why do they use carbon dioxide in fire extinguishers

Carbon dioxide is used in fire extinguishers for its ability to put out flames. (Image: fire extinguisher image by Mat Hayward from
Carbon dioxide, otherwise known by its chemical shorthand CO2, is a naturally occurring gas that's present in the air we breathe. This gas is essential to life on earth: It's a vital component of both photosynthesis and cellular respiration. Among its many uses, CO2 has the ability to put out flames and is therefore used as the extinguishing agent in both portable and fixed-installation fire extinguishers. An examination of its properties and uses shows why it's a popular choice for suppression of most, but not all, types of fire. CO2 is a colorless and, in normal concentrations, odorless gas. It doesn't react with burning materials, so it doesn't create any toxic or other by-products when used to suppress a fire. It's therefore a clean gas, meaning it leaves no trace of its use when suppressing a fire. Carbon dioxide doesn't conduct electricity, making it an ideal fire suppressant for use in computer rooms, electrical distribution stations and other locations where a large amount of electricity may be present. CO2 acts on fires in two ways: The release of the gas under pressure has a cooling effect, as can be seen by the resulting mist cloud and ice particles; the gas also displaces the oxygen that's necessary to maintain combustion. Fires are classified by the type of material that's burning.

There's broad international agreement on fire classifications in Australia, Asia and Europe. American classifications differ, as shown in the following table: CO2 is effective on liquid fires, gaseous fires and electrical fires (American classes B and C and International classes B, C and E). The gas is particularly useful for electrical fires as it's non-conductive and leaves no trace after its use (as would be the case for foams or other gases). However, CO2 is not recommended for general fires, as it's emitted under high pressure; light burning material, such as paper and wood, could get blown around, possibly worsening the situation. Fixed CO2 fire-suppression systems are often installed where the risk of fire from burning liquids or gases is high, such as at gas stations. Businesses will install CO2 fire-suppression systems in enclosed areas such as computer rooms or electrical switch rooms. Many ships have these systems installed in their engine rooms. CO2 may have health risks. At high concentrations, CO2 can become toxic or even lethal. According to the Environmental Protection Agency, in an enclosed space that has been subject to fire suppression, the typical resulting concentrations of CO2 of 17 percent or more can cause loss of function, unconsciousness and death within a minute; lower concentrations can cause confusion, dizziness, headaches and nausea. It's important that any enclosed area that has been subject to a fire-suppression incident is evacuated immediately.

A CO2 fire extinguisher that's discharging will cool quickly, creating a frostbite risk to the operator, who should take care to touch only the handle and the nozzle and avoid the metal parts of the extinguisher. What is fire? Ask most people what a fire is and they'll tell you it's something frightening and destructive involving flames. But to a scientist, a fire is something much more precise. A fire is actually a chemical reaction called combustion. When combustion happens, substances like, oil, or coal (all of which are made from chemicals, even if you don't immediately think of them that way) combine with oxygen in the air to produce water, carbon dioxide, waste gases and an awful lot of. Combustion doesn't normally happen all by itself: things don't burst into flames without help. It usually takes some activation energy (provided by a spark or a match) to kick off the reaction. Once combustion is underway, the fire seems to continue all by itself. Photo: You need to to take away one or more of heat, air (oxygen), or fuel to break the triangle and put the fire. That's not quite true. Fire happens when three things are in the same place at the same time: Fuel (something to burn such as or coal). Oxygen (usually from the air). Heat. A fire can burn when all these things are present; it will stop when at least one of them is removed.

As any fire-fighter will tell you, putting out a fire involves breaking the fire triangle which means removing either the fuel, the heat, or the oxygen. Suppose a fire breaks out in a pan on top of your cooker, the first thing you normally do is switch off the heat. If that doesn't work, you might soak a towel with water and place it very carefully over the pan (or, better still, use a ). The towel is designed to block off the supply of oxygen to the fire (the water stops the towel from catching fire and making things worse). Every fire-fighting technique you can think of involves removing heat, oxygen, or fuel sometimes more than one of those things at the same time. Photo: Some extinguishers have pressure gauges on top so you can check they're correctly pressurized and safe to operate. If the pressure is either too high or too low, the needle moves into the upper or lower red zone. On this dry-powder extinguisher, the needle is right in the middle: still safely in the green zone, pressurized to about 14 times atmospheric pressure (the normal pressure of the air around us). Water extinguishers, which are the most common, are essentially tanks full of with compressed (tightly squeezed) air as the propellant to make them come out. Water extinguishers work by removing heat from the fire. Dry chemical extinguishers are tanks of foam or dry powder with compressed nitrogen as the propellant.

They work by smothering the fire: when you put a layer of powder or foam on the fire, you cut the fuel off from the oxygen around it, and the fire goes out. ) extinguishers contain a mixture of liquid and gaseous carbon dioxide (a nonflammable gas). CO is normally a gas at room temperature and pressure. It has to be stored under high pressure to make it a liquid. When you release the pressure, the gas expands enormously and cools to make a huge white jet. CO attacks the fire triangle in two ways: it smothers the oxygen and, because it's so cold, it also removes heat. That classifies extinguishers by what they contain. You'll also find fire extinguishers classified by the types of fires you can use them on. This gives us five different kinds: A: Green: For wood, cloth, and paper. B: Red: For combustible and flammable liquids such as oil, gasoline, and paint. C: Blue: For electrical equipment and tools. D: Orange: For flammable metals. K: Black: For animal or vegetable oils or cooking facts. It's important always to use the right extinguisher for the fire. Using the wrong extinguisher can put your life in danger and make the fire worse. For example, you must never use water extinguishers on electrical fires because you could electrocute yourself and the people nearby. If you're in the slightest doubt about tackling a fire, leave it alone and get yourself to safety.

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