from "A Reporter's Guide to Fire and the NFPA"

What is fire?
The ancient Greeks believed that fire was one the four basic elements that composed all things in the universe. In the mythology of virtually every culture, fire is a sacred substance that gives life or power. Fire is not, in fact, a substance. When you gaze at the leaping flames of a campfire, you’re observing not an object, but a process – a chemical reaction. It’s the same chemical reaction that occurs when a cut apple left on the counter turns brown, when silver tarnishes or when an iron nail rusts.

That process is oxidation: combining oxygen with another substance. The defining difference between a fire and your half-eaten apple is speed: fire is an oxidation process that happens very fast, so that light, heat and sound are released — often with enough force and majesty to justify the ancients’ reverence.The sudden release of energy causes temperatures to rise, sometimes by thousands of degrees. And it also results in smoke, the toxic waste of fire’s leftovers.

The fire triangle and the fire tetrahedron
Fire TetrahedronThe fire triangle identifies the three needed components of fire:
  • fuel (something that will burn)
  •  heat (enough to make the fuel burn)
  • and air (oxygen)

All three components must be present to have a fire. Fire will burn until one or more of the components are removed. Traditional fire extinguishing methods involve removing the fuel, heat, or oxygen.

In more recent years, a fourth component – the uninhibited chain reaction – has been added to explain fire. This chain reaction is the feedback of heat to the fuel to produce the gaseous fuel used in the flame. In other words, the chain reaction provides the heat necessary to maintain the fire. The addition of this fourth component (which forms what is called the "fire tetrahedron ") more accurately describes the mechanism for fire suppression by clean agent halon replacements which break up the uninhibited chain reaction of combustion.

Stages of fire

  • Ignition: Fuel, oxygen and heat join together in a sustained chemical reaction. At this stage, a fire extinguisher can control the fire.
  • Growth: With the initial flame as a heat source, additional fuel ignites. Convection and radiation ignite more surfaces. The size of the fire increases and the plume reaches the ceiling. Hot gases collecting at the ceiling transfer heat, allowing all fuels in a room to come closer to their ignition temperature at the same time.
  • Fully developed: Fire has spread over much if not all the available fuel; temperatures reach their peak, resulting in heat damage. Oxygen is consumed rapidly.
  • Decay (Burnout): The fire consumes available fuel, temperatures decrease, fire gets less intense.

How fire spreads
Fire spreads by transferring the heat energy from the flames in three different ways.

  • Conduction: The passage of heat energy through or within a material because of direct contact, such as a burning wastebasket heating a nearby couch, which ignites and heats the drapes hanging behind, until they too burst into flames.
  • Convection: The flow of fluid or gas from hot areas to cooler areas. The heated air is less dense, and rises, while cooler air descends. A large fire in an open area produces plume or column of hot gas and smoke high into the air. But inside a room, those rising gases encounter the ceiling. They travel horizontally along the ceiling forming a thick layer of heated air, which then moves downward.
  • Radiation: Heat traveling via electromagnetic waves, without objects or gases carrying it along. Radiated heat goes out in all directions, unnoticed until it strikes an object. Burning buildings can radiate heat to surrounding structures, sometimes even passing through glass windows and igniting objects inside.

Four ways to put out a fire

  1. Cool the burning material
  2. Exclude oxygen
  3. Remove the fuel
  4. Break the chemical reaction

Special circumstances

  • Rollover occurs when ignited fire gases, or incompletely burned fuels, rise to the ceiling, and spread out horizontally. Then smoke appears to suddenly start burning. If nothing is done to ventilate the room or cool the air, this condition leads to flashover.
  • Flashover is the sudden, simultaneous ignition of everything in a room. This is how it happens:
    • Hot gases rise to the ceiling and spread out across to the walls.
    • Heat radiates downward and intensifies until all combustible items reach their ignition temperatures and burst into flames.
    • Temperatures soar to as much as 1,000 degrees Fahrenheit in a few seconds. Even a firefighter in full protective gear is unlikely survive a flashover.
    • Firefighters are trained to recognize the signs that flashover is about to occur: dense black smoke with tightly packed curls ("black fire"); dense, black smoke that pushes out of a doorway or window opening; smoke that has accumulated as low as a doorknob, with the fire seen below.
  • Backdraft is an explosion that occurs when oxygen is introduced into a room full of hot gases.
    • A fire burning in a confined area consumes all the oxygen.
    • Visible flames disappear. Solid fuels smolder, and hot flammable gases accumulate and fill the room.
    • The temperature increases, the gases expand, and pressure builds, pulsing against doors and windows. From outside, the building may look like it is breathing or throbbing.
    • If an opening is made to admit oxygen, the hot vaporized fuel bursts into flames, and the pressurized gases explode through the opening, resulting in a rolling fireball.

Classifying fire
Fire classifications based on fuel type:

  • Class A: Ordinary combustible materials, such as wood, cloth, paper, rubber and many plastics. They burn with an ember and leave an ash. Extinguish by cooling the fuel to a temperature that is below the ignition temp. Water and other extinguishing agents are effective.
  • Class B: Flammable liquids (burn at room temperature) and combustible liquids (require heat to ignite). Petroleum greases, tars, oils, oil-based paints, solvents, lacquers, alcohols, and flammable gases. High fire hazard; water may not extinguish. Extinguish by creating a barrier between the fuel and the oxygen, such as layer of foam.
  • Class C: Fuels that would be A or B except that they involve energized electrical equipment. Special techniques and agents required to extinguish, most commonly carbon dioxide or dry chemical agents. Use of water is very dangerous because water conducts electricity.
  • Class D: Combustible metals, such as magnesium, titanium, zirconium, sodium, lithium and potassium. Most cars contain numerous such metals. Because of extremely high flame temperatures, water can break down into hydrogen and oxygen, enhancing burning or exploding. Extinguish with special powders based on sodium chloride or other salts; also clean dry sand.
  • Class K: Fires in cooking appliances that involve combustible cooking media (vegetable or animal oils and fats).