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fifteen minutes the metal begins to run down. As the charges sink, suc-
cessive additions are made in the order named. Melting is facilitated by
breaking the pig and scrap into small pieces. As fusion is confined to the
area immediately above the tuyeres, extending therefrom to a height of
about 30 in., metal of different grades, harder and softer, can be charged in
the same cupola at the same time if separated with charges of coke. The
k, N. * metal accumulates in the
vxv>* bed charge, and must be
tapped before it rises to
the tuyere holes.
The Melting Ratio.—
Most of the modifications
that have been made in
cupola design have for
their object an increased
melting ratio, which is
Fig 55.—Cupola, with Air-belt A and three rows of Tuyeres B arranged spirally. C, Receiver.
D, Slag holes. E, Tapping hole. F, Drop bottom.
accomplished by supplying enough oxygen in the right place to secure the
nearest approximation possible to complete combustion. If a ton of iron
is melted with from 2 to 3 cwt. of coke, that represents good average practice.
To use less than 2 cwt. of coke is exceptional. This is only possible in
lengthy fusions, using: (i) clean iron that throws out little slag; (2) good
furnace coke; (3) a deep bed charge; (4) suitable proportioning of fuel and
iron; (5) an adequate supply of blast at proper pressure and volume, with
variations made when necessary as the melting proceeds.
Since the supply of oxygen in the right locality is the master key to
economical melting, this explains the very numerous variations that have