Four elements must be presentin order for fire to exist. These elements are
HEAT, FUEL, OXYGENand CHAIN REACTION.
While not everything is known about the combustion process, it is generally accepted that fire is a chemical reaction. This reaction is dependent upon a material rapidly oxidizing, or uniting with oxygen so rapidly that it produces heat and flame.
Until the advent of newer fire extinguishing agents, fire was thought of as a triangle with the three sides represented by heat, fuel, and oxygen. If any one of the three sides were to be taken away, the fire would cease to exist.
Studies of modern fire extinguishing agents have revealed a fourth element - a self propagating chain reaction in the combustion process. As a result, the basic elements of fire are represented by the fire tetrahedron - HEAT, FUEL, OXYGEN and CHAIN
REACTION.
Fire Tetrahedron
The theory of fire extinguishment is based on removing any one or more of the four elements in the fire tetrahedron to suppress the fire.
REMOVING THE HEAT
In order to remove the heat, something must be applied to the fire to absorb the heat or act as a heat exchanger. Water is not the only agent used to accomplish this, but it is the most common.
REMOVING THE FUEL
Under many circumstances, it is not practical to attempt to remove the fuel from the fire.
When dealing with flammable liquid fires, valves can be shut off and storage vessels pumped to safe areas to help eliminate the supply of fuel to the fire. Flammable gas fires are completely extinguished by shutting off the fuel supply.
REMOVE THE OXYGEN
Oxygen as it exists in our atmosphere (21%) is sufficient to support combustion in most fire situations. Removal of the air or oxygen can be accomplished by separating it from the fuel source or by displacing it with an inert gas. Examples of separation would be foam on a flammable liquid fire, a wet blanket on a trash fire, or a tight fitting lid on a skillet fire.
Agents such as CO2, nitrogen, and steam are used to displace the oxygen.
INTERRUPT THE CHAIN REACTION
Modern extinguishing agents, such as dry chemical and halons, have proven to be effective on various fires even though these agents do not remove heat, fuel, or oxygen. Dry chemical and halogenated agents are thought to suspend or bond with "free radicals" that are created in the combustion process and thus prevent them from continuing the chain reaction.
It must be noted that Halon is now banned in most country or cities. The agreement banning Halon Production is called The Montreal Protocol.
Click on the following link to see a nice video on fire fighting and extinguishing agents, it cover key information you need to know for the exam.
Resume of the class of Fires:
Class of Fires
All of the other answers are incorrect:
References:
Fire and Fire Extinguishment
and
http://code7700.com/fire.html
