SCREEN PLATE

Abstract: The invention relates to a screen plate for screening or fractionating fibre suspensions, especially suspensions of papermaking pulp, which screen plate is of the type where the screen holes are arranged in rows substantially perpendicular to the flow direction of the suspension. In order to increase the efficiency of the screen, the screen according to the invention comprises plane unperforated surfaces (3, 4, 5, 6, 8) for ejecting shives and other impurities, and the distances between the rows are substantially greater than the mutual distances between the holes in the rows.

Description: Screen plate This invention relates to a screen plate to be used in apparatuses for screening or fractionating fibre material suspensions, especially suspensions of paper- making pulp. The screen plate is of the type provided with screen holes arranged in rows substantially per¬ pendicular to the flow direction in the apparatus. Several such screen plates are already known for a long time. The European patent application with publication number 2 6 8 3 discloses a screen plate, which is provided with circular screen holes located in the bottom of parallel grooves in the plate. The essential feature of the screen plate according to said application is, that the grooves shall be formed in a special way, viz. so that their "walls" shall form a certain definite angle with the surface of the plate, and the bottom of the grooves shall have a certain width in relation to the diameter of the screen holes. The European patent application with publication number 79 8ll also discloses a screen plate with the screen holes arranged in parallel grooves. At least one wall in the grooves extends in radial direction perpendicular to the flow direction of the suspension. In Swedish patent application available to the public No 84.00330 a screen plate is described, where the screen holes are located in the bottom of grooves, the direction of which deviates from the flow direction of the incoming suspension. Also in this case the form of the grooves is said to be important. The bottom of .the grooves, thus, shall be plane and in parallel with the surface of the screen plate. Both sides of the grooves, further, shall be inclined and form an agle of βθ-5 degrees with the screen surface. The object of the arr- angement according to said application is to bring about micro turbulences, whereby fibre flocculation is slushed and thickening is prevented. Other examples of previously known screen plates are shown in US-PS 2 827 169 with waved screen plate, in SE-PS with publication number 374 1*^8 with a cleaning drum provided with warts or eyelid-shaped protrusions, and in SE-PS with publication number 374 00 with screen holes, the edges of which located downstream are recessed in relation to the surface of the screen plate. The previously known screen plates, however, have several disadvantages. It is, for example, not possible with the previously known screen plates to achieve a sufficiently selective separation of the material. It is not possible, either, to obtain a satisfactory shives separation. Known screen baskets often comprise holes close-packed in either more or less square or uniformly triangular patterns. The flμidization effected by the grooves in the hole rows results in the acceptance of impurities and shives, which properly should be removed as reject. This is due to the fact, that there is no space for the orientation of shives and oblong impurities. The present invention eliminates these disadvantages. According to the invention, the screen plate comprises between the rows of screen holes substantially plane unperforated surfaces for the erection of shives and other impurities. The distance between the rows, calc¬ ulated in the flow direction, shall be substantially greater than the mutual distances between the holes in the rows, so that the following conditions are met tl - D = 0.5 mm, preferably 1-3 mm and t2 - D = 4 - 12 mm, preferably 5-8 mm where tl is equal to the distance between the holes, t2 is equal to the distance between the rows, and D is equal to the diameter in a circular hole with a cross-sectional area equal to the cross-sectional area of the screen holes. According to a special embodiment of the screen plate according to the invention, the holes have substanti¬ ally circular cross-section and are located in the bottom of parallel grooves. According to another important embodiment, the surfaces, the so-called erection distances, are provided with turbulence-forming and thereby capacity-increasing grooves or protuberances, so-called warts. It was found that these grooves should be arranged substantially in parallel and should extend in the flow direction of the suspension or at an angle there¬ to. The grooves can be arched, for example by milling. The screen holes, as mentioned, can be circular, but may also have another cross-sectional shape, for ex¬ ample be formed as slits. The invention is described in greater detail in the following by way of some embodiments and with reference to the accompanying drawings, in which Fig. 1 schemat¬ ically shows a screen plate with inserted designations tl, t2 and D, Fig. 2 also schematically shows how the grooves can be arranged, Figs ' . 3 and 4 show an arrange¬ ment where the screen holes are located in the bottom of grooves, and Fig; 5 is a diagram showing the technical effect obtained according to the invention. In Fig. 1 a screen plate 1 with circular holes 2 is shown. The designations tl, t2 and D are identical with the ones specified above. In Fig. 2 is shown how the grooves can be arranged in the erection distances, either in parallel with the flow direction of the sus¬ pension as at 6, perpendicular thereto as at 3 or at a certain arbitrary angle as at 4 and 5. In Figs. 3 and 4 a particularly advantageous embodiment is shown. The screen holes 2 here are located in the bottom of grooves 7 3 which in the Figure are shown to be parallel. The grooves 8 according to this embodiment have saw-toothed cross-section, but may, of course, have another shape and be located at greater distances and may have varying depth. The paper technology develops toward processes, which require fibre compositions of greater homogeneity. The screening technique in this respect is of decisive im¬ portance, and the perforation and surface geometry of the screen plates used in the screens is essential for the result achieved. The screen plate according to the invention renders it possible to separate shives and other oblong impurities with high selectivity. At fibre fractionation long fibres easily can be separated from short ones. The erection distances arranged on the screen plate according to the invention imply improved shives separ¬ ation and increase fine-scale turbulence at the screen plate surface. Shives and oblong impurities are erected and oriented between the rotating cleaning drum and stationary screen basket. The shives most often are sep¬ arated due to their length and, therefore, are to be oriented so as to be located over the screen hole. Such erection and orientation is facilitated substant¬ ially by the screen plate according to the invention, at the same time as the turbulence intensity and thereby the capacity increase. The members producing turbulence in the orientation distances give rise to fluidization of the fibre sus¬ pension. Fluidization implies that the fibres are freely movable in relation to each other and prevent mechanical binding of impurities to the fibre material. Owing to the invention, shives orientate in the fluidized fibre suspension between the hole rows. The unperfor¬ ated distances prevent thickening of the suspension when long shives orientate in the shear field in front of the screen basket. According to the invention, the screening process is divided into two phases, viz. the orientation phase and the phase of passage through the holes. In the diagram according to Fig. 5 a result of tests with a screen plate according to the invention is shown, where the number of shives with the length 3-6 mm in the accept is indicated as a function of (t2 - D). The number of shives, as can be seen, de¬ creases considerably at a higher (t2 -D) value. The invention is not restricted to the embodiments shown, but can be varied within the scope of the in¬ vention idea.

Claims: Claims 1. A screen plate for screening or fractionating fibre material suspensions, especially papermaking pulp, comprising a plate (1) provided with screen holes (2) arranged in rows substantially perpendicular to the flow direction of the pulp, which holes are intended for the accept separated from the suspension, c h a r a c t e r i z e d i n that the plate between the rows comprises substantially plane unper¬ forated surfaces (3,4,5,6,8) for the erection of shives and other impurities, and that the distances between the rows, calculated in the flow direction, is substantially greater than the mutual distances between the holes in the rows, so that tl - D = 0.5 - 4 mm. preferably i - 3 mm and t2 - D = 4 - 12 mm, preferably 5 - 8 mm where tl is equal to the distance between the holes, t2 is equal to the distance between the rows, and D is equal to the diameter in a circular hole with a cross-sectional area equal to the cross-sectional area of the screen holes. 2. A screen plate as defined in claim 1, c h a r a c t e r i z e d i n that the holes (2) have substantially circular cross-section and are arranged in the bottom of substantially parallel grooves (7 in Figs. 3 and 4). 3. A screen plate as defined in claim 1, c h a r a c t e r i z e d i n that the surfaces, the so-called erection distances, are provided with turbulence-forming and thereby capacity-increasing grooves or protuberances (8), for example warts. 4. A screen plate as defined in claim 3, c h a r a c t e r i z e d i n that the grooves are arranged substantially in parallel and extend in the flow direction of the suspension or at an angle therewith.

Inventors: GRUNDSTROEM KARL JOHAN, GRUNDSTROEM, KARL, JOHAN
Applicants: KNUTSILPLAOTAR AB, AKTIEBOLAGET KNUTSILPLAATAR
Application year: 1988
Main classification: D21D5
IPC codes: B01D29, D21D5