GB2137188A - Ceramic Hob - Google Patents

Abstract

A ceramic support hob for a wire wound resistive electric heating element comprises a ceramic blanket impregnated with an inorganic binder, channels for holding the heating element being impressed in the blanket. The support is produced by impregnating a ceramic blanket with binder solution (suitably colloidal silica), moulding channels in the wet impregnated blanket and subsequently firing the moulded blanket. The blanket is preferably thin in comparison with the depth of the channels. The support can be laid on a thin layer of thermal insulation such as agglomerated amorphous silica in a slimline hob.

Description

SPECIFICATION Electric Heater The present invention relates to supports for electric heaters of the wire-wound resistive type and especially to ceramic hob units which are used in electric cookers and the like.

Ceramic hob units and other electric heater units commonly employ helical open wound wire heating elements supported within channels in a ceramic support. The heating element is generally stapled to the ceramic support and the support accommodated in a steel casing. A soft ceramic blanket is interposed between the ceramic support and the steel casing in order to prevent the ends of the staples from touching the latter.

Hitherto, the ceramic support has generally been formed with the appropriate channels for the heating element by vacuum moulding a slurry of ceramic particles which is subsequently dried and hardened. Such a process is relatively expensive, and requires a relatively large amount of low thermal conductivity material in order to keep the temperature of the steel casing reasonably low in use. The thermal conductivity of the ceramic support must be particularly low in modern "slimline" hobs which are only about 20 mm thick. Relatively sophisticated ceramic materials have therefore been used, but in such supports the ceramic material lying between the channels does not form a thermal barrier between the heating element and the casing and is therefore effectively wasted.

According to one aspect of the present invention, an electric heater comprises a helical open wound wire heating element supported in a channel in a ceramic blanket impregnated with an inorganic binder. The binder is suitably silica.

Preferably the thickness of the support is not more than three times the depth of the channel.

We have found that a ceramic blanket impregnated with inorganic binder may be moulded when wet by an ordinary press and that no special precautions are needed when handling the wet moulded blanket before it is fired.

Although the moulded support has a higher thermal conductivity than amorphous silica materials, this is not important provided the support is not much thicker than the depth of the channels.

According to another aspect of the invention, a method of making ceramic support for a heating element comprises the steps of impregnating a soft ceramic blanket with inorganic binder, impressing a ridged moulding tool on the impregnated blanket to form a channel and subsequently firing the moulded impregnated blanket. The inorganic binder is preferably colloidal silica.

The invention will now be described by way of example with reference to Figures 1 to 3 of the accompanying drawings, of which: Figure 1 is a plan view of a radiant ceramic hob in accordance with the invention; Figure 2 is a section taken on Il-Il of Figure 1: and Figure 3 is a schematic section of a solid ceramic hob in accordance with the invention.

The hob units illustrated both incorporate a ceramic heater element support 1 which is made by a process in accordance with the invention as follows: i) A soft blanket of ceramic material of uniform thickness (approximately 10 mm) and having a density of approximately 100 kg/m3 is supported on a gauze beneath a fly press or power press.

The blanket is impregnated with a colloidal solution of silica for approximately 5 seconds (for example Syton X30 as supplied by Monsanto diluted with water in a volume ratio of 5 of Syton X30:2 of water has been found to be satisfactory) and a generally disc-shaped press tool provided with an appropriately shaped continuous raised projection is lowered onto the blanket to form a corresponding channel of the appropriate depth for the heating element. The press tool may be perforated to allow colloidal silica to escape upwards through the ceramic blanket and ensure even impregnation. The thickness of the compressed impregnated blanket is suitably 7 to 8 mm. The blanket is held in position during this process by a clamping ring. The press tool may be of aluminium, high density plastic or any other non-ferrous material. Excess colloidal silica is collected and subsequently re-used.It is important to keep the colloidal silica out of contact with ferrous materials since these tend to cause precipitation of the silica.

ii) The wet pressing is transferred to an infra red oven for 30 minutes to drive off the bulk of the moisture and then heated at 700 C to complete the drying process. The electrical insulation resistance is then measured as a safety check.

A helical open wound wire heating element 2 is stapled into the channel 4 in the support by staples 3, as shown in Figures 1 and 2. The hob shown in Figure 4 is provided with an identical heater element/ceramic support assembly, but differs in other respects.

Referring first to Figures 1 and 2, face F of the radiant hob shown is adapted to be fitted underneath a glass cooking surface (not shown).

The ceramic support 1 is held within a steel casing 5 and is supported by a base layer of agglomerated amorphorus silica 6. Support 1 and layer 6 are both approximately 7 to 8 mm thick.

Agglomerated amorphous silica has a much lower thermal conductivity than conventional ceramics.

The support 1 is held against the layer 6 by a ceramic ring 7 which is clipped to the casing by clips 10 (Figure 1) secured by selftapping screws 11. A temperature probe 8 of conventional form is supported diametrically by holes in ring 7 and is attached to a conventional thermal cut-out assembly 12, which is itself supported from casing 5 by a bracket 9. Insulated current-supply terminals 13 and 14 are mounted on the casing and are electrically connected to the ends of the heating element.

The solid hob shown in Figure 3 comprises a cast iron shell which accommodates a ceramic support 1 and element 2 between two ceramic locating rings 4 and 5. The shell is tapped internally to receive a clamping bolt 6 which holds a plate 7 against the lower ceramic ring 4.

Thermal contact between the shell 3 and element 2 is provided by a layer of cement 8.

Claims (13)

1. A method of making a ceramic support for a heating element comprising the steps of impregnating a soft ceramic blanket with inorganic binder, impressing a ridged moulding tool on the impregnated blanket to form a channel and subsequently firing the moulded impregnated blanket.

2. A method as claimed in Claim 1, wherein said inorganic binder is colloidal silica.

3. A method of making a heating assembly incorporating a ceramic support wherein the ceramic support is made by a method as claimed in Claim 1 or Claim 2 and wherein a helical wire heating element is fixed within said channel.

4. A method as claimed in Claim 1 substantially as described hereinabove.

5. A ceramic hob incorporating a ceramic heating element support made by a method as claimed in any preceding claim.

6. An electric heater comprising a helical open wound wire heating element supported in a channel in a ceramic blanket impregnated with an inorganic binder.

7. An electric heater as claimed in Claim 6 wherein the binder consists essentially of colloidal silica.

8. An electric heater as claimed in Claim 6 or Claim 7 wherein the thickness of the support is not more than three times the thickness of the channel.

9. A ceramic hob incorporating an electric heater as claimed in any of Claims 6, 7 or 8.

10. A ceramic hob as claimed in Claim 9 wherein the impregnated ceramic blanket is supported on a layer of material having a lower thermal conductivity than said ceramic blanket.

11. A ceramic hob as claimed in Claim 10 wherein said material is agglomerated amorphous silica.

12. A ceramic hob substantially as described hereinabove with reference to Figures 1 and 2 of the accompanying drawings.

13. A ceramic hob substantially as described hereinabove with reference to Figure 3 of the accompanying drawings.