Channel inductor and melt furnace comprising such channel inductor

A channel inductor and a furnace for melting, holding or refining metal. The channel inductor has a closed-loop core, a primary winding arranged around the core and a refractory lining in which the winding and core is partly enclosed and in which an inductor channel is formed. The channel inductor also has a detector to detect metal penetration of the refractory lining.

TECHNICAL FIELD
 The present invention relates to a channel inductor and to a furnace for
 melting, holding or refining of metal comprising such channel inductor.
 BACKGROUND ART
 A channel inductor is an electric device for melting and holding of metal.
 The inductor comprises a first primary winding, e.g. a multi-turn coil
 wound around a magnetic core. Around this core and the coil is a channel,
 normally called inductor channel, arranged. The channel opens at both its
 ends into a furnace vat. The inductor and the channel is normally
 contained in a removable inductor housing such that the inductor can be
 replaced without the need to reline the complete furnace vat.
 The inductor channel, which during operation is filled with molten metal,
 constitutes a closed circuit. As the primary winding during operation is
 fed with an alternating current the melt in the inductor channel acts as a
 short-circuited secondary winding of a transformer. Power is thus induced
 in the melt which is heated and a flow pattern is developed in the
 channel. Due to the good stirring effect provided by the inductor a good
 homogenization as to temperature and composition will be achieved in the
 melt rendering this type of furnace suitable for many type of refining and
 alloying treatments. However the flow pattern generated in the channel,
 which normally is a two-loop flow over the channel cross-section, might
 also create erosion of the lining in the inductor or in some cases
 deposition of refining agents, solid particulate matter formed in the melt
 or other particles on the walls in the inductor channel resulting in a
 clogging of the channel. Such clogging will disturb the flow in the
 channel and thus the efficiency of the inductor.
 A channel inductor is normally equipped with a cooling jacket for cooling
 of both the housing and the coil. The cooling jacket is arranged within
 the refractory lining provided around the coil, i.e. between the coil and
 the inductor channel and will shield the coil from any moisture given off
 by the lining material during sintering of the lining but will also
 constitute a protective barrier or shield around the coil which any melt
 which happens to penetrate the lining have to pass. The cooling jacket
 will, however, cause substantial thermal and electric losses. These losses
 will show as the heating of the water passing through the cooling jacket.
 Today refractory lining is normally applied as dried masses which are
 formed around templates without any water-additions. As the masses contain
 essentially no added water and there is no longer a need to protect the
 coil from moisture in the refractory lining and the primary object of the
 cooling jacket is in installations using this type of linings to protect
 the coil from any metal penetrating through the lining. Thereby has it
 become advantageous to design a channel inductor without the cooling
 jacket, giving the following advantages;
 the inductive losses to the cooling jacket are eliminated;
 possibility to reduce the thermal losses by an increasing the distance from
 the hot melt to the cooling system comprised in the coil;
 possibility to increase the overall efficiency of the inductor;
 possibility to increase melt and/or superheat capacity; and
 reduced maintenance as the corrosion situation in the cooling jacket is
 eliminated as will all water couplings etc. and supply hoses or tubes for
 the cooling jacket.
 The inductor and especially the coil must however be safe-guarded against
 melt penetrating through the lining and damaging the coil and also against
 excessive wear especially in cases with increased superheat or melt
 capacity which are likely to increase the temperature in the interface
 melt/refractory lining and possible also the flow rate in the inductor
 channel. It is also an object to reduce the thermal and mechanical
 stresses, which the lining around the coil is subjected to.
 It is therefore the object of the present invention to provide a channel
 inductor with an improved thermal efficiency and reduced need for
 maintenance while maintaining or improving the operational safety. It is
 one object that the cooling jacket shall be removed but that the inductor
 still shall be safe guarded from damage to the coil due to metal
 penetration. That is any metal penetration shall be prevented to reach the
 inductor coil. Further it is also an object to improve the flow
 characteristics of the inductor channel to reduce wear and to improve the
 control of supplied electrical power to reduce depositions and clogging
 and also measures will taken reduce losses to the mechanical structure and
 cooling system.
 SUMMARY OF THE INVENTION
 The present invention provides a channel inductor which comprises a winding
 wound around a core, and a refractory lining, in which at least a part of
 the core and the coil are arranged enclosed and embedded and an inductor
 channel formed around and encircling the core in the refractory lining
 such that it when filled with melt constitutes a secondary winding. The
 channel inductor further includes detection means for detecting any melt
 penetration through the refractory lining arranged in the refractory
 lining between the coil and the inductor channel.
 Preferably the winding is a multi-turn coil with conductors in the form of
 copper tubes in which water or other suitable coolant is flowing during
 operation. The core is preferably a laminated iron core which for
 installation purposes is divided. When assembled the core normally forms a
 four-legged square or rectangular core. A refractory lining mass are
 rammed or in other way formed around part of the core and coil after the
 core have been assembled and placed within a channel template. The used
 ramming mix preferably essentially free from water additions but can also
 be a cast lining with high water-additions provided the lining is dried
 before the coil is mounted. The coil, the core and the template are
 mounted in an inductor housing and positioned relative each other in a
 desired manner within the inductor housing. The housing is thereafter
 filled with the refractory mix. The refractory mix is rammed around the
 coil and the template in such a way that an inductor channel with openings
 at two ends is formed around the coil and the core.
 According to an embodiment of the invention the inductor comprises
 detection means in the form of a detection wall or fire wall, such as a
 cylindrical tube-like wall made from a mesh or net of an electrically
 conductive material, such as a metal, arranged around the coil in the
 lining between the coil and the channel. The detection wall is connected
 to indication means for indicating any melt penetration into the lining as
 it reaches the detection wall. With the detection means will it also be
 possible to indicate other disturbances in the lining which are likely to
 affect the performance of the inductor, such as moisture in the refractory
 lining.
 According to one preferred embodiment of the invention the inductor
 comprises detection means in which two walls are arranged in the
 refractory lining between the inductor channel and the coil. A first
 essentially cylindrical detection wall or fire wall is arranged at a
 suitable distance from the inductor channel. The fire wall is exhibits an
 open structure and comprises an electrically conductive material.
 Preferably the fire wall is backed on either side or both sides with a
 backing wall made from an electrically insulating material such as a
 material based on mica. The first or fire detection wall indicates any
 metal penetration reaching this far in the insulation. The first detection
 wall is placed at such a distance from the coil that metal penetration
 reaching the first wall do not constitute an immediate danger but the
 inductor can be taken out for relining and other suitable repair at a
 planned coming stop in the production. This first wall is also arranged to
 interact with a second wall to measure the resistance in the refractory
 lining between these two walls. By measuring the resistance in the
 refractory lining between these two walls it is possible to monitor the
 metal penetration to see if it continues beyond the first wall, should the
 resistance be reduced under a preset value the inductor is disconnected
 from its power supply. This measurement of the resistance can also be used
 for monitoring the condition of the lining and indicate the moisture in
 the lining. To high moisture content in the lining increases the risk for
 flash-over or leakage currents in the lining. The second wall is often
 made of a heavier gauge wire material and will thus provide reinforcement
 to the refractory lining.
 To further reduce the risks of metal penetrating the lining improvements
 have according to the present invention been made aimed at reducing wear,
 deposition and clogging in the inductor channel, these improvements will
 also show in reduced energy consumption and is characterized by the
 features of additional claims. Other developments have reduced the
 electrical losses in the channel inductor and there some of the strains
 put on the lining.
 According to one embodiment the inductor channel is designed according to
 the following criteria;
 the width of the channel which exhibits an essentially oval or rectangular
 cross-section with a width to radial height ratio of 1.5 or larger;
 the radial height shall vary along the channel;
 and preferably shall the inner-wall of the channel in area between the two
 openings of the inductor channel show an angle across half the channel
 width that is 0 degrees at the openings and at least 30 degrees at a
 center point between the openings. A channel designed according to these
 criteria will exhibit an improved flow with essentially no zones of
 stagnation and dead-water in a cross-section at any point along the
 channel. Preferably the variation of the radial height shall comprise
 sectors where the height is increased alternating with sectors where the
 height is decreased. The change in relative height along the channel will
 along the whole channel length both exhibit sectors with increasing radial
 height and sectors with decreasing radial height. The changes shall over
 such a sector correspond to a change of the radial height with at least
 25% within a sector of one-eight of the periphery of the channel. As
 essentially all zones of stagnation or dead-water is eliminated in the
 cross-section flow pattern in the channel the deposition and clogging is
 substantially reduced. The changes in the cross-section flow pattern will
 also substantially reduce wear.
 According to another embodiment a similar improvement in flow
 characteristics and substantial reduction in deposition, clogging and wear
 is achieved by the use of thyristor-controlled power supply. The thyristor
 shall be in a mode controlling the pulse duration, i.e. a pulse-length
 modulation mode. The most frequently used way to control the power supply
 to an inductor is to use a tap- or step-transformer giving different
 voltages at different taps. Dependent on the power need the inductor is
 connected to a suitable voltage. Alternatively step-less power supply can
 be used, using an alternation between to voltage steps of the transformer.
 The duration of the connected time at the different voltages is control by
 a clock relay automatically switching between the steps to supply the
 desired average power. The use of a thyristor-controlled power supply
 offers a step-less control between zero and hundred percent of the rated
 power, but the normally used the phase angle firing mode will create
 transient overtones on the distribution net to which the inductor is
 connected. Therefore shall according to this embodiment of the invention a
 thyristor in a pulse-length modulation mode, i.e. a mode controlling the
 duration of the pulses, by controlling the number of complete cycles for
 which the thyristor is on and the number of complete cycles for which it
 is off. This frequent switching off and on of the power supply creates
 forces acting the flow in the channel which frequent changes between
 maximal during the on periods and zero during the off periods. This
 results in variations in the flow whereby stagnation zones and so called
 dead-water zones never develops. Hereby is deposition and clogging
 essentially eliminated and as the flow is constantly changing also a
 likely reduction in wear. Further the pulse-length is according to one
 embodiment chosen such that the flow-velocity during the on-periods
 exceeds a critical value where it tends to break loose any newly-deposited
 relatively loose bonded particles on the wall. By choosing a suitable
 length for the off-periods can further advantages be obtained as all
 non-metallic particles will show a tendency to float up and out of the
 channel during these period of reduced flow.
 According to one further improved embodiment have the energy losses in the
 inductor been substantially reduced while at the same time improving the
 flow in the channel by the introduction of air-gaps in the mechanical
 structure supporting the refractory lining, the core and the coil, i.e.
 the inductor housing. The introduction of air-gaps or slits in the housing
 and other parts of the supporting structure will reduce the inductive
 losses in these parts and thereby increase the overall efficiency of the
 inductor.

DESCRIPTION OF PREFERRED EMBODIMENTS
 The channel inductor according to the prior art shown in FIG. 1 and the one
 according to one embodiment of the invention shown in FIG. 2 both
 comprises a multi-turn coil 11 with tube-conductors in the form of copper
 tubes in which water or other suitable coolant flows during operation. The
 coil 11 is wound around a core 12. The core 12 is a laminated iron core
 which for installation purposes is divided. When assembled the core 12
 normally forms a four-legged square or rectangular core of which only part
 of one leg is shown in the figures. The coil 11 and core 12 is arranged in
 a refractory lining 13 in such a way that part of the core 12 and the coil
 11 are enclosed and embedded in the lining 13. An inductor channel 14 is
 formed in the lining 13. The inductor channel 14 is formed to encircle the
 core 12 such that the channel 14 when filled with a metallic melt or other
 electrically conductive material constitutes a secondary winding. The
 inductor channel 14 is during operation filled with molten metal,
 constitutes a closed circuit. As the primary winding during operation is
 fed with an alternating current the melt in the inductor channel 14 acts
 as a short-circuited secondary winding of a transformer. Power is thus
 induced in the melt which is heated and made to flow in the channel 14.
 Due to the good stirring effect provided by the inductor a good
 homogenization as to temperature and composition will be achieved in the
 melt rendering this type of furnace suitable for many type of refining and
 alloying treatments. However the flow pattern generated in the channel 14,
 normally a two-loop flow over the channel cross-section as shown with the
 dotted lines in FIG. 3, might also create erosion of the lining in the
 inductor channel 14 or in some cases deposition of refining agents, solid
 particulate matter formed in the melt or other particles on the walls in
 the inductor channel resulting in a clogging of the channel 14. Such
 clogging of the channel 14 will disturb the flow in the channel 14 and
 thus the efficiency of the inductor. Channel inductors are normally
 equipped with a cooling jacket 15 as shown in FIG. 1. The purpose of the
 cooling jacket 15 is to provide cooling of both the housing 16 and the
 coil 11. The housing 16 is a structure for mechanical support arranged
 around the inductor. The cooling jacket 15 is arranged in the refractory
 lining 13 between the coil 11 and the inductor channel 14. The cooling
 jacket 15 is arranged to protect the coil 11 from any moisture given off
 by the lining material during sintering of the lining but will also
 constitute a protective barrier or shield around the coil which any melt
 which happens to penetrate the lining have to pass. The cooling jacket 15
 will, however, cause substantial thermal and electric losses. These losses
 will e.g. show as the heating of the water passing through the cooling
 jacket 15. Because of these losses has it become advantageous to design a
 channel inductor without the cooling jacket 15, giving the following
 advantages;
 the inductive losses to the cooling jacket are eliminated;
 possibility to reduce the thermal losses by an increasing the distance from
 the hot melt to the cooling system comprised in the coil;
 possibility to increase the overall efficiency of the inductor;
 possibility to increase melt and/or superheat capacity; and
 reduced maintenance as the corrosion situation in the cooling jacket is
 eliminated as will all water couplings etc. and supply hoses or tubes for
 the cooling jacket.
 The inductor and especially the coil 11 must however be safe-guarded
 against melt penetrating through the lining and damaging the coil and also
 against excessive wear especially in cases with increased superheat or
 melt capacity which are likely to increase the temperature in the
 interface melt/refractory lining and possible also the flow rate in the
 inductor channel 14. The channel inductor shown in FIG. 2 is arranged
 without cooling jacket but with two fire walls 22, 22 to safe guard the
 inductor coil 11 from being reached by metal penetrating the lining. The
 fire walls are arranged to detect any melt penetration through the
 refractory lining 13 and placed in the refractory lining 13 between the
 coil 11 and the inductor channel 14. The first fire wall 21 or detection
 wall is essentially cylindrical and coaxially arranged around the coil 13
 arranged at a suitable distance from the inductor channel 14. The fire
 wall 21 has an open structure and is made in electrically conductive
 material. The fire wall is backed on both sides with a sheet of mica
 insulation 211,212. The first detection wall 21 or fire wall is arranged
 to indicate any metal penetration reaching this far in the lining 13 and
 is placed at such a distance from the coil 11 that metal penetration
 reaching the first wall 21 do not constitute an immediate danger but the
 inductor can be taken out for relining and other suitable repair at a
 planned coming stop in the production. This first wall 21 is also arranged
 to interact with the second wall 22 to measure the resistance in the
 refractory lining 13 between these two walls 21,22. By measuring the
 resistance in the refractory lining 13 between these two walls 21,22 it is
 possible to monitor the metal penetration to see if it continues beyond
 the first wall 21, should the resistance be reduced under a preset value
 the inductor is disconnected from its power supply. This measurement of
 the resistance can also be used for monitoring the condition of the lining
 13 and especially to indicate any change in moisture content in the
 lining. To high moisture content in the lining increases the risk for
 flash-over or leakage currents in the lining. The second wall 22 is often
 made of a heavier gauge wire material and will thus provide reinforcement
 to the refractory lining 13.
 According to the embodiment of the inductor illustrated in FIGS. 3, 4 and 5
 the inductor channel is designed according to the following criteria;
 the width W of the channel which exhibits an essentially oval or
 rectangular cross-section with a width to radial height ratio of 1.5 or
 larger, W/H.sub.rad ;
 the radial height, H.sub.rad, shall vary along the channel;
 and preferably shall the inner-wall 35 of the channel in area between the
 two openings of the inductor channel show an angle a across half the
 channel width that is 0 degrees at the openings and at least 30 degrees at
 a center point between the openings. The channel designed according to
 this embodiment exhibit an improved flow with essentially no zones of
 stagnation and dead-water in a cross-section at any point along the
 channel as the two-loop flow pattern, the thin lined circles 2a,2b in FIG.
 3, that normally is developed in a channel designed according to prior art
 is changed into a flow pattern exhibiting an essentially one-loop flow the
 thicker line 1, in FIG. 3. The variation in radial height H.sub.rad is
 accomplished with sectors where the height 36a, 36c, 36e is decreased
 alternating with sectors where the height is increased 36b,36d,36f. The
 change in relative height over such a sector correspond 36a, 36b 36c, 36d
 36e, 36f which corresponds to one-eight of the periphery is at least 25%,
 i.e. the ratio H.sub.max /H.sub.min is 1.25 or more. Essentially all zones
 of stagnation or dead-water is eliminated in the cross-section flow
 pattern in the channel 14 according to this embodiment thereby
 substantially reducing the deposition and clogging in the channel 14. The
 changes in the cross-section flow pattern will also substantially reduce
 wear in the channel 14.