WO2000065151A1

WO2000065151A1 - Edge wire contour with enhanced functionality

Info

Publication number: WO2000065151A1
Application number: PCT/US2000/011071
Authority: WO
Inventors: Frey A. Frejborg
Original assignee: Cae Screenplates, Inc.
Priority date: 1999-04-27
Filing date: 2000-04-26
Publication date: 2000-11-02
Also published as: WO2000065151A8

Abstract

A screen cylinder (125) formed from discrete elements (128) (a wedge wire type screen cylinder) is constructed to accurately simulate the form of a substantially sharp edge milled screen cylinder having substantially the same contour. The simulation is preferably accomplished by providing at the transition between the upstream and downstream surfaces (132, 133) of any particular (128) bar or wire a portion (44) substantially parallel to the tangential direction (14) of movement of the cylinder or slurry moving therepast, and a substantially sharp edge (45) between the upstream surface and the transition. Protuberances (38, 40) defining the slot (130) are also preferably provided on both the upstream and downstream bars or wires, the protuberances (38, 40) having substantially parallel surfaces (41, 42) that are substantially co-extensive and are at least as long as the slot is wide, and effect a plug phenomena during the negative pulse phase. A substantially sharp edge (39) and substantially rectangular cross section protuberance (38) may extend outwardly from the upstream surface of each bar or wire defining a screen slot. A cylinder is used for screening cellulose pulp slurries, and has a debris removal efficiency at least about 10 % greater than a comparable wedge wire screen cylinder without sharp edge protuberances.

Description

EDGE WIRE CONTOUR WITH ENHANCED FUNCTIONALITY

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon provisional application Serial No. 60/131 ,141 , filed April 27, 1999, the disclosure of which is incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

There are two main types of screen cylinders with slotted apertures that are used for screening cellulosic fibrous material pulp, in the pulp and paper industry, milled screen cylinders, and screen cylinders fabricated from discrete elements, such as bars or wires. The screen cylinders formed from discrete elements, including wedge wire screen cylinders, are known to have higher capacity than milled screen cylinders because there is more potential open area. However, it has been recognized that even for screen cylinders having substantially the same contour, those formed of discrete elements have the significantly lower debris removal efficiency. For example, in one test conducted between a discrete element cylinder (a wedge wire cylinder) and a milled cylinder, which had similar configurations and were manufactured by the same company, the milled cylinder had a debris removal efficiency of over 77%, while the wedge wire cylinder had a debris removal efficiency of about 40%, using the same furnish.

According to the present invention it has been recognized for the first time that a major difference between the debris removal efficiency of wedge wire screen cylinders and milled screen cylinders has to do with the fact that the earlier-perceived minor differences - - which are provided for ease of construction -- of the bars or wires used in the wedge wire construction compared to their counterparts in a milled screen cylinder, have a very substantial affect on debris removal efficiency. However, it has also been recognized according to the present invention that by forming the bars or wires in a particular manner, the wedge wire screen cylinders may more accurately simulate the milled screen cylinders, and therefore have an increased debris removal efficiency, e.g. increased at least about 10% compared to a conventional wedge wire cylinder of substantially the same construction.

In conventional wedge wire cylinders rounded edges, particularly at the top of each bar or wire, as a result of the Coanda effect, allow unwanted debris to be induced through the slots between the bars or wires. The provision of a sharp edge destroys the Coanda effect, making it more difficult for debris to enter the slots because the debris wants to keep on going straight once it leaves the flat sharp edge instead of following a curve sur ace which would direct or induce the debris into the slot, thus exhibiting the Coanda effect. In one test that was done, a wedge wire screen cylinder having 0.15 mm slots on approximately 3.2 mm slot spacing was compared to a milled cylinder with the same slot size and spacing. The wedge wire cylinder had continuous slots and 1.63 dm² open area and the milled cylinder had 1.28 dm² open area. The pressure screen in which the test was performed had a rotor with four 105 mm wide foils, operating at a tip speed of about 19 m/sec, giving a time for a negative pulse in one slot of about .2 thousandths of a second. The wedge wire cylinder was found to have about 50% more shives in the accepts than the milled cylinder. Also, a wedge wire screen cylinder typically has about a 15% higher power consumption.

Another (besides the Coanda effect) reason for the poorer performance of the wedge wire screen cylinders compared to milled screen cylinders, as indicated above, is the shape of the relief portion of the cylinders. The instantaneous negative pulse in a milled cylinder more or less forms a fiber plug in the relief groove (opening), which prevents an excessive volume of reverse flow. This "plug phenomena" does not occur in a conventional wedge wire construction because of the shape of the relief opening. Therefore, a conventional wedge wire cylinder allows an excessive reverse flow volume (water and fibers) to re-enter the feed side. When pushed back into the accept side by the pressure drop the reverse flow causes an increase in absolute accept flow (passing velocity), which pulls more debris through the slots into the accepts.

According to the present invention, a wedge wire screen cylinder construction is provided that significantly mitigates or substantially eliminates the comparative differences between wedge wire and milled cylinders related to the Coanda effect and the plug phenomena.

According to one specific aspect of the present invention a screen cylinder formed from discrete elements, as opposed to being milled, is provided which comprises the following components: A screen cylinder frame having a screen surface. The screen surface comprising a plurality of bars or wires mounted to form screening slots between the bars and wires. Each screening slot defined by an upstream bar or wire surface of one bar or wire and a downstream surface of another bar or wire, cooperating to define the slot. And at least one of the upstream bar or wire surface and associated downstream bar or wire surface being contoured to define the slot in a manner which simulates the form of a substantially sharp edge milled screen cylinder with substantially the same contour.

The upstream surface may comprise a substantially sharp edge substantially rectangular cross section protuberance extending outwardly from the upstream surface toward the immediately adjacent downstream bar wire, the screening slot being defined between the protuberance and the downstream bar wire. The upstream bar or wire element preferably has a flat surface substantially parallel to the envelope surface ending at a sharp angle substantially perpendicular to the envelope surface. This substantially perpendicular surface forms the upstream side plane of the Contoured Profile™ as shown in U.S. patent 4,529,520. The height or depth of the side plane surface will depend upon the contour depth necessary to induce the pulses or turbulence required by the screening process.

The leading edge of the upstream bar has a very small radius connecting 10 a substantially straight surface, which are at an angle of 5 to 60° from the envelope surface. This angle surface becomes the inclined surface of the proceeding contour. From the angled surface the wire or bar has a slight taper down to the bottom of the wire, which has a radius, or curved bottom connecting to the side plane surface forming a wedge shape form except having a rounded wedge instead of a sharp point. Again the overall wire height and thickness will depend upon both the mechanical strength and contour depth required by the process. The downstream bar or wire has substantially the same shape and is placed substantially parallel to the upstream bar with the inclined surface facing the side plane of the upstream bar The spacing or gap between the two substantially parallel bars forms an open slot, which is the barrier for the debris or contaminants within the stream of fibers

Support members hold :he substantially parallel contoured bars cr wires in place by either resistance welding, fillet welding, or by mechanical locking of bars or wires into predetermined interference openings or fittings After the contoured bars or wires are held in place the flat leading surfaces essentially parallel to the envelope surface can be machined either when the plate is in a flat state before roiling into cylinder or after rolling and machining either the I.D. or O.D of the cylinder depending if it is an outward or inward flow cylinder Machining the flat surface also has the advantage of controlling the contour depth, which is a very important variable in screening performance (capacity screening efficiency fractionation of long fibers, reject concentration, etc ) Machining the flat surface and controlling the contour depth eliminates the number of wire or bar shapes necessary for varying the contour depth for different screening applications Another advantage with this performance improving grinding/machining operation of drawn wires is the possibility to not only modify the top of the wire contour but also provide wedge wire cylinder contours of different heights U S patent 5,624,558 (incorporated by reference herein) discusses the advantages of varying heights (typically in a repeating pattern) in different axial sections of screen cylinders and how this technology can optimize the performance of screen cylinders This grinding/machining method for providing different contour heights in the same wire, which represents the full axial length of a screen cylinder, cannot easily be duplicated with conventional cold-drawing equipment

The downstream bar wire also may include a protuberance having a substantially right triangle configuration with a leg of the right triangle substantially paralleling the upstream surface protuberance to define the screening slot Also, each bar or wire may include a transition between the upstream and downstream surfaces thereof, and the transition may include a portion substantially parallel to the tangential direction of movement of the cylinder or slurry moving therepast, and a substantially sharp edge between the upstream surface and the transition The contour of the bars or wires substantially simulating the contour of a milled cylinder increases the debris removal efficiency of the discrete element cylinder at least about 10% compared to the same screen cylinder without the milled cylinder contour simulation, substantially by effecting a plug phenomena that is similar to that in a milled cylinder.

According to another aspect of the present invention a screen cylinder formed from discrete elements as opposed to being milled, which moves in a path or has slurry moving therepast in a path, is provided comprising the following components: A screen cylinder frame having a slotted screen surface. A plurality of substantially parallel bars or wires mounted to define screening slots therebetween. Each bar or wire comprising a downstream surface having a slope making an angle of between about 5-60° with respect to the tangential direction of movement of the cylinder or slurry moving therepast over at least the majority of the extent thereof, and an upstream side plane making an angle of between about 70-95 ^D with respect to the tangential direction of movement or slurry moving therepast. Each downstream surface comprising a closest point, which is closest to a the slot defined thereby, at a point most remote from the slot. And a substantially sharp edge protuberance extending outwardly from the upstream surface of each of the bars or wires adjacent the closest point of an immediately adjacent bar or wire, the protuberance and the closest point defining the screening slot therebetween.

In the screening cylinder of the invention, each downstream surface may include a protuberance adjacent the closest point, which protuberance substantially effects a plug phenomena during negative pulsing (the negative pulse phase). For example, the protuberance is one having a substantially right triangle configuration with a leg of the right triangle substantially paralleling the upstream surface protuberance to provide another substantially sharp edge to define the screening slot. The transition between the upstream and downstream surfaces is preferably also as described above. The upstream protuberance may comprise a substantially rectangular protuberance. Alternatively, the upstream surface may comprise a continuation of a substantially sharp edge at the top of the bar or wire and extending past where the slot forms to the same depth that the downstream surface protuberance extends, to define substantially parallel slot walls having a length dimension at least as great as the width of the slot. The downstream protuberance may simulate a general triangle, isosceles triangle, or an equi-lateral triangle, and need not have a bottom-most point of the continuation of the downstream surface that is a substantially sharp edge. The protuberances and the cooperation of the protuberances with the closest point increases the debris removal efficiency of the cylinder at least about 10% compared to if the protuberances and the cooperation thereof with the closest points were net present in an otherwise identical cylinder.

According to another aspect of the present invention, a method of utilizing the screen cylinder such as described above is provided. The method comprises: (a) Causing a slurry of comminuted cellulosic fibrous material (e.g. at a consistency of about .5-5%) to flow with respect to the screen cylinder (e.g. at an average velocity of about 1.5-2.0 meters/second when passing the slots) so that a slurry flows first past the downstream surface of any particular bar or wire, and then past the upstream surface of that particular bar or wire, so that turbulence is formed adjacent the slot between the upstream and downstream surfaces, and so that the Coanda effect at the slot is substantially avoided. (b) Causing accepts to be drawn through the slots. And (c) causing rejects to continue to flow with the slurry and to ultimately be discharged from adjacent the screen cylinder. The method may also comprise (d) providing a plug phenomena at the relief opening during negative pulsing.

Typically, (a), (c), and/or (d) are practiced so as to increase the debris removal efficiency of the screen cylinder at least about 10% (e.g. at least about 15%, and often about 20% or more) compared to a substantially identical cylinder, including slot width, without the protuberances and associated closest points. In one embodiment of the invention (a) is practiced by causing the slurry to flow (powered by a rotor or foil) while the screen cylinder remains substantially stationary. In another embodiment (a) is practiced at least in part by causing the cylinder to rotate and having the foils or pulse protrusions stationary.

It is the primary object of the present invention to provide a wedge wire type screen cylinder having enhanced debris removal efficiency without sacrificing the inherent capacity and other advantages of wedge wire screen cylinders. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

FIGURES 1 and 2 are an enlarged schematic cross sectional views of an exemplary milled cylinder contour, and wedge wire contour, respectively, in the prior art,

FIGURES 3A-3C are views like that of FIGURE 2 only showing the contour of a wedge wire screen cylinder according to three different embodiments of the invention

FIGURE 4 is a schematic top perspective view of an exemplary screen cylinder according to the invention, and

FIGURE 5 is a longitudinal cross sectional view showing the screen cylinder of the invention in a pressure screen for normal operation

DETAILED DESCRIPTION O^ THE DRAWINGS

FIGURE 1 schematically illustrates a cross sectional configuration (greatly enlarged for clarity of illustration) of an exemplary high debris efficiency removal screen cylinder contour, such as shown in U S patents 4,529,520, 5 524,770, and 5,607,589, the disclosures of which are incorporated by reference herein Commercial versions of this screen cylinder are sold by CAE ScreenPlates Inc of Glens Falls, New York under the trademark "PROFILE®" The screen cylinder contour illustrated in FIGURE 1 is milled into a piece of metal, and comprises a cylinder body 10 having a screening surface 1 and an accepts surface 12 Normally the surface 11 is on the interior of the cylinder 10 but may be on the exterior The screening surface 1 1 comprises a plurality of grooves 13 provided in repeating patterns along the surface 1 1 and preferably substantially completely covering the screening surface 1 The grooves 13 are substantially transverse to the genera! tangential flow direction 14 of cellulose pulp being screened The grooves 13 may extend substantially the entire length (height) of the cylinder 10 or more typically are interrupted at various points along the cylinder 10 by load bearing bands

Each of the grooves 13 is formed by an upstream (with respect to the tangential direction of pulp flow 14) surface 15, and a downstream surface 16 The surface 15 is substantially perpendicular to the flow direction 14 (e g preferably has an angle α of between about 70-1 10^;, e.g. about 85°). and the downstream surface 16 is sloped over at least a majority of the extent thereof, preferably having an angle β of between about 5-60° (e.g. aoout 15~-30^cj A screening slot 17 is defined between the surfaces 15, 16, opening up into an enlarged opening (relief slot, groove, or opening) 18 providing communication between the surfaces 11 , 12 The screening slots 17 have the width thereof as the critical dimension, that is a dimension parallel to the general flow path 14; typical widths for the slots 17 are .002-.024 inches A slot 17 may take up the entire transition of the surfaces 15, 16 to each other at the bottom of the groove 13, or a substantially flat (not sloped) continuation of the downstream surface 16 may be provided in which the groove 17 is formed. In any event, each of the grooves 17 has a substantially sharp edge portion 19 at the upstream edge of the screening slot 17, and another substantially sharp edge portion 20 at the downstream edge of the screening slot 17

Further, the surface 1 1 typically is also defined by an upper transition 21 between adjacent surfaces 15, 16. The transition 21 preferably includes a portion substantially parallel to the tangential flow direction 14 of the pulp, and also includes a substantially sharp edge 22. By "substantially sharp edge" is meant an edge having no radius of curvature, or a radius of curvature of less than about 3 mm, and one which avoids the Coanda effect The configuration of the relief opening 18 (particularly at the portion 18' thereof) provides a plug phenomena preventing excess reverse flow of fibers during negative pulsing

In the operation of the screen cylinder 10, either the screen cylinder is rotateα so that it moves in the tangential direction 14, or the screen cylinder 10 remains fixed while the pulp is moved in the tangential direction 14 (e g with a conventional rotor) The typical desired velocity of the pulp as it passes the slots 17 is aoout 1 5-2 0 m/sec In any event pulp enters each of the grooves 13 and because of the contour thereof, the pulp is subjected to micro turbulence, so that a very high percentage of the debris in the pulD flowing in direction 14 is removed that is precluded from passing through the slot 17, while the desired pulp fibers do pass through the slot 17 Utilizing the milled screen cylinder 10 as illustrated in FIGURE 1 , it is not unusual to get debris removal efficiencies of 75% or greater FIGURE 2 is a view like that of FIGURE 1 only showing a conventional wedge wire screen cylinder, shown generally by reference numeral 25, and including a screen cylinder frame 26 and having a screen surface shown generally by reference numeral 27. The wedge wire screen cylinder 25 is formed of a plurality of discrete elements, as opposed to the milled configuration for the screen 10. That is, the screen surface 27 is defined by a plurality of metal bars or wires 28, which are adhesively, by mechanical locking, by welding, or a combination thereof, attached to the frame 26. The frame 26 comprises a plurality of widely spaced rings. The bars or wires are cut to axial cylinder lengths in mechanical locking or welded designs and when of resistance welded wedge wire style, the wires are continuous and wrapped around supports 26. Both of these techniques are conventional.

Like the milled screen cylinder 10, the general contour of the bars or wires 28 simulates a plurality of grooves 29, slots 30 opening up into a wide volume 31 , with the slots 30 defined between substantially perpendicular (i.e. angle a about 70-110°) upstream surface 32, and a downstream surface 33 having an angle β of about 5-60°, in both cases the angles α, β being measured with respect to the tangential pulp flow direction 14.

While the wedge wire screen cylinder 25 attempts to generally simulate the milled contour of screen cylinder 10, because of the configuration of the bars or wires 28 typically used, and their mounting in the rings 26, there are no substantially sharp edges, such as provided at 19, 20, and 22 in the milled screen 10 of FIGURE 1. Rather, the upstream surface 32 is substantially continuously and slightly curved or flat, extending from the groove simulation 29 into the open area 31 , while the edges 34, 35 at the transitions between adjacent surfaces 32, 33 at both the closest point to the slot 30 and the furthest point from the slot 30 are typically rounded, e.g. not substantially sharp edges. Because of this configuration the screen cylinder 25 suffers from the Coanda effect, allowing a greater amount of debris than desired to enter slots 30. Even if a substantially sharp edge is provided, however, such as schematically illustrated in U.S. patent 5,255,790, no substantially flat surface (compared to the tangential movement of the pulp 14) is provided at the transition 35. Despite the fact that the conventional wedge wire cylinders 25 have similar contour to the milled screen cylinders 10, and have a greater capacity, the debris removal efficiency of the wedge wire cylinders 25 (at comparable operating conditions) is much lower than for the milled cylinders 10 For example, in one test in which a milled cylinder 10 and a wedge wire cylinder 25 made by the same company and having similar contours (such as schematically illustrated in FIGURES 1 and 2) were tested using substantially the same furnish and other relevant conditions, the milled cylinder 10 had a debris removal efficiency of over 77%, while the wedge wire screen cylinder 25 had a debris removal efficiency of about 40% It has been recognized according to the present invention that a very significant reason for the wide differences in debris removal efficiency between milled cylinders 10 and wedge wire cylinders 25 is because of the lack of substantially sharp edges at critical locations In order to enhance debris removal efficiency of the wedge wire screen cylinder configuration (e g enhance the debris removal efficiency by at least about 10%), according to the present invention very simple changes are made in the configuration of the bars or wires 28 of the wedge wire cylinder 25

FIGURE 3A is a view like that of FIGURE 2 only showing a wedge wire screen cylinder 125 according to the present invention In FIGURE 3A components comparable to FIGURE 2 are shown by same reference numeral only preceded by a "1 " As can be seen by a comparison of the construction of FIGURE 3A to that of FIGURE 2 and then the construction of FIGURE 3 to that of FIGURE 1 the construction of FIGURE 3A much more accurately simulates the contour and configuration of the milled screen cylinder 10 than does the wedge wire cylinder 25

A very significant difference between the screen cylinder 125 compareα to the cylinder 25 is the prevision of an elongated substantially sharp edge protuberance 38 on the upstream surface 132 of each bar or wire 128 the protuberance 38 defining -- with the closest point 132 of the downstream surface 133 - tne slot 130 That is the protuberance 38 has a substantially sharp edge 39 which greatly increases micro turbulence and therefore increases debris removal efficiency In the preferred embodiment illustrated in FIGURE 3A, the protuberance 38 is substantially triangular in cross section, extending outwardly from the upstream surface 132 toward the immediately adjacent downstream (in the direction 14) bar or wire 128 with the screen slot 130 defined between the protuberance 38 -- particularly the substantially sharp edge 39 thereof -- and the immediately adjacent downstream bar or wire 128. The slot 130 may have a width between about .002-.024 inches, e.g. .004-.010 inches, or about 0.10-0.15 mm. The main function of the protuberance 38 on the upstream surface 139 in FIGURE

3A is to add an additional hindrance (for debris or shives/stiff fibers) along the otherwise almost straight surface 42 in FIGURE 3B. The protuberance 38 is also helpful in fractionation applications where the objective is to separate long and stiff fibers from short fibers. The debris removal efficiency can be enhanced even more, according to the screen cylinder formed from discrete elements 125 according to the invention, by also providing a protuberance 40 cooperating with the protuberance 38 to define the screening slot 130. The protuberance 40 changes the radius of curvature of the edge 134 so that it becomes a substantially sharp edge. Preferably, the protuberance 40 -- as illustrated in FIGURE 3A -- has a substantially right triangle configuration (in cross section) with a slot-defining surface 41 substantially paralleling the outermost slot defining surface 42 of the protuberance 38, so that essentially the slot 130 is defined between the protuberances 38, 40.

In order to increase the debris removal efficiency even more, according to the invention the screen cylinder 125 is provided at the transition 135 between the surfaces 132, 133 (that is the furthest point from the slot 130 in a direction perpendicular to the tangential flow direction 14) with a short substantially planar surface 44 substantially parallel to the flow direction 14, and with a substantially sharp edge 45.

The protuberances 38, 40 are preferably constructed so as to provide a relief opening construction similar to that of the milled cylinder (FIGURE 1 ) relief grooves 18, at the portion 18' thereof. This construction substantially effects a plug phenomena, minimizing reverse flow of fibers and water during the negative pulse phase.

The protuberances 38, 40, and the surface 44 with a substantially sharp edge 45, can be added to the bars or wires 128 by welding or other techniques. However, in the preferred embodiment according to the invention when the bars or wires 128 are cast, extruded, or otherwise formed, they are made integral with the bars or wires 128 (and have substantially the same length or extent), and are of the same metal. In the preferred embodiment the dimension of the protuberance 38 substantially parallel to the direction 14 -- that is the surface 46 illustrated in FIGURE 3A - is about 20-400 microns, while the length of each of the slot-defining surfaces 41 , 42 is about 500-1000 microns, and longer than the surface 46 and the width of the slot 130, and the surfaces 41 , 42 are preferably substantially coextensive as seen in FIGURE 3A. The surface 44 preferably has a length (parallel to the flow direction 14) of about 200-1000 microns.

FIGURE 3B is of an embodiment similar to that of FIGURE 3A, with changes made in order to better accommodate practical manufacturing requirements and tolerances when producing the wires or bars. The configuration of FIGURE 3B provides a reasonable depth of the residual material clearing the slot opening 130. In FIGURE 3B components comparable to those in FIGURE 3A are shown by the same reference numeral, only where structure is changed in any way a follows the reference numeral.

The major differences between the FIGURE 3B and FIGURE 3A embodiments are that in the FIGURE 3B embodiment the protuberance 38' is substantially a continuation of the upstream surface 132 from the substantially sharp edge 45 at the top of the bar or wire 128. This creates a surface 42' which is substantially parallel to the surface 41 ' of the protuberance 40'. The protuberance 40' is substantially the same as the protuberance 40 except that it does not have the configuration of a right triangle, but rather has more of a general triangle (or perhaps isosceles or equi-lateral triangle) configuration, although it need not be exactly triangular, and the bottom-most point 48 of the surface 41 ' and its opposite point at surface 42' each need to have a radius smaller than 0.020 inches, preferably about 0.010 inches. The width of the relief groove at the point of the outmost end of said radiuses is at least four times the slot width or a minimum of 0.020 inches. The surfaces 41 ', 42' preferably extend substantially co-extensively below the surface 134, defining the slot 130, a distance at least as great as the width of the slot 130. FIGURE 3C is an embodiment where the tops of conventional bars or wires 128 have merely been ground or machined to provide the surfaces 44 with the sharp edges 45. The grinding or machining is preferably while the bars or wires 128 are flat (as in FIGURE 3C), i.e. before being rolled into the cylinder 125 (FIGURE 4), although the grinding or machining could alternatively be done after cylinder construction. The structures such as 38, 38', 40, 40' from FIGURES 3A and 3B may or may not be utilized. FIGURE 4 schematically illustrates -- looking in on the surface 49 (the outer surface of the cylinder 125 in FIGURE 4) -- a portion of the exemplary screen cylinder 125 according to the invention FIGURE 5 illustrates one exemplary complete construction of the cylinder 125 schematically illustrated in association with a pressure screen 55 of conventional design, including a housing 56 in which the screen cylinder 125 is mounted In this embodiment the cylinder 125 is essentially stationary, and is mounted on the stationary mounting element 57 within the housing 56. Mounted within the screen cylinder 125 is a foil or rotor 58 which is rotated about a substantially vertical axis defined by the shaft 59 so that there is relative movement between the screening surface 127 and the foil or rotor 58 causing the pulp to flow (in direction 14) past the screen surface 127 to separate accepts from rejects e g at a passing velocity of about 1 5-2 0 m/sec Alternatively or in addition, the cylinder 125 can be rotated about the axis of shaft 58

The housing 56 includes an inlet 60 for the pulp, an accepts outlet 61 , for pulp that has passed through the slots 130, and a rejects outlet 62 for reject material does not pass through the screen 125 By utilizing the screen 125 according to the invention, the reject rate is minimized and the debris removal efficiency is enhanced while the screen cylinder 125 still maintains the substantially open configuration of conventional wedge wire screens (those formed of discrete elements), e g 60-80% more open area and capacity than a conventional milled cylinder 10 In the above disclosure it is to be understood that all broad ranges include all specific ranges within a broad range For example, 60-80% means 61 -66%, 62-78% 77-80%, and all other narrow ranges with the broad range

It will thus be seen that according to the present invention a highly advantageous wedge wire type screen cylinder, and method of utilization thereof, which typically increase the debris removal efficiency by at least about 10% (e g about 20% to approaching that of a milled screen cylinder) compared to otherwise identical screen cylinders but not having the protuberances and the like according to the invention are provided While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiments thereof it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent structures, devices and methods

Claims

WHAT IS CLAIMED IS:

1. A screen cylinder formed from discrete elements as opposed to being milled, comprising: a screen cylinder frame having a screen surface; said screen surface comprising a plurality of bars or wires mounted to form screening slots between said bars and wires; each screening slot defined by an upstream bar or wire surface of one bar or wire and a downstream surface of another bar or wire, cooperating to define said slot; and at least one of said upstream bar or wire surface and associated downstream bar or wire surface being contoured to define said slot in a manner which simulates the form of a substantially sharp edge milled screen cylinder with substantially the same contour.

2. A screen cylinder as recited in claim 1 wherein said upstream surface comprises a substantially sharp edge and substantially rectangular cross section protuberance extending outwardly from said upstream surface toward the immediately adjacent downstream bar or wire, said screening slot being defined between said protuberance and said downstream bar or wire.

3. A screen cylinder as recited in claim 2 wherein said downstream bar or wire includes a protuberance having a substantially right triangle configuration with a leg of the right triangle substantially paralleling said upstream surface protuberance to define said screening slot.

4. A screen cylinder as recited in claim 3 wherein each said bar or wire includes a transition between said upstream and downstream surfaces thereof; and wherein said transition includes a portion substantially parallel to the tangential direction of movement of said cylinder or slurry moving therepast, and a substantially sharp edge between said upstream surface and said transition.

5. A screen cylinder as recited in claim 1 wherein each said bar or wire includes a transition between said upstream and downstream surfaces thereof; and wherein said transition includes a portion substantially parallel to the tangential direction of movement of said cylinder or slurry moving therepast, and a substantially sharp edge between said upstream surface and said transition.

6. A screen cylinder as recited in claim 5 wherein said slots are defined by substantially parallel and substantially co-extensive surfaces that have a length that is at least as great as the width of a slot defined thereby.

7. A screen cylinder as recited in claim 1 wherein said contour of said bars or wires substantially simulating the contour of a milled cylinder so simulates a milled cylinder contour that the debris removal efficiency of said discrete element cylinder is increased at least about 10% compared to said screen cylinder without said milled cylinder contour simulation.

8. A screen cylinder as recited in claim 1 further comprising a relief opening, said relief opening constructed to provide a plug phenomena.

9. A screen cylinder formed from discrete elements as opposed to being milled, which moves in a path or has slurry moving therepast in a path; comprising: a screen cylinder frame having a slotted screen surface; a plurality of substantially parallel bars or wires mounted to define screening slots therebetween; each bar or wire comprising a downstream surface having a slope making an angle of between about 5-60° with respect to the tangential direction of movement of said cylinder or slurry moving therepast over at least the majority of the extent thereof, and an upstream side plane making an angle of between about 70-1 10° with respect to said tangential direction of movement or slurry moving therepast; each downstream surface comprising a closest point, which is closest to a said slot defined thereby, at a point most remote from said slot: and a substantially sharp edge protuberance extending outwardly from said upstream surface of each of said bars or wires adjacent said closest point of an immediately adjacent bar or wire, said protuberance and said closest point defining said screening slot therebetween.

10. A screening cylinder as recited in claim 9 wherein each said downstream surface includes a protuberance adjacent said closest point having a substantially right triangle configuration with a leg of the right triangle substantially paralleling said upstream surface protuberance to provide another substantially sharp edge to define said screening slot.

11. A screen cylinder as recited in claim 9 wherein each said bar or wire includes a transition between said upstream and downstream surfaces thereof; and wherein said transition includes a portion substantially parallel to the tangential direction of movement of said cylinder or slurry moving therepast, and a substantially sharp edge between said upstream surface and said transition.

12. A screen cylinder as recited in claim 9 further comprising a relief opening, said relief opening constructed to provide a plug phenomena.

13. A screen cylinder formed from discrete elements as opposed to being milled, which moves in a path or has slurry moving therepast in a path; comprising: a screen cylinder frame having a slotted screen surface; a plurality of substantially parallel bars or wires mounted to define screening slots therebetween; each bar or wire comprising a downstream surface having a slope making an angle of between about 5-60 ^α with respect to the tangential direction of movement of said cylinder or slurry moving therepast over at least the majority of the extent thereof, and an upstream side plane making an angle of between about 70-95° with respect to said tangential direction of movement or slurry moving therepast; each downstream surface comprising a closest point, which is closest to a said slot defined thereby, at a point most remote from said slot; and wherein each said bar or wire includes a transition between said upstream and downstream surfaces thereof; and wherein said transition includes a portion substantially parallel to the tangential direction of movement of said cylinder or slurry moving therepast, and a substantially sharp edge between said upstream surface and said transition.

14. A screen cylinder as recited in claim 13 wherein said slots are defined by substantially parallel and substantially co-extensive surfaces that have a length that is at least as great as the width of a slot defined thereby.

15. A screen cylinder as recited in claim 13 wherein said downstream surface includes a protuberance adjacent said closest point having a configuration simulating a general or equi-lateral or isosceles triangle, and further comprising a protuberance extending outwardly from said upstream surface, said protuberances having substantially parallel surfaces that are substantially co-extensive and longer than said slot is wide, and defining said slot.

16. A screen cylinder as recited in claim 15 wherein said protuberances and the cooperation of said protuberances with said closest points increase the debris removal efficiency of said cylinder at least about 10% compared to if said protuberances and the cooperation thereof with said closest portions were not present in an otherwise identical cylinder.

17. A screen cylinder as recited in claim 13 further comprising a relief opening, said relief opening constructed to provide a plug phenomena.

18. A screen cylinder formed from discrete elements as opposed to being milled, comprising: a screen cylinder frame having a screen surface; said screen surface comprising a plurality of bars or wires mounted to form screening slots between said bars and wires; each screening slot defined by an upstream bar or wire surface of one bar or wire and a downstream surface of another bar or wire, cooperating to define said slot; and a relief opening, said relief opening constructed to provide a plug phenomena.

19. A method of utilizing the screen cylinder of claim 9, comprising: (a) causing a slurry of comminuted cellulosic fibrous material to flow with respect to said screen cylinder so that a slurry flows first past the downstream surface of any particular bar or wire, and then past the upstream surface of that particular bar or wire, so that turbulence is formed adjacent the slot between the upstream and downstream surfaces and the Coanda effect at the slot is substantially avoided; (b) causing accepts to be drawn through the slots; and (c) causing rejects to continue to flow with the slurry and to ultimately be discharged from adjacent the screen cylinder.

20. A method as recited in claim 19 wherein the screen cylinder has a relief opening; said method further comprising (d) providing a plug phenomena at the relief opening during a negative pulse phase.

21. A method as recited in claim 19 wherein (a) - (c) are practiced so as to increase the debris removal efficiency of the screen cylinder at least about 10% compared to a substantially identical cylinder, including slot width, without the protuberances and associated closest points.

22. The method as recited in claim 21 wherein (a)-(d) are practiced so as to increase the debris removal efficiency of the screen cylinder at least about 10% compared to a substantially identical cylinder, including slot width, without the protuberances and associated closest points.

23. A method as recited in claim 19 wherein (a) is practiced by causing the slurry to flow while the screen cylinder remains substantially stationary.

24. A method as recited in claim 19 wherein (a) is practiced at least in part by causing the cylinder to rotate,

25. A method as recited in claim 19 wherein (a) is practiced by causing the slurry to flow under the influence of a foil or rotor while the screen cylinder remains substantially stationary, at an average velocity of about 1.5-2.0 m/sec.

26. A method of utilizing the screen cylinder as recited in claim 18 comprising: (a) causing a slurry of comminuted cellulosic fibrous material to flow with respect to said screen cylinder so that a slurry flows first past the downstream surface of any particular bar or wire, and then past the upstream surface of that particular bar or wire; (b) causing accepts to be drawn through the slots; (c) causing rejects to continue to flow with the slurry and to ultimately be discharged from adjacent the screen cylinder; and (d) providing a plug phenomena at the relief opening during a negative pulse phase.

27. A method of making the screen cylinder of claim 1 by grinding and/or machining the tops of the bars or wires to make them substantially flat and to define the substantially sharp edges at the upstream surfaces thereof.

28. A method as recited in claim 27 wherein the grinding and/or machining is practiced while the bars or wires are in a substantially flat plate configuration, and wherein the cylinder is formed by rolling the plate after grinding and/or machining.

29. A method as recited in claim 27 wherein the grinding and/or machining is practiced to form contours of different depth in a repeating pattern.

30. A method of making the screen cylinder of claim 9 by grinding and/or machining the tops of the bars or wires to make them substantially flat and to define the substantially sharp edges at the upstream surfaces thereof.