EP1045616A2

EP1045616A2 - Radiant electric heater and method of manufacture

Info

Publication number: EP1045616A2
Application number: EP00302846A
Authority: EP
Inventors: George Anthony Higgins
Original assignee: Ceramaspeed Ltd
Current assignee: Ceramaspeed Ltd
Priority date: 1999-04-16
Filing date: 2000-04-04
Publication date: 2000-10-18
Anticipated expiration: 2020-04-04
Also published as: ATE349878T1; EP1045616B1; GB9908609D0; EP1045616A3; GB2349059B; DE60032539T2; DE60032539D1; GB2349059A; ES2278579T3

Abstract

A radiant electric heater (1) comprises a heating element (5) in the form of a geometrical pattern separated from a metal foil and overlying a base (3) of thermal and electrical insulation material. The pattern includes a plurality of strips (6) in lateral spaced disposition traversing the heater (1) and each having a width considerably greater than its thickness. The strips (6) each have a major face facing the base (3) and are formed with undulations providing alternating peaks (7) and troughs (8). For any strip, the peaks (7) of the undulations are substantially laterally aligned with troughs (8) of the undulations in an adjacent strip. The heating element (5) is arranged with peaks (7) of the undulations in the strip (6) supported on protrusions (4) provided on the base (3).

Description

This invention relates to radiant electric heaters and a method of manufacture.
More particularly the invention relates to radiant electric heaters, particularly for use in glass-ceramic cooking appliances, comprising a heating element in the form of a geometrical pattern separated from a metal foil and overlying a base of thermal and electrical insulation material, the pattern including a plurality of strips in lateral spaced disposition, traversing the heater. Each of the strips has a width considerably greater than its thickness and has a major face facing the base.
It is known to provide such heaters in which the heating element is peripherally secured to the base and the laterally disposed strips are suspended above the base, with a support being provided in the middle of the heater. When the heater is energised for operation, readily-observable sagging of the strips occurs as a result of thermal expansion thereof. Visible rising and falling of the element occurs during energising and de-energising thereof and this may cause concern to the user of the heater, since heating elements generally do not exhibit discernible movement.
With this arrangement there is also a risk that because the strips are unsupported over relatively long lengths thereof, and are very close together, undesirable lateral displacement may also occur, resulting in contact, and hence short-circuiting, between adjacent strips.
It is an object of the present invention to overcome or minimise these problems.
According to one aspect of the invention there is provided a radiant electric heater comprising a heating element in the form of a geometrical pattern separated from a metal foil and overlying a base of thermal and electrical insulation material, the pattern including a plurality strips in lateral spaced disposition traversing the heater and each having a width considerably greater than its thickness, the strips each having a major face facing the base, the strips being formed with undulations providing alternating peaks and troughs and such that, for any strip, peaks of the undulations thereof are substantially laterally aligned with troughs of the undulations in an adjacent strip, the heating element being arranged with peaks of the undulations in the strips supported on protrusions provided on the base.
According to a further aspect of the invention there is provided a method of manufacturing a radiant electric heater comprising the steps of providing a base of thermal and electrical insulation material; providing a heating element in the form of a geometrical pattern separated from a metal foil and overlying the base of thermal and electrical insulation material, the pattern including a plurality of strips in lateral spaced disposition traversing the heater and each having a width considerably greater than its thickness, the strips each having a major face facing the base; forming the strips with undulations providing alternating peaks and troughs and such that, for any strip, peaks of the undulations thereof are substantially laterally aligned with troughs of the undulations in an adjacent strip; providing protrusions on the base; and arranging the heating element with peaks of the undulations in the strips supported on the protrusions.
The method may further comprise temporarily locating a heat-resistant electrically insulating member securely against the supported heating element and electrically energising the heating element for a predetermined period of time.
The protrusions may be provided integrally with the base.
The protrusions may be provided in rows and may be arranged to maintain spacing between adjacent strips.
The heating element may be supported with troughs of the undulations in the strips free from contact with the base.
The strips may be arranged substantially parallel to one another.
Means may be provided for securing opposite end regions of the strips to prevent longitudinal movement of the strips.
Such means for securing may be at a peripheral region of the base and may comprise clamping or hooking means, or staples or pins or the like.
A peripheral wall of insulation material may be provided for the heater.
The peripheral wall may overlie opposite end regions of the strips.
The base of thermal and electrical insulation material may be provided in a supporting dish, such as of metal, and may comprise microporous thermal and electrical insulation material.
The heating element in the form of the geometrical pattern may be separated from the metal foil by means such as etching, stamping, cutting, or milling.
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;
Figure 2 is a cross-sectional view of the heater of Figure 1; and
Figure 3 is a cross-sectional view of part of an alternative arrangement of a radiant electric heater according to the present invention.
Referring to Figures 1 and 2, a radiant electric heater 1, suitable for use under a glass-ceramic surface in a cooking appliance, has a metal supporting dish 2 in which is provided a base 3 of compacted microporous thermal and electrical insulation material of well-known form. The base 3 is provided with rows of protrusions 4, suitably formed integral with the base 3. The protrusions 4 in one row are offset with regard to those in the next row such that a protrusion in one row is next to the midway point between two protrusions in an adjacent row.
A heating element 5 is produced in the form of a desired geometrical pattern by separation from a metal foil. Such a metal foil may, for example, comprise an iron-chromium-aluminium alloy of thickness between about 40 and 200 microns and preferably between about 50 and 100 microns. The geometrical pattern of the heating element 5 shown in Figure 1 is of meandering or zig-zag form, but other forms could be considered. It is suitably produced from a metal foil by a well known operation, such as etching, stamping, cutting, or milling. The resulting pattern of the heating element 5 includes a plurality of strips 6 in lateral spaced disposition, preferably but not essentially parallel to each other, each having a width considerably greater than its thickness. By way of example, a typical width for each strip 6 would be about 3 mm. The strips 6 each have a major face which is to face the base 3 having the protrusions 4 thereon.
As shown in Figure 2, the strips 6 are formed with undulations providing alternating peaks 7 and troughs 8. The peaks and troughs in adjacent strips are staggered such that for any strip 6, peaks 7 of the undulations are substantially laterally aligned with troughs 8 of the undulations in an adjacent strip.
The resulting heating element 5 is arranged with respect to the base 3 of the heater, such that the strips 6 traverse the heater with peaks 7 of the undulations supported on the protrusions 4 provided on the base. The arrangement is such that the troughs 8 of the undulations on the strip 6 locate between protrusions 4 on either side thereof and sideways movement of the strips 6 is thereby prevented or minimised. In this way lateral contact between adjacent strips 6 is prevented at all times so that during subsequent operation of the heater there is no risk of lateral distortion of the strips 6 or of short-circuiting between adjacent strips.
The troughs 8 of the undulations of the strips 6 are preferably arranged such that, at least when the heating element 5 is in an unenergised condition, and optionally also in an energised condition, they are free from contact with the surface 9 of the base 3 between the protrusions 4, a gap being provided between the troughs 8 of the undulations and the surface 9 of the base.
Each strip 6 merges with an adjacent strip at the ends thereof to form the meandering heating element 5 and means are provided for securing opposite end regions 10, 11 of each strip 6 to prevent movement of the strips in a longitudinal direction. This may be achieved in a variety of ways. A peripheral wall 12 of thermal insulation material is provided in the heater. The heater is located for operation with the top surface of the wall 12 in contact with the underside of a glass-ceramic cooking surface (not shown). The end regions 10, 11 of the strips 6 may be arranged to be clamped between the bottom of the wall 12 and the base 3 of thermal insulation material.
Alternatively, or additionally, pins or staples 13 may be provided, passing through holes in the ends 10, 11 of the strips 6 and either downwards into the base 3 or upwards into the wall 12.
A further means of securing the ends of the strip is shown in Figure 3. The base 3 of thermal insulation material is provided with a step 14 at a location inboard of the peripheral wall 12.
A complementary step 15 is provided at the ends of the strips 6 of the heating element and the element is effectively hooked at these ends onto the step 14. The remaining end portions 10, 11 of the strips 6 are arranged to be trapped under the peripheral wall 12 to prevent lifting of the heating element.
Terminals 16, 17 are provided for connecting the heating element 5 to a voltage supply (not shown) for energising the heater.
It is advantageous during the manufacture of the heater to temporarily locate a heat-resistant electrically insulating member, such as a ceramic, securely against the top of the supported heating element 5 and electrically energise the heating element for a predetermined time, for example about 5 to 30 seconds. This process anneals and heat-sets the element to ensure minimum tendency for any lifting or distortion of the element to occur during subsequent operation of the heater in service.
When the heater is operating, thermal expansion of the strips 6 occurs, causing the troughs 8 of the undulations to move downwards towards the surface 9 of the base 3, this being possible as a result of the clearance provided at these regions during manufacture of the heater. However, because of the relatively close spacing of the protrusions, such movement is sufficiently small as to be substantially indiscernible to the user of the heater.
It may be desirable to provide non-uniform power per surface area of the heater, for example to provide higher power per surface area around the edge of the heater than at the centre. This can be achieved by providing the strips 6 of non-uniform, as opposed to uniform, cross sectional area. For example, the cross sectional area may be smaller at the end regions of the strips than at the central regions.
Alternatively, the ratio of the length of the strips between the protrusions to the distance between the protrusions may be varied, for example to be greater at the edge of the heater than at the centre.

Claims

A radiant electric heater comprising a heating element (5) in the form of a geometrical pattern separated from a metal foil and overlying a base (3) of thermal and electrical insulation material, the pattern including a plurality of strips (6) in lateral spaced disposition traversing the heater (1) and each having a width considerably greater than its thickness, the strips (6) each having a major face facing the base (3), the strips being formed with undulations providing alternating peaks (7) and troughs (8) and such that, for any strip, peaks of the undulations thereof are substantially laterally aligned with troughs of the undulations in an adjacent strip, the heating element (5) being arranged with peaks (7) of the undulations in the strip (6) supported on protrusions (4) provided on the base (3).
A heater according to claim 1, characterised in that the protrusions (4) are provided integrally with the base (3).
A heater according to claim 1 or 2, characterised in that the protrusions (4) are provided in rows.
A heater according to claim 3, characterised in that the protrusions (4) are arranged to maintain spacing between adjacent strips (6).
A heater according to any preceding claim, characterised in that the heating element (5) is supported with troughs (8) of the undulations in the strips (6) free from contact with the base (3).
A heater according to any preceding claim, characterised in that the strips (6) are arranged substantially parallel to one another.
A heater according to any preceding claim, characterised in that means (12, 13) are provided for securing opposite end regions of the strips (6) to prevent longitudinal movement of the strips.
A heater according to claim 7, characterised in that securing of the opposite end regions of the strips (6) is at a peripheral region of the base (3).
A heater according to claim 7 or 8, characterised in that the means for securing comprise clamping or hooking means (12), or staples or pins (13).
A heater according to any preceding claim, characterised in that a peripheral wall (12) of insulation material is provided, the peripheral wall preferably overlying opposite end regions of the strips (6).
A method of manufacturing a radiant electric heater characterised by the steps of: providing a base (3) of thermal and electrical insulation material; providing a heating element (5) in the form of a geometrical pattern separated from a metal foil and overlying the base (3) of thermal and electrical insulation material, the pattern including a plurality of strips (6) in lateral spaced disposition traversing the heater (1) and each having a width considerably greater than its thickness, the strips (6) each having a major face facing the base (3); forming the strips (6) with undulations providing alternating peaks (7) and troughs (8) and such that, for any strip, peaks of the undulations thereof are substantially laterally aligned with troughs of the undulations in an adjacent strip; providing protrusions (4) on the base (3); and arranging the heating element (5) with peaks (7) of the undulations in the strips (6) supported on the protrusions (4).
A method according to claim 11, characterised by the further step of temporarily locating a heat-resistant electrically insulating member securely against the supported heating element (5) and electrically energising the heating element for a predetermined period of time.
A method according to claim 11 or 12, characterised in that the protrusions (4) are provided integrally with the base (3).
A method according to claim 11, 12, or 13, characterised in that the protrusions (4) are provided in rows.
A method according to claim 14, characterised in that the protrusions (4) are arranged to maintain spacing between adjacent strips (6).
A method according to any of claims 11 to 15, characterised in that the heating element (5) is supported with troughs (8) of the undulations in the strips (6) free from contact with the base (3).
A method according to any of claims 11 to 16, characterised in that the strips (6) are arranged substantially parallel to one another.
A method according to any of claims 11 to 17, characterised in that means (12, 13) are provided for securing opposite end regions of the strips (6) to prevent longitudinal movement of the strips, the securing means being provided, for example, at a peripheral region of the base (3).
A method according to claim 18, characterised in that the means for securing comprise clamping or hooking means (12), or staples or pins (13).
A method according to any of claims 11 to 19, characterised in that a peripheral wall (12) of insulation material is provided, the peripheral wall preferably overlying opposite end regions of the strips (6).