Abstract:
A screen, for cleaning a fiber suspension, includes at least one separating unit containing a housing, a parabolic rotor, a screen basket, an accept chamber, and a reject outlet. The reject outlet is located in the vicinity of the maximum rotor diameter. The screen also includes one or several devices for interrupting the axial flow located in the vicinity of the maximum rotor diameter.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a screen for cleaning a fiber suspension. 
   Screens are machines used in the paper industry to clean a pulp suspension comprising water, fibers, and dirt particles. Here a feed flow runs through a screening device, where the accept flow, consisting of water and fibers, flows through the screen. A partial flow, known as the reject and consisting of water, fibers, and dirt particles, is generally removed at the opposite end to the feed flow. Thus, the solids particles present in the liquid are separated from one another in the screens. By contrast, in filtration processes the liquid is separated from the solids. 
   In general, a screen of this type is rotationally symmetrical and consists of a housing with a feed device mounted at a tangent, a cylindrical screen basket, normally with perforations or vertical slots, and a rotating rotor. The purpose of the rotor is to keep the screen slots clear, achieved by the vanes rotating close to the screen surface. The accept is collected in a so-called accept chamber, which often has a conical design, and drawn off from here in radial direction. The reject flow is generally brought to a reject chamber, which is usually annular, located at the opposite side of the screen basket to the inlet, and drawn off from here at a tangent. 
   A screen of this type is known, for example from U.S. Pat. No. 4,268,381. 
   Other screens known are described in, for example, EP 1 122 358 A2, EP 1 124 002 A2, and EP 1 124 003 A2. 
   In the screens according to EP 1 122 358 A2, EP 1 124 002 A2, and EP 1 124 003 A2, the following measures are implemented, particularly in order to improve flow conditions:
         An additional screen basket is provided in the feed area for pre-screening.   In the feed area between the pipe socket and the freely accessible end of the rotor there is a stationary mounting, particularly a cone, truncated cone, hemisphere, spherical segment, spherical segment between two parallel circles, paraboloid, or a hyperboloid of two sheets.   The accept chamber is designed as twin cones, widening in flow direction of the pulp suspension and tapering again from the mouth of the accept outlet in a conical shape towards the reject outlet.       

   In these known screens the rotor is designed for even flow onto the screen and is parabolic in shape so that the axial flow speed inside the screen basket remains constant at an assumed uniform flow through the screen basket. As an alternative, a cone shape can be used to come closer to the parabolic shape of the rotor. 
   It is also known that screens can be designed as multi-stage units, comprising several separation stages one after another. 
   The screens known from the state of the art, however, still hold disadvantages. In particular, the flow conditions at the reject outlet leave much to be desired. 
   SUMMARY OF THE INVENTION 
   The present invention provides a screen in which a further improvement can be attained in the flow conditions and thus, a reduction in the energy applied, while increasing production and dirt separation. 
   The screen according to the invention is characterised by the reject outlet being located in the vicinity of the maximum rotor diameter and by one or several devices to interrupt the axial flow being located in the vicinity of the maximum rotor diameter. 
   In the following, the term “devices” (plural) is used, relating also to screens according to the invention which have only one device to interrupt axial flow. 
   Depending on their origin and type (recycled fibers, fresh fibers, etc.), pulps contain differing amounts of dirt particles. To ensure stable screen operations, certain minimum amounts of carrier medium (reject amounts) must be set as a function of the dirt and flake content, and of the suspension&#39;s rheological characteristics. 
   It has proved favorable to mount devices to interrupt the axial flow at the same height as the maximum rotor diameter in order to guarantee stable screen operations. 
   The devices to interrupt axial flow can be mounted at the housing of the separation unit or at the screen basket and/or at the rotor of the screen. Thus, a design in which devices to interrupt the axial flow are provided on both sides (i.e. both at the housing and at the rotor) is also possible. 
   The devices should preferably be one or several axial flow interruption rings. Depending on its design, the flow interruption ring can either be continuous or in the form of individual segments, or have gaps. 
   The flow interruption ring (or flow interruption rings) can be of adjustable design, such that the size of the opening created by the flow interruption ring for the reject can be modified. 
   The flow interruption ring can be of adjustable design, for example in the same way as an iris diaphragm. In addition, the flow interruption ring can be adjustable statically (e.g. in the form of statically adjustable ring segments). 
   The outer diameter of a flow interruption ring on the rotor side preferably has a toothed profile. 
   A further preferred configuration of the screen according to the invention is characterised by at least one feed for dilution water being located in the vicinity of the reject outlet, particularly directly below it. 
   As a result, the reject leaving the screen is diluted with water. This dilution is favorable particularly in a multi-stage screen configuration where the reject from one stage is also the feed to the following stage. 
   One or more feed points can be provided for dilution water, which can be located at the housing of the separation unit or at the screen basket and/or at the rotor. If a feed for dilution water is located at the rotor, this feed is supplied preferably through a pipe mounted inside the rotor. 
   The feed point—if necessary, several—for dilution water can be oriented such that dilution water can enter in rotor running direction and/or in the opposite direction to rotation of the rotor. 
   Thus, the rotating movement of the pulp suspension can be reduced. By causing turbulence in the suspension, loosening of the suspension can be improved. 
   In a further preferred configuration of the screen according to the invention, at least one feed for dilution water is coupled to a device for interrupting the axial flow. For example, the feed of dilution water can protrude into the area between housing and rotor and thus, serve as a device for interrupting the axial flow. 
   Particularly in multi-stage screens, thickening of the suspension takes place on the one hand in the inflow area to the screen surface as the suspension flows between the first and the final screening stage, and on the other hand, the flake content becomes more concentrated. 
   In order to maintain the screening effect, the suspension consistency, as described above, is set by means of intermediate dilution. It has proved favorable to counteract this concentration of the flake content by inserting a deflaking unit. 
   Thus, the separating unit of the screen according to the invention should preferably contain a deflaking unit. Advantageously, the deflaker should take the form of one or several rings mounted on the housing or screen basket and/or on the rotor. The shape of the mountings used corresponds to models that are already known in themselves, while additional hydraulic guiding elements can be included in order to set differential pressures. 
   The screen according to the invention can preferably comprise two or more separation units located one after another in a manner already known, where all separation units have one common rotor, which has a parabolic or parabolic segment shape for each separation unit, adapted to the flow conditions in the separation unit in each case. 
   The height of each separation unit should preferably be at least twice the sum of the heights of all separation units adjoining the separation unit in question, i.e. in a screen with three separation units, the height of the first stage is at least ⅔ the overall height of the unit and the height of the second stage is at least 2/9 of the overall height. 
   Each separation unit of a multi-stage screen according to the invention should preferably contain one or more devices to interrupt the axial flow, as described above, in the vicinity of the maximum diameter. 
   Similarly, it is preferable to have at least one inlet for dilution water in each separation unit in the vicinity of the reject outlet or underneath it. 
   In a multi-stage screen, the feed for dilution water can be located in the lower delimitation of the rotor segment of a separation unit so that the dilution water is discharged into the space beneath the rotor segment (and thus into the vicinity of the reject outlet or the area below it). As an alternative or additionally, the feed for dilution water can be mounted in the upper part of the rotor segment of the following separation unit. 
   In a multi-stage screen according to the present invention with at least three separation units, a minimum of one deflaking unit should preferably be provided, particularly at the transition from the second to the third separation unit. 
   In addition to the features described above, the screen according to the invention should preferably contain one or several features of the screens described in EP 1 122 358 A2, EP 1 124 002 A2, and EP 1 124 003 A2. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings in which: 
       FIG. 1  is a view of a conventional screen; 
       FIG. 2  is a view of a multi-stage screen according to a preferred configuration of the present invention; 
       FIG. 3  is an enlarged section of a reject outlet from the screen according to  FIG. 2 ; and 
       FIG. 4  is an enlarged section of an alternative design of a reject outlet. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The screen according to  FIG. 1  comprises, in a way already known, a feed branch  2 , through which a pulp suspension if fed for cleaning purposes. In the feed area, a mounting  3  is provided, which is shown here as a truncated cone. The pulp suspension enters the space between the parabolic rotor  4  and the screen  5  and is conveyed through the screen into the accept chamber  6 . The housing of the accept chamber is designed as a double cone in this configuration and in a way which is generally known. The accept outlet is marked with reference number  7 . The reject is removed through a reject outlet  8 . 
   In  FIG. 2 , those devices or parts of devices that are identical to the configuration which is state of the art and shown in  FIG. 1  are marked with the same reference numbers. In the preferred configuration of a screen according to the invention and as shown in  FIG. 2 , the screen  1  consists of three separation units  1 ′,  1 ″ and  1 ′″. 
   The three separation units  1 ′,  1 ″ and  1 ′″ have one common rotor, whose sections  4 ′,  4 ″ and  4 ′″, respectively, adapted to the flow conditions in the corresponding separation unit, are parabolic or have the shape of a truncated paraboloid. As an alternative, the sections of the rotor can also be shaped similar to a truncated cone or a parabola. 
   Each separation unit has a reject outlet ( 9 ′,  9 ″ and  9 ′″). The reject from the first and second separation units is thus also the feed to the next separation unit in each case. The reject from the third and final separation unit is drawn off through the reject outlet  8 . 
   In  FIG. 2 , a pipe for dilution water mounted inside the rotor is marked  10  and the outlets from the pipe will be described in more detail below. 
   A deflaking unit  13  is provided at the transition from the second to the third separation unit. 
     FIGS. 3 and 4  show preferred configurations of a reject outlet (in this case reject outlet  9 ′) in an enlargement of the section marked with a chain-dot line in  FIG. 2 . 
   According to the configuration shown in  FIG. 3 , an adjusting ring  12   a ′ is mounted at the lower end of the rotor section  4 ′. The adjusting ring can have an adjustable mounting, as explained above, e.g. in the shape of an iris diaphragm  14  (indicated by the double arrow). The outer diameter of the adjusting ring or its segments should preferably have a toothed profile  15 . 
   With the adjustable ring  12   a ′, the axial throughput can be controlled by means of the reject outlet  9 ′. 
   Furthermore, in the configuration according to  FIG. 3 , feed points for dilution water  10   a ′,  10   b ′, and  10   c ′ are provided on the housing, as well as at rotor sections  4 ′ and  4 ″ in the vicinity of the reject outlet  9 ′ and beneath it. 
   The feed point  10   a ′ is located in the lower delimitation of the rotor segment  4 ′ of the first separation unit  1 ′. The feed point  10   b ′ is placed in the upper section of the rotor segment  4 ″ of the second separation unit  1 ″. The feed points  10   a ′ and  10   b ′ can be supplied through a pipe  10  (see  FIG. 2 ) mounted inside the rotor. 
   The feed point  10   c ′, for example, is located in the vicinity of a flange  11  between the first separation unit  1 ′ and the second separation unit  1 ″ and is supplied through a pipe not shown in this illustration. 
   With the feed pipes for dilution water  10   a ′,  10   b ′ and  10   c ′, the consistency of the pulp suspension flowing to the next separation unit can be controlled effectively. 
   The configuration of the reject outlet  9 ′ shown in  FIG. 4  differs from the configuration shown in  FIG. 3  in that a flow interruption ring  12   b ′ is mounted on the housing in addition to the adjusting ring  12   a ′. The housing side feed  10   c ′ for dilution water is also located in the flow interruption ring  12   b ′, i.e. the feed for dilution water and the flow interruption ring are coupled to one another. Of course, the configuration in  FIG. 4  can also include additional feed lines for dilution water at the rotor, as shown in  FIG. 3 . 
   The height of each separation unit should preferably be at least twice the sum of the heights of all separation units adjoining the separation unit in question, i.e. in a screen with three separation units,  1 ′,  1 ″,  1 ′″, the height of the first stage  1 ′ is at least ⅔ the overall height of the unit and the height of the second stage  1 ″ is at least 2/9 of the overall height. 
   While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.