GB2335833A - Support for coiled wire heating element - Google Patents

Abstract

A heating unit comprises a coiled wire heating element 4 supported on a substrate 1, the substrate having at least one recess 10 formed in a surface 8 thereof characterised in that the heating element 4 and the substrate 1 are arranged such that a section of the heating element 4 is supported at two points C, D by the substrate 1 and bridges the recess 10.

Description

1 A Heating Unit 2335833 The present invention relates to a heating unit

and more particularly, although not exclusively, to a heating unit for an electric hob.

Electric hobs of the type having glass-ceramic heating surfaces generally utilise a coiled wire or ribbon heating element mounted on and supported by a substrate in a heating unit to provide cooking heat. Commonly used to form this substrate are compacted microporous thermally insulating materials which can be shaped by pressing in a mould.

In electric hob heating units, such material is often pressed into a shallow open-cylinder shaped metal container which acts as a mould as well as comprising a canister for the heating unit assembly. This method also allows for a surface of the substrate nearest the open end of the canister to have press-mounted thereon a groove, in which the heating element can be subsequently located and stapled in place.

The presence of such a groove limits movement of the heating element which can occur due to thermal expansion of the element when electrical power is applied thereto. It is known for this groove to have a cross-sectional profile which assumes generally an arc of a circle having a diameter greater than the cold cross-sectional diameter of the heating element so that the element can both move and increase in diameter when electrical power is applied without leaving the groove.

As the heating element ages with use, sections thereof between stapling, points can distort W C 2 to the extent that contact between adjacent sections of heating element occurs resulting in a short-circuit. Short-circuits and the consequent increase in current usually result in the heating element overheating and failing. The groove, in tending to constrain the heating element to its original layout, reduces the risk of short-circuits occurring.

In accordance with a first aspect of the present invention, a heating unit comprising a coiled wire heating element supported on a substrate, the substrate having at least one recess formed in a surface thereof, is characterised in that the heating element and the subs are arranged such that a section of the heating element is supported at two points by the substrate and bridges the recess therebetween.

This permits air to flow over or against a surface of the bridging section of the heating element nearest to the recess.

Preferably the recess comprises a first groove which, more preferably, is lengthways contiguous to the heating element. These features allow the flow of air beneath the heatina element.

c Furthermore, the heating element may be supported by andlor bedded into lips of the first groove.

The heating element preferably has a cross-sectional diameter between two and ten times the width of the first aroove. which may also have a profile which is approximately 3 arcuate.

A preferred heating unit further comprises a second groove of larger dimensions than the heating element abutting the first groove from above. This feature allows heat radiation from the heating element to be reflected substantially upwards from the substrate surface.

The second groove may have a profile which is substantially arcuate in part, the diameter of the arc being larger than a cross-sectional diameter of the heating element, and may also connect with a primary plane of the upper surface by a rounded profile.

The reduced transfer of heat energy from the heating element to the substrate which can be achieved by the use of the present invention results in more of the thermal energy provided by the heating element being transferred to air in the heating unit, and can be equated to improved heater performance if all other factors remain constant.

It has been found also that, because the heating element can be better ventilated and the substrate can be generally cooler, the temperature of the heating element can be reduced, for the same heating power output, and its life thus increased.

In accordance with a second aspect of the present invention, a hob includes a heating unit in accordance with any of the first aspects above.

An embodiment of the present invention will now be described with reference to the 4 accompanying drawings of which:

Figure 1 shows a plan view of a heating unit in accordance with the present invention suitable for use in an electric hob; and Figure 2 shows in cross-section a composite groove of a substrate having a heating element supported therein forming part of the Figure 1 heating unit.

Referring firstly to Figure 1, a substrate 1 is located in the bottom of a metal canister 2 of the heating unit 3. A coiled single-strand wire heating element 4, mounted on and supported by the substrate, 1, is located lengthwise contiguous with a long recess or groove (shown in Figure 2, described below) which has been press-moulded on the upper surface of the substrate 1. The heating element 4 is thermally insulated from the metal canister 2 by the substrate 1 and an insulating annular wall 5. The heating element 4 is affixed to the substrate 1 by a number of distributed staples 6. The heating element 4 is electrically connected to a connection block 7, to allow easy connection of an electrical power supply (not shown) to the heating element 4 upon installation of the heating unlit 3 in a hob (not shown).

Referring now to Figure 2, the generally planar substrate 1 is shown with an upper surface 8 having forrned thereon the composite groove 9 comprising a first groove or channel 10 having a profile 11 and a second groove or channel 12 having a profile 13, 14. The profile C 11 of the first -roove 10 forms approximately the arc of a semi-circle having a diameter L- t of 3 mm. The intersections of the first groove 10 with the second groove 12 form first and second lips, shown in cross-section as C and D. The lips C and D form the points of contact between the substrate 1 and the heating element 4, which has a cross-sectional diameter of 9 mm. The profile of the second groove 12 has two main geometric components indicated at 13 and 14 respectively. Firstly, sections 13 between the lip C and a point E and the lip D and a point F form arcs of a circle having a diameter of 10 nim; and, secondly, sections 14 between the points A and E and B and F form an arcuate rounding of the profile of the intersections between the planar surface of the substrate and the arcuate sections 13.

The first groove 10 allows the circulation of air from beneath the heating element 4, and thus allows improved cooling thereof in comparison with heating elements which are mounted in a substrate groove of the type described above as prior art. It should be noted that the wire of the heating element 4 may "bed into" the lips C and D, especially when first heated. This would increase the contact area between the substrate 1 and the heating element 4 and thereby increase heat conduction therebetween. However, bedding in can be advantageous in that it may provide support for the heating element 4, especially with regard to lessening the extent of, or substantially preventing, creeping and subsequent bunching of the heating element 4.

The sections of the second groove 12 having the profile 13 have the primary function of reflecting heat radiated by the heating element 4 upward from the substrate surface 8, although the resultinggroove will also assist positioning of the heating element onto the 6 lips C and D during heating unit assembly. The sections 14 are present primarily for ease of substrate press-moulding, although the surface provided by the profile 14 will also reflect heat radiation upward from the substrate surface 8.

Although the embodiment has been described with reference to a singlestrand wire heating element, it will be appreciated that the invention is applicable also to coiled multi stranded wire heating elements, to coiled ribbon heating elements, which can be formed t> by flattening a single-strand wire, and the like. Heating units in accordance with the present invention may find utility in areas other than hobs, such as grills, domestic electric heaters and industrial heaters.

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Claims (1)

1. Claims

   1.

   3.

   4.

   5.

   6.

   8.

   2.

   A heating unit comprising a coiled wire heating element supported on a substrate, the substrate having at least one recess formed in a surface thereof characterised m that the heating element and the substrate are arranged such that a section of the heating element is supported at two points by the substrate and bridges the recess therebetween.

   A heating unit in accordance with Claim 1 in which the recess comprises a first groove.

   A heating unit in accordance with Claim 1 or Claim 2 in which the first groove is lengthways contiguous to the heating element.

   A heating unit in accordance with Claim 2 or Claim 3 in which the heating element is supported by lips of the first groove.

   A heating unit in accordance with any of Claims 2 to 4 in which the heating element is bedded into lips of the first groove.

   A heating unit in accordance with any of Claims 2 to 5 in which the heating element has a cross-sectional diameter between two and ten times the width of the first groove.

   7. A heating unit in accordance with any of Claims 2 to 6 in which the first groove has a profile which is approximately arcuate. A heating unit in accordance with any of Claims 2 to 7 further comprising a second groove of larger dimensions than the heating element abutting the first groove from above.

   9. A heating unit in accordance with Claim 8 and one of Claims 4 and 5 in which said lips comprise the connections of the first groove and the second groove.

   10. A heating unit in accordance with Claim 8 or Claim 9 in which the second groove has a profile which is substantially arcuate in part, the diameter of the arc being larger than a cross-sectional diameter of the heating element.

   1.A heating unit in accordance with any of Claims 8 to 10 in which the second groove connects with a primary plane of the upper surface by a rounded profile.

   12. A heating unit having on a substrate surface thereof a groove with a profile substantially as herein described with reference to or as shown in Figure 2 of the accompanying drawings. A hob including a heating unit in accordance with any preceding Claim.

   1 13.