Combustion

For a fire to exist, you need three things: fuel, oxygen, and high temperatures (Fig. 41-1). Fires begin when supplies of fuel and oxygen are brought together at a sufficiently high temperature for combustion. The fire consumes fuel and oxygen as it burns, and gives off gases, particles, and large quantities of heat.

Oxidation is a process in which molecules of fuel are combined with molecules of oxygen. The result is a mixture of gases and the release of energy. Oxidation is how our bodies turn food into energy. Rust is the oxidation of iron. The process of combustion involves a chemical change that releases energy as heat and light. When oxygen mixes with a combustible substance rapidly and continually, you have a fire. Smoke is produced when incompletely burned particles are suspended in the air.

The best way to avoid a tragic fire loss is to prevent fires from starting. Fires need fuel, heat, and oxygen. Break that fire triangle, and a fire can't ignite. Limiting one element of the fire triangle (fuel, oxygen, or high temperature) prevents the fire from starting or puts it out. Fire suppression systems can work by covering the fuel or by displacing the oxygen with another gas, thus limiting the supply of oxygen. High temperatures can be controlled by cold water from sprinkler systems. However, the primary way that we strive to prevent and control fire in a building is by controlling the fuel: the building's structure and contents.

There are a variety of ways to ignite a fire. Chemical combustion, also known as spontaneous combustion, occurs when some chemicals within a building reach the point where they can ignite at ordinary temperatures. Chemical combustion happens when combustible ma-

Figure 41-1 The fire triangle.

terials are saturated with chemicals and rapidly produce heat. The process of chemical combustion depends on having enough oxygen to support the fire but not enough to lower the temperature. When the chemical-saturated materials are protected by their immediate surroundings, as for example oily rags in a metal garbage can in the sun, the fumes are contained while the temperature rises to reach combustible levels.

Electrical fires are usually caused by resistance heating appliances or space-heating equipment, although induction, arcing, static electricity, or other electrical processes ignite some electrical fires. Heating specialists blame most fires in which space heaters play a role on the building's wiring. Low-income housing stock, in particular, often has old, inadequate wiring, and people tend to overload the outlets they have. Extension cords running all over the house can result in a lot of problems. The situation is compounded when a space heater draws a huge amount of amperage and overloads old wiring. Fires can also start when a space heater is placed too close to combustible material like curtains or bedding. The heaters should never be left on when everyone is asleep or has left the building.

Lightning is an infrequent source of ignition, but is enormously destructive. Tall buildings and buildings in exposed locations are particularly vulnerable. A lightning strike averages 200 million V and 30,000 A. It goes through a grounded object in less than one one-thousandth of a second.

A lightning protection system safeguards a building in the event of a lightning strike by providing a continuous metallic path for the high-voltage static electrical charge into the solid ground. A typical residential lightning system consists of three parts: air terminals (lightning rods), conductors, and ground terminals. In many areas, it should be supplemented with a surge arrester installed at the electrical service panel. Manufacturers of lightning protection equipment sell only to Underwriters Laboratories (UL) listed installers.

Building codes and zoning ordinances regulate the combustibility of materials in different areas of a city, and also the conditions for storage of flammable and explosive substances in or near buildings. Building maintenance personnel must make sure that rubbish is stored safely and removed frequently. Firefighters and fire underwriters (insurers) inspect buildings periodically, looking for accumulated combustible materials. Heating devices, chimneys, electrical systems, electrical devices, and hazardous industrial processes are controlled especially tightly. Smoking is now prohibited by law in many kinds of places, including gas stations, some industrial plants, and auditoriums.

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