DESIGN OF OPEN-HEARTH FURNACES 249
with two chambers at each end, that they may be converted with
little difficulty to producer-gas firing.
Those furnaces in which only the air is preheated have a slight
advantage over those in which both the gas and the air are pre-
heated, in that no unburned gas has to be wasted up the stack at
reversal. The amount of fuel lost in this manner depends upon the
gas-filled volume between the reversing valve and the port, and
the frequency of reversal. When the gas is preheated this loss
cannot be avoided.
This gas likewise creates an explosion hazard; when the condi-
tions are right, it burns and passes up the stack as a puff of flame.
Again it may become mixed with air, the mixture being below the
ignition temperature; when a portion of this mixture is suddenly
ignited an explosion of more or less violence occurs. These
explosions damage the walls of regenerative chambers, flues and
waste-heat boilers so that large amounts of air are drawn into the
system, reducing the stack draft and the output of the boiler by
reason of the lowering of the temperature of the waste gases.
One of the factors in regard to valve area that meets with
little consideration is the velocity of the gases passing through the
valve. In addition, most valves involve a change in direction of
flow, totaling 360°, 180° in the valve and two 90° changes in the
flues. When a stream of flowing gases passes through passages
involving changes in area, velocity changes are involved. The
velocity of flow in the normal section of the flue may be repre-
sented by Fmin and in the contracted area of the valve by Fmax.
The corresponding velocity heads will be Amin and Amax- That is,
a velocity head = Amax—Amin will be required to produce the increase
in velocity. The pressure, in kilograms per square meter or in
millimeters of water, required to produce the increase in velocity
8 = (ftmax — hmln) A*,
in which d = pressure in kilograms per square meter or millimeters
Aj=the weight of 1 cu m of the gas in motion at a tem-
perature of t°.
When the pressure is desired in inches of water, the weight of
the gas being in pounds per cubic foot, the formula is
5 = 0.192(ftmax-~ Amln) At*