Thread: Low Pressure Nozzles
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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