As illustrated in Figure 7, the small wastebasket does not release sufficient heat energy to cause a 16' x 20' (4.88 m x 6.10 m) room to reach flashover. However, the rate of heat release from the wood and polyurethane sofa is more than sufficient to result in flashover.
A number of factors influence ignitability and heat release rate of solid fuels, in addition to the fuel type; thermal thickness, and configuration are also significant. When heated, the temperature of thermally thin materials will rise quickly. With thermally thick materials, surface temperature will increase, but the internal temperature will rise more slowly. Thermal thickness is dependent on physical thickness and thermal conductivity. Materials that are physically thin and/or have high thermal conductivity are thermally thin. Materials that are physically thick and/or have low thermal conductivity are thermally thick. Thermally thin materials heat and reach their ignition temperature more quickly than materials that are thermally thick. Regardless of the thermal characteristics of the fuel material, the more surface area exposed to heating the more rapidly it will reach its ignition temperature. As previously discussed in relation to building construction (e.g., heavy timbers vs. light weight trusses), surface to mass ratio can be a significant factor in fire development.
In the wildland environment, firefighters recognize the significance of horizontal and vertical continuity of fuel materials. When fuel consistently covers the ground, fire can spread more readily. When there is continuous fuel from the ground up to the level of trees fire can quickly spread vertically from the ground to aerial fuels. Structural firefighters face the same issues. However, in a compartment it is critical to examine not only fuel on the floor (the ground), but also fuel on the walls and ceiling (contents, interior finish, and structural materials).
At the most basic level, size involves area and height. However, it is important to consider the impact of compartmentation. For example, a building is 50 feet deep x 100 feet wide and 30 feet tall. Would the fire problem presented be different if this was a single compartment as compared to a three-story building? What if each floor was open as compared to being subdivided into small rooms?
Compartmentation (or a lack thereof) is often recognized as a factor in high rise buildings. However, many modern homes have considerably less compartmentation in living areas such as kitchen, dining room, living room, and family room etc. than older buildings. What impact might this have on fire development and fire control operations? When examining compartmentation within a building, don't limit your view to habitable spaces. Void spaces are compartments too!
Size is really about the volume of the compartment, not simply floor area. High ceilings increase compartment volume, providing increased oxygen for fire development and often masking developing fire conditions from firefighters operating at floor level. For example two Chicago, firefighters were killed by a backdraft in a high ceiling commercial building in 1998. Firefighters were operating in a large commercial building with a 20-foot ceiling height (bow truss roof). Firefighters inside the building observed smoke overhead, but conditions at floor level were dark (nighttime fire) but not smoky or hot. Unknown to the firefighters, backdraft conditions had developed in the hot smoke layer under the buildings bow truss roof. A change in ventilation resulted in a backdraft that killed two firefighters and injured three others (NIOSH, 1998).
Building and compartment size has a significant influence on both fire development and fire control requirements. Large compartments contain more oxygen and require more fuel and a higher rate of heat release to reach flashover. Fire development in a larger compartment may be slower (depending on the type and quantity of fuel available). When a fire in a large compartment progresses beyond the incipient stage, an increased flow rate will be required (fire flow requirements will be addressed in a subsequent article).