Continuous furnace and plant for the production of wood-based panels

Abstract

Continuous furnace (4) for the continuous heating of a pressed mat (1), particularly in the course of the production of wood-based panels, with a tunnel-shaped housing (5) through whose interior (7) the pressing mat (1) can be passed, wherein one or more waveguide slot antennas (8a, 8b) are arranged in or on the housing (5), each having several exit slots (9) distributed along the longitudinal direction of the antenna in an antenna wall (19) for the emission of microwaves into the interior (7), characterized in that the tunnel-shaped housing (5) has a surface arranged above the pressing mat (1) or pressing mat level (P). upper wall (10) and one below the pressing mat (1) respectively. has a bottom wall (11) arranged in the press mat level (P) and two side walls (12) arranged next to the press mat (1), wherein the side walls (12) are curved.

Description

[0001] The invention relates to a continuous oven for the continuous heating (in particular preheating) of a pressed material mat, especially in the course of the (continuous) production of wood-based panels, with a tunnel-shaped housing through whose interior the pressed material mat can be passed and with one or more microwave generators for generating microwaves which can be radiated into the interior of the housing, e.g. via one or more waveguides.

[0002] Within the scope of the invention, "pressed material mat" preferably refers to a mat or web of material made from (glued) particles, e.g., chips or fibers, preferably wood chips or wood fibers, during the production of wood-based panels. The particles, e.g., wood chips or wood fibers, are generally spread onto a spreading belt conveyor or the like using a spreading device, forming a pressed material mat. The pressed material mat thus produced then passes through a press, in particular a continuously operating press, which may, for example, be designed as a double belt press. In the press, the pressed material mats are compressed into a panel or a strip of panels, in particular a wood-based panel (e.g., particleboard or fiberboard), by applying pressure and / or heat. To optimize the pressing process, the material being pressed is preheated.The pressing mat is preheated using a pressing mat preheating device designed as a continuous microwave oven. The pressing mat is thus preheated by means of microwave radiation. According to the invention, microwave radiation refers to electromagnetic radiation in a frequency range of 100 MHz to 300 GHz, preferably 300 MHz to 100 GHz. Particularly preferred is conventional microwave radiation with a frequency of 900 to 950 MHz, e.g., approximately 915 MHz, or a frequency of approximately 2.4 to 2.5 GHz, e.g., approximately 2.45 GHz. The microwave radiation is generated in one or more microwave generators, e.g., magnetrons, and preferably radiated or coupled into the interior of the housing via waveguides.

[0003] A continuous furnace for preheating a pressed material mat of the type described above is known, for example, from EP 2 247 418 B1. Microwaves in a frequency range of 2400 to 2500 MHz are used to heat the mat, with the microwaves being generated for each pressing surface by 20 to 300 microwave generators with magnetrons, providing a power output of 3 to 50 kW. The inlet and outlet of the continuous furnace are designed to be variable in height and / or width. Movable absorption elements, such as absorber blocks and / or water tanks, can be provided to adjust the inlet or outlet.

[0004] DE 697 37 417 T2 describes a device and a method for manufacturing products from wood or wood fibers, wherein microwaves are used for preheating a binder. The aim is to produce veneer wood in particular.

[0005] German utility model DE 20 2015 102 422 U1 describes a device for the continuous heating of materials made primarily of non-metallic material, comprising a continuous furnace for the continuous heating of material on an endlessly circulating conveyor belt. The continuous furnace includes a plurality of magnetrons for generating electromagnetic waves and waveguides with outlet openings for feeding the waves into a radiation chamber. At least two outlet openings, which are arranged in and / or perpendicular to the production direction as nearest neighbors, have their principal axes forming an angle of <0° and / or the line connecting the centroids of the surfaces of the outlet openings forming an angle of <0° with the perpendicular to the production direction. This measure is intended to ensure uniform heating of the material.

[0006] Furthermore, WO 2008 / 067996 A1 describes a microwave heating device specifically designed for ceramic materials and molded parts, featuring several microwave generators for emitting microwaves with a frequency of 300 MHz to 5.8 GHz. The high- and low-frequency microwaves are coupled in via several coupling elements embedded in the ceiling and floor of the drying chamber. These are slot antennas tuned to the emission frequency. To achieve a particularly uniform microwave distribution, several field guides are arranged in the ceiling of the drying chamber. The focus of this publication is on the industrial drying of ceramic materials and mineral insulation materials. These considerations had no bearing on the design of preheating devices for the wood-based panel industry.

[0007] Furthermore, DE 10 2004 052 871 A1 describes a device for joining thin, planar elements using an adhesive, wherein the adhesive is cured by microwaves. The microwaves are generated by a magnetron and guided into a waveguide, the waveguide having at least one slot, wherein the butt joint of the planar elements containing the adhesive is guided past the slot.

[0008] A continuous furnace for the continuous heating of a pressed material mat of the type described above is known, for example, from DE 10 2016 110 808 A1, wherein the tunnel-shaped housing has an oval cross-section and preferably a width greater than its height. The housing can have a substantially elliptical cross-section. The microwaves can be radiated into the interior via coupling windows, these coupling windows being arranged on the oval circumference of the housing and optionally distributed along its length. Alternatively, the microwave generators can also be arranged separately at a distance from the housing and connected to the housing or the coupling windows via waveguides.

[0009] Finally, DE 100 846 93 T1 describes a plant and a process for manufacturing a pressed wood product consisting of several interconnected veneer strips. The veneer strips can be bonded together in a press using pressure and heat, with shaft guides for irradiating microwaves into the product being provided between each individual press roller. The shaft guides can be provided with longitudinal slot openings.

[0010] The invention is based on the invention of creating a continuous furnace with which a pressing mat, in particular for the production of wood-based panels, can be heated efficiently and economically and in particular preheated for a continuous pressing process.

[0011] To solve this problem, the invention teaches a continuous furnace with the features of claim 1. It is provided that one or more waveguide slot antennas are arranged in or on the housing, each having several exit slots distributed along the longitudinal direction of the antenna in an antenna wall (facing the pressing mat) for emitting microwaves into the interior (i.e., onto the pressing mat). The tunnel-shaped housing has a (preferably straight / flat) upper wall arranged above the pressing mat (or pressing mat plane) and a (preferably straight / flat) lower wall arranged below the pressing mat (or pressing mat plane), and two side walls arranged next to the pressing mat. The side walls are curved, preferably about an axis oriented parallel to the direction of travel. The cylinder walls are particularly preferably cylindrical shell-shaped.They have a circular or semi-circular cross-section, with the cylinder axis oriented parallel to the direction of travel. It is advantageous if the side walls are not formed by cylindrical half-shells, but rather by smaller cylindrical segments, the segment angle being 10° to 180°, preferably 20° to 90°, e.g., 30° to 60°. Furthermore, it is advantageous if the curvature of the side walls is adapted to the wavelength of the microwave radiation and / or to the waveguide wavelength of the waveguide slot antennas. It is preferably proposed that the radius of curvature be 0.5 to 3 times the wavelength (or the waveguide wavelength), e.g., 1.5 times the wavelength of the microwaves (or the waveguide wavelength).

[0012] The invention is based on the understanding that microwaves can be used to achieve particularly efficient and economical heating of a pressable mat, and especially preheating of a pressable mat during the production of wood-based panels, if the microwaves are radiated into the interior of the continuous oven and consequently onto the pressable mat via waveguide slot antennas. The microwaves are thus coupled into the interior of the housing from the microwave generators in a generally known manner via waveguides, but according to the invention, these waveguides are designed, at least partially, as waveguide slot antennas, each having a slot antenna section with several exit slots for coupling the microwaves into the interior. The slot antennas with the exit slots according to the invention can therefore form a part or a section of a waveguide.A waveguide is, in a generally known manner, a waveguide for electromagnetic waves (here: microwaves). The waveguide is designed as a metal tube with a preferably rectangular (optionally also circular or elliptical) cross-section. Such waveguides are used in the prior art for transporting the microwaves generated in the microwave generator into the oven when the microwave generators are not directly connected to the housing. While in the prior art the microwaves generally exit the open ends of the waveguide and are radiated into the interior of the oven, the invention proposes that the waveguides (at least partially) be designed as waveguide slot antennas, each having a plurality of exit slots in the antenna wall. Preferably, the waveguide or...The waveguide slot antenna is closed at one end, specifically at the end facing away from the microwave generator, by an end wall. Consequently, the microwaves do not exit the waveguide at the end face, but are radiated via a longitudinal wall, the so-called antenna wall, of the waveguide slot antenna through the exit slots located there. The microwaves thus enter the waveguide, or rather the waveguide slot antenna, on the side facing the microwave generator and are reflected at the opposite closed end, or end wall, so that a standing wave with the so-called waveguide wavelength forms within the waveguide slot antenna; that is, two antinodes are created for each waveguide wavelength.The field created in this way is strongly disturbed by the slots in the antenna wall, and through this disturbance the field escapes from the waveguide slot antenna and spreads from there into the room, i.e. into the interior of the oven.

[0013] The invention is based on the understanding that with conventional radiation via open-end waveguides, reflections occur when the microwaves enter the interior of the oven housing, causing the radiation to enter the interior in a non-directional manner and resulting in uneven heating. The waveguide slot antenna, on the other hand, provides directional radiation to the pressing mat, meaning the energy input is directed onto the mat and reflections are avoided. This improves the "illumination" of the pressing mat. Such slot antennas are generally known from communications technology for the uniform and targeted radio communication of specific sectors within a service area. The invention applies these considerations to the microwave heating of pressing mats, particularly for the wood-based materials industry. Preferably, the waveguide slot antennas have a rectangular cross-section.The waveguide slot antenna extends along a longitudinal direction, such that the waveguide slot antenna forms a predetermined section of the waveguide. This slot antenna section has an antenna wall extending along the antenna's longitudinal direction, in which the exit slots are arranged. The waveguide can also, in principle, include a (conventional) waveguide section without slots. Starting from the microwave generator, the waveguide can therefore initially comprise a waveguide section without slots and a subsequent slot antenna section with slots. The waveguide (with its waveguide section and antenna section) can extend in a single direction and with essentially identical cross-sections.However, it is also within the scope of the invention that the waveguide section or a waveguide section extends in a different direction than the slotted antenna section (and consequently the waveguide-slotted antenna), so that a spatial deflection can occur within the waveguide. This is generally only advantageous if the arrangement of the microwave generators in space requires it.

[0014] Building on these considerations and the advantages of waveguide slot antennas, the invention proposes the curved side walls of the oven already described. Surprisingly, the illumination of the mat passing through the interior of the oven can be optimized and made more uniform by such a geometry, so that uniform heating is achieved in a particularly efficient manner, especially in combination with the waveguide slot antennas according to the invention.

[0015] Within the scope of the invention, a uniform, box-shaped housing for the continuous furnace is provided, comprising a (non-curved) upper wall, a (non-curved) lower wall, and two curved side walls according to the invention. Preferably, several slotted antennas are distributed along the direction of flow within such a box-shaped housing. It is preferred to provide several slotted antennas above the pressing mat or pressing mat level and / or several slotted antennas below the pressing mat or pressing mat level. Irradiation can be achieved solely from above or solely from below. Particularly preferred, in combination with the curved side walls, is irradiation from both above and below.

[0016] In a preferred embodiment, the tunnel-shaped housing comprises several box-shaped individual cavities, each with a (non-curved) upper wall, a (non-curved) lower wall, side walls, and end walls. A connecting tunnel is arranged between the end walls of each individual cavity, through which the mat passes from one cavity to the next. The connecting tunnel preferably has a reduced height compared to the individual cavities. According to the invention, one or more optionally obliquely oriented waveguide slot antennas are arranged in each individual cavity. Preferably, some, and particularly preferably all, of the side walls of these individual cavities are equipped with curved side walls.

[0017] In an embodiment with multiple individual cavities, it is also advantageous to provide several slotted antennas arranged directly one behind the other in the housing. It is fundamentally within the scope of the invention to provide only a single slotted antenna in each individual cavity, or even just a single upper slotted antenna and a single lower slotted antenna. However, it is equally possible to provide at least two slotted antennas arranged directly one behind the other in each individual cavity, which, in the case of an inclined arrangement of the individual cavities, are then preferably arranged at different heights, i.e., at different distances from the pressing mat along the radiation direction, for structural reasons.

[0018] Furthermore, the problem is solved by a continuous furnace with the features of claim 8. Consequently, according to a further independent aspect of the invention, a continuous furnace of the type described above is provided for in or on the housing of several waveguide slot antennas with different radiation characteristics and different antenna wall geometries, which can be selectively operated for heating differently shaped pressed material mats. This means that in such a continuous furnace, either one or more waveguide slot antennas with a first radiation characteristic or one or more waveguide slot antennas with a second radiation characteristic are operated for processing different types of pressed material mats. The continuous furnace can therefore be structurally designed to provide different antenna types that can be selectively switched on or off.

[0019] These measures allow for the optimal processing of different types of pressed material mats by selecting and operating the appropriate slotted antennas depending on the mat type. This enables the continuous oven to be adapted to different mat widths; that is, for a pressed material mat of one width, slotted antennas of the first type are activated, and for a mat of a second width, slotted antennas of a second type are activated.

[0020] Several slotted antennas of the same length (i.e., the same length of the antenna section), the same slot length, and the same spacing of the slots along the longitudinal direction are used in or on the housing. However, the slotted antennas have a different number of slots. Consequently, in several antennas of the continuous furnace, preferably in all antennas of the continuous furnace, slots with identical slot lengths and identical spacing along the longitudinal direction are distributed over a different length section of the slotted antenna.In this way, optimal adaptation to different mat widths can be achieved very easily by selecting the appropriate slot antenna, because with one antenna type the slots extend over a first total width and with a second antenna type only over a reduced total width, but with otherwise identical geometry.

[0021] In an embodiment with multiple individual cavities, it is proposed that each individual cavity contain several slotted antennas with different radiation characteristics, each with a different antenna wall geometry and the same length and slot length, but with a different number of slots. The slots in all antennas are distributed at identical intervals along the longitudinal direction over a different section of the waveguide slotted antenna. Each individual cavity thus provides at least two different antenna types, so that one antenna for a specific mat type can be selected in each individual cavity. However, it is also possible to configure individual cavities entirely for a specific mat type, so that a first cavity can be selected for a first mat type and a second cavity can be selected for a second mat type.

[0022] Furthermore, it is possible to provide one or more antenna pairs along the feed direction, each featuring a slotted antenna positioned above and below the surface of the pressed material mat. These antennas are equipped with a parallel radiation pattern facing the mat but with different radiation characteristics. This means, for example, that one antenna radiating downwards onto the mat is designed for a first mat type, and a second antenna opposite it is designed for a second mat type. Several such antenna pairs can be provided in a single cavity, but they must be positioned opposite each other in pairs, maintaining the same distance along the radiation direction. For further details, please refer to the figure description.

[0023] Furthermore, the slots are designed to be oriented parallel to the antenna's longitudinal direction. The slot length can be adjusted appropriately based on the wavelength of the microwave radiation. Preferably, the slot length along the antenna's longitudinal direction should correspond (approximately) to half the waveguide wavelength. Considering the wavelengths typically used, it is advantageous for the slot length to be approximately 50 mm to 200 mm, e.g., 100 mm to 200 mm.

[0024] Furthermore, according to the invention, the waveguide slot antennas are positioned at a relatively large distance from the surface of the pressing mat so that a truly planar wave field can form in the area of ​​the pressing mat, thus achieving particularly uniform heating. For this purpose, it is advantageous if the distance of the waveguide slot antenna (or the antenna wall) from the pressing mat (along the direction of radiation) is greater than the wavelength of the microwave radiation used or the waveguide wavelength. Preferably, the distance is greater than 100 mm, more preferably greater than 200 mm, e.g., greater than 400 mm.

[0025] It is within the scope of the invention that the waveguide slot antennas project into the interior of the housing, i.e., they penetrate the housing wall, particularly the side wall. Consequently, they do not terminate upon entering the housing, but extend through the housing wall and project into the housing as waveguide slot antennas, so that they are arranged above and / or below the pressing mat and irradiate the pressing mat selectively from above and / or below (directed). In an alternative embodiment, the waveguide slot antennas can be connected to or attached to the outside of the housing, e.g., mounted on the top and / or bottom wall, in which case the antenna wall can optionally be formed by a section of the top or bottom wall.

[0026] The invention also relates to a plant with the features of claim 13 for the production of wood-based panels, in particular fiberboard or particleboard, wherein particleboard also includes OSB panels. Such a plant comprises a spreading device for producing a mat of material to be pressed and a continuously operating press in which the mat of material is pressed into the wood-based panel by applying pressure and / or heat. According to the invention, a continuous oven of the type described is arranged between the spreading device and the press.

[0027] With such a system, a process for the production of wood-based panels can be implemented in the manner described, wherein the inventive preheating of the pressing mat is used within the framework of such a process.

[0028] The invention will now be explained in more detail with reference to drawings illustrating only one embodiment. These drawings show... Fig. 1. A plant for the production of wood-based panels with a continuous furnace in a highly simplified side view, Fig. 2 a continuous furnace according to the invention of the device according to Fig. 1 in a modified embodiment in a simplified side view, Fig. 3 a cut AA from Fig. 2, Fig. 4 a top view of the object according to Fig. 3, Fig. 5 a continuous furnace according to Fig. 2 in a modified embodiment, : Fig. 6a, Fig. 6b a simplified (comparative) side view of the designs according to Fig. 2 and Fig. 5 in comparison, Fig. 7a, Fig. 7b another aspect of the invention, Fig. 8 A slit wall of a waveguide slot antenna in a top view.

[0029] In Fig. Figure 1 is a simplified representation of a continuous production line for wood-based panels. This line includes a spreading device (not shown) that spreads the material to be compressed (e.g., wood fibers or wood chips) onto a spreading belt conveyor 2, forming a mat of material 1. The resulting mat of material 1 is then compressed into a wood-based panel (e.g., particleboard or fiberboard) in a continuously operating press 3 using pressure and heat. Such a press 3 is typically a double-belt press, comprising an upper and a lower heating plate and continuously circulating press belts (e.g., steel press belts) in both the upper and lower press sections. These press belts are supported on the press plates / heating plates by means of rolling element assemblies (e.g., roller bars).One or both heating plates are subjected to pressure by press cylinders that are supported on the press frame (e.g., on press frames).

[0030] To optimize the pressing process within the press 3, according to the invention the pressing mat 1 is preheated using a Fig. 1. A continuous oven 4, only indicated by a diagram. To preheat the pressing mat 1, it passes through the continuous oven 4, which has a tunnel-shaped housing 5. The continuous oven 4 also has a multitude of microwave generators 6, which generate microwaves so that the pressing mat 1 is exposed to microwaves and thus heated in the interior 7 of the housing 5. The microwave generators 6 can be magnetrons, or the generators can contain such magnetrons. The microwave generators 6 are connected to the housing 5 via waveguides 8, so that the microwaves are radiated into the interior 7 of the housing 5 via the waveguides 8.

[0031] Fig. Figure 1 shows a first embodiment of a continuous furnace 4 with a uniform tunnel-shaped housing consisting of only a single cavity.

[0032] In the illustrated embodiment according to Fig. 2 The tunnel-shaped housing 5 consists of several box-shaped individual cavities 5a arranged one behind the other in the direction of flow D. The tunnel-shaped housing 5, or its individual cavities 5a, each has a top wall 10, a bottom wall 11, side walls 12, an inlet end wall 13, and an outlet end wall 14. The press material mat enters the housing 5 through an inlet tunnel 15 and consequently the first individual cavity 5a, and exits the housing 5 or the last individual cavity 5a through an outlet tunnel 16. A connecting tunnel 17 is arranged between each of the individual cavities 5a or their end walls 13, 14. The inlet tunnel 15, outlet tunnel 16, and connecting tunnel 17 each have a reduced height compared to the individual cavities 5a. In principle, it is possible that the inlet tunnel 15, the outlet tunnel 16 and the connecting tunnels 17 have the same width as the housing 5 or the individual cavities 5a.Preferably, at least the inlet tunnel 15 and the outlet tunnel 16 have a reduced width compared to the housing 5 and the individual cavities 5a, respectively. They are preferably designed to be only as wide as necessary to achieve maximum damping and for design reasons. The connecting tunnels may optionally have a different width than the housing or the individual cavities, for example, for design or technological reasons.

[0033] The pressing mat 1 passes through the continuous oven 4 on a forming belt or conveyor belt 18, which is made of a non-conductive material, so that it can be guided through the microwave oven 4 without problems during operation. This can, in principle, be the same forming belt onto which the pressing mat is spread. However, it is also within the scope of the invention to provide a separate, continuously circulating forming belt 18 through the continuous oven 4.

[0034] According to the invention, the microwaves are irradiated into the interior 7 or onto the pressing mat by means of waveguide slot antennas 8a, 8b. These slot antennas 8a, 8b are formed by the end sections and consequently slot antenna sections 8a, 8b of the waveguide 8. The slot antennas 8a, 8b or the antenna section 8a, 8b of the waveguide 8 therefore have a length L, in which the exit slots 9 are arranged. The exit slots 9 are arranged in a wall, namely in the antenna wall 19. In the exemplary embodiment, the waveguides or waveguide slot antennas 8a, 8b have a rectangular cross-section, wherein the antenna wall 19 with the exit slots 9 (and its opposite wall) has a greater width B than the walls running transversely to it, which have a width or height H.In the exemplary embodiment, the width B of the waveguide slot antennas, as well as the waveguide itself, is approximately 1.5 to 2 times the height H. The end wall 20 closes off the waveguide slot antennas 8a, 8b at the end of the waveguide 8 opposite the microwave generator 6. In this way, a standing wave forms in the waveguide 8 and, in particular, in the slot antennas 8a, 8b, the field of which is disturbed by the slots 9 provided in the antenna wall 19, so that the microwaves enter the interior 7 of the oven 4 via the slots 9 and heat the pressing mat 1.

[0035] The slots 9 incorporated into the antenna wall 19 are in Fig. Figure 8 shows that such a slotted antenna 8a, 8b, or its antenna wall 19, has (at least) two parallel rows of slots 9', each row having several slots 9 spaced one behind the other. The two rows of slots 9' are spaced A apart, and the individual slots 9 in a row of slots 9' are spaced a apart. The slots 9 of the two rows 9' are offset from each other along the longitudinal direction of the waveguide. Furthermore, it is evident that the two rows of slots 9' are offset from the center line X of the antenna wall 19, i.e., they have a distance V offset from the center line X. The slots 9 themselves have a length I, with the length I of the slots in the exemplary embodiment being aligned parallel to the longitudinal direction of the waveguide slotted antenna. The slots 9 are, for example,rectangular in shape, whereby such a design also includes an elongated hole-like design with rounded ends.

[0036] The Fig. 1 and Fig. Figure 2 shows embodiments in which the box-shaped housings 5, 5a are oriented essentially with a horizontal (planar) upper wall 10 and a horizontal (planar) lower wall 11, or with the upper and lower walls parallel to the pressed material mat plane P. The waveguide slot antennas 8a are oriented with their longitudinal antenna direction transversely and preferably perpendicular to the direction of travel D, and the longitudinal antenna direction is otherwise oriented parallel to the pressed material mat plane P. Furthermore, in the embodiments according to Fig. 1 and Fig. 2 provided that the waveguide slot antennas are oriented parallel to the direction of travel D such that the radiation direction R of the waveguide slot antennas 8a, 8b, oriented perpendicular to the antenna wall, is vertical and consequently oriented at an angle α of 90° to the direction of travel D.

[0037] In contrast, in Fig. Figure 5 shows an embodiment in which the waveguide slot antennas 8a, 8b are oriented obliquely to the direction of travel D such that the radiation direction R of the slot antennas, oriented perpendicular to the antenna wall 19, is oriented obliquely at an angle α other than 90° to the direction of travel D. The slot antennas 8a, 8b are oriented with their antenna longitudinal direction (along the length L) transversely and preferably perpendicularly to the direction of travel D. Furthermore, the slot antennas 8a, 8b are oriented with their antenna longitudinal direction parallel to the plane of the pressed material mat P. It can also be seen that the housing sections or the individual cavities 5a are similarly oriented obliquely at an angle α' to the direction of travel D. This means that both the inlet-side end wall 13 and the outlet-side end wall 14 of each individual cavity 5a are oriented obliquely at an angle α' other than 90° to the flow direction D.In the illustrated embodiment, the angles α and α' are identical, i.e., the inclination of the antennas 8a, 8b is determined by the inclination of the cavities 5a.

[0038] In the Fig. 6a and Fig. Figure 6b simplifies the effects and advantages achieved by tilting the object. Fig. Figure 6a shows a vertical orientation (according to Fig. 2) the slot antennas 8a, 8b and thus vertical radiation of the microwaves and Fig. In contrast, 6b shows the inclined position of the slot antennas 8a, 8b and thus the oblique radiation of the microwaves into the pressing mat 1 relative to the working direction D (according to Fig. 5) It is evident that the microwave field, in the variant according to the invention, Fig. 6b penetrates a longer section of the pressing mat 1 and, secondly, that the microwave field does not extend exclusively transversely, but also with a directional component in the direction of the longitudinal direction of the pressing mat or the working direction, so that the microwaves propagate with their horizontal propagation component in the direction of travel D or opposite direction of travel D.

[0039] Furthermore, it can be seen in the figures that several obliquely oriented waveguide slot antennas 8a, 8b are arranged one after the other in the continuous furnace 4 along the flow direction D, both above and below the pressing mat level P. An embodiment is shown in which several slot antennas 8a, 8b are provided in each individual cavity 5a, specifically at least one slot antenna 8a, 8b above the pressing mat 1 and one slot antenna 8a, 8b below the pressing mat 1 (or pressing mat level P), so that each individual cavity 5a receives or can receive radiation from both above and below.

[0040] However, in the case of the Fig. In the embodiment shown in Figure 5, it should be taken into account that several slotted antennas 8a, 8b with different radiation characteristics are arranged in the continuous oven, namely with different antenna wall geometries 19. On the one hand, slotted antennas 8a of a first type with a first radiation characteristic and, on the other hand, slotted antennas 8b with a second geometry and consequently a second radiation characteristic are provided, which can, for example, be arranged alternately along the direction of passage in the housing 5. Figure 5 shows that the slotted antennas have different radiation characteristics. Fig. 5 An embodiment in which one or more slotted antennas 8a of the first type and one or more slotted antennas 8b of the second type are provided in each individual cavity 5a. This makes it possible to operate the continuous furnace 4 either with the slotted antennas 8a of the first type or with the slotted antennas 8b of the second type and thus to easily adapt the continuous furnace 4 to different mat types and, in particular, press mats with different mat widths. For this purpose, reference is made to the Fig. 7a and Fig. Reference is made to Figure 7b, which, in simplified terms, shows on the one hand the slotted antennas 8a of the first type and on the other hand the slotted antennas 8b of the second type and the radiation characteristics achieved with each, so that pressed material mats of a first (larger width) are processed with the antennas 8a and pressed material mats of a second, smaller width with the antennas 8b. It is interesting to note that the two antenna types 8a and 8b each have exit slots 9 with identical slot length I and also identical spacing and otherwise identical geometry according to the Fig. 8 exhibit. Types 8a and 8b differ solely in that the slots 9 are distributed over different length sections A1 and A2 of the slot antenna in the two types, as it can be seen that in antennas 8a according to Fig. 7a the slots 9 or the rows of slots 9' extend over the length section A1, while the slots 9 or the row of slots 9' at the antenna 8b according to Fig. 7b extends only over the reduced length section A2 compared to A1, with otherwise identical slot row geometry and slot geometry, and in particular identical length L of the antennas and also identical width of the cavities or the housing. Thus, antenna 8b can be used according to Fig. 7b irradiate a pressed material mat with a reduced width without causing undesirable overheating of the mat in the edge areas. This aspect of the invention can, moreover, be implemented not only with the inclined slot antennas and inclined housings, but also with other embodiments. Thus, in the embodiment according to Fig. 2. Antennas 8a on the one hand and antennas 8b on the other hand are provided, as shown in [reference to relevant document]. Fig. 2 is shown.

[0041] In the embodiment according to Fig. Figure 5 provides that each cavity 5a has two antenna pairs, each antenna pair comprising a slotted antenna 8a, 8b arranged above and below the plane of the pressed material mat, which are arranged opposite each other with parallel and antiparallel radiation directions, respectively. Each antenna pair has one antenna 8a with the first geometry and one antenna 8b with the second geometry, so that in operation only one antenna (8a or 8b) of the respective antenna pair can be selected and thus an adjustment to the mat width can be made.

[0042] Furthermore, it can be seen in the figures that, according to the invention, the side walls 12 of the housing or the cavities are not flat, but curved. In the exemplary embodiment, the side walls 12 are each (partially) cylindrical shell-shaped, i.e., they have a semicircular cross-section, with the cylinder axis (not shown) oriented parallel to the direction of travel D. The two side walls 12 are not formed by cylindrical half-shells, but only by smaller cylindrical segments, with the segment angle β in the exemplary embodiment being approximately 30° to 60°. Moreover, with such curved side walls, it is provided that the curvature is adapted to the wavelength of the microwave radiation or the waveguide wavelength, for example, by the radius r of the curvature being 0.5 to 3 times the wavelength (or waveguide wavelength), e.g., 1.5 times the wavelength of the microwaves (or waveguide wavelength).

[0043] Furthermore, in the illustrated embodiment according to Fig. 5 with several box-shaped individual cavities 5a provided that each of the individual cavities 5a is equipped with curved side walls 12.

Claims

[1] Continuous furnace (4) for the continuous heating of a pressed mat (1), especially in the course of the production of wood-based panels, with a tunnel-shaped housing (5) through whose interior (7) the pressing mat (1) can be passed, wherein one or more waveguide slot antennas (8a, 8b) are arranged in or on the housing (5), each having several exit slots (9) distributed along the longitudinal direction of the antenna in an antenna wall (19) for the emission of microwaves into the interior (7), characterized by , that the tunnel-shaped housing (5) is arranged above the pressing mat (1) or pressing mat level (P) upper wall (10) and one below the pressing mat (1) respectively. has a bottom wall (11) arranged in the press mat level (P) and two side walls (12) arranged next to the press mat (1), wherein the side walls (12) are curved. [2] Continuous furnace according to claim 1, wherein the side walls (12) are cylindrical shell-shaped, the cylinder axis preferably being oriented parallel to the direction of flow. [3] Continuous furnace according to claim 2, wherein the cylindrical shell-shaped side wall extends over a cylinder segment angle (β) of 10° to 180°, preferably 20° to 90°, e.g. 30° to 60°. [4] Continuous furnace according to one of claims 1 to 3, wherein the radius (r) of the curvature is 0.5 to 3 times the wavelength of the microwaves or the waveguide wavelength and / or wherein the radius (r) of the curvature is 10 cm to 100 cm. [5] Continuous furnace according to any one of claims 1 to 4, characterized by , that the waveguide slot antennas (8a, 8b) are oriented with their antenna longitudinal direction transversely and preferably perpendicular to the through-direction (D) and / or are oriented with their antenna longitudinal direction parallel to the pressed material mat plane (P). [6] Continuous furnace according to any one of claims 1 to 5, characterized by , that the tunnel-shaped housing (5) has several box-shaped individual cavities (5a) each with upper wall (10), lower wall (11), side walls (12) and end walls (13, 14), between which a connecting tunnel (17) is arranged, wherein in the area of ​​each individual cavity (5a) one or more waveguide slot antennas (8a, 8b) are arranged, wherein each of the side walls (12) of the individual cavities (5a) is curved. [7] Continuous furnace according to any one of claims 1 to 6 characterized by , that several waveguide slot antennas (8a, 8b) with different radiation characteristics are arranged in or on the housing (5), e.g. with different geometry of the antenna wall (19), which can be selectively operated for heating differently designed pressed mats. [8] Continuous furnace (4) for the continuous heating of a pressed mat (1), especially in the course of the production of wood-based panels, with a tunnel-shaped housing (5) through whose interior (7) the pressing mat (1) can be passed, characterized by , that several waveguide slot antennas (8a, 8b) with different radiation characteristics are arranged in or on the housing (5), which can be selectively operated for heating differently designed pressed mats, wherein several waveguide slot antennas (8a, 8b) of the same length (L) and same slot length (I), but with different numbers of slots, are arranged in or on the housing (5), wherein the slots in all antennas are distributed at identical intervals along the longitudinal direction over a different length section of the slot antenna. [9] Continuous furnace according to claim 8 , characterized by, that in each individual cavity (5a) several waveguide slot antennas (8a, 8b) with different radiation characteristics are arranged, namely with the same length and the same slot length but different numbers of slots, wherein the slots in all antennas are distributed at identical intervals along the longitudinal direction over a different length section of the waveguide slot antenna. [10] Continuous furnace according to one of claims 8 or 9, characterized by, that along the direction of travel (D) one or more pairs of antennas are arranged, each having a waveguide slot antenna (8a, 8b) arranged above and below the plane of the pressed material mat, which are arranged with a facing, parallel radiation direction, but with different radiation characteristics, wherein preferably the waveguide slot antennas of the pairs opposite each other have the same distance along the direction of radiation. [11] Plant for the production of wood-based panels, in particular fiberboard or particleboard, comprising a spreading device for producing a pressed mat (1) and a continuously operating press (3) in which the pressed mat is pressed into the wood-based panel by applying pressure and heat, characterized by , that a continuous furnace (4) according to one of claims 1 to 10 is arranged between the spreading device and the press (3).

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