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258                                  APPENDIX VII
wider chamber and 149 m3 50 (5280 cu ft) for the narrower cham-
ber. The gases will remain in the chamber approximately 1.50
seconds, which, with a temperature drop of 200 C. (360 F.) per
second, means a temperature of approximately 1500 C. (2732 F.)
for the gases leaving the chamber. Referring to the curve
(Fig. 184) and allowing for a drop in calorific intensity of about
200, it will be seen that a preheat of the air and gas of between
800 and 1000 C. (1472 to 1832 F.) will be required. Allowing
for a possible loss of temperature in the necks, cinder pockets and
uptakes, the checkerwork will be proportioned to supply a preheat
of 1200 C. (2192 F.).
The distance the jet of flame must drop below the bridge or
port sill, in order to permit 'the sintering of the bottom, has been
fixed at 1200 mm (48 in). A resultant velocity of the air and gas
can be assumed, and this will fix the resultant angle of the two
jets. The higher the resultant velocity is assumed, the less the
resultant angle will be, and the further beyond the center of the
chamber will be the point where the maximum depression of the
jet of flame occurs.
High velocities, in addition, cause the incoming end of the
chamber to work cold and the outgoing end to work hot, while
it is desirable that both ends of the chambers work as uniformly
as possible. High velocities for either the gas or the air mean
reduced port areas, and high velocities for the outgoing products
of combustion, which in turn call for an increased draft depression
in order to pull the gases through the ports. This draft depression
creates a suction acting to pull air in through the valves, flues
and chamber walls. At the same time, it is necessary to have
sufficient draft to draw the waste gases out of the chamber and
down through the checkerwork, but the lower this draft depression
the less the tendency to induce air leakage or infiltration.
As the flame has to drop 1200 mm (48 in) in one-half of 14 m 00
(46.00 ft), which is the length of the furnace, an angle somewhat
greater than 10 must be allowed for the trajectory of the jet.
Yesmann's formula for this case is
.., , .                          Tr   *>2sin2 8
Metric:                       H = ------X
2g        m-t
r?    r u                        TT    ^2sin2 5    459+*,
English:                      H =             X ---*,