Are fog nozzles not the ans
Too Little or Too Much Water
What happens if less than the right amount of water is used on a confined fire, or if more than the right amount of water is used? One answer is just common sense. However, the other answer may surprise you.
First, less than the right amount of water. Research at Iowa State University by Royer & Nelson established that less than the right amount of water has less than the desired effect in the fire room. The temperature is this room levels off for a number of minutes but continues to burn at this level. In adjacent areas (hallway, attic, or room) are not affected and the temperature remains at the same level as before the attack and then continues to increase. This is just common sense. Too little water has little effect upon a confined fire Those of us who have attempted to fight a room size fire with a garden hose know exactly how frustrating it is to use too little water on a fire.
Thermal imbalance can be avoided easily by using the right amount of water to control the fireSecond, what about applying greater than the right amount of water to a confined fire? Common sense tells you that the more water applied to a confined fire, the quicker the fire is brought under control. Not so, for confined fires. Too much water creates thermal imbalance which will disrupt an effective attack and delay bringing the fire under control. The research done by Royer & Nelson at Iowa State University and the research done at the U.S. Naval Research Laboratory both confirm the validity of this statement. What happens is that thermal imbalance, or turbulence, causes temperature spikes both at the lower and at higher levels in the room. This can continue for ten minutes or more. All this blocks visibility, delays entrance in the fire area, because part of the area is too hot and other parts are too cool. Thermal imbalance can be avoided easily by using the right amount of water to control the fire. In other words, there is more to fighting a fire than just opening a nozzle to maximum flow each time you fight a fire. That is robotic fire fighting. A robot acts without thinking.
So the 3 D attack must apply a very small amount of water to protect the firefighters in the hallway and at the same time prevent a flashover of backdraft from happeningNow let’s shift to another method of attack, the 3D pulse fog attack. For this attack it is critical that the right amount of water be used. The purpose of this attack is to make it safe for an interior crew to approach a fire so as to be able to make direct attack on the fire. If, for example, a crew is proceeding in a hallway toward a fire room, and a rollover threatens their safety, then the 3 D pulse fog attack can remove this threat. However, the right amount of water must be used. Using too much water would fill the hallway full of steam that could scald the firefighters .If the hallway is 25 feet by 4 feet by 8 feet high (7,6 meters by 1,2 meters, by 2,4 meters high) with a volume of 800 cubic feet (22,6 cubic meters), only 4 gallons (15 liters) of water is needed to fill the hallway full of steam. So the 3 D attack must apply a very small amount of water to protect the firefighters in the hallway and at the same time prevent a flashover of backdraft from happening. Thus we are back to the fundamental question
How much water is needed to eliminate the threat of flashover or backdraft?
This contraction must be greater than the steam expansion in order to produce a net contraction in the overheadThe answer is certainly less than 4 gallons (15 liters) and no more than one or two gallons (3,78 or 7,56 liters). One gallon (3,785 liters) expands to 227 cubic feet (6,3 cubic meters) of steam. At the same time the fire gases are being cooled by the fog attack which results in a contraction of these gases as the temperature falls from around 1,000° F (537° C) to 300° F (149° C). This contraction must be greater than the steam expansion in order to produce a net contraction in the overhead. Calculations have been made to show that this is actually what happens. The 3D pulse attack does work if the nozzle is opened for no more than 2 seconds
For example, if the r.o.f. is Gpm = 30 (113,5 Lpm) and t = 1/30 minute or 2 seconds, then
| Gpm x t = Gal | Lpm x t = L |
| 30 x 1/30 = Gal | 113,5 x 1/30 = L |
| 1 = Gal | 3.8 = L |
This amount of water will accomplish the purpose of the 3D pulse attack, prevent flashover or backdraft, and protect the firefighters as they get in position to make a direct attack.
So how much water is needed to make a 3 D pulse fog attack? Very little indeed! One of two gallons (3,78 or 7,57 liters) is surely sufficient to accomplish the purpose of the 3 D attack.
