Abstract
The present invention relates to a screen cylinder that is particularly suitable for screening, filtering, fractionating, or sorting cellulose pulp or fiber suspensions of the pulp and paper industry or other similar suspensions. The present invention relates more particularly to screening devices of the type comprising a plurality of screen wires positioned axially and at a small spacing parallel to each other, and a plurality of bar-wires arranged in a reverse orientation to the screen wires.
Claims
1. A screen cylinder comprising a support structure, a plurality of wire sections, and an axis, the plurality of wire sections including at least a plurality of screen-wire sections and a plurality of bar-wire sections, each screen-wire section comprising at least one screen-wire, each bar-wire section comprising at least one bar-wire, the support structure having a radially inner and a radially outer circumference, the support structure being provided with means for fastening the screen-wires and bar-wires at one of the outer circumference and the inner circumference, the screen wires and the bar wires being fastened to the support structure axially and at a small spacing parallel to each other by the fastening means, the screen-wires and the bar wires forming a screening surface facing away from the support structure, the small spacing between the screen wires and the bar-wires forming the screening openings for allowing an accept portion of a pulp or fiber suspension to flow therethrough, at least a majority of the screen-wires having a head surface facing away from the support structure, the head surface being formed of a first surface part and a second surface part, the head surface of the at least the majority of the screen wires having a peak at the second surface part and a peak circumference with a radius, Rps, relative to the cylinder axis, at least one bar-wire having a head surface facing away from the support structure, the head surface being formed of a first surface part and a second surface part, the head surface of the at least one bar-wire having a peak and a peak circumference with a radius, Rpb, relative to the cylinder axis, the peak circumference of the head surface of the at least one bar-wire extending, in a direction away from the support structure, at a distance h1 from the peak circumference of the head surface of the at least the majority of the screen wires, wherein the peak of the head surface of the at least one bar-wire is located at the first surface part, the head surfaces of the screen wires has an average angle α of slope and the head surfaces of the bar wires have an average angle β of slope, the angles α and β opening in opposite directions, wherein the bar-wire has, at a side of the peak of the head surface, a side surface, when in use, facing a flow of pulp suspension, and a leading edge between the side surface and the head surface, and wherein the leading edge has a radius, the radius being at most one half of the distance h1.
2. The screen cylinder as recited in claim 1, wherein, when the screen cylinder comprises an inflow cylinder, the peak of the head surface of the at least one bar-wire has a radius relative to the cylinder axis greater than that of the peak of the at least the majority of the screen wires, and, when the screen cylinder comprises an outflow cylinder, the peak of the head surface of the at least one bar-wire has a radius relative to the cylinder axis less than that of the peak of the at least the majority of the screen wires.
3. The screen cylinder as recited in claim 1, wherein the leading edge has a radius, the radius at most one fourth of the distance h1.
4. The screen cylinder as recited in claim 1, wherein the second surface part of the head surface of the bar-wire slopes from the first surface part of the head surface towards the support structure.
5. The screen cylinder as recited in claim 1, wherein the bar-wire sections are evenly spaced between the screen-wire sections.
6. The screen cylinder as recited in claim 1, wherein two or more bar-wires are next to one another in the bar-wire section.
7. The screen cylinder as recited in claim 1, wherein the bar-wire sections cover 1-20% of a screen cylinder circumference.
8. The screen cylinder as recited in claim 1, wherein the peak of each bar-wire in each bar-wire section has an equal elevation from the peaks of the at least the majority of the screen wires.
9. The screen cylinder as recited in claim 1, wherein the peak of one bar-wire in at least one bar-wire section has a different elevation than another bar-wire from the peaks of the at least the majority of the screen wires.
10. The screen cylinder as recited in claim 1, wherein the bar wires have a cross-section of equal shape with the at least the majority of the screen wires.
11. The screen cylinder as recited in claim 1, wherein the bar wires are tilted or installed in a higher location in the support structure than the screen wires.
12. The screen cylinder as recited in claim 1, wherein the support structure comprises a series of support rings and a frame cylinder.
13. The screen cylinder as recited in claim 1, wherein the bar-wires have a taller cross-section than the screen wires.
14. The screen cylinder as recited in claim 9, wherein the bar-wires are arranged in notches having a smaller radial depth within the support structure than notches in which the at least the majority of the screen wires are installed.
15. The screen cylinder as recited in claim 1, wherein the peak of each bar-wire extends, in a radial direction away from the support structure, 1-8 mm from the peak circumference of the at least the majority of the screen-wires.
16. The screen cylinder as recited in claim 1, wherein at least one of the screen-wires and the bar-wires are coated with a wear-resistant coating.
17. The screen cylinder as recited in claim 16, wherein the bar-wires are coated with a more wear-resistant coating than the screen-wires.
18. The screen cylinder as recited in claim 1, wherein at least one of the screen-wires and the bar-wires are made of a wear-resistant material.
19. The screen cylinder as recited in claim 18, wherein the at least one of the bar-wires is made of a more wear-resistant material than the at least one screen-wire.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] In the following, the screen cylinder will be explained in a more detailed manner with reference to the accompanying drawings of which:
[0034] FIG. 1 illustrates schematically a wire screen cylinder of prior art;
[0035] FIG. 2 illustrates schematically a section of a screen cylinder of prior art showing the dissymmetric screen wires and contours and the circumferential flow induced by the rotor;
[0036] FIG. 3 illustrates schematically a section of a screen cylinder of prior art showing a bar attached to the plurality of screen-wires;
[0037] FIG. 4 illustrates schematically a section of a screen cylinder in accordance with a first preferred embodiment of the present invention;
[0038] FIG. 5 illustrates schematically and in an enlarged scale a section of a screen cylinder in accordance with a first preferred embodiment of the present invention;
[0039] FIG. 6 illustrates schematically a section of a prior screen cylinder typically used for filtration and formed of symmetric screen-wires and a symmetric bar-wire therebetween;
[0040] FIG. 7 illustrates schematically a section of a screen cylinder in accordance with a second preferred embodiment of the present invention;
[0041] FIG. 8 illustrates various alternatives for the cross section of the screen-wires or bar-wires used in the present invention;
[0042] FIG. 9 illustrates schematically a section of a screen cylinder in accordance with a third preferred embodiment of the present invention;
[0043] FIG. 10 illustrates schematically a section of a screen cylinder in accordance with a fourth preferred embodiment of the present invention;
[0044] FIG. 11 illustrates schematically a section of a screen cylinder in accordance with a fifth preferred embodiment of the present invention; and
[0045] FIG. 12 illustrates schematically a section of a screen cylinder in accordance with a sixth preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0046] FIG. 1 shows, in a very schematic and simplified manner, a wedge wire screen cylinder, 10, of prior art about a central axis, 12. The end rings, or the top and bottom rings of the screen cylinder, are shown as 14 and 16 respectively. Three support elements, here rings, 18, are shown, but there will more typically be many such support elements, 18, spaced axially. The prior art screen cylinder, 10, is made of substantially axially-oriented screen wires, 20, which are the so-called “wedge wires”. Originally the generally triangular wire cross-section resembled a wedge, and most often still do. These screen wires are attached to support elements, 18, and, on the other hand, at their axial ends either directly or via the outermost support rings to the end rings, 14 and 16, situated at the opposite ends of the screen cylinder, 10. Note that in this schematic drawing, the screen wires, 20, have not been sketched in detail or to scale and only a few of the screen wires are shown, while a typical screen cylinder would have a plurality of screen wires extending essentially around the full circumference of the screen cylinder. Most often, the wedgewire screen cylinder, 10, is of the so-called “outflow” type like in FIG. 1. This means that the screening surface facing the pulp suspension to be screened is the inner surface of the screen cylinder, 10, and the flow of accept pulp proceeds radially outward through the cylinder openings. To make this operation possible, the screen wires are normally attached to the radially inward rim of the support elements, i.e. the support rings, 18, in this case. However, so-called “inflow” type wedgewire screen cylinders are also known whereby the screening surface facing the pulp suspension to be screened is the outward surface of the screen cylinder, 10, and the accept flow proceeds radially inward through the cylinder openings. In either the outflow or the inflow configuration, elements of the support structure, in this case the support rings, 18, are arranged along the length of the screen wires, 20, in such a manner that the axial distance between the support rings, 18, is about 20 to 100 mm depending on the size and the application of the screen cylinder, 10.
[0047] FIG. 2 illustrates schematically a section of a screen cylinder, 10, of prior art showing the dissymmetric screen wires, 20, with dissymmetric contours and the circumferential flow, F, induced by the rotor and, in particular, by the rotor foil, 24. The distance between the adjacent screen wires, 20, defines screen cylinder openings, or screen slots, 22. The slot width is normally set at some particular value in the range of 0.10 to 0.30 mm depending on the application of the screen cylinder, 10. However, in coarse screening applications, slot widths as large as 0.80 mm may be used. Conversely, future design and manufacturing improvements may make slot widths less than 0.10 mm practical. A common way of fastening and properly positioning the screen wires, 20, to the support elements or support rings, 18, is to provide transverse notches or recesses or openings in the support elements, 18, where the screen wires, 20, are inserted. The screen wires, 20, may include a feature on the aforementioned foot part of the wire whereby this foot-part feature fits into the notches, recesses or openings. A further manufacturing step, such as welding, gluing, soldering, riveting or clamping, is then typically taken after the wires, 20, are installed in the support elements, 18, to attach the wires even more securely, and especially to avoid any axial movement of the wire. The support elements, 18, may have a simple rectangular cross-section or they may have a substantially more complex shape to support a clamping or riveting action, for example. The screen wires, 20, may be installed into the support elements, 18, while the support elements are in a circular form, i.e. as a support ring. Alternatively, the screen wires, 20, may be installed in the support elements, 18, while the support elements are flat and this assembled mat of screen wires, 20, and support elements, 18, is then formed into a cylinder.
[0048] FIG. 3 illustrates schematically a section of a screen cylinder, 10, of prior art (U.S. Pat. No. 5,472,095) showing bars, 26, attached to the plurality of screen wires, 20, and in particular, to the surface of the screen cylinder facing the pulp suspension to be screened. The bars, 26, extend the full length of the cylinder, 10, although only a small section of the cylinder is shown in FIG. 3. The bars are aligned either parallel to the cylinder axis, 12, and thus parallel to the screen wires, 20, or at a relatively small angle to the cylinder axis, as is shown in FIG. 3. There will be many times fewer bars, 26, than cylinder wires, 20. The bars, 26, are typically rectangular in cross-section. They can be applied to cylinders made of a plurality of wires either by attaching the bars to the surface of the wires facing the pulp to be screened, or by installing the bars on top of wires that have been modified to receive the bars, or in place of certain wires. The most typical approach, however, is to install the bars by welding them onto the surface of the wires facing the pulp to be screened using either a fillet or stitch weld along the sides of the bar that extend more or less axially.
[0049] FIG. 4 illustrates schematically a section 100 of a screen cylinder in accordance with a first preferred embodiment of the present invention. The screen cylinder section 100 is made of a plurality of wire sections, which include a plurality of screen-wire sections and a plurality of bar-wire sections (shown in FIGS. 4-12). Each screen-wire section is formed of at least one and preferably a plurality of screen-wires, 30. The bar-wire sections comprise at least one (shown in FIGS. 4, 5, 7) and typically several (shown in FIGS. 9-12) bar-wires, 32. The screen-wires and the bar-wires are fastened to a support structure, 34. Here the support structure is formed of a plurality of support rings, 34, provided with transverse notches, 36, into which the foot, 38, of each of the screen-wires, 30, and each of the bar-wires, 32, is fitted. The support structure may be, in addition to support rings, whatever type is applicable with wedge wires like, for instance, a skeleton (CA-A1-2 609 881) or a frame cylinder construction (U.S. Pat. No. 6,915,910) to which the wedge wires are either directly attached or via the support rings supported.
[0050] An essential feature of the screen cylinder of the present invention, as shown in FIGS. 4, 5 and 7, is that the a majority of the screen-wires, 30, 130, and the bar-wires, 32, 132, have dissymmetric (in relation to a radial centreline plane) wire head surfaces, 40, 140 and 42, 142 as opposed to a prior art screen cylinder illustrated in FIG. 6, where the head surfaces are symmetric in relation to a radial centreline plane. The wire head is defined here as the part of the wire above a line that connects the entry to the openings on either side of the wire. The openings, in turn, are defined as the location of the minimum gap between adjacent wires.
[0051] The wire head surface can be defined by the changing radius relative to the central axis, 12 (shown in FIG. 1), of the screen cylinder as one moves along the surface circumferentially from one opening to the next. Different wire shapes create different changes in radius, with the radius instantaneously increasing, decreasing, or remaining constant as a trace is made circumferentially. For a symmetric wire surface, the values of the radius relative to the location of the slots are the same regardless of whether one moves clockwise or counter clockwise along the surface. For a dissymmetric surface the values are not independent of the direction of motion, not at least for the entire width of the wire.
[0052] The dissymmetry of the screen-wires, 30, and the bar-wires, 32, is expressed in the radius of the various parts of the head surfaces, 40 and 42. The radius is measured, naturally, from the axis of the screen cylinder. Here, in FIG. 4, an inflow screen cylinder is shown, i.e. a screen cylinder where the pulp to be screened is fed to the outside of the screen cylinder and the accepts pass the cylinder slots to a direction towards the axis of the cylinder. Thus, the screen-wire 30 has two radii between which the screen-wire fits, i.e. a foot radius, Rfs, and a radius, Rps, of the peak circumference, i.e. the radius of the point or peak, 40p, at the head surface, 40, farthest away from the foot, 38. In a corresponding manner, the bar-wire, 32, has two radii between which the bar-wire fits, i.e. a foot radius, Rfb, (here it happens to be the same as the foot radius, Rfs, of the screen-wire, but the, Rfb, may be either smaller or greater than, Rfs) and a radius, Rpb, of the peak circumference, i.e. the radius of the point or peak, 42p, at the head surface, 42, farthest away from the foot, 38.
[0053] As to the screen-wire, 30, it has on its head surface, 40, a circumferential mid-point, Mps, i.e. a point that is located by drawing a circumferential arc between the entrances to two adjacent openings (defining a circumferential width of a wire at the level of the entries) and drawing a perpendicular bisector thereto, whereby the circumferential mid-point is the crossing point of the bisector and the head surface, 40. The mid-point, Mps, divides the head surface, 40, into two parts: a first surface part, 40l, and a second surface part, 40t. The first surface part, 40l, may also be called a leading surface part as it is the first surface part receiving the flow of pulp or fibre suspension. The second surface part, 40t, may also be called a trailing surface part, as it is the surface part allowing the flow of pulp or fibre suspension to be discharged from above the screen-wire. In one particular embodiment, when considering an outflow screen, the average radius of the first surface part, 40l, of the screen-wire, 30, is greater than that of the second surface part, i.e. the trailing surface part, 40t, of the head surface, 40. In another particular embodiment, when considering an inflow screen, the average radius of the first surface part, 40l, of the screen-wire, 30, is less than that of the second surface part, i.e. the trailing surface part, 40t, of the head surface, 40.
[0054] As to the bar-wire, 32, it has on its head surface 42 a circumferential mid-point Mpb, i.e. a point that is located by drawing a circumferential arc between the entrance to two adjacent openings (defining a circumferential width of a wire at the level of the entries) and drawing a perpendicular bisector thereto, whereby the circumferential mid-point Mpb is the crossing point of the bisector and the head surface 42. The mid-point Mpb divides the head surface, 42, into two parts: a first surface part, 42l, and a second surface part, 42t. The first surface part, 42l, may also be called a leading surface part as it is the first surface part receiving the flow of pulp or fibre suspension. The second surface part, 42t, may also be called a trailing surface part, as it is the surface part allowing the flow of pulp or fibre suspension to be discharged from above the bar-wire. In one particular embodiment, when considering an outflow screen, the average radius of the first surface part, 42l, of the bar-wire, 32, is less than that of the second surface part, i.e. the trailing surface part, 42t, of the head surface, 42. In another particular embodiment, when concerning an inflow screen, the average radius of the first surface part, 42l, of the bar-wire, 32, is greater than that of the second surface part, i.e. the trailing surface part, 42t, of the head surface, 42.
[0055] Another essential feature of the invention is that the peak 40p of the screen-wire 30 is at the second or trailing surface part 40t thereof, whereas the peak 42p of the bar-wire 32 is at the leading or first surface part 42l thereof. However, in case the peak/s of the screen-wire and/or bar-wire is at the mid-point Mps and/or Mpb, the peak/s is/are considered to be at the above mentioned surface parts. But in such a case, naturally, the average radius of the surface part in question defines the required dissymmetry of the screen-wire or bar-wire as discussed on the two closest paragraphs above.
[0056] A further essential feature of the present invention is discussed in FIG. 5 where a bar-wire 32, two screen-wires 30 and the direction of rotation of the rotor by means of arrow F are shown. The feature essential in view of breaking the pulp flakes is the sharp leading edge 42e of the bar-wire 32. The leading edge 42e is located between the head surface 42 and the side surface 42s of the bar-wire. The side surface 42s is the surface at the wire head being positioned at a side of the head surface, and, when in use, facing the flow of pulp suspension. The leading edge 42e could be perfectly sharp but it has, in practice always for manufacturing reasons, a small radius r, (or the radial extension of a bevel) normally of the order of from one tenth to a few tenths of a millimeter. However, to define the required sharpness of the leading edge 42e the dimension of the radius is compared to the radial height h1, i.e. a radial distance between the peaks 40p of the screen-wire 30 and the peak 42p of the bar wire 32. The sharpness of the leading edge 42e is defined as the radius r being at most one half of the radial height h1, preferably at most one quarter of the radial height. Additionally, the proper operation of the bar-wire 32 requires that the trailing part 42t of the head surface 42 of the bar-wire 32 is slanting from the leading part 42l. Thus, preferably but not necessarily, to guarantee efficient breaking up of fiber flakes at the leading edge 42e the leading edge angle γ, i.e. the angle between the leading part 42l of the head surface 42 and the side surface 42s of the bar wire 32 is between 45 and 90 degrees.
[0057] To clarify that the same approach applies to wires having themselves a symmetric cross-section FIGS. 6 and 7 have been sketched. FIG. 6 illustrates schematically a section of a prior art screen cylinder of the type used in filtration having symmetric screen-wires and a symmetric bar-wire therebetween. Since both the screen-wires and the bar-wire have been fastened to the support structure such that their centreline plane, i.e. plane of symmetry (shown by vertical lines), is radial, the screen surface facing the pulp that is to be screened is flat, i.e. non-contoured, except for the bar-wire elevated from the level of the screen-wires. However, now that the head surface of the bar-wire is not slanting the head surface of the bar-wire guides most of the flow past the first screening slot immediately following the bar-wire, and, additionally, creates a strong field of turbulence that subjects a strong wearing action to the first screen-wire downstream of the bar-wire.
[0058] In FIG. 7 a section of a screen cylinder in accordance with a second preferred embodiment of the present invention is schematically illustrated. Here the screen-wires, 130, and the bar-wire, 132, have, again, a symmetric cross section, but, as the axis or plane of symmetry (shown by inclined lines) is not in radial direction, i.e. the wires, 130 and 132, are installed to the support structure, 34, in a tilted position, the screen surface has a contour. Now that the screen-wires, 130, are tilted to the right and the bar-wire, 132, is tilted to the left, i.e. to the opposite or reverse direction in relation to the screen-wires, an abrupt upward step is created in the flow direction F. In this embodiment, too, the heads of the screen-wires, 130, have a circumferential mid-point, Mps, a first or leading surface part, 140l, and a second or trailing surface part, 140t. In a corresponding manner, the heads of the bar-wires, 132, have a circumferential mid-point, Mpb, a first or leading surface part, 142l, and a second or trailing surface part, 142t. Thus, in accordance with the present invention, the peak of the screen-wires, 130, is at the trailing or second surface part, 140t, whereas the peak of the bar-wire, 132, is located at the first or leading surface part, 142l.
[0059] In FIG. 8 a few cross-sections of screen-wires or bar-wires are shown with their centreline planes. The three first wires from the left are not symmetrical in relation to the centreline plane of the wire, whereas the rightmost wire is symmetrical (requiring, when taken into use, tilting). There are a few features in common to all shown variations of the bar-wire. Firstly, the second or trailing surface part of the head surface of the bar-wire, i.e. the surface part to the left from the vertical line representing the centreline plane of the wire, is sloping from the first or leading surface part of the head surface towards the support structure represented by the horizontal line. The angle of slope, i.e. the angle in a radial plane between the second or trailing surface and the circumferential direction, is, preferably but not necessarily, of the order of 15 to 45 degrees, more preferably between 15 and 35 degrees. Secondly, the peak of the head surface of the bar-wire is located at the first or leading surface part of the bar-wire. Thirdly, the leading edge between the first or leading surface part of the head surface of the bar-wire and the side surface is sharp, though for manufacturing reasons rounded (or bevelled). And fourthly, the peak may, however, be located at a distance from the side surface, as shown by the leftmost bar-wire, or the first or leading surface part of the head surface may be flat, i.e. positioned in circumferential direction, that is, in a direction perpendicular to the centreline plane of the bar-wire. The latter option is, in a way, a preferred one, as it offers more bar-wire material to wear out, i.e. increases the lifetime of the bar-wire and the entire screen cylinder. Thus it is clear that all both disclosed and non-disclosed non-symmetrical wire cross sections may be used in the invention in both tilted and non-tilted (centreline plane in radial direction) configuration, and that all such wires that have a symmetrical cross-section in relation to its centreline plane may be arranged in tilted position to fulfil the requirements of the present invention. Also, the cross-sections of the screen-wires and the bar-wires of a screen cylinder may be similar, but it is as well possible to use different cross sections.
[0060] As has been discussed above in connection with FIGS. 4, 5 and 7, the dissymmetry of the contour of the bar-wire, 32/132, is opposite, or in reverse orientation, to that of more common screen-wire, 30/130, i.e. the leading or first surface part, 42l/142l of the head surface, 42/142, is at a shorter radial average distance from the axis of the screen cylinder in an outflow screen cylinder than the trailing or second surface part, 42t/142t, of the head surface, 42/142, for this typical example. In an inflow cylinder the leading or first surface part, 42l/142l, of the head surface, 42/142, is at a greater radial average distance from the axis of the screen cylinder than the trailing or second surface part, 42t/142t, of the head surface, 42/142.
[0061] Thus, the peak, 42p, of the head surface, 42/142, i.e. the highest part or point thereof, is located on the leading surface part, 42l/142l, of the head surface, 42/142. In other words, and in general terms, the bar-wires, 32/132, have a reverse orientation to the more common screen-wires, 30/130. The “reverse orientation” above means that the screen wires have at their head, i.e. the surface facing away from the support structure, an inclined slope generally facing the impinging pulp suspension flow along the screen surface for the particular wire shapes shown in FIGS. 4, 5 and 7, whereas the bar-wires have at their head surface facing away from the support structure an average inclined slope facing away from the impinging pulp suspension flow along the screen surface. In other words, the average angle α of slope of the screen wires open in the direction of the pulp flow along the screen surface, whereas the average angle β of slope of the bar-wires opens against the direction of the pulp flow, i.e. the average angles α and β of slope of the screen-wires and the bar-wires open in opposite directions for the particular wire shapes shown in FIGS. 4, 5 and 7.
[0062] The leading part 42l of the head surface 42 of the bar-wire 32 joins, at a preferred but not necessary angle γ of 45 to 90 degrees, preferably of 60-85 degrees, to a side surface 42s of the head 42 (when not taking into account the rounding or bevel), for the particular wire shape shown in FIG. 4. The side surface 42s is, preferably, at an angle of 70-110 degrees to the circumferential direction represented by the flow, F, or at an angle of ±20 degrees to the radial plane of the bar-wire, 32, established by the cylinder axis.
[0063] By means of this configuration of the head, 42, of the bar-wire, 32, the flow of the pulp suspension in the direction, F, meets the side surface, 42s, which creates a substantially more aggressive mechanical action than any screen-wire, 30. The bar-wire, 32, with its side surface, 42s, leading edge 42e, and the leading surface part, 421, generates macro-turbulence, shearing forces and particle impact, and thus provides a distinct and complementary function to the function of the more common screen-wire contours.
[0064] FIGS. 9-12 illustrate schematically screen cylinder designs of other preferred embodiments of the present invention, where a bar-wire section, comprising several bar-wires, 32, arranged in series, is located among the more common screen-wires, 30, of screen-wire sections. The bar-wire sections, comprising at least one but often several bar-wires, 32, are preferably, but not necessarily, evenly spaced within the screen cylinder circumference. The percentage of the circumference occupied by bar-wires is in the range of 1 to 20%, and typically between 5 and 15%. The intent of arranging several bar-wires in series may be to provide: a) a saw-tooth arrangement for a more complex action by, for instance, three bar-wires, 32, arranged at the same height with one another (FIG. 9); b) a more gradual downstream slope among the series of bar-wires, 32, (FIG. 10); c) a higher upward step on the upstream side of the collection of bar-wires, 32 (FIG. 11), where the third (the right-hand side) bar-wire could as well be arranged somewhat higher in the series, whereby an even higher step would be provided between the screen-wires and the leading (right-hand side) bar-wire, or d) an arrangement (FIG. 12), where the central bar-wire, in the bar-wire section, is not reversed in relation to the screen-wires 30.
[0065] As to dimensioning the bar-wires, and especially the radial elevation h1 (FIG. 5) of the bar-wire peak relative to the peak of the screen-wires, the elevation h1 is between 1 and 8 mm, preferably between 1 and 6 mm, and more preferably between 1 and 4 mm. When referring to FIG. 4 and the discussion in connection therewith, the above elevation h1 may be calculated as the difference between radii Rpb and Rps, i.e. Rpb-Rps (for an inflow screen) or Rps-Rpb (for an outflow screen).
[0066] The use of several adjacent bar-wires may also follow from the need to create a stronger support structure given that the bar-wires may be subjected to the impact of large and hard contaminants. The use of several bar-wires rather than a single bar-wire provides an additional degree-of-freedom for designers seeking to use an existing inventory of wires and wire shapes. Regardless of whether one or several bar-wires are used in a bar-wire circumferential section, a majority of the screen-wire and bar-wire heads are both generally dissymmetric and, in particular, at least one of the bar-wire heads in the bar-wire section has a reverse orientation to the screen-wire heads. Alternatively, the screen-wires and the bar-wires may be tilted but with the same final result where at least one of the bar-wire surfaces has a reverse orientation to the screen-wire surfaces.
[0067] In addition to solving all of the aforementioned problems with the current design of a cylinder with bars, the proposed invention also minimizes the required inventories of wire types, since one may be able to simply reverse the direction of a screen-wire to create a bar-wire. It will typically be advantageous to have the bar-wires appear as larger than the screen-wires, but this can be achieved in the following ways or some combination thereof: First, in cases where different wires are maintained in inventory to provide different screen-wire contour depths for different cylinders, a larger wire, with increased contour depth, may be selected for use as the bar-wire. Second, in cases where different contour depths are achieved by wire tilting, the bar-wire would be both reversed and installed with a reduced amount of tilt. Finally, the means of attaching the wire to the support structure could be modified so as to make the bar-wire appear higher. For example, where the screen wires, including the bar-wires, are installed in notches in a support ring, the notches for the bar-wires would be formed at a location closer to the notched edge of the support ring than for the screen-wires.
[0068] Hard chrome plating and alternate wear-resistant surface treatments have been traditionally applied to cylinders to minimize wear and extend lifetime. In addition, the bars have sometimes been made of materials which are harder and more wear resistant, such as Stellite™, than the 316L stainless steel material commonly used for the wires in respect of the especially high-wear environment of the bars.
[0069] As can be seen from the above description, a new screen cylinder has been developed, eliminating at least some disadvantages of the prior art screen cylinders. While the invention has been herein described by way of examples in connection with what are at present considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations and/or modifications of its features and other applications within the scope of the invention as defined in the appended claims.
