TUNNEL FURNACE FOR CONTINUOUSLY HEATING PRESSED MAT

Abstract

A tunnel furnace for continuously heating a pressed mat for making wood-based panels has a housing forming a tunnel and a conveyor for passing the pressed mat in a travel direction through the tunnel. A microwave generator for producing microwaves is connected to a waveguide extending from the generator and forming a slot antenna having a plurality of antenna slots opening into the tunnel for irradiating the mat therein with microwaves produced by the generator.

Claims

1. A tunnel furnace for continuously heating a pressed mat, the furnace comprising: a housing forming a tunnel; conveyor means for passing the pressed mat in a travel direction through the tunnel; a microwave generator for producing microwaves; and a waveguide extending from the generator and forming a slot antenna having a plurality of antenna slots opening into the tunnel for irradiating the mat therein with microwaves produced by the generator.

2. The tunnel furnace defined in claim 1, wherein the antenna slots each have at least one antenna wall in which the outlet slots are arranged.

3. The tunnel furnace defined in claim 1, wherein each of the waveguides is basically tubular and has an intake end connected to the respective generator and an output end provided with a cover, the slots being between the ends.

4. The tunnel furnace defined in claim 1, wherein the slot antennas project into an interior of the housing.

5. The tunnel furnace defined in claim 1, wherein the housing forma a side wall of each of the waveguides.

6. The tunnel furnace defined in claim 1, wherein each of the antenna slots extends transverse to the travel direction.

7. The tunnel furnace defined in claim 1, wherein the antenna slots extend parallel to the travel direction.

8. The tunnel furnace defined in claim 1, wherein the waveguides and the slot antennas are spaced from one another in the travel direction.

9. The tunnel furnace defined in claim 1, wherein the waveguide is of substantially rectangular cross section with a width measured horizontally equal to between 1.5 and 2.5 times a height measured vertically of the waveguide.

10. The tunnel furnace defined in claim 1, wherein the waveguide has two parallel rows of the antenna slots, the slots of each row being staggered relative to the slot of an adjacent row.

11. The tunnel furnace defined in claim 10, wherein the slots are offset with to a center line of the waveguide.

12. The tunnel furnace defined in claim 1, wherein the slots have a length from 100 mm to 200 mm.

13. A method for preheating a pressed mat with a continuous furnace as defined in claim 1, wherein: the pressed mat is passed through the interior of the housing and irradiated with the microwaves emerging from the slot antennas and thereby heated.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0027] The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

[0028] FIG. 1 is a simplified side schematic view of an apparatus for making wood-based panels with a continuous furnace;

[0029] FIG. 2 is a schematic perspective view of just the tunnel furnace of FIG. 1;

[0030] FIG. 3 is a large-scale view of a detail of a slot antenna near the pressed mat to be heated;

[0031] FIG. 4 is a schematic top view of the slot wall of a slot antenna;

[0032] FIG. 5 is a modified version of the structure of FIG. 3;

[0033] FIG. 6 is a block diagram illustrating operation of the furnace of this invention; and

[0034] FIG. 7 is another view of a detail of a modified embodiment of an inventive continuous furnace.

SPECIFIC DESCRIPTION OF THE INVENTION

[0035] FIG. 1 shows a simplified view of a system for making wood-based panels in a continuous cycle. First, loose fibrous and/or particulate material, typically of wood, to be pressed is deposited from a supply onto the upper reach of a normally mesh belt conveyor 2 to form a loose mat 1. This loose mat 1 manufactured in this manner is pressed into a wood-based panel (for example particle board or fiber panel) in a continuously operating press 3 under application of pressure and heat. Such a press 3 is generally formed as a dual-belt press having a heated upper platen and a heated lower platen as well as endlessly circulating press belts (for example of steel) in the upper and lower parts of the press. These press belts are supported on the press platens via roller element assemblies (for example wooden bars). One or both of the heating platens is engaged by cylinders supported on the press frame to press the platens together.

[0036] In order to optimize the pressing process within the press 3, the pressed mat 1 is preheated according to the invention with the aid of a continuous tunnel furnace 4 shown only schematically in FIG. 1. For preheating, the pressed web 1 thus passes through the continuous furnace 4 that has a tunnel-shaped housing 5. In addition, the continuous furnace 4 has a plurality of microwave generators 6 so that the web 1 is exposed to microwaves in an interior 7 of the housing 5 and thereby heated. The microwave generators 6 can be magnetrons, or the generators can have such magnetrons. The microwave generators 6 are connected by waveguides 8 to the housing 5 so that the microwaves are conducted and radiated via the waveguides 8 into the interior 7 of the housing.

[0037] The tunnel-shaped housing 5 has a parallelepipedal housing 10 that has an upstream and inlet-forming wall 11 and a downstream outlet-forming wall 12. The wall 11 forms a rectangular inlet opening 13 and the wall 12 forms a similarly rectangular outlet opening 14 through which the pressed mat 1 enters and exits the housing 5. Here, in order to prevent or reduce leakage of microwaves from the interior of the housing, an inlet tunnel extension 15 is connected to the inlet opening 13 and an outlet tunnel extension 16 is connected to the outlet opening 14. For this purpose, the inlet tunnel 15 and the outlet tunnel 16 can be formed like waveguides, for example as rectangular tubes, but dimensioned such that the microwave radiation used inside the housing 5 is suppressed.

[0038] The pressed mat 1 can pass through the continuous furnace 4 on the belt 2 so that it can pass through the microwave furnace 4 without problems during operation. It also lies within the scope of the invention to provide a separate, endlessly circulating forming belt 17 for the continuous furnace, so that the pressed mat 1 previously scattered onto a first forming belt 2 is subsequently transferred to a second forming belt 17 that passes through the continuous furnace 4. According to the invention, the waveguides 8 are formed at least partially as slot antennas 8a that each have a plurality of outlet slots 9 for projecting the microwaves into the furnace interior 7.

[0039] The waveguides 8, in an inherently known manner, have a section to which a slot antenna section is then connected, thus forming the slot antenna 8a. With respect to the longitudinal direction of the waveguide 8, the slot antenna 8a is therefore a part or portion of the waveguide 8 that defines the slot antenna section 8a of the waveguide 8, with the slot antenna 8a formed by the waveguide having a length L, and with the outlet slots 9 being arranged in this longitudinal portion with the length L. The outlet slots 9 are arranged in a wall, specifically in the antenna wall 18. Here, the waveguides or slot antennas 8a are of rectangular cross section, with the antenna wall 18 with the outlet slots 9 (and opposing wall thereof) having a larger width B than the walls that extend transversely thereto and that have a width or height H. The width B of the slot antenna (as well as of the waveguides) is here approximately 2 times the height H. The front wall 19 closes off the end of the slot antenna 8a at the end of the waveguide 8 that is opposite the microwave generator 6. In this way, a standing wave is formed in the waveguide 8 and particularly in the slot antenna 8a whose field is disrupted by the slots 9 formed in the antenna wall 18, so that the microwaves enter the interior of the furnace directionally via the slots 9 and heat the pressed mat 1.

[0040] In FIG. 2, the slot antennas 8a formed as sections of the waveguides 8 project into the interior 7 of the housing 5 through the housing wall 10. The waveguides 8 thus project with their antenna section that forms the slot antennas by a predetermined amount, for example by the length L of the slot antenna, into the interior of the housing. In the embodiment of FIG. 2, the slot antennas 8a extend transverse to the travel direction D that defines the longitudinal process or travel direction through the furnace. Several slot antennas 8a that extend transverse to the travel direction are arranged successively along the travel direction D. A single one of these slot antennas is shown in FIG. 3. The microwave field M is radiated directionally out of the outlet slots onto the pressed mat 1.

[0041] The slots 9 in the antenna wall 18 of the slot antenna 8a are shown in FIG. 4 where the slot antenna 8a, more particularly the antenna wall 18 thereof, has (at least) two rows of slots 9 running parallel to one another, each having several slots 9 that are arranged successively at spacings. The two rows of slots 9 are set at a transverse spacing A from one another, and the individual slots 9 of a row of slots 9 are staggered at a distance a from one another. The slots 9 of the two rows 9 are offset from one another along the longitudinal direction of the waveguide. It can also be seen that the two rows of slots 9 are offset relative to a centerline X of the antenna wall 18, i.e. they have a spacing V of offset from the center line X. The slots 9 themselves are rectangular with a length 1.

[0042] FIG. 5 shows a simplified view of a modified embodiment of the invention in which the slot antennas 8a are arranged so as to not be transverse to, but rather parallel to the travel direction D, so that they extend along the travel direction D. Here as well, it is possible for several slot antennas 8a to project out, in which case they are preferably arranged next to one another transverse to the travel direction. This is not shown in FIG. 5.

[0043] In the preferred embodiment of FIGS. 2 to 5, the slot antennas 8a project through the housing cover 10 into the interior 7 of the housing 5, so that the slot antennas 8a have an antenna housing that is separate from the housing 5.

[0044] FIG. 7 shows a simplified view of a modified embodiment in which the slot antenna 8a is connected on the outside to the housing 5, so that the antenna wall 18 is formed by a region of the housing 5 or of the housing cover 10 and the antenna wall itself forms part of the housing or of the housing cover. In this embodiment, the slots 8 are in the housing jacket or cover 10. This can be achieved, for example, by placing a rectangularly U-shaped piece of sheet metal against or on the housing 5, 10, so that, together with the housing wall, a waveguide having a rectangular cross section is formed, and the slots 9 are then formed by the housing wall. Such an embodiment can also be implemented in the case of a housing 5 that does not have a rectangular cross section, but rather an oval cross section, for example, in which case the slot antenna is then curved and can be adapted to the outer periphery of the oval-shaped housing. This is not shown in the figures.

[0045] Incidentally, FIG. 2 shows that six microwave generators are provided with six waveguides, so that six slot antennas thus project into the housing. Each microwave generator can produce an output of 100 kW. The pressed mat 1 can enter the furnace at a temperature from 20 C. to 40 C., for example 35 C., and be preheated to a temperature from 70 C. to 100 C., for example 80 to 90 C.

[0046] FIG. 6 shows a schematically simplified illustration of the generation of the microwaves and the coupling thereof. Every single microwave generator 6 has a magnetron 20 and a heating voltage generator 21 as well as an anode voltage generator 22 and a cooler 23 and a insulator 24. In addition, a cooler and/or ventilator 25 for the furnace is also indicated.

[0047] In this embodiment, the irradiation occurs only from above, that is, the slot antennas are arranged above the mat. Alternatively or in addition, however, slot antennas with which the mat is irradiated from below can also be under the mat.