Fire sprinklers

Last updated: May 6, 2009.

If your job is to run a department store or a warehouse, fire is perhaps your ultimate nightmare. If a fire breaks out at night, when there's no-one around, and your building is stocked with furniture or flammable chemicals, the flames can spread in no time. Even if fire claims no lives, it can still be devastating: lose your stock or your building and you might lose a business that's taken years or decades to build up. It makes sense to have a fire-fighting system that can react the moment trouble strikes, not just sounding an alarm but automatically putting out a fire as quickly as possible. That's exactly what fire sprinklers do. Sprinklers aren't just for business buildings: they're also well worth having in homes. According to the Home Fire Sprinker Coalition, a sprinkler adds about 1 percent to the cost of a building but (fitted alongside a smoke alarm) can reduce the risk of death in a home fire by 82 percent. Let's take a closer look at how these superb gadgets work!

Photo: A fire sprinkler system under test onboard a US Navy aircraft carrier. Normally, only one sprinkler head (the one directly above the fire) is designed to trigger at a time. Photo by Dustin Howell courtesy of US Navy and Defense Imagery.

How to put out fires... automatically

Photo: Remember the fire triangle: if you can remove either the heat, the air (oxygen), or the fuel, you can usually put out a fire.

Suppose your mission is to design a system that can put out a fire automatically, even when there's no-one around. Where would you start? You probably know that water is one of the best, all-round substances for tackling fires; that's why firefighters use it, after all. Why is water so good? First, because it's cold when it's piped out of the ground, it removes the heat from a fire—breaking what's known as the fire triangle by taking away one of the three key ingredients (heat, oxygen, and fuel) that all fires need. (Water-based fire extinguishers work the same way, while fire blankets and fire beaters put fires out by removing air.) Second, because water has what's called a high specific heat capacity, it removes heat more effectively. Kilogram per kilogram (or pound per pound), water can hold more heat than almost any other everyday substance. (That's why it's used to ferry heat around our homes in central heating systems.)

Okay, so your automatic fire-fighting system is going to use water. How will it work? If you've seen firefighters tackling a blaze, you might have noticed them firing water up into the air so it falls as a spray over a wide area. Maybe what you need is something like an automatic fire hose attached to the ceiling of your building that could work the same way? Unfortunately, what you don't have at your disposal is lots of highly trained firefighters: you can't have people sitting around all day and night on the off-chance that a fire might break out. So what you need is a fire-hose that switches on automatically when there's a fire nearby—and, ideally, only in the immediate vicinity of the fire itself. The last thing you want is the entire building doused with water if your only problem is a small fire in a wastepaper bin. The water could do more damage than the fire!

Right, so how will the fire hose switch on automatically? If you've read our article on smoke detectors, you'll know there are some pretty clever ways of detecting fires by using electronic circuits to sense the smoke they give off. But we're going to make things harder: your fire-fighting system can't use any electrical or electronic components. Electrical systems can fail, especially in fires. Think about lightning. It can knock out electrical systems and start fires too, so any fire-fighting system that depends on an electrical or electronic sensor could prove useless in a thunderstorm. It's far better to use something simple and mechanical instead. What we need is a basic, mechanical device fitted to a fire hose, up in the ceiling, that will work just like a faucet (tap), but opening to release water only when a fire breaks out underneath it. Step forward the fire sprinkler!