Factors that affect to the fire spread

When a fire occurs, there are a number of conditions that can affect the spread and therefore we must know them. The heat generated in a fire can be transmitted in 3 ways:

  • Convection: the transmission of heat occurs through the movement of smoke, gases, air and hot particles. Smoke and hot gases rise. The air, close to the fire, heats and also rises, and then it transports particles away from the fire. As these gases and solids warm up, the colder air turns to fire. This generates streams that accelerate the convection process, which, in turn, it accelerates as combustion speed increases.
  • Conduction: It is the transmission of heat through a solid. Metallic objects, such as beams, columns, pipes, nails and cables are excellent conductors of heat.
  • Radiation: The heat transmitted by radiation does not require a material medium as a vehicle. All materials radiate heat energy in the form of electromagnetic waves. When this energy hits another body, it can be partially reflected, transmitted or absorbed. The energy absorbed is that manifests itself in the form of heat in the body.

In the first phase of a fire, heat is transmitted by radiation and only a small part of the heat is transmitted by convection. This phase is mainly applied to the contents of the site or room where the fire has been originated. As the fire develops, the heat transmitted by the radiation decreases and the heat transferred by convection rises. Only a small part of the heat is transmitted by conduction between the metallic elements present in the fire place.

Once the full development phase of the fire has been reached, the process of spreading the fire to the rest of the building begins. The fire usually is propagated by the action of convection and thermal radiation mechanisms caused by the flames, also by contact or direct radiation to other products or construction elements.

On one hand, the fire and smokes movement during a fire depends on mainly of the geometry of the building and the potential routes that constitute elements such as: doors and windows, ventilation ducts, cavities between the building elements, stairwells, hidden spaces on suspended ceilings, etc

On the other hand, so that the fire spreads and evolves, heat, fuel and oxygen must be available and the chain reaction must be produced, that is, the tetrahedron of fire. In the absence of any of these elements the fire will be extinguished.

Therefore and considering the information explained above, the factors that may affect the spread will be the following:

  • Oxygen/fuel concentration.
  • Specific surface exposed to fire attack.
  • Environment conditions and geometry of the scenario.
  • Weather conditions.
  • Size, number and distribution of the openings.
  • Thermal properties of the enclosure.
  • Size, composition and location of fuel sources that it could be ignited in the first place.
  • Availability and location of additional fuel sources.
  • Correct operation of the active and passive protection measures.

A fire is a very complex phenomenon that it has been affected by a large set of factors. In fact, fires cannot be simulated, due to the large number of parameters that influence a fire. For this reason, a perfect simulation program has not yet been found.

Antonio Galán Penalva

Fire Safety Consultant

Diseño de Instalaciones de Protección contra Incendios (PCI)