WO1992011410A1

WO1992011410A1 - High consistency foil

Info

Publication number: WO1992011410A1
Application number: PCT/US1991/008812
Authority: WO
Inventors: Peter E. Leblanc
Original assignee: Beloit Corporation
Priority date: 1990-12-20
Filing date: 1991-11-25
Publication date: 1992-07-09
Also published as: MX9102662A; AU9102691A

Abstract

An apparatus and method for screening paper machine stock using a fabricated foil (16) and moving the foil along the arcuate surface of a cylindrical screen (14), with the foil formed of curved inner and outer plates (20 and 21) with supporting flat plates (24 and 25) with arcuate edges therebetween and a blunt leading surface (23) connecting leading edges of the curved inner and outer plates.

Description

"HIGH CONSISTENCY FOIL"

BACKGROUND OF THE INVENTION

The present invention relates to improvements in apparatus and method for screening paper stock for removing knots, shives, dirt, specks, and other undesirable materials.

In the development of stock screening mechanisms, various forms of screening structures have been utilized, and one acceptable structure has been a cylindrical screen with a rotor being rotated past the inner surface of the screen. An important factor in obtaining continual screening operation without plugging of the screen and with satisfactory screening operation has been in the design of the rotor. One theory of operation has been to provide foil rotors which are designed to slip through the stock smoothly, without generating significant turbulence. Screening is effected by pulse generation from the rotating foils. The foils or vanes of the rotor are of generally airfoil shape, to easily slip through the stock while generating pulses in the stock. A curved front surface is provided, curving radially away and rearwardly relative to the rotational direction. U.S. Patent 4,919,797 illustrates the generally airfoil shape used in rotors of this type.

Other rotors have been used employing structures that provide turbulence in the stock, with minimal pulse. These turbulence generating rotors are often used with screen baskets having profiled or contoured surfaces. Conventional thinking has been that pulse-type screens should minimize turbulence, and turbulence generating screens should minimize pulse, for effective screening.

In the screening of pulp, it is important that the screening be effectively handled with a minimum fiber loss. One difficulty has been in the screening of high consistency pulp, which has handling advantages in part because of lower volumes for a given quantity of pulp, but presents screening problems, not the least of which is plugging.

In my U.S. patent 4,855,038, I have disclosed a drum¬ like rotor which combines turbulence and pulse generating features. The rotor has been effective in screening pulps of higher consistency than can be handled with conventional rotors. ith lower consistency pulps, hydraulic inefficiencies result in a decrease of overall screen efficiency relative to foil-type screens.

An object of the present invention is to provide an improved pulp screening apparatus capable of continual effective screening of high consistency paper pulp, while maintaining acceptable efficiency with lower consistency pulps.

Another object of the invention is to provide an improved pulp screen using a cylindrical screen with a rotating foil within the screen, wherein the foil generates both turbulence and pulse to obtain screening advantages from both.

FEATURES OF THE INVENTION

The concepts of the screening structure are to provide a cylindrical screen with a foil rotor therein, wherein the foil rotor generates both turbulence and pulse. The rotating foils have blunt leading surfaces and are fabricated with curved plates both on the inner facing away from the screen and on the outer surface facing the screen. The plates are mounted on side plates, and the lead surface of the foil is provided by a flat plate extending substantially normal to the surface of the screen and connected to the forward most edges of the curved plates. The foil is arranged to run adjustably close to the screen, and the foil drop off rearwardly from the leading edge is likewise adjustable. The reject end of the screen is arranged within a housing, so that rejects are discharged through a relatively small single outlet rather than using an annular reject passage. The single outlet produces a reject flow that has a higher debris content and is at lower consistency with less fiber loss.

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 embodiment thereof in the specification, claims and drawings, in which:

DESCRIPTION OF THE DRAWINGS

Figure 1 is a vertical sectional view taken through a screening apparatus embodying the principles of the present invention;

Figure 2 is a vertical sectional view taken substantially along line II-II of Figure 1;

Figure 3 is an exploded detailed view illustrating features of a foil of the rotor; and

Figure 4 is a sectional view taken substantially along line IV-IV of Figure 2. DESCRIPTION OF THE PREFERRED EMBODIMENT

As illustrated in Figure 1, the screening apparatus of the present invention includes a housing 10 with an outlet 11 for the acceptable fibers. The unscreened stock enters the housing at 12 and flows into the center of the screening apparatus. Rejects pass out of the housing through a rejects line 13. The line 13 has a single opening such as a 2" pipe, and the single 2" outlet appears to produce a reject flow that has a higher debris content and lower fiber content than devices heretofore available which discharged over the entire periphery of the housing.

within the housing is a stationary screen 14, and the stock flows into the center area of the screen, generally within a rotor indicated by numeral 17. The screen 14 can be of different types, including those having profiled surfaces. The acceptable stock fibers, or accepts, flow out through openings in the screen 14 and into the annular space radially outward from the screen 14, to be discharged from the housing through the line 11. Knots, shives, dirt and other undesirable elements do not pass through the screen 14 and are discharged through the rejects line 13.

The rotor 17 rotates within the stationary screen 14 and carries a plurality of foils 16 which are designed to generate both pulsations and turbulence in the stock within the screen. The rotor is supported on a shaft 15 driven in rotation by suitable drive means. At the ends of the rotor are rings 18 and ι9 which support the foils at spaced circumferential locations, as indicated by the foils 16 in Figure 2. The rings are supported on spokes attached to the shaft 15.

In Figure 2, a series of four foils is shown, each foil having the same construction. Depending on the size and design capacity of the screen, the physical properties of the slurry and the like, a rotor may have more than or fewer than the four foils shown.

Figure 3 illustrates one of the foils in detail. The foils are constructed of a curved outer plate 20 which faces the surface of the screen 14, and a curved inner plate 21 which faces the space within the rotor. The curved plates are joined at their trailing ends by a curved, joining, tail piece 22. At the lead end of the foil is a flat leading plate 23, which extends at substantially right angles to the screen, and connects the leading edges of the curved plates 20 and 21. The flat leading plate defines a blunt leading surface facing in the direction of rotation, and helps generate turbulence within the stock. As used in the specification and claims hereof "blunt", when used in reference to the rotor, refers to a surface so shaped as to be capable of capturing a volume of stock and accelerating it to near the foil velocity. Thus, the leading surface can be substantially at right angles to the screen, can be forwardly inclined such that the outer most edge precedes the inner most edge in rotation or can be concave in shape.

The curved plates 20 and 21 are joined at their edges by side plates 24 and 25. The assembly, as shown in Figure 3, is welded in its entirety, with the edges of the flat leading plate 23 welded to the forward edges of the curved plates 20 and 21. The curved plates are welded to the joining tail piece 22. The curved plates are welded at their ends to the side plates 24 and 25.

The side plates 24 and 25 have threaded openings 26 to receive securing bolts 28. The securing bolts pass through openings in the end rings 18 and 19 to support the foils at a predetermined desired angle. The bolts 28 pass through elongate slots such as shown at 29 in Figure 2 for the lead end of a foil and such as shown at 30 for the trailing end of a foil. By loosening the bolts, the foil can be set at the desired location relative to the screen and can be set at the desired angle. The angular relationship between a foil and the screen 14 and the spacing of the foil from the screen are broadly adjustable. Preferably, the foils will be set with a small screen clearance, on the order of 1/4 inch. The trailing end of the foil will be set with a fairly rapid drop off, on the order of a slope of 2. Adjustment of the individual foils is made so that all foils are at the same attitude relative to the screen. The bolts passing into the end plates 24 and 25 are tightened to clamp the end plates firmly against the rings 18 and 19.

In operation of the screen, stock is supplied through the inlet 12, Figure 1, to pass into the center of the rotor 17. As the rotor rotates, some of the stock is brought into rotation to pass over the openings in the stationary stock screen 14. The foils rotating past the openings in the screen create turbulence within the stock and also create pulsations as they pass the screen openings. Reject material does not pass through the screen openings and is discharged through a discharge hole 13. The acceptable fibers from the stock pass through the openings into the annular space and are discharged from the housing 10 through the accepts conduit 11.

As the foils rotate through the stock, significant turbulence is generated in the stock as the blunt lead surface of a foil approaches. This turbulence along the surface of the screen, particularly with profiled screens, fluidizes the fibers in the stock, aiding in screening. As the foil passes a given location, a period of positive pulsation in the accepts direction is generated, which is followed by a period of negative pulsation as the foil continues to pass and the foil-to-screen spacing increases toward the trailing end of the foil. During this period of negative pulsation, it is believed that water is drawn back through the screen, thereby resulting in accepts thickening and rejects dilution. This back flush of liquid through the screen also tends to clean the screen openings, flushing debris therefrom. The back flushing thereby reduces plugging.

With many of the basic features of a foil-type screen remaining, resulting from the generally airfoil shape behind the blunt leading edge, a rotor having the present foil design can be rotated through the stock efficiently, and many of the advantages of foil screens in lower consistency screening are obtained while the advantages of turbulence along a profiled screen surface can be added. Thereby, greater screening flexibility and efficiency can be realized in a screen having the modified foils of the present invention.

Thus, it will be seen that I have provided an improved, unique, high consistency pulp screen which meets the objectives and advantages above set forth and provides advantages over structures and methods heretofore available.

Claims

I CLAIM AS MY INVENTION:

1. A stock screening mechanism for paper making stock fibers in a slurry comprising in combination: an annular screen (14) forming a surface with openings of a size for separating foreign materials from a stock slurry; an individual foil (16) positioned opposite the screen surface and movable therealong through the slurry in rotation along the surface of the annular screen; a blunt leading surface (23) on the foil substantially normal to the screen surface; and a tapered trailing edge (20,21,22) on the foil following said blunt face so that the foil generates the combined effect of pulse and turbulence.

2. A stock screening mechanism for paper making stock fibers in a slurry constructed in accordance with claim 1: including a plurality of individual foils (16) arranged in sequential positions to move in sequence along the surface of the screen (14) .

3. A stock screening mechanism for paper making stock fibers in a slurry constructed in accordance with claim 1: including means for adjusting the angle of the foil (16) relative to the screen surface (14).

4. A stock screening mechanism for paper making stock fibers in a slurry constructed in accordance with claim 1: wherein said foil (16) has a surface (20) facing the screen and tapered so as to lead away from the screen.

5. A stock screening mechanism fcr paper making stock fibers in a slurry constructed in accordance with claim 1: wherein the foil (16) is positioned within the annular screen (14) and rotates about an axis coaxially with the annular screen.

6. A stock screening mechanism for paper making stock fibers in a slurry constructed in accordance with claim 1: including a plurality of foils (16) supported on end rings (18,19) movable coaxial with the annular screen.

7. A stock screening mechanism for paper making stock fibers in a slurry constructed in accordance with claim 6: wherein said end rings (18,19) are supported on radial spokes.

8. A stock screen mechanism for paper making stock fibers in a slurry comprising in combination: an annular screen (14) having a surface with openings of a size for separating foreign material from a stock slurry; individual foils (16) movable along the surface of the screen (14); each said foil (16) having inner and outer surfaces formed by arcuate plates (21,20), end plates (24,25) on the foil with curved edges supporting the arcuate plates to form a curved foil, and each said foil further including a blunt leading surface provided by a plate (23) between the leading edges of said arcuate plates.

9. The method of screening stock fibers in a slurry for a paper machine comprising the steps: exposing the stock to the arcuate face of an annular stock screen (14); moving a foil (16) along the screen and generating both turbulence and pulses; and preceding the leading edge of the foil with a flat plate (23) extending substantially normal to the screen surface.

10. The method of screening stock fibers in a slurry for a paper machine in accordance with the steps of claim 9: including moving the foil (16) with a screen clearance between the screen surface (14) and the foil on the order of 1/4 inch.

11. The method of screening stock fibers in a slurry for a paper machine in accordance with the steps of claim 12: including positioning the foil (16) so that the drop off of the rotating foil leading to a trailing edge relative to the screen surface is on the order of 2 to 1.

12. In a paper making pulp screening apparatus wherein a plurality of generally airfoil shaped vanes (16) rotate through stock adjacent an annular screen plate (14) to generate pulses in a stock for enhancing screening efficiency, the improvement comprising: a blunt lead surface (23) disposed on at least some of said vanes (16) for capturing a certain volume of stock and accelerating the captured stock substantially to rotor velocity for generating significant turbulence in the slurry.

13. The improvement for a paper making stock screening apparatus as defined in claim 12, and further comprising: each of said vanes (16) having a blunt lead surface

(23) for capturing and accelerating stock to rotor velocity.

14. An improved foil (16) for a paper stock screening apparatus wherein the foil includes a first curved surface (20) facing an annular screen plate (14), and a second curved surface (21) oppositely facing said first curved surface, said surfaces formed by plates joined to define a generally airfoil shaped vane, the improvement comprising: a blunt lead surface formed by a plate (23) connecting said first and second surfaces at the leading edges thereof.

AMENDED CLAIMS

[received by the International Bureau on 04 May 1992 (04.05.92); original claims 4 and 8 deleted; original claims 1, 9 and 12 amended; remaining claims unchanged (4 pages) ]

1. A stock screening mechanism for paper making stock fibers in a slurry comprising in combination, an annular screen (14) forming a surface with openings of a size for separating foreign materials from a stock slurry, and at least one individual foil (16) positioned opposite the screen surface and movable therealong through the slurry in rotation along the surface of the annular screen, characterized in that: said foil includes a blunt leading surface (23) substantially normal to the screen surface; and a trailing edge (20,21,22) on the foil following said blunt face tapering away from the screen surface so that the foil generates the combined effects of turbulence and positive and negative pulsation.

2. A stock screening mechanism for paper making stock fibers in a slurry constructed in accordance with claim 1: including a plurality of individual foils (16) arranged in sequential positions to move in sequence along the surface of the screen (14).

5. A stock screening mechanism for paper making stock fibers in a slurry constructed in accordance with claim 1: wherein the foil (16) is positioned within the annular screen (14) and rotates about an axis coaxially with the annular screen.

9. The method of screening stock fibers in a slurry for a paper machine comprising the steps of exposing the stock to the arcuate face of an annular stock screen (14), and moving a foil (16) along the screen, characterized by: generating turbulence and positive and negative pulses in the stock.

12. A paper making pulp screening apparatus wherein a plurality of generally airfoil shaped vanes (16) rotate through stock adjacent an annular screen plate (14) to generate pulses in a stock for enhancing screening efficiency, characterized in that: a blunt lead surface (23) is disposed on at least some of said vanes (16) for capturing a certain volume of stock and accelerating the captured stock substantially to rotor velocity for generating significant turbulence in the slurry; and a second surface of said vanes (16) extends generally along but angularly with respect to said screen plate (14) so that said vane generates both positive and negative pulses in stock adjacent said screen plate.

(23) for capturing and accelerating stock to rotor velocity. 14. An improved foil (16) for a paper stock screening apparatus wherein the foil includes a first curved surface

STATEME T UNDER ARTICLE 19(1)

In response to the International Search Report, and to more fully comply with PCT preferences, the claims in the pending application have been amended.

More specifically, the number of independent claims have been reduced through cancellation, and the remaining independent claims have been amended to be in two-part format, including pre-characterizing clauses defining the general state of the art and post-characterizing clauses more accurately defining the invention.

The claims have been amended to emphasize the distinctions between the present invention and AT B 362 226 (Canadian Ingersoil-Rand Company) , which was cited as a document of particular relevance. This reference clearly states that the hydrofoil taught therein is designed to have an outer surface conforming to the inner surface of the screen, that is the space between the screen and the outer surface of the foil remains constant. This is done to achieve the goal specified in the reference, which is to prevent negative pulses in the stock. Pulsations are generated only in one direction.

In contrast, the present invention seeks to utilize the combined effects of turbulence, positive pulsation and negative pulsation. The claims have been amended to more accurately and clearly recite this distinction.