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Effective fire control and extinguishment requires a basic understanding of the chemical and physical nature of fire. What is fire? How does it work?

The Fire Triangle

Fire is a chemical process. This process is known as “Combustion”. Combustion is a chain reaction chemically similar to photosynthesis in reverse. In order for something to burn, three elements must be present simultaneously. Three things are required in proper combination before ignition and combustion can take place

·         Heat, Oxygen and Fuel.

Fire is a process that requires that a combustible material (fuel) is subjected to sufficient heat and has access to oxygen. To illustrate how these three elements are dependent on one another, we can put them together in a triangle, which we call the fire triangle.

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·         There must be FUEL to burn.

·         There must be AIR to supply oxygen.

·         There must be HEAT (ignition temperature) to start and continue the combustion process.

Without one of these elements a fire cannot start or continue. Therefore to extinguish a fire, one or more of the sides of the triangle must be removed. In other words, one must either remove the combustible material, lower the temperature or reduce access to oxygen.

Stages of Fire ; burning process

The burning process occurs in clearly defined stages. By recognizing the different stages, a fire fighter can better understand the process of burning and fighting the fire at different levels and with different tactics and tools. Each stage is characterized by differences in room temperature and atmospheric composition. The stages of a fire are described below.

11.    Incipient Stage : Initial Fire Stage
In the first stage, the oxygen content in the air has not been significantly reduced and the fire is producing water vapor, carbon dioxide, perhaps a small quantity of sulfur dioxide, carbon monoxide and other gases. Some heat is being generated, and the amount will increase with the progress of the fire. The fire may be producing a flame temperature well above 1,0000F (5370C). 

12.    Fully Developed Stage
The second stage of burning encompasses all of the free-burning activities of the fire. During this phase, oxygen-rich air is drawn into the flame as convection (the rise of heated gases) carries the heat to the upper most regions of the confined area. The heated gases spread out laterally from the top downward, forcing the cooler air to seek lower levels, and eventually igniting all the combustible material in the upper levels of the room. At this point, the temperature in the upper regions can exceed 1,3000F (7000C). 

13.    Decay Stage
In the third stage, flame may cease to exist if the area of confinement is sufficiently airtight. In this instance, burning is reduced to glowing embers. The room becomes completely filled with dense smoke and gases to the extent that it is forced from all cracks under pressure. The fire will continue to smolder, and the room will completely fill with dense smoke and gases of combustion at a temperature of well over 1,0000F (5370C).

Time Temperature Curve

The demarcations between the three stages can be identified by a “time temperature curve”. During the incipient (or growth) phase of a fire, shown below as the upward curve, the time can vary depending on the type of fuel, the size of the room, and the amount of oxygen supplying the fire. Flashover occurs at the end of the incipient (or growth) phase and start of the free burning (or fully developed) stage. Backdraft can occur in the smoldering (or decay) phase.

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[Time Temperature Curve]