CA1248348A - Microwave vacuum dryer apparatus and method for microwave vacuum drying - Google Patents

Abstract

Abstract of the Disclosure Slot-array antennas are connected by waveguides to microwave generators and are disposed within a vacuum drying tank of a drying apparatus.
Blocking plates are mounted in the waveguides so as to keep the wveguides airtight thereby maintaining an atmospheric pressure within the waveguide portions near the microwave generator during operation of the apparatus.
The slot-array antennas have heat pipes attached to the sides of said antennas so that a heat radiation may be effected simultaneously with the microwave heating.

Description

~83~

This invention relates to a vacuum ~reeze dryer apparatus or a vacuum dryer apparatus, equipp~d with microwave heater apparatuses.

As is well known, it is possible by freeze-drying foods to obtain dried products which are in no substan-tial degree inferior as to their color, flavor, taste and vitamins in comparison with those in the raw materials, and yet further it is possible, by adding water or hot water to the dried products, to restore same substantially completely to the state that was observed prior to the drying.

However, in ~the case of freeze-drying the foods, there has been a drawback in that it requires long time to dry same to the predetermined state when only the conven-tional method either of radiator heating or conduction heating, is utilized, and in conseguence thereof that the drying cost gets very high.

For shortening the drying time, we have previously proposed a drying method in which a combined heating system is designed to include a microwave heating and a heating apparatus for the combined heating.

In the case of freeze-drying the foods generally prevailing is the drying under reduced pressure, thus the so-called vacuum drying, and also known is the fact that it is important in order to accelerate the drying rate under reduced pressure to improve a heat transmission to the goods to be dried.
Such as good heat transmission will also be important in case of microwave heating. On the other hand, when the ~reeze-drying apparatuses on the lndustrial scale is requlred to handle a large amount of the foods, it is imperative to inject a hlgh microwave energy necessary and sufficient to the great treating amount, the high energy thereby causing the following problem to take place.
When gradually heightening -the output level of ..... ~ : , . , ~Z4~3~1~

microwaves radia-ted into a field of free space under a reduced pressure, the insulation of a gas at the location where the electric field is strong will be broken to thus induce microwave discharge at such location so that the heating energy is accordingly consumed without avail, it has therefore not been able to effect efficient microwave heating even by raising up the power injected.

The above problem will be explained in more detail hereinafter. The intensity of electrical field for initiation of electric discharge in case of where the microwaves are emitted in a free space under a reduced pressure varies according to the kind of the gas existing in the free space, oscillation-exciting frequency of the microwaves and so forth. Said ~0 ~5 : - 2 -~ ....

,,, ~.

~ 3 ~ ~

intenslty varies ln a typlcal manner in lts relatlonship partlcu-larly with the pressure. Fig. 1 shows a correlative relationship between the pressure and the electrlcal field intensity wherein the oscillation-exciting frequency o~ the microwaves is set at 2450 MHz, the gas within said space being a mixture of air and water vapor. Since the air or water vapor in general the main ingredient of the gas within the space in the operation of the freeze-drying of the foods, Fig. 1 serves as the basls for a design for the vacuum dryer apparatus or the vacuum freeze dryer appara~us provided with a microwave heater apparatus.

When the foods are freeze-dried, operation is generally carried out at a vacuum state of or below 200 Pa. This state is, however, the worst ambient condition, as seen from Fig. 1, because the discharge lnltiatlng lntenslty of el~ctrical fleld ls minimum, namely most likely to cause the electric discharge.
However, the known conventional apparatuses have never adopted such a structure of the microwave transmltting clrcuit that could be reasonable from the view of electric discharge property.
Therefore, none of the known apparatuses has become to be used practically on a commercial base.

According to the present invention there is provided a microwave vacuum dryer apparatus comprising: a vacuum drying tank, slot-array antenna means dlsposed in said vacuum drying tank, microwave generating means disposed outwardly of said vacutum~ ~rying tank and under amblent pressure, waveguide means D l~a~inglsaid slot-array antenna means from sald microwave gener-ating means, and blocking plates mounted between said microwave generating means and said slot-array antenna means, said blocklng plates each comprisir.g a window formed of a material permitting easy passage of microwaves and a metal window frame, sald window having a strength to withstand a differential pressure between a vacuum ln said vacuum drying tank and the ambient pressure out-side thereof, said metal window frame belng disposed between thewaveguide means and the antenna means, and further comprlsing at \

, .

~133~

least two irises, each iris provided an end portion of the slot-array antenna in such a manner that reflected waves from ~K~ ~
short-circuit wall and the window frame of the blocking plate are reflected by the irises so as to advance forward again into the antenna wherein said window, said metal window frame and said slot-array antenna means togetherwith operate to set up a closed type resonator.

The present invention also provides a method for microwave vacuum drying comprising the steps of: maintaining a vacuum drying tank in a relative vacuum state to facilitate dry-ing; maintaining waveguides leading to the vacuum dryer at a higher pressure than that in the vacuum drying tank to minimize electrical discharge from the waveguides; mlnimizlng microwave discharge by setting up a closed-type resonator, having a glass window and a metal window frame, the metal window frame being disposed between the waveguide and an antenna means, transmitting microwaves from the antenna means along the waveguides into the vacuum drying tank to dry material in the drying tank, and pro-viding a plurality of irises to reflect microwave from the direc-tion of the antenna means back toward the antenna means.

Thus, th~ present invention provides a dryer apparatus that has a slot-array antenna within a vacuum drying tank, a waveguide connectlng - 3a -:

.

.

said slot-array antenna to a microwave generating apparatus, and a blocking plate made of a material through which it is easy for the microwav~ to pass, the pla-te being located ak a connecting portion between the slot-array antenna and waveguide or the waveguide per se near the connecting portion to thereby seal up the waveguide interior. ~ccording to the in~ention the microwaves are transmitted to the slot-array antenna or to a vicinity closest thereto under the atmospheric pressure or a pressure near the atmospheric pressure so that the electric field intensity for initiation of the electric discharge is improved to thereby make it more difficult for the microwave transmitting waveguides to electrically discharge whereby the necessary level of the microwave energy is maintained within the invented apparatus.

Further, if the blocking plate is formed with a window frame made of a metal and a window made of a material through which it is easy for the microwave to pass, then the blocking plate will constitute a closed type resonator apparatus integral with the slot-array antenna, and the loss of the microwaves passing through the blocking plate will be reduced.

The present invention will be further illustrated by way of the accompanying drawings, in which:-Fig. 1 is a graph showing the correlative relationshipbetween pressure and electrical field intensity for initiation of electric discharge;

Fig. 2 is a front view of an embodiment of this invention with its portion shown in vertical section;

Fig. 3 is a plan view thereof;

Fig. 4 is a plan view of a slot-array antenna with a portion cut away;

.

~.

~ Z ~ ~ 3 ~ ~

Fig. 5 is an enlarged sectional view of a connecting portion be-tween the slo-t-array antenna and a waveguide;

Fig. 6 is a righthand side elevation of Fig. 5;

Fig. 7 is a sectional vi.ew per line ~-A; and Fig. 8 is a front view of a different embodiment, with its portion shown in vertical section.

Details of this invention will be hereinafter described referring to the accompanying drawings.

A vertical section of an embodiment of this invention and a plan view thereof are respectively shown in Fig.s 2 and 3, in which there are provided main waveguides (1) to ~3) outside a tank, the main waveguides consisting of rectangular waveguides to which are connected microwave oscillators t4) to ~6) at the righthand ends and T-shaped branching waveguides t8) to piercing through the vacuum-dryer tank wall (7), and located within the tank at the lefthand ends.

To the outlet side of the respective T-shaped branching waveguides (8) to (10) within the tank, th re are connected primary branching waveguides (11) to (13) respectively having their ends closed (short-circuited), and to each of the primary branchlng waveguides (11) to (13) there in turn are connected a plurality of, for instance I b : :

_ 5 _ - , : :
. .

~ . .
, ..
-.
.~ :

~24133~8 four secondary branch~ng waveguldes (14) to (17~. The end por-tions of the prlmary branchlng waveguldes arranged In thre0 layers and four llnes and those o~ the secondary branching waveguldes also arranged In three layers and ~our llnes are connected so as to overlap each other. The secondary waveguldes are arranged In confIguratlon as comb-teeth at Intervals ~ f equal to the wave-length of the transmltted waves wlthln the guldes. Slots (18) for connectlon are leadlng the mlcrowaves from the prImary branc~lng waveguldes to the secondary branchlng waveguldes Slot-array antennas (19) to (2Z) are connected vla blocklng plates ~23) to the secondary branchlng waveguldes (14) to (17), respectlvely, wlth thelr sectlonal shape as shown In Flg. 7. Slots (24) ~or mlcrowave radlatlon have the same shape and are arranged In conflguratlon of stepplngstones at regular Intervals. on the lower surfaces of each slot-array antenna In the upper tler, on the upper surfaces of each slot-array antenna In the lower tler as well as on both the upper and lower surfaces of each slot-array antenna In the mlddle tler shown In Flg. 2.
Thus, each of slot-array antenna In the upper tler and lower tler Is a type of slngle-surface-radlatlon whlle each of slot-array antenna In the mlddle tler belng a type of both-surface-radlatlon.

2~
Fach of the blocklng plates (23) Is fIxedly secured, as shown In Flg. 5, sandwlched between the secondary branchlng waveguldes (14) and the slot-array antennas (19), and Is formed wlth a wlndow frame ~23a) alrtlght to prevent any vaccum leakage as well as a wlndow t23b) I~

3~3 made of a material such as Teflon (a trademark), ceramic, glass, quartz glass, borosilicate glass or polysulphone. The windows have an induc-tion characteristlcs to allow the microwaves to pass through them without resulting in a great energy loss. Since the borosilicate glass in particular is substantially of the same thermal expansion coefficient as that of Kovar (a trademark) metal, jointing fusion thereof will be easy when the window frame is made of Kovar metal.

Each of abovesaid blocking plates (23) thus constitutes a closed type resonator (R) integral with the slot-array antenna.

Circulation pipes (25) serve the transfer a heating medium and are disposed in parallel with the primary branching waveguides (11) to (13). Heat-transmission-relaying bodies (26) made of aluminum alloy, are casted on the outer surfaces of the pipe~ (25) in shape like skewered dumplings according to the separate gap spaces between the secondary branching waveguides.

Heat pipes (27) are fitted in protrusion grooves (28) (see Fig. 7) on both sides of the slot-array antennas, and secured by means of adhesive agent of good thermal conductivity.
End portions of the heat pipes are inserted into holes provided in the heat-transmission-relaying bodies (2~), and fixedly secured by means of adhesive agent of good thermal conductivity.

It may be possible to omit the heat-transmission-. . .

' 83~8 relaying bodies (26) so as to connect the heat-pipes directly to the circulation pipes for the heating medium.
By the way, the heat pipes referred to above may be such that the heating medium passes through the pipe interior, or heating medium is contained therein.
Moreover, it is also possible to use solid rods or heater apparatuses such as electric heaters and the like, in place of the abovesaid heat-pipes.
As shown in Fig. 5 and Fig. 6, irises (29) (adaptor apparatuses for retroflexion of reflected microwaves) are provided in the secondary branching haveguides (14) for the purpose of retroflexion of the microwaves reflected from the slot-array antennas (19).
B Recipient trays (30) adapted to contain the foods ~-t~
~ike to be dried are held between the slot-array antennas located thereabove and therebeneath by means of a transferring-holder (31), in such locations as to scarcely cause hindrance against the radiation of the microwaves emitted by the slot-array antennas.
The recipient trays (30) are made of a material such as Teflon, polypropylene, polysulphone or the like, of small dlelectrlc loss and small reflectioli coefficient.
Strength and junctions of the waveguides of the microwave transmitting circuit arranged ~ithin the vacuum-dryer tank are so carefully de~signed not to cause .~ any vacuum leakage. The inlet-side end portionfi of the ~,J
:

- , :', . `: `

: ~ ': ' ~' '- .
, ' ~' . ' ~4~33~15 T-shaped branching waveguides (8) to (10) within the tank are mounted to the vacuum-dryer tank wall (7) by means of vacuum gaskets.
Though it is not illustrated in the drawings, both the microwave and radiation heating apparatuses shown in Fig. 2 and Fig. 3 are provided on the left and right sides symmetrically with regard to the axis of the transferring-holder (31), all of them being mounted in the vacuum-dryer tank.
Having given hereinabove the description on the structure of the embodiment, it is noted here that the ~ acteristics~
primaryYof this embodiment resides in a fact that the interior of the waveguides on the side ofthe microwave oscillator can be maintained at a pressure higher than that within the microwave antennas which are under reduced pressure, preferably at the atmospheric pressure _ t , by means of providing the blocking plates ____ . __ _ (23) intermediately of the waveguide circuit in the microwave heating apparatuses.
Besides, since the blocking plate (23) is formed to construct a close type resonator apparatus (R) together with the microwave antenna portion the microwaves are able to pass through this blocking plate at only a slight loss.
On the other llalld, the microwa~ve-tr.lnsniitting circllit lS designed as follows.

- _ 9 . . . .

., - , - .. : ,, ,. . -,: . , : :

,:

~834L~3 First, with regard -to the minimum electrical field intensity Vm ~ 180 Volts/cm for initiatior of electric discharge, the microwave-transmitting circuit which comprises the waveguide circuit consisting of the main waveguides outside the tank, T-shaped branching waveguides, primary and secondary waveguides are so constructed that the electrical field intensity Vw at the inlet ends of the slot-array antennas (19) to (22) is smaller than the Vm, namely Vw< Vm.
By virtue of this, it is possible to unify the shape and size of the waveguides constituting all of the slot-array antennas while suppressing electric discharging all over the entire system of the slot-array antennas.
Besides dimensions and locations of the slots (2~) for microwave radiation give influences to the directive characteristics of the microwaves radiated from the slots and the electric field intensity distribution of the radiated electric waves, said characteristic and distribut1on also have much to do with the electrtc field intensity prevailing at the locations of the slots.
If~electrical field intensity in regions near the slots becomes~higher than~that for initiation of electric dischar~e,then~electric discharging will occur at sucl-regions.
~ ccordingly, the d1mensions, 1Oc.lt1ons and nu~ er : ~ :

. . : . , ., ~ :, , ,: , -:
::::: : :
- : :

. - ~ : :

~839L~3 of the slots are designed so that the directivity of the radiated electric waves and the intensity distribution of the radiation field can be optimized by virtu ~preferable relations between the juxtaposed slots from a point of view that these directivity and distribution characteristics are the imperative conditions for uniform heating and drying of the foods or the like stationarily placed over the wide range.
In view particularly of the directional characteristics of the wall surface current flowing in the pipe wall of the antenna, the slots are disposed alternately with respect to the axis of the antenna, and the distance between the centers of the alternate slots are made equal to a half of the wave-length /\g of the transmitted wave within the guide. Hence, all the electric currents flowing to the respective slots have tl-e same phase, and it results in that the electric waves radiated fronn the slots in a direction normal to the pipe axis of the antenna.
Besides, the radiation impedance of the respective slots are made equal~to each other by disposlng them at the interval exactly equal to 1/2 of the wave-length Ag of the transmitted~wave within the guide.
Further, lt is noted that each tip en~ of the slot-array antenna is short-circuited ~ ile makine the distance from the tip end to t11e center line of tlie slot tllat is at the shortest distance thereflom, cqllal ~o ~/4 of the :

:` : : : :

'` ^`'`'```'' ` ` ~' `:.`:: : : .- .

:, . : : : :: .
-~ , : : . . :

:~LZ4~334~

wave-length A of the transmitted wave within the guide.
Induction impedance of each is thus caused to be ,infinity (~) so that the feeble reflection waves as occurring at the short-circuit wall and the window frame (23a) of the blocking plate ~23) are turned around by the function of the irises (29) provided at the end portion of the slot-array antenna and are successively radiated to the outer space (within the vacuum-dryer tank) from the slot (24), like the progressing waves. The microwaves which have jumped into the slot-array antenna from the outer space are also subjected to the function of the irises (29) to be re-radiated to the outer space.
Stubs can be used in place of the irises (29).
In this manner, the fact that the radiation impedances of the respective slots are of the same value characteristics while blocking up reflection waves at the portion of the single circuit of each slot-array antenna, will thlls cause the microwaves to be radiated to the outer space passing through the slots at the rate of substantially equal electric power ratios.
It has hereinbefore been mentioned that electric discharging is blocked up by maintaining the interior of a series of the transmission pipe circuit at the atmospheric pressure where the electrical field intellsity for initiation of e~lectric~discharge is l-igll. ~Anot;her inventive effor~t resides in the othor fact tha~t tl~e ~ imarv I'L~ - ;

~: : : : :

: - ` ::
: , . ~ . ~ : , :

3~

and secondary branching waveguides are connected eacl-other by means of the slot-type junction so that radiation of equal-electric-power may be effected.
The locations where directional components of swirling electric currents flowing in the broad-width guide wall surfaces of the rectangular waveguide as used in this embodiment get to be equal, are arranged to assume an appearance like as steppingstones located at intervals equal to the wave-length of the transmitted wave within the pipe.

In this embodiment of the invention, the secondary connecte branching waveguides (14) to (17) juxtapos ~ right angle to the primary waveguide (11) are at the distance ~f equal to the wave-length of the transmitted wave within the guide. In the portion of the electrical connection between the primarv and secondary branching waveguides, a slot is provided, at the point tl-at is spaced apart from the center line of the primary branching~waveguide (11) a certain predetermined d1stance x as shown in Fig. 4.
The connection slot is oriented in parallel to the center line of the branching waveguide and has a length Oe 1/2 ~ .
g A certain normalized conductance Go per each slot is unpolysemously def1ned by the transmission tlleory in accordance with the number of the seconclnrv ~ranching waveguides.
On the other l~and5 tlle actual normali~e(1 co1ldllctance ::
.. . . , :

~ ~ :
, ~ , :ILZ~33~

G of the secondary branching waveguide (1~) connected to the primary branching waveguide (11) with its righthand end short-circuited as shown in Fig. 4 will vary complexly in accordance with the short-circuiting distance of the secondary branching waveguide (14) and the 2-dimensional location of the slot ~18~. In view hereof, the distance x is experimentally determined in this embodiment so that the normalized conductance G defined by the 2-dimentional location of the slot (18) may come to be Go ~ G, to thus establish the optimal connection by the slot.
In the branching waveguide circuit in which all the normalized conductances G of the respective slots possess the same value, the microwave electric power which is transmitted to the primary branching waveguide (11) is then transferred to the secondary branching waveguides (14) to (17) at the equal ratio of electric power and in the state of the equal phase.
In the other aspect, it is necessary that -the electric power passing through the slot (18) for the connection affords the electric power to be supplied to the slot-array antennas which are connected to the secondary branclling waveguide.
The necessary electric power, wllen evaluated under the aforementioned normal conditions, may be about 150 watts for one single slot-anray antenna.

, If such large electric power shollld be trallslllitted .

under such a reduced pressure as in the known freeze-drying process, the electrical field intensity exerted to the single slot of the electric wave radiation would become extremely high far beyond the discharge initiation intensity of electrical field and thus electric discharging would occur as a matter of course at such location.
However as mentioned hereinbefore the blocking plates (23) are sandwitched between the slot-array antennas and the secondary branching waveguides, in such a manner that said plates constitute the closed type resonator apparatuses (R) integral with the slot-array antennas to thereby keep the resonator airtight to maintain atmosphenic pressure within, the waveguide circuit interior leading to the slot-array antennas. The necessary and sufficient microwave electric power is thus transmitted while avoiding any electric discharge under such a structure that can retain the discharge initiation intensity of electrical field on a high level.
Now, the functlon and effect of the above apparatus will be described in vacuum-freeze-drying the frozen foods.
B The foods _ that are frozen beforehalld are put into the recipient trays (30). The trays are then put on the transferring-holder (31), which in turll is transferred into the vacuum-dryer tank, its door l)eing closed thereafter.
After evacuation of the vacuum-dryel tank to a - 15 - ~

. :

.
;
.

~Z~83~

pressure near tlle operational pressure of the freeze-drying, the heating medium such as hot air, steam, heating oil or the like is then circulated within the circulation pipes (25) by means of a heating ~ontrol apparatus (not shown) so as to obtain an optimum temperature pattern for radiator-heating operation. By this operation, the heat-ipes (27) provided on both sides of the slot-array antennas via the heat-transmission-relaying bodies (26) are heated to heat the slot-array antennas (19) to (22) substantially to a temperature of the heating medium so that the slot-array antennas (l9) to (22) themselves may function as the heat radiators.
Here, with the temperature difference between the slot array antenna and the goods to be heated (both the frozen foods and the recipient trays) functioning as the motive power of the heat transfer, ncat travels to surfaces of the goods to sublimingly dry same that have been in the frozen state, wherein heat radiation plays a dominant roll in the heat transmission.
On the other hand, microwave electric po~ers emitted by the microwave oscillators (4) to (6) are transmitted from the main waveguides (1) to (3) disposed outside the tank) throtlgh the respective T-shaped branchin~ pipes (8) to (10) into the primary branching wave~uides ~11) to (13) connected thereto on the leEthand and~rialltll~and sldes, at the equal splittlng ratio of 1/2. Tlllls, fol~ instance :

3~

in case where microwave energy having a power of unit i.e. (1) is supplied to the T-shaped branching pipe (8) a microwave energy having half a unit power i.e. 1/2 is transmitted to each first branching waveguide (11).
Microwave electric power fed to the primary branching waveguide will then be divided into branches of the same phase and the same power by virtue of the function of the slots (18) provided at the connecting portions between the primary and secondary branching waveguides, and is thus transmitted through the respective blocking plates (23) between the second branching waveguides and tlle closed type resonators (R) finally into the slot-array antennas (19) to (22) whereby the microwaves are radiated from a plurality of the slots (24) towards the goods to be heated, under the condition of the good inherent directivity and as well as tlle homogeneous distribution.
Majority of the microwaves radiated from the slots (24) travel stright in the free space so as to be penetrate into deep portions of the foods to thereby be consumed as absorption heat within the foods wllile being repeatedly reflected or refracted at the boundary surfaces of the foods or the recipient trays.
Microwaves that have not been consIlmed within the foods form up higher mod~es within the ~ree space ~herein a 3-d1mensional transn-ission takes pl~ce to~calIse said residual microwaves to be transferred t;o the variolIs electro-conductive walls within the vaclIllm-dryer tank : . , .: , .
. . . ~ :
, - :,: .. ~ :

33~3 or to the foods or the like while being consumed as heat energy.
If the waveguide circuit is supplied with an electric power the level of which exceeds that of such microwave electric power as consumed under the load, the reflection waves as well as the electric field intensity will increase in general to thus excite initial electrons and initial ions existing in the interveing gas which are likely to induce electic discharging.
Accordingly, there is provided in the embodiment a photosensor means (not shown) adapted to detect the electric discharging which will occur due to the action of the excessively high power of microwave fed to the apparatus. Thus the input level of the microwave electric power can be controlled to an optimum level in accordance with the actual load so that an efficient freeze-drying operation may always be guaranteed.
In the known or conventional freeze-dryirlg process utilizing radiator or conduction heating system, t~he drying proceeds in such a manner that water content of the foods decreases to some degree thereby forming a dried layer or outer surface which is resistant to the heat transmission resulting ln a drastically reduced drying rate. It has therefore been necessary to continue the operati~on for quite a long time in order to seek a slight reduction of the water content.

. - 18 -,, : , .

, ~83~8 However, with the composite apparatus of the embocliment provided with the radiation heating and the microwave heating systems, a remarkable shortening of the drying time as well as a better homogenization of the heating and drying are now afforded advantageously by either effecting the initial opera-tion depending only upon the radiator heating system so as to operate the microwave heating system in a compound way from the period when the drying speed just begins to be decelerated or by operating both the heating systems in a compound way from the beginning of the operation. As a result hereof, proof is obtained which demonstrates a higher productivity and a lower drying cost.
It is also to be noted that since the slot-array antennas serve dually as the microwave emitter and as the heat radiator radiator heating. it is possible to make the apparauts more compact in comparison with the case a heat radiator is simply added to the apparatus.
Despite the exemplified arrangement in wllicll the blocking plate (23)~is positioned at~the lnlet end of the slot-array antenna, to constitutes the closed type resonator (R) together with the slot-array antenlla, this location Gf the plate can o~f cour~e be altered arbitrarily.

, ~` However,~ th1s location~is advanta~eous iri manllfacturing and in obtaining a good function ~?f the apparnt:lis.

:: :
::

- . ..

~Z~3~

B0sides, it is also possible to give to the slots (24) a shape of slits extending in the direction of microwaves without arranging the slots to form the shape like as steppingstones.

Furthermore, though both the microwave and radiator heating systems are disposed on one side of the transferring-holder (31) the slot-array antennas may be located in a manner such that they alternately extend toward each other in opposite directions as shown in Fig. 8.

Besides, though the T-shaped branching waveguides, the primary and secondary branching waveguides as well as the slot-array antennas are disposed within the vacuum-dryer tank ln the embodiment, it may be modified to protrude only the slot-array antennas into the vacuum-dryer tank.
As above, the dryer apparatus according to this invention is widely availa~le on a commercial scale to the vacuum-freeze drying and to the vacuum dr~ing of the foods and pharmaceuticals and more particularly to the vacuum-freeze-drying of the frozen foods.

......... . . . .
.
, -~ , .

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A microwave vacuum dryer apparatus comprising: a vacuum drying tank, slot-array antenna means disposed in said vacuum drying tank, microwave generating means disposed outwardly of said vacuum drying tank and under ambient pressure, waveguide means to guide said slot-array antenna means from said microwave generating means, and blocking plates mounted between said microwave generating means and said slot-array antenna means, said blocking plates each comprising a window formed of a mate-rial permitting easy passage of microwaves and a metal window frame, said window having a strength to withstand a differential pressure between a vacuum in said vacuum drying tank and the ambient pressure outside thereof, said metal window frame being disposed between the waveguide means and the antenna means, and further comprising at least two irises, each iris provided an end portion of the slot-array antenna in such a manner that reflected waves from a short-circuit wall and the window frame of the blocking plate are reflected by the irises so as to advance forward again into the antenna wherein said window, said metal window frame and said slot-array antenna means co-operate to set up a closed type resonator.

2. The dryer apparatus as claimed in claim 1 further comprising heating means mounted peripherally of said slot-array antenna means.

3. The dryer apparatus of claim 2, in which the window is made of borosilicate glass.

4. The dryer apparatus of claim 3, wherein the slot-array antenna comprises slots arranged therein at suitable inter-vals so as to form a shape of steppingstones.

5. The dryer apparatus of claim 2, wherein the heating means comprises heat pipes.

6. The dryer apparatus of claim 5, in which the heat pipes are fixedly fitted in protrusion grooves provided on the outer surfaces of the slot-array antennas.

7. A method for microwave vacuum drying comprising the steps of: maintaining a vacuum drying tank in a relative vacuum state to facilitate drying; maintaining waveguides leading to the vacuum at a higher pressure than that in the vacuum drying tank to minimize electrical discharge from the waveguides; mini-mizing microwave discharge by setting up a closed-type resonator, having a glass window and a metal window frame, the metal window frame being disposed between the waveguide and an antenna means, transmitting microwaves from the antenna means along the wave-guides into the vacuum drying tank to dry material in the drying tank, and providing a plurality of irises to reflect microwave from the direction of the antenna means back toward the antenna means.

8. The method according to claim 7, wherein the wave-guides are maintained at ambient pressure.