International research has shown the complexities of heat energy and the effects of ventilation positively or negatively influence fire growth. Within this article, I plan to explain some of the basics of fire dynamics that will enable you to interpret flow paths at your next structure fire in order to provide you with a safer fire attack.
Rescue Through Recognition and Mitigation
It is said, “the best firefighter rescue we can make is the one that never happens.” We accomplish this by recognizing and reading the fire characteristics on arrival and observing it throughout the deployment of various fireground tasks. It should be each firefighter’s responsibility to locate the source of the fire and be on the lookout for “fireground cues.” Cues can be the changes in smoke volume, velocity, color, as well as listening for glass breakage. Glass failure can indicate the location of the fire and may result in changes in the flow path direction and increase in fire growth. Early recognition as to the type of flow path, location of inlets/outlets and emphasis on controlling openings can make or break a successful outcome. Recognizing the appearances of bi-directional, uni-directional, and vertical flow paths will save your life! Mitigation through controlling inlets and outlets — where air enters or escapes — reduces the ability for the heat energy to efficiently sustain combustion, therefore, slow fire progression. Mandatory use of your thermal imaging camera as a fire attack tool is a must! Pay attention to the fire gases and prohibit crews from entering any temperature above 450 degrees until they have cooled the fire corridor to within acceptable temperatures from a distance either from the doorway or yard. Surface and gas cooling from a safe distance not only reduces temperatures but also pre-wets remote fuel sources to prevent ignition surrounding the advancing crews. In some hose-handling courses, students are taught to overwhelm the fire by constantly flowing as you advance the attack line. While this seems like a good idea and appears to make sense, you should be mindful of a few concerns. Is there a secured and reliable water source to sustain hundreds of gallons per minute? Will your on-board tank size permit for this flow as you do maneuvers? Try conducting a training session to determine how much actual time you can sustain pumping — one or two 1¾” or even one 2½” continuously flowing lines before running out of water. In some cases, you have less than 90 seconds of water to get the job done.
In any case, remotely applying water from a protected location to the source of the fire or by gas cooling will reduce temperatures and the possibility of being caught in a flashover by advancing crews.
Reading the Flow Path
Flow Paths — air tracks — result from very complex heat energy and molecular reactions and air travel within a structure. To understand how a flow path occurs one can observe the process by watching a candle burn. When the combustion process occurs, it requires oxygen which usually is drawn towards the base of a flame. As this air is consumed, the atmospheric air pressure drops around the fuel source — candle — and creates a negative air pressure drop known as the “Under-pressure.”Under-pressure is observed as the air induction side of the flow path and travels towards the fire. Air, like the other portions of the fire tetrahedron, influences the effectiveness of combustion. Without it the efficiency is decreased and it slows fire growth. Thus, the reason for controlling inlets like doors or windows.
The combustion process is seen as flaming in the candle scenario. Combustion produces high energy fire gases that become buoyant and rise and travel along the ceiling and they follow the path of least resistance looking for an avenue of escape. Like a hot air balloon, as heated gases travel further from their heat source they lose energy and cool. As heat energy is lost, the gases lose their ability to rise and ignite. These buoyant gases are considered above atmospheric air pressure — positive pressure— known as the “Over-pressure.”Over-pressure is observed as the exhaust side of the flow path and travels away from the fire source.
Air Pressures and Types of Flow Paths
Air pressures can fluctuate within a structure fire based upon fire growth and the number openings to the exterior. Physics tells us higher air pressures will always flow towards lower air pressures. Like a bicycle tire, when the valve stem is pushed the higher air pressure on the inside of the tire will go towards lower air pressure outside. So, how does this effect the fire attack? As a fire continues to grow it creates more pressure, as that pressure increases — like the tire — arriving firefighters going to work creating openings for rescue, fire attack, or ventilation unknowingly create a path of least resistance due to the existence of a lower air pressure when opening a door, window or the roof. When a fire exists on a lower floor and an opening is creating above the fire, this provides a lower pressure and produces a vertical flow path or “overpressure relief” for escaping gases. This could be the front door on a basement fire, a roof or window opening on an upper story as well. The significance of a vertical opening cannot be over-emphasized. Vertical flow paths have resulted in many firefighter line-of-duty deaths from being trapped above the fire source when an opening is made from below. Vertical flow paths will result in an increased velocity or speed of the smoke and heat movement throughout a structure similar to the flue of a chimney.
As mentioned previously, fire research is demonstrating the intervention of fireground operations, coupled with the design characteristics of a structure, will create the air entrainment needed to sustain and enhance combustion. Controlling openings allows us to control the fire and reduce fire growth and progression. Thus the term, “control the air … control the fire.” In controlling the openings, not only do we reduce the airflow but also cause the smoke — fuel — to remain within the structure essentially creating an atmosphere that is “fuel rich” also referred as a Ventilation Limited Fire.
Vent Limited Fires
Vent limited fires are the majority of fires we encounter. Today’s energy efficient homes and modern fuel packages are vastly different from years ago. They burn hotter and faster due to higher heat release rates that result in achieving flashover sooner. However, as these fuels lose their heat energy following combustion, the further they travel from the source the more they will cool. As they cool they take the form of smoke, as we know it, although cooler smoke is still ignitable. The only thing missing is for the smoke/fuel to be reheated, along with proper mixing of air. Remember that the smoke is already pre-heated so it may not take much heat. With the inability for smoke to escape the structure, the more the container fills with smoke — fuel — and eventually banks to the floor. The container becomes too rich to burn — or less efficient — and begins to decay. We can recognize a vent-limited fire by heavy smoke conditions to the floor, high or moderate heat and possibly cracked windows. When the fire department arrives and initiates operations, opening doors and windows changes the air pressures within the compartment. Why is this important? We must remember that “Mass equals Mass.” When making an opening, smoke leaves a structure and nature attempts to balance out the air pressure loss — or mass — it will replace it with mass — in this case air. So as smoke (mass) leaves the structure; air (mass) is being drawn inward to replace it.
Bi-Directional Flow Path and Neutral Zone/Plane
So, as gases flow along the ceiling and an outlet — i.e. door or window — it provides an avenue for buoyant heated gases (smoke) to escape via the over-pressure and air to be entrained via the under-pressure. This is known as the Bi-directional flow path when smoke/gases are seen leaving out of the top of an opening and air enters along the bottom. Bi-directional flows are less efficient fires due to the two flows competing for one opening. Because of the two opposing pressures and flow paths, the atmospheric pressure attempts to achieve a “neutral balance” or zero atmospheric pressure. This thin layer between the pressures is referred as the Neutral Zone or Neutral Plane (NP). We can recognize the NP as the bottom layer of the over-pressure flow path. Along the Neutral Zone are shear forces resulting from the two opposing flow paths. This shear force causes mixing along the neutral zone comprising of air and unburned fuel. This provides an ignitable mixture just above our heads. All that is needed is the mixing and an ignition source, which may be down the hallway.
Uni-directional Flow Path
When the air pressure on one side of the structure is higher than the pressures on the opposite side of the structure, we often refer to this as the windward to leeward side of the structure. In Fire Dynamics it is referred to as anUni-directional Flow Path. These types of flow paths are considered firefighter killers due to their abrupt changes in fire growth. Uni-directional flow paths are more efficient fires compared to the Bi-directional flow path because of the creation of a dedicated inlet and a dedicated outlet which provide more air influx thus allowing for exponential fire growth. Firefighters can find themselves in a catastrophic situation when they place themselves where the fire is located and where the fire wants to go; especially during a wind-driven event. During structure fires the potential for a uni-directional flow path exist even with a bi-directional flow path. Should a window/glass failure occur on the windward side of the structure, the lower floor can transition in seconds to a uni-directional flow path. Depending upon the size of the opening, location and proximity of the opening, the fire dictates the grace period it takes for the fire growth to respond. The larger the opening the more air influx — the closer the opening, the faster the response.
All fireground personnel should be alert for life-threatening changes in flow paths during wind-driven fires. Increased velocities of wind are game changers and traditional fireground attack methods may require attacking the fire from the inlet side to avoid exposing crews to overpressure exhaust. In addition, watch for changes in the neutral plane rising or falling. A dropping neutral plane can indicate a rapidly growing fire or if it’s rising it could indicate an outlet has been created elsewhere. When operating on the fireground, the sound of glass breaking should alert you to possible flow path changes as well. Recognizing and mitigating these flow paths through cooling or door control is the key towards preventing yourself from being caught in a flashover event. Remember that flashovers typically occur where the air is located. Incident commanders and nozzle teams should make every effort to neutralize the flow path from extreme temperatures and creating ideal mixtures.