Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to three-phase electric arc
furnaces. More particularly the invention is concerned with
arc smelting furances, and with reducing furnaces for producing
ferro-alloys, silicon alloys, or the like.
The furnace of the present invention has an elongated,
non-tilting rectangular furnace vessel, the walls of which are
at right angles to one another or have slightly rounded corners.
The electrical power is supplied to the furnace through at least
six electrodes arranged longitudinally within the furnace vessel
at equal intervals, pairs of adjacent electrodes being inter-
connected to form respective independent single-phase circuits,
and a supply transformer is connected to each such pair of
electrodes, through high-tension conductors, which form out-
going and return leads wound non-inductively close together and
constituting a bifilar cable run.
In the smelting of steel, also more especially in the
production of ferro- and silicon-alloys and calcium-carbide,
it has hitherto been preferred to use a furnace of circular plan
with a triangular arrangement of electrodes. Such an arrangement
is disclosed in the periodical "Technische Rundschau", Berne,
No. 48, November 1978, pages 21 to 23: U. Becker-Barbrock,
W. Felix and G. Papachristos: "Lichtbogenofen Hochleitungsanlagen
under ihre Baugruppen".
As a result of expansion of the steel industry, there is
a marked increasing need for ferro-alloys. Rapidly increasing
the costs of power, raw materials, and labour are subjecting the
producers of ferro-alloys to an increasing cost squeeze. In
order to take as much advantage as possible of reduced product-
related costs resulting from reduced investment costs and lower
wage bills, installations of larger capacity than before are
continually being designed and built.
The increase in the amount of heat required for
metallurgical processes, represented as effective electrical
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power of furnace installations, leads to the requirement for
electro~sand furnace hearths of ever increasing diameter.
Moreover, the relationship between the amount of power to be
transferred through the electrodes, and the possible~or necessary
conversion of energy in the hearth, and the increasing need for
heat, imposes a technico-economic limit on the power of the
circular three-electrode furnace.
From the structural point of view, there are two limita-
tions to be observed in the design of high power furnaces. One
of these is the maximum available electrode diameter (in the case
of graphite electrodes about 560 mm; for carbon electrodes about
1400 mm; and for Soderberg electrodes about 2000 mm). The other
limitation is imposed by the maxiumum diameter of the circular
furnace, since the efficiency of the circular furnace decreases
with increasing size.
It has been found that, in the case of a circular
three-electrode furnace, these limits may be exceeded by doubling
the number of electrodes and adapting the capacity of the hearth
accordingly, but this is not economical.
It is therefore the main object of the present invention
to provide a three-phase electric furnace of the type mentioned
initially which will preferably have a higher productive capacity
and an output which exceeds the limits imposed by the production
techniques and economics of conventional electric furnaces and
which, while retaining the advantages of known rectangular
furnaces having six electrodes, will achieve increased electro-
thermal efficiency by improved utilization of the entire hearth-
surface as a process-active surface, defined as the quotient of
the power supplied less the total of electrical and thermal losses
and the power supplied. At the same time, the electrical and
metallurgical conditions of the process are to be improved by
more uniform loading of the bath and by extensive electrical
balancing.
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The furnace is also to have lower operating costs, and
the installation is to be of simpler design.
According to the invention there is provided a three-
phase electric arc furnace comprising an elongated non-ti~ting
rectangular vessel having longitudinal and transverse walls,
and having six, or an integral multiple of six, electrodes
arranged in two longitudinally extending parallel rows within
the vessel~ Pairs of adjacent electrodes are connected in
respective independent single-phase circuits, each circuit being
connected to a respective supply transformer by a non-inductively
wound pair of high tension conductors. The electrodes are
disposed in a square array with the same number of electrodes
in each said row, the spacing of the electrodes with respect to
one another and with respect to the walls of the vessel being
such that the individual active bath surfaces surrounding the
electrodes constitute a coherent bath surface of maximum horizon-
tal area, and the pairs of high tension conductors are symmet-
rically arranged with respect to the longitudinal axis of the
vessel .
German Patent No. 677 279 discloses an arc furnace having
a rectangular vessel, but this is a typical tilting arc furnace
for producing steel products only, the six electrodes being
arranged in two rows and being connected in a star circuit
arrangement to two three-phase alternating current systems. The
electrodes are arranged on the longitudinal sides of the furnace
vessel. During tilting, the vessel is rotated about a vertical
axis, so that the narrow side, upon which a tap hole is located,
assumes a sharply inclined position. The use of six electrodes
is presumably intended to improve the distribution of the power
introduced. On the other hand, since the vessel rotates about a
vertical axis, the elctrodes must be arranged centrally. As a
result of this, large inactive areas are produced at the two ends
of the vessel, and these impair the electrothermal efficiency of
S
the furnace. In this known furnace, furthermore, there is a
sharp increase in the negative effect of power asymmetry.
German OS 25 35 207 discloses an electrical furnace for
obtaining steel from pre-reduced material, in small lumps,
containing iron, more particularly pellets, lump-ore, and sponge-
iron. This furnace is of elongated rectangular shape, with more
than three electrodes arranged in one or more rows upon the
longitudinal axis of the vessel. The purpose of this invention
is to produce a flow of material in the furnace vessel, which
passes through different processing zones.
Various arrangements of transformers and cable runs for
furnaces having three or six electrodes are described in an
article b~ Dr.-Ing. Mollenkamp and Dr.-Ing. Kallfelz: "Moderne
Elektro-Reduktionsofen fur die Gewinning von Ferro-Leg;erungen,
Roheisen und Kalzium-Karbid", Electrowarme International, Vol.B,
2/79. The arrangement of transformers for a six-electrode
furnace is shown in Fig. 4. However, this is a furnace arrange-
ment with six electrodes connected in a row, the total power
being divided between three single-phase transformers. The row
of transformers extends parallel with the longitudinal side of
the furnace.
An arrangement of this kind would have been unsatisfactory
for the production of ferro- and silicon-alloys. The book by
Durrer/Volkert: "Metallurgie der Ferro-Legierungen", 2nd Edition,
Springer-Verlag, Berlin 1972, pages 130 and 131, describes cable
runs to three-electrode furnaces, paying particular attention to
the problems of electrical asymmetry, which impairs the distri-
bution of total power.
The above publications contain no mention of measures
whereby the thermal efficiency of a furnace may be increased by
improved utilization of the total hearth area and by the
functional relationship between the shape of the furnace vessel
and the electrode arrangement. Nor do they contain any
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instructions for improving the process-related and electrical
conditions under which the furnace operates, whereas it is
precisely upon these considerations that the present invention
is based.
Norwegian Patent Publication 139 796 describes a current
supply circuit for a direct current reducing furnace having four
electrodes arranged in a square. This does not deal with
problems related to alternating-current furnaces.
The invention is based upon the following considerations:
In the case of a circular furnace, the hearth resistance
of the furnace vessel, in which the power required for the process
is converted under the electrodes, decreases with increasing
electrode diameter. The current must be increased as a function
of the decreasing hearth resistance, in order to obtain the
necessary power. This means that the effective force of the
current decreases with increasing electrode diameter.
In view of these relationships, a rectangular furnace
having six electrodes, for example, according to the above-
mentioned article by Drs. Mollenkamp and Kallfelz: "Moderne
20 Elektro-Reduktionsofen ", has the following advantages:
- as compared with a circular furnace having three electrodes
and of comparable effective power, the rectangular six-
electrode furnace operates with smaller electrodes and there-
for has higher electrical efficiency;
- with smaller electrode diameter, smaller conductor cross
sections are used in the vicinity of the electrodes;
- since the height of the hearth is dependent upon electrode
diameter, among other things, the height is less in a six-
electrode furnace and the system conductors in the vicinity
of the electrodes are shorter;
- since this furnace permits shorter high tension conductors of
smaller cross section, the inductance of the conduc~or system
is substantially reduced;
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- the reduced inductance achieved (and therefore the reduced
inductive-resistance~ leads, in conjunction with comparably
lower electrode currents, to lower reactive powers;
- the use of single-phase systems permits a bifilar conductor
arrangement from transformer to electrode tips;
- by a suitable choice of phase-sequence, and thus predeter-
mination of the direction of the current in the electrodes,
the whole six-electrode system may be designed as a bifilar
conductor system;
- the considerable reduction in impedance losses makes it possible
to equip a furnace according to the invention, which has an
improved power factor, with lower power transformers and
reactive power compensating installations;
- the bifilar arrangement, by lowering the inductance, avoids a
rotating field whereby the bath in conventional furnaces is
caused to rotate, thus making it more difficult to separate the
metal and the slag, especially in the boundary area;
- a rectangular furnace vessel is structually simpler to build
and less costly than a circular furnace; this applies both to
the steel structure and to the refractory lining.
As compared with the last-mentioned known rectangular
furnace having six electrodes, a furnace according to the present
invention of comparable effective power provides a number of
additional advantages which have the effect of increasing the
electrothermal efficiency.
The configuration of the furnace vessel, and the arrange-
ment of the electrodes, together provide reaction zones which
cover almost the whole horizontal cross-sectional area of the
vessel.
The active reaction surface, together with the previously
mentioned reduced hearth height, produces substantially smaller
radiation surfaces. From the metallurgical point of view, this
leads to better utilization of the hearth area and to improved
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electrical conditions; this has, above all, a positive effect
upon the electrothermal efficiency of the furnace.
Utilizing the whole hearth area as a process-active area
makes it possible to eliminate the rotation or movement of the
furnace vessel which in the case of circular furnaces is required
in order to distribute the power over the hearth.
By dividing the total power among three (,or a multiple
of three~ single-phase systems arranged symmetrically in
relation to the longitudinal axis of the furnace, and making use
of the above-mentioned extensive bifilarity of the system as a
whole, the disruptive effects of geometrical and electrical
asymmetry are eliminated.
The symmetrical arrangement of the transformers in
relation to the longitudinal axis of the furnace vessel make it,,
possible to keep one transverse wall of the furnace vessel free
for tapping equipment and the like.
The structural arrangement of the entire high tension
installation is greatly simplified in that charging hoppers may
be arranged in two parallel rows; where two furnaces are arranged
side by side, the charging hoppers may be arranged in only three
rows running parallel with the rows of electrodes.
Investigations and measurements carried out in the course
of testing have shown that a 40 MW furnace according to the
invention has substantially more favourable economic factors
than a 40 MW circular furnace with three electrode or a 40 MW
rectangular furnace with six electrodes arranged in a row.
The results of the comparison measurements are given in
Table 1.
Three embodiments of the inventîon will now be described
by way of example with reference to the accompanying diagrammati-
cal drawings, in which:
Fig. 1 is a plan view of an electric furnace according
to the invention having six electrodes;
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Fig. 2 is a plan view of a modification having twelve
electrodes; and
Fig. 3 is a plan view of another design of the twelve-
electrode furnace according to the invention, with another
arrangement of high-tension conductors.
Fig. 1 represents a closed reducing furnace having a
stationary, elongated, rectangular furnace vessel 7. As shown
in the figure, 8iX electrodes 1 to 6 are spaced equally apart
in two rows running parallel with the longitudinal axis of the
furnace vessel, in such a manner that each row contains three
electrodes. As seen in plan view, electrodes 1, 2, 5, 6 and
2, 3, 4, 5 are located at the corners of imaginary squares,
that i8, they are disposed in a square array.
In order to achieve optimal power distribution over
the bath, the electrodes are spaced from each other, and from
the walls of the vessel in such a manner that the surfaces of
the bath which are technically active in the process, and which
extend around the electrodes 1 to 6, form a coherent surface.
Adjacent electrodes 1 and 2, 3 and 4, 5 and 6 are combined into
respective, independent single-phase circuits. Electrodes 1 and
2 are connected, through high tension conductors, to a supply
transformer A; the pair of electrodes 3, 4 are connected to a
transformer B; and pair of electrodes 5 and 6 are connected to
a transformer C.
The supply transformers A and C, with their respective
high tension conductors, are positioned at the longitudinal sides
of the furnace vessel, and the supply transformer B at one of the
; transverse sides. As may be seen in the figure, the arrangement
of the high tension conductors is symmetrical with respect to
30 the longitudinal axis of the furnace vessel 7. The tap-hole,
not shown, is located on the free transverse side of vessel 7.
As may be seen in all of the figures, the outgoing and
return conductors between each transformer and the electrodes
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connected thereto are laid close together, thus forming a non-
inductive bifilar cable-run, which considerably reduces the
inductive reactive power from electromagnetic fields with
currents flowing in opposite directîons.
In the example illustrated, the supply transformers A,
B and C have their primary sides connected to a three-phase
system, the secondary sides of the single-phase circuits
connected to the transformers being connected, through the
metallic product in the furnace, to a three-phase system which
is balance with respect to the pr~mary system.
For the purpose of regulating the voltage in each of
the single-phase systems, each supply transformer is provided
with a regulating power circuit-breaker. Each electrode may be
adjustable individually according to its process-effective
power-input. Power is supplied to the furnace illustrated in
Fig. 2 by twelve electrodes 21 to 32, the spacing of which, in
relation to each other and to the walls of the vessel is similar
to the arrangement in Fig. 1. Three transformers with high
tension conductors 8 are provided at each longitudinal side of
vessel 7.
The arrangement in Fig. 3 is similar to that in Fig. 2,
except that of the six transformers D, E, F, G, H, K, four,
namely D, E and H, G, are arranged at the longitudinal sides of
the vessel, whereas transformers F and K, with their high tension
leads, are located at the transverse sides.
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TABLE 1
Comparison of data for 40 MW furnaces of circular design (with
3 electrodes) and of rectangular design (with 6 electrodes).
_ ,.
Designation Design Data
ISymbol Dim. (a~ (b~ (c~
_ r
~onnected Furnace Load s MVA 83 62 68
Primary Current Il - 1380 1040 1100
Necessary Capacitor
Power for Reactive- P MVar 5129 35
Power Compensation C
. ,
rurraentoS all Secondary I2 kA 430 475 545
Elect. Power Loss Pv kW335030803140
In
otal Furnace Design A m2 990 1090 1350
onverted Area of VG m389002285028700
Furnace Building
C as Activation f VH Nm3/m2h 46 49 40
earth Surface
_ _
eat-Radiation Area in
he Vicinity of Bottom ABM m2277 271 339
nd Bath of Metal
C Radiation Surface in 2
. icinity of Burden AMO m 150 153 197
o adiation Surface of A m2 246 228 282
essel Cover D
_
~a~* Circular design: three electrodes.
(b)* Rectangular design: six electrodes in two rows.
(c)* Rectangular design: six electrodes in single row.
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