|This graphic explains the movement of fire gases through a two-storey home as the gases are drawn from high-pressure areas to low-pressure areas.
“Based on varying building design and the available ventilation openings (doors, windows, etc.), there may be several flow paths within a structure. Operations conducted in the flow path can place firefighters at significant risk due to the increased flow of fire, heat and smoke toward their position.”
■ The why factor
It is essential for all firefighters to understand why flames and hot smoke move and behave the way they do. In addition to heat, a developing fire creates various hot fire gases and byproducts of combustion, which we generally refer to as smoke. Although this smoke consists of a wide range of gases, aerosols and particulates, these fire gases behave similarly to other gases. For this reason, the various gas laws established as far back as the 17th century by scientists such as Irish physicist Robert Boyle can help us understand the fundamental behaviour of fire gases. Quite simply, these gas laws show us how the volume, pressure and temperature of gases are connected to one another.
Since the density of fire gases decreases when heated, the gases move vertically through the fire plume and accumulate in the upper areas of the compartment of origin. This accumulation and confinement of the fire gases, coupled with their continued increase in temperature and natural tendency to expand when heated, creates an area of high-pressure in the hot fire gas layer. When an area connected to the fire compartment is available, such as a hallway, room or stairwell, the lower pressure that is present in these connected spaces causes the hot fire gases to move and spread as the gases follow the path of least resistance – that is, from high-pressure areas to low-pressure areas.
When fire crews preform interior fire operations, hot fire gases increase the temperature of firefighters’ turnout gear as the gear absorbs the fire gases’ thermal energy. However, the drastic increase in the rate of convective heat transfer (which refers to the transfer of heat by the movement of gas or liquid) when velocity, or speed, is added to these fire gases is not commonly considered. Firefighters experience the effect of convective heat transfer if they are positioned in a flow path, between where the fire is and where it is going as the fire gases move over the interior fire crews to low-pressure areas. Much like the effect of convection ovens or hair dryers, when velocity is added to hot gases, the rapidly moving molecules are able to quickly run over a surface and deposit thermal energy. The higher the temperature and velocity of the fire gases, the greater the convective heat transfer. This incredible energy transfer is one of the main reasons so many firefighters have been injured or killed in a flow path.
■ Every opening is a potential flow path
Consider, for example, arriving on the scene of a working fire located on the ground floor of a two-storey residential home. If smoke or flames exit a failed window on the ground level (see position 1 on graphic), at least one flow path is already established as hot gases and fire move to the low-pressure point available outside of the window. When crews force the front door and smoke exits through the doorway, another flow path is established (see position 2 on graphic). By opening the front door, fire crews provide another low-pressure point for the hot fire gases to move toward.
“Firefighters in a flow path can be convectively heated and cooled,” Kerber explains. “It is important to understand the direction of gas flow in the flow path. It can be bidirectional or unidirectional depending on the relation to the fire’s location. When below or on the same level as a fire, firefighters can find themselves being cooled as fresh air is entrained into the fire, or heated if they are in the outflow, which depends of the height of the neutral plane (smoke layer).”
Furthermore, if a crew advances to the second floor to perform search operations and a window is opened or tactically ventilated prior to fire control, yet another flow path is created (see position 3 on graphic). It is this situation in which firefighters are essentially working in the chimney of the fire due to the buoyant nature of the fire gases.
“The firefighter in this situation may not see flame but will be subjected to high heat conditions,” says Kerber. “When a firefighter is in a flow path above the fire the flow will be unidirectional. This situation has claimed the lives of many firefighters. Gas flow in the flow path has been measured to be 16-32 km/h. The higher the gas velocity and higher the temperature of the fire gases, the less time a firefighter has to escape as the heat will transfer quickly through their turnout gear.”
Fire crews on the second floor above the fire in a flow path are in an extremely dangerous position.
■ Controlling the flow paths
A sound understanding of this concept before arriving on the fire ground is the first step firefighters can take to reduce the threat of being injured or killed in a flow path. The second step is a proper size-up, identifying all current flow paths and any future flow paths that may result from fire growth or tactical actions. Finally, consider closing windows and doors to control the movement of air and fire gases in a way that is consistent with the operational objectives.
Doors are not just entry points into rooms and structures. They also serve as an exceptionally efficient ventilation point. Crews are able to interrupt a flow path either on the downstream or upstream side of the fire, simply by closing a door. This action can be life saving. For crews performing vent-enter-search operations, for example, isolating themselves from the flow path is vitally important. When a window is breached for firefighter access, a new flow path is created, drawing heat, smoke and fire toward that location. However, by quickly closing the door to the room the firefighters just entered, before beginning their search, firefighters can cut themselves off from the flow path, improving conditions for both themselves and for any potential victims in that area.
Door control methods must also be considered when crews make primary entry to a structure to perform fire attack operations. Although this technique is somewhat new to North American fire departments, firefighters in other parts of the world have been doing this for decades. For this technique to be successful, an additional firefighter must remain at the entry door to close the door as much as possible while assisting with hose advancement. With proper door control actions, the flow path created between the fire and the front door can be reduced, once again improving firefighter safety.
For more information on flow paths, have a look at the video (https://vimeo.com/89428277), titled Why Thermal Flow Paths are the Key to Successful Firefighter, by Dan Madrzykowski, a fire protection engineer with the Fire Research Division of the National Institute of Standards and Technology.