DESIGN OF OPEN-HEARTH FURNACES 289
Additional stack height will be required to cover the friction in the
stack; this additional height may be approximated as follows:
Assume about 5 m 00 in height required for this purpose; then
h=-------------------L__-----------------_ = 5 m 15.
The total height of the chimney will be
63 m 50+5 m 15 = 68 m 65 = 225 ft.
Additional chimney height might be desirable in order to pro-
vide a larger margin for increased draft resistance toward the end
of a campaign, due to blocking up of the checkerwork. A dif-
ferent fuel, a different method of operation, or a reduction or
increase in fuel consumption will modify these figures, but this
method of analysis may be applied^ not only to such cases, but
likewise to the design of other types of regenerative furnaces.
In connection with the reversing valves, a factor of importance
is the depth of the water seal. In the case of the gas valve, this
seal is exposed upon one side to the pressure within the gas main,
while the other is influenced by the draft depression at the valve.
The air-valve seal is influenced by the stack depression. In the
case of the gas valve, the gas pressure will be approximately 0.50 in
of water and the draft depression about 1.50 in of water, making
a total differential of 2.00 in of water. Gases, when their flow is
interrupted, have a tendency to produce a rise in pressure, as
water does. Therefore, with a 3-in water seal, only 1 in. of which
is effective, there is the possibility of a blow-through.
Practically all reversing valves present more or less of a direct
connection between the gas main and the stack flue or the air,
during reversal, unless special dampers or other means are pro-
vided to prevent this. Some of the devices designed to operate
dampers for this purpose are so arranged that the water seal on
the valve is broken before the dampers have cut off either the gas
main or the chimney flue. There is, however, no particular
difficulty in arranging a mechanism or a valve which will com-
pletely cut the furnace and flues off from the stack and the air
One of the main reasons why open-hearth progress has been
very slow is that space around the valves is limited, and it is