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Full text of "The Flow Of Gases In Furnaces"

DESIGN OF OPEN-HEARTH FURNACES               211
It is a basic principle of furnace design that satisfactory results
cannot be obtained unless the flame licks the sole of the furnace.
It is likewise well known that bottom cannot be made in an open-
hearth furnace unless the flame can drop down far enough to
sinter the bottom in place, no matter how luminous the flame may
be. One of the hottest flames is that of the oxy-acetylene torch.
Acetylene burned in air gives a very luminous flame, but when this
flame is supplied with oxygen it becomes a blue non-luminous
flame. The luminous acetylene flame does not emit an excessive
amount of radiant heat to any recipient surface. It is rather
noticeable that when the oxy-acetylene torch is used in welding
it is necessary for the flame to impinge upon the work, and that
the work has a tendency to become luminous, while the flame
itself has a very slight luminosity, and that at the tip only. The
mixed producer gas which is used in numerous Siemens furnaces
derives its heating value mainly from carbon monoxide and
hydrogen. These two combustibles are the main constituents of
blue water gas. In fact, blue water gas bears a fairly close resem-
blance to a good mixed producer gas from which the nitrogen has
been removed.
Radiant heat from a luminous flame may also be considered
from another viewpoint. The roof, side walls and bath are at a
high temperature and emit a certain amount of light, depending
upon their temperature and emissivity. The flame is sufficiently
transparent, when the furnace is at a high temperature, to permit
the opposite wall and the slag surface to be seen. The red and
yellow portion of the flame, which emits the most visible light, is
not transparent and is at a lower temperature than the blue
transparent portion. A smoky flame is caused by the presence
of soot or unignited carbon. Soot may be caused by the disso-
ciation of carbon monoxide when this gas is chilled by impinging
upon cold metal.
Stratification is frequently observed in the flame. Cool gases
tend to collect below hotter gases. The hottest gases tend to
rise to the roof where they are cooled. The coldest gases would
have a tendency to collect on the surface of the bath, but the
jet of flame from the ports tends to sweep them away. Convec-
tion currents are rarely appreciated at their true value. A
temperature difference of 1 C. is sufficient to impress a velocity
of 0.268 m (0.88 ft) per second and this velocity will increase as