RADIANT HEATERS

Abstract: In the radiant heater for an electric appliance such as a cooker, the heating element is secured by glue on stakes (8) automatically inserted into and held by friction in a compacted layer (4) of microporous thermal insulation.

Description: The present invention relates to a radiant heater for an electric cooker or other electric appliance. A radiant heater for an electric cooker comprises in general a base layer of thermal insulation material typically having an upstanding peripheral wall, and a heating element in the form of a coil of bare resistance wire secured to the base layer of thermal insulation material. It is known to secure the heating element in position on the base layer by means of refractory glue. It is also known to secure the heating element in position on the base layer by means of staples as described in DE-A-2 806 367. However, although staples are effective where the base layer is a compacted particulate material capable of exerting sufficient frictional force to prevent easy withdrawal of the staples, staples are not sufficiently firmly held where the base layer is a fibrous material. In addition, the use of staples renders difficult any automation of the securing of the heating element to the base layer because it is necessary to apply a stapling gun between adjacent turns of the heating element so as to avoid crushing the heating element and to locate accurately that portion of the heating element which bears on the base layer. In practice, the pitch of the turns of the heating element varies along its length which makes it impossible to predict the precise location of the turns for applying the stapling gun between the turns without human assistance. On the other hand, the use of a refractory glue is effective where the base layer is a fibrous material and also facilitates automation of the securing of the heating element because glue can be applied accurately to the base layer. However, refractory glues when used with base layers in the form of compacted particulate material can interfere with the cohesive strength of the material so as to lead to an unreliable joint between the heating element and the base layer. It is an object of the present invention to provide a radiant heater for an electric appliance in which a heating element can be secured to a base layer of compacted particulate material by means of an adhesive material such as glue. According to one aspect of the present invention there is provided a radiant heater for an electric appliance comprising a dish, a layer of compacted particulate electrical and thermal insulation material disposed within the dish, a bare wire heating element disposed within the dish, and means for securing the heating element to the layer of insulation material wherein the securing means comprises a plurality of stakes disposed in the layer in a predetermined configuration, each stake having a portion accessible from outside the layer, and adhesive material disposed on each accessible portion such that the bare wire heating element is at least partly immersed in the adhesive material. Preferably, the adhesive material comprises a refractory glue. The stakes may be inserted into the layer and may extend out of the layer. The stakes may be made of metal or of refractory material. According to another aspect of the present invention there is provided a method of manufacturing a radiant heater for an electric appliance comprising the steps of compacting particulate electrical and thermal insulating material to form a layer and securing a bare wire heating element to the layer and including the additional steps of disposing a plurality of stakes in the layer in a predetermined configuration, each stake having a portion accessible from outside the layer, and securing the heating element to each stake by means of adhesive material disposed on each accessible portion such that the bare wire heating element is at least partly immersed in the adhesive material. Radiant heaters in accordance with this invention for an electric appliance will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic cross-sectional view of a radiant heater; Figure 2 is a plan view of the heater of Figure 1; Figure 1 is an enlarged view of a portion of the heater of Figure 1; and Figures 4 to 8 illustrate various modifications to the heater shown in Figure 1. Figures 1, 2 and 3 show a radiant heater which comprises a metal dish 2 containing a base layer 4 of compacted particulate electrical and thermal insulation material. Preferably, the material forming the base layer 4 is a compacted microporous thermal insulation material which comprises silica aerogel or pyrogenic silica, an infra-red opacifier (such as titanium dioxide) and a reinforcing fibre (such as ceramic fibre). Such a material is marketed by Micropore International Limited under the Registered Trade Mark MICROTHERM. Positioned on the base layer 4 is a peripheral wall 6 of thermal insulation material such as ceramic fibre material. Set in grooves 7 (shown in dashed outline in Figure 2) formed in the base layer 4 are a number of stakes 8 which are pressed into the compacted particulate material such that the concealed ends of the stakes terminate short of the metal dish 2. The stakes may be inserted automatically, at a spacing of the order of one to two centimetres. The stakes may be made of a refractory ceramic such as alumina or steatite, or a metal such as steel. The stakes should have a low thermal capacity in order not to function as heat sinks. The stakes can be single legged or multi-legged, and may be barbed. Applied to the exposed head of each of the stakes 8 is a bead 10 of glue or other adhesive material. The glue may be applied by an automated dispenser (not shown) and may be any refractory glue which preferably sets at room temperature, for example water glass. A heating element 12, in the form of a coil of bare resistance wire 14, is located in the grooves 7 formed in the base layer 4 and is partly immersed in each of the beads 10 of glue. (The heating element has been omitted from Figure 2 for the sake of clarity). We have found that in such a radiant heater the heating element is firmly held in place against the base layer and does not become dislodged even when the heater is subjected to vibration. A radiant heater as shown in Figures 1 and 2 can be manufactured, for example, by pressing the dish 2 from a metal blank and spreading the particulate insulation material inside it. The insulation material is compacted in a press, the grooves 7 being formed by a suitably shaped moulding die. The peripheral wall 6 is similarly formed by moulding ceramic fibre. The stakes 8 are pressed into the layer 4 and a bead 10 of refractory glue applied to the exposed head of each stake 8. Then the heating element 12, which has preferably already been formed into its desired configuration, is placed in the grooves 7 so as to be partly immersed in each bead 10 of glue. The element 12 is retained in place until the glue has set sufficiently for the element 12 not to be dislodged. Various alternatives and modifications to the arrangement described above are possible. Depending on the strength of the adhesive material, it may not be necessary for the wire 14 to be completely immersed in the adhesive material, as shown in Figure 3; in this case the wire 14 may be partly immersed, or a layer of adhesive material sandwiched between the wire 14 and each stake 8 may be sufficient as shown in Figure 4. The stakes 8 need not extend out of the layer 4 of insulation material. Instead their upper ends may be flush with the surface of the layer 4, as shown in Figure 5, or even countersunk into the layer 4 as shown in Figure 6, the adhesive material flowing down the countersink to adhere to the top of the stakes 8. If the stakes 8 or the dish 2 are made of electrically non-conductive material, the stakes 8 may project from the dish 2. If the stakes 8 and the dish 2 are electrically non-conductive, the stakes 8 may be made integral with the dish 2 (for example by moulding) as shown in Figure 7. It is also possible to mould the layer 4 of insulation material alone, insert the stakes 8 from the rear of the layer 4 and then place this assembly into the dish 2. Figure 8 shows the possible use of a material 16 which does not adhere to the wire 14 but can nonetheless secure it to the stakes 8, by arranging for the material 16 to engage with the element 12 by extending in a loop encircling the wire 14 forming the element 12. The material 16 could likewise be arranged to en gage with the stakes 8, for example around an enlarged head portion provided on the stakes 8. Instead of applying the glue 10 to the stakes 8 before the element 12 is placed in position, the element 12 may be positioned and then the glue 10 may be applied over the juxtaposed stakes 8 and element 12. To facilitate this the stakes 8 may be made long enough to protrude above the element 12 when it is located on the layer 4. In this case also a material which does not adhere to the element 12 may be used.

Claims: A radiant heater for an electric appliance comprising a dish (2), a layer (4) of compacted particulate electrical and thermal insulation material disposed within the dish, a bare wire heating element (12) disposed within the dish, and means for securing the heating element to the layer of insulation material characterised in that the securing means comprises a plurality of stakes (8) disposed in the layer (4) in a predetermined configuration, each stake having a portion accessible from outside the layer, and adhesive material (10) disposed on each accessible portion such that the bare wire heating element (12) is at least partly immersed in the adhesive material. A radiant heater as claimed in claim 1, characterised in that the adhesive material (10) comprises a refractory glue. A radiant heater as claimed in claim 1 or 2, characterised in that the stakes (8) are inserted into the layer (4). A radiant heater as claimed in any preceding claim, characterised in that the stakes (8) extend out of the layer (4). A radiant heater as claimed in any preceding claim, characterised in that the stakes (8) are made of metal. A radiant heater as claimed in any one of claims 1 to 4, characterised in that the stakes (8) are made of refractory material. A method of manufacturing a radiant heater for an electric appliance comprising the steps of compacting particulate electrical and thermal insulating material to form a layer (4) and securing a bare wire heating element (12) to the layer (4) characterised by the additional steps of disposing a plurality of stakes (8) in the layer (4) in a predetermined configuration, each stake (8) having a portion accessible from outside the layer (4), and securing the heating element (12) to each stake (8) by means of adhesive material (10) disposed on each accessible portion such that the bare wire heating element (12) is at least partly immersed in the adhesive material.

Inventors: JACKSON, JAMES DAVID JOSEPH, MORGAN, DEREK EDWARD
Applicants: MICROPORE INTERNATIONAL LIMITED
Application year: 1987
Main classification: H05B3
IPC codes: H05B3