Fundamental Fire Physics

Fundamental Fire Physics

Photo credit: djwtwo / Foter / CC BY-NC-SA

Photo credit: djwtwo / Foter / CC BY-NC-SA

Fire protection personnel must understand fundamental fire physics if they want to know how building and fire codes should be applied and how fire protection systems work.

Six key terms in fire dynamics are

1. Heat Release Rate–a measurement of the energy released from a burning material over a specific time period. It describes the material’s potential fire severity. Heat release rate is described in watts, British Thermal units (Btu’s) per second or joules per second. For example, a burning upholstered chair has a heat release rate between 80kW and 2.5 MW.

2. Heat of Combustion–the total potential heat output from a fire, measured in Btu’s or joules. For example, wood and cellulose products have a heat of combustion of about 8,000 Btu’s per pound, while hydrocarbons have a range from 16,000 to 24,000 Btu’s per pound.

3. Pyrolysis–the heat-created decomposition of combustible materials into simpler compounds that can be ignited. It is the process of converting solid materials into combustible vapors.

4. Piloted Ignition — the ignition of a gas- or vapor-air mixture by the introduction of an outside source such as a flame, spark, or hot spot. Piloted ignition occurs at the lower flammable limit of vapor-air mixtures. Piloted ignition is employed when flammable and combustible liquids are tested for fire points by ASTM D92, Test Method for Flash and Fire Points by Cleveland Open Cup.

5. Heat Flux — the rate of heat transfer per unit of time in the crosssectional area of a material. Heat flux is measure of the material’s thermal conductivity. It often is measured in Btus/second/ft2 (Watt/ m2 ) and is represented in formulas by the symbol “Q”. In the built environment, carpet and other floor coverings are tested for the heat flux needed to ignite them in accordance with NFPA 253, Standard Method of Test for Critical Radiant Flux of Floor Covering Systems Using a Radiant Heat Energy Source.

6. Flame Spread — the continuous progress of piloted ignition along surfaces or through porous solids where the flames provide the heat source. The flame spread of building materials is measured in the Steiner Tunnel apparatus using ASTM E84, Test Method for Surface Burning Characteristics of Building Materials.

For additional information, refer to the SFPE Handbook of Fire Protection Engineering, the NFPA Fire Protection Handbook, or perform a keyword search for these terms on the World Wide Web.

Knowledge Check: Can you explain the terms heat release rate, heat of combustion, pyrolysis, piloted ignition, heat flux, and flame spread?