Ventilation Controlled Fires
Under ventilation controlled conditions, excess pyrolizate and flammable products of combustion present in smoke are a significant hazard to firefighters. Let's 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.
When the fire is burning in a ventilation controlled state, any increase in the supply of oxygen to the fire will result in an increase in heat release rate. Increase in ventilation may result from firefighters making entry into the building (the access point is a ventilation opening), tactical ventilation (performed by firefighters), or unplanned ventilation (e.g., failure of window glazing due to elevated temperature).
It is essential to recognize when the fire is, or may be ventilation controlled and the influence of planned and unplanned changes in ventilation profile. Most compartment fires that progress into the growth stage are ventilation controlled when the fire department arrives. A bi-directional air track (smoke out the top and air in the bottom) is often a significant indicator of a ventilation controlled fire. As illustrated in Figure 5, as the neutral plane drops, smoke exiting through the opening reduces the size of the inlet opening, further restricting the air available for combustion and extent to which the fire is ventilation controlled.
The series of photos in Figure 5 also illustrates the influence of decreased oxygen supply on the color and optical density of smoke. As combustion becomes more incomplete, smoke production increases, color darkens, and optical density increases. When working inside a burning building, flames moving through the hot gas layer are also a strong indicator of a ventilation controlled fire.
Ventilation Induced Extreme Fire Behavior
When the fire is ventilation controlled, increased air supply to the fire will result in increased heat release rate and depending on conditions may result in extreme fire behavior such as flashover or backdraft. While similar, vent induced flashover and backdraft are different phenomena. When the fire is ventilation controlled and fuel gas and vapor in the smoke is below its ignition temperature, increased ventilation is likely to result in a vent induced flashover (see Figure 6). If 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 backdraft (see Figure 7).
While these phenomena 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.
Table 5 lists the fire behavior indicators related to ventilation controlled decay stage conditions and the potential for ventilation induced extreme fire behavior. It is important to note that there are not always clear distinctions in the visual indicators for vent induced flashover and backdraft.
It is often assumed (incorrectly) that ventilation induced extreme fire behavior (flashover or backdraft) will occur immediately after an increase in ventilation. Depending on fire conditions and building configuration there may be a significant time lag between ventilation and resulting changes in fire behavior. When ventilation controlled decay conditions are indicated (or suspected), firefighters should move cautiously and take action to change conditions inside the building or compartment (e.g., gas cooling, ventilation).