Screening apparatus for paper making stock

Description

[0001] Paper mills have for many years made extensive use, for the cleaning of paper making stock, of screening apparatus embodying a cylindrical perforated screening member defining supply and accepts chambers on the opposite sides thereof in a closed housing, and including a rotor member which operates in one of the chambers to keep the screening perforations open and free from solid material tending to cling to the screening surface. Commonly, the stock or furnish is delivered to the supply chamber adjacent one end of the screening cylinder, and the material rejected by the screening cylinder is collected and discharged from the opposite end of the supply chamber.

[0002] The present applicants have manufactured and sold many such screens in accordance with a series of U.S. patents. commencing with Staege No. 2,347,716, and followed by Martindale No. 2,835,173, Seifert Nos. 3,849,302 and 4,105,543, and Chupka-Seifert No. 4,155,841. Starting with the construction shown in the Martindale patent, all such screens manufactured and sold by applicants have been characterized by a rotor comprising bars or vanes of airfoil section moving in closely spaced but non-contacting relation with the surface of the screening cylinder for the purpose of creating alternating positive and negative pressure waves effective on the perforations in the screening cylinder to prevent plugging thereof.

[0003] The art has experimented widely with detailed variations in screens of the above type, including variations in the vane shape and other forms of rotor, and also in the size configuration, and spacing of the perforations in the screening cylinder. Thus since the era of the Staege patent in the mid-1940's, many screening cylinders have been fabricated with multiple uniformly cylindrical drilled perforations, which commonly range in diameter from approximately 1.27 to 3.17 mm (0.050 inch to 0.125 inch).

[0004] In more recent years, the trade has been offered pressure screens generally of the above type wherein the perforations in the screening cylinder are elongated slots rather than round holes, with the slots running either circumferentially or axially of the cylinder. Typical such constructions are shown in Lamort U.S. Patent No. 3,617,008, Holz U.S. patent No. 3,581,903, and the above noted Seifert '302 and Chupka-Seifert patents.

[0005] Both of the Lamort and Chupka-Seifert patents also show, in addition to slotted cylinders, a plurality of shoulders or small bars running generally axially of the screen cylinder in circumferentially spaced relation around the inlet side of the cylinder. The rotor arrangement is described by Lamort as preventing clogging of the screening slots by fiber, albeit in an undescribed manner. In the Chupka-Seifert patent, the purpose of the bars is described as to generate a field of high intensity, fine scale turbulence in the stock adjacent the inlet side of the screen cylinder and thereby to effect screening of paper fiber stock with minimum fractionation thereof on the basis of fiber length.

[0006] The disclosure of the Chupka-Seiffert patent is limited to screen cylinders provided with circumferentially extending slots of a width range of only 0.025 to 0.203 mm (0.001-0.008 inch). A later Chupka-Seiffert U.S. application, Serial No. 145,654 filed May 2, 1980, discloses the application of a similar multi-bar arrangement to a screen cylinder having circumferentially extending slots of a substantially greater range of widths, i.e. as wide as 0.762 mm (0.030 inch) although the preferred width range is stated to be 0.356 to 0.559 mm (0.014-0.022 inch), with resulting increase in the capacity of the screen in terms of both tonnage per unit of time and the power requirements per unit of accepted fiber.

[0007] EP-A-0046687 discloses that when a pressure screen is equipped with a screening cylinder provided with screening holes of circular or vertically slotted shape and multiple bars spaced circumferentially about the inlet side thereof, dramatic improvements of the capabilities of the screen resulted. More particularly, when compared with standard screens having round holes in the screening cylinder in accordance with the above noted Seiffert patents, a screen having such an arrangement of circumferentially spaced bars on its inlet side will produce a higher tonnage of accepted fiber per day while screening stock of higher consistency through smaller holes, and also will draw less power per unit of accepted fiber.

[0008] Full realization of the advantages of the multiple bar screening cylinders disclosed in the above noted Chupka-Seiffert '841 patent and EP-A-0046687 has not been possible to achieve because it has not previously been practically possible to produce screening cylinders with as small circular holes or as narrow slots as are needed to reject the maximum number of contaminant particles greater in cross section than paper fibers, e.g. holes and slots having a minimum dimension as small as 0.025 mm (.001 inch).

[0009] More specifically, while the Chupka-Seifert '841 patent discloses screen cylinders provided with circumferentially extending slots of a width range of 0.025 to 0.203 mm (.001-.008 inch), it is extremely difficult to obtain such narrow slots of uniform width even when the cylinder is formed of spirally wound wire as disclosed in that patent. Further, since slots parallel with the axis of a screen cylinder are most conveniently produced by means of a saw, the minimum thickness of saws suitable for such use in steel or other metal adequately stiff for use as a screen cylinder establishes the practical minimum of such slots.

[0010] In the development of the present invention, it has been found that as a practical matter, the minimum width which can be maintained in a circumferentially slotted screen produced from spirally wound wire is not less than 0.256 mm (.0101 inch), and the smallest width obtainable with a saw in a screen cylinder having slots parallel with its axis is of the order of 0.152 mm (.006 inch). Similarly, it has been found that the practical minimum diameter of a cylindrical hole drilled in typical screen cylinder steel plate is approximately 1.27 mm (.050 inch), while even in stainless steel as thin as 1.59 mm (¹1,₆ inch), it is not possible to punch holes less than approximately 0.94 mm (.037 inch) in diameter.

[0011] The present invention has solved this problem by an entirely different approach to its ultimate objective of obtaining uniformly sized screening slots and circular holes of the smallest practical width or diameter. This approach involves, as a first step, producing a base shell having the desired slot arrangement or circular holes of as small a minimum dimension as can be uniformly maintained - which has been found to be approximately 0.152 mm (.006 inch) width for slots parallel with the axis, 0.254 to 0.356 mm (.010-.014 inch) for circumferentially extending slots, and a diameter of 1.27 mm (.050 inch) for circular holes. These dimensions for circular holes and axially extending slots apply to steel plate of a thickness of 4.76 to 7.94 mm (³/,c, to ⁵/,s inch), and the drilling and sawing operations are preferably carried out on the flat plate which is then rolled and welded to cylindrical form, either before or after having multiple bars welded to its inlet side.

[0012] According to the invention, a screening member according to the preamble of claim 1, comprising the resulting cylindrical shell having multiple perforations therethrough of a predetermined minimum cross sectional dimension, is provided with a coating which forms a lining for the wall of each of the perforations which reduces said cross sectional dimension of each perforation to a predetermined value. Conveniently the coating is of predetermined uniform thickness and covers the inner and outer surfaces of the shell as well as forming a lining for the wall of each of the perforations.

[0013] Preferred results in the practice of the-invention have been obtained with a coating of electroless nickel applied by conventional techniques such as are described in Gutzeit et al. U.S. Patent No. 2,658,841 of 1953 or the section entitled "Electroless Nickel coatings" by G. Gutzeit in Encyclopedia of Engineering Materials and Processes, published in 1963 by Reinhold Publishing Corp. Such coatings have the desired adherence to steel and controlled uniform thickness to provide screening cylinders with holes of a uniform minimum diameter of 0.025 mm (.001 inch) and slots of a uniform minimum width of 0.025 mm (.001 inch) when the circular holes were initially as large as 1.27 mm (.050 inch) in diameter and the slots were axially extending sawed slots of a width of 0.152 mm (.006 inch) or circumferentially extending slots of a minimum width as great as 0.356 mm (.014 inch).

[0014] Screening perforations of these small sizes offer outstanding advantages from the standpoint of the degree of cleanliness of the accepted stock, because of the greatly increased capability of such perforations to reject contaminant particles almost as small as good paper fiber. In addition, while it would seem that such small perforations would have strong tendencies to clogging or to being blocked by fiber felting over their inlet ends, it has been established in the development of the invention that when screens having such small perforations are provided with multiple bars along their inlet side in accordance with EP-A-0046687, the capacity of the resulting screen is astonishingly high. Test results supporting this summary are set forth below in conjunction with the description of the illustrated preferred embodiments of the invention.

[0015] In order that the invention may be more readily understood, reference will now be made to the accompanying drawings, in which:

Fig. 1 is a perspective view, partly broken away, of pressure screening apparatus having a screening cylinder embodying the invention and provided with circular screening holes;

Fig. 2 is a somewhat diagrammatic top view of the screening cylinder 7 rotor in the apparatus of Fig. 1;

Fig. 3 is an enlarged fragmentary section through the screening cylinder in the screen of Fig. 1;

Fig. 4 is a fragmentary view illustrating a screening cylinder in accordance with the invention provided with slotted screening perforations extending axially of the cylinder;

Fig. 5 is an enlarged fragmentary section taken radially through the screening cylinder of Fig. 4;

Fig. 6 is a fragmentary isometric view showing a screening cylinder in accordance with the invention wherein the screening perforations are slots which run circumferentially of the cylinder; and

Fig. 7 is an enlarged fragmentary section on the line 7-7 of Fig. 6.

[0016] The screening apparatus shown in Fig. 1 is constructed generally in accordance with Seifert U.S. patent No. 4,105,543, with certain exceptions in accordance with the invention. It comprises a main housing 10 on a base 11, and in the upper end of the housing is an inlet chamber 12 having a tangential inlet port 13 to which the furnish is fed under pressure as is customary with such screening apparatus. A cylindrical screening member 15 provided with multiple substantially circular holes 16 divides the interior of the housing below chamber 12 into a center supply chamber 17 and an accepts chamber 18 having an outlet port 19.

[0017] The bottom wall 20 of the supply chamber includes a trough 21 leading to a discharge port 22 provided with a control valve assembly 23 which can be preset to provide a desired continual bleed of reject-rich stock. Heavy particles which settle into the trough 21 drop therefrom to the heavy trash collection box 24 by way of manually controlled valve 25 for intermittent removal.

[0018] A rotor 30 is supported on a drive shaft 31 in the center of the supply chamber 17 and is driven through suitable gearing or belts by a motor 33 also mounted on the base 11. Vanes or bars 35 are mounted on the rotor 30 by support rods 36, and adjustable connections 37 between the inner ends of rods 36 and rotor 30 provide for positioning the vanes 35 in properly spaced relation with the inner surface of screening member 15.

[0019] The vanes 35 extend the full length of the screening surface of screen member 15, and they are preferably helically curved and so arranged that the upper end of each vane is spaced forwardly of the lower end in the direction of rotation of the rotor. In fig. 1 two vanes 35 are shown, but other numbers can be used, and in general a greater number, e.g., four, may make possible improved operation at higher consistencies.

[0020] The screening cylinder 15 is provided along its inner (inlet) side with a plurality of bars 40, shown as of essentially square section, which extend generally axially thereof in circumferentially spaced relation, and cooperate with the surface portions of the member 15 therebetween to form a series of shallow pockets 42. The radial dimension between the vanes 35 and the radially inner surfaces of the bars 40 should be relatively small, preferably of the order of 1.59 mm (¹1,₆ inch), but it may vary within a range of approximately 0.254 to 0.953 mm (0.010-0.375 inch).

[0021] Outstanding results in the testing of the invention have been obtained utilizing square bars 6.3 mm (¹4 inch) wide on each side and spaced 50.8 mm (2 inches) apart, but these values are subject to substantial variation. For example, the spacing between adjacent bars may range from 6.3 mm to 127 mm (4 inch to 5 inches), with resulting change in the number of bars, and the size of rectangular bars may range from 1.59 to 12.7 mm (¹1,₆ to inch) on a side. When rectangular bars are used, their larger dimension should preferably extend radially of the screening cylinder to minimize reduction in the open screening area.

[0022] As previously pointed out, the present invention is particularly concerned with the production of screening holes 16 of uniformly much smaller size than have previously been possible, e.g. cylindrical holes as small as 0.254 mm (.010 inch) in diameter. Referring particularly to Fig. 2, the technique of the invention is to drill holes 16 in the steel plate shell 15 by means of as small a drill as can be used successfully in steel plate as thick as 7.94 mm (5/,fi inch). Practical experience has established that the minimum such diameter is approximately 1.27 mm (0.050 inch), with the outlet end of each hole preferably relieved by countersinking as shown.

[0023] Then after the drilled plate has been provided with the desired bars 40, and has been rolled and welded into the proper cylindrical form, it is subjected to an electroless nickel coating process, during which a coating 44 is deposited in a uniform thickness over the entire surface of the cylindrical shell, thereby also forming a lining for each hole 16 which will correspondingly reduce the diameter of the hole. For example, if the holes 16 are drilled with an initial diameter of 1.27 mm (.050 inch) the thickness of the coating 44 may be controlled to 0.318 mm (.0125 inch) to provide each hole 16 with a final minimum diameter of 0.635 mm (.025 inch).

[0024] In operation, the natural effect of the rotational movement of the vanes 35 in the circular path which they define will tend to create similar circulation of an annular layer of stock adjacent the inner surface of the cylinder 15. This movement of the stock, however, will be interrupted by the successive bars 40, which will in effect peel off the outer portion of the circulating layer and divert it into the associated pockets 42.

[0025] The stock diverted into each of the pockets 42 will have a substantial circulatory momentum and will therefore tend to reverse its direction of flow within the pocket into which it is diverted. Since the action of the vanes is also to create alternating pressure waves through the holes 16, which are radially outward while each vane approaches each pocket and radially inward as each vane passes the pocket, the result of the combination of forces is the development of a high degree of local turbulence in each pocket.

[0026] At the same time, the interior of the supply chamber is under continuous pressure from the pump by which stock is supplied to the screen, and there is therefore a force continuously urging stock to discharge through the holes 16 into the accepts chamber 18. The turbulence within the pockets has the effect of keeping the fibers within each pocket in a continuous condition of changing random orientation and thereby promotes their passage through the holes 16, particularly the long fibers which otherwise tend to become aligned tangentially of the cylinder and thereby to flow past the holes 16 instead of through them.

[0027] The validity of this theoretical explanation of the operation of the screen of the invention, as well as the practical importance of screening cylinders having exceptionally small sized holes, has been adequately established by test. Thus in one series of tests, a screen constructed as shown in the above Secor application and provided with screening holes 1.27 mm (.050 inch) in diameter was compared with a screen in accordance with the invention wherein the screen cylinder was provided with holes of a diameter of only 0.635 mm (.025 inch) produced as described above. Both screen cylinders were 610 mm (24 inches) in diameter and were provided with 39 6.3 x 6.3 mm (4 x inch) bars 40 evenly spaced around its inner surface.

[0028] The same two feed stocks were used with each screen, one being clean corrugated stock and the other being the same corrugated stock to which reject materials were added in the amount of 2.5 percent of solids. The operating conditions for both screens were the same for all runs, namely a rotor speed of 680 r.p.m. and a pressure drop of 8 psi. In the following summary, the screen with the 1.27 mm (.050 inch) holes was used with the clean stock during Runs 1-3 and the contaminated stock during Runs 4-6, while the screen with the 0.635 mm (.025 inch) holes was used with clean stock on Runs 7-9 and contaminated stock on Runs 10-12.

[0029] Such test results are astonishing even when consideration is given only to comparison of the Runs with clean stock. In fact, it is astonishing as to Run 7 that the screen worked at all and did not almost immediately plug or otherwise block passage of any accepted stock. Yet in Run 7, the screen of the invention accepted almost 80 percent of the feed stock at a consistency of 3.29 percent solids, which was a reject rate only approximately twice as high as with the screen wherein the flow area of each hole was four times as great, and at a feed rate equal to approximately % as high as with the larger perforations.

[0030] Similarly in handling the dirty stock, the screen of the invention could handle almost one-half the feed rate of the screen with perforations four times larger in flow area at a consistency of more than 3.50 percent, and its acceptance/reject ratio was still approximately 70 percent as compared with approximately 83 percent for the larger perforations which necessarily accepted many contaminant particles too large for acceptance by the 0.635 mm (.025 inch) holes in the screen of the invention. The resulting greatly increased cleanliness of the accepted stock would more than compensate for the comparatively small decrease in capacity.

[0031] Figs. 4-5 illustrate the application of the invention to a screen cylinder which has a pattern of axially arranged slots like that shown in Seifert 3,849,302. The cylinder 50 is formed like the cylinder 15 except that it is provided with multiple screening slots 51 which extend vertically, i.e. axially of the cylinder, in axially spaced circumferentially extending rows. This screening cylinder also has multiple bars 52 welded along its inlet side, which may be of the same dimensions described in connection with Figs. 1-2, but which are straight and extend axially of the cylinder.

[0032] In constructing a cylinder 50 in accordance with the invention, the screening slots 51 are initially formed as saw cuts using a circular saw as thin as is practical for use in steel plate as thick as 7.94 mm (%₆ inch). Practical experience has established that it is possible to maintain a consistent slot width of 0.152 mm (.006 inch) by such sawing method, and the cylinder is then subjected to electroless nickel coating until the coating 55 has reached a thickness providing the desired minimum width for the slots 51, e.g. a coating 0.064 mm (.0025 inch) in thickness to provide a slot width of 0.025 mm (.001 inch).

[0033] Figs. 4-7 illustrate the application of the invention to a screening cylinder 60 constructed as disclosed in the above-identified Chupka-Seifert patent and application from a series of rings 61 of wire of generally triangular section arranged in spaced relation axially of the cylinder to define screening slots 62 therebetween. In accordance with the Chupka-Seifert patent and application, such a screening cylinder can be made either by laying up a series of rings 62 in a suitable jig, or by winding the wire in a spiral pattern. With either of those production techniques, however, it has been found difficult to maintain uniform slot widths of less than about 0.254 mm (.010 inch), and it is easier if they are somewhat larger.

[0034] In accordance with the invention, either of the above techniques for forming the cylinder 60 may be employed to produce a screening cylinder having circumferentially extending slots of as small a width as can be practically maintained uniform, e.g. from 0.254 to 0.356 mm (.010 to .014 inch). The completed cylinder, with the bars 63 welded in place, is then subjected to electroless nickel coating until a coating 65 of the desired uniform thickness has been produced. For example, if the initial slot width is 0.356 mm (.014 inch) and the desired slot width is 0.051 mm (.002 inch), the coating should be 0.152 mm (.006 inch) thick.

[0035] The invention is not limited in principle to use with electroless nickel coatings, but preferred results have been obtained thereby because of the particular properties of electroless nickel coatings, including strong adhesion to steel, easily controlled uniformity of thickness and smoothness, and fidelity to the detail of the coated material. Because of this latter characteristic, such a coating will accurately reproduce the hole or slot which it lines, to provide the desired final minimum dimension for the inlet ends of the screening perforations.

[0036] It will be apparent that the principle of the invention is applicable to materials other than steel as the base shell of the screening cylinder, and to other coating materials. For example, the screening cylinder could be of any material other than nickel which is appropriately stiff and strong, as well as certain plastics, e.g. polypropylene. Similarly the coating may be of any material which will adhere to the base shell with the desired strength and will reproduce in adequately accurate detail the outlines of the coated shell. The choice of both the material of the base shell and the material with which is coated is accordingly open to those skilled in the selection and use of such materials, guided by the principle of the invention as set forth above and in the claims.

[0037] It should also be noted that while in general it is simpler, and therefore preferred, to coat the entire inner and outer surface of the shell as well as the walls of the screening perforations, it is also possible, by means of suitable masking, to coat only the walls of the perforations to whatever thickness is necessary to reduce the minimum dimension of each perforation to the desired value in accordance with the principle of the invention as described above.

[0038] While the forms of apparatus herein described constitutes preferred embodiments of this invention, it is to be understood that the invention is not limited to these precise forms of apparatus, and that changes may be made therein without departing from the scope of the invention as defined in the appended claims.

Claims

1. A screen member for installation in the housing (10) of screening apparatus for paper fiber stock to separate the interior thereof into a supply chamber and an accepts chamber, said screen member comprising a cylindrical base shell (15, 50, 60) having multiple perforations (16, 51, 62) therethrough of a predetermined minimum cross sectional dimension, characterised in that a coating (44, 55, 65) forms a lining for the wall of each of said perforations which reduces said cross sectional dimension of each said perforation to a predetermined value.

2. A screen member as defined in claim 1, characterised in that said coating (44, 55, 65) is of predetermined uniform thickness and covers the inner and outer surfaces of said shell as well as forming a lining for the wall of each of said perforations.

3. A screen member as defined in claim 1 or 2, wherein said shell (15, 50, 60) is composed of a metal, and said coating is composed of electroless nickel.

4. A screen member as defined in claim 1, 2 or 3, wherein said perforations (16) are circular in radial section and said predetermined dimension is the minimum diameter of each said perforation.

5. A screen member as defined in claim 1, 2 or 3, wherein said perforations are parallel slots (51, 62), and said predetermined dimension is the minimum width of each said slot.

6. A screen member as defined in claim 5, wherein said slots (51) extend parallel with the axis of said shell.

7. A screen member as defined in claim 6, wherein said slots (62) extend substantially circumferentially of said shell.

8. A screen member as defined in any preceding claim, wherein said base shell has secured to the inlet side thereof a plurality of bars (40, 63) extending generally axially thereof in circumferentially relatively closely and uniformly spaced relation to define with the adjacent surface of said shell a series of pockets (42), the dimension of each of said bars (40, 63) measured radially of said shell being sufficient to effect interruption of the circulation of stock immediately adjacent the inlet side of said screen member, and said coating covering said bars.

Ansprüche

1. Sortierteil für den Einbau im Gehäuse (10) einer Sortiervorrichtung für Papierfaserstoff zur Abtrennung des Gehäuseinneren in eine Zuführkammer und eine Gutstoffkammer, wobei das Sortierteil eine zylindrische Basisschale (15, 50, 60) mit Vielfachperforationen (16, 51, 62) eines vorbestimmten minimalen Querschnittsmaßes durch die Schale hindurch aufweist, dadurch gekennzeichnet, daß eine Schicht (44, 55, 65) eine Verkleidung für die Wand jeder der Perforationen bildet, welche das Querschnittsmaß jeder Perforation auf einen vorbestimmten Wert verringert.

2. Sortierteil nach Anspruch 1, dadurch gekennzeichnet, daß die Schicht (44, 55, 65) eine bestimmte gleichmäßige Dicke hat und die innere und äußere Oberfläche der Schale abdeckt sowie eine Verkleidung für die Wand jeder Perforation bildet.

3. Sortierteil nach Anspruch 1 oder 2, wobei die Schale (15, 50, 60) aus einem Metall zusammengesetzt ist und die Schicht aus stromlos aufgebrachtem Nickel zusammengesetzt ist.

4. Sortierteil nach Anspruch 1, 2 oder 3, wobei die Perforationen (16) in radialem Schnitt kreisförmig sind und das vorbestimmte Maß der Minimaldurchmesser jeder Perforation ist.

5. Sortierteil nach Anspruch 1, oder 3, wobei die Perforationen parallele Schlitze (51, 62) sind und das vorbestimmte Maß die minimale Breite jedes Schlitzes ist.

6. Sortierteil nach Anspruch 5, wobei die Schlitze (51) sich zur Achse der Schale parallel erstrecken.

7. Sortierteil nach Anspruch 6, wobei sich die Schlitze (62) im wesentlichen über den Umfang der Schale erstrekken.

8. Sortierteil nach einem vorhergehenden Anspruch, wobei an der Einlaßseite der Basisschale eine Mehrzahl von Stangen (40, 63) befestigt sind, die sich im allgemeinen axial zur Schale in am Umfang relativ dichten und gleichmäßigen Abständen erstrecken, um mit der benachbarten Oberfläche der Schale eine Reihe von Taschen (42) zu bilden, wobei das Maß jeder Stange (40, 63), radial zur Schale gemessen, ausreicht, um eine Unterbrechung der Zirkulation des Stoffes unmittelbar neben der Einlaßseite des Sortierteils zu bewirken, und die Schicht die Stangen bedeckt.

Revendications

1. Elément de tamis destiné à être installé dans le logement (10) d'un dispositif de tamisage pour pâte à papier pour séparer l'intérieur de celui-ci en une chambre d'alimentation et une chambre des matières acceptées, ledit élément de tamis comprenant une coque de base cylindrique (15, 50, 60) ayant de multiples perforations (16, 51, 62) à travers, d'une dimension minimale prédéterminée en coupe transversale, caractérisé en ce qu'une couche (44, 55, 65) forme un revêtement pour la paroi de chacune desdites perforations qui réduit ladite dimension en coupe transversale de chacune desdites perforations à une valeur prédéterminée.

2. Elément de tamis selon la revendication 1, caractérisé en ce que ladite couche (44, 55, 65) est d'une épaisseur uniforme prédéterminée et en ce qu'elle recouvre les.surfaces intérieure et extérieure de ladite coque tout en formant un revêtement pour la paroi de chacune desdites perforations.

3. Elément de tamis selon la revendication 1 ou la revendication 2, caractérisé en ce que ladite coque (15, 50, 60) est composée de métal, et en ce que ladite couche est composée de nickel chimique.

4. Elément de tamis selon les revendications 1, 2 ou 3, caractérisé en ce que lesdites perforations (16) sont circulaires en section radiale et en ce que ladite dimension prédéterminée est le diamètre minimal de chacune desdites perforations.

5. Elément de tamis selon les revendications 1, 2 ou 3, caractérisé en ce que lesdites perforations sont des fentes parallèles (51, 62), et en ce que ladite dimension prédéterminée est la largeur minimale de chacune desdites fentes.

6. Elément de tamis selon la revendication 5, caractérisé en ce que lesdites fentes (51) s'étendent parallèlement à l'axe de ladite coque.

7. Elément de tamis selon la revendication 6, caractérisé en ce que lesdites fentes (62) s'étendent sensiblement circonférentiellement par rapport à ladite coque.

8. Elément de tamis selon l'une quelconque des revendications précédentes, caractérisé en ce que ladite coque de base a une pluralité de barres (40, 63) fixées à son côté d'admission, s'étendant sensiblement axialement par rapport à elle dans une relation d'espacement circonférentiel relativement proche et uniforme pour délimiter avec la surface adjacente de ladite coque une série de poches (42), la dimension de chacune desdites barres (40, 63) mesurées radialement par rapport à ladite coque étant suffisante pour interrompre effectivement la circulation de matière immédiatement adjacente au côté de l'admission dudit élément de tamis, ladite couche recouvrant lesdites barres.

Drawing