GB2323922A

GB2323922A - A heating unit for a hob

Info

Publication number: GB2323922A
Application number: GB9706716A
Authority: GB
Inventors: Maurice Hugh Carter
Original assignee: Redring Electric Ltd
Current assignee: Redring Electric Ltd
Priority date: 1997-04-02
Filing date: 1997-04-02
Publication date: 1998-10-07
Anticipated expiration: 2017-04-02
Also published as: GB2351553B; GB9706716D0; GB2351553A; GB2323922B; GB0017826D0

Abstract

A drop-in heating unit for a hob is formed from a metal can 14, a substrate 15, a resilient annular spacer 16, a radiant heating element 17 and a moulded glass-ceramic insert 1. An overtemperature thermal limiter 18 acts to prevent the unit becoming overheated. A metal compression bad 19 extends around a downwardly protruding skirt 8 of the insert 1, increasing its resistance to shock. Glass-ceramic insert 1 has reinforcing radial spars 2-7 and a profile which generally has a greater thickness of glass towards its periphery and a generally lesser thickness towards its centre, the transition being provided by a curved profile. Such shaping of the insert improves both its transmissive and its conductive properties without a reduction in strength.

Description

A Heating Unit for a Hob The present invention relates to a heating unit for a hob and more particularly to a heating unit having a glass-ceramic insert.

"Drop-in" heating units for hobs having glass-ceramic cooking surfaces and radiant heating elements are known for example from WO 94/24490. This document describes a hob having a circular aperture into which a moulded ceramic-glass insert is located, the hob also having a tungsten halogen or inductive heating source located beneath the insert.

The applicant has found that, although the arrangement described in the above mentioned document works well, improvements could be made to the ceramic-glass insert, especially with regard to the volume of glass-ceramic used, and hence cost per insert, and to the thermal conduction properties of the insert.

In accordance with a first aspect of the present invention, there is provided a heating unit for a hob comprising a container, a radiant heating element positioned within the container, and a moulded glass-ceramic insert having a substantially planar upper surface and a lower surface shaped to reinforce the insert against damage from vertically applied forces.

Advantageously, the said lower surface is shaped so as to have a generally greater thickness of glass towards its periphery and a generally lesser thickness towards its centre wherein the transition from a greater to a lesser thickness is provided by a curved profile. By providing the insert with such a cross section, the central portion, through which portion it is desired to transmit the majority of heat, can be thinner than would be possible employing an insert of uniform thickness. It is not important that the insert has a thickness of glass greater than that at the centre of the insert for the entire circumference of its periphery so long as there is generally a greater thickness of glass on its periphery relative to the centre of the insert.

Preferably the lower surface is shaped to form a plurality of radial reinforcing spars, which greatly increase the resistance to shattering of the insert on impact. The spars preferably meet at a central node. Such features have been found to also be aesthetically pleasing, which is an important factor in the design of a hob.

In accordance with a second aspect of the present invention, there is provided a heating unit for a hob comprising a container, a radiant heating element positioned within the container, a moulded glass-ceramic insert having a substantially planar upper surface and a lower surface having a downwardly protruding skirt formed thereon, and a metal band held in tension by such as to act to compress the skirt and thereby reinforce the insert against damage from vertically applied forces.

Employing the present invention, the thickness of the glass-ceramic can be reduced which reduces the quantity of material required to produce the insert. More importantly, the reduced thickness considerably improves the thermal transmission properties of the glass, for it has been found that in addition to the attenuation of radiant transmission being reduced, conduction through the glass is greatly increased and this contributes significantly to the efficiency of the heating unit.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 shows in inverted plan view a glass-ceramic insert of a heating unit in accordance with the present invention, Figure 2 shows section B-B of the glass-ceramic insert of Figure 1, Figure 3 shows section A-A of the glass-ceramic insert of Figure 1, Figure 4 shows a plan view of a heating unit in accordance with the present invention, and Figure 5 shows section A-A of the heating unit of Figure 4.

Referring now to Figures 1, 2 and 3, a glass ceramic insert 1 of a heating unit in accordance with the present invention is shown comprising a central node 2, reinforcing spars 3-7, a skirt 8, a flange 9 and lugs 10-12.

Insert 1 has a substantially planar upper surface, which constitutes a cooking surface. Flange 9 extends around the entire circumference of the insert 1 as does skirt 8. The lowermost surfaces of reinforcing spars 3-7 and central node 2 lie in a plane substantially parallel to that of the cooking surface. The spars 3-7 provide insert 1 with a substantially stronger structure than would be achieved by a uniform thickness insert having the same volume of glass. Thus, a glass-ceramic insert having the same strength as that of the insert of WO 94/24490 can be formed from less glass with the resultant reduction in weight and cost of manufacture.

Insert 1 is formed by pressing in a mould from a material such as a lithiumalumino-silicate. The areas 13 bordered by skirt 8 and the reinforcing spars 3-7 on the upper face of insert 9 have a surface pattern moulded thereon to form a lens. This pattern comprises a series of concentric circles, originating from the geometric centre of insert 1, and has a substantially sinusoidal profile. Such patterns are sometimes referred to as Fresnel lenses, although the pattern is not a true Fresnel lens as it is not optically precise and does not have a focal point. The lens formed by this pattern has the effect of preventing objects being seen clearly through the substantially transparent insert 1 and also acts to evenly disperse energy radiated through the insert.

Reinforcing of insert 1 is also provided by forming the lower surface of the insert with a relatively large radius curved surface abating skirt 8, as can be seen in Figure 2.

Referring now to Figures 4 and 5, a heating unit in accordance with the present invention is shown comprising an insert 1, a metal can 14, a planar substrate 15, an annular spacer 16, a radiant heating element 17, a conventional overtemperature thermal limiter 18 and a compression band 19.

Substrate 15 is formed in the base of the metal can 14, the substrate 15 preferably having a continuous indentation formed on its uppermost surface for containing the coiled heating element 17, stapled to the substrate 15 by staples 20.

The resilient annular spacer 16 separates substrate 15 from insert 1, the spacer 16 having a hole formed therein to allow the overtemperature thermal limiter 18 to extend into the space above heating element 17.

On assembly, the insert is pushed into the can 14 whereupon the lugs 10-12 latchingly engage with holes (not shown) in the sidewall of can 14. These holes preferably allow some downward movement of insert 1 in can 14 so that the resilience of spacer 16 can be used to provide an amount of shock absorbing movement between the can 14 and the insert 1. The compliance of the spacer also acts to maintain the insert 1 fixed relative to the can 14 by forcing the lugs 10-12 into contact with the uppermost edge of the holes in the can 14.

The metal band 19 is located on the outside surface of can 14 and forces can 14 against skirt 8 of insert 1. The metal band is held in tension by any suitable clamping arrangement, in turn keeping the insert in compression which greatly increases its resistance to breakage on impact. The metal band may be of the "gaiter clamp" type.

The hob element is secured, in use, to a sheet metal cooker top or the like (not shown) by a seal (not shown) formed from a mixture of alumina/glass fibre and silicone sealant. This seal is located between the upper surface of the cooker top, which encircles can 14 or skirt 8, and the lowermost surface of flange 9.

Claims

1. A heating unit for a hob comprising a container, a radiant heating element positioned within the container, and a moulded glass-ceramic insert having a substantially planar upper surface and a lower surface shaped to reinforce the insert against damage from vertically applied forces.

2. A heating unit in accordance with Claim 1 in which said lower surface is shaped so as to have a generally greater thickness of glass towards its periphery and a generally lesser thickness towards its centre, wherein the transition from a greater to a lesser thickness is provided by a curved profile.

3. A heating unit in accordance with either preceding claim in which said lower surface is shaped to form a plurality of radial reinforcing spars.

4. A heating unit in accordance with Claim 3 in which the reinforcing spars each extend from a central node toward the periphery of the insert.

5. A heating unit in accordance with any preceding claim in which the insert is formed with a skirt on its lower surface, the outermost dimensions of the skirt being such as to fit the innermost dimensions of the container.

6. A heating unit in accordance with claim 5 wherein the insert comprises a plurality of protrusions extending from the skirt which protrusions latch in corresponding engagement portions formed in the container when the insert is inserted into the container.

7. A heating unit in accordance with any preceding claim in which the heating element is mounted on a substrate at the bottom of the container.

8. A heating unit in accordance with Claim 7 further comprising an annular spacer positioned between the substrate and the insert, and adjacent to the container.

9. A heating unit in accordance with Claim 8 in which shock absorbing movement between the insert and the substrate is allowed by virtue of the resilience of the spacer.

10. A heating unit as claimed in any preceding claim further comprising a metal band extending around a downwardly protruding skirt of the insert, which band is in tension.

11. An insert for a heating unit as claimed in any preceding claim.

12. A heating unit for a hob comprising a container, a radiant heating element positioned within the container, a moulded glass-ceramic insert having a substantially planar upper surface and a lower surface having a downwardly protruding skirt formed thereon, and a metal band held in tension by such as to act to compress the skirt and thereby reinforce the insert against damage from vertically applied forces.

13. A heating unit substantially as shown in or described with reference to Figures 4 and 5 of the accompanying drawings.