Show Menu

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.

Download the Fire Triangle/Tetrahedron & Combustion Cheat Sheet

2 Pages
//media.cheatography.com/storage/thumb/davidpol_fire-triangle-tetrahedron-and-combustion.750.jpg

PDF (recommended)

Alternative Downloads

Share This Cheat Sheet!

Like this cheat sheet? Check out our sponsors!

Readable.io is a collection of tools to make your writing better. More readable content means higher conversion rates and better reader engagement. Measure website and document readability, measure keyword density and more!

Click Here To Get Started!

 

Comments

No comments yet. Add yours below!

Add a Comment

Your Comment

Please enter your name.

    Please enter your email address

      Please enter your Comment.

          Related Cheat Sheets

          First Aid Kit Cheat Sheet

          More Cheat Sheets by Davidpol