WO1990013986A1

WO1990013986A1 - Flat electric heating arrangement

Info

Publication number: WO1990013986A1
Application number: PCT/NO1990/000077
Authority: WO
Inventors: Per Wesseltoft
Original assignee: Per Wesseltoft
Priority date: 1989-05-08
Filing date: 1990-05-07
Publication date: 1990-11-15
Also published as: NO166910B; NO891868L; NO166910C; NO891868D0; AU5646390A

Abstract

Flat electric heating arrangement comprising a long extended, insulated heating element (1.1, 1.2, 1.3, 1.4) and at least one heat-distributing metal foil (1.5a, 1.5b) which in tight-fitting heat contact surrounds at least a part of the heating element's outer surface. The heating element comprises an insulated core (1.1), an electric resistance element (1.2) fitted on the core, and insulated on the outside. Three individual insulating layers (1.3, 1.4) are applied outside the resistance element, each of which has a minimum thickness sufficient to withstand a given test voltage, where at least the two innermost insulating layers (1.3) consist of plastic film which is wound so tightly around the insulated core with its resistance element that practically no air or gas is included under the plastic film. The outermost insulating layer (1.4) or possibly a surrounding covering provides the necessary mechanical protection and seal against the penetration of water, etc.

Description

FLAT ELECTRIC HEATING ARRANGEMENT

This invention concerns a flat electric heating arrangement based on thin, almost flexible flat electric heating elements. The intention is to obtain an electric heater for heating surfaces, which have the necessary safety built in to satisfy the authorities' requirements regarding waterproofness, fire, mechanical strength, electrical insulation, is double insulated, can be made without magnetic fields and voltage fields, and can be made for all voltages, outputs and sizes.

There is an increasing need for flat electric heating arrangments, either for the transfer of heat to the heat receiving medium in the form of radiant heat, or by contact heat. Several such flat electric heating arrangements have been developed, and characteristic of these is that they produce heat over the whole surface, e.g. by the heating element being made of a metal foil which is cut into a pattern to obtain the desired electrical resistance. These metal foils are then welded or glued between two plastic films. The plastic film then acts both as backing and mechanical protection for the metal foil, and as electrical insulation.

In newer elements the metal foil may be replaced by conductive plastic film, or a conductive carbon/metal layer may be impressed upon one of the insulating films.

All these elements have the weakness that they must be electrically insulated over the whole surface and that the whole internal element surface has an electrical voltage, something which can be expensive when the element must be double insulated and waterproof. It is usual with these types of elements to obtain sufficient electrical resistance to make small elements with low outputs and for 230 V, without e.g. in the case of an etched metal foil element to having use expensive metal foils with high specific resistance as the base material, or having to reduce the cross-section of the current path so much that the element withstands little mechanical stress.

This type of element, where the whole heating surface is electrically conductive, also creates a large electrical voltage field which in many applications can be annoying, if not directly harmful for sensitive people. All such foil elements must because of their mechanical strength have an outer insulating layer for protection against contact and damage.

In those cases where the element must be waterproof the whole covering sheet, the outer insulation and not least all joints between the two layers of covering must be waterproof.

Another way of arranging the flat electric heating elements is to place the electrical resistance wires so close to each other between two plastic films that the surface is heated almost evenl .

Another problem with this type of element is that if there is air inside the element, the air will expand when the element heats up and prevent the heat from the element reaching the surroundings with the result that the element becomes overheated and can cause damage to the surroundings or burn out.

In most applications of such flat heating elements there is a temperature limitation system on the surface of the element, either because the element's insulation does not withstand higher temperatures, or because the heated surfaces only withstand a limited temperature. Normally it is aimed to have the lowest possible surface temperature with a given specific specific output to achieve the longest possible life of the element and the heated surface.

If the heated surface has good horizontal conductivity this is no problem, but if the heated surface has good conductivity vertically to the element and poor conductivity horizontally with the element, (e.g. in the case of heating elements for water beds ) , there will be warm parts at the resistance wires or conductive covering with cold parts in-between. It is obvious that the warm part must then be at a higher temperature than if the whole surface had the same temperature. One way of solving this is to cover the element with e.g. aluminium foil to obtain good horizintal conductivity. It emerges however that the conductivity of aluminium foil is so good that it is possible to increase the spacing between the heat producing parts considerably. This good horizontal thermal conductance makes it unnecessary to produce heat over the whole surface, on which the elements with metal foil and conducting plastic films are based, and makes it advantageous to place insulated heating cables in direct contact with the aluminium foil. This has been done in an element for water beds, cf. Norwegian patent No. 85.1506.

Here a double insulated heating cable is used, laminated between two sheets of aluminium foil. The weaknesses with this design include the following:

Because of the double insulation which consists of firstly an inner insulation which is approx. 0.8 mm thick and an outer additional insulation which must be at least 1 mm thick, it is difficult to make heating cables under 4.5 mm in diameter.

This means that a meander-shaped track must be impressed in the aluminium foil, deep enough for the cable to fit, something which is both complicated and expensive, while at the same time the finished element becomes too thick (approx. 4.5-5 mm), something which limits the use of this type of element.

Secondly the transport of heat from the heat producing resistance wire through the electrical insulation, which is approx. 1.8-2 mm thick, is a limiting factor on how much heat can be obtained from each metre of cable at a given maximum temperature for the electrical insulation. Measurements show that there is a temperature difference of up to 20-25 degrees Celsius between the temperature inside the insulation (against the resistance wires) and on the outside of the insulation (against the aluminium foil) .

As plastic materials in the insulation are normally not approved for more than 80°C it will not be possible for the outer temperature to be more than 55-60°C. This means that the maximum specified load on such cables is approx. 28 W per metre of cable, and this will for a 360 water bed element give a cable length of 13 metres. Measurements show that if it is possible to raise the temperature outside the cable against the aluminium foil, it will be possible to increase the specific output significantly, but then the temperature inside the cable will become too high.

This means that even if it is possible to use greater spacing between the parallel heating cables in such a heating element, this is limited by the fact that it is also necessary to find room for sufficient cable length, something which is particularly the case with elements with an output over 10-12 W/cm-^! of element surface. This is the specific output which is used e.g. for heating elements for water beds - a very large application for flat elements. Conventional heating cables, therefore, are not suitable for use in flat heating arrangements of the type in question here, and it will be seen from the following that this invention is to a great extent based on use of a heating element of special construction.

Another factor in connection with electrical heating elements is the voltage field which surrounds this type of element, and which when heating water beds induces a voltage field throughout the water bed. This can be particularly annoying and harmful for sensitive people.

A way of removing this voltage field is to have an earthed metal screen outside the element. Measurements show that this is an effective way of solving the problem by e.g. connecting an earthing wire to the outer aluminium foil on the actual heating element. This as such is a method that can be used in Norway and those countries where the electricity network does not have an earthed neutral conductor.

This invention is aimed at solving all the shortcomings and weaknesses mentioned, and is primarily based on a new type of heating element where the aim has been to have a maximum outside diameter of 3 mm, one of reasons being to be able to impress the track in the aluminium foil at the same time as putting down the element. The element is also built up of resistance wires wound around glassfibre cables in order to be able to load the resistance wires to the maximum. (Max. loading of resistance wires without glassfibre cores is 1 W/crn^ of resistance wire surface, while with glassfibre cores this can be increased to 5 W/cm-- of surface) . The insulation between the resistance wire and the aluminium foil is made of a heat-resistant material, the thickness at the same time being reduced to a minimum. It is also possible to insert a metal screen inside the insulation sufficiently insulated from both to remove the voltage field and also to satisfy the Equipment Inspectorate's safety requirements.

More precisely then the invention concerns a flat electric heating arrangement of the type which comprises a long extended, insulated heating element and at least one heat-distributing metal foil which in tight-fitting heat contact surrounds at leas one part of the heating element's outer surface, and where the heating element comprises an insulated core, an electric resistance element fitted on the core, and insulated on the outside. What is new and characteristic about the arrangement in accordance with the invention consists primarily in there being three individual insulating layers applied outside the resistance element, each of which has a minimum thickness sufficient to withstand a given test voltage, and in at least the two innermost insulating layers consisting of plastic film which is wound so tightly around the insulated core with its resistance element that practically no air or gas is included under the plastic film, the outermost insulating layer or possibly a surrounding covering providing the necessary mechanical protection and seal against the penetration of water, etc.

Such heating elements may be loaded with such a high specific load (W/m) that the advantage with the outer thermally conducting aluminium foil is used to best effect from the point of view of both heating and economy.

In the following description of this invention it will be seen that this objective has been achieved, and this is demonstrated by a number of experiments and long-term tests .

The essential point of the invention is in being able to reduce the necessary thickness of insulation between the resistance wire and the aluminium foil, while waterproofness, insulation property and mechanical strength are retained or improved. The resistance wire is also wound on glassfibre cores so that the load on the resistance wire can be high as possible and also to get so much resistance wire per metre that the requirement for max. 5 W/cm^ of surface is met.

In this invention it has been chosen to use plastic film in the two first insulating layers and extruded silicone, plastic or thermoplastic rubber as the outer, waterproof insulating layer. When an element is insulated as described and is placed between two sheets of aluminium foil the requirement of safety against contact is met.

A polyester sheet 25 μm thick has a dielectric strength of over 5000 V and thus provides sufficient insulation. Polyester film also withstands up to 150°C and is thus significantly better than e.g. cross-linked polyester (Pex) or PVC, which withstand max. 80°C and are usually used in heating cables. There is also film which withstands higher temperatures and which also has a higher dielectric strength which can be used in those cases where this is necessary. By using two layers of film wound round a glassfibre core with resistance wires spun round it and extruding a layer of plastic, rubber or silicone, etc. on outside, the overall insulation thickness can be reduced to approx. 0.6 mm, which means that the thermal resistance in the insulating layer is reduced considerably. As the two layers of film also withstand higher temperatures than the materials in ordinary heating elements, it is the temperature at the outer extruded insulating layer that determines the heating element's specific output.

Experiments show that elements such as these can be loaded with up to 70 W/m if the heat is transported off through the surrounding aluminium foil and to e.g. a water bag in a water bed. Long-term tests have been carried out with a specific load of 60 W/m, i.e. twice that compared with a traditional cable.

This design also means that the thickness of the heating element can be reduced to approx. 3 mm, which was the requiremen for the maximum outside diameter when allowing for lamination between two sheets of aluminium foil without a pre-pressed track

Reducing the thickness of the insulation in this way also means a saving on materials which in turn means lower costs, while the length of the heating element can be reduced by 40-50%

Time will also be saved in production when placing this heating element in aluminium foil, while the finished element construction is only approx. 3 mm thick. This means that the element, apart from as an element for water beds, also has a number of other applications, e.g. for heating bathroom mirrors to prevent condensation with high air humidity, and a number of other products where another heating foil is used today.

The invention is explained in more detail below with reference to practical examples shown in the drawings, where:

^'Fig. 1 shows diagrammatically the construction of a special form of heating cable consisting of a long extended insulated heating element in a flat electric heating arrangement in accordance with the invention,

Fig. 2 shows in horizontal projection an arrangement of heating element and accompanying electrical circuits and components in an arrangement in accordance with the invention,

Fig. 3 shows in cross-section a part of a flat electric heating arrangement specially designed for e.g. heatin mirrors in bathrooms, Fig. 4 shows diagrammatically the construction of another special version of the heating element and Fig. 5 shows in principle connection of a heating element as in Fig. 4 in a flat electric heating arrangement where unwanted, external voltage fields are to a great exten removed.

Reference is first made to Fig. 1 of the drawings. Here 1.1 is the inner glassfibre core on which is wound the resistance wire 1.2. On the outside is wound with 60% overlap a polyester film or similar, and outside this again is extruded a surroundin waterproof cover of e.g. silicone rubber. The whole lot thus laminated between two layers of aluminium foil 1.5a and 1.5b is glued together with a heat-resistant adhesive 1.6 to form a complete laminated arrangement. The invention also covers extrusion of two layers of insulation and one layer of film or three extruded layers, when the only requirement is for electrical insulation and not for thickness.

The heating element is placed in loops as shown in Fig. 2, and a safety thermostat and leads can be laminated in at the same time to obtain a complete flat heating arrangement. 2.1 are supply leads, while 2.2a, b, and c are connections between supply and heating element and safety thermostat 2.3 respectively. It is a requirement that connections 2.2 and the safety thermostat have sufficient electrical insulation e.g. by using two layers of polyester tape, with e.g. a shrunk-on plastic sleeve outside to make it waterproof, and the outer electrical insulating layer. In the same way the aluminium foil can be treated with lacquer, be plastic coated or otherwise made as mechanically strong and attractive as is necessary with regard to the application.

In Fig. 3 is shown another form of element specially designed to warm up a flat surface, e.g. a bathroom mirror to prevent condensation during bathing. The construction of the element is not described in more detail here. A heating element 3.1 as described earlier, is pressed into one of the sheets of aluminium foil 3.2 which then envelops most of the heating element. 3.3 shows the layer of adhesive between the sheets of aluminium foil, while 3.4 shows the other sheet of aluminium foil which is completely flat. This aluminium foil can have a coating of adhesive 3.5 which may be an aluminium tape with covering paper which when mounting at the user's can be stuck to e.g. the back or a mirror surface 3.6. 0.1. Other methods of lamination are also covered by this invention, e.g. one of the sheets of aluminium foil can be replaced by other materials which may be flexible or stiff, depending on what the element is to be used for.

In Fig. 4 is described an element where the electrical voltage field is also removed, especially for electricity networks where an earthed neutral conductor is used and where the element is connected between neutral conductor and phase. Here 4.1 is the inner glassfibre core. 4.2 is the resistance wire. 4.3 is a layer of polyester film with 10-20% overlap. 4.4 is a resistance wire, metal foil or semi-conductive plastic film with a resistance per metre significantly greater than the resistance per metre in the heating element itself. If the resistance is particularly high it is possible to use a longitudinal resistance wire 4.41 which ensures contact with the semi-conductive film along the heating element. Outside is wound a polyester film 4.5 with 60% overlap, with an outer extruded cover 4.6, the whole lot laminated between aluminium foils 4.7a and 4.7b. The intention with the resistance wire 4.4 and wire 4.41 is to form a screen outside the inner electrical circuit, but under the outer double insulation.

Fig. 5 shows how the wires or screen 4.4/4.4.1 is to be connected in order to remove the voltage field. The heating cable is thus connected to the earthed neutral conductor at 5.1 and phase at 5.2. The screen 4.4/4.41 is connected to the neutra conductor at 5.1 and will then carry off the voltage field to earth.

To ensure against short-circuits in the event of an insulation fault in the rest of the insulation 4.3 (Fig. 4) it is an advantage for the resistance per linear metre to be significantly higher than the element's resistance (4.2) so that a short-circuit between wires 4.2 and 4.4 will not change the complete element's resistance value and thereby the emitted output per linear metre by more than what is permissible.

In this way it is possible to make a heating element withou electrical voltage fields for those electricity networks which have an earthed neutral conductor and then without using separate earthing. This design will also satisfy the authorities' requirements.

The earlier mentioned magnetic field can be reduced to an accepted minimum by placing two such heating elements beside each other and with opposite current direction so that the magnetic fields cancel each other out.

Claims

Patent Claims

1. Flat electric heating arrangement comprising a long extended, insulated heating element (1.1, 1.2, 1.3, 1.4) and at least one heat-distributing metal foil (1.5a, 1.5b) which in tight-fitting heat contact surrounds at least a part of the heating element's outer surface, and where the heating element comprises an insulated core (1.1), an electric resistance elemen

(1.2) fitted on the core, and insulated on the outside, characterized in that three individual insulating layers (1.3, 1.4) are applied outside the resistance element, each of which has a minimum thickness sufficient to withstand a given test voltage, and in at least the two innermost insulating layers

(1.3) consisting of plastic film which is wound so tightly aroun the insulated core with its resistance element that practically no air or gas is included under the plastic film, the outermost insulating layer (1.4) or possibly a surrounding covering providing the necessary mechanical protection and seal against the penetration of water, etc.

2. Flat electric heating arrangement as in Claim 1, characterized in that the two innermost insulation layers consis of a plastic film wound with an overlap of 60% so that in all there will be at least two layers of plastic film around the resistance element, preferably with an extruded outermost layer on top.

3 Flat electric heating arrangement as in Claim 1 or 2, chaxacterized in that the outermost insulation layer is extruded outside two layers of plastic film in such a way that practicall no air or gas occurs between the three insulating layers or between the innermost insulating layer and the insulated core with the resistance element.

4. Flat electric heating arrangement as in Claim 1, characterized in that the three insulating layers are built up of three layers of plastic film wound crosswise around the insulated core with heating element.

5. Flat electric heating arrangement as in Claims 1-4, chaxacterized in that one or more of the layers of plastic film is arranged in the longitudinal direction of the heating element

6. Flat electric heating arrangement as in Claims 1-5, chaxacterized in that the heating element is laminated between two outer heat-spreading sheets of aluminium foil with the heating element lying preferably symmetrically between the sheet of aluminium foil.

7. Flat electric heating arrangement as in Claim 6, characterized in that one or both of the sheets of aluminium foil has a coating which increases its mechanical strength, gives it a better appearance or the like.

8 Flat electric heating arrangement as in Claims 1-7, characterized in that the sheet of metal or aluminium foil is self-adhesive on one side in order to be stuck to the surface to be heated.

9. Flat electric heating arrangement as in Claims 1-8, characterized in that additional leads and possibly a safety thermostat and the like are laminated together with the heating element between the two outer sheets of metal or aluminium foil.

10. Flat electric heating arrangement as in Claims 1-9, characterized in that between the insulating layers and the resistance element is inserted a conductive layer, e.g. wire, conductive film or metal foil insulated from the resistance element along most of the length.

11. Flat electric heating arrangement as in Claim 10, charactexized in that the conductive layer is electrically connected to one end of the resistance element.

12. Flat electric heating arrangement as in Claim 10 or 11, chaxacterized in that the conductive layer has a specific resistance along the element that is significantly greater than the resistance element.

13. Flat electric heating arrangement as in one of Claims 1-12, chaxacterized in that in addition to the said sheet(s) of metal or aluminium foil there is encapsulated another material, e.g. etal, plastic, woven cloth, wood and the like, and that this ma be soft, stiff or have the character and shape that is suitable for the actual area of use.

14. Mirror heater comprising a flat electric heater arrangement as in one of claims 1-8, charactexized in that a surface (3.4) on the flat electric heate arrangement which is designed to lie against the back of the mirror (3.6) has an adhesive coating (3.6) applied to at least a significant part of its area for fixing to the back of the mirror. ( ig. 3) .