79T
90 RATIONAL CONSTRUCTION OF FURNACES
It is frequently necessary to subdivide a current of cold air or
gas which is being heated, as in the hot-blast stove or in furnace
regenerators. This problem may be solved as follows : Figs. 48 and
49 show a current of .a cold gas circulating through a channel,
having walls heated to incandescence. Assume that the stream
of cold gas being heated has been equally divided between two
velocities of these two streams are denoted by ui and u>> and the friction in
the two branches in millimeters of water is & and &.
The condition necessary for the maintenance of equilibrium, in this case,
is that the increase of the hydrostatic pressure in the two branches q\ and r/2
shall be equal. If there were no loss of hydrostatic pressure in impressing
the velocities ?/i and u? upon the two branches and in overcoming the fric-
tional resistance & and & of the two channels to the passage of the gas, the
hydrostatic pressure in millimeters of water in the channel q\ of a height h
would be, taking 1.29 kg as the weight of a cubic meter of the furnace gas
at°°
J1.
I 1.29A
<>
f }\ For the branch qz, the hydrostatic pressure would be
*°
A part of these increases in the hydrostatic pressure will be expended in
overcoming the frictional resistances & and £>, and in impressing the velocities
HI and uz upon the columns of gas. These last losses, in millimeters of water,
may be expressed in the following manner :
wi2 1.29 , uS 1.29
—X: - and —
and the condition for the equality of the increases in hydrostatic pressure
in the two branches is given by the following equation:
In this equation there are six variables; five of these must be known in
order to fix the value of the sixth.
For example, the checker openings around the outside of the checker-
work of a Cowper hot-blast stove lose a great deal more heat by radiation
and by the cooling effect of the outside of the stove than the central passes.
By reason of this they have a much greater cooling effect upon the current
, of hot gases, and therefore the current of gases flowing downward through
|l these openings is reinforced, since if fe<*i, ii2>%. By measuring fe and th
it is not difficult to find —.
Us