By P.J. Norwood and Sean Gray
In the world of firefighting, one thing remains constant: change. As we move into an era where knowledge about fires and their behavior is more accessible through research, our ability to understand and adapt to these dynamic situations becomes paramount. Fire behavior is a continuously evolving field, offering us the keys to smarter and safer firefighting tactics. It is imperative that we embrace this evolution, evaluating research and applying it to our fireground operations. The synergy between researchers and firefighters is crucial to our profession’s evolution, and its high time we bridge that gap.
- Today’s Evolving Fire Attack
- The Impact of Ventilation at Structural Fires: Breaking Down the Fire Triangle
- The Fire Attack-Ventilation Connection: Street Considerations
Firefighters are known for their adaptability and their ability to overcome challenges swiftly. We are the ones who rush into danger, determined to save lives and property. We take pride in our profession and continuously strive to master our craft. Yet, when it comes to understanding fire behavior, have we truly grasped the depths of this critical aspect of our work? Traditionally, we were taught the basics in recruit school, often by instructors who may not have fully comprehended fire behavior themselves. It is time to change that narrative.
Let us start by distinguishing between two fundamental terms: fire dynamics and fire behavior.
Fire dynamics is the study of how fires ignite, spread, and develop. It delves into the scientific measurements and calculations that explain fire behavior. On the other hand, fire behavior is precisely what it sounds like–it’s how fire reacts and behaves in its environment.
Temperature vs. Heat-Release Rate
Another essential distinction is between temperature and heat release rate. You may have heard claims that fires burn hotter in modern structures, but that is not entirely accurate. The temperature of a candle flame in 1776 is the same as that of a candle flame burning in 2018. The key in today’s fires is the heat release rate rather than temperature. Imagine one candle compared to 10 candles–they may have the same flame temperature, but the 10 candles release 10 times the heat. Heat release rate (HRR) is the rate at which heat energy is generated by burning. This impacts us and how the heat energy transfers through our personal protective gear.
Heat Transfer
Understanding heat transfer mechanisms is critical. Heat moves from hotter objects to colder ones, and there are three primary ways this happens: conduction, convection, and radiation. Conduction is the heat transferred to the fire’s immediate surroundings, while convection involves heat transfer through moving gases or liquids. Radiation, on the other hand, is the heat transferred via electromagnetic waves, and the closer you get, the hotter it feels. These mechanisms apply to the ceiling, walls, and floor of the fire compartment.
Pressure
Fires generate pressure, and pressure moves from areas of high to low pressure. The high-pressure zone is the fire room, created by the fire itself, while the low-pressure area can be any opening away from the fire. When this pressure gradient forms, it sets up a flow path. High-pressure combustible gases, like smoke, are forced across the upper parts of a compartment, while the fire draws in lower-pressure air from a different opening. Controlling or stopping the air from being drawn into the fire reduces temperatures, slowing down the combustion process. This reduces the size of the fire and the amount of combustible gases produced, improving interior conditions (except visibility, which worsens).
Consider the flammable range of combustible gases. This concept can be likened to the fuel-air mixture needed in a car engine. The flammable range consists of the upper explosive limit (UEL) and the lower explosive limit (LEL). The UEL is the rich end, while the LEL is the lean end. Fires can’t burn if they’re above the UEL or below the LEL because the mixture is either too lean or too rich.
Fire Development
Fire development can be categorized into two types: legacy and modern. Legacy fire development is related to fuel-limited fires. In these fires, the flame is not limited by a lack of oxygen. The fire grows as long as it has fuel to burn, and once all the fuel is consumed, the fire reaches its fully developed stage
In contrast, modern fire development often relates to ventilation-limited fires. In the early stages, there is enough air to allow complete flaming combustion of gases. Some might consider this flashover. However, as the fire starts to run out of air, temperatures decrease. The introduction of air, such as when firefighters arrive and open a door, can rapidly reignite the fire, pushing it into a fully developed compartment fire.
Understanding fire behavior involves recognizing and addressing hazardous situations, such as flashovers, backdrafts, and flow paths.
Flashover and Backdrafts
Flashover is a perilous transition between the growth and fully developed stages of a fire. It occurs when fire gases fully ignite, and the radiant heat from the initial stages ignites room contents, leading to rapid fire growth. Do not mistaken your training in a flashover simulator as a true flashover. Flashover simulators do not subject you to a flashover—they only show you how a flashover develops. Flashover simulators will also demonstrate the precursors to flashover. However, you also need to remember the rollover that is shown in a simulator may be masked by the dark, dense volatile smoke of today’s modern furnishings.
Backdraft, once defined by temperatures exceeding 1,600°F, now relates to the sudden introduction of air into an oxygen-deficient space containing incomplete combustion products. It is a phenomenon that is becoming more common and can catch firefighters off guard.
Flow Paths
Flow paths are crucial spaces through which fire, heat, and smoke progress, moving from high-pressure fire areas toward lower-pressure oxygen sources. Understanding and controlling flow paths is vital for firefighter safety and effective fire control. Flow paths provide clues to where in the building the fire is located. They will also show you where the fire is receiving its air to maintain combustion from. Flow paths can be dangerous when firefighters are operating within them, since you are between where the fire is and where the fire will transition to. Operating in the flow path is one of those necessary evils on the fireground. However, by identification and understanding flow paths you can use them to your benefit to control the fire environment and operate successfully.
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Recognizing and comprehending fire behavior is crucial for firefighter safety and effective firefighting. It is time to embrace the evolving field of fire dynamics, as it offers us the insights needed to adapt our tactics and keep ourselves and the communities, we serve safe. By understanding the principles of temperature, heat release rate, heat transfer, pressure, flammable ranges, and legacy vs. modern fire development, we become better equipped to face the challenges of today’s fire environment.
As firefighters, we must always strive to educate ourselves and stay ahead of the curve. Let’s close the gap between research and practice, evolve our profession, and enhance our ability to protect lives and property. We owe it to ourselves, our fellow firefighters, and the communities we protect to continuously seek knowledge and apply it to our lifesaving mission. Fire behavior may be a complex subject, but by delving into its intricacies, we are taking a significant step toward becoming smarter, safer, and more effective firefighters.
For additional information, please refer to The Evolving Fireground: Research-Based Tactics.
P.J. Norwood is the director of training for the Connecticut Fire Academy. He is a retired deputy chief from the East Haven (CT) Fire Department. He is on the Fire Engineering/FDIC Advisory Board and serves as UL FSRI technical panel member, fire dynamics instructor, and training advisory committee member.
Sean Gray is a captain with Cobb County (GA) Fire and Emergency Services and has been in the fire service since 1993. He has been a member of multiple technical panels involving fire safety research and is an appointed member of the UL FSRI Advisory Board. He has co-authored the book The Evolving Fireground (Fire Engineering) and speaks nationally on research-based tactics.
