GB2279855A

GB2279855A - Resistance tubular heating wire

Info

Publication number: GB2279855A
Application number: GB9313688A
Authority: GB
Inventors: John Thomas Hughes
Original assignee: Zortech International Ltd
Current assignee: Zortech International Ltd
Priority date: 1993-07-02
Filing date: 1993-07-02
Publication date: 1995-01-11
Also published as: GB9313688D0

Abstract

An electrical resistance heating wire (1) for use in an electric heating element (4) of a radiant heater, comprises a tube (2) of one or more metals or metal alloys enclosing a core (3) of electrically insulating material. The core (3) may suitably comprise air or a particulate solid such as one or more powdered oxides of metals or metalloids. Electric current is passed through the tube (2) which tube may be wound into a spiral, and this heats up to full heat much more rapidly than a solid wire of the prior art. Additional heating or sensing wires 10 may be embedded in the core. <IMAGE>

Description

Electric Heating Wire, Heating Element and Heater This invention relates to electrical resistance heating wires, to electrical heating elements formed by the wires and to radiant electric heaters incorporating such elements. Such wires, elements and heaters find application in a wide variety of products ranging from domestic electric cooking apparatus to industrial furnaces.

Electrical resistance wire used in electric heating elements is normally formed of solid metal or metal alloy with circular cross-section and with a suitable electrical resistance per unit length such that a voltage applied to the ends of a selected length of wire results in appropriate generation of heat in the wire. The wire is often formed into a heating element by winding it into a helical coil which can be stretched somewhat to ensure that adjacent turns are separated from one another.

The operating temperature of the heating wire depends upon the ultimate use to which it is to be put as an element in a heater. In cooking apparatus and in furnaces the wire is often used at a very high temperature, e.g. 10000C or higher.

There may be a need to make a heating element which is as small and/or lightweight as possible in which case the operating temperature could be approaching the extreme limit of the thermal/physical properties of the material of which the wire is composed. When such limits are approached, careful design of an element in the form of a wire coil is necessary to maximise the operating life. If too high an operating temperature is reached, an undesirable level of oxidation of the wire can take place. If a wire coil is formed having a diameter which is too large compared with the diameter of the wire, distortion and possibly collapse of the coil may occur.

A problem consequently occurs when it is required to design a coiled wire heating element which must provide a specific heat density with a fixed applied voltage.

Such a requirement may, for example, occur in a heating coil for use in a radiant heater in domestic electrical cooking apparatus, where the radiant heater is located underneath a glass-ceramic cooking surface. The need to supply, for example, a heat output of 1500 watts over a circular area of 150 mm diameter demands the use of a long length of very thin wire when the well-known solid wires composed of nickel-chromium or iron-chromium-aluminium are employed.

The mass of the heating element wire affects the time taken for the element to reach full heat. The greater the mass of wire, the longer is the time required for the element to reach full heat. This can be a disadvantage. It is more often the case that it is desirable to have a short heat-up time so that maximum heat is provided as quickly as possible. It can be a further benefit if the wire in the element can be seen to heat quickly so that the element glows red soon after the power is switched on.

It is an object of this invention to provide an electrical resistance heating wire which is capable of being used to form a heating element for a radiant heater in the same way as with wires of the prior art, but which heats up more rapidly and provides heating element designers with a greater ability to vary coil diameter or weight or length when providing a specific heat load at a fixed voltage.

The present invention provides an electrical resistance heating wire comprising a tube of electrically conducting material adapted for heating by passage therethrough of electric current, the tube enclosing a core of electrically insulating material.

The tube is suitably continuous (e.g. like a pipe) and may comprise one or more metals, or metal alloys, for example an iron-chromium-aluminium alloy or a nickelchromium alloy.

The tube may comprise a plurality of layers of different metals or metal alloys. Such a tube may, for example, comprise an inner layer of, or including, iron coated with aluminium, the aluminium subsequently thermally oxidising to form a protective layer of aluminium oxide on the inner layer.

The tube is selected with regard to internal bore size and with regard to wall thickness to provide a desired electrical resistance value per unit length thereof.

The tube may conveniently be of circular cross-section, although other crosssectional shapes may be considered.

The core may comprise an electrically insulating fluid or solid.

The fluid is preferably one or more gases, particularly air.

The solid may be of particulate form, such as one or more powdered oxides of metals or metalloids. such as for example MgO, Awl203, SiO2.

The particulate solid may be suitably compressed or compacted inside the tube.

One or more elongate electrical conductors may be provided in the core of the tube along the length thereof and electrically insulated from the tube. Such one or more elongate electrical conductors may be adapted to be used for additional heating purposes, by passage of electric current therethrough or may be used for other purposes such as temperature sensing, by appropriate selection of the material or materials thereof.

The present invention also provides an electric heating element constructed from the above electrical resistance heating wire. The wire in the element may suitably be provided in the form of an elongate helical coil.

The present invention further provides a radiant electric heater incorporating the above electric heating element.

A coiled wire heating element according to the invention is found to fully heat up in a considerably shorter time (e.g. one third of the time) than that taken by an element of similar physical dimensions and overall electrical resistance, manufactured from conventional solid wire of the prior art.

The invention is now described by way of example with reference to the accompanying drawings, in which: Figure 1 represents an enlarged perspective view of a portion of an electrical resistance heating wire according to the invention; Figure 2 represents an enlarged perspective view of a portion of a coiled wire electric heating element formed with the wire of Figure 1; Figure 3 represents a plan view of a radiant electric heater incorporating the heating element of Figure 2; and Figure 4 represents an enlarged perspective view of a portion of an alternative electrical resistance heating wire according to the invention.

Referring to Figure 1, an electrical resistance heating wire 1 comprises a tube 2 of electrically conducting material, of circular cross-section, enclosing a core 3 of electrically insulating material. The tube 2 is suitably continuous and is formed of a metal or metal alloy, such as a nickel-chromium or an iron-chromium-aluminium alloy. The core 3, enclosed by the tube comprises an electrically insulating fluid or solid. It may comprise air or a particulate solid material such as one or more powdered electrically insulating metal or metalloid oxides, for example MgO, Al203 or SiO2.

The wire 1 is produced by well-known drawing techniques such that a required overall diameter, d, and wall thickness, t, are obtained. The overall diameter, d, and wall thickness, t, are selected in association with the material of the tube 2 such that a desired electrical resistance value per unit length is obtained.

The drawing technique used to form the wire 1 can, for example, be similar to that which is typically employed to form metal-clad thermocouples in which a pair of thermocouple wires are embedded in mineral insulation material, such as MgO, inside a metal tube of narrow bore and thin wall. When employed to form a heating wire 1 according to the present invention, a tube 2 having, for example, an overall diameter, d, of about 0.75 mm and a wall thickness, t, of about 0.05 mm can be readily produced, surrounding the electrically insulating core 3. This would have a resistance per unit length of about twice the value of a solid wire of 0.5 mm diameter of the prior art.

The resulting wire 1 is conveniently formed into an elongate helical coil 4 for use as a heating element, as shown in Figure 2. Such a helical coil heating element 4 may be utilised, for example, in furnaces or in radiant heaters for glass-ceramic cooking appliances, a typical such heater being shown in plan view in Figure 3. In Figure 3, the heater comprises a metal supporting dish 5, of well-known form, containing a layer 6 of thermal and electrical insulation material, e.g. of known microporous form, on which the coiled wire heating element 4 is arranged. A known form of peripheral wall 7 of thermal insulation material is provided, whose top surface is arranged to contact the underside of a glass-ceramic cook top (not shown) of well-known form.

A terminal block 8 is also provided for making electrical connection from an electricity supply to the heating element 4 e.g. by welding to the exterior of the tube 2. A probe-type thermal cut-out device 9 is mounted on the heater and used in known manner to switch off the electricity supply to the heating element 4 when a predetermined temperature is reached.

When the element 4 is electrically energised, the outer tube 2 (Figure 1) of the wire 1 which forms it and through which electric current passes, heats up very rapidly to visible radiance as a result of its low mass, and much more rapidly than would a conventional solid wire of similar overall physical dimensions and electrical resistance. For example a wire formed with a tube having the dimensions stated above would heat up in about one third of the time required for a solid wire of 0.5 mm diameter of the prior art.

By means of a heating wire according to the invention a higher electrical resistance per unit length can be obtained compared with a solid wire of the same external diameter, of the prior art. Altematively the same resistance per unit length as a solid wire of the prior art can be achieved but with the advantage that the overall diameter, and hence the surface area, of the wire according to the invention will then be greater than that of the solid wire of the prior art.

A further embodiment of heating wire according to the invention is illustrated in Figure 4. In addition to the tube 2 of electrically conducting material and the core 3 of electrically insulating material, as shown in and described with reference to Figure 1, one or more fine wire conductors 10 may be provided through the core 3. In this case the core 3 comprises a solid insulation material, such as MgO, which serves to electrically insulate the one or more fine wire conductors 4 from one another and from the tube 2. The one or more fine wire conductors 10 is or are arranged to extend from the end of the tube 2 and may be connected to the electricity supply to generate additional heat or may be suitably connected for temperature sensing purposes. In this latter application two such fine wire conductors 10 may be provided to form a thermocouple. Altematively, one or more wire conductors 10 of appropriately high temperature coefficient of electrical resistance may be provided and connected to suitable circuitry for temperature sensing purposes.

Claims

1. An electrical resistance heating wire comprising a tube of electrically conducting material adapted for heating by passage therethrough of electric current, the tube enclosing a core of electrically insulating material.

2. A heating wire according to Claim 1, in which the tube is continuous and comprises one or more metals or metal alloys.

3. A heating wire according to Claim 2, in which the metal alloy comprises an iron chromium-aluminium alloy or a nickel-chromium alloy.

4. A heating wire according to Claim 2, in which the tube comprises a plurality of layers of different metals or metal alloys.

5. A heating wire according to Claim 4, in which the tube comprises an inner layer of, or including, iron, coated with aluminium, the aluminium subsequently thermally oxidising to form a protective layer of aluminium oxide on the inner layer.

6. A heating wire according to any preceding Claim in which the tube is selected with regard to intemal bore size and with regard to wall thickness to provide a desired electrical resistance value per unit length thereof.

7. A heating wire according to any preceding Claim, in which the tube is of circular cross-section.

8. A heating wire according to any preceding Claim, in which the core comprises an electrically insulating fluid or solid.

9. A heating wire according to Claim 8, in which the fluid comprises one or more gases.

10. A heating wire according to Claim 9, in which the gases comprise air.

11. A heating wire according to Claim 8, in which the solid is of particulate form.

12. A heating wire according to Claim 11, in which the solid comprises one or more powdered oxides of metals or metalloids.

13. A heating wire according to Claim 12, in which the one or more powdered oxides are selected from: MgO; At03; SiO2.

14. A heating wire according to Claim 11, 12, or 13, in which the particulate solid is compressed or compacted inside the tube.

15. A heating wire according to any preceding Claim, in which one or more elongate conductors are provided in the core of the tube along the length thereof and electrically insulated from the tube.

16. A heating wire according to Claim 15, in which the one or more elongate conductors are adapted to be used for additional heating purposes, by passage of electric current therethrough.

17. A heating wire according to Claim 15, in which the one or more elongate conductors are adapted to be used for temperature sensing purposes.

18. An electric heating element constructed from an electrical resistance heating wire according to any preceding Claim.

19. An electric heating element according to Claim 18, in which the wire therein is provided in the form of an elongate helical coil.

20. A radiant electric heater incorporating an electric heating element according to Claim 18 or 19.

21. An electrical resistance heating wire constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

22. An electric heating element constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

23. A radiant electric heater constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.