Fire Protection Fundamentals and Fire Science
The combustion model
Fire requires four elements - fuel, oxygen, heat, and the uninhibited radical chain reaction. Removing any one extinguishes combustion, which is why cooling, smothering, separation, and chemical inhibition are the four families of extinguishment. Solid fuels must first pyrolyze into combustible gases; flaming combustion occurs in the gas phase, not in the solid.
Key properties
Distinguish flash point (lowest temperature producing ignitable vapor), fire point (vapors sustain burning), and autoignition temperature (no pilot needed). Flammability limits (LFL/UFL) define the vapor-in-air band that burns; below the LFL the mixture is too lean, above the UFL too rich. Heat release rate (HRR, kW) drives hazard far more than total heat of combustion.
Fire development
Incipient → growth → fully developed → decay. Growth is often modeled as a t² fire: Q = αt². Flashover is the transition to full-room involvement (ceiling gas temperatures near 600 °C, floor radiant flux roughly 20 kW/m²). Backdraft is different: an oxygen-starved, fuel-rich compartment deflagrates when air is admitted. Ventilation-limited fires respond dramatically to new openings.
Heat transfer and smoke
Conduction, convection, and radiation all matter; radiation scales with T⁴ and dominates large fires. Hot gases form a ceiling jet, spread laterally, and descend to form a neutral plane. Smoke is the primary killer - most fatalities are from inhalation, not burns.
Worked example
A fast-growth t² fire uses α ≈ 0.047 kW/s². At t = 200 s: Q = 0.047 × 200² = 1,880 kW ≈ 1.9 MW. Rule of thumb: a fast fire roughly doubles every 30 seconds, so a small wastebasket fire becomes a room-threatening fire in a few minutes - the basis for response-time planning.
Traps
Questions swap flashover and backdraft, or flash point and autoignition temperature. Watch for fire class confusion (A solids, B flammable liquids, C energized electrical, D metals, K cooking oils).