Induction generator to heat metallic pipes with a continuous process under a controlled atmosphere

Abstract: An induction heating generator is described for braze-welding metallic pipes (37) with a continuous process under a controlled atmosphere, comprising at least one heating inductor (33) composed of two half-inductors (33',33") in series whose winding directions for their windings are one opposite to the other for removing residual induced voltages on the pipes. The generator may be further equipped with a quartz tube (39) placed inside the inductors (33) and in which the metallic pipes (37) pass together with the controlled atmosphere gas (35). The generator may further include at least one removable sealing insert (41) to join two contiguous inductors (33). For every medium- and/or high-frequency section of the generator, a load matching system may finally be provided to compensate for external diameter variations of the pipes (37), the system being interlacedly handled by a programmable controller (P.L.C.).

Description: The present invention deals with an induction generator to heat metallic pipes with a continuous process under a controlled atmosphere, and particularly with double-walled metallic pipes for motor vehicle braking systems or metallic pipes with a small diameter for refrigerating applications and the like. In the current art, plants are known for braze-welding, in ovens with a controlled atmosphere, crop ends of bimetallic pipes or for braze-welding with a continuous process double-walled pipes by means of resistance heating, and moreover plants for induction braze-welding double-walled pipes with a continous process. These latter ones employ heating inductors immersed into a controlled gas atmosphere, through which inductors the bimetallic pipe to be braze-welded continuously passes. During this passage, however, residual voltages induced on the product are generated, these voltages being dangerous since they can create electric discharges between bimetallic pipe and inductor and/or between bimetallic pipe and driving rollers. Furthermore, there are strong explosion risks as regards the hydrogen-based controlled atmosphere, due to gas ionization for these residual induced voltages. A solution to the above drawback has been proposed in JP-A-60124482 and JP-A-5261526 which disclose induction heating generators suitable for braze welding with a continuous process under a controlled atmosphere, comprising at least one heating inductor consisting of a first and second coil in series having opposite winding directions so as to remove residual induced voltages on the metallic pipe. Document GB-A-2253214 discloses an induction heating generator for annealing metallic pipes with a continuous process under a controlled atmosphere comprising a glass chamber for containing inert purge gas and the tube being annealed, said glass chamber being encircled by an electromagnetic coil of an induction heating system. These known plants have maintenance problems, because, when there is the need of cleaning them or of replacing a damaged inductor, it is necessary to disassemble and re-assemble the gas-sealed flanged tubular sections with lengthy and cumbersome operations. Moreover, when the external diameter of the produced pipe changes, it is necessary to replace heating inductors through other problematic operations. Object of the present invention is providing an effective arrangement to join containing tunnel sections that makes maintenance operations easier, and an electronic arrangement that does not oblige replacing the inductors at every diameter change for the produced pipe. The above-mentioned and other objects and advantages of the invention, as will appear from the following description, are obtained by means of an induction heating generator for braze-welding metallic pipes with a continuous process under a controlled atmosphere as claimed in Claim 1, with the constructive and improving embodiments claimed in Claims 2 to 11. The present invention will be better described by some preferred embodiments thereof, given as a non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a first embodiment of the induction heating generator according to the present invention; Figure 2 is a block diagram of a second embodiment of the induction heating generator according to the present invention; Figure 3 is a circuit diagram of the medium-frequency section of an embodiment of the present invention; Figure 4 is a circuit diagram of the high-frequency section of an embodiment of the present invention; Figure 5 is a top view of the sealing insert between inductor sections of the present invention; and Figure 6 is a top view of one of the heating inductors according to the present invention. Fig. 1 shows a first embodiment of the induction generator of the present invention: in particular, there are shown here two induction generators with different frequencies for braze-welding double-walled metallic pipes with a continuous process, under a controlled atmosphere, especially for motor vehicle braking systems. These generators are composed of a medium-frequency section 1 and of a high-frequency section 3; these sections 1 and 3 respectively include a power supply stage 5 and another power supply stage 7 both containing thyristor converters for the linear adjustment with partialization of the braze-welding output power. The power supply stages 5 and 7 also include a Programmable Logic Controller (P.L.C.), that will be described in detail below, to handle interlaced systems that allow adjusting the load depending on its diameter variation without the need of replacing the inductors. Connected in cascade with the power supply stages 5 and 7, there are two frequency generator stages 9 and 11, respectively: the stage 9 is with ad inverter bridge, operating, in the described embodiment, at 1⊘ ÷ 3⊘ kHz to pre-heat the product with a continuous process, while the stage 11 is equipped with an electronic oscillator tube, operating, always in the described embodiment, at 3⊘⊘ ÷ 5⊘⊘ kHz to finally heat the product at the braze-welding temperature thereof. Downstream of the stages 9 and 11 there are the inductor banks 13 and 15, including at least one heating inductor (in the described embodiment, the inductors are four per bank, and their number changes depending on the power output by generators and on the pipes to be produced). The bimetallic pipe produced moves on the line along the direction shown by arrow A and, after having passed the bank 13, is controlled by an optical pyrometer 17, equipped with optical fibers, whose purpose is monitoring process temperatures and feed-backing in a loop the automatic correction signals. The product then enters the bank 15 and is afterwards controlled by the optical pyrometer 19, quite similar to the pyrometer 17. Fig. 2 shows a second embodiment of the induction generator of the present invention: in particular, there is shown here a high-frequency induction generator for annealing metallic pipes with a small diameter with a continuous process, under a controlled atmosphere, for refrigerating applications and the like. This generator is composed of a single high-frequency section 21 that includes a power supply stage 23 containing thyristor converters and a programmable controller, whose functions are the same as those previously described with reference to Fig. 1. Connected in cascade with the power supply stage 23, there is the high-frequency generator stage 11, that is always with an electronic metallic-ceramic tube oscillator for industrial applications. Downstream of the stage 11 there is the inductor bank 27, including two inductors. The produced pipe to be annealed moves on the line along the direction shown by arrow B and, after having passed the bank 27, is controlled by an optical pyrometer 29 quite similar to the previous pyrometers 17 and 19. Both the embodiments shown in Fig. 1 and 2 provide, for the various inductor banks 13, 15 and 27, a single type of heating inductor with constant length and internal diameter for the whole range of pipes to be annealed and braze-welded, that, at the current state of the art and of the commercial standards, is included (but not limited to) between the 4.76-mm external diameter and the 1⊘-mm one. In the following the embodiment of the induction generator shown in Fig. 1 will be described in detail, omitting the one shown in Fig. 2, that is practically a sub-set of the previous embodiment, being wholly identical thereto in the part thereof related to the high-frequency section, that is the only one appearing therein. The embodiment in Fig. 2, in the end, uses the high-frequency generator (3⊘⊘ ÷ 5⊘⊘ kHz) only for annealing metallic pipes with a small diameter with a continuous process, under a controlled atmosphere, for refrigerating applications and the like. The embodiment in Fig. 1, instead, uses, as a matter of fact, two induction generator in cascade: the first one has a frequency included, in the described embodiment, between 1⊘ and 3⊘ kHz, to pre-heat the material up to the Curie temperature thereof; the second one has a frequency that is preferably included between 3⊘⊘ and 5⊘⊘ kHz, to finally heat the material up to the braze-welding process temperature thereof equal to 1⊘8⊘ ÷ 11⊘⊘ °C. Fig. 3 shows the arrangement adopted for the first generator with a frequency between 1⊘ and 3⊘ kHz: therein, function of the unit L1⊘ is filtering the residual ripple of the direct supply voltage, and this unit is preceded by a three-phase thyristor converter bridge (not shown, since it is well known) for rectifying network voltage with a linear control with partialization. The circuit further includes an inverter bridge, with fast thyristors Th1 ÷ Th4, or with transistors of the I.G.B.T. (Insulated Gate Bipolar Transistor) type to generate the medium-frequency voltage. There are also components RC3 ÷ RC6, Z1 ÷ Z4 to protect the thyristors Th1 ÷ Th4, and a remote oscillating circuit 31, that is the characterizing part of this circuit, and is composed of the heating inductors 33 and of a capacitor bank Cmf. The oscillating circuit 31 can be matched to the different load conditions by means of a variable-taps autotransformer AUTOmf, equipped with an electropneumatic switching device to suit the output voltage of the oscillating circuit 31 to varying load conditions. Matching to the load through the selection of the autotransformer AUTOmf tap is carried out by the programmable controller (P.L.C.) together with the capacity C11 change (Fig. 4) for the high-frequency generating section 11. In this way, that is through the programmable controller (not shown), interlacing is guaranteed between the two medium-frequency 9 and high-frequency 11 generators as regards their matching to varying load conditions or in order to compensate for the change in external product diameters. By means of this arrangement, it is possible to braze-weld the provided range of double-walled pipes with an external diameter variation from the current 4.76 mm to the current 1⊘ mm, without the need of replacing heating inductors 33, both for the medium-frequency heating section, and for the high-frequency one. These inductors 33, as better shown in Fig. 6, are each one composed of two half-inductors 33', 33", laid in series and in such a way that the winding direction of the first half-inductor 33' is opposite to the winding direction of the second half-inductor 33": this feature, that is the main one of the present invention, allows neutralizing the residual induced voltage in the pipe to be braze-welded. The presence of this residual voltage, as already seen, is the cause of product sparkings against mechanical guides and feeding rollers. Since there is provided a process that needs an hydrogen-based controlled atmosphere, the danger generated by these sparkings is immediately apparent, these sparkings producing gas ionization phenomena and ultimately explosion risks. Thanks to the arrangement of the present invention, insulations between inductor and restraining pipe and between this latter one and the pipe to be braze-welded are minimally stressed from the dielectric point of view, guaranteeing a longer and safer working life. Moreover, such arrangement allows adopting another improvement, that is a further characteristic of the present invention, that consists in confining the controlled atmosphere composed of the gas 35 only around the bimetallic pipe 37 to be braze-welded: this confinement is carried out through quartz tubes 39, that form the gas chamber 35 around the product 37, and are inserted inside the heating inductors 33 for the whole length thereof (Fig. 6), both in the medium-frequency section 1, and in the high-frequency one 3. As further feature of the present invention, the heating inductors 33, that are no more immersed into the controlled atmosphere environment like in the prior art plants, are cooled by a forced water circulation and furthermore through a forced air circulation (that in the described, non limiting embodiment occurs along a vertical direction from the bottom upwards, as shown by arrow C in Fig. 6), which also performs a self-cleaning function as regards undesired metallic dust deposits present in the working environment. This external arrangement of the inductors 33 with respect to the controlled atmosphere area implies total safety against gas explosion risks, following possible electric discharges of the inductors 33 or between inductors 33 and produced pipe 37. To be able to guarantee the controlled atmosphere seal between the different heating sections containing the inductors 33, a special arrangement has been adopted, that is a further feature of the present invention. As better shown in Fig. 5, this is composed of a gas-proof insert 41 with plane and parallel faces 43, 45, including a three-axial system composed of two metallic bellows 47 and 49 and of an internal driving through-bushing 51 made of through-holed ceramics and with a conical shape. The pipe 37 to be braze-welded passes through this bushing 51 that functions as centering member when passing from an inductor 33 to the following one. Both the ceramic bushing 51, and the pipe 37 are immersed into the controlled atmosphere environment, and the confinement thereof, in this section of the plant, is carried out by the internal metallic bellows 49. Inside the chamber between internal bellows 49 and external bellows 47 the cooling water for the whole gas-proof insert 41 circulates in a loop, along the direction shown by arrow D in Fig. 5. The hydraulic pressure of the cooling circuit generates an elastic elongation of the metallic bellows 47, 49, that is transformed into an elongation for the whole insert 41. This elongation under pressure guarantees a seal between the end faces 53, 54 of two contiguous inductors 33 wherein the insert 41 is housed with a simple sliding along the guides 55. In order to have access then inside the controlled atmosphere area for normal cleaning and routine maintenance operations, it is sufficient to stop the cooling water pump (not shown) for the plant. Through the following pressure drop of the hydraulic circuit, the insert 41 returns to its previous size (the hydraulic pressure being absent) and can therefore be removed, through a manual vertical sliding (with respect to the paper plane) from the seat thereof, thus freeing two contiguous sections 57 and 59 of the inductor bank 33, for example to replace the quartz tubes 39 in case of breakage or for their internal cleaning. With this arrangement, the common sealed, flanged junctions with screws and gaskets are avoided, that are typical in a tubular, gas-proof process pipe, highly reducing access times for maintenance interventions and making the related operations easier. Fig. 4 shows the circuit arrangement adopted for the second, high-frequency generator (3⊘⊘ ÷ 5⊘⊘ kHz) as regards the innovative part only, that is the oscillating stage 11. This high-frequency oscillating stage 11 is equipped with a metallic-ceramic triode V1 for industrial applications, used as Hartley oscillator with reverse grid feedback Ctg, realized through the LC network composed of L4 and the capacitor bank C11 of the high-frequency oscillating circuit. The output circuit of the oscillating stage 11 is provided with a high impedance, that is the inductive component of the high-frequency oscillating circuit is directly composed of the heating inductors 33 and therefore does not use transformers to match the output impedance. From the triode V1, on the anode A, the inductor bank 33 is supplied through L5 (matching inductance for the output impedance) and a reverse feedback signal is provided taken from the point P in the network C11-L4 and sent, through Ctg and L3-R7 (that is a parasitic oscillation suppressor for the grid circuit) to the grid G of the triode V1. The other components in the oscillating stage are: T5, transformer turning on the triode V1 filament; R12-C12, parasitic oscillation suppressor between grid G and cathode K; L1, high-frequency locking inductance of the cathode circuit K; L2, high-frequency locking inductance of the grid circuit G; Ctk, transfer capacitor of the cathode K; C9, locking capacitor for radiofrequency returns towards grid biasing resistors R11; R11, grid G biasing resistor bank; Ag, amperometer for the grid current G; C4, locking capacitor against radiofrequency returns towards the d.c. power supply stage 7. The innovative feature of the circuit arrangement adopted in Fig. 4 consists in having referred to ground the central tap P1 of the heating inductors 33, while, as regards the oscillating triode V1, none of the three electrodes thereof (cathode K, grid G and anode A) is directly referred to ground. That is, they are referred to the insulated negative pole N of the high supply voltage, apart from the anode A that is supplied by the positive grounded pole corresponding to the geometric center P1 of the heating inductors 33. This arrangement is therefore different both from a classic circuitry with grounded cathode, and from an arrangement with grounded grid. Since the ground has been referred to the center P1 of the inductors 33, half of the supply voltage being present at the oscillating circuit ends is obtained at the inductor ends and to ground. This voltage value will be, however, the one useful for the type of application, that does provide an high-impedance output for the oscillating stage. This arrangement is usually realized with a "push-pull" circuitry, normally equipped with two electronic tubes. It then follows, due to the use of a single oscillating electronic tube, that there is a greater overall system efficiency, with following savings in terms of energy consumptions. Moreover, there is a higher intrinsic operating safety of the circuit, since it uses practically half the number of components used by a classic push-pull circuit equipped with two electronic tubes. In this high-frequency circuit too, matching the generator to varying load conditions is obtained through an electropneumatic switching of the total number of capacitors C11 that compose the bank in the high-frequency oscillating generator circuit 11, this switching being handled by the programmable controlled (P.L.C.) in an interlaced way with the medium-frequency heating section 1. As for the previous inductor bank 33 of the medium-frequency generator 1, in this case too the inductors 33 are divided into two half-inductors 33' and 33" and there are both the quartz tube 39, and the sealing inserts 41 described above.

Claims: Induction heating generator for either braze-welding or annealing metallic pipes (37) with a continuous process under a controlled atmosphere, comprising at least two contiguous heating inductors (33) each composed of two half-inductors (33', 33") in series, the winding direction of the first one (33') of said two half-inductors being opposite with respect to the winding direction of the second one (33") of said two half-inductors, so as to remove the residual induced voltages on said metallic pipes (37) and a quartz tube (39) placed inside said heating inductors (33), said metallic pipes (37) passing, together with the controlled atmosphere gas (35), inside said quartz tube (39) during manufacturing the generator further including at least one sealing insert (41) removably placed in the fitting point of two sections (57, 59) of said quartz tube (39) corresponding to said two inductors (33). Induction heating generator according to Claim 1, characterized in that said sealing insert (41) is composed of a pair of parallel plane surfaces (43, 45) containing therein a three-axial system with metallic bellows (47, 49) and a ceramic driving through-bushing (51) in an internal and coaxial position with respect to said metallic bellows (47, 49). Induction heating generator according to any one of the previous Claims, characterized in that said heating inductors (33) are not striken by the controlled atmosphere (35), but by an external, forced ambient air circulation. Induction heating generator according to any one of the previous Claims, characterized in that it is composed of two induction generators, a medium-frequency one and a high-frequency one, to braze-weld double-walled bimetallic pipes (37) with a continuous process under a controlled atmosphere, particularly for motor vehicle braking circuits. Induction heating generator according to Claim 4, characterized in that they include a medium-frequency section (1) and a high-frequency section (3), each one of said sections (1, 3) being equipped with a load matching system suitable to compensate for external diameter variations in said metallic pipes (37), by always using the same inductors (33). Induction heating generator according to Claim 5, characterized in that said load matching system is composed of electropneumatic switches, said switches in said medium-frequency section (1) switching the taps on an autotransformer (AUTOmf) that supplies a first oscillating circuit (31) and modifying the voltage in said first oscillating circuit (31), said switches in said high-frequency section (3) switching the total number of capacitors in a bank (C11) of a second oscillating circuit (11) and modifying the frequency of said second oscillating circuit (11). Induction heating generator according to any one of Claims 4 to 6, characterized in that said high-frequency section (3) is composed of an Hartley-type oscillating stage circuitry equipped with a single oscillating tube (V1) with a high-impedance output, said high-frequency section (3) using as ground point the geometric center (P1) of said heating inductors (33) without directly referring to ground any other point in said high-frequency oscillating stage, thereby halving the voltage between heating inductors (33) and metallic pipes (37). Induction heating generator according to any one of Claims 1 to 3, characterized in that it is a high-frequency induction generator to anneal metallic pipes (37) with a small diameter with a continuous process under a controlled atmosphere, particularly pipes for refrigerating applications. High-frequency induction heating generator according to Claim 8, characterized in that it includes a high-frequency section (21), said high-frequency section (21) being equipped with a load matching system suitable to compensate for the external diameter variations of said metallic pipes (37). High-frequency induction heating generator according to Claim 9, characterized in that said load matching system is composed of electropneumatic switches, said switches switching the total number of capacitors in a flywheel bank (Cll) of the oscillating circuit (11) and modifying the frequency of said oscillating circuit (11). High-frequency induction heating generator according to any one of Claims 8 to 10, characterized in that said high-frequency section (21) is composed of an Hartley-type oscillating stage circuitry equipped with a single oscillating tube (V1) with a high-impedance output, said high-frequency section (21) using as ground point the geometric center (P1) of said heating inductors (33) without directly referring to ground any other point in said oscillating circuit (11).

Inventors: DOMINICI ERASMO, DOMINICI, ERASMO
Applicants: EMMEDI S P A, EMMEDI S.P.A.
Application year: 1996
Main classification: B23K3
IPC codes: H05B6, B23K1