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TUNNEL FURNACES OR KILNS                     145
to force these gases down to the hearth of the kiln the dampers si
and 52 are opened more or less.C1)
By forcing the hot gases to circulate through piles of briquets,
the frictional resistance to the flow of the gases is increased; it
becomes necessary, accordingly, to increase the pressure of the
blower, forcing the primary air in below the grate of the gas
producer and the gas into the furnace, and to increase the height
of the chimney. At the same time, the clay-filled joints, which
are not air-tight, tend to prevent any such increase in the pressure
from the blower.
In order to overcome this difficulty, Groendal forces a stream
of air under the cars in the kiln for the purpose of cooling them.
It is probable that supplementary air from this source enters the
chamber of the kiln and has a tendency to turn the *hot gases
back to the roof. As a consequence of this, these kilns must be .
made very long (from 50 to 70 m).
In his design of a tunnel kiln, Groendal commits the obvious
error of carrying away the waste gases by ports in the roof of the
tunnel. By reason of this location of the waste-gas outlet, the
train of cars carrying the briquets is plunged into a pocket of cold
gases. The second fault which he commits consists of seeking to
force the hot gases to pass through the pile of briquets for the
entire length of the tunnel. This is very difficult and in order to
accomplish it the inventor limits the charge of brick upon the
cars to two tiers.
The author believes that, in the design of the tunnel kiln, it is
necessary to place one feature of the problem above all the others.
It is not possible to force the stream of hot gases to pass through
the piles of briquets, which are 50 m long. It is more advan-
tageous to heat the briquets by a descending current of hot
The difference between these two methods—that of Groendal
and that proposed by the author—may be studied by comparing
the cross-sections of the tunnel kiln designed by Groendal (Fig. 125)
and the cross-section as corrected by the author (Fig. 126).
Groendal placed a flat roof at a distance of 150 mm above the
(1> It is claimed that the manipulation of the dampers ss and s4 improves
the preheating of the air by forcing it down on the hearth of the kiln. These
dampers are evidently useless for this purpose, as the cold air will naturally
seek the lowest portion of the chamber.