by Chris Woodford. Last updated: December 29, 2019.
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 (fire-suppression systems) do.
Sprinklers aren't just for business buildings: they're
also well worth having in homes. According to the
Home Fire Sprinkler Coalition
(an educational nonprofit that has campaigned to improve awareness of the issue for over 20 years),
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!
Animation: This fire sprinkler head opens when heat causes the red liquid-filled capsulse to break.
That opens a valve, allowing water to spurt through, bouncing off the flower-shaped deflector to make a wide spray.
Why water puts out fires
Artwork: 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.)
Photo: Slow to heat, quick to cool: It takes a long time to boil water in an open pan sitting on a stove because it has such a high specific heat capacity: even this small amount of water needs a lot of energy to raise its temperature. It's a real nuisance when we're cooking, but incredibly helpful when we're fighting fires. If we use water to cool something that's burning, it can remove a great deal of heat energy very quickly. That's why water is so good at putting out fires.
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
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
A sprinkler system is a network of pipes running through the
ceiling of a building holding water under pressure. 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. The artwork below
shows how the glass-bulb version works.
Photo: The network of water pipes feeding a fire sprinkler system.
Photo from US Library of Congress.
Artwork: The key parts of a typical fire sprinkler. Water under pressure in a pipe (1) is held in place by a small plug (2), itself held in place by a glass bulb filled with glycerin (3). When a fire breaks out, the bulb breaks, releasing the plug, allowing the water to hit the flower-shaped deflector, which spreads it around in a spray.
The Wood's metal version works in a slightly different way. The array of sprinklers is fed by water pipes
in the ceiling (1). 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).
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.
According to the Home Fire Sprinkler Coalition, sprinklers can extinguish
small fires in as little as 90 seconds using less than one tenth as much
water as a fire hose, greatly limiting water damage.
Types of fire sprinklers
Artwork: Top: In a wet-pipe sprinkler, the pipes above the ceiling are all filled with cold water (blue). When a fire breaks out and the sprinkler opens, water can flow out immediately, but the pipes can also freeze and burst in cold weather if the building is unheated. Bottom: In a dry-pipe sprinkler, the ceiling pipes are filled with compressed air (yellow). When a fire breaks out, the pressurized air rushes out of the sprinkler, lowering the pressure in the pipe and opening the clapper valve (red). This lets water (blue) flow along the same pipe to put out the fire. It takes longer for the water to reach the fire in this case.
The setup I've described above is what's called a wet-pipe system, which means
all the pipes above our warehouse or store ceiling hold cold, pressurized water, ready to release it
the moment the Wood's metal melts. They respond very quickly and effectively, dousing flames and
helping to reduce the effects of smoke and toxic gases billowing out from a fire. Not all sprinklers work like this, however.
In another design, called a dry-pipe system, the ceiling pipes contain compressed air (or nitrogen gas) instead of water.
When the Wood's metal melts, the air leaks out of the pipes, causing a sudden drop in pressure. That opens
a valve further down the system that allows water to flow into the same network of pipes from the main, which then flows
out of the open sprinkler heads in the usual way. Systems like this are great in cold (typically unoccupied and unheated)
buildings where water shut inside pipes might freeze and burst, causing a devastating flood. The drawback is that it takes more time for a dry-pipe system to activate—literally every second counts when you're fighting a fire—which is why sprinklers like this are used only where really necessary. While a dry-pipe system can be installed in a very cold
building, the valve that opens through air pressure to release the water must be protected from freezing by a heater of some kind.
Automatic sprinklers might all look the same, at a glance, but they come in many different varieties,
designed to respond at different temperatures and speeds and work in all kinds of different buildings and environments.
They can be mounted traditionally, on a ceiling pointing down; in a loft-space pointing up; or on a side wall
pointing inward. Some have special coatings (such as Teflon or polyester) to protect them in corrosive or other challenging settings. They're also available in a range of colors, designs, and finishes so they don't look too obtrusive.
Residential sprinklers, for example, often have a neat "flush" design or cover cap so they're partly or completely recessed into a ceiling or wall.
What else do you need in a fire sprinkler system?
Photo: What a sprinkler system looks like from the outside: here's a typical fire department connection, drain valve, and other valves. Photo from US Library of Congress: Historic American Buildings Survey.
Apart from the automatic sprinklers themselves, there's the network of pipes in the ceiling, inspection or test valves
(so the system can be checked out periodically), a drain connection (for emptying the pipes for routine maintenance), and
a fire department connection (also called an FDC or Siamese connection)—a standardized connector to which the fire department can attach hoses and pump extra water into the sprinkler system if necessary, typically located in a very
accessible place on the outside of the building or in a nearby parking lot.
Who invented fire sprinklers?
Modern-style sprinklers like those described up above, based on an alloy that melts in a fire,
were invented in the 1870s by Henry S. Parmelee of New Haven, Connecticut, USA.
Browse through the records at the US Patent and Trademark
Office and you'll find Parmelee filed quite a few different patents for fire sprinkler heads
(or "automatic fire-extinguishers," as he called them), including the very simple one shown on the left
of the diagram. It's simply a metal cap fixed to the end of a pipe with a solder specifically designed to melt in
a fire. According to Parmelee's US patent #218,564 (issued August 12, 1879), from which this artwork is taken, the "solder, or even a disk, can be made by a
proper alloy of tin or other metal with bismuth, the [melting] point... of which is as low as
160° Fahrenheit [71°C]."
The artwork on the right is from a more modern-looking sprinkler designed by William Neracher about a decade
later. You can clearly see the spring arms and the plug that holds the water pipe closed. Over a century
later, modern sprinklers still work much the same way. Good ideas never go out of fashion!
Note: There is some inconsistency in the literature in the spelling of Parmelee's name, with some sources calling him "Paramalee" and others "Parmelee." I've chosen to follow the spelling given on his patents; even though some
people believe this is incorrect, it's as good a source as any.
Diagrams: Left: US Patent #218,564: Automatic Fire-Extinguisher by Henry S. Parmelee of New Haven, Connecticut. The red seal stops water flowing until a fire breaks out. Then the red seal melts, water flows upward through the blue pipe, and out through the green top (perforated with holes like the rose of a watering can). Right:
US patent #388,905: Automatic Fire Sprinkler by William Neracher of Cleveland, Ohio. This looks much more like a modern sprinkler and works exactly the same way. In a fire, the red seal melts and the orange supporting structure falls away. Water gushes down from the blue pipe, hits the green deflector, and sprinkles over the fire. Artworks courtesy of US Patent and Trademark Office (with our own coloring added for clarity).