EP0014121B1

EP0014121B1 - Microwave heating apparatus

Info

Publication number: EP0014121B1
Application number: EP19800400045
Authority: EP
Inventors: Jerome R. White; Jacques Thuery
Original assignee: JD Technologie AG
Current assignee: JD Technologie AG
Priority date: 1979-01-22
Filing date: 1980-01-14
Publication date: 1987-04-22
Also published as: DE3071956D1; EP0014121A1

Description

Background of the invention

The present invention relates to microwave heating apparatus for processing material enclosed in an evacuated chamber.
Heretofore, it is known to provide in industry a continuous process system for heating and drying subject material moved on a conveyor through a microwave chamber that is sealed and evacuated and into which microwave energy is fed. These systems are now of large physical size and feed many kilowatts of continuous microwave power into the chamber from a multitude of sources. A system of this sort is described in US-A-4,045,639. The function of such apparatus is to feed microwave energy con-: tinuously into the process material with as uniform distribution of the energy as can be achieved, while the material is moved through the chamber on a conveyor. Large systems of this sort may use six or more magnetrons, each operating continuously and producing many kilowatts of power. Hence, the total microwave power fed into the chamber is large and may be in the range of 50 to 100 kilowatts continuous power.
- Operating at such high power levels clearly requires the use of high power sources in order to avoid having an excessively large number of sources. Magnetrons of such high power have a narrow limitation on the allowable amount of power that may reenter them, either as a reflection of the magnetrons' own output or as "cross-talk" from other magnetrons in the system. It has been suggested by some to use circulators to avoid this problem; however, circulators are relatively expensive and they do not solve other problems that accompany high power delivery into a microwave chamber. Also, where vacuum drying is carried out simultaneously with microwave heating, it is very difficult to prevent ionization of gas in the chamber by the microwave electric fields. Ionization permits arcing that wastes the microwave energy and, it can also degrade the quality of the process material and is generally destructive of the apparatus.
It has been suggested to use arc detectors within the chamber that detect the occurrance of an arc and quickly interrupt microwave power to minimize the harm that is done by the arcing. However, frequent interruptions make the heating process very inefficient and in the processing of some materials the interruptions are not at all acceptable.
Some of the objects of the present invention are to overcome these problems of non-uniform high power, discontinuous heating, reflection into the magnetrons, as well as cross-talk. Another object is to reduce ionization and arcing problems where microwaves are used within an evacuated chamber.

Summary of the invention

The uniform distribution of continuous high microwave power into the process material is accomplished by radiating, preferably at regular places along the inside of the chamber, defined beams of circularly polarized microwave radiation, each beam being directed to illuminate a prescribed portion of the process material or a prescribed area through which the process material is moved. Microwave beams are weaker near their edges than at the center and so they are overlapped to compensate for this and provide illumination that is essentially uniform across the width of the path of the process material. Overlapping may also be effective to compensate for non-uniform distribution of the process material on the conveyor. Thus, the illumination does not depend upon the use of mode stirrers or turntables or any other equipment designed to reflect the microwave energy about the chamber and it does not depend particularly upon the geometry of the chamber. It permits the chamber to be designed in view of the pressure or vacuum conditions of operation: For example, it permits-a cylindrical chamber which is the preferred shape for withstanding vacuum and still provide for the spatially extended continuous processing of the subject materials on a conveyor belt.
In the past where microwave heating has been accomplished in a cylindrical chamber, the process material was not directly illuminated by microwave energy beamed into the chambers. As a consequence, the tendancy was to concentrate the microwave energy near the walls of the cylindrical chamber and there would be a deficiency of energy along the cylinder axis (along the center of the conveyor). Thus, material carried through the chamber on the conveyor would be heated more along the edges of the conveyor than at the center. Where the process of microwave heating occurs while at the same time vacuum drying occurs, the vacuum pressure produced in the chamber is often in the range in which ionization by the microwave electric fields and arcing are very difficult to prevent. In the present invention a structure is provided so that the peak values of electric fields of the microwave energy at a window where the microwave energy enters the chamber from the ambient pressure exterior are severely limited. Experience as well as theory show that high intensity microwave electric fields are produced at the "bottle neck" where the microwave is transmitted from the exterior region through some sort of microwave transparent, pressure withstanding window, into the evacuated interior of the chamber. Elsewhere, within the chamber, the intensity of the electric fields is usually much lower and ionization problems are reduced.
In an embodiment of the present invention described herein, a cylindrical microwave chamber is provided through which a conveyor belt moves the process material at about the. center of the cylinder. Opposite the material along the walls of the chamber, at regularly spaced places, separate sources radiate beams of microwave energy into the chamber, directed toward the process material. Each beam illuminates a prescribed area of the conveyor belt and the beams overlap particularly along the center of the belt so as to produce a uniform or an intentionally "tailored" heating pattern. The configuration of each beam is modified by a dielectric lens that redirects the microwave energy just before it enters the chamber through the window and directs the beam toward the conveyor.
In order to reduce arcing in the microwave chamber, it is necessary to minimize the intensity of the microwave electric fields and, thereby minimize ionization within the chamber, which occurs when the chamber is evacuated, as in a heating-drying operation. This is done by radiating beams of microwave energy that are circularly polarized and in the TE mode over substantially the whole area of the relatively large window through which the microwave beam is radiated into the chamber.
In order to heat uniformally and avoid those problems in prior systems where heating is excessive at the edges of the conveyor and deficient at the center, the radiation is re-directed by the lens and radiated directly at the process material, impinging upon the material at the "first pass". Another design feature that contributes to uniform heating is to provide separate microwave power sources for the sources that are not mutually coherent.
In the preferred embodiment, each of the individual beams is generated outside of the chamber, re-directed by the lens and radiated into the chamber through a transparent window at the chamber wall. Thus, there are a multitude of lenses and transparent windows arranged along the chamber wall through each of which a formed beam of circularly polarized microwave radiation is radiated directly toward the process material. The window is a half wave length of the microwave in thickness to minimize reflections from the window.
In order to minimize the likelihood of ionization around the window on the inside of the chamber, the area of the window is made as large as practical, the beam radiated through it is predominantly TE mode waves rather than TM mode waves and is essentially a single mode beam rather than a mixture of modes and the single mode is circularly polarized. One advantage of circular polarization is that twice as much power is transmitted as for a plane polarized beam for a given peak value of the microwave electric field.
It is an advantage in some embodiments of the present invention to provide a dielectric lens or prism near the dielectric window. This allows the source apparatus that produces the circularly- polarized, single-mode TE beam spread over the entire area of the relatively large window to be vertically oriented above the chamber (and above the conveyor), with the source magnetron upright and not "tipped". Magnetron performance and life is best when operated upright. Furthermore this orientation of the source apparatus and the magnetron therein is often preferred from the standpoint of: total floor area required for the heating apparatus; ease and convenience of maintenance; and cost of manufacture.
Vertical orientation of the sources on top of the cylindrical chamber also facilitates changing the direction of a beam so that the beam therefrom can be directed to one side or the other of vertical and so pointed as needed at one region or another of the conveyor. The vertical orientation of sources can be along the axial plane of symmetry of the cylindrical chamber and conveyor therein; or the sources can be set off to either side of that plane by the effect of a prism lens, and send skewed beams toward the conveyor. The skewed beam does not define a figure of revolution and does not illuminate the conveyor uniformally everywhere it strikes, but equal skewed beams from opposite sides of that plane can balance each other and even provide combined illumination that is uniform across the conveyor.
It is an object of the present invention to provide high power microwave apparatus for continuously heating process material uniformly throughout, even when the material. is spread over a relatively large surface.
It is a further object to provide microwave heating and drying apparatus wherein problems of arcing are reduced or avoided.
It is another object to provide microwave heating and drying apparatus wherein the problems of reflection into the source of microwave energy, cross-talk, uniform high power continuous heating, ionization and arcing are reduced and/or avoided.
It is another object to provide a microwave lens for shaping and/or directing a pattern of microwave energy radiated into microwave heating apparatus.
It is a further object to provide improved microwave heating apparatus.

Description of the drawings

Figure 1 is a cross section of continuous process microwave heating and vacuum drying apparatus for treating a process material;
Figure 2 shows the same apparatus in cross section taken longitudinally through the apparatus;
Figures 3 and 4 show in detail the structure 6 for converting linearly polarized radiation to circularly polarized radiation, and:
Figure 5 is a side cross section view of structure for providing a dielectric prism lens adjacent to the outside of the chamber window for directing and/or shaping, the circularly polarized microwave radiation as a skewed beam toward the conveyor.
Figure 6 is the top view of the dielectric prism lens structure 9;
Figure 7 is the top view of the layers of dielectric showing construction of the prism lens; and
Figures 8 and 9 are a side cross section view and top view of the layers of dielectric for another lens configuration for forming a controllably spread beam that is not skewed as the beam formed by the lens shown by Figures 5, 6 and 7.

Description of an embodiment of the invention

Turning first to the Figures 1 and 2, there is shown a section of cylindrical microwave chamber. The chamber is defined by the chamber wall 12 that is electrically conductive. It is cylindrical in shape, because that shape is intrinsically strong and best able to withstand the pressures produced on the chamber when it is evacuated as, for example, in a microwave heating and vacuum drying process. Within the chamber is a conveyor belt 21, a section of which is shown in the Figures, positioned substantially at the center of the chamber and extending longitudinally therethrough. The conveyor belt carries the process material 22 usually distributed evenly along and across the belt.
Only a section of the complete chamber is shown in these Figures. It is the section where the process material is irradiated by the high power directional microwave beams to heat the material. There are well known structures and techniques for vacuum sealing the ends of such a cylindrical chamber, for mounting and powering a conveyor system within the chamber and for feeding the process material on-to and off-of the conveyor belt. Also, there are many known techniques for sealing the chamber against leakage of microwave energy from the chamber that could be hazardous and/or wasteful. In as much as none of those techniques are the particular subject of the present invention, they are not described herein.
Along the section of the cylindrical chamber disclosed in the drawings, all on the same side of the conveyor belt 21, six beams of microwave energy are radiated into the chamber, each beam coming from a different source and directed to a specific area of the belt (presuming the belt to be stationary for the moment). The spatial arrangement of the beam is regular; they are uniformly spaced longitudinally along the belt in pairs, each pair including the left beam and the right beam (as viewed in the direction of the moving belt--Fig. 1). Thus, the arrangement of the beams along the chamber is symmetrical about the plane defined by line 23 shown in Figure 1.
The microwave radiation contained within each beam is preferably of substantially uniform intensity across the beam-prior to entering the cylindrical chamber-and is circularly polarized. Furthermore, where the beams enter the cylindrical chamber, defined by walls 12, through an opening 13 in the chamber, the beam substantially completely fills that opening. As shown in Figures 1 and 2, the beams are defined by broken lines and are denoted 24, 26 and 28 along the left side of the chamber, and 25 along the right side of the chamber. Each beam preferably overlaps the adjacent beams and so insures that along this microwave heating section of the chamber substantially the whole area of the conveyor belt 21 is illuminated by the sources. Furthermore, by overlapping the beams, the tendency is to compensate for the reduced intensity of radiation at the edges of the beams.
The uniform arrangement of equal beams is preferred. If unequal beams are used and/or the beam spacing is not regular, particular heating effect could be achieved; however the problem becomes complicated. The particular embodiment described herein suggests using equal beams all of the same intensity and size, uniformly spaced along the chamber and overlapping where they illuminate the conveyor belt enough to insure adequate illumination along and from side to side thereof in the microwave heating section of the apparatus. Furthermore, each of the beams is from a different source wherein the microwave energy is generated, converted to a linearly polarized TE mode, then converted to circular polarization, spread, redirected and radiated into the chamber. The separate sources, denoted 34, 36 and 38 .on the left side and 35, on the right (one for each beam), may be constructed - su.bstantially identical to each other as a matter of convenience..
The process material is carried through the microwave heating section shown in Figures 1 and 2 on the conveyor belt and heated by the direct radiation of the beams. Thus, the process material is primarily heated by the first pass of microwave energy from the microwave sources and there is no primary dependance upon reflections of the microwave energy within the chamber to direct it to the process material. Hence, there is no need of microwave mode stirrers within the chamber. In the preferred embodiment, each beam is derived from a different source, and there is no phase coherence between the beams; so, again, there is no need for mode stirrers within the chamber. In the preferred embodiment there is no direct radiation from one source into another source, and since the radiation is circularly polarized, a negligible amount of radiation from any source that may reflect within the chamber will reenter the same or any - other source.
Thus, re-entrant, cross-talk, and moding problems, that have occurred with prior systems, are avoided.

Sources

The beams 24, 26, 28 and 25 respectively are produced by sources 34, 36, 38 and 35 respectively. These sources may be constructed substantially all the same and so only one of them, source 34, is described in detail herein below.
The source consists of a microwave power generator, a mode-forming launcher, a polarization convertor, a conical expansion in diameter, and an assembly, which includes a sealed transparent dielectric window in the chamber and a dielectric lens. The microwave power generator is a,magnetron 1. The coaxial output section 2 from the magnetron terminates in a probe 4 that feeds a standard waveguide section 3, also called a launcher, that launches the microwave energy into the waveguide and thence into the polarizing and beam-forming portions of the source.
The polarizing section of the source includes a quarter wavelength transformer section 5 between the launcher 3 and the polarization converter 6. The converter 6 consists of a square waveguide and means, denoted 7, within the square waveguide for converting linearly polarized radiation from the launcher into circularly polarized radiation. Thus, the microwave radiation flowing out of the converter 6 is circularly polarized and flows into the top end of conical wave guide coupling 8 that contains a dielectric lens 9 at the wide circular bottom end thereof. The lens is immediately adjacent and above the dielectric window 10 that is of minimally larger diameter than the lens, in order to be captive in and sealed to holder 19 that, in turn, seals to cylindrical chimney 11, connected directly to an opening 13 in the wall ofthe chamber 12. Suitable flanges at 14,15,16,17 and 18 connect these various parts and sections together as shown in the Figures. All of these flanges must provide contiguous conductive connections between the parts to insure ideal operation without arcing or leakage of microwave energy or mode transformation. In addition, flange 18 must seal against the vacuum within the chamber when the equipment is used for vacuum drying and microwave heating and, as mentioned above, the window must be sealed to its holder 19.
The curve 9a of the dielectric lens, the electrical thickness of the dielectric window 10 and the dimensions of the cylindrical chimney 11 are all designed to produce the particular beam direction and shape that is desired. More particularly, it is generally desired that the beam 24 from source 34 be directed to illuminate an area of the conveyor 21 that begins at the outside edge 21 a of the conveyor and extends across the conveyor past the center at 21 b. Similarly, beacon 35 produces the beam 25 that illuminates the conveyor from the opposite edge 21 c somewhat past the middle at 21 b; and so at the middle, where the two beams 24 and 25 are weaker, they overlap to some wxtent. This insures complete illumination from side 21 a to side 21 b of the conveyor.
Source 35 can be constructed identical to beacon 34, but would be a mirror image of it, as viewed in Figure 1. Thus, these sources tend to produce beams directed radially inward from the edge of the cylindrical chamber 12 toward the center of the chamber and the subsequent pairs of sources 36 and 37 and 38 and 39 do the same. Clearly, sources 34, 36 and 38 can be constructed identical to each other and sources 35, 37 and 39 can be constructed identical to each other and the even numbered sources are mirror images of the odd numbered sources.
Figures 3 and 4 show details of construction of the polarization converter 6. It consists of a square wave guide section 6a in which are mounted dielectric plates 7a and 7b contiguous to each other across a diagonal of the square. These plates are longitudinally staggered along the length of the square wave guide. They are the same length, (L1 plus L2) and one is staggered ahead of the other by the dimensions L2. L1 is of such a length that:
where
and m is greater than n.
X'g is wavelength with only plate 7a or 7b. λ"g is with both plates 7a and 7b. Subscript ┴ is with electric field perpendicular to the plates. Subscript 11 is with electric field parallel the plates.
The dielectric prism lens 9 may be constructed as illustrated by Figures 5, 6 and 7. The experimentally shaped curve 9a-of this lens is established by a stack of dielectric plates 9a' to 9j, shown in the side cross section view by Figures 5 and a plan view by Figures.7. The purpose of the dielectric lens in the source is to re-directthe beam emerging from the conical wave guide 8, to be skewed toward the center of the chamber.
This prism lens structure is clearly not a figure of revolution about the beacon axis 30. It is symmetrical with respect to the plane through the beacon axis, perpendicular to the chamber axis. The thickness of this lens varies around the edge or periphery of the stack. Thus, it is generally prism- shaped. It is thicker at the inside edge toward the chamber plane of symmetry 23, than at its edge away from plane 23. The effect of this lens is to redirect the beam to one side of the beacon axis 30, toward the center 21 b of the conveyor in the chamber.
The dielectric lens is immediately adjacent dielectric window 10. The -thickness D of the window is one half wave length of the microwave radiation in the dielectric material of which the window is made. Hence, D is expressed as follows:
where K=relative dielectric permittivity of the material.
The conical wave guide connects to the dielectric window holder 19 by flange 17 and the window holder connects to the cylindrical chimney 11 by flange 18. At flange 18 the window seals to the chimney by an 0-ring gasket vacuum seal 20 and so the interior of the chamber is vacuum sealed from the exterior ambient and is also sealed from the source structure.
Between the flanges 17 and 18, completely enclosing the dielectric window, the holder 19 provides electrical continuity between the wide end of the conical wave guide and the cylindrical chimney 11. Thus, microwave currents conducted by the conical wave guide 8, holder 19 and chimney 11 bound the propagating fields of the microwave energy that flows into the cylindrical treatment chamber 12, and is radiated as the beam 24. By this construction, the beam of microwave radiation is substantially all in a single TE mode, circularly polarized and fully blankets the dielectric window. As a consequence of these conditions, the instantaneous peak value of the microwave electric field at the dielectric window on the vacuum side (the chamber side) is maintained low. It is maintained sufficiently low that it does not cause ionization of the gas inside the chamber even at the low operating pressures desirable for vacuum drying.
Another dielectric lens structure is shown by Figures 8 and 9. This structure is symmetrical with respect to all planes through the beacon axis 30. It defines a figure of revolution about the axis 30 and so that axis of the pattern of radiation that defines the beam issuing from this lens is a continuation of the axis 30. This lens is spheroidal and has no prism. Using this. lens to direct the beam toward the center of the conveyor may require that the source be located or oriented with its axis toward the center of the conveyor and so the source axis would be substantially radial with respect to the cylindrical chamber in which the center of the conveyor is at the center of the chamber cylinder.
Beacons each with a lens constructed as in Figures 8 and 9 are preferably all in line along the cylindrical chamber directly above the conveyor and so all such beacons would have the beacon axis in the symmetrical plane 23 of the cylindrical chamber. This lens is made up of a stack of dielectric plates 31 a, b, c each being ring-shaped. The outer peripheries of these plates are all about the same and their inner peripheries are successively larger and so the stack defines the dielectric lens curve 32 and spreads the radiation into a wider directional beam directed along the source axis 30.
The embodiment of the invention described hereinabove includes and incorporates all of the features of the invention and this embodiment is capable of microwave heating of materials in a continuous process, while, at the same tjme, (or at least in the same chamber), exposing the materials to a low pressure for vacuum drying. Thus, the apparatus is suitable for microwave heating and vacuum drying and many of the features of construction provide an advantage for both heating by microwaves and vacuum drying.
This apparatus is also usable in a discontinuous process.

Claims

1. Microwave heating apparatus for processing material enclosed in an evacuated chamber, comprising at least one source of microwave energy mounted on the chamber and radiating into it, characterized by a column comprising a microwave generator, followed by a circular- polarizing section, followed itself by a conical waveguide section, containing a dielectric divergent lens at the outlet window, which is designed in such a way as.to spread the radiation, in order to homogenize the density of power at product level and minimize the risk of arcing and corona effect.

2. Apparatus as in Claim 1, comprising a plurality of such sources.

3. Apparatus as in Claim 1, in which the product is on a moving conveyor, in order to allow continuous microwave processing.

4. Apparatus as in Claim 1, in which the circular polarizer is a section of square waveguide, containing diagonally-oriented dielectric material to differentially retard the two space quadrature components of the entering wave and of that length in which the cumulative differential retardation is π/2 radians, so that there is also time quadrature of the two components, thereby . providing circular polarization.

5. Apparatus as in Claim 1, in which the outlet window is a dielectric of thickness equal to half a wavelength inside the dielectric.

6. Apparatus as in Claim 1, in which the dielectric divergent lens is symmetric with respect to the axis of the column.

7. Apparatus as in Claim 1, in which the dielectric divergent lens is asymmetric with respect to the axis of the column, when irradiation is done by sources symmetrically spaced above and across the conveyor's axis, so that the beams overlap, in order to insure homogeneous irradiation of the subject material.

8. Apparatus as in any pair of claims 2, 3, 4, 5 and 6 or 7.

9. Apparatus as in a triplet of claims 2, 3, 4, 5 and 6 or 7.

10. Apparatus as in any quadruplet of claims 2, 3, 4, 5 and 6 or 7.