METHOD AND FURNACE FOR CONTINUOUSLY HEATING A STRIP WORKPIECE

Abstract

A method of continuously heating a moving strip workpiece entails the steps of passing the strip workpiece in a horizontal travel direction through a tunnel furnace with a tunnel housing of oval cross section, providing the oval cross section of greater horizontal width than vertical height, and irradiating the workpiece with microwave radiation in the tunnel housing.

Claims

1. A method of continuously heating a moving strip workpiece, the method comprising the step of: passing the strip workpiece in a horizontal travel direction through a tunnel furnace with a tunnel housing of oval cross section; providing the oval cross section of greater horizontal width than vertical height; and irradiating the workpiece with microwave radiation in the tunnel housing.

2. The method defined in claim 1, wherein the housing has a substantially elliptical cross section.

3. The method defined in claim 1, further comprising the steps of: radiating the microwave radiation into an interior of the tunnel housing via apertures uniformly distributed around and along in the tunnel housing.

4. The method defined in claim 1, further comprising the step of: supporting the strip workpiece on a conveyor belt in the tunnel housing.

5. The method defined in claim 1, wherein the strip workpiece is a pressed wood-particle mat.

6. A tunnel furnace for continuously heating a moving strip workpiece, the furnace comprising: an oval-section tunnel housing having an interior through which the workpiece passes in a horizontal travel direction, the tunnel having a predetermined vertical height and a predetermined horizontal width; and a plurality of microwave generators on the housing for irradiating the workpiece in the interior of the tunnel with microwave radiation.

7. The tunnel furnace defined in claim 6, wherein the tunnel is formed with a plurality of apertures each aligned with a respective one of the microwave generators, the apertures being distributed uniformly around and along the tunnel housing.

8. The tunnel furnace defined in claim 6, wherein the microwave generators are fixed on an outer surface of the tunnel at the respective apertures.

9. The tunnel furnace defined in claim 6, wherein each of the generators has a respective waveguide fitted to the respective aperture.

10. The tunnel furnace defined in claim 6, wherein the housing has a pair of horizontally spaced end walls each formed with a respective port slot through which the work piece passes.

11. The tunnel furnace defined in claim 6, wherein the tunnel housing is provided with a horizontally outwardly extending tubular extension at at least one of the ports.

12. The tunnel furnace defined in claim 11, wherein there is such an extension at each of the ports.

13. The tunnel furnace defined in claim 12, wherein each of the tunnels and the respective port has a cross section conforming to a cross section of the workpiece and having an inner periphery fitting closely around the workpiece.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0018] 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:

[0019] FIG. 1 is a small-scale schematic side view of an apparatus for treating and heating an elongated moving strip workpiece according to the invention;

[0020] FIG. 2 is a simplified perspective view of the tunnel furnace of FIG. 1; and

[0021] FIG. 3 is a schematic vertical cross section through the FIG. 2 furnace.

SPECIFIC DESCRIPTION OF THE INVENTION

[0022] FIG. 1 shows a simplified view of a system for manufacturing wood-based panels in a continuous cycle. First, the loose material to be pressed (consisting of wood fibers or wood shavings, for example) is scattered with the aid of a scattering device onto a belt conveyor 2 to form a scattered material mat 1. The scattered material mat 1 manufactured in this way is pressed into a finished wood-based panel (for example particle or fiber board) in a continuously operating press 3 under application of pressure and heat. Such a press 3 is generally embodied as a dual-belt press having an upper heated platen and a lower heated platen as well as endlessly circulating press belts (for example of steel) in the upper part and lower part of the press, with these press belts being supported on the pressing plates/heated platens with interposition of rolling element assemblies (for example roller bars). At least one of the heated platens is biased by press cylinders that are supported on the press structure (for example on the press frame).

[0023] In order to optimize the pressing process within the press 3, the pressed mat 1 is preheated in the context of the invention with the aid of a tunnel furnace 4 shown only schematically in FIG. 1. For the purpose of preheating the strip workpiece 1, the pressed mat 1 thus passes through the tunnel furnace 4 that has a tunnel housing 5. Moreover, the tunnel furnace 4 has a plurality of microwave generators 6 with which microwaves M are generated, so that the strip workpiece 1 is exposed to microwaves in the interior 7 of the housing 5 and thereby heated. The microwave generators 6 can be magnetrons, or the generators can have such magnetrons. According to the invention, the tunnel housing 5 have an oval and preferably elliptical cross section, with the width B of the housing 5 being greater than the height H of the housing. In this illustrated embodiment, the width B and height H refer to the (maximum) width B and the (maximum) height H of the interior 7 of the housing 5.

[0024] Through these measures, and thus the oval or elliptical design of the tunnel housing 5, an especially efficient connecting of the microwaves M and improved field distribution in the area to be heated, and consequently within the pressed mat 1, is achieved. This is indicated in FIG. 3. It can be seen that the microwaves M radiate through feedthrough apertures 8 into the interior 7, these feedthrough apertures of the housing being arranged on the oval periphery of the housing 5. A plurality of microwave generators 6 can be distributed over the periphery of the housing. These generators 6 are preferably arranged above and below the mat 1 to be heated, so that the upper and lower sides of the mat are irradiated and thus heated. In addition, a plurality of microwave generators 6 can be distributed over the length of the housing 5.

[0025] In the illustrated embodiment, the microwave generators 6 themselves are directly on the housing 5 and thus distributed on the elliptical outer surface so that the described irradiation occurs via the elliptical or oval periphery.

[0026] It also lies within the scope of the invention, however, to arrange the microwave generators 5 separately at a spacing from the housing, for instance, beside the housing, under the housing, and/or over the housing, and connect the microwave generators with waveguides to the housing or feedthrough apertures. Even if the microwave generators themselves are not on an oval periphery in such an embodiment, which is not shown in the figures, the coupling of the microwaves does occur nonetheless in the inventive manner via an oval periphery of the housing.

[0027] Furthermore, the drawing shows that the housing has a jacket 10 that has the described oval cross section. Moreover, the housing 5 has an intake-end front wall 11 and an output-end back wall 12, with the intake wall 11 having an intake port 13 and the output wall having an output port 14 through which the pressed mat 1 enters and exits the housing 5.

[0028] In the illustrated embodiment, in order to prevent or reduce leakage of microwave radiation M from the interior of the housing, an outwardly projecting intake extension 15 is connected to the intake port 13 and an outwardly projecting output extension 16 is connected to the output port 14. For this purpose, the intake and output extension 15 and 16 can be shaped as waveguides, for example as rectangular tubes, but dimensioned such that the corresponding modes of the microwave radiation are suppressed.

[0029] The pressed mat 1 passes through the tunnel furnace 4 on a forming belt or conveyor belt 17 that is made of a nonconductive material so that it can pass through the microwave furnace 4 without problems during operation. In principle, it can be the same forming belt onto which the pressed mat is scattered. However, it also lies within the scope of the invention to provide a separate, endlessly circulating forming belt 17 for the tunnel 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 tunnel furnace 4.