|Flames exiting a window are an indication that the fire is creating more ignitable gas-phase fuel than can be burned with available oxygen within the fire area and that it is ventilation-limited. Here, the fire gases leave the fire compartment and mix with the required air in order to burn.
Unfortunately, this curve depicts a fuel-limited fire that more accurately reflects the fire stages of a couch burning in an open parking lot; it is not a fair representation of how fires develop when confined in structures. Fundamentally, fuel-limited fires are those in which the heat-release rate (the amount of energy released over time) of the fire is limited primarily by the chemical and physical characteristics of the available fuels. In most structure fires, it is only during incipient and early growth stages that fuel is the limiting factor in the development and spread of the fire.
|At a doorway, the existence of a mid-level neutral plane with air being drawn into the structure at a considerable velocity is a strong indicator that the fire is in need of more air to burn optimally and is ventilation-limited.
“Ventilation-limited fires are more typical of the fires you are going to encounter on the streets today,” said Dan Madrzykowski, a fire-protection engineer in the fire research group of National Institute of Standards and Technology (NIST), in a presentation to firefighters at the 2013 IAFF Redmonds Symposium on occupational health and safety. Fires burning within this state will be drastically affected by any increase in ventilation to the fire. It should also be noted that ventilation within a building is always occurring: lack of ventilation does not mean lack of tactical ventilation performed by firefighters, but 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 megajoules of heat energy is created. This concept allows us to make some basic calculations to determine how long it will take for any given room or structure to run out of the oxygen required for a fire to continue to develop. In a large living space measuring 2.5 metres by 6 metres by 12 metres in area, with a fire burning at a steady state of two megawatts, the oxygen would be depleted in about five minutes and 24 seconds if all windows and door were closed. This is, however, a considerable oversimplification, as fires begin to burn at a lower heat-release rate and grow until they are limited by oxygen, and as levels often fall below the 15 per cent required for fires to burn efficiently. Furthermore, there is often leakage through doors and windows that allows for some smoke and air exchange. Regardless, the impact of oxygen and the critical impact it has on fire production of thermal energy has long been misunderstood or somewhat dismissed by many firefighters.
■ Today’s ventilation-limited timeline
The highly combustible fuel loads found in homes today have changed the rate at which fires transition to ventilation-limited from fuel-limited. Underwriters Laboratories Firefighter Safety Research Institute (ULFSRI) has compared the difference in the time it takes for fire to become ventilation-limited in a home with older furniture made of natural products circa 1970s, and another home filled with modern, synthetic-based furniture. The results were staggering. The natural-material based furniture, referred to as legacy furnishings, reach ventilation-limited conditions 20 minutes after ignition. The modern fire reached ventilation-limited conditions in just five minutes. Furthermore, when tactical ventilation tactics were introduced – opening the front door and removing a window – the time in which the two fires flashed over and reached the fully developed stage was also alarming: while fire in the legacy unit took eight minutes and 30 seconds to reach flashover, the fire in the modern setting did so in just two minutes and 15 seconds, demonstrating how much more quickly today’s fires react to oxygen.
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) as the fire is in the decay stage and can no longer release sufficient thermal energy. When this occurs, the existing hot fire gases begin to contract in volume, which often results in the structure entering a state of negative pressure. Subsequently, previously visible smoke that may have been exiting from windows or other areas will stop, and the negative pressure created by the contracting fire gases will result in air being drawn into the structure via 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.
|This red line represents the fire-development curve of a fuel-limited fire, such as a couch burning in an open parking lot. Unfortunately, this model does not accurately represent how fires develop in confined structures.
(For videos of ULFSRI and NIST test demonstrating these concepts, visit https://vimeo.com/80730910 )
■ Recognizing the danger
In order for fire crews to make appropriate decisions on the fire ground, the prevailing fire condition must 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 are:
- 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 are:
- Fire is beyond incipient stage
- 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, but will be declining if in decay stage
- All common backdraft indicators
Assumptions are generally bad. If crews assume a fire is fuel-limited upon arrival and makes inappropriate decisions, potentially negative results may occur. Assuming every working fire is ventilation-limited upon arrival is likely correct and 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 upon arrival, the fire may be ventilation-limited and in decay stage as the structure is in a state negative pressure. For this reason, nothing showing upon 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 opening a door also allows fresh air in to feed a developing fire. Closing an open door to a house immediately upon arrival – until there is a tactical advantage to opening it – will help to control the heat-release rate of the fire. As well, in some situations, firefighters may want to consider door-control tactics while a fire crew advances a line in search of the fire.
For further information about ventilation-limited fires, and the impact of oxygen on fire conditions, visit http://ulfirefightersafety.com and view the various ULFSRI reports on horizontal and vertical ventilation.