SOURCES OF HEAT ENERGY

SOURCES OF HEAT ENERGY

The third factor necessary for the initiation of the reaction between a fuel and anoxidant is a source of heat energy. Only in some exceptionally rare circumstances will the sole contact between the fuel and oxygen lead to a spontaneous combustion. In the great majority of cases, an outside source of heat energy is necessary for the initiation of the combustion. The source of heat energy often behaves as a catalyst, especially in the piloted ignition. Its presence is necessary for the promotion of the combustion reaction, but only up to the point where the reaction becomes self-sustaining.

There are several distinct means of generating heat energy in the quantities and duration required for the initiation of fire. They may conveniently be divided into three broad categories, namely electrical, chemical and mechanical sources of heat energy (Hartzell G.E, 2008). Each of these is further described below.

1. Electrical Heat Energy

Electricity is repeatedly quoted as a major cause of many fires. Fire statistics regularly show electricity to be responsible for between 20% and 40% of all fires of known origin (Purser D, 2009). Although formally it is true that electrical heat energy does cause a significant number of fires, it is also undisputable that fires will seldom be initiated by the electricity if the electrical installations and equipment’s are made, used and maintained in accordance with the corresponding codes and standards.

Electrical heat energy is always generated when an electric conductor carries a current (Kealy et al, 2003). From classical physics, it is known that the greater the current and resistance, and the longer the current flows through the conductor, the greater the heat energy liberated. For safe use of electrical conductors, electrical codes specify the maximum current which will not overheat them.

The resistance of conductors is determined by the kind of material employed. Materials which are good conductors, such as copper, silver and aluminium are preferred and generally used. Finally, the time the current takes to pass through the conductor is determined by the intended use of electrical appliances, unless the appliance is left on unintentionally, causing an accidental fire.

2. Chemical Heat Energy

Heat energy necessary for the initiation of fires may be produced by several types of chemical reactions. Among them of primary concern to fire protection are combustion reactions. These reactions when brought to a completion release aconsiderable amount of heat known as the heat of combustion (Kobes, 2009). Examples of common sources of heat energy formed by combustion reactions are open flames, a lighted cigarette and acetylene torch used for cutting and welding, a hot fuel- fired boiler, a drier or a furnace surface. Other types of chemical reactions which release heat are spontaneous heating (self-heating) and spontaneous ignition (self- ignition)

3. Mechanical Heat Energy

It is well known that some forms of mechanical energy can be converted to heat energy (Margrethe et al, 2009). The greater the mechanical work, the greater the amount of heat released. The mechanical heat energy, formed as result of mechanical failure or malfunction of equipment, is the source of ignition for a significant number of fires (5 – 10%) (Krasny, 2001). Furthermore, when two moving solids are in contact, the resistance to relative motion is manifested in the form of heat due to friction. If heat due to friction is not dissipated as rapidly as it is formed, there will be overheating which may ignite the combustible material.