This is the fourth installment in a series exploring the impact of our new understanding of the effects of ventilation on compartment fires, and suggesting modifications to that tactic that incorporate that knowledge. Rather than fill this space with links, any reader wishing to catch up by binge-reading my prior posts on this topic should start athttp://community.fireengineering.com/profiles/blog/show?xg_source=activity&id=1219672%3ABlogPost%3A626990 and proceed from there.
We have been discussing the need to re-evaluate our use of ventilation in light of the findings from fire dynamics research, which showed that additional air flow to an interior fire resulted in more heat increase than heat release. While intensifying a fire is never a good idea, releasing the heat and smoke after fire control is accomplished will make things easier on firefighters and any victims within the structure, so we still have a need for this tactic, despite its newly highlighted dangers. In order to distinguish these two situations – still burning vs. stopped burning – with their opposite reactions to increased air flow – worse vs. better – I introduced the concept of considering Tactical Ventilation in terms of whether it affects combustion (Vent Fire) or not (Vent Smoke). This post will attempt to further shift our paradigm regarding this tactic so that we might continue to reap its benefits while avoiding its hazards.
(Note: Dr. Michael Reick, a Regional Fire Chief in Germany, and a ventilation control advocate in his own right, commented on my last posting, pointing out that only ventilation limited fires [his term: “Ventilation Controlled Fires”] will react to increased air flow with increased heat output, while fuel limited fires [“Fuel Controlled Fires”] will not. He is correct, in a practical sense, as fuel limited fires are typically small, incipient fires that have not yet consumed all of the available oxygen in a compartment. Using my suggested schema, fuel limited [controlled] fires would usually fall into the “Vent Smoke” category. Ventilation limited fires, on the other hand, were the focus of recent fire dynamics research, and therefore my previous discussions. This is a key point that I had neglected to emphasize, and I appreciate Dr. Reick’s feedback spawning that clarification. Keep in mind, though, that adding sufficient air flow to any fire, big or small, will increase its intensity, as when PPV is established; just not as quickly as if the fire were actually looking for more air.)
North American firefighters have typically used the word “ventilation” to refer to the process of removing smoke and heat from a burning building. Firefighters given that tactic as an assignment would interpret it to mean creating openings in the involved building, to include clearing windows, opening doors, and cutting holes in roofs. Some would even take a leap further and utilize fans to enhance the effect (Positive Pressure Ventilation [PPV]). This is not to suggest that the execution of this process was haphazard or ill-advised. It requires training, discipline, skill, and coordination in order to perform these operations as they are intended. And operating, as we were, with the belief that this process improved conditions in a burning building, our attempts to enhance the movement of air into and out of structures on fire were a natural interpretation and utilization of ventilation as a tactical concept. That is, at the time, it seemed like a good idea.
Unexpectedly, at least from the perspective of firefighters on this continent, fire dynamics researchers demonstrated that air flow to a fire invariably causes more heat to be produced, whether ventilation was the intention (e.g., breaking a window to release smoke) or not (e.g., opening a door to gain entry). That increased heat not only spreads outward, if an exhaust route is available, but also inward, causing a rise in temperature in communicating areas of the building. Those same series of experiments also demonstrated the opposite effect, in that decreasing air flow to a fire reduced the level of heat output by the burning fuel. With the predictable results of more ventilation now proven to be contrary to what we previously believed, and the utility of reducing the amount of air reaching a fire now evident, we need to expand our use of that term so that it is more inclusive, accurate, and useful.
Now, we have long been aware of the ability of increased air flow to a fire to result in its enlargement, even if we were surprised by the intensity of that effect. Any competent and mindful firefighter will delay “opening up” until hose lines are at least in place (good), if not flowing (better), in expectation of this consequence. We are also quickly embracing the new concept of flow paths, in recognition of the dangers that such zones of heat movement represent. Similarly, the practice of door control, a logical and simple measure to minimize/delay fire enlargement, is becoming routine. So, it is actually not a radical stretch for the fire service to begin to embrace a new, comprehensive model for the management of air flow at a compartment fire.
We should start by taking a more holistic view of ventilation, considering all of its facets and techniques, rather than focusing our attention just on how to increase it. Recall its definition from a previous post: “Any action that allows smoke out and air into the structure”. This “action” is certainly not limited to those we carry out, but includes wind effects, windows that fail due to fire damage, and doors left open by fleeing occupants (or cunning arsonists). Ventilation is not merely something we do, but the very process by which any compartment fire obtains oxygen. This can be “bidirectional flow” through a door or window, with cool, fresh air coming in low and smoke and heat exiting above; or “unidirectional flow” when air intake and smoke output occur at different sites. Lacking both of these – the “no ventilation” situation – combustion will be reduced, and heat output will decrease. (Keep in mind, though, that your average building is not designed to withstand the interior temperature insult from even a “dampened down” fire, which might remain 600-800F+, so that, usually and eventually, structural degradation will lead to the “re-establishment” of ventilation.)
So, instead of ventilation being a standard assignment, leading to the natural conclusion that it is something that needs to be “done”, a more appropriate approach would be to first size up the situation, ascertaining the location of the fire, any flow paths, and modifiable barriers (windows, doors, walls, roofs), whether already open or not. Next, the expected effects of various firefighting activities, such as forcible entry, VEIS, or hoseline deployment, must be layered over this milieu. This is not just the Incident Commander’s responsibility but, in fact, every firefighter must be constantly aware of how their actions might affect the air flow to the fire, even if their assignment is search, extinguishment, or salvage. This mandates the careful coordination of every task that can result in air introduction to a compartment containing fire. Considering ventilation during all facets of fire control is actually more practical than attempting to tease it out as a stand-alone factor in such a complex and dynamic environment as a burning building, often with multiple, concurrent interventions, players, and sites involved. It is only after such a careful and comprehensive evaluation that the best ventilation intervention, if any, can be determined and undertaken to improve the situation.
Even though an integrated view of ventilation makes more sense, interconnected as it is to so many other elements and actions, making calculations and predictions regarding its consequences remains a complex task. Still, we firefighters are used to making time-critical assessments with limited information, developing plans to stabilize even evolving situations, and then implementing those measures expertly. We are able to do so, in large part, by having available a limited number of strategies that can be applied to a wide variety of different incidents, allowing us to fall back on a general plan that is known to all involved. For instance, our options for fire control are investigative, offensive, or defensive, describing in broad but clear terms that our plan is to, respectively, examine, attack, or contain the fire. In the case of tactical ventilation, the choices can be categorized as maintain, increase, or decrease air flow.
At some incidents, the existing ventilation profile suits our attack strategy nicely, and we maintain it, leaving it as found. An example would be an upper story room-and-contents fire with an open window. In that instance, we can approach from below, in the cooler intake flow path; extinguish the fire; and then direct the hose stream out the window to help exhaust the remaining products of combustion. Similarly, attic fires, which research shows us are typically ventilation-limited, need “only” water application from below (easier said than done), and merely worsen when the roof is opened above. Other scenarios may call for us to create flow paths to facilitate extinguishment efforts, and thereby increase ventilation, as in the first example of a second floor compartment fire if the window were not already open on our arrival. Going a step beyond, many fires can be more easily managed by augmenting air flow (PPV), as long as a path is available to safely exhaust the additional products of combustion that will be produced. Finally, there will be situations when we need to decrease or halt ventilation that is already occurring in order to improve conditions, usually just temporarily, while preparations are made for water application and firefighter entry. This can be as simple as closing a door, or the placement of specially designed smoke curtains across the site of air entry.
These choices illustrate a wider perspective of ventilation, showing it to be a process that we might enhance, reduce, or just leave alone, as best suits our needs and the setting at hand. This requires looking beyond ventilation as something we merely “perform”, and instead as a key factor affecting the rate of combustion that we must do our best to “control”. Tactical ventilation, then, encompasses a more extensive variety of factors and techniques than those with which we have long been accustomed. And, although decisions regarding this parameter will challenge our skills and judgement, entangled as it is with so many other fireground activities that can be happening almost simultaneously, this enhanced perspective will offer more opportunities to manipulate its effects to our benefit.
To be sure, this “radical” change in our use of this facet of fire control is not one I can take credit for, despite having brought us to this point through a rather circuitous discussion of the topic. In the SLICE-RS acronym, the “I” represents “Identify and control flow paths”, nicely paraphrasing the new ventilation paradigm here described. By shifting our consideration of ventilation to its effects on combustion and the distribution of its products, we can better manage it as a factor in accomplishing fire control.
Well, enough theory already! It’s time to show how these ideas translate into action.
Next installment: MFA #29: Moving Products – Effective ventilation techniques