220px|right|thumb|The fire triangle
The fire triangle or combustion triangle is a simple model for understanding the necessary ingredients for most fires.
The triangle illustrates the three elements a fire needs to ignite: heat, fuel, and an oxidizing agent (usually oxygen). A fire naturally occurs when the elements are present and combined in the right mixture. A fire can be prevented or extinguished by removing any one of the elements in the fire triangle. For example, covering a fire with a fire blanket blocks oxygen and can extinguish a fire. In large fires where firefighters are called in, decreasing the amount of oxygen is not usually an option because there is no effective way to make that happen in an extended area.
Fire tetrahedron
220px|right|thumb|The fire [[tetrahedron]]
The fire tetrahedron represents the addition of the chemical chain reaction to the three already present in the fire triangle.
Combustion is a chemical reaction that feeds a fire more heat and allows it to continue. Once a fire has started, the resulting exothermic chain reaction sustains the fire and allows it to continue until or unless at least one of the elements of the fire is blocked:
- foam can be used to deny the fire the oxygen it needs
- water can be used to lower the temperature of the fuel below the ignition point, or to remove or disperse the fuel.
- halon can be used to remove free radicals and create a barrier of inert gas in a direct attack on the chemical reaction responsible for the fire.
- carbon dioxide, being denser than air, can displace oxygen away from the fire
When the fire involves burning metals like lithium, magnesium, titanium, etc. (known as a class-D fire), it becomes even more important to consider the energy release. Because 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. Carbon dioxide extinguishers are ineffective against certain metals such as titanium. Thus, while heat is important to ignite a flame, topography is important for aiding fire spread especially by preheating upslope fuels, and ignitions sources are important to help explain recurrence on longer time scales. Similarly, while oxygen is relevant to sustain a flame, weather and associated winds feed oxygen into a spreading fire, and the longer-term pattern of weather is summarized as climate. Lastly, fuels is the term to describe what burns in a single flame to the range of materials burnt in a spreading wildfire, but fuels vary over larger space and time scales in what is called vegetation.
At the smallest scale, the combustion fire triangle, individual fuel particles are ignited once at a critical temperature, and fire transfers energy to the nearest surroundings. Combustion events range on the scale of several seconds to a couple of days and their effects are monitored at the quadrant scale. The largest scale, in contrast, describes the fire regime concept. Global climate change drives many of the factors involved in the 'wildfire' and the 'fire regime' triangles. For example, with respect to the fire regime, a particular vegetation type will support a characteristic fire in terms of recurrence, intensity, seasonality and biological effects; a change in vegetation type will have implications for a changing fire regime.
See also
- Glossary of firefighting terms
- Fire classes