GB1562905A - Electric resistance heating utilising the skin effect in magnetic metal shapes - Google Patents

Description

(54) ELECTRIC RESISTANCE HEATING UTILISING THE SKIN EFFECT IN MAGNETIC METAL SHAPES (71) I, DONALD FREDERICK OTHMER, 333 Jay Street, Brooklyn, New York, 11201, United States of America, a citizen of the United States of America, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the use of an insulated electrical conductor wire carrying alternating current (a.c.) in an "out" leg of a circuit; and the a.c. flows back through an adjacent and substantially parallel, magnetic and electrically conductive shape to supply the return or "back" leg of the circuit.A "skin effect" localized in the surface of the shape which is in the form of a band immediately adjacent to the wire, is developed by induction and magnetic effects and causes two quite important phenomena; (a) the substantial reduction and localization of the path of flow of the a.c. both as to depth and width of the effective conductor path in the cross-section of the shape, so as to increase greatly the effective resistance of the said shape, and (b) the effective insulation against any electrical contact with the remainder of the shape in two dimensions-both depth and width-and particularly on the surface away from the band on both sides thereof. The proximity relation of the two conductors- out and back-and proper electromagnetic shielding increases further these effects, the basis of the the present advantageous system.

Hereinafter the magnetic and electrically conductive shape will be exemplified by reference to a steel pipe, since steel is the most common material from which a pipe, the most common shape, is manufactured.

Alternating current flows only along a band of the skin of the pipe acting as a very specialized conductor under these conditions. (The word "steel" is used hereinafter to represent any magnetic, electrical conductive material, since steel is the most common such material).

As an example, a pipe may be considered which has a minimum wall thickness of at least twice the skin depth (depth or thickness being the depth in the surface of the pipe to which the ac. has diffused.) or, preferably about 1/8", less for many steels; a.c. may be conducted out to the far end of the pipe by an adjacent, external and insulated wire which is connected there to the wall. Due to what is called the "skin" effect, substantially all of the a.c. flows back on that part of the outside surface or skin of the pipe which is immediately adjacent and parallel to the conductor wire. This band of the steel surface subtended from the wire becomes a specialised conductor called a skin effect "conductor-resistor", which expression is hereinafter used to refer to such a specialised conductor.The remainder of the surface of the pipe, both inside and out, is for all practical purposes completely insulated electrically from the said band or conductor-resistor. This considerable reduces tion of what is normally regarded as the effective cross-section of an electrical conductor (the entire pipe), greatly increased the effective resistance of what otherwise would be entirely a conductor. Thus, steel pipes through which oil or other utilitarian liquid is transported, may be of such very substantial cross-section of metal, compared to that of the usual copper wire conductor, that they could not practically be used as a heating conductor. In the present invention, a.c.

flow is limited to this narrow band of skin; the pipe thereby offers greatly increased resistance; and this new technique allows such pipes to be used for resistance heating with a.c. for which they would be quite unsuitable with direct current.

Accordingly, therefore from one aspect the present invention provides a heat generating system comprising (a) a shape made of a metal having magnetic properties and electrical conductivity and having a first, near, portion and a second, remote portion; (b) means whereby a utilitarian fluid is caused to flow in direct contact with a surface of said shape; (c) an elongate electrically conducting means, positioned in closely adjacent but electrically insulated relationship with respect to a surface of said shape opposite said surface in contact with the fluid and extending from said near portion to said remote portion, said conducting means having a first, near, end adjacent said first portion of said shape and a second, remote end adjacent said remote portion of said shape; said conducting means being uncovered throughout those portions of its outer surface not directly confronting said shape by any electrically conductive material which would shield the electromagnetic field generated around said conducting means in a direction away from said shape; (d) a source of a.c. having a first supply terminal and a second supply terminal; (e) an electrical connection directly between said first a.c. terminal and said first, near portion of said shape; (f) an electrical connection between said second a.c. terminal and said first, near end of said electrically conducting means; (f) an electrical connection between said second, remote, portion of said shape and said second remote, end of said electrically conducting means; (h) an a.c. circuit established: (i) from said second terminal of said a.c.

source through the length of said electrically conducting means, (ii) then back through an elongate band portion of said shape formed in a skin portion thereof and adjacent to said electrically conducting means such that a skin effect current is created in said band portion of said shape which acts as a conductor-resistor whereby heat is produced in said shape, said skin portion being less than half of the thickness of said shape; and (iii) finally back to said first a.c. terminal, to complete said a.c circuit whereby at least some part of said heat produced in said shape is transferred directly to said utilitarian fluid without passing through any other material.

From another aspect the present invention provides a heat generating system comprising: (a) a three elongate shapes, a first, a second and a third, each of which is a pipe made of a metal having magnetic properties and electrical conductivity and having a first, near, end and a second, remote end; each of said three pipes being positioned in closely adjacent but electrically insulated relationship with respect to the respective exterior surfaces of the other two; and with the respective near ends of said three pipes adjacent to each other, and with the respective far ends of said three pipes adjacent to each other, the outer surfaces of the pipes which do not confront each other being uncovered by any electrically conductive material which would shield the electromagnetic fluid generated around the pipes; (b) means by which a utilitarian fluid is caused to flow through each of said pipes; (c) a source of three phase a.c. provided with a first supply terminal, a second supply terminal, and a third supply terminal; (d) means provided for an electrical connection directly between said near end of said first pipe with said first terminal, said near end of said second pipe with said second terminal, and said near end of said third pipe, with said third terminal;; (e) means provided for electrically connecting together the remote ends of all three of said pipes in a "Y" connection, whereby (f) a three phase a.c. circuit is established, of which: (i) a first single phase a.c. flows from said first terminal through the length of the wall of said first pipe in a longitudinal band adjacent to the wall of said second pipe, and also in a longitudinal band which is adjcent to the wall of said third pipe; (ii) a second single phase a.c. flows from said second terminal through the length of the wall of said second pipe in a longitudinal band adjacent to the wall of said first pipe, and also in a longitudinal band adjacent to the wall of said third pipe;; (iii) a third single phase a.c. flows from said third terminal through the length of the wall of said third pipe in a longitudinal band adjacent to the wall of said first pipe, and also of in a longitudinal band which is adjacent to the wall of said second pipe; (iv) and then each of these three circuits for the respective single phases passes through said "Y" connection at the far ends of all three of the respective pipes; (v) finally each of these respective three flows of single phase a.c. returns back to the other two of said three terminals through the lengths of the walls of the other two respective pipes in longitudinal bands adjacent to the respective pipe carrying said a.c. single phase out from its respective terminals; ; (g) each particular one of said three phases of ac. produces by its flow out from its respective terminal a skin effect current concentrated in the respective longitudinal bands of those parts of the skin of the wall of the particular pipe which are adjacent to the walls of the other two pipes; and the same particular one of the three phases produces by its flow from the "Y" connection back to the other two terminals a skin effect current concentrated in the longitudinal bands of those parts of the wall of each of the other two pipes which are adjacent to the wall of the particular pipe; (h) each of said three pipes having a wall at least twice the thickness of said skin through which said a.c. flows to produce heat in each of said pipes; whereby at least some part of said heat produced in each of said pipes is transferred directly to said utilitarian fluid without passing through any other material.

It has been found that pipes used in accordance with this invention have considerably more resistance than those of United States Patent 3,777,117 which may depend upon the skin effect of the entire inside circumferential surface of a pipe transporting a liquid. Since according to the present invention a.c. flows only in a band on the outside surface of a pipe i.e., that part immediately adjacent to the external adjacent insulated wire, the inner wall of the steel pipe is, for practical purposes, perfectly insulated from the a.c. and may carry liquids which conduct electricity. Most of the outer pipe wall is also effectively insulated from the a.c.

and the pipe may be earthed and even touched without shock, even though it is of considerable size. The pipe thus may be used for transport of liquids which must be kept heated in transit or reheated after a shutdown, because of the greatly reduced crosssection, and decrease of effective area of this conductor-resistor. In normal use, the utilitarian fluid, usually a liquid, is forced through the pipe by a pump or other positive means.

If such an insulated conductor wire is extended along the length of an element of a steel pipe and covered with a concave covering strip, which should be a non-conductor of electricity, e.g., a plastic strip such as might be formed by extrusion of a thermoplastic resin, all of the heat from the a.c. flow is generated within the pipe wall, in a band subtended by the wire as the skin effect conductor-resistor, and the plastic strip protects the wire.

In many cases, the wire may be wrapped as a helical coil around and along the pipe; and may be covered with a concave cover, such as that refered to above. The flow of return a.c. is in the skin band subtended by the wire. The return current follows the band and the shape of the wire, which may be a much longer path than the shortest distance, i.e. the length of the pipe.

In the use of this invention, a utilitarian fluid, as opposed to some casual fluid in the vicinity, is heated by the steel pipe against which the fluid is forced by pump, blower, gravity, convection, or other means, the steel pipe being heated by the flow of a.c. along a skin effect conductor-resistor therein, which conductor resistor is limited in cross-sectional area by the skin effect and the proximity effect (hereinafter described), which thereby offers increased resistance to, and hence heat from, the return leg of the a.c. circuit.

While the shape often may be a steel pipe, as hereinafter exemplified and the utilitarian fluid may be a liquid being forced therethrough, in other cases, the shape may be other than tubular--e.g., planar, conical or spheroidal; and the utilitarian fluid may be heated by being passed or forced into contact therewith, rather than transported thereby.

Long distance pipelines which require heat to give a lower viscosity to the heavy oils at higher temperatures, particularly at start-ups, have used heat-tubes as described in U.S. Patent No. 3,617,699, but with low heat fluxes e.g. a maximum of from 10 to 15 watts per foot. Skin effect heating of pipelines, even with the low performance of the prior art, has large advantages over other systems which have been used with steam or other hot fluids. Heat tubes can be heated economically throughout their length by the skin effect, using a.c., which has frequencies of the standard 50 to 60 cycles of conventional a.c. generation and lower and higher, as described in U.S. Patent No: 3,777,117 and U.S. Patent No: 3,617,699.These patents develop the theory and practice of the effect tive use of the skin effect, in various designs and size of pipelines and other heat-generating equipment; also the influence on their design and operation of the proximity effect, the change in the number of cycles of the a.c.

used, and of other parameters. Particularly, the influence of these factors on the effective electrical resistance of large steel pipes has been demonstrated; and it becomes increasingly important to increase this resistance as pipeline sizes increase, and the cross-section of which would be the normal conductor, in the absence of the skin effect becomes so very large.

In a heat-tube, an internal electric wire forms one leg-out-of the a.c. circuit; and its other terminal is at the far end of the heat-tube on the inside thereof. The return leg of the circuit is the inside wall of the heattube because of the skin effect, with very little current flowing on the outside wall if the steel tube is more than about twice the socalled effective skin depth of about 0 04 inches; in usual steels, the total thickness need not be more than about 3 times this-or about 1/8 inch. The other junction to the source of a.c. is a point at the near end of the inner surface of the heat-tube adjacent to the entrance of the insulated copper wire into the heat-tube to carry the a.c. to its far end.

In the prior art, the internal conductor or wire has been completely enclosed or encompassed throughout its length in a steel tube.

"Skin effect" is a phenomenon of an a.c.

circuit which restricts the a.c. flow to the surfaces of iron and steel conductors-resistors which are operating in electromagnetic fields. Commerical a.c. frequencies of 50 to 60 cycles per second are used with another adjacent conductor carrying a.c. so as to generate surface magnetic and induction effects. With appropriate circuitry long known in the art, all three phases of standard a.c.

current generation may be used to advantage, as may frequencies in the range of from 10 to 1000 or more per second, which range may be produced with conventional alternators.

The electromagnetic flux surrounding a wire carrying an a.c. extends until shielded by another metal. Thus, the transporter pipe itself was used as a heat-tube in U.S. Patent 3,777,117, with an internal wire on the axis or the bottom of the pipe.

To ensure that there is no current leakage or danger from the high voltage a.c. which flows on the inside skin, the minimum thickness of the steel pipe wall was found to be at least two times, and usually preferably three times, the skin thickness, or about 1/8 inches, with usual steels under the usual characteristics of the a.c. flow used. There will then be no practical voltage or power loss, even when the outside of the heat-tube is earthed or submerged in salt water. Conventionally, unburied pipelines are earthed at reasonable distances and they should be in the present invention; also, if installed in corrosive conditions, they may have the conventional sacrificial cathodic protection system with no interference with the skin effect heating.

The "proximity effect" is a very important aspect of the electromagnetic field produced by a.c. passing through an inner wire conductor in penetrating the return leg of the circuit, the steel wall of the pipe. This was analyzed in U.S. Patent 3,777,177, for a steel heat-tube having a diameter which is large compared to that of the electrical conductor wire. Increase of the effective resistance up to twenty-five times or more of that when the wire was on the pipe axis, was found for the pipe wall when the insulated wire was simply laid on the bottom.

The combination of the "skin effect" and the "proximity effect" is equally important in some examples of the present invention, which has, however, a different geometry of the arrangement of the conductor wire and of the return conductor of steel. Usually, this steel conductor is a transport pipe, the effective resistance of which must be increased greatly for practical usage.In the example of a circular conductor wire, the insulation of which contacts the outside element of a steel pipe, the effective resistance of the return circuit has been found to be substantially that of a skin effect conductor-resistor, which for practical purposes may be considered as cut out of the steel wall, having the depth of the skin and an effective width of only a narrow band of the pipe immediately below and adjacent to the insulated wire, The thickness of the insulation on the wire limits this effective distance of the wire from the pipe wall in the present invention, as does also the shape of the cross-section of the wire.

A flat conductor ribbon tightly held against the outer pipe wall, with a minimum thickness of suitable insulation between, would give the least distance between; and the proximity effect would be most effective.

Similarly, in the co-pending application now U.S. Patent 3,777,117 a flat wire, when placed against the inner surface of the pipe, gave the greatest effective resistance of the skin of the pipe wall. In the described embodiment of the present invention, the convexity of the exterior wall of the pipe against the wire (of any shape) accentuates greatly the increase of resistance of the skin of the said band, as compared to the concavity of the inner surface of the pipe wall. This is because in addition to (a) the skin effect, and (b) the proximity effect as has been previously demonstrated, there is also (c) a "shielding" effect which does not pertain when the wire is entirely encompassed by the pipe.

Now it has been found desirable in scm cases to have a flat wire when possible as close to the surface of the metal as not more than about 5 to 12 times the so-called penetration depth. With usual steels, this would amount to about 0-2 to 0 5 inches. This distance includes the thickness of the electrical insulation. Since the flatter the wire, if of the same cross-sectional area, the greater its width will have to be for the same electrical capacity-and therefore the greater the width will be of the band of the skin effect conductor resistor subtended by the electromagnetic flux generated by the wire.

In general, however, the effective width of this band which returns the current carried in a flat insulated conductor wire closely held against the wall will be the sum of the width of the flat wire plus about two or three times the skin penetration or depth on each side, i.e., the total may be the wire width plus about 4 to 6 times the skin penetration. This depends on the effective distance the conductor wire is from the steel.

If the pipe has a flat surface throughout its length on which the insulated wire is laid, the proximity effect may be most pronounced, this is particularly effective, for flat wires, but is also appreciable for conventional wires.

If flat steel wire is used as the conductor, the desirability of taking advantage of the skin effect in this as well as in the pipe wall has been considered in the co-pending application. If a thickness of the flat steel band is greater than about twice that of the skin depth, all of the rest of the thickness of the steel wall of the band becomes an insulator, more effective as the thickness increases. This aspect of the use of steel as an insulator must be balanced against the use of a more or less standard insulation, which would have to be used-applied, coated, drawn on, etc. on the inner or pipe side in any case.

Movement of the wire in relation to the pipe changes greatly the proximity effect, the pipe's resistance, and the heat generated.

Shielding An electromagnetic flux, such as that generated by the a.c. flow in the conductor wire, is shielded or arrested by an electrical conductor. This shielding is what reduces the penetration of the flux into the steel wall of the pipe and causes the skin effect. Shielding is also closely related to the proximity effect, particularly in the convex outer portion of the pipe wall having a wire carrying a.c. adjacent thereto.

In some cases, it may be desired to narrow the width of the band in the pipe wall of this skin effect conductor-resistor which returns the a.c. As indicated above, for a conductor very near the wall, this may extend about twice the value of the skin penetration depth, or about 2 x 0 04 or 0 08 inches, on each side beyond the width of the wire, depending on the distance of the wire from the wall. This much of the wall of the pipe on either side of the pipe wall will carry a.c. rapidly decreasing to a very low voltage and amperage as the distance increases from either side of the flat conductor wire.

If the insulation around this flat wire extends more than two or three times the value of the penetration depth on either side of the wire itself, it insulates very effectively the balance of the pipe wall from any possible contact or short circuit.

As an additional precaution, the insulation may be formed even wider, so that it extends more than 3 or more times the penetration depth on each side of the wire, with either continuous or discontinuous conductor ribbons formed in, along the sides. These will shield the steel pipe wall and give an effective width of the band of skin which returns the a.c. current, which width need be no wider than that of the conductor ribbon itself, thus amply insulated by the insulation of the ribbon from external contact.

The invention will now be more fully described with reference to the following drawings in which: Figures 1 and 2 are cross-sections of a conventional, insulated conductor wire running adjacent to the wall of a transport pipe, and concave shapes of insulating materials covering the electric wire.

Figure 3 is the cross-section of a conventional insulated electric wire against the outside of a transport pipe.

Figure 4 is the cross-section of an insulated electric wire of rectangular cross-section against the outside of a transport pipe, with means for increasing the distance of the wire from the pipe.

Figure SA is the cross-section of the insulated wire of Figure 4 but with shielding strips of metal imbedded in the insulation.

Figure SB is an enlargement of the crosssection of the special wire of Figure SA showing the insulated wire outside of à layer of thermal insulation.

Figure 6A is a cross-section parallel to the axis of a special conductor wire cable electrically connected to a special steel shape, also in cross-section along the line A-A of Figure 6-B.

Figure 6B is a cross-section at right anglers to the axis of Figure 6A, along the line B-B.

Figure 7 is a cross-section of two transport pipes, each of which acts also as the conductor wire for the other.

Figure 8 is a cross-section of three transport pipes, each of which acts also as the conductor wire for the other two in a 3-phase circuit.

Figure 9A is a plan view of a steel sheet heated by a simple insulated wire as one leg of an AC circuit; and a subtended band of a skin effect conductor-resistor of the steel is the return leg.

Figure 9B is the cross section of Figure 9A.

Figure 10 is a diagram of an a.c. circuit wherein an insulated wire is wrapped spirally around a transport pipe.

Figure 11 is a diagram of an a.c. circuit wherein an insulated wire on a pipe wall is protected by a plastics cover of a concave cross-section.

Figure 12 is a diagram of an a.c. circuit wherein an insulated wire on a pipe wall is protected by a plastics cover of an angular cross-section.

Figure 13 is a diagram of an a.c. circuit wherein an insulated wire runs adjacent of a pipe wall.

Figure 14 is a side elevation and the circuit of Figure 4.

Figure 15 is a side elevation and the circuit of Figure 5A.

Figure 16 is a side elevation and the circuit of Figure 5B.

Figure 17 is a side elevation of the two pipes and the circuit of Figure 7.

Figure 18 is a side elevation of the three pipes and the circuit of Figure 8.

In this invention and in the figures, only a.c. is used. All figures, including electrical wiring, are diagrammatic, without scale; and in the wiring diagram insulation of the wires is usually not shown. Thermal insulation is not shown usually in the figures as it is not a part of this invention by itself. Circulation of fluid as indicated merely by arrows which show direction of flow; and means for causing such flow, e.g. a pump is not shown.

Figure 1 shows the cross-section of a heattube formed by the pipe wall, 1, a skin effect conductor-resistor. The wire, 3 with its insulation, 4, is laid along an element of the pipe and is covered by a concave strip or shape 2, of plastic or other non-conductive material. This may, or may not, have flanges 6, to conform generally with the pipe wall. A thin crack 7, between the pipe and the two flanges of the cover strip, is present throughout the length of both.

The electrical connections are shown in Figure 11 and are analogous to those for a heat-tube:-- the wire is connected at the near end to one terminal of an a.c. supply 5, on the far end to the external surface of the pipe. The near end of the pipe is connected to the other terminal 5 of the a.c. supply. A skin effect develops in the band of steel under the wire.

The cover 13 made wide enough to cover that band of the wall of the pipe subtended by the wire as a skin effect conductor-resistor. The current intensity, along the sides of this band decays rapidly as the distance from the longitudinal axis of the band increases. The effective total width depends principally on the size of the wire, and thus its effective distance from the steel, also particularly therefor the thickness of its insulation. For round wire, the effective width of the skin effect conductorresistor band usually is not more than about 25 to 50 times the value of the penetration or skin depth. Heat generated within the pipe wall passes directly to the rest of the wall and thence to the fluid.

Thus, the system of Figure 1, may be used either with or without a plastic cover over the conductor wires, properly connected, running along one or more elements of the pipeline.

If more than one, they should usually be spaced apart by a distance equal to at least 100 times the value of the penetration depth.

The wire may also be wrapped around the transport pipe in a helix of whatever pitch may be desired-with, however, a desired minimum spacing of coils of not less than about 100 times the value of the penetration depth. The use of a helical heat-tube is described more fully in U.S. Patent 3,617,699 and the present invention adds to that in the use of a simple insulated wire as the only part required to be added to the pipe itself to give the skin effect conductor resistor hitherto found only in a heat-tube encompassing the wire throughout its length.

Similarly, any desired pattern within the aforementioned spacing may be laid out on a steel shape other than that of a pipe, either flat or curvilinear, for various other heating purposes, such as heating fluids passed in contact therewith, etc.

Figure 2 illustrates another of the many elongated concave cover shapes of plastics which may be used. Here it is an angle used as described above for Figure 1. A standard angle shape is conveniently used-as may also many other strips of plastics which are available in concave shapes. The plastics material may be used in a helix around a pipe, or on a flat plate or other steel sheet, as described with reference to Figure 1. Figure 10 shows the wiring diagram of the helical curve on the pipe surface of the wire, either with or without the cover.

Figure 3 shows in cross-section the simplest embodiment of the invention, and is a conventional insulated wire held in place against the pipe's outer surface, either parallel to the axis of an element, or as a helix or in any other desired pattern ; the electrical circuitry as shown in Figure 13 is the same, and, the return path, that of the skin effect conductorresistor, is limited to the band formed by the permeation of the electromagnetic flux into the outer surface of the steel pipe-dependant on both the proximity effect and the skin effect, with an effective width of about 25 to 50 times to value of the permeation depth, depending upon the size of the wire and the thickness of its insulation.The wire shown in this and other figures may be used either parallel to the longitudinal axis of a pipe, or as a helix as shown in Figure 10 or other pattern of proper spacing on the pipe wall.

This wire, by itself, may similarly be used to generate heat in a steel shape other than a pipe wall, as described with reference to Figures 1 and 2.

A valuable variation of the conventional wire of Figure 3 is the flattened, insulated wire or ribbon of Figure 4 which has a width greater than its thickness. To secure the maximum benefit of the proximity effect, i.e. the minimum width of the skin effect conductor resistor band and thus its maximum resistance, the conductor's effective centre should be as close to the pipe wall as possible. This may be approached by using a flat wire or metal ribbon. However, as it becomes wider, it subtends a wider band of the steel skin for reverse flow, with a corresponding lessened resistance in the steel conductor for return.

The optimum ratio of these dimensions depend on the electrical characteristics required of the circuit. The electrical insulation is applied in a conventional manner, and if made somewhat wider than otherwise necessary, i.e. 2 to 6 times the value of the penetration depth of each side of the wire ribbon, it covers the entire width of the conductorresistor band and thus insulates it against possible mechanical contact and shock or short circuit.

This wire design and usage lends itself best to the application of thermal insulation to the transport pipe over the wire, because of the minimal protuberance of the heating wire.

Figure 14 is an elevation view of Figure 4 and also shows the conventional wiring diagram, the special fittings 14, 15 and 16 are described below.

Conventionally, copper or aluminium conductor wires or ribbons would be used, but steel may also be used, particularly here as a ribbon, where heat generated by usual line loss or that due to skin effect in the steel conductor often may be transferrred readily through the thin electrical insulation to the pipe wall. If the steel ribbon or band is thicker than the effective penetration depth or skin thickness, then only the skin thickness is the effective thickness for carrying a.c. Any greater thickness of the steel ribbon becomes that much electrical insulation. As noted in U.S. Patent 3,777,117 this insulating effect in steel increases extremely rapidly with thickness (as an exponential function) as compared with the first power function for ordinary insulating materials.

Figure 5A is a modification of Figure 4 and includes, as an additional feature a shielding border, 8, on each side of the flat wire to limit the effective band width of the skin conductor resistor beneath. Thus, this band cannot extend beyond a predetermined width, the width of the shielding border, thereby preventing exposure of some part of the pipe surface which has an appreciable current flowing therein. These shielding strips are surrounded by the molded insulation and may be either single pieces, thin ribbons of copper of aluminium, or made up of a mass of lengths of metal fibres or wires. Either serves to shield the steel pipe and definately to fix the width of the band of the skin effect conductor-resistor beneath. A view of Figure 5A and 'its conventional wiring diagram is shown in Figure 15.

Figure 5B is an enlargement of a crosssection of this special flat wire and shielding strips made up of a molded or extruded assembly of suitable insulation material. This may be applied, as is indicated in Figure 5A and the previous figures, directly against the pipe wall. However, this form of the wire may be used-as also those of the others of this invention, particularly as shown in Figures 3 and 4 outside of the conventional thermal insulation as shown in Figure 5B and with the conventional circuit of Figure 16. The thermal insulation 12, is of any usual thickness from i to 6 inches, and the conductor wire is simply held on its outside, with the return a.c.

flow and the useful heat generated in the skin effect conductor-resistor subtended below and within the pipe wall.

The conductor which supplies one leg of an a.c. circuit with a band of skin effect conductor resistor as the return leg, may be made advantageously of one of the specialized types of cable which increases in resistance and therefore lower the current flow, as the temperature increases. This is usually formed as a pair of wires in a cable with a special material between the two wires. This special material may be a synthetic plastics material with a special filler. The composite has a high electrical specific resistivity compared to that of the wires and increases its electrical resistance-thus decreasing its heat generating ability-as its temperature increases. Such a material is said to have a positive temperature co-efficient of resistant (PTC). The special cable thus acts as its own thermostat or control of the input of a.c.An outer insulation covers both of the parallel wires and the connecting, PTC material. In the past two wires have always before been connected at one end of the cable to a source of either a.c. or d.c. As the temperature of the cable, and the PTC material between the two wires increases, its resistance increases and the flow of current between the two, i.e. through the PTC material and through these wires themselves, decreases. With lower heat input, the cable cools until a point of temperature is reached where the conductivity of the PTC material, which is uniformly distributed along the length of the two wires, is increased, more current flows, more heat is produced, and the cycle is repeated.

Such heating cable has always had its two parallel wires on opposite sides of the material connected to the power source at the same end.

Now it has been found that if this heating cable is used in a somewhat different circuit as the conductor wire of a skin effect heating system, the thermostatic control of the heat produced is obtained not only in the cable itself, as in the prior art, but simultaneously the current flow and heat produced in the return leg of the circuit, i.e. the coductorresistor portion of the shape is also controlled.

This novel connection, as shown in longitudinal cross-section in Figure 6A and in transverse cross-section in Figure 6B, is for heating a steel pipe, 1, by means of the skin effect conductor-resistor band therein induced by a.c. flowing in the parallel conductor wires, 3' and 3 ", with general electrical insulation 4. (For clarity in drawing, the wires are shown in elevation). The PTC material, 11, in this case is a synthetic resin, which has a special filler added. It is part of an integrally molded cable in which are included the two conductor wires, and the general insulation. Any similar system may be used which electrically connects the two wires uniformly throughout their length to increase the resistance between the wires and thus reduce the current flow as the cable is heated.

The composition of this material, 11, and its method of electrical connection between the two wires, 3' and 3", is not a part of this invention, nor is the fact that per se the electrical resistance and hence heat generated in this PTC material, 11, is utilized to heat the steel shape, 1, as is the "line loss" of any other conductor wire which would be used.

Furthermore, as in other embodiments of this invention, the conductor wires, here 3' and 3", may be made of a metal which is an excellent conductor of electricity such as copper or aluminium, or of one as steel which is an intermediate conductor, or of one which is a poor conductor, such as one of the metals normally used for electrical resistance heating wire. In any case, these wires themselves will have a greater or less electrical resistance and heat generated as what is usually called the "line loss". This heat is added to a greater or lesser extent to the steel shape, 1, to add to that generated within the steel by the band of skin effect conductor resistor.

Basically, when cold, the two parallel wires 3' and 3 " with the material, 11, act as a single element, conventional conductor wire in carrying AC from the live (+) terminal of the a.c. supply to the connection, 6, with a far end of the steel shape, 1, back through the band of skin effect resistance-conductor in 1, and thence through the connecting wire, 13, to the other neutral (-) terminal of the a.c.

supply.

As the system-and particularly the PTC material, 11,-heats up, it becomes more and more resistant to the flow of a.c., and less current flows. Less heat is generated as what may be regarded here as line loss, as it is in the conventional use of this special type cable used for heating pipes directly. (In conventional use, both ends of two wires are connected directly from one end of the cable to the a.c. terminals, and one is not connected to the pipe as in this novel circuit).

In the present invention, as the PTC material, 11, heats, its electrical conductivity decreases as does the electrical flow, not only in this, the out-leg of the electrical circuit, but particularly in the skin effect conductorresistor band in the immediately adjacent steel shape which forms the return leg of the a.c. circuit and gives the substantial heat generating ability within the steel shape itself.

The thermostatic effect of the cable itself (as hitherto used in the prior art) thus may be multiplied many times by controlling the skin effect current flow, and the heat effect thereby developed within the pipe or other steel shape itself.

Figures 6A and 6B illustrate, in section, the steel to be heated 1, and the adjacent double wire cable assembly. It is obvious that the cable assembly may be either inside or outside a pipe wall, also that this steel section may be of other shape than a pipe wall, as hereinafter discussed for other wire conductors of this invention. Also, the cable assembly may be covered with, as in Figures 1 or 2, plastic covers of the type exemplified in these figures, which may be used for insulation or protection of the band of skin effect conductorresistor beneath. Also the cable assembly may usually be placed with the flat side of the molded insulation against the steel shape in order to increase the proximity effect as indicated above.

Figure 7 shows the cross-section of two transport pipes, 31 and 31', each of which acts also as the conductor wire adjacent to the other. An elevation view and the circuit is shown in Figure 17. There is thus a dual function for each, a mutual relation of wire and skin effect conductor-resistor. The number 31 is used as the combination of 3 for the conductor and 1 for the steel pipe, since both functions are realized here by a transport pipe. An electrical insulation means, 4, is shown as a flat ribbon of suitable material tangential to the two pipes; but such a ribbon may also be applied to the surface of one or both pipes, or it may be moulded to the contour of the space between the two pipes which are nearly tangential.It serves to insulate from each other the skin effect conductor-resistor bands in the steel wall of each pipe, one, 31, acting as the "out" leg of the circuit, and the other, 31', acting as the "return" leg.

Terminals connected to the a.c. source are shown at 5, connected to the near end of 31, and at 5' connected to the near end of 31'. A jumper wire electrically connects the far ends of the two pipes to each other at junction 6 for 31, and 6' for 31'. The circuit thus established carries AC from the terminal 5 to the near end, then through and out the conductor-resistor in 31 to the far end, through the jumper wire from 6 to 6', then back through the skin conductor-resistor band in 31' to its near end, thence to the terminal 5'.

Both pipes are heated by the heat generated in the respective bands of skin effect conductor-resistors in that part of their respective wall sections closest to the other throughout their co-extensive lengths; and each of these bands acts as the usual conductor wire in developing the electromagnetic flux causing the skin effect and proximity effect pheno mena in the other.

Because of the expanse of the two convex walls of the pipes adjacent to each other, this band in each pipe may be wider than desired; and the corresponding resistance less than desired. Shielding may be used to reduce the widths of these skin effect conductor bands as by two thin ribbons of conductor metal, 8, imbedded in the insulation, 4 as shown for example in Figures 5A and 5B.

While 31 and 31' are indicated as crosssections of transport pipes, and this may be the most useful application of this embodiment of the invention, other steel shapes or elongated strips may also be used with the same results; and the fluid being heated may be forced on the outside of the two shapes, rather than being transported inside the pipes as illustrated in Figure 7. If means are provided for moving the (pipes farther apart, the skin effect and proximity effect serve to increase the effective resistance of the circuit, and less heat is generated as shown with an ordinary conductor wire in Figure 4. While in most cases it may-be desirable to use shapes of uniform cross-section, there may be others where non-similar, elongated shapes in substantially parallel relation are more useful.

Figure 8 is an expansion of the system of Figures 7, applied to a supply of 3-phase a.c., with three co-extensive steel pipes, each of which has a band of skin effect conductorresistor adjacent each of the other two pipes.

Each band acts as the conductor wire for developing the corresponding and adjacent band of skin effect conductor in the adjacent pipes. Again, there is provided insulation, 4, here shown as a single molded shape, between each line of contact of the respective pairs of pipes; and again shielding may be provided in the insulators or otherwise although not shown in Figure 8.

The a.c. circuit for Figure 8 is shown in Figure 18; and it is analogous to that of Figure 17, expanded to include 3-phase a.c., each phase 120O out of phase with the other two. The three terminals respectively 5, 5' and 5" of the a.c. source are indicated as small circles at the ends of the three wires, 13, 13' and 13" connected at the near ends of pipes, 31, 31' and 31", at points 10, 10' + 10". The far ends of each of the three pipe wire combinations are connected by jumper wires between 6 and 6', 6' and 6", also 6" 6, although only any two of these three jumper wires would suffice. Thus, the familiar "Y" connection is established at the far ends of 31', 31", and 31"' with balanced loading of each of three uniform pipes.

Here again, the three elongate shapes of Figure 8 like those of Figure 7, may be other than tubular. They may be the same in crosssection or of different cross-section and the fluid being heated may be forced against any part or all of their internal surfaces, and they may be adjustable as to distance apart by pairs, so as to increase the resistance to a.c.

flow through the circuit of a pair and thus the heat generated in the adjacent skin effect conductor-resistors.

Figures 9A and 9B show a steel sheet, here assumed flat, although any other surface: cylindrical, conical, even spherical, or other warped surface, may be used. The surface may be a part of a heater; and means then may be provided to pass a fluid to be heated against a surface. The sheet of steel, 1, corresponding to the transport pipe wall of the other figures, has attached thereto on its opposite surface an insulated wire, 3. The wire also may have a plastic cover, as shown in Figures 1 and 2, again with narrow cracks between the cover and the steel substantially parallel to the wire and on its either side. The surface of the steel plate may be covered uniformly or otherwise with straight runs, zig zag meander or other patterns of the wire However, no section of the wire is closer to another than about 50-100 times the value of the penetration depth.The one end of the wire is connected to the steel plate at the point 9; and at a far corner, it leaves its attachment to the plate to be connected to the neutral terminal (-) of the a.c. source. The live terminal (+) of the a.c. is connected to the steel plate at a point, 10, very near the point where the insulated wire leaves contact therewith.

All of the other embodiments of this invention, as those illustrated in all of the other Figures, may also be applied to one or more steel shapes other than pipe walls in conjunction with that diagrammed in Figures 9A and 9B; and utilitarian fluid may be flowed against a surface of such steel shape or shapes as indicated by arrows in Figures 9A and 9B.

Numerous means are available for holding to the pipe wall or plate the insulated conductor wires shown in the several figures, also the covers of Figures 1 and 2. All have been used in the prior art and none are a part of this invention. These methods of attachment may depend on wiring, bolting, taping, clamping, cementing, etc. None should interfere more than a minimal amount with the thermal insulation which is invariably applied to a transport pipe, either in a pre-fabrication or in a field operation. In the pre-fabrication of thermally insulated and coated lengths of pipe, stub wires are left for electrical connection in the field following welding of the pipe joints.

The materials for electrical insulation of the conductor wire are discussed in U.S. Patents 3,617,699 and 3,777,117. Insulation of the conductor wires of this invention does not present the problems mentioned in Patent specification No. 3,777,177, because of re latively lower temperature, and freedom from contact with the fluids being transported in the present installations where the wire is located outside the transport pipe, Similarly, the temperatures which the electrical insulation of the conductor wires must resist are very much less than those encountered in the insulation of conventional electric tracing wires used for heating. These are outside the pipe, and all of the heat generated must pass through the insulation at a temperature sufficiently high so as to transfer the heat into the pipe wall.In the present invention, much or substantially all of the heat produced is generated within the pipe wall itself; and thus only the heat from the relatively small line loss of the copper, aluminium, or steel conductor wire must be transferred through its insulation to the pipe wall. Correspondingly, there is a much lower temperature in the insulation and less difficulty in its design and operation.

In some cases, where particularly high temperature are involved, and especially if the pipe is pre-insulated and pre-coated in a factory operation, a special electrical insulation system may be used. This may be applied directly to the pipe wall as a mineral insulation of the types specified in the Co- pending application, or as glass fibres specially impregnated with a high temperatureresistant, insulating resin, or others. The wire, preferably in ribbon form, may then be attached to the pipe on the outside of this electrical insulation and then covered with further electrical and thermal insulation, as specified.

THERMAL INSULATION The type and specification of the thermal insulation used on a transport pipe heated by this system is, in general, outside of this invention, however it has been found that pipelines already thermally insulated by many of the standard materials, when they are not an effective shield for an electromagnetic flux, may be heated by the method of this invention, in effect-through the insulation.

Thus, if an electrical conductor is laid on the outside of the existing insulation, and the circuit is wired as described above, a band of skin effect conductor-resistor effectively heats the subtended part of the steel pipe wall, thus the entire wall and then the contents of the pipe. This is not nearly as efficient as when the wire is next to the pipe wall, since practically all of the line loss of the conductor wire is lost to the surroundings rather than being kept inside to heat the pipe itself.

However, this system may be applied in an existing, insulated pipeline in some cases of emergency, or when only a small amount of heating time is necessary per year. In these cases, any inefficiency in the use of power may be relatively unimportant. The wire may be applied either parallel to the axis or in a helical winding. The insulating covers of Figures 1 and 2 might be used; but the flat wires of Figures 4 and 5 do not show up so relatively advantageous at this distance from the pipe wall. The simple insulated wire of Figure 3 would almost always be used, although Figure 5B shows the application with a flat conductor wire.

In this particular usage, particularly if the pipe is above, but close to the ground, a somewhat greater efficiency may be gained if the wire is laid on the top of the insulation.

Thus, the entire pipe circumference acts as a shield against the mangetic field from the conductor wire penetrating the earth to cause any possible current losses thereto. The same is true, but to a much lesser degree, with other wires laid directly on the pipe wall.

As above mentioned, the position of the conductor wire relative to the pipe changes the proximity effect and resistance of the skin effect conductor-resistor band. Moving the wire or the pipe relative to the other; or of another pipe which is part of the a.c. circuit, controls the resistance, the heat generated, and the temperature. In some cases this movement of pipe or wire may be done during operation by supplying suitable means to provide this relative movement of the parts.

Figure 4 shows such a means; and Figure 14 shows the circuit. A lever arm, 14, is attached rigidly to the insulation, 4. The other end of the lever arm is pivoted in a bearing block, 16, firmly attached by welding or otherwise to the pipe wall, 1. The lever arm moves on a pin, 15, in the bearing block to allow a rotary movement. A number of such arms spaced along the insulated wire, 4, moves it away from the pipe through an arc as shown. The electromagnetic field changes with increased distance as well as changing angle with the pipe wall of the redtangular wire, 4; and the skin effect conductorresistor may have its resistance increased many times by swinging the lever arm and thus the wire, 3, away from the pipe wall to reduce, accordingly, the heat generated in the pipe. If the wire is of steel, the variance and hence control range may be expanded even more.If the transport pipe is moved relative to the wire, or relative to another transport pipe wherein the skin effect conductor-resistor band is formed along the adjacent surface of the co-extensive lengths, the effect is the same; and the important point is in securing such increase of resistance and hence control of heat input by the separation of the two co-extensive legs of the a.c.

circuits.

WHAT I CLAIM IS: 1. A heat generating system comprising: (a) shape made of a metal having magnetic properties and electrical conductivity and having a first, near, portion and a second, remote portion; (b) means whereby a utilitarian fluid is caused to flow in direct contact with a surface of said shape; (c) an elongate electrically conducting means, positioned in closely adjacent but electrically insulated relationship with respect to a surface of said shape opposite said surface in contact with the fluid and extending from said near portion to said remote portion, said conducting means having a first, near, end adjacent said first portion of said shape and a second, remote end adjacent said remote portion of said shape; said conducting means being uncovered throughout those portions of its outer surface not directly confronting said shape by any electrically conductive material which would shield the electromagnetic field generated around said conducting means in a direction away from said shape; (d) a source of a.c. having a first supply terminal and a second supply terminal; (e) an electrical connection directly between said first a.c. terminal and said first, near, portion of said shape; (f) an electrical connection between said

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (16)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   pending application, or as glass fibres specially impregnated with a high temperatureresistant, insulating resin, or others. The wire, preferably in ribbon form, may then be attached to the pipe on the outside of this electrical insulation and then covered with further electrical and thermal insulation, as specified.

   THERMAL INSULATION The type and specification of the thermal insulation used on a transport pipe heated by this system is, in general, outside of this invention, however it has been found that pipelines already thermally insulated by many of the standard materials, when they are not an effective shield for an electromagnetic flux, may be heated by the method of this invention, in effect-through the insulation.

   Thus, if an electrical conductor is laid on the outside of the existing insulation, and the circuit is wired as described above, a band of skin effect conductor-resistor effectively heats the subtended part of the steel pipe wall, thus the entire wall and then the contents of the pipe. This is not nearly as efficient as when the wire is next to the pipe wall, since practically all of the line loss of the conductor wire is lost to the surroundings rather than being kept inside to heat the pipe itself.

   However, this system may be applied in an existing, insulated pipeline in some cases of emergency, or when only a small amount of heating time is necessary per year. In these cases, any inefficiency in the use of power may be relatively unimportant. The wire may be applied either parallel to the axis or in a helical winding. The insulating covers of Figures 1 and 2 might be used; but the flat wires of Figures 4 and 5 do not show up so relatively advantageous at this distance from the pipe wall. The simple insulated wire of Figure 3 would almost always be used, although Figure 5B shows the application with a flat conductor wire.

   In this particular usage, particularly if the pipe is above, but close to the ground, a somewhat greater efficiency may be gained if the wire is laid on the top of the insulation.

   Thus, the entire pipe circumference acts as a shield against the mangetic field from the conductor wire penetrating the earth to cause any possible current losses thereto. The same is true, but to a much lesser degree, with other wires laid directly on the pipe wall.

   As above mentioned, the position of the conductor wire relative to the pipe changes the proximity effect and resistance of the skin effect conductor-resistor band. Moving the wire or the pipe relative to the other; or of another pipe which is part of the a.c. circuit, controls the resistance, the heat generated, and the temperature. In some cases this movement of pipe or wire may be done during operation by supplying suitable means to provide this relative movement of the parts.

   Figure 4 shows such a means; and Figure

   14 shows the circuit. A lever arm, 14, is attached rigidly to the insulation, 4. The other end of the lever arm is pivoted in a bearing block, 16, firmly attached by welding or otherwise to the pipe wall, 1. The lever arm moves on a pin, 15, in the bearing block to allow a rotary movement. A number of such arms spaced along the insulated wire, 4, moves it away from the pipe through an arc as shown. The electromagnetic field changes with increased distance as well as changing angle with the pipe wall of the redtangular wire, 4; and the skin effect conductorresistor may have its resistance increased many times by swinging the lever arm and thus the wire, 3, away from the pipe wall to reduce, accordingly, the heat generated in the pipe. If the wire is of steel, the variance and hence control range may be expanded even more.If the transport pipe is moved relative to the wire, or relative to another transport pipe wherein the skin effect conductor-resistor band is formed along the adjacent surface of the co-extensive lengths, the effect is the same; and the important point is in securing such increase of resistance and hence control of heat input by the separation of the two co-extensive legs of the a.c.

   circuits.

   WHAT I CLAIM IS: 1. A heat generating system comprising: (a) shape made of a metal having magnetic properties and electrical conductivity and having a first, near, portion and a second, remote portion; (b) means whereby a utilitarian fluid is caused to flow in direct contact with a surface of said shape; (c) an elongate electrically conducting means, positioned in closely adjacent but electrically insulated relationship with respect to a surface of said shape opposite said surface in contact with the fluid and extending from said near portion to said remote portion, said conducting means having a first, near, end adjacent said first portion of said shape and a second, remote end adjacent said remote portion of said shape; said conducting means being uncovered throughout those portions of its outer surface not directly confronting said shape by any electrically conductive material which would shield the electromagnetic field generated around said conducting means in a direction away from said shape; (d) a source of a.c. having a first supply terminal and a second supply terminal; (e) an electrical connection directly between said first a.c. terminal and said first, near, portion of said shape; (f) an electrical connection between said

   second a.c. terminal and said first, near end of said electricallv conducting means; (g) an electrical connection between said second, remote, portion of said shape and said second remote, end of said electrically conducting means; (h) an a.c. circuit established: (i) from said second terminal of said a.c. source through the length of said electrically conducting means, (ii) then back through an elongate band portion of said shape formed in a skin portion thereof and adjacent to said electrically conducting means such that a skin effect current is created in said band portion of said shape which acts as a conductor-resistor whereby heat is produced in said shape, said skin portion being less than half of the thickness of said shape; and (iii) finally back to said first a.c. terminal, to complete said a.c. circuit whereby at least some part of said heat produced in said shape is transferred directly to said utilitarian fluid without passing through any other material.
2. 2. The system of claim 1 wherein said shape is a pipe through which said utilitarian fluid is caused to flow in contact with its inner surface.
3. 3. The system of claims 1 or 2 wherein an elongate shape made of an electrical insulating material is placed to cover said electrically conducting means and the adjacent said elongate band of said skin portion of said shape, acting as a skin effect conductor-resistor, thereby electrically insulating said elongate band.
4. 4. The system of any one of claims 1 to 3 wherein said shape has a planar surface in that part of its surface which is most closely adjacent to said elongated insulated electrically conducting means.
5. 5. The system of any one of claims 1 to 4 wherein said elongate electrically conducting means comprises a metal strip electrically insulated throughout its length and having a width greater than its thickness.
6. 6. The system of any one of the preceding claims wherein a layer of thermal insulating material is placed between said elongate electrically conducting means and said shape.
7. 7. The system of any one of the preceding claims wherein said elongate electrically conducting means is made of metal.
8. 8. The system of any one of claims 2 to 7 wherein said elongate electrically conducting means follows a three-dimensional curved path in being formed along the wall of said pipe.
9. 9. The system of claim 8 wherein said electrically conducting means follows a helical pattern along the wall of said pipe.
10. 10. The system of any one of the preceding claims wherein means is provided for adjustably spacing said electrically conducting means away from said shape along at least some part of their coextensive lengths, so as to increase the resistance in said elongate band and thus lower the amount of a.c. flow and heat generation therein.
11. 11. The system of any one of claims 1 to 9 wherein an elongate ribbon of an electrical conducting material is positioned parallel to and on at least one side of said electrically conducting means, said ribbon being between said shape and said electrically conducting means, and being electri ally insulated from both said shape and said electrically conducting means, whereby said ribbon shields said shape from the electromagnetic field engendered by said a.c. current flow and thus narrows said elongate band acting as a skin effect conductor resistor, whereby the electrical resistance of said band is increased.
12. 12. The system of claim 7 wherein both said shape and said electrically conducting means are the same in transverse crosssection.
13. 13. The system of any one of claims 1 to 11 wherein said elongated electrical conductor means comprises:- (a) two electrically conductive wires, a first and a second, adjacent to each other and substantially parallel; (b) means for electrically connecting said first wire to said second wire uniformly throughout their respective lengths, said means being made of a material with a positive temperature coefficient of resistance and having a relatively high electrical specific resistivity compared to that of the wires, and a higher electrical resistance at a higher temperature; (c) an electrical connection between said second a.c. terminal and the end of said first wire which is near the electrical connection at said first portion of said shape and said first a.c. terminal; ; (d) an electrical connection between said second portion of said shape and said second wire remote from the electrical connection of said first wire with said second a.c. terminal; (e) an a.c. circuit established through said conductor means: (i) from said second terminal of said a.c. source through the length of said first wire; (ii) through said means for electrically connecting said first wire to said second wire uniformly throughout their respective lengths; (iii) to and through the length of said second wire; and (iv) to an electrical connection with said second portion of said elongate shape, whereby (f) said positive temperature coefficient of resistance and highly specific resistivity of said means for electrically connection said first wire to said second wire reduces the flow of a.c. as the temperature increases in said electrically connecting means of said first wire to said second wire and thus reduces the heat produced in, and the resulting temperature of, said shape.
14. 14. A heat generating system comprising: (a) three elongated shapes, a first, a second, and a third, each of which is a pipe made of a metal having magnetic properties and electrical conductivity and having a first, near, end and a second, remote end; each of said three pipes being positioned in closely adjacent but electrically insulated relationship with respect to the respective exterior surfaces of the other two; and with the respective near ends of said three pipes adjacent to each other, and with the respective far ends of said three pipes adjacent to each other, the outer surfaces of the pipes which do not confront each other being uncovered by any electrically conductive material which would shield the electro-magnet field generated around the pipes; (b) means by which a utilitarinan fluid is caused to flow through each of said pipes; (c) a source of three phase a.c. provided with a first supply terminal, a second supply terminal and a third supply terminal; (d) means provided for an electrical connection directly between said near end of said first pipe with said first terminal; said near end of said second pipe with said second terminal, and said near end of said third pipe, with said third terminal; (e) means provided for electrically connecting together the remote ends of all three of said pipes in a "Y" connection; whereby (f) a three phase a.c. circuit is established, of which:: (i) a first single phase a.c. flows from said first terminal through the length of the wall of said first pipe in a longitudinal band adjacent to the wall of said second pipe, and also in a longitudinal band which is adjacent to the wall of said third pipe; (ii) a second single phase a.c. flows from said second terminal through the length of the wall of said second pipe in a longitudinal band adjacent to the wall of said first pipe, and also in a longitudinal band adjacent to wall of said third pipe; (iii) a third single phase a.c. flows from said third terminal through the length of the wall of said third pipe in a longitudinal band adjacent to the wall of said first pipe, and also in a longitudinal band which is adjacent to the wall of said second pipe;; (iv) and then each of these three circuits for the respective single phases passes through said "Y" connection at the far ends of all three of the respective pipes; (v) finally each of these respective three flows of single phase a.c. returns back to the other two of said three terminals through the length of the walls of the other two respective pipes in longitudinal bands adjacent to the respective pipe carrying said a.c. single phase out from its respective terminals; (g) each particular one of said three phases of a.c. produces by its flow out from its respective terminal a skin effect current concentrated in the respective longitudinal bands of those parts of the skin of the wall of the particular pipe which are adjacent to the walls of the other two pipes; and the same particular one of the three phases produces by its flow from the "Y" connection back to the other two terminals a skin effect current concentrated in the longitudinal bands of those parts of the wall of each of the other two pipes which are adjacent to the wall of the particular pipe; (h) each of said three pipes having a wall at least twice the thickness of said skin through which said a.c. flows to produce heat in each of said pipes; whereby at least some part of said heat produced in each of said pipes is transferred directly to said utilitarian fluid without passing through any other material.
15. 15. The system of claim 14 wherein the cross-section of each of said three elongated steel pipes are substantially identical; the same utilitarian liquid flows in each of said pipes; and, at any transverse cross-section of said three pipes, their centres are approximately equidistant.
16. 16. A heat generating system substantially as described herein with reference to and as shown in the accompanying drawings.