical and/or chemical, a boiler explosion, or a chemical reaction of a gas/pa­rticle mixture. Our discussion will focus on chemical reaction explos­ions.

The process of a chemical reaction explosion is similar to the combustion process whereby a fuel and oxidant have premixed prior to ignition such as petroleum vapor or fine particles of grain dust mixed with air. However, in an explosion the oxidation process proceeds at a greatly accele­rated rate. The oxidation process is usually, but not always, confined within an enclosure such as a tank, grain silo, so that a rapid high-p­ressure rise occurs with an associated flame front. Generally, it is this high-p­ressure shock wave that causes the damaging effects from an explosion.

Resultant shock waves that propagate from the point of ignition at a velocity less than the speed of sound are termed deflag­ration. Shock wave velocities in excess of the speed of sound are termed detona­tions.

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