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Full text of "Modern Mechanical Engineering Vol-I"

THE  FURNACES

153

supply the additional oxygen to the CO formed lower down by the com-
bustion of the coke. The same result is accomplished by additional height,
since a more prolonged contact of the carbonic oxide with the heated blast
is assured. For it must be remembered that the blast is cold when it enters
the furnace, and its oxygen must be highly heated before it will enter into
combination. Much heat is wasted in warming the upper charges, and the
large proportion of inert nitrogen in the blast.

Blowers and Fans.—The first named (fig. 58) are used now more often
than the second, because the action is positive, the air being driven out under
definite pressure. Good results are alsd obtained from fans if they are
selected and used with judgment, but generally they are more suitable for
the lower pressures, say not exceeding 8 oz. per square inch. The fan has
to revolve at a very high rate of
speed; that of the blower is
moderate, and the pressure and
volume are under better control.
The speed of a fan cannot be in-
creased beyond that for which it
is rated without absorbing power
that increases with the cube of
the number of revolutions. Hence
one of large diameter should be
selected to allow for contingencies.
In either case the supply pipes
must be large, free from quick
bends, and of minimum length
possible from the machine to the
cupola. A blast gauge is necessary
as a check upon the working. It
reads to z Ib. pressure, and is sub-
divided into ounces. It is necessary to regulate the blast at different stages
of melting. This is done by varying the amount of opening of the blast
gate. At the normal pressure of from f Ib. to i Ib. per square inch, the
blast must supply from 30,000 to 40,000 cu. ft. per ton of iron melted per
hour. The makers of blowers and fans give the capacities for different
sizes. From 3! to 4 b.h.p. per ton melted per hour are required.

Ladles.—These, up to about 3 cwt. capacity, are carried by hand, by one,
two, or three menu hence termed " hand shank ladles". Larger sizes are
slung in the cranes, or run on carriages on rail tracks. All are tipped when
pouring. Fig. 59 shows a common form, where the tipping is done through
bevel and worm gears. It is effected similarly in fig. 60. This type can be
run on tracks, or lifted in a crane. Both have two pouring lips, to be tipped
to either side. The bodies of ladles are formed of pressed steel plates,
stiffened with belts. Capacities are reckoned inside the fire-clay lining with
which they are daubed each morning. A cubic foot of ladle capacity is the
equivalent of 3 cwt. of iron.

Fig. 58.—-Section across Pressure Blower