US3622733A

US3622733A - Method and apparatus for drying sheet materials

Info

Publication number: US3622733A
Application number: US6499A
Authority: US
Inventors: Franklin J Smith; Klaus Silbermann
Original assignee: Cryodry Corp
Current assignee: MICRODRY Inc A CORP OF KENTUCKY; Cryodry Corp
Priority date: 1970-01-28
Filing date: 1970-01-28
Publication date: 1971-11-23
Anticipated expiration: 1988-11-23

Abstract

Wood veneers or other flat materials are passed through successive sections of energized meander waveguides and through jets of heated air from air knives situated between the waveguide sections to effect a very rapid drying action in which relatively moist areas are dried more strongly than dryer areas to produce a finished product of uniform moisture content. The waveguide and air knife structure is disposed within an electrical conductor walled, thermally insulated cabinet with continuous feed conveyor means and an air recirculation system in an arrangement which optimizes operating efficiency and convenience.

Description

United States Patent [72] inventors Franklin J. Smith Diablo; Klaus Silbermann, Danville, both of Calif. [21] Appl. No. 6,499 [22] Filed Jan. 28, 1970 [45] Patented Nov. 23, 1971 [73] Assignee Cryodry Corporation San Ramon, Calif. Original application Nov. 14, 1967, Ser. No. 682,903. Divided and this application Jan. 28, 1970, Ser. No. 6,499

[54] METHOD AND APPARATUS FOR DRYING SHEET MATERIALS 7 Claims, 12 Drawing Figs.

[52] US. Cl 219/1055, 219/ 10.69 [51] Int. Cl 1105b 9/06, 1105b 5/00 [50] Field of Search 219/1055 [56] References Cited UNITED STATES PATENTS 3,235,702 2/1966 Timmermann et a1 219/1055 23 23 23 EATED AIR HEA ED A R 3,304,399 2/1967 Timmermann et a1 219/1055 3,413,483 11/1968 Timmermann et a1 219/1055 3,449,836 6/1969 Schreiber et al 219/1055 3,471,672 10/1969 White 219/1055 3,474,210 12/1969 Silbermann et a1. 219/1055 3,474,213 10/1969 Hilton et a1. 219/1055 3,507,050 4/1970 Smith et a1 219/1055 X Primary Examiner-J. V. Truhe Assistant Examiner-1. H. Bender Attorneys-Carl C. Batz and Fryer, Tjensvold, Feix, Phillips and Lempio MICROWAVE MICROWAVE TMC ROWAVE HEATED AIR PATENTEDHUY 2 I 71 3. 622 7 3 3 saw 1 or 7 MICROWAVE MICROWAVE HEATED AIR) HEATED AIR HEATED AIR M lCROWAVE III INVHNTORS FRANKLIN J. SMlTH KLAUS SILBERMANN ATTORNE PATENTEDNnv 23 ISYI SHEET 2 BF 7 FIG 3 INVI'INTORS FRANKLIN J. SMITH KLAUS SILBERMANN WM/Z' ATTORNE PATENTEDunv 23 I971 SHEET 3 BF 7 me vm mm v wz - mvramons FRANKLIN J. 3mm

KLAUS SILBERMANN M 9 AT TORN PATENTEDSnv 23 19h SHEEI b [1F 7 FIG 5 INVFN'H )RS FRANKLIN J, SMITH KLAUS SILBERMANN Wag ATTORNEY PATENTEDNuv 23 ml 3.622.733

SHEET 5 BF 7 INVI'INTORS F G 6 FRANKLIN .1. SMITH KLAUS SILBERMANN ATTORNEY PATENTEDunv 2 3 an SHEET 8 OF 7 INVENTORS FRANKLIN J. SMITH KLAUS SILBERMANN ATTORNEY PATENTEU 2 3, 622 7 33 FIG 1O INVENTURS FRANKLIN J. SMITH KLAUS SILBERMANN ATTORNEY METHOD AND APPARATUS FOR DRYING SHEET MATERIALS This application is a division of the application by Smith et al., Ser. No. 682,903 entitled Method and Apparatus for Drying Sheet Materials, filed Nov. 14, 1967.

a This invention relates to the drying of sheet materials and more particularly to a method and apparatus for rapidly and efficiently drying such materials with microwave energy on a continuous process basis.

. Many industrial products which must be dried in the course of manufacture have a thin, flat configuration and may be treated by specialized techniques which differ from those employed for processing bulky objects, ,granular materials or products having other configurations. Wood veneers of the type which are laminated together to form plywood are an example of a sheet material which is customarily subjected to a drying operation. The present invention was initially developed to facilitate the drying of veneers and accordingly will be described with reference thereto, it being understood that the'invention is also applicable to the drying of many other similarly shaped products.

Veneers are typically about 1/10 inch to 7/32 inch thick and are about 50 inches wide and 8 feet long. To form plywood, several of the dried veneers must be bonded together and it is preferable'that a hot resin be used for bonding and that pressure be applied to the veneers as the resin hardens. Product quality is very dependent upon the moisture content of the veneers at the time of bonding. The moisture content should typically be from about 3 percent to about 8 percent and it is important that this moisture content be as uniform as possible throughout all areas of each veneer.

If there are localized areas of relatively high moisture content in any of the veneers during bonding a phenomenon known as blow-up may occur when the pressure is relieved. This is a physical disruption of the adjacent surfaces of the veneers at the wet areas and is believed to result from steam pressure generated at themoisture concentration. Conversely, if there are localized areas which have a moistureicontent below the optimum range, checking, brittleness and loss of strength may result. The problem of moisture leveling, i.e., obtaining'a uniform moisture content throughout the veneers, has been particularly troublesome in the plywood industry.

' While veneers have been dried by being pressed between heated platens and by various other means, the technique most widely employed at present makes use of an air drying chamber. The usual structure includes a cabinet in which roller conveyors, known as nip rolls, carry the veneers through the drying, region. Heated air is continuously passed through the cabinet and insome machines, air knives are used to direct high velocity jets of air against the veneer surfaces. These aretubes into which heated air is pumped and which are disposed transverse to the path of the veneers and in close proximity'thereto. The tubes are usually arranged in pairs so that the veneers pass between the two members of each pair. Each tube has a long slit opening facing the veneers to direct the jet of heated air thereagain'st.

Several problems are encountered in utilizing an air dryer of this kind for drying veneers. Processing time, for example, is undesirably long. Drying, in these systems, is dependent on heat transfer from the surface of the veneers into the interior thereof and several factors limit the rate at which such heat transfer can be made to take place. In particular, the surface temperature of the veneers'm ust be kept belowa fixed limit to avoidscorching or otherwise degrading the wood. Secondly, wood is a poor thermal conductor, particularly after partial drying. In conventional systems, these factors have resulted in minimum processing times of the order of minutes. If the air temperature is maintained sufficiently low to avoid any adverse effects on product quality, much longer processing time is required.

Another consequence of long processing times is that several parallel columns of veneers must be passed through the dryer at any given time, if reasonably efficient operation is to be realized without an extremely long dryer. This in turn requires multiple conveyor systems and complex product input and output mechanisms.

Still another major problem area in the use of conventional air dryers is that it is very difficult to obtain dried veneers with the preferred uniform moisture content. Green veneers may differ in their overall moisture content and localized areas of each individual veneer may have different moisture contents. Knots, for example, are typically more moist than the surrounding areas of the veneer. Using conventional systems, the relatively moist veneers and the'relatively moist areas of individual veneers, remain in' this condition after drying. Veneers which have an uneven moisture distribution, or an overall moisture level outside of the desired range, are detected at the output of the dryer and must be recycled or rejected. Successful recycling is, in turn, very difficult. In reducing the moist areas to the desired range, the other areas of the veneers tend to become too dry. This problem of moisture leveling has been a major source of operating inefficiency and process complication, not only in the drying of wood veneers but in drying many other sheetlike products as well.

To avoid certain of the difficulties discussed above, microwave heating, rather than air drying, may be used for treating various materials. This form of heating is unique in that it does not depend upon heat transfer inward from the surface of the material but very rapidly generates heat directly .within the interior region thereof. Further, microwave energy interacts primarily with the internal water content of the material. "This provides for a moisture leveling-effect in that proportionately more heat is generated in the regions of the material which contain the most moisture. However, the treatment of industrial products with microwave is a relatively new art and the techniques and equipment which have heretofore been utilized are not optimally adapted to the drying of veneers or many other sheetlike materials.

A sheet material can be heated rapidly by being passed through an energized waveguide. If the waveguide is energized in one of certain modes, longitudinal slots can be provided along'the' sides of the waveguide to provide for passage of the sheet material therethrough. Further the material may be passed through successive sections of a single waveguide while traveling along a linear path if the waveguide is formed with a serpentine or meander configuration which repeatedly transects the ,path. In practice, there are many serious problems involved in efficiently utilizing such a system for the drying of veneers and the like.

Microwave irradiation both heats the veneers and tends to drive the moisture thereof outward toward the surfaces of the material. This results in the formation of a layer of moisture at the surfaces which can adversely affect the drying operation in several respects. First, the surface moisture concentration must be dissipated if drying is to be accomplished and if evaporation by microwave heating is relied upon for this purpose, a very high microwave power input or undesirably long processing time is required. The process is further complicated in that such evaporation tends to maintain the surface regions of the veneers in a relatively cool condition.

A very significant problem which is encountered in meander waveguide drying systems arises from the fact that the microwave. energy density is at a maximum at the input end of the waveguide and diminishes in a nonlinear manner therealong. Since in the meander configuration the successive sections of the waveguide extend transversely relative to the veneers, each waveguide section is characterized by a diminishing energy density in a direction transverse to the veneers, with the heating effect being greatest at the end of each waveguide section which is'electrically closestto the power input. This energy density gradient is reversedat each successive transverse section of the meander waveguide and thus each suchsection provides some compensation for the uneven heating effect of the preceding sectionrl-lowever, this is insufficient to fully compensate for the effect since the aggregate energy density within each waveguide section is substantially less than that within the preceding section. The net effect is that an uneven heat input may have occurred across the veneers after passage through the entire system with the maximum heating having occurred. along the edge of the veneers which was adjacent to the power input end of the meander waveguide.

Still other difficulties are encountered in making efficient use of the meander waveguide system in connection with conveying and'supporting the thin semiflexible veneers within the necessarily intricate apparatus and with respect to matching the localized energy density along the waveguide with the progressive decrease in the moisture content of the wood as it passes through the system. Further, difficulties are encountered with respect to suppressing the release of microwave energy from the system, with electrical arcing and in providing for convenient access to various regions of the apparatus.

This invention provides apparatus and techniques which resolve the problems discussed above by providing for drying veneers and other sheet materials in a fast, convenient, and highly efficient manner and under conditions which produce a product of uniform moisture content. The invention uses a combination of microwave heating and air drying in which the veneers or the like are continuously passed through a succession of spaced high intensity microwave regions which may be successive sections of one or more meander waveguides. Surface moisture is efficiently removed by a series of gas flows which are impinged against the veneers at spaced zones along the path of travel thereof.

In addition to effecting a very rapid and uniform drying, cost efficiencies result, in that this system does not depend upon microwave heating to supply all of the heat needed to vaporize the moisture content of the product. The microwave heating functions in part to drive internal moisture toward the surface of the product where it may be more economically removed by the gas flow.

The invention provides several structural innovations which jointly contribute to the improved drying operation.

In a preferred form, the invention utilizes two separately energized meander waveguides which interleave in a structurally simple configuration that matches the diminishing energy intensity along each waveguide with the progressive decrease in product moisture content during the course of drying. Further, and very importantly, the interleaved multiple meander waveguide arrangement provides for power input to the two waveguides at opposite sides of the veneer path with the result that the transverse energy density gradients discussed above effectively compensate each other and a fairly uniform energy density across the veneers is realized for the system as a whole.

The gas flows between the successive waveguide sections are preferably provided by pairs of air knives situated therebetween and which may be physically joined to an adjacent waveguide section to produce several structural and functional advantages. For example, a portion of the gas flow is caused to enter the adjacent waveguide section to scavenge moisture and debris therefrom.

According to still another aspect of the invention, the waveguide and air knife assembly is divisible approximately along the plane defined by the path traveled by the veneers so that one portion of the assembly may be separated from the other, preferably by motorized lifting means, to provide access to the interior of the assembly for cleaning and maintenance. To provide for such separation without manual disengagement of connectors, choke couplers of unique design are used to connect the movable portions of the waveguide assembly with the microwave source and waveguide terminators.

In still another aspect of the invention the waveguide and air knife assembly, together with a roller conveyor system, is disposed within a cabinet having an air recirculation system associated therewith to reduce air heating costs. Preferably the cabinet is thermally insulated to reduce heat losses and has electrically conducting wall members to confine microwave energy which may be released from the waveguides. Veneer input and output passages at the ends of the cabinet have electrically lossy material therein to suppress energy emission therethrough.

A further aspect of the invention provides for lip projections along the longitudinal slots in the waveguide wall through which the veneers pass to provide for wide slots while inhibiting the release of microwave energy therethrough. Arcing at such slots is suppressed by specialized lip configurations.

The invention embodies several specific refinements of the features described generally above as well as still other features to be hereinafter described.

Accordingly, it is an object of this invention to provide a method and apparatus for more rapidly and efficiently drying wood veneers and other sheet materials.

The invention, together with further objects and advantages thereof, will best be understood by reference to the following specification in conjunction with the accompanying drawings of which:

FIG. I is a diagrammatic perspective view illustrating the drying of a sheet material in accordance with the invention and showing certain elements of apparatus which may be utilized for this purpose;

FIG. 2 is a perspective view of a waveguide construction which may be utilized in the practice of the invention and which is characterized by a broad slot opening for passing the sheet material through the waveguide while suppressing the emission of microwave energy therefrom;

FIG. 3 is a side elevation view of a veneer dryer embodying the invention;

FIG. 4 is a plan section view of the veneer dryer of FIG. 3 taken along line 4-4 thereof;

FIG. 5 is an elevation section view of the product input end of the veneer dryer taken along line 5-5 of FIG. 4;

FIG. 6 is an elevation cross section view of the veneer dryer taken along line 6-6 of FIG. 4;

FIG. 7 is a section view taken along 77 of FIG. 5 illustrating a disengageable air coupling utilized in the veneer dryer;

FIG. 8 is a plan section view taken along line 8-8 of FIG. 6 showing a chain drive system utilized for retracting certain elements of the veneer dryer from other elements thereof to provide access to interior regions of the structure;

FIG. 9 is an elevation section view taken along line 99 of FIG. 6, with the central region of the apparatus omitted, illustrating additional internal components of the veneer dryer including a conveyor system for carrying veneers therethrough;

FIG. 10 is a perspective view of a choke coupling utilized for electrically connecting relatively movable elements of the waveguide structure without requiring a mechanical attachment therebetween;

FIG. 11 is a side elevation view of the choke coupling of FIG. 10; and

FIG. 12 is a plan section view of the choke coupling taken along line l2l2 thereof.

Referring first to FIG. 1, veneers 21 are dried, in accordance with the present invention, while being conveyed along a predetermined path as indicated by arrow 22 and by applying microwave energy to the veneers at a plurality of zones 23 which are spaced apart along the path of travel. In combination with these operations, high velocity flows of air or other gas are directed against the surfaces of the veneers at regions 24 which are also spaced along the path of veneer travel. A very rapid drying results in that the microwave energy penetrates into the veneers and interacts primarily with the internal moisture therein to generate heat throughout the in terior of the wood. In addition to the heating effect, this tends to drive the internal moisture towards the veneer surfaces.

One result of the microwave heating at zones 23 is that a layer of such moisture tends to form on the surface of the veneers. In the absence of the air flows at regions 24 this effect would slow the drying process as the surface regions of the veneers tend to remain relatively cool, probably as a result of the cooling effect of evaporation at the moisture film. This slowing of the drying process is avoided in the present invention by the high velocity gas flows which are impinged against the veneers at regions 24 to break up and remove the superfcial moisture layers. Thus, moisture which has accumulated at the veneer surfaces after each passage through a microwave zone 23 is removed from the vicinity of the veneer prior to the passage of the veneer through a subsequent microwave zone.

It is possible to use cold dry gas, generally air, at regions 24 and in some cases this is a preferred technique since heat induced deterioration of the veneer surfaces is avoided and an extremely high quality product is obtained. However, it is usually preferable to employ heated air to expedite the drying action and toreduce processing cost. By this means, a substantial portion of the heat needed for drying is supplied through the airflow.

This provides cost savings in that it is generally more economical to apply heat through a gas flow, to the extent possible, rather than relying on the more expensive microwave heating alone-However the temperature of the air which is employed in the present invention may be substantially lower than is the case in conventional heatedair dryers. Where the air temperatures in conventional dryers are typically in the range, from about 300 F. to about 450 F., the air in the present invention may typically be at about 225 F. Using the relatively low air temperature in the present invention, deleterious effects on the veneers are minimal. Thus it should be understood that various airflow temperatures may be used in the practice of the present invention as determined by the desired product qualityon the one hand and processing-time and cost considerations on the other hand. However, in substantially all instances the air temperatures employed are lower than those which have been customary in conventional air dryers and product quality is thereby benefited.

An important advantage of this drying technique is a markedly reduced processing time. Whereas veneers typically require about 5 minutes in a conventional air dryer, processing times of as little as 20 seconds are possible using the techniques of the present invention. This has indirect benefits additional to the general desirability of reducing processing time. If high volume output is to be achieved in a reasonably sized conventional dryer, complex veneer input,

output and conveyor. mechanisms are needed to provide for the simultaneous handling of many veneers within the dryer. These mechanisms can be-greatly simplified by utilizing the techniques of the present invention, in that high capacity output is possible in a.compact system in which veneers may be continuously passed through the processing region along a single path if desired.

Still another very important advantage of the invention is the moisture leveling effect which is inherent in the above described combination of steps. As the heating effect of microwave energy is primarily due to interaction with the internal moisture of the veneers, heating and drying is more intense at the relatively moist regions. Consequently, the finished veneers then have a much more uniform moisture content than has heretofore been the case.

Considering now suitable structurefor accomplishing the above described operations, the microwave irradiation regions 23 are established and defined by a series of spaced apart parallel waveguide sections 26 which transect the path 22 of veneer travel and which in this instance are of rectangular cross section. Toprovide for passage of the veneers through the waveguide sections 26, longitudinal slots, including input and

output slots

27 and 28 respectively, are provided in the walls thereof. Provided that the waveguide sections 26 are excited in one of certain specific modes, such as the TB 010 mode,

slots

27 and 28 oriented in this manner will not interfere with the operation of the waveguide.

The airflows at zones 24 are most advantageously defined by pairs of air knives 29 of which one pair is situated between each successive pair of waveguide sections 26 in parallel relationship thereto. The membersof each pair of air knives 29 are disposed on opposite sides of the path of travel 22 of the veneers and are tubular conduits 31 each having a longitudinal slit 32 facing the veneers to direct a planar jet of air thereagainst at high velocity.

in order to support the veneers and to carry the veneers along the path of travel 22, roller conveyors 33 are utilized, one pair of rollers being disposed between each pair of air knives 29 and the subsequent waveguide section 26 in parallel relationship therewith with the members of each pair of rollers being on opposite sides of the veneers.

Thus the veneers are processed in accordance with the inventionby being passed through a repeated sequence of elements of which each unit is comprised of a pair of conveyor rollers 33, a waveguide section.26, and a pair of air knives 29. The advantages of the invention are most fully realized if these elements are interrelated in a specific manner and have certain structural characteristics which jointly contribute to the desired results. A first very desirable characteristic is that the waveguides 26 be formed of two

separable members

34 and 36 with the boundary therebetween preferably lying in the plane defined by the path of the veneers. The division between the two

members

34 and 36 of each waveguide section 26 is thus similar to the input and

output slots

27 and 28 in that the transmission of energy through the waveguides is not interfered with provided the-waveguides are excited in a compatible mode. This split waveguide structure provides for easy access to the interior of the waveguide sections 26 for the purpose of removing knots, sawdust, and other debris which may tend to accumulate therein and for other purposes.

Further advantages are realized by employingspecialized configurations for the input and

output slots

27 and 28 through which the veneers pass into and out of the waveguide sections 26. If the width of these slots in a direction normal to the veneers is a sizable portion of one wavelength of the microwave energy, and in the absence of corrective structure to be hereinafter described, a substantial leakage of energy through the slots can occur. To avoid this undesirable result, the input slots 27 may be formed to have a width which isvery small in relation to the microwave wavelength. However, with many veneers as well as other materials which may be treated in this apparatus, there may be -a tendency to buckle and in the absence of corrective measures, scraping or breakage of the veneers against the edges of the input slot 27 might occur where the slot is of very narrow width. To forestall this problem, the pair of roller conveyors 33 which precedes each waveguide section 26 is situated very close to the input slot.

Electrical arcing-often causes substantial problems where divisions or openings are present in the wall of a waveguide .and to avoid this difi'rculty, at the ends of the product input slots 27,.a tapered design is utilized in which the edges which define the slot converge gradually as illustrated at 37.

The product output slots 28 of the waveguide sections 26 preferably have ,a difi'ering construction for several reasons. First, the output slots 28 cannot be made as narrow as the input slots 27 without difficulties from jamming of the veneers inasmuch as it is not generally practical to dispose a pair of the rollers 33 adjacent the output slot. Second, as will hereinafter be discussed in more detail, it is desirable to dispose the subsequent pair of air knives 29 immediately adjacent the output slot 28 and to electrically and mechanically connect the air knife structure with the waveguide structure at the output slot. Further, it is desirable to provide for the directing of a portion of the gas flow from the air knives 29 into the waveguidesections 26 through the output slots.28.

Accordingly the product output slots 28 are made relatively wide in a direction normal to the veneers and a lip 38 extends outward-from each edge of the slot. The lips 38 are slightly convergent inthe example illustrated in FIG. 1 and in combination with the relatively wide width of slot 28 facilitate the passage of the veneers out of the waveguide without jamming or damage. The lips 38 are comprised of conductive material and are preferably formed as an integral portion of the

waveguide section members

34 and 36. The lips 38 also form a shelf against which the adjacent air knife conduits 31 may be secured to form an integral assembly of each

waveguide section member

34 and 36 and one of the associated pair of air knives. This arrangement provides for locating the slits 32 of the air knives 29 adjacent the outer edges of the lips 38 so that a substantial portion of the airflow will be transmitted into the adjacent waveguide section 26 through the space between lips 38 and through output slot 28. This extends the zone of interaction between air flow and the veneers into the waveguide sections 26 and provides for the scavenging of moisture from the interior of the waveguide sections to facilitate drying and to reduce the possibility of arcing therein. The exhaust of such air from the waveguide sections 26 is provided for by a plurality of ap .rtures 39 in the walls thereof which are of sufficiently small diameter that no microwave leakage can occur.

A very important function of the lips 38 is to suppress energy leakage through the relatively broad output slot 28. The field distributions in a waveguide of this type are such that currents travel in an axial direction along the center of the broadwalls of the waveguide and the transverse currents are zero at this point. Thus a longitudinal slot 28 along the center of the broadwall does not interrupt currents and therefore will not radiate energy provided the slot is very narrow. in a waveguide operating at 915 megacycles, for example, a slot having a width less than approximately 1 inch will exhibit very little leakage. However, as the slot is made wider as may be desirable in the present instance for the reasons discussed above, the field distribution is significantly modified and leakage may occur. Further, the presence of a dielectric material, such as the veneers 21, within the slot, will tend to couple energy out of the slot. This effect is counteracted in the present invention by the lips 38. Energy which leaks through the slot 28 couples into the space between the lips 38 in a higher order mode as in a parallel plane transmission line. If the lips 38 extend away from the waveguide wall for approximately one-quarter wavelength or an odd multiple thereof, maximum suppression of energy release is accomplished. Under this condition, the lips 38 act as quarter wave choke assemblies with respect to energy propagation normal to the waveguide. In addition, any dielectric material between the lips 38, such as the veneers 21, aids in producing attenuation by absorbing energy in the space between the lips.

In a 915 ,megacycle system for veneer drying, the width of the output slots 28 may typically be 3 inches and the lips 38 may extend outward for a similar distance which is approximately one-fourth of the 13-inch free space wavelength.

For maximum effectiveness, specialized constructions may be utilized at each end of each pair of lips 38 to suppress leakage in these areas and to inhibit electrical arcing. The ends of the product output slot 28 in the waveguide wall are tapered to converge gradually as in the case of the previously described input slots 27 and the lips 38 are arranged to be similarly convergent but without being joined or closely spaced inasmuch as any such contact or near contact may promote sparking. This may be accomplished in several different ways, the lips 38 illustrated in FIG. 1 being convergent towards each other and being convergent towards the midplane of the associated waveguide section 26 as illustrated at 41. FIG. 2 illustrates a modified construction for the lips. In this modified embodiment the waveguide section 26a is again formed of two separable members 34 and 36' which are divisible along a central longitudinal plane through the waveguide. The lips 38' in this example are parallel rather than convergent and extend outward from the waveguidesection 26a for a uniform distance at all points along each lip including the end sections 41 thereof. The lips 38' at end sections 41' converge in correspondence with the convergent tapered end of output slot 28 but are terminated short of the apex thereof so that the ends of the lips are spaced apart forming a gap sufficiently wide to avoid arcing. In a waveguide construction having the dimensions discussed above, wherein the central portions of the lips 38' are separated by a 3-inch gap, the spacings of the ends of the lips 38' may typically be about 1 inch.

Referring now again to FIG. 1, by forming the air knife conduits 31 of electrically conducting material and by securing the air knives to the adjacent waveguide sections 26 by electrically conductive means such as by welding, further suppression of sparking is accomplished and the air knives are in effect an extension of the lips 38 to assist in the several functions thereof. In this connection, since the length of the lips 38 should be one-quarter wavelength to suppress microwave emission as described above, the air knives should be located so that the slits 32 are situated one-quarter wavelength away from the wall of the associated waveguide. This is desirable since it is the air knife slit 32 which defines the end of the associated lip 38 insofar as electrical effects are concerned.

Considering now the detailed construction of a veneer dryer in accordance with the invention, with reference to F108. 3 and 4 in combination, there is shown a generally rectangular cabinet 43 having a product input end 44 and product output end 46. As best shown in FIG. 4 in particular, cabinet 43 is formed of electrically conductive walls 47 to aid in confining stray microwave energy. To reduce air heating cost, the cabinet walls 47 are formed of a double layer of electrical conductor with a suitable thermal insulation 48 disposed therebetween except at the product input and

output sections

44 and 46. The walls 47 ofthe input and output sections may be a single layer of electrical conductor as transverse internal walls 49, also formed of thermal insulation disposed between electrically conducting layers, are situated between the product input and output sections and the central section 52 of the cabinet.

The walls of cabinet 43 may be attached to and supported by suitable structural members 51 and the sidewalls of the central section 52 of the cabinet are preferably formed as a continuous series of doors 53 supported by hinges 54 and having latches 56 to provide for convenient access to the interior of the cabinet 43 at any point therealong on either side thereof. To insure good electrical contact between the edges of each door 53 and the adjacent fixed portion of walls 47 and to prevent air leakage therebetween, R.F. seals, such as copper gaskets 57, are disposed thereat.

The product input and

output end sections

44 and 46 respectively of cabinet 43 provide for access for feeding veneers into the cabinet and for passing the veneers out of the cabinet while suppressing the emission of stray microwave energy through the necessary openings. To attenuate the microwave energy both the input section 44 and the output section 46 are loaded by means of a pair of flattened helical nonconductive coils 58 containing water or other electrically lossy material, the coils being supported by transverse tubular members 59 formed of dielectric material.

Considering now the detailed construction of the product input section 44 of cabinet 43 and with reference to FIGS. 4 and 5 in combination, a slot opening 61 in the conducting end wall 62 of cabinet 43 provides for admittance of the veneers and is of sufficient height to avoid any difficulties from buckling of the veneers. To facilitate the feeding of veneers through slot 61, a short shelf 63 extends outwardly therefrom. A generally similar slot opening 64 provides for passage of the veneers through the internal thermally insulated end wall 49. To facilitate the passage of the veneers through both slots 61 and 64 and to inhibit the passage of microwave therethrough, a conducting sloping lip 66 may be provided along the upper edge of each slot with the lips extending in a direction corresponding to the travel of the veneers and being converged towards the plane defined by the veneer path. To support the veneers in passage between slots 61 and 64, a flat dielectric member 67 extends therebetween beneath the veneer path. The pair of helical water-loaded coils 58 are vertically spaced within product input section 44 with one being above the veneer path and the other being below the path and below member 67. The effect of the water or other lossy substance contained within coils 58 is to absorb and attenuate microwave energy attempting to pass from slot 64 to slot 61. Only a very small proportion of such energy will be directed parallel to the path of the veneers in a way which will enable the energy to pass from slot 64 to slot 61 without traversing the water within coils 58. Most such energy will be directed obliquely with respect to the veneer path and will be repeatedly reflected between the conducting walls of the product input section 44 before reaching the outermost slot 61 and accordingly will make multiple passes through the lossy coils 58 and will be attenuated thereby. A system of this general type for suppressing microwave emission through an opening in a multimode microwave chamber is described in detail in copending application Ser. No. 589,149 of Morris R. .Ieppson for Apparatus and Process for Microwave Treatment filed Sept. 7, 1966 and assigned to the assignee of the present application.

Referring now to FIG. 9, the product output end section 46 of cabinet 43 also has electrically conducting walls 47 and

slot openings

69 and 71 in the end wall and internal thermally insulated wall 49 respectively. To guide the veneers into the output end section 46, upper and lower electrically conductive members 72 are disposed in slot opening 71 and are convergent in the direction of veneer travel. For similar purposes, flat dielectric members 73 extend from the upper and lower ones of the members 72 to the upper and lower edges of slot 69 at the end wall of the product output end section. The pair of water-loaded coils 58 of the product output end section are disposed one below and one above the dielectric members 73 to attenuate stray microwave energy as hereinbefore described.

Referring now again to FIG. 4, the waveguide sections 26 and air knives 29 which provide the drying effect are situated within the central section 52 of cabinet 43 between the internal thermally insulated walls 49. Two meander waveguide assemblies, including a first waveguide assembly 76 and second waveguide assembly 77, are disposed at the path of travel of the veneers within cabinet 43, sections of the first waveguide assembly being designated in FIGS. 4 to 9 of the drawings by reference numeral 26 and sections of second waveguide assembly being designated by number 26". All

sections

26 and 26" of both waveguide assemblies are parallel and spaced apart along the veneer path in transecting relationship thereto.

Advantageous results are realized by alternating pairs of the sections 26 of the first waveguide assembly 76 with pairs of sections '26" of the second waveguide assembly. This arrangement provides for a mechanically simple overall construction in that the successive sections of each of the waveguide assemblies 76 and 77 may be coupled at the sides of the veneer path by U-shaped side connectors 78 and 78' which may be coplanar with the primary waveguide sections 26' and 26". In some other possible sequences of waveguide sections in a system having a plurality of meander waveguide assemblies, much more complicated coupling structure is needed at the sides of the assemblies. In some such arrangements, for example, such connectors would have to extend vertically in order to go over or under adjacent sections of the other waveguide assembly. Still another advantage of this sequencing of waveguide sections 26' and 26" is that the diminishing microwave intensity along each waveguide assembly 76 and 77 from the power input end towards the opposite end thereof is correlated with the diminishing moisture content of the veneers as the veneers travel through the dryer. Thus, the veneers are exposed to a progressively diminishing microwave power level in passage from the input to the output end of the dryer. In other possible arrangements utilizing multiple meander waveguide assemblies ofthis "general type, regions of relatively strong microwave field must necessarily follow regions of lesser power density.

Referring now to FIG. 6 in combination with FIG. 4, microwave power is supplied to the two waveguide assemblies 76 and 77 through input waveguide sections 79 and 81 respectively which extend upwardly, through the top wall 80 of cabinet 43, from the forward end of the waveguide assemblies at opposite sides of the veneer path to connect with two separate microwave sources-82 and 83 respectively as shown in FIG.-.3. By providing for power input to the two waveguide assemblies 76 and 77 at opposite sides of the veneer path and at the initial waveguide sections 26' and 26" thereof, the energy density gradient at the veneers along sections 26' of one assembly is compensated for by the reversed but equal magnitude gradients along the sections 26" of the other asseml and a more uniform drying effect is accomplished across t veneers.

Near the output end section 46 of the cabinet,

waveguide end sections

84 and 86 extend upwardly through the top wall of the cabinet to connect waveguide assemblies 76 and 77 with conventional water loaded

waveguide terminations

87 and 88 respectively, also shown in FIG. 3.

Referring now to FIGS. 5 and 6, each of the waveguide sections 26' and 26" has the slot and lip construction hereinbefore described to provide for the passage of the veneers therethrough. This includes the relatively narrow slot openings 27 with a tapered end configuration 37 for admitting the veneers and the wider slot openings 28 through which veneers emerge from the waveguide sections, the slot openings 28 having the previously described lips 38.

Each waveguide section 26' and 26" is formed of an upper and

lower member

34 and 36 respectively which are separable at the plane defined by the veneer path as hereinbefore described to provide for access to the interior of the waveguide structure and to expose the veneers in transit therethrough if desired. For such purposes, it is highly advantageous if the upper members 34 of all waveguide sections can be lifted away from the lower members 36 simultaneously and by motorized means. Referring still to FIGS. 5 and 6 in conjunction, this is provided for by securing the upper member 34 of all waveguide sections to a supradjacent rectangular frame 85. Frame carrying all of the upper waveguide sections members 34 is liftable directly upward to a position adjacent the underside of the top wall 80 of the dryer as indicated by dash lines at 85 in FIG. 5. To provide for this movement, the U-shaped side connectors 78 and 78' which couple successive waveguide sections 26' and 26" are also divisible along the plane defined by the veneer path so that the upper half of each such connector moves with frame 85.

To guide the frame 85 during this movement, four vertical guide posts 89 are mounted within the cabinet 43 in proximity to the four corners of the frame and extend through guide passages 91 therein. To effect the upward and downward movement of the frame 85, a vertical screw 92 is situated adjacent each of the posts 89 and such screw engages a threaded bushing 93 in frame 85 to lift the frame along posts 89 as the screws are turned. It will be appreciated that the rotation of each of the four screws 92 must be carefully synchronized;to avoid misalignment and possible jamming of the frame 85. For this purpose, a gear 94 is mounted at the top of each screw 92 above cabinet top wall 80 and all four gears 94 are coupled by a chain 96 which extends through a drive motor-housing 97 on the top of the cabinet 43. Referring now to FIG. 8, housing 97 has slots 98 at opposite ends to provide for passage of the chain 96 therethrough and a loop is formed in the chain therein by a pair of idlers 99 and drive gear 101. Gear 101 is turned by an electrical drive motor 102 through a speed reduction device 103. Thus, the frame 85 and upper members 34 of the waveguide sections may be lifted and lowered as desired by operation of the motor 102.

Referring now to FIG. 5 in particular, the lower members 36 of the waveguide sections are stationary and are secured to a pair of beams 104 which extend longitudinally through the central section 52 of the dryer immediately below the waveguide sections. To facilitate the removal of condensed moisture, a drain pan 106 is disposed between the beams 104 below the waveguide assemblies and, as shown in FIG. 6, is provided with an outlet 107 to a drain trough 108.

Inasmuch as the lower waveguide section members 36 are fixed in position, the coupling to the microwave power

input waveguide sections

81 and 86 may be made through elbowshaped 'Lwaveguide portions 109 which have conventional flange couplings 111, including bolts 113, to the lower members 36 of the initial sections 26' and 26" of the two waveguide assemblies. However, as hereinbefore described, the upper members 34 of the initial waveguide sections may be lifted in an upward direction as illustrated in FIG. or lowered against the lower waveguide section members 36 as illustrated in FIG. 11. Thus, the type of flanged joint used to couple lower members 36 to elbows 109 is not adaptable to coupling the upper members 34 to the elbows. Such a coupling would require the disengagement of bolts before the upper waveguide member 34 could be retracted and the reengagement of such bolts when the upper waveguide section inembt was lowered. Further, the requirement that such flanges be abutted for good electrical contact would require an extreme degree of precision in the mechanism which raises, lowers and guides the upper waveguide section members 34. To avoid these problems, the upper member 34 of the initial section of each of the two waveguide assemblies 76 and 77 is not mechanically coupled to the corresponding portion of the associated elbow 109 and a small clearance space 114 is present therebetween as best shown in FIGS. 11 and 12. Notwithstanding the lack of physical contact, electrical continuity between the elbow 109 and upper waveguide section member 34 is established by a specialized choke coupling I12.

Considering now the construction of the choke coupling 112, with reference to FIGS. 10 to 12 in conjunction, the upper waveguide section member 34 has a flange 116 which includes a relatively thick base portion 117 adjacent the waveguide section member and further includes thin rectangular side flange projections 118 which are integral therewith and which extend normal to the waveguide axis to define one boundary of the hereinbefore described clearance space 114. To define the opposite boundary of the clearance space 114, a pair of flange projections 119 extend sidewardly from the adjacent end of elbow 109 in parallel relationship to flange projections 118. As best shown in FIG. 12 in particular, both the flange projections 118 of waveguide section member 34 and flange projections 119 of elbow 109 extend outwardly for a distance slightly greater than one-quarter wavelength of the microwave energy which is transmitted through the system.

Each side flange projection 119 of the elbow 109 is formed with a gap 121, as best illustrated in FIG. 12, with the center of the gap being situated substantially one-quarter wavelength outward from the inner surface of the adjacent wall of elbow I09. The gaps 121 are defined by convolutions 122 in the flange projections 119 which extend parallel to the adjacent wall of the elbow 109 and the gaps have a length, measured from the center of the clearance space 114, of substantially one-quarter wavelength. It should be noted that short outer sections 123 of the flange projections 119 extend outward from the gap 121 for a small distance to conform with the facing flange projections 118 of member 34.

The top and bottom of gap 121 are closed and the flanged structure at elbow 109 strengthened, by wing plates 124 which extend outwardly from the elbow above and below the flange projections I19 and which are secured thereto by electrically conductive means such as by soldering.

Flange projections

118 and 119 as well as wing plates 124 are all formed of electrical conductor. Thus both clearance spaces 114 and gaps 121 are defined and bounded by electrically conductive means.

Referring now to FIG. 10 in particular, difficulties from slight misalignments during lowering of the movable waveguide section member 34 towards lower waveguide section member 36 are avoided by a ramp projection 126 which extends upward from the end of elbow 109. Ramp projection 126 has a tapered surface 127 facing the movable waveguide section member 34 and carries a roller wheel 128 at each side to engage the flange base portion 117 of movable member 34 as the movable member approaches the fixed waveguide section member 36. Rollers 128 are situated above the level of movable member flange 116 when the movable member is at the fully lowered position thereof so that no physical contact exists at such time.

Microwave energy is transmitted from elbow 109 to the fixed lower waveguide section member 36 in the usual manner in that flanges ll] define a direct electrically conductive connection therebetween and insofar as wall currents and field configurations are concerned the two members are in effect an integral conductor. Considering now the manner in which the microwave energy is transmitted from the elbow 109 to the upper waveguide section member 34 despite the clearance space 114 therebetween, the configuration of the

flange projections

118 and 119, including gaps 121, causes the flange structure to behave electrically in a manner equivalent to a direct conducting connection between elbow 109 and member 34. Microwave energy being transmitted along the elbow 109 is coupled into the inner portion of each clearance space 114; and since the clearance space extends outward for a distance greater than one-quarter wavelength, field nodes occur at the entrances to gaps 121. Inasmuch as the gaps I2I have a depth of substantially one-quarter wavelength and are terminated by electrical conductors 129, energy is coupled into the gaps rather than being transmitted further along clearance spaces 114 where a much greater impedance is present. Thus, the conductive connections 129 across the ends of gaps 121 constitute short circuits which are situated onehalf wavelength from the waveguide walls and therefore reflect energy back towards the waveguide. This, in effect, creates a virtual short across the inner end of each clearance space 114 at the inner walls of elbow 109 and waveguide section member 34.

Referring now again to FIG. 4, it may be seen that an additional similar specialized choke coupling 112 is employed at the output end of each waveguide assembly 76 and 77 to couple the final section 26 of each assembly to the fixed

sections

84 and 86 which extend upwardly to the

waveguide terminations

87 and 88 as previously described with reference to FIG. 3.

Considering now the detailed construction of the veneer conveyor mechanism, with reference to FIGS. 6 and 9 in conjunction, an upper conveyor roller 33' and a lower roller 33 are disposed adjacent the veneer input slot 27 of each waveguide section 26 in parallel relationship thereto to support and drive the veneers as hereinbefore described. Both

rollers

33 and 33" of each such pair are mounted on brackets 131 which extend upward from beams 104 at the ends of each pair of rollers. The lower roller 33" of each pair is journaled in the associated bracket 131 for rotation about a fixed axis while the upper rollers 33' are engaged in vertical slots 132 at the top of the brackets whereby the upper rollers may lift or drop as necessary in response to variations in the thickness of veneers. Each lower roller 33" has a drive gear 133 at one end and an additional gear 134 at the opposite end which engages a gear 136 at the corresponding end of the associated upper roller 33 to transmit rotary motion thereto.

As shown in FIG. 9 in particular, all lower rollers 33" are driven by a single continuous chain 137 which is operated by a motor 138 situated in the product output end section 46 of the dryer. Chain 137 extends from motor 138 along beam 104 to a tensioner assembly 139 in the product input end section 44 of the dryer, slots 141 being provided in the internal thermally insulated walls 49 to provide for passage of the chain therethrough. At tensioner assembly 139 the chain engages two spaced idlers 142 which have fixed axes of rotation and the portion of the chain which extends between the two idlers passes around a third idler 143 which is movable. An adjustable tensioner spring 144 draws the movable idler I43 downwardly from idlers 142 to maintain the chain under a selected degree of tension.

At each of the brackets 13] along beam 104, chain 137 engages two spaced idlers 146 situated above the chain and a gear 146 which is situated therebetween but below the chain. Gear 146' turns a coaxial gear 145 which is coupled to roller drive gear 133 by a continuous chain 150.

Considering now the detailed structure of the air circulation system, with reference again to FIG. 9, a pair of the air knives 29 are disposed adjacent the veneer output slot 28 of each waveguide section 26 in the manner hereinbefore described. In particular, the upper air knife conduit 31 and lower air knife conduit 31' are secured to the upper and

lower members

34 and 36 respectively of the associated waveguide sections 26 at the lips 38 of the output slot 28 and each has a slit opening 32 to direct air against the adjacent surface of the veneers. As shown in FIG. 3, air is pumped to the air knives by three air blowers 147 which are disposed along one side of the dryer near the base thereof with each being operated by a separate electrical motor 148.

Referring now to FIG. 6, an angled wall 149 of thermally insulated material separates the blowers 147 and motors 148 from the interior region of the dryer and each blower has an air intake conduit 151 which extends upwardly through such wall. Thus each such blower 147 draws air which has been exhausted from the air knives for recirculation thereto, thereby minimizing air heating costs. As shown in FIG. 3, an air heater 152 which may be of the type having a gas flame 153 therein is disposed between each air intake 151 and the associated blower 147. Since the oxygen supply in the recirculating air may be diminished, each intake conduit 151 also has air inlet openings 155 for admitting a selected amount of external air to heaters 152 to support combustion at flames 153.

Referring now again to FIG. 6 in particular, the heated air outlet 154 of each blower 147 extends through wall 149 to communicate with a manifold conduit 156 which extends longitudinally within the dryer below drain pan 108. A transverse rectangular air conduit 157 below each pair of air knives 29 transmits heated air thereto from manifold 156. To couple each such transverse conduit 157 to the associated pair of air knives, an elbow 158 is disposed at the end opposite manifold 156. Manifold conduit 156 may supply all air knives 29 from the combined outputs of the three blowers 147. However, it is frequently advantageous to adjust air temperature or velocity or both to different values at different stages along the veneer path. For this purpose, partitions 160 may be disposed in the manifold conduit to confine the output of each blower to a separate group of air knives. Referring now to FIG. 7, the lowermost air knife conduit 31' of each pair has an angled end portion 159 which extends down to connect with the associated elbow 158. However, the retractability of the upper air knife conduit 31 requires a coupling to the elbow which automatically engages and disengages if complications are to be avoided in connection with the retracting of the upper waveguide member 34 as hereinbefore described. For this purpose, the end portion 161 of the upper air knife conduit 31 is angled downwardly and provided with an enlarged lower end 162, having a reentrant lip 163. A sleeve 164 projects upward from elbow 158 and is received in end 162 of the upper knife conduit 31 when the air knife is lowered to the operating position thereof.

To avoid a progressive accumulation of moisture in the recirculating airflow, a selected proportion of the output of blowers 147 is exhausted from the dryer. Referring to FIGS. 6 and 9, exhaust conduits 166 extend beneath beams 104 from manifold 156 to a vertical exhaust stack 167 at the opposite side of dryer. To control the proportion of the output of the blowers 147 which is exhausted rather than being recirculated to the air knives, a valve 168 of the damper type is situated within stack 167.

In operation, with reference to FIG. 5, veneers are continuously fed onto shelf 63 at the product input end section 44 of the dryer and are passed through slots 61 and 64 thereof to the initial pair of conveyor rollers 33. The successive pairs of rollers 33 then carry the veneers through successive ones of the waveguide sections 26 and between successive pairs of air knives 29. At each passage through a waveguide section 26 internal heat is rapidly generated within the veneers driving the moisture content outward toward the veneer surfaces. Moisture whichaccumulates at the veneer surfaces at each waveguide section 26 is then removed by the subsequent pair of air knives 29 with the airflow therefrom having the further beneficial effect of supplying additional heat to the veneers. Thus the veneers become progressively dryer while passing through the cabinet 43 and ultimately are carried out of the dryer between the members 73 of the product output end section 46. As has been previously discussed, the drying effect is proportionately greater in the more moist areas of the veneers so that the finished product has a uniform low moisture content.

Control of the degree of drying to accommodate to variations between batches of green veneers and other variable factors may be effected by adjusting one or more of several operating parameters. These include the microwave power input to one or both of the meander waveguide assemblies 76 and 77, airflow velocity and temperature as determined by blowers 147 and heater 152 respectively in conjunction with valve 168 and conveyor speed as determined by motor 138. Process control is facilitated by a very significant characteristic of the above described system. Specifically, a curve of the moisture content of the veneers plotted against time (or longitudinal position within the dryer) is characterized by periods of rapidly diminishing moisture alternated with plateaus at which moisture tends to remain fairly constant. Thus a localized area of a veneer which is at one such plateau may temporarily undergo little further drying while more moist areas are rapidly brought down to the plateau. For many woods, one such plateau occurs at a moisture content of about 6 percent to 7 percent and in many instances this is a very suitable moisture level for bonding the veneers to form plywood provided that the veneers have been processed in accordance with the present invention.

In one example of the practice of the above described method, partially dried veneers comprising is, 54-inch by 10 I inch pieces of 0.1-inch thick douglas fir were conveyed through a dryer of the type described above in which a sequence of 19 spaced-apart waveguide sections were energized from two separate 28 kilowatt microwave sources, operating at a frequency of 915 megacycles. The veneers were subjected to air flows between successive waveguide sections which had a temperature of 300 F. and a velocity at the air knife slits of 3,000 feet per minute. Veneer transit time through the system was 48 seconds. Prior to this treatment, the veneers had an average moisture content of 8.1 percent with substantial variations at different areas of individual ones of the veneers and sizable differences in the moisture content of separate veneers the standard deviation recorded being 4.9. After drying as described above, all veneers had an average moisture content of 3.9 percent and localized variations in the moisture content of individual veneers did not exceed about plus or minus 1 percent for which a standard deviation of 0.4 was recorded. No significant surface deterioration from the process was discernible.

While the invention has been described with respect to specific embodiments, it will be apparent that numerous variations and modifications are possible and it is not intended to limit the invention except as defined in the following claims.

We claim:

1. In a meander waveguide-apparatus for drying materials which are moved along a plane path, the combination comprising first and second meander waveguide assemblies each having a plurality of spaced-apart sections extending across said plane path, pairs of successive sections of said first waveguide assembly being alternated with pairs of successive sections of said second waveguide assembly along said path.

2. The combination defined in claim 1 further comprising first and second groups of U-shaped waveguide connectors, the connectors of said second group being of greater length and radius than the connectors of said first group and wherein'the sections of said first waveguide assembly are coupled by connectors of said first group at a first side of said plane path and are coupled by connectors of said second group at the opposite second side of said plane path, and

wherein said sections of said second waveguide assembly are coupled by connectors of said second group at said first side of said path and are coupled by connectors of said first group at said second other side thereof.

3 The combination defined in claim 1 further comprising a pair of microwave energy sources, and wherein each of said waveguide assemblies is energized by a separate one of said sources and at opposite sides of said path.

4. The combination defined in claim 1 comprising at least seven of said waveguide sections and wherein said first meander waveguide is comprised of the first and third and fourth and seventh of said waveguide sections and wherein the second of said meander waveguide assemblies is comprised of the second and fifth and sixth of said waveguide sections.

5. Apparatus for heating sheet materials wherein said materials are conveyed along a plane path and having a meander waveguide assembly with sections which extend across said plane path at spaced-apart locations therealong, said waveguide assembly being formed of at least two separable components having the juncture therebetween at said plane path, a plurality of linear guide means extending in a direction generally normal to said plane path, one of said separable components of said waveguide assembly engaging said guide means and being slidable therealong, said apparatus further comprising power means for retracting one of said separable of said waveguide assembly away from the other thereof.

6. The apparatus defined in claim 5 wherein said power means for retracting said one of said separable components of said waveguide assembly comprises at least one rotatable lead screw extending parallel to said guide means and engaging said one of said separable components of said waveguide assembly and a motor for electrically rotating said lead screw.

7. An apparatus forheating sheet materials wherein said materials are conveyed along a plane path and having a meander waveguide assembly with sections which extend across said plane path at spaced-apart locations therealong, said waveguide assembly being formed of first and second separable members having the juncture therebetween at said plane path, one end of said waveguide coupled to a microwave source through a fixed waveguide portion, said first waveguide member having an end spaced a small distance from the end of said fixed waveguide portion and being electrically connected thereto through a choke coupling comprising a first electrically conducting flange secured to said end of said first waveguide member, a second electrically conducting flange secured to said end of said fixed waveguide portion and being spaced from said first flange to define a clearance space having an effective electrical length of one-half wavelength and said second waveguide member and said fixed waveguide portions being secured together to be electrically and mechanically continuous.

Claims

1. In a meander waveguide apparatus for drying materials which are moved along a plane path, the combination comprising first and second meander waveguide assemblies each having a plurality of spaced-apart sections extending across said plane path, pairs of successive sections of said first waveguide assembly being alternated with pairs of successive sections of said second waveguide assembly along said path.

2. The combination defined in claim 1 further comprising first and second groups of U-shaped waveguide connectors, the connectors of said second group being of greater length and radius than the connectors of said first group and wherein the sections of said first waveguide assembly are coupled by connectors of said first group at a first side of said plane path and are coupled by connectors of said second group at the opposite second side of said plane path, and wherein said sections of said second waveguide assembly are coupled by connectors of said second group at said first side of said path and are coupled by connectors of said first group at said second other side thereof.

3. The combination defined in claim 1 further comprising a pair of microwave energy sources, and wherein each of said waveguide assemblies is energized by a separate one of said sources and at opposite sides of said path.

7. An apparatus for heating sheet materials wherein said materials are conveyed along a plane path and having a meander waveguide assembly with sections which extend across said plane path at spaced-apart locations therealong, said waveguide assembly being formed of first and second separable members having the juncture therebetween at said plane path, one end of said waveguide coupled to a microwave source through a fixed waveguide portion, said first waveguide member having an end spaced a small distance from the end of said fixed waveguide portion and being electrically connected thereto through a choKe coupling comprising a first electrically conducting flange secured to said end of said first waveguide member, a second electrically conducting flange secured to said end of said fixed waveguide portion and being spaced from said first flange to define a clearance space having an effective electrical length of one half wavelength and said second waveguide member and said fixed waveguide portions being secured together to be electrically and mechanically continuous.