Are fog nozzles not the answer?
The main thrust of these articles is to highlight the differences between smooth bore nozzles and fog nozzles with respect to the amount of air entrained by each type nozzle. The authors did conclude that there was little difference between a smooth bore nozzles and a fog nozzle set on a straight stream. They quote Chief Andrew O’Donnell as saying: “The difference between smooth bore and straight stream in terms of direct attack effectiveness is negligible.” (1-74) Air flow, however, from a fog nozzle set at 300 is characterized as “extreme”.
The key to these nozzle tests is the rate-of-flow of 180 gpm. Here is where the author’s make a fatal mistake. They assume that a safe and effective fog attack must be made with maximum flow from a fog nozzle. This is simply not true. The volume of the room used in the nozzle tests was 1,250 cubic feet. Let’s calculate the amount of water needed to fill this room full of steam. Using the formula, Volume/200, this equals 1,250/200 or 6.25 gallons of water. For further calculations, let’s use 6 gallons. This formula is based upon the fact that one gallon of water expands to at least 200 cubic feet of steam at 2120 F. This formula also assumes that at least 90% of the liquid water is changed to steam.
The authors do not provide the data on time, but presumably a flow of more than one minute is needed for smooth bore nozzles to be effective. Six gallons can be distributed equally in a fire area by a flow of 30 gpm for 12 seconds. Now how much air is entrained by a flow of 30 gpm for 12 seconds? Using Walter Haessler’s data of 5,400 cfm for 180 gpm flow from a fog nozzle at 300 with 100 psi nozzle pressure, dividing 5,400 by 6 to produce a rate of flow of 30 gpm, and dividing again by 5 to get 12 seconds (1/5 minute) produces 180 cubic feet of air. This number is well below the range of airflows produced by smooth bore nozzles. The authors do not provide the data on time, but presumably a flow of more than one minute is needed for smooth bore nozzles to be effective.
A fog nozzle used properly and effectively entrains less air that a smooth bore nozzle used properly and effectively. The conclusion is immediate. A fog nozzle used properly and effectively entrains less air that a smooth bore nozzle used properly and effectively. In other words, these nozzle tests are largely irrelevant as far as fire ground realities are concerned. The shorter time needed for an effective fog attack using little drops of water does not require the entrainment of massive amounts of air into the fire area. The little drops of water with a surface area as much as 1,000 times greater than the surface area of a solid slug of water from a smooth bore nozzle truly have a remarkable fire fighting power. This is the real reason for choosing fog nozzles over a smooth bore nozzle. No other nozzle can come close to matching the capability of a fog nozzle to absorb heat.
A 1,250 cubic foot room is a small room. The authors make a rather unusual statement with respect to one room fires. They say almost anything works for a one room fire! (1-76)
I have never heard or read a statement anywhere or anytime that even remotely resembles the author’s statement. The only incident cited to justify this statement is an incident observed by the authors where a one room fully involved kitchen fire was extinguished by a fire fighter using a booster line from outside the kitchen. I have never heard or read a statement anywhere or anytime that even remotely resembles the author’s statement. Remember that we are talking about the most frequent type of fire encountered by fire departments in the United States. The NFPA reports that 75% of all structure fires occur in one or two family detached dwellings, and that almost 75% of all structure fires are extinguished with a single attack line that confines the fire to the room of origin.
