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.

Furnace Volume
The furnace portion of the boiler and its cubical volume include that portion between the heat sources (grates for coal, jets for pulverized coal, oil and gas) and the fi rst place of entry into or between the first bank of boiler tubes.

The most suitable furnace volume of a boiler is largely influenced by the following:

1. Kind of fuel;
2. Rate of combustion;
3. Excess air; and,
4. Method of air admission.

Kind of Fuel
Anthracite coal needs no special provision for a combustion volume until the boiler is forced to high ratings. The fixed carbon, which comprises a large percentage of the coal, is burned near or on the grate. The carbon monoxide gas rising from the fuel bed requires some combustion space to mix with the air thoroughly, but not much volume in comparison with the highly volatile ingredients found in oil and gas.

Bituminous coal is high in volatile content and requires considerable furnace volume because a large portion of the volatile combustibles must be burned above the fuel bed. In burning high volatile coal, the distillation of the volatile matter takes place at a comparatively low temperature.

This condition is favorable to the light hydrocarbons which are more readily oxidized than the heavier compounds which distill off at higher temperatures. Slow and gradual heating of the coal is necessary to bring about the desired results. Once the volatile matter starts distilling off, it must be completely oxidized by the proper amount of air in order to approach smokeless combustion.

In burning pulverized coal, oil or gaseous substances, the important factors are the volume of fuel to be burned, the length of the flame travel, and turbulence. In general, the greater the percentage of volatile matter present in fuel, the larger must be the combustion space but these two factors are not directly proportional.

Rate of Combustion
When a boiler is operating at a low rating, the fuel and air have quite a period in which to mix and burn completely. As the rating is increased, both the fuel and air are increased. A proportion of the uniting of oxygen and carbon monoxide takes place in the furnace chamber.

If the boiler rating is increased to such an extent that the mixing has too short a duration in the furnace volume, the gases will enter the tube area and ignite, producing what is known as secondary combustion, unless they have been cooled below ignition temperatures by the ex- change to the tubes. The higher rating expected from a boiler, therefore, the larger should be the combustion space.

Excess Air
To produce effi cient combustion each certain grade of fuel requires a defi nite amount of air to unite with a pound of the fuel. The amount of air varies according to the ingredients of the fuel. The fur- nace volume must be such that the air required has sufficient time to unite with the fuel as well as to take care of the expanded gas at the furnace temperature.

Method of Admission
The method of air admission is dependent upon the air required which, in turn, depends upon the kind of fuel. The furnace volume for pulverized coal, oil or gas is large in comparison with a stoker installation not only to admit additional air properly but to provide for the long flames.

The different ducts required for air admission necessarily have an effect on the shape of the furnace volume.

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