Abstract:
A microwave heating apparatus with a tubular waveguide applicator and reactive and resistive chokes to decrease leakage. Microwave-transparent centering elements maintain articles to be treated centered in the applicator. Articles, such as individual cylindrical articles or continuous cylindrical strands, advance through the applicator in a direction in or opposite to the direction of propagation of microwaves. The resistive chokes have conductive vanes coated with a dielectric material that absorbs microwave energy that leaks through the reactive chokes to allow for large openings for large-diameter articles. The waveguide applicator is operated in the TE 01  mode to concentrate microwave heating energy along the outer circumferences of the articles.

Description:
BACKGROUND 
     The invention relates generally to microwave heating apparatus and more particularly to waveguide applicators for heating or drying products with microwaves. 
     Microwaves are often used in industrial processes to heat or dry products. For example, U.S. Pat. No. 4,497,759 describes a waveguide system for dielectrically heating a crystalline polymer drawn into a rod fed continuously through a circular waveguide applicator along its centerline. The TM 01  mode is used to concentrate the heating along the centerline. The narrow waveguide applicator has an inner diameter of 95.6 mm, which limits its use to small-diameter products, such as drawn polymer rods. For continuous heating and drying processes in which individual products or a product strand is fed continuously through a waveguide applicator, openings are provided at opposite ends of the applicator for product entry and exit. But microwave radiation can also leak through the openings, especially if the openings are large to accommodate large-diameter products. 
     SUMMARY 
     One version of a microwave heating apparatus embodying features of the invention comprises a tubular waveguide applicator forming a heating chamber between a first end and an opposite second end. The applicator has a circular cross section and an axis along its centerline. A waveguide feed connected between a microwave source and the tubular waveguide applicator at the first end propagates microwaves through the tubular waveguide applicator from the first end to the second end with a dominant TE 01  field pattern in the heating chamber. A first resistive choke is connected in series with the tubular waveguide applicator at the first end. A second resistive choke is connected in series the tubular waveguide applicator at the second end. Each of the resistive chokes includes a plurality of conductive vanes covered with a microwave-absorbent material and spaced apart along the axis in a chevron pattern. The vanes have central apertures aligned with openings in the opposite ends of the resistive chokes and with the heating chamber to guide articles to be treated in the heating chamber through the resistive chokes. 
     Another version of a microwave heating apparatus comprises a tubular waveguide applicator that has a cylindrical outer wall terminating in a first end and an opposite second end to form a heating chamber with a circular cross section between the first and second ends with an axis along the heating chamber&#39;s centerline. A microwave source supplies microwave energy into the tubular waveguide applicator. A first reactive choke is disposed in series with the tubular waveguide applicator at the first end of the tubular waveguide applicator. A second reactive choke is disposed in series with the tubular waveguide applicator at the second end of the tubular waveguide applicator. A first resistive choke is connected in series with the tubular waveguide applicator and the first reactive choke. A second resistive choke is connected in series with the tubular waveguide applicator and the second reactive choke. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These features of the invention are described in more detail in the following description, appended claims, and accompanying drawings, in which: 
         FIG. 1  is an isometric view of a tubular waveguide applicator embodying features of the invention; 
         FIG. 2  is a cross section of the waveguide applicator of  FIG. 1 ; 
         FIG. 3  is an enlarged cross section of a reactive choke in the applicator of  FIG. 1 ; 
         FIG. 4  is an enlarged cross section of a resistive choke in the application of  FIG. 1 ; and; 
         FIG. 5  is a cross section of the tubular applicator of  FIG. 1  showing the electric-field pattern. 
     
    
    
     DETAILED DESCRIPTION 
     A microwave heating apparatus embodying features of the invention, including a tubular waveguide applicator, is shown in  FIGS. 1 and 2 . The applicator  10  shown in this example is constructed of a single circular waveguide section having a cylindrical outer wall  11  forming a heating chamber. But the applicator could be constructed of a series of individual circular waveguide sections connected end to end. The applicator  10  has circular flanges  12  at each end. Plastic or teflon ribs  14  extend radially inward from the inside surface of the metal waveguide walls. The ribs  14 , which extend along the length of the applicator  10 , are spaced apart circumferentially around the inner circumference of the applicator. The plastic or teflon ribs  14  are transparent to microwaves. The ribs extend radially inward a distance sufficient to bound a central bore  16  through the heating chamber through which articles, such as individual cylindrical items or a continuous cylindrical strand, can pass. The ribs  14  center and guide the articles through the central bore  16 . 
     A microwave source  17  injects microwaves  18 , for example, at 915 MHz or 2540 MHz, into the waveguide applicator  10  through a rectangular waveguide feed  20  at an entrance end  22  of the applicator. The microwaves propagate along the waveguide applicator  10  from the entrance end  22  to an exit end  23 . The microwaves travel through the interior of the applicator  10  in a direction of propagation  24  parallel to the axis  25  of the applicator. Microwave energy unabsorbed by the articles to be treated in the heating chamber exits the exit end  23  through a rectangular waveguide segment  21  to a dummy load  26 , which prevents reflections back into the applicator. But it would also be possible to operate without a dummy load and allow the microwave energy to reflect back through the applicator  10  toward the entrance end  22  and, in that way, double the effective length of the applicator. The shorter sides  27  of the rectangular waveguide feed  20 , which define the feed&#39;s E plane, are perpendicular to the axis  25  of the applicator  10  to produce an electric field pattern in which the TE 01  mode is dominant. 
     As shown in  FIG. 5 , the TE 01  mode produces an electric field with circular symmetry in the applicator  10  and with its maximum electric-field intensity midway between the centerline and the cylindrical outer wall  11  of the applicator. This increased field intensity between the center and the wall is indicated by the bolder and denser arrows  28  concentrically circling the centerline in the electric-field pattern shown in  FIG. 5 . The magnitude of the electric field at any position along the applicator varies sinusoidally with the passing traveling microwave with reversals of direction every half cycle. Because the field intensity is greatest near the inner ends  30  of the guide ribs  14 , the applicator  10  is especially useful in applications that require the outer circumference of the cylindrical article to be heated. 
     As shown in  FIG. 2 , cylindrical articles  32  enter the vertically oriented applicator  10  at the upper end and fall through the applicator aided by gravity. The articles  32  advance through the applicator  10  in or opposite to the direction of propagation  24  of the microwaves. The articles could be advanced through the applicator by an injected air stream instead of or in addition to gravity. As the articles fall, the microwaves heat the outer portions. For large-diameter articles the central bore has to be relatively large with respect to the cross-sectional dimensions of the waveguide applicator  10 . For that reason leakage of microwave energy through the large openings at the ends  22 ,  23  of the applicator is reduced by two chokes  34 ,  42  at each end. 
     The chokes  34  closer to the applicator are reactive chokes that reflect microwave energy back into the applicator. The reactive chokes  34  are positioned at the ends  22 ,  23  of the applicator  10 . The reactive chokes  34  shown in  FIG. 3  in more detail are constructed of four metal circular waveguide segments  36 ,  37 A,  37 B,  38 . Each segment has a flange  40  at each end to attach to the flange of another segment, of the applicator  10 , or of a choke box  42  ( FIG. 1 ) with screws, for example. The left-most segment  38  in  FIG. 3  is a flanged cylindrical metallic tube having a circular bore. The identical interior metallic waveguide segments  37 A,  37 B are flanged at each end and have a stepped bore formed by a small-diameter section  44  and a large-diameter section  45 . The small-diameter section  44  has the same inner diameter as the left-most segment  38 . The right-most segment  36  is the same as the interior segments  37 A,  37 B, except that the small-diameter section  44 ′ is elongated. A plastic or teflon microwave-transparent ring  46  having the same inner diameter as the small-diameter sections  44 ,  44 ′ is retained in the large-diameter end of each interior waveguide segment  37 A,  37 B and the right-most segment  36 . When the waveguide segments are fastened to each other, the rings  46  are clamped in place and form a continuous smooth bore with the small-diameter sections  44 ,  44 ′ and the bore of the left-most segment  38 . The smooth bore allows cylindrical articles to pass through without snagging. Air gaps  48  are formed between the walls of the large-diameter sections  45  and the rings  46 . The air gaps  48  are spaced apart axially on quarter-wavelength centers (about 2.9 cm at 2540 MHz). The quarter-wavelength spacing of the steps in the waveguide&#39;s diameter provides choking that reduces the leakage of microwave energy. 
     Because of the large opening required to accommodate large-diameter articles entering and exiting the reactive chokes  34 , the reactive chokes may not reduce leakage enough. So resistive, absorbing choke boxes  42  ( FIG. 1 ) are connected in series with the reactive chokes  34 . The resistive chokes  42  are shown in more detail in  FIG. 4 . The choke box  42  is shown as a rectangular box in  FIG. 4 , but it could be another shape, such as circular or elliptic cylindrical. The dimensions of the choke box  42  are greater than the diameter of the bore formed in a plastic or teflon tube  50  extending centrally through the choke box. V-shaped, conductive metallic vanes  52  arranged in a chevron pattern have central apertures  54  to receive the microwave-transparent tube  50  that guides the articles centrally through the choke box  42 . The vanes  52  are attached at their opposite ends to one pair of side walls  56  of the choke box. Openings  57  in end walls  58  are aligned with central apertures  54  in the vanes to admit the tube  50  and guide articles centrally through the choke and into the applicator. The metallic vanes are coated with a dielectric material, such as Eccosorb, that absorbs microwave energy. Like the steps in the reactive chokes  34 , the vanes are spaced apart in the axial direction by a quarter of the wavelength of the microwave radiation. The combination of the reactive and resistive chokes reduces the leakage to a level 60 dB below the power level of the microwave source  17  ( FIG. 1 ).