Fire triangle
From Wikipedia, the free encyclopedia
The fire triangle or combustion triangle is a simple model, from the practitioners of firefighting, for understanding the ingredients necessary for most fires.[1] It has largely been replaced in the industry by the fire tetrahedron, which add further aspects, but still does not explain fire fighting completely. It is also described below.
The triangle illustrates the rule that in order to ignite and burn, a fire requires three elements: heat, fuel, and an oxidizing agent (usually oxygen). The fire is prevented or extinguished by removing any one of them. A fire naturally occurs when the elements are combined in the right mixture.
- Without sufficient heat, a fire cannot begin, and it cannot continue. Heat can be removed by dousing with water; the water turns to steam and the steam is further heated, taking the heat with it. Introducing particles of powder or any gas in the flame removes heat in the same manner. Separating burning fuels from each other also reduces the heat. In forest fires, burning logs are separated and placed into safe areas where there is no other fuel. Scraping embers from a burning structure also removes the heat source. Turning off the electricity in an electrical fire removes the ignition source.
- Without fuel, a fire will stop. Fuel can be removed naturally, as where the fire has consumed all the burnable fuel, or manually, by mechanically or chemically removing the fuel from the fire. Fuel separation is an important factor in wildland fire suppression, and is the basis for most major tactics, such as controlled burns. Note, however, that the fire stops because a lower concentration of fuel vapor in the flame leads to a decrease in energy release and a lower temperature. Removing the fuel thereby decreases the heat.
- Without sufficient oxygen, a fire cannot begin, and it cannot continue. With a decreased oxygen concentration, the combustion velocity gets lower. But, in most cases there is plenty of air left when the fire goes out so this is commonly not a major factor. An example of this is that when adding inert gases, you need about 30% carbon dioxide, but about 40% nitrogen to suppress a flame. If the removal of oxygen was the point, the concentrations should be equal. The explanation is that carbon dioxide requires more energy to be heated and therefore a lower concentration. So, once again, it is all about reducing the temperature in the flame below the level where radicals can exist.
[edit] Fire tetrahedron
The removal of heat explains fire suppression in a majority of cases. However, the suppression effect of Halon can be explained only to approximately 95% by regarding the thermal theory. The remaining 5% can be explained by its chemical inhibition. That means that it reacts chemically to reduce the concentration of radicals in the flame. The radicals are essential in the chemical reactions. This has led to development of the fire tetrahedron: a triangular pyramid having four sides including the bottom representing the sustaining of chemical reactions.
Combustion is the chemical reaction that feeds a fire more heat and allows it to continue. When the fire involves burning metals like lithium, magnesium, titanium,[2] etc. (known as a class-D fire), it becomes even more important to consider the energy release. The metals react faster with water than with oxygen and thereby more energy is released. Putting water on such a fire results in the fire getting hotter or even exploding because the metals react with water in an exothermic reaction. Some metals also burn in carbon dioxide.[2] Therefore, inert agents (e.g. dry sand) must be used to break the chain reaction of metallic combustion.
