Understanding Fire: Combustion, Classification, and Extinguishing Methods

Posted on Nov 27, 2024 in Chemistry

Triangle of Fire

Combustion requires three elements: a substance that is oxidized, a substance that is reduced, and energy (activation energy, usually heat) to initiate the reaction. If any of these elements is missing, combustion will not occur.

Oxidant: Oxygen is the primary oxidizer in most combustion processes.

Fuel: Any substance capable of undergoing combustion or combining with oxygen to produce an exothermic reaction.

Heat: Energy required to start the reaction, often provided by ignition sources like overloads, short circuits, friction, stoves, or chemical reactions. Heat increases molecular agitation, raising the temperature.

Heat Transfer

Heat transfer occurs from a higher energy body to a lower energy body and can occur through:

  • Conduction: Heat transfer by direct contact or through a conductive medium.
  • Convection: Heat transfer by moving air; hot air rises.
  • Radiation: Heat transfer through space via heat rays, absorbed by air or opaque bodies.
  • Direct Flame Contact: Heat transfer by direct contact with flames, igniting flammable vapors.

Fire Classification

Fires are classified based on the fuel type:

  1. Class A: Solid fuels like wood, charcoal, fabric, and carbonaceous materials.
  2. Class B: Flammable liquids like gasoline, diesel, oils, and solids that melt into liquids like asphalt and waxes.
  3. Class C: Flammable gases like propane, butane, and natural gas.
  4. Class D: Combustible metals like magnesium, uranium, and aluminum powder. These fires are highly dangerous and difficult to extinguish.

Fire Fighting

The essential elements for fire are fuel, oxidant, activation energy, and a chain reaction. Removing any element extinguishes the fire.

  1. Fuel Elimination (Starvation): Preventing fire by removing fuel, either partially or totally.
  2. Cooling: Reducing heat below the fire point, preventing vapor production and combustion. Water is the most effective cooling agent, absorbing 540 calories when vaporizing. CO2, halon, and powders also have cooling effects.
  3. Suffocation (Oxygen Displacement): Eliminating or diluting oxygen contact with fuel. CO2 and water vapor can act as suffocating agents. Foam also suffocates by isolating fuel from the oxidizer.
  4. Chain Reaction Inhibition: Interrupting the chemical chain reaction using catalysts or other chemical means.

Extinguishing Equipment

  1. Extinguishers
    1. By Size
      • Portable: Max 20 kg (handheld) and max 30 kg (dorsal).
      • Wheeled: Larger extinguishers on wheels.
    2. By Pressurization System
      • Stored Pressure: Dry powder and halon extinguishers with a pressure gauge indicating internal pressure.
      • Cartridge Operated: The propellant gas is stored in a separate cartridge.
      • Self-Pressurized: CO2 extinguishers, which don’t require a separate propellant.
    3. By Extinguishing Agent
      • Water and Water-Based Agents
      • Foam (Chemical or Physical)
      • BC Powder (Dry Chemical) or ABC (Multipurpose)
      • Carbon Dioxide (CO2)
      • Halogenated Hydrocarbons Replacements
  2. Hydrants (Equipped)

    Fixed systems in buildings for manual firefighting. They include hoses, nozzles, valves, pressure gauges, and a protective cabinet.

    45mm Hose Reel: Hose wound on a reel, requiring two people to operate. The hose must be fully unreeled before opening the valve.

    25mm Hose Reel: Hose wound on a reel, operable by one person. Requires a minimum pressure of 2 bar and a flow of 100 l/m for one hour. Tank capacity should be 12,000 liters, or half if only one hose is used.

  3. Hydrants
  4. Dry Column
  5. Automatic Extinguishing Systems