One of the basic concepts taught in both basic firefighter training and fire investigation training is the theory of heat transfer. This article will address the three most common methods of heat transfer.
Why is heat transfer so important to understand? It helps us to understand how a fire may have spread from one area of a structure to another through no readily visible path. It also indicates how a fire may start in one building and spread to another, again with no direct contact being made by flame.
Heat transfer is defined by NFPA 921 Guide to Fire and Explosion Investigation as "the transport of heat energy from one point to another caused by a temperature difference between those points."(1)
There are three basic methods of heat transfer: Conduction, Convection, and Radiation.
"Conduction is the transfer of heat within a solid mass traveling from the hotter end to the cooler end."
An example would be a metal I-Beam or truss located in a room and passing through a wall to another room that is being impinged upon by a fire. As the beam or truss heats, the heat starts to pass down the I-Beam or truss to the cooler end. The end opposite of the fire heats up and if it reaches the auto ignition temperature of combustible materials close to it a second fire will start. The subsequent second fire may lead one to identify two separate points of origin.
"Convection is the transfer of heat through the movement of heated gases or liquids." "(2)
An example of convection is using a hair dryer to dry your hair. Air currents that are blown out of the hairdryer are transferring heat.
Convection plays a significant role in fire investigation because it is one of the initial means for fire to spread by the hot or super heated gases and products of combustion spreading out into the upper portions of the room or area of origin.
This is why we see soot and other products of combustion spread throughout a structure and even in rooms that are a significant distance away and have no direct flame impingement.
"Radiation is the transfer of heat energy from a hot surface material to a cooler material by electromagnetic waves without the need of an intervening medium. This method of heat transfer is by direct sight only and any intervening material or obstacle will reduce or block its effects.""(3)
A good example of radiation heat transfer is the "heat from the sun being radiated to the earth through a vacuum." Another example of radiant heating is the heat that you feel that is being radiated from a fireplace when you stand in front of it.
In conclusion, imagine a fire originating on a stove. The burner heats a frying pan through conductive heating, which is also heating the oil, located in the frying pan. As the oil reaches its ignition temperature and begins flaming combustion, it produces gases, which start to spread throughout the area through convective currents. As the oil ignites and produces flaming combustion, the fire will radiate heat to other combustible materials in the area, which subsequently reach their ignition temperature and in turn burst into flames.
1. National Fire Protection Association, NFPA 921 Guide to Fire and Explosion Investigations, NFPA, Quincy, MA, 2004 edition, Page 17, Section 5.2.1
2. National Fire Protection Association, NFPA 921 Guide to Fire and Explosion Investigations, NFPA, Quincy, MA, 2004, Page 17, Section 5.2.3
3. National Fire Protection Association, NFPA 921 Guide to Fire and Explosion Investigations, NFPA, Quincy, MA. 2004, Page 18, Section 5.2.4