Tubing shape, particularly for fabricating an induction coil

Abstract: Electrically conductive tubing ( 26 ) for realizing induction coils ( 24 ) having improved efficiencies comprises an outer peripheral portion ( 28 ) of conductive material defined by a plurality of side walls ( 30 , 32 , 34 ) and a single hollow passageway ( 40 ). Each side wall has inner surfaces and outer surfaces ( 42 , 44 ). At least one side wall has a curved outer surface ( 42 ). The hollow passageway ( 40 ) is defined by the inner surfaces and can be either entered within the tubing ( 26 ) or spaced apart from the center of the tubing ( 26 ). The present invention also defines an induction coil ( 24 ) and an induction furnace ( 10 ) having an induction coil ( 24 ) which utilizes such tubing ( 26 ). The curved outer surfaces ( 42 , 44 ) have radius of curvatures which are defined substantially by a function of the tubing width in the axial direction, and spacing between turns of the induction coil. The cross section of the outer surface of the outer peripheral portion of the tubing taken along a transverse axis of the tubing defines a geometric figure. The perimeter of the figure is substantially polygonal with at least one curved side wall ( 34 ).

Description: This invention relates to conductive tubing and to the use of such tubing for fabricating an induction coil. The invention is described in the context of, but is not limited to, induction heating apparatus. Induction heating apparatus such as induction furnaces for heating or melting metals operate on the principle of inducing eddy currents in a workpiece (sometimes referred to as the load) to be heated. The eddy currents cause the load to act as its own heat source by the P=I 2 R heating principle. The eddy currents are induced in the load by passing alternating current through a generally helical induction coil disposed near or around the load. In a "coreless" induction furnace, the load is typically disposed inside the induction coil, so that the load itself acts as the core. Coreless induction furnaces in common use today often include induction coils of copper tubing adapted to allow a liquid coolant to flow therethrough. The copper tubing conducts the alternating current which produces the electromagnetic field inside the furnace to create the eddy currents in the load. Running water or other liquid coolant flows through the copper tubing of the coil to remove the heat conducted through the refractory material and the heat generated by the coil current. The efficiency of an induction furnace depends, in part, on the amount of energy (in the form of electromagnetic energy) which couples from the induction coil to the load and is converted into heat energy in the load. One overall goal in designing such furnaces is to maximise this efficiency. The efficiency is a function of many different design parameters. One parameter which affects the efficiency is the tubing used to fabricate the induction coil. Different tubing shapes, sizes and dimensions, when wound into a helical coil, will produce different electromagnetic flux patterns. Different patterns will cause more or less of the electromagnetic energy to couple into the load, thereby resulting in greater or lesser furnace efficiency. In the prior art, the induction coil tubing typically has a rectangular cross-sectional profile with a rectangular opening for cooling fluid to flow therethrough. The outer side walls of the rectangular tubing are typically straight, although sometimes the outer concerns may be slightly rounded. Another well-known form of tubing has a circular cross-sectional profile with a circular opening therethrough. It is also known to curve the outer surface of the side walls such that curved surface is concave and conforms with the curvature of the load (see US-A-1983 242). Oval-shaped tubing with an oval-shaped opening therethrough is also well known. DE-A-3405119 describes D-shaped induction tubing, the straight side of which faces the load. GB-A-0274008 describes the use of flattened induction coils with the tubing having convex round outer walls facing and opposite to the load which are of uniform thickness. One prior art attempt to increase the efficiently of an induction coil by changing the geometry of the tubing involved displacing the opening of the tubing away from the center axis of the tubing. In other words, instead of the geometric center of the opening being centered on the center axis of the tubing, the geometric center of the opening was spaced apart from the center axis. The displacement resulted in a reduction of losses due to an increased amount of electromagnetic flux being able to couple to the load. In spite of extensive research and exhaustive attempts to further improve the efficiency of induction melting furnaces, there is still a need for further improvements in efficiency of an induction coil so as to maximise the proportion of energy supplied to the induction coil which couples to the load and heats it through induced eddy currents. Specifically, there is a need for further improvements in the shape of the tubing used in the induction coil which will lead to higher efficiencies. The present invention fills that need. According to the present invention there is provided an electrically conductive tubing for realizing induction coils comprising: an outer peripheral portion of conductive material defined by a plurality of side walls, each side wall having inner and outer surfaces; and a single hollow passageway defined by the inner surfaces, wherein at least one side wall has a convex outer surface, said at least one side wall having a non-uniform thickness between the convex outer surface and the inner surface of said side wall. The cross section of the outer surface of the outer peripheral portion of the tubing taken along a transverse axis thereof may define a geometric figure, the perimeter of the figure being substantially polygonal. The polygon may be quadrilateral, preferably being a rectangle or a square. The side walls, other than the convex side wall, may be substantially straight. Alternatively, two adjacent side walls may have a convex outer surface with the remaining outer surfaces being substantially straight. The cross section of the hollow passageway of the tubing, taken along a transverse axis thereof, may be substantially polygonal. The polygon may be rectangle. Preferably, at least two sets of adjacent side walls of the tubing are perpendicular to one another. The geometric center of the hollow passageway may be spaced apart from the center of the tubing. Alternatively, the geometric center of the hollow passageway may be generally centered within the tubing. The conductive material of the outer peripheral portion of the tubing is preferably of solid copper. The induction tubing may form turns of an induction coil, the convex surface of the helical tubing facing the axis of the helical turns. The induction coil may form part of an induction furnace for applying heat energy to a load in the furnace by inducing eddy currents therein wherein the convex surface of the tubing faces the load. For the purposes of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. Figure 1 is a cross-section of an induction melting furnace which shows an induction coil fabricated from the novel tubing according to the present invention. Figure 2 is an enlarged fragmentary of Figure 1, showing geometric features of the tubing in greater detail. Figure 3 is an enlarged fragmentary view of an alternative embodiment of the tubing shown in Figure 1. Figure 4 is an enlarged fragmentary view of another alternative embodiment of the tubing shown in Figure 1. Figure 5 is an enlarged view of two adjacent turns of the induction coil shown in Figure 1, illustrating dimensions used for determining the radius of curvature of the convex side walls. While the invention will be described in connection with presently preferred embodiments, it will be understood that it is not intended to limit the invention to any one disclosed embodiment. Referring now to the drawings, wherein like numerals indicate like elements, there is shown in Figure 1 selected components of an induction melting furnace 10 which are visible in cross-section. Only those elements of an induction furnace necessary to illustrate the present invention have been shown, and the rest have been omitted for the sake of clarity. Those skilled in the art will have no difficulty in understanding the invention from the simplified illustrations and the accompanying description. Furnace 10 comprises a crucible 12 which holds a workpiece or load 14. Load 14 typically consists of a conductive material such as metal but can also comprise nonmetallic conductive materials. The crucible 12 is surrounded by insulating refractory 16, which in turn is surrounded by refractory cement 18. The refractory cement 18 may in turn be surrounded by a shell 20 which gives added mechanical strength to furnace 10. For explanation purposes, the combination of the crucible 12, insulating refractory 16, refractory cement 20 and shell 18 are refereed hereinafter as assembly 22. An induction coil 24 surrounds the entire assembly 22. The induction coil 24 is formed from conductive tubing 26 wound in a helical coil. The simplified view in Figure 1 depicts seven windings (sometimes called "turned") of tubing for illustration purposes. However, it should be understood that the invention is not limited to any particular number of windings but, rather, encompasses any number of windings as may be desired for a particular coil. Figure 2 is an enlarged fragmentary view of Figure 1 and illustrates the novel aspects of the tubing shape. The tubing 26 in Figure 2 is defined by an outer peripheral portion 28 defined by a plurality of side walls 30, 32 and 34, each of which has an inner surface and an outer surface. The outer surfaces can be either planar or non-planar , ie. curved. The outer surfaces of the side walls define the outer perimeter 36 (shown as a dotted line surrounding the lowermost turn of tubing 26) which is substantially polygonal. That is, tubing 10 has a substantially polygonal cross-section when taken along a transverse axis of the tubing 26, except for one side which is convex as described below. The inner surfaces of the side walls define inner perimeter 38 (also shown as dotted line surrounding the lowermost turn of tubing 26) which is also substantially polygonal in cross-section. The inner perimeter 38 defines the dimensions of opening 40, through which coolant fluid flows. In the exemplary embodiment, the polygon is a rectangle. However, the invention is not limited only to rectangular polygonal shapes. One novel feature of the invention is that the tubing 26 comprising the induction coil 24 is shaped such that at least one of its side walls (denoted in Figure 2 as 30, 32 and 34) which faces the load 14 has a convex outer surface. In the embodiment depicted in Figure 2, side wall 34 which faces load 14 has a convex outer surface 42. Alternatively, two adjacent side walls can have convex outer surfaces. For example, Figure 2 depicts adjacent side walls 34 and 32 having convex outer surfaces 42 and 44 respectively. However, it is a novel feature of the invention that at least the side wall facing the load 14 has a convex outer surface. It should be understood that the windings of tubing 26 depicted in cross-section in FIgure 1 are typically, but not necessarily, part of a continuous piece of tubing. Thus, the induction coil can be fabricated by using tubing with one side wall have a convex outer surface and winding the tubing into a coil while always keeping the convex outer surface 42 facing inward (toward the center axis of the coil 24). Another novel feature of the invention is that tubing 26 having at least one side wall with a convex outer surface 42 facing the load 14 has in internal opening 40 displaced from the center axis of tubing 26. In the exemplary embodiment depicted in Figure 2, tubing 26 has an opening 40 with a substantially polygonal cross-section taking along the transverse axis of the tubing. Instead of the geometric center of the opening 40 being centered on the center axis of the tubing 26, the geometric center of the opening 40 is spaced apart (or displaced) from the center axis. As noted above, this displacement results in a reduction of losses from the coil 24 because of increased coupling of electromagnetic flux to the load 14 when compared to tubing in which the opening 40 is centered on the center axis of tubing 26. Figure 3 illustrates an enlarged view of an alternative embodiment of the tubing 26 shown in Figure 1. Focusing on the uppermost turn of tubing 26, outer perimeter 36 of the outer peripheral portion 28 of tubing 26 which is defined by three side walls 30 and one side wall 34 has a substantially polygonal cross-section when taking along the transverse axis of the tubing, except for side wall 34 whose outer surface is convex. Inner perimeter 38 fo the outer peripheral portion 28 also has a substantially polygonal cross-section when taken along the transverse axis of the tubing 26. The inner perimeter 38 defines the dimensions of opening 40. In the exemplary embodiment, the polygon is a rectangle, and more particulary, a square. However, the invention is not limited only to rectangular polygonal shapes. In the Figure 3 embodiment, the geometric center of the opening 40 is centered with in the tubing 26, as is common in the prior art. More importantly with respect to Figure 3, the convex side wall 34 faces assembly 22, thereby also facing load 14 (not shown). Figure 3 also illustrates a lowermost tube 46 forming the bottom turn of induction coil 24 which has two adjacent convex side walls, one of which faces the load 14 (not shown). Tube 46 also has a center opening displaced from the transverse axis of the tube. One advantage of having a convexity on two adjacent side walls in combination with a displaced opening is that the same rectangular piece of tubing can be wound in either of two directions depending on the coil spacing. By carefully selecting the dimensions of the substantially rectangular tubing, one would need to stock only one-half as many shapes of tubing. Figure 4 illustrates yet another embodiment of the invention wherein uppermost tube 48 which forms the coil's top turn and lowermost tube 46 which forms the coil's bottom turn have a geometric shape which is different from the tubes of intermediate turns 50. The tubes of intermediate turns 50 are similar in geometric shape to the tubes 26 described with respect to Figure 3. The uppermost tube 48 has an outer peripheral portion 28. Outer perimeter 36 of this peripheral portion 28 has a substantially polygonal cross-section when taken along the transverse axis of the tubing 28, expect for one side wall 52 which has at least one convexity with an exaggerated or high degree of curvature. In contrast, convex side wall 34 of the tubes of intermediate turns 50 preferably has only one convexity. Referring again to uppermost tube 48, inner perimeter 38 of the outer peripheral portion 28 has a substantially polygonal cross-section when taken along the transverse axis of the tubing 28. The inner perimeter 38 defines the dimensions of opening 40. In this exemplary embodiment, the polygon is a rectangle. As noted above, the side wall 52 is defined by at least one convexity having an exaggerated or high degree of curvature. In the exemplary embodiment, the side wall 52 is defined by at least two different adjacent curvatures, one of which is exaggerated with respect to the other. More specifically, the depicted convexity has a first portion 54 with a gradual curvature followed by a second portion 56 with an exaggerated or high degree of curvature. It should also be recognised that side wall 52 can also be defined by a single convexity having an exaggerated or high degree of curvature. Figure 4 also depicts lowermost tube 46 which forms the coil's bottom turn. This tube 46 has a convex side wall 58 whose shape is a mirror image of side wall 52. Again, all of the side walls 30 of the tubes in Figure 4 which either directly or partially face assembly 22, and which thereby also directly or partially face load 14 (not shown), have a convexity. Figure 4 requires the use of a different geometric shape for the tubing which forms the top and bottom end turns. However, in certain applications, the increased efficiently gained by the exaggerated or high degree of curvature at the ends may offset the disadvantages associated with using the two different shapes to form the coil. The convexities referred to above are mathematically defined by the inverse of the curvature, called the "radius of curvature", R. The radius of curvature R, is a function of the tubing width, w, in the axial direction and the spacing between turns, x, of the induction coil. Mathematically, this can be expressed as: R = f(w, x) This is best illustrated with respect to Figure 5, which is an exaggerated view to two adjacent portions of tubing 26 generally depicted in Figures 1 and 2. The radius of curvature of side wall 34 facing the load, labelled as R1, is a function of the tubing width, w, in the axial direction and the spacing between adjacent turns of the coil, represented as x in cross-section of the coil. The radius of curvature of adjacent side wall 32, labelled as R2, is preferably, but not necessarily equal to R1. Geometrically speaking, the curvature of a space curve, at a point on a curve is the derivative of the inclination of the tangent with respect to arc length, also expressed as the rate of change of direction of the tangent with respect to the arc length, ie. whether K is the curvature, represents the change in direction of the tangent, and s is the length. The radius of curvature, R, can be then be expressed as the inverse of that function, or the inverse of K. In one design example using copper tubing, the tubing width, w, is 0.035m (1 3 / 8 inches) and the spacing between adjacent turns of the coil, x, is 0.0095m ( 3 / 8 of an inch) yielding a radius of curvature of 0.114m (4 1 / 2 inches). When tubing having these dimensions were used to form a coil with a single convex side wall facing the load, losses in coil conductors were decreased by 8% in a comparison with identically shaped tubing not having a convexity. Opening 40 in the variously shaped tubes is depicted in the preferred embodiments as being substantially rectangular. However, it should be understood that the opening can be any geometric shape that achieves the desired function of acting as a cooling channel. The disclosed embodiments all depict outer peripheral portions with polygonal cross-sectional shapes. However, it should be understood that the ends of adjacent side walls need not necessarily meet one another exactly at the ends. One end may overlap an adjacent end so as to stick out from the adjacent end when viewed in cross-section. The novel tubing shape for induction coils described above provides significant advantages not contemplated by the prior art. By merely altering the geometric shape of a portion of the tubing, induction heating furnaces with greater efficiencies can now be constructed. This greater efficiently allows one to achieve either faster heating of the load with the same input of electrical energy and cooling energy into the induction coil, or the same amount of heating of the load but with less electrical energy and/or cooling energy input into the induction coil.

Claims: Electrically conductive tubing for realizing induction coils comprising: (a) an outer peripheral portion (28) of conductive material defined by a plurality of side walls, (30,32,34) each side wall having inner and outer surfaces; and (b) a single hollow passageway (40) defined by the inner surfaces, at least one side wall (34) having a convex outer surface (42), characterised in that said at least one side wall has a non-uniform thickness between the convex outer surface (42) and the inner surface of said wall. The apparatus of claim 1, wherein the cross section of the outer surface of the outer peripheral portion (28) of the tubing taken along a transverse axis of the tubing defines a geometric figure, the perimeter of the figure being substantially polygonal with at least one side wall (34) being convex. The apparatus of claim 2, wherein the polygon is a quadrilateral having three straight side walls (30) and one convex side wall (34). The apparatus of claim 3, wherein the quadrilateral is a rectangle with three straight side walls (30) and one convex side wall (34). The apparatus of claim 4, wherein the rectangle is a square with three straight side walls (30) and one convex side wall (34). The apparatus of claim 2, wherein the remaining side walls are substantially straight. The apparatus of claim 1 or 2, wherein at least two adjacent side walls (52,56) have a convex outer surface. The apparatus of claim 7, wherein the remaining two outer surfaces are substantially straight. The apparatus according to any one of claims 1 to 8, wherein the cross section of the hollow passageway (40) of the tubing taken along a transverse axis of the tubing is substantially polygonal. The apparatus of claim 9, wherein the polygon is a rectangle. The apparatus of claim 1, wherein at least two sets of adjacent side walls are perpendicular to one another. The apparatus according to any one of claims 1 to 11, wherein the geometric center of the hollow passageway (40) is spaced apart from the center of the tubing. The apparatus according to any one of claims 1 to 11, wherein the geometric center of the hollow passageway (40) is generally centered within the tubing. The apparatus according to any one of the preceding claims, wherein the conductive material is solid copper. An induction coil (24) fabricated from tubing (26) which forms helical turns of the induction coil, the tubing being as claimed in any one of claims 1 to 14, the convex surface of the tubing facing the axis of the induction coil. An induction furnace (10) for applying heat energy to a load in the furnace by inducing eddy currents therein, including an induction coil as claimed in claim 15, wherein the convex surface of the tubing faces the load.

Inventors: FISHMAN OLEG, PEYSAKHOVICH VITALY, PRABHU SATYEN N, FISHMAN,OLEG, PEYSAKHOVICH, VITALY, PRABHU,SATYEN N., PEYSAKHOVICH, VITALY, MOORESTOWN WOODS
Applicants: INDUCTOTHERM CORP, INDUCTOTHERM CORP.
Application year: 1993
Main classification: H05B6
IPC codes: H01F27, H05B6