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Full text of "The Flow Of Gases In Furnaces"

METHODS OF COMPUTING FOR FURNACES             77
According to calculations based upon various furnaces in
service, it was found that the time during which the waste gases
remained in the furnace varies from a fraction of a second to 3, 5,
7, and up to 10 seconds. The determination of the length of time
which the gases must remain in the furnace chamber is based
upon the following considerations :
Computations may be made which will give the theoretical
calorific intensity obtained by the combustion of a coal with an
assumed excess of air—for example, the combustion of the best
grade of Donietz basin coal, with an excess of 73.3 per cent of air
over that theoretically required will give a calorific intensity of
1400°. If, for the designing of a continuous ingot heating furnace,
it is assumed that there will be a temperature of 700° for the gases
at the exit port, the total drop of temperature in the heating
chamber will be 700°. If the drop in temperature of these gases
per second is known, it is comparatively simple to determine the
time the gases should remain in the heating chamber of the
furnace.
According to those computations which have already been
made—computations which at the best are only approximate—
the following values have been found for the drop in temperature
per second:
Open-hearth furnaces...........           200° per second
Continuous reheating furnaces. ..   150°-200°
Annealing furnaces.............   100°—150°
Chamber brick kilns...........             80°
It should not be considered that these values are perfectly
established. Exact data covering the drop in temperature of the
gases per second can only be obtained by a series of observa-
tions for each of the varieties of furnaces, using for this purpose
furnaces which are correctly designed and constructed. This
last point presents the principal difficulty which exists at
present.
It is evident that the drop in temperature per second of the
gases in a furnace heating chamber is a function of:
(a) The losses by radiation, which are a function of the
exposed surface of the furnace, the conductivity of the walls and
the temperature of the hot gases on the inside of the chamber;