US3493724A - Infra-red concentrator - Google Patents

Description

Feb. 3, 1970 H. D. WELLS INFM-RED coNcENTRAToR Filed Aug. 5, 1967 2 Sheets-Sheet l FIGI H. D. WELLS INFRA-RED CONCENTRATOR Feb. 3, 1970 2 Sheets-Sheet 2 Filed Aug.

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United States Patent Oce Patented Feb. 3, 1970 3,493,724 INFRA-RED CONCENTRATOR Harold D. Wells, 3 Shari Drive, St. Louis, Mo. ,6,3122

Filed Aug. 3, 1967, Ser. No. 658,106 Int. Cl. H05b 1/00 U.S. Cl. 219-345 v 7 Claims ABSTRACT 0F THE DISCLOSURE Infrared emitter having a directly heated refractory panel with a gridwork of non-reliective cells in front of it. The gridwork, which is contiguous with the panel, absorbs and remits the infrared rays.

The invention relates to infrared emitters.

Infrared emitters are in wide use for supplying heat necessary to bake, fuse, cure, or plasticize industrial products and components therefor, or finishes thereon. Such emitters as heretofore known waste considerable energy. Such waste arises in several ways.

First,v the infrared rays emitted from every point source are scattered in all directions, and when those intercepted by a reflector (on the lside of the point source opposite that on which is located the object to be heated) are reflected or re-mitted, they, too, scatter. Considering only the half of the emitted field which emanates from the side of the point source that is addressed toward the object to be heated: such rays travel on every radius of a virtual hemisphere whose center is at the point source. For example, assuming an ideal situation, virtually impossible in practice, that the areaand contour of the object to be heated is identical with that of the emitter, and that they are parallel, but spaced apart a distance equal to the square root of their area, about seventy percent of the rays are wide of the target.

Second, an exposed emitting surface constantly loses heat by the travel thereacross of convection currents.

Wherefore, it is the general object of the invention to provide an infrared emitter which reduces the aforesaid waste of energy.

Generally stated, the invention contemplates the provision of an infrared emitter having ja suitable heating element, such as an electrical resistor, and a front or emitting surface which is provided, either integrally or separately but contignously, with a gridwork defining a multiplicity of open cells whose individual apertures have a cross-sectional area (substantially parallel to the total expanse of the heating element) which is relatively small as compared to the area encompassed by the heating element, and `whose dimension, perpendicular to the lastmentioned area, is about half the square root of the cross-sectional area of an individual cell. Each of the cells is surrounded and separated from its neighbor by walls which are as thin as practical, and which have low reflectivity, e.g., 25%, preferably being black. The cross-sectional shape of the cells may be polygonal or curvilienar, or a combination o f both, but is preferably substantially rectangular or square.

Such a gridwork may be made of metal which has a melting point safely above the-maximum operating tempertaure of the emitter, or of refractory material capable of withstanding repeated change in temperature between the maximum temperature of operation and ambient temperature. Such a gridwork, disposed as a veil across the front or emitting surface of the heating element, intercepts and absorbs, preferably without reecting, much of the infrared rays which are emitted by the heating element in directions which are wide of the object to be heated, but having been thus absorbed, the gridwork itself become an auxiliary emitter. Furthermore, the gridwork not only improves the uniformity of emission (obviating or minimizing hot spots), but also shields the front or emitting surface of the heating element from convection currents -which otherwise would waste heat from the refractory panel.

When the emitter is energized electrically, it is preferable to encase the resistance wires in material which is both dielectric and refractory; as, for example, by sandwiching the resistance wires between the two sheets of refractory material, and appropriately unifying the sheets -together about the wires to produce a refractory panel which becomes the heating element.

Preferably, the cells are square in the cross-section parallel with their apertures and circumscribed by walls whose height is about half the cross-dimension of the cells, and whose thickness is the least practical in the case of an emitter4 which is to be operated for relatively short periods of time, but in the case of an emitter which is to be operated for long periods of time, thicker walls, with greater emitting surface about each aperture and greater heat storage capacity may be desirable. For example, in the case of an emitter intended for use on a continuous production line, where large heat storage capacity in the gridwork reduces the power consumption, it may bedesirable to make the cells circular in crosssection, thereby increasing the thickness of the wall at locations between the points where a cell is most nearly tangent to its neighbors. On the other hand, where the emitter is for use in situations which, for the sake of economy, require that the emitter be energized and deenergized several times per hour, the loss of heat by the largements in wall thickness between circular cells may be more than offset by the loss due to convection currents, and in such cases, square or rectangular cells, whose bounding walls are thin enough that lthe average edge area bounding a cell is less than sixty-five percent of the aperature area, are preferred. The finite value of the aperture area of the individual cells is essentially a small fraction of the emitting area of the refractory panel, but, consistent with that criterion, the cells may be as large as two inches square and one inch deep, or as small as a half inch square and a quarter inch deep.

In the illustrative embodiment shown in the drawings and described in detail hereinafter, an intermediate cell size is selected for illustration, to wit: a gridwork whose cells are one inch square in cross-section, parallel with the front surface of the refractory panel, and a half inch deep perpendicular to said front surface. In the drawings:

FIGURE l is an isometric view of one embodiment of the emitter with supporting stand, as seen from the front or emitting surface;

FIGURE 2 is an isometric view of the emitter as seen from the rear or non-emitting surface thereof;

FIGURE 3 is a sectional view taken along plane 3-3 of FIGURE 2; and

FIGURE 4 is an exploded view of the refractory panel or heating element contained in the emitter shown in FIGURE 1.

The embodiment shown in the drawings is a two-unit emitter consisting of identical units 1 and 2, permanently connected together by a frame 3, so that the respective units are disposed one above the other, as shown in FIG- URE 1. It will be understood, of course, that the emitter may have only one unit, and that it may have more than two units arranged either edge to edge or end to end, depending upon the size and shape of the target area, and the distance between the target area and the emitter.

The front -face of each of units 1 and 2, as seen in FIGURE l, is a gridwork consisting of a series of parallel, equi-spaced, vertical slats 4 intersected by a series of equi-spaced horizontals S. In the embodiment shown, the spacing between the verticals-4 and the horizontals 5 is equal so that the gridwork defines a multiplicity of cells 6, each having a square opening or aperture addressed toward the front, and a corresponding opening or aperture addressed toward the rear. Where, as illustrated in the drawings, the apertures of the respective cells are one inch square, the depth of the cells (i.e., the Width of the respective vertical and horizontal slats) is preferably about one-half inch.

As shown in FIGURE 3, the rear of the gridwork is contiguous with a panel 7 which closes the rear opening of the respective cells, and is the source of infrared radiation. The panel 7 is a sandwich structure which consists of two sub-panels 8 and 9, as shown in FIGURE 4. The sub-panels are preferably made of refractory material, as, for example, that known commercially as Fiberfrax fiber, made by the Carborundum Company, which is composed of high purity alumina and silica, with small additions of modifiers, such as soda, borax, or zirconia. Other mineral fibers, which have a melting or softening point above the operating temperature of the heating element in the emitter of the invention, may be employed in lieu of the Fiberfrax fiber. In fact, the refractory material need not be fibrous if other means are employed for rendering it resistant to cracking or breaking as a result of repeated heating and cooling, or other abuse to which the panel may be subjected in use. In any event, the refractory material should have substantial dielectric strength, and a relatively low coeflicient of thermal expansion consistent with sufficient thermal conductivity to transmit heat fr-om one face thereof to the opposite face.

Such fibrous refractory material is available on the market in the form of paper sheets ranging in thickness up to about an eighth inch. The invention contemplates that there be mounted on one such sheet of refractory material, e.g., that which formed sub-panel 9, one or more resistance elements 10 which, in the form illustrated, is a length of electrical resistance wire, such as Nichrome, arranged in zigzag fashion, with its terminals 11 and 12 extending through holes 13 and 14, respectively, in the sub-panel. The electrical resistance wire may be bent into zigzag shape, such as that shown in FIGURE 4, and cemented in place with refractory cement. When the refractory cement is applied over the entire surface of sub-panel 9, on which the resistance ele-ment 10 is lo cated, it can, and preferably does, serve the dual purpose of maintaining the resistance wire 10 in fixed location, as well as securing sub-panel 8 to sub-panel 9, so that the resistance element is sandwiched between the two subpanels 8 and 9.

The composite sandwich of sub-panels 8 and 9 sealed together about the resistance wire 10 preferably has the fface thereof which is to be addressed toward the gridwork coated with an appropriate composition which endures the operating temperature of the device, which withstands repeated beatings and coolings and which leaves the coated face totally black. A suitable composition is a mixture of colloidal silica and carbon black in a suitable vehicle, such as water. When dried, the colloidal-silicacarbon-black solids produce a black surface which is the most efficient for infrared emission, as well as absorption. While it is less important that the surface of panel 7, which is addressed away from the cells 6, be blackened then that the surface thereof which is addressed t-oward the cells 6 be so blackened, both surfaces of the panel may be blackened if desired.

Likewise, it is desirable that all surfaces of the gridwork, consisting of vertical slats 4 and horizontal slats 5, be blackened, and this can be done by coating them with the same colloidal-silica-carbon-black composition, regardless of whether the slats -be made of metal, or of re- :fractory material, such as that of which sub-panels 8 and 9 are made.

Given the completed panel 7, it is assembled with the other components in the manner shown best by FIGURE 3, wherein the frame 3 has an overhanging flange 15 at the front thereof. The gridwork, consisting of slats 4 and 5, is disposed within theframe so that the outer edges of the slats at the margins of the gridwork abut against the flange 15. Immediately behind the gridwork, and abutted against the inward edges of the slats therein, the refractory panel 7 is disposed with the leads from terminals 11 and 12 of the resistance wire 10 extending through holes 13 and 14, as previously described. Any suitable protective sleeve 16, of dielectric material which is resistant to the temperatures encountered in use, may be inserted in each of the holes 13 and 14 so as to form a conduit for the lead wires from resistance element 10. Thereupon, a block 17, of any suitable heat insulating material and having a hole of cross-section such as to accommodate the sleeve 16, is ybrought into position against the rear side of panel 7. Other strips 18 of such insulating material are positioned adjacent the balance of the periphery of panel 7 within frame 3, and may be appropriately secured against either frame 3 or panel 7, so as to define an air space 19 between. the rear lsurface of panel 7, and a reflective panel 20 parallel with panel 7.

The reflective panel 20 may be a steel sheet having at least one surface which is reflective. Both aluminized steel and stainless steel are fappropriate, and when such steel has but one surface which is highly reflective, that surface is arranged to face toward panel 7.

Next behind the reflective panel 20 is a layer of heat insulating material 21 -which fills the space between reflective panel 20, and another parallel panel 22 which is not necessarily, but is preferably, also of metal having at least one reflective surface which, as was the case with panel 20, is addressed toward panel 7.

Behind panel 22 there is arranged a series of channel sections 23, 24, 25, and 26, which serve to stiffen the panels 22 of the respective units 1 and 1, and also serve to space from panel 22 a pair of ducts 27 and 28 for the units 1 and 2, respectively.

The ducts 27 and 28 are secured t0 the frame 3 in any appropriate manner, and each preferably has a removable cover accessible from the back of the emitter. The ducts 27 and 28 are also secured to the several reinforcing channels 23-26, and serve to house the electrical supply lines which energize the heating elements of the respective units, as Well as to accommodate, when desired, such thermal control devices as may be dictated by the circumstances of use in response to a sensor located either within the emitter, or adjacent the target which is the object to be heated.

One or both of ducts 27 and 28 may, as shown, be provided with a plug-in receptacle 29 for connection with electrical service.

While, in practice, the emitter of the invention may be mounted in any desired way, the embodiment shown in the drawings is mounted upon a stand consisting of legs 32 and 33, on each of which there is an adjustable bracket 34 secured to frame 3 by means of a stud 35 having a wing nut 36. The arrangement provides not only for vertical adjustment, but for adjustment about the axis of opposite studs 35, s0 that the emitter may be addressed either horizontally or vertically, or at any position therebetween.

In operation, the respective sub-panels of refractory panel 7 are directly heated by the energization of the resistance wire 10. Infrared rays radiate from every increment of the surface, both front and back, of panel 7. As shown in FIGURE 3, for one of cells 6, rays 30 and 31 represent the extremes of the field of infrared rays emanating from the center of a cell and which are not intercepted by the cell walls. Infrared rays emanating from the point source at lesser angles with the front surface of panel 7 are intercepted by those walls and absorbed by them until the temperature of the gridwork attains a value at which the edges and sides of the gridwork slats become emitters of radiant energy. Some of the infrared rays reemitted by the walls of the cell are addressed toward the target area, some are addressed toward panel 7, and some are addressed toward other cell walls. Consequently, when the cell walls are blackened, and hence substantially unreective, they reduce the waste of infrared energy which, without the refractory panel 7 being veiled by the gridwork, would be directed outwardly in a direction which would be wide of the target. Furthermore, when the emitter is in use, the veiling of the front face of refractory panel 7 by the gridwork largely reduces the dissipation of heat by the circulation of air thereacross, as the only surfaces of the gridwork which so dissipate heat to convection currents are the front edges thereof, whose total area is a relatively small fraction of the total area of the front face of panel 7. In a typical case, where the several cells are one inch square, and the slats 4 and 5 are an eighth inch thick, the total edge area surrounding the given cell is 0.5 square inch, but, save for the marginal ones, such walls each serve to partially bound an adjacent cell so that, per cell, the effective: edge area is only 0.25 square inch per one inch square cell.

Turning no wto the infrared energy which is emitted by the rear surface of panel 7; all rays, except those which travel in the direction of the insulating material 17 and 18, are intercepted by the reflective panel 20, and reflected backwards into panel 7 where they are re-absorbed and serve to maintain the temperature of panel 7, thereby conserving electrical energy. The greater the reflectivity of panel 20, the greater will be the degree of such reection of energy, but that energy which is not rellected by panel 20 traverses insulation 21, and may be reflected by panel 22 back toward panel 20 and panel 7. The vertical and horizontal slats 4 and 5, respectively, are preferably made of the same refractory material as that of which subpanels 8 and 9 are composed, but may, if desired, be made of metal. Regardless of the material of which such slats are made, however, it is preferred that they be blackened, or at least substantially non-reflective.

From the foregoing description, those skilled in the art should readily understand the invention, and realize that it accomplishes its objects by so controlling the infrared radiation from panel 7 that that which is emitted initially in a direction which is Wide of the target is large- 1y conserved by absorption in, and re-emission from, the gridwork, and, by reflection from panel 20, thereby reabsorbed and re-emitted by panel 7. Moreover, the radiation from the entire surface of panel 7, and the gridwork which veils the front of it, is substantially uniform, and devoid of the hot spots which characterize infrared emitters of the character heretofore employed.

While one complete embodiment of the invention has been shown in the drawings and described in detail hereinbefore, it is not to be understood that the invention is limited to the embodiment so disclosed, but, on the contrary, that the principles of the invention are applicable at large to infrared emitters where it is desired to make most eicient use of the energy supplied.

Having thus described the invention, what is claimed and desired to be secured by Letters Patent is:

1. An infrared emitter comprising a heating element having a front surface and a rear surface, and a gridwork overlying said front surface and contiguous therewith, said gridwork defining a multiplicity of open cells separated from each other by side walls whose aggregate cross-sectional area, parallel with said front surface, is substantially less than the aggregate cross-sectional area of the cells, said walls having a reflectivity of less than twenty-tive percent, lwhereby hot spots are minimized and said front surface of the heating element is shielded from convection currents.

2. The emitter of claim 1 wherein the heating element is a panel of dielectric material.

3. The emitter of claim 1 wherein each of said cells is parallelepipedonal and has:

(a) an aperture whose area, parallel with said front surface, is between 0.0625 and 4.0 square inches; and

(b) four side walls Whose aggregate area perpendicular to said front surface is about twice the area of said aperture.

4. The emitter of claim 1 wherein each of said cells is parallelepipedonal and has:

(a) an aperture whose area is parallel with said front surface, is between 0.0625 and 4.0 square inches; and

(b) walls whose aggregate cross-sectional area, parallel with said front surface, is less than sixty-live percent of the aperture area.

5. The emitter of claim 3 wherein said grid and said front surface are black.

6. The emitter of claim 2 wherein said panel is refractory material, a at reflector, and said rear surface being addressed toward said fiat rellector.

7. The emitter of claim 1 having a reflective shield behind the rear surface of said heating element.

References Cited UNITED STATES PATENTS 1,701,096 2/1929 Bowling et al 219-376 X 1,923,083 8/1933 Fisher 219-365 2,545,805 3/ 1951 Callender 219-345 2,717,950 9/ 1955 Nathanson 219-345 3,031,739 5/1962 Boggs 219-345 X FOREIGN PATENTS 1,078,795 5/ 1954 France.

JOSEPH V. TRUHE, Primary Examiner MARTIN C. FLIESLER, Assistant Examiner U.S. C1. X.R. 219-347, 354

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,493,724 February 3, 1970 Harold D. Wells It is certified that error appears in the a'bove identified patent and that said Letters Patent are hereby corrected as shown below:

Column l, line 47, "contgnously" should read contiguousl Column 2, line 33, "largements" should read enlargements Column 3, line 65, "then" should read than Column 4, line 37, "units l and l," should read units l and 2, Colu 5, line Z0, "effective: edge" should read effective edge line 22, "no wto" should read now to Signed and sealed this 17th day of November 1970.

(SEAL) lAttest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, 1R.

Attesting Officer Commissioner of Patents

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