20
APPLICATION OF THE LAWS OF HYDRAULICS
the velocity with which the gases and the flame flow, and which
also acts as a reserve force through their inertia.
In a number of types of furnaces this live force or velocity
is utilized to direct and force the flame or hot gases from the top
down upon the hearth of the furnace. It has been shown that
the cold external air has a tendency to enter the furnace through
the cracks or openings below the working doors; since this cold
air settles upon the hearth it acts as a heavy liquid flowing within
a chamber filled with a light liquid, the hot gases. However, the
material to be heated (ingots, faggots, billets, bars, etc., in the
furnaces for the reheating of small pieces of metal, the molten
metal of the open-hearth furnace, etc.) is always placed upon- the
FIG. 16.
hearth. Therefore it is necessary to overcome the formation of
such a current of cold air. There are two means of doing this:
1. Increasing the velocity of the flame by increasing the
height h.
2. Bending the flame sharply down upon the hearth as it issues
from the firebox, and in this way pushing back the cold air and
preventing it from entering the furnace through the openings below
the doors.
A furnace constructed to employ this second method is shown
in Fig. 16. The increasing of the head h provides a possibility
of increasing the thickness of the bed of fuel and of obtaining in
the firebox a producer gas at a slightly lower temperature. This
increase in the head of the column of gases also permits the mixing
of the producer gas from the firebox with a supply of preheated
secondary air, supplied through channels in the bridge wall.
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