PRIOR ART In cellulose pulp processing, screw presses are used in certain applications for the purpose of increasing the dry substance content of the fibre pulp by means of dewatering. These screw presses are available in various designs. A common feature of these is that a screw rotates in a cylinder consisting of a screen plate. The cylinder is filled with a pulp suspension of a certain consistency. By means of the rotation of the screw, water is pressed out of the pulp and removed through holes in the plate cylinder.

The perforated screen plate in the screen cylinder is exposed to high axial and radial forces.

Existing constructions of screen cylinders have hitherto been based on a thick screen plate (ca 12 mm) which, because of its thickness, is provided with double-bored holes, which is costly. Double boring (with a greater diameter on the drainage side of the plate) must be carried out in order to prevent clogging of holes through thick plate. On production of a screen cylinder, certain operations are made more difficult and more costly on account of the thickness of the screen plate. In addition, the great thickness itself entails greater cost. A large proportion of the overall cost of a screw press is accounted for by the cost of the screen cylinder.

The considerable loading and wear of the screen plate means that the screen plate has to be replaced at regular intervals. In the constructions used at present, this is complicated and is associated with considerable costs. The thick plate makes the screen

cylinder heavy and difficult to work on. The design of the cylinder is such that the screen cylinder including the support structures has to be removed. This is time- consuming and complicated. describes a known arrangement for treating and removing chemicals from fibre pulp.

The arrangement is associated with problems of the type indicated above. The arrangement consists of a screw press in which a press screw is surrounded by a thick- walled, perforated cylindrical plate through which the liquid being removed is pressed out. The thick-walled, perforated plate lies in a housing and is anchored in the housing by means of intermediate ribs. These ribs are placed symmetrically around the circumferential surface of the perforated plate and have the main purpose of separating the arrangement into sections A- D, the screen plate in each section being completely free between the ribs. The thickness of the plate in this construction is necessarily considerable since the plate in each section must itself be able to cope with the stresses to which it is exposed. It is clear from the drawings that replacement of the screen plate would be complicated. The construction is expensive and it is costly to maintain.

US-A-3,943,034 describes another screw press for dewatering and homogenizing pulp. The arrangement consists of a perforated, hollow screw surrounded by a thick-walled screen drum. The liquid being dewatered from the pulp can pass out through the screen drum and also in through the screw. The screen drum surrounding the screw and the screw itself have a conical shape.

The housing surrounding the screen drum is divided into two sections and the screen drum is anchored in the housing. The specification does not say anything about the thickness of the screen plate. Judging from its structure, the screen plate is exposed to considerable forces, for which reason it ought to be made of sheet metal of considerable thickness, and is therefore expensive. The screen plate surrounding the screw is

placed in such a way that it passes through two compartments which are separate from each other. In one compartment, liquid passes out through the screen plate. In the other compartment, steam is added to the pulp and liquid passes out through the hollow, perforated screw. This division into two compartments presumably makes it considerably difficult to replace the screen plate.

US-A-4,582,568 moreover describes a screw press for controlling the water content of a fibre suspension. The arrangement consists of two parts: a thickener and a pulp press of the screen type. The thickener consists of a screw surrounded by a cylindrical, thick-walled screen plate which lies in a cylindrical outer housing. The water which leaves the pulp passes through the screen plate. The pulp is driven from the thickener onwards to the pulp press, which is made up of a conical screw surrounded by a screen plate. The housing which surrounds the pulp press has a sloping floor so that the water leaving through the screen plate will be able to run off. The screen plate is anchored in the surrounding housing.

The thickness of the screen plate is not specified.

Since as small as distance as possible is sought between screw and plate, it may be assumed that the plate is exposed to considerable forces as the water content of the pulp drops. The thickness of the plate in this construction must therefore also be considerable. Changing the plate in both of the said parts of the arrangement is relatively complicated. A number of pipes are connected to the parts. Moreover, the screen plates appear to be well built-in, which also makes changing the screen plate more complicated.

Finally, reference is made to the prior art described in Finnish laid-open specification 54719, which concerns an arrangement for separating liquids and solid substances. The arrangement comprises a cylindrical, thick-walled drum with a perforated wall.

A screw conveyor is placed in the screen drum. Liquid

is added to the liquid mixture in the drum via nozzles on the screw conveyor. No support structures are shown in the drawings, for which reason the plate thickness must be great and the cost high. With such a plate thickness, changing the screen plate is associated with a number of time-consuming operations.

BRIEF DESCRIPTION OF THE INVENTION The object of the present invention is to make available a screen cylinder which eliminates or at least minimizes the abovementioned problems. One aim of the invention is to make available a screen cylinder for dewatering pulp, and a method for producing the screen cylinder, which screen cylinder and method ensure complete functionality of the screen cylinder at less cost and permit economic production.

A further aim of the invention is to make available a screen cylinder whose screen plate is easier to replace upon servicing. According to the invention, this is achieved by means of the features specified in the characterizing clause of Claim 1.

An important advantage of the invention is that the screen plate can be made very thin, which permits low costs for materials and easier working and also eliminates the need for double-bored holes. The great advantage in terms of maintenance of the screen cylinder is that the whole structure together with support rings can be re-used. When the screen plate is worn, it can be removed and replaced, while the actual frame can be used again as a support for the new screen plate. This makes replacement and maintenance of the screen press much less expensive.

Further characteristics and aspects of the invention will become evident from the following description of a preferred embodiment and from the attached patent claims.

BRIEF DESCRIPTION OF THE FIGURES In the following description of a preferred embodiment, reference is made to the attached drawings in which: Fig. 1 is a perspective view of a screen cylinder, Fig. 2 is a coaxial view of a support ring, Fig. 3 is a perspective view of a spacer, Fig. 4 shows a detail from Fig. 1, with the support rings supporting a screen plate, Fig. 5 shows a dewatering arrangement with screen plates according to the prior art, Fig. 6 shows an alternative screen cylinder in divided form, Fig. 7 shows a detailed view of the connection element 71a from Figure 6, and Fig. 8 shows a side view of the connection element shown in Figures 6 and 7.

DETAILED DESCRIPTION The field of application of the invention is that of arrangements for dewatering woodpulp suspensions in accordance with Figure 5, comprising a displacement screw 2 arranged inside a screen cylinder.

The screen cylinder comprises, in a conventional manner, a number of sections with screen cylinders 1 which are combined to give the necessary dewatering length. The woodpulp suspension is introduced via the inlet 3 and, under displacement of the rotating screw 2, liquid is pressed out radially through the screen plates for onward delivery to the liquid outlet 5. The dewatered woodpulp suspension is obtained at the outlet 4. In a final section lb, a final dewatering is carried out at very high pressure. To ensure that the dewatering arrangement can withstand the very high pressure in the pulp, the screen plate is made very thick, usually around 10-15 mm, preferably 12 mm, resulting in a heavy and expensive construction.

Referring first to Fig. 1, a screen cylinder is designated generally by 1. The screen cylinder 1 comprises a screen plate 20 in the form of a cylinder.

Support rings 30 lie on the screen plate 20. The support rings 30 enclose the screen cylinder 1 about its circumferential surface. At the start and end of the assembly of support rings 30 along the screen cylinder 1, there is an end ring 60. The support rings 30 are held axially apart by means of spacers 50. The support rings 30 are anchored in the end rings 60 by means of bars 40 which run coaxial with the screen cylinder 1, and by means of spacers 50 and corresponding holes in each support ring 30. The external diameter of the bars is generally between 13 mm and 21 mm. The bars are preferably hollow with an internal diameter generally of between 12 mm and 20 mm.

The screen plate 20 consists of a sheet of suitable metal, preferably stainless steel, provided with single-bored circular holes 21. The screen plate 20 has a thickness of 2 to 6 mm. It is welded together along a weld seam 22 which runs axially along the screen cylinder 1. A screen plate 20 forms a screen cylinder unit which can have a length of between 1 m and 2 m. The screen cylinder 1 generally has an internal diameter (d) of between 0.7 m and 1.5 m, preferably 0.25 m to 2 m.

The support rings 30 (see Fig. 2) are preferably laser-cut sheet metal rings for the purpose of achieving high precision. The thickness of the support rings 30 is generally between 3 mm and 8 mm.

Formed in these support rings 30 there are a number of holes 31 which are spaced along the entire ring 30 at a mutual spacing of between 100 mm and 400 mm. The internal diameter of the support ring 30 constitutes the size of the screen cylinder. The height (h) of the support ring is between 50 mm and 100 mm. The support rings 30 are arranged in a row about the circumferential surface of the screen cylinder 1. The

support rings 30 are separated from each other (by spacers 50) with a spacing of between 30 mm and 150 mm.

The number of support rings 30 per metre of screen cylinder is 5-35, preferably 10-25, and most preferably 15-20. In this way, it is possible to use a very thin screen plate which is unsupported across a relatively short distance between two adjacent support rings.

Fig. 3 shows the spacers 50 which consist of cylinders with a circular hole 51. The spacer 50 has an internal diameter which is greater than the said bar 40 and an external diameter which is greater than the hole 31 in the support ring 30. The internal diameter of the spacer 50 is the external diameter of the bar 40 plus 1 mm. The internal diameter of the spacer 50 is generally between 14 mm and 22 mm. The spacers 50 are preferably made of stainless steel and preferably have a thickness of at least 2 mm.

The bar 40 consists of a pipe or a rod and is preferably made of stainless steel, for example 2333, 2343 or duplex. The bar 40 runs through the holes 31 in the support rings 30 and through the holes 51 in the spacers 50. The bar 40 is fixed at each end to an end ring 60 via a threaded connection or a welded connection.

Fig. 1 shows the end ring 60, which preferably consists of a ring of the same material, i. e. stainless steel. It has an internal diameter adapted to the screen plate 20. The end ring 60 preferably has a thickness of between 10 mm and 30 mm. In the end ring 60, there are bolt holes 61 which are placed uniformly about the entire end ring. The distance between the bolt holes 61 on the end ring 60 is between 100 mm and 250 mm. The bars 40 passing through the support rings 30 are fixed in these holes 61.

The support arrangement 10 for a screen cylinder 1, comprising an end ring 60 at each end of the screen plate 20 and support rings 30 separated by spacers 50, is held together by means of these bars 40.

The bars 40 are fixed in the end rings 60 by screws or are fixed by welding.

Figure 4 shows an enlarged detail from Figure 1, with the support rings 30 arranged on the outside of the assembled screen plate 20. The bars 40 pass through all the support rings, and the spacers 50 are arranged coaxially on the bars. The spacers ensure that the support rings 30 are situated at a suitable distance from each other and that the screen plate thereby has the necessary support in the radial direction from the screen cylinder. In the embodiment shown, the screen plate 20 has been provided with double bores 21a and 21b, with a slightly larger bore size arranged furthest from the woodpulp suspension which is to be dewatered (the woodpulp suspension is conveyed underneath the screen plate in Figure 4).

The invention means that each individual screen section can be built up with unsupported areas between the end rings 60. The support rings do not require any radially external support if the screen plate and support rings are suitably dimensioned.

The production of the screen cylinder 1 is illustrated on the basis of two different methods. Both of these methods for producing the screen cylinder 1 are divided into a number of stages. In the first method, the screen plate 20 is welded together, after which the screen plate 20 is shrunk radially, for example by cooling or local buckling, or, alternatively, the support rings are heated. Support rings 30 are arranged on the outside of the screen plate 20, after which the screen plate 20 is expanded to engage them. This engagement can be obtained by the heated support ring structure being cooled, or by the locally deformed screen plate being allowed to resume its original shape in the radial direction. Welding can be effected between support rings and screen plate before removal of a mandrel underpinning the screen plate.

In the second method, the support arrangement 10 is pre-assembled, after which the screen plate is rolled up. The rolled-up screen plate 20 is inserted into the hollow space in the free support arrangement 10, after which the screen plate 20 is expanded until support rings 30 of the support arrangement 10 bear against the entire circumferential surface of the fully expanded screen plate 20. The screen plate 20 is then joined together by means of welding.

Figure 6 shows an alternative embodiment with a divided screen cylinder according to the invention, which facilitates repairs and replacement of individual screen cylinders in paper pulp dewatering constructions made up of a number of screen cylinders. The basic construction is similar to previously illustrated embodiments, but with the difference that a connection element 71 comprising a flat iron member 34a, 34b is welded into place 37 on both sides of a dividing plane 32 in each support ring half 30a, 30b. The screen plate halves 20a, 20b are also welded in each support ring half 30a, 30b with intermittent welds 70 of suitable number on the dividing plane. The screen plate halves are held together by a bolt and nut connection 35,36 through the flat iron members 34a, 34b, preferably between every second support ring, as is shown in Figure 8. The flat iron members can preferably be centred by means of a round rod 33 which is clamped securely between V-shaped grooves in the flat iron member. Other types of known centring arrangements for circle-forming halves can also be used. The screen arrangement of the type having a divided construction is preferably produced in a similar manner to the previously illustrated embodiment. In the divided construction, each screen plate is arranged in the shrunk state on pre-assembled support rings, after which the screen plate is expanded against the support ring, followed by application of an intermittent weld 70 in association with the dividing plane 32.

The invention is not limited to the above embodiments, and instead it can be varied within the scope of the attached patent claims. The bars can be made of different materials and with different geometrical cross sections. They can be solid or hollow. The spacers can be integrated with the bars or can consist of separate elements which are fixed to the said bars. The screen plate can of course be provided with holes other than by single boring. The support ring can, for example, have bevels inwards towards the surface bearing against the screen plate in order, if so required, to reduce the bearing surface against the screen plate, which could be an obstacle for the liquid flowing out through the screen. The attachment of the bars in the end rings can of course also be varied in a number of different ways. They can be fixed by welding or by screws. It will also be appreciated that a finished screw press according to the invention can consist of one or more combined screen sections, and not all the sections need necessarily be designed according to the invention.