US 3525850 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
8- 0 N. E. HAGER, .JR 3,525,850-
HIGH OUTPUT, QUICK RESPONSE, RADIANT HEATER Filed March 15, 1968 INVENT OR MAX-71w ATTORNEY NATHANIEL E. HAGERIJR.
United States Patent 3,525,850 HIGH OUTPUT, QUICK RESPONSE, RADIANT HEATER Nathaniel E. Hager, Jr., Lancaster, Pa., assignor to Armstrong Cork Company, Lancaster, Pa., a corporation of Pennsylvania Filed Mar. 15, 1968, Ser. No. 713,509 Int. Cl. H0511 3/26 US. Cl. 219-357 5 Claims ABSTRACT OF THE DISCLOSURE A high intensity, quick response, electrical resistance, foil radiant heater is described having a heating element in the form of a corrugated metallic foil ribbon adapted to be heated by electricity to a temperature in the range of about 12001800 F. The heating element is backed by a thermally insulating backing preferably of a ceramic material, the backing having a thermal conductivity at 1500 F. in the range of about 0.07-0.15 B.t.u./hr.-ft.- F. The backing may be made of an assemblage of blocks or rectangles to minimize warping or fracturing at the high temperatures of operation of the heater. Metallic mounting means are provided in the nature of heat resistant wires, nails, or pins which hold the heating element to the backing. These mounting means support the heating element and extend from the heating element to the back or reverse side of the backing. The mounting means are adapted to convey heat from the heating element to the back of the backing, and lose heat there.
BACKGROUND OF THE INVENTION Field of the invention The invention relates generally to electrical heaters, particularly to heaters having a large area, which operate at high temperatures in a range centering around 1500 F. and which are quick response heaters in the sense that they heat up to and cool down from their operating temperatures in times on the order of a fraction of a second.
Description of the prior art Metallic foil heaters on a backing are known as is illustrated in US. Pat. No. 2,682,596, Cox et al. Such laminated heaters, however, cannot function at the relatively high temperatures contemplated for use by the heaters of the present invention. High temperature heaters, particularly those having a relatively broad surface, as compared witht he surface of a resistant wire, are normally made by imbedding wire heating elements or rod heating elements in a thick plate in order to achieve the requisite tempera ture in the thick plate. Other resistant heaters designed to function at high temperatures are made of relatively thick bars or bands which glow when electricity is passed through the resistance heating element. All such heaters have large thermal inertia in that they cool slowly when turned off, and heat up slowly when turned on. When such heaters are used to heat objects on a moving line such as the waxed surface on a paper backing, filled bottles on a lbottling line, or thermoplastics, troublesome diificulties occur when the line must be stopped due to a malfunction somewhere along the production line. Stoppage of the objects to be heated in front of such heaters causes the objects to be overheated resulting in fires, scorched products, or exploded bottles, depending on the nature of the obpects being heated. These high-temperature, largemass heaters cannot cool sufficiently fast when the power is cut off to prevent ruination of the objects being heated when those objects are brought to a sudden stop in front of the heater.
While foil heaters are known to have low thermal inertia, high output foil heaters have not been used due to the difficulty of maintaining the structural integrity of the heaters. The backing of the heater must be such to resist stresses produced by off-and-on operation at the high temperatures. Conductivity of the backing must be sufficiently low that heat losses are minimized through the back and so the structure for holding the backing is protected. Even worse, the mounting for the high temperature metallic foil ribbon heating element must be such that hot spots are not created. Such hot spots unduly shorten the life of an already fragile heating element. Additionally, the mounting means themselves easily burn out. The;
support problem can be overcome by the use of metallic mounting means for the heating element which conduct heat away from the heating element and radiate the heat out the back of the backing on which the heating element ribbon is mounted. The mounting means themselves are kept strong by such cooling.
SUMMARY OF THE INVENTION The problems mentioned above are solved by a heater having a heating element in the form of a corrugated mettalic foil ribbon to maintain the foil under tension. The foil is adapted to be heated by the passage of electricity therethrough to a temperature in the range of about 1200-1800 F. A thermally insulating backing of the backing either by passing through the backing orby passing around the edges of the backing to the back side thereof.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simplified three-dimensional view of the heater of the present invention showing three different types of heat element mounting means;
FIG. 2 is a section through the lines 2-2 of FIG. 1;
FIG. 3 is a section through the lines 3-3 of FIG. 1;
FIG. 4 is a section through the lines 4-4 of FIG. 1; and
FIG. 5 is a simplified view of another embodiment of the heater.
DESCRIPTION OF THE PREFERRED I EMBODIMENTS Referring to FIG. 1, the ribbon heating element 1 is shown in corrugated form. Corrugations are necessary to accommodate expansion of foil due to changes in temperature without losing tension as the heating element cools and heats in response to power input. The heating element 1 is in the form of a ribbon and must be made of a metallic foil. The foil may be made of any metal which withstands the rigors of the required heating. It may be, for example, a stainless steel foil of the austenitic type. Other suitable alloys are sold under the known names of Hastelloy, Inconel, Waspalloy, and others of the cobalt-chromium-nickel class of refractory alloys. The foils will have a thickness in the range of about 0.0005"- 0.01", and preferably on the order of 0.00 Such foils are readily obtainable in various widths, which can be cut to 1"2", which are convenient widths for a variety of applications. If desired, the foil may have a serpentine configuration in known manner, as shown in FIG. 5, in
which the current passing through the foil traverses a serpentine path.
The thermally insulating backing 2 must be resistant to the temperatures in the stated range achieved by the heating element 1. Blocks or sections of insulating brick may be used, as may certain cellular asbestos cement compositions, mineral wool with a clay binder formed into rigid ceramic-like sheets, and glass fiber compositions which can withstand these temperatures. Thickness of the backing may vary, and normally it will run from 0.5"-2" in thickness. The thermal conductivity of the backing must be in the range of about 0.07-0.15 B.t.u./hr.-ft.- F. in order to minimize heat loss out the back. If the backing is assembled from a series of adjacent blocks or rectangles of insulating backing material, they may be held in position by means of a suitable frame 3 shown in IFIG 5.
The metallic mounting means for the heating element 1 may have any of several configurations. They may simply be heat resistant wires 4 as shown in FIG. 1 and FIG. 2, twisted or otherwise fastened at the rear. The metallic mounting means may consist of flattened U-shaped supports 5 as shown in FIG. 1 and FIG. 3 which hold the heating element 1, pass through the backing 2, and protrude from the back of the backing at 6 as shown in FIG. 3. The metallic mounting means may also consist of nails or nail-like supports 7 as shown in FIG. 1 and 'FIG. 4 which penetrate the backing 2 and protrude from the rear of the backing 2. All of these configurations of metallic mounting means must conduct heat from the heating element to the back of the backing 2 in order to radiate heat at the back. If no heat is radiated at the back, the region where the metallic mounting means press against the heating element 1 will grow hotter until an undue hot spot is created, the hot spot eventually burning through and allowing the heating element to droop or fall away from the backing 2. The mounting means themselves may burn away unless the heat is radiated out the back. The materials from which the metallic mounting means are constructed will be those materials which will resist high temperatures in air. Several of the stainless steels are suitable, as are the nickel-chromium alloys. The wire-like metallic mounting means 4 may simply be Nichrome wires.
Power junctions 8 are connected to the ends of the ribbon heating element 1 in convenient manner. These, too, may protrude through the backing 2 in order to cool the site of the power input. The ends 9 of the ribbon heating element 1 may be coated with silver solder or welded between heavier platelets of oxidation-resistant metal at the ends to give good electrical contact and to lower the electrical resistance and thus the 'heating eifect in that portion of the ribbon. Power leads 10 are connected to a suitable source of power, not shown.
Referring to FIG. 5, the end turn 11 may be coated with silver solder or welded between heavier plates to lower the electrical resistance and prevent heat buildup there.
Power requirements for a 12" long heater having a width of 1", running at 1750 F., will be about 1000 watts, running at or close to Volts and 50 amperes. At this temperature these heaters radiate at about 10 kilowatts per square foot. Such heaters melt the wax on a wax paper in less than 0.2 second and hence allow use of the heater on a fast wrapping line in which portions of the wax on the wax paper are melted just before wrapping in order to bring about sealing of the object in the wax paper. At the same time, if the packaging line should jam or otherwise stop due to malfunction, and the power of the heater is turned off simultaneously with the power driving the conveyor, heater temperature will fall sufl-lciently in a few tenths of a second to prevent ignition of the wax or paper. Since such conveyor lines normally require at least a few tenths of a second to stop, the possibility of fire or damage is held to a minimum.
It is sometimes advantageous to coat the ribbon heating element to raise its emissivity above the normal value for stainless steel. In such cases the heater emits the maximum flux at its operating temperature. These coatings are usually refractory non-metal coatings, in the nature of metallic oxides.
1. A high intensity, quick response, electrical resistance, foil, radiant heater having a heating element in the form of a transversely corrugated metallic foil ribbon adapted to be heated by the passage of electricity therethrough to a temperature in the range of about l200- 1'800 F., a thermally insulating backing for said ribbon, said backing being at least coextensive in area with said ribbon and having a thermal conductivity at 1500 F. in the range of about 0.=070.15 B.t.u./hr.-ft.- F., and metallic mounting means at each end of said ribbon to hold it in contact with said backing, said mounting means being in supporting contact with said ribbon and extending integrally to the back of said backing and adapted to convey heat from said ribbon to said back of said backing.
2. A heater according to claim '1 wherein said ribbon is a stainless steel.
3. A heater according to claim 1 in which said backing has a thermal conductivity at 15 00" F. of about 0.11 B.t.u./hr.-ft.- F.
4. A heater according to claim 1 wherein said metallic mounting means comprise wires.
5. A heater according to claim 1 in which said ribbon has a serpentine configuration.
References Cited UNITED STATES PATENTS 1,695,860 12/1928 Rohn l325 3,342,977 9/ 1967 Anderson 2i19548 3,431,705 3/ 1969 Schmermund 3 38287 2,682,596 6/1954 Cox et al. 338293 2,857,499 10/1958 'Fearn 219354 3,327,093 6/1967 Hager et al. 219--354 X FOREIGN PATENTS 196,480 4/1923 Great Britain. 975,03 8 10/ 1950 France.
ANTHONY BARTIS, Primary Examiner US. Cl. X.R.