Science Made Simple: How a Fire Extinguisher Works
Have you ever wondered how a fire extinguisher works? How exactly does the chemical inside the container put out a fire? Find out why one particular chemical works so well on three different types of fires.
To start, it’s important to understand that there are three things fires need: fuel, heat and oxygen, according to the National Fire Protection Association (NFPA). A fire will burn until at least one of these components is taken away, the NFPA explains.
There are several types of fire extinguishers, each designed to put out certain kinds of fires. The most widely used is the ABC, or multipurpose dry chemical, fire extinguisher, which is effective on Class A, B and C fires, according to the Fire Equipment Manufacturers’ Association.
- Class A fires involve materials like wood, paper and cloth.
- Class B fires involve combustible and flammable liquids like gasoline, grease, oil and oil-based paints.
- Class C fires involve electrical equipment like appliances, tools or other devices that are plugged in, according to the U.S. Fire Administration.
Home improvement stores often carry ABC fire extinguishers, the site adds.
How an ABC Fire Extinguisher Works
The first thing you do when using a fire extinguisher is pull the pin located near the top of the device, says the NFPA.
Inside the extinguisher is a firefighting chemical and a canister of highly pressurized nitrogen gas, explains Kate Biberdorf, Ph.D., a chemistry lecturer and director of demonstrations and outreach for The University of Texas at Austin.
Nitrogen is used in fire extinguishers because, “our Earth’s atmosphere is 78 percent nitrogen. So it’s really easy for us to use that,“ she says. “And what’s beautiful about nitrogen is it’s what we call an inert gas.”
An inert gas is stable and nonexplosive, Dr. Biberdorf explains. “It’s a good gas to have in a dangerous situation because you know there’s almost nothing you could do to force a reaction to happen.”
The nitrogen is highly pressurized, which means that its molecules are moving constantly, hitting the walls of the container. When you squeeze the lever on a fire extinguisher, a valve on the canister of nitrogen opens up. This allows the nitrogen gas to leave. It immediately expands to fill the interior of the extinguisher, pushing down on the chemical inside the extinguisher. As this chemical is forced down by the nitrogen, it goes up a tube that is near the bottom of the extinguisher. The chemical then sprays out of the nozzle.
Imagine blowing bubbles into a drink, Dr. Biberdorf says. If you blow down too hard, liquid splashes up in your face. “It’s that same type of process,” she explains. “You’re forcing pressure down, which is going to then force everything else to go up.”
Class A and C Fires
As mentioned, Class A fires involve materials like wood, paper and cloth, while Class C fires involve electrical equipment. The chemical found inside an ABC extinguisher is often a powder of monoammonium phosphate, Dr. Biberdorf notes. Although temperatures of burning objects vary, Class A and C fires generate enough heat to melt the monoammonium phosphate powder and turn it into a liquid. Fires need oxygen as fuel, so when this liquid flows over the fire, it cuts off the oxygen and smothers the flames, Dr. Biberdorf explains.
Class B Fires
Class B fires, which involve liquids, work differently, though, Dr. Biberdorf continues. For one thing, liquid fires don’t burn as hot as Class A or C fires. This is usually because a smaller area is burning — when a liquid is on fire, only its top layer is burning.
In addition, liquids can be volatile — meaning the molecules are switching back and forth between liquid and gas states, Dr. Biberdorf says. “With a volatile liquid, there’s more flammable gas hovering directly above the liquid,” she explains. “The vapors ignite the liquid, and then continuously fuel the fire.”
With a liquid fire, it’s not hot enough for the monoammonium phosphate to melt. Instead, it remains a solid, which covers the liquid and puts a barrier between it and those volatile vapors. The vapors can no longer keep igniting the liquid, and the fire is extinguished, Dr. Biberdorf explains.
Now you know how an ABC fire extinguisher works. Take the time to brush up on how to use these safety devices correctly to help protect your family.