US3522415A

US3522415A - Electric heating devices

Info

Publication number: US3522415A
Application number: US658785A
Authority: US
Inventors: Paul Eisler
Original assignee: Individual
Current assignee: Individual
Priority date: 1966-08-08
Filing date: 1967-08-07
Publication date: 1970-08-04
Anticipated expiration: 1987-08-04
Also published as: GB1187562A

Description

2 Sheets-Sheet 1 Filed Au 7, 1967 Ira c572 i0 Pal/Z 5415261 Filed Aug. 7, 1967 g 4, 70 P. EISLER 3,522,415

ELECTRIC HEATING DEVICES 2 Sheets-Sheet 2 3,522,415 ELECTRIC HEATING DEVICES Paul Eisler, 57 Exeter Road, London, NW. 2, England Filed Aug. 7, 1967, Ser. No. 658,785 Claims priority, application Great Britain, Aug. 8, 1966, 5,416/ 66 Int. Cl. H05b 3/36 US. Cl. 219528 16 Claims ABSTRACT OF THE DISCLOSURE An electric heating device comprises a plurality of limbs, generally in zig-zag disposition, of flat section sheathed electric conductor, the width of the limbs being substantially coplanar and the limbs being held under longitudinal tension. The apices of the zig-zag may be supported by smooth good insulators rotatable on two supporting rods one fixed and the other pulled by tension springs, so that all limbs are under substantially the same tension. Insulating strips may be interwoven to produce a loose fabric and such fabric may be stored in the roll from which pieces can be cut. The sheaths need not be impervious or of adequate insulating quality in themselves and moisture may be driven off by preliminary operation at sub-normal voltage. Various uses and applications are described.

This invention relates to electric heating devices and an object is to provide a structure which can readily be adapted to various applications and loading and to provide a material which can be stored in long lengths from which a piece or pieces can be cut for the production of a heating device for particular cases.

According to the invention an electric heating device United States Patent 0 comprises a plurality of flexible limbs each forming a heating element and each being constituted by a generally flat section, sheathed electric conductor, the limbs being spaced and disposed with their flat surfaces substantially parallel so that when energised all radiate heat in the same general direction, and means exerting longitudinal tension solely on the sheaths of the limbs whereby the limbs are held suspended in the said disposition. In particular a plurality of adjacent limbs of the device may be continuous and disposed in a zig-zag formation, the tension exerting means supporting the zigzag at the apices thereof with negiligible friction. Then the tension is carried over from one limb to the next and they will all be under substantially the same longitudinal tension. Briefly the supporting means may be smooth freely rotatable rollers or the like disposed within the apices.

In the simplest case the flat section of the limbs will define a more or less plane area (ignoring inevitable slight sag) from which heat will be radiated but if the limbs are spaced in the direction of their widths the atmosphere in which the device is used can flow over and between them and be heated by convection. This spacing may differ in different parts of the area and it may also be adjustable to enable the ratio of heat dissipated by radiation to heat dissipated by convection to be varied.

In the case of a zig-zag disposition, smooth rotatable rollers or the like may be disposed in the two sets of apices those in each set being carried by a respective support. One support may be fixed in an external skeleton framework and the other be secured in the framework by tension springs at intervals between the individual apices, serving to draw this support away from the other and so to impose tension on the sheath; or both supports may be secured by tension springs.

Flexible strips, to which longitudinal tension is applied may be interwoven with the limbs. Also a loosely woven "Ice framework to which the limbs are suspended, may be insuflicient of themselves to effect adequate electrical insulation of the conductors under all conditions of operation to be expected. For example if the sheaths are porous, under humid conditions upon switching on the moisture might permit leakage under the normal working voltage, but the moisture can be driven off by starting at low voltage, as will be further explained. A particularly suitable sheath is a single layer of glass fibre fabric.

A cover may be used on one side of the area occupied by at least some of the limbs, disposed to reflect back radiated heat so that the greater part of the total radiation will be in a direction away from the cover. The spacing of the cover from the limbs can be chosen according to the desired convective flow around the limbs.

Again at least two sets of limbs can be superposed spaced apart, in order to increase the total emission over a given area without increasing the load/ area factor of the individual limbs.

The invention is not limited to such conductors or to any particular type of conductors provided the sheathed whole is a flat structure. The constructions shown in my US. Pats. Nos. 2,971,073, 3,020,378, 3,033,970, 3,149.- 406, 3,317,657 and 3,283,284 can all be used and other possibilities are carbon or graphite coated textile tapes, and carbon layers between overlapping pieces of metal foil as in FIG. 11 of 3,033,970.

The production of electric surface heating devices having relatively large areas emitting heat from narrow elements has hitherto usually been effected either by producing sheet materials with heating elements laminated in them or stuck, pressed or otherwise fixed to them, or by interweaving heating wires with insulating strands. The present invention on the other hand makes use of surface heating elements which inter alia permits the same emission at lower surface temperature.

In many applications such as drying, baking or curing ovens and tunnels run at mains voltage or other voltage subject to safety regulations the heating elements used have hitherto either been provided with an insulating sheath as prescribed by the regulations or been hermetically shielded by metal cladding or the like. Air or vapour has been guided to pass over these heating elements by suitable trunking, baflles, blowers, etc. In view of the high temperature of the heating elements the insulation and other features of the construction have been expensive, and the cyclic expansion and contraction of the heating element has led to difficulties and failure.

The present invention avoids the difiiculties of expansion and contraction, and since the limbs are suspended, usually in air, the sheath need not be of high insulating quality provided the suspending means introduces adequate insulation at the points of suspension.

The conductors used in the present invention can be enclosed in an inexpensive, preferably fibrous sheath, e.g. non-impermeable woven or felted glass fibre tapes. These sheath materials in the thin thicknesses used would not constitute an adequate insulation according to regulations, that is they would not maintain the required high resistance to earth of the heating conductor when subject to high humidity, although they show this high resistance when dry, and they prevent actual contact between the heating conductor and any solid object. A short circuit however would have to be expected on switching on if water condensed on the cold tape and an earthed metallic body touched the tape. 1

The present invention avoids this danger and also permits the use of heating elements with meander-type conductive strands wherein neighbouring strands are at different potential and insulated from each other substantially only by being spaced from each other by their fibrous support or sheath, such as where they are sewn on to or into nonimpregnated glass fibre tapes.

The invention will be further described with reference to the accompanying diagrammatic drawings in which:

FIG. 1 is a perspective view of part of a heating device according to the present invention,

FIG. 2 is a plan view of part of the device of FIG. 1,

FIG. 3 is a detail end view of FIG. 2,

FIG. 4 illustrates a roll of fabric in accordance with the invention, and

FIGS. 5 and 6 are details of two forms of sheathed conductor which can be used in the invention, part broken away; and FIG. 7 is a diagrammatic view.

Referring first to FIGS. 1 and 2 a framework generally indicated at 11 serves to support the limbs 12 of sheathed conductor. These limbs are continuous and form a zigzag the apices 13 of which are supported by being passed over glass or ceramic bobbins or tubes 14 constituting smooth surfaced cylindrical insulators of high insulating value. They are rotatably on supports formed by

rods

15, 16, suitably of steel. The rod 15 is fixed in the framework 11 (which can be built for example of slotted angle steel) by the aid of brackets 17. The rod 16 is drawn away from the rod 15 by tension springs 18 anchored at suitable intervals to the framework at a spacing which bring them clear of the limbs 12. The weight acting on the rod 16 is carried by horizontal support brackets 19.

The springs 18 impose longitudinal tension on the sheathed conductors, and since the tubes or bobbins 14 are smooth and can rotate with negligible friction on the

rods

15, 16 the tension is carried over from one limb to the next by sliding of the sheathed conductors and/ or rotation of the tubes or bobbins and the whole zig-zag is under substantially the same tension throughout. Also this mode of suspension applies the tension exclusively to the sheaths. If the construction of the limbs is suitable (as will be described) no substantial tension will be transmitted to the conductors themselves.

There may be a single tube or bobbin 14 at each apex, or the tubes or bobbins or some of them may be of such length that several apices are supported at each. In the former case rotation alone is sufiicient to ensure transmission of substantially constant tension from one limb to the next. In the latter case some slip of some of the apices on the tube or bobbin may also be involved. The tubes or bobbins may be slid along the rods as may be necessary in adjusting the spacing of the limbs. Some form of adjustable stop (not shown) e.g. tubular ferrules with grub screws, may be fitted to the rods between the tubes or bobbins to prevent accidental axial movement.

The rod 15 could also be mounted by the aid of springs and supporting brackets but this is usually unnecessary.

With the zig-zag so laid out, the width of each limb is substantially parallel to the general area of the zig-zag and when the conductors are energised heat will be radiated generally in directions perpendicular to the general area. Also with the apices 13 well spaced there is ample room for convective flow of air round the limbs.

If radiation mainly in one direction from the general area of the zig-zag is wanted, e.g. in the case of a drying tunnel, a cover may be used on the opposite side of the zig-zag e.g. constituted by the tunnel wall which reflects the radiated heat back. The spacing of the cover from the zig-zag controls convective flow and may be adjustable. Part of such a cover is indicated in FIG. 1 at 21.

If increased heat dissipation over a given area is wanted without increasing the load/area factor of the sheathed conductor and without bringing the limbs closer together, a plurality of zig-zags may be superposed, sufficiently spaced to permit the desired convective flow. By way of example FIG. 1 shows two superposed zig-zags. The second zig-zag has the parts marked with the same numerals as used in the preceding general description.

The second zig-zag may equally be regarded as forming a second heater of an oven or drying tunnel, the material to be heated constituting or being carried by a web which travels between the two zig-zags so that it is irradiated from opposite sides. Such a web may travel in several successive passes in opposite directions between a series of superposed zig-zags or pairs of zig-zags carried in a single framework, thus shortening the overall length of the oven or tunnel. If there is a pair of zig-zags per pass there may be dividing walls between the pairs, constituting covers having the action above described, though usually they will not be used because free vertical air fiow is desirable.

To assist in maintaining the desired disposition of the limbs of the zig-zag and reduce sag, well-spaced tapes 22 may be interwoven with the limbs of the zig-zag and held under longitudinal tension e.g. by anchoring one end of each to the framework 11 and attaching the other end by a tension spring not shown. These tapes may be of similar material to the sheaths of the limbs.

These tapes 22 convert the zig-zag into a species of loosely woven fabric. As shown in FIG. 4 such prepared fabric comprising sheathed conductors 12a. as the weft and tapes 22a as the warps can be stored in the roll. Heaters may then be prepared by cutting off a length of the fabric and making connection to the ends of the conductors, or to the ends and to intermediate points at which the conductors are if necessary severed, to enable the conductors to be connected in parallel groups or other arrangements as may be necessary for the particular case.

FIGS. 5 and 6 show by way of example two possible constructions of sheathed conductor which can be used.

In FIG. 5 there is a single wide conductor strip 25 e.g. of crimped metal foil such as steel, aluminium or alloy foil, the crimps 26 running transverse to the length of the strip 25. The width and thickness of the strip will be chosen according to the required operating conditions and the material of the strip which may be of other material than metal as mentioned earlier herein.

The strip 25 is enclosed in a two layer sheath, consisting of an inner covering 27 and an outer covering 28. Preferably the strip 25 is loose within the covering 27, and the latter loose Within the covering 28; then, in conjunction with the elasticity conferred by the crimping of the strip, no substantial tension will be transmitted to the conductor strip while if the conductor strip should break, the elasticity conferred by crimping will give it a tendency for the two ends to retract at the break and convert this into a gap.

The

coverings

27, 28 can be of plastics material or textile fabric, e.g. fibre glass fabric or one of one material and the other of the other material. Plastics materials can be extruded or applied as separated strips and joined e.g. welded, struck, or sewn along the edge; or a single strip may be folded over the conductor strip and the margins be simply overlapped or sewn welded or stuck together. Similarly textile fabrics can be woven or knitted in tubular form around the conductor strip or be produced as tapes which are sewn along the edges, or a single tape may be folded over with the margins overlapped or sewn or stuck together. Textile fabrics can be impregnated with plastics materials.

In FIG. 6 a plurality of conductive strip 29 are used with transverse crimps 31. The sheath again comprises a loose inner covering 32 and a loose outer covering 33. The top and bottom of the inner covering 32 are joined together between the strips 29 as indicated by dotted lines 34. These lines may represent stitching, or in the case of plastics, welding, or they may represent lines of adhesive. The effect is to prevent contact between adjacent strips 29 enabling them to be at different potentials as may be necessitated by the scheme of electrical connection necessary for the desired heating effect; the several conductor strips in one limb or series of limbs may for example be connected to provide a meander path (or several meander paths in parallel).

The outer covering 33 is here assumed to be of woven textile film but all the variations in material above described with reference to the

coverings

27, 28 of FIG. 5 are equally applicable to the construction of FIG. 6. Also in either case one covering may be omitted so that the sheath is a single layer covering as in the case of FIG. 6, it is the covering 32 which would be omitted since it is usually necessary to keep the individual strips 29 separated.

The choice of materials above described with reference to the coverings of FIGS. 5 and 6 also apply to the sheaths of FIGS. 1 to 3 and 4. Again instead of tapes 22, 22a. cords could also be used, for example glass fibre cords.

The above description with reference to the drawings by no means exhausts the possibilities of the invention and some further details will now be given.

As regards the tubes or bobbins of FIGS. 1 to 3 these may be mounted on metal (steel) tubes which are rotatable on the rods 15, 1-6. Such metal tubes serve as bushings. Equally the

rods

15, 16 may themselves be tubular. The tension of the springs 18 may be adjustable e.g. by screws and nuts. Usually bobbins will be used where only a single apex passes over the bobbin, the flanges axially locating the sheathed conductors while single tubes will be used where several apices are supported by the one tube.

In the case of a fabric such as that of FIG. 4 the edges of the fabric are made especially strong, for example by using stouter tapes or cords for the warps at the edges. The edges can then be fixed to helical springs which are provided with a glass fibre sleeve over their ends or some other gripping device which comes between the sheathed conductor and the metal of the framework and constitutes an electrical insulator which is reliable electrically under all the conditions including temperature, humidity and liability to chemical attack prevailing at the position of use Such a fabric having glass fibre insulation and glass fibre warps (whether produced when installed as in FIGS. 1 to 3 or premanufactured as in FIG. 4) is capable of running at high temperature while the spacing of the successive weft limbs determines the average maximum temperature or heat dissipation per unit area of the fabric.

By using quartz fibres still higher temperatures are available while by using plastic film or fibrous nylon or cotton sheaths and warps a cheaper fabric can be produced if the temperatures required are low enough to permit.

When the heating fabric has to be decorative as well, the warps and/or weft can be coloured, printed, crimped or embossed. For low wattage per square foot requirements the sheaths and warps can be extruded P.V.C., and with such constructions the insulation of the tensioning and holding provisions operates essentially as a safeguard in case of softening of the plastics material along the edges of the fabric where the sheathed conductor is folded and over a small area is in effect under double watt loading.

The weakness of the insulation of the sheathed conductor used for the fabric of the invention is in most cascsparticularly in the case of textile sheaths, comprising fabrics of glass and quartz fibres as well as those of nylon, terylene or cottonthe breakdown of their insulating value when water condenses on them or when they are needed to operate in a vapour or in highly ionised atmospheres. The sheathed conductors may be designed so that they can be connected when starting from cold to such a low voltage that they get sufficiently hot while still on that low voltage to evaporate the water condensed on them. Thus they may be run, if in free air, just over C. in a low voltage circuit and switched over to their normal running conditions after the water, if present, must have evaporated or boiled off.

If the heating fabric has to operate in a space with an atmosphere of high inflammability or in a space where vapour threatens the breakdown of a permeable unim pregnated textile insulation the heating fabric may be encased in a flat thin perforated box or bag of metal or other material, and air or inert gas be pumped into the box with at least slightly higher pressure than exists in the space filled with inflammable or highly ionised vapour or gas. Thus the chance of these vapours or gases reaching the tape is eliminated and the need for usual expensive precautionary measures for electric heating in spaces where inflammable vapours are present is avoided.

The features described so far, particularly the permeability of the flexible fabric to air, the ease of adjustment or assembly in a frame for any desired shape and Watt/ sq. ft. loading, the safety provisions arranged for, the low cost and robust nature of the fabric make it suitable for use as a large area heater not only in ovens and drying tunnels, but generally as a mobile, readily erected heater anywhere and for open air heating of either goods, human beings or animals.

It can be used lying on the floor or ground or supported off the ground for drying wheat, corn, hay and grain piled on it or as a blanket put over heaps of any agricultural products requiring drying while air can percolate through them and water can flow off. It can be used suspended under the roof in passages and stables, as an acoustic ceiling supporting a layer of sound deadening glass Wool, as a retractable canopy over a terrace, pavement, outdoor seating arrangement (e.g. of a stadium or coffee house, if desired underneath a usual canopy which would protect it from rain and reflect heat), as a curtain fabric for hanging on wet walls for drying purposes or over scaffolding on building sites for frost protection, as a blanket cover for accelerated curing of concrete, as a temporary cover over roofing or other surfaces to warm them prior to pouring viscous compounds and thereby ease their flow, for pre-warming or thawing frozen equipment or engines, etc.

The mobile, ready for use nature of the fabric can be used for improvising many types of surface heaters just when needed. The same fabric, fed preferably from a variable transformer, can be used as a drum heater for instance to melt solid wax to enable it to be poured out of the drum, it can be hung over a metal sheet to dry and cure wet paint and be wrapped around a build-up of resin impregnated laminated sheets clamped in a mould for a boat hull, the curing of this reinforced plastic structure being accelerated by the heat from the fabric. The same fabric can be laid under propagation boxes in the greenhouse or spread out in the garden and wet laundry placed on it, or perform many other heating tasks, even indoors or in camping.

Being a flexible strong woven fabric it can be used as a heatable support for bedding or seats and when made with narrow components in decorative colours as a furnishing and curtaining fabric and it can even be used for sleeping blankets and heatable coats.

The industrial application of the fabric is not restricted to ovens and drying tunnels where the frames holding the fabric can be positioned and moved readily to suit any desired drying path or object and permit application of uniform radiant heat as well as convection of heat by air flow through the fabric. It can perform almost all conventional heating tasks where heating of a large surface is desirable by a flexible uniform low thermal inertia heater which can readily follow the surface to be heated in close proximity and can be brought to this surface when required, necessitating no fixed installation. A particular application of the heating fabric is its use as conveyor belting essentially for the simultaneous transport and heating of flat goods lying on it. The heating and cooling of flat objects under pressure while they move continuously is an example of this application. The objects are placed between two heatable belts of the heatable fabric which run in an arcuate path and thereby, or by additional means, hold the objects under pressure. The sheathed conductors form the weft in the heating fabric of the belts and are connected while they pass over the first part of the arc; they are switched off when they run over the second part. The objects are therefore kept under pressure constantly, both during the heating and during the cooling period without a pressure release at the transition. This double web heating and cooling under pressure is suitable for welding of plastic films to foils, for laminating sheets, films, foils, etc., Connection can be effected by bushes or fixed conductors arranged on the supporting frame, with which connections to the limbs of the conductors make contact at the appropriate times.

FIG. 7 illustrates an example of two conveyors used to heat objects under pressure.

Two

conveyors

35 and 36 are essentially heating fabrics as above described. They are superposed and driven by

rollers

37, 38. The inner conveyor 35 is guided over an arcuate support 39. The outer conveyor is guided over

guide rollers

41, 42, 43, 44. The return runs of the two conveyors move at the same speed; the flexibility of the fabrics allows for the small differences in the operative runs due to the different lengths they occupy. The roller 41 guides the conveyor 36 so far from the conveyor 35 that the objects to be heated and pressed can be pushed in between them from the side (either manually or automatically). The rollers 42, 43 are positioned so that the tangent to both while clearing the objects at the location of the rollers is closer to the conveyor 35 than the height of the objects. Thus immediately after the objects pass the roller 42 the conveyor 36 presses on them. The roller 44 guides the conveyor 36 in a path which is clear of the objects so that they can easily be removed to the side.

Both conveyors are energised over part of the path from the roller pair 41, 42 to the

roller pair

43, 44. Over this part of the path heat is supplied to them. Over the remainder they cool down and this may be assisted by an air current. Energisation can be by means of fixed contact rails one of which is marked 44. Against this spring brushes indicated at 45 on the conveyor, connected each to a limb or group of limbs of the conveyor 35 make contact with the rails over a corresponding part of the path of the conveyor. Similar provision, rail 46, brushes 47, is made for the conveyor 36. The rails and brushes must be at least in duplicate, suitably on opposite sides, to enable a complete circuit to be made through the respective conductors of the conveyors.

What I claim is:

1. An electric heating device comprising a plurality of flexible limbs each constituted by a generally flat section electric conductor forming a heating element and a sheath of electrically non-conductive material loosely enclosing the conductor, the adjacent limbs being continuous with one another and disposed in a zig-zag formation with the limbs spaced apart and disposed with their flat surfaces substantially parallel so that when energized all radiate heat in the same general direction, terminal means at least at the ends of the zigzag formation whereby the heating elements constituted by the limbs can be connected to a supply of electrical energy, and means of electrically non-conductive material supporting the zigzag formation at the apices thereof with negligible friction and exerting longitudinal tension solely on the sheaths of the limbs whereby the sheaths of the limbs are held suspended in the said disposition all under substantially the same longitudinal tension.

2. An electric heating device according to claim I having the limbs spaced apart in a direction generally perpendicular to the length and in the direction of their widths whereby the atmosphere in which the heater is used can flow over and between them and be heated by convection.

3. An electric heating device according to claim 2 in which the spacing apart of said limbs is adjustable thereby enabling the ratio of heat dissipated by radiation to heat dissipated by convection to be varied.

4. An electric heating device according to claim 2 in which the spacing apart of said limbs differs in different parts of the area occupied by the device.

5. An electric heating device according to claim 1 in which the apices of the zigzag formation are supported by insulating members of circular cross section freely rotatable on supporting means.

6. An electric heating device according to claim 1 in which the two sets of apices of the zigzag formation are in a respective straight line, each apex being supported by an insulator of cylindrical form, the insulators at each set of apices being freely rotatable on a respective sup port, at least one of the supports being under a constant force tending to draw it away from the other support.

'7. An electric heating device according to claim 6 in which the respective supports are mounted in an external skeleton framework one support being fixed in the framework and the other being attached to the framework by a plurality of tension springs disposed at intervals between the limbs.

8. An electric heating device according to claim 1, in which spaced flexible supporting strips of electrically nonconducting material are interwoven with the limbs.

9. An electric heating device according to claim 8 in which the supporting strips are held under longitudinal tension.

10. An electric heating device according to claim 1 in which the sheaths are of insulating character and adequate to space the conductors from one another and from any framework in which the limbs are suspended, but are insufficient of themselves to effect adequate electric insulation of the conductors under all conditions of operation to be expected.

11. An electric heating device according to claim 1 in which the sheaths are porous.

12. An electric heating device according to claim 1 in which the sheaths consist each of a single layer of glass fibre fabric.

13. An electric heating device according to claim 1 in which the conductor is a crimped metal foil with the crimps running transverse to the length.

14. An electric heating device according to claim 1 in which each limb includes a plurality of distinct conductors each being a strip of crimped metallic foil with the crimps running transverse to the length.

15. An electric heating device according to claim 1 also including a cover on one side of the area occupied by at least some of the limbs, disposed to reflect back radiated heat so that the greater part of the total radiation will be in a direction away from the cover.

16. An electric heating device according to claim 1 in which at least two sets of limbs are superposed spaced apart.

References Cited UNITED STATES PATENTS 1,065,015 6/1913 Youmans 338-280 X 2,462,607 2/1949 Browne 338-316 X 2,482,050 9/1949 Yejeski 219-545 X 2,665,356 1/1954 Du Bois 338-316 X 2,817,737 12/1957 Morris 338-208 (Other references on following page) References Cited UNITED STATES PATENTS Duffy et a1. 13-22 Balestrini 338-58 Shoup 991 Eisler 174117 Lund et a1. 219529 Murphy 219-528 Cox 338212 Burdge et a1. 338316 10 FOREIGN PATENTS 369,225 6/1963 Switzerland.

VOLODYMYR Y. MAYEWSKY, Primary Examiner US. Cl. X.R.