AND PREFERABLY USABLE IN THE WOOD-WORKING INDUSTRY

The object of the present invention is a screen, preferably making a circular trajectory, especially for use in the wood-working industry, the screen being implemented by means of meshes of different mesh sizes and comprising a number of mesh levels one above another, with which screen wood-based raw material is screened into different-sized fractions which screen comprises a frame part and a moving part.

This invention is especially prepared for using material of the wood-working industry and its contamination with dust after the chipping and/or drying.

Therefore one of the main issues to be dealt is the prevention of damages due to dust explosions.

The invention thus relates to a screen, preferably making a circular motion, for use in chipboard factories, with which wood-based raw material is screened into different fractions, usually after chipping and drying.

By means of the device, the raw material is screened into face particles and core particles for use in the wood-working industry, especially in a chipboard manufacturing process. Screens making a movement, especially circular, for use in a chipboard factory are typically large steel structures in which a welded steel structure forms the frame of the device. The screens in question conventionally comprise 2-4 screening levels formed from screen meshes that are inclined slightly with respect to the horizontal plane.

The angle of inclination is generally 5-10°. The material being flows in the direction of the inclination. The screen meshes are detachable and replaceable via doors or hatches in the outer sides of the screen. The screen structure is arranged to move on an essentially circular path on the horizontal plane. The screen in question is not compellingly a screen vibrating back and forth, but preferably it is a screen making a circular motion, in which a constant centripetal acceleration rotating around an essentially vertical axis gets the material to be screened on top of and through the mesh levels. A typical diameter of an orbit is 40-80 mm and the circulation speed 150-200 r/min. The screens are supported on the frame structure either with suspension means, which are e.g. cables or hinged rods. Another typical support structure is supporting from below on top of flexibly fastened supports that allow the relatively small circular screening motion of the screen with a relatively small resisting force.

These types of screens are typically manufactured as single-layered or double layered constructions. A screen of double-layered structure is formed from two structures, each functioning essentially as a single-layer screen, one above the other. By means of a double-layered screen, the screening capacity can be substantially increased, in principle doubled, for a certain-sized area.

The screens making a circular movement for use in a chipboard factory are typically large steel structures in which the width of the frame exceeds 3 m and the length of the frame is generally between 5-10 m. Such large steel structures are demanding to fabricate and, because of their large dimensions, expensive to transport. With the screen that is the object of the invention, material is processed in the wood-working industry, especially after drying, i.e. the wood-based material is dry and it contains dust. It is possible that a dust explosion will occur inside the screen and for that reason the structure must be designed and manufactured to function safely in a possible dust explosion. Typically, in the top part of a single layered screen, i.e. in the roof structure of the screen, there are explosion hatches, via which explosion pressure is released in a controlled manner. On the other hand, no solution has as yet been found for the safe and economic management of dust explosion occurring in the lower screen section of a double-layered screen.

An object of the present invention is to provide a new type of screen for a chipboard factory, with which screen a cheaper manufacturing price and more advantageous transportation are achieved.

Another object of the present invention is to provide a new type of screen for a chipboard factory with improved screen safety and economic and improved dust explosion management.

The above and other objects are achieved by a screen as defined in claim 1. Preferred embodiments are set forth in the dependent claims.

In a first aspect, there is provided a screen, preferably making a circular trajectory, for use in chipboard factories, the screen being implemented by means of meshes having different mesh sizes and comprising a number of mesh levels one above another, with which screen wood-based raw material is screened into different-sized fractions after chipping and drying, which screen comprises a frame part and a moving part, wherein the moving part of the screen comprises at least two screen modules and at least one frame module. Using a modular design allows for easier and thus faster and more economic assembly. In particular, because the center frame and the screen modules are different structures and are also manufactured in different ways, the

manufacturing of them can be done in a factory specialized in the relevant manufacturing method for reducing manufacturing costs. Also, the auxiliary tools and means for use in manufacture, e.g. welding jigs, are smaller and more ergonomic. Correspondingly, repairing a screen comprised of separate modules in the event of possible damage is less expensive than in screens according to prior art.

Preferably, there is one frame module and two screen modules, and the frame module is disposed between the screen modules. More preferably, there is one frame module and four screen modules, wherein the four screen modules are arranged in two pairs of screen modules, wherein in each pair of screen modules, one screen module is mounted above the other screen module, and in that the frame module is disposed between the two pairs screen modules.

Such a design allows for a high level of modularization, and moreover allows for economic scaling of a screen design e.g. for different demands in workload, by utilizing two or four screening modules, thus allowing re-use of components and subassemblies. This also allows for easier maintenance, and reduces the number of spare parts that need to be kept on-site.

Preferably, the screen modules have essentially the same volume.

In one particularly preferred embodiment of the screen according to the invention, the walls facing towards the frame module of the screen modules are provided with explosion hatches or burst discs for directing the pressure of a possible dust explosion into the space between the screen modules and from there either upwards or downwards into a free space. ln a case that a dust explosion occurs in one of the screen modules, the resulting pressure will cause the explosion hatches or burst discs associated with this screen module to break open, and thus the explosion gases are vented out of the screen module in a lateral direction into the frame module, and via the frame module in vertical direction towards the environment. This lateral venting of the screen modules makes it possible, also in case of a“stacked”

arrangement of screen modules one above another, to allow for a safe and controlled pressure release for each and all screen modules. Moreover, due to each screen module being provided with individual explosion hatches or burst discs, a dust explosion occurring in one screen module, possibly causing hot and burning gases to be vented into the frame module, will not be able to ignite dust in other screen modules. For this reason, even if a dust explosion occurs, the effect of such a dust explosion is limited to a single screen module. As each screen module comprises only a volume that is a partial volume (e.g. 1/2 in case of two screen modules, or 1/4 in case of four screen modules) of the entire volume of the screen, accordingly, also the risks and dangers associated with a potential dust explosion are reduced and limited accordingly.

Preferably, the pressure of a dust explosion possibly occurring in a screen module is conducted in the frame module via at least one flame extinguisher.

Providing a flame extinguisher achieves a flameless pressure release, so that there is no risk of igniting e.g. wooden dust in an environment surrounding the screen, and thus further substantially reducing the risks and dangers associated with a potential dust explosion.

With such a reduced risk, it may e.g. be possible to, in economic fashion, erect a screen inside a building, instead of in open outside space, so the screen is no longer exposed to outside weather conditions, reducing machine wear and improving operating quality. Preferably, the frame module comprises at least one drive machinery that is eccentric in terms of its mass and that has an essentially vertical axis of rotation, and that produces in the screen a rotating force in relation to the vertical axis and thus an essentially rotating trajectory on the horizontal plane. The drive machinery may in particular be a rotary cylinder, having additional mass attached to part of the outer circumference, e.g. in form of steel or aluminium structures or weights affixed the rotary cylinder over only a part of the circumference.

The frame module thus serves a dual purpose of providing pressure relase space in case of dust explosions, and of housing the drive machinery. This achieves a very space efficient design.

Preferably, the center of mass of the one or more rotating drive shafts disposed in the frame module is on essentially the same plane as the center of mass of the screen modules, which is the plane in the direction of the mesh levels of the screen module.

With such a design, the oscillating forces imparted by the rotating eccentric masses may be transferred to the moving part essentially in the plane of the rotating movement, resulting in a smooth movement of the moving part with no or only little tilting forces.

Preferably, the screen has two drive shafts. Further preferably, the two drive shafts have rotating masses of different magnitudes to each other, in which case the resultant of the forces producing a rotational movement is directed at the center of mass of the moving structure.

The moving part of the screen is preferably supported by a support structure below or above it that allows a circular trajectory. The moving part may be supported for example in hanging resp. suspended fashion by means of rods or steel cables, which are fastened to the support structure.

The screen module 4 preferably comprises one screening chamber or a number of screening chambers essentially one above another and separated

structurally from each other, e.g. with a solid intermediate level, in each of which chambers the material for screening is screened into at least two fractions.

The screen modules 4 and the frame module are preferably dimensioned for transporting in a standard-sized sea container.

Leveraging the modular and space-efficient design as described above, and as further described elsewhere herein, allows to achieve a very compact screen design, which in turn allows a design that fits in a standard-sized sea container. This allows for cheaper and easier shipping to a customer’s location.

In another embodiment, the angle of the inclination is automatically altered, preferably by at least a servo drive.

In the following, the invention will be described in more detail by the aid of some preferred embodiments with reference to the attached drawings:

Fig. 1 presents an oblique view from above of a screen according to an embodiment of the invention.

Fig. 2 presents the frame module of the screen according to the embodiment, and the drive devices of the frame module.

Fig. 3 presents the screen according to the embodiment as viewed from the end. Fig. 4 presents the screen according to another embodiment as viewed from the end.

Figure 1 presents the screen according to one embodiment of the invention. It comprises a frame structure and a moving part suspended from it. The moving part of the screen could also be supported from below instead of being suspended. The suspension here is implemented with hinged rods 3. Instead of hinged rods 3, it is also possible to implement a suspension by means of steel cables. The moving part comprises two screen modules 4 and a frame module 5 between them. A screen module 4 comprises one screening chamber or a number of screening chambers essentially one above another separated structurally from each other, e.g. with a solid intermediate level, in each of which chambers the material for screening is screened into at least two fractions. In the drawings an embodiment is presented wherein the screen is double layered, in other words there are two screening chambers, one above the other, in each screen module 4.

The material to be screened is fed into the feeder hoppers in the manner indicated by the arrows 6 and the screened fractions are removed from the other end of the screen in a manner that is per se known in the art. Each fraction to its own conveyor.

The screen modules 4 comprise screening levels 7 (Fig. 3) comprising screen meshes in a manner known in the art. The screens and their screening levels 7 are inclined e.g. approx. 5-10° with respect to the horizontal plane, in which case the material being screened flows in the direction of the inclination when a horizontal rotating movement is induced in the moving part of the screen.

Suspension from above or support from below permits the rotating movement in question. The rotating movement can be brought about with at least one, but in the embodiment according to Fig. 2 with two, drive machineries 8 that are eccentric in their mass and at least mostly situated inside the frame module 5, the drive machineries having rotating shafts 9 that are essentially vertical and that produce in the screen a rotating force in relation to the vertical axis and thus a rotating trajectory on essentially the horizontal plane. The eccentric masses of the rotation shafts 9 are marked with the reference number and they are situated inside the frame module 5 thus reducing the space required by the screen.

Additionally, the center of mass of the one or more rotating drive shafts 9 disposed in the frame module 5 can be on essentially the same plane as the center of mass of the screen modules 4, which is the plane in the direction of mesh levels 7 of the screen module 4.

The screen in Fig. 3 is presented as viewed from the end and partly

opened/sectioned. There is a dust explosion risk in screens screening wood- based products. For this reason, the screening chambers of the screen modules 4 are provided with explosion hatches 11 that are per se known in the art. The screen structure according to the invention, having a frame module 5 in the center and screen modules 4 on both sides of the frame module, makes it possible that the walls facing towards the frame module 5 of the screen modules 4 or towards their screening chambers can be provided with explosion hatches 11 for directing the pressure of a possible dust explosion into the space between the screen modules 4 (into the frame module 5) and from there either upwards or downwards into a free space. In the embodiment of Fig. 3 it is directed upwards, as the arrows indicate.

The pressure of a dust explosion can additionally be conducted via at least one flame extinguisher 12 in the frame module 5. As will be understood, the space between the screen modules 4, defined between the screen modules 4 and inside the frame module 5, is preferably forming an enclosed space that is opened to the environment only through a path passing through the flame extinguisher 12.

The screen modules 4 and the frame module 5 are dimensioned for transporting in a standard-sized sea container, which is advantageous for transporting them.

Although in the above there is described an embodiment comprising one frame module 5 and two screen modules 4, this is not limiting, and it is also possible to utilize more than two screen modules 4 in combination with one or more frame modules 5. One such example is shown in Fig. 4, which depicts an embodiment in which the moving part 2 comprises four screen modules 4 and one frame module 5. The four screen modules 4 are arranged in two pairs, wherein, in each pair of screen modules 4, one screen module 4 is mounted above the other screen module 4. These two pairs of screen modules 4 are disposed at the two opposing sides of the frame module 5.

It is obvious to the person skilled in the art that the invention is not limited to the embodiments described above, but that it can be varied within the scope of the claims presented below. One alternative is that the screen has two drive shafts 9 that have rotating masses of different magnitudes to each other, in which case the resultant of the forces producing a rotational movement is directed at the center of mass of the moving structure.

The characteristic features possibly presented in the description in conjunction with other characteristic features can also, if necessary, be used separately to each other.