The Flow Of Gases In Furnaces

METHODS OF COMPUTING FOR FURNACES

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(6) Height of Bridge Wall.—Considered with reference to the
velocity of the gases over the bridge wall, it is theoretically possible
to decrease this velocity over the hearth by increasing the thickness
of the stream of flowing gases by one-half their thickness over
the bridge wall.(1) As it is necessary that the velocity should not
be too great over the hearth, the roof will be given a downward
slope and the height of the bridge wall will be fixed at not more than
one-half the height of the opening over the bridge wall. In the
construction of the bridge wall this proportion will be reduced to
one-third (140 mm), and the roof will be given a downward slope
of the same amount toward the exit port for the gases; a general
longitudinal outline of the furnace will, therefore, appear as in
Fig. 44.

The working chamber will be supplied with two working doors,

FIG. 44.
each having a clear opening 400 mm in height. The hearth will be
given a slight grade or slope toward the gas-exit port, in order to
permit the cinder deposited upon the bottom to drain off into the
cinder pocket in the flue. The waste gas flue will be dropped
below the level of the hearth, giving any cold air or gases which
may enter the working chamber a chance to drain out of the
chamber. With the usual construction, it is impossible to prevent
small amounts of cold air from entering the furnace below the
doors.
(c) Dimensions of Grate.—These will be based upon the
assumption that 75 kg of coal can be burned per square meter
per hour (p. 75) (chimney draft):
= 1 m2 72 or approximately 2 m 0X0 m 90.
^ According to Yesmann h =•§//.