JPH0327277Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to an improvement of a drying device equipped with a microwave heating device.

[Conventional technology and its problems]

As is well known, when freeze-drying foods, etc., a dried product is obtained whose color, aroma, taste, and vitamins are comparable to those in the raw materials.Moreover, when water or hot water is added to the dried product, It can be almost completely restored to its pre-drying state.

However, when freeze-drying foods, etc., conventional radiation heating or conduction heating methods alone require a long time to dry to a predetermined state, resulting in extremely high drying costs.

In order to shorten this drying time, JP-A-56-23879
We have previously proposed a drying method using combined heating using microwave heating as shown in Japanese Patent Publication No. 56-22086, and a heating device using combined heating shown in Japanese Patent Application Laid-Open No. 56-22086. However, when trying to put into practical use an industrial-scale freeze-drying device that handles large quantities of food, etc.,
Since necessary and sufficient microwave energy must be input according to the amount of processing, there are problems as described below, and to date, no industrial-scale freeze-drying equipment using microwaves has been seen.

It is well known that in order to speed up drying under reduced pressure, not only in freeze drying but also in vacuum drying, it is important to effectively transfer heat to the material to be dried. The case of drying by microwave heating is no exception.

However, when microwaves are radiated into a field in free space under a certain reduced pressure and the output is gradually increased, the insulation of the gas in the area where the electric field is strongly acting breaks down, causing microwave discharge in that area. Even if the input power was increased, efficient microwave heating could not be achieved because heating energy was wasted. This is a major problem that has prevented conventional drying by microwave heating under reduced pressure from being put to practical use.

As a result of various studies, the present inventors obtained the following knowledge, found a solution to the problem, and succeeded in commercializing a drying device that uses microwave heating on an industrial scale.

In this way, the strength of the discharge starting electric field when microwaves are radiated into free space under reduced pressure varies depending on the type of gas present in free space, the excitation frequency of the microwave, etc. Transition in a typical manner in a relationship. Figure 1 shows the intervening gases as air and water vapor, and the excitation frequency of the microwave as
This figure shows the correlation between pressure and discharge initiation electric field strength under the condition of 2450MHz.

In the freeze-drying operation of foods, etc., air or water vapor generally constitutes the main component of the intervening gas, so Figure 1 shows how to design a vacuum dryer or vacuum freeze-dryer that uses a microwave heating device. becomes the basis of

When freeze-drying foods, etc., it is generally operated in a vacuum region of 200 Pa or less, so as you can see from Figure 1, the strength of the discharge starting electric field is the smallest, and therefore the most likely to cause a discharge. Despite the poor environmental conditions, the conventional devices failed to be put into practical use because they did not adopt a microwave transmission circuit structure that took such discharge characteristics into consideration.

This idea was made based on this knowledge, and improved the structure of the microwave transmission circuit to make it difficult to discharge, making it possible to put into practical use a drying device that uses a microwave heating method with excellent functionality. It is.

[Means for solving problems]

The present invention was made to solve the above problems, and consists of a plurality of slot array antennas arranged in parallel in a vacuum drying tank, and a single slot array antenna connected to a microwave generator. In addition to connecting to the secondary branching waveguide which is arranged in parallel at right angles to the secondary branching waveguide, a resonance window is formed using a low-loss material at the connection between the slot array antenna and the secondary branching waveguide. A closed resonator is provided, and a matching device such as an iris or stub for reversing the reflected microwaves is provided on the inlet side of the resonator, and the antenna is heated to atmospheric pressure or atmospheric pressure to the slot array antenna or as close as possible. By transmitting microwaves under a pressure close to that of In order to increase the size, the structure is devised when a plurality of slot array antennas are provided in a comb-like shape.

That is, a plurality of secondary branch waveguides are provided at right angles to a primary branch waveguide connected to a microwave generator, and a slot array antenna is connected to each of the secondary branch waveguides. At the joint between the primary branch waveguide and the secondary branch waveguide, a point parallel to the center line of the primary branch waveguide is placed at a certain distance away from the center line of the primary branch waveguide. It has a structure in which a coupling slot with a length of 1/2λg is provided.

〔Example〕

The embodiments of this invention will be described below based on the drawings.

FIG. 2 is a longitudinal sectional view of an embodiment of this invention, and FIG. 3 is a plan view thereof, in which microwave oscillators 4 to 6 are connected to the right end, and the vacuum drying tank wall 7 is connected to the left end. This is an external main waveguide consisting of a rectangular waveguide to which inner T-shaped branch waveguides 8 to 10 are connected, respectively.

On the outlet side of each T-shaped branch waveguide 8 to 10 in each tank, there is a primary branch waveguide 11 whose end is closed (short-circuited).
~13 are connected, and its primary branch waveguides 11~1
3, a plurality of secondary branching waveguides 14 to 17 are arranged at an interval lf equal to the internal wavelength of the transmitted wave, and in the embodiment, four secondary branching waveguides 14 to 17 are arranged in a comb with their ends overlapped. connected in the form of teeth. Note that 18 is a coupling slot for transmitting the transmission wave from the primary branch waveguide to the secondary branch waveguide.

Reference numerals 19 to 22 denote slot array antennas connected to the secondary branch waveguides 14 to 17 through closed resonance devices 23, respectively, and have a cross-sectional shape as shown in FIG. In the figure, each slot array antenna in the upper row has microwave radiation slots 24 of the same shape on the bottom surface, each slot array antenna in the lower row on the upper surface, and the slot array antennas in the middle row each having the same shape on the upper and lower surfaces. The slot array antennas in the upper and lower rows are of the single-sided radiation type, and the slot array antennas in the interrupted slots are of the double-sided radiation type.

The sealed resonator 23 includes a secondary branch waveguide 14 and a slot array antenna 1, as shown in FIG.
A shielding plate 23a made of a low-loss material with inductive properties such as Teflon, ceramics, quartz, borosilicate glass, etc. is tightly attached to the resonance window 23a in a perfect state with no vacuum leakage. It is.

Reference numeral 25 denotes a circulating flow tube for heat medium arranged in parallel with the primary branch waveguides 11 to 13, and a heat transfer tube made of aluminum alloy is arranged outside the tube in accordance with the intermittent arrangement space of the secondary branch waveguides. The relay body 26 is fixed by casting in the shape of a skewer dumpling.

27 is a heat pipe that is inserted into the protrusion grooves 28 (see Fig. 7) provided on both sides of the slot array antenna and fixed with a thermally conductive adhesive, and its end is connected to the through hole provided in the heat transfer relay body 26. and fixed with thermally conductive adhesive.

As shown in FIGS. 5 and 6, 29 is an iris (matching device for inverting reflected microwaves) provided in the secondary branch waveguide 14 to invert the reflected waves from the slot array antenna 19; is a tray for storing food to be dried, etc., and is placed between the slot array antennas positioned above and below by means of the conveyance support 31, so that the radiation characteristics of the microwaves radiated from the slot array antennas are unlikely to be affected. held in position.

The saucer 30 is made of a material with low dielectric loss and low reflection coefficient, such as Teflon or polypropylene.

The strength of the waveguide of the microwave transmission circuit installed in the vacuum drying tank and its joints are carefully considered to prevent vacuum leakage. The inlet side end of is attached to the vacuum drying tank wall 7 via a vacuum gasket.

Although not shown in the drawings, both the microwave and radiation heating devices shown in FIGS. 2 and 3 are configured symmetrically with respect to the axis of the transport support 31, and they are all placed in a vacuum. It is installed inside the drying tank.

The structure of the embodiment has been explained above, and the first feature of this embodiment is that the sealed resonator 23 is provided in the middle of the waveguide circuit in the microwave heating device, and the sealed resonator 23 is provided inside the waveguide on the microwave oscillator side. can be maintained at a higher pressure than in the microwave antenna under reduced pressure, at atmospheric pressure in the embodiment, and the microwave transmission circuit is designed as follows.

First, the minimum discharge starting electric field strength Vm ~ 180 Voltscm
The microwave transmission circuit, i.e., the main waveguide outside the tank, the T-shaped branch waveguide, the primary and secondary guides, is set so that the electric field strength Vw at the input end of the slot array antennas 19 to 22 becomes Vw<Vm. A waveguide circuit consisting of wave tubes is constructed.

By configuring it in this way, it is possible to unify the shape and dimensions of the waveguides that make up all slot array antennas, and it is also possible to unify the conditions for suppressing discharge over the entire system of slot array antennas. Become satisfied.

The size and position of the microwave radiation slot 24 not only affect the directivity characteristics of the microwave radiated from the slot and the electric field strength distribution characteristics of the radiated radio wave, but also affect the electric field strength existing at the slot. greatly related. If the electric field strength at the slot portion exceeds the discharge starting electric field strength,
Electric discharge will occur at that location.

Therefore, in this example, the directivity of the radiated radio waves and the intensity distribution of the radiated electric field are considered as composite characteristics of the slots arranged side by side, and these characteristics uniformly heat food, etc. placed still over a wide range. Since this can be an absolute condition for drying, the size, position, and number of slots are designed to be optimized.

In particular, in view of the directional characteristics of the wall current flowing through the tube wall of the antenna, the slots are arranged alternately with respect to the center line of the antenna, and the distance between the centers of the slots is made equal to 1/2 of the tube wavelength λg of the transmitted wave. By doing this, the currents flowing through each slot are all in phase, and the radio waves radiated from each slot are radiated in a direction perpendicular to the tube axis of the antenna.

Further, the slots are arranged at intervals exactly equal to 1/2 of the tube wavelength λg of the transmitted wave so that the radiation impedance of each slot is made equal.

Then, the tip of the slot array antenna is short-circuited, and the distance from the tip to the center line of the slot at the shortest distance is made equal to 3/4 of the channel wavelength λg of the transmitted wave. By doing this, the induced impedance of each slot becomes infinite, so
The weak reflected waves generated by the short-circuit wall are reversed by the action of the iris 29 provided at the input end of the slot array antenna, and are sequentially radiated from the slot 24 to the outside world in the same way as the traveling waves. .
Note that a stub may be used instead of the iris 29.

In this way, by making the radiation impedance of each slot have equal characteristics and by taking measures to block reflected waves in the single circuit part of each slot array antenna, microwaves have approximately equal power. It will be radiated to the outside world (inside the vacuum drying tank) through the slot at a ratio of

As mentioned above, discharge is prevented by maintaining the series of transmission tube circuits at atmospheric pressure with a high discharge starting electric field strength. One of the ideas is that the next step is to achieve the objective by connecting the branch waveguides using a slot method.

As used in this embodiment, the positions where the directional components of the swirling current flowing on the wide wall surface of the rectangular waveguide are equal are located like stepping stones at intervals equal to the wavelength in the tube of the transmitted wave.

In the embodiment of this invention, a secondary branch waveguide 14 is arranged parallel to the primary branch waveguide 11 at right angles.
~17 are also coupled at a distance lf equal to the pipe wavelength of the transmitted wave. At this coupling part, as shown in FIG. 4, the primary branch waveguide 11
At a point a certain distance x away from the center line of
The length is 1/2 parallel to the center line of the branch waveguide.
A slot of λg is cut to electrically connect the primary and secondary branch waveguides.

Depending on the number of branches of the secondary branch waveguide, a certain normalized conductance for each slot is uniquely determined from transmission theory.

On the other hand, in FIG. 4, the actual normalized conductance when the secondary branch waveguide 14 whose right end is short-circuited is connected to the primary branch waveguide 11 is the short-circuit distance of the secondary branch waveguide 14, It varies in a complicated manner depending on the two-dimensional position of the slot 18. Therefore, in this embodiment, the normalized conductance determined by the two-dimensional position of the slot 18 is
The distance x is experimentally determined to be G to establish the optimal degree of connection of the slots.

In this way, in the branch waveguide circuit in which the normalized conductances of each slot all have equal value characteristics, the microwave power transmitted to the primary branch waveguide 11 is proportional to the equal power ratio. , and is transmitted to each of the secondary branch waveguides 14 to 17 in an equal phase state.

On the other hand, the power passing through the coupling slot 18 is 2
It is necessary to cover the power supplied to the slot array antenna connected to the secondary branch waveguide.

However, when this required power is quantified under the normal conditions mentioned above, it can be as much as 150 watts for a single slot array antenna.

If such a large amount of power is to be transmitted under the reduced pressure atmosphere in which freeze-drying is performed, the strength of the electric field acting on a single slot of radio wave radiation will far exceed the strength of the electric field at which the discharge starts, and Naturally, a discharge will occur.

However, in this embodiment, as described above, the sealed resonator 23 is placed between the slot array antenna and the secondary branch waveguide, and during operation, the inside of the waveguide circuit up to the slot array antenna is closed. Since the pressure is always maintained at atmospheric pressure, discharge is prevented and necessary and sufficient microwave power is transmitted with a structure that can maintain the strength of the discharge starting electric field at a high level.

This drying device can be widely used for vacuum freeze-drying of foods, pharmaceuticals, etc., and for vacuum drying. Explain the effects.

Pre-frozen food or the like is placed in the tray 30, placed on the transport support 31, the transport support 31 is sent into the vacuum drying tank, and the door is closed.

After evacuating the inside of the vacuum drying tank to around the operating pressure for freeze drying, first, hot air, steam, heat medium oil, etc. are heated in the circulation flow pipe 25 in an optimal temperature pattern for radiant heating operation by a heating control device (not shown). circulating the heat medium. Through this operation, the heat pipes 27 provided on both sides of the slot array antenna are heated via the heat transfer relay body 26, and then the slot array antennas 19 to 22 are heated to approximately the same level as the heat medium. Therefore, the slot array antennas 19 to 22 themselves serve as heat radiators.

Then, the temperature difference between the slot array antenna and the object to be heated (both the frozen food and the saucer) becomes the driving force for heat transfer, and heat is transferred to the surface of the object to be heated at a rate controlled by radiation heat transfer. Foods, etc. that have been frozen by heat are sublimated and dried.

On the other hand, the microwave power oscillated from the microwave oscillators 4 to 6 is transmitted from the main waveguides 1 to 3 outside the tank through the T-shaped branch pipes 8 to 10 to the left and right primary branch waveguides connected thereto. It is transmitted to 11 to 13 at an equal ratio of 1/2. That is, for example, when one microwave power is supplied to the T-shaped branch pipe 8, one half of the microwave power is transmitted to the primary branch waveguide 11.

The microwave power transmitted to the primary branch waveguide obtains characteristics of equal phase and equal power due to the action of the slot 18 provided at the joint between the primary and secondary branch waveguides, and The microwaves are transmitted to the slot array antennas 19 to 22 through the secondary branch waveguide and the resonant window 23a of the sealed resonator 23, and the microwaves are transmitted from the plurality of slots 24 with specific directivity and equal power distribution characteristics. is emitted toward the heated object.

Most of the microwaves emitted from the slot 24 travel straight through free space, and the straight waves are repeatedly reflected at the interface of the food or the saucer, or penetrate deep into the food while undergoing complex refraction development. While passing through, most of it is consumed inside the food as absorbed heat.

Microwaves that are not consumed inside the food are
It forms a high-order mode in free space and propagates to various conductive walls in the vacuum drying tank, food, etc. while repeating three-dimensional flight, and is consumed as thermal energy.

If more power than the microwave power consumed under these loads is injected into the waveguide circuit, the number of reflected waves will increase, the overall electric field will increase, and the initial energy present in the intervening gas will increase. This excites electrons and initial ions and induces a discharge.

Therefore, in the embodiment of this invention, although not shown in the drawings, an optical detector is used to detect the discharge caused by the action of the excessively input microwave power. and perform optimal control according to the load at that time.
Efficient freeze-drying operations are always carried out.

With conventional freeze-drying methods using radiation or heat conduction heating, when the moisture content of foods, etc. decreases to a certain extent as drying progresses, the heat transfer resistance of the dried layer on the surface increases, resulting in extremely slow drying speeds. becomes smaller. Therefore, it was necessary to continue the operation for a long time even in order to reduce the water content even slightly.

However, according to an example device to which this idea is applied, which operates both radiant and microwave heating in a combined manner, the initial operation relies only on radiant heating, and microwave heating is started from the time when the drying rate begins to decline. By performing multiple operations, or by performing both heating operations in combination from the beginning, we have made it possible to significantly shorten the drying time and make heating drying more uniform compared to conventional methods. As a result, it has been demonstrated that productivity can be improved and drying costs can be reduced compared to conventional methods.

Furthermore, since the slot array antenna serves both as a microwave radiator and a radiant heat radiator, the device can be made more compact than when a radiant heating device is simply used in combination.

In the embodiment shown here, the closed type matching plate 2
3 is provided at the input end of the slot array antenna, but the position is not limited to this.

However, this position is advantageous both in terms of manufacturing and functionality.

Further, instead of having the shape of the slot 24 as a stepping stone, it may be a long slit in the direction of propagation of the microwave.

Furthermore, in this embodiment, both the microwave and radiation heating devices provided on both sides were provided so that the slot array antennas at each stage were located on the same plane with the transport support 31 as the center. The slot array antennas may be intermeshed as in the different embodiments shown.

In addition, in this example, the T-shaped branch waveguide, the primary and secondary branch waveguides, and the slot array antenna were installed in the vacuum drying tank, but only the slot array antenna was installed in the vacuum drying tank. Of course, there are some design changes, such as making it a type of entry.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the correlation between pressure and the strength of the discharge starting electric field, Fig. 2 is a longitudinal sectional front view of an embodiment of this invention, Fig. 3 is a plan view thereof, and Fig. 4 is a slot array antenna. A partially cutaway plan view, Fig. 5 is an enlarged sectional view of the connection between the slot array antenna and the waveguide, Fig. 6 is a right side view of Fig. 5, and Fig. 7 is a sectional view taken along line A-A. , FIG. 8 is a longitudinal sectional front view of a different embodiment. 1-3... Main waveguide outside the tank, 4-6... Microwave oscillator, 7... Vacuum drying tank wall, 8-10...T
type branch waveguide, 11 to 13...primary branch waveguide,
14-17... Secondary branch waveguide, 18... Coupling slot, 19-22... Slot array antenna, 23... Sealed resonator, 23a... Resonance window, 23b... Shielding plate, 24 ...Microwave radiation slot, 25...Circulating flow pipe, 26...Heat transfer relay body, 27...Heat pipe, 28...Protrusion groove, 29...Iris, 30...Saucer, 31...
Transport support.

Claims (1)

[Claims for Utility Model Registration] (1) A plurality of slot array antennas are arranged in parallel in a vacuum drying tank, and each of these slot array antennas is connected to a primary branch waveguide connected to a microwave generator. A closed resonator device that is connected to secondary branch waveguides that are arranged in parallel at right angles, and that has a resonance window closed with a low-loss material at the connection portion between the slot array antenna and the secondary branch waveguide. established,
A matching device such as an iris or a stub for reversing reflected microwaves is provided on the entrance side of the sealed resonator,
In addition, at the joint between the primary branch waveguide and the secondary branch waveguide, a point located a certain distance away from the center line of the primary branch waveguide is connected to the center line of the primary branch waveguide. A drying device characterized by having parallel coupling slots with a length of 1/2λg. (2) The drying device according to claim 1, wherein the slot array antenna has slots arranged in the shape of stepping stones. (3) The slot array antenna is equipped with a heat pipe, a part of which is connected to a heat medium circulation pipe via a heat transfer relay, as claimed in claim 1 or 2 of the utility model registration. drying equipment.