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284                                 APPENDIX VII
is given a high velocity as compared to the air. The angle neces-
sary for the jet of flame to sinter in the bottom will be fixed by the
resultant of the air and gas angles and velocities. That is, either
the air or the gas may be given a maximum velocity; but any
change in air velocity will entail a corresponding change in gas
velocity, and vice versa, as well as a change in port angles, in order
to produce the fixed resultant velocity and angle.
For instance, if the gas-port is lined on the resultant angle,
the gas velocity will be maximum and the air velocity a minimum.
The desirable condition is a velocity which will make the furnace
hot throughout its full length. Frequently the trouble with over-
heated port blocks at the outgoing end is due to the high velocity
of the incoming gases. With a high velocity at the incoming end
a portion of the jet is practically shot into the outgoing port.
In most American furnaces, the air-port forms a segment over
the gas-port; it is claimed that this construction forms a blanket
of cooler air between the jet of flame and the roof. As the entering
air is several hundred degrees cooler than the flame itself or the
hot products of combustion, it is probable that this blanketing
action is largely a matter of the imagination. Cooler gases have
a tendency to sink below hotter gases; but any swift-moving jet
has a tendency to hang together and will cany a layer of colder
gases on top of it; these colder gases will have a tendency to move
diagonally to the edges of the jet, while they are absorbing heat.
The hottest gases, as soon as they escape from the influence of the
jet, lose their velocity and tend to seek the roof of the furnace.
The air pressure available for creating velocity in the port is
3 mm 830 of water (0.15 in water column) due to the stack effect
of the checkers and flues; it is not, however, advisable to utilize
the full pressure available. In the case of the gas, 4 kg 236
(0.166 in of water) are due to the stack effect of the checkers and
flues, but in addition there is the pressure created in the producer,
which may be varied, amounting to one-half inch to one inch of
water (12 to 25 kg per square meter). However, for every unit of
pressure expended in creating velocity in the gas-port, a unit of
pressure must be available at the outgoing port in order to pull a
proportional part of the waste gases through the port. If this
is not the case, the quantity of waste gases passing through the
gas regenerator will be reduced and the amount of heat available
for preheating the gas will be reduced.