GB2313994A

GB2313994A - Radiant electric heater

Info

Publication number: GB2313994A
Application number: GB9611778A
Authority: GB
Inventors: Kevin Ronald Mcwilliams
Original assignee: Ceramaspeed Ltd
Current assignee: Ceramaspeed Ltd
Priority date: 1996-06-06
Filing date: 1996-06-06
Publication date: 1997-12-10
Anticipated expiration: 2016-06-06
Also published as: GB9611778D0; GB2313994B; DE29709356U1

Abstract

A radiant electric heater comprises an elongate electrical resistance heating element (4) supported in a heat-withstanding tube (1). A layer (2) of thermal and electrical insulation material in the tube (1) has a surface (3) to which the heating element (4) is secured along its length and particularly such that the heating element does not contact the tube (1). Radiant energy is transmitted in a preferential direction from the heater.

Description

Radiant Electric Heater This invention relates to a radiant electric heater and more particularly to a radiant electric heater comprising an elongate electrical resistance heating element inside a tube.

Such radiant heaters are known in which an elongate coiled wire or coiled ribbon electrical resistance heating element is arranged loosely inside a heat-resistant glass tube such as of quartz glass, which is at least partially transmissive of radiation from the heating element, or a heating element of ribbon form is bent into zig-zag form at intervals along its length and slotted into a heat-resistant glass tube. These prior art heaters suffer from a disadvantage that the heating element makes contact with the tube, even though such contact has been reduced in some cases by means of the zig-zag configuration of the element or by complex cutting of a coiled ribbon element and bending out tab-like portions therefrom to form spacers. Contact with the tube can result in degradation of the material thereof during high temperature operation of the heater.

Such prior art heaters also radiate in all directions therefrom and if required to provide radiation in a preferential direction, it has hitherto been necessary to add an external reflector, such as of aluminium. It is desirable to be able to provide a heater in which the radiant energy therefrom is able to be transmitted in a preferential direction, without the requirement for such an external reflector.

The present invention provides a radiant electric heater comprising: an elongate electrical resistance heating element supported in a heat-withstanding tube; a layer of thermal and electrical insulation material provided in the tube lengthwise thereof, the layer having a surface securedly supporting the heating element.

The tube may comprise a material which is at least partially transmissive of radiation from the heating element.

The heating element is preferably dimensioned and supported such that it does not contact the tube.

Various kinds of insulation material may be selected. For example, the insulation material may comprise microporous insulation material or a pressed granular inorganic material such as vermiculite. Alternatively it may comprise a fibrous inorganic material such as a ceramic fibre material, or may comprise a formed ceramic material.

The insulation material may incorporate an opacifier material to enhance reflection, by the insulation layer, of thermal radiation produced by the heating element.

The tube suitably comprises a heat-resistant glass material, such as quartz or fused silica, or may comprise a ceramic or glass-ceramic material. A tube comprised wholly or partly of metal may also be considered.

Although the tube may be of any desired cross-sectional shape, a tube of substantially circular cross-section may be generally preferred.

With a tube of substantially circular cross-section, the layer of insulation material is preferably arranged such that it partially fills the tube to a level at least corresponding to, and preferably slightly beyond, a diameter of the tube, to minimise risk of displacement of the insulation material upon orientation of the tube such that the insulation material is uppermost.

Irrespective of the shape of the tube, the tube may be provided with means, such as one or more suitable projections, indentations or deformities, to secure the layer of insulation material against displacement.

If desired, the tube may be formed of two parts of channel form, secured together. Such tcvo parts may comprise the same, or different, materials.

One or more perforations or apertures may be provided in the tube.

The elongate electrical resistance heating element is suitably of wire or ribbon form.

A well-known form of coiled wire or ribbon element may be securedly supported on the surface of the layer of insulation material by known means, such as by staples, adhesive material or partial embedment in the surface of the layer of insulation material.

Alternatively an element of corrugated ribbon form may be securedly supported on edge on the surface of the layer of insulation material. The ribbon-form element may be secured to the layer of insulation material by partial embedment in the surface thereof.

The ribbon-form element may be provided with tabs extending from an edge thereof for embedment in the layer of insulation material for securement of the element to the insulation material.

The layer of insulation material may be pre-formed, such as by moulding, and subsequently slotted into the tube, the heating element being preferably secured to the pre-formed insulation material prior to slotting into the tube.

The pre-formed layer of insulation material may be provided in a temporary or pennanent support, such as of metal foil, to facilitate slotting into the tube.

When a tube is formed of two parts of channel form, the layer of insulation material may be provided directly in one of the channel-form parts, the heating element being secured to the layer, the tube being completed by addition of the other channel-form part.

The tube is suitably provided with closure means at opposite ends thereof. Such closure means may comprise one or more caps, suitably of ceramic material. At least one of the closure means may include or accommodate electrical terminal means connected to the heating element for enabling the heater to be electrically connected to a voltage supply for operation.

The surface of the layer of insulation material may be substantially planar, or may be profiled to provide increased angle of spread of radiant energy from the heater.

The tube may be hermetically sealed if required and, optionally, evacuated or filled with air or one or more other gases.

The radiant electric heater of the invention may be used singly or in plurality (for example in arrays) according to the application for which it is required.

The invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 is a perspective view of part of a radiant electric heater according to the invention; Figure 2 is a perspective view of a pre-formed layer of insulation material with a heating element, for use in the radiant electric heater of Figure 1; Figure 3 is a sectional view of part of a radiant electric heater according to the invention; Figure 4 is a side view of the radiant electric heater of Figure 3 ; Figure 5 is a perspective view of part of a radiant electric heater according to the invention having a layer of insulation material with a modified surface profile; Figure 6 is a perspective view of part of a further embodiment of radiant electric heater according to the invention.

As shown in Figure 1, a radiant electric heater according to the invention comprises a tube 1 of a thermal radiation transmissive material such as quartz, fused silica or glass-ceramic. A layer 2 of compacted microporous thermal and electrical insulation material of well-known composition is provided in the tube, along the length of the tube, the insulation material being, for example, as described in GB-A-1580909. The insulation layer 2 has a surface 3 securedly supporting an elongate electrical resistance heating element 4. The heating element 4 comprises a corrugated ribbon of a material such as iron-chromium-aluminium alloy which is supported on edge and secured by partial embedment in the insulation layer 2. As shown in Figure 3. the ribbon-form heating element 4 may be formed with integral tabs or legs 5 which are embedded in the insulation layer 2. The supported heating element 4 is arranged to be devoid of contact with the tube 1. Instead of the heating element 4 comprising a corrugated ribbon, other well-known forms of electrical resistance heating element could be adopted, such as a coiled wire or ribbon heating element. A coiled wire or ribbon heating element, if selected. could be secured to the insulation layer 2 by well-known means such as staples or adhesive means, or by partial embedment in the surface of the insulation layer 2.

The insulation layer 2 is suitably pre-formed as shown in Figure 2 and subsequently slotted inside the tube 1, the heating element 4 being also secured to the insulation layer 2, prior to slotting the latter into the tube. For ease of handling, the compacted microporous insulation layer 2 may be formed by moulding in a substantially U-shaped metal foil support 6 having a contour conforming to the tube 1. The corrugated ribbon heating element 4 may be partially embedded in the surface of the insulation layer 2 during moulding of the layer 2, or may be subsequently pressed into the surface of the layer 2. It is preferred that the insulation layer 2 is arranged such that it partly fills the tube 1 to a level at which its surface 3 is at least up to, and preferably slightly beyond, a level corresponding to a diameter 7 of the tube 1. This means that the layer 2 should extend at least half way, preferably more than half way, across the tube.

In this way it is ensured that displacement of the insulation will not occur if the heater is operated in an orientation such that the insulation layer 2 is uppermost in the tube 1 with the heating element 4 extending downwards therefrom.

Securement of the insulation layer 2 against such displacement could alternatively be arranged by providing the tube 1 with projections 8 on its inside surface, or by suitably indenting or deforming the tube during manufacture.

End caps 9, suitably of ceramic material, are provided to close the ends of the tube 1 and terminal members 10, suitably of metal strip form, are electrically connected to opposite ends of the heating element 4, such as by welding, and are arranged to pass through holes in the end caps 9. The insulation layer 2 is suitably profiled at the end regions of the tube to accommodate the end caps 9 and the terminal members 10. Lead wires 11 are connected to the terminal members 10 to enable the heating element 4 to be electrically energised from a voltage supply (not shown).

If the heating element 4 were to be provided of U-shaped form, both terminal members 10 could be provided at one end of the tube and passing through two separate holes in one end cap 9.

When the resulting heater, as shown in Figure 4, is electrically energised for operation, the heating element 4 self-heats to radiance and radiant energy is transmitted though the tube 1 to fulfil a desired heating function. Such heating function may be cooking, or space heating, or any thermal processing operation. The heater may be operated in any desired orientation, with the heating element pointing upwards, downwards, or sideways, as required. The microporous material of the insulation layer 2, usually contains an opacifier, such as titanium dioxide (Rutile). This means that the layer 2 serves as a diffuse reflector to optimise the transmitted radiant energy, in the directions shown by the arrows in Figure 1 by reflecting thermal energy produced by the heating element 4.

As shown in Figure 5, the angle of spread of the radiant energy from the heater, as indicated by the arrows, may be increased by adopting an arrangement in which, instead of the insulation layer 2 having a planar surface 3 as shown in Figure 1 it is profiled such that it has a top surface 3A, on which the heating element 4 is supported, and sloping side surfaces 3B, 3C.

The tube I could alternatively be formed of ceramic material, or could be formed wholly or partially of metal. Such a metal tube could, if required, be provided with one or more perforations or apertures therein. One or more such perforations or apertures could also be provided in a tube I formed of a non-metallic material.

The tube 1 could be formed of two parts 1A, 1B as shown in Figure 6. Parts 1A and 18 are suitably of channel form and may be provided with flanges 12, 13 to enable them to be secured together. With such an arrangement the insulation layer 2 could be moulded directly into one part 1 A of the tube and the heating element 4 also secured, followed by applying auld securing the other part 1B to complete the tube. The two parts IA, 1B of the tube could be formed of the same, or different, materials as required. For example, the part 1A could be of metal and the part 1B of a heat-resistant glass, such as quartz or fused silica, or of a glass-ceramic or ceramic material.

The tube 1 may be hermetically sealed if required and either evacuated or filled with air or one or more other gases.

Instead of the layer 2 comprising microporous insulation material it could comprise other thermal and electrical insulation material. For example, it could comprise a fibrous material. such as a ceramic fibre material, a particular example being a material available from the Carborundum Company Limited under the Trade Mark FI13ERFRAX.

Alternatively, the layer 2 could comprise a pressed granular material. such as vermiculite, or could comprise a formed ceramic material.

An opacifier material could be incorporated with any of these other insulation materials to enhance reflection, by the layer 2, of thermal radiation produced by the heating element.

Claims

Claims 1. A radiant electric heater comprising: an elongate electrical resistance heating element supported in a heat-withstanding tube; a layer of thermal and electrical insulation material provided in the tube lengthwise thereof, the layer having a surface securedly supporting the heating element.
2. A heater according to claim 1, in which the tube comprises a material which is at least partially transmissive of radiation from the heating element
3. A heater according to claim 1 or 2, in which the heating element is dimensioned and supported such that it does not contact the tube.
4. A heater according to claim 1, 2 or 3, in which the insulation material is selected from: microporous insulation materials; pressed granular inorganic materials, fibrous inorganic materials; formed ceramic materials.
5. A heater according to claim 4, in which the pressed granular inorganic material comprises vermiculite.
6. A heater according to claim 4, in which the fibrous inorganic material comprises a ceramic fibre material.
7. A heater according to any preceding claim, in which the insulation material incorporates an opacifier material to enhance reflection, by the insulation layer, of thermal radiation produced by the heating element.
8. A heater according to any preceding claim, in which the tube comprises a heat-resistant glass material, or a ceramic material, or a glass-ceramic material, or comprises a metal at least in part.
9. A heater according to claim 8, in which the tube comprises quartz or fused silica.
10. A heater according to any preceding claim, in which the tube is of substantially circular cross-section.
I 1. A heater according to claim 10, in which the layer of insulation material is arranged such that it partially fills the tube to a level at least corresponding to a diameter of the tube.
12. A heater according to claim 11, in which the layer of insulation material is arranged such that it partially fills the tube to a level slightly beyond that corresponding to a diameter of the tube.
13. A heater according to any preceding claim, in which the tube is provided with means to secure the layer of insulation material against displacement.
14. A heater according to claim 13, in which the means to secure the layer of insulation material against displacement comprises one or more projections, indentations or deformities of the tube.
15. A heater according to any preceding claim, in which the tube is formed of two parts of channel form, secured together.
16. A heater according to claim 15, in which the two parts comprise the same, or different. materials.
17. A heater according to any preceding claim, in which the tube is provided with one or more perforations or apertures.
18. A heater according to any preceding claim, in which the elongate electrical resistance heating element is of wire or ribbon form.
19. A heater according to claim 18, in which the heating element is of coiled wire or ribbon form, securedly supported on the surface of the layer of insulation material by staples, or bv adhesive material, or by partial embedment in the surface of the layer of insulation material.
20. A heater according to claim 18, in which the heating element is of corrugated ribbon form and is securely supported on edge on the surface of the layer of insulation material.
21. A heater according to claim 20. in which the corrugated ribbon is secured to the layer of insulation material by partial embedment in the surface thereof.
22. A heater according to claim 21 in which the ribbon is provided with tabs extending from an edge thereof for embedment in the layer of insulation material for securement of the element to the insulation material.
23. A heater according to any preceding claim, in which the layer of insulation material is pre formed and subsequently slotted into the tube.
24. A heater according to claim 23, in which the heating element is secured to the pre-formed insulation prior to slotting into the tube.
25. A heater according to claim 23 or 24, in which the pre-formed layer of insulation material is provided in a temporary or permanent support, to facilitate slotting into the tube.
26. A heater according to claim 25, in which the support is of metal foil.
27. A heater according to any of claims 15 to 22, in which the tube is formed of two parts of channel form and the layer of insulation material is provided directly in one of the channel form parts, the heating element being secured thereto, the tube being completed by addition of the other channel-form part.
28. A heater according to any preceding claim, in which the tube is provided with closure means at opposite ends thereof.
29. A heater according to claim 28, in which the closure means comprise one or more caps.
30. A heater according to claim 29, in which the one or more caps comprise ceramic material.
31. A heater according to claim 28, 29 or 30, in which at least one of the closure means includes or accommodates electrical terminal means connected to the heating element for enabling the heater to be electrically connected to a voltage supply for operation.
32. A heater according to any preceding claim, in which the surface of the layer of insulation is substantially planar.
33. A heater according to any of claims 1 to 31, in which the surface of the layer of insulation material is profiled to provide increased angle of spread of radiant energy from the heater.
34. A heater according to any preceding claim, in which the tube is hermetically sealed.
35. A heater according to claim 34, in which the tube is evacuated or filled with air or one or more other gases.
36. A radiant electric heater constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.