MFA #29: Moving Products - Effective ventilation techniques

Winding up my treatise on ventilation in the modern era, this fifth and final (for now) blog on the topic will focus on the practical changes we need to make for controlling the air flow to a fire, as inspired by our new knowledge of its effects.  Links to sources for additional information on specific techniques will be provided at the conclusion of this installment.  Previous postings have discussed how fire dynamics research findings have required us to re-visit our use of this tactic, suggested that we should keep first in mind its effects on combustion, and described a wider view of the process, to encompass both its enhancement and reduction.  For those of you just joining, the first of this series can be found at http://community.fireengineering.com/profiles/blog/show?xg_source=a..., and the interested reader can proceed from there.

So, imagine we have before us a structure fire; already completed a size-up that has informed us, to the extent possible, of the location of the fire, potential victims, and any flow paths; developed and communicated to everyone involved a strategy (offensive, defensive); and are now at the point of considering whether we need to do something to change the fire’s ventilation profile.  What should be our approach?

Keeping in mind the standard caveats that every fire presents a unique set of circumstances to consider and manage, every fire department has different capabilities, and there's almost always more than one way to accomplish anything, here's my interpretation of how our new understanding of the effects of ventilation on a structure fire should guide our use of this tactic:  

1. Limit Ventilation
2. Cool the fire
3. Go with the Flow

While a step-wise method, it is not rigidly so, in that some components can be skipped or combined, as will be explained below. 

1. Limit ventilation - The default approach for a generation of firefighters has been quite the opposite, with the prototypical fire attack consisting of near simultaneous opening of the structure, stretching of hoselines, and interior search.  While fire dynamics research has shown that fires actually react to increased ventilation with increased heat production, we were usually successful in accomplishing extinguishment regardless, though likely not because of this practiced sequence.  Since the effects of ventilation, both good and bad, can be delayed, and most structures are relatively small, our PPE usually provided sufficient protection to reach the seat of the fire before conditions deteriorated sufficiently to drive us out.   

That said, many LODDs have been caused by firefighters being overcome by flashovers or caught in flow paths when ventilation efforts were poorly-timed and/or -positioned.  In my opinion and experience, many, many, many more close calls and generally worsening of conditions have resulted from the misapplication of this tactic.  If the stretch to the fire takes too long, or the vent is made too soon, things quickly take a turn for the worse, with the “too” being dangerously uncertain.  What is certain is that this potentially catastrophic change in a compartment fire's heat release rate now occurs much faster than in the past.  Modern building materials and contents contribute to fires becoming ventilation limited, and therefore almost violently reactive to a rise in available oxygen, just minutes after ignition.  Working from the premise that increasing ventilation will make things worse, it is logical that we should instead focus our efforts, at least initially, toward preventing further air flow into the fire area.  It’s best to try to minimize any hazard that is both unpredictable and lethal. 

Mentioned only to be condemned is the practice of increasing ventilation prior to the cooling of the fire, except in conjunction with Positive Pressure Attack (PPA).  Otherwise, it is vital that we limit air introduction to the building interior until after water has been applied to the burning material.  True, we are increasing ventilation whenever we break a window in order to direct a water stream inside, perform VEIS, or open a door to enter, or just peer inside, a burning building, and must be mindful of the effects of even these inadvertent increases in air flow, but in such instances ventilation is not the intent, and the resulting increase in combustion can be minimized.  What I am warning against is the intended enhancement of air flow in the mistaken effort to "clear out the smoke" in advance of interior crews performing searches or stretching hoselines.

Vertical ventilation had been the "go to" tactic for "improving" interior conditions for a generation of firefighters until its surprisingly poor performance when subjected to careful measurement and study, with real-time instrumentation and video correlation.  While there was shown to be an initial clearing of the products of combustion within a building when a vent opening was created, it was quickly replaced by more products of combustion, so that the practical benefit of this brief "lifting" was non-existent.  In order to take advantage of the temporary improvement provided by ventilation, while preventing the almost immediate worsening of conditions that will follow, hoselines would need to be in place inside the structure before the vent is opened.  Once so positioned, though, what would be the benefit of delaying water application?  The results of extinguishment (reduced temperature, initially worse visibility) are always better than continued combustion.  Fire extinguishment before ventilation might be uncomfortable; ventilation without extinguishment might be lethal. 

So, if the building is already closed up, leave it that way.  If merely a door is open, close it (after looking for any victims near the doorway, of course).  And, if other openings are providing ventilation, close them too, if possible, and by whatever means necessary.  An "old school” method to contain a fire that has burned through the door to a room is by removing another door nearby from its hinges and using it to block the entry to the fire room.  Now, we also have available specially made barriers (smoke curtains) that can perform the same function, but which are much easier to deploy.  Even without handy doors or such function-specific tools, the goal should be to minimize the amount of air available to a fire until water application begins.

2. Cool the fire - This may not be considered a ventilation tactic, but it certainly and significantly modifies the effect of ventilation on interior conditions.  It is inserted in this sequence to emphasize that air flow to a fire should not be increased without first attempting to decrease the energy that fire will thereby create.  That is, try extinguishment before ventilation.  Reducing the size of the fire prevents, or at least significantly slows, it from being worsened by more ventilation, and is therefore an important pre-ventilation technique.  This might mean playing straight streams from the exterior of the building into the fire area, a tactic that has the benefits of safety, speed, and effectiveness; but may also be accomplished by attacking the fire from the interior while maintaining door control. 

Water application has a benefit even before our streams are able to reach the seat of the fire, by cooling hot gases in the compartment, from whichever direction the fire is attacked (i.e., directly at the flames, or from the “unburned side”).  This reduction in the heat (energy) of the fire is transmitted back to its source, effectively reducing the intensity of the fire even without direct contact with the water, and should be continued until water application to the burning fuel itself can be accomplished.  The traditional concerns about the effects of steam production on interior conditions have been debunked, with the contraction of the gases produced by cooling outweighing any such expansion.  Since unplanned ventilation, as from the failure of a window, can cause sudden deterioration of interior conditions, the sooner the fire can be rendered less potent, the better, so the full reach of the hose stream should be utilized.  There is no benefit, and much risk, in delaying water flow.  

3. Go with the Flow - Except for a purely defensive operation, where firefighters never enter the interior of the structure to attack the fire, we will eventually need to utilize ventilation to facilitate our activities, and should do so by creating and/or using flow paths to our advantage.  The options and components are the same whether we are planning to Vent Smoke or Vent Fire, though the expected results, complexity, and potential for worsening conditions are vastly different.  We need both intake and exhaust sites, using openings already present (windows, doors, scuttles) or those we make ourselves (through windows, walls, roofs); and can rely on smoke and heat movement that occurs naturally (wind and/or the temperature-driven exchange of light, hot smoke with cool, heavier fresh air) or which is “enhanced” (Positive Pressure Attack [PPA] or Ventilation [PPV], hose streams).  While the menu of interventions is limited (available vs. created openings, natural vs. enhanced flow), the infinite number of associated variables (to include staffing, building geometry and construction, fire location and size, etc.) renders the process anything but simple.

For Vent Smoke, as in fires that we have successfully "knocked down", use whatever method is the most efficient - that is, the right balance of ease and effectiveness.  If smoke removal is merely serving the purpose of overhaul, it would be prudent to choose an exhaust path that can be established while causing the least amount of collateral damage.  After all, the fire, in this instance, is no longer causing damage itself, so open windows and doors instead of breaking through them.  Also, in such a less-urgent situation, allowing natural ventilation may be adequate.  On the other hand, there may be an urgency to remove smoke if victims remain inside, or (more likely) if the absence of victims has not yet been confirmed, and in either case more aggressive measures are indicated.  This might include breaking windows or (rarely, in my opinion) cutting holes in the roof or walls, as well as increasing the velocity of air flow in some manner. 

In the absence of an opportune wind blowing in the right direction, accelerating air flow by "pushing" it with fans, or "pulling" it with hose streams, will clear the structure much more quickly than natural ventilation.  PPV might be the most effective method to accomplish this, but if fans have not yet been set up, and a charged line is in hand after its use for fire extinguishment, you have a ready tool for initiating smoke removal.  Directing a hose stream out a window, with the nozzle set on fog pattern, and the majority of the window filled with the stream, can move over 10,000 cubic feet of air per minute, quickly refreshing an interior atmosphere.  In either instance, manipulating the building - opening and/or closing doors and windows - to direct air flow, will maximize the results.  For example, the research on Positive Pressure Ventilation performed by UL found that smoke is best cleared when openings (exhaust paths) are made in all affected areas, rather than one room at a time.

If the fire compartment cannot be reached from the exterior for water application, extinguishment via the interior with door control is unsuccessful, and ventilation is to be used in order to facilitate the fire attack, then we are in a Vent Fire situation, and must be prepared to manage the increased heat release that will inevitably occur.  We do so by arranging for the hose team to approach the fire via a flow path in the direction of air entry.  In that way, the interior firefighters are operating in a more tenable environment - cooler, and maybe a little clearer - as they move toward the fire.  

As with Vent Smoke, once an intake and exhaust are established, air flow in this case might be allowed to occur naturally, as in an upper floor fire that can be reached from a stair landing, or (if really lucky) when wind entering from an opening on the unburned side of the structure is driving the fire away from the attack team.  Usually, though, air flow along the direction of the fire attack needs to be facilitated, as whenever we have a fire on the same level as the attack route.  The ideal method for creating a favorable flow path is PPA, which will force products of combustion from the higher pressure created indoors to the lower pressure outdoors, while simultaneously drawing in cooler air at the intake site.  Despite its intensification of combustion, when properly performed, with an adequate exhaust path, the heat and smoke thereby produced will be propelled out of the structure, instead of spread within, allowing for a better (cooler, clearer, safer) route for firefighters to reach the seat of the fire.

This is a sequence, not a recipe, and needs to act as a guideline, not a rigid process.  Its application will vary with the circumstances, just as any of our other tactics must.  For instance, when an open window has fire blowing out and we immediately direct a hose stream to knock it down, we can safely skip the Limit Ventilation step.  Or, a fire showing from multiple portals may be too far gone to effectively stop air entry, and we would proceed to the next consideration.  When we cannot adequately Cool the Fire before firefighter entry, as when its location is not accessible from the exterior and/or interior conditions are too severe for firefighters in full PPE, yet we are not ready to "write off" the building to a defensive strategy, we next try to Go with the Flow to reach the fire.  Finally, in the fully-involved, defensive strategy scenario, we may not be able to implement any component of tactical ventilation unless/until the fire burns and/or is cooled down to a size and location that allows for these measures to be effective, so the entire process is delayed or bypassed altogether.

This discussion, while focusing on technique, was intentionally short on details of those techniques.  Besides being unable to fit that much information in anything short of a textbook chapter, there are many other expert sources for such education, two of which are here provided:

UL recently posted the reports on its experiments on Positive Pressure Attack (PPA) and Ventilation (PPV), as well as an accompanying training program on the subject:

http://ulfirefightersafety.com/projects_blog/ul-fsri-launches-posit...

Dr. Michael Reick, a Regional Fire Chief in Germany, provided this comprehensive discussion on Smoke Curtains, a topic that he also covered at FDIC in 2016:

http://www.smokeblockingdevice.eu/RSS_e.pdf ;

The preceding series is a reinterpretation of tactical ventilation, which should now be understood as the control of air flow to facilitate firefighting operations.  It acknowledges that the application of water must be integrated with this tactic in order to have the desired effect.  While this new perspective is intended to improve our understanding of these concepts and their implementation, performing these and the many other fireground functions (forcible entry, search, water supply, salvage, overhaul, etc.), in a coordinated manner, under extreme conditions (urgency, uncertainty, chaos), remains a challenging and dangerous activity.  It is hoped that that this discussion will help to reduce its inherent risks.

MJC

The author can be reached at markjcotter@comcast.net