Radiant electric heaters

Abstract: An electric radiant heater comprises an infra-red source, such as an infra-red lamp (22) and a coiled wire heating element (212), such that the coiled wire heating element can be arranged in an annular zone surrounding a ring-shaped infra-red source. The central zone within the infra-red source can remain unheated.

Description: This invention relates to radiant electric heaters, and in particular relates to heaters of the kind incorporating an infra-red lamp. Infra-red radiant electric heaters incorporating infrared lamps have been described, for example in GB-A-1 273 023, EP-A-0 117 346 and GB-A-2 146 431. Such heaters are typically incorporated in cookers and cooktops having a flat, glass ceramic cooking surface. The type of lamp used comprises a tungsten filament supported inside a tubular envelope of fused silica, with electrical connections brought out through hermetic pinch seals at the ends of the envelope. In these heaters the infra-red lamps extend above a reflecting surface and are fixed in position by sandwiching both ends of the lamp envelope in the heater periphery or by bolting the electrical connections to a bracket. Although sandwiching the ends of the lamp envelope is a satisfactory arrangement in the case of relatively short, straight lamps, it would not provide sufficient support in other configurations such as the generally circular lamp shown in GB-A-1 273 023. In this latter case the lamp would be cantilevered, with the risk of excessive stress being applied to the lamp envelope. Bolting the electrical connections is even less desirable, since it subjects the hermetic pinch seal between the connections and the envelope to considerable stress, especially in the configuration of GB-A-1 273 023. Nonetheless, a need exists for lamp configurations other than the existing short, straight arrangements. Such arrangements suffer from the disadvantage of emitting the radiation only from restricted portions of the overall heater area, and these portions do not have an optimum shape. This makes it difficult to provide a heated area with an appropriate distribution of radiation. DE-A-3 406 604 describes a radiant heater for a glass ceramic top cooker which shows in Figure 4 thereof a high temperature heating element in the form of a circular infra-red lamp having one normal-temperature heating element positioned in an annulus around the outer periphery of the lamp and another normal-temperature heating element positioned in the central area of the heater. EP-A-0 206 597 describes a radiant heater for a glass ceramic top cooker in which one or two straight infra-red lamps extend across the heater and a coiled wire heating element extends around the inner periphery of an outer wall of insulating material. It is an object of this invention to provide a heater incorporating an infra-red lamp arranged in an annular region of the heater together with a coiled wire heating element and which provides a favourable distribution of radiation throughout the heated area. According to the present invention there is provided an electric radiant heater, particularly for heating a ceramic plate such as a glass ceramic plate, comprising an insulating support with an infra-red lamp which surrounds a central zone of the radiant heater and with at least one coiled wire heating element which extends around the outer periphery of the infra-red lamp, wherein the extent of the said central zone of the heater is defined by the infra-red lamp, and this central zone does not contain a heating element. The coiled wire heating element and/or the infra-red lamp may have a substantially circular basic shape. The insulating support may incorporate an outer wall which preferably extends towards the glass plate, the coiled wire heating element being arranged in an annular zone between the outer wall and the infra-red lamp. The infra-red lamp and the coiled wire heating element may be arranged with their portions nearest to the glass plate substantially in the same plane with the infra-red lamp preferably lying in a depression of the insulating support. The coiled wire heating element may be secured on a shoulder formed inside a peripheral wall of the insulating support. The insulating support may be in the form of a shaped insulating layer in its central zone and in the region of the infra-red lamp, and on whose outer section a peripheral wall lies. The insulating support may be raised in its central zone facing the area of the infra-red lamp. A peripheral wall of the insulating support may form a ring within which is the region of the coiled wire heating element, radially within which is arranged the infra-red lamp, the peripheral wall preferably being made of a mechanically solid insulating material, preferably containing fibre material. A rod-shaped temperature sensor may extend through the space between the heating elements in the form of the infra-red lamp and the coiled wire heating element and the plate at a distance from each and may extend through a peripheral wall of the insulating support. In order to provide an electrical connection to the infrared lamp, the region of the coiled wire heating element may be formed as an open ring-shaped curve in which the coiled wire heating element is formed with a double bend with its contacts closely adjoining one another on one side of the contacts for the infra-red lamp. Radiant electric heaters for use in a glass ceramic top domestic cooker will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a plan view of a heater with an infra-red lamp; Figure 2 is a sectional view of the heater of Figure 1; Figure 3 is a plan view of a heater with an infra-red lamp and a coiled wire heating element; Figure 4 is a sectional view of part of the heater of Figure 3; Figure 5 is a plan view of a heater according to the present invention; and Figure 6 is a plan view of a heater showing a modified form of lamp. Referring to Figures 1 and 2, a radiant electric heater 10 has a container in the form of a metal dish 12 with an upstanding rim 14 and containing a layer of electrical and thermal insulating material 16. This material is for example a microporous insulation which comprises a silica aerogel powder mixed with a ceramic fibre reinforcement, titanium dioxide opacifier and a small quantity of alumina to resist shrinkage, and which is pressed into the dish 12. A ring-shaped wall 18 of ceramic fibre extends around the inside of the rim 14 of the dish 12, on top of the layer 16 and protruding slightly above the edge of the rim 14. When installed in a glass ceramic top cooker, the wall 18 is pressed against the underside of a glass ceramic cooking surface, shown in dashed outline at 20 in Figure 2, the heater being held in position by a spring or other mounting device (not shown). Prior to installation, the wall 18 is retained in position by pins or staples (not shown) extending into the layer 16. A heat source is provided in the form of a tungsten-halogen infra-red lamp 22. This lamp is generally circular in configuration and contains a tungsten filament 24 supported substantially axially within an infra-red transmissive fused silica envelope 26 on spacers (not shown). These spacers are arranged closely enough together to maintain the filament 24 at the desired distance from the envelope 26 in between each pair of spacers despite the curvature of the envelope 26. The filament 24 is secured at each end to connections brought out through flattened hermetic pinch seals 30 at the ends of the envelope 26. These ends are adjacent one another, and the pinch seals 30 extend generally radially of the heater 10 through recesses provided in the underside of the ceramic fibre wall 18 and in the layer 16, and through holes in the rim 14 of the dish 12. The surface of the layer 16 is contoured, as shown in Figure 2, to reduce the concentration of heat on the glass ceramic cooking surface 20 immediately above the lamp 22, and to maintain an adequate thickness for the layer 16. Thus under the lamp 22 there is an annular depression 32. Although the presence of this depression also helps to minimise the overall height of the heater, it is considerably broader than is required for this purpose alone, extending from the ceramic wall 18 to well within the inner circumference of the lamp 22. In addition to the depression 32, the central region 34 of the layer 16 is made slightly convex. As is customary with heaters for glass ceramic top cookers, a temperature sensitive rod limiter 36 is provided with its probe 38 extending across the heater 10 above the lamp 22. This probe typically comprises a silica tube containing a metal rod, which is preferably plated with a reflective material, such as silver, as described in GB-A-2 146 431. A snap-action switch 40, controlled by the probe 38, is connected in series with the lamp 22, which in turn is connected to an electrical connector block 42 mounted at the edge of the dish 12. To restrain the lamp 22 against movement relative to the layer 16, it is secured to the layer 16 at two spaced positions. Thus, where the pinch seals 30 extend under the wall 18, small raised pads 44 of the insulating material are formed in the layer 16 to engage the underside of the lamp 22, the upperside of which engages the wall 18. A second support point is provided diametrically opposite the pinch seals 30, in the form of an elongate exhaust tube 46. The lamp 22 contains an atmosphere with a halogen gas to help prolong the working life of the lamp and to reduce blackening of the inside surface of the envelope 26. To this end the envelope 26 has at least one protrusion, in this case in the form of the tube 46, which is used to evacuate the envelope 26 and back-fill it with gas containing halogen during manufacture of the lamp 22. Thereafter the tube 46 is softened by heating and closed to seal the interior of the envelope 26. As shown in Figures 1 and 2, the exhaust tube 46 is made long enough to extend across the space between the lamp 22 and the ceramic wall 18 and part way between the adjacent surfaces of the wall 18 and the layer 16. In this instance the exhaust tube 46 is positioned mid-way along the lamp 22. The tube 46 is restrained against movement by the wall 18 and the layer 16, and this restraint, together with the clamping of the pinch seals 30 between the wall 18 and the layer 16 supports the lamp 22 against side-to-side and up-and-down movement at diametrically opposed points. In addition, at positions mid-way between the tube 46 and each pinch seal 30, small ridges 48 of insulating material extend up from the layer 16 in the annular depression 32 towards the underside of the lamp 22. The ridges have the shape of a truncated elongate pyramid, with a concave top surface. Although the surface of the lamp 22 becomes very hot, the heat per unit length is less than in the case of straight lamps in a heater of equivalent power rating. This helps avoid excessive heating of the insulating material forming the ridges 48. The dimension of each ridge 48 measured along the circumference of the lamp 22 is minimised to limits its effect on the temperature distribution along the lamp. Thus it is possible to rely on heat conduction along the lamp 22 to limit the increased temperature of the lamp envelope 26 in the vicinity of each ridge 48. The effect of the tube 46 is to limit potential movement of the lamp 22 to rotation about the axis extending from the tube 46 towards the pinch seals 30. However, such rotation would require upward movement of one side of the lamp and downward movement of the other side. Any such downward movement of either side of the lamp 22 is limited by the ridges 48. The height of the ridges 48 is chosen so that there is a small clearance (for example 1-1.5 mm) between the top of each ridge and the underside of the lamp 22. This clearance reduces the risk of stress being imposed on the lamp 22 or the tube 46 owing to manufacturing tolerances in the dimensions and shape of the lamp 22. Instead of using an exhaust tube to form a support as shown in Figure 1, or in the absence of such a tube, a separate tube could be fused to the envelope 26 in such a manner as to maintain the sealing of the envelope 26 undisturbed. Modifications are also possible to the ridges 48. Thus for example they may be in the form of ceramic pieces embedded in the layer 16, rather than formed in the layer 16 directly. Instead of isolated ridges, ribs extending across the layer 16 from its outer periphery may be provided. In the case of a relatively small heater the tube 46 alone may be sufficient, and the ridges 48 may be omitted. Figures 3 and 4 show a heater 110 in which parts corresponding to those of Figure 1 have like reference numbers. Referring to Figure 3, the heater 110 includes a heating source additional to the lamp 22, in the form of a coiled bare resistance wire heating element 112. This element is disposed on the layer 16, within the area encompassed by the lamp 22, in a generally circular serpentine configuration to provide an aesthetically pleasing appearance whilst at the same time accommodating the required length of wire and promoting uniform heat distribution. The coiled element 112 is secured to the insulating material 16 by, for example, staples held by friction in the insulating material 16, or by gluing to the insulating material 16 or to stakes inserted therein. The ends of the wire heating element 112 are coupled to the electrical connector block 42. Where the wire of the coiled element 112 crosses under the lamp 22 it is provided with insulating sleeves 114 to limit the temperature of the wire at that point and also to provide additional electrical isolation. In the case of the heater 110 in Figure 3, the exhaust tube 46 is positioned approximately one third the way around the circumference of the lamp 22 from one end. A second protrusion 116 in the form of another exhaust tube, or a length of silica tube or rod fused to the envelope 26, is located approximately one third the way around from the other end of the lamp 22. This second protrusion is of similar length to the exhaust tube 46, and is attached to the envelope 26 in such a way that it does not disturb the sealing of the interior of the lamp 22. The protrusion is restrained against movement by the wall 18 and the insulating material 16 in the same manner as the tube 46. Thus the lamp 22 is firmly supported at three points spaced evenly around its circumference, avoiding cantilevering and providing protection against undue stress on the envelope 26 for example in the vicinity of the pinch seals 30. Therefore the ridges 48 are in this case omitted. The surface of the insulating material 16 is contoured as with the heater 10 and as shown in Figure 4, to promote uniform distribution of heat around the heater 110. Thus under the lamp there is an annular depression 118, and the insulating material 16 in the region within the coiled heating element 112 is made slightly convex. The number, position and nature of the protrusions such as 46 and 116 can be varied. Thus more than two protrusions may be provided, for example three spaced from each other by a quarter of the circumference of the lamp 22. For additional strength the or each protrusion may be made long enough to extend completely across the ceramic fibre wall 18, into clearance holes provided in the rim 14 of the dish 12. In this way there is increased contact area and less risk of the ceramic fibre wall being locally deformed. Each protrusion may be an addition to the lamp 22, instead of using an exhaust tube as a protrusion. A protrusion need not extend out towards the ceramic wall 18; it may extend downwardly onto the insulating material 16, providing a support leg for the lamp 22 as shown in dashed line at 120 in Figure 4. However, if every protrusion extends downwardly, some additional form of retention (such as a form of clip) may be desirable to restrain the lamp 22 against upward movement. In the heater 210 according to the invention shown in Figure 5, the lamp 22 is disposed within the region inside a coiled heating element 212. In this case, it may be preferable to arrange for short straight sections of the element 212 to extend under the protrusions 46 and 116, to provide space for the protrusions and to limit the heat dissipation immediately under them. The protrusions supporting the infra-red lamp need not be secured between the ceramic fibre wall 18 and the insulating material 16. A protrusion could be located in a recess in a plateau formed in the insulating material. The protrusion may be secured for example by glue or by means of a staple straddling the protrusion. A downwardly directed protrusion may have an enlarged end designed to be embedded in the insulating material to provide resistance to upward movement of the lamp. For aesthetic reasons, it is preferred that the illuminated part of the filament 24 in the lamp 22 of Figure 1, for example, should not extend into the ends of the envelope 26. Accordingly the filament 24 is connected to a rigid support wire 68, curved to match the bend in the envelope 26. This support wire is welded to a section of molybdenum foil which is sealed within the pinch seal 30 and is connected to an external electrical connection. It is also preferred that the non-illuminated arc between the ends of the filament should be as small as possible, and this constrains the bend between each end and the main body of the lamp 22 to have a small radius. However, if an attempt is made to manufacture a lamp with an acceptably small radius, a problem is encountered. The lamp is made by first forming and bending the fused silica envelope 26 to the required shape. One of the electrical connections and one of the rigid wires are welded to a piece of molybdenum foil, and this assembly is connected to one end of the filament 24. The second support wire 68 is connected to the second end of the filament 24, and this end is threaded into one end of the envelope 26. The filament 24 is worked around the envelope 26 until it reaches the far end. In order to weld the molybdenum foil and the external connection to the support wire 68 at the second end of the filament 24, this support wire must be brought a short distance out of the end of the envelope 26. This in turn requires the end of the filament 24 already carrying its support wire and foil to pass temporarily into the envelope 26 and around the bend at that end. It has been found that with a bend of the desired small radius of curvature it is very difficult or impossible for a support wire 68 of the desired length, with the foil and external connection, to travel sufficiently far into the envelope 26. The lamp 22 can be made, but only by increasing the radius of the bend for at least one end of the lamp (although for aesthetic reasons the bend would typically be made the same on both sides of the lamp). As noted above, this increases the length of the non-illuminated arc in the energised lamp and is therefore considered undesirable. Figure 6 shows a heater 1210 incorporating a lamp 1212 which avoids this problem. In making this lamp the envelope 26 is formed with two concave-outwards bends 1214 and 1216 in place of a single concave-outwards bend. The bends 1214 and 1216 are both of smaller radius than the bend in the lamp 22 of Figure 1, so the length of the non-illuminated arc is smaller than would be the case with a single bend of practicable radius. Nevertheless, we have found that the support wire 68 and the foil will travel further along the envelope 26 than would be the case with a single bend. In particular, they will travel far enough for the foil to be connected to the support wire 68 at the second end of the filament 24. Each of the support wires 68 is preferably made with a small right-angled bend at the end to be welded to the molybdenum foil, in the same plane as the bends in the wire which match the bends 1214 and 1216 in the envelope 26. These bends are arranged to be coplanar prior to attachment of the filament 24. Thus, when the molybdenum foils are welded to the support wires 68, the foils and the bends in the support wires 68 will all be in the correct common plane. The bend 1214 should be as close as possible to the end extremity of the envelope 26 while leaving sufficient room for the pinch seal 30. The second bend 1216 may be positioned as necessary, depending on the dimensions of the lamp 1212 and the envelope 26. The configuration of the envelope 26 between the bends 1214 and 1216 is not critical, but it can conveniently be generally straight as shown in the figure. The radii of the bends 1214 and 1216 may be made as small as practicable, within the constraints imposed by the manipulation of the fused silica material. The angle of each bend 1214 and 1216 may be approximately half the angle that would be required for a single bend; however, it is advantageous to make the bend 1216 larger than the bend 1214, since the angle of the bend 1214 has a significant effect on the length of the non-illuminated arc. Various modifications may be made to the embodiments of the invention described above. Thus, instead of using a geometrically circular lamp, it is possible to use a polygonal lamp made by heating and bending the envelope of a straight lamp at points spaced (preferably regularly) along it. With such a lamp the locations of support protrusions, ridges or clips, may be determined to coincide with, for example, the mid-point of a straight segment of the lamp.

Claims: An electric radiant heater, particularly for heating a ceramic plate such as a glass ceramic plate, comprising an insulating support (16, 18) with an infra-red lamp (22) which surrounds a central zone of the radiant heater (210) and with at least one coiled wire heating element (212) which extends around the outer periphery of the infra-red lamp, characterised in that the extent of the said central zone of the heater (210) is defined by the infra-red lamp (22), and that this central zone does not contain a heating element. A radiant heater as claimed in claim 1, characterised in that the coiled wire heating element (212) and/or the infra-red lamp (22) have a substantially circular basic shape. A radiant heater as claimed in claim 1 or 2, characterised in that the insulating support (16) incorporates an outer wall (18) which preferably extends towards the glass plate, the coiled wire heating element (212) being arranged in an annular zone between the outer wall (18) and the infra-red lamp (22). A radiant heater as claimed in any preceding claim, characterised in that the infra-red lamp (22) and the coiled wire heating element (212) are arranged with their portions nearest to the glass plate substantially in the same plane and the infra-red lamp (22) preferably lies in a depression of the insulating support (16, 18). A radiant heater as claimed in any preceding claim, characterised in that the coiled wire heating element (212) is secured on a shoulder formed inside a peripheral wall (18) of the insulating support (16, 18). A radiant heater as claimed in any preceding claim, characterised in that the insulating support (16, 18) is in the form of a shaped insulating layer (16) in its central zone and in the region of the infra-red lamp (22), and on whose outer section a peripheral wall (18) lies. A radiant heater as claimed in any preceding claim, characterised in that the insulating support (16) is raised in its central zone facing the area of the infra-red lamp (22). A radiant heater as claimed in any preceding claim, characterised in that a peripheral wall (18) of the insulating support (16, 18) forms a ring within which is the region of the coiled wire heating element (212), radially within which is arranged the infra-red lamp (22), the peripheral wall preferably being made of a mechanically solid insulating material, preferably containing fibre material. A radiant heater as claimed in any preceding claim, characterised in that a rod-shaped temperature sensor extends through the space between the heating elements in the form of the infra-red lamp (22) and the coiled wire heating element (212) and the plate at a distance from each and extends through a peripheral wall (18) of the insulating support (16, 18). A radiant heater as claimed in any preceding claim, characterised in that, in order to provide an electrical connection to the infra-red lamp (22), the region of the coiled wire heating element (212) is formed as an open ring-shaped curve in which the coiled wire heating element is formed with a double bend and with its contacts closely adjoining one another on one side of the contacts for the infra-red lamp.

Inventors: HIGGINS GEORGE ANTHONY, MCWILLIAMS KEVIN RONALD, HIGGINS, GEORGE ANTHONY, MCWILLIAMS, KEVIN RONALD
Applicants: CERAMASPEED LTD, CERAMASPEED LIMITED
Application year: 1989
Main classification: H05B3
IPC codes: H05B3