How does a foam fire extinguisher put out a fire?

Answer 1

Water removes heat by converting into steam and may reduce the heat below the ignition temperature of the fuel. It may also INCREASE the ignition temperature of the fuel itself by soaking in. Thus, the fuel cannot continue to burn until all the water has been converted to steam, which requires more heat.

Answer 2

Most CO2, foam and dry chemical fire extinguishers smother the burning material so that it cannot get enough air (oxygen) for the fire to continue to burn. Also, water-type extinguishers cool the burning material enough that there isn't enough heat to support the fire.

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What was said above is true of most fires. But in recent years some extinguishing agents have been developed that actually change the chemical reaction going on in a fire. These are mostly used on chemical fires, especially combustible metals. But the can be very effective for other types of fires as well.

Answer 3

depends on the type or class of fire, and the extinguisher. All fires need four things to burn: fuel (wood, gas, fumes), oxygen, heat, and chain reaction. This is appropriately called the fire tetrahedron. Without one piece the tetrahedron collapses and the fire dies.

Class A is burning paper, wood etc. The extinguisher to use here is water or foam. This works by cooling the fuel. Foam extinguishers also smother fires, excluding oxygen. A fire without oxygen can't burn. Dry chemical extinguishers disperse a powdered agent onto the fire, separating and disrupting the four parts of the fire tetrahedron. Halon extinguishers interfere with the chemical chain reactions of combustion. Larger halon units are effective on class A fires. A CO2 extinguisher can knock down or reduce a class A fire, but is unlikely to displace enough oxygen to entirely extinguish the fire.

Class B is burning fluids. Water is ineffective on burning oils, as oil floats on water. Worse, the force of the water can spread the fuel and fire. Foam extinguishers get around this problem because the air in the foam allows it to float on the oil, forming a floating seal over the fluid and cutting it off from oxygen and making the fuel difficult to reignite. If the fluid is alcohol based, it will disrupt the foam, necessitating use of a specialized foaming agent. Halon will disrupt the chemical chain reaction of combustion. If the burning fluid is very shallow, dry chemical extinguishers are also effective on this type, but the fluid can be easily reignited. CO2 extinguishers will also extinguish this type of fire by displacing oxygen above the fire and cooling the fuel.

Class C is any other type of fire with the added complication of charged electrical components. TURN OFF THE POWER. Often this will kill the fire or turn it into an ordinary class A or B fire. If turning off the power is not an option, dry chemical or halon extinguishers can be used to disrupt the fire tetrahedron. CO2 extinguishers work very well displacing oxygen from the enclosed spaces within most electronics. Halon-replacement clean agents work like CO2, but won't thermally shock sensitive (expensive) components. Water mis (not to be confused with water) extinguishers can be used on AC class fires. The use purified water, which is a poor conductor when devoid of salts, and break up the flow into a mist which won't carry the shock back to the user. The water cools the fuel and doesn't contaminate the surrounding area the way dry chemicals will. Water mist extinguishers are common in hospitals but rare everywhere else. Never use a water or foam extinguisher on a class C fire.

Class D is a burning metal like magnesium, titanium, or lithium. With more of these materials being used for lighter car components or in rechargeable batteries, these are becoming more common. If you encounter a class D fire, it is probably best to leave it to the professionals. Sand will usually put out small class D fires by cooling the fuel and excluding oxygen. Most class D extinguishers are simply the agent contained in a bucket that is shoveled onto the fire, though the agents are also sometimes found in more typical housings which will spray it onto the fire. Class D fire extinguishers are usually dry powder (not to be confused with dry chemical). Copper-based of this type work by disrupting the chemical reactions. Salt and thermoplastic mix work by forming a shell that excludes oxygen. Burning magnesium and lithium react explosively with water, so neither foam nor water-based extinguishers should be used. There is one foam concentrate on the market that is rated for class D, but this is a rare exception to the rule.

Class K is burning cooking fats, oils, or greases. Until 2008, these were included in class B. Most class K fires can be extinguished by simply covering the cooking vessel and turning off the heat, excluding oxygen and heat to the fire. Class K extinguishers are a foam type that is also effective on classes A and B. The extinguishing agent includes a soap-like emulsifier that puts the burning lipids into solution with the water, rendering them non-flammable. Class K can also be extinguished by any class B rated extinguisher.

So to sum up, fires need four things to burn, and without one of those things the fire will die. Common extinguishers are water, dry chemical, and CO2. some more exotic extinguishers are Aqueous Film Forming Foam (AFFF), dry powder, and Halon gas. All of these put out a fire by removing part of the fire tetrahedron.

Answer 4

Water raises the ignition point of the fuel source eliminating one leg of the fire triangle. Water also reduces the HEAT in the fire by converting liquid water to steam, which expands into the surrounding air. The increased humidity in the air then reduces the amount of oxygen available to the fire.

Answer 5

The foam smothers the fire by cutting off the supply of oxygen to the fire.

Answer 6

They suffocate it and prevent it from using the oxygen around it to burn. To put it simply: fire extinguishers choke fires, which eventually die.