More on nozzles
(Editor`s note: Bill Clark sent me this letter shortly before he passed away. I wanted to share it with you now.)
It has bothered me that I haven`t been able to follow up on my previous article, “Using Water Wisely” (September 1995), which ended with the promise to disclose why fog is not actually as effective as it is supposed to be in theory [for interior attack]. So, let me tell you now.
One time I was watching as the stream from a garden hose was played across a swimming pool. The water falling from the hose stream made a pattern on the surface of the water in the pool. It was obvious that the more narrow the spray pattern, the farther the reach of the stream. With the straight-stream position, nearly all of the water reached the far end of the pool; with spray patterns, none of it did.
Consequently, we set up a series of experiments at the Florida State Fire College, where I was superintendent at the time. Briefly, we discovered–using 60- and 95-gpm fog nozzles–that a surprisingly large amount of the water fell to the floor within a few feet of the nozzle. With the target 30 feet from the nozzle, only the straight stream reached it. Measurements showed the percentage of the total discharge failing to get far from the nozzle was very high in wide spray patterns and quite high in narrower patterns.
Evidently, the small, separated drops of the spray are quite susceptible to both friction and gravity. The friction is caused by the air; and inasmuch as water is relatively heavy, gravity has considerable effect. Furthermore, the forward projection of the stream gets less as the pattern is widened.
We did not use “solid” (smooth-bore) stream nozzles in these tests. The nozzles used were “constant flow” variable pattern fog nozzles that we had tested for flow in advance.
Some of our experiments involved actual fires. The straight stream was more effective in all cases except those in which the fog nozzle was within five feet of the burning material (wood).
William E. Clark
