US 2939807 A
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June 7, 1950 B. A. NEEDHAM 2,939,807
METHOD OF MAKING A HEATING PANEL Filed June 29. 1956 /7 f TQFNIK United States O.
METHOD OFMAKING A HEATING PANEL Basil A. Needham, Bayside, N .Y., assgnor to Thermway Industries, Inc., New York, N.Y., a company of Delaware Filed June 29, 1956, ser. No.s94,s34 t 4 claims. (ci. 111-212) This invention relates to heating devices and particularly to an improved electrically energized radiant heating element and the method of making it.
An object of the invention is to provide an improved radiant heating element suitable for use in space heating panels adapted to be mounted on the walls or ceilings of buildings, in portable heaters, and 'as a heating element for water heaters, ranges, etc. My improved element is also suitable for use in industrial applications.
Another object is to provide an improved radiant heating element whichv is self supporting, has inherent rigidity, yet is rugged in construction and not easily subject to damage by mechanical or heat shock.
Still another object is to provide an improved electrical heating element wherein the resistor is fused on a self supporting backing member which may be llat or curved and wherein the resistor presents a relatively large area of heating surface.
Another object of the invention is to provide an improved electrically tenergized heating element of sheet form which includes ya metal sheet with a resistor element mounted thereon suitably insulated from the metal sheet, and which has a vitreous or porcelain enamel coating on the front surface of the sheet for diffusing the radiation thereof.
Other objects and advantages of the invention will become apparent from the following specification which describes preferred embodiments of the invention.
In the drawing accompanying the specification,
Fig. l is a rear view of my improved heating element with a portion of the protecting coating broken away to show the resistor;
Fig. 2 is 1an enlarged section taken on the line 2-2 of Fig. 1; and
Fig. 3 is a section, similar to Fig. 2, of a modified form of element.
Referring to Figs. l and 2 of the drawing, it may be seen that my improved heating element comprises a sheet which is preferably of metal but may be of other suitable material. The sheet 10 may be of any desired thickness so long as it is self supporting and substantially rigid. Generally, met-al of sufficient section to provide the desired characteristics with lightest weight is used. While the sheet 10 is shown as at and rectangular, any regular or irregular shape may be used and the sheet may be formed into a cylinder or other shape.
The sheet 10 is coated with a layer of frit, vitreous enamel or porcelain enamel 12, and an electrical resistor 14 is applied to the refractory coating as indicated at 14.
Terminal electrodes 16 are suitably soldered or otherwise attached to the resistor as shown. The other surface of the sheet 10 has a coating 18 of frit, vitreous or porcelain enamel, and preferably the rear (element carrying) side is coated with a layer 20 of corrosion retardant material such as silicone enamel, melamine resin, or the like.
The coating 20 protects the rear surface of the panel generally and prevents corrosion of the resistor 14 which might occur from exposure to air, water or a corrosive atmosphere. Such corrosion sometimes causes 4localized high resistance which, in turn, causes localized over heating and failure of the resistor.
Application of the vvitreous coating and the resistor presents some difiiculty because the element in use is subjected to rapid heating and cooling and it has been found that unless the various coatings are applied under careilly controlled conditions, chipping and spalling may result. Furthermore, unless the vitreous coatings are applied correctly warping of the element results and this is highly objectionable. Heating elements made in accordance with the method outlined herein have proven completely satisfactory.
A metal backing sheet 10 is first heated uniformly to approximately 1500 F. and the vitreous layers 12 and 18 are applied, preferably by spraying from a spray gun or nozzle, while the sheet lisrnaintained at the indicated temperature. The coated sheet is then carefully annealed and brought to room temperature slowly such that the coating or coatings do not spall and no warping of the element takes place. It is of prime importance that the vitreous coating be uniform in thickness, and if coatings are applied to both sides of the sheet, the thickness should be the sa'me for best results.
I have found that steel coated with porcelain to a uniform thickness of one-sixteenth of an inch on each surface is ideal for the heating element herein described.
The coated sheet is then cleaned, preferably in a vaporizing oven with a vaporized solvent, `and is reheated to a uniform temperature of 550-700 F. or somewhat higher and maintained at this temperature while the resistor 14 is applied. The latter may be pure aluminum or an alloy and is applied by spraying molten metal from a gun or nozzle to a thickness of .O05 inch. During application of the metal coating, the sheet is moved at a uniform rate past the gun which is oscillated vertically or horizontally.
The metallic coating 14 m-ay be sprayed through a mask to provide the grid-like path desired, but I prefer to lapply a uniform coating of metal on the porcelain then etch the grid pattern. Accordingly, a silk screen I pattern is placed on the hardened metal coating and a coating of silicone Varnish or resin is squeezed over the entire surface of the sheet. The element is then placed in an etching bath, such as a 2% nitric acid solution, for a time sufficient to etch the metal completely away in the places not covered by the enamel.
Next, the element is removed from the etching bath and dried whereupon it is complete, the silicone still adhering to the resistor pattern forming an anti-corrosive layer.
As illustrated in Fig. l, a border 11 of bare metal is preferably left around the margin of the sheet 10 on both sides. This uncoated border protects the coatings from being chipped at their marginal edges dur-ing handling and provides space for holes 13 which can be used to fasten the element in a frame or other support.
Fig. 3 shows a modification wherein the element above described is provided with an additional coating 22 on its rear side. This coating 22 may be of rock wool, foam glass, fibre glass, or other suitable heat insulating material, and improves the heat radiating qualities of the element by reducing rad-iation from the rear side which is generally against or close to the mounting surface, such ias a wall or ceiling.
In either form of the invention, the metal base sheet 10 may be of any suitable metal or alloy such as copper, aluminum, steel, etc. Stainless steel has approximately the same coecient of expansion as the vitreous enamel and therefore works very well. In general, it may be `PatentedJune7,1960
stated that the coeicients of expansion of the metal base y and the refractory coatings used should be approximately ment is heated and cooled. This requirement is, ofi
course, not so critical if it is intended to operatethe element at'fairly low temperature. For space'heating, electrically energized panels of the sheet-element type are :operated at temperatures of approximately 150" to 375 F. In industrial use, much higher temperatures are utilized.
' I claim:
1. The method of making an electrical radiant heater.
comprising, heating a metal sheet to approximately 15 00v F. and While maintaining the sheet at such temperature? applying lan insulating, hard, vitreous coating to bothv surfaces of said sheet, annealing said coated sheet, re-
heating said coated sheet to a temperature of approxi-A 4 surfaces of said sheet, annealing said coated sheet, reheating said coated sheet to a temperature of approximately 550 F. and While maintaining the coated sheet at such temperature spraying one of the coated surfaces with molten metal, then etching said metal to provide a grid-like pattern.
4. The method of claim 3k including the stepv of Y applying facoating of silicone varnish to said grid-like pattern.
References Cited in the ile of this patent UNITED STATES PATENTS 617,375 'Voigt Jan. 10, 1899 2,061,107 Schellenger Nov. 17, 1936 2,119,680 Long June 7, 1938 2,179,566 Stoekle Nov. 14, 1939 2,357,473 Jira Sept. 5, 1944 2,432,800 Reichold Dec. 16, 1947 2,441,960 Eisler May 25, 1948 2,486,148 Glynn et al. Oct. 25, 1949 2,502,291 Taylor Mar. 28, 1950 2,542,726l Sullivan Feb. 20, 1951 2,679,569 Hall May 25, 1954 2,695,351 Beck Nov. 23, 1954 2,739,085 McBride f.. Mar. 20, 1956 FOREIGN PATENTS 162,632 Australia Apr. 29,