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Construction electric arc furnace
eafh.com
Construction
An electric arc furnace used for steelmaking consists of a refractory-lined
vessel, usually water-cooled in larger sizes, covered with a retractable roof,
and through which one or more graphite electrodes enter the furnace. The furnace
is primarily split into three sections: the shell, which consists of the
sidewalls and lower steel 'bowl'; the hearth, which consists of the refractory
that lines the lower bowl; and the roof, which may be refractory-lined or
water-cooled, and can be shaped as a section of a sphere, or as a frustum
(conical section). The roof also supports the refractory delta in its centre,
through which one or more graphite electrodes enter. In modern meltshops, the
furnace is often raised off the ground floor, so that ladles and slag pots can
easily be manouvered under either end of the furnace. Separate to the furnace
structure is the electrode support and electrical system, and the tilting
platform on which the furnace rests. Two configurations are possible: the
electrode supports and the roof tilt with the furnace, or are fixed to the
raised platform.
A typical alternating current furnace has three electrodes. Electrodes are round
in section, and typically in segments with threaded couplings, so that as the
electrodes wear, new segments can be added. The arc forms between the charged
material and the electrode, and the charge is heated both by current passing
through the charge and by the radiant energy evolved by the arc. The electrodes
are automatically raised and lowered by a positioning system, which may use
either electric winch hoists or hydraulic cylinders. The regulating system
maintains an approximately constant current and power input during the melting
of the charge, even though scrap may move under the electrodes while it melts.
The mast arms holding the electrodes carry heavy busbars, which may be hollow
water-cooled copper pipes, used to convey current to the electrode holders.
Modern systems use 'hot arms', where the whole arm carries the current,
increasing efficiency. Since the electrodes move up and down automatically for
regulation of the arc, and are raised to allow removal of the furnace roof,
heavy water-cooled cables connect the bus tubes with the transformer located
adjacent to the furnace. To protect the transformer from the heat of the
furnace, it is installed in a vault.
The furnace is built on a tilting platform so that the liquid steel can be
poured into another vessel for transport in the steel making process. The
operation of tilting the furnace to pour off molten steel is called "tapping".
Originally, all steelmaking furnaces had a tapping spout closed with refractory
that washed out when the furnace was tilted, but often modern furnaces have a
bottom tap-hole on the spout to reduce inclusion of nitrogen and slag in the
liquid steel. Modern plants may have two shells with a single set of electrodes
that can be transferred between the two; one shell preheats scrap while the
other shell is utilised for meltdown. Other DC-based furnaces have a similar
arrangement, but have electrodes for each shell and one set of electronics.
A mid-sized modern steelmaking furnace would have a transformer rated about
60,000,000 volt-amperes (60 MVA), with a secondary voltage around 800 volts and
a secondary current in excess of 44,000 amperes. In a modern shop such a furnace
would be expected to produce a quantity of 55 metric tons of liquid steel in
approximately 70 minutes from charging with cold scrap to tapping the furnace.
Each batch is called a "heat". Enormous variations exist in furnace design
details and operations, depending on the end product and local conditions, as
well as ongoing research to improve furnace efficiency.
To produce a ton of steel in an electric arc furnace requires on the close order
of 400 kilowatt-hours per short ton of electrical energy, or about 440kWh per
metric tonne; the theoretical minimum amount of energy required to melt a tonne
of scrap steel is 300kWh (melting point 1520°C/2768°F). Electric arc furnace
steelmaking is only economical where there is a plentiful supply of electric
power, with a well-developed electrical grid.
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