Homestead mailing list

[Gene GeRue <genegerue AT ruralize.com>]

A bit of research shows that this is a big and controversial  
discussion among many who know far more than I. Here is one post I  
found that refers to an earlier post that postulates, it is all about  
gallons per minute. Not so, says this guy:


"It's all about GPM's, and not pressure always. GPM's v. BTU's is a  
great arguement to use, pressure doesn't play much of a role in that,  
other than getting the water to the seat of the fire."

I probably wouldn't use that arguement with someone how knows the  
science & engineering behind fog attacks and/or high-pressure fog  
unless you want to have you have your hat handed to you.

One gallon of water, for each degree in tempature rise, by  
definition, will absorb 1 BTU of heat.

Let's your apparatus bay is at 65 degrees, so that's the temp of the  
water in your tank. And just about the temp of the water coming out  
of the end of your nozzle.

212-65=147.

If you're applying water at a rate the water gets as hot as possible,  
without turning to steam, it will absorb 147 BTUs per gallon.

Let's say you're applying 250gpm to a fire, and most of it is running  
off the fire floor hot enough it's uncomfortable for you to be  
crawling in it -- that's probably around 150 degrees, or in other  
words it's absorbing 85 BTUs per gallon...after all if the water was  
actually coming off the fire at 212 degrees you'd be scalding anyone  
crawling in it (yourself), or underneath it's dripping on. 250gpm x  
85BTUs = 21,250 BTUs/minute.

Now let's take the same situation and fog it.
Fog present smaller droplets to the fire, raising the possibility any  
given droplet is converted to steam.

One gallon of steam, raised one degree from 212 degrees, turns into  
steam. Conversion to steam absorbs 970 BTUs. Let's take the same size  
fire you where controlling above with a 250gpm stream, and hit it  
with a conventional 100gpm fog stream. Let's even figure only half  
the water is converted to steam -- that'll absorb 50gpm x 970 BTUs =  
46,000 BTUs plus another 5,000 BTUs or so for the other 50 gpm left  
on the floor. You're already sucking more than twice as many BTUs  
than the 250gpm solid bore.

The Steam also has two additional advantages -- it physically  
displaces oxygen suffocating the fire, and it physically disrupts  
airflow in the fire area (ever blow out a candle?)

High-pressure fog takes the principal one step further. Using the  
extra high pressure to produce a particulary fine fog, even more if  
not all the fog is converted to steam. Here, 30gpm x 970BTU =  
29,100BTUs/min, or still more than a 250gpm line with most of the  
water running out the front door.

The "Danger" in any of these isn't with the science or engineering  
behind them -- low pressure and high pressure fog both work. You can  
have hoses and pumps that safely handle the pressure. Fog WILL absorb  
more heat, more quickly, and put out most fires more effectively than  
solid bores.

The "Danger" comes from mixing tactics.

In recent years, many FDs have wanted to emulate "the big boys" like  
FDNY and other old, urban cities. These cities have significantly  
higher life hazards, and higher manpower levels, than your average  
suburban or rural fire district. There is nothing wrong with their  
tactics, they work well. But they are incompatible with using fog  
attack early on in the fire. So they advocate smooth bores.

In many suburban and rural areas given lower life risks, lower  
staffing, and often poorer water supplies using fog streams is an  
effective way to put out the fire quicker. Stop the fire, stop the  
hazard. Then complete the search.

Nor are fog operations dangerous on the interior. They are dangerous  
if you stay in an unvented room with the nozzle open -- you'll steam  
yourself. With a vented room, open up a narrow fog pattern (30  
degrees) and push the fire out the vent hole -- it's an old fashion  
version of Positive Pressure Ventilation. With an unvented room, open  
the door, fog the ceiling quickly, and close the door as the steam  
starts to come out. Wait a few minutes, and repeat. The fire will  
smother out. Very little water is used, and the fire is no longer  
threatening to extend to other parts of the house.

Using high-pressure fog, there is a caveat. It turns to steam SOONER  
than a low-pressure fog. This means you have to use more caution with  
it to avoid steaming yourself. It also may mean you have to get  
closer to the seat of a fire. While solid-bores and low-pressure fog  
aren't as efficient, you can use them further from the literally "hot  
zone." In this case, pressure indeeds reduces the penetration of the  
water -- it vaporizes before it might reach the seat (of course, it  
may very well smother the seat of the fire!)

The "raw facts" really have to do more with TACTICS and how your  
department operates, because from a scientific standpoint, high- 
pressure fog is nearly 10 times as effective as smoothbores in terms  
of GPM, and more than adequate for most single, two, and three family  
residential buildings.

http://forums.firehouse.com/showthread.php?t=15815