FW

una

Introduction to Fire Protection and Arson Investigation

  • Fire: a state, process, or instance of combustion in which fuel or other material is ignited and combined with oxygen, giving off light, heat and flame.

    • Rapid oxidation with evolution of heat and light.

  • Cold Fire: fire that burns below a temperature of 400^\u00b0F. These are cooler-than-normal fires. Example: Alcohol produces a cooler flame than acetylene.

Elements of Fire

  • Classic Fire Triangle: HEAT, OXYGEN, FUEL.

    • Fire occurs when fuel reaches its ignition point (via heat) and reacts with oxygen in surrounding air, causing combustion.

    • The fire continues to burn until at least one of the three elements runs out.

FIRE TRIANGLE

  • Elements:

    • FUEL

    • HEAT

    • OXYGEN

  • Notes on FUEL:

    • Fuels can be solid, liquid, or gaseous.

    • A fuel is anything that can burn; removing fuel or having too little fuel causes the fire to go out.

    • Example: paper or wood.

  • Notes on HEAT:

    • The flash point of the fuel determines how much heat is necessary for the chemical reaction.

    • Heat can be generated by various means (e.g., friction, chemical reactions).

    • Lowering the material temperature is a method of fire suppression (e.g., water or chemical extinguishing agents).

  • Notes on OXYGEN:

    • Fire typically requires oxygen; air contains about 21\% oxygen.

    • Extinguishers and blankets reduce or cut off oxygen to the fire.

WHY OXYGEN IS IMPORTANT TO FIRE

  • The chemical process when a fuel is ignited requires oxygen for the reactions that cause burning.

  • Oxidation generates heat and combustion byproducts (e.g., smoke, gases).

  • The fire continues as long as there is oxygen in the air.

  • Some extinguishing agents work by reducing oxygen concentration (e.g., CO₂ or inert gases).

FIRE TETRAHEDRON

  • Adds a fourth element to the Fire Triangle: chemical chain reaction.

  • This chemical chain reaction provides adequate heat to sustain the fire.

  • Fire grows and burns as long as the chain reaction is sustained.

  • Suppression occurs when at least one element of the fire tetrahedron is removed.

STAGES OF FIRE DEVELOPMENT

1) Ignition – when the four components combine and combustion starts.
2) Growth – a fire plume develops above the burning fuel; surrounding air is entrained into the plume.
3) Fully-Developed – all combustible materials in the compartment are involved.
4) Decay – heat release declines as fuel is consumed.

  • Note: Flashover is not a stage; it is a rapid transition between growth and fully developed stages.

COMBUSTION

  • Combustion: an exothermic sequence of chemical reactions between fuel and an oxidant, producing heat and changing chemical species; can present as heat with glowing or flame.

  • A complex reaction requiring: a fuel (gas/vapor), an oxidizer (oxygen), and heat to proceed.

TYPES OF COMBUSTION

A. Glowing Combustion

  • Occurs when solid fuels do not produce enough gases during pyrolysis to sustain a flame.

  • If access to oxidant is limited, glowing combustion may occur.

B. Flaming Combustion

  • Commonly recognized type; occurs with gaseous fuel sources.

  • Flame color can indicate fuel composition.

C. Spontaneous Combustion

  • Ignition of organic matter without apparent cause, typically via internally generated heat from rapid oxidation (self-heat).

D. Explosive Combustion

  • Occurs when vapors, dust, or gases are premixed with air in the right proportions and ignite.