The Flow Of Gases In Furnaces

CHAMBER FURNACES, BRICK AND POTTERY KILNS 113

In a preceding chapter the principles governing the computa-
tion for the updraft chamber furnace -have been stated. By
partially closing the smoke hole in the dome it is possible to
force the free lower surface of the hot gases in the kiln chamber
down to the hearth level, and thus force the heating of ware
placed upon the hearth. Nevertheless such a method of heating
is very imperfect and is not uniform. Its mechanics are as
follows:

The incandescent and burning gases issuing from the firebox
(Fig. 83) rise immediately to the
highest point in the chamber;
therefore the central portion of
the heating chamber does not
receive any direct action from
these ascending currents, and is
accordingly filled by heavier and
colder gases than those coming
from the fireboxes. These heavy
gases gradually drop to the
hearth of the chamber as the
upper parts of the chamber fill
with the hotter gases. Portions
of these colder gases in the bot-
tom of the chamber become
mixed with the heated gases

flowing from the fireboxes and rise with them. Finally a circu-
lation of gases is established within the chamber, so that all
portions of the charge are gradually heated.

Moreover, only a small portion of the heated gases circulate in
this manner. The largest portion of the hottest gases escape
immediately to the chimney through the smoke holes in the roof
of the kiln. For these reasons, all the ware, which is set where it
comes into direct contact with the hot gases is hard burned,
whereas the burn of the ware which only comes in contact with the
colder currents of gases will be less hard and portions of the ware
will receive a very slight burn.

While updraft or direct-draft kilns continue to be used in many
of the clay products plants, the principles of the downdraft kiln

Note by English translator.—Considerable heat is carried down to the
hearth of the kiln by conduction through the charge and the walls.

FIG. 83.