Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B6/00—Heating by electric, magnetic or electromagnetic fields
  • H05B6/64—Heating using microwaves
  • H05B6/78—Arrangements for continuous movement of material

Definitions

• TITLE IMPROVED MICROWAVE TREATMENT APPARATUS Field of the Invention: This invention relates to improvements in microwave treatment apparatus of the type described in our Australian Patent No. 565393 and corresponding United States Patent No. 4,631,380 Tran. Background of the Invention: In the above Australian and United States patents, we describe an improved system for the microwave treatment of materials in which the material to be treated is confined in a generally circular form and microwave sources are arranged around the material to be treated in a generally offset manner, the radiation emitted by said microwave sources being essentially non-coherent.
• the invention in one aspect provides a system for the microwave treatment of materials comprising a treatment zone including means to confine the material to be treated in at least a partly cylindrical form, radiation means spaced from and outside said treatment zone for directing microwave radiation into said zone along at least three different axes of radiation, said radiation means being arranged in spaced relation around said zone with said axes of radiation being in spaced relation to each other at the radiation means, means for reflecting the microwave radiation directed along each axis back into ' the treatment zone after it passes through said zone, said microwave radiation being non-coherent, characterized in that said reflecting means has a generally concave curved form having at least one radius of curvature which lies within said treatment zone.
• the radiation passing through the treatment zone is more effectively reflected back into the treatment zone, thereby improving the efficiency of the treatment system.
• the use of generally circular reflectors also enables the radiation means to be supported in closer proximity to the material to be treated in the treatment zone, thereby further improving the efficiency of operation of the system.
• the generally circular reflectors may be provided by a portion of a circular wall which serves to support the radiation means in spaced relation around the treatment zone containing the material to be treated. This form of the invention provides a particularly simple supporting structure which also provides the advantages of generally circular reflectors as discussed above.
• the treatment zone may be defined by a length of low-loss tubing which allows the material to be treated to be contained and transported through the treatment zone.
• the tubing may be supported for rotation about any desired axis and may include agitating means such as short angularly portioned vanes or auger means supported by the inner surface of the tube.
• the thickness and dielectric properties of the tubing may be selected to reduce reflection to thereby improve the matching of the microwave energy to the material to be treated. Al ernatively, the thickness of the tubing is selected so as to be sufficiently thin that the thickness of the tubing has little effect on microwave reflection.
• the thickness of the material is selected to be one-half of the wavelength of the microwave radiation treating the material contained in the tubing.
• the tube may be a composite tube comprising two or more different materials, which may be separated by a further material, including air.
• the dielectric properties of the material to be heated may be matched to the microwave applicator supplying microwave energy to the composite tube.
• the dielectric properties of the different materials may increase from the outer tube to the inner tube or alternatively they may decrease from the outer tube to the inner tube.
• the material may be a liquid, such as a non-absorbing saline solution or oil, or may be a solid material such as polystyrene.
• the strip or conveyor In the treatment of strips or other generally planar lengths of material, or in the treatment of materials capable of being supported by a generally microwave transparent conveyor, it is common to pass the strip or conveyor under a support on which one or more microwave applicators are already entered such that the long axis of the radiation pattern (usually elliptical in shape) extends in the direction of travel of the strip or conveyor. This results in non-uniform heating of the strip or conveyed material such that undesirable hot spots or cold spots develop.
• the invention provides a microwave heating system for a continuously moving strip or other generally planar object or for a conveyor supporting a material to be heated, comprising means for supporting one or more microwave applicators in an overlying and/or underlying relationship to said strip or conveyor, characterised in that at least said one microwave applicator is supported such that the long axis of the microwave energy pattern produced thereby is oriented in a direction which is transverse to the direction of travel of the material to be heated.
• one or more microwave heaters having a multi-sided microwave applicator supporting structure is arranged so that one part of the supporting structure is positioned on one side of the strip or conveyor belt while the other part of the supporting structure is positioned on the other side of the strip or conveyor belt, each microwave applicator being supported by one side of said support structure in a manner such that the axis of radiation of each applicator intersects the material to be heated before intersecting the axis of radiation of another microwave applicator.
• a multiplicity of spaced microwave applicators are supported along each side of the support structure, the applicators on adjacent sides of the supporting structure being offset with respect to preceding applicators by an amount equivalent to an appropriate fraction such as one- quarter of the wavelength of the microwave radiation generated by each microwave applicator.
• the applicators of adjacent heaters may be offset by one-quarter of the wavelength of the radiation emitted by each applicator to ensure uniformity of heating.
• Figure 1 is a sectional plan view of a three sided treatment system
• Figure 2 is a sectional plan view of a five sided treatment system
• Figure 3 is a sectional elevation of the treatment systems of Fig. 2 showing the method of transport of the material to be treated
• Figures 4 to 7 are schematic perspective views showing various forms of microwave heating systems for heating strips or material carried by conveyors
• Figure 8 is a sectional end view showing a composite tube for the material to be heated.
• the microwave treatment system shown schematically will be seen to comprise a three sided supporting structure 1 supporting three microwave applicators 2, 3 and 4, capable of emitting coherent or non-coherent microwave radiation into the structure 1."
• the supporting structure has three part- circular reflectors 5, 6 and 7 positioned opposite the applicators 2, 3 and 4 respectively.
• the reflectors may be integral parts of the wall or, as shown in Fig. 1, separate items.
• Each reflector 5, 6 and 7 has a radius of curvature r which has its centre located coincident with the centre of the treatment zone T, which in the present embodiment, is defined by a dielectric tube 8, although a radius of curvature having its centre located anywhere within the zone T will produce acceptable results.
• the tube 8 is made of low loss dielectric material such as glass, fused quartz, fibreglass, Teflon (registered trade mark), alumina and the like.
• the part-circular reflectors 5, 6 and 7 operate to reflect any radiation passing through the treatment zone T directly back into the treatment zone, focusing at the centre of the treatment zone. In this way, interference between the microwave applicators is kept to a minimum.
• the thickness of the tube should be one-half of the wavelength of the radiation emitted by the applicator.
• one preferred embodiment of the system comprises a support structure having five sides 9 to 13, each of which centrally supports a microwave applicator 14 to 18.
• Part-circular reflectors 19 to 23 are positioned within the structure opposite the applicators 14 to 18, with the focus of each reflector being coincident with the centre of the treatment zone T defined by a tube 24 of low loss dielectric material such as glass, fused quartz, fibreglass, Teflon (registered trade mark), alumina and the like.
• the tube 24 contains a further tube 24a having a multiplicity of angularly oriented vanes 24c attached to the inner surface of the tube 24a, The vanes 24c also assist in preventing agglomeration or packing of moist materials to be heated.
• An intermediate low-loss material 24b which may be air or any one of the other materials described below in relation to Figure 8 of the drawings, is positioned between the tubes 24 and 24a to match the applicators 14 to 18 with the material to be treated in tube 24a, as described in greater detail in relation to Figure 8 of the drawings.
• the tube 24a is supported by rollers R rotatably mounted on a support frame F and engaging a bearing ring B attached to the tube 24a, and a drive collar C which is engaged by a belt drive or - 7 - gear drive (not shown) by means of which the tube 24a may be rotated in the direction shown in Figure 2 of the drawings.
• the vanes 24c are oriented angularly with respect to the axis of the tube 24a so that the material to be heated in the tube 24a is both circulated within the tube 24a and drawn into the tube 24a from a hopper H or the like, and discharged from the other end of the tube, the hopper may additionally have an auger (not shown) for assisting the feeding of certain materials, particularly moist materials such as soil and coal.
• an auger not shown
• a suitable residence time for the material to be heated may be achieved.
• the vanes 24c shown may be replaced by one or more continuous auger flights to achieve a similar results.
• the material from which the vanes 24c or auger flight are made is similar or identical to the material of the tube 24a, so as not to interfere with heating process. While the tube 24a is shown in Figure 3 to be generally horizontally positioned, the tube may be vertically oriented to allow gravity feed of the material, in which case the vanes need only perform a material circulating function.
• the space between the tubes may be used to cool the inner tube 24a by means of circulating air or some other suitable cooling liquid or gas such as nitrogen.
• suitable cooling liquid or gas such as nitrogen.
• Such an arrangement is suitable for the treatment of minerals at high temperatures, for example 1000°C, such as smelting operations, sintering materials such as SiC, ceramics or superconducting materials.
• the space between the walls of the support structure and the tube(s) is filled with a suitable insulating material.
• the tube(s)/materia 1 s must be selected to allow for the additional material.
• the space may be evacuated to provide an insulating effect.
• the vanes are removed to enable generally cylindrical objects, such as logs and the like, to be dried or heated for other purposes.
• the log or the like will be pushed through the inner tube 24a to provide the required residence time. Rotation of the tubes is still desirable to ensure uniform heating.
• Such an arrangement may be used to initially dry a green log and to subsequently dry a log after chemical treatment.
• the use of such an apparatus has been found to provide a particularly efficient means of drying green and treated logs.
• the supporting structure for the microwave applicators is circular
• the part-circular reflectors are provided by portions of the support.
• the material to be treated is confined within a low loss tube, which may be in the form shown in Fig. 1 or in Figs. 2 and 3.
• several alternative methods of causing absorption of the microwave radiation passing through the treatment zone T may be applied at each reflecting surface.
• the absorbing material may be solid, liquid, powder or gas and may be deployed permanently or in a removable manner when load conditions are expected to be discontinuous.
• the material is preferably selected so as to cause matching of the impedance of the applicator to the material to be heated, and making such a selection would be within the knowledge of a person skilled in the art.
• the positioning of absorbing material reduces the amount of reflection by the reflectors and thereby minimises damage to the magnetron of the microwave applicator caused by reflected radiation.
• the reflectors may be constructed with passages into which liquid is pumped to increase the absorbing properties of the reflector, to achieve similar results.
• a means of matching the microwave source and the material comprises a composite tube including outer and inner tubes 40, 41 of suitable low-loss material having dielectric properties or complex permiti ities c ⁇ and £, and a dissimilar material 42 of dielectric property .
• the composite tube will act as a matching device between the microwave energy applicator on the outside and the material to be heated inside the inner tube 40 to ensure that reflection is minimised and transmission of energy into the material to be heated is maximised.
• the composite tube should be designed for each material to be heated where efficiency is important. The composite tube therefore facilitates better transmission of the microwave energy and provides greater flexibility in the heating of different materials.
• the tubes 40 and 41 will for convenience usually be made of the same material, such as glass, Teflon, or any other of the low-loss materials described above, although the materials may be different to achieve different results.
• the material 42 is most conveniently (and cheaply) air, although the material may be polystyrene, Teflon, a non- absorbing liquid such as saline solution, oil or the like. If Teflon is used, the tube 41 could be of fused quartz, to match the tubes in the material to be heated.
• ⁇ 0°.
• the reflection from the tube will be zero for a tube thickness equal to "j" or A, or any multiple thereof.
• the function performed by the composite tube is to reduce the large step or increase in permitivity between the air or other material in the support structures such as 1 or 9 to 13 above between the applicators and the tube 8 or 24 into a series of smaller steps or a smooth or gradual increase to reduce reflection of the radiation where the material to be heated has a low dielectric constant, a single tube will usually be adequate, although a composite tube would produce superior results.
• FIG. 4 to 7 of the drawings an alternative form of microwave heating system is shown in which a strip or other planar material to to be heated, or a conveyor belt carrying a material to be heated, is passed through slots formed in a series of microwave heaters of the type generally described above, modified to exclude the central tube.
• the conveyor is made from some suitable non- absorbing material such as cotton, Teflon, fibreglass, terylene or polypropylene to prevent reflection and to allow the lower portion of each heater to heat the material through the conveyor.
• the strip or conveyor belt S passes between opposed halves 30, 31 of a series of supporting structures, similar to that shown in Figure 2 of the drawings, having a multiplicity of spaced microwave applicators 32 supported on each side 33 of each supporting structure half 30, 31. While the ends of the structures 30, 31 are shown in Figures 4 to 7 as being open, it will be appreciated that in practice the ends would be closed by suitable shielding material.
• Each microwave applicator 32 is supported by its supporting structure 30, 31 in such a manner that the long axis of the microwave energy pattern thereof (usually elliptical in shape) is directed transversely to the direction of travel of the strip or conveyor S. In this way, more uniform heating of the strip or material supported by the conveyor is achieved.
• the applicators 32 supported by adjacent sides, or by adjacent heaters are spaced by an amount equal to one-quarter of the wavelength of the radiation emitted by each applicator 32. This is shown most clearly in Figure 4 by the first microwave heater having its applicators 32 spaced by a distance d, while the next microwave heater has its applicators 32 spaced by a distance d + ⁇ , while the third heater has its applicators 32 spaced by a distance d - ⁇ . This arrangement ensures that the strip or material supported by the conveyor is uniformly heated across the entire width of the strip or conveyor.
• each supporting structure 30, 31 is preferably fitted with a part-circular reflector, or the type described above, preferably having its radius of curvature approximatel coincident with the material being heated.
• partial reflectors may be positioned where the strip S passes between the supports 30, 31, since the material to be heated partially occupies this position in any event, it is presently thought to be unnecessary to position partial reflectors at these locations. Modifications of the heating arrangement shown in Figure 4 will be seen in Figures 5 to 7 of the drawings. In Figure 5, the arrangement shown in Figure 4 is inverted; in Figure 6, the lower support structure is generally se i- circular, while in Figure 7, the upper support structure is generally semi-circular.
• the pairs of rollers would normally be used to feed the material through the heaters.
• supporting rollers may be positioned at either end of the series of heaters and/or between the heaters, and the panel conveyed through the heaters by means of a pusher or puller of some suitable form.
• the tube containing the material to be treated may be partially evacuated for drying sensitive materials such as pharmaceutical products.
• the tube may be flushed with an inert gas to prevent combustion of inflammable material or may be flushed with oxygen or a fuel for material processing such as mineral smelting.

Landscapes

• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Constitution Of High-Frequency Heating (AREA)

Applications Claiming Priority (2)

Publications (2)

Family

ID=3773042

Family Applications (1)

Country Status (2)

Families Citing this family (8)

Citations (3)

Family Cites Families (5)

• 1989
  • 1989-04-19 WO PCT/AU1989/000171 patent/WO1989010678A1/en not_active Ceased
  • 1989-04-19 EP EP19890905041 patent/EP0364571A4/en not_active Withdrawn

Patent Citations (3)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events