Fire is a rapid chemical reaction known as combustion, a process that releases energy in the form of heat and light. For this reaction to start and continue, a specific set of conditions must be met, summarized by the Fire Triangle model. This model illustrates that combustion requires the simultaneous presence of three distinct elements. If even one component is removed, the chemical process of fire cannot be sustained. Removing any element of the triangle is the basis for all fire suppression techniques.
Defining the Fire Triangle Components
The first element is Fuel, the material that burns during combustion. This includes any combustible substance, whether it is a solid like wood, a liquid like gasoline, or a gas like propane. The fuel’s physical state often affects how quickly it ignites, as liquids and gases release flammable vapors more readily than solids.
The second element is Heat, which provides the energy required to raise the fuel to its ignition temperature. The third component is an Oxidizing Agent, typically the oxygen found in the surrounding air. Oxygen supports the chemical reaction by combining with the fuel.
The Result of Removing Heat
Removing the heat element is known as cooling or quenching the fire. The goal is to reduce the fuel’s temperature below the point required to sustain combustion. Once the temperature drops beneath the ignition point, the chemical reaction cannot continue, and the fire ceases.
The most common way to achieve this is by applying water, an effective cooling agent due to its high heat capacity. When water is sprayed onto a fire, it absorbs thermal energy as it heats up and vaporizes into steam. The process of vaporizing one kilogram of water requires approximately 2,260 kilojoules of energy, pulling heat directly from the burning material.
The Result of Removing Fuel
When the fuel source is removed, the fire is extinguished through a process called starvation. Without a material to oxidize and burn, the combustion reaction consumes all available fuel and then dies out.
One practical application is shutting off a gas supply valve, which immediately cuts the flow of fuel to a gas fire. In wildland firefighting, this principle is applied through the creation of fire breaks. These strips of land are cleared of vegetation and combustible materials, isolating the fire and forcing it to burn itself out.
The Result of Removing Oxygen
Removing the oxidizing agent, oxygen, results in the fire being extinguished by smothering or suffocation. Combustion requires a minimum concentration of oxygen, typically around 16%, to sustain the flame. Reducing the oxygen level below this threshold prevents the chemical reaction from continuing.
A common technique is using a fire blanket, which creates a physical barrier between the burning material and the surrounding air. Carbon dioxide (\(CO_2\)) extinguishers work by releasing a dense, non-combustible gas that displaces the air and dilutes the oxygen concentration. Closing off an area by shutting doors and windows also limits the inflow of fresh air, effectively suffocating the fire.