Patent Publication Number: US-7721895-B2

Title: Screening device for wet screening of paper fiber suspensions

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority under 35 U.S.C. §119 of German Patent Application No. 10 2005 049 943.0, filed on Oct. 19, 2005, the disclosure of which is expressly incorporated by reference herein in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a screening device for the wet screening of paper fiber suspensions. The device includes at least one screen plate provided with a plurality of screen openings, a support structure to which the screen plate is to be attached in a detachable manner with its back and a rotor that can be moved closely along the screen plate. An outlet opening is provided for the part of the paper fiber suspension passing through the screen openings. 
   2. Discussion of Background Information 
   Screening devices are used in paper stock preparation, e.g., in sorters and pulpers of the pulp and paper industry, in order to treat the fiber suspension during a wet screening. In most cases, non-fibrous impurities and not yet disintegrated paper are thereby to be retained at a screen as a result of their size and then to be discharged. The passing of fiber suspension through the screen openings involves a considerable loss of pressure. This leads to strong forces pressing against the screen, which are absorbed as compressive forces by the support structure. The screens of such devices are usually made of metal, e.g., alloyed high-grade steel. 
   In order to prevent a clogging of the screen openings, typically scrapers attached to a rotor are used, which move closely along the openings, thereby creating hydraulic pressure impulses and hydraulic suction impulses. These impulses can cause the screen plate to vibrate, which stresses both the support structure and the screen plate. Other hydraulic impulses (thrusts) that can endanger said components originate in the periphery of a screening device, e.g., actuated sliders and pumps in the connecting lines. Even welded components are endangered by the vibrations. 
   The mentioned stress conditions during wet screening necessitate a complex and sturdy attachment of the screen plates, which implements in most cases a reduction of free screen area. 
   It is known that the effect of the scrapers can be improved by protruding strips, as these cause turbulences that prevent solid materials from depositing. Mounted strips of this type are disclosed, e.g., in DE 195 06 084A1. 
   SUMMARY OF THE INVENTION 
   This invention is directed to screening devices for the wet screening of paper fiber suspensions, characterized by a simple structure that can well tolerate vibrations and hydraulic impulses occurring during wet screening. The screen plates of the device can be attached in a reliable and easy manner. Losses in free screen area through the means of attachment are kept to a minimum. 
   The screening device comprises a screen plate, which is attached in a detachable manner, preferably screwed together, to an outer support ring and an inner support ring and the screen plate lies against at least one support element arranged radially between inner support ring and outer support ring. Elastic pre-stressing against the screen plate is generated, and as a result of the elastic pre-stressing, the screen plate exerts compressive forces on at least one radially intermediate support element and respectively exerts tensile forces both on the inner support ring and on the outer support ring. 
   The screening device of the invention is characterized by a specific support structure, which comprises an outer support ring and an inner support ring and at least one middle support element arranged radially between the inner and outer support ring. In the mounted state of the screen plate, elastic pre-stressing is generated against the screen plate, causing compressive forces directed against the middle support element and tensile forces against the inner and outer support rings, whereby said generated forces exceed the tensile forces produced by vibrations and thrusts occurring against the screen plate during wet screening. 
   The measures make it possible to attach the screen plate—irrespective of whether it is produced from one piece or assembled from various segments—with simple elements on a simplified support structure such that a durable connection is ensured even with vibrations occurring inevitably. According to the invention a screen plate pre-stressed in the manner described is insusceptible to the vibrations and hydraulic thrusts inevitably occurring with screening devices of this type. The measures described make it possible to simplify the support structure, in particular by reducing the surface area needed for the screen plate to rest on. This not only simplifies the structure and mounting, but also has the effect that less screen area of the screen plate is blocked by the support structure. The smaller number of the screw joints hitherto required means also a considerable simplification and reduction in cost. 
   Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
       FIG. 1 : Top view of the screen of a screening device according to the invention; 
       FIG. 2 : Section through the screen area of a screening device according to the invention; 
       FIG. 3 : A varied support ring; 
       FIG. 4 : A screen attachment variation; 
       FIG. 5 : A further screen plate suitable for the screening device according to the invention (shown in section); 
       FIG. 6 : A secondary pulper embodied according to the invention; 
       FIG. 7 : A pulper embodied according to the invention. 
   

   DETAILED DESCRIPTION OF THE PRESENT INVENTION 
   The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice. 
     FIG. 1  shows the view of a screen plate  1  (half of which is drawn) and the way it is installed with respect to the support structure  3 . The support structure  3  comprises an inner support ring  4 , a middle support ring  5  and an outer support ring  6 . These three and preferably concentric support rings serve to attach the screen plate  1  of the screening device according to the invention. It is possible to make the screen plate  1  in form of a complete ring or to assemble it from individual annular segments during the construction of the screening device. In a manner known per se, the screen plate  1  separates the inflow chamber  19  ( FIG. 6 ) from the accepted-stock chamber  9  of the screening device. In the bottom part of  FIG. 1 , the accepted-stock chamber  9  is openly visible, as the screen plate  1  is not drawn there. 
   In the exemplary embodiment of the invention, the support structure  3  has a single middle support ring  5 . This support ring may have a closed surface towards the screen ( FIG. 3 ), or can be provided with radially open recesses  25 , e.g., with radial grooves ( FIG. 1 , drawn with dashed lines). 
   The three support rings are connected to one another by a radial brace  7 . This brace serves to hold the three support rings together, which facilitates the mounting of the support structure  3 . The radial brace  7  is advantageously arranged in the area of the outlet opening  18 , where it least obstructs the flow in the accepted-stock chamber  9 . In the mounted state the screen plate  1  is connected to the outer support ring  6  by screws  13  and to the inner support ring  4  by screws  11 . Advantageously, the screen plate  1  and the middle support ring  5  may not be connected by screw joints, which greatly simplify the mounting. However, in special cases, i.e., with a particularly high dynamic load, a screw joint may be also utilized, as indicated, by way of example, in  FIG. 1  by a screw  12 . 
   As mentioned above, screen plates  1  in wet screening devices are often provided with strips  8  that can be arranged radially or at an angle to the radius and improve the effect of the screening device in particular when an additional disintegrating effect is desired in the screening device, which is the case, e.g., with pulpers and secondary pulpers. 
     FIG. 2  demonstrates how the screen plate  1  is mounted in the screening device. Thereby, the middle support ring  5  protrudes over, i.e., axially above, the inner support ring  4  and the outer support ring  6  by a projecting length A in the direction of the screen plate  1 . During the mounting of the screen plate  1 —i.e., when the screws  11  and  13  are tightened—the screen plate  1  is elastically pre-stressed, thereby producing compressive forces directed against the middle support ring  5 . The projecting length A preferably lies in the range between 0.5 and 1 mm (drawn disproportionately large in  FIG. 2 ). The protruding part of the middle support ring  5  does not have any disadvantageous effect on the function of the screen plate  1 , although a certain buckling of the screen plate  1  cannot be prevented. 
   In  FIG. 2 , a part of the rotor  20  is also indicated by a dashed line. The rotor  20  and the screen plate  1  have the same center line  17 . The support rings  4  and  6  are directly attached to the housing  10  and thus seal off the accepted-stock chamber  9  radially inward and outward. The middle support ring  5  is supported in the housing  10  through a single ring or several spacers  14 , e.g., welded studs, which are located in the accepted-stock chamber  9 . 
   In special embodiments (see  FIG. 3 ), the middle support ring  5  is provided with a number of projections radially towards the screen side. Between said projections are further radially open recesses  25 , e.g., radial grooves. Thus, additional free screen area is made possible without any loss of strength at the attachment. 
   As noted above, instead of a complete middle ring, the spacers  14  could also extend up to the screen and thus serve as support elements (not drawn). 
   It is also possible to create the elastic pre-stressing by using a screen plate  1  that is slightly curved in the non-mounted state, indicated in  FIG. 4  by a curvature line  16 . It is preferred that such curvature be applied through individual screen ring elements rather than a complete screen ring. Moreover, the middle support ring  5  can then end towards the screen plate  1  in the same plane as the inner support ring  4  and the outer support ring  6 . Although this embodiment is somewhat more complex, it has the advantage that the screen plate  1  is exactly level towards the rotor. 
     FIG. 5  shows a further option of embodying the screen plate according to the invention. In this embodiment the screen plate  1 ″ is provided with a projection  15  on the side facing the middle support ring  5 , which projection protrudes by the dimension A′ with respect to the remaining screen area and amounts to preferably 0.1 to 1 mm. Alternatively, the projection can have a projection length between 0.2 and 5 mm or between 0.5 and 1 mm. The projection  15  can be annular, corresponding to the middle support ring  5 , or also be composed of annular arranged spacer pieces, e.g. knobs. Alternatively, an intermediate layer, in particular a spacer ring, can also be used instead of the projection  15 . 
   An example of the invention applied in a secondary pulper is shown in  FIG. 6 . A secondary pulper is a screening device, in which stress problems occur in a particularly severe manner. Secondary pulpers are customarily in the closed system of a paper stock preparation. Pumps are installed in the supply and discharge lines, and sliders are actuated during operation, causing the hydraulic thrusts described above. In addition, the rotor  20  causes vibrations in the area of the screen plate  1 . The secondary pulper shown in  FIG. 6  has a rotationally symmetric container  23  with an inflow chamber  19  for the paper fiber suspension S, wherein the chamber is provided with an inflow pipe  22  and a reject outflow  21 . The container  23  is closed by a lid  24  at one front side, while the screen plate  1  is located on the opposite side, separating the inflow chamber  19  from the accepted-stock chamber  9 . A rotor  20  serves to scrape the screen plate  1  in the manner described above, ensures the necessary movement in the inflow chamber  19 , and further disintegration of the fibrous material flowing in. The screen plate  1  is attached to a support structure  3  with an inner support ring  4 , a middle support ring  5  and an outer support ring  6 . 
   The invention can also be applied in flat-screen pressurized sorters having a similar structure as secondary pulpers, however the disintegration effect of the rotor plays a much less important role or none at all. 
   Another application of the invention, shown in an exemplary manner in  FIG. 7 , is the screen area of a pulper with a rotationally symmetric container  23  that is open at the top. In this container, e.g., air-dry fibrous material F is mixed with water W and brought into suspension S. As is known, pulpers of this type also have a mostly level screen plate in the area of the material outlet, which screen plate is also subject to vibrations and hydraulic thrusts and can be advantageously attached according to this invention. 
   It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to an exemplary embodiment, it is understood that the words, which have been used herein, are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. 
   LIST OF REFERENCE NUMBERS 
   
       
         1  Screen plate 
         2  Screen opening 
         3  Support structure 
         4  Inner support ring 
         5  Middle support ring 
         6  Outer support ring 
         7  Radial brace 
         8  Strip 
         9  Accepted-stock chamber 
         10  Housing 
         11  Screw 
         12  Screw 
         13  Screw 
         14  Spacer 
         15  Projection 
         16  Curvature line 
         17  Center line 
         18  Outlet opening 
         19  Inflow chamber 
         20  Rotor 
         21  Reject outflow 
         22  Inflow pipe 
         23  Container 
         24  Lid 
         25  Recesses 
       A Projecting length 
       A′ Dimension 
       F Fibrous material 
       S Paper fiber suspension 
       W Water