Chapter 5: Fire Behavior

When will Nomex burn?

At normal oxygen levels, Nomex® does not burn. When placed in an oxygen-enriched atmosphere of approximately 31 percent oxygen, however, Nomex® ignites and burns vigorously.

Conduction

Conduction is the transfer of heat through and between solids (Figure 5.12). Conduction occurs when a material is heated as a result of direct contact with a heat source. Conduction results from increased molecular motion and collisions between the molecules of a substance resulting in the transfer of energy through the sub- stance. The more closely packed the molecules of a substance are, the more readily it will conduct heat. For example, if a metal pipe is heated by a fire on one side of a wall, heat conducted through the pipe can ignite wooden framing components in the wall or nearby combustibles on the other side of the wall.

Convection

Convection is the transfer of thermal energy by the circulation or movement of a fluid (liquid or gas). In the fire environment, this usually involves transfer of heat through the movement of hot smoke and fire gases. As with all heat transfer, the flow of heat is from the hot fire gases to the cooler structural surfaces, building contents, and air.

Flaming combustion

Flaming combustion occurs when a gaseous fuel mixes with oxygen in the correct ratio and is heated to ignition temperature. Flaming combustion requires liquid or solid fuels to be vaporized or converted to the gas phase through the addition of heat. When heated, both liquid and solid fuels will emit vapors that mix with oxygen producing flames above the material's surface if the gases are ignited. The fire tetra- hedron accurately reflects the conditions required for flaming combustion. Each element of the tetrahedron must be in place for flaming combustion to occur. Removing any element of the tetrahedron interrupts the chemical chain reaction and stops flaming combustion. However, the fire may continue to smolder depending on the characteristics of the fuel.

Ambient Conditions that effect fire growth

If a window fails or a door is opened on the windward side of a structure, fire intensity and spread can increase significantly, creating a "blowtorch" effect

British thermal unit

In the customary system, the unit of measure for heat is the British thermal unit (Btu). A British thermal unit is the amount of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit. While not used in scientific and engineering texts, the Btu is still frequently used in the fire service. When comparing joules and Btu, 1055 J = 1 Btu

Non flaming combustion

Nonflaming combustion occurs when burning is localized on or near the fuel's surface where it is in contact with oxygen. Examples of nonflaming or smoldering combustion include burning charcoal or smoldering wood or fabric. The fire triangle is a simple model that can be used to illustrate the elements/conditions required for this mode of combustion.

Radiation

Radiation is the transmission of energy as an electromagnetic wave, such as light waves, radio waves, or X-rays, without an intervening medium. Radiant heat can become the dominant mode of heat transfer when the fire grows in size and can have a significant effect on the ignition of objects located some distance from the fire. Radiant heat transfer is also a significant factor in fire development and spread in compartments.

Does fire spread faster on vertical or horizontal surfaces?

Vertical

Hydrogen cyanide

a toxic and flammable substance produced in the com- bustion of materials containing nitrogen, is also commonly encountered in smoke, although at lower concentrations than CO. HCN also acts as a chemical asphyxiant

Carbon dioxide

is a product of complete combustion of organic materials. It is not toxic in the same manner as CO or HCN, but it acts as a simple asphyxiant by displacing oxygen. Carbon dioxide also acts as a respiratory stimulant, increasing respiratory rate.

Carbon monoxide

is a toxic and flammable product of the incomplete combustion of organic (carbon-containing) materials. This gas is probably the most common product of combustion encountered in structure fires. Exposure to it is frequently identified as the cause of death for both civilian and firefighter fatalities. Carbon monoxide acts as a chemical asphyxiant. It binds with hemoglobin in the blood preventing these cells from distributing oxygen to the body.

Backdraft indicators

• Building indicators — Fire confined to a single compartment or void space, building contents have a high heat release rate. • Smoke indicators — Optically dense smoke, light colored or black becoming dense gray-yellow, although the smoke color alone is not a reliable indicator. Neutral plane rising and lowering similar to a pulsing or breathing movement. • Air flow indicators — High velocity, turbulent smoke discharge, sometimes appearing to pulse or breathe. • Heat indicators — High heat, smoke-stained windows. • Flame indicators — Little or no visible flame.

Flashover indicators

• Building indicators — Flashover can occur in any building; interior configuration, fuel load, thermal properties, and ventilation will determine how rapidly it can occur • Smoke indicators — Rapidly increasing volume, turbulence, darkening color, optical density, and lowering of the hot gas level • Air flow indicators — High velocity and turbulence, bi-directional movement with smoke exiting at top of doorway and fresh air moving in at the bottom, or pulsing air movement • Heat indicators — Rapidly increasing temperature in the compartment, pyrolysis of contents or fuel packages located away from the fire, darkened windows, or hot surfaces • Flame indicators — Isolated flames in the hot gas layers or near the ceiling

Thermal Properties

• Insulation - Contains heat within the compartment causing a localized increase in the temperature and fire growth • Heat reflectivity - Increases fire spread through the transfer of radiant heat from wall surfaces to adjacent fuel sources •Retention - Maintains temperature by absorbing and releasing large amounts of heat slowly

Effects on radiant heat transfer

• Nature of the exposed surfaces — Dark materials emit and absorb heat more ef- fectively than lighter color materials; smooth or highly-polished surfaces reflect more radiant heat than rough surfaces. • Distance between the heat source and the exposed surfaces — Increasing dis- tance reduces the effect of radiant heat • Temperature difference between the heat source and exposed surfaces — Temperature difference has a major effect on heat transfer through radiation. As the temperature of the heat source increases, the radiant energy increases by a fac- tor to the fourth power

How can electric heating occur

• Resistance heating — When electric current flows through a conductor, heat is produced. Some electrical appliances, such as incandescent lamps, ranges, ovens, or portable heaters, are designed to make use of resistance heating. Other elec- trical equipment is designed to limit resistance heating under normal operating conditions. • Overcurrent or overload — When the current flowing through a conductor ex- ceeds its design limits, it may overheat and present an ignition hazard. Overcurrent or overload is unintended resistance heating. • Arcing — In general, an arc is a high-temperature luminous electric discharge across a gap or through a medium such as charred insulation. Arcs may be gener- ated when a conductor is separated (such as in an electric motor or switch) or by high voltage, static electricity, or lightning. • Sparking — When an electric arc occurs, luminous (glowing) particles can be formed and spatter away from the point of arcing. In electrical terms, sparking refers to this spatter, while an arc is the luminous electric discharge.

common elements of flashover

• Transition in fire development — Flashover represents a transition from the growth stage to the fully developed stage. • Rapidity — Although it is not an instantaneous event, flashover happens rap- idly, often in a matter of seconds, to spread complete fire involvement within the compartment. • Compartment — There must be an enclosed space such as a single room or enclosure. • Ignition of all exposed surfaces — Virtually all combustible surfaces in the en- closed space become ignited.