When a fire is unconfined, much of the heat produced by the burning fuel escapes through radiation and convection. Think about a pile of wood pallets burning is an open parking lot. While you can feel the radiant heat as you approach the fire, convection moves smoke and hot gases up and away from the burning pallets. What changes when the fire occurs in a compartment?
Other materials in the compartment as well as the walls, ceiling and floor absorb some of the radiant heat produced by the fire. Radiant heat energy that is not absorbed is reflected back, continuing to increase the temperature of the fuel and rate of combustion. Hot smoke and air heated by the fire become more buoyant and rise, on contact with cooler materials such as the ceiling and walls of the compartment; heat is conducted to the cooler materials, raising their temperature. This heat transfer process raises the temperature of all materials in the compartment. As nearby fuel is heated, it begins to pyrolize. Eventually the rate of pyrolysis can reach a point where flaming combustion can be supported and the fire extends. In addition to containing heat energy, fires in compartments are influenced by the ventilation profile. The size of the compartment and the number and size of the openings that can provide a source of oxygen for continued combustion also influence fire development.
Stages of Fire
While the "stages of fire" are been described differently in fire service textbooks the phenomenon of fire development is the same. For our purposes, the stages of fire development in a compartment will be described as incipient, growth, fully developed and decay (see Figure 2). Despite dividing fire development into four "stages" the actual process is continuous with "stages" flowing from one to the next. While it may be possible to clearly define these transitions in the laboratory, in the field it is often difficult to tell when one ends and the next begins.
Incipient: This stage of fire development can be defined in two ways. The simplest definition is a small fire that has not yet significantly impacted the environment inside the compartment (i.e. heat, toxicity, visibility). Occupational Safety and Health Administration (OSHA) regulations dealing with fire protection (OSHA, 1993) identify an incipient fire in terms of risk. This regulation states that a fire which is in the initial or beginning stage and which can be controlled or extinguished by portable fire extinguishers or a small hoseline without the need for protective clothing or breathing apparatus.
Going back to the basics of fire behavior, ignition requires heat, fuel, and oxygen. Once combustion begins, development of an incipient fire is largely dependent on the characteristics and configuration of the fuel involved (fuel controlled fire). Air in the compartment provides adequate oxygen to continue fire development. During this initial phase of fire development, radiant heat warms adjacent fuel and continues the process of pyrolysis. A plume of hot gases and flame rises from the fire and mixes with the cooler air within the room. This transfer of energy begins to increase the overall temperature in the room. As this plume reaches the ceiling, hot gases begin to spread horizontally across the ceiling. Transition beyond the incipient stage is difficult to define in precise terms. However, as flames near the ceiling, the layer of hot gases becomes more clearly defined and increases in volume, the fire has moved beyond its incipient phase and (given adequate oxygen) will continue to grow more quickly. The fire now presents an immediately dangerous to life and health (IDLH) threat and the OSHA requirements for two-in/two-out apply (OSHA, 1993).