The elements of fire are the basic factors that let combustion happen and a fire take hold. They are usually explained as the fire triangle, which becomes the fire tetrahedron once a fourth factor is added.
What are the elements of fire?
Fire has four basic elements: fuel, heat, oxygen and a chemical chain reaction. The general principle of putting a fire out is to take at least one of them away from the combustion process, by cooling it, cutting off the oxygen, removing the fuel, or interrupting the chain reaction.
1. Fuel
Any material or substance that can catch fire. It comes in three states:
Solid: wood, paper, cloth, plastic, rubber
Liquid: gasoline, diesel, alcohol, thinner
Gas: liquefied petroleum gas (LPG), natural gas (NGV/CNG), hydrogen
2. Oxygen
The oxygen in the air around us. Air normally holds about 21% oxygen, which is enough for combustion. Below roughly 16%, most fires cannot keep burning.
3. Heat
A temperature high enough to ignite the fuel, from an electrical spark, an open flame, friction, sunlight, or electrical equipment running too hot.
4. Chemical chain reaction
The fourth factor, and the one that keeps a fire going. When fuel is heated and mixes with oxygen, free radicals form and feed heat back into the remaining fuel. Without that reaction, a fire would go out on its own very quickly.
How fires are put out by disrupting the elements
The principle is simple: take one of the elements away.
Starvation, removing the fuel: moving combustible material away from the fire, or shutting off a gas valve.
Smothering, cutting off the oxygen: covering the fire with a fire blanket, using foam, or discharging CO2 to displace the air.
Cooling, removing the heat: lowering the temperature, by spraying water to draw heat out of the fuel for instance.
Interruption, breaking the chemical reaction: using an agent such as a clean agent or dry chemical powder to inhibit the free radicals driving the combustion.
How the elements of fire shape the choice of agent
The elements of fire decide which agent you choose, because putting a fire out means disrupting at least one element of the fire tetrahedron: fuel, oxygen, heat, or the chain reaction. Each type of agent is built to attack a different one.
1. Cooling, removing heat
Draws heat out of the fuel until its temperature drops below the ignition point.
Agent used: water, or stored pressure water.
How it relates: suits ordinary solid combustibles (Class A) such as wood, paper and cloth, which need deep cooling. Do not use it on fires involving oil or electricity, though, because water conducts electricity and makes oil splatter.
2. Smothering, cutting off oxygen
Blankets the fuel so air cannot reach it, or dilutes the oxygen in the area below 16%.
Agents used:
Foam: floats on and covers the surface of flammable liquids (Class B), keeping oxygen away from the fuel vapors.
Carbon dioxide (CO2): displaces the oxygen around the fire. Suits Class B and Class C fires, meaning electrical equipment.
3. Interrupting the chemical chain reaction
Delivers a chemical that inhibits the free radicals produced by combustion, so the fire cannot keep burning even with oxygen and heat still present.
Agents used:
Dry chemical (ABC/BC): the powder disrupts the chemical reaction and forms a film that blankets the fuel.
Clean agents: FK-5-1-12, HFC-227ea (FM-200), HFC-236fa or Halotron. These put a fire out quickly by interrupting the chain reaction and drawing heat out of the flame at the same time, and they leave no residue, so they can be used on electrical equipment that is still running.
4. Fuel-specific reactions
Some fuels have properties that mean the general methods will not work.
Class K fires, cooking oils: use wet chemical, which reacts with vegetable and animal oils to form a soapy foam layer, a process called saponification, that covers the surface to cut off oxygen and reduce heat at the same time.
Class D fires, combustible metals: use Class D powder, sodium chloride or graphite based, to create an insulating layer over the extremely hot metal without reacting violently with it.
Examples of risk in different areas
1. Server rooms, data centers and electrical control rooms
Primary fuel: cables, electronic devices, circuit boards, plastic wire jackets. Class C and Class A fires.
Heat sources: short circuits, equipment overheating under heavy workloads, electrical power surges.
Key risks:
Fires that start quietly in cabling or a server rack and spread fast.
Damage from the extinguishing agent. Using water or dry chemical powder here damages electronic equipment for good, and the loss can run higher than the damage the fire itself caused.
2. Factories and warehouses
Primary fuel: wooden pallets, cardboard boxes, plastic packaging, chemicals, machine oils. Class A, Class B and Class D fires.
Heat sources: hot work such as metal welding and cutting, machines overheating from continuous operation, combustible dust.
Key risks:
Large quantities of fuel across vast areas, which makes fires spread rapidly and hard to control.
Toxic smoke, and heat building up under a high ceiling.
3. Commercial kitchens
Primary fuel: vegetable oil, and animal fats accumulated in exhaust ducts. Class K and Class B fires.
Heat sources: gas stoves, open burners and deep fryers.
Key risks:
Auto-ignition. Cooking oil can ignite on its own above roughly 340°C, without an open flame.
The wrong suppression method. Spraying water into a deep fryer throws up an immediate fireball.
4. Offices and residential buildings
Primary fuel: paper documents, furniture, carpets, curtains, electrical appliances. Class A and Class C fires.
Heat sources: overloaded power strips, old or damaged wiring, batteries left charging unattended.
Key risks:
Evacuation held up by the number of people present, or by fire exits that are locked or blocked.
Toxic smoke from carbon monoxide and burning plastic.
What to know before choosing an agent
Before choosing an extinguisher or a suppression system, ask these questions:
What is the primary type of fuel in the area? A, B, C, D or K.
Is the equipment in the area sensitive to damage? If there are computers or electrical cabinets, look at a clean agent, or CO2 where the space and the people in it allow for it.
Is the space enclosed or open? Avoid light gases such as CO2 in open areas, where the wind will carry them away.
How experienced are the people who would use it? The extinguisher has to be easy to operate and safe for whoever is holding it.
What to read next
Clean Agent Fire Extinguishers
For anyone choosing an extinguisher for electrical equipment, or for areas where chemical powder residue is unwanted.What is PAF-222
PAF GLOBAL's clean agent fire extinguishing products.What is FK5112 / What is FK-5-1-12
The agent names and technical terms used for clean agents.Clean Agent vs CO2
Comparing clean agent extinguishers with CO2 extinguishers.Clean Agent vs Dry Chemical
How clean agent and dry chemical extinguishers differ.Fire Extinguishers for Data Centers / Server Rooms / Electrical Rooms
For organizations choosing extinguishers or systems for areas holding critical equipment.Clean Agent Fire Suppression Systems
For anyone looking at fixed installations rather than portable extinguishers alone.Contact for Consultation
To ask PAF GLOBAL for more information.
Frequently Asked Questions (FAQ)
1. What are the elements of fire?
The basic elements are fuel, heat and oxygen. Explained as a fire tetrahedron, a chemical chain reaction is added as the fourth.
2. Why do you have to understand the elements of fire before choosing an extinguisher?
Because each type of extinguisher puts a fire out by a different mechanism and suits a different class of fire. Choose the wrong one and it either does not work, or it adds to the damage to the equipment and the area.
3. What type of extinguisher should be chosen for electrical areas?
Electrical areas need extinguishers or systems that suit electrical equipment and keep residue to a minimum, PAF-222 Clean Agent FK-5-1-12 for instance.