US3089940A - Radiant heater - Google Patents

Description

May 14, 1963 w. H. NORTON RADIANT HEATER Filed May 11, 1960 sigiii A A \s United States Patent Office .'igdl) Patented May 14, 1963 3,089,940 RADEANT HEATER William H. Norton, Mundelein, lll., assigner to Thermal lneorporated, Franklin Park, lll., a corporation of Delaware Eiietl May 11, 1960, Ser. No. 28,455 1 Claim.. (Cl. E9n-3d) The present invention relates to improved heating devices and more particularly to a radiant heating unit.

The present invention contemplates the provision of a heating unit employing an infra-red heat radiating surface having improved emissivity to increase the capacity of the heating unit with which the surface is used, and having an improved dispersion pattern of heat transfer to improve its heating effect and broaden its adaptability. The features of the invention are particularly useful in a heating unit having an elongated electrical resistor contained within a tubular sleeve or sheath of thermally conductive material such as metal with an electrically insulative refractory particulate material filling the sheath and maintaining the resistor therein in closely spaced relation to the sheath. The sheath is in the form of a cylindrical tube with its outer surface having a pattern of projections thereon with each projection having sloping side surface areas facing outwardly, such as are provided in a preferred form -by knurling the outer surface of the tube. The surface areas each provide an infra-red emission surface area and the entire surface emits the infra-red rays outwardly in a dispersed pattern and each surface area is positioned relative to other emitting areas so that the infra-red rays will not strike the surfaces of adjacent areas or adjacent projections.

It is accordingly an object of the invention to provide an improved heating unit with a surface capable of emitting increased infra-red rays to increase its heat radiation capacity.

Another object of the invention is to provide an improved heating unit having a cylindrical outer surface wherein the dispersion pattern of heat transfer is improved and heat is radiated to a work piece over a greater surface area and the area is more uniformly heated.

A feature of the invention is the provision of a tubular type of heating element having a plurality of infra-red emitting surface such as are formed by knurling to improve the emissivity of the element and thereby increase its capacity 'while maintaining the surface at a temperature for obtaining optimum or maximum infra-red radiation. Over the range of wavelengths for infra-red emission there are optimum wavelength spans wherein energy transmission is at a maximum. The wavelength at which infrared radiation occurs from the surface of the heater is dependent on the heater surface temperature. Therefore for a radiant heater with a given size of heater emission area, the electrical input is limited by the capacity of the area to emit infra-red heat energy. To further increase temperature merely results in moving out of the range for optimum infra-red emission.

A feature of the present invention is to provide an increase in emissivity of a radiant heat surface to thereby increase the electrical input for a given heater, thereby increasing its capacity without requiring an increase in size and the accompanying increase in space consumed and increase in expense. The invention particularly contemplates increasing the emissivity of a heating element of the tubular sheath type.

Another feature of the invention obtained from the provision of a pattern of heat emitting projections with infra-red radiating surfaces on the outer surface of the heating unit is the improved heat dispersion pattern. With tubular heating units heretofore provided the heat was directed downwardly along a narrow line so that obtaining uniform heating over a substantial area with a plurality of units was difficult to achieve. Uniform heating is important in a number of environments particularly in such equipment as vacuum forming machines where 4broad surface areas are to be heated. This feature of the invention per-mits the entire heater to be moved closer to the material to be processed thereby increasing without detriment the total intensity of the radiant pattern.

Other features and objects will become more apparent with the teaching of the principles of the present invention in connection with the disclosure of the preferred embodiments thereof in the specification, claim, and drawings, in which:

FIGURE l is a side elevational view of a heating unit of the type in which the principles of the present invention may vbe embodied;

FIGURE 2 is an enlarged vertical sectional view taken substantially along line Il--lI of FIGURE l;

FIGURE 3 is a detailed fragmentary view, shown partially in section, of a length of heater tube constructed in accordance with the principles of the invention;

Fi-GURE 4 is a greatly enlarged plan View of a fragmentary section of the surface of the heater tube; and

FIGURE 5 is a schematic end view of a bank of heater tulbes arranged to heat a work surface.

As shown on the drawings:

As illustrated in FIGURES l and 2, a heating unit 10 includes a tubular type heating element 11, which is shown in greater detail in `FIGURE 3.

The tubular element `11 is suitably mounted at its ends in electrical boxes 12 and 13 with an electrical cable 14 leading into the box l2. A conduit 15 extends across between the boxes for containing a wire leading to the box 13.

Attached to the conduit 15 and the boxes 12 and 13 are end plates 16 and 17. The end plates, as illustrated by the plate 16 in FGURE 2, have openings 18 to slide the plates over the conduit 15 and have set screws 19 for locking to the conduit.

The end plates have downwardly extending legs 2@ and 21 which are notched to provide a support for a heat reflector 22 which is concavely shaped and extends across above the heater element 1l.

As illustrated in FlGURE 3, the heater element 11 includes an electrical resistance wire 23 contained within an outer metal sleeve or sheath 25. Within the sheath is an electrically insulative compacted refractory particulate material 24 maintaining the resistance wire Z3 in closely spaced relation to the sleeve. The tubular sleeve 25 is formed of a heat conducting material, preferably such as those known as Inconel or Incolloy, or other suitable metals or materials capable of withstanding ternperatures of 9001100 F.

On the outer surface of the sleeve 25 are a plurality of raised surface projections 26 preferably formed by knurling the outer surface of the tube 25. Each of the surface projections has infra-red radiating side areas facing generally outwardly from the tube, shown at 27, 2S, 29 and 3ft for the projection 25. Each of the areas radiates infra-red with the surfaces shown as being at an angle to the outer cylindrical plane of the tube and to each other to provide a diffused pattern. The side surfaces of the raised projections are at an angle so that the infra-red rays emitted therefrom do not strike adjacent projections but project outwardly to a work surface. The projections increase the infra-red radiating surface to increase the total ernissivity of the tube.

The projections 26 are shown as formed by a diamond lrnurling tool and are fully raised to a peak. Knurling of a less depth will work although the side surface areas will not be as large and emissivity will not Ibe increased as much.

'The diffused pattern of infra-red rays formed by the projections avoids a concentration of heat pattern as occurs with a tube having a smooth outer surface. Because of the increase in surface area and the diffused pattern the entire heater can be moved closer to material being processed, thereby increasing, without detriment the total intensity of the radiant pattern. For example, as illustrated in FGURE 5, tubular heating elements 3f, 32, and 33 are shown arranged in a bank above a work surface 34. With the use of ordinary tubular heating elements a concentration of the heat pattern will occur in a straight line below each tube inasmuch as the infrared rays will be emitted radially and the intensity varies inversely as the square of the distance, so that uniform intensity could be achieved only along an arcuate surface at constant radius from the tube. With present arrangement the heater banks can be moved more closely to the work surface while obtaining a more uniformly heated area.

Another factor obtained from the present improved arrangement permits the heaters to operate cooler and a greater power input can be used within the same connes than as heretofore been possible with a smooth tube. The more power which can be concentrated and directed on a uniform basis, the faster and more effective and more eiicient the entire system becomes.

Another advantage in the present structure which has been discovered is that long tubes do not sag or warp out of shape under radiant temperature. This is of importance particularly when the present arrangement permits bringing the tubes closer to the work so that any sag would be highly objectionable and intensities occasioned by a sag could be critical. With the present knurled eiement long spans up to 4 or 5 feet have shown no tendency to sag and there is absolute uniformity of the heater. Furthermore the need for an interim support is eliminated and an interim support is objectionable inasmuch as it will normally create a cold spot on the surface of the element.

While a pattern of projections on the heater element surface formed by a standard diamond knurling tool is preferred for convenience in obtaining a good diffusion pattern, other projection shapes may be employed. A plurality of conical surfaces would be advantageous although in mass production these projections would be expensive to achieve. Another operation which would obtain the projections and the infra-red radiation surfaces would be to roll cross threads on the surface. A right hand thread rolled in the surface and then repassed with a left hand thread would give a double crossing. However, it has been found that a single standard thread is unsatisfactory inasmuch as the radiant rays emitted from the surfaces of the threads strike adjacent threads. In some instances it will be satisfactory to form the projections by a very coarse sand blasting or shot blasting of the surface thereby forming a pattern of projections with infra-red emitting areas. This will not be as uniform or as effective in forming surfaces although it may be less expensive to accomplish.

Tests have indicated that increased emissivity due to providing projections by knurling on a tube surface decreases the temperature of the surface on the order of 100 to 150 F. when normally running in the range of 900 F. to 1150 F. This indicates an efficiency between 15% `and 20% greater emissivity. Operations have also shown that a tubular element having a smooth outer surface will create a straight line intense pattern of heating which will burn a surface at a given distance Whereas the same tubular element when knurled will operate a much broader pattern of heat of uniform temperature without burning the surface thereby illustrating the feasibility of positioning the element closer to the work surface.

The structure thus finds advantage in use in environment such as in vacuum forming machines, heat banks for paint dryers, physiotherapy infra-red, comfort heaters, cooking, and uses where conductive or convective transfer of heat is impractical or inefficient.

Thus it will be seen that l have provided an improved radiant heater for infra-red radiation which meets the objectives and advantages hereinbefore set forth. The provision of the surface with the projections and the infra-red radiating surface areas, such as provided by knurling, increases the available surface area and provides a greater emissivity directly in proportion to area increase; provides a greatly dispersed heat transfer pattern avoiding concentration of heat transfer; permits moving the heater closer to the work surface to be processed permitting an increase in the total intensity of radiant pattern; obtains increased capacity and energy input for a heating unit of a given size operating at optimum infra-red temperature radiation; and permits the use of longer heating elements without support.

The drawings and specification present a detailed disclosure of the preferred embodiments of the invention, and it is to be understood that the invention is not limited to the specific forms disclosed, but covers all modifications, changes and alternative constructions and methods faliing within the scope of the principles taught by the invention.

l claim as my invention:

in electrical heater comprising an electrical resistance wire, a heat resistant electrically insulating material surrounding said wire, an elongated tube of high temperature resistant heat conducting material enclosing said Wire and said insulating material with the material compacted therein and having a cylindrical outer heat emitting surface with a plurality of pyramidally shaped radial projections having planar' side surfaces facing away from the tube forming a uniform pattern on said surface extending for the full length of the surface and for the full circumference thereof so that the infra-red emission is increased and the size of the tube is reduced for a given heat emission capacity, said projections having planar side surfaces at an angle relative to the planar side surfaces of the adjacent pyramidal projections so that infrared rays emitted normal to the surfaces Will not strike surfaces of adjacent projections, and a heat reliector positioned behind said tube and extending parallel thereto to reiiect heat from one side of the circumference in the same direction as the heat is emitted from the other side of the circumference.

References Cited in the file of this patent UNlTED STATES PATENTS 1,644,255 Kercher et al. Oct. 4, 1927 152,934 Trent Apr. 4, 1939 2,627,314 Kolb Ian. 27, 1953 2,860,225 Steen Nov. 1l, 1958

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