When air is supplied to a fuel, the temperature must be high enough or ignition will not take place and burning will not be sustained. Nothing will burn until it is in a gaseous state.

For example, the wax of a candle cannot be ignited directly; the wick, heated by the flame of a match, draws up a little of the melted wax by capillary action until it can be vaporized and ignited. Fuels that liquefy on heating usually will melt at a temperature below that at which they ignite.

Solid fuels must be heated to a temperature at which the top layers will gasify before they will burn. The ignition temperatures of fuels depend on their compositions; since the greatest part is carbon, the temperature given in the table, 870°F, will not be far wrong.

Heat must be given to the fuel to raise it to the temperature of combustion. If there is moisture in the fuel, more heat must be supplied before it will ignite, since practically all of the moisture must be evaporated and driven out before the fuel will burn.

Temperature may be measured by its effect in expanding and contracting some material, and is usually measured in degrees. The mercury thermometer is a familiar instrument in which a column of mercury is enclosed in a sealed glass tube and its expansion and contraction measured on an accompanying scale.

Two such scales are in common use, the Fahrenheit (F) and the Centigrade or Celsius (C). The former has the number 32 at the freezing point of water and 212 at the boiling point; thus 180 divisions, or degrees, separate the freezing and boiling points or temperatures of water.

The latter has the number zero (0) at the freezing point of water and 100 at the boiling point; thus 100 at the boiling points thus 100 divisions or degrees separate the freezing and boiling points or temperatures of water. Both scales may be extended above the boiling points and below the freezing points of water.

Other instruments may employ other liquids, gases, or metals, registering their expansion and contraction in degrees similar to those for mercury.

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