btain perfect combustion with the
theoretical amount of air, and an excess may be required, amounting to
sometimes double the theoretical supply, depending upon the nature of
the fuel to be burned and the method of burning it. The reason for this
is that it is impossible to bring each particle of oxygen in the air
into intimate contact with the particles in the fuel that are to be
oxidized, due not only to the dilution of the oxygen in the air by
nitrogen, but because of such factors as the irregular thickness of the
fire, the varying resistance to the passage of the air through the fire
in separate parts on account of ash, clinker, etc. Where the
difficulties of drawing air uniformly through a fuel bed are eliminated,
as in the case of burning oil fuel or gas, the air supply may be
materially less than would be required for coal. Experiment has shown
that coal will usually require 50 per cent more than the theoretical net
calculated amount of air, or about 18 pounds per pound of fuel either
under natural or forced draft, though this amount may vary widely with
the type of furnace, the nature of the coal, and the method of firing.
If less than this amount of air is supplied, the carbon burns to
monoxide instead of dioxide and its full heat value is not developed.

An excess of air is also a source of waste, as the products of
combustion will be diluted and carry off an excessive amount of heat in
the chimney gases, or the air will so lower the temperature of the
furnace gases as to delay the combustion to an extent that will cause
carbon monoxide to pass off unburned from the furnace. A sufficient
amount of carbon monoxide in the gases may cause the action known as
secondary combustion, by igniting or mingling with air after leaving the
furnace or in the flues or stack. Such secondary combustion which takes
place either within the setting after leaving the furnace or in the
flues or stack always leads to a loss of efficiency and, in some
instances, leads to overheating of the flues and stack.

Table 32 gives the theoretical amount of air required for various fuels
calculated from formula (10) assuming the analyses of the fuels given in
the table.

The process of combustion of different fuels and the effect of variation
in the air supply for their combustion is treated in detail in the
chapters dealing with the various fuels.

TABLE 32

CALCULATED THEORETICAL AMOUNT OF AIR