Fire Behavior

This is a re-post of one of my previous posts on my older blog

Kind of got a request via Twitter, and in answering this request I had to break out my old fire behavior books from college.

I'm a big "back to basics" guy, as I think sometimes we focus too much on the advanced stuff rather than mastering the basics. This seems to be true in most aspects of where I work. We want to play with the big toys, which are the most fun, without mastering the reason why we have those fun toys. I remember taking Firefighter II at Sinclair Community College in Dayton, OH, and spending most of my physical strength and a good couple hours wailing away at cars with hand tools. It was a good exercise, it reinforced the reason WHY we have hydraulic tools, but also WHAT to do if those tools should fail. The same is true in EMS, we want to get that intubation, but forget that an OPA or NPA is almost just as effective in most cases.

In keeping with the back to basics theme, I've done ventilation already, but neglected to tell you WHY we do ventilation in the first place. Such a concept is not easily grasped without a firm understanding of how fire behaves. This will be a two part episode, as the concepts are complex but easy to understand when presented correctly. Without all the background in an IFSTA manual, I will define and explain fire behavior in a manner that should be easily understandable. You should refer to Essentials of Firefighting 5th Edition for a brisk review should you need one.

So what is fire? Simply, fire is the result of a chemical reaction involving fuel and an oxidizer which on Earth is typically oxygen, although some other oxidizers exist. This process is called cumbustion. In order for it to be called fire, light and heat must be produced, although the creation of a flame is not always present. Typically a fire will involve hydrocarbon fuels.Remembering the Law of Conservation of Mass and Energy, matter can be neither created nor destroyed. As a result of the chemical reaction involved in combustion, the fuel changes to energy.

The fire triangle is often used to describe the combustion process, using the elements that are necessary for a fire to sustain itself. Fire is sustained when oxygen mixes with fuel, and heat creates fuel vaporization in order to burn. A common misconception is that the fire burns the matter as it is. Vaporization is when the heat vaporizes the fuel into a gas in order to burn. This is called pyrolysis, and that is where you get the fuel mix, in a space called the reaction zone. Take an ordinary candle flame. If you view the flame from the side you will notice that the flame does not actually touch the wick, this is because the heat is vaporizing the fuel (wax, a hydrocarbon) and as it travels up due to buoyancy into the reaction zone it burns. This is the "hot air rises, cool air descends" effect, because hotter gases have less density in air and are therefore lighter.

Now as the energy is generated it has to go somewhere, right? We are considering the idea of heat transfer. As you know, if you place your hand on a cold object it will get warm. This is called conduction, because the less dense molecules of your hand are flowing to an area of more dense molecules in an attempt to achieve equilibrium. This same concept can propagate a fire in the same manner. As heat flows along a steel beam, for example, it will absorb the heat being transmitted to it as will any other object that is touching the beam. Once the other mass reaches ignition temperature (the amount of energy it takes for the matter to auto-ignite), it will start a fire.

The process where the steel beam is heated by hot air is called convention. It works like your oven. The gas (air) in the oven is superheated, causing the air to move (because of buoyancy, more on this concept later) and transferred through the gas to a solid. Eventually the solid will reach the same temperature as the gas. This is useful to know when using a thermal imaging camera, because the thermal conductivity of all materials are different, and the camera is sensing the temperature differences. You can use this to discern a human body from a lamp, or a couch from a table, etc.

Radiation is a concept that it somewhat difficult to explain. Heat not only travels through solids or gases but also through the electromagnetic spectrum as well. You're thinking "this sounds like convection." And you would be correct, partially. Radiation is the travel of charged particles from a point to another point in space in a direction other than up, although it is included. Think X-Ray. If you emit charged particles upon a target it will warm a little, considering the source, distance, medium of travel, etc. Take a building fire for example. When the heat travels through space and heats the side of an adjacent structure (called an exposure, for those not knowing the lingo) that's radiated heat.

Now that we know how heat is transfered, we can talk about phases of fire. There are five, and they can be expressed as a graph that ties heat release, fire growth, and time together which you will see in a minute. The first phase is the ignition or incipient phase. At this phase, the fire is relatively small and the heat release rate is small as well, but it is steady. From here, the fire can go in two different directions. If the heat release is too small, the fire will fail to grow and smolder out. If the heat release rate is enough to radiate to other fuels and ignite them, the fire goes into a growth phase.

During the growth phase, the heat release rate grows and the fire spreads to consume other fuels around it. If the fire is contained inside a container (a structure in our case) you will get radiation feedback, meaning that the heat from other fire sources is radiating back upon other fuels that are not ignited. This is a dangerous situation and left unchecked will progress to the most dangerous phase of fire growth: flashover.
Flashover. The very sound of it strikes fear into the hearts of even the most stout firefighter. It is unsurvivable for both firefighters and victims. It is the very moment that the radiation feedback loop created by the fire in an enclosed space simultaneously ignites all the contents of the space. This is the transition period into the fire stage known as free burning. Free burning, or fully developed, simply means that the fire is now at it's maximum heat release and consuming all the fuel it can. At this point the fire is only limited in two ways. Fuel limited means that the fire can only consume the fuel available to it, this is typically true for compartment fires. Ventilation limited means that there is enough fuel available but not enough oxygen available to support combustion. This is true for enclosed spaces. In the video you will see the phases of fire, and note what happens just before flashover, you will see off-gassing of vapor from fuel at floor level, this is truly an excellent video if you want to see how fire works. You can find more at NIST's Building and Fire Research Laboratory.

In decay, the heat release rate drops off, and the flames diminish in size. This is caused by the limiting factors above. At this point the fire is at a point that is unable to sustain combustion unless one of the limiting factors is relieved (addition of more fuel or increase in oxygen flow) and the fire becomes smoldering, where the heat release or fuel is of too small a size to sustain free burning combustion.

At left is the heat release curve, crudely drawn on Microsoft Paint, but enough to understand the point. You should take note of this, because I will refer to it later when I discuss extinguishment theory.

So by now you are wondering "what does this mean to me??" It means A LOT. The rookie firefighter is done a serious disservice by not being taught how fire works outside of the fire triangle because using a knowledge of fire behavior you can predict how fire will react given certain conditions. Next time we will discuss different types of phenomena at a fire and how you can keep yourself and your crew safe.

Resources
IFSTA. Essentials of Firefighting, 5th edition, 2007.
Quintere, James G. Principles of Fire Behavior, 1997