Patent Publication Number: US-2006000764-A1

Title: Pressurized screen for screening a fibrous suspension

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      The present application claims priority under 35 U.S.C. §119 of German Patent Application No. 10 2004 031 622.8, filed on Jun. 30, 2004, the disclosure of which is expressly incorporated by reference herein in its entirety.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a pressurized screen for screening a fibrous suspension with at least one wire basket inserted in a housing and surrounding a wire chamber. The wire basket is provided with a plurality of wire openings, through which a part of the fibrous suspension, which is guided through an intake into an intake chamber, then into the wire chamber, and can reach an accepted-stock outlet of an accepted-stock chamber located radially outwardly of the wire chamber. Another part of the fibrous suspension is rejected at the wire openings and discharged separately from the pressurized screen as reject through at least one reject outlet. A wire scraper, located in the wire chamber, features a preferably cylindrical rotor body having scraper elements located on the circumference of the rotor body, and a drive runs through the intake chamber.  
      2. Discussion of Background Information  
      Pressurized screens of this type are used in the preparation of paper fiber stock suspensions to process the fibrous suspension in a wet screening. To this end, such a pressurized screen contains at least one wire basket that is provided with a plurality of openings. The fibers contained in the suspension are to pass through the openings, whereas the undesirable solid components are rejected there and guided out of the screen again. Thereby, a centrifugal operation mode is used in which the suspension passes through the wire openings radially from the inside outwards. Round holes or slots generally serve as screening openings. Pressurized screens of the type under consideration here are equipped with wire scrapers that feature scraper elements which can be moved closely past the wire basket. Thus the clogging of the wire openings is prevented in a manner known per se.  
      The wire scraper is embodied as a rotor that is generally supported concentrically to the wire basket. The bearing housing of the wire scraper must be sealed against the paper fiber stock suspension. Thereby, the sealing device required hereto is loaded in particular by the contaminants entrained in the paper fiber stock suspension. In particular heavy particles, such as, e.g., wire pieces, broken glass, sand and stones can penetrate to the seal and damage it. Also chains of paper clips and tangled foils or strings can cause problems at the seal. In order to keep this load to a minimum, the rotor with pressurized screens of the type under consideration here is driven from the intake side, so that the sealing device can come into contact only with the inflowing suspension that is not yet concentrated with contaminants as a result of the screening process. U.S. Pat. No. 6,679,384 B1 shows an example. The situation would be different with pressurized screens driven from the reject side, such as, e.g., according to Gerrnan Application No. DE-A-199 11 884.  
      The separating effect of a pressurized screen is thus attributable to the fact that at least a part of the impurities contained in the fed paper fiber stock suspension cannot pass through the wire, i.e., is separated from the paper fibers on account of the size, shape or flexibility. Also known are pressurized screens in which an additional separation takes place that targets the density of the contaminants by utilizing the different forces of the contaminants in a centrifugal field. Although a majority of the heavy particles would not fit through the wire openings typically used anyway, i.e., would be rejected there, there is still the danger of damage or wear if they come into contact with the wire.  
     SUMMARY OF THE INVENTION  
      The present invention improves the operational safety and effect of pressurized screens.  
      According to the invention, a pressurized screen of the aforementioned type includes a front surface of the rotor body facing the drive featuring a rotationally symmetrical projection concentric to the rotor body, which projection extends into the intake chamber at its radially inner end with a defined axial extension and the smallest diameter of this projection amounts to a maximum of 60%, preferably a maximum of 30% of the largest diameter of the projection.  
      With a pressurized screen in accordance with the invention, substantially more favorable flow conditions can be produced in the intake area of the housing through the measures according to the invention. The intake chamber is essentially arranged in a ring-shaped manner around the drive shaft of the wire scraper and is supplied with the fibrous suspension to be screened through, e.g., a tangentially attached intake. As a result of the rotor rotation and, where applicable, the tangential intake, a rotational flow forms in the intake chamber. Through an exemplary embodiment according to the invention, the accumulation of air and/or light particles is prevented in the intake chamber, as these can flow off upward along the projection on account of their tendency to rise. Large heavy particles, such as, e.g., stones and wire pieces, are centrifuged radially outward as a result of the centrifugal forces and can be removed in advantageous embodiments through a special heavy-particle outlet, so that they do not come into contact with the wire.  
      The present invention is directed to a pressurized screen for screening a fibrous suspension including a housing, at least one wire basket, having a plurality of wire openings, inserted in the a housing to surround a wire chamber. An intake chamber is arranged to receive a fibrous suspension through an intake, an accepted-stock chamber, coupled to an accepted-stock outlet, is located radially outside of the at least one wire basket, and at least one reject outlet is structured and arranged to discharge a portion of the suspension rejected at the at least one wire basket. A wire scraper, located in the at least one wire chamber, has a rotor body with scraper elements coupled thereto and a drive arranged to run through the intake chamber. The rotor body has a front surface arranged to face the drive, and the front surface has a rotationally symmetrical projection concentric to the rotor body that extends into a radially inner end of the intake chamber having a defined axial extension. A smallest diameter of the projection is a maximum of 60% of a largest diameter of the projection.  
      According to a feature of the invention, the smallest diameter of the projection can be a maximum of 30% of a largest diameter of the projection.  
      In accordance with another feature of the present invention, the rotor body may include a cylindrical rotor body. The scraper elements can be arranged on a circumference of the rotor body.  
      According to still another feature of the instant invention, the axial extension can be at least 20% of the axial extension of the intake chamber.  
      Further, the axial extension can be at least 40% of the axial extension of the intake chamber.  
      According to a further feature of the invention, the projection may have a truncated cone shape, the oblique angle of which is between 15° and 45°.  
      Moreover, a wall of the intake chamber located opposite the at least one wire chamber can be provided with an intake chamber projection extending into the intake chamber with a defined axial extension.  
      In accordance with a further feature, an intake may be tangentially attached to the intake chamber.  
      According to a still further feature, a heavy-particle outlet can be coupled to the intake chamber. Further, a heavy-particle sluice may be attached to the heavy-particle outlet, and the heavy-particle sluice may be intermittently activatable.  
      The at least one wire basket can include a cylindrical wire basket arranged to separate the wire chamber from the accepted-stock chamber. The wire chamber may be located radially inside the cylindrical wire basket and the accepted-stock outlet can be located radially outside the cylindrical wire basket.  
      According to a feature of the invention, a center line of the wire basket can be perpendicular in the position of use.  
      Further, a center line of the wire basket may be vertically oriented in an operating position. The reject outlet may be arranged above the wire basket, and the accepted-stock outlet can be arranged above the intake.  
      The heavy-particle outlet can be attached to a lower part of the intake chamber.  
      According to a further feature of the invention, an air and light-particle outlet can be coupled to the wire chamber from above.  
      In accordance with still yet another feature of the present invention, the wire scraper may be embodied to emit pressure and suction surges towards the at least one wire basket through relative movement to a surrounding liquid.  
      Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing. 
    
    
     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  diagrammatically represents a sectional side view of a pressurized screen according to the invention;  
       FIG. 2  illustrates a more detailed view of the intake chamber depicted in  FIG. 1 ;  
       FIG. 3  diagrammatically represents a sectional top view of the pressurized screen depicted in  FIG. 1 ,  
       FIG. 4  illustrates an alternative embodiment of the invention having a changed reject management. 
    
    
     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 a typical embodiment of the pressurized screen according to the invention. This pressurized screen contains in a housing  2  a cylindrical wire basket  1 , the center line of which is perpendicular (vertical) in the position of use of the device. This is then also referred to as a vertical screen. The fibrous suspension S to be screened is guided into housing  2  through an intake  8 . The suspension first reaches the intake chamber  4 , which extends in an essentially ring-shaped manner around drive shaft  16  of wire scraper  7 , and then wire chamber  5  located above it. As  FIG. 3  shows, intake  8  is attached to intake chamber  4  in a tangential manner here, which reinforces the rotational flow in intake chamber  4  during the operation of the machine. The accepted stock that, coming from wire chamber  5 , has passed through wire openings  19  of wire basket  1 , reaches accepted-stock chamber  3  and is discharged from there through accepted-stock outlet  9 . Accepted-stock outlet  9  is arranged above intake  8 , so that the main flow runs from the bottom upward. Between accepted-stock chamber  3  and intake chamber  4  there is a dividing wall  20 .  
      In wire chamber  5  there is wire scraper  7  with a rotor body  28  that is provided with a number of scraper elements  6 . Through relative movement to the liquid surrounding them they produce pressure and suction surges, with which the wire openings are kept clear. In the lower part of housing  2 , wire scraper  7  is in an overhung position (bearing unit  17 ) and is driven for rotation. The larger heavy particles collect in the lower part of intake chamber  4  and are discharged here via heavy-particle outlet  13  into heavy-particle sluice  15 . The heavy-particle outlet is provided with an easily detachable lid  21  or a cleaning flap to allow for cleaning, if necessary. Hardly any fiber losses occur on account of the use of a heavy-particle sluice  15 . The largest part of the suspension rejected at wire basket  1 , in particular the contaminants, are collected in reject chamber  24 , located above wire basket  1 , and are removed as reject R from housing  2  via reject outlet  10 . Reject R is generally fine-screened in order to prevent fiber losses.  
      In its topmost part, wire chamber  5  features a ventilation opening  14  through which air and light particles can be removed, which have flowed past wire basket  1  without having passed through wire openings  19 .  
      The features with which the pressurized screen is equipped according to the invention can be discerned in somewhat more detail in  FIG. 2 . Rotor body  28  is displayed only in its lower part and it can be seen that the front surface (here the one located below) facing the drive is provided with a truncated conical projection  11  that extends into intake chamber  4  with its pointed end. Projection  11  is dimensioned such that its smallest diameter D 1  amounts to approximately 30% of the value of largest diameter D 2 . It is advantageous to keep the smallest diameter of projection  11  small, so that the inclined surface extends as far as possible into the central area, which improves the discharge of light particles. The oblique angle α thus forming is advantageously larger than 15°. The axial extension A 1  of projection  11  can amount to at least 20% of axial extension B of the intake chamber  4 . Also the wall of intake chamber  4  located opposite wire chamber  5  is provided with a projection  12 , which extends into intake chamber  4  with a defined axial extension A 2 . The interaction of these two projections  11  and  12  causes a central narrowing of the flow cross-section in intake area  4 , which leads to a considerable improvement of the discharge of light materials and air as well as of heavy particles. In the example displayed here, this narrowing continues up to the sealing unit  18  with smallest outer diameter D 3 . Because of its low space requirement, the sealing unit  18  can be accommodated in intake chamber  4 . Thereby, a tangential arrangement of intake  8  has the advantage that the pulse of the intake flow is not directed at sealing unit  18 . On account of the immediately active centrifugal forces in intake chamber  4 , heavy particles, such as, e.g., wire pieces, are furthermore prevented from entering the central area where sealing unit  18  is located. By contrast, bearing unit  17  is located largely or (as here) completely outside intake chamber  4 , which in particular has the advantage that intake chamber  4  can be optimized in terms of flow technology.  
       FIG. 3  shows a top view of the pressurized screen according to the invention with a tangential intake  8  and a radial accepted-stock outlet  9 . Dividing wall  20  between accepted-stock chamber  3  and intake chamber  4  is drawn open in two places. On the right side of this representation heavy-particle outlet  13  is discernible, which is attached to housing  2  outside in the form of a lateral pocket, whereby the tangential centrifuging out of the heavy particles is promoted through appropriate shaping.  
      As  FIG. 4  shows, a reject line  22  may open into the laterally attached heavy-particle outlet, which line is connected outside to reject chamber  24 . The reject concentrated with heavy particles is thus discharged into heavy-particle outlet  13 . If necessary, the pressure in reject chamber  24  can be increased by scoops  25  mounted on wire scraper  7 . The remaining part R′ of the reject can be discharged through a laterally attached reject outlet  10  or via a reject outlet  10 ′ (drawn in dashed line) leading upward above reject line  22 .  
      As already mentioned, the typical case of use of this device is the prescreening or course screening of newly dissolved paper stock. Wire openings  19  of the wire basket are then mostly round and have a diameter of between one and three millimeters.  
      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.