GB2275405A

GB2275405A - Mounting radiant electrical heating element

Info

Publication number: GB2275405A
Application number: GB9319785A
Authority: GB
Inventors: Joseph Anthony Mcwilliams
Original assignee: Ceramaspeed Ltd
Current assignee: Ceramaspeed Ltd
Priority date: 1993-02-11
Filing date: 1993-09-24
Publication date: 1994-08-24
Anticipated expiration: 2013-09-24
Also published as: GB9302693D0; GB9319785D0; GB2275405B

Abstract

A radiant electrical heating element comprises an elongate electrically conductive strip (1) adapted for edgewise mounting on a base (3) of thermal and electrical insulation material, the strip including a a plurality of mounting tabs (2) extending in a direction away from the continuous portion at spaced-apart locations, for securement to or in the base. Provided in each tab is at least one aperture such as a hole (5C) or slot (5B) or slit (5A), whereby thermal conduction into the tabs (2) from the continuous portion is reduced or minimised and also variation in electric current density along the continuous portion of the strip (1) is reduced or minimised when the strip is electrically connected for operation in a radiant electric heater. <IMAGE>

Description

Electric Heatina Element This invention relates to an electrical heating element for a radiant electric heater, particularly, although not exclusively, for use with a glass-ceramic smooth top cooker. More particularly, it relates to a heating element in the form of an elongate strip of electrically conductive material, such as a metal or a metal alloy, arranged for edgewise mounting on a base of thermal and electrical insulation material by means of integral tabs extending edgewise therefrom in the plane of the strip and secured to or in the insulating base. When secured in this way and brought to radiance for heating purposes by passing an electric current through it, there is a disadvantage in that the strip glows less brightly in the regions adjacent to the tabs. The reason for this is two-fold.Firstly, heat is conducted from the strip into the mounting tabs and then into the base. Thermal conduction into the base can be reduced somewhat by employing a base with very good thermal insulation properties, a particular example of such being microporous thermal insulation material. Microporous thermal insulation materials are well known in the art. Secondly, it is found that the electric current density in the strip varies along the length of the strip, being lower in the regions adjacent the tabs than elsewhere. In such regions the strip is effectively wider on account of the tabs and electric current flows though the tabs in addition to the strip, so resulting in the lower current density referred to above.

It is an object of the present invention to overcome or minimise this problem.

The present invention provides an electrical heating element for a radiant electric heater, said element comprising an elongate electrically conductive strip adapted for edgewise mounting on a base of thermal and electrical insulation material, said strip having integral therewith a plurality of mounting tabs extending from an edge thereof at spaced-apart locations, for securement to or in said base, there being provided in each tab at least one hole, slot or slit, whereby thermal conduction into said tabs is reduced or minimised and also variation in electric current density along said strip is reduced or minimised when said strip is electrically connected for operation in a radiant electric heater.

By appropriate selection of the size and location of the holes, slots or slits in relation to the tabs, it can be arranged to reduce or minimise reduction in temperature in the strip in the regions of the tabs during electrical self-heating of the strip, where such reduction in temperature would be caused by thermal conduction from the strip into the tabs and by an effective increase in cross-section of the strip at the region of the tabs resulting from the tabs.

The said at least one slot or slit is provided extending into and along said tab from the extremity thereof. It may be required for the or each slot or slit to extend beyond its associated tab and into the strip itself.

The or each said slot or slit may be straight, bent or curved and of uniform or non-uniform shape.

A plurality of said slots or slits may be provided in each tab, either parallel to, or at an angle to, each other.

When a said hole is provided for each tab this may suitably be of a size so as to be as large as possible commensurate with its associated tab. It may be necessary for the or each said hole to extend beyond the associated tab and into the strip itself.

The optimum length and shape and relative configuration of the slots or slits and/or the size and location of the hole can be readily determined by simple experiment for any particular strip with tabs.

By means of the invention a strip-form radiant heating element mounted on an insulating base by means of tabs integral therewith can be provided which demonstrates improved uniformity of visible radiance along its length when in operation.

The strip may, if required, be provided of sinuous (sometimes also known as serpentine or corrugated or convoluted) form along its length.

The said strip and tabs suitably comprise a metal, or a metal alloy, such as, for example, an iron-chromium-aluminium alloy. However the invention is not limited to any particular material or configuration of the strip.

For economy of manufacture of the heating element, the tabs are preferably uniformly spaced apart along the strip and with dimensions such that gaps between the tabs form upside down profiles of the tabs. As a result, at least two lengths of heating element can be produced by separation from a single conductive strip, with minimum wastage of strip material, by means of a cutting, punching or stamping operation, the two lengths being side by side on the strip with the tabs of one element being effectively interdigitated with the tabs of the other element as the separating operation is effected.

It may be advantageous to provide the heating element of the invention with one or more selected regions which operate in a heater at a lower temperature and/or with lower visible radiance than the remainder of the element. This may be required, for example, at terminal regions of the element, particularly where these are long, such as is necessary in terminating the inner heating element of a concentric arrangement of separate inner and surrounding outer heating elements. It may also be required for visually isolating regions of a heating element e.g. to produce one or more isolated patterns in the heating element.

Such selected regions for operating at a lower temperature and/or with lower visible radiance may be suitably provided by localised increase in effective cross-sectional area over a required length, or required lengths, of the conductive strip. This may be effected by localised increase in thickness of the strip and/or increase in effective width of the strip.

The slots or slits or holes may be omitted from appropriate tabs in the or each selected region. Alternatively the spaced-apart tabs may be replaced in the or each selected region by an effectively elongate tab extending along the or each selected region and without slots, slits or holes therein. As a further alternative, an additional piece of electrically conductive material may be bonded to the strip at the or each selected region to effectively locally increase the thickness of the strip and optionally the width of the strip. Such additional piece of conductive material may conveniently be formed of the same material as the strip, although it may comprise any other suitable metal or metal alloy.

Advantageously, the said base of thermal and electrical insulation material may comprise microporous thermal and electrical insulation material.

The tabs may conveniently be provided for embedment in said base of insulation material.

An additional advantage resulting from the invention is that the slits, slots or holes in the mounting tabs provide enhanced securing of the strip when the strip is mounted with the tabs embedded in the base of insulation material.

The present invention also provides a radiant electric heater incorporating an electrical heating element as described above.

The invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 represents a side elevational view of an electrical heating element according to the invention having mounting tabs incorporating slits; Figure 2 represents a side elevational view of an electrical heating element according to the invention having mounting tabs incorporating slots; Figure 3 represents a side elevational view of an electrical heating element according to the invention having mounting tabs incorporating holes; Figure 4 represents a plan view of a radiant electric heater incorporating a heating element of Figures 1 to 3; Figure 5 represents a plan view of portions of electrical heating elements of the invention during manufacture;; Figures 6A to 6C represent side elevational views of portions of an electrical heating element of the invention having alternative arrangements for providing one or more selected regions for operation in a heater at a lower temperature and/or with lower visible radiance than the remainder of the element.

An electrical heating element for a radiant electric heater comprises an elongate electrically conductive strip 1, of a metal or a metal alloy which has integral mounting tabs 2 spaced apart along its length and extending from an edge thereof in the plane of the strip. The strip 1 is intended to be mounted edgewise on a base 3 of thermal and electrical insulation material by embedding the tabs 2 in the base 3 to a level shown by the dotted line 4. By way of example, the base 3 suitably comprises microporous thermal and electrical insulation material such as described in GB 1 580 909. As shown in Figure 1, the tabs 2 are provided with slits 5A extending from the lower edge thereof. As shown in Figure 2, a slot 5B is provided in each tab 2, extending from the lower edge thereof.As shown in Figure 3, a hole 5C is provided in each tab 2. Although the hole 5C is shown as circular in Figure 3, it may be another shape such as rectangular or oval, according to the shape of the tabs 2.

The slits 5A, slots 5B and holes 5C fulfil a common purpose. In their absence, if the strip 1 were to be operated as a radiant heating element by passing an electric current along it, it would be observed that the region of the strip 1 immediately above each tab 2 would glow less brightly than the remainder of the strip 1. The reason for this is two fold. In addition to the electric current flowing along the strip 1, it would also flow through the tabs 2, and the effective increased width of the strip 1 in the regions of the tabs 2 caused by the tabs 2 results in a lower electric current density in these regions compared with that in the remainder of the strip. Hence slower and less heating of the strip occurs in these regions.

Furthermore heat would also be lost from the strip into the tabs by conduction. By means of the present invention, the provision of the slits 5A, slots 5B, or holes 5C in the tabs 2 reduces thermal conduction into the tabs from the strip and also compensates for or reduces or eliminates flow of electric current through the tabs 2. Hence the flow of current along the strip 1 is as if the tabs 2 were substantially not present. Consequently the electric current density is substantially uniform along the strip 1. The strip 1 glows with greater uniformity of brightness along the length thereof when in operation as a heating element, compared with a similar strip without the holes, slots or slits.

The length, shape and number of the slits 5A or slots 5B and the size and location of the holes 5C required for maximum effect can be readily determined by simple experiment. It may be required for the slits 5A, slots 5B or holes 5C to extend somewhat beyond the tabs 2 and into the body of the strip 1 itself as shown in Figures 1, 2 and 3. Generally speaking, each hole 5C should be made as large as is reasonably possible in relation to the size of its associated tab 2. The slits 5A or slots 5B could, if required be provided of non-uniform shape or width and could be bent or curved.A plurality of slits or slots, when provided in each tab, could be arranged non-parallel to each other, if required.

Figure 4 illustrates an application of a heating element according to the invention to a radiant electric heater for use under a glass-ceramic plate of a smooth top cooker. A heating element comprising an elongate electrically conductive strip 1 of a metal or metal alloy, such as an iron-chromium-aluminium alloy, constructed as shown in Figures 1, 2 or 3 and described above with reference thereto, is made into sinuous form and is mounted edgewise on the surface of a base layer 3 of microporous thermal and electrical insulation material compacted into a metal dish 6. Tabs 2 (Figures 1, 2, 3) on the strip 1 are embedded in the base layer 3, e.g. by pressing them into the base layer 3. After mounting the strip 1, the surface of the base layer 3 is arranged to be substantially at a level shown by reference numeral 4 in Figures 1, 2, 3.Against the side of the dish 6 is located a peripheral wall 7 of thermal insulation material, such as ceramic fibre material or microporous insulation material.

A terminal connector 8 is provided for electrically connecting the heating element strip 1 to an electrical supply.

A well-known form of thermal cut-out device 9 is provided, extending over the heating element to switch off the heating element in the event of overheating when the heater is installed and operating in the cooker.

The heating element strip 1 operates advantageously as described above with reference to Figures 1, 2 and 3, the presence of slits, slots or holes (5A, 5B, 5C in Figures 1, 2, 3 respectively) resulting in improved uniformity of brightness of radiation from the strip 1 and also providing a secondary advantage of enhancing securement of the tabs 2 in the insulation material 3.

Economy of manufacture of the heating elements illustrated in Figures 1, 2 and 3 can be improved if the gaps 10 between the tabs 2 are made somewhat narrower than shown and are shaped and dimensioned such that they represent in form exact upside-down profiles of the tabs 2. Lengths of elements of this form can be produced with minimum wastage of material as illustrated in Figure 5. In Figure 5 four lengths of heating elements 1A, 1B, 1C and 1D are simultaneously produced from a single strip of material by a suitable cutting, punching or stamping operation, holes 5C (or alternatively slots or slits similar to those denoted by reference numerals 5A, 5B in Figures 1 and 2) being appropriately located therein.The cutting, punching or stamping operation results in separation of the strip along the lines of demarcation 11, into the four lengths of heating element 1A, 1B, 1C, ID. Apart from the material removed to form the holes 5C, substantially no other wastage occurs in this process.

There may be requirements for providing the heating element of the invention with one or more selected regions which operate in a heater at a lower temperature andlor with lower visible radiance than the remainder of the element. This may be required, for example, at terminal regions of the heater, e.g. at the regions 12 in Figure 4. Such terminal regions may be quite long, particularly in a heater where two or more separately connected concentric heating elements are provided and it is necessary to provide low-radiating terminal portions extending from a heating element in a central zone of the heater to a connector at the periphery of the heater. It may also be required to provide visual isolation of selected regions of a heating element, e.g. to give the visual impression that a portion of the heating element 1 in Figure 4 is separate from the remainder of the heating element 1.

Such selected regions may be provided by localised increase in the effective crosssectional area of the strip element 1 over one or more required lengths thereof. Examples of ways in which this may be achieved are illustrated in Figures 6A, 6B and 6C. Referring to Figure 6A, the holes 5C have been omitted in tabs 21, 22 in the strip 1 to form the selected regions of lower operating temperature. Referring to Figure 6B, the tabs 2 with holes 5C are replaced by an effectively elongate tab 23 without any holes, at each required selected region. If desired, the width W1 of the region of the strip with this elongate tab 23 may be extended to be greater than the width W2 of the remainder of the strip.

As a further alternative, the strip I may be increased in thickness at the one or more selected regions. This is preferably achieved, as illustrated in Figure 6C by bonding an additional piece of electrically conducting material 13 to the surface of the strip 1, covering each required selected region of the strip 1. This measure may also be carried out as an addition to those described with reference to Figures 6A and 6B. The width of the additional piece of material 13 may be such that the additional piece of material either covers only the portion of the strip excluding the tabs 2 (W3), or covers the portion of the strip including the tabs 2 (W4), or somewhat exceeds the width of the strip and tabs (W5).

The additional piece of material 13 is conveniently formed of the same material as the strip 1, but may comprise any other appropriate metal or metal alloy.

The arrangements shown in Figures 6A, 6B, 6C also apply where the tabs are provided with slots or slits, as shown in Figures 1 and 2, instead of holes.

Claims

1. An electrical heating element for a radiant electric heater, said element comprising an elongate electrically conductive strip adapted for edgewise mounting on a base of thermal and electrical insulation material, said strip having integral therewith a plurality of mounting tabs extending from an edge thereof at spaced-apart locations, for securement to or in said base, there being provided in each tab at least one hole or slot or slit, whereby thermal conduction into said tabs is reduced or minimised and also variation in electric current density along said strip is reduced or minimised when said strip is electrically connected for operation in a radiant electric heater.

2. An electrical heating element according to Claim 1, in which said at least one slot or slit extends into and along said tab from the extremity thereof.

3. An electrical heating element according to Claim 1 or 2, in which the or each said slot or slit is straight or bent or curved.

4. An electrical heating element according to Claim 1, 2 or 3, in which the or each said slot or slit is of uniform or non-uniform shape.

5. An electrical heating element according to Claim 2, 3, or 4, in which the or each said slot or slit extends beyond its associated tab and into the strip itself.

6. An electrical heating element according to Claim 1, in which a single said hole is provided in each strip, said hole being as large as possible commensurate with its associated tab.

7. An electrical heating element according to Claim I or 6, in which said hole extends beyond its associated tab and into the strip itself.

8. An electrical heating element according to any preceding Claim, in which the said strip is provided of sinuous form along its length.

9. An electrical heating element according to any preceding Claim, in which said strip and tabs comprise a metal, or a metal alloy.

10. An electrical heating element according to any preceding Claim, in which the tabs are uniformly spaced apart along the strip and with dimensions such that gaps between the tabs form upside down profiles of the tabs.

11.An electrical heating element according to Claim 10, in which at least two lengths thereof have been produced by separation from a single conductive strip, by means of a cutting, punching or stamping operation, the at least two lengths being side by side on the strip with the tabs of one element being effectively interdigitated with the tabs of the other element as the separating operation is effected.

12. An electrical heating element according to any preceding Claim, provided with one or more selected regions which operate in a heater at a lower temperature and/or with lower visible radiance than the remainder of the element.

13. An electrical heating element according to Claim 12, in which said one or more selected regions is or are provided by localised increase in effective cross-sectional area over a required length, or required lengths, of the conductive strip.

14. An electrical heating element according to Claim 13, in which at the one or more selected regions localised increase in thickness of the strip and/or effective width of the strip is effected.

15. An electrical heating element according to Claim 14, in which the slots or slits or holes are omitted from the tabs in the or each selected region.

16. An electrical heating element according to Claim 14, in which the spaced-apart tabs at the or each selected region are replaced by an effectively elongate tab extending along the or each selected region and without slots, slits or holes therein.

17. An electrical heating element according to Claim 14, in which an additional piece of electrically conductive material is bonded to the strip at the or each selected region to effectively locally increase the thickness of the strip and optionally the width of the strip.

18. An electrical heating element according to Claim 17, in which the additional piece of conductive material is formed of the same material as the strip.

19. An electrical heating element according to any preceding Claim, in which said insulation material comprises microporous thermal and electrical insulation material.

20. An electrical heating element according to any preceding Claim, in which the said tabs are to be embedded in said base of insulation material.

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

22. A radiant electric heater incorporating an electrical heating element according to any preceding Claim.