NO166910B - ELECTRIC AIR HEATING DEVICE. - Google Patents

Description

The present invention relates to an electric surface heating device based on thin, almost flexible electric surface heating elements. The purpose is to achieve

an electric heating device for heating surfaces that has built-in the necessary safety to satisfy the authorities' requirements for water tightness, fire, mechanical strength, electrical insulation, is double insulated, can be made without magnetic fields and without voltage fields, and can be made for all voltages, effects and sizes.

There is a growing need for electric surface heating devices, either for the transfer of heat to the heat-receiving medium in the form of heat rays, or by contact heating. Several such electric surface heating devices have been developed, and a characteristic of these is that they produce heat over the entire surface, e.g. in that the heating element is made of a metal foil that is slit in a pattern, to obtain the desired electrical resistance. These metal foils are then welded or glued between two plastic foils. The plastic foil then covers the function both as carrier and mechanical protection of the metal foil, and as an electrical insulator.

In newer elements, the metal foil may have been replaced

of conductive plastic foil, or a conductive carbon/metal layer can be printed on one insulation foil.

All these elements have the weakness that they must have electrical insulation over the entire surface as the entire inner element surface has electrical voltage, which can be expensive when the element is to be double insulated and waterproof. It is difficult with these types of elements to get sufficient electrical resistance to make small elements with low effects and for 230 volts, without e.g. in the case of an etched metal foil element, expensive metal foils with a high specific resistance must be used as the base material, or the cross-section of the current path must be reduced so much that the element can withstand little mechanical stress.

This type of element, where the entire heating surface is electrically conductive, also creates a large electric voltage field which in many applications can be troublesome, if not directly harmful to sensitive people.

All such foil elements must, due to the mechanical strength have an outer insulating layer for protection against contact and damage.

In cases where the element is to be waterproof, the entire cover foil, the outer insulation and not least all joints between the two layers of cover foil must be waterproof.

Another way to make electric surface heating elements is to lay electric counter-current wires so close to each other between two plastic foils that the surface is heated almost evenly.

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

In most applications of such surface heating elements, there is a temperature limitation on the surface of the element, either because the element insulation cannot withstand higher temperatures, or because the heated surfaces can only withstand a limited temperature. Usually one seeks to have the lowest possible surface temperature for a given specific effect in order to get the longest possible delivery time for the element and the heated surface.

If the heated surface has good horizontal thermal conductivity is. this is not a problem, but if the heated surface has good thermal conductivity vertically on the heating element and poorly horizontally with the element, (e.g. in the case of heating elements for water beds), you will get hot areas near the resistance wires or the conductive coating with cold areas in between. It goes without saying that the hot part must therefore have a higher temperature than if the entire surface had the same temperature. A way to

to solve this, is to cover the element with e.g.

Al foil to thereby obtain a good horizontal heat conduction. However, it turns out that the thermal conductivity i

Alfolie is so good that you can increase the center distance between

the heat-producing parts significantly. This good horizontal heat conduction makes it unnecessary to produce the heat over the entire surface, on which elements with metal foil and conductive plastic foils are based, and makes it advantageous to lay insulated heating cables directly in contact with the Alfolien. This has been done in an element for water beds, see Norwegian patent application no. 85.1506.

A double-insulated heating cable laminated between two aluminum foils is used here. The weakness of this construction is i.a. the following: Due to the double insulation which consists of first an inner insulation which is approx. 0.8 mm thick and an external additional insulation that must be min. 1 mm thick, it is difficult to make heating cables under 4.5 mm diameter.

This means that a meander-shaped groove must be embossed in the Al foil, deep enough for the cable to fit, which is both complicated and expensive, while the finished element will be too thick (approx. 4.5-5 mm), which limits the use of this type of elements.

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

As the plastic materials in the insulation are not normally approved for more than 80°C, the cable's external temperature cannot be more than 55-60°C. This means that the maximum specified load on such cables is approx. 28 W per meter of cable, and for a water bed element of 360 W, that will give a cable length of 13 metres. Measurements show that if you can increase the temperature on the outer side of the cable towards the Alfolien, you will be able to increase this specific effect significantly, but then the temperature inside the cable would be too high.

This means that even if you could use a larger center distance between the parallel heating cables in such a heating element, this is limited by the fact that you also have to make room for sufficient cable length, which is especially the case with elements with an output of more than 10-12 W/cm<2 >element surface. This is the specific effect that i.a. used for heating elements for water beds, which is a very large area of application for surface elements.

Conventional heating cables are thus not suitable for use in surface heating devices of the type in question here,

and it will appear from what follows that this invention is largely based on the use of a heating element with a special structure.

Another factor in connection with electric heating elements is the voltage field with which this type of element surrounds itself, and which, in connection with heating water beds, induces a voltage field in the entire water bed. This can be extremely distressing and harmful to sensitive people.

One way to remove this voltage field is to have a grounded metal shield outside the element. Measurements show that this is an effective way to solve the problem, e.g. by connecting an earthing wire to the outer Al foil on the heating element in question. This is without further ado a usable method in Norway and countries where there is no electricity grid with a grounded neutral conductor.

The present invention aims to solve all the aforementioned shortcomings and weaknesses, and is primarily based on a. new type of heating element where the aim has been to have a maximum outer diameter of 3 mm, i.a. to be able to mark traces in the Alfoil at the same time as you lay down the element. Furthermore, the element is built up with resistance wires wound on a glass fiber core in order to thereby be able to load the resistance wire to the maximum. (Maximum load of resistance wires without glass fiber core is 1 W/cm<2 >surface of resistance wires, while with glass fiber core this can be increased to 5 W/cm<2> surface.) The insulation between the resistance wire and the Al foil is made of one heat-resistant material, at the same time as the thickness is reduced to a minimum. Furthermore, a metal screen can be placed inside the insulation sufficiently insulated to both remove the voltage field and also satisfy the safety requirements of the material control.

More specifically, the invention thus relates to an electric surface heating device of the kind that comprises an elongated, insulated heating element and at least one heat-distributing metal foil which, in tight heating contact, surrounds at least part of the heating element's outer surface, and where the heating element comprises a core of insulating material, an electrical resistance element placed on the core, and an external insulation. The new and distinctive feature of the device according to the invention consists primarily in the fact that three separate insulation layers are placed on the outside of the resistance element, each of which has a minimum thickness sufficient to withstand a given test voltage, that at least the two innermost insulation layers consist of plastic foil that is wound so tightly around the core with associated resistance element, that practically no air or gas is trapped under the plastic foil, as the outermost insulation layer or possibly a surrounding mantle provides the necessary mechanical protection and sealing against ingress of water etc.

Such heating elements must be able to be loaded with such a high specific load (W/m) that optimal utilization of the advantage of the external heat-conducting

Alfoile both heat-technically and economically.

In the subsequent description of the present invention, it will appear that this objective has been achieved, which is also shown by a number of trials and long-term tests.

The core point of the invention lies in being able to reduce the necessary insulation thickness between the resistance wire and the Alfoil, while at the same time retaining or improving water tightness, insulation ability and mechanical strength. Furthermore, the resistance wire is wound on a fiberglass core in order to be able to load the resistance wire as much as possible and also to be able to get as much resistance wire per meters that the requirement for a maximum of 5 W/cm<2> surface is met.

In the present invention, it has been chosen to use plastic foil in the first two insulation layers and extruded silicone, plastic or thermoplastic rubber as the outer, waterproof insulation layer. When an element is insulated as described and placed between two Alfolie, the requirement for touch safety is met.

A polyester foil with a thickness of 25 /xm has an impact strength of over 5000 V and thus provides sufficient insulation. Polyester foil can also withstand up to 150°C and is therefore significantly better than e.g. cross-linked polyester (Pex) or PVC, which can withstand a maximum of 80°C, which is usually used in heating cables. There is also foil which can withstand higher temperatures and which also has higher impact resistance, which can be used in cases where this is necessary.

By using two layers of foil wrapped around a fiberglass core with resistance wires spun around, and then extruding a layer of plastic, rubber or silicone etc. on the outside, the total insulation thickness can be reduced to approx. 0.6 mm, which means that the heat resistance in the insulation layer is significantly reduced. Since at the same time the two foil layers can withstand a higher temperature than the materials in ordinary heating elements, it is the temperature of the outer extruded insulation layer that determines the heating element's specific effect.

Tests show that elements like this can be loaded with up to 70 W/m if the heat is transported away through the surrounding Al 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. a doubling compared to traditional heating cable.

This construction also means that the thickness of the heating element is reduced to approx. 3 mm, which was the requirement for the maximum outer diameter when lamination between two Alfoils without a pre-embossed groove is taken into account.

Such a reduction of the insulation thickness also results in material savings, which in turn means lower costs, while the length of the heating element can be reduced by

40-50%.

Furthermore, production time will be saved by laying this heating element in Alfolie at the same time that the finished element construction is only approx. 3 mm thick. This means that the element, in addition to being an element for water beds, also has a number of other uses, e.g. for heating bathroom mirrors to prevent fogging in high humidity, and a number of other products where today other heating foils are used.

The invention is explained in more detail below with reference to examples of execution shown in the drawings, where: Fig. 1 schematically shows the structure of a special form of heating cable which forms an elongated insulated heating element in an electric surface heating device according to the invention,

fig. 2 shows in plan view an arrangement of heating element and associated electrical circuits and components in a device according to the invention,

fig. 3 shows in cross-section a part of an electric surface heating device specially intended for, among other things

heating mirrors in bathrooms,

fig. 4 schematically shows the structure of another

special design of the heating element, and

fig. 5 shows, in principle, a connection of a heating element as in fig. 4 in an electric surface heating device where unwanted, external voltage fields have been largely removed.

Reference is now made first to the drawings' fig. 1. Here 1.1 is the inner fiberglass core around which the resistance wire 1.2 is wound. On the outside, a polyester foil etc. is wrapped with a 60% overlap, and on the outside again, a surrounding waterproof jacket of e.g. silicone rubber. The whole thing is then laminated between two layers of Alfolie 1.5a and 1.5b glued together with a heat-resistant glue 1.6 to form a complete laminated device. The invention also includes the extrusion of two layers of insulation and a layer of foil or three extruded ones, where requirements are only made for the electrical insulation and not the thickness.

The heating element is placed in loops as shown in fig.

2nd, and at the same time you can laminate in the safety thermostat and connections so that you get a complete surface heating device. 2.1 are supply lines, while 2.2a, b, and c are connections between supply and heating element, respectively the safety thermostat 2,3. It is a prerequisite that the connections 2.2 and safety thermostat have sufficient electrical insulation, e.g. when using two layers of polyester foil tape, with e.g. a shrink-wrapped plastic stocking on the outside to achieve water tightness, and the outer electrical insulation layer. In the same way, the Alfolien can be surface-treated with varnish, be coated with plastic or in some other way be made as mechanically strong and attractive as is necessary for reasons of

the application.

In fig. 3 shows another form of element specially designed to heat a flat surface, e.g. a bathroom mirror to prevent dew in connection with bathing.

The element structure itself is not described in more detail here. A heating element 3.1, as previously described, is pressed down into the one Alfolien 3.2, which then encloses most of the heating element. 3.3 shows the adhesive layer between the Alfoils, while 3.4 shows the second Alfoil which is then completely flat. This Alfolien can have an adhesive coating 3.5. such as an aluminum tape with cover paper which, when installed by the user, can be glued to e.g. the back of a mirror surface 3.6. beer. Other lamination methods are also covered by the present invention, e.g. the one Alfolie can be replaced by other materials which can be flexible or rigid, depending on the application of the element.

In fig. 4 describes an element where the electric voltage field is also removed, especially for those electricity networks where a grounded neutral conductor is used and where the element is connected between neutral conductor and phase.

Here, 4.1 is the inner fiberglass core. 4.2 is the resistance wire. 4.3 a layer of polyester foil with 10-20% overlap. 4.4 is a resistance wire, metal foil or semi-conducting plastic foil with a resistance per meters significantly greater than the resistance per meters in the heating element itself. If the resistance is particularly high, a longitudinal resistance wire 4.41 can be used, which ensures contact with the semi-conducting foil along the heating element. On the outside, a polyester foil 4.5 with 60% overlap is wound, with an outer extruded sheath 4.6, the whole laminated between the Al foils 4.7a and 4.7b. The purpose of the resistance wire 4.4 and the wire 4.41 is to form a shield outside the internal electrical circuit, but below the outer double insulation.

Fig.5 shows how the wires or screen 4.4/4.41 should be connected to remove the voltage field. The heating cable is thus connected to the grounded neutral conductor in 5.1 and phase in 5.2. The screen 4.4/4.41 is connected to the neutral conductor in point 5.1 and will then lead away the voltage field to earth.

In order to protect against short circuits in the event of an insulation fault in the inner insulation 4.3 (fig.4), it is an advantage that the resistance per running meter is significantly greater than the resistance of the element (4.2) so that a short circuit between the wires 4.2 and 4.4 will not change the resistance value of the complete heating element and thereby the output per running meters more than what can be allowed.

In this way, a heating element can be made without electric voltage fields for those electricity networks that have a grounded neutral conductor and then without the use of separate grounding. This construction will also satisfy the authorities' requirements.

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

Claims (13)

1. Electric surface heating device comprising an elongated, insulated heating element (1.1, 1.2, 1.3, 1.4) and at least one heat-distributing metal foil (1.5a, 1.5b) which surrounds at least a part of the heating element's outer surface in a tight heating contact, and where the heating element comprises a core of an insulating material (1.1), an electrical resistance element (1.2) placed on the core, and an overlying insulation, characterized in that three separate layers of insulation (1.3, 1.4) each of which has a minimum thickness sufficient to withstand a given test voltage, that at least the two innermost insulation layers (1.3) consist of plastic foil that is wound so tightly around the core with the associated resistance element that practically no air or gas is trapped underneath the plastic foil, as the outermost insulation layer (1.4) or possibly a surrounding mantle provides the necessary mechanical protection and sealing against ingress of water e.1. 2. Electric surface heating device according to claim 1, characterized in that the two innermost insulation layers consist of a plastic foil wrapped with an overlap of 60%, so that there are at least two layers of plastic foil everywhere around the resistance element, with a preferably extruded outermost insulation layer on the outside . 3. Electric surface heating device according to claim 1 or 2, characterized in that the outermost insulation layer is extruded on the outside of two plastic foil layers in such a way that practically no air or gas occurs between the three insulation layers or between the innermost insulation layer and the core with resistance element. 4. Electric surface heating device according to claim 1, characterized in that the three insulation layers are made up of three layers of plastic foil wrapped crosswise around the core with a resistance element. 5. Electric surface heating device according to one of claims 1-4, characterized in that one or more of the plastic foil layers run exclusively in the longitudinal direction of the heating element with purely longitudinal overlapping and joining of the longitudinal edges of the relevant plastic foil layer. 6. Electric surface heating device according to one of claims 1-5, characterized in that the heating element is laminated between two outer heat-dissipating aluminum foils with the heating element lying preferably symmetrically between the aluminum foils. 7. Electric surface heating device according to claim 6, characterized in that one or both aluminum foils have a coating that increases the mechanical strength, gives a better appearance or the like. 8. Electric surface heating device according to one of claims 1-7, characterized in that the metal or aluminum foil (3.4) on one side is self-adhesive over a significant part of the area, with the aim of sticking (3.5) to the surface to be heated, in particular the back of a mirror (3.6). (Fig. 3). 9. Electric surface heating device according to one of claims 1-8, characterized in that additional cables, possibly a safety thermostat and the like, are laminated together with the heating element between two outer metal or aluminum foils. 10. Electric surface heating device according to one of claims 1-9, characterized by the fact that a conductive layer has been inserted between the insulation layers, e.g. wire, conductive foil or metal foil isolated from the resistance element along most of its length. 11. Electric surface heating device according to claim 10, characterized in that the conductive layer is electrically connected to one end of the resistance element. 12. Electric surface heating device according to claim 10 or 11/ characterized in that the conductive layer has a specific resistance along the length of the element which is substantially greater than the resistance element. 13. Electric surface heating device according to one of claims 1-12, characterized in that, in addition to said metal or aluminum foil(s), an encapsulation of other material is provided, e.g. metal, plastic, woven cloth, wood and the like, and that this can be soft, rigid or have the character and shape that suits the area of use in question.