Electric food warmer.

The present invention relates to an electric heating device. While the invention is primarily directed to an electric food warmer, especially in the form of a tray, it has applications in other areas e.g. in the construction of wall mounted space heaters or radiators.

Electric food warmers in the form of trays in which the upper heating surface is defined by a glass plate having a plain front surface and an electrical heating element in the form of a patterned layer attached to the rear surface, have been known for many years. Such a food warmer is described e.g. in U.S. Patents 2,563,875 and 2,976,386. A covered tray is described in U.S. Patent 3,098,924. The heating element may be in the form of a conductive frit which is fused onto the surface of the plate.

It is normally necessary to shield the rear surface of the glass plate both to protect the conductive layer and to prevent radiation from the heating element spoiling surfaces near which the plate is placed, such as a table top or, a case of a wall heater, the wall behind. Conventionally the glass plate is mounted in a structure including a surrounding frame and a metal plate spaced from the rear surface of the glass plate.

The conventional mounting arrangement is seriously limiting as regards the production of attactive designs to which the glass heating panel lends itself. Also the known mounting arrangements are relatively inefficient. There is heat loss through the metal shielding means which can endanger adjacent sufaces e.g. of furniture and often necessitate a comparatively thick encasing structure or the provision of legs of undue length. Furthermore the heat efficiency is reduced by the heat loss.

The invention is directed to the provision of a structure for an electric heating device of the glass panel type which lends itself to greatly improved design possibilities without sacrifice of safety factors or efficiency and in which these factors are preferably improved upon.

According to the invention there is provided an electric heating device comprising a first glass plate having a front surface and an electrical heating element in the form of a patterned layer attached to the rear surface, and a mounting structure including electrical connecting means for operating the element and heat shielding means extending over the rear surface of the glass plate, characterised in that the heat shielding means comprises a second glass plate generally co- extensive with and spaced from the first glass plate.

Preferably the intervening space between the glass plates is sealed.

By utilising a second glass plate in accordance with the invention it has been found that not only can an extremely attractive appearance be produced by e.g. making the device transparent over the whole surface, thus enabling attractive features of furniture to be revealed, but the safety factor and efficiency can be positively improved.

An embodiment of the invention is hereafter described with reference to the accompanying drawings in which: <ul id="ul0001" list-style="none"><li>Figure 1 is a perspective view of a food warming tray in accordance with the invention,</li><li>Figure 2 is a plan view, partly cut away, of the right hand side of the tray of Figure 1,</li><li>Figure 3 is a longitudinal sectional view of the right-hand side portion of the tray of Figure 1,</li><li>Figure 4 is a transverse sectional view corresponding with Figure 3, and</li><li>Figure 5 is a sectional view corresponding generally to part of Figure 4 showing an alternative means of sealing the side edges of the plates.</li></ul>

In the drawings there is shown a food warming tray generally indicated at 10 comprising a first (upper) glass plate 11 having a plain upper or front surface and an electrical heating element 12 in the form of a patterned layer attached to the rear surface. Since the plate 11 is transparent, the patterned heating element is visible and is arranged in an attractive pattern with an extensive rectangular generally lower temperature area 12a and a generally higher temperature area 12b in a circular arrangement. The heating element may be conventionally formed by fusing a conductive glass frit on the rear surface of the plate 11.

The device includes a mounting structure in the form of end pieces 13,14 each being of two-component plastics mouldings bolted together and housing an electrical connecting means for the heating element terminating in a pronged connector 15.

A second (lower) glass plate 16 is positioned by the mounting mens so as to be generally coextensive with and spaced from the first glass plate 11. The intervening space between the glass plates is sealed.

The sealing is accomplished at the longitudinal side edges by the interposition between the plates of a compound sealing strip. As shown more particularly in Figure 4 a first resiliently compressible sealing strip 17 has a tongue portion 18 frictionally engaged between plates 11,16 and flange portions 19 extending against the adjacent edge surfaces of the plates. The tongue portion 18 is hollow, having a cavity 22 extending longitudinally and exposed between the flanges 19. A rigid strip 20 has a tongue portion 21 engaged within the cavity 22 of the compressible strip. The tongue portion 18 of the compressible strip is provided with longitudinally extending barbed projections 23 on its outer surfaces. When the strip is inserted in place the barbed projections 23 are bent over towards the flanges 19 thus presenting an extensive sealing surface against the interior surfaces of the glass plates and sealing the interior space against exchange of heated air and the possible ingress of water e.g. during household cleaning operations.

The rigid strip 20 has a continuous flange 24 which generally covers the flanges 19, the flanges 19 and 24 being chamfered to provide a pleasing appearance. The inner surface of at least the upper of the flanges 19 may be partly cut-away towards the tongue so as to ensure a continuous seal at its upper edge 25.

It will be apparent from Figure 4 that the flanges 19 and 24 do not extend as far as the edges of the plates. This gives a positive advantage that the edge of the upper glass plate is exposed to allow easy cleaning and to avoid the entrapment of spilt liquid.

The end pieces 13 and 14 are in mirror image relationship, end piece 13 enclosing the electrical connections, and only this end piece will be described in detail with reference to Figures 3 and 4. As shown, the end piece 13 is formed as separate upper and lower mouldings respectively 13a and 13b which have complementary moulded formations 26a,26b for receiving three screws 27, attached by means of threaded washers 28.

The narrow edges of the pair of plates 11,16 are each enclosed in a steel generally U-sectioned extrusion 29 which may have an interior lining 30 of softer material. The compressible flange 17 terminates at the edges of the plates but the rigid flange 20 extends within the hollow interior of the moulding 13 or 14 as shown in Figure 2 and two of the screws 27 project through apertures in the tongue portion 21. When the glass sheets are fitted in place within the mould halves and the mould halves screwed together, the plates are squeezed together compressing the projections 23 of the compressible strip 17.

The electrical connections are generally conventional. As shown in Figure 2 a plate 31 provides connections with the ends of the heating element which are connected within the hollow interior of the end piece 13 to the prongs 15 which extend within a cavity 32 formed within the end piece 13.

For a medium sized tray of overall length about 60cm, the element may suitably be of 150 watts, generating a surface temperature of 120°C. Since the element will normally have a positive temperature coefficient the wattage given is based upon the hot current. This compares very favourably with a similar tray of the type having a metal casing and metal bottom shielding where to achieve the same surface temperature a power rating of 180 watts (hot current) is required.

The compressible strip 17 may be formed of any suitable plastics material such as PVC. It is not necessary for the strip to be specially heat resistant since lateral conduction of heat through the glass is poor. The rigid strip 20 is conveniently of aluminium. The end pieces 13,14 may be of any suitable moulded plastics material. Preferably both plates are of tempered glass with polished edges but glass of lower specification or heat resistant glass may be utilised where appropriate for the upper or lower plate. The upper glass plate is preferably of 6mm thickness and the lower plate of 4mm thickness. The spacing is e.g. 5mm.

Although not shown, further sealing strips may be inserted between the plates 11,16 extending along the lateral edges within the channel piece 29. A switch, fuse and indicator light may be incorporated in the cavity 32.

Figure 5 shows an alternative and more preferable method of spacing and sealing the longitudinal side edges of the plates at each side. The aluminium spacer at 21 of Figure 4 is modified as shown at 40. The tongue 41 is formed with a longitudinally extending web 42 having as shown in cross-section spaced limbs 43, 44 projecting from each side so as to abut and space respective plates 11, 16. Flanges 47 projecting on each side of the tongue 41 have oblique portions 48 abutting respective plates.

Within the spaces 45 on each side of the web 42 is located a bead 46 of adhesive. When applied, the bead of adhesive, which may be of about 2mm diameter, stands proud of the outer edges of the limbs 43, 44 and is compressed to the thickness or depth of the spaces 45, which may be e.g. 1.5mm, when the plates 11, 16 are positioned. The adhesive is preferably a silicone- cellastic rubber adhesive which cures by absorption of humidity and remains resilient while adhering to the glass and the aluminium tongue. Suitable forms of adhesive are: (Trade Mark) EVODE general purpose adhesive, Silicon Products adhesive RTV 732 and Engineering Polymers adhesive RTV 702

In the tray shown, both plates are transparent and a completely novel appearance is produced. Features of the table surface underneath can be viewed through the food warmer which is generally less obtrusive and blends in with the surroundings. The construction is also very much simplified with two end mouldings which serve to mount the plates and provide leg formations 33. If it desired to increase the efficiency, the lower plate 16 may be rendered upwardly reflective e.g. by silvering its bottom surface, or heat reflective glass may be employed. This greatly reduces the loss of heat downwardly by radiation and increases the heat available for warming food.

It is also possible to provide the spaced plates as a completely sealed, bonded and at least partly evacuated unit which reduces the heat loss downwards very considerably. Silvering may again be employed. This feature may be employed with advantage where higher temperatures are employed e.g. in portable cooking units.

While the invention has been described generally with reference to a food warmer, the invention has notable application in the construction of wall mounted space heaters or radiators, since the heating element can be made very unobtrusive as indeed can the surrounding mounting so that if transparent plates are used there is very little apparent obstruction of the wall surface behind.