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(                              130           RATIONAL CONSTRUCTION  OF FURNACES
impossible to attain the necessary working temperature in the
A furnace is a very delicate mechanism, which cannot be suc-
cessfully constructed unless the two following conditions are
successfully met:
1. The mixture of the air and the combustible gas must be
,,'                               2. The size of the combustion chamber must be such that the
-' -                                  jet of flame may form in the fore part- of the heating chamber.
In addition, as the working temperature of the furnace is very
! | '<                                  nearly equal to the instantaneous calorific intensity computed for
the fuel, the time during which the hot gases remain in the heating
chamber is less than one second; the supplying of flaming hot
! i |     }                            gases to the heating chamber and the withdrawal of the waste
I          j                            gases must be accomplished with uniformity.
I1                                              From the foregoing it is evident that the art of proportioning
^                                     these furnaces is, above all, a question of combustion, and that
this must be completed -before the gases reach the rear of the
heating chamber. It will be necessary to take up and analyze
I                                      this question in order that the direction to be given the current of
I                                      hot gases may be fixed.
I                                           In one of the preceding sections mention was made of the
;     i                            conditions under which combustion may be best effected.    In
!   . '                            postponing   the   examination   of   these   conditions   the   author
remarked that, for the better mixture of the combustible gases
with the eomburent, resource was had to a strangulation of the
opening over the bridge wall, just as a kerosene lamp is prevented
from smoking by the contraction of the chimney.
The only result of this contraction in the opening over the
bridge wall is that of increasing the velocity of the hot gases
passing that point. On account of this lowering of the roofs,
these gases, after passing into the heating chamber, are slowed
down and fill the chamber over the hearth, from which they are
drawn off through the waste-gas or chimney port. This shows
why it is necessary to place the waste-gas or chimney port at the
level of the hearth of the furnace, and exposes the error in the
construction of those furnaces which have an updraft and from
i |                           which the waste gases pass away at the highest point.
Practice completely confirms these deductions; in all reheating
furnaces the waste gases should be carried away from the heating