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Full text of "The Flow Of Gases In Furnaces"

COMBUSTION AND BOILER SETTINGS                321
was found that the computed combustion temperatures derived
from the heat capacity of the gases in the fuel bed agreed very
closely with the observed temperatures. This would appear to
indicate that a portion of the air supply, instead of passing up
through the fuel, passed up the walls of the firebox. This con-
clusion was borne out by the writer's recollections of fires which
seemed to burn faster close to the wall than they did in the center
of the grate and of clinkers occurring along the walls, and fused
to them, when the central portion of the fuel bed was free from
clinkers. This last would seem to indicate a sort of an air-blast
or oxidizing-zone effect close to the walls, strong enough to solidify
the viscous ashes.
Another indication of the possibility of the air passing up
between the wall and the fuel bed lies in the fact that the CC>2
in a number of the tests had a curve with two peaks. One was a
high peak with low CO, after which came a steady decrease with
an increase in CO, this valley being followed by an increase in
C02 (and a decrease in CO) to a low peak; then there was another
drop in the C02. The last drop in C02 only showed in a few
cases, and might be an error as it was very slight.
The tabulations on p. 322, of the results with a 12-inch fuel
bed summarize the combustion results attained:
The total weight of air required for the burning of 1 Ib of com-
bustible will vary from 11.5 to 15 Ib and will have a volume at
ordinary temperatures of from 144 to 187 cu ft. But the amount
of air that can be forced up through the fuel bed is much less.
The specific gravity of coal will average about 1.20 and a cubic
foot of coal as fired will weigh from 40 to 50 Ib. From this it will
be seen that the volume of the voids will be between 33 and 45
per cent of the mass and this will also give the proportion of the
fuel bed area through which the air must be forced. The tempera-
ture of the fuel bed and of the gases leaving it will be about
1400 C. and the gases will have an impressed velocity. A formula
might be devised to give the ash pit pressure required for burning
a given weight of fuel per unit of area, but there are so many
variables that it would be of comparatively little use. The limit
of the ash pit pressure is reached when the ascending volume of air
forced through the fuel is sufficient to keep it floating. Such an
intense draft pressure, however, would tend to carry unburned
fuel away from the grate.