WO2009056682A1

WO2009056682A1 - Screen, screen cylinder, flat screen and methods for manufacturing screen cylinder

Info

Publication number: WO2009056682A1
Application number: PCT/FI2008/050613
Authority: WO
Inventors: Jukka Virtanen
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2007-11-02
Filing date: 2008-10-30
Publication date: 2009-05-07
Anticipated expiration: 2010-05-02
Also published as: FI20070833A0

Abstract

A screen cylinder (1) for purifying or classifying pulp mixture. The screen cylinder (1) comprises screen wires (3) forming a cylindrical screening surface (2) of the screen cylinder (1) and at least one screen wire (3) comprises apertures (9) provided through at least one side (3c, 3d) of the screen wire (3), through which apertures the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1) to the accept side (7) of the screen cylinder (1). In addition, methods for manufacturing a screen sylinder (1), a flat screen and a screen.

Description

SCREEN, SCREEN CYLINDER, FLAT SCREEN AND METHODS FOR MANUFACTURING SCREEN CYLINDER

BACKGROUND OF THE INVENTION

[0001] The invention relates to a screen cylinder for purifying or classifying pulp mixture, in which screen cylinder there are screen wires forming a cylindrical screening surface of the screen cylinder.

[0002] The invention further relates to a method for manufacturing a screen cylinder intended for purifying or classifying pulp mixture, the method comprising mounting screen wires, forming the screening surface of the screen cylinder, side by side on support bars, bending the support bars into rings and connecting the ends of the support bars bent into rings such that the screen wires form a screening surface in the axial direction of the screen cylinder.

[0003] The invention further relates to a second method for manufacturing a screen cylinder intended for purifying or classifying pulp mixture, the method comprising mounting screen wires, forming the screening surface of the screen cylinder, side by side on support bars, bending the support bars into rings and connecting the ends of the support bars bent into rings such that the screen wires form a screening surface in the axial direction of the screen cylinder.

[0004] The invention further relates to a flat screen for purifying or classifying pulp mixture, the flat screen comprising wires forming a planar screening surface of the screen.

[0005] The invention further relates to a screen for purifying or classifying pulp mixture, the screen comprising screen wires forming the screening surface.

[0006] Screen cylinders are used, among other things, for purifying and classifying pulp mixtures. Screen cylinders formed of screen wires are typically manufactured by attaching parallel screen wires, forming the screening surface of the screen cylinder, side by side close to one another to form a cylindrical shape such that a slot of desired magnitude, i.e. a screen slot, will be provided therebetween. This may be performed, for instance, by attaching the screen wires first either by laser welding or hard soldering directly to the surface of the support bars or to grooves provided in the support bars for the screen wires. Subsequently the produced planar screen panel is bent into a cylindrical shape by rolling the support bars into rings. The ends of the support bars that have been bent into rings are connected to one another by welding, for instance. Around the support bars it is further possible to mount particular supporting rims to reinforce the structure of the screen cylinder.

[0007] The screen wires of the screen cylinder form the screening surface of the screen cylinder comprising structures through which the fluid in the pulp mixture and the portion of fibres defined by the size of said structures may flow from the screen cylinder feed side to the screen cylinder accept side and further onwards in the pulp making process. The pulp mixture is fed into the screen cylinder through one end thereof and the portion of pulp mixture that has not flown from the screen cylinder feed side to the screen cylinder accept side will be discharged from the opposite end of the screen cylinder for further processing.

BRIEF DESCRIPTION OF THE INVENTION

[0008] The object of the present invention is to provide a novel screen cylinder and a novel method for manufacturing a screen cylinder.

[0009] The screen cylinder of the invention is characterized in that at least one screen wire comprises apertures formed through at least one side of the screen wire, through which apertures the fluid in the pulp mixture and the portion of fibres defined by the aperture size may flow from the feed side of the screen cylinder to the accept side of the screen cylinder.

[0010] The method of the invention for manufacturing the screen cylinder is characterized in that in a screen cylinder manufacturing step there are formed apertures through at least one side of at least one screen wire, through which apertures the fluid in the pulp mixture and the portion of fibres defined by the aperture size may flow from the screen cylinder feed side to the screen cylinder accept side.

[0011] The second method of the invention for manufacturing a screen cylinder is characterized by mounting screen wires, which form the screening surface of the screen cylinder, side by side close to one another in support bars of the screen cylinder, in which screen wires there are provided in advance in a screen wire manufacturing step apertures through at least one side of the screen wire, through which apertures the fluid in the pulp mixture and part of the fibres defined by the aperture size may flow from the screen cylinder feed side to the screen cylinder accept side.

[0012] The flat screen of the invention is characterized in that at least one screen wire comprises apertures formed through at least one side of the screen wire, through which apertures the fluid in the pulp mixture and the portion of the fibres defined by the aperture size may flow from the screen cylinder feed side to the screen cylinder accept side.

[0013] The screen of the invention is characterized in that at least one screen wire of the screening surface comprises apertures formed through at least one side of the screen wire, through which apertures the fluid in the pulp mixture and the portion of the fibres defined by the aperture size may flow from the screen cylinder feed side to the screen cylinder accept side.

[0014] The screen cylinder for purifying or classifying a pulp mixture comprises screen wires forming a cylindrical screening surface of the screen cylinder. Further, at least one screen wire of the screen cylinder comprises apertures formed through at least one side of the screen wire, through which apertures the fluid in the pulp mixture and the portion of the fibres defined by the aperture size may flow from the screen cylinder feed side to the screen cylinder accept side.

[0015] The presented screen cylinder combines the structure that resembles the structure of a conventional perforated screen cylinder and a modern manufacturing technique of a wire screen cylinder. The presented screen cylinder typically provides a structure that resembles the structure of a perforated screen cylinder having a good fractioning capability with an efficient manufacturing technique characteristic of a wire screen cylinder, when screen wires are placed side by side one close to the other such that there will be no particular screen slot between the screen wires, but all the flow from the screen cylinder feed side to the screen cylinder accept side passes through said apertures provided in the screen wire sides. It is possible to machine or otherwise produce apertures or channels of desired size and shape in the wire sides, through which part of the water and fibres in the pulp mixture to be fed into the screen cylinder will be able to flow from the screen cylinder feed side to the screen cylinder accept side.

[0016] In accordance with an embodiment, at least one screen wire comprises apertures provided through the side directed against the feed flow direction of the pulp mixture, through which apertures the fluid in the pulp mixture and the portion of fibres defined by the aperture size may flow from the screen cylinder feed side to the screen cylinder accept side. Said disposition of the apertures gives the screen cylinder a good fractionating accuracy, in particular, if the surface of the screen wire that is directed to the screen cylinder feed side is inclined and ascending towards the feeding direction of the pulp mixture, whereby said inclined surface guides large particles away from vicinity of the apertures.

[0017] In accordance with a second embodiment, the screen wires are arranged adjacently at a distance from one another such that between the adjacent screen wires there will be provided a slot such that the fluid in the pulp mixture and the portion of the fibres defined by the apertures and the slot size may flow from the screen cylinder feed side to the screen cylinder accept side both via the slots between the screen wires and via said apertures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Some embodiments of the invention will be described in greater detail in connection with the attached drawings, in which

Figure 1 shows a schematic cross-sectional view of a general basic structure of a screen cylinder, seen from the end of the screen cylinder,

Figure 2 shows schematically a screen cylinder, seen from the end of the screen wires and crosscut,

Figure 3 shows schematically the screen cylinder of Figure 2, seen in the direction of a screening surface,

Figure 4 shows schematically a second screen cylinder, seen from the end of the screen wires and crosscut,

Figure 5 shows schematically the screen cylinder of Figure 4, seen in the direction of the screening surface,

Figure 6 shows schematically a third screen cylinder, seen from the end of the screen wires and crosscut,

Figure 7 shows schematically the screen cylinder of Figure 6, seen in the direction of the screening surface,

Figure 8 shows schematically a fourth screen cylinder, seen from the end of the screen wires and crosscut,

Figure 9 shows schematically the screen cylinder of Figure 8, seen in the direction of the screening surface,

Figure 10 shows schematically a fifth screen cylinder, seen from the end of the screen wires and crosscut,

Figure 11 shows schematically the screen cylinder of Figure 10, seen in the direction of the screening surface, Figure 12 shows schematically a flat screen seen from the end of the screen wires and crosscut,

Figure 13 shows schematically the flat screen of Figure 12, seen in the direction of the screening surface,

Figure 14 shows schematically a sixth screen cylinder, seen from the end of the screen wires and crosscut,

Figure 15 shows schematically a seventh screen cylinder, seen from the end of the screen wires and crosscut,

Figure 16 shows schematically an eighth screen cylinder, seen from the end of the screen wires and crosscut, and

Figure 17 shows schematically the screen cylinder of Figure 16, seen in the direction of the screening surface.

[0019] In the figures some embodiments of the invention have been shown in a simplified manner, for the sake of clarity. Like reference numerals refer to like parts in the figures.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Figure 1 shows schematically a general basic structure of a screen cylinder in the axial direction of the screen cylinder, i.e. seen from the end of the screen cylinder and crosscut. Throughout the inner circumference of the screen cylinder 1 there are arranged screen wires 3 side by side that constitute the screening surface 2 of the screen cylinder 1. The screen wires 3 are typically mounted on support bars 4 prior to bending the support bars 4 into rings such that a screen cylinder 1 of a suitable size in diameter is produced. In Figure 1 , the ends 4' and 4" of the support bars 4 are attached to one another by welding at an attachment point 4a such that the support bars 4 form a ring- shaped structure. Support bars 4 are located in the axial direction of the screen cylinder 1 , i.e. in the longitudinal direction of the screen cylinder, at suitable intervals such that the screen wires 3 will keep sufficiently rigidly and firmly in place. As screen wires 3 are mounted on the support bars 4, the screen wires 3 may be secured to the surface of the support bars 4, for instance, either by hard soldering or laser welding. It is also possible that on the inner surface of the support bars 4, i.e. on the surface facing the interior of the screen cylinder 1 , there are provided particular attachment grooves, to which the screen wires 3 are secured, whereby the screen wires may be secured to the grooves, for instance, by laser welding or only through bending the support bars 4 such that the bending of the support bar 4 provides shape-locking between a screen wire and said groove. Further, Figure 1 shows schematically support rims 5 arranged around the support bars 4, which support rims are intended to further reinforce the structure of the screen cylinder 1. The support rims 5 may be arranged either around all the support bars 4 of the screen cylinder 1 or only around some of the support bars 4. Typically, the interior of the screen cylinder 1 constitutes the feed side 6 of the screen cylinder 1 , where the pulp mixture to be purified or classified is fed and the exterior of the screen cylinder 1 constitutes an accept side 7 of the screen cylinder 1 , where the fibres of desired magnitude are able to flow or move along with the fluid through the screening surface 2. In some cases it is possible, however, that the interior of the screen cylinder 1 constitutes the accept side of the screen cylinder 1 and the exterior of the screen cylinder 1 constitutes the feed side 6 of the screen cylinder. The pulp mixture to be purified or classified is fed into the screen cylinder 1 through one end, i.e. the feed end, and the portion of the pulp mixture that has not flown through the screening surface 2 of the screen cylinder 1 from the screen cylinder feed side 6 to the accept side 7, is removed from the opposite end of the screen cylinder 1 , i.e. from the discharge end, for further processing. The general structure and operating principle of the screen cylinders are known per se to the persons skilled in the art, and therefore they are not discussed here in any greater detail.

[0021] Figure 2 shows schematically a screen cylinder 1 seen from the end of a screen wire 3 and crosscut along the line A - A of Figure 3, and Figure 3 shows schematically the screen cylinder 1 of Figure 2 seen in the direction of the screening surface 2. Figures 2 and 3 show two screen wires 3 of the screen cylinder 1. For the sake of clarity, Figure 2 does not show a support bar 4. The feed flow direction of the pulp mixture in the screen cylinder 1 is indicated by arrow F. The screen wire 3 comprises a feed side surface 3a directed to the feed side 6 of the screen cylinder 1 , an accept side surface 3b directed to the accept side 7 of the screen cylinder 1 , a first side 3c of the screen wire 1 directed against the feed flow direction F of the pulp mixture and a second side 3d of the screen wire 1 in the same direction with the feed flow direction F of the pulp mixture.

[0022] In the screen wire 3 of Figures 2 and 3, as well as in screen wires 3 of Figures 4 to 9 to be described later in the text, the feed side surface 3a of the screen wire 3 directed to the feed side of the screen cylinder 1 is formed to be inclined such that the feed side surface 3a of the screen wire 1 is an ascending surface in the feed flow direction F of the pulp mixture. Thanks to the screen wire's 3 feed side surface 3a ascending in the feed flow direction F of the pulp mixture the screening surface 2 of the screen cylinder 1 forms a gradual screening surface 2 whose advantages will be discussed later. In elevation of the screen wire 3, the first side 3c and the second side 3d of the screen wire 3 are formed such that when proceeding along the first side 3c and the second side 3d of the screen wire from the feed side of the screen cylinder 1 towards the accept side of the screen cylinder 1 , the sides 3c and 3d are arranged, for at least part in the elevational direction of the screen wire 3, to orient towards the centre line of the screen wire 3. In other words, the screen wire 3 is formed such that the upper part of the screen wire 3 is broader than its lower part. Consequently, as the screen wires 3 are mounted in the manner shown in Figures 2 to 9, the upper parts of the screen wires 3 are in close connection to one another, and in the lower part of the screen wires 3 there will be provided an accept channel 8 between them, via which the portion of the pulp mixture that has passed the screening surface 2 will be able to flow to the accept side 7 of the screen cylinder 1. In the screen wires of Figures 2 to 9, the accept side surface 3b of the screen wires 3 is, in turn, formed to be an arc of a semi-circle, which contributes to the flow of pulp mixture from the accept channel 8 to the accept side 7 of the screen cylinder. The accept side surface 3b of the screen wire 3 may also be straight, acute-angled or of some other shape, of course.

[0023] The screen wires 3 shown in Figures 2 to 9 are so-called profiled screen wires 3 in shape, and they are thus arranged one beside the other such that the upper part of the first side 3c of the screen wire 3 coming behind in the feed flow direction F of the pulp mixture is arranged to come close to the upper part of the second side 3d of the screen wire 3 ahead in the feed flow direction F of the pulp mixture. In the embodiment of Figures 2 to 9, there is no screen slot characteristic of screen cylinders implemented in conventional wire techniques.

[0024] In the screen cylinder 1 of Figures 2 and 3, in the first side 3c of the screen wires 3, i.e. the side that is directed against the feed flow direction F of the pulp mixture, there are provided apertures 9 or channels 9 side by side in the longitudinal direction of the screen wires 3 indicated by arrow L, which apertures extend from the feed side 6 of the screen cylinder 1 to the ac- cept channel 8. Said apertures 9 are provided in the first side 3c of the screen wire 3, in other words, such that the apertures 9 are open in the direction against the feed flow direction F of the pulp mixture, i.e. at the apertures 9 the screen wire 3 comprises no screen wire manufacturing material between the aperture 9 and the second side 3d of the adjacent screen wire in the feed flow direction F of the pulp mixture. The fluid in the pulp mixture to be purified or classified and the fibres belonging to the class defined by the size of apertures 9, i.e. the portion of fibres 9 defined by the apertures 9 may flow through the apertures 9 from the feed side 6 of the screen cylinder 1 to the accept side 7 at the same time as slivers, excessively large fibres, fibre bunches and other matter to be separated remain in the feed side of the screen cylinder 1 to be removed from the screen cylinder 1 for further processing. In the embodiment of Figures 2 and 3, the apertures 9 in the first sides 3c of the screen wires are semi-circular in cross section and form semi-circular screen apertures in the screening surface 2 of the screen cylinder 1 , when the screen wires 3 are placed in the manner shown in Figures 2 to 9 side by side close to one another on the side directed to the feed side 6 of the feed cylinder 1 such that there will remain no screen slot characteristic of a conventional wire cylinder between the screen wires 3.

[0025] Figure 4 shows schematically a second screen cylinder 1 , seen from the end of the screen cylinder 1 and crosscut along the crosscut line B - B appearing in Figure 5, and Figure 5 shows schematically the screen cylinder 1 of Figure 4 seen in the direction of the screening surface 2. For the sake of clarity, Figure 4 does not show a support bar 4. The screen cylinder 1 of Figures 4 and 5 corresponds to the screen cylinder 1 of Figures 2 and 3 in all other respects but in the screen cylinder 1 of Figures 4 and 5 the apertures 9, i.e. the screen apertures of the screening surface 2 in the screen cylinder 1 , have semi-oval cross sections.

[0026] The presented screen cylinder combines the structure resembling the structure of a conventional perforated cylinder and the modern manufacturing technique of a wire cylinder. The presented screen cylinder thus provides a structure that resembles the structure of a perforated cylinder typically having a good fractioning capability with an efficient manufacturing technique characteristic of wire cylinders. It is considered that the best characteristic of the perforated screen cylinders is typically a good fractioning capability and an important characteristic is a large, open area, which is now possible to implement by an efficient manufacturing technique characteristic of wire screen cylinders. In the first side 3c of the screen wires 3 it is thus possible to machine or otherwise produce apertures of desired size and shape, through which part of the water and fibres in the pulp mixture to be fed into the screen cylinder 1 is able to flow from the feed side 6 of the screen cylinder 1 to the accept side 7 of the screen cylinder 1. Said apertures 9 may be machined in the screen wires 3, for instance, by milling prior to mounting the screen wires 3 in connection with the support bars 4 or after the screen wires 3 have been mounted in connection with the support bars 4 but prior to bending the support bars 4 into rings. The apertures 9 in the first side of the screen wire 3 may be produced also in connection with mangling, in general cold rolling, of the screen wire 3, in advance while manufacturing the screen wire 3 before the screen wires 3 for the screen cylinder 1 are cut to size corresponding the length of the screen cylinder 1 to be manufactured. The presented solution has also an advantage that the solution allows screen cylinders with smaller perforations to be manufactured more economically than before, because previously perforations had to be produced in a completely solid screen cylinder made of a plate by cumbersome and time-consuming manufacturing techniques, such as drilling, electron jets or laser beams.

[0027] In connection with the above embodiments it was set forth that the aperture 9 may be semi-circular or semi-oval in cross section. Naturally, the cross sectional shape of the aperture 9 is not limited to these shapes, but the shape and size of the cross section of the aperture 9 may vary.

[0028] In the embodiments of Figures 2 to 5 the apertures 9 are very close to one another in the longitudinal direction L of the screen wire 3, but the spacing therebetween in the longitudinal direction L of the screen wire 3 may be even larger, as schematically shown in Figure 7. The spacing between the apertures 9 in the longitudinal direction of the screen wire 3 may also be different in the feed end of the screen cylinder from that in the discharge end. The cross sections of the apertures 9 may also be different in size in the feed end and in the discharge end of the screen cylinder 1 and the cross sectional area of the apertures 9 may also become larger when proceeding along the aperture 9 from the feed side 6 to the accept side 7 of the screen cylinder 1.

[0029] When fractioning classification is desired, the size of the apertures 9 may be reduced and the spacing therebetween in the longitudinal direction L of the screen wire 3 may be reduced towards the discharge end of the screen cylinder 1 , where the pulp is more tainted and contains a proportionally larger amount of large particles and slivers, which will thus remain on the screening surface 2, i.e. on the classifying surface, and will not be allowed to the accept side 7 of the screen cylinder 1 , which improves pulp fractioning. When large yield is particularly desired and a slightly poorer quality of pulp transferred to the accept side is acceptable, it is possible to reduce the spacing between the apertures 9 in the longitudinal direction of the screen wire 3 from the feed end of the screen cylinder 1 towards the discharge end such that the apertures 9 are equal in size throughout the length of the screen cylinder 1 , or they become even larger from the feed end of the screen cylinder 1 towards the discharge end of the screen cylinder 1.

[0030] Figure 6 shows schematically a third screen cylinder 1 , seen from the end of the screen cylinder 1 and crosscut along the line C - C in Figure 7, and Figure 7 shows schematically the screen cylinder 1 of Figure 6, seen in the direction of the screening surface 2. In the screen cylinder 1 of Figures 6 and 7, in the second side 3d of the screen wires 3, i.e. in the side that is in the same direction with the feed flow direction F of the pulp mixture, there are provided apertures 9, or channels 9 side by side close to one another in the longitudinal direction indicated by L of the screen wires 3, which apertures extend from the feed side 6 of the screen cylinder 1 to the accept channel 8. Said apertures 9 are provided in the second side 3d of the screen wire 3 such that the apertures 9 are open in the same direction with the feed flow direction F of the pulp mixture. In the embodiment of Figures 6 and 7, the apertures in the second sides 3d of the screen wires 3 are semi-oval apertures in cross section that form semi-oval screen apertures on the screening surface 2 of the screen cylinder 1 , when the screen wires 3 are placed side by side close to one another on the side directed to the feed side 6 of the screen cylinder 1 , as shown in Figures 2 to 9, such that between the screen wires 3 there will be no screen slot typical of conventional wire cylinders.

[0031] In the embodiment of Figures 2 to 5, the apertures 9 are provided in the screen wire's 3 first side 3c, which is directed against the feed flow direction F of the pulp mixture. The screen wire 3 being a profiled screen wire 3, in which the feed side surface 3a of the screen wire 3 is ascending in the feed flow direction F of the pulp mixture, the step or ridge between the feed side surfaces 3a of two successive screen wires throws large particles con- tained in the pulp mixture away from the vicinity of the apertures 9, whereby the separation efficiency of the screen cylinder 1 improves. If the apertures 9 are placed as shown in Figures 6 and 7 at the highest point on the feed side surface 3a of the screen wire 3, the quality of pulp may slightly deteriorate as compared with the embodiment of Figures 2 to 5.

[0032] Figure 8 shows schematically a fourth screen cylinder 1 , seen from the end of the screen cylinder 1 and crosscut along the line D - D of Figure 9, and Figure 9 shows schematically the screen cylinder 1 of Figure 8 seen in the direction of the screening surface 2. In the screen cylinder 1 of Figures 8 and 9, in both the first side 3c and the second side 3d of the screen wire 3 there are provided apertures 9 or channels 9 in the longitudinal direction of the screen wires 3 indicated by arrow L, which apertures extend from the feed side 6 of the screen cylinder 1 to the accept channel 8. In a case like this, it is possible that the apertures 9 in different sides 3c, 3d of the screen wire 3 may be of the same or different size, as shown in Figures 8 and 9. When apertures 9 are provided on both sides 3c and 3d of the screen wire 3, the apertures 9 on sides 3c and 3d are advantageously provided in offset locations in the longitudinal direction L of the screen wires 3 in order that the screen wire 3 would not break in use or in order that larger apertures 9 could be made.

[0033] Figure 10 shows schematically a fifth screen cylinder 1 , seen from the end of the screen cylinder 1 and crosscut along the line E - E of Figure 11 , and Figure 11 shows schematically the screen cylinder 1 of Figure 10, seen in the direction of the screening surface 2. The structure of the screen cylinder 1 of Figures 10 and 11 resembles the screen cylinder 1 of Figures 2 and 3, in which the first side of the screen wires 3 comprises apertures 9, but in addition to apertures 9, there is provided a slot 10, i.e. a screen slot 10, typical of conventional wire cylinders. Hence, the fluid in the pulp mixture fed into the screen cylinder 1 and the portion of fibres defined by the size of the apertures 9 and the slots 10 may flow through the apertures 9 and the slots 10 from the feed side 6 to the accept side 7 of the screen cylinder 1.

[0034] The screen cylinder 11 of Figures 10 and 11 thus comprises both apertures 9 in the first side 3c of the screen wires 3 and slots 10 between the screen wires. This concerns a combination of perforated and slot screen cylinders, by means of which it is possible to achieve a higher yield than by means of solutions currently in use without the quality being any lower. Typically, it is considered that the best characteristic of the slot screen cylinders is good sliver reduction, which has been more difficult to achieve with conventional perforated screen cylinders. By providing a screen cylinder having both apertures 9 and slots 10 in the manner shown in Figures 10 and 11 it is possible to combine good characteristics of both perforated and slot screen cylinders, and hence to provide a higher yield without the quality being any lower.

[0035] A screen cylinder may also be made of screen wires that are not profiled as the screen wires shown in Figures 2 to 11 , whereby a flat screening surface will be achieved. In that case the screen wires may be rectangular, quadrangular or triangular in cross section, for instance. The apertures may be provided in one or both sides of the screen wire, for instance, alternately, whereby it is possible to form larger apertures of any shape, and thus a screen cylinder having apertures and flat screening surface will be provided. For instance, when both sides of the screen wire are provided with corresponding semi-circular apertures and when the adjacent screen wires are arranged in close contact to one another such that the apertures of the adjacent screen wires will be at the same point in the longitudinal direction of the screen cylinder, a screen cylinder having circular apertures will be provided.

[0036] In Figures 1 to 11 , the screen wires 3 of the screen cylinder 1 run in the axial direction of the screen cylinder 1 , but the screen cylinder may also be manufactured such that the screen wires 3 run in the circumferential direction of the screen cylinder 1 and the support bars 3 run in the axial direction of the screen cylinder.

[0037] The screen cylinder 1 should be understood in a broad sense such that the cylindrical screening surface 2 of the screen cylinder 1 may be a cylinder or a truncated cone in shape, i.e. the term screen cylinder refers to both screen structures having a cylindrical shape and screen structures of having a truncated cone shape.

[0038] Figure 12 shows schematically a flat screen 11 , i.e. a planar screen plate, seen from the end of the screen wires 3 and crosscut along the line G - G appearing in Figure 13, and Figure 13 shows schematically the flat screen 11 of Figure 12, seen in the direction of the screening surface 2. The flat screen 11 of Figures 12 and 13 comprises rectangular screen wires 3 having no particular profiling on the surface directed to the feed side of the flat screen 1 , which wires constitute a planar screening surface 2 such that the screen wires 3 form a planar screen structure and, for at least a portion of the screen wire surface 3a, the surfaces of the screen wires 3 belong to said pla- nar screening surface 2. On both sides 3c, 3d of the screen wires 3 there are provided semi-circular apertures 9 such that in combination the apertures 9 of the screen wires 3 placed one against the other form circular screen apertures, through which the fluid in the pulp mixture to be purified or classified and the portion of fibres defined by the size of the apertures 9 may flow from the feed side 6 of the screen cylinder 1 to the accept side 7 of the screen cylinder 1. In Figure 12, the feed flow direction of the pulp mixture is towards the screening surface 2, as indicated by arrow F. In flat screens the feed flow direction of the pulp mixture may also be in the direction of the planar structure of the flat screen, however.

[0039] The flat screen 11 may be manufactured in a manner resembling that of a screen cylinder by mounting screen wires 3 to attachment grooves provided in a support bar 4 and by attaching the screen wires to said grooves by welding or soldering. The screen wires 3 may be attached to the grooves provided in the support bars 4 also by utilizing shape-locking, for instance, such that the support bars are produced in the shape of an arc of a circle and the grooves are provided in the outer circumference of the arc- shaped support bar, whereby, when the support bars are straightened, the screen wires mounted in the grooves will lock in the grooves of said support bars. Figure 12 shows schematically a support bar 4 drawn in a broken line. The support bars 4 may also be without the mounting grooves for the screen wires 3, whereby the screen wires 3 may be attached directly onto the support bars 4 by welding, for instance. The flat screen may be applied, for instance, when manufactured in a circular shape in the flow direction of the pulp mixture placed in the upstream of the screen cylinder acting as a pre-purifying screen. The flat screen may also be used in a separate screening device.

[0040] In the same way as set forth in connection with the screen cylinder, the shape of the apertures 9 in the screening surface of the flat screen 11 may also vary. In addition, apertures 9 may be on one side of the screen wires alone. Between the screen wires 3 there may also be a slot such that the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures 9 and the slot 10 may flow from the feed side 6 to the accept side 7 of the screen both through the slots 10 between the screen wires 3 and through said apertures 9. Correspondingly, the cross sectional shape of the screen wires 3 may be, apart from a rectangle, a square, a triangle or a circle, for instance, or they may have a profiled shape. [0041] Screen cylinders 1 and flat screens 11 may be advantageously employed, apart from various screens, as pulper screen plates. In pulper screen plate applications the diameter of the apertures may be 8 to 10 mm, for instance, and in screen plate applications in pulpers used in primary screening step the aperture size may be about 3 to 5 mm, for instance. Because of the relatively large aperture size required for pulper applications the thickness of the screen wires must be correspondingly increased such that in some applications the presented solutions does not necessarily give so great an economic advantage on the manufacturing costs over those achievable by the conventional screen applications separate from the pulpers. In conventional screen applications separate from the pulpers the aperture size of the screening surface of the screens may be 0.5 to 3 mm, for instance, advantageously 1 to 2 mm. When an electronic jet or a laser beam is used for making the apertures, the aperture size may be as small as 0.15 to 0.2 mm.

[0042] In accordance with the solution, one or more, or possibly all the screen wires are provided with apertures either in one or both sides of the screen wire, through which apertures the fluid in the pulp mixture to be fed into the screen cylinder or the flat screen and the portion of fibres defined by the aperture size may flow from the feed side of the screen to the accept side of the screen cylinder.

[0043] Figure 14 further shows schematically a sixth screen cylinder 1 , seen from the end of the screen wires 3 and crosscut. Figure 14 shows, by way of example, three of the screen wires 3 in the screen cylinder 1. For the sake of clarity, Figure 14 does not show a support bar 4. An exemplary feed flow direction of the pulp mixture in the screen cylinder 1 is indicated by arrow F. The screen wire 3 comprises a feed side surface 3a directed to the feed side 6 of the screen cylinder 1 , an accept side surface 3b directed to the accept side 7 of the screen cylinder 1 , a first side 3c of the screen wire 3 directed against the feed flow direction F of the pulp mixture and a second side 3d of the screen wire 3 directed in the same direction with the feed flow direction F of the pulp mixture.

[0044] In the screen wire 3 of Figure 14, the feed side surface 3a of the screen wire 3 directed to the feed side of the screen cylinder 1 is provided straight, whereby said screen wire 3 is neutral with respect to the feed flow direction of the pulp mixture, but the feed side surface 3a of the screen wire 3 of Figure 14 could also be given an ascending shape in the feed flow direction F in the same manner as is shown in Figure 2, for instance.

[0045] The first side 3c of the screen wire in accordance with Figure 14 comprises a first side flange 12 or a first side branch 12, and the second side 3d comprises a second side flange 12' or a second side branch 12'. The side flanges 12 and 12' are thus placed at a distance from the feed side surface 3a and the accept side surface 3b of the screen wire 3 such that the screen wire 3 of Figure 14 forms a profiled screen wire 3 having a cross-like cross section. The apertures 9 or the channels 9 are arranged in the side flanges 12 and 12' in the sides 3c and 3d of the screen wires 3 such that they extend from the feed side 6 of the screen cylinder 1 to the accept channel 8. In the longitudinal direction of the screen wires 3, said apertures 9 may be provided in the side flanges 12 and 12' side by side in the above-described manner. Apertures 9 are provided in the first side flange 12 such that the apertures 9 are open in the direction against the feed flow direction F of the pulp mixture, i.e. at the apertures 9 the screen wire 3 does not comprise screen wire manufacturing material between the aperture 9 and the second side flange 12' in the adjacent screen wire 3. Apertures 9 are also provided in the second side flange 12' such that the apertures 9 are open in the feed flow direction F of the pulp mixture, i.e. at the apertures 9 the screen wire 3 does not comprise screen wire manufacturing material between the aperture 9 and the first side flange 12 in the adjacent screen wire 3. In the screen cylinder of Figure 14 the screen wires 3 are placed side by side to one another such that the side flanges of the screen wires will be in contact with one another on the finished screening surface such that a screening surface having apertures is formed.

[0046] Figure 15 shows schematically a seventh screen cylinder, seen from the end of the screen wires and crosscut. The structure of the screen cylinder 1 in Figure 15 corresponds to that of the screen cylinder shown in Figure 14. However, the structure of the screen cylinder in Figure 15 differs from the screen cylinder in Figure 14 in that the side flanges of the screen wires 3 are placed at a distance from one another such that, in addition to the apertures 9 provided in the sides 3c, 3d of the screen wire profile 3, the finished screening surface comprises slots 10 between the screen wires 10.

[0047] In the embodiments of Figures 14 and 15, the apertures 9 may be, for instance, circular, as in Figure 13, or semi-circular, as in Figure 9, in shape. Also other cross-sectional shapes are applicable, for instance, oval or elliptical shape or halves of these shapes. It is possible that the apertures 9 in the screen wire 3 are located, as in Figures 14 and 15, in both side flanges 12, 12' of the screen wires 3 or only either in the first side flange 12 or in the second side flange 12'. The extent or depth of the apertures 9 in the longitudinal direction of the side flanges, i.e. substantially in the feed flow direction F of the pulp mixture may be arranged such that the depth of the apertures 9 correspond to the length of the side flanges or such that they extend only to a portion of the length of the side flanges 12, 12' in accordance with Figures 14 and 15. The portion of the screen wire locating above the side flanges 12 and 12' as seen in Figures 14 and 15 or the top flange of the screen wire having a cross-like cross sectional profile may be relatively high as in Figures 14 and 15 in relation to the total height of the screen wire 3, but the top flange may also be very low. The apertures 9 may extend in the longitudinal direction of the side flange also to the area of the vertical portion of the screen wire.

[0048] The screen wire 3 having a cross-like cross sectional shape as shown in Figures 14 and 15 cleans the screen effectively. The top flange of the screen wire 3 brushes the surface of a rotor or foil preventing spinning formations. The screen wire having a cross-like cross section generates, on the other hand, a strong turbulence in the feed flow of the screen or classifier and thus prevents accumulations on the surfaces of the screen wire 3 and thus prevents the screen cylinder or screen basket from being clogged. The screen wire having a cross-like cross section enables a screen wire having large and high dimensions that resists well mechanical stresses. The mechanical resistance of the screen wire is further improved by the symmetrical structure of the screen wire. In addition, even though individual screen wires 3 are fairly broad because of the side flanges 12 and 12', the structure of the screen cylinder encompasses, however, relatively large open screening area. The large, open screening area of the screening surface translates to large yield as the screen cylinder is used.

[0049] Figure 16 shows schematically an eighth screen cylinder 1 , seen from the end of the screen wires 3 and crosscut along the line H - H appearing in Figure 17, and Figure 17 shows schematically the screen cylinder 1 of Figure 16, seen in the direction of the screening surface. Figures 16 and 17 show, by way of example, three of the screen wires 3 of the screen cylinder 1. For the sake of clarity, Figures 16 and 17 do not show the support bar 4. The exemplary feed flow direction of the pulp mixture in the screen cylinder 1 is indicated by arrow F. The screen wire 3 comprises a feed side surface 3a directed to the feed side 6 of the screen cylinder 1 , an accept side surface 3b directed to the accept side 7 of the screen cylinder 1 , a first side 3c of the screen wire 3 directed against the feed flow direction F of the pulp mixture and a second side 3d of the screen wire 3 directed in the same direction with the feed flow direction F of the pulp mixture. In the screen wire 3 of Figure 16 the feed side surface 3a directed to the feed side of the screen cylinder 1 is formed wavy, but the feed side surface 3a of the feed side of the screen wire 3 could also be formed flat.

[0050] The first side 3c of the screen wire of Figure 16 comprises a first side flange 12 or a first side branch 12 and the second side 3d comprises a second side flange 12' or a second side branch 12' such that the portion of side flanges 12 and 12' that is directed substantially to the feed side of the screen cylinder 1 constitutes part of the feed side surface 3a of the screen wire 3. The screen wire 3 of Figure 14 forms thus a profiled screen wire 3 having a substantially T-shaped cross section. The apertures 9 or channels 9 are arranged in the side flanges 12 and 12' of the sides 3c and 3d of the screen wires 3 such that they extend from the feed side 6 of the screen cylinder 1 to the accept channel 8. In the embodiment of Figures 16 and 17 the apertures 9 are semi-circular and they are placed at different points in the sides 3c and 3d of the screen wire 3 in the longitudinal direction L of the screen wire 3, whereby a structure resisting well to mechanical stress is obtained. Apertures 9 are formed in the first side flange 12 such that the apertures 9 are open in the direction against the feed flow direction F of the pulp mixture, i.e. at the apertures 9 the screen wire 3 does not comprise screen wire manufacturing material between the aperture 9 and the second side flange 12' of the adjacent screen wire 3. Apertures 9 are also provided in the second side flange 12' such that the apertures 9 are open in the feed flow direction F of the pulp mixture, i.e. at the apertures 9 the screen wire 3 does not comprise screen wire manufacturing material between the aperture 9 and the first side flange 12 of the adjacent screen wire. The adjacent screen wires 3 may be placed in close contact with one another such that the adjacent screen wires' 3 apertures 9 facing one another are in the same line, whereby circular apertures will be provided in the finished screening surface, or in accordance with Figures 16 and 17, such that elongated, circular apertures will be provided in the finished screening surface due to screen slots 10 between the screen wires. [0051] The screen cylinder consisting of screen wires having a T- shaped cross section is an alterative solution to the conventional perforated plate screen basket, in which a purifying, turbulent flow is provided by selecting a suitable shape for the feed side surface 3a of the screen wire 3. Further, the screen wire having a T-shaped cross section may provide a broad and high screen wire, which endures well mechanical stresses. The symmetrical structure of the screen wire 3 improves the structural endurance of the screen wire and further of the whole screen cylinder. Even though the screen wire 3 with the side flanges 12 and 12' of Figures 16 and 17 will be relatively broad, the structure may comprise, however, a relatively large open screening area. In one or both side flanges of the T-profile it is possible to implement a suitable number of apertures of suitable size and depth for the purpose, which may also extend to the vertical portion of the T-profile.

[0052] In the embodiment of Figures 16 and 17 it is shown how apertures 9 are placed on the level with the feed side surface in the feed flow direction F of the pulp mixture or therebelow such that the flow in the direction of the feed side surface will not be able to collide with the back wall of the aperture 9, whereby the aperture keeps clean and its capability of passing the flow remains high.

[0053] In the height direction of the screen wire 3, i.e. in the direction that is substantially transverse to the feed flow direction F of the pulp mixture suspension in Figure 16, the thickness or extent of the side flanges 12 and 12' may be slight, because the vertical portion of the screen wire 3 may be dimensioned to receive the major part of mechanical stress loading the screen wire 3. In addition, the side flanges 12 and 12' having a thin structure resist the flow less than thick side flanges, whereby reduction in the thickness of the side flanges 12 and 12' makes it possible to increase the amount and proportion of accepted fractions flowing through the screening surface.

[0054] The corners of the screen wire 3 profile may be rounded, but they are often manufactured relatively sharp, because sharp edges produce more turbulence in the pulp mixture suspension flow. The T-profile of Figures 16 and 17 comprises a substantially sharp-cornered profile. Relatively great turbulence is often desirable, because the material to be screened or classified can be kept in motion and the material is prevented from being stopped on the surfaces of the screen wire 3, and thus, clogging will be prevented. [0055] In the above examples, a screen wire profile having apertures through at least one screen wire profile side or side flange is applied for providing a screen cylinder or a screening surface of a flat screen. The application of the solution is not restricted, however, to the implementations of the examples, but the screen wire profiles of the solution allow a cylindrical, conical or planar screening surface to be formed. The screen wire profiles of the solution also allow a curved screening surface to be formed, whereby the solution may also be applied in the curved screens.

[0056] In some cases the features described in the present application may be employed as such, irrespective of other features. On the other hand, features described in the present application may be combined, when necessary, to provide various combinations.

[0057] The drawings and the relating description are only intended to illustrate the inventive idea. The details of the invention may vary within the scope of the claims.

Claims

1. A screen cylinder (1) for purifying or classifying pulp mixture, the screen cylinder (1) comprising screen wires (3) forming a cylindrical screening surface (2) of the screen cylinder (1), characterized in that at least in one screen wire (3) there are provided apertures (9) through at least one side (3c, 3d) of the screen wire (3), through which apertures the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1) to the accept side (7) of the screen cylinder (1 ).

2. The screen cylinder of claim 1, characterized in that all the screen wires (3) of the screen cylinder (1) comprise apertures (9) provided through at least one side (3c, 3d) of the screen wire (3), through which apertures the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1) to the accept side (7) of the screen cylinder (1 ).

3. The screen cylinder of claim 1, characterized in that at least one screen wire comprises apertures (9) provided through the side (3c) directed against the feed flow direction (F) of the pulp mixture, through which apertures the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1 ) to the accept side (7) of the screen cylinder (1 ).

4. The screen cylinder of any one of the preceding claims 1, characterized in that all screen wires (3) comprise apertures (9) provided through the side (3c) directed against the feed flow direction (F) of the pulp mixture, through which apertures the fluid in the pulp mixture and the proportion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1) to the accept side (7) of the screen cylinder

(1).

5. The screen cylinder of any one of the preceding claims, characterized in that the screen wires (3) are arranged side by side in contact with one another such that the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1 ) to the accept side (7) of the screen cylinder (1 ) only through said apertures (9).

6. The screen cylinder of any one of claims 1 to 4, characterize d in that the screen wires (3) are arranged side by side with a mutual spacing such that a slot (10) will be provided between adjacent screen wires such that the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) and the slot (10) may flow from the feed side (6) of the screen cylinder (1) to the accept side (7) of the screen cylinder (1) both through the slots (10) between the screen wires (3) and through said apertures (9).

7. The screen cylinder of any one of the preceding claims, characterized in that the apertures (9) are semi-circular in cross section.

8. The screen cylinder of any one of claims 1 to 6, characterize d in that the apertures (9) are semi-oval in cross section.

9. The screen cylinder of any one of the preceding claims, characterized in that the screen wire (3) comprises, in the side (3c) directed against the feed flow direction (F) of the pulp mixture, a side flange (12) and that the side (3d) in the feed flow direction (F) of the pulp mixture comprises a side flange (12') directed in the feed flow direction (F) of the pulp mixture and that said apertures (9) are provided through at least one said side flange (12, 12').

10. A method for manufacturing a screen cylinder (1) intended for purifying or classifying pulp mixture, the method comprising mounting screen wires (3), forming the screening surface (2) of the screen cylinder (1 ), side by side onto support bars (4) of the screen cylinder (1), bending the support bars (4) into rings and attaching the ends (4', 4") of the support bars (4) bent into rings to one another such that the screen wires (3) form a cylindrical screening surface (2) in the axial direction of the screen cylinder (1), characterized in that at a manufacturing stage of the screen cylinder (1 ) there are provided apertures through at least one side (3c, 3d) of at least one screen wire (3), through which apertures (9) the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1 ) to the accept side (7) of the screen cylinder (1 ).

11. The method of claim 10, characterized in that at a manufacturing stage of the screen cylinder (1) there are provided apertures (9) through at least one side (3c, 3d) of all screen wires (3), through which apertures the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1 ) to the accept side (7) of the screen cylinder (1 ).

12. The method of claim 10, characterized in that at a manufacturing stage of the screen cylinder (1) there are provided apertures (9) through the side (3c) of at least one screen wire (3) directed against the feed flow direction (F) of the pulp mixture, through which apertures the fluid in the pulp mixture and the proportion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1 ) to the accept side (7) of the screen cylinder (1 ).

13. The method of any one of claims 10 to 12, characterized in that at a manufacturing stage of the screen cylinder (1) there are provided apertures (9) through the sides (3c) of all screen wires (3) directed against the feed flow direction (F) of the pulp mixture, through which apertures the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1) to the accept side (7) of the screen cylinder (1 ).

14. The method of any one of claims 10 to 13, characterized in that as the screen wires (3) are mounted, the screen wires (3) are arranged side by side in contact with one another such that the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1) to the accept side (7) of the screen cylinder (1) only through said apertures (9).

15. The method of any one of claims 10 to 13, characterized in that as the screen wires (3) are mounted, the screen wires (3) are arranged side by side with a mutual spacing such that a slot (10) will be provided between the adjacent screen wires (3) such that the fluid in the pulp mixture and the proportion of fibres defined by the size of the apertures (9) and the slot (10) may flow from the feed side (6) of the screen cylinder (1 ) to the accept side (7) of the screen cylinder (1) both through the slots (10) between the screen wires (3) and through said apertures (9).

16. The method of any one of claims 10 to 15, characterized by providing said apertures (9) in the screen wires (3) prior to mounting the screen wires (3) in connection with the support bars (4).

17. The method of any one of claims 10 to 15, ch a ra cte rize d by providing said apertures (9) in the screen wires (3) after the screen wires (3) have been mounted in connection with the support bars (4) but before bending the support bars (4) into rings.

18. The method of any one of claims 10 to 17, characterized in that the apertures (9) are semi-circular in cross section.

19. The method of any one of claims 10 to 17, ch a ra cte rized in that the apertures (9) are semi-oval in cross section.

20. The method of any one claims 10 to 19, characterized by providing said apertures (9) in the sides (3c, 3d) of the screen wire (3) by milling.

21. The method of any one of claims 10 to 20, characterized in that the screen tread (3) comprises, in the side (3c) directed against the feed flow direction (F) of the pulp mixture, a side flange (12) directed substantially against the feed flow direction (F) of the pulp mixture and that the screen wire (3) comprises, in the side (3d) in the feed flow direction (F) of the pulp mixture, a side flange (12') directed substantially in the feed flow direction (F) of the pulp mixture and that said apertures (9) are provided through at least one of said side flanges (12, 12').

22. A method for manufacturing a screen cylinder (1) intended for purifying or classifying pulp mixture, the method comprising mounting screen wires (3) forming a screening surface (2) of the screen cylinder (1) side by side to support bars (4) of the screen cylinder (1 ), bending the support bars (4) into rings and attaching the ends (4', 4") of the support bars (4) bent into rings to one another such that the screen wires (3) form a screening surface (2) in the axial direction of the screen cylinder (1), characterized by mounting screen wires (3) forming the screening surface (2) of the screen cylinder (1) side by side to the support bars (4) of the screen cylinder (1), in which screen wires (3) there are formed in advance, at a manufacturing stage of the screen wire (3), apertures (9) through at least one side (3c, 3d) of the screen wire (3), through which apertures the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1 ) to the accept side (7) of the screen cylinder (1 ).

23. The method of claim 22, characterized in that the apertures (9) are provided in the screen wire (3) in connection with the mangling of the screen wire (3), in particular in connection with the cold rolling of the screen wire (3).

24. The method of claim 22 or 23, c h a r a c t e r i z e d in that in the side (3c) of the screen wire (3) directed against the feed flow direction (F) of the pulp mixture there is a side flange (12) directed substantially against the feed flow direction (F) of the pulp mixture and that in the side (3d) of the screen wire (3) directed in the feed flow direction (F) of the pulp mixture there is a side flange (12') directed substantially in the feed flow direction of the pulp mixture and that said apertures (9) are provided through at least one of said side flanges (12, 12').

25. A flat screen (11 ) for purifying or classifying pulp mixture, which flat screen (11 ) comprises screen wires (3) forming a planar screening surface (2), c h a r a ct e r i z e d in that at least one screen wire (3) comprises apertures (9) provided through at least one side (3c, 3d) of the screen wire (3), through which apertures the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1 ) to the accept side (7) of the screen cylinder (1 ).

26. The flat screen of claim 25, c h a r a c t e r i z e d in that all the screen wires (3) of the screening surface (2) in the flat screen (11) comprise apertures (9) provided through at least one side (3c, 3d) of the screen wire (3), through which apertures the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1 ) to the accept side (7) of the screen cylinder (1 ).

27. The flat screen of claim 25 or 26, c h a r a c t e r i z e d in that the screen wires (3) are arranged side by side in contact with one another such that the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen cylinder (1 ) to the accept side (7) of the screen cylinder (1 ) only through said apertures (9).

28. The flat screen of claim 25 or 26, c h a r a c t e r i z e d in that the screen wires (3) are arranged side by side with a mutual spacing such that a slot (10) is provided between the adjacent screen wires (3) such that the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) and the slot (10) may flow from the feed side (6) of the screen cylinder (1 ) to the accept side (7) of the screen cylinder (1 ) both through the slots (10) between the screen wires (3) and said apertures (9).

29. The flat screen of any one of the claims, c h a r a c t e r i z e d in that the apertures (9) are semi-circular in cross section.

30. A screen for purifying or classifying pulp mixture, which screen (1, 11) comprises screen wires (3) forming a screening surface (2) of the screen (1, 11), characterized in that at least one screen wire (3) of the screening surface (2) comprises apertures (9) provided through at least one side (3c, 3d) of the screen wire (3), through which apertures the fluid in the pulp mixture and the portion of fibres defined by the size of the apertures (9) may flow from the feed side (6) of the screen (1 ) to the accept side (7) of the screen (1 ).

31. The screen of claim 30, characterized in that the screen wire (3) comprises a first side (3c) and a second side (3d) and that the first side (3c) comprises a first side flange (12) and that the second side (3d) comprises a second side flange (12') and that said apertures (9) are provided through at least one side flange (12, 12¹).

32. The screen of claim 30 or 31, characterized in that the screening surface (2) is cylindrical, conical, arcuate or planar.