FIRE

From the prehistorical era, fire has generated the energy that allowed human beings to warm themselves and their surroundings, illuminate the darkness, prepare food, and create artifacts with both utilitarian and aesthetic value. More recently, fire has powered transportation and manufacturing and served as an object of and means for research.

These dual aspects of fire are represented in its deeply symbolic character. From the earliest periods fire has served as a symbol for moral and intellectual achievement. Many religions use fire in ceremonies, as in candles and funeral pyres. Fire is also common in celebrations, such as birthday candles and fireworks.

Although fire is indispensable to human beings and civilization, it also can kill and destroy or be used as a means for intentional destruction and warfare. The great library at Alexandria was destroyed by fire in 47 b.c.e. and the Chicago fire of October 1871 forced the city to rebuild itself. The World War II firebombings of Dresden and Tokyo were more destructive than the atomic bombings of Hiroshima and Nagasaki.

The Science of Fire

Chemically, fire is an exothermic reaction involving the rapid oxidation of fuel. For example, the burning of red oak could be approximated as follows: This reaction liberates approximately 12.7 megajoules of energy for each kilogram of red oak burned. (One joule of energy is equal to one watt of power generated for one second; a megajoule represents one million joules.) This liberated energy raises the temperature of the reaction products and emits thermal radiation. Most fires occur in a normal air atmosphere, which is approximately 21 percent oxygen and 78 percent nitrogen. Thus, the burning of red oak in air is written more correctly as follows: Because the nitrogen in the air plays no role in the reaction, the temperatures of flames in air are lower than the temperatures in pure oxygen; some of the energy that is liberated by the fire heats the ambient nitrogen.

Fires can be characterized as two different types of flaming: premixed and diffusion. Premixed flaming occurs when fuel and air are mixed before combustion; a diffusion flame exists when burning occurs as fuel and air are being mixed. Both types can be illustrated with a standard laboratory Bunsen burner. Typically, when a Bunsen burner is used, a blue flame is desired. That flame is premixed because air is entrained into the fuel stream through openings in the burner before the flame zone is established. However, if the openings where air is entrained into the burner are closed, the flame loses its regular shape and changes color from blue to yellow. This is a diffusion flame, where the gas feeding the burner must mix with the surrounding air in the area of flaming.

In general, all fires involve the combustion of gaseous fuel. If the fuel is a gas, such as the fuel that feeds a Bunsen burner, it only needs to mix with air for burning to occur. If the fuel is a liquid, it must be heated sufficiently to release vapors. The temperature at which a liquid fuel releases sufficient vapors for combustion to occur is called its flashpoint, and it occurs at a temperature lower than the boiling point of the liquid.

Solid fuels similarly must be heated to a temperature at which sufficient vapors are released to support combustion; however, solid fuels do not necessarily vaporize as liquids do. Some solid fuels melt and then subsequently vaporize before combustion. Others, such as wood, do not melt before releasing combustible vapors. Upon heating, these solids decompose into simpler compounds that are distinct from the original material in a process called pyrolization. The temperature at which a solid fuel releases sufficient vapors for combustion is called its ignition temperature.

Ignition of a fire requires the introduction of energy. For a gaseous fuel or a liquid fuel that is at a temperature above its flashpoint, a spark or small flame may be sufficient to provide that energy. For solid fuels or liquid fuels that are at a temperature below their flashpoint, the fuel first must be heated to a temperature at or above its flashpoint or ignition temperature.

Once a fuel is ignited, the heat liberated form the fire can transfer back to the fuel, causing the fire to sustain itself or grow. However, if the energy feedback to the fuel is not sufficient, the fire will decay and eventually go out. This can be illustrated by looking at logs in a fireplace. If there is only one wood log in a fireplace, so much of the energy liberated by burning the log is lost to the environment that the fire will go out. However, if more logs are added, some of the energy that would have been lost is transferred to the other logs, and the fire will be sustained.

Fire and Technology

Humans are the only creatures that have the ability to control and harness fire. Fire has been indispensable to technological progress. It is no accident that Prometheus is said to have stolen fire from the gods to make it possible for human beings to live. Early humanity learned how to start fires that could serve very simple uses. As humanity evolved, the heat generated by fire was used for more complex tasks, such as hardening clay and molding metals. Later, the energy liberated by fires created steam to power moving equipment.

As the understanding of fire increased, so did the efficiency with which it was used to generate energy. Instead of using fires to heat water to create steam, fires could be ignited under controlled conditions in cylinders, allowing the potential energy in the fuel to be converted more directly to kinetic energy. It is through these types of processes that fire can be used to power generators that create electricity or to create mechanical energy to power airplanes and boats directly.

However, just as fire can serve benign purposes, it can be used in more destructive tasks. Fire applied to people or their environments either intentionally or unintentionally can cause death, injury, and the loss of property. Entire cities have been lost to fire; although the rate of death, injury, and destruction caused by fire has decreased steadily, fire continues to take a serious toll. It is likely this dichotomy of purpose that prompted fire as a symbol of attractive self-destruction, as when the moth flies into the flame.

The Ethics of Fire

The technological advances that have been made possible by the ability to harness fire also have created risks to society. As the Industrial Revolution created an environment in which manufacturing facilities were placed closely together, the flammability of the items inside those facilities, coupled with the closeness of buildings, allowed for fires that could destroy entire cities. This caused society to look for ways to protect people and the community from the hazards of fire.

As a result of fires that caused the loss of whole cities, people began to look for means of limiting the effects of a fire to a single building. This was accomplished by controlling the materials from which buildings were constructed, the spacing of buildings, and the types of openings, such as windows, installed in buildings.

As people learned ways to limit the impact of a fire to a single building, the goal of fire safety changed to limiting that impact to a single portion of a building. As methods of protection against fire improved, the maximum tolerable effects of fire became smaller.

Society has devised a number of ways to prevent fire that can be divided into two broad areas: prevention of fire ignition and management of fire impact. By controlling the methods of creating and distributing energy, it is possible to make it less likely that a fire will be ignited. Examples of means to prevent fire ignition include the electrical protections that typically are required in a home or business: the use of minimum wire sizes, electrical insulation on wires, and the use of fuses or circuit breakers.

Management of fire impact can involve means of managing a fire once it begins or ways to manage the things that are intended to be protected from fire. Fires can be managed by controlling the fire combustion process, suppressing fires, and controlling fires by means of the types of construction used. Examples of these means include controlling the type of fuel present, the use of fire suppression systems such as automatic sprinkler systems, and the use of building materials that resist the spread of fires, such as fire walls and doors.

In light of the fact that almost all human pursuits create fire risk, society has an obligation to ensure that the risks that are created are controlled. Developing better means of fire protection requires the development of a better understanding of fire and of the way fire affects buildings, people, and property. To this end there is a branch of science that is dedicated to the study of fire. Fire science involves scientific study of how fires start, how they grow, how they can be extinguished or suppressed, and the amount of heat and chemical compounds that are created when fires occur. Fire scientists also create models, or methods of simulating, fires. Fire models range in complexity from sophisticated computer programs to relatively simple equations that can be solved with a calculator.

Similarly, there is a branch of engineering that is dedicated to the application of scientific principles to protect people, property, and the environment. Fire protection engineers apply the scientific understanding of fire to reduce the risks of fire to reduce the likelihood of unwanted fires and manage the impact to society when unwanted fires occur.

MORGAN J. HURLEY

SEE ALSO Air;Earth;Environmental Ethics;Water.

BIBLIOGRAPHY

Bachelard, Gaston. (1964 [1938]). The Psychoanalysis of Fire, trans. Alan C. M. Ross. Boston: Beacon.

Drysdale, Dougal. (2002). "Chemistry of Physics and Fire." In Fire Protection Handbook, ed. by Arthur E. Cote. Quincy, MA: National Fire Protection Association.

Lyons, John. (1995). Fire. New York: Scientific American Books. Provides an overview of the hazards that fire pose to society, the science of fire, and the methods that are used to combat unwanted fire.

National Fire Protection Association Publication 550. (2002). Guide to the Fire Safety Concepts Tree. Quincy, MA: National Fire Protection Association. The fire safety concepts tree provides a method of communicating fire safety concepts to people who do not have specialized knowledge of fire protection.

Pyne, Stephen J. (2001). Fire: A Brief History. Seattle: University of Washington Press.

Tewarson, Archibald. (2002). "Generation of Heat and Chemical Compounds in Fires." In SFPE Handbook of Fire Protection Engineering. Quincy, MA: National Fire Protection Association.