FIREEXTINGUISHER
Fire is a chemical reaction between the oxygen in the atmosphere and a fuel source. Fuels do not catch fire simply because they are surrounded by oxygen; the fuel must be heated for the combustion to take place. When a fuel is heated to its ignition temperature, the heat starts to decompose the fuel and release volatile gases. These gases formed from the decomposition of complex molecules react with oxygen to form water, carbon dioxide, and other products. The gases rise up through the air and make up the flame that emits heat and light. The heat produced by combustion keeps the fuel at the ignition temperature, so the fire continues to burn as long as fuel and oxygen are still present. The three essential elements to the fire are heat, oxygen, and fuel. Fire extinguishers are designed to remove one or more of these essential elements so the fire will cease this self-sustaining combustion. One of the best ways to remove heat is by dumping large amounts of water onto the fire. This cools the fuel below the ignition temperature as the water is converted to steam, interrupting the combustion cycle. It is also possible to remove the oxy- gen surrounding the fire by covering the fuel with a nonflammable mate- rial such as baking soda. This smothering effect separates the fuel from the atmospheric oxygen, again interrupting the combustion process.
Modern fire extinguishers operate on these two principles: removing heat or oxygen from the fire. A fire extinguisher is a metal cylinder that is filled with water or a smothering material with a mechanism to deliver the material from a safe distance. The extinguisher usually expels the water or smothering agent by pressure. A siphon tube leads from the bottom of the metal cylinder to the top of the extinguisher. The area above the active agent is pressurized with carbon dioxide. When the handle of the extinguisher is depressed, the carbon dioxide pressure forces the active agent through the siphon tube and out the nozzle of the extinguisher. If the extinguisher is aimed toward the fire, the water or smothering agent is delivered to cool or smother the fire.
Water is by far the most commonly used fire suppression material and one of the most effective. However, it is important that water not be used on fires involving electricity or flammable liquids. Most flammable liquids are less dense than water and will continue to burn while floating on top of the water. In these situations, the water serves only to spread the fire and will never effectively remove the heat as intended. Since water will also conduct electricity, there is the potential of electrocution should a water extinguisher be used on an electrical fire. Water extinguishers are intended to extinguish fires in which wood, plastic, or paper are the pri- mary fuels and should not be used on electrical or flammable liquid fires.
Another popular extinguishing material is ordinary compressed carbon dioxide. Because carbon dioxide is heavier than air, it displaces the oxy- gen surrounding the fire, halting the combustion of the fuel. However, carbon dioxide extinguishers do little to cool the temperature of the fuel, and the carbon dioxide quickly dissipates. This can result in the fire reig- niting. Ordinary baking soda (sodium bicarbonate), potassium bicarbon- ate, and ammonium phosphate are common smothering agents used in “dry chemical” fire extinguishers. These bicarbonates decompose and re- lease carbon dioxide that displaces oxygen. Combined with the insula- tion from the dry material, it works by smothering the fire.
Most fire extinguishers contain small amounts of fire suppressant ma- terial and should only be used on relatively small fires. Some of these agents work by displacing oxygen from the atmosphere surrounding the fire. This can be hazardous to persons in the area, who may become as- phyxiated because of the lack of breathable oxygen. The active agents in dry chemical extinguishers can be extremely hazardous if inhaled.
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