Fire is a chemical process that rapidly transforms its surroundings. Understanding how fire progresses from its initial spark to its final decline is important for general awareness and safety. This progression follows a predictable sequence of stages, each with distinct characteristics that influence its behavior and impact.
The Nature of Fire
Fire is a chemical reaction known as combustion, which generates heat and light. This process requires a specific combination of elements to ignite and sustain itself. The “Fire Triangle” illustrates these three components: fuel, oxygen, and heat. Fuel is any combustible material, oxygen acts as an oxidizer, and heat provides the energy to initiate and continue the reaction.
The “Fire Tetrahedron” expands on this concept by adding a fourth element: the chemical chain reaction. This continuous exothermic reaction sustains the fire by generating more heat, which ignites more fuel. If any one of these four elements is removed, the fire cannot begin or continue to burn.
Incipient Stage
The incipient stage marks the beginning of a fire. At this point, the fire is small, localized, and confined to its initial fuel source. It has low heat release and a distinct flame.
During this phase, sufficient oxygen is available, and visibility is not obscured by smoke. The incipient stage is the easiest to extinguish, making early detection and intervention effective in preventing further growth.
Growth Stage
Following the incipient phase, a fire enters the growth stage, intensifying and spreading beyond its initial point. Heat production increases, and flames grow larger as the fire consumes more fuel and involves a wider area. The generated heat causes nearby combustible materials to reach their ignition temperature, rapidly increasing fire size and intensity. Hot gases and smoke accumulate, often forming a defined layer at the ceiling.
An event that can occur during this stage is “flashover.” Flashover is a sudden, near-simultaneous ignition of most combustible materials within an enclosed area due to radiant heat. Temperatures can rapidly soar to 1,000 degrees Fahrenheit in seconds, making conditions hazardous. This phenomenon signifies a rapid transition to the next, more dangerous stage.
Fully Developed Stage
After flashover, the fire reaches its fully developed stage, marking its peak intensity. All available fuel in the compartment is actively burning, and the fire generates its maximum heat release rate. Temperatures within the affected area reach their highest, reaching 2,000 degrees Fahrenheit.
Conditions throughout the compartment become uniform and hazardous due to the widespread involvement of combustible materials. This stage is characterized by destructive power and high temperatures, threatening property and life. The fire continues to burn at this intensity until it depletes its fuel supply or oxygen levels decrease.
Decay Stage
The final stage of fire progression is the decay stage, where the fire begins to diminish in intensity. This reduction occurs due to the depletion of available fuel or a reduction in oxygen within the compartment. It is characterized by decreasing heat and shrinking flames.
While visible flames may lessen, the fire can still pose hazards through smoldering materials. A phenomenon known as “backdraft” can occur if oxygen is suddenly reintroduced into a fuel-rich, oxygen-depleted environment during this stage. This sudden influx of oxygen can cause accumulated hot, unburned gases to ignite explosively.