6.2: Flowpath

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Page ID: 66918

Robert Smith
Western Technical College

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Flowpath refers to the movement of hot, high-pressure gases out of the structure and the movement of colder, low-pressure gases into the structure of a compartment.

Figure \(\PageIndex{1}\): Gasoline Can.
Copyright: "Gasoline-can" by Morewrite? licensed CC BY-SA 4.0.

Flowpath can be compared to the openings of a gas can, or a soda can. If only one opening is used, and the vessel is tipped slightly, the liquid will flow out the bottom of the opening, and fresh air will flow in through the top of the opening. If the can is tipped further, the liquid will flow out of the entire opening until the pressure inside falls below the outside pressure; then air will be sucked in. This results in the “glug-glug” motion you may be familiar with. However, if the vent is opened, or we poke a hole in the can, the “glug-glug” stops because air can flow in one opening, and out the other. This situation also allows the most liquid to flow out of the container.

This is the same thing that happens inside a room. The pressure of the heated gasses is similar to tipping the can. Openings in the room or structure function similarly to the opening in the cans.

Heat Release

Here is a great video provided by the Fire Safety Research Institute.

Neutral Plane

The neutral plane refers to the horizontal plane that separates the high-pressure gases from the low-pressure gases. In the gas can analogy, this would be where the liquid and air pass by each other when the can is tipped. The neutral plane is often visible on bi-directional openings and can give firefighters valuable size-up information.

Uni-Directional Opening

A uni-directional opening refers to an opening where gasses in the entire opening are moving in one direction. Uni-directional openings can either be inlets or outlets. Typically, if a uni-directional opening exists, there is another uni-directional opening or bi-directional opening on the structure somewhere. It is important to identify the location of these other openings prior to entering the structure.

Bi-Directional Opening

A bi-directional opening has both an outlet and an inlet on the same opening. Bi-directional openings can be identified by the existence of the neutral plane.

Figure \(\PageIndex{1}\): Flowpath.
Copyright: "Flow Path: Single Compartment, Bidirectional Vent Opening" by UL Research Institutes’ Fire Safety Research Institute. Copyright: All rights reserved, used with permission from Underwriters Laboratory; see Terms of Use.

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