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Full text of "The Flow Of Gases In Furnaces"

210

APPENDIX VII

Regardless of whether the fuel is used cold or preheated it must bo
brought into contact with the preheated air so that the flame
formed will permit the sintering of the bottom, and the heads of
the furnace must be designed to obtain this result.

One of the reasons why blue water gas and blast-furnace gas
have not been considered on their heating possibilities is that they
burn with a non-luminous flame, it being considered that to obtain
high temperatures in the open-hearth and reverberatory furnace a
flame with a so-called high radiating effect is necessary. By this
is meant a luminous flame. Ifc is well known that the transmission
of heat by radiation varies as the difference between the fourth
powers of the temperatures of the radiating and recipient surfaces
and a coefficient varying from unity for the ideal black body to a
very small fraction of unity for a polished surface. Conduction
varies with the temperature difference. Convection depends
upon the temperature head or difference in temperature and the
flow of the fluid.

In the early designs of Siemens furnaces the roof was depressed
from each end to the center, as shown in Fig. 147. It was sup-
posed that this type of roof enhanced the heating effect by forcing
the flame into contact with the bath and assisted in the sintering
of the bottom. This type of roof had a short life, as it had a
tendency to burn out, and it was frequently damaged when
charging the furnace. Its worst defect was that it choked the
furnace. Later designers were obsessed by the radiant-heat idea
and this resulted in the forms of roof shown in Figs. 148 and 149.

It was soon found, however, that this
type of roof resulted in an increased
fuel consumption, and the straight
roof (Fig. 150) is now used. Fig. 151
shows a form of skewback designed to
prevent the wall expansion from in-
terfering with the roof. Twelve-inch
roofs are widely used and many
American furnaces employ the Orth
FIG. I51.-Skewback Construe- roof which permits the use of a repair

fannto?irdedatt°opPernlltblock Whe* the intermediate shapes

burn out.   It is possible that a roof

with cooling ribs spaced closer together than in the Orth roof
would be more satisfactory.

Cooling Rib.

-S«t after Eoof
is Completed