WO2002009844A1

WO2002009844A1 - Screening method and apparatus

Info

Publication number: WO2002009844A1
Application number: PCT/US2001/024003
Authority: WO
Inventors: Christopher E. Mccarthy
Original assignee: Kadant Black Clawson, Inc.
Priority date: 2000-08-01
Filing date: 2001-07-31
Publication date: 2002-02-07

Abstract

An improved pressure screen (10) includes a screen cylinder (12) and a rotor (14). The screen cylinder (12) defines an axis (30); an inlet side (32); and a flow velocity retarding zone (18), most preferably including an annular chamber (46). Preferably, a single stator tooth (48) is positioned in the annular chamber (46). The rotor (14) preferably mounts one or more foils (60, 62, 64, 66) near the inlet side (32) of the screen cylinder (12) as well as an annular lip (70) extending radially outwardly from the rotor (14) in alignment with the annular chamber (46). The annular lip (70) preferably includes one ore more bypass channels (100) communicate with the inlet side and are adapted to direct a portion of the flowing stock suspension around the flow retarding zone. An improved pressure screening method includes the steps of (a) inducing a flow of the suspension against the inlet side (32) of said screen cylinder (12); (b) rotating the rotor (14) about the axis (30) to induce microturbulence in the suspension; (c) propelling a portion of the flow radially outwardly into the flow velocity retarding zone (18); and (d) bypassing a portion of the flow past the flow velocity retarding zone (18). The preferred pressure screen and method serve to provide more uniform screening efficiency over the axial length of the screen cylinder (12) while minimizing the axial forces on the bearings (92) supporting the rotor (14).

Description

SCREENING METHOD AND APPARATUS

FIELD OF THE INVENTION This invention relates to methods and apparatus for the screening of paper making stock, and, more particularly, to methods and apparatus wherein the rotational component of the flow of paper making stock is controlled in a flow velocity retarding zone so as to improve downstream screening efficiency and wherein an axial pressure differential across the zone is minimized so as to reduce the wear on one or more bearings which support the rotor.

BACKGROUND OF THE INVENTION Paper making stock is a suspension which typically includes liquid, desirable paper making pulp and undesirable particles. These undesirable particles are capable of interfering with the flow of the stock through the paper making system, or of causing defects in the paper sheet, if not removed before the paper making stock reaches the head box. A significant quantity of these particles will have specific gravities too similar to the specific gravity of desirable paper making pulp to be removed efficiently by centrifugal cleaning. Therefore, it is common practice in the paper making industry to install one or more pressure screens upstream of the head box.

A conventional pressure screen includes a screen cylinder or basket defining an axis, an inlet side and an accepts side; and a rotor mounting a plurality of foils for generating microturbulence near the inlet side of the screen cylinder. The screen cylinder and the rotor are positioned in a fluid-tight housing assembly. The housing assembly includes spaced inlet and rejects ports communicating with the inlet side of the screen cylinder, as well as an accepts port communicating with the accepts side of the screen cylinder. Typically, the housing assembly also includes bearings which support the rotor for driven rotation about the axis of the screen cylinder. A pressure screen separates, by size, "accepts," which ideally consist primarily of desirable paper making fibers, from "rejects," which ideally consist primarily of undesirable particles. For example, a screen cylinder for separating paper making fibers from larger undesirable particles will have openings sufficiently large to conduct the paper making fibers from the inlet side to the accepts side while impeding the passage of larger particles.

During a screening operation, the accepts are driven through the openings in the screen by a fluid pressure differential between the sides of the screen cylinder. The paper making stock enters the housing assembly through the inlet port and flows over the inlet side of the screen cylinder at a fluid pressure greater than the fluid pressure immediately adjacent the accepts side. As the paper making stock flows over the inlet side of the screen cylinder, the higher pressure on the inlet side drives the accepts, and a portion of the fluid, through the openings to the accepts side. The accepts, along with the portion of the fluid driven through the openings, exit through the accepts port. The rejects and the remaining fluid exit through the rejects port for reprocessing or disposal.

The microturbulence generated by the foils improves the efficiency of the screening process. During the pressure screening process, both paper making fibers and undesirable particles tend to accumulate over the openings through the screen cylinder so as to obstruct the movement of accepts through the openings. This can lead to incomplete separation of the desirable paper making fibers from the rejects, thereby increasing manufacturing costs through the loss of desirable paper making fibers or the necessity of additional separation steps to recover those fibers. The microturbulence generated by the foils breaks up the accumulations of fibers and other materials over the openings, thereby decreasing the likelihood that desirable paper making fibers will remain with the rejects.

In conventional pressure screens, the efficacy of the intended screening function decreases the further one proceeds downstream along the axially disposed screening path. This is because the rotational speed of the stock flow and the rotational speed of the foils tend to approach each other. Stated differently, high efficacy screening occurs along those locations of the screening path where a substantial difference exists between the foil speed and the stock speed. As the stock proceeds in the downstream direction along the screening path, its rotational speed approaches that of the foils, thereby resulting in screening inefficiencies. Priem U.S. Patent 1,537,691 proposes a centrifugal screen including a main frame, two perforated shells and two rotated runners having wings which throw the stock against the perforated shells by centrifugal action. The upstream perforated shell terminates in an enlarged housing section defining an overflow spout which leads, through a second inlet, to the interior of the downstream perforated shell. Schδn et al. U.S. Patent 4,356,085 proposes a rotary screening machine for pulp suspensions including an annular rotatable drum surrounded by an annular porous screen basket. Fig. 3 of Schόn et al. proposes a vertically-oriented rotary screening machine in which the downstream part of the annular rotatable drum is made of a larger diameter than the upper part of the drum. This construction provides a weir or damming wall to prevent the suspension from moving too quickly through the screen basket under the influence of gravity.

UK Patent 2 222 967 proposes a screening machine including a wire cage, a rotor drum and screening vanes mounted on the rotor drum. The wire cage includes a radial baffle ring. According to the reference, the baffle ring decreases the peripheral velocity of the fiber suspension so as to maintain a differential velocity of at least 5 m/sec between the screening vanes and the peripheral velocity components of the suspension.

French Application No. 2 613 390 proposes an improvement to screens used for the treatment of paper pulp in which the adjacent stages of the screen are connected by enlarged annular gutters, each having an upstream surface perpendicular to the axis of the screen and an inclined downstream surface. Each gutter is provided with a radial water inlet which opens through the bottom of the gutter. These water inlets inject water against the direction of the agitators. In French patent application 9902425, filed February 26, 1999, the problem pertaining to gradual reduction of the screening efficacy along the stock flow path is addressed. In this patent, areas of "high deflocculation and/or areas of considerable slowing of the pulp" are provided along the flow path of the stock along the screen. In one embodiment, a plurality of "chicane" passages is provided to slow the rotational speed of the stock as it flows downstream along the intended screening path. Stock flow is diverted by an annular partition that is carried by the rotor into one or more chicanes that are linked to the screen. This structure purportedly causes an increase in the relative speed between the pulp and the rotating vanes so as to enhance deflocculation of the stock.

In those instances in which the chicane and annular partition are provided, excessive back pressure upstream from'the partition/chicane interface causes undue axial thrust on the rotor, possibly leading to rotor bearing failure.

Therefore, a need in the art exists for improvement in the type of screens set forth in the aforementioned French Patent, and in other devices, wherein stock is diverted by an annular lip or the like into an enlarged chicane or other velocity retarding zone to increase deflocculation action. More specifically, in such devices, there is a need for the provision of mechanisms for decreasing the large pressure differential that may exist across these deflocculation zones.

SUMMARY OF THE INVENTION This need and others are addressed in a screen including a screen cylinder and a rotor. The screen cylinder defines an axis, an inlet side and an accepts side. The preferred screen cylinder also includes a flow velocity retarding zone, most preferably including an enlarged chamber positioned along the inlet side of the screen cylinder. The rotor preferably carries one or more foils near the inlet side of the screen and includes an annular impeller lip extending radially outwardly from the rotor to pump stock into the enlarged chamber.

In accordance with a preferred method, the stock suspension comprising liquid, paper making fibers and undesirable rejects is induced to flow against the inlet side of the screen cylinder, that is, most preferably, along an inner surface of the cylinder. The rotor is caused to rotate about the axis of the screen cylinder so that the foils induce microturbulence in the suspension near the inlet side. When the stock suspension reaches the velocity retarding zone, it is diverted radially outwardly, most preferably by the annular impeller lip into the enlarged chamber. As such the tangential velocity of the flow is slowed relative to the foil speed.

As noted earlier, one drawback to prior art pressure screens is that screening efficacy is diminished when the rotational speed of the stock approaches the rotational speed of the foils. The present invention tends to minimize this problem by slowing the rotational component of the flow. By slowing the rotational component or speed of the stock suspension relative to the angular speed of the rotor, screening efficiency is improved.

In an especially preferred form of the invention, a single stator tooth is positioned in the enlarged chamber to further retard the rotational component of the stock.

Often, as noted earlier, the axially directed thrust forces acting on the rotor bearings tend to wear or "freeze" these bearings. A pressure differential likely will develop across the velocity retarding zone. This pressure differential will manifest itself as an axial force acting on the annular impeller lip, which force will be conducted to the rotor bearings.

Preferably, to reduce the likelihood of additional wear on the rotor bearings, this axial force is partly offset through the step of bypassing a portion of the stock flow past the velocity retarding zone. More specifically, in one embodiment, the annular impeller lip includes one or more bypass channels extending from an upstream side of the annular lip to a downstream side. These bypass channels allow high pressure fluid from the upstream side of the bypass channels to flow directly to the downstream side without diversion into the velocity retarding zone, thereby at least partially helping to equalize the pressure differential across the velocity retarding zone. In accordance with this first embodiment, the one or more bypass channels extend in a direction oblique to the annular velocity retarding chamber with the upstream entrance and downstream exit from the bypass positioned to extend from entrance to exit in the opposite direction from the rotational direction of the rotor. This presents a negative angle of attack when the rotor is turned. In this orientation, the bypass channels tend to "scoop" or lift the stock suspension into the bypass channel, providing sufficient pressure to dislodge lightweight particles such as styrofoam and the like which might otherwise become lodged under the impeller lip.

Pressure screens in accordance with the invention may comprise a vertically-oriented, screen cylinder, a rotor positioned in the screen cylinder, a housing assembly surrounding the screen cylinder and at least one bearing rotatably supporting the rotor in the housing assembly. The screen cylinder defines an axis, an inwardly-facing inlet side and an outwardly-facing accepts side. It comprises at least two cylinder stages or sections affixed to a channeled ring, in the preferred embodiment, the channeled ring defining an enlarged chamber concave with respect to the inlet side and serves as the velocity retarding zone for the stock.

In one embodiment, the rotor includes a cylindrical drum mounting one or more foils near the inlet side of the screen cylinder and an annular lip aligned with the enlarged chamber. The annular impeller lip has an upstream side and a downstream side, the upstream side continuously flaring in a radially outward direction from an outer surface of the cylindrical drum and the downstream side having a substantially flat axial surface. It includes one or more bypass channels extending obliquely to the vertical axis from its upstream side to its downstream side. Other rotors such as open-ended cylinders and disk type rotors may be used. In these cases, the bypass channels need not be formed in the annular impeller lip, but fluid bypass can be provided through apertures or the like in the disk or through the open ended cylinder itself.

From the foregoing, it is one object of the invention to provide screening methods and apparatus in which the flow of the suspension in the screen cylinder is diverted radially outwardly into a flow velocity retarding zone so as to reduce the rotational speed of the flow. It is a another object of the invention to position a single stator tooth in the velocity retarding zone to further slow the rotational speed of the flow. It is yet another object of the invention to provide for a portion of the flow to bypass the velocity slow down or retarding zone so as to reduce the pressure differential across the zone, thereby reducing axial forces otherwise imposed on the bearings supporting the rotor by this pressure differential. Other objects of the invention will be apparent from the follow description, the accompanying drawings and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic perspective view of one embodiment of a pressure screen in accordance with the invention, in which a housing assembly and screen cylinder are cut away to show the configuration of a rotor, shaft and thrust therein;

Fig. 2 is a sectional view through a flow velocity retarding zone of the pressure screen, taken along the line 2-2 in Fig. 1;

Fig. 3 is a side elevation of another embodiment of the invention with certain parts omitted for clarity, showing an open-ended rotor configuration; Fig. 4 is a top plan view of the embodiment shown in Fig. 3; and

Fig. 5 is a perspective view of another rotor embodiment that may be used in accordance with the invention.

DETAILED DESCRIPTION Fig. 1 shows a vertical pressure screen 10 embodying the present invention. The pressure screen 10 comprises a screen cylinder 12 and a drum rotor 14 enclosed in a housing assembly 16. Preferably, the screen cylinder 12 defines at least one flow velocity retarding zone 18 capable of slowing the rotational component of a flow of paper making stock (not shown) within the screen cylinder 12. The drum rotor comprises a top housing, 7, bottom housing section 9 and circumferential, cylindrical housing 11.

The screen cylinder 12 is formed from an upstream cylinder section or stage 20 and a downstream section or stage 22, each of which is welded or otherwise affixed to a channeled ring 24. The cylinder stages 20, 22 together define an axis 30; a inwardly-facing inlet side 32; and an outwardly-facing accepts side 34. Each of the cylinder stages 20, 22 includes a plurality of small openings or holes (not shown) passing through the screen cylinder 12 from the inlet side 32 to the outlet side 34. Preferably, the openings are so dimensioned as to permit desirable paper making fibers (not shown) to move through the openings from the inlet side 32 to the accepts side 34 while impeding the movement of undesirable paiticles or rejects (not shown) through the openings. As shown, the channeled ring 24 includes a web 40, an upstream flange 42 and a downstream flange 44 which cooperate to define an enlarged chamber 46 which, in this embodiment, serves as the velocity retarding zone. The enlarged chamber 46 preferably has a maximum inner radius, measured from the axis 30 to the web 40, greater than a radius measured from the axis 30 to the inlet side 32 of the screen cylinder 12, so that the enlarged chamber 46 is concave with respect to the inlet side 32.

Most preferably, the enlarged chamber 46 includes a single stator tooth 48. As best shown in Fig. 2, the stator tooth 48 preferably takes the form of a substantially prismatic metal piece affixed in the channeled ring 24 so as to obstruct rotational flow in the chamber 46.

Returning to Fig. 1, each of the cylinder stages 20, 22 is preferably formed of a metal such as stainless steel. Techniques for forming screen cylinders for pressure screens are well known to those of ordinary skill in the art, and the particular technique used is not critical to the present invention. Additionally, it should be mentioned that although the described preferred apparatus comprises an "in to out" screen system wherein the stock flows along a path from a radially inward location to a radially outward location, the stock path and structural parts could easily be reversed so as to provide an "out to in" path. All such designs are within the ambit of this invention.

As shown in Fig. 1, the rotor 14 is a hollow cylindrical drum, most preferably formed of a metal such as stainless steel. The gap between the inlet side 32 of the screen cylinder 12 and an outer surface 50 of the preferred rotor 14 defines an annular inlet chamber 52. Techniques for forming rotors for pressure screens are well known to those of ordinary skill in the art, and the particular technique used is not critical to the present invention.

The rotor 14 carries one or more foils 60, 62, 64 and 66 (four shown in Fig. 1) which sweep the inlet chamber 52. The foils 60, 62, 64, 66 have smooth outer contours which are asymmetric along a radial direction relative to the axis 30 so as to enable the foils 60, 62, 64, 66 to generate microturbulence in paper making stock (not shown) flowing over the inlet side 32 of the screen cylinder 12. Techniques for the construction and mounting of foils such as those shown in Fig. 1 are well known to those of ordinary skill in the art and are not critical to the present invention. Likewise, while the rotor 14 shown in Fig. 1 mounts two sets of three foils, one set consisting of foils 60, 62, 64 and the other consisting of foil 66 and two others hidden from view; and while the foils within each set shown in Fig. 1 are symmetrically arranged about the axis 30, neither the number of foils nor their arrangement on the rotor 14 is critical to the present invention.

The rotor 14 carries an impeller, shown here in the form of an annular lip 70, which extends radially from the outer surface 11 of the rotor 14 in alignment with the enlarged chamber 46. In Fig. 1, an upstream side 72 of the preferred annular lip wall 70 is shown continuously flaring in a radially outward direction from the outer surface of the rotor 14, while a downstream side 74 of the annular lip 70 has a substantially flat axial surface. Although the foils 60, 62, 64, 66 and the annular lip 70 are preferably formed as separate parts and welded or otherwise affixed to the rotor 14, it is within the contemplation of the invention to fabricate the rotor 14; the foils 60, 62, 64, 66; and the annular lip 70 as a unit.

The housing assembly 16 shown in Fig. 1 is cylindrical in shape and provides a fluid-tight enclosure about the screen cylinder 12. The gap between the rejects side 34 of the screen cylinder 12 and the inner surface of the housing assembly 16 defines an annular accepts chamber 80. Techniques for forming housing assemblies for pressure screens are well known to those of ordinary skill in the art, and the particular technique used is not critical to the present invention.

As shown in Fig. 1, the cylinder stages 20, 22 may include structure 82, such as ribs, bars or rings, extending into the accepts chamber 80 (or, alternatively, into the inlet chamber 52) for added strength and flow control. The rotor 14 is preferably mounted on a shaft 90 supported, in turn, by one or more bearings, such as the rotor bearing 92 shown in Fig. 1, for rotation about the axis 30. The shaft 90 may be keyed into a hub or the like in the lower surface 9 of the drum.

A preferred method for screening a suspension (not shown) of stock suspension comprising liquid, paper making fibers and undesirable rejects to remove a substantial portion of the rejects therefrom includes the steps of (a) introducing a flow of the suspension over the inlet side 32 of the screen cylinder 12; (b) rotating the rotor 14 about the axis 30 to induce turbulence in the suspension near the inlet side 32; and (c) forwarding the flow of the suspension radially outwardly by the lip 70 into the flow velocity retarding zone 18.

More specifically, the step of introducing a flow of the paper making stock or suspension (not shown) over the inlet side 32 of the screen cylinder 12 preferably comprises pumping or otherwise injecting the suspension into the inlet chamber 52 under sufficient pressure to induce the suspension to flow through the inlet chamber 52. Most preferably, in a vertical pressure screen such as the pressure screen 10 shown in Fig. 1, the suspension enters the housing assembly 16 through an inlet port (not shown) near the lower end of the housing assembly 16 and flows through the housing assembly 16 into the lower portion of the inlet chamber 52. Under these conditions, it is desirable that the suspension enter the lower portion of the inlet chamber 52 under a pressure sufficiently greater than the sum of the pressure in the accepts chamber 80 and the head loss in the inlet chamber 52 to force desirable paper making fibers (not shown) and a portion of the liquid (not shown) in the stock suspension through the openings (not shown) in the screen cylinder 12 along the entire working length of the screen cylinder 12.

The step of rotating the rotor 14 about the axis 30 preferably comprises coupling the rotor 14 or the shaft 90 to a motor (not shown) and activating the motor. The rotor 14 shown in Fig. 1 preferably rotates in a clockwise direction. The foils 60, 62, 64, 66 are oriented on the rotor 14 so as to generate fluid pulses. The microturbulence resulting from the combination of these pulses serves to break up accumulations of paper making fibers and other materials (not shown) over the openings (not shown) in the screen cylinder 12. The step of forwarding the flow radially outwardly into the flow velocity retarding zone 18 comprises interposing the annular impeller lip 70 in the inlet chamber 52 so as to deflect or divert the flow of suspension through the inlet chamber radially outwardly into the enlarged chamber 46. This diversion slows the rotational component of the flow. It is preferred to provide structure such that the chamber 46 comprises a diameter that is about 20% greater than the inner diameter of the screen cylinder. The slowing of the rotational component of the flow improves the downstream screening efficiency of the screen 10. More specifically, the slowing of the rotational component of the flow in the flow velocity retarding zone 18 increases the rotational speed of the rotor 14 relative to the rotational component of the flow. This increases the efficacy of the screening action leading to enhanced breaking or deflocculation of large agglomerates. In turn, screening efficiency is made more uniform along the axial length of the screen cylinder 12.

On the other hand, the step of directing the stock into the zone 18 induces an axial force on the annular lip 70, which would be directed vertically upwardly in the embodiment 10 of Fig. 1. This axial force, which is conveyed to the bearing 92 through the rotor 14 and the shaft 90, aggravates wear in the bearing 92 and thereby decreases the useful life of the bearing.

The method in accordance with the invention includes the step of bypassing a portion of the flow past the flow velocity retarding zone 18 to reduce the pressure differential across the zone 18. In accordance with the embodiment shown in Figs. 1 and 2, the annular impeller lip 70 includes one or more bypass channels 100 communicating between its upstream side 72 and its downstream side 74.

Preferably, as shown in Fig. 1, the bypass channels 100 extend in a direction oblique to the direction of the axis 30 so as to present a negative angle of attack to the suspension (not shown) as the rotor 14 is rotated. More specifically, the bypass channel 100 shown in Fig. 1 slopes upwardly (i.e. from upstream entrance to downstream exit) through the annular lip 70 in a counterclockwise direction, so that clockwise rotation of the rotor 14 tends to lift a portion of the suspension into the bypass channel 100. This action serves to dislodge lightweight particles which may become lodged in the bypass channel 100 and impede the equalization of pressure across the zone 18.

Fig. 2 illustrates a symmetrical arrangement of three bypass channels 100 through the annular impeller lip 70 around the axis 30 of the concentrically arranged screen cylinder 12, rotor 14 and housing assembly 16. The number and arrangement of the bypass channels 100 is not critical to the present invention.

As suggested by Fig. 2, the bypass channels 100 preferably ensure that a portion of the stock suspension bypasses the flow velocity retarding zone 18 (Fig. 1). The flow through the bypass channels 100 serve to equalize the pressure between the upstream and downstream sides 72, 74 (Fig. 1) of the annular lip 70, and reduce the resulting axial reaction force on the thrust bearing 92 (Fig. 1).

Therefore, in one embodiment, the pressure screen 10 and the method of the present invention serves to reduce the rotational component of the flow downstream in the flow velocity retarding zone 18 by means of a radially outward diversion of the flow into an enlarged chamber. The positioning of a single stator tooth 48 in the zone 18 further slows the rotational component of the flow. At the same time, inducing a portion of the flow to bypass the flow velocity retarding zone 18 serves to reduce the pressure differential or head loss across the zone, thereby reducing the axial forces imposed on the bearing 92 supporting the rotor 14 so as to extend the useful life of the bearing 92.

Preliminary data suggest that the embodiment shown in Figs. 1 and 2 will be highly effective in the screening of chemical pulp and some recycle pulps. For other pulps such as recycles, deinked, OCC 's etc., preliminary observations indicate that the embodiments shown in Figs. 3-5 may be most effective.

Turning first to Figs. 3 and 4, there is shown an open ended, cylindrical rotor 210. Four foils 266 a, b, c, d are carried by the rotor and affixed thereto by conventional means such as threaded studs and bolts or by welded support lugs. Similar to the embodiment shown in Figs 1 and 2, an annular impeller lip 270 is fixed to the outer circumference of the rotor and serves to propel stock into the enlarged chamber 246 (shown only schematically in the left hand side of Fig. 3) to slow the tangential velocity of the stock.

In the embodiment of Figs. 3 and 4, the bypass channel 300 is defined by the cylindrical wall of the rotor 210. That is, stock may bypass the enlarged chamber 246 simply by passing through the interior of the open ended rotor cylinder. Similarly, and with regard to Fig. 5, a disk like rotor 610 may also be used. As shown, the rotor carries four vanes 666a, b, c, d as is conventional in the art. The rotor may be attached to drive shaft 98 through keyed hub 692. Cut-outs 600a, b, c as shown in the drawing may be formed through the disk rotor to serve as stock flow bypass channels. As shown, impeller lip 670 surrounds the circumference of the propellers and the disk and is adapted to pump pulp stock into an enlarged chamber associated with the screen cylinder (such as the chamber 62 shown in Fig.

1).

While the method herein described, and the form of apparatus for carrying this method into effect, constitutes a preferred embodiment of this invention, it is to be understood that the invention is not limited to this precise method and form of apparatus, and that changes may be made in either without departing fi-om the scope of the invention, which is defined in the appended claims.

What is claimed is:

Claims

1. In a screen cylinder including a central axis, an inlet side, an outlet side, a flow velocity retarding zone, and a rotor carrying foils near said inlet side, a method for screening a stock suspension of liquid, paper making fibers and undesirable rejects to remove a portion of the rejects therefrom, said method comprising the steps of:

(a) introducing a flow of said stock suspension against said inlet side of said screen cylinder;

(b) rotating said rotor about said axis to create turbulence in said stock suspension near said inlet side of said screen cylinder; (c) directing a portion of said stock suspension to flow into said flow velocity retarding zone; and

(d) directing another portion of said stock suspension to bypass said flow velocity retarding zone.

2. The method as recited in claim 1 wherein said step (d) includes bypassing the portion of the flow past said flow velocity retarding zone along a direction oblique to said axis.

3. The method as recited in claim 1 including the additional step of (e) positioning a stator tooth in said flow velocity retarding zone.

4. The method as recited in claim 1 wherein said flow velocity retarding zone includes an enlarged chamber concave with respect to said inlet side and said rotor carries an annular lip aligned with said enlarged chamber for directing said suspension into said enlarged chamber.

5. The method as recited in claim 4 wherein said annular lip includes at least one bypass channel formed therein; said step (c) includes impinging said portion of said flow against said annular lip so as to direct said flow into said enlarged chamber; and said step (d) includes allowing said another portion of said flow to pass through said at least one bypass channel.

6. The method as recited in claim 4 wherein said rotor comprises an open ended cylinder, said step (c) comprises impinging said portion of said flow against said annular lip so as to direct said flow into said enlarged chamber and said step (d) includes allowing said another portion of stock suspension to flow through said open ended cylinder.

7. The method as recited in claim 4 wherein said rotor is a disk rotor having bypass channels therethrough, said step (c) comprising impinging said portion of said flow against said annular lip so as to direct said flow into said enlarged chamber and said step (d) includes allowing said another portion of stock suspension to flow through said bypass channels formed in said disk.

8. In a screen having a screen cylinder defining an axis, an inlet side, an accepts side and a flow velocity retarding zone; a rotor carrying foils near said inlet side of said screen cylinder; and at least one bearing axially and rotatably supporting said rotor on said axis, a method for reducing an axial load on said bearing while screening a stock suspension of liquid, paper making fibers and undesirable rejects to remove a substantial portion of the rejects therefrom, said method comprising the steps of:

(a) providing a flow of said stock suspension against said inlet side of said screen cylinder; (b) rotating said rotor about said axis to induce turbulence in said suspension near said inlet side of said screen cylinder;

(c) directing a portion of said stock suspension to flow into said flow velocity retarding zone; and

(d) directing another portion of said stock suspension to bypass said flow velocity retarding zone to reduce a pressure differential across said flow velocity retarding zone.

9. The method as recited in claim 8 wherein said step (d) includes bypassing said portion of the flow past said flow velocity retarding zone along a direction oblique to said axial direction.

10. The method as recited in claim 8 wherein said flow velocity retarding zone includes an annular chamber concave with respect to said inlet side; said rotor carries an annular lip aligned with said annular chamber for directing said suspension into said annular chamber, said annular lip including at least one bypass channel; said step (c) includes impinging said portion of said stock suspension against said annular lip so as to direct said portion into said annular chamber.

11. The method as recited in claim 8 wherein said flow velocity retarding zone includes an annular chamber concave with respect to said inlet side, said rotor comprises a disk carrying an annular lip aligned with said annular chamber for directing said suspension into said annular chamber, said disk being provided with at least one pass channel therein, said step (c) includes impinging said portion of said stock suspension against said lip so as to direct said portion into said annular chamber, and said step (d) includes allowing another portion of said stock suspension to pass through said at least one bypass channel formed in said disk.

12. In a screen including a screen cylinder defining an axis, an inlet side, an accepts side and a flow velocity retarding zone; and a rotor carrying foils near said inlet side of said screen cylinder, a method for screening a stock suspension of liquid, paper making fibers and undesirable rejects to remove a portion of the rejects therefrom, said method comprising the steps of:

(a) providing a flow of said stock suspension against said inlet side of said screen cylinder;

(b) rotating said rotor about said axis to induce turbulence in said stock suspension near said inlet side of said screen cylinder; (c) directing a portion of said stock suspension to flow radially outwardly into said flow velocity retarding zone; and

(d) further retarding the speed of said portion of said stock suspension in said flow velocity retarding zone by positioning a single stator tooth in said flow velocity retarding zone.

13. The method as recited in claim 12 wherein said flow velocity retarding zone includes an annular chamber concave with respect to said inlet side and said rotor carrying an annular lip aligned with said annular chamber for directing said portion of said stock suspension into said annular chamber.

14. In a pressure screen having a screen cylinder defining an axis, an inlet side, an accepts side, a flow velocity retarding zone comprising an annular chamber associated with said inlet side; a rotor positioned in said screen cylinder, said rotor mounting foils near said inlet side of said screen cylinder, a propeller lip adjacent said flow velocity retarding zone, at least one bypass channel communicating with said inlet side, and at least one bearing axially and rotatably supporting said rotor on said axis, a method for reducing an axial load on said bearing while screening a stock suspension of liquid, paper making fibers and undesirable rejects to remove a portion of the rejects therefrom, said method comprising the steps of: (a) providing a flow of said stock suspension against said inlet side of said screen cylinder;

(b) rotating said rotor about said axis to induce turbulence in said stock suspension near said inlet side of said screen cylinder;

(c) directing a portion of said stock suspension to flow against said propeller lip so as to direct said flow into said annular chamber; and

(d) directing another portion of said stock suspension to pass through said at least one bypass channel to reduce a pressure differential across said flow velocity retarding zone.

15. A method as recited in claim 14 further comprising the step (e) of positioning a stator tooth in said annular chamber

16. A pressure screen for screening a stock suspension of liquid, paper making fibers and undesirable rejects to remove a portion of the rejects therefrom, said screening apparatus comprising:

(a) a screen cylinder defining an axis, an inlet side, and an accepts side; (b) a rotor mounting foils near said inlet side of said screen cylinder;

(c) a flow velocity retarding zone positioned along said inlet side;

(d) an impeller aligned with said flow velocity zone and adapted to direct a portion of said stock suspension into said flow velocity retarding zone; and

(e) at least one bypass channel communicating with said inlet side adapted to allow another portion of said stock suspension to bypass said flow velocity retarding chamber.

17. The pressure screen as recited in claim 16 wherein said flow velocity retarding zone comprises an annular chamber communicating with said inlet side.

18. The pressure screen as recited in claim 16 wherein said screen cylinder includes at least two cylinder stages affixed to a channeled ring, said channeled ring having a diameter greater than said two cylinder stages and defining said flow retarding chamber.

19. The pressure screen as recited in claim 18 wherein said rotor includes a cylindrical drum mounted on a shaft, said impeller comprising an annular lip carried by said rotor and wherein said lip includes said at least one bypass channel (e) extending through said lip.

20. The pressure screen as recited in claim 16 wherein said rotor comprises an open ended cylinder, said open ended cylinder including said at least one bypass channel (e) therein, and wherein said impeller is carried by said rotor.

21. The pressure screen as recited in claim 16 wherein said rotor comprises a disk carrying said impeller, said disk including said at least one bypass channel (e) formed therein.

22. A pressure screen for screening a stock suspension of liquid, paper making fibers and undesirable rejects to remove a substantial portion of the rejects therefrom, said screening apparatus comprising:

(a) a screen cylinder defining an axis, an inlet side, and an accepts side, said screen cylinder including an annular chamber concave with respect to said inlet side;

(b) a rotor mounting foils near said inlet side of said screen cylinder for generating turbulence near said inlet side, said rotor mounting an annular lip aligned with said annular chamber and adapted to direct said stock suspension radially outwardly into said annular chamber; and

(c) a stator tooth positioned in said annular chamber.

23. The pressure screen as recited in claim 22 wherein said screen cylinder includes at least two cylinder stages affixed to a channeled ring, said channeled ring defining said annular chamber.

24. The pressure screen as recited in claim 22 wherein said rotor includes a cylindrical drum and said annular lip has an upstream side and a downstream side, said upstream side continuously flaring in a radially outward direction from an outer surface of said rotor and said downstream side having a substantially flat axial surface.

25. A pressure screen for screening a stock suspension of liquid, paper making fibers and undesirable rejects to remove a portion of the rejects therefrom, said screening apparatus comprising:

(a) a vertically-oriented screen cylinder defining a vertical axis, an inlet side, and an accepts side, said screen cylinder including at least two cylinder stages affixed to a channeled ring, said channeled ring defining an annular flow retarding chamber concave with respect to said inlet side;

(b) a stator tooth positioned in said annular flow retarding chamber; (c) a rotor positioned in said screening cylinder, said rotor including foils near said inlet side of said screen cylinder and an annular lip aligned with said annular flow retarding chamber;

(d) at least one bypass channel communicating with said inlet side and adapted to direct flow of a portion of said stock suspension to bypass said flow retarding chamber;

(e) a housing assembly surrounding said screen cylinder; and

(f) at least one bearing rotatably supporting said shaft in said housing assembly.

26. A pressure screen for screening a stock suspension of liquid, paper making fibers and undesirable rejects to remove a portion of the rejects therefrom, said screening apparatus comprising:

(a) a screen cylinder defining an axis, an inlet side and an accepts side;

(b) a rotor carrying foil members thereon located proximate said inlet side of said screen cylinder;

(c) a flow retarding zone communicating with said screen cylinder; (d) means for directing a portion of said stock suspension into said flow retarding zone; and

(e) means for directing another portion of said stock suspension to bypass said flow retarding zone.