We humans have had a very strange relationship with fire. It has both life giving and life taking powers. Sometimes we crave it for cooking and warmth but cannot have it. Other times it swallows our homes and lives.
While fires can be deadly and incredibly difficult to put out, they do require three specific elements to take place. So, they can also be incredibly difficult to start as well!
With knowledge of these three elements we can learn how to control fire so we can light it, keep it lit, and put it out … on our own terms.
What is the Fire Triangle?
The fire triangle is a model that demonstrates the three elements required for a fire.
The three elements of the fire triangle are:
Every element of the triangle is required for a fire to start and for it to continue burning.
The Fire Tetrahedron
Sometimes we call the fire triangle a ‘fire tetrahedron’.
A tetrahedron is a 3D triangle. The extra point in the tetrahedron is the ‘chemical reaction’ (combustion) that occurs when the three elements of the fire triangle are present in enough quantity.
A chemical reaction (more specifically an “exothermic” reaction) is always taking place during a fire. The effects of this chemical reaction are the heat and light that we see as ‘fire’. The stored energy in the fuel reacts with the oxygen and heat to let off excess heat and light.
Three Elements of the Fire Triangle
Oxygen is usually required for combustion (the fire’s chemical reaction) to occur. In most cases, this is simply the oxygen in the air.
So, air in our atmosphere is a sufficient source of oxygen for fires to take off.
During large wild fires, the oxygen is sucked out of the air at an incredible rate. Firefighters often report a sense that they are unable to breathe when stuck in raging fires. This is because all the oxygen in the air has been extracted by the fire to fuel its combustion.
In very special circumstances, elements other than oxygen can be used instead of oxygen to start a fire. We call these elements ‘oxidizers’ or ‘oxidizing agents’.
Oxidizing agents can create the same chemical reaction as oxygen during a fire. That chemical reaction involves the transfer of oxygen atoms or electrons which causes the chemical reaction in fire known as combustion.
We will often use oxidizing agents when oxygen in the atmosphere is not present, such as in space. Space ships need to carry their own oxidizing agents to cause their rockets to fire.
Another example of the use of an oxidizing agent to cause fires is the use of NOS in drag racing cars. NOS (Nitrous oxide) is injected into a car’s engine to increase the concentrated amounts of oxygen present during combustion. This makes the engine’s combustion stronger and forces the pistons to move even faster than they normally would.
Here they are using NOS in the Fast and Furious movie:
Common oxidizing agents include:
- Oxygen (O2)
- Ozone (O3)
- Nitrous oxide (N2O)
- Hydrogen peroxide (H2O2)
- Sulfuric acid (H2SO4)
Heat is required to start a fire. But, once the fire has gotten started, it often generates enough of its own heat to keep the fire going without the need for more heat to be continually added.
There are three temperatures for a fuel that will demonstrate when combustion will occur:
- Flash point: The temperature at which a substance will burn when exposed to an open flame (such as a match).
- Autoignition temperature: The temperature at which a substance will burn without exposure to an open flame.
- Fire point: The temperature at which a substance will continue to burn on its own after ignition (usually a few degrees higher than the flash point).
The amount of heat needed to get something burning is called the ‘autoignition temperature‘. This is the point at which enough vapor molecules are extracted from the fuel to cause the fuel and oxygen to combust (without a fire such as a match being used). It is the point where oxygen atoms or electrons are transferred between the oxygen and fuel source and stored energy is released in the form of energy, heat and light (e.g. a fire).
The ‘flash point‘ is another temperature at which combustion may occur. However, this is the temperature at which something will burn if exposed to an open flame, like a match. It’s lower than the substance’s autoignition temperature.
The ‘fire point‘ is the heat point at which combustion will self-sustain for at least 5 seconds without the need for more heat to be added. This point is usually a few degrees above the flash point of the fuel source.
Here are the flash points and autoignition temperatures of common fuel sources:
Fuel burns when exposed to a flame at this temperature.
Fuel burns without exposure to flame (spontaneous combustion).
|Ethanol||16.6 °C (61.9 °F)||363 °C (685 °F)|
|Gasoline||−43 °C (−45 °F)||280 °C (536 °F)|
|Vegetable Oil||327 °C (621 °F)||424 °C (795 °F)|
|Jet Fuel||38 °C (100 °F)||210 °C (410 °F)|
|Diesel||52 °C (126 °F)||210 °C (410 °F)|
When we rub sticks together to light a fire, we are generating heat through friction.
Fuel is the heart and soul of a fire. The fuel is the thing that provides the stored energy that is released in the form of heat and light during combustion.
Fuels for fire are different to other forms of fuel like batteries which release electrical energy or springs which release mechanical energy. While all these different types of fuels all release stored energy, fuels for fires release stored energy in the form heat and light.
The traditional form of fuel for a fire is wood. Humans have controlled wood fuel for fires for more than 2 million years.
Fossil fuels such as coke have been used since 800 CE when they were used by Persian chemists. But it wasn’t until the Industrial Revolution in the 1700s that fossil fuels were used at a large scale. It was in this era that the stored energy in fire fuels was translated into mechanical and electrical energy through the use of steam and combustion engines.
Common fuels for fires include:
|Gas Fuels||Liquid Fuels||Solid Fuels|
Implications for Fire Control
The fire triangle is a model for firefighters to conceive of ways to extinguish fires that are out of control.
Most firefighting and fire extinguishing equipment is designed to minimize the presence of one of the three elements of a fire.
Here are some examples:
Reduces the element of fuel.
When a wild fire is out of control, firefighters are often unable to put it out. There is just too much heat, oxygen and fuel for the fire so it will keep going. We don’t have the tools to extinguish the fire.
Backburning is a strategy that involves moving ahead of a fire and removing the fuels that are in the fire’s path. Usually this will involve running controlled burns ahead of the main fire to burn away any potential fuels for the main fire. This fire is then extinguished before the main (out of control) fire arrives.
When the fire arrives, it finds it has no fuel to burn, so it stops in its tracks.
An alternative to backburning is bulldozing trees in a strip of land and carrying them away, creating a gap in the forest that the main fire cannot leap.
Reduces the element of oxygen.
Water from a fire fighting pump or sprinkler can smother a fire to reduce its access to oxygen.
Similarly, if you plunge a burning log into a river, the log will be extinguished because it has had its access to the oxygen in the atmosphere cut off.
Water can also lower the temperature of a fire, helping us to bring it under control.
Fire extinguishers that use water, also known as Air-Pressurized water Extinguishers, are not very common. This is because water is not actually all that good at putting out a fire.
Here are some issues with water extinguishers:
- Water will often simply run off a fuel source, so it does not suppress oxygen access for all that long.
- For grease fires, grease floats on top of water, so the fire will continue to burn ‘on’ the water.
- For electrical fires, water is an electrical conduit – so the water will become electrified and can cause severe electrocution of the firefighters.
3. Sand / Soil
Reduces the element of oxygen.
When you put out a campfire, you’ll often throw sand and soil over the fire. By effectively burying the fire, you’re depriving it of access to the oxygen in the atmosphere, causing it to extinguish. The fire could continue to burn for a while after it has been buried, so care must be taken to monitor the fire even after it is buried.
Reduces the element of oxygen.
The fire extinguisher you will find lying around your home will probably be an ABC fire extinguisher.
These are extinguishers that put out most types of common household fires – Class A, Class B, and Class C fires. They put out trash, wood, and paper fires (Class A), liquid and gas fires (Class B), and energized electrical fires (Class C).
An ABC fire extinguisher works by shooting out a dry chemical called monoammonium phosphate. This chemical smothers a fire and deprives it of oxygen.
5. Fire Blanket
Reduces the element of oxygen.
A fire blanket smothers a fire source. These are commonly used for small house fires. They’re made of woven glass fiber fabrics that are very fire resistant. They are also very finely woven so air doesn’t pass through them.
When the fire blanket is properly thrown over the whole of the fire, it will create a barrier between the fire and the oxygen, putting it out.
It works in a very similar way to when you put a saucepan over a candle holder to put out a candle. You can see the candle splutter for a second as it sucks up all the last little bits of oxygen under the saucer before it finally subsides.
The fire triangle is a great model for learning about what a fire needs in order for it to burn. It might be a complicated mix of different fuels that determine how strongly a fire burns, but at its basic there are only three requirements for a fire: oxygen, fuel and heat.
This model helps us think about what we need to start and extinguish a fire. All modern firefighting methods work to deprive a fire of at least one element on the fire triangle.