FW

1

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.

PRODUCTS OF COMBUSTION

A. Fire Gases

  • Byproducts of combustion in gas phase (e.g., CO, CO₂, SO₂, soot).

B. Heat

  • The energy released by combustion that propagates the fire; causes burns and heat-related injuries.

C. Smoke

  • Visible product of incomplete combustion; typically a mixture of O₂, N₂, CO₂, CO, soot, and other products.

D. Flame

  • Incandescence of gases accompanying rapid oxidation; the luminous portion of a burning gas.

PROPERTIES OF FIRE

A. Physical Properties

  1. Specific Gravity – ratio of the weight of a solid/substance to the weight of an equal volume of water.

  2. Vapor Density – weight of a volume of pure gas compared to weight of a volume of dry air at the same temp/pressure.

  3. Vapor Pressure – force exerted by molecules on the surface of the liquid at equilibrium.

  4. Temperature – measure of thermal agitation of molecules in a substance.

  5. Boiling Point – constant temperature at which a liquid's vapor pressure equals atmospheric pressure.

  6. Fire Points – lowest temperature at which vapors evolve fast enough to sustain continuous combustion; higher than flash point.

  7. Flash Point – temperature at which a liquid gives off enough vapor to form an ignitable mixture; e.g., gasoline around -50^\u00b0F; kerosene around 100^\u00b0F.

  8. Auto-ignition Point / Kindling Temperature – temperature at which a flammable liquid forms a vapor–air mixture that ignites.

B. Chemical Properties

  1. Endothermic Reaction – chemical change where energy/heat is absorbed before reaction proceeds.

  2. Exothermic Reaction – release of energy/heat; products have less energy than reactants.

  3. Oxidation – chemical change where a fuel reacts with an oxidizer (e.g., oxygen).

  4. Pyrolysis – production of incandescent gases and heat; a combustion product when solid fuel thermally decomposes.

ELEMENTS AND THEIR USES

FUEL

  • Fuels can be solid, liquid, or gaseous; any material that can burn.

  • Example: paper or wood.

GENERAL CATEGORIES OF FUEL

  1. Solid Combustible Materials

  • Includes organic/inorganic, natural or synthetic solids, including metals.

  • Common solids: wood, paper, cloth; dust can burn faster than bulky material.

  • Types of Flammable Solids:
    a. Pyrolyzable Solid Fuels – burn readily; vapors released during chemical breakdown mix with air to form flames.
    b. Non-Pyrolyzable Fuels – harder to ignite; e.g., charcoal; no pyrolyzable components; no fumes released; gas-to-solid flame interaction.

  1. Liquid Combustible Materials

  • All flammable liquid fuels and chemicals.

  • Rate of vaporization is greater for liquids than for solids because of looser molecular packing.

  1. Gaseous Substances

  • Examples: acetylene, propane, butanes.

  • Properties: compressibility, expandability, permeability, diffusion.

GROUPS OF SOLID FUELS

A. Biomass – replaceable organic matter (e.g., wood, garbage, animal manure) used for energy.

  • Factors affecting combustibility of wood/wood-based products:

    • Physical Form (smaller pieces ignite more easily than large pieces)

    • Moisture Content (dry wood ignites more easily than wet wood)

    • Heat Conductivity (good heat conductor ignites more easily)

    • Rate and Period of Heating (less-flammable materials require direct contact)

    • Rate of Combustion (more oxygen increases burn rate)

    • Ignition Temperature (higher temperature reaches ignition faster)

B. Fabrics and Textiles – most fibers are combustible; fabrics are twisted/woven; textiles may be machine-woven/knitted.

  • Classification of Fibers:
    a. Natural Fibers – from plants, animals, minerals.
    b. Synthetic/Artificial Fibers – organic/cellulose fibers, acetate; non-cellulose/inorganic fibers (e.g., fiberglass, steel).

  • Factors affecting combustibility of fibers: chemical composition, fiber finish/coating, fabric weight, weave tightness, flame retardant treatment.

LIQUID COMBUSTIBLE MATERIALS

  • Includes flammable liquids and chemicals.

  • Rate of vaporization is greater for liquids than solids; liquids release a wide range of vapors, making gasoline a major fire hazard due to flammable vapor at normal temperatures.

  • General characteristics of liquids: definite volume but no definite shape; assumes vessel shape; slightly compressible but not indefinitely expandable.

  1. General Groups of Liquid Fuels

  • Flammable Liquids – flash point around 37.8^\u00b0F and vapor pressure around 40\;psia.

  • Combustible Liquids – flash point at or above 37.8^\u00b0F.

  • Factors affecting flame propagation and burning of liquids: wind velocity, temperature, heat of combustion, latent heat of evaporation, atmospheric pressure.

  • Latent heat – heat absorbed by a substance when changing phase (e.g., solid to liquid to gas); heat released during gas–to–liquid or liquid–to–solid transitions.

  1. Gaseous Substances

  • Examples: acetylene, propane, butanes.

  • Gaseous properties: compressibility, expandability, permeability, diffusion.

CHARACTERISTICS OF GAS FUELS

  • Gas molecules are in rapid movement and random motion; no definite shape.

  • Molecular collisions occur with container walls.

1) Classification of Gases

  • Based on Source:
    a. Natural Gas – mainly methane; odorized for leaks; often mixed with small amounts of butane/propane (LPG/LNG).
    b. Manufacture Gas – synthetic fuels derived from coal, petroleum, or biomass.

2) According to Physical Properties

  • a. Compressed Gas – gas at normal temperature inside a container remains gaseous under pressure (e.g., oxygen tanks).

  • b. Liquefied Gas – gas exists partly as liquid and partly as gas under pressure.

  • c. Cryogenic Gas – gas at very low temperatures (far below ambient), typically near its boiling point with moderate pressure (e.g., nitrogen).

3) According to Usage

  • a. Fuel Gases – flammable gases used with air to produce heat, power, light, or process energy.

  • b. Industrial Gases – used in industrial processes (e.g., oxygen, acetylene for welding/cutting; refrigerants like Freon, ammonia, sulfur dioxide).

  • c. Medical Gases – used for treatment (e.g., anesthesia, respiratory therapy).

FLAMES

  • Flames are incandescent gases, bright and hot; a combustion product in gas-phase combustion.

  • Difference between Fire and Flame:

    • Flame: a stream of hot, burning gas from something on fire.

    • Fire: the state of burning that produces flames, heat, light, and might produce smoke.

TYPES OF FLAMES

A. Based on Color and Completeness of Combustibility of Fuel

  1. Luminous Flame – orange-red; deposits soot; incomplete combustion; lower temperature (center part of a candle flame).

  2. Non-Luminous Flame – blue; complete combustion; higher temperature (outer layer of candle flame).

B. Based on Fuel and Air Mixture

  1. Premixed Flame – air thoroughly mixed with hydrocarbons before entering flame (e.g., Bunsen burner; low flame velocity).

  2. Diffused Flame – gas confected into atmosphere, diffuses with air; examples: candle flame, oxyacetylene torch; diffusion controlled by molecular diffusion.

C. Based on Smoothness

  • Laminar Flame – particle follows a smooth path through flame.

  • Turbulent Flame – erratic, irregular flows; all laminar flows can become turbulent with increasing physical size, gas density, or velocity.

HEAT

  • Thermal Energy: energy possessed by a material due to molecular activity; energy in transit due to temperature difference.

  • Thermal Balance: natural condition created by fire or normal movement of fire, smoke, and gases within a structure.

  • Thermal Imbalance: turbulent circulation of steam and smoke that can mislead investigators about origin; hot spots may appear.

  • Heat transfer: moving heat from warmer to cooler objects.