US3474210A - Access opening construction for microwave chambers - Google Patents

Description

Oct. 21, 1969 K. SILBERMANN ET AL 3,474,210

ACCESS OPENING CONSTRUCTION FOR MICROWAVE CHAMBERS Filed July 13, 1967 2 Sheets-Sheet 1 IN V ENTOR3 Wm R U Y R E EMH m B O l W S 0 A E %D W E LR KF V. B

Oct. 21, 1969 K. SILBERMANN ET AL 3,474,210

ACCESS OPENING CONSTRUCTION FOR MICROWAVE CHAMBERS Filed July 15, 1967 2 Sheets-Sheet 2 FlG 2 FlG 3 KLAUS SILBERMANN BY FREDERICK MAURER (La/J 6.6

ATTORNEY United States Patent 3,474,210 ACCESS OPENING CONSTRUCTION FOR MICROWAVE CHAMBERS Klaus Silbermann, Danville, and Frederick Maurer, Walnut Creek, Calif., assignors to Cryodry Corporation,

San Ramon, Califi, a corporation of California Filed July 13, 1967, Ser. No. 653,092 Int. Cl. Hb 9/06, 5/00 US. Cl. 219-1055 17 Claims ABSTRACT OF THE DISCLOSURE This invention relates to high frequency systems for heat treating substances and more particularly to access opening structure for microwave chamber apparatus.

Electromagnetic energy in the microwave range of frequencies has many unique advantages as a means for heating substances such as foods, lumber, plastics and diverse other commercial products. A very rapid and uniform heating can be effected, often with results that cannot readily be duplicated by any other technique. In order to utilize this advantageous form of heat treatment, a processing chamber structure is used to confine the microwave and precautions must be taken to prevent the escape of any significant amount of such energy through access openmgs.

Several factors combine to make fairly strict containment of the microwave energy mandatory. Such energy may be injurious to personnel and can interfere with communications equipment and other electrical apparatus if broadcast into space. For these reasons, governmental regulations in most areas prohibit the release of anything more than a minimal power level. Accordingly, microwave heating is virtually always performed in a processing chamber assembly having electrically conductive walls which reflect the energy rather than transmitting or absorbing any sizable proportion thereof and such chamber assemblies must have access openings which are also arranged to suppress microwave emission.

Where the chamber assembly is of the type used for batch heating operations, access can often be provided for by means of a hinged door in the chamber wall together with arrangements, generally in the form of an automatic interlock, for insuring that the microwave power input to the chamber is cutoff when the door is opened. However, a much more difficult access problem is present in continuous process operations in which the material to be treated is continually passed through the microwave region. This type of chamber assembly, which may have a conductor walled tunnel with a conveyor belt or the like extending therethrough, should have product input and output ports which can be continually open without releasing significant amounts of microwave from the tunnel.

One form of access port for a continuous process chamber assembly which meets these requirements is disclosed in co-pending application Ser. No. 589,149 now Patent No. 3,365,562 of Morris R. Jeppson entitled Apparatus and Process for Microwave Treatment, filed Sept. 7, 1966, and assigned to the assignee of the present invention. In the structure of this co-opending application, a lossy material, such as Water, is maintained adjacent the in- 3,474,210 Patented Oct. 21, 1969 ner walls of the chamber assembly end terminations. The tunnel and power injection system configuration is such that microwave propagates towards the ends of the tunnel by repeated reflections between opposite walls and subsequently enters the terminations. 'In passing along the terminations in a similar manner, the microwave must repeatedly pass through the lossy material and is attenuated thereby to the point where no significant release of energy occurs from the ends of the chamber assembly. In this construction, the degree of microwave attenuation is, in part, a function of the size of the terminations.

In addition to providing a large continuously open access passage while suppressing microwave emission therethrough, the termination with lossy material therein has the further property of providing the necessary electrical load when the amount of product in the chamber assembly is insufficient for this purpose or when no product is present.

An important further advantage of the access port construction of co-pending application Ser. No. 589,149 now Patent No. 3,365,562 is that the passage may be unobstructed to the extent necessary to accommodate products of large unit size. However, some products are of relatively small unit size relative to the total passage area and in this situation a more compact construction may be preferred.

It is advantageous, where possible, to reduce the size of the terminations and the length of the conveyor and thereby achieve an overall reduction in the size of the chamber assembly as a whole. In addition, other operations may be required at the ends of the chamber assembly, and it is advantageous if the same access opening construction which suppresses energy emission is made to serve these other purposes. Product leveling, i.e., insuring that the material to be treated is distributed on a conveyor belt in the preferred manner, is one such operation.

The present invention is a compact construction providing for access to a microwave region while suppressing microwave emission through the access opening. The structure may serve the further purpose of distributing product on a conveyor belt or other means for passing the product through the microwave region.

In accordance with the invention, the access passage is defined by an electrically conductive housing which may have dimensions, transverse to the direction of product travel, which exceed the microwave wavelength. To suppress the emission of microwave through the passage under this condition, at least a portion of the passage is subdivided by conductive means into a plurality of channels each of which has transverse dimensions which are sufficiently small to define a cut-off waveguide.

In many instances, the above described conductive housing may advantageously be an inclined chute to provide a simple and reliable structure for feeding material onto a conveyor or the like. In these instances, the conductive means which subdivides the access passage may also be arranged to provide for a more even distribution of product on the conveyor. Under certain conditions, deflectors may be used to facilitate the passage of product through the access passage and vibrators may be coupled to the structure for the same purpose.

Accordingly, it is an object of this invention to provide a compact, eificient, and structurally uncomplicated access opening construction for a microwave chamber assembly.

It is a further object of the invention to facilitate the feeding of product substances into continuous process microwave chamber assemblies.

The invention, together with further objects and advantages thereof, will be better understood by reference to the following description of preferred embodiments, and by reference to the accompanying drawings.

In the drawings, FIGURE 1 is a side elevation view of a continuous process microwave chamber assembly having product input and output passage constructions at opposite ends in accordance with the invention, portions of the apparatus being broken out to illustrate internal elements;

FIGURE 2 is a plan view of the product input end of the chamber assembly of FIGURE 1 taken along line II II thereof with certain areas of the structure broken out to illustrate underlying elements; and

FIGURE 3 is a plan section view taken along line III- III of FIGURE 1 showing a vibrating product deflector within the product output passage of the chamber assembly of FIGURE 1.

The term microwave chamber assembly is used herein to refer collectively to all components of the apparatus hereinafter described.

Referring now to the drawing and specifically to FIG- URE 1 thereof, one of the components of a typical microwave chamber assembly 11 of the type to which the in- L vention is applicable is a tunnel 12 with walls formed of electrical conductor and which may have a rectangular cross-section. Substances or products which are to be irradiated with microwave energy are transported through the tunnel 12 by means such as a conveyor belt 13 car- 4 ried on a rotary drum 14 at each end of the chamber assembly and which turn in the direction indicated by arrow 16. Microwave energy from a suitable source 17 is injected into tunnel 12 through a. waveguide 18 that extends along one wall thereof.

Waveguide 18 has a series of transverse slots 19 which provide for a distributed emission of energy into the tunnel 12. The detailed structure and operation of a suitable microwave injection system of this type is described in US. Patent No. 3,263,052 issued July 26, 1966 and entitled Power Distribution System for Microwave Process Chambers.

In a continuous process microwave chamber assembly 11 of this general type, the energy which is injected into the tunnel 12 from the waveguide 18 propagates gradually toward both ends of the tunnel by repetitive reflections between opposite walls thereof. To provide for access while preventing the release of significant amounts of such energy from the ends of the tunnel, and to effect certain other results, terminations 21 and 22 are provided adjacent the input and output ends respectively of the tunnel. The supplementary functions of the terminations 21 and 22 may include that of providing an electrical load for the microwave source 17 to avoid a severe mismatch, and possible tube damage, when there is insufficient product on the belt 13 to provide adequate loading. Both terminations 21 and 22 may serve as chutes for physically guiding product substances and the input termination may also function to distribute such products evenly on the conveyor belt 13 for movement into the input end of the tunnel 12.

The detailed construction of the input and output terminations 21 and 22 may differ while embodying essentially similar basic principles. Considering first the input termination 21, a conductive housing 23 of the termination forms a region 24 which is communicated with the tunnel 12. To absorb microwave energy which reaches the input end of tunnel 12 and thereby assures that an adequate electrical load is always present irrespective of the amount of product in the chamber assembly 11, a lossy material 26 is disposed along interior surfaces of the termination housing 23 so that the energy which propagates through the region 24 of the housing by repetitive reflections between opposing walls must repeatedly pass through the lossy material. Water constitutes an excellent lossy material 26 for this purpose.

Various different constructions may be employed to retain a liquid lossy material 26 in position provided that at least part of the components of the construction are dielectrics transparent to microwave energy. Referring now to FIGURE 2 in conjunction with FIGURE 1, this example of the invention uses a particularly advantageous construction in that the water is contained within plastic pipes 27 which have been formed into a flattened helix with the two broad parallel sections thereof being spaced from the inner wall of housing 23 by distances corresponding to about one-quarter wavelength and three-quarter wavelength of the microwave energy. This concentrates the water bulk at locations at which mazimized interaction between the water and the microwave occurs. This results in that the wall of housing 23 is a conductor so that the electrical vector of the microwave is necessarily minimal thereat with electrical field maxima occurring at distances equal to odd multiples of one-quarter wavelength.

In addition to providing an electrical load to avoid a mismatch in the system, the lossy material 26 also aids in suppressing the emission of energy from the chamber assembly 11 in the manner described in the hereinbefore identified co-pending application Ser. No. 589,149 now Patent No. 3,365,562. However, the grid structure 29 to be hereinafter described is capable of suppressing energy emission without being associated with a lossy material 26.

Referring still to FIGURE 2 in conjunction with FIG- URE -1, the access passage 28 into the input termination 21 is defined by a housing 31 which is of rectangular cross section in this instance. At least the inner surfaces of housing 31, which face the passage 28, are formed of electrically conductive material to co-act with a grid structure 29 therein for suppressing microwave emission as will hereinafter be discussed in more detail.

While the access passage 28 may be oriented in any of various ways relative to the input termination 21, it is particularly advantageous if the housing 31 constitutes an upwardly directed, and preferably inclined, chute leading to the conveyor 13 within the input termination. Thus in this example, the housing 31 transpierces the upper wall of termination housing 23 at an obtuse angle with respect to the direction of travel of the conveyor belt 13. With this arrangement, product from a supply conveyor 32 or the like may readily drop through the access passage 28, past grid structure 29, and be deposited on the internal conveyor belt 13. In order to guide the product material between the lower end of housing 31 and the conveyor 13, a chute extension 30 extends downward from the lower end of housing 31. Chute extension 30 is open at the side facing tunnel 12 and is formed of a plastic transparent to microwave energy.

The grid structure 29 is formed of planar electrically conductive thin gauge material and subdivides the access passage section 28 into a plurality of smaller parallel channels 34. Generally, it will be most convenient to arrange the baffles to provide channels 34 of square cross section as in this example so that two groups 36 and 37 of grid members are present with each group being at right angles to the others. The total number of channels 34 and thus the number of grid members 36 and 37 is dependent upon the overall configuration of the housing 31 which defines the access passage 28, the present example having two of the channels 34 in one direction and seven channels in the other in order to accommodate to the use of a broad conveyor belt 13 in the chamber assembly 11. It should be understood that a suitable grid structure 29 may be provided for any other access passage configuration by varying the number of the channels 34 in each direction provided that the dimensions of each channel are related to the microwave wavelength as will hereinafter be described.

The extent to which the grid structure 29 suppresses microwave emission is dependent on the dimensions of the channels 34 relative to the microwave wavelength. In general, these dimensions are selected so that each channel 34 is equivalent to a cut-off waveguide. The transverse channel dimensions, indicated by letters a and b in FIG- URES 1 and 2 respectively should be less than the wavelength and preferably should not exceed about one-third thereof. Conversely, attenuation increases as the length of the channels, indicated by letters 0 in FIGURE 1, is increased. Generally, the channel length c should be at least one-half wavelength. Where, as in this example, the grid members 36 have a parallelogram configuration due to the inclination of housing 31, dimension a should be considered as taken along a line normal to grid members 37, while dimension 0 is measured between similarly normal lines which intersect the obtuse angles of the parallelogram defined by each grid member 36.

At a frequency of 915 megacycles, for example, the attenuation is approximately 7 db per inch of channel length 0 if the transverse dimensions a and b are both five inches. However, the attenuation drops to 1.7 db per inch of channel length 0 for transverse dimensions a and b of six inches. Thus, the channels 34 are preferably made small in the transverse direction and of maximum length consistent with the need to avoid impeding the movement of product through the access passage 28 and the desirability of avoiding an inconvenient height for the microwave chamber assembly 11 as a whole.

In addition to providing continuous access to the microwave region while suppressing energy release, a further effect of the grid structure 29 is to promote a more even distribution of product across the width of the conveyor belt 13 in that if material entering the access passage 28 from the supply conveyor 32 tends to concentrate at one section of the total passage, the channels 34 thereat will rapidly fill, causing material to spill over into the adjacent channels. To facilitate this leveling action, a hopper 38 may be disposed at the top of passage 28, below the end of supply conveyor 32.

Most granular or particulate products which may be treated in the chamber assembly 11 may be passed through the access passage 28, past grid structure 29, without difficulty particularly when the particle size is very small in relation to the transverse dimensions a and b of the channels 34. However, in processing larger unit sized products and particularly soft, somewhat adhesive products such as potato slices for example, there may be some tendency for individual pieces to catch on the thin upper edges of the grid members 36 and 37 if the edges are exposed. To avoid this, a deflector 39 may be disposed in the access passage 28 immediately above the upper end of the grid structure 29. Deflector 39, in this example, has linear rods 40 situated above the upper edges of grid members 36 and extending parallel thereto and has an additional rod 41 situated above rods 40 and extending parallel to the upper edges of grid members 37. The rounded upper surfaces of rods 40 and 41 deflect incoming product from the subjacent edges of the grid members. Preferably, the deflector 39 is formed from a material having a low co-efiicient of friction such as tetrafluoroethylene plastic in order to avoid the retention of wet or adhesive materials thereon.

In those instances where there is still some tendency for product material to stick or jam within the access passage 28, an electrical vibrator 35, connected with a current source 45 through a control switch 50, may be coupled to housing 31 to oscillate the structure disposed therein.

Referring now to FIGURE 3 in conjunction with FIG- URE 1, the termination 22 adjacent the output end of tunnel 12 may embody essentially similar principles, but may differ in that the output passage 28 will generally be directed downwardly in order that product released from the end of the tunnel conveyor belt 13 may drop through the output passage, which contains a grid structure 29', to a third conveyor 42 or other means for carrying the treated product from the chamber assembly 11.

Output termination 22 has a conductive housing 23' which defines the output region 24' adjacent the end of tunnel 12 and which has a quantity of water or other lossy material 26' held along opposite walls and the endwall in plastic tubing 27 in an arrangement essentially similar to that of the input termination 21 as hereinbefore described. Housing 23 has a section 43 below the end of conveyor belt 13 with a sloped wall 44 that forms a hopper to direct product from conveyor 13 into the upper end of the output passage 28' of the termination 22. A dielectric wall 48 extends upward from passage 28 opposite sloped wall 44 to prevent any accumulation of product at the end of housing 23'. The output passage 28 extends directly downward from hopper 43 to the product removal conveyor belt 42 and is defined by a conductor walled housing 46 with an opening 47 at the lower end and at the side toward which the conveyor 42 travels to allow product to be carried away from the output termination 22.

The grid structure 29', formed of electrically conducting grid members 36' and 37, is disposed within the upper portion of passage 28' to subdivide the passage into channels 34 which have dimensions related to the microwave wavelength as hereinbefore discussed with reference to the input termination 21.

Thus, the output termination 22 functions as hereinbefore described with reference to the input termination 21 to provide loading in the region 24' adjacent the end of tunnel 12 and to suppress the emission of microwave energy therefrom while providing a large area opening for transferring product from conveyor 13 to the product removal conveyor 42. It will be noted that the grid structure 29 within output termination 22 will also serve the function of providing for a uniform distribution of product across the product removal conveyor 42.

As in the case of the input termination 21, a deflector 39' is disposed adjacent the upper end of grid structure 29' to prevent the impalement or adherence of product thereon. As best shown in FIGURE 4, the deflector 39 may again have component rod members 40 and 41 similar to those hereinbefore described and disposed in a similar manner relative to the upper edges of the grid members 36 and 37. Since the output end deflector 39 is situated within the microwave region, it should be formed of a low loss material, such as polycarbonate plastic, which also has the further properties hereinbefore described.

Jamming or adherence of material at the top of the grid structure 29 is avoided, in the output termination 22 of this example, by oscillating the deflector 39'. Various mechanisms may be utilized to oscillate the deflector 39 for this purpose. In this instance, the deflector 39' is disposed in passage 28' in a manner which provides for limited motion in a direction transverse to the axis of tunnel 12. The ends of the component members 40' of the deflector 39', along opposite sides thereof, extend into a pair of parallel grooves 51 formed in the inner surfaces of housing 46 while the component member 41' of the deflector has one end extending into a guide well 52 at the side of the housing and has an opposite end 53 which extends through a small opening 54 at the opposite side thereof. Thus, the entire deflector 39 is capable of a limited amount of transverse movement along grooves 51. A compression spring 56 bears against a flange 57 on end 53 of deflector member 41 and against the outer wall of housing 46 to urge the deflector 39' towards the adjacent side of the housing. An eccentrically mounted cam 58 rides against an end cap 59 on end 53 of deflector member 41' to move the deflector 39' against the action of spring 56 so that rotation of the cam 58 results in a systematic oscillation of the deflector as a whole. Cam 58 may be turned by an electrical motor 61 secured to housing 46 by a suitable bracket 62.

While the microwave chamber assembly 11 as de-- scribed herein utilizes termination constructions embodying the invention at both ends of the tunnel 12, it should be understood that such a termination may be used only at one end of the tunnel, if desired, with other means being used at the opposite end. Further, the physical configuration of the terminations may be varied considerably to adapt to different forms of microwave chamber assembly. Thus, while the invention has been described with respect to certain specific embodiments, it should be understood that many modifications are possi- 'ble within the scope of the invention, and it is not intened to limit the invention except as defined in the following claims.

What is claimed is:

1. In a processing chamber assembly of the class having an electrically conductive wall defining a region into which microwave energy is injected, the combination comprising:

electrically conductive means defining an access passage communicating said microwave region with the region exterior thereto; and

at least one conductive member disposed in said access passage and subdividing at least a portion of said passage into a plurality of smaller channels each having a dimension transverse to said passage which is less than the wavelength of said microwave energy and a dimension along said passage which is at least one-half the wavelength of said microwave energy, whereby the emission of microwave energy through said passage is suppressed.

2. The combination defined in claim 1 wherein said channels have transverse dimensions limited to about onethird of said wavelength.

3. The combination defined in claim 1 wherein a plurality of said conductive members are provided, said members being thin planar elements which intersect to define said plurality of channels.

4. The combination defined in claim 3 wherein said conductive members extend longitudinally in said passage, with a first group of said conductive members being disposed normal to a second group thereof whereby said plurality of channels are parallel and of rectangular cross section.

5. The combination defined in claim 1, further comprising a vibrator coupled to said means defining said access passage.

6. The combination defined in claim 1 wherein said conductive member is a thin, fiat element and further comprising a deflector disposed in said passage and extending along the edge of said conductive member and being of greater thickness than said conductive member to shield said edge from material passing through said passage.

7. The combination defined in claim 6 wherein said deflector has a rounded surface opposite from said edge of said conductive member.

8. The combination defined in claim 6 wherein said deflector is formed of a material having a low coefiicient of friction.

9. The combination defined in claim 6 wherein said deflector is formed of material having a low electrical loss factor.

10. The combination defined in claim 6 further comprised of means for oscillating said deflector.

11. The combination defined in claim 1 wherein one end of said access passage is higher than the other end thereof whereby product substances may drop through said passage and said channels therein.

12. The combination defined in claim 11 further comprising a hopper disposed above said passage to facilitate the distribution of material between said channels.

13. The combination defined in claim 11 wherein said processing chamber assembly is of the class having means for carrying product through said microwave region, and wherein said conductive means defining said access passage is a chute extending in a generally upward direction relative to said means for carrying product, said conductive member having a lower end spaced above said means for carrying product whereby product which drops through said passage is deposited on said means for carrying product with said conductive member acting to promote a uniform distribution of said product thereon.

14. The combination defined in claim 13 further comprising a product guiding chute extension disposed between said chute and said means for carrying product, said chute extension being formed of dielectric material.

15. The combination defined in claim 1 further comprising a lossy material disposed in said microwave region whereby microwave energy is partially attenuated before reaching said conductive member in said access passage.

16. In a microwave chamber assembly of the class having a conductive wall forming a microwave region and having means for continuously passing product substances to be treated through said microwave region, a product input comprising a chute extending substantially upwardly from said means for passing product substances through said microwave region, said chute defining an access passage which extends through said conductive wall.

17. The combination defined in claim 16 wherein said means for continuously passing product substances through said microwave region is comprised of a conveyor wholly contained within the region Within said conductive wall of said chamber assembly.

References Cited UNITED STATES PATENTS 2,597,825 5/1952 Schroeder 219-1055 X 3,083,528 5/1959 Brown 21910.55 X 3,263,052 7/1966 Jeppson et al. 219-1055 3,422,242 1/1969 Miyata 219-10.55

JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner US. Cl. X.R. 19--10.61

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