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June 2025
If you were to ask a police officer about their service weapon, they would not only be able to demonstrate competence in handling it but also describe its features in detail. They know their weapon intimately because they understand it can be the difference between life and death. In the fire service, our weapon is the nozzle, and it deserves the same level of familiarity and respect. Arriving on scene of a structure fire is a lot more than just “putting wet stuff on the red stuff.” Whether you're paid or volunteer, as professional firefighters we must go beyond simply knowing how to open and close the bale; we must understand how to apply water with intention, because like a firearm, the nozzle can have life-or-death consequences for firefighters and the citizens we serve.
This article is a follow-up to “Water Mapping During the Interior Fire Attack,” published in July 2024. That article intentionally left out a crucial part of water application: air entrainment. This article explores that topic and explains why it matters.
In short, air entrainment occurs when water moves through air and pulls air along with it. But why does this happen? As water droplets travel, they compress air in front of them and create a low-pressure zone behind them. This combination of low pressure and turbulence causes air to be drug along in the direction of the water flow. The extent to which this occurs depends largely on how much the fire stream breaks up. Firefighters can increase air entrainment by breaking-up the stream into smaller droplets, or conversely, they can minimize it by keeping the stream as intact as possible. Depending on the tactical objective—whether it is to hydraulically ventilate a structure or to suppress a fire while improving conditions for potential victims—firefighters must tactfully control how much air is entrained by manipulating the stream pattern, length, and movement.
Stream Pattern
The stream pattern selected plays a major role in air entrainment. A tightly held stream, such as a solid or straight stream, remains relatively cohesive and entrains very little air. As the stream pattern becomes wider and more broken—such as with a fog pattern—the stream breaks into more droplets and significantly increases the amount of air entrained. (Picture 1 on the left – A straight stream is being flowed down the hallway. Notice that the orange streamers remain still, indicating minimal air movement.) This can be visualized as a continuum, with solid and straight streams on one end producing minimal air movement, and widest possible fog patterns on the other end creating the most air entrainment. (Picture 2 on the right – A wide fog stream is being flowed down the hallway. Note how the orange streamers are now being drawn into the hallway due to the air entrainment created by the fog pattern.)
picture credit UL-FSRI
picture credit UL-FSRI
Stream Length
As water exits the nozzle, it immediately begins to lose momentum due to air resistance and gravity. Over distance, the stream begins to break apart into smaller droplets. The farther the stream travels, the more it fragments and the more air it entrains. This means that air entrainment is much lower near the nozzle and higher farther downrange, where the stream has significantly broken up.
graph from UL-FSRI stduy "Impact of Fire Attack Utilizing Interior and Exterior Streams on Firefighter Safety and Occupant Survival: Full Scale Experiments"
Stream Movement
When the nozzle is moved during operation, the stream once again is broken up even, thus, once again increasing the volume of entrained air. The faster the stream is moved, the greater the air entrainment becomes.
graph from UL-FSRI stduy "Impact of Fire Attack Utilizing Interior and Exterior Streams on Firefighter Safety and Occupant Survival: Full Scale Experiments"
On the other hand, there are three factors that have no significant effect on air entrainment: nozzle type, movement pattern, and flow rate or pressure. Although a fog nozzle operating in a wide fog pattern will entrain more air than a smooth bore operating in a solid stream, when the fog nozzle is set to straight stream, it will entrain the same amount of air as the solid stream produced by the smooth bore. While movement increases entrainment, the specific pattern—whether O, Z, or inverted U—has no measurable impact on how much air is entrained, assuming the speed of movement is the same. Finally, changes in flow rate or nozzle pressure have negligible impact. A 1 ¾” hose line flowing 160 gpm at 50 psi through a ⅞-inch smooth bore tip entrains the same amount of air as it would if the pressure was increased to 70 psi flowing 190 gpm.
This information has real consequences for how firefighters apply water during fire attack. A nozzle firefighter who understands these principles can use them to meet tactical goals, while one who does not may unintentionally create dangerous flow paths, and worsen conditions for potential victims.
One practical example of using this knowledge to minimize air entrainment is during an exterior water application (ie. transitional attack). When operating from outside the structure, the nozzle firefighter should position themselves as close to the opening—typically a window—as safely possible. Using a solid or straight stream, they should aim the water toward the ceiling of the fire room, keeping the line as stationary as possible while flowing. This technique maximizes surface cooling and water coverage within the compartment while minimizing the amount of air being pushed into the structure. By limiting air entrainment, this approach avoids forcing hot fuel gases or smoke into adjoining hallways or rooms while still allowing gasses to vent out the exterior opening. Using interior and exterior camera views, the YouTube video “Pushing Fire” by the Firefighter Safety Research Institute visually illustrates the implications of both fog and straight/solid streams being applied from outside the structure.
“Put the fire out and everything gets better” is true for the firefighters wearing an SCBA. However, for the victim with an unprotected airway, things don’t truly get better until the structure is ventilated (or they are removed from the smoke filled environment). Therefore coordinated ventilation is critical, and few methods are more coordinated than the nozzle firefighter hydraulically ventilating the fire room immediately following suppression. In this case, the nozzle firefighter can use the air entrainment principles to move as much air from the interior through an exterior opening. To do this they should position themselves as far back from the exterior opening as practical while remaining in the fire room. They should direct their stream through the window or opening and move it rapidly in a deliberate pattern. If using a fog nozzle, the pattern should be opened as wide as possible while still projecting water through the opening. Although not as effective as a fog nozzle, the smooth bore nozzle can be used to hydraulically ventilate by partially closing the bale to effectively break-up the stream into droplets. Listening to the sound of the water exiting the nozzle can help maximize the effectiveness of this technique––slowly opening the bale until the stream sounds most turbulent (ie. sounds the worst), indicates maximum stream breakup, signaling optimal air entrainment for ventilation. Additionally, if the smooth bore nozzle has a break-apart tip, removing the tip can further disrupt the stream and enhance air entrainment.
In the end, knowledge of air entrainment isn’t just academic. It’s a tool that can help you control conditions on the fire ground. Understanding how and when to increase or decrease air movement through nozzle technique is a vital part of mastering the craft. Firefighters who understand how to manage their stream can improve survivability for victims, increase tactical effectiveness, and reduce risk to themselves and their crews.
Know your weapon.
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Classroom Training:
Using the Firefighter Safety Research Institute's YouTube video “Pushing Fire,” analyze the interior and exterior effects of applying fog versus straight/solid streams when applying water from the exterior. Emphasize how nozzle selection and technique impact air entrainment, fire spread, flow path, and victim survivability.
Hands-On Training:
Conduct a hands-on drill where firefighters flow water from one end of a hallway, adjusting stream pattern, length, and movement to explore how each affects air entrainment. While changes in airflow can be felt, hanging streamers at hallway openings offers a clear visual reference. This drill can also be modified for exterior water application through a window.
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