US3495328A - Electric heating unit - Google Patents

Description

Feb. 17, 1970 e. M. ZIVER I ELECTRIC HEATING UNIT 2 Sheets-Sheet 1 I Filed'a l 7. 1967 Y r w INVENTOR. GARO M. ZIVER AGENT I Feb. 17, 1970 e. M. ZIVER 3,495,323

ELECTRIC HEATING UNIT Filed July 7. 1967 2 Sheets-Sheei 2 -l2 ELECTRICAL T ELECTRICALLY RESISTANCE NONCONDUCTING FOIL SURFACE 0 CUT FOIL IN PATTERN POSITION CUT FOIL ON 0F HEATING ELEMENT NONCONDUCTING SURFACE OR ELEMENTS PLUS OF ARTICLE I0 NARRow SUPPORTING LINKS POSITION PAD OF NONCONDUCTING AND HEAT RESISTANT BACKING MATERIAL AGAINST CUT FOIL PRESS PAD AGAINST CUT FOIL AND FOIL AGAINST SURFACE II BY SECURING COVER PLATE ADJACENT PAD AND TO ARTICLE IO SUPPLY ELECTRIC POWER IN SERIES WITH HEATING ELEMENT OR ELEMENTS TO MELT AND DESTROY SUPPORTING LINKS DETERMINE ELECTRICAL RESISTANCE OF HEATING ELEMENT OR ELEMENTS TO ASCERTAIN WHETHER ALL LINKS ARE DESTROYED (OPTIONAL) EACH TIME AT LEAST ONE LINK IS FOUND TOT REMAIN INTACT INCREASING SAID ELECTRIC. POWER AND REPEATING TWO STEPS JUST PRECEDING IN VE N TOR 4 GARO M. ZIVER AGENT United States Patent US. Cl. 29-611 4 Claims ABSTRACT OF THE DISCLOSURE A method of fabricating an electric heating unit comprising an article such as a panel having an electrically nonconducting surface against which one or more electric heating elements are to be supported, each such heating element or elements comprising a sheet of electrical resistance foil which is cut in a sinuous pattern to provide a sinuously winding ribbon heating element having narrow temporary supporting links of foil extending between the sinuations of the ribbon element. A plurality of protrusions in a pattern corresponding to that of said element or elements may be deposited on said non conducting surface for positioning of the element or elements against such surface, the element or elements being held against the nonconducting surface by a pad of resilient heat resistant material. Said supporting links are thereafter destroyed by supplying in series to each said element electrical power of a sufficient wattage to melt and destroy such links.

Background of the invention The present invention relates to electric heating units and, more particularly, to electric heating units of the type employing electric heating elements comprising ribbons of an electrical resistance material. Still more specifically the present invention relates to a method of fabricating electric heating units comprising an article such as a panel of an electrically nonconducting material or having an electrically nonconducting surface against which one or more heating elements comprising ribbons of a suitable electrical resistance material are provided.

Many electric heating units presently employed for electric heating comprise a card of an electrically nonconducting material having one or more electrical resistance heating elements in the form of one or more ribbons of an electrical resistance heating material sinuously or serpentinely wound back and forth across one side of said card in substantially parallel and equally spaced paths, such ribbons reversing their directions at opposite edges of said card and extending through notches provided in and equally spaced along such opposite edges for support by such card. One example of such an electric heating unit is shown and described in copending US. patent application Ser. No. 598,307, filed Dec. 1, 1966 for Electrical Heating Units and assigned to the same assignee as the present application.

Heating units of the type shown and discussed in the cited copending application are relatively expensive since the ribbon resistance heating elements of the units are either entirely manually wound on their supporting cards or are manually wound on such cards with a minimum of assistance by mechanical means. Furthermore, as discussed in said copending application, heating units comprising ribbon resistance heating elements wound on electrically noncoducting supporting cards are subject to hot spots which increase the possibility of burn-out of the heating elements at such spots. The invention covered by said copending application is, of course, intended to provide a method of eliminating such hot spots. However, as is readily apparent, the elimination of the hot 3,495,328 Patented Feb. 17, 1970 spots by such method adds to the cost of fabricating heating units of the type discussed.

In view of the economic factors pointed out above, it has oftentimes been found expedient to cut a sheet of relatively thin electrical resistance heating foil in a pattern comprising one or more sinuously winding ribbon heating elements and to support such element or elements directly on and against an electrically nonconducting surface of a panel or other similar article which is to be heated. However, due to the thinness of said sheets of foil, the sinuate ribbon heating elements cut therefrom are difiicult to handle since they tend to droop or sag under their own weight or otherwise become distorted during the handling thereof. Accordingly, the method of the present invention was developed for the purpose of facilitating the fabrication of heating units of the type employing ribbon heating elements cut from relatively thin sheets of foil of a resistance material.

Summary of the invention The invention herein disclosed comprises the cutting of sheets of relatively thin electrical resistance foil in patterns comprising sinuate or sinuously winding ribbon heating elements with very narrow temporary supporting links of the foil extending between adjacent sinuations of the heating elements at selected points thereon. Such links provide short-circuit current flow paths between said sinuations and also temporarily support the ribbon heating element or elements during the handling thereof in fabricating a heating unit or units using such element or elements. Following the fabrication of each such heating unit, the supporting links are destroyed by connecting the ribbon heating element or elements across a source of electrical power of a sufficient value or wattage to melt and destroy the links. Whether any of said links remain intact following the supplying of said power to the heating elements may be determined by measuring the power delivered to the element at a suitable reference voltage or, alternatively, by directly measuring the electrical resistance of the heating elements following the supplying of the electrical power thereto.

Brief description of the drawings FIGS. 1, 2 and 3 of the drawings are plan views of several configurations for resistance heater elemnts which may be used in practicing the invention, and

FIG. 4 is a diagram illustrating, sequential steps in practicing the inventive method disclosed.

Description of the preferred embodiment It is believed that it will be expedient prior to the outset of the detailed description of the invention to point out that the term cutting as employed herein is intended to include stamping or die cutting, shearing as with shears or other sharp instruments, and chemical cutting such as by etching with acids or other suitable etchants. It is also pointed out that, although the invention is described in conjunction with a panel having an electrically nonconducting surface against which electrical resistance heating elements are to be disposed, the electric heating units could just as well comprise other similar articles such as skillets, sauce pans etc. each provided with such an electrically nonconducting surface. One example of a heating or warming unit for the fabrication of which the present invention could be conveniently employed is disclosed in Letters Patent of the United States 3,191,004, issued June 22, 1965, to James P. Hocker for Electrically Heated Immersible Warming Unit.

Referring to FIG. 4 of the drawings, an article such as a relatively thin panel 10 of a heat-resistant electrically nonconducting material, or provided with or having an electrically nonconducting surface 11, is provided. Panel may, for example, be a panel of a heat-resistant glass or glass-ceramic material. While panels of such materials are usually electrically nonconducting at relatively low temperatures, at increased temperatures the materials of some of such panels become increasingly electrically conductive. Accordingly, it is sometimes expedient and oftentimes necessary to provide an electrically insulating or nonconducting coating on the surface of the panel against which an electric heating element is to be provided. This may be accomplished, for example, by flame-spraying a coating of alumina on said surface when the material of the panel, such as 10, is or may become electrically conducting, or by providing a thin sheet of a thermal resistant and an electrically insulating material against said surface when the material of the panel has said conducting characteristics. However, such a step does not, per se, form a part of the present invention.

A relatively thin sheet of foil 12 of an electrical resistant heating material and of a size corresponding to an electrical heating element or elements to be mounted on the panel, such as 10, is also provided. The thickness of the sheet of foil 12 may, for example, be on the order of 0.005 inch and the material of such foil may, for example, be Nichrome or other suitable nickel-chrome or iron-chromium-aluminum electrically resistant alloy. The sheet of foil 12 is cut in a pattern comprising one or more sinuously or serpentinely winding ribbon heating elements such as elements 13 and 14 shown in FIGS. 1 and 3, re-

spectively, or elements 15 and 16 shown in FIG. 2. The pattern in which the sheet of foil 12 is cut depends, of course, on the configuration desired for the ribbon heating elements and the patterns shown in FIGS. 1, 2 and 3 are merely illustrative of three of many possible configurations for the heating elements. FIG. 1 illustrates a single heating element sinuously winding in back and forth parallel paths. FIG. 2 illustrates a pair of heating elements winding parallel with each other in a pattern similar to that of FIG. 1, and FIG. 3 illustrates a single sinuous heating element spirally winding in parallel paths.

During the cutting of sheet 12 in the pattern desired for the heating element or elements, very narrow links of the foil material, such as links 17, 18 and 19 in FIGS. 1, 2 and 3, respectively, are permitted to remain between adjacent windings of the element or elements at selected points thereon. Such links provide support for the sinuations or windings of the respective ribbon heating element or elements during subsequent handling thereof in fabricating a heating unit and, as previously mentioned and hereinafter further discussed, also provide short-circuit current flow paths between the sinuations of the ribbon heating element or elements. As also previously mentioned, the cutting of the sheet of foil, such as 12, in the selected pattern for the ribbon heating elements with the supporting links of foil material remaining therebetween may be performed by die cutting or shearing of the sheet of foil. However, it is preferred to mask the sheet of foil in the pattern desired for said heating element or elements and said supporting links and to etch away the unmasked portions of the foil. Such masking and etching procedures are now well known and, when the material of the sheet of foil, such as 12, is Nichrome as previously mentioned, the foil may, for example, be masked and etched as described in detail in a publication of the Eastman Kodak Company, Rochester 4, N.Y., such publication being entitled Kodak Photosensitive Resists for Industry and being obtainable from the Sales Service Division of such company by ordering Kodak publication No. P-7. The procedure for masking and etching Nichrome materials is outlined on page 45 of said publication. However, as previously mentioned, other wellknown selective etching procedures may be employed for cutting sheets of foil, such as 12, into the patterns desired for the ribbon heating elements.

In order to aid in correctly positioning and supporting a selectively cut pattern of a ribbon heating element or elements on the electrically insulated surface such as 11 of an article such as a panel 10, a plurality of protrusions such as protrusions 21, 22 and 23 shown in FIGS. 1, 2 and 3 of the drawings, respectively, may be provided or deposited on said surface in a pattern corresponding to that of the respective pattern of the heating element or elements to be disposed on such surface. Said protrusions preferably protrude from said surface a distance at least equal to or exceeding the thickness of said ribbon heating elements and may be deposited on the electrically insulated surface as discussed below.

There is provided a stencil comprising a sheet of a thin resilient material having holes or perforations extending therethrough from one planar surface thereof to the other in a pattern corresponding to that of the respective ribbon heating element or elements. Such sheet of resilient material may be a sheet of silicone rubber, for example, having a thickness commensurate with the distance said protrusions are to protrude from said elec trically nonconducting surface. One side of the perforate resilient sheet of material is disposed against said surface in a position corresponding to the desired location of the ribbon heating element or elements to be disposed on and against such surface and a coating comprising a slurry of a high temperature heat insulating cementitious material is applied to the remaining exposed side of the perforate sheet. Such coating may, for example, comprise Fiberfrax coating cement which is manufactured and sold by The Carborundum Company, Refractories Division, PO. Box 337, Niagara Falls, NY. Following the application of the coating cement to the exposed surface of said perforate sheet a squeegee is run over the coated sheet to assure that the cement fills the holes or perforations in said sheet and contacts the surface of the article, such as panel 10, covered by such sheet. The excess coating cement is also removed from the surface of the perforate sheet by such use of the squeegee. The perforate sheet is then removed from the coated surface, and the protrusions of coating cement so applied to said surface of the article, such as panel 10, are then permitted to air dry at ordinary ambient temperatures. If recommended by the manufacturer of the coating cement employed, the cement may then be further hardened by suitable heating thereof, the time-temperature cycles for such heating depending upon the recommendations of the manufacturer of the specific coating cement used.

It is pointed out that the protrusions deposited on the nonconducting surface, such as 11, of an article, such as panel 10, need not necessarily be circular as illustrated in FIGS. 1, 2 and 3 of the drawings but may be square or may form channels having substantially continuous walls for receipt of the ribbon heating element or elements therebetween. Furthermore, the provision of said protrusions to facilitate the positioning of the ribbon heating element or elements is not necessarily considered a critical part of the invention since the ribbon heating element or elements could be correctly positioned on said nonconducting surface by many other procedures such as by the use of a suitable bonding agent, or by the use of a suitable adhesive as discussed in the previously cited patent to James P. Hocker, for example.

As shown in FIGS. 1, 2 and 3 of the drawings, suitable electrical conductors or lead wires are attached to the ends of the heating elements by any suitable method known in the art, such as by welding, soldering or the like, for example. The leads or conductors in FIGS. 1 and 3, respectively, are designated by the reference characters 24 and 25, and 29 and 30, respectively. The lead wires or conductors in FIG. 2 are designated 26, 27 and 28. It will be noted that lead Wire or conductor 28 in FIG. 2' provides for a common electrical connection to the pair of heating elements 15 and .16 shown in such drawing figure. The electrical leads or conductors connected to the respective heating elements can be so connected either prior to the positioning of the respective element or elements against the insulated surfaces of their respective panels or following such positioning. Such leads or conductors are intended, of course, to be later connected to suitable terminals for energization of the heating element or elements from sources of power suitable for such energizationduring actual usev or'operation of the heating unit or units embodying such element or elements.

As diagrammatically shown in FIG. 4, the cut foil of material in the configuration of a ribbon heating element or elements is positioned on the insulated surface, such as 11, of its respective panel, such as 10, with the sinuous windings of the heating element or elements located between adjacent rows of the corresponding pattern of protrusions when such are provided on said surface and, with the free ends of said electrical leads or conductors remaining free, a resilient pad or blanket of thermal resistant and electrical nonconducting material is then disposed against the remaining exposed surface of the heating element or elements as a backing therefor. A cover plate is then disposed adjacent the remaining exposed surface of the heating element or elements as a backing therefor. A cover plate is then disposed adjacent the remaining exposed surface of said pad and is secured, in any convenient manner, to the panel such as so that it will press said pad of material into intimate contact with said exposed surface of the heating element or elements and, thereby such elements into intimate contact with the insulated surface of the panel such as 10. The resilient pad of material may, for example, be a piece of a Fiberfrax Lo Con blanket, such material also being manufacture and sold by the refractories division of The Carborundum Company, previously mentioned. The cover plate may be secured or fastened to the plate, such as 10, by suitable clamping devices or other such suitable means.

Following the steps discussed above and as further shown in FIG. 4, the ribbon heating element or elements are connected in series with a source of electrical power of a value sufiicient to rapidly melt and destroy the temporary supporting links of foil such as links 17, 18 or 19 shown in FIGS. 1, 2 and 3. That is to say, as previously mentioned, said temporary supporting links of foil provide short-circuit current flow paths between the sinuations of their respective heating element or elements and such links are destroyed by supplying in series to the respective element or elements electrical power of a wattageselected to rapidly melt and destroy the supporting links of foil. With heating elements such as shown in FIGS. 1, 2 and 3, respectively, the first and second terminals of the respective sources of power are connected to the previously-mentioned electrical leads or conductors 24 and 25, 26 and 27, 29 and 30, respectively. In order to ascertain that none of said links remain intact, that is, whether all of said links have been destroyed, the resistance of the heating elements may be determined either dufing or following the supply of power from each said source across the respective heating element or elements. That is to say, during the supply of power from each said source across the respective heating element or elements, the amount of power delivered to said element or elements may be measured at a reference supply voltage and, if the value of such power is not in accordance with that known to be correct for the respective heating element or elements without the connecting links, that is, if said measured amount of power is too high, it will be known that the resistance of the respective element or elements is too low and that one or more of said links remain intact. Alternatively, following the supply of power from each said source across the respective heating element or elements, the resistance of the element or elements may be measured directly, as by an ohmmeter, to determine if the value of such resistance is in accordance with that previously known to be correct for the respective heating element or elements without the connecting links. If the measured resistance value is too low, it will be known that at least one of said supporting links was not destroyed, that is, that at least one of such links remains intact. With either procedure the value or wattage of the power from the respective supply source is increased and supplied across: the respective heating element or elements. These procedures may be successively performed until, by said power or direct resistance measurements, a determination is made that all of the supporting links of a heating element or an associated plurality of such elements as shown in FIG. 2 are destroyed. It will be noted that in FIG. 4 of the drawings the step of determining the electrical resistance of the heating element or elements, during or following the supplying of the selected value of power thereto, is indicated as being optional. This is because the value of power which should melt and destroy the supporting links provided between the heating element or elements should be known or is readily ascertainable. However, it is preferred to determine said electrical resistance, by one of the procedures described above and as diagrammatically illustrated in FIG. 4, in order to assure that none of said links remain intact.

Although there is herein specifically shown and described only a single example of the practice of the method embodying the invention, it will be understood that such is not intended to be in any way limiting but that the exclusive rights that are desired for the protection of the invention are intended to be limited only by the spirit and scope of the appended claims.

What is claimed is:

1. A method of fabricating an electric heating unit comprising an article such as a panel having a top surface to be heated for cooking purposes and an electrically nonconducting bottom surface against which an electric heating element is to be supported, said method comprising,

(A) cutting a sheet of relatively thin electrical resistance foil in a pattern comprising a sinuate ribbon heating element of a known resistance and narrow temporary supporting links of foil extending between adjacent sinuations of the ribbon heating element to provide short-circuit current flow paths between such sinuations,

(B) depositing on said bottom surface of said article a pattern of protrusions corresponding to the sinuous pattern of said ribbon heating element, said protrusions being of a thermal resistant material and each protruding from said bottom surface a distance at least equal to the thickness of said foil,

(C) disposing said sinuate heating element between said protrusions with one surface of such element bearing against said bottom surface of said article,

.(D) disposing a resilient pad of thermal resistant and electrically nonconducting material against the exposed surface of said heating element,

(E) securing a cover plate adjacent said bottom surface of said article to press said pad of material into intimate contact with said exposed surface of said heating element and, thereby, said one surface of such element into intimate contact with said bottom surface of said article, and

(F) supplying in series to said heating element electrical power of a wattage selected to rapidly melt and destroy said supporting links of foil due to the current flow through said short-circuit current flow paths.

2. The method in accordance with claim 1 and further including the steps of determining the resistance of said heating element to ascertain whether any of said links remain intact and, each time it is found that at least one of said links remains intact, increasing the wattage of said electrical power supplied to said heating element.

3. The method in accordance with claim 1 and in which the cutting of said sheet of electrical resistance foil is performed by selectively etching the foil in said pattern.

4. The method in accordance with claim 2 and in which the cutting of said sheet of electrical resistance foil is performed by selectively etching the foil in said pattern.

References Cited UNITED STATES PATENTS 8 2,842,653 7/1958 Clemons 2925.42 X 3,071,749 1/ 1963 Starr. 3,134,953 5/1964 Eisler 29620 X JOHN F. CAMPBELL, Primary Examiner R. J. SHOR-E, Assistant Examiner US. Cl. X.R. 29620, 625

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