HEATING DEVICES

Abstract: Heating devices of improved efficiency are provided comprising either:- i) a coil (1) supported and retained on a base (2) of castable refractory material by ribs (12) moulded with the base (2) and around part of the periphery of the coil (1), the material of the base (2) being moulded between adjacent turns of the coil (1), the core (3) of the coil (1) being free of refractory and open to the surface of the device; or ii) a coil (1) embedded in a base of refractory comprising a region(6,9) of high thermal conductivity refractory in which the coil (1) is embedded backed by a support (7) of low thermal conductivity refractory, the coil may optionally be raised above the general surface of the base in ridges (8). A common form of mould is also provided for making these forms of heating device.

Description: This invention relates to electric heating devices in which an electric resistance heating element is fully embedded in a refractory support. Such devices are made as plane panels, curved panels, muffles, or in more complicated geometries and the present invention is not restricted to any particular shape of device. For simplicity the following description will refer to manufacture of plane panels, though the invention is not restricted to this geometry. Electric heating panels have been made in the past by pressing a heating element, usually in the form of a coiled wire element, into a wet mix of, thermally insulating, castable refractory material which then sets around the element. The element is only partially pressed into the castable refractory material so that part of the coil is exposed and this form of panel is referred to as a 'partially embedded panel'. An alternative form of panel is made by casting a thin layer of castable refractory material (of the kind used for partially embedded panels) into a mould, laying the coiled wire element on the castable refractory material, and then adding further castable refractory material so as to completely embed the element in the castable refractory material. Such panels are referred to as 'fully embedded panels'. The moulds currently used are of simple form to suit the shape of the end product, (generally rectangular) and are made of wood or zinc coated steel. Such panels are widely used in the construction of furnaces and as heaters in metallurgical processing. These panels have a number of disadvantages. The fully embedded panel gives the element protection from e.g. metal splashes but because the element is embedded in an insulating refractory a temperature gradient exists between the element and the surface of the panel so that the effective surface temperature at which the panel can be used is below the maximum working temperature of the heating element. Higher temperatures can be obtained with the partially embedded panel, but the element is then exposed to the atmosphere and is vulnerable to metal splashes or corrosive gases; additionally the part of the element that is embedded in the thermally insulating panel will, in use, be hotter than the part of the element that is exposed and this can lead to failure of the element. United Kingdom Patent Specification No. 1441577 (Albert George Docx) proposes a heating panel for muffle furnaces comprising a coiled wire element fixed in a filter cast ceramic fibre base, the inside of the coil being substantially free of ceramic fibre, a gap being provided between the back of the coil and the ceramic fibre base. This construction has only part of the elements exposed to the surface, the gaps between windings being filled with ceramic fibres, (see page 2 lines 55-58 of specification). U.K.Patent Specification 1441577 also shows a second form of construction in which the core of the element is exposed to the surface, but this embodiment is made by cementing the coil into a channel in an existing panel and some of the cement can flow into the core of the coil covering the element in places, so leading to hot spots. Further a disadvantage of using ceramic fibre for open coil systems is than problems of creep arise at high temperature, the windings bunching and distorting. German Patent Specification 3206508 discloses an open-cored coil of wire embedded in a ceramic panel, the core of the coil being open to the surface of the panel. The coil lies completely below the panel surface. French Patent Specification No. 2499060 discloses similar structures, in which the refractory support comprises layers of differing thermal conductivity, with the layer of highest thermal conductivity in contact with the heating elements (Figures 2 and 6; Table 3). This layer contains open channels exposing the heating elements to the atmosphere, following the accepted practice of the prior art, to ensure that the effective surface temperature of the panel is as close as possible to the temperature of the heating elements. The applicants have realised that to lessen the risk of localised heating it is advantageous to have the heating element fully embedded in a region of thermally conductive, electrically non-conductive refractory backed by thermally insulating refractory material. They have further realised that embedding the heating element in such a thermally conductive region enables the surface temperature of the panel to more closely approach the temperature of the heating element without the need to expose the heating element to the atmosphere. Accordingly this invention provides a heating device comprising an electrical heating element embedded in a support of refractory material characterised in that the element is fully embedded in a region of high thermal conductivity refractory material backed by low thermal conductivity refractory material. (Throughout this specification 'low thermal conductivity' and 'high thermal conductivity' are to be taken as relative terms only and do not imply an absolute value of thermal conductivity). The region of high thermal conductivity refractory material can comprise silicon carbide in a refractory matrix up to such a proportion that the bulk refractory is electrically non-conductive, e.g. up to 70%. Further refractories that can be used are oxide refractories such as e.g. magnesium oxide. This invention further provides methods for forming heating devices as set out in the following description and as claimed in the appended claims. The following description is by way of example only and refers to the drawings in which:- Figs. 1 - 3 are sectional views of prior art heating panels; Fig. 4 is a plan view of the heating panel of Fig. 2; Figs 5 - 6 are sectional views of various heating panels falling within the present invention; Fig. 7 is a sectional view of a mould in accordance with one aspect of the present invention. Fig. 1 shows a fully embedded panel as described above formed from a castable refractory material. Figs. 2 and 4 show a partially embedded panel formed by the coil being partially pressed into wet castable refractory material. In typical examples of this construction the wire heating element would be made of iron-chromium-aluminium alloy e.g. Kanthal (Trade Mark) Grade A1 which has a manufacturer's nominal composition of 22% chromium, 5.8% aluminium, balance iron; or Kanthal (Trade Mark) Grade AF which has a manufacturer's nominal composition of 22% chromium, 5.3% aluminium, balance iron (all percentages being weight per cent). The refractory material can comprise 2 parts mullite (-22 mesh), 1 part Secar 71 (Trade Mark) a hydraulic cement containing approximately 71% A1₂0₃, the balance being CaO. Fully embedded panels of this form can be used up to furnace temperatures of around 1100°C and partially embedded panels using these materials can be used up to approximately 1200°C. These temperatures correspond to element temperatures some 50°C or more higher. Fig. 3 shows a fully embedded panel as described in United Kingdom Patent Specification No. 1441577. Performance figures for such a panel are not available. Fig. 5 shows a heating device in the form of a panel according to the present invention. The panel comprises a heating element (1) fully embedded in a layer (6) of thermally conductive, electrically insulating refractory material, in this case silicon carbide refractory comprising e.g. 70% silicon carbide, 30% refractory cement. This layer is backed by a thermally insulating layer (7) which may be of castable refractory material as previously described. The panel is made by casting a layer of thermally conductive refractory, putting the element (1) in place, casting more thermally conductive refractory to cover the element (1), allowing this to set and then casting the thermally insulating refractory (7) to form a backing. Alternatively the procedure can be reversed, the backing being cast first. Use of a thermally conductive, electrically nonconductive layer results in improved heat transfer from the heating element to the surrounding refractory. This has several important advantages. Firstly there is an increase in heating efficiency; this is made evident by the reduced back face temperatures given in Table 1 resulting from improved heat loss from the front face of the panel. Tests comprising using panels to heat a furnace to 1000°C. Panel Front Face Temperature Back Face Temperature Standard Panel (Fig. 1) 1036°C 882°C Silicon Carbide Front Face (Fig. 6) 1038°C 824°C Secondly improved heat transfer results in a more even production of heat across the face of a panel (for a given furnace design) which may help to prolong the life of the elements (1) and also allow high temperatures to be reached with this fully embedded panel while remaining within the element manufacturer's specified wire temperatures. For example a fully embedded panel using Kanthal A1 wire and a silicon carbide refractory front face may be run at 1200°C which is some 100°C higher than the previously known fully embedded panels and equals the temperature of the known partially embedded panels. In view of the protection given by embedding this is a substantial advantage. To increase the radiative efficiency of this form of panel elements (1) may be partially or fully embedded in ridges (8) raised from the surface of the panel base. Fig. 6 shows the elements (1) fully raised in ridges (8) of thermally conductive, electrically insulating material. Such ridges (8) can either be raised from a layer of that same material (9) or can form separate islands on the thermally insulating backing (7). Such a panel can be made using the mould of Fig. 7 by casting a small amount of the thermally conductive refractory into the base of the channels (5), inserting the elements (1) into the channels (5), casting further thermally conductive refractory to embed the elements (1), and then casting the thermally insulating refractory (7) to form the backing. The mould (4) is made of vacuum formed plastics material such as ABS (acrylonitrile butadiene styrene). The material has to be sufficiently thick at its walls (10) to support the sideways pressure of the wet refractory mix and a suitable thickness is of the order of 2.4 mm. A peripheral flange (11) assists in giving resistance to deformation during moulding. Moulding these panels by using such a mould offers several advantages, firstly that the 'hot' face of the panel has a smoother finish than existing products, secondly more complex profiles are possible and thirdly that the moulds are easily freed from the panel after casting.

Claims: A heating device comprising an electrical heating element (1) embedded in a support of refractory material characterised in that the element is fully embedded in a region (6) of high thermal conductivity refractory material integrally backed by low thermal conductivity refractory material (7). A heating device as claimed in claim 1 in which the region (6) of high thermal conductivity comprises silicon carbide in a castable refractory matrix. A heating device as claimed in claim 1 or claim 2 in which the regions of high-thermal conductivity refractory material surrounding the element form surface areas (8) raised above the general surface (9) of the support. A method of making a heating device as claimed in claim 3 comprising using a mould (4) comprising one or more surfaces to define the heating device and channels (5) in the surface(s) to define heating element geometry, casting a layer of high-thermal conductivity refractory into the channels (5) of the mould (4), placing the element in the channels (5) adjacent the layer of high-thermal conductivity refractory, casting high-thermal conductivity refractory to embed the element, and casting a backing layer of low-thermal conductivity refractory.

Inventors: HOWGATE, ADRIAN GEOFFREY, MOUG, STANLEY BARNETT, ROEBUCK, PETER HAMISH ATHEY
Applicants: KANTHAL LIMITED
Application year: 1987
Main classification: H05B3
IPC codes: H05B3, F27D11