PULB SCREEN FOIL AND METHOD OF USE

CROSS REFERENCE TO RELATED APPLICATION [0001] The present application claims priority under 35 U. S. C. §119(e) to U.S. Provisional Patent Application No. 60/773,839, filed February 16, 2006.

FIELD OF INVENTION

[0002] The present invention relates to an improved screening apparatus and method useful in the paper industry and to a hydrofoil in combination with the screen to pump pulp through the screen and to clean the screen.

BACKGROUND OF THE INVENTION

[0003] Current designs overlook a critical pressure screen operational consideration that, in turn, contributes to limiting the performance of current screen foils and therefore the pressure screens themselves. Specifically:

1. The contours of existing screen foils are generally configured in a similar manner to those of conventional hydrofoils or airfoils in ground effect. See Prior Art Fig. 1. These designs have been refined over the years and therefore provide a generally acceptable foundation upon which to base a screen foil design. However, the designs have been constrained because of the additional requirement that the flow fields of conventional hydrofoils, etc. be stable (i.e., the location of the flow field center of pressure (CP) remains relatively constant as the foils themselves experience operational attitude changes). The CP will generally maintain its location (within acceptable limits) if the locations of the foil pressure profile maximum and minimum pressures remain constant. This requires that the location of the minimum gap (between the bottom of the foil and the "ground") remain constant with attitude change which, in turn, eliminates the possibility of utilizing a constant gap region (or channel) in the foil design. In such a case the minimum gap location would move from one end of the channel to the other (a significant distance) hi response to clockwise and counterclockwise rotation of the

foil. This would then cause commensurate CP movement and hence significant instability.

2. Flow stability with attitude change is not an important issue for screen foil operation and design. A screen foil is not subject to significant attitude changes during its operation (due to the rigidity of its mounting hardware) and, even if it were, center of pressure variation would not significantly effect foil screening or structural performance.

3. Yet the thinking behind current design philosophy does not take advantage of this fact to extract maximum hydraulic performance from contemporary screen foil configurations.

[0004] A typical prior art pulp screen hydrofoil is shown in Fig. 1. Here, hydrofoil 100 includes leading edge 102 and trailing edge 104. A face side 106 is provided opposite to the camber surface 108 of the hydrofoil. The camber surface is positioned in proximity to screen surface 110.

[0005] The screening surface 110 is shown diagrammatically here and can comprise any perforated surface in which accepts fibers are passed through the surface with reject trash and other contaminants remaining on the hydrofoil side. The surface may be a slotted cylindrical or other screen wherein slots or other openings are machined, laser cut, hydraulically cut or cut via electro-discharge or other mechanism. Also, the screen surface may comprise a wedge-wire or other design.

[0006] As shown, the hydrofoil is adapted to travel in the clockwise direction with the camber surface 108 closely spaced from screening surface 110. In these typical designs, the minimum clearance between the camber surface and the screen surface is designated as gap 125. This gap is the minimum distance between the lowest extremity 130 of the camber surface and the screening surface.

SUMMARY OF THE INVENTION

[0007] One object of the invention is to develop an improved screen foil for operation at relatively low rotation tip speeds (15 to 16 meters per second). In order to achieve screen operation at these low speeds it is necessary to improve the overall performance/efficiency of the screen foil design. More specifically, to increase the foil energy efficiency (as compared to that of existing screen foils), we want to create (with a minimum expenditure of energy) a high magnitude, long duration suction (negative) pressure pulse (between the foil and the screen cylinder) while minimizing the magnitude and duration of the foil positive pressure pulse. Also, during this process we want to maximize the fluidization of the pulp slurry.

[0008] The unique use of a constant (minimum) height dimension passage or channel (between the underside of the foil and the screen cylinder) to enhance the magnitude and extend the duration of the lowest pressure region generated under the foil is disclosed. In addition, a greater degree of fluid shear is imparted to the flow over the screen cylinder inner surface which increases slurry fluidization associated with the cylinder surface profile. These factors then combine to improve the overall efficiency of the screening process.

[0009] It is possible to utilize this relatively unstable (with respect to foil attitude change) design in this specific application for two reasons: (1) the foil is rigidly attached to the rotor hub assembly which maintains the relationship of the foil to the screen cylinder (no significant attitude changes possible) and (2) the screening process itself does not require the stability (with attitude change) demanded by the applications into which most hydrofoil designs are integrated.

BRIEF DESCRIPTION OF THE DRAWINGS [0010] The invention will be further described in connection with various exemplary embodiments that are illustrated in the appended drawings.

[0011] Figure 1 is a schematic cross-sectional view of a prior art hydrofoil shown in proximity to a screening surface;

[0012] Figure 2 is a schematic cross-sectional view of a hydrofoil in accordance with the invention shown in proximity to a screening surface;

[0013] Figure 3 is a schematic cross-sectional view of a hydrofoil in accordance with the invention and associated screen surface highlighting a screen surface portion adjacent to the hydrofoil during hydrofoil travel along the screen surface;

[0014] Figure 4 is a graph resulting from a CFD analysis of a hydrofoil operating in accordance with the invention showing the pressures encountered along the screen surface as the hydrofoil passes thereover showing positive pressure values and pressure during the negative or suction phase of hydrofoil passage over the screening;

[0015] Figure 5 is a schematic, CFD presentation showing pulp slurry flow direction and velocities along the screen surface during the throughput phase of the screening operation; and

[0016] Figure 6 is a schematic, CFD presentation showing pulp slurry flow direction and velocities along the screen surface during the backfiow phase of the screening operation.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0017] The criteria that have been utilized for assessing screen foil performance for CFD evaluations of various foil designs are as described below.

[0018] The screen foil's primary purpose in a pressure screen is to prevent the pulp slurry at the inner surface of the screen cylinder from creating a fiber mat which would immediately stop the screening process from continuing. It does this in two ways: (1) by fluidizing the slurry in the vicinity of the screen cylinder and (2) by creating a backfiow of the slurry through the screen cylinder. Fluidization mixes the slurry fibers, keeping them in motion so that they cannot adhere to each other to form a mat. Backfiow enhances the fluidization process by extracting pulp fibers that are wedged (or about to be wedged) in the screen cylinder openings into a region where fluidization is taking place. And due to these two operations there will be no net mat formation and the screening process can continue.

[0019] CFD simulations were conducted during the analysis/design phase of the development program as a means of assessing foil design efficacy.

The six parameters below associated with the fluid flow in the pressure screen were specifically monitored to determine pressure screen performance. The parameters are the following:

1. Area under the Negative Pressure (suction) portion of the pressure profile curve (see Fig. 4) = A _p -

2. Area under the Positive Pressure portion of the pressure profile curve (see Fig. 4) = A _p +

3. Sum of Vorticity Magnitude squared under the foil and across the inner surface of the screen cylinder (see Fig. 3) = ∑ \ξ\ ²

4. Average Fluid Shear Rate under the foil and across the inner surface of the screen cylinder (see Fig. 3 where the applicable screen surface portion is d shown at 300) = m ( — u ) dy

5. Average Fluid Speed under the foil and across the inner surface of the screen cylinder = m (S)

6. Foil Power Consumption = Pf where ξ = Vorticity = V x V

V - Velocity Vector = u i + vj + w k

V = Del Operator = — / + — j + — k dx dy dz

S = Speed = VM + v ² + vv ²

[0020] Clearly, consistent with the discussion above, we want parameters 1 , 3, 4 and 5 to be as high as possible and parameters 2 and 6 to be as low as possible. This will give us our desired high backflow/slurry dilution, fluidization (leading to high screen throughput) plus contaminant removal efficiency coupled with low power consumption requirements.

[0021] Note that the third of our performance parameters is calculated by summing the square of the magnitude of vorticity. We know that vorticity has both positive and negative values (denoting fluid spin direction) either of which will disrupt the flow to fluidize the slurry. We can therefore envision a situation in which the sum of the vorticity magnitudes could be "zero", indicating minimal fluid mixing, while in reality high ± values of the vorticity magnitude would exist in the flow that were actually creating fluidization and improving performance. This possibility is eliminated by summing the square of the vorticity magnitudes.

[0022] One or more of the following advantages may then be associated with an exemplary embodiment of the invention.

1. Negative Pressure Profile: The negative portion of the channel foil pressure profile (see Fig. 4) is significantly wider (and therefore the area under its curve, Ap- is greater) than that of a more conventional foil. The additional suction thus created produces more backfϊow through the screen cylinder which in turn leads to a greater degree of fiber mat disruption, slurry fluidization and pulp dilution than would occur otherwise and therefore greater overall pressure screen throughput.

2. Lower High Pressure Pulse: The average positive pressure in this zone has a relatively low value and therefore reduces the potential for slurry contaminant throughflow or plugging that might occur under more typical positive pressure conditions (see Fig. 4).

3. Fluid Shear/Speed Transfer to Screen Cylinder: Greater levels (as compared to those of conventional foils) of fluid shear and thus fluid speed flowing tangentially over the screen cylinder profiles are imparted to the screen cylinder surface below the foil underside region. This enhances the fluidization process as the momentum transfer to the recirculation zone (due to the increased fluid speed

across its upper region) increases its strength and hence its mixing potential (see Figs. 5 and 6).

4. Higher Vorticity/ Mixing Levels: Fluidization is further enhanced by the addition of relatively high amounts of vorticity to the slurry above the screen cylinder surface and below the foil underside region. Here the fluid rotation associated with the vorticity aids in the mixing and thus fluidization of the pulp slurry. Note that this effect is in addition to that discussed in 3 above.

5. Design Easily Modifiable: The channel foil design can be changed easily to alter the suction pressure profile and therefore the creation of slurry backflow (and to a lesser degree the shear/fluid speed transfer to the screen cylinder). Such a change is made merely by (1) sectioning the foil in the channel region, (2) rotating the resulting halves around the foil axis of rotation until the new desired channel length is achieved and (3) appropriately re-contouring the foil top surface via a gradual curve. The underside contours of the foil leading and trailing the channel zone remain the same. This eliminates the need for the complex analysis/testing required when varying the underside design of conventional foils.

6. Low Power/Performance Efficiency: While power requirements similar to those of our preferred foil configuration can be achieved via an efficient hydrodynamic foil design without a channel, the channel itself does not significantly increase power requirements, if it increases them at all. Therefore (due to the performance increases realized by use of the channel) the ratio of performance benefits to power of the channel foil is significantly improved over that of more conventional designs.

[0023] Turning now to Fig. 2 in accordance with the invention, the foil rotates at a fixed gap with respect to the inner surface of the screen cylinder (filtering element) 8. In the foil frame of reference, the fluid flow approaching the foil splits into two streams; one over the foil upper side 13 and one along the under side 7. The flow enters the foil along stream 7 at location 1 where it is accelerated until it reaches the channel region 3 (i.e., the area between boundaries A and B). The average positive pressure in this zone has a relatively low value and therefore the potential for slurry contaminant throughflow or plugging (that would most

likely occur under more typical positive pressure conditions) is reduced. The flow then enters the constant height channel zone 3 in which the pressure changes from nominal (the value at the screen cylinder inner surface 8 outboard of the foil underside) to slightly more than the maximum negative pressure achieved under the foil. See Fig. 4. As a result, slurry backflow usually begins in this region.

[0024] Upon leaving the channel region as shown at 4 the flow enters the diffusion zone 9 where it decelerates to subsequently exit this zone at 5 at substantially nominal pressure (again the value at the screen cylinder inner surface 8 outboard of the foil underside. See Fig. 4 again. In order for the flow to satisfy continuity (conservation of mass) in the diffusion zone 9 (and temporarily neglecting the effects of cylinder backflow) the maximum negative pressure becomes located just inboard (or downstream) from the diffusion zone entrance as shown at 4. Backflow through the screen cylinder 8 also occurs in this region 9 due to the low pressure profile values there. Finally the flow leaves the foil tinder side region at its exit 5.

[0025] During this process relatively high levels of voiticity and fluid shear are imparted to the pulp slurry in the region under the foil. This enhances fluidization both directly and in combination with the screen cylinder backflow. More specifically the vorticity shed from the foil acts to stir the slurry fibers, the fluid shear acts to drag the slurry (in a tangential manner) more quickly over the screen cylinder profiles where the slurry fibers are disrupted in the profile recirculation zone (Figs. 5 and 6). This is enhanced as the strength of the recirculation created by the backward flowing slurry is increased by the shear driven tangential flow (Figs. 5 and 6).

[0026] It is noted that while the invention has been described in conjunction with a hydrofoil adapted for rotation along the inside surfaces of a screening surface, it is also within the scope of the invention to apply this concept to apparatus in which a hydrofoil rotates about an outer surface of a screen as shown for example in U.S. Patent 3,759,392. Conversely, the screen could move relative to the hydrofoil. Also, although generally cylindrically shaped screen surfaces are shown other geometric configurations of "screening surfaces may also benefit from the invention.

[0027] The hydrofoil 200 of the invention has a leading edge 202 and a trailing edge 204. A face side 206 is disposed opposite from the camber surface 208 of the foil with the camber surface rotating in close spatial proximity to the perforated screen surface 8. As shown in this drawing, the hydrofoil will rotate in the clockwise direction. In the foil frame of reference, the pulp will therefore enter the space between the hydrofoil and the screen surface from the leading edge and proceed toward its exit at the trailing edge side as explained above.

[0028] The camber surface has a total length (TL) as measured from 202 to 204. A channel surface portion 212 of the camber surface is provided intermediate the leading and trailing edges. (The length of the channel surface portion is defined as the length between boundaries A and B). This channel surface portion is configured so that it is essentially parallel to the shape of the screen surface 8. Accordingly, a channel 3 is provided between the surfaces 212 and 8 that is of substantially even height throughout and represents the narrowest clearance between the camber surface and the screen surface 8. Stated differently, along the length of the channel region 3 the radial clearance between the surfaces 212 and 8 is substantially equal and is the minimum clearance presented between the camber surface and the screen surface 8.

[0029] In one exemplary embodiment, this channel surface portion 212 and corresponding channel 3 extend along a continuous portion of the TL. Preferably, the length (L) of the channel portion measured along the camber surface is from about 5 to 100% TL and most preferably 10 to 60% TL. The artisan can appreciate that a protuberance, bump, or other irregular surface could be provided along the camber surface of the foil that would be of narrower dimension or clearance between the surfaces 8, 212. Such discontinuous or irregular surfaces would not constitute a channel as defined herein.

[0030] Fig. 3 illustrates the section of the screen zone surface for which fluid shear, speed, and vorticity have been determined by computational fluid dynamics (CFD). As shown, hydrofoil 200 rotates in the clockwise direction and underlying area 300 on the screen surface corresponds to the screen surface area juxtaposed along the length of the camber surface 208 passing thereover that may be referred to as the arc length.

[0031] In Fig. 4, one can see that a positive pressure is encountered at the cylinder screen location juxtaposed along the leading edge 202 of the foil. As the channel section 212 of the foil passes over the screen a negative (suction) force is applied with the pressure returning toward zero at the screen surface underlying the face slightly downstream from the trailing edge 204 of the hydrofoil.

[0032] When Fig. 4 is reviewed and compared to some conventional prior art hydrofoil/screen arrangements, the suction pressure in section 420 of the graph is greater than that of a corresponding suction section in many prior art devices. This demonstrates that use of the channel region as defined herein improves the suctional forces on the screen cylinder slots by increasing the negative pressure and duration of the suction pulse. This is important as improvement in the suction phase helps to clean the slot area from fiber mats and debris that may otherwise form to prepare that slot area for the throughput stage. Also, as shown in Fig. 4, the area on the graph representing the positive pressure pulse 480 may exhibit lower pressure than in conventional designs and therefore it can inhibit undesirable push through or "extrusion" of contaminants such as "stickies" and the like through the slot to the accepts side of the screen.

[0033] Fig. 5 depicts the pulp slurry velocity and direction existing in the area of screen slot 510 at the throughput stage of the process. A robust pulp recirculation zone 520 is provided over the slot 510 to fluidize the slurry. This large recirculation zone contributes to slurry mixing. In this analysis, the velocity of the particles may be on the order of about 70 cm/sec in the recirculation zone. A very high speed pulp slurry section 530 overlies intermediate speed section 540 and is located immediately beneath the camber surface of the hydrofoil. In accordance with one exemplary embodiment, particle velocity may be on the order of 550 cm/sec in the section 530 with particle velocity in section 540 approximately at about 400 cm/sec. A lower velocity section 550 exists intermediate section 530 and the recirculation zone 520. Particle velocity in section 550 may be on the order of about 300 cm/sec. Thus the zones 530, 540, and 550 provide stacked zones of increasing velocity as measured from the screen surface .toward the camber side of the hydrofoil. These zones and the different

particle velocities existing therein provide improvement in shear to help defiber the slurry.

[0034] Fig. 6 shows the pulp suspension overlying slot 510 during the suction or backflow stage of the process. Again, an ample recirculating zone 520 is shown adjacent slot 510. This again shows good mixing of the slurry while it helps to inhibit mat formation. In the exemplary embodiment shown, the fluid velocity in zone 590 over the slots does not exhibit discrete bands of differing velocities as shown in Fig. 5.

[0035] It is accordingly apparent then that the invention provides a hydrofoil that is adapted for relative movement along a perforated screen surface to promote flow of a pulp suspension along the screen surface and aid in pulp suspension separation including passage of accepts through the perforated surface. The foil has a leading edge and a trailing edge and a camber surface and an opposing face surface. The camber surface is adapted for relative movement along the screen surface and in proximity thereto.

[0036] The camber surface comprises a channel surface portion located intermediate the leading edge and the trailing edge. The channel surface portion is substantially parallel to the cylinder surface. In one exemplary embodiment, the distance between the channel surface portion and the corresponding screen cylinder portion proximate the channel region portion is substantially the minimum distance or clearance between the camber surface and the cylinder surface. Further, the channel surface portion extends along a length (1) of the camber surface. The camber surface itself has a total length (TL) as measured from the leading edge to the trailing edge. The channel surface portion extends along only a portion of the TL, preferably from about 10 to 60% of the TL.

[0037] Turning back again to Fig. 2, an elongated channel 3 is provided along the total length of the hydrofoil as measured from the leading edge 202 to the trailing edge 204. This channel can be shown distinctly in Fig. 2 as the area existing between boundary lines A and B. The channel also defines the minimum clearance existing between the camber surface and the screening surface. Upon leaving the channel region as shown by reference 4, a diffusion zone is provided

between boundary lines B and C in Fig. 2. The maximum negative pressure is located just downstream from the boundary B.

[0038] Turning back for contrast to Fig. 1, it can be seen that the lowest extremity 130 of the camber surface 108 the prior art depiction is provided at a point 130. In contrast, the lowest portion of the hydrofoil 200 as shown in fig. 2 exists as a continuous curvilinear surface provided between the points A and B. The curvilinear configuration of the surface here is substantially parallel to the surface of the screen cylinder and defines a continuous channel provided along the TL of the hydrofoil 200.

[0039] While certain embodiments of the invention have been shown and described herein, it is intended that there be covered as well any change or modification therein which may be made without departing from the spirit and scope of the invention as defined in the appended claims.

[0040] What is claimed is: