Fire Basics

Physics and Chemistry of Fire

Fire Triangle

There are three main components to the fire triangle as shown in Figure 1 below. These components are fuel, heat, and oxygen. Without at least these three components, fire cannot exist.

Matter States

Matter exists in three basic states: solid, liquid, or gas. A substance experiences a phase change when the physical characteristics of that substance change from one state to another state. Perhaps the most recognizable examples of phase changes are those changes from a solid to a liquid or a liquid to a gas. When a substance goes through a phase change, there is a change in the internal energy of the substance but not the temperature of the substance (Serway, et al. 611).

Combustion

Combustion is a rapid, continuous reaction that usually takes place in the gas phase. Wood is an organic compound primarily comprised of cellulose. For wood, the phase change from a solid to gas is almost instantaneous as combustion occurs (Coleman, et al. 95). Ignition occurs when an outside source is no longer needed to sustain combustion (Coleman, et al. 87).

Heat

Heat is a physical characteristic of energy that is produced as energy changes form. It is characterized by the transfer of energy resulting from a difference in temperature between a system and its surroundings (Serway, et al. 605). Energy cannot be created or destroyed; it only changes form. There are several sources of heat. These sources are chemical, mechanical, electrical, and nuclear.

Chemical

The most common source of heat for the purpose of firefighting comes from chemical reactions. When something burns, it is the result of a chemical reaction where heat is being released. Pyrolysis is the "decomposition or transformation of a compound caused by heat."(Coleman, et al. 81)



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Mechanical

The mechanical source of heat deals the heat that is created from friction. Friction is a resistive force that occurs from the rubbing or compression of materials. For firefighters, this source of fire is seen mostly in industrial machinery (Coleman, et al. 81).

Electrical

An electric current exists when there is a net flow of charge through a given area. Electrons flow from the negatively charged region to the positively charged region. This is usually accomplished by using an electrical conductor which allows the electrons to pass freely through the material (Serway, et al. 709). This flow of electrons can generate heat when collisions between the molecules and electrons occur. These collisions then cause the molecules to break apart thus releasing heat. Some common examples of electrical energy are lightning, static electricity, and wiring outlets (Coleman, et al. 82). For the wildland firefighter, lightning is probably the most common of electrical sources of fire, if not one of the most dreaded during an extreme fire season.



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Nuclear

Probably the least common source firefighters will encounter is nuclear energy. Nuclear energy deals with the breakdown of radioactive materials. These reactions take place in a confined environment where the release of heat or energy from the process can be precisely controlled. In the event of a nuclear accident, little can be done by firefights on scene. Rather efforts to evacuate the surrounding area and minimize the exposed are pursued while trained technicians handle the on-site emergency (Coleman, et al. 83).

Modes of Heat Transfer

Conduction

The transfer of heat through an object or material is referred to as conduction (Coleman, et al. 92). Conduction will only occur if a difference in temperature exists between the two sides of the conducting material (Serway, et al. 623). Heat will travel from the hot source to the cold source.

Convection

Convection refers to the "energy transferred by the movement of a warm substance (Serway, et al. 628)." As air is heated by a fire, it becomes less dense and begins to rise above the cooler air surrounding it which is more dense. This cooler air then begins to sink. This process will continue until a new, equilibrium temperature has been reached. Convection resulting from the changes of air density, such as above, is called natural convection (Serway, et al. 628). An excellent example of natural convection is shown in the smoke plume below.



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Radiation

Radiation is the continuous transmission of electromagnetic waves which are caused by thermal variations between molecules. Every object radiates energy in this manner. Stefan's Law, shown below in equation form, is a method used to determine the rate at which this radiation occurs (Serway, et al. 628).

P=(5.669x10^-8 W/m²*K⁴)AeT⁴

Where:
P = Power
A = Surface Area
e = emissivity
T = Surface Temperature in Kelvin

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