Interestingly the individuals who post in support of solid stream nozzles seem to understand the physics of nozzle reaction, but not the physics related to absorption of heat. Note that this is not an attempt to start a debate about fog vs. solid stream, both can be used to put out fires. However, surface area of the stream has a significant impact on the efficiency with which the stream absorbs water. The greater the surface area, the greater the efficiency. An effective stream is one that maximizes heat absorption at the location where the nozzle operator wants it to occur. Sometimes this involves the use of a straight (or solid) stream to cool surfaces and other times it involves fog to cool hot gases.
The decision to use low pressure nozzles should not be taken lightly. There are a number of fairly solid reasons to consider their use, but equal number in support of maintaining 100 psi nozzle pressure. These include increased cooling efficiency, stream density, and less kinking.
While I advocate 100 psi with combination nozzles, each agency needs to to an objective and scientifically based assessment on what they want to do and make their choice on that basis. Unfortunately the decision is often made on unsupported opinion.
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Thread: Low Pressure Nozzles
07-28-2007, 06:45 PM #21
Simple PhysicsEd Hartin, MS, EFO, MIFireE
07-28-2007, 07:09 PM #22
- Join Date
- May 1999
- Here, There, Everywhere
We aren't talking about fighting fires on Navy ships here.
When during a push down a hallway do we want to use this fog to cool the gasses? Why not just continue the push with the stream out and ahead breaking it up against the ceiling and upper wall surfaces while occasionally cooling the floor and working to extinguish the main body of fire that is producing those gasses.
Have you ever actually opened a fog stream while moving in on a fire in a building...I did when I was a young and inexpereinced FF with little leadership in the dept I was with at the time...it isn't pleasant and it does NOTHING to contain or control the fire.
I also hope you still don't believe in this "protection" afforded by fogs nonsense.
Your thinking is about 3 decades out of date....and misapplied to structure fires. There is plenty of science and written research along with plenty of experience to support our positon...I haven't seen much from the other side that makes much sense...or that works outside of a labratory setting.
07-28-2007, 11:06 PM #23
- Join Date
- Dec 2002
- Central NJ
07-29-2007, 08:53 AM #24
Science and Experience
I hear what you are saying and do not dispute that your approach can be effective at extinguishing the fire and I would use the same approach under some circumstances. However, when faced with a shielded fire that cannot be attacked directly, I would prefer to cool the gases overhead to reduce pyrolysis (in the immediate work area) and limit the possibility that the smoke (fuel) overhead will light up. This is a fire control, not extinguishment technique that supports, not replaces use of a straight stream. In this application, the droplets of water don't need to go that far. When reaching a location where the stream can be applied to the burning fuel, a straight stream provides the most effective way of extinguishing the fire.
There has been quite a bit of research that supports this approach outside that done by the US Navy that deals with fires in buildings. Much of this work has been done in Europe and not widely reported in our fire service publications. However, one example of research that supports this approach was conducted by the National Research Council of Canada (http://www.firetactics.com/NRC%203D.pdf). If there is research that supports your position I would be interested in having a look. Can you tell me where it has been published?
This is not just theory, While not commonly used here in the US, this approach is being used around the world (and even here in the US).Ed Hartin, MS, EFO, MIFireE
07-29-2007, 10:26 AM #25
- Join Date
- Sep 2006
- Northeast Coast
To me kinking is the only real concern when considering switching to low pressure. With reduced personnel keeping lines kink free is more difficult. But through proper training and technique this can be overcome. But, kinking is one reason that smoothbores are best suited for low pressure work (actually for almost all work, but that's another matter). Using a nice new TFT or Akronmatic low pressure nozzle, you'll have the same great looking stream without the gpm. With smoothbores you immediately know that you have a reduced flow due to the stream quality. So while it is rare, once in a while the best answer may be the easiest and cheapest.
While the solid stream off the ceiling may produce larger droplets than the fog, they are effective in cooling superheated fire gases. The bonus is that they are actually more forgiving of over use of the stream. Too much fog and the ceiling drops and we cook. Too much solid stream and things ahead of us get wet, with little steam. At the minimum: Right is Right and Left's for Lobster!
07-29-2007, 11:17 AM #26
In response to your question, droplet size is a varies with nozzle pressure. As nozzle pressure. As nozzle pressure increases, droplet size decreases. Smaller droplets have increased surface area and absorb heat more efficiently. This is the same concept of surface to mass ratio with fuel. However, there is a trade-off. As observed in earlier posts, the droplets need to be big enough to reach where you want them to work. The fog pattern of most 100 psi combination nozzles have droplets with an average size of .3 mm (300 micron) which works well when cooling gases. While this is not measurable in the field, you can look at hang time (put a short pulse of water fog into the air and see how long it hangs, 4-5 seconds is a good indicator of small enough droplets for good gas cooling efficiency). This droplet size is effective at cooling and large enough to reach well into the hot gas layer. Low pressure nozzles will have a shorter hang time (indicating larger droplets). Shifting to a straight stream packs these close enough to have good penetration under most circumstances for interior firefighting.
Gas cooling is used in compartments that have pre-flashover growth stage conditions. Compartments with a fully developed, post-flashover fire require other tactics (such as a direct attack with a straight stream).
I agree with your concern about loss of flow rate with automatic nozzles. The operator should be able to tell the difference between 70 gpm and 150 gpm by the feel, but may miss this in the heat of battle. Fixed or variable flow (non-automatic) nozzles provide more obvious feedback.
If given a solid stream nozzle, I would use it much as you describe. Directing the stream off the ceiling will cool the hot gas layer to some extent (a positive effect). I would also agree that excess application of water fog is a bad thing. I have experienced this as well, particularly early in my career with poorly trained nozzle operators. While I agree that the solid stream may be a bit more forgiving (I have seen excess steam production with this type of stream as well), I think that the key is having well trained nozzle operators regardless of what type of nozzle or tactics are used.Ed Hartin, MS, EFO, MIFireE
07-29-2007, 04:01 PM #27
- Join Date
- May 2006
The nozzle you want is the one that offers the biggest punch with the manpower you run with.
For us it's the Akron Assault at 75/175. Running a 3 man company is the norm for us and on a good day 4 or 5 (rarely). Currently we are looking to change to smoothbores on our highrise packs to allow us the bigger punch. The 15/16 tip has a penetration of 14 feet more than the assaults (fog) nozzles we currently have and if the engineer gives us 75 psi at the tip we will get 229 gpm, 187 if pumped at 50 psi. I know this sounds like an Akron commercial but it's not it just what we use and are testing. Another thing to look at is the Akron Saberjet Single Shut Off. The redesigned nozzle isn't like the 2 shut off ones. It is a true smoothbore that has a fog nozzle "attachment". The one we are sampling is a 15/16" SB. The nozzle can be used with foam or cafs. We are trying it out and found some training issues, but for the most part no complaints.
The hose we are currently using in Angus Ultima and Ultima Lite(High Rise) along with a hodge podge of National and Firestream.
Good luck with you research.
07-30-2007, 08:04 AM #28
- Join Date
- Sep 2006
- Northeast Coast
Hartin: Thanks, that was a good explanation. I believe I understand that you're talking about the efficeincy of the stream to cool the overhead and prevent flashover vs. the efficeincy of the stream to directly extinguish the burning material. I certainly wouldn't begin to say that fog nozzles can't be used safely and effectively as I have first hand knowledge with positive results. My views in this thread are based on switching to low pressure and justifying the need for more expensive and complex nozzles that often firefighters do not truly understand. At least one person mentioned going with the 50 psi type so all lines are figured the same, making it easier to ensure the line is pumped correctly. It just means you need to invest in decent hose (to avoid kinks) and decent training.
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