Finely perforated undulating screen cylinder

Abstract

 A paper pulp screen construction including a housing for receiving a flow of paper stock to be screened with a cylindrical screen therein and a rotor movable past the screen plate with the screen plate formed in multiple layers and the layers shaped in undulations, in one form with smaller holes in a screen layer between inner and outer layers having holes of equal size, and in another form with the inner layer first receiving the stock and being formed with small openings and the outer layer with larger openings.

Description

BACKGROUND OF THE INVENTION:

[0001] The present invention relates to improvements in pulp preparation apparatuses, and more particularly to apparatus for screening wood pulp to pass clean fibers and reject undesirable elements.

[0002] In processing wood pulp, screens are utilized to separate acceptable fiber from undesirable constituents in a slurry. In a typical screening structure, a housing is provided with an inlet for pulp slurry which may have been prepared by cooking wood chips or by repulping secondary pulp fibers, so that the wood fibers are suspended in a slurry. The slurry may include undesirable elements, such as knots, shives, particles of bark, dirt and other undesirable constituents.

[0003] Within the screen housing, the slurry flows adjacent a cylindrical screen, often referred to as a plate or basket, with the acceptable fibers flowing through the screen plate and the undesirable material remaining on the first side of the screen plate to flow out of a reject opening in the housing. A rotor having pulse or turbulence generating structures normally is moved past the surface of the screen plate, to enhance the flow of slurry through the screen plate, and enhance the screening operation. The screen has openings which are formed in the shape of holes or slots of a size required for the screening operation and in combinations for optimizing screening performance. The screen plate must have screening openings of the proper size to pass the acceptable fibers and to prevent passage of the undesirable materials. Yet, with the necessity of these small openings, the screen plate must have sufficient strength to withstand the pressure differential across the surface, particularly when an arrangement is provided where pulsations are caused by moving members along the surface of the screen.

[0004] Different manufacturing processes have involved machining the screen plates, which is a relatively time consuming and expensive process. A construction has been proposed which involves shaping the screen without expensive machining. This construction is disclosed in U.S. patent 4,954,249 wherein the screen is formed of a flat material shaped in an undulating shape to increase the amount of screening surface exposed to the stock, and to strengthen the screen against pressure differential across the screening surface. Rings and tie members complete the modular construction.

[0005] An object of the present invention is to provide still further improvements in screen plates which reduce manufacturing costs and yet provide a screening structure which has substantially improved strength over structures heretofore available.

[0006] A further object of the invention is to provide a screen structure which is capable of being made with closely spaced, fine screening holes, which normally would weaken the strength of the screen plate and yet which provides a large screening area and accomplishes substantial strength across the screening surface. A further object of the invention is to provide a screen plate construction which accommodates a great variety of arrangements for providing the screening perforations in the plate.

FEATURES OF THE INVENTION

[0007] In accordance with the foregoing, a screen plate is presented wherein relatively thin material is formed into an undulating shape or contour. The relatively thin material can be utilized because multiple layers are arranged parallel to each other to, in essence, form a sandwich of layers. The sandwich allows for the use of very thin material wherein screening perforations can be cut, such as with a laser beam, and yet the sandwiched material provides for unexpected strength, preventing any collapse of the screen material during continuous screening operation with pressure differentials across the screen. The screen structure is capable of handling highly abrasive material, and provides a substantially increased screening area not fully dependent on the cylindrical size of the overall screen. In different forms, the plural layers of the screen assembly can be arranged so that the inner surface does the principal screening operation; or arranged so that, in a sandwich construction, the intermediate layer provides a screen plate to perform the screening operation, with outer layers having openings so as to freely pass the screened material and yet provide a monolithic sandwich layer which has substantial strength.

[0008] Other objects, advantages and features will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments in the specification, claims and drawings in which:

DESCRIPTION OF THE DRAWINGS:

[0009] 

Figure 1 is a perspective view of a screening mechanism constructed in accordance with the principles of the present invention, shown with a portion of the housing broken away;

Figure 2 is an enlarged detailed fragmentary sectional view taken substantially along line II-II of Figure 1;

Figure 3 is an enlarged perspective view taken from one edge of a screen structure such as used in Figures 1 and 2, showing details of one form of screen cylinder;

Figure 4 is an enlarged fragmentary plan view of one of the openings in the screen arrangement of Figure 3;

Figure 5 is a perspective view, similar to Figure 3 showing another form of screen arrangement; and

Figure 6 is a fragmentary plan view illustrating one of the openings in the screen of Figure 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Figure 1 illustrates a screening apparatus 8 wherein previously treated pulp is screened to remove undesirable elements such as shives, bark knots, particles of wood, dirt, glass, plastic and the like. A screen plate assembly is generally shown at 10, defining in the apparatus 8 an interior chamber 11 where the pulp to be screened flows in and an exterior chamber 12 where the screened pulp flows out after passing through the screen plate assembly. The assembly is enclosed in a housing 13 which has an inlet, not shown, for the entrance of pulp to be screened into the chamber 11, and an outlet, not shown, leading from the chamber 11 for the undesirable material such as the shives, bark and dirt. The accepted pulp flows out through an outlet 14.

[0011] The screen plate assembly 10 is stationary within the housing 13, and for aid in passing the liquid stock with pulp through the screen plate, and to help inhibit plugging, pulsation generators 18 are mounted for rotation within the cylindrical screen plate assembly. The pulsation generators 18 are supported on arms of a rotary driven shaft 19, and rotate in a clockwise direction, as viewed in Figure 1. The pulsation generators in the form of the hydrofoils shown are merely illustrative of a suitable type, and it should be understood that the present invention can be used for screen plates with various types of pulse, turbulence and combination pulse and turbulence generating rotors.

[0012] The screen plate assembly 10 includes cylindrical screen sections 16 and 17 which, without support, are essentially flexible and require rigidifying or strengthening for use in the pressurized environment of screen apparatus 8. The necessary support and strengthening is provided by end rings 20 and 20a and an intermediate support ring 21. Each of the rings has grooves, such as illustrated by the grooves 23 and 24 in the ring 21 shown in Figure 2. The grooves 23 and 24 are circular, to hold the screen sections in a substantially cylindrical shape. The grooves 23 and 24 have a radial dimension substantially equal to the radial thickness of the shaped screen plates.

[0013] While the preferred arrangement is illustrated for supporting the screen plates, it will be understood that other forms and arrangements may be utilized for supporting the plates. The plates are not necessarily segmented but may be arranged as a full annular assembly appropriately and adequately supported to remain stationary within the housing and to withstand the pressures of continuous flow and continuous movement of the pulse generator 18.

[0014] During assembly, each of the shaped screen plates is positioned into the grooves in the end ring 20 or 20a and the intermediate ring 21, and the rings are pulled together to force the screen plates into the grooves. For this purpose, axially extending rods 22 are provided, spaced circumferentially from each other, and the rods are provided at their ends with threads and nuts 22a, so that the nuts can be tightened to pull the end rings toward each other and force the ends of the screen plates into the grooves. The grooves are preferably tapered so that the slot becomes narrower in a direction toward the bottom of the groove, as indicated by the illustration of Figure 2. When the nuts are threaded onto the rods, the screen plates are pushed tightly into the tapered grooves, so that the screen plates are held firmly in a fixed position, circumferentially. With screen assemblies of different lengths, the screens can be longer or shorter, or even greater in number, and additional reinforcing intermediate rings such as 21 may be employed between the ends of each of the adjacent screen plate sections.

[0015] Screening openings are provided in the screen plates, such as shown at 25 in Figure 2, which is primarily a schematic showing of the opening inasmuch as the screen plates have a unique construction, with varying arrangements for the openings 25 such as will be described in connection with Figures 3 through 6.

[0016] As shown in Figures 3 and 4, a screen structure 26 is comprised of multiple layers. A first layer 27 faces the incoming pulp. This layer is the inner layer of an outward flowing screen, or the outer layer of an inwardly flowing screen, as one skilled in the art will recognize. The first layer 27 is juxtaposed against and supported by a second layer 28. In Figures 3 and 4, the screen shown is an outwardly flowing screen and the first layer described herein is the inner layer, with the second layer being the outer layer. The inner layer is provided with a series of screen perforations 29, preferably evenly distributed over the area of the inner layer 27.

[0017] The outer layer 28 has a series of flow passage holes 30, which are arranged so that the pulp being screened can continue flow past the outer layer to the opposite side of the screen. Thus, on the inner side of the inner layer will remain the undesirable material, to flow out of the rejects opening in the housing. The flow through the multilayered screen will consist of the clean fibers. So that a substantially unimpeded flow occurs, the larger openings 30 in the outer layer 28, Figure 4, are arranged so that they align with a maximum number of screening perforations 29 of the first layer 27. As illustrated, the outer layer has holes 30 which register with 5 of the smaller screening holes 29, but it will be understood that other arrangements can be employed, so that the larger openings 30 register with any number of perforations 29. The function of the outer layer 28 is to reinforce or support the inner layer, so that a principal requirement of the outer layer is one of strength. The outer layer may be thicker than the inner layer, or may be made of different material, to provide the necessary strength. The outer layer affords this strength even though the openings are provided to permit the flow of clean, acceptable fibers.

[0018] In a preferred construction, the sandwich or combination of layers is formed in an undulated shape. Preferably, these layers are formed with their individual holes before they are joined, and then are joined to each other and shaped in the undulating shape in unison. This insures a continued surface-to-surface contact for maximum strength and the maximum support function afforded by the outer layer 28.

[0019] In a preferred arrangement, the undulations are shaped in smooth curvatures to give maximum strength and to afford maximum screening surface area. By way of illustration, the layers are formed with ribs or crowns 31 facing inwardly, similar ribs or crowns 33 facing outwardly and vertical walls 32 and 34 to join the inner crown or ribs 31 with outer crowns or ribs 33. This arrangement will provide a multilayered screen of superior strength and of exceptional screening capacity.

[0020] As illustrated in Figures 5 and 6, a screen structure is formed with a strong reinforcing inner layer 44 and a strong reinforcing outer layer 45. Between the layers is sandwiched a screening layer which consists primarily of a fine mesh screen 47 of a size to perform the screening operation. The fine mesh screen does not have substantial strength, but as will be seen, the only area of the screen which is exposed is through openings 51 and 52 in the inner and outer layers respectively. The openings are preferably formed as circular holes for maximum strength and afford a substantial screening area. The layers are assembled by the holes first being formed in the inner and outer layers 44 and 45, which are then placed with the screen sandwiched therebetween, and shaped or bent to the undulations illustrated in the drawing, Figure 5. The undulations are preferably curved to form inner ribs and outer ribs joined by straight wall areas as described for Figure 3. This structural arrangement affords very substantial strength, as well as providing good screening area, while allowing very fine perforation of the screening layer 47. In each of the structures of Figures 3 and 5, the multilayered screen assembly will preferably be arranged in cylindrical form, whether in one piece or in sectional plates.

[0021] In that environment, the pulse generators will be moved past the inner surface, that is, past the inner layer 27 in the structure of Figure 3 and the inner layer 44 of Figure 5. The stock flowing into the cylinder of the screen assembly will be screened, with the clean fibers passing through the screening structure and out toward the outer side of the screen plate. The undesirable materials will remain inside, to flow toward the reject opening.

[0022] In operation and in assembly of the structure, the composite screen structures of Figures 3 and 5 preferably are formed by first arranging flat sheets and providing the openings therethrough. The flat sheets are then brought together in sandwich fashion and bent into the undulating shape shown in the Figures or into other desirable shapes. The undulating shaped screens are then arranged in cylindrical form, fully supported and positioned in the housing of Figure 1 for a continuous screening operation.

[0023] Thus, it will be seen we have provided an improved structure which meets the objectives and advantages above set forth, and which attains an equal amount or greater screening area than heretofore available plus substantially greater strength in a finely perforated screen. The arrangement also permits utilizing individual thinner layers, with the individual layers performing their own individual function in a better manner. That is, the reinforcing layers can be constructed of a material which provides the maximum strength, and the screening layers can be made very thin to provide well defined openings of a uniform and correct size for the screening operation.

Claims

1. A paper pulp screening construction comprising in combination:
   a housing having an inlet for receiving stock to be screened for the removal of undesirable elements and having an outlet for clean fiber screened material and an outlet for the undesirable elements;
   a cylindrical screen plate within the housing for screening the stock to pass to the clean fiber outlet and rejecting undesirable material to flow to the undesirable elements outlet;
   and a rotor mounted for movement near the surface of the cylindrical screen for aiding in passing fiber through the screen;
   said screen plate having a plurality of overlapping layers each with openings for the flow of acceptable fibers through the screen to a clean fiber side of the screen.

2. A paper pulp screen construction constructed in accordance with claim 1:
   wherein said layers are parallel to each other.

3. A paper pulp screen construction constructed in accordance with claim 1:
   wherein said layers are coextensive and in surface-to-surface contact with each other.

4. A paper pulp screen construction constructed in accordance with claim 1:
   wherein said layers extend in a repeating undulating shape.

5. A paper pulp screen construction constructed in accordance with claim 4:
   wherein the undulations form smooth curves.

6. A paper pulp screen construction constructed in accordance with claim 1:
   wherein said screen plate comprises three layers.

7. A paper pulp screen construction constructed in accordance with claim 1:
   wherein the openings in the screen layer on an upstream side of the flow are smaller than the openings of a layer on the downstream side of the flow.

8. A paper pulp screen construction constructed in accordance with claim 1:
   wherein said screen plate has inner and outer layers with a screening layer sandwiched therebetween and the screening layer has smaller openings than the inner and outer layers.

9. A paper pulp screen construction constructed in accordance with claim 1:
   wherein the openings in each of the layers are in alignment with each other.

10. A screen plate construction for use in a paper pulp preparation apparatus comprising in combination:
   an inner layer for being located adjacent incoming pulp to be screened and having openings for admitting a flow of pulp toward a clean fiber side;
   an outer layer having openings for receiving fiber pulp flow from the inner layer; and
   screening means positioned in a flow stream flowing through said openings in said layers accommodating clean fiber flow through the openings and rejecting undesirable material.

11. A screen construction for use in a paper pulp preparation apparatus constructed in accordance with claim 10:
   wherein the layers are shaped to form undulations so that an increased flow is provided.

12. A screen construction for use in a paper pulp preparation apparatus construction in accordance with claim 11:
   wherein said undulations are formed in a smooth curve.

13. A screen construction for use in a paper pulp preparation apparatus constructed in accordance with claim 10:
   wherein said layers are formed parallel to each other.

14. A screen construction for use in a paper pulp preparation apparatus constructed in accordance with claim 10:
   wherein said layers have said screening means sandwiched therebetween.

15. A screen construction for use in a paper pulp preparation apparatus constructed in accordance with claim 10:
   wherein said inner layer has openings of a size to provide said screening means.

16. A screen construction for use in a paper pulp preparation apparatus constructed in accordance with claim 10:
   wherein said inner and outer layers have openings of equal size.

17. A screen construction for use in a paper pulp preparation apparatus constructed in accordance with claim 10:
   wherein said layers are arranged in a cylindrical form.

18. A screen construction for use in a paper pulp preparation apparatus constructed in accordance with claim 10:
   wherein the outer layer is formed of a thicker, stronger material than the inner layer.

19. A screen plate construction for use in a paper pulp preparation apparatus comprising in combination:
   a screening layer having screening openings therethrough of a size for admitting a flow of pulp toward a clean fiber side and for rejecting undesirable material; and
   a reinforcement layer in supporting contact with the screening layer so that the reinforcement layer provides strength, preventing collapse of the screening layer due to pressure differentials across the screen layer.

20. A screen construction for use in a paper pulp preparation apparatus constructed in accordance with claim 19:
   wherein the outer support is a layer extending coextensive with the screening layer.

21. A screen construction for use in a paper pulp preparation apparatus constructed in accordance with claim 19:
   wherein the support layer has openings with each opening embracing a plurality of openings in the screening layer.

22. In a screen plate assembly made of relatively thin flexible sheet-like material formed by bending into an undulating pattern and shaped into a cylinder secured at its ends by end rings and rigidified by tie members, the improvement comprising:
   said sheet-like material including a first screening layer having holes therein for passing first size materials and restricting the passage therethrough of second size materials; and
   at least one reinforcing layer of greater strength than said first screening layer, and having openings therein larger than said openings in said first screening layer.

23. The improvement defined in claim 22 further comprising:
   a second reinforcing layer coextensive with said first two layers, with said first screening layer being sandwiched between said reinforcing layers.

Drawing