Ventilation-Limited Fires and the Influence of Oxygen

By Ian Bolton

Fire engineers have long understood in detail the true impact of oxygen on fire growth and spread. For decades now, these engineers have known that when a fire develops within a room or structure, the fire will burn within one of two burning regimes.  These two regimes are often referred to as “fuel-limited” or “fuel-controlled” and “ventilation-limited” or “ventilation-controlled.”  Since North American fire services have often faced a great disconnect between them and fire research, this important concept has only recently begun to find its way into their vocabulary, training, and operational considerations.

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Most firefighters are well aware of the fire development curve represented by the red line on the graphic in Figure 1, the model usually introduced during basic firefighter recruit training. It is likely that this curve still represents their understanding of how fires develop.  But, the fire of today has changed from the one in the graphic. The curve is a depiction of a fuel-limited fire that more accurately reflects the fire stages of a couch burning in an open parking lot.  The curve is not a fair representation of how fires develop when confined in the built environment.  Fundamentally, the heat release rate (amount of energy released over time) is limited primarily by the available fuels, chemical, and physical characteristics.  In most structure fires, only during incipient and early growth stages is fuel the limiting factor of a fire’s rate of development and spread.

During these early stages of fire development, fires often begin to become less affected by the available fuel characteristics and more affected by the limited air available within the fire compartment and the connecting structure (see position #1 on graph). When this occurs, the fire’s growth will be largely dictated by ventilation. At this time, the fire is said to have transitioned from fuel-limited to ventilation-limited.

Figure 1: Ventilation’s Impact on Fire

“Ventilation-limited fires are more typical of the fires you are going to encounter on the streets today,” said Dan Madrzykowski of the National Institute of Standards and Technology, while presenting to firefighters at the 2013 Redmond Symposium. Fires burning within this state will then drastically be affected by any increase in ventilation to the fire.  It should also be noted that ventilation is always occurring within a building. Lack of ventilation does not mean lack of tactical ventilation performed by firefighters; it refers solely to the lack of air available for the developing fire.  

In 1917, British scientist William Thornton discovered that the consumption of oxygen by a fire is directly related to the amount of energy created. Furthermore, it was found that regardless of what product may be burning, whether it is a piece of wood or a polyurethane sofa, for each kilogram of oxygen used for combustion, approximately 13.1 MJ of heat energy will be created.  Based on this concept, it is possible to make some basic calculations to determine how long it would take for any given room or structure to run out of the required oxygen for a fire to continue to develop.  For example, if we consider a large living space measuring 2.5m × 6m × 12m in area, with a fire burning at a steady state of 2MW, it could be found that the oxygen in this space would be depleted in approximately five minutes and 24 seconds if all windows and doors are closed.  This is, however, a considerable oversimplification, as fires begin burning at a lower heat-release rate and grow until limited by oxygen as levels fall often below 15 percent.  Furthermore, there is often leakage through doors and windows that would allow some smoke and air exchange. Regardless, many firefighters have somewhat dismissed or misunderstood the critical impact oxygen has on producing thermal energy.    

Today’s Ventilation-Limited Timeline

Evolving fuel loads found in today’s homes have changed the timeline in which fires become ventilation-limited. Recently, Underwriter Laboratories Firefighter Safety Research Institute (ULFSRI) compared the time differences between fires becoming ventilation-limited in two homes—one filled with older furniture constructed of natural products indicative of the home furniture of the 1970s and the other home filled with modern synthetic-based furniture found in homes today.  The results were staggering.  The natural material-based furniture, referred to as a legacy fire, reach ventilation-limited conditions in 20 minutes after ignition.  Conversely, the modern fire reached ventilation-limited conditions in only five minutes.  Furthermore, after initiating tactical ventilation (resulting from opening the front door and removing a window),  the home with the legacy furnishings took eight minutes, 30 seconds to reach flashover; the modern fire took only two minutes, 15 seconds.

NIST and ULFSRI tests have also shown that once a fire runs out of oxygen and becomes ventilation-limited, temperatures inside the structure start to decline rapidly (see position #2 on graph); the fire is in decay stage and can no longer release sufficient thermal energy.  When this occurs, the existing hot fire gases begin to contract in volume, often resulting in the structure’s entering a state of negative pressure.  Subsequently, previously visible smoke that may have been exiting from windows or other leakage areas of the structure will stop, and the negative pressure created by the contracting fire gases will result in air being drawn into the structure through any available ventilation openings or gaps.  Fire crews arriving on scene at this time may see no smoke showing even though the structure is charged with hot ignitable smoke.

Based on the average response time of fire service personnel, it is common for crews to arrive on scene after a fire has transitioned into the ventilation-limited state.  Even if a fire has self-vented and flames are showing out a window, it is highly likely the fire is still ventilation-limited.  In these cases, flames are burning outside the fire compartment because there is not sufficient air inside.  Furthermore, the rest of the structure may also be full of hot fire gases but unable to burn because of a lack of oxygen. At this point, crews need to recognize the potential impact any further supply of air will have on the fire.  As internationally recognized fire behavior instructor Chief Ed Hartin explains, “If a developing compartment fire becomes ventilation-controlled [limited] with the heat release rate limited by the oxygen available in the compartment, any increased ventilation at this point (see position #3 on graph) may cause the fire to quickly transition to the fully developed stage” (position #4 on graph).

Videos of ULFSRI and NIST tests demonstrating these concepts are at https://vimeo.com/80730910

Recognizing the Danger

For fire crews to make the appropriate decisions on the fireground, the prevailing fire condition most be recognized and understood. Identifying a fire that is ventilation-limited is critical to safe and effective operations.

Some common signs and indicators of a fuel-limited fire follow:

·       An incipient or early growth fire

·       High neutral plane (smoke layer)

·       Better visibility

·       Relatively low temperatures

Some common signs and indicators of a ventilation-limited fire follow:

·       Fire beyond the incipient stage

·       A mid to low neutral plane (smoke layer)

·       Poor visibility

·       Flames exiting outside a window or door

·       Air being drawn into the fire when a door is opened

·       Increased temperatures, which will decline in the decay stage

·       All common backdraft indicators

Tactical Considerations

Assumptions are generally bad. If crews assume a fire is fuel-limited on arrival and make inappropriate decisions, potentially negative results may occur.  However, assuming every working fire is ventilation-limited on arrival is not only likely to be correct, but it will also improve firefighter safety as members will control available air to the fire until it will serve as a tactical advantage to do otherwise.

If no smoke is issuing from a structure on arrival, the fire may be ventilation-limited and in decay stage, as the structure is in a state of negative pressure. For this reason, ­nothing showing on arrival means nothing!  A proper size-up is necessary to determine whether or not crews have a working fire.

The front door must be considered as ventilation. Doors provide access for fire crews to perform fire attack and search, but also they also allow fresh air in to feed a developing fire.  Furthermore, closing an open door to a house immediately on arrival until it will serve as a tactical advantage to do otherwise will help control the heat-release rate of the fire.  Also, in some situations, firefighters may want to consider door-control tactics while a fire crew advances a line in search of the fire.

Further information regarding ventilation-limited fires and the impact oxygen has on fire conditions may be found in various ULFSRI reports on horizontal and vertical ventilation @ http://ulfirefightersafety.com.

BIO

Ian Bolton has been a member of the fire service for 12 years. While working in Sydney, Australia, he was trained as a fire behavior and tactical ventilation instructor and has received additional training in these areas through the Swedish Civil Contingency Agency in Revinge, Sweden. He serves as a firefighter, lead fire behavior instructor, and director of fire behavior training for the District of North Vancouver Fire Rescue Service, where he is responsible for program design and implementation. He is a member of a technical panel at Underwriters Laboratories Firefighter Safety Research Institute and is pursuing a degree in fire science from Western Oregon University in Monmouth. He is a contributing writer to fire service magazines in Canada.

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