The new information produced by recent fire science research demands a significant reassessment of the role of ventilation for fire suppression. MFA #25 The Ventilation Revolution - New information brings new understanding (http://community.fireengineering.com/profiles/blog/show?xg_source=a...) provided the introduction to this discussion, while this installment explores how our improved understanding of its effects prompts new considerations regarding its implementation, all in an attempt to further bridge the gap between the lab and the fireground.
There are many definitions of ventilation as it relates to firefighting, but the one I prefer is "Any action that allows smoke out and air into the structure”. It is simultaneously simple and comprehensive. Included are both intentional and spontaneous actions, as we can open or close ventilation (flow) paths, with predictable effects, but it can occur even without our intervention, as when fires burn through to the exterior or cause windows to fail, or doors are left open by occupants. This description also acknowledges that the intake of air is as important a component as the exhaust of smoke. (In fact, they are equal, as the only way for a gas to exit a compartment is for one to enter to take its place.) This basic and unavoidable scientific balance - smoke out equals fresh air in - is not new information to any firefighter, and has been part of our intent with this tactic all along. We were always trying to replace the bad smoke with good air. The enlightenment provided by the research was that the increased intensity of the combustion process that resulted was more significant than the amount of heat exhausted. In other words, ventilating a fire produces more energy than it releases.
Tactical Ventilation is the utilization and/or manipulation of this “action” for our purposes. While we traditionally considered it a process of creating openings in a burning structure, our new knowledge of its effects has caused our efforts to expand to include closing some of those same openings. In MFA parlance, this is known as “managing flow paths”, but it is merely blocking the intake and/or output of gases to decrease the intensity of a fire. This suppression of ventilation can be active - placing smoke curtains or closing doors - or passive - not opening doors, windows, roofs or walls that would create flow paths. Finally, this tactic can allow the process to occur naturally, with hot smoke exiting above while cool, fresh air replaces it from below; or can be “enhanced”, with fans pushing or pulling from outside, or fog streams pulling from within, to accelerate its removal. The almost infinite combinations of planned/spontaneous, opening/closing, active/passive, and enhanced/natural, layered upon such other variables as staffing, fire location, and building construction and geometry, render Tactical Ventilation a complex process to implement and manage.
It has long been taught that there are several approaches to this tactic, most commonly “Vent for Life” and “Vent for Fire”, but “Vent for Safety” and “Vent for Property” are other variations, the difference between each being related to their intended effects. In my opinion, focusing on narrow end results creates unnecessary and impractical categories. In reality, anything we do to clear the products of combustion from within a structure serves any and all of our goals of life safety, property conservation, and fire extinguishment. Instead of purpose-directed classifications, I would like to propose a different categorization altogether (I know; big surprise!): since we now know that supplying fresh air to burning material results in a significant increase in heat production, yet still need to remove that and other products of combustion in order to do our jobs, I would suggest that the most important decision regarding the utilization of this tactic is how it will affect the combustion process. That is, before we initiate any form of ventilation, we need to determine if it will cause more heat to be created, or not, and adjust our expectations, and efforts, accordingly.
Following this “theme” of focusing on the expected heat production reaction to ventilation, the terminology I would recommend be utilized to describe the two versions of this tactic are “Vent Fire” and “Vent Smoke”, with the former signifying that the actions will result in increased combustion, and the latter merely the removal of products of combustion without affecting the burning process. Certainly, other words could be just as explanatory, or even clearer (and I welcome your suggestions), but the point is that we need to recognize these significantly different circumstances due their predictable, and essentially opposite, responses to ventilation. This distinction should be our primary consideration in decisions regarding the utility, timing, and performance of this tactic as part of an incident action plan.
Now, viewing Tactical Ventilation in the context of how it will affect the fire is certainly not a new approach. In fact, ventilation prior to fire control has always been performed with the intent of taking advantage of those effects, at least as we had previously understood them. Unfortunately, we had believed that the increase in fire intensity after “opening up” was counterbalanced by the release of heat and smoke, at least long enough for us to take action. Traditional interior firefighting methods were actually premised on ventilation causing rapid cooling and clearing to facilitate fire attack and search. With the correction in our understanding of the nature, magnitude, and timing of the effects of ventilation having been provided by recent fire dynamics research, which essentially demonstrated that the reality was quite the opposite of our assumptions, we find ourselves needing to re-evaluate this tactic altogether.
Given our continued need to release heat and smoke from burning buildings, the process of integrating our new knowledge of ventilation with our actual practices will be continued in my next posting when we explore in more detail these two approaches to Tactical Ventilation.
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