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18 APPLICATION OF THE LAWS OF HYDRAULICS
drawn into the furnace, whereas from the upper opening a tongue
of flame will escape. From this it might be concluded that if
an opening m were made halfway between the top and the
bottom of the wall of this furnace, there would be no tendency for
the air to be drawn into the furnace nor for tongues of flame to
In reality the phenomenon which occurs is slightly different.
At this opening m, the outer air will sometimes be drawn into the
furnace and at other times small jets of flame will escape, as the
pressure within the furnace varies. That is, the level at which
the pressure in the furnace is in equilibrium with the atmospheric
pressure shifts vertically, now above and now below the level of
the opening m.
These simple observations show that the pressure of the hot
gases within the laboratory of a metallurgical furnace provided
with working openings or doors is, on the average, equal to
atmospheric pressure. These pressures may be directly measured
by the use of a manometer.(1)
Let it now be considered whether it would be possible for a
metallurgical furnace to work in a regular and uniform manner if
the pressure of the hot gases within the furnace were less than the
atmospheric pressure. If the foregoing occurred, an enormous
quantity of cold air would be drawn in through the working doors.
When this occurred, the depression due to the chimney draft
would be entirely overcome. And, further, by reason of this
inrush of cold air, the temperature of the furnace would be lowered
to such an extent as to produce a very bad effect upon the working
of the furnace. In addition, this would be likely to damage the
brickwork of the furnace.
In order to understand the working of those furnaces which
operate with a natural current of air it is not necessary to take
into account what is called the " draft of the chimney," the only
function of the chimney being to remove the burned gases from the
heating chamber, in order to provide space for the new or burning
gases. By placing this construction upon the question of chimney
(1) Extensive observations covering the temperatures and the pressures at
various points in an open-hearth furnace have been made by E. Juon, at the
Donetz-Jurjewka works, Russia. M. Juon's paper appeared in Stahl und
Eisen, Oct. 24 and Nov. 7,1912. It was abstracted in The Iron Age, Dec. 26.