Continuing this series of reviews of the UL’s Impact of Fire Attack Utilizing Interior and Exterior Streams on Firefighter Safety and Occupant Survival, having previously reviewed the results of Water Mapping (https://ulfirefightersafety.org/docs/DHS2013_Part_I_Water_Mapping.pdf), this installment will cover Air Entrainment (https://ulfirefightersafety.org/docs/DHS2013_Part_II_Air_Entrainmen...). These studies, which I have characterized as “water stream behavior” experiments, were performed without the presence of fire, or even furniture, in order to focus specifically on the attributes of the movement of water alone, and provided preliminary and background data for the Full Scale Experiments (https://ulfirefightersafety.org/docs/DHS2013_Part_III_Full_Scale.pdf) that followed, and which will be the subject of my next post. While the previously reviewed study on Water Mapping showed where hose streams go after exiting a nozzle and entering a building, this installment quantifies the air movement that follows. The first experiments looked at the destination, and this one examines the baggage.
The ability of hose streams to move air had been examined previously, with the most-cited study being that by Knapp, Pillsworth, and White performed in 2002 (http://www.fireengineering.com/content/dam/fe/online-articles/docum...). While its results were valid and enlightening, they were limited by the testing mechanism utilized, a repurposed HVAC air flow gauge that maxed out at 2,000 cubic feet per minute (cfm), and looked only at streams directed out of the interior of a structure, as for hydraulic ventilation. Researchers from the National Institute of Standards and Technology (NIST) performed a similar study in 2005, that time examining the effects of hose streams flowed into a building from outside, simulating an exterior attack. While better instrumented, that second study was also limited in its scope. In this series of tests, the UL researchers built upon the knowledge from these previous studies, using more sensitive, robust, and expensive instruments than the first, and encompassing more scenarios and variables than the second, including the examination of different nozzle types, pressures, flows, and movement; and from both the inside and outside.
The testing was performed in a two-story structure, the interior dimensions of each level being 20 feet wide by 36 feet long by 8 feet high, with double doors at one end of each floor, each 3 feet wide by 6 feet 8 inches high. With all of the water flows performed through the opening on the ground floor, the enclosed area above was utilized for placement of the air movement measurement probes, which were installed at the top of the single stairwell that connected the levels. This positioned them remote from any water streams, contact with which would have rendered them inaccurate, yet in the flow path of air pushed up the stairs from the ground floor by streams directed in, or pulled down from the second level by the force of water flowed out. The structure, in essence, was used as a water resistant wind tunnel.
Experiments were performed on sixteen different hose streams, varying the hose diameter (1 3/4- or 2 1/2-inch), orientation (interior vs. exterior), nozzle type (smooth bore vs. combination), and nozzle characteristics (tip size for smooth; gpm and nozzle pressure for combination). In turn, these sixteen different water flows were examined using different amounts and patterns of stream movement for each and, for the combination nozzles, different patterns (straight, narrow fog [described in Part 1 as a 30 degree pattern], and wide fog [undefined, but obviously more than 30 degrees]). The comparison measurements were all made with the nozzles positioned 12 feet from the doors, whether from the interior or exterior, but then the performance of selected hose streams were further analyzed by varying the distance from the opening. Also, the potential for variability of performance between different brands was investigated by repeating many of the experiments on nozzles from three nozzle producers. As no significant variability was found amongst the manufacturers examined, the remaining tests were all conducted using the nozzles from just one of the companies (none of which were named). Again, as with the Water Mapping and Full Scale Experiments portion of this series (or any other research report, for that matter), reviewing the description of the test methods and data analysis is well worth the time spent and will answer many more questions than this cursory review can address.
Summary of findings:
The amount of air moved by streams from smooth bore nozzles, also referred to as “solid”, and combination nozzles set on straight, were equivalent.
- Solid/straight streams can cause a collateral airflow of about 1,500 cfm even if held still. Using the example of a 10 foot by 12 foot room with an 8 foot ceiling, containing 960 cubic feet, this flow is sufficient to exchange its atmosphere in less than a minute. (It won’t, of course, given the angled geometry of a room and the inability to flush the smoke from its corners and edges, but it provides some perspective for the significant amount of air that can be moved in this fashion.)
- Narrow fog entrained about 6 times more than straight/solid streams, over 8,000 cfm, or the entire volume of the test structure in about a minute and a half.
- Wide fog moved about twice as much as narrow fog.
Moving the direction of the stream about the opening, as if trying to methodically wet the entire area, significantly increased entrainment.
- Solid/straight streams could push/pull 5,000 to 8,000 cfm when moved.
- Narrow fog streams increased air movement by about 50%, to 12,000 cfm, with rapid nozzle motion. (Wide fogs were not tested with movement as they already completely filled the test opening.)
The pattern of nozzle movement, or even a lack thereof, did not matter, but the rate of movement did (faster nozzle movement=greater air movement).
- The “T”, “O”, “Z”, and “n” movements were tested, as well as “Spray and Pray”, a vigorous but random repositioning, and were found to be similar in their effects on air movement.
- Air entrainment almost doubled when the rate of nozzle movement, which was synchronized using a metronome, increased from 50 rpm to 150 rpm.
Somewhat surprisingly, larger hoselines and higher nozzle pressures lead to minimal increases in air entrainment.
The further away from the entry/exhaust, the more air was entrained by the hose stream.
- When spraying outward, airflow was doubled by moving from 3 feet back to 9 feet from the opening.
- When spraying inward, moving from 3 feet back to 12 feet away from the opening doubled the entrainment.
- The air exhaust from a narrow fog 10’ from a window, moved in an “O” pattern, is comparable to that of a PPV fan (15,000 cfm) with a 2:1 exhaust/inlet ratio, and double that of a 1:1 ratio.
When directing a hoseline out of the interior of a structure, other than in the rare instance where you are attempting to extinguish a burning building next door, the intent is to maximize the outward flow of air, a process known as “Hydraulic Ventilation”. When flowing a solid stream, positioning the nozzle as far back from the window as possible, and rapidly moving of the nozzle, will best accomplish this objective. (While not reported in this project, Knapp et al showed that about halfway closing the bail of a smoothbore nozzle resulted in a broken stream that, in turn, increased associated air flow - from 510 cfm to 960 cfm - as measured by their makeshift instrument.)
For hydraulic ventilation with a combination nozzle, simply open the pattern as wide as possible. Using a narrow fog with rapid movement moves less air than a wide fog kept still, and would not be necessary unless circumstances prevented the nozzle operator from positioning near the window, as might occur if debris blocked the way. In summary, either type of nozzle should be positioned as far back from the exhaust opening as possible, with the combination nozzle set as wide as possible ("Far and Wide") and the solid stream moved as much as possible ("Far and Wild").
Exterior streams - those directed into a burning building - should be managed in such as way that air entrainment is minimized. Failure to do so can cause products of combustion to be propelled further into the structure, the very hazard that for decades dissuaded us from using this route for water application, as well as prevent those hot gases from being released. (While this is not a significant concern if the target compartment is closed off from the remainder of the structure, as into a room with a closed door, expecting to be able to determine that condition from the outside is, to say the least, unrealistic.)
The ideal exterior stream, therefore, is one that is positioned close to the opening, with a narrow pattern, and no rapid movement ("Close and Tight”), at least until after the fire is knocked down. As with interior streams, circumstances, in this case such things as falling debris, exhausting flames, and landscaping, can prevent achieving the nozzle location "sweet spot", but the nearer to the best position, the less air entrainment and subsequent ill effects. Despite the usual speed and effectiveness of exterior streams, some fires are better fought via an interior route. On the other hand, even an exterior stream that fails all of these characteristics can still be the best choice, as when a deck gun is used from the curbside to knock down a raging fire in an attached garage. As usual, this evidence helps support our decision making, but does not eliminate the need to consider the entire situation and all of the variables.
So, we now have been provided with much more information regarding the behavior and effects of our most basic instrument: the nozzle. Interpreting and operationalizing this knowledge will enhance our ability to accomplish our mission, as will be demonstrated and explored in Part 3 of this series about the results of the Full Scale Experiments.
The author can be reached at firstname.lastname@example.org