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  • From: Gene GeRue <genegerue AT ruralize.com>
  • To: Homestead <homestead AT lists.ibiblio.org>
  • Subject: [Homestead] Rural home firefighting
  • Date: Wed, 12 Sep 2007 07:43:42 -0500

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




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