Electric heating – Heating devices – With heating unit structure
Reexamination Certificate
1999-07-01
2001-06-05
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
With heating unit structure
C118S725000, C219S462100, C219S466100
Reexamination Certificate
active
06242722
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the use of thin conductive films in resistance heating applications and, more particularly, to printed heating elements for surface heating applications, such as counter top stoves, which are constructed with large-area circular heating panels that provide even, low-power density, efficient heating.
BACKGROUND ART
U.S. Pat. No. 5,616,266, entitled “Resistance Heating Element with Large Area, Thin Film and Method,” issued Apr. 1, 1997 and co-pending patent application, Ser. No. 08/874,524, entitled “Method and Apparatus for Edge Heating of Thin Film Heating Element,” filed Jun. 13, 1997, both assigned to assignee of the present patent application, disclose thin film resistance heating elements for use in a variety of oven and space heater applications. The present invention improves upon the design of the thin film heaters disclosed in these patents.
The '266 patent discloses a thin film heater having a metal substrate with a ceramic layer thermally bonded across one side of the metal substrate. An electrically conductive, large area thin metallic film is deposited on the ceramic layer, isolated from the metal substrate. A pair of spaced apart electrical terminals are provided at the ends of the conductive film. Preferably, the conductive film is stannic oxide (tin-oxide) and is deposited onto the ceramic layer as a very thin film of, for example, 2 microns or less. Large area heaters constructed in this manner have been found to be capable of temperatures in excess of 500° F. while allowing operation at high power levels, but lower power densities. Low power densities produce an extremely even heat at lower temperatures without significant hot spots or excessive thermal gradients over the area of the panel.
Co-pending application Ser. No. 08/874,524 discloses a method and apparatus for controlling heat loss at the peripheral edges of heaters of the type disclosed in the '266 patent. A thin film conductive edge heater strip is formed around the peripheral edges of a large area thin film heater and is separately controlled to adjust its heat loss in order to compensate for heat loss at the outer edges of the large area heater. This design can be used in combination with the improved thin film heater of the present invention.
Circular heating elements are conventionally made in the form of a spiral, such as the spiral heating elements of electric stove-top heaters. Circular heaters are employed because most cooking utensils are circular and because a round shape improves the efficiency of the heater by matching more closely the geometry of the load. Thick film heaters, typically 0.001″ thick, provide a relatively uniform, low-temperature operating surface. However, a more uniform and lower operating temperature heater is the thin film heater, such as disclosed in the '266 patent. Thin film heaters made from metal oxides, such as tin-oxide, provide long term durability and stability up to approximately 500° C., which is hot enough not only for cooking purposes but for many other purposes as well.
DISCLOSURE OF INVENTION
Briefly described, a first embodiment of the present invention comprises a circular resistance heating element that includes an annular, electrically conductive, thin film outer heat region and an annular, electrically conductive, thin film inner heat region within the outer heat region. A first buss bar separates and electrically connects the inner and outer heat regions and a second buss bar electrically connects to and extends around the outer peripheral edge of the outer heat region and electrically connects to and extends around the inner edge of the inner heat region. With this design, a voltage applied across the first and second buss bars applies the same voltage across the outer heat region and across the inner heat region. In addition, the relative widths of the inner heat region and the outer heat region are determined so that the power dissipated per unit area for the inner and outer heat region film are approximately equal. In this manner, the resistance heating of the circular heater is kept relatively uniform across the surface of the heater and thus the temperature is more uniform than for a single region film.
The invention contemplates the provision of at least two annular heat regions, but provision of additional annular regions increases the uniformity of the heat gradient radially of the heating surface and reduces the current density at the inner diameter of any region.
A second embodiment of the resistance heater of the present invention comprises annular outer and inner heat regions that do not necessarily have to be circular as with the first embodiment, but which are each divided into at least two radially divided sections. The inner and outer heat regions are electrically connected in parallel and each include a first buss bar extending around outer edge of a first section of the heat region (or alternatively around the inner edge of the first section of the heat region), one or more intermediate buss bars electrically interconnecting the first section with subsequent sections, and a final buss bar extending around one of the inner and outer edges of the preceding section and the inner and outer edges of the last subsequent section. The initial intermediate buss bar extends around the edge of the first section that the first buss bar does not extend around, i.e. if the first buss bar extends around the outer edge, then the initial intermediate buss bar extends around the inner edge. The initial intermediate buss bar also extends around the same edge of the next subsequent section, and any additional intermediate buss bars extend around the inner or outer edge of a preceding section not occupied by a preceding buss bar and extend around one of the inner and outer edges of a subsequent section. In other words, the buss bars alternate from inner edges to outer edges of each section so that all sections making up a region are electrically connected in series. A voltage applied across the first and final buss bars applies a fraction of the total voltage across each section of the inner and outer heat regions, first through the first section of each region, and then through subsequent sections. This has the advantage of uniform heat distribution of the first embodiment and also the advantage of lower voltage and resistance per unit square for each section of heating element.
These and other features, objects, and advantages of the present invention will become apparent from the following description of the best mode for carrying out the invention, when read in conjunction with the accompanying drawings, and the claims, which are all incorporated herein as part of the disclosure of the invention.
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Feldman Bernard
Provancha Kenneth M.
Dahbour Fadi H.
Flehr Hohbach Test Albritton & Herbert LLP
Thermostone USA, LLC
Walberg Teresa
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