48 APPLICATION OF THE LAWS OF HYDRAULICS
I [j Moreover, according to this principle, the use of a roof sloping
i I' ^ 1 upward tends to increase the velocity of flow of the hot gases and
' (? > | in this manner to decrease the thickness or depth of their stream.
1 . I, They are thus kept away from the hearth, and the fuel consump-
I ' [' tion is increased, as it is necessary to increase the quantity of gases
flowing in order to force them to lick the hearth. At the same
time, these statements should not lead to the conclusion that con-
temporary continuous reheating furnaces cannot be reconstructed
so that they will work in a satisfactory manner.
Figs. 29 and 30 show a model of the " N " furnace immersed
in water; a stream of colored kerosene flows into the heating
chamber from the firebox. The two half-tones show the outflow
of the waste gases through openings pierced in the hearth of the
furnace. Fig. 29 shows that with this method of outflow of the
waste gases there remains upon the sole or hearth of the furnace
a layer of cold air. The rise of the hearth forms a pocket from
which the colder and heavier gases cannot flow by gravity. This is
a very serious defect of continuous reheating furnaces having the
roof and the hearth rising from the firebox end toward the charging
end. The ingots or billets heating on the hearth of the furnace
chill the gases, and these chilled gases cannot by any possibility
reach the chimney, since they are heavier than the hot gases.
In Fig. 30 the current of colored kerosene has been increased,
but it has not been possible to force out the water from the pocket
on the hearth, as the pressure has not yet become higher than the
water pressure, that is to say, the atmospheric pressure. When
this point is reached a portion of the kerosene escapes through the
interstice or small opening between the sheets of.glass forming the
sides of the model. It is evident that this is not a desirable
manner of working as the outflow of the chilled gases from the
hearth will take place through the working doors, instead of
through the waste gas opening to the chimney.
In practice, therefore, a furnace of this design will work under
the conditions shown in Fig. 29, and the ingots or billets will not
be well heated - except on their upper portion, since their lower
portion will rest in the layer of chilled gas on the hearth of the
furnace and the cold air drawn in through the working doors.
This prolongs the time required to heat the ingots, increases the
defects in the metal, etc.
The problem of the rational construction of the continuous