Flow Paths and Fire Behavior


The term “flow path” has evolved into one of our industry’s buzzwords. Recently, many studies have been conducted on flow paths with regard to fire spread. But what exactly is a flow path? How do we control it? Does it matter? What dangers does it present to firefighters?

Fire needs four components to survive: heat, fuel, oxygen (O2), and an uninterrupted chemical chain reaction. This is known as the “fire tetrahedron”; to veteran firefighters, it is the “fire triangle.” Our atmosphere is comprised of 21 percent O2. If that percentage increases, objects will burn much easier. And, what exactly is it that is burning?

If we introduce heat to a pot of water, the water would begin to bubble and boil. At 100°C (212°F), the water starts to convert to steam. The temperature that converts water to steam is known as the vapor (or gasification) temperature; every object has a vapor or gasification temperature. Applying heat to a solid fuel breaks down the fuel, resulting in gases being giving off by that fuel; this a process known as pyrolysis. These vapors, however, need to reach what is known as a “flash point.” This is the point where vapors given off by a fuel source meet a temperature in which they will combust, reaching the proper fuel-to-air mixture. If the O2 content is too low or too high, combustion may not be supported.

(1) Note the bi-directional airflow in the window with a good view of the neutral plane. <i>(Photo by Lori Washburn.)</i>
(1) Note the bi-directional airflow in the window with a good view of the neutral plane. (Photo by Lori Washburn.)

Home Furnishings

Traditionally, home furnishings were manufactured using organic materials such as wood. When they burned, they were able to burn slower and more completely. The energy released from these furnishings was lower than that of today’s furnishings. Today’s home furnishings are made out of synthetic materials and use petroleum-based materials, which are full of hydrocarbons that, when they burn, burn incompletely. These hydrocarbons are released in the form of smoke, creating fuel-rich smoke. The amount of energy released from these furnishings is much higher than that from traditional furnishings and will quickly consume the O2 in the atmosphere of the room. The fuel-rich smoke is high in heat and lacks the proper oxygenation to support combustion; as a result, it will seek a new O2 source.

Smoke moves from areas of high pressure to low pressure or high temperature to low temperature, and it is moving constantly. When a room is on fire, heat is released, creating an area of expanding gases and, thus, high pressure. Generally, this results in an area of high pressure at the ceiling-where higher temperatures reside-and an area of lower pressure near the floor-where cool air resides. This movement of hot to cold is known as convection heat transfer. You can see this very easily. As an example, dye an ice cube blue and place it in a clear bowl filled with water. The dye will sink to the bottom as the cube melts. If you warm the water, the blue dye warms and moves to the top.

A Response Scenario

Your engine arrives at a one-story wood-frame structure with fire showing out of a window on the B/C corner. When you look at the window, you see that its upper half has fire blowing out of it; this is the high-pressure area. The bottom half of the window looks clear; this is the low-pressure area. This dichotomy is called bi-directional movement (photo 1). The gases at the window opening are moving in two different directions. The dividing line between the high-pressure, hot exhaust gases and the denser, cooler inlet air is known as the neutral plane. The fire inside the room is acting as a “pump.” When the higher-pressure smoke is moving out of the window, the lower-pressure air is getting pulled in, bringing O2 with it as the O2 mixes with the hot fuel gases and burns.

What does all this mean to firefighters? When you arrive at a working fire, you (generally) stretch a line, conduct a size-up, open the front door and go in, find the seat of the fire, and put it out from the unburned to the burned side; this works most of the time. How can you become more effective? Let’s look at this process, starting at the beginning, and going back to the above scenario.

So, your engine arrives at a one-story, wood-frame structure with fire showing out of a window on the B/C. What do you have? You have a fire in the B/C corner in a one-story house. Generally, you would stretch a line to the front door; open the door; and go in and locate and extinguish the fire. But is there a better way? Let’s take a look.

Whether you accept it or not, opening a door to a structure is a form of ventilation. In fact, a door is the best way to ventilate because it extends from near the ceiling to the floor. It allows for large volumes of high heat to escape while bringing in cooler air from the bottom at the floor level. As previously discussed, gas moves from high pressure to low pressure and from hot to cool. When you open the door, you provide a low-pressure vent to the volume of high-pressure gases inside the structure. The smoke (with the heat and unburned hydrocarbons) moves toward this opening. You have created what is known as a flow path through the structure; a flow of heat and fuel from the fire room is now moving toward the lower-pressure, O2-rich environment.

Firefighters tend to operate in the flow path quite frequently. In the past, with the traditional furnishings, operating in the flow path wasn’t as dangerous as it is today. With the incomplete combustion of today’s fuel sources, you must recognize that smoke is actually fuel. When you are operating in the flow path, you are operating in an area that is looking for the correct concentration of O2 to ignite.

Scenario Tactics

What are some ways we can keep ourselves safe? Control the door. Often, you enter a structure and keep the door wide open. By closing the door at least two-thirds of the way, you can control and reduce the flow of O2 entering the structure. A firefighter stays at the door and opens it fully once water is applied to the seat of the fire.

Another method for staying safe is to rapidly cool the fire room. The term “hitting it hard from the yard” means hitting the fire room hard with water from the exterior by bouncing a straight stream off the ceiling and cooling the fire gases and the contents. Firefighters then enter the structure and extinguish the rest of the fire. This rapid cooling action reduces the heat from the fire, thereby decreasing the flow velocity in the flow path. Studies by Underwriters Laboratories (UL) and the National Institute of Standards and Technology (NIST) have shown that after aggressive exterior attack, firefighters have to make entry to extinguish the seat of the fire. Some will argue that you will “push the fire” by using this method. Studies conducted by NIST and UL have shown that the rapid cooling action on the fire is very beneficial. When there is only one exterior vent, fire cannot be pushed. If multiple low-pressure vents exist in the structure, fire gases will tend to flow toward the low-pressure vents. If hot fuel gases flow to an O2-rich area, there is a possibility of flaming. If high pressure is provided with a fan or a fog nozzle, the hot gases will flow to a lower-pressure area. By using a straight stream directed at the ceiling, fire gases are cooled rapidly as the water is converted to steam, pulling the heat out of the fire and stopping its spread.

You can also cool the smoke. Smoke is fuel, and if you cool a fuel source, you reduce its potential to combust. Use short bursts of a straight stream at the ceiling to cool the layer above you as you advance through a structure to the fire room. This has a twofold purpose. If water droplets come back down, the area above you is cool. If it converts to steam, the area is hot. Both actions also take heat away from the fuel in the smoke, reducing its chance of combustion bringing us in the flow path.

You must also coordinate ventilation. When you create an opening, whether it is on a roof or from a window, you create a flow path to that area. The fire will want to move in this direction as it seeks cooler O2-rich air. There have been many firefighter close calls and line-of-duty deaths from getting caught in the flow path created by ventilation. The attack and ventilation teams must coordinate tactics as well as be vitally aware of the location of the ventilation.

Our fireground is ever changing. Look at your current strategies and tactics as well as the studies being conducted. Adapt your techniques to the environment in which you will be working. Firefighters are facing higher heat as well as fuel-rich smoke to a greater degree than ever before. New synthetics mixed with lightweight construction create a recipe for disaster. Be mindful of the flow paths inside a structure, and do not put yourself in the exhaust portion of the flow path. You must operate in a safe and effective manner and understand the “why” before you can implement the “how.”

JONATHAN MONAGHAN is a firefighter with the Troy (NY) Fire Department.

The Job: Flow Path and Door Control
Searching in the Flow Path  
Ventilation Limited Fire: Keeping it Rich and Other Tactics Based Off Science

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