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xii                 PREFACE  TO THE FRENCH EDITION
it uniformly, when in reality the hot gases may circulate only in
I                         the upper portion of the heating chamber, without coming into
I                         any contact with the material which has been placed upon the
I                         hearth.    The utilization of the heat in this case will be very
'                         inefficient.    The stream of fire which flows against the roof will
I                         carry all of its heat to the chimney.
|                              The reason for this lies in the high coefficient of expansion of
'                         gases, that is to say, in the fact that their density diminishes
I                         very  rapidly  as   their   temperature  increases.    Therefore,   the
I                         hydrostatic equilibrium of the different layers of gases, which are
                         at different temperatures, is accomplished with great rapidity,
f                         and the hottest gases tend to accumulate at the highest point in
,'                         the furnace.    Take, for example, the case of a kiln for bricks,
I                        having a height of 5 meters and an average temperature of 1000
f                        and assume that the hot gas issuing from the fireboxes has, where
I                        it enters the kiln, a temperature 500 higher, or 1500.    The
motive pressure caused by this difference in temperature, acting
upon a column of gases 5 meters in height, tends to produce, in the
column of hot gases a vertical velocity of 5 meters per second,
while the average velocity of the gases, as referred to the total
area of the kiln chamber, is perhaps one-fiftieth of this velocity.
The gases, therefore, have ample time to arrange themselves in
layers according to their relative density. If the opening for the
escape of the waste gases is located at the highest point of the
roof of the furnace, the greatest portion of the heat will be lost.
It is necessary, in order to avoid such heat loss, to employ the
downdraft principle of heating, which has been employed for a
long time in the manufacture of porcelain and faience, but which
still remains unknown in a great many other industries.
By numerous examples, the author of this volume shows that
at least one-half of the furnaces designed for high-temperature
heating are arranged in such a manner that the currents of hot
gases tend to isolate themselves in the heating chamber, thus
giving an extremely poor utilization of the heat released by the
The formation of these streams of hot gases in the furnace is
the cause of another difficulty; they tend to retard the completion
of the reaction of combustion. Above a fire burning upon a
grate, there are a number of parallel currents of gas, some of which
contain an excess of oxygen, while others contain an excess of