MULTI-WIRE INDUCTION HEATING

Abstract: In the induction heating of thin (e.g. 0.5 to 3 mm diameter) steel wires (W) in continuous heating processes, relatively low field strengths of e.g. up 20,000 Am⁻¹ are employed but are chosen so as to set the relative magnetic permeability µr at 40 or more. The wire are passed through coil blocks (1), which are wound flat around a row of ceramic tubes (3) to guide and insulate the wires. The coil blocks are energized at frequencies typically up to 30 kHz. The direction of winding and/or the phase of the current is changed between adjacent coil blocks (1), or groups of blocks, to reduce voltage build-up along the wires.

Description: The present invention relates to an induction heating appara­ tus comprising a coil, said coil comprising at least two adja­ cent lengths, the direction or the phase of the energizing current being changed between adjacent lengths. There is frequently a need to heat steel wires to low or intermediate temperatures. Examples of thermal treatments are thermal diffusion, stress relieving, tempering, partial or full annealing, and so forth. The heating step may be carried out separately or in line with other stages such as surface treatments, coating processes, deformation processes etc. The present invention is especially concerned with the heating of steel wires of a diameter less than 5 mm, and preferably in the range 0.5 to 3 mm, to temperatures not exceeding the Curie temperature (750-760°C). The invention may be applied to other processes, however. For production purposes, it is generally desirable to heat a plurality of wires simultaneous­ ly while they move continuously in parallel paths and the invention is particularly applicable to such a context. Conventional devices in use for simultaneously heating a plu­ rality of steel wires include for example fuel-fired or elec­ trical furnaces, molten lead or salt baths and resistive heating lines making use of direct electrocontact wire heating and the like. The performance of conventional furnaces is limited with regard to high speed precise heating of wires up to 750°C, because of rather slow heat transfer rates in this low to intermediate temperature range. As a consequence, furnaces of considerably increased length are necessary for achieving high wire throughput speeds, which is both technically and economically an unattractive solution. Direct immersion heating in molten salt and lead, to the contrary, is rapid and more effective, but the major drawbacks here are wire sur­ face contamination, demanding extra cleaning, and inferior working conditions. Direct electroresistive devices, feeding the heating current to the wires by means of electrocontact rollers, have been adopted by the industry for several wire heating applica­ tions. With increasing speed and power density, however, it has been found that the risks of sparking and surface defects may become very critical. In the production of high quality and fine wires, this is highly undesirably. A promising solution is afforded by induction heating methods which produce rapid non-contact heating. Induction heating is nowadays a widely spread technology for heating large-sized workpieces such as billets, bars and rods prior to forging or working etc. and for surface hardening articles such as axles, sucker rods, etc. The technique is already applied in some specific areas of the wire industry, for example for stress relieving of prestressed concrete wire and strands. It has also been employed to a given extent for the annealing of drawn copper wire in line with the wire drawing machine. In this way FR-A-2.323.290 discloses an induction furnace where the electrical current is reversed between some adjacent parts of the coil. Induction heating is not, however, widely used in multiwire installations, which at present can treat a large number of wires (5, 10, 20, 30 and even 40 or more) simultaneously. The concept of non-contact inductive heating of plurality of wires has long been regarded as being technically impractical and/or too wasteful in energy and capital, in particular for smaller wire sizes. It has been stated that induction heating is too expensive because of the reputed need for distinct induction heating coils for each wire track, and the complexi­ ty of branching all the inductors to current supplies of suitable frequency. Similarly, it is a common belief and expe­ rience that electrical and/or thermal efficiency remain inade­ quate when comparing induction heating of wires to other heating methods. In this respect it has been believed that inductively heating small diameter wires is not only highly unsatisfactory but may become even impossible below a certain diameter limit of, say, 1 to 1.5 mm. It will be appreciated that the induction heat input may be less than for conventional arrangements, although by virtue of the invention it will be at practical levels. This means that the length of time required for heating will be increased. The invention is particularly concerned with the heating of continuously moving wires and thus, in comparison with what would be the case in a conventional heating arrange­ ment, greater coil lengths will be required. This results in a potential problem to be solved. The heating requirements will vary according to wire type and diameter, and the coil length will also depend on the wire speed which may, for example, be in the range of 10 to 100 m.min _1 or more. A typical coil length may, for exam­ ple, be about 2 or 3 m to 5 m. We have found that this can result in unacceptably high induced potential differences over the wire length. This is normally not a problem in induc­ tion heating, but in this particular case _ when using long coils and continuous wire lines _ the accumulated voltage in the longitudinal wire direction may attain a critical value (up to more than 50 V) and thereby cause sparking between the wire and earthed components of the installation such as guiding members, take-up reels and the like, which may result in intolerable wire surface marks or defects. It is believed that a main cause of excessive voltage accumu­ lation in the longitudinal wire direction is the fact that, due to pitch of the coil windings relative to the wire axis, a spatial electric field is induced in the wire, which is slightly inclined to the wire direction. Said field can be resolved into a large radial component and a small axial com­ ponent the vector sum of which creates a gradual potential increase along the wire length. Dividing the induction coil into separate parts between which the electrical current is reversed, as taught by FR-A-2.323.290, causes dead spots in the magnetic field due to the interaction of the ends of adjacent parts. It is an object of the present invention to avoid this draw­ back. According to the present invention there is provided an induc­ tion heating apparatus comprising a coil, said coil com­ prising at least two adjacent lengths, the direction or the phase of the energizing current being changed between adja­ cent lengths, characterized in that the direction of coil winding is also reversed between adjacent lengths. In this way dead spots are reduced between adjacent lengths. In a preferable embodiment of the present invention the coil has a generally flat cross-section and the winding of the coil closely envelopes a planar array of a plurality of elon­ gate steel elements. A plurality of guides may be provided for receiving the elongate steel elements. The guides may be in the form of tubes of an insulating material. Most preferably the windings of the coil are also substantial­ ly flat. It will be appreciated that, in general, the reversal of coil winding results in a reversal of the angle of inclination of the windings to the axis of the coil. Some embodiments of various aspects of the invention will now be described by way of example and with reference to the accompanying drawings, in which : Figure 1 is a plan view of a general embodiment of a heating apparatus comprising a multiwire induction coil connected to a medium-frequency generator; Figure 2 is an enlarged cross-sectional view of the inductor taken along the ine A-A of Figure 1; Figure 3 is an enlarged cross-sectional view taken along line B-B of Figure 2 and showing one preferred embodiment of the coil winding configuration in the longitudinal wire direc­ tion; Figure 4 shows another preferred embodiment of the coil winding construction; Figure 5a and 5b show a more detailed view of a specific embodiment of a multiwire induction heating coil construction comprising copper strip windings with brazed-on copper cooling tubes; Figure 6 is a schematic representation of a preferred embodi­ ment of an induction heating line featuring a modular building block concept with a sequence of identical multiwire inductors, connected in parallel to the supply; Figure 7a gives a graphical representation of the working range (magnetic field strength and permeability) in the method according to the invention in comparison with conven­ tional practice; Figure 7b shows in particular the measured µr-values of common ferrous wires versus ampere turns per meter; Figure 8 shows different possibilities of serially connected heating coil arrangements to accommodate longitudinal voltage build-up in the wire direction; Figure 9 illustrates several embodiments of parallel con­ nected heating coil arrangements for minimizing the voltage build-up in the axial direction of inductively heated wires; Figure 10 shows an embodiment of a multi-wire induction heating block concept containing also an improved current sup­ ply connection; and Figure 11 shows some specific heating line set-ups permitting heating and soaking (11a), and heating above the Curie-temperature (11b). Referring now to Fig. 1, giving a general view of an induc­ tive heating system, there is shown a multiwire induction heating coil 1 energized by a medium (or high) frequency cur­ rent supplied by a suitable generator 2. The electrical cir­ cuitry of generator 2 normally comprises a power supply trans­ former (Tr) and a 3-phase Thyristor rectifier bridge (Rf), choke coils (CH), a thyristor (Th1 to Th4) controlled frequen­ cy current generating section and a capacitor array (C) of appropriate capacitance (to compensate the impedance of the load circuit in relation to the electrical characteristics of the generator). A plurality of wires W travel continuously through a flat inductor 1 and more in particular, each wire passes through a distinct heat and electrically insulating tubular guide 3. The guides are preferably ceramic tubes of suitable cross-section and are arranged here as a one-layer planar row, enveloped by the windings 4 of the heating coil. For round wires one normally uses cylindrical tubes, whereas for shaped wires such as e.g. flat wires, a tube with rectangular cross-section can be envisaged. The coil windings may be made of solid high-conductivity copper wire, cable or strip, pro­ vided suitable cooling means are incorporated in the coil outer construction (e.g. a water mantle separated from the coil interior and directly contacting the windings) or of tubular copper windings with inner water cooling. Figure 2 gives a cross-sectional view, perpendicular to the wire direction, of a flat multiwire heating coil clearly showing the location of coil windings (4), ceramic tubes (3) and wires W. Figures 3 and 4 give further details by showing a longitu­ dinal cross-section of the heating coil through the wire axis (along line B-B of fig.2). In Fig. 3 an embodiment is shown wherein the coil windings consist of solid copper strip (5) which is water-cooled by means of a continuous copper tube (6) brazed on the exterior surface of the strip windings. This type of copper windings makes it possible to fabricate very flat heating coils with an interior inductor tunnel height of even below 20 mm. Figure 4 shows a heating coil type formed of water cooled copper tubes (7), which preferably have an oval shaped, flat-rounded or flattened cross-section for the purpose of being coilable to low inductor heights (closely matching the outer periphery of the tubular wire guide array on the inside). In constructing and using the heating coils, it is important to bring inductor height as close as possible to the interior conveying tubes and to hold this height constant in the lateral and longitudinal coil direction during ser­ vice. Among the several possibilities for fabricating a multiwire coil one may choose the following method. First the copper conductor is wound on a mandrel of suitable dimension to form a heating coil of desired geometry (height, width, length and number of turns per meter). The coil is stress relieved for form stability, and coated with a polymeric or other material to insulate the individual inductor turns (e.g. stretching the coil helix as a spring and dipping it in a suitable varnish, curing / drying, releasing the helix). Thereafter the bare coil is provided with appropriate mechanical fixing means and with the necessary electrical connections and cooling accessories. Alternatively the coil windings can also be permanently stabilized by means of a rigid mass, e.g. by mouldforming in a suitable plastic material (epoxy and the like) or in a fibrous cementitious material such as fibre cement or fibre concrete. The interior tunnel space of the finished inductor is pro­ vided with the prescribed plurality of suitable guiding chan­ nels, which could be provided by a multi-groove refractory plate or a multi-hole muffle but in this case comprises the plurality of ceramic tubes 3 to convey the wires individually through the inductor. Between the ceramic tubes and the copper windings, one can also dispose an extra heat-insulating barrier layer, e.g. a mica sheet, a refractory fibre felt or the like, for the pur­ pose of further reducing heat loss of the wires to the windings. This layer also increases system security in case of an accidental break of a tubular guide, and eliminates risk of direct contact between wire and coil. The ceramic tubes inserted in the flat rectangular inductor channel (having a height of a few millimeters larger than the tube diameter) are fixed at a coil entry and exit by appro­ priate clamping means. When the total heating length is com­ posed of a plurality of separate adjacent coil modules, the ceramic tubes may either be continuous over the entire heating paths or they can extend over the length of one coil block only and then be connected in line with the subsequent modules by suitable flanges. The advantage of the latter arrangement relies upon the fact that a broken tube and/or a faulty coil module is quickly removed and replaced by a new component. Figure 5 gives a more detailed view of an inductor comprising a continuous coil length subdivided in two serially connected coil sections, having opposite winding pitch directions (change of winding direction in the coil middle 8). In this embodiment the coil windings are made of copper strip 5 bearing a brazed-on cooling pipe 6, which is shown in more detail in the deployed cross-section of Fig. 5b. This coil has a length of about 2.4 metres and comprises 37 turns of copper strip 2 mm × 55 mm. The interior coil height is 20-­ 22 mm and in the inductor channel, having a width of less than 60 cm, 36 ceramic tubes of 15 mm outer diameter and 2 mm thickness can be fitted. This means that in this case wire spacing is nearly 15 mm. When using smaller tubes, wire spacing can still be lowered to increase the inductor filling factor. In a production line, however, the minimum attainable spacing will also depend on practical considerations such as the difficulty of pulling the wires through very small holes, which can lead to wire breakages, availability of small-diameter ceramic tubes, possibility of wire entanglement, etc. When the induction heating step is integrated in a wire processing line, the actual wire inter­ distance can also be imposed by the minimum spacing attaina­ ble in the process step prior or subsequent to the heating step. In Figure 6 a multiwire induction heating line is schema­ tized, comprising a sequence of separate coil blocks (1′), each featuring a multiwire coil construction. The coil is made of oval shaped tubular copper windings 7 (36 turns of roughly 10 mm width) as depicted in Fig. 6a. Here the conduc­ tor windings are separated from the ceramic tubes by means of a heat-resistant insulating layer 10 of a few mm thickness. In Fig. 6b the coil blocks (1′) are connected in parallel to the current supply by means of 2 parallel bus bars 9. This arrangement is very favourable for maintaining the useful capacity of the generator at a high level (versus installed power capacity) because ohmic and inductive voltage losses over bus bars and heating coils can be kept low as compared to serially connected coil blocks featuring considerably longer current supply lines. Figure 7a illustrates the dependence of relative magnetic per­ meability µr on applied field strength H based on the mag­ netic measurements carried out on several ferrous wire types (band A relates to unalloyed low and high carbon steel wires, curve B relates to ferritic high-Cr steel wire). The selected working range corresponds to µr values above 40-50, prefera­ bly above 80 and most preferably from 100 to 200. To achieve these conditions, coil design (turns per m) and energizing current are adapted to cover a broad working range from 3,000 to 35,000 A/m, preferably from 5,000 to 20,000 A/m. It will be noted that for µr values above about 200, the rapid change in µr for a change in H can lead to undesirable fluctuations in working conditions. This approach considerably departs from the working range close to or beyond the saturation magnetization Hs (µr far below 40 and usually around 10-20), which is the normal prior art procedure of induction heating, as indicated in Fig. 7b. In using the multiwire flat coils and in applying the ener­ gizing conditions prescribed, coil efficiencies from 60 to 90% (according to wire diameter) are obtainable, typically from 70 to 85% for the wire diameter range 0.75-2 mm at generator frequencies from 8,000 to 25,000 herz. Total actual energy efficiencies (which also depends on actual generator efficiency and its efficiency versus load curve) in all cases tried exceeded 50%, and in a majority of practical processing situations the system total energy efficiency ranged from 60 to 75%, which are surprisingly high levels for the wire diameters concerned. Fig. 8 shows various heating coil arrangements for reducing voltage build up, whereby either a continuous long coil is subdivided into at least two coil sections which are wound in opposite senses (Fig. 8a), or whereby the total heating length is composed of a varying number of serially connected distinct coil blocks (Fig. 8b-e) which are either wound in opposite directions and have the same instantaneous current flow direction (Fig. 8b), or interconnected to the supply in such a way that the current flow direction is reversed in some coil blocks (Fig. 8c-e). The coil block may also be reversed in terms of winding direction. In Fig. 8 the current direction at any instant is illustrated by arrow i. Another arrangement is represented in Fig. 9, showing an induction heater subdivided into a number of coil sections or in distinct coil modules, which are connected in parallel to the supply and whereby the voltage build-up along the wires is reduced either by reversing the current direction, or the coil winding direction or both. Fig. 9a and 9b show a coil subdivided in 2 sections with cen­ tre tapping of the supply, either with the same windings (9a) or with opposite windings (9b). The arrow i gives the instan­ taneous direction of the current. In the modular building block concept (Fig. 9 c-d-e) one can change the current direc­ tion from block to block (Fig. 9c) or from block assembly to assembly (Fig. 9d) and maintain the same coil block winding direction, or also change both the winding and current direc­ tion such as represented in Fig. 9e, showing an embodiment wherein current flow and winding direction are each time reversed simultaneously between two adjacent coil blocks. The most preferred embodiments of the examples shown in Fig. 8 and 9, are these multi-coil arrangements whereby simulta­ neously coil winding direction and current flow are reversed. In this way, potential differences between the wires and the earthed installation components are effectively suppressed to negligible levels (1 to 5 V max.) and also the local poten­ tial differences over the heating length are largely reduced. Moreover, all the ampere-turns of the successive coils remain active because the reversing arrangement also rules out the negative interaction between two adjacent coils, which interaction otherwise lowers the effective heating capacity of the heating line. In Fig. 10, a preferred embodiment of a multicoil building block (connected in parallel) is shown, which is additionally improved in that current supply is effected by means of a novel design of bus bar connection, comprising two parallel conductor bars of suitable form (e.g. U or L-profile) at close interdistance (max. 2 mm) and separated by a thin in­ sulation layer, for example in the form of a 3-layer com­ posite bar. This current supply arrangement is surprisingly beneficial in that it largely prevents the undesirable decrease in available work voltage (terminal voltage of generator versus effective voltage over the heating coil con­ nections), not only by reducing current supply distance and related ohmic voltage drop (through parallel tapping to the supply), but in particular by suppressing or compensating the inductive field (and related voltage drop) effects along the current supply bars and/or between the supply and the coils. The preferred embodiment shown in Fig. 10 is an induction heating line of modular building block concept, comprising a sequence of multi-wire coils (1′), connected in parallel to the supply (with current reversal between the coils) which features the improved bus bar design comprising two parallel U-profiled conductors 11, with insulating interlayer 12. An additional advantage of the bus bar construction is that it enables quick and simple attachment/replacement of standard coil blocks. In Fig. 11 two additional induction heating arrangements are illustrated, aimed at carrying out specific heat treatments. Fig. 11a shows a heating line for heating and soaking, com­ prising a first set of coils with impedance Z1 and a second set of coils with different impedance Z2, with all the coils being preferably connected in parallel to the same generator. Impedance Z2 is adapted to hold the temperature level attained after rapid linear heating with coils Z1. Other tem­ perature profiles are of course feasible. In Fig. 11b, a pre­ ferred embodiment is shown to simultaneously heat a plurality of ferrous wire above the Curie point. The induction line is constituted of respectively a medium-frequency heating path (coils C1 connected to generator G1) and of a high-frequency heating path (C2-G2). As shown schematically in the accompa­ nying heating diagrams, effective high-temperature inductive heating is possible (provided the required high level of generator frequency is applied and available at sufficient power) and a major part of total heating (up to approximately 760°C) still occurs at optimum electrical efficiency. It is further obvious that the multiwire induction heating apparatus being equipped with individual ceramic tubular chan­ nels, enables the use of any protective atmosphere. Gas con­ sumption can be kept low because of the small tube diameter and gas volumes involved. The following examples will further illustrate the practical advantages and versatility of the preferred multiwire heating apparatus and methods. Example 1 A multiwire induction coil having a total length of 4.6 m is used for the simultaneous heating of 36 steel wires of 1.20 mm diameter. The inductor (similar type as shown in Fig. 5) is coiled from copper strip 100 x 2 mm (water cooled by a copper pipe of 10 mm diameter brazed on the strip windings′ outer surface), enclosing an inner inductor channel with a flat rectangular cross-section of 22 mm (height) × 560 mm (width). It contains 36 ceramic tubes (in planar parallel pattern) for guiding each wire individually. The coil is divided into two coil sections, each with 17 windings, respec­ tively right-hand and left-hand windings. A 200 kW-10 kHz generator is connected to the coil. All the wires can be uni­ formly heated to a temperature of up 700°C and at speeds up to and exceeding 50 meters per minute. In a practical example the multiwire inductor heater was used in line with a patenting and electrogalvanizing process installation to carry out a thermodiffusion step on copper and zinc-coated steel wire at a uniform line speed of approximately 40 m/minute. The heating conditions were as follows : induced field strength : 14-15 . 10³ A/m relative magnetic permeability: µr = 95-100 net dissipated heat in the wires : 70 kW generator efficiency : 90.2% coil efficiency : 65-72% total heating efficiency : 60-64% average wire temperature : 596°C standard deviation : 14°C (standard deviation : scatter between the wires related to their relative position in heating coil and as result of slight variations in wire surface humidity electroplating, rinsing and drying.) max. potential difference in longitudinal wire direction 32 V (coil entry to exit) voltage drop from wire to earthed guides reel : 26-29 V Example 2 A multiwire induction heating apparatus was designed to treat simultaneously 40 ferrous wires up to a temperature of 700°C. The apparatus comprises two heating coils of 2.4 m length and 0.62 m width. Each coil has 37 copper solid strip windings of 55 × 2 mm (provided with water cooled copper tube) which form an inductor channel of about 20 mm height wherein 40 ceramic tubes (14 mm outer diameter, 10 mm inner diameter) are inserted. The distinct coils are connected each to a 80 kW-­ 25 kHz generator. The 37 windings are subdivided into 18 right and 18 left-wound turns. The induction heating installa­ tion was used to treat 40 wires of 0.7 to 1.4 mm diameter at a speed of 35 to 65 m/min. In carrying out a thermodiffusion treatment on previously plated and patented wires, the working conditions were as follows : wire 0.70% C of 1.04 mm diameter ; throughput speed 48 m/min. applied magnetic field strength : 8500-9000 A/m relative magnetic permeability µr of wire material : 140-160 working frequency : 20,600 herz Heating Results : heating power dissipated in wire : 66 kW (generator 1 + 2) heating coil efficiency : 70-76% total heating efficiency : 58-67% average wire temperature : 585°C mean spread (40 wires) : s = 12.3°C max. potential difference on wires between entry exit of heating coils : 17 V max. voltage drop from wires to earthed reeling means : 15.2 V Example 3 In this example the usefulness of the multiwire induction heating system is demonstrated for tempering of oil quenched (martensitic) steel wire, and more in particular flat wire with 5 × 1 mm section. For this purpose an inductor building block has been designed of 2 m length, constituted of 4 coils of 0.50 m in series, each comprising 50 windings of tubular copper. The coils were connected to a 40 kW-generator working at 25 kHz. In the heating coil tunnel 10 ceramic conduits of rectan­ gular cross-section 16 a 10 mm (wall thickness 2 mm) were arranged to pass each of the 10 flat wires through its respec­ tive ceramic channel. The shaped wires were tempered at a tem­ perature of 480°C at a speed of 10 to 25 m/min. Mean spread of wire temperature was smaller than 10°C. The system, which replaces a conventional lead tempering bath, is particularly advantageous in that it is compact and clean in terms of working environment in that the lead con­ tamination, which sometimes affects wire surface quality, is absent and in that total energy consumption is markedly lower _ i.e. negligible heat loss to surroundings ; compact system with zero heat content, no stand-by heating, etc. To demonstrate the versatility of the multiwire coil provided with flat tubular ceramic guides, it was tested for simulta­ neously heating a plurality of multiwire tapes, each tape com­ prising 10 parallel wires of 0.5 mm held together by an adhe­ sive compound. The test revealed that it was perfectly possi­ ble to dry and cure the composte wire tapes at a constant tem­ perature (200 to 300°C) and at elevated velocities of up to 300 m/min. Energy efficiency amounted to 55-70% (coil efficiency up to 80%) according to generator type and load factor. Example 4 In this example the versatility of multiwire heating coils is illustrated with regard to various heating applications. Dif­ ferent coil winding materials have been used to fabricate the desired multi-wire heating coils. In a first series of tests a 10 wire induction heater was pro­ vided with one or more heating coils of a total heating length ranging from 1 to 2 m. Steel wires with a diameter from 0.5 to 2 mm were heated to different temperature levels below 750°C. Wire throughput speed was varied from 20 to 100 m/min. Two generator types were used : a 40 kW-25 kHz device and a 80 kW-10 kHz device. Inner coil height was 22 mm, filled with one row of ceramic tubes of 15 mm diameter. Among the tested coil windings were : solid copper strip (20 × 2 mm), layered copper foil (10 layers of 20 × 0.2 mm), copper cable (7 × 4 × 0.30 mm), copper strip (55 × 2 mm) with exterior cooling tube and flattened copper tube (10 × 6 mm). The volumetric power density Q (total heat dissipated in the wires divided by number of wires, coil length and wire cross-section) was varied from 0.3 to 3 kW/m/mm² and the actual heating efficiencies were compared, calculated from power supplied by the mains and from the amount of coil losses and generator losses (measured by determining heat con­ tent of cooling water). The general conclusions of the tests are as follows : · the multiwire induction heating coils made it possible to carry out rapid heating of medium to small wire diameters with reasonably good total energy efficiency. According to wire diameter, velocity (applied energy density) and generator type the total efficiencies range from 50 to 75%. · coil efficiencies are higher than 65% and mostly range from 70 up to 85% (lowest value for highest energy density of 3 kW/m/mm²). · for highest wire speeds improved coil designs (optimum length and number of windings per m to allow optimum energy density conditions) allow one to increase coil efficiency by 5-15%. · generator type, in particular generator efficiency curve vs. load factor, is important to attain improved "total" ener­ gy efficiencies. · wire velocities in the 150-250 m/min. range are perfectly sustainable. · max. rate of wire speed in multiwire induction heating can in principle be raised to above 500 m/min. (still with satis­ factory heating efficiency) when enlarging the total heating length of a modular inductor building block by adding addi­ tional coil modules with all the coils connected in parallel to a current source (one or more generators) of sufficient power. Example 5 In this example, referring also to Figures 8 and 9, a large number of coil arrangements has been investigated for the pur­ pose of controlling the axially induced voltage build-up in the moving wires. A heating length of 4.5 to 5 m was chosen and provided with either continuous coils, subdivided coils or modular coil blocks (either serially or connected in parallel to the supply), whereby the winding direction and/or the phase of the current were changed. The unexpected results of these investigations was the fact that current flow direc­ tion in the coil section or coil block is comparatively of greater effect and importance in controlling and compensating the induced voltage in the longitudinal wire direction than the change in magnetic flux direction by using opposite windings. In theory both measures should have similar effects. We observed however in imposing a current direction reversal between 2 coil sections or 2 adjacent coil blocks, that total heating capacity is lowered to a significant extent. The most preferred arrangement, which we found to be surprisingly effective in practice when using a squence of replacable coil blocks (connected in parallel to the supply), is the simultaneous reversal of both the current flow direction and the winding direction. Some typical results are given below : It is evident that many modifications are possible, such as for example in the choice of the most appropriate coil geome­ tries, inductor tunnel design and work path configuration and the like for a given application. In addition the induction systems as described hereinbefore may be provided with suitable temperature sensors and appropriate current regu­ lating devices aimed at subjecting the workpieces to automati­ cally controlled heating profiles such as for example a pro­ grammed heating schedule. Whilst the embodiments have been particularly concerned with thin wires, the apparatus and processes could be used for much thicker wires, e.g. 3 to 6 mm diameter or larger, as well as non-ferrous e.g. copper wires, stainless steel wires, or shaped wires of non-circular cross-section. Additionally, high temperature heating is possible _ e.g. above the Curie point when ferrous materials become non-magnetic. The induc­ tor configuration and frequency should be adapted for each specific purpose. Considering now various aspects and advantages of the pre­ ferred embodiments of the invention, without limiting the broader aspects thereof, the apparatus permits heating of a number of elements in which each element is to be subjected to the same heating profiel extending along the length thereof and is guided along a distinct insulated path through the apparatus, the individual paths being arranged in such a way as to form a regular array of straight and parallel chan­ nels contained in the interior space of a common inductor and enveloped by the windings thereof. In addition to being energy efficient and being capable of uniform, effective and economic heating up to high wire speeds and down to small wire sizes, the apparatus permits identical heating of each individual wire to an adaptable heat profile, eliminates the need to stop the whole line in case of an incidental wire break (even at elevated wire velocities) and prevents the occurrence of wire surface defects such as e.g. contacting and/or sparking marks. The preferred induction heating apparatus comprises suitable means for feeding and energizing the inductor with a regu­ lable current of desired frequency and further comprises suitable means for conveying the plurality of elements at the same adjustable rate of speed longitudinally along their respective parallel work paths leading to and through the heating coil(s) of the inductor, the apparatus being provided more specifically with a flat shaped inductor constituted of one or more induction heating coils disposed adjacently in the longitudinal wire direction, which coils form an induc­ tion heating tunnel of adaptable length with flat rectangular cross-section containing all the work paths therein. The work paths are arranged in one or more horizontal rows of closely spaced insulated channels through which the metal elements are individually guided and along the length of which the ele­ ments are brought to the same prescribed temperature. From another aspect, the preferred apparatus comprises a flat shaped inductor device for inductively heating metal wires, more in particular a multiwire inductor for heating simulta­ neously a plurality of ferrous metal wires (for example to a temperature not exceeding their Curie point; approximately 750°C), whereby the wires are guided individually through identical refractory channels disposed in the inductor channel preferably as a one-layer sheet, which channels extend over the full length of the respective heating coils of the inductor. According to a specific design possibility of the multiwire coil, the channels can be in the form of a multihole ceramic muffle, and most preferably in the form of distinct (replacable) ceramic tubes of suitable cross-section, disposed as a closely packed one-layer configu­ ration and tightly enveloped by the coil windings. From a still further aspect, the preferred apparatus provides an improved inductor system for simultaneously heating a plu­ rality of identical wires with a high energy efficiency (exceeding 50%) and elevated wire capacity (heating up to 40 and even more wires simultaneously at velocities ranging from 10 to more than 100 m/min. according to wire type and pro­ cess), which inductor line may be constituted of one con­ tinuously wound coil unit of desired coil length or of a num­ ber of separate coil modules (as replacable blocks) of shorter length arranged together (in a so-called building block concept) longitudinally and electrically connected in series or in parallel so as to form a multi-wire heating path of any desirable length thereby allowing progressive power input and effective heating at any desired rate and further allowing, by selecting the optimum combination of coil parame­ ters and energizing conditions (current density and fre­ quency), one to enhance energy efficiency and wire throughput well above prior art methods for induction heating of wires. The coil or coil module construction may be made in the form of an integrated preassembled building block containing all necessary electrical and cooling attachments and having the insulated coil windings rigidized in an electrically non-conductive moulding mass or rigidized by other suitable mechanical means so as to maintain in service a constant height of the flat interior inductor tunnel, in which a plu­ rality of replacable ceramic tubes is fitted for conveying the individual wires in parallel straight paths from entry to exit, whereby the tubes may extend uninterruptedly over the entire heating length or are only continuous per coil module length and then interconnected in line from one coil block to another so as to provide a continuous wire path. In accordance with a preferred feature there is provided a specific arrangement of coil windings and of the electrical connection of the successive heating coils to the current sup­ ply so as to minimize voltage build-up along the length of the inductively heated wires and thereby avoiding possible sparking between the wires and the guidance or take-up mem­ bers. A further preferred feature relates to the current supply connection between generator and working coils which connec­ tion is designed in such a way that ohmic losses, and in particular inductive voltage losses, are substantially reduced so that actual available power (ratio of coil working voltage to terminal voltage of generator) remains the highest possible. Accordingly, there is provided a bus bar connection of composite construction comprising two parallel conductor profiles at very close distance, having a thin insulation layer separating said profiles, which composite bus bar is disposed parallel to the longitudinal inductor direction. This bus bar, and particularly in combination with induction coils, may be used in contexts other than those disclosed herein. In accordance with the preferred process, there is provided an improved method for inductively heating a plurality of ferromagnetic metal wires, such as carbon steel wires, which improvement relies upon the judicious selection of coil geometry and coil energizing conditions, in which the active magnetic field is kept below a level of 30 to 35,000 A/m and the relative magnetic permeability of the heated wires exceeds a value of 40-50, so as to minimize coil losses for a given wire diameter range without imparting heating capaci­ ty and wire throughput. In the preferred embodiments there are provided reliable and economic methods and an improved multiwire induction appara­ tus for use in uniformly heating a plurality of carbon steel wires of 0.3 to 3 mm diameter at elevated travelling speeds and at high efficiencies, without the need to stop the line in case of an incidental wire break and without causing sparking defects. A general advantage of the preferred embodiment featuring a particular inductor design, a flexible coil block concept and an improved energizing arrangement, is the possibility of car­ rying out contactless, rapid and economic heating of ferro­ magnetic steel wires at high rates, which heating step may be carried out separately or continuously in line with a pre­ ceding or subsequent wire treatment stage (depending on the type of wire process or multiwire processing installation) and which step can be adapted in velocity to be consistent with the optimum processing speed of the entire manufacturing line.

Claims: 1. An induction heating apparatus comprising a coil (1) said coil (1) comprising at least two adjacent lengths, the direction or the phase of the energising current being changed between adjacent lengths, characterized in that the direction of coil winding is also reversed between adjacent lengths. 2. An induction heating apparatus according to claim 1 characterized in that said coil (1) has a generally flat cross-section and the winding (4) of the coil closely envelopes a planar array of a plurality of elongate steel elements (W). 3. An induction heating apparatus according to claim 2 characterized in that a plurality of guides (3) are provided for receiving said elongate steel elements (W). 4. An induction heating apparatus according to claim 3 characterized in that said guides (3) are in the form of tubes. 5. An induction heating apparatus according to claim 4 characterized in that said guides (3) are of an insulating material. 6. An induction heating apparatus according to any one of claims 1 to 4 characterized in that the windings (7) of said coil (1) are substantially flat.

Inventors: NEIRYNCK, MICHEL, VANNESTE, GODFRIED
Applicants: N.V. BEKAERT S.A.
Application year: 1986
Main classification: H05B6
IPC codes: C21D9, H05B6, C21D1