Fire sprinklers

by Chris Woodford. Last updated: September 16, 2011.

If your job is to run a department store or a warehouse, fire is 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 Sprinkler 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!

How sprinklers work

Close-up of a fire sprinkler ceiling on a ceiling

Photo: The head of a fire sprinkler (or, as shown here, an agricultural sprayer) has a flower-shaped diffuser on the bottom. Water from a pipe above it hits the diffuser and bounces off into a fine spray that spreads over a wide area. Photo by David Nance courtesy of US Department of Agriculture/Agricultural Research Service.

The diagram down below summarizes how everything works. A sprinkler system is a network of water pipes running through the ceiling of a building (1). Each sprinkler is nothing but a faucet (or "tap," as they call it in some countries)—a hole in the pipe through which water can escape into the building below. In a normal faucet, you turn a screw to open up a valve that allows water to escape. In a sprinkler, the hand-operated faucet is replaced by a heat-sensitive plug designed to open automatically when fire breaks out. In some sprinklers, the plug is made of an alloy called Wood's metal, a mixture of bismuth, lead, tin, and cadmium that melts at a relatively low temperature. In other sprinklers, the plug is a small glass bulb full of a glycerin-based liquid designed to expand and shatter when it gets hot. The basic idea is the same in both cases: the plug is meant to break and open the sprinkler as soon as a fire breaks out.

Simple diagram showing how a ceiling fire sprinkler system works

Here's how the Wood's metal version works. Each sprinkler has two spring-like metal arms (2) held together by a slug of the Wood's metal (3). When the Wood's metal is intact, the spring arms are locked together and clamp the water pipe closed so no water can escape. Directly beneath each sprinkler, you'll notice there's a flower-shaped piece of metal called a deflector (4), but it doesn't do anything useful at this stage.

If a fire breaks out beneath a sprinkler (5), hot gases swirl upward toward the ceiling (6). When the temperature reaches about 70°C (160°F), the Wood's metal melts, allowing the two metal arms to spring open (7). (In the other design of sprinkler, the glass bulb breaks instead, opening up a hole in the water pipe above it.) Water can now escape from the pipe just as it does from an open faucet. It pours down from the pipe in the ceiling, hits the flower-shaped deflector head directly beneath, and falls to the ground in a gentle spray (8)—hopefully extinguishing the fire. If the fire is small, only the sprinkler directly above it will trigger and other nearby sprinklers will remain switched off to limit water damage (9). However, if the fire spreads, nearby sprinklers will soon be triggered as well until either the fire goes out or the firefighters show up to help out.