Figs. 94 and 95 show sketches of this type of kiln. Combustion
occurs in chamber III; the air is being preheated in chambers I
and II, and the bricks are being preheated in chambers IV and V.
The gases which are cooling have a downward flow, while the air
which is being heated flows upward; the circulation of the gases
is therefore correct. The change in direction of the flow of the
gaseous currents is effected by a system of channels or flues in the
walls separating the chambers: channels 6161, 6262, &3&s ... for
the air and d\d\, d^dz, feds ... for the products of combustion.
When the products of combustion are passing into a chamber
(for example, IV), the flues dd are open and the flues bb are closed.
The opening and closing of the flues bb and dd is accomplished in a
very simple manner by the damper brick shown in dotted lines.
The kiln which is designed according to this method is very
simple in construction, and the author will enter into correspond-
ence with any reader who may be interested in it. From this
short description of the ring tunnel or chamber kiln it may be
seen that it is not difficult to construct a continuous kiln which
will operate in a satisfactory manner through the rational flow of
the air and the products of combustion.
XI. MUFFLE FURNACES
For tne annealing and tempering of steel, the annealing of
brass and bronze and for other purposes, muffle furnaces possess
many advantages. A uniform temperature, the absence of jets
of flame, the small amount of oxidization and the protection of the
surface of the material being heated (a con-
dition very important in the stamping of
metals) make these furnaces a type much
favored in these works, despite the fact that
the muffles deteriorate quite rapidly. If the
currents of hot gases are circulated in a
rational manner, muffles of steel castings
will give a service of several years. Un-
fortunately, the great importance which
attaches to the correct circulation of the
hot gases is not well known. j?IG 95
Fig. 96 shows a muffle furnace which is
widely used. The flame and hot gases are produced in a firebox