WO2004028210A1

WO2004028210A1 - Radiant electric heater

Info

Publication number: WO2004028210A1
Application number: PCT/GB2003/003955
Authority: WO
Inventors: George Anthony Higgins
Original assignee: Ceramaspeed Limited
Priority date: 2002-09-17
Filing date: 2003-09-11
Publication date: 2004-04-01
Also published as: AU2003263359A1; GB0221519D0

Abstract

A radiant electric heater (2) comprises at least first and second heating zones (14, 12; 20, 22; 24, 26; 28, 30), the second heating zone (12; 22; 26; 30) at least partially surrounding the first heating zone (14; 20; 24; 28). Each heating zone has at least one electrical resistance heating element (10) comprising crinkled ribbon of corrugated form, having a corrugation pitch (p) and a corrugation amplitude (a). Each heating element (10) is formed from a length of flat ribbon and arranged in at least one turn in each of the heating zones. A first length (L2) of crinkled ribbon has been formed from a second length (L1) of flat ribbon and a crinkle factor, defined as a ratio of the second length to the first length, is different in the second heating zone as compared with the first heating zone, whereby a variation in power density between the first and second heating zones of the heater results.

Description

RADIANT ELECTRIC HEATER

This invention relates to a radiant electric heater, in particular for use in a cooking appliance such as a hob or oven.

It is known to manufacture radiant electric heaters for cooking appliances, such as hobs or ovens, in which a dish-like support, such as of metal, has a layer of thermal and electrical insulation material provided therein and at least one electrical resistance heating element of crinkled ribbon form supported relative to the layer of insulation material. The crinkled ribbon is of corrugated, substantially sinusoidal, form, having a corrugation pitch and a corrugation amplitude and is generally supported edgewise on the base of insulation material. The crinkled ribbon is formed from a basic flat ribbon of a predetermined breadth and thickness.

In order to achieve a desired heating power, it is necessary to accommodate a particular length of ribbon of predetermined breadth and thickness into the available space within a heater. The crinkling process is selected such that a required length of flat ribbon is compressed into an overall crinkled length suitable for the dimensions of the particular heater. This is effected in practice by applying what is known as a crinkle factor and which is defined as a ratio of the selected length of the basic flat ribbon to the required overall length of the crinkled ribbon. The achievable crinkle factor is limited by mechanical constraints of the crinkling process and is primarily determined by the corrugation pitch and the corrugation amplitude. Hitherto, it has been common practice to manufacture, for a particular heater, a crinkled ribbon heating element having a predetermined uniform crinkle factor along its length, as produced by the crinkling machine. When such a heating element is provided in a heater, in the form of a plurality of turns substantially equally spaced apart, it results in a substantially uniform power density across the entire heater. For some applications it is required to provide a heater in which the power density is higher at one or more regions than at one or more other regions. This has necessitated spacing the turns of the uniformly-crinkled ribbon heating element closer together where higher power density is required and wider apart where lower power density is required. This may be required, for example, to provide what is known as edge weighting in power to a heater, in which a higher power density is required at an outer peripheral region of the heater than at an inner region of the heater. It may also be required where a heater has a plurality of heating zones and where different power densities are required in the different heating zones . Such an arrangement for varying the power density by varying the spacing between the turns of the crinkled ribbon heating element is disadvantageous in that it prevents the maximum available space within a heater from being utilised by the heating element, and particularly where space is at a premium for accommodating a required particularly long length of crinkled ribbon heating element .

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

According to the present invention there is provided a radiant electric heater comprising at least first and second heating zones, the second heating zone at least partially surrounding the first heating zone, each heating zone having at least one electrical resistance heating element comprising crinkled ribbon of corrugated form, having a corrugation pitch and a corrugation amplitude, formed from a length of flat ribbon and arranged in at least one turn in each of the heating zones, wherein a first length portion of crinkled ribbon has been formed from a second length portion of flat ribbon, and wherein a crinkle factor, defined as a ratio of the second length portion to the first length portion, is different in the second heating zone as compared with the first heating zone, whereby a variation in power density between the first and second heating zones of the heater results.

The crinkle factor in at least one of the heating zones may vary continuously along the length of the ribbon. If desired, the crinkle factor in both the first and second heating zones may vary continuously along the length of the ribbon.

Alternatively, the crinkle factor may vary in stages along the length of the ribbon. Each stage may comprise substantially at least one turn.

The at least one electrical resistance heating element may be provided on a base of thermal and electrical insulation material, such as microporous thermal and electrical insulation material, and may be provided in a dish-like support.

The crinkled ribbon may be supported edgewise on the base of thermal and electrical insulation material. The crinkled ribbon may be formed of ribbon material of substantially constant breadth and thickness.

The crinkle factor may vary by variation of the corrugation pitch and/or the corrugation amplitude and/or the curvature of the corrugations (i.e., whether the corrugations are relatively rounded or relatively sawtooth in configuration) .

The plurality of turns of crinkled ribbon may be uniformly or non-uniformly spaced from one another.

The first and second heating zones may be undivided, or may be divided from one another such as by a wall of thermal insulation material.

The first heating zone may be energisable independently of the second heating zone.

A single continuous crinkled ribbon may be provided for the first and second heating zones and may be provided with at least one tapping point whereby the first and second heating zones may be connected to a power supply for independent energising thereof.

In a region where the ribbon passes from one heating zone to another, the ribbon may be arranged with a crinkle factor lower than that on either side of the region, whereby temperature of the ribbon is reduced in such region.

The electric heater may be adapted for use in a cooking appliance, such as an oven or a cooking hob and such as behind a glass-ceramic plate. For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made by way of example to the accompanying drawings in which:

Figure 1 is a plan view of one embodiment of a radiant electric heater according to the present invention;

Figures 2A and 2B are perspective views respectively of a portion of flat ribbon heating element and a corresponding portion of crinkled ribbon heating element formed therefrom;

Figure 3 is a plan view of another embodiment of a radiant electric heater according to the present invention, having two heating zones divided from one another;

Figure 4 is a plan view of a further embodiment of a radiant electric heater according to the present invention, having two undivided heating zones; and

Figure 5 is a perspective view from above of a still further embodiment of a radiant electric heater according to the present invention, having two undivided heating zones .

Referring to Figure 1, a radiant electric heater 2 is provided that is suitable for use in a cooking appliance, for example an oven or a cooking hob, such as behind a glass-ceramic plate. The heater 2 comprises a metal dish-like support 4 having therein a base layer 6 of thermal and electrical insulation material, such as microporous thermal and electrical insulation material. A peripheral wall 8 of thermal insulation material surrounds the heater 2.

An electrical resistance heating element 10 is provided in the heater 2 and comprises a plurality of turns of crinkled metal ribbon of corrugated form supported edgewise on the base layer 6.

As shown in Figures 2A and 2B, the crinkled ribbon heating element 10 is formed from a length Ll of flat ribbon material of breadth or height h and thickness t (Figure 2A) , which may or may not have legs 11. By way of example, the ribbon may have a breadth or height h of about 5.0 mm and a thickness t of about 50 microns. The flat ribbon 10 is passed between appropriate gear wheels in a machine of known form to produce a crinkled ribbon heating element 10 of corrugated, substantially sinusoidal, form and length L2, as shown in Figure 2B. The crinkled ribbon heating element 10 has a corrugation pitch p and a corrugation amplitude a, determined by the nature and separation of the gears in the crinkling machine. The length L2 of the crinkled heating element 10 is less than the length Ll of the flat ribbon material and an expression known as the crinkle factor is defined as the ratio of Ll to L2. The value of the crinkle factor is determined by the corrugation amplitude a and the corrugation pitch p and can be varied by adjusting the corrugation amplitude and/or the corrugation pitch. A typical range of values for the crinkle factor is between about 1.1 and about 3.0.

In the heater of Figure 1, the crinkled ribbon heating element 10 is arranged with its plurality of turns substantially uniformly spaced apart. However, during manufacture of the crinkled ribbon heating element 10, at least the outer turn 12, and possibly the outer two turns, is or are formed with a higher crinkle factor than the inner turns 14. That is, the outer turn or turns 12 form an outer heating zone surrounding the inner turn 14, which form a first heating zone. This is particularly advantageous as it means that when the heater 10 is energised, the power density in the region of the outer turn or turns 12 will be higher than that in the region of the inner turns 14. This results in advantageous edge weighting of heating power in the heater 2 while enabling uniform pitch between adjacent turns of the crinkled ribbon heating element 10.

The heating element 10 is electrically connected to a terminal block 16 at the edge of the heater and a known form of temperature limiter 18 is provided, overlying the heater.

By way of example, the outermost turn 12 of the heating element 10, between points A and C, may have a crinkle factor of about 1.60. Between points C and B, a reduced crinkle factor of about 1.49 may be provided. It may be preferred not to stop the crinkling machine and make an abrupt change in crinkle factor at point C. Instead, between a point E and a point D the crinkle factor may be gradually reduced from about 1.60 to about 1.49, continuing thereafter at about 1.49 to point B.

Instead of arranging for the crinkle factor to change to provide the two stages as shown, it may be arranged for the crinkle factor to be reduced continuously or stepwise between terminal point A and a central point Y and then to be increased continuously or stepwise between the central point Y and terminal point B. When such changes in crinkle factor are stepwise, each step substantially comprises at least one turn of the crinkled ribbon heating element 10.

Referring now to Figure 3, a radiant electric heater 2A also comprises a metal dish-like support 4 having therein a base layer 6 of thermal and electrical insulation material. A peripheral wall 8 of thermal insulation material surrounds the heater 2A and a section of wall 8A divides the heater into two heating zones 20 and 22.

A terminal block 16 is provided at the edge of the heater and a known form of temperature limiter 18 overlies the heating zone 20.

A crinkled ribbon heating element is provided having a portion 10A arranged as a plurality of spaced-apart turns in heating zone 20 and having a portion 10B arranged as a plurality of spaced-apart turns in heating zone 22. The heating element 10A, 10B is formed by substantially the same processes as described with reference to the heating element 10 of Figures 1 and 2 and is supported edgewise on the base 6.

The crinkled ribbon heating element 10A, 10B is arranged with a tapping point X connected by welding to a terminal in the terminal block 16 and such that the portions 10A, 10B of the heating element can be energised independently, but usually such that the portion 10A is either energised alone or together with the portion 10B. In this way, the heating zone 20 is operated alone or together with the additional heating zone 22.

It is arranged for the crinkle factor of the portion 10A of the crinkled ribbon heating element (that is between points A and X) to be lower than the crinkle factor of the portion 10B of the crinkled ribbon heating element (that is between points X and D) . This enables a suitable power (for example 1100 watts) to be obtained in the heating zone 20 and a suitable power (for example 900 watts) to be obtained in the additional heating zone 22.

Although not essential, it is advantageous for the crinkle factor of the heating element 10A, 10B between points B and C to be reduced to a value lower than that for the portion 10A of the heating element, whereby a low temperature of the heating element is achieved in that portion thereof which passes through or below the section of wall 8A.

If desired, it may also be arranged for the crinkle factor of the portions 10A and/or 10B of the ribbon element within the heating zones 20 and/or 22 to vary.

Referring now to Figure 4, a rectangular radiant electric heater 2B, which is particularly suitable as a grill heater, has a metal dish-like support 4 having therein a base layer 6 of thermal and electrical insulation material. A peripheral wall 8 of thermal insulation material surrounds the heater and a terminal block 16 is provided at the edge of the heater.

A crinkled ribbon heating element is provided edgewise on the base layer 6, having a plurality of turns and arranged with a portion 10A surrounded by a portion 10B, and forming two undivided heating zones 24 and 26 surrounding one another. The heating element 10A, 10B has end terminal points A and B and a central tapping point X welded to a terminal in the terminal block 16. The two portions 10A and 10B of the heating element can be energised separately or together. - lo osing substantially the same techniques as described with reference to Figures 1 to 3, the portion 10A of the crinkled ribbon heating element (between points B and X) is formed during the crinkling operation with a crinkle factor which is different from that of the portion 10B of the heating element (between points A and X) . This means that the power density of the portion 10A is different from that of the portion 10B. For example, if the crinkle factor for the portion 10B is higher than that for the portion 10A, the portion 10B will have a higher power density than the portion 10A.

Referring now to Figure 5, a circular radiant electric heater 2C has a metal dish-like support 4 having therein a base layer 6 of thermal and electrical insulation material, such as microporous thermal and electrical insulation material. A peripheral wall 8 of thermal insulation material surrounds the heater.

A terminal block 16 is provided at the edge of the heater and a known form of temperature limiter 18 overlies the heater.

A crinkled ribbon heating element is provided edgewise on the base layer 6, having a plurality of turns and arranged with two concentric portions 10A and 10B forming two undivided heating zones 28 and 30. The heating element 10A, 10B has end terminal points A and B and a central tapping point X that is welded to a terminal in the terminal block 16. The two portions 10A and 10B of the heating element can be energised separately or together.

The portion 10A of the crinkled heating element (between points B and X) is formed during the crinkling operation with a crinkle factor that is different from that of the portion 10B of the heating element (between points A and X) . This means that the power density of the portion 10A is different from that of the portion 10B and can be higher or lower depending upon whether the crinkle factor of portion 10A is higher or lower than that of portion 10B.

Claims

1. A radiant electric heater comprising at least first and second heating zones (14, 12; 20, 22; 24, 26; 28, 30), the second heating zone (12; 22; 26; 30) at least partially surrounding the first heating zone (14; 20; 24; 28), each heating zone having at least one electrical resistance heating element (10) comprising crinkled ribbon of corrugated form, having a corrugation pitch (p) and a corrugation amplitude (a) , formed from a length of flat ribbon and arranged in at least one turn in each of the heating zones, wherein a first length portion (L2) of crinkled ribbon has been formed from a second length portion (Ll) of flat ribbon, characterised in that a crinkle factor, defined as a ratio of the second length portion to the first length portion, is different in the second heating zone as compared with the first heating zone, whereby a variation in power density between the first and second heating zones of the heater results.

2. A heater as claimed in claim 1, characterised in that the crinkle factor in at least one of the zones (14, 12; 20, 22; 24, 26; 28, 30) varies continuously along the length of the ribbon.

3. A heater as claimed in claim 2, characterised in that the crinkle factor in both the first and second zones (14, 12; 20, 22; 24, 26; 28, 30) varies continuously along the length of the ribbon.

4. A heater as claimed in claim 1, characterised in that the crinkle factor varies in stages along the length of the ribbon.

5. A heater as claimed in claim 4, characterised in that each stage comprises substantially at least one turn.

6. A heater as claimed in any preceding claim, characterised in that the at least one electrical resistance heating element (10) is provided on a base (6) of thermal and electrical insulation material.

7. A heater as claimed in claim 6, characterised in that the insulation material (6) comprises microporous thermal and electrical insulation material.

8. A heater as claimed in claim 6 or 7, characterised in that the base (6) of thermal and electrical insulation material is provided in a dish-like support (4) .

9. A heater as claimed in claim 6, 7 or 8, characterised in that the crinkled ribbon is supported edgewise on the base (6) of thermal and electrical insulation material.

10. A heater as claimed in any preceding claim, characterised in that the crinkled ribbon is formed of ribbon material of substantially constant breadth and thickness .

11. A heater as claimed in any preceding claim, characterised in that the crinkle factor varies by variation of the corrugation pitch (p) .

12. A heater as claimed in any preceding claim, characterised in that the crinkle factor varies by variation of the corrugation amplitude (a) .

13. A heater as claimed in any preceding claim, characterised in that the crinkle factor varies by variation of the corrugation curvature.

14. A heater as claimed in any preceding claim, characterised in that the turns of crinkled ribbon are uniformly spaced from one another.

15. A heater as claimed in any one of claims 1 to 13, characterised in that the turns of crinkled ribbon are non-uniformly spaced from one another.

16. A heater as claimed in any preceding claim, characterised in that the first and second heating zones (14, 12; 20, 22; 24, 26; 28, 30) are undivided from one another.

17. A heater as claimed in any one of claims 1 to 15, characterised in that the first and second heating zones (20, 22) are divided from one another.

18. A heater as claimed in claim 17, characterised in that the first and second heating zones (20, 22) are divided by a wall (8A) of thermal insulation material.

19. A heater as claimed in any preceding claim, characterised in that the first heating zone (20; 24; 28) is energisable independently of the second heating zone (22; 26; 30) .

20. A heater as claimed in any preceding claim, characterised in that a single continuous crinkled ribbon is provided for the first and second heating zones (14, 12; 20, 22; 24, 26; 28, 30) .

21. A heater as claimed in claim 20, characterised in that the ribbon is provided with at least one tapping point (X) .

22. A heater as claimed in claim 21, characterised in that the first and second heating zones (20, 22; 24, 26; 28, 30) are connected by the at least one tapping point (X) to a power supply for independent energising thereof.

23. A heater as claimed in any preceding claim, characterised in that in a region where the ribbon passes from one heating zone to another, the ribbon is arranged with a crinkle factor lower than that on either side of the region, whereby temperature of the ribbon is reduced in such region.