Returning to the foundations — ventilation during a fire


CarolinaFireJournal - Mike Coffey
Mike Coffey
10/14/2011 -
If we as humans give the chemical process of combustion human-like characteristics, we find that the process uses solids, liquids and gases as fuel and oxidizers combined with a heat component in the right combinations — it eats, drinks and breathes just as we consume solids or liquids and breathe. If we apply those concepts within a compartment — room or structure — the same results happens to combustion as would to us under water — a limit of the available oxygen needed to sustain the living process — the air within our lungs vs. the air available within the enclosure. image

Ventilation by definition is the systematic removal of heated air, smoke, and gases from a structure, followed by the replacement of these products of combustion with a supply of cooler air, which facilitates other fire fighting priorities. I have noticed that fire departments tend to vent too little rather than too much, and often too late. Most of the time — ventilation is an afterthought once suppression attack lines knock down the fire.

With what’s in our homes, offices and work environments, ventilation needs to become a higher priority due to the plastics and other manmade products producing more BTUs (double to triple) than what used to be in those structures, such as wood with natural fiber padding inside the natural fiber covering.

Building construction has become more energy-efficient allowing for little air, if any, to enter from the outside or let any out from inside the structure. Sending crews into these superheated atmospheres that are oxygen-starved for fire attack and search for occupants without ventilation efforts is asking for trouble.

Ventilation is broken into two categories: natural and forced ventilation. Natural ventilation allows the natural currents and air movement to take products of combustion out of the structure through the natural openings — doors and windows. With the natural tendency of hot gases to go up and out, do not forget to use what is already on the roof — ventilators, skylights, removable gables — it will make life easier.

Forced ventilation uses blowers, ejectors and fog nozzles to move air in or out of the structure. Types of forced ventilation we are used to hearing about are actually misnomer terms. Positive pressure — pressure is always positive — entails blowing of air from outside a confined space into that space. Negative pressure — absence of pressure = vacuum — involves ejecting/exhausting air from within a space to the outside. Hydraulic ventilation assumes the venturi effect draws smoke and fire gases out an opening — not accurate description of entrainment of air ejected from confined space.

Positive pressure ventilation is the newest form on the block. The ability to understand that air pushed into a compartment must have a way out. Using blowers to push out contaminants and toxic gases is easier than the older method of ejecting the products of combustion out with smoke ejectors. For positive pressure ventilation to work, the exit opening should be three-quarter the size of the entry opening or larger. The cone of air the fan produces should cover the entry opening completely. Since ventilation is systematic, removing contaminants room by room is the most efficient method — positive pressure ventilation will not work as effectively if the whole building is opened at once and takes longer to get the task done.

Negative pressure ventilation involves a staple that is dwindling in use — the smoke ejector. Designed to bring contaminated air inside a room/building to the outside by drawing air through the fan creates a “negative” pressure inside the room. Openings are important here as well — replacement air has to come from somewhere. Placement of the ejector for the best results is inside the space either to be vented approximately six feet back from the exit opening, on the floor or up to four feet high — depending if the contaminants are heavier or lighter than air. Smoke ejectors used to hang from blocked open doors on a ratcheting bar. When this tactic is used — the rest of the opening must be covered to prevent contaminants from circling back around the ejector and back into the building. We still look for maximum airflow from the fan to remove the maximum amount of smoke and contaminants.

Hydraulic ventilation involves the use of a fog stream on a wider pattern than used for firefighting. As the pattern opens, air entrained into the stream makes it very effective in bringing the smoke and other byproducts of combustion with it and exhausting them outside through a window or door opening. This is a tried and true method, but some of the drawbacks are water damage created by the stream, limited water supply does not allow for full use — water supply needed may not be able to keep up — the crew doing hydraulic ventilation must be in the contaminated atmosphere = full personal protective equipment in place and using the SCBA.

Making the decision for ventilation is based on several ideals including stage of fire (growth, flashover, fully developed and decay), location of fire (where in the building), fire spread from convection and radiation, smoke conditions (volume, velocity, color and density), building construction (fire-resistive, noncombustible, ordinary, heavy timber, wood frame) including lightweight components (trusses, wooden I-beams, composite materials) and age of the building.

With void spaces all the rage in some of the construction styles, knowing where the potential spots can be for releasing hot gases and smoke will pay dividends. Occupancy also plays into the ventilation decision. What is in the structure to burn (contents) along with construction components. Sprinkler systems will hold the fire in check or put it out, but the cooler smoke will cause it to hang lower and be a vision problem. Forced ventilation is the preferred method here as natural wind currents may not be effective.

Coordination of the vent group with the fire attack group and search group is essential. Being able to release those superheated gases, smoke and steam produced from the fire stream, either horizontally or vertically, allows for more aggressive fire attack and search for victims and gives victims better chances for survival. Channeling those deadly gases and unburned fuel outside the structure also reduces further property damage and reduces the possibility for backdraft, rollover or flashover.

One simple act assists in the three major tactical considerations: life safety (for firefighters and occupants — atmosphere is cooler and tenable), incident stabilization (reduces property damage by directing unburned fuel and fire gases outside the structure) and property conservation (less damage to structure).

Horizontal and vertical ventilation practices are as debatable as fog nozzles and smooth bore nozzles. Opening attics with a hole for an attic fire reduces the possibility of flashover or further damage to the trusses — already under heat stress and additional live and dead loads. Opening the compartment to release dangerous and combustible byproducts of combustion is the only way to maintain the fire in a localized area or making a stand not to let any further damage happen.

Horizontal ventilation is the easiest method encountered and is very useful in a room and contents fire. This way perceives to create less damage and works well with light to moderate smoke conditions. Allowing the natural (or forced) movement of air to do what Mother Nature intended helps us to accomplish fireground objectives. There are circumstances where vertical ventilation is the only way to ventilate — below-grade spaces  such as basements, cellars, subbasements, etc.

Vertical venting should not be the tool left in the toolbox when needed. It is tougher to accomplish, fire and products of combustion looking for a way out and are damaging the property more. The ability to recognize which system to use benefits those we send in to do battle and gives the occupants a fighting chance to be rescued and removed from deadly atmospheres.

These brief points can open the doors for more application and review. Not all the techniques are covered here, but knowing the individuals I work with, there are more ways to get things done. Ventilation is not a one and done subject. Continue to work to better the service we provide and understand that one way is not the only way. Be careful and be safe in the job!

Captain Coffey currently serves as one of the training officers of the High Point Fire Department. He has been an instructor with the NC Community College system and the Office of the State Fire Marshal since 1990. He can be reached at [email protected].
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