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.