METHODS OF COMPUTING FOR FURNACES 85
give the waste gas flue sufficient area to reduce its frictional
resistance to the passage of the gases to a minimum.
Assuming for the waste gas flue, considered as an inverted
weir, a width of 1 m, its height will be determined by YesmamVs
12X120Q==Q inm 636>
In order to insure the contact of the hot gases with the hearth
of the furnace, the waste gas flue is inclined downward from the
hearth level. This provides, at the same time, a pocket into
which the cinders from the hearth may drain.
It is now necessary to check the time period of the hot gases
in the heating chamber of the furnace and to determine the drop
in the temperature of these gases per second.
The average temperature of the hot gases will be
from which t Qiaoo = 2 m3 96.
The volume of the heating chamber of the furnace is equal,
according to the dimensions given, to 2 m3 80. The time during
which the hot gases remain in this chamber is therefore in the
neighborhood of one second, and the drop in temperature of the
gases is in the neighborhood of 200° per second, which may be
considered normal for this type of furnace.