Circular film heater and porcelain enamel cooktop

Abstract: A range cook top is provided with circular heating zones having a layer of resistive thin film thereon. Slots in the cook top separate the heating zones from the surrounding areas of the cook top. Tongues of cook top material interrupt the slots and support the heating zones. The slots have beveled or stepped edges and beveled or stepped plastic inserts fill the slots to provide a smooth cook top. Sealer also fills the slots. The resistive layer in the heating zone is divided into arcuate segments by insulating partitions. Power supply bus bars are disposed around the outside edge of the resistive layer. A common bus bar interconnects the segments at the inner edges. A dual heater has separately controlled inner and outer resistive layers.

Description: BACKGROUND OF THE INVENTION This invention relates generally to the field of heating and cooking and specifically to a resistance heater. Electrical resistance heating films are used in various applications. Typically, the resistive film is applied on a substrate, which may provide a heating surface or may be the surface to be heated. A controlled voltage or current is applied to the film to effect the creation of heat energy. Examples of film heaters and controllers therefor are described in U.S. Patents Nos. 4,233,497 to Lowell, 4,384,192 to Lowell, 4,973,826 to Baudry, 5,160,830 to Kicherer and 5,616,266 to Cooper. Range cook tops for cooking food use electric heaters. It is desirable to provide a durable surface for supporting objects so that the objects can be heated efficiently and reliably. Heating of the surface should be limited to a desired area. The patent publication US-A-5 679 273 discloses a flush-mountable cooktop comprising a support table with openings to which burners are fixedly flush-mounted by mounting means which provide a seal. Each burner is comprised of a support layer of an electrically insulating material and an electrically conducting layer having associated current-supply strips to connect it to an external source of energy. The patent publication WO 97 21326 discloses a resistive heating element for a cooker comprising of a conductive film applied to a substrate, e.g. an enamelled steel plate which is imbedded flush in a cooktop. The patent publication US-A-4 384 192 discloses an electric heating element for a cooking apparatus in which a heating surface is divided into sections separately connectable to an electric energy source. Also patent publication US-A-4 233 497 discloses an electric heating element for a cooking appliance, wherein the heating element comprises a resistive layer divided into sections. BRIEF SUMMARY OF THE INVENTION The present invention provides a heater including a substrate and a slot in the substrate to define a heating zone. A resistive layer is applied to the substrate over at least part of the heating zone. Bus bars are connected to provide a voltage across the resistive layer. The slot is through the substrate. Tongues interrupt the slot and support the heating zone. The tongues are formed by substrate material remaining when the slot is formed. A thermally insulating insert is disposed in the slot. A sealant is disposed between the insert and the substrate. The slot has beveled or stepped edges and the insert is dovetailed or stepped to complement the slot. A sealing layer is disposed over the resistive film. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING Fig. 1 shows a schematic elevational view of a heating element according to the invention; Fig. 2 shows a top view of a range cook top according to the invention; Figs. 3A and 3B show section views of the cook top at an edge of a heating zone taken from line 3-3 of Fig. 2 according to different aspects of the invention. DESCRIPTION OF THE INVENTION Referring to Fig. 1, a heating apparatus, such a range cook top 10, includes a generally horizontal planar surface forming a substrate 12. A heating zone is formed on the substrate 12 and includes a resistive film layer 14 deposited on the substrate. A dielectric layer (not shown) can be disposed between the resistive film layer 14 and the substrate 12. A sealing layer 18 can be disposed over the resistive film 14. Fig. 1 is schematic and the relative thicknesses of the layers do not represent actual thicknesses. The substrate 12 is preferably a thermal shock resistant, rigid, and planar structure having a low electrical conductivity and suitable for supporting objects to be heated. Preferably, the substrate 12 is porcelain enameled (P-E) steel about 2.5mm thick, that is, 2.0mm of steel 12a with about 0.25mm of porcelain enamel 12b on each side. Other materials having the desired properties may also be suitable. For example, LAS glass ceramic or Si 3 N 4 ceramic about 4.0mm thick can be used in some cases. In a domestic range cook top application, for example, the substrate 12 is supported by a frame of the range and forms the base of the cook top. The resistive film 14 is preferably a thin film of atmospheric chemical vapor deposition (ACVD) applied F-doped or Sb-doped SnO 2 able to withstand a power density of 1.5 to 13 W/cm 2 and a current density between 11,000 and 90,000 A/cm 2 . A voltage applied across the film causes a current to flow through the film thereby heating the film. Preferably, the thin film has a positive temperature coefficient (PTC) to prevent thermal run away. A PTC film also provides even heating because cold spots draw more current and hot spots draw less current. Other materials having the desired properties may also be suitable. Because its resistance varies as a function of temperature, the resistive film can also be used as a temperature sensor. Alternatively, a separate temperature sensor can be located at the heating zone for closed loop temperature control. One layer of the porcelain enamel 12b acts as the dielectric layer. When the substrate is glass ceramic, the dielectric layer is preferably a sol gel applied SiO 2 /AlN or a screen printed and fired glass layer. The dielectric layer preferably insulates the substrate from currents flowing in the resistive film 14 and has a dielectric constant of about 5 to 8 (at room temperature and 50-60Hz). The dielectric constant should be as low and as stable as possible over the operating temperature range of the heater, which is about 20°C to 500°C. The dielectric layer should not substantially limit heat conduction from the resistive film to the substrate. Other materials having the desired properties may also be suitable. The sealing layer 18 is a heat resistant, rigid material having high electrical insulating properties and high heat conductivity. Preferably, glass or a sol gel applied ceramic, such as SiO 2 /AlN is used. Electrically conductive bus bars 20, such as cermet based silver thick film, are disposed on the resistive film layer 14 and preferably covered by the sealing layer 18. The bus bars 20 are connected to a power supply for providing a controlled current or voltage to the resistive film 14. The bus bar configurations and connections are discussed below. Referring to Fig. 2, the cook top 10 includes several heating zones 22. Each heating zone 22 includes resistive film disposed on the substrate as discussed above. Preferably, the heating zones 22 are circular and correspond in size with conventional large and small cook top element sizes, for example, about 235mm and 160mm in diameter. The heating zone 22 is separated from the remaining area of the cook top 10 by a circumferential slot 24. The slot 24 thermally insulates the cook top 10 from the heating zone 22. The resistive film does not extend past the slot. The slot 24 is discontinuous, interrupted by circumferentially spaced tongues 25. The tongues provide mechanical support for the heating zone and can provide a path for running electrical connections, such as conductive bus bar layers. Preferably, the tongues 25 are formed by leaving substrate material when the slot 24 is formed. Thus, the tongues have the same thickness as the substrate, but do not have porcelain enamel applied thereto expect where a path is provided for electrical conductors, wherein the enamel provides electrical insulation between the substrate and electrical conductors. One of the tongues 25a extends directly across the slot to serve as a bridge for simple routing of the bus bars. The other tongues 25 follow a serpentine path across the slot. The serpentine tongues allow for thermal expansion of the cook top elements. The width and number of tongues are selected to provide support for the physical loads placed on the heating zone. For example, four evenly spaced tongues 25, three having a width of about 4.0 to 4.5mm, an offset of 12mm, and thickness of 2.0mm, and the bridge tongue 25a having a width of about 20mm for a 2.5mm substrate thickness, are adequate. Alternatively, the tongues 25 can be separate parts, such as insulating fasteners, added to secure the heating zone to the cook top. Referring to Figs. 2 and 3A, an insert 26 is provided in the slot. Preferably, the insert 26 is made from a heat resistant and thermally insulating material, such as a molded ceramic or heat resistant plastic. A heat resistant sealer 28, such as silicone, seals the slot and retains the insert 26 in place. The sealer 28 prevents passage of liquid through the slot and allows for expansion and contraction of components. The insert 26 is dovetailed and the slot is provided with beveled edges 27 to support the insert. Referring to Figs. 2 and 3B, a different insert 26a can be provided in the slot 24a. Edges of the substrate defining the slot 24a are bent to form steps defined by two right angle bends in each edge. Preferably, the steps 29 are formed by stamping when the slot is formed. The insert 26a has a T-shaped cross-section having a stepped bottom complementing the steps of the slot. The insert 26 or 26a has a top surface substantially coplanar with the top surface of the substrate so that the cook top is smooth. A heat resistant sealant can be applied over the entire cook top to provide a uniform surface. Preferably, the slot 24 is spaced from the heating zone 22 to provide a circumferential ring of substrate that does not have a resistive heating layer applied thereon. Alternatively, the slot can be located at the edge of heating zone to define the boundary of the heating zone.

Claims: A heater comprising: a substrate (12); a through slot (24) in the substrate (12) to define a heating zone (22); a resistive layer (14) applied to the substrate (12) over at least part of the heating zone (22); and bus bars (20) connected to provide a voltage across the resistive layer (14), characterized by tongues (25) interrupting the slot (24) and supporting the heating zone (22). A heater according to claim 1, characterized in that the tongues (25) are formed by substrate material remaining when the slot (24) is formed. A heater according to claim 1 or claim 2, characterized in that the tongues (25) are serpentine. A heater according to any of the preceding claims, characterized in that a thermally insulating insert 26) is disposed in the slot (24). A heater according to claim 4, characterized in that the top surface of the insert (26) is substantially coplanar with a top surface of the substrate (12) to form a smooth cook top. A heater according to claim 4 or claim 5, characterized in that a sealant (28) is provided between the insert (26) and the substrate (12). A heater according to any of claims 4 through 6, characterized in that edges (29) of the slot (24) support the insert (26). A heater according to any of claims 4 through 7, characterized in that the slot (24) is bevelled. A heater according to claim 8, characterized in that the insert (26) is dovetailed to complement the slot (24). A heater according to any of claims 4 trough 9, characterized in that the edges (29) of the slot (24) are stepped. A heater according to claim 10, characterized in that the insert (26) is stepped to complement the slot (24). A heater according to any of the preceding claims, characterized in that a sealing layer 18) is disposed over the resistive layer (14).

Inventors: KALLGREN JOHAN, KALLGREN, JOHAN
Applicants: WHITE CONSOLIDATED IND INC, WHITE CONSOLIDATED INDUSTRIES, INC.
Application year: 1998
Main classification: H05B3
IPC codes: H05B3