Most fires that progress beyond the incipient stage are ventilation controlled at the point where the fire department arrives. This means that if the ventilation profile changes to increase ventilation (i.e., a window fails or firefighters make an opening for ventilation and/or access); the fire can increase in intensity.
Hazards of Ventilation Controlled Fires
Graphic Under ventilation controlled conditions excess pyrolizate and flammable products of combustion present in smoke are a significant hazard to firefighters. Lets go back to the fire triangle (see Figure 4) to examine the nature of this threat. While fuel, heat, and oxygen are present in proportion to support combustion where the fire is burning, the heat of the fire is pyrolyzing more fuel vapor than the fire can consume. In addition, incomplete combustion results in production of flammable gases such as carbon monoxide. The speed of fire development is limited by the availability of atmospheric oxygen provided by the current ventilation profile of the compartment or building.
Firefighters generally think of ventilation as a life safety or fire control strategy or in terms of the specific tactics used to accomplish these strategies. However, buildings are always ventilated (to one extent or another). If this was not the case, the occupants could consume the atmospheric oxygen and make the space uninhabitable. Ventilation is the exchange of the atmosphere inside the structure with outside air. The term ventilation profile refers to the actual and potential ventilation of a structure based on structural openings, construction methods, and building ventilation systems at the present time (before or during fire conditions).
Under fire conditions, the ventilation profile may change in a variety of ways. The fire may cause structural materials such as window glazing to fail, increasing ventilation. Occupants exiting the structure may either close doors (reducing ventilation) or leave them open (increasing ventilation). These of changes are called unplanned ventilation. Firefighters can also influence ventilation by simply making entry (opening a door for access creates a ventilation opening) or by specific tactical action to change the ventilation profile. When ventilation is changed intentionally as part of the incident action plan, this is tactical ventilation.
When a compartment fire is ventilation controlled, what happens when the ventilation profile changes? Smoke contains a large quantity of unburned pyrolizate and flammable products of incomplete combustion as illustrated in Figure 5. Increased ventilation provides the oxygen necessary for fire development to increase and for the fire to quickly extend to the readily available fuel present in smoke. This can lead to one of two extreme fire behavior phenomenon; ventilation induced flashover or backdraft.
In a backdraft the smoke is above its ignition temperature and simply needs sufficient air to come into the flammable range for ignition to occur. However, the smoke is not above its ignition temperature, an external source of heat is required. In this case increased ventilation allows a rapid increase in combustion and transition to a fully developed fire (flashover). This process of ventilation induced flashover is illustrated in Figure 6.
It is important to note that this is not a backdraft. In a backdraft the fuel gas and vapor in the smoke is above its ignition temperature and the rate of combustion is generally much faster (deflagration) producing a more violent reaction as illustrated (see Figure 7).
While these two phenomenon are different, both present a significant threat to firefighters. Rapid fire progress due to ventilation induced flashover or backdraft is not an instantaneous process. Depending on a number of variables such as the location of the fire, current level of involvement, temperature of the smoke (hot gas) layer, and extent of the increase in ventilation these rapid fire progress phenomenon may take some time to occur. However, when it does, fire development will be extremely rapid! Firefighters entering a compartment or building containing an under ventilated fire must be aware of and manage the hazards presented by the potential for rapid fire progress. Remember, many if not most fires that have progressed beyond the incipient stage before firefighters arrival are ventilation controlled and present the potential for rapid fire progress with increased ventilation.