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Full text of "The Flow Of Gases In Furnaces"

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