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Full text of "The Flow Of Gases In Furnaces"

330                                   APPENDIX IX
result in its decomposition, the formation of carbon and CO2,
which is an exothermic reaction releasing 38.8 calories per molecular
volume of CC>2 formed, and the deposition of soot on the tubes.
Another source of soot lies in the hydrocarbons. The high firebox
temperature has a tendency to break these up, forming soot, CO
and hydrogen (Efo). The luminosity of the flame in the firebox
is due to the heating of this soot to incandescence, and unless this
floating carbon is burned in a properly designed combustion
chamber a considerable portion of it will be carried to and deposited
on the tubes.
Combustion, like all chemical reactions, follows the law of
mass action and the laws of chemical equilibrium. The velocity
of the reaction is affected by the temperature of the reacting
masses, by the mixing of the combustibles and the comburent
and by the dilution with inert gas. The velocity of the reaction
increases very rapidly with the temperature. Tor example, a
mixture of hydrogen and oxygen at 200 will require months to
combine, while at 2200 the explosion wave indicates that combina-
tion occurs in one ten-millionth of a second. The ratio between
the two velocities of the reaction is that of 1 to 1014. Increases
of pressure increase the velocity of the reaction. This factor,
while important in internal combustion motors, does not amount
to very much at the slight differences of pressure that exist in
furnace fireboxes used in industrial heating. Dilution with inert
gases slows down the reaction velocity by reason of the increase
of the mass. The mixing has a considerable effect on the velocity
of the reaction as may be readily verified by observing the flame
of a Bunsen burner while altering the air supply. The slower the
reaction velocity the longer the flame length and the distance
the gases travel in burning. The temperature will be lower with
a long flame than ifc will be with a short flame.
In a boiler it is absolutely necessary that combustion should
be completed before the water-cooled surfaces are reached, as
complete combustion will result in the practical elimination of the
soot trouble, with a resultant increase in the efficiency of the heat-
ing surface. Most boilers and settings, however, are so designed
that the incandescent gases are shot up against a refrigerating
surface before combustion can be completed. The so-called
combustion arches and the baffling being so arranged that this
refrigeration will be promoted, instead of combustion.