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Full text of "Modern Mechanical Engineering Vol-I"

THE  FURNACES                              I57
For large installations, the preference should be given to furnaces that
are fired with oil or gas, or with a mixture of each. There is no large con-
sumption of fuel in the preliminary heating up, and no waste of fuel unburnt,
at the end of the melt, as there is with coke. The temperature is under
precise regulation, and ashes have not to be removed. On the other hand,
oil storage reservoirs and supply tanks have to be installed, with pipes,
cocks, gauge glasses, and, if the oil is sprayed under high pressure, a supply
of compressed air is necessary. If used with a low pressure, a fan or a blower
is employed. A low-pressure burner works with air at about 12 oz. per
square inch, a high-pressure one at from 20 to 25 Ib. per square inch. In
the American " Rockwell " furnaces the pressure for the oil is 5 Ib. or more,
and that for the air is 2 Ib. per square inch. Gas may be used instead of
oil, with burners and pipe connections modified. 100 Ib. cf brass can be
melted with from 2 to 3 gall, of oil, and after a furnace has been heated
with a first charge, 400 Ib. of metal can be melted in about 45 minutes.
Electric furnaces are being used in increasing numbers when large
quantities of brass are being melted, but chiefly in the United States.
Whether they are more economical than the oil-fired designs depends mainly
on the relative costs of power, attendance, and upkeep. But there is one
important fact in favour of the electrical designs, that the metal is melted in
a closed vessel, in a non-oxidizing atmosphere, and that there is then hardly
any loss due to the volatilization of zinc, of dirty borings, and of fine scrap.
This loss often amounts to 5 or 6 per cent in the fuel furnaces. As a result,
alloys can be graded and duplicated with such precision that the average
deviation is only about 0-25 per cent. Electric furnaces will not deal eco-
nomically with small charges, since, with their necessary equipment, they are
costly to install, so that, like the oil-fired designs, they are only suitable for
the large foundries.
Electrical energy is applied to the melting of brass by two methods: by
means of the electric arc drawn between electrodes, or by the resistance
offered to a current by its passage through liquid metal, on the same principle
as that of the heating of an incandescent lamp. Each design has its advocates,
and each has its application in several furnaces that are in successful operation,
melting quantities that may range from 200 to 2000 Ib. weight. Some of
the furnaces are stationary, some tilt for pouring. A few are rocked through
an arc to maintain a uniform temperature, and prevent surface superheating,
while a perfect mixture of the metals that form the alloy is produced.
Generally, the mixture is contained in a bath in the bottom of the furnace.
This method is better for heavy charges, but for moderate casts a crucible
design of furnace is made, in which the metal is melted by the passage of
electric currents through the crucible walls.
The arc furnaces may have the arc drawn between two electrodes of
graphite, or of amorphous carbon, provision for the adjustment of which is
made by hand or electrically. The heat is transmitted to the metal below
by radiation chiefly, although in one design it is directed downwards by
a third electrode, placed vertically above, which forces the flame of the arc