How do dry chemical fire extinguishers work?

by | Oct 4, 2009 08:00 PM ET

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Hi, I'm Marshall Brain with today's question.

How do dry chemical fire extinguishers work?

If you ask any firefighter what it takes to create a fire, you will learn that you will have to have three things. First, you need a fuel. You need some sort of combustible solid, liquid or gas. Next, you need oxygen to react with the fuel, and finally you need heat. There must be enough heat to get the fuel above its flash point. If there is paper on your desk right now, that's a fuel and is surrounded by oxygen, but it does not burn unless you get the paper hot enough.

If you want to put out a fire, you need to remove one of those three elements. When you watch firefighters battling a forest fire, they generally try to remove fuel or heat. Either they pour water on the fire to reduce the temperature, or they try to bulldoze strips of bare earth to eliminate the fuel. A carbon dioxide fire extinguisher works by eliminating oxygen and replacing it with carbon dioxide. You can do the same thing with just about any non-oxidizing gas, nitrogen, for example, but carbon dioxide is inexpensive, and is easy to store.

Another way to cutoff oxygen is to throw a blanket over the fire. Covering the fire with dirt or sand does the same thing as a blanket. You may have heard that you can put out a kitchen fire by throwing baking soda or salt on the fire. Throwing sand would do the same thing, but most people have more salt in their kitchens than they have sand. Baking soda also releases carbon dioxide when it gets hot, so that makes it even more effective. A dry, chemical fire extinguisher is essentially just a fancy way of throwing baking soda on a fire. Dry chemical fire extinguishers are by far, the most common fire extinguishers in the home. They can handle all three types of fires that you would find in a kitchen or workshop, combustible solids like water or paper, combustible liquids like gasoline or grease, and electrical fires.

The idea behind a dry chemical fire extinguisher is to blanket the fuel with baking soda. A dry chemical extinguisher sprays a very fine powder of sodium bicarbonate, which is baking soda, or potassium bicarbonate, which is nearly identical to baking soda, or monoammonium phosphate, and these solids coat the fuel to smother the fire.

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