There has been and continues to be great work performed in comparing and contrasting the differences between fires in today’s modern fire environment versus those that were seen in the past decades referred to as the legacy environment. It can be said that the fire itself does not behave any differently than it’s supposed to. The fire does not think therefore it is extremely predictable!
The chemistry and physics of fire remain the same yet the environment or the compartments that these fires are burning have significantly changed, thus changing the dynamic in which we chose to measure fire severity. The basic mechanism in which fires spread has increased solely because of the products that are within our environment therefore we must improve our understanding of our surroundings as it relates to fire propagation.
The Legacy Fire Environment
The time period of structures built between 1900 and 1949 can be labeled as legacy construction. The use of true dimensional lumber was the norm for the frame of residential structures. In taking a look back to the products that made up the environment, the all-natural wood, cloth, and organic fibers were the mainstays of these furnishings. These materials were the prevalent resource used in the construction of the contents that were inside compartments. In legacy fires, the fire initiation was slower because the breakdown in the chemical bonds of the materials burning did not create a high exothermic condition.
Gann and Friedman state (2015) “Smoldering is the most common initial stage of combustion in fires that lead to injury or death” (p.83). In legacy fires, the smoldering stage lasted much longer in duration because of the natural chemical bonds of the furnishings and surrounding environment. In legacy fires, combustion is impacted directly upon the pyrolysis of the aforementioned environment. It can be argued that the basic mechanism of fire spread in legacy fires is through conduction.
The direct heating of one material because it is in contact with another burning or flaming material. The unburned fuel particles did not generate enough enthalpy to spread flaming combustion via convective currents as fast as they do now in modern fires. This delay of rapid combustion allowed firefighters to over ventilate to create lift and sparingly use water for extinguishment.
The Modern Fire Environment
The current construction environment that we find ourselves in is extremely dangerous. The use of cheaper more factory-made structural components that are constructed with glues and frozen hydrocarbons directly increases enthalpy whereas the natural stick-built home does not. The process of pyrolysis is significantly increased thus lowering the amount of time we as firefighters have to initiate the break in the chemical chain reaction. “Pyrolysis is different from smoldering in that pyrolysis stops when the heat source is removed, while smoldering generates sufficient heat to continue without external heat input” (Gann & Friedman, 2015, p. 83).
The external heat output of modern-day fires within the compartment are constantly generating enough enthalpy to continue the smoldering process even without a direct smoldering source. The basic mechanism of fire spread in this environment can be seen through all three heat transfer processes. The noticeable difference of conduction as seen within legacy heat transfer is now magnified with convection and the movement of superheated smoke which is unburned frozen hydrocarbons at the molecular level.
In legacy fires, the main extinguishment process was to surface cool. Now with the addition of what I call fluid combustion; gas cooling and the efficacy of water placement are now the order of the day. Until we, as educators can improve upon how we train the US Fire Service at the street level in the laws of thermodynamics I fear that the civil war will continue among us and continue to grow.
One of the many reasons for fire propagation we see today in modern-day fires is because we the fire department, have failed to embrace science as a tool. I cannot remember the last time I myself have gone to fire as we made the fire bigger, hotter, and move faster because of our actions. I’m certain that we can improve upon this chain of events. Our attitudes, behaviors, and cultures have not evolved as quickly as the built environment. We are still fighting modern-day fires with a legacy mindset!
Gann, R., & Friedman, R. (2015). Principles of fire behavior and combustion (4th ed.). Burlington, MA: Jones & Bartlett.
Photo Credit: Brett M. Dzadik