Flow Path

A flow path refers to the route through which oxygen enters a building from the outside and the path through which fire, smoke, or heat exits the building. The direction and intensity of this flow are determined by the fire’s behavior, our actions, and the movement of wind.

Understanding the flow path is crucial for safe evacuation and effective firefighting during a fire. Fires grow by consuming oxygen, which enters through openings in the building. At the same time, heat and smoke generated by the fire escape through the path of least resistance to the outside. Wind direction and strength can also significantly influence this flow, affecting how fire spreads and how smoke travels.

For this reason, accurately assessing the situation at a fire scene and responding with consideration of the flow path is essential to minimizing damage and saving lives.

Flow Path Animation

A flow path refers to the movement of heat, smoke, and air from high-pressure areas to low-pressure areas inside and outside a building.

When a fire starts, it releases heat, increasing the pressure near the fire. Due to this pressure difference, heated gases move upward and outward.

As these hot gases rise and move outward, cooler, oxygen-rich air is drawn toward the base of the fire. This marks the beginning of the flow path. When the hot gases spread near the ceiling, they create a high-pressure area that moves downward until it finds an escape through doors or windows. When cool air enters through the lower part of a window while hot gases exit through the upper part, a neutral plane is formed.

This arrangement, called a bidirectional flow path, typically occurs in a closed compartment with only one opening.

When multiple openings exist, a unidirectional flow path may form. Factors such as wind, the size and position of openings, and height differences influence this flow. A straightforward example of a unidirectional flow path happens during vertical ventilation: the open front door acts as the air inlet, and the roof vent becomes the outlet.

In multi-story buildings, another type of unidirectional flow path can occur. For example, if a window on the upper floor breaks, air enters through a lower-level door and exits through the upper-level window, establishing a low-to-high flow.

Special wind-driven flow paths may occur on the ground floor if wind breaks a window on the windward side of the structure. This increases pressure inside, forcing air out through leeward openings. Positive-pressure ventilation fans can also create wind-driven flow paths. Once a flow path is established, fire grows and spreads in its direction.

Controlled Flow Path Demonstration

A controlled experiment offers a closer look at flow paths. Designed by Adam St. John, a fire research engineer at the ATF Fire Research Laboratory, the setup consists of two stacked rooms connected by a third room resembling a stairwell. Both rooms have movable doors connecting them to the exterior or the stairwell. The stairwell has two roof-level exits to mimic vertical ventilation. Initially, all openings are closed except for the lower-left door.

When a fire is lit, air enters low and exits high through the single opening, creating a bidirectional flow where cool air flows in, and hot air flows out. When the stairwell door is opened, this bidirectional flow continues.

However, when the upper stairwell door is opened, the flow path changes instantly to a unidirectional flow: cool air enters through the lower opening, and hot air exits through the upper opening. This unidirectional path is driven by rising hot air.

Fire Attack and Flow Path

Attacking a fire against the flow path makes delivering water to the fire challenging. Conversely, introducing water through the lower air inlet allows water and mist to follow the flow paths to the fire effectively.

If the upper door is closed and a roof vent is opened, the unidirectional flow becomes even stronger, demonstrating how improper roof ventilation can worsen conditions.

When the upper door and roof vent are closed and another door to the upper room is opened, the flow path remains unidirectional but loops through the upper room, igniting it and its contents. This shows that flow paths can change direction unexpectedly, though they generally follow the principle of moving from low to high pressure.