The function of the boiler furnace is to convert into heat all the latent chemical energy of the fuel. External heat is applied to the fuel to cause its ignition initially; subsequently the heat is generally supplied by the furnace walls and, in the case of coal, from the bed of glowing fuel.

While combustion is taking place, if the temperature of the elements is lowered, by whatever means, below that of ignition, combustion will become imperfect or cease. Gases developed in a furnace passing too quickly among the tubes of a boiler may be similarly chilled and thus combustion be stopped, causing a waste of fuel and production of large deposits of soot.

Part of the heat developed in the furnace goes from the fuel bed or flame directly into the metal of the tubes by radiation. The rest of the heat raises the temperature of the gases resulting from the combustion—carbon dioxide, nitrogen and water.

These gases pass among the tubes transmitting their heat through the tube walls to the water and steam. Thus the gases are cooled and, since they cannot leave the boiler at a lower temperature than that of the water and steam in the tubes, the amount of heat which can be released by the gases is directly dependent on the temperature of the gases when they enter among the tubes.

It is important, therefore, that the gases be raised to as high a temperature as possible in the furnace. Hence, every factor affecting this temperature should be considered carefully. The maximum temperature attained depends on compromises:

1. Excess air is required to achieve complete combustion of the fuel, but as more excess air is supplied, the temperature tends to decrease; if the amount of excess air is decreased to too low a point, the amount of heat liberated will be decreased since incomplete combustion results.

2. So much heat can be generated even with the lowest possible excess air that the temperatures reached may breakdown the enclosing refractory brick of the furnace. The absorption of heat by the enclosing brick together with a large area of water-cooled surface exposed to the heat, may lower the temperature of the furnace and result in poor efficiency.

3. Since the quantity of heat radiated from a burning fuel is dependent upon the duration as well as the temperature, the temperature of the fire will increase as the rate of combustion increases (the relation of fuel to air remaining constant).

Rates of combustion of fuel must be matched by appropriate amounts of excess air so as not to produce excessively high temperatures that may cause rapid deterioration of the refractory brick of the furnace. To protect the brickwork, temperatures are held down by water screens surrounding the furnace, or by water circulated in piping behind or within the brickwork.

This permits higher temperatures of combustion to exist with greater heat absorption by the boiler heating surfaces (carrying the water to be turned into steam) both by radiation and by convection from the hotter gases emanating from the fire. The result is greater boiler efficiency.

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