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How Fire Extinguishers Work


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Channel: BrainStuff - HowStuffWorks
Categories: Chemistry   |   Science   |   Technology  
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Fire extinguishers are nearly ubiquitous, and they can be an invaluable, life-saving device. But what’s inside those little red cylinders, and how does it stop a fire?

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If you’re like me, then you constantly worry about buildings catching on fire when someone plays your newest mixtape.

And since you can’t compromise your artistic integrity because of a few exploding sound systems, you have to be prepared for the blazes that inevitably ensue once the beat drops.

A fire extinguisher is one of your best options – but what are these things, anyway? How do they work?

First, let’s talk about fire. Fire is the result of a chemical combustion reaction, typically between oxygen and some sort of fuel, like wood or gasoline. For the reaction to occur, the fuel has to reach its ignition temperature.

For wood, that’s about 500 degrees F (or 260 C). The heat decomposes some of the wood’s cellulose. The decomposed stuff is released as volatile gases, like a compound of hydrogen, carbon and oxygen. When the gas is hot enough, its molecules break apart. The atoms recombine with the oxygen to form water, carbon dioxide and other products. The heat keeps the fuel at the ignition temperature, so it keeps burning as long as there is fuel and oxygen.

There you have it. Fire, the result of extreme heat, oxygen and fuel. Fire extinguishers remove at least one of these elements from the equation.

Fire extinguishers are metal cylinders filled with water or a smothering material. When you depress a lever at the top here, the material is expelled by high pressure, kind of like an aerosol can. Imagine a fire extinguisher cut in half – see it?

OK, that plastic siphon tube leads from the bottom of the fire-suppressant reservoir to the top of the extinguisher. A spring-mounted valve blocks the passageway from the siphon to the nozzle. At the top of the cylinder, there is a smaller cylinder filled with a compressed gas -- liquid carbon dioxide, for example. A release valve keeps the compressed gas from escaping.

When you pull out the safety pin and depress the operating lever, it pushes on an actuating rod. The rod presses the spring-mounted valve down to open up the passage to the nozzle.

The bottom of the actuating rod has a sharp point, which pierces the gas cylinder release valve. The compressed gas escapes, applying downward pressure on the fire-suppressant material. This drives the material up the siphon and out the nozzle.

The proper way to use the extinguisher is to aim it directly at the fuel, not the flames, and spray in a sweeping motion.

There are three main types of extinguisher. A water extinguisher can put out things like burning wood, paper or cardboard – but it doesn’t work well on electrical fires or fires involving inflammable liquids.

In an electrical fire, water may conduct the current, which could electrocute you. Water will only spread an inflammable liquid, making the fire worse. Not a good look.

Then there’s the carbon dioxide extinguisher. The CO2 is kept in a pressurized liquid form. When the container is opened, the CO2 forms a gas. This gas is heavier than oxygen, so it displaces the oxygen surrounding the fuel.

This extinguisher is common in restaurants because it won't contaminate cooking equipment or food.

The most popular extinguisher material is the dry chemical extinguisher. These cylinders contain foam or powder, typically made of sodium bicarbonate (you know, baking soda), potassium bicarbonate or monoammonium phosphate.

Baking soda starts to decompose at only 158 degrees F (70 degrees C), and when it decomposes, it releases CO2. The CO2, along with the insulation of the foam, smothers fire.

But be careful. Most fire extinguishers contain a small amount of suppressant -- you can use it up in seconds. That’s why they’re only effective on relatively small fires. To put out a bigger fire, you need more equipment, like a fire engine, as well as trained professionals. But for the blazes that can pop up in your house, a fire extinguisher is a lifesaver.


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