JP2005071724A - Microwave heating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an improved microwave heating device capable of obtaining uniform and highly efficient microwave heating for a sheet-like object to be heated. <P>SOLUTION: Only one component of an electric field exists in one direction in a cavity 1, and a resonance mode such as one having a fixed size is excited with respect to that direction. The length c in one direction z of the resonance cavity 1 is not less than three times the standard length of a wave guide. Metallic short-circuited plates 13, 14 are disposed at both ends in the x direction of an aperture 12 for coupling with the wave guide of the resonance cavity 1, and a prescribed gap is provided between both of them. A path for a current flowing in a side wall 4 is secured by the short-circuited plates 13, 14. Thereby, a uniform electric field is generated in the cavity 1, and highly efficient and uniform heating of an sheet-like object W to be heated can be obtained. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a microwave heating apparatus, and more particularly to uniformizing the degree of heating of a sheet-like object to be heated.

Microwave heating allows the heated object to be directly heated by absorbing the microwave in the heated object, enabling efficient heating in a short time, and industrial heating equipment including household microwave ovens. Widely used in However, when the object to be heated has a sheet shape in particular, it is difficult to efficiently absorb the microwave into the sheet and to uniformly heat the entire sheet. In other words, the electric field at the position of the sheet must be strengthened, and a uniform electric field distribution must be obtained particularly in the width direction of the sheet. However, it is extremely difficult to satisfy such conditions. There is no satisfactory answer for sheet heating.
Microwave technology is highly advanced, and various types of resonant cavities and matching elements that can be used as microwave circuit elements of devices have been widely developed. However, it is not specialized for sheet heating and does not meet the requirements for sheet heating.
For example, Patent Document 1 discloses a technique for drying a ceramic green sheet with a microwave heating and drying apparatus. In this method, the ceramic green sheet before drying is dried by being irradiated with microwaves while being transported on a belt in a microwave heating and drying apparatus. However, in general, when the heated object is a thin sheet, since the thickness of the sheet is thin, the microwave power absorbed by the sheet is reduced, so that it is difficult to heat efficiently. The same applies to the microwave heating and drying apparatus described in Patent Document 1, and sufficient uniform and efficient heating cannot be expected. For this reason, there is a strong demand for high-efficiency and uniform heating using microwaves.
JP 7-294122 A

  An object of the present invention is to solve the above-mentioned disadvantages in the prior art and to provide an improved microwave heating apparatus capable of uniformly heating a sheet-like object to be heated with high efficiency.

  In the present invention, in order to achieve efficient heating, only the electric field component in the width direction exists for the sheet-like object to be heated having a relatively large width and a small thickness, and further, with respect to the width direction. A cavity that can generate a microwave resonance mode in which the magnitude of the electric field does not change is used. When a microwave electric field of this mode is used, the electric field strength is extremely high and uniform, so that highly efficient and uniform heating can be realized.

  In general, a microwave resonance cavity for heating a sheet needs to be large enough to accommodate a sheet having a certain width. In order to satisfy this requirement, the width direction of the sheet corresponds to the longitudinal direction of the cavity, and the sheet can be moved in a direction perpendicular to the longitudinal direction of the cavity. The article to be heated W is a long sheet or an aggregate having a small thickness such as a plurality of short sheets arranged uniformly on a belt conveyor. In this configuration, when the resonance mode is in the longitudinal direction of the cavity and has a constant electric field strength in that direction, for example, the resonance cavity has a rectangular parallelepiped shape, TM110 mode resonance is generated. In order to excite such a mode, microwave power is supplied from the microwave oscillator through the waveguide into the cavity. A circulator that prevents the microwaves reflected from the cavity from returning to the oscillator and a matching unit for sending as much power as possible to the cavity are provided in the waveguide. In the case of sheet heating, in order to meet the demand for heating a relatively wide sheet, the length of the cavity is set so that if the waveguide used for coupling is a standard waveguide with a rectangular cross section, the waveguide It is common to design the tube so that it is at least several times the height of the tube (the dimension between the opposing long side walls in a rectangular cross section). If a waveguide is simply coupled to such a long cavity, only a mode with a distribution that deviates significantly from the desired mode that provides a uniform electric field can be obtained. The electric field in this mode is generally toward the longitudinal direction of the cavity, but it bends particularly near the side wall, and the strength is generally strong at both ends of the cavity and weak at the center. End up. The reason for this is that an opening for coupling the waveguide is provided on the wall surface of the cavity. That is, in the desired mode, the current that should flow in the longitudinal direction of the cavity on the side wall surface is obstructed by this opening.

  As a means for eliminating this inconvenience, the length of the cavity opening in the width direction may be narrowed to widen the path through which current flows. Specifically, both ends of the opening are short-circuited, and correction is made so that current flows through this portion. This is indistinguishable in shape from the waveguide-type inductive window used to improve load matching. Waveguide type inductive windows improve the impedance matching by neutralizing the capacitance of the capacitive load with the inductance of the window. On the other hand, the purpose of using the short-circuit plate in the present invention is to form a current path blocked by the opening, and its purpose and function are completely different from those of the waveguide type inductive window. In general, the coupling between the cavity and the waveguide is designed from the standpoint of matching so that the impedance of the waveguide is matched with the impedance of the cavity. Since the cavity impedance is neither capacitive nor inductive in the resonant state, it does not make sense to use inductive windows or capacitive windows. Usually, a flat waveguide having a reduced height is used to couple the two.

  Another way to overcome the inconvenience of the mode being disturbed is to place the aperture in series resonance so that an equivalent resonant short circuit current flows.

  In the present invention, short-circuit plates 13 and 14 are added or a ferroelectric member 17 that causes series resonance is disposed for blocking current through the opening 12 that is a coupling portion between the cavity 1 and the waveguide 8. By using such means, a uniform electric field is generated in the cavity 1 by securing the path of the current flowing through the side wall 4, thereby obtaining high-efficiency and uniform heating of the sheet-like object to be heated W. Has the effect of being

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic perspective view showing a microwave sheet heating apparatus, and FIG. 2 is a schematic perspective view showing a resonance cavity which is a basic part of the microwave sheet heating apparatus.

  The resonant cavity 1 of the microwave heating apparatus of this embodiment has front, rear, upper, lower, left and right side walls 2, 3, 4, 5, 6, and 7, and has a substantially rectangular parallelepiped shape. Microwaves are supplied from above in the figure through a waveguide 8 connected to a microwave power source 9.

  In FIG. 2, slots 10 for passing an object to be heated are provided on the front side wall 2 and the rear side wall 3 of the resonance cavity 1 so as to face each other in parallel to the z axis that is the center of the cavity. The slot 10 is disposed at a position slightly shifted downward in the y direction from the center z axis of the cavity 1. When the microwave absorption of the sheet-like object to be heated is small, the sheet may be allowed to pass by providing the slot 10 at the opposing portion across the z-axis. However, if the loss is relatively large, the sheet is removed from the z-axis. It is better to pass through the eccentric position. The sheet-like object to be heated W is transferred by the transfer means 11 through the two slots 10 at a substantially constant speed in the x direction.

  The mode of the electromagnetic wave excited in the cavity 1 is TM110 or close to this mode. The electric field of the TM110 mode is in the z direction, and the x and y components are zero. Further, the electric field changes in a sine half-wave shape with respect to the x direction and the y direction, and is zero on each side wall surface and has a maximum value at the center. With respect to the z direction, there is no loss in the cavity itself, the sheet-like heated object W is also lossless, its relative dielectric constant is equal to 1, and the structure of the coupling portion between the cavity 1 and the waveguide 8 If is appropriate, it remains constant without any change. In the embodiment, the dimensions of the cavity 1 are a = 109.2 mm, b = 73.8 mm, and c = 540 mm in FIG.

  FIG. 3 is a longitudinal sectional view of the cavity. If the opening 12 of the side wall 4 for coupling the microwave power to the cavity 1 through the waveguide 8 is an opening that coincides with the width direction and the height direction of the waveguide 8, it is actually significant. A problem occurs. That is, when the TM110 mode is excited in the cavity, a current flowing in the z direction flows through the side walls 2, 3, and 5. The current flowing in the side wall 4 should originally flow in the z direction without being cut off, but the opening 12 obstructs the flow. Since the width of the waveguide is a, the portion of the width a is in a circuit cut state. In this state, the desired TM110 mode is greatly deformed. When this happens, the electric field in the cavity, particularly near the side wall 4, is bent, resulting in an undesirable distribution in which the electric field is weak at the center in the z direction and strong near the end. When measured, the electric field at the edge is 1.5-2 times that at the center.

  3 is provided with a thin metal short-circuit plate as shown by 13 and 14 in FIG. The short-circuit plates 13 and 14 are disposed on both sides of the opening 12 in the width a direction, and only the center portion is a relatively narrow opening 12 a that communicates with the cavity 1. The structure of this opening takes the same form as an inductive window used for impedance matching purposes. When the width of the short-circuit plates 13 and 14 is wide, the path of current is widened, and a mode closer to TM110 can be excited. When the width is made extremely narrow, the inductance generated thereby becomes close to zero, impedance matching with the waveguide 8 becomes worse, and the microwave power transmitted to the cavity is reduced. Therefore, an appropriate value exists for the width dimension of the short-circuit plates 13 and 14. Here, the opening 12a formed between the short-circuit plates 13 and 14 is referred to as a short-circuit window. This is functionally quite different from an inductive window that performs impedance matching.

  Another method for generating a mode close to TM110 is to employ a structure as shown in FIG. Reference numerals 15 and 16 in FIG. 5 denote capacity plates extending in the width a direction so as to face each other in a direction orthogonal to the width a. The capacitance plates 15 and 16 generate capacitances determined by their areas and mutual gaps. When this capacitive plate is formed of a stripped metal plate, the interval between the capacitive plates 15 and 16 is very narrow. Therefore, the area of the capacitive plates 15 and 16 needs to be increased to some extent.

  If the interval between the capacitor plates 15 and 16 is very narrow, high frequency discharge occurs in the gap, which is not realistic. In order to avoid this, a ferroelectric member 17 such as alumina is inserted into the gap. When a ferroelectric member is inserted into the gap, it is possible to approach series resonance with a reasonable structure in which the capacity plates 15 and 16 have a thickness of about 10 mm and an interval of about 10 mm. When series resonance occurs, the impedance viewed from the waveguide side becomes zero, so the parameters are designed so that an appropriate value of capacitance can be obtained. As a result, almost the same current flows as when the side wall 4 has no opening. Here, the thickness of the capacitor plates 15 and 16 and the thickness of the ferroelectric member 17 are the same. For convenience, this coupling structure is called a series resonance window, which is completely different from the capacitive window used for impedance matching purposes, and is a special window that is short-circuited in the resonance state.

  In the above description, a rectangular parallelepiped resonant cavity is used, but the cross section of the cavity cut in a direction perpendicular to the z-axis is not necessarily rectangular, and other shapes such as an ellipse, a circle, and a polygon are used. Even if it takes a shape, the same purpose can be achieved. Further, the short-circuit plates 13 and 14, the capacitor plates 15 and 16, the ferroelectric member 17 and the like can be appropriately modified from a simple rectangular parallelepiped shape.

  The present invention heats, for example, a sheet-like object to be heated that is impregnated with water or the like, which has a large microwave loss and greatly disturbs the electric field in the cavity when directly inserted into the resonant cavity. And can be used as an apparatus for drying.

It is a schematic perspective view of a microwave heating device. It is a schematic perspective view of the resonance cavity which is a fundamental part of a microwave heating device. It is a schematic longitudinal cross-sectional view of a resonance cavity. It is the schematic of the cavity coupling | bond opening which added the path of the electric current which flows into a side wall. It is the schematic of the coupling opening which ensured the way of the electric current which flows into a side wall by raise | generating a series resonance.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Microwave resonance cavity 2 Front side wall 3 Rear side wall 4 Upper side wall 5 Lower side wall 6 Left side wall 7 Right side wall 8 Waveguide 9 Microwave power supply 10 Slot 11 Transfer means 12 Coupling opening 12a Opening 13 Shorting board 14 Shorting board 15 Capacity Plate 16 Capacitance plate 17 Ferroelectric member W Sheet-like object to be heated

Claims (2)

  A microwave resonant cavity that excites a resonant mode in which only one component of the electric field exists in one direction in the cavity and the magnitude is constant in that direction, and microwave power is supplied to this resonant cavity. A waveguide to be supplied, wherein a length of the resonant cavity in the one direction is not less than three times a standard height of the waveguide, the waveguide of the resonant cavity A microwave heating apparatus, wherein a metal short-circuit plate is disposed at both ends of the opening for coupling to the one direction orthogonal to the one direction, and a predetermined gap is provided between the both short-circuit plates.   A microwave resonant cavity that excites a resonant mode in which only one component of the electric field exists in one direction in the cavity and the magnitude is constant in that direction, and microwave power is supplied to this resonant cavity. A waveguide to be supplied, wherein a length of the resonator in the one direction is not less than three times a standard height of the waveguide, the resonance cavity, the waveguide, A microwave heating apparatus comprising: a metal capacitive plate disposed at both ends in one direction of the opening for coupling; and a ferroelectric member interposed between the capacitive plates.

Images