Patent Application
20050045529
This invention relates generally to machinery for screening paper-making pulp and, more particularly, to a screening apparatus having an enhanced rotor for promoting screening efficiency together with power conservation.
The Pulp and Paper Industry uses pressure screens to separate undesirable materials from usable fiber in the Industries various processes. The ideal embodiment of a pressure screen would remove all undesirable materials without the loss of desired fibers, without requiring any additional dilution of the fiber suspension, and with the minimum possible power consumption. As actually practiced in the Industry, screens with apertures sufficiently small to remove the undesirable particles are prone to plugging with fiber, fiber flocs, and debris. Plugging of the screen apertures eliminates the passage of all fiber. To allow the screen to operate, the screen must be cleared of potentially blocking materials.
Numerous variations of pressure screen design exist. Most pressure screens are fed with the fiber suspension to be cleaned in the interior of the screen plate, which is cylindrical in shape. The rotor, containing pulse generating elements, rotates inside the cylindrical screen. Other variations include rotating screen plates with stationary rotor elements, and pressure screens that are fed from the outside with the cleaned fiber suspension collected in the interior of the cylindrical screen.
Rotor construction also varies greatly based on the particular screen design, but all rotors have in common an element or elements that pass in close proximity to the surface of the screen cylinder. In all cases there is a differential in speed between the rotating component and the fixed component in the rotor—screen cylinder combination. The established design practice is to configure the rotor element such that the speed differential generates a positive pressure pulse preceding the passage of the rotor and a negative pressure pulse beginning at the point of closest proximity of the rotor to the screen cylinder surface. The pulse is measured at the surface of the screen cylinder directly over the rotor element as it passes the measurement sensor. Additionally, for some screening applications, the pressure trace is also measured several pipe diameters away from the screen body in the screen accepts piping.
Conventional thinking and prior art practice has utilized the positive pressure pulse trace to provide turbulence and disruptive shear stress to break pulp flocs so individual fibers could pass through the screen cylinder apertures. This disruption of flocs is necessary since flocs are 2-5 millimeters in diameter and typical screen cylinder apertures are 1.5 millimeters or smaller for holed screens and frequently as small as 0.15 millimeters in width for slotted screens.
The negative pulse component of the pressure trace is utilized to create a flow reversal through the screen cylinder. This has the function of drawing fluid back to flush the apertures and to disrupt fiber mat formation on the surface of the screen. But such pulse generation can have several negative effects. For example, significant energy is required in order to produce the pulses, and the rotor structure to create such pulses can add to the expense of making the rotors.
In some instances, pulses can have a negative influence on the screening process. For example, a final pulp screening occurs prior to the headbox where the pulp is about to enter the final stages of the paper making process. Any pressure variations caused by pressure pulses can cause uneven forming of the pulp, with the result that the paper can become alternately thick and thin in spots. A rotor that aids the screening process prior to the headbox without any pressure pulses would produce an improvement in the even forming of the paper.
To reduce the magnitude of the effects described above, many machines are now made with closed rotors, that is, rotors having a full cylindrical surface on which projections and depressions are directly attached without support arms to generate localized pressure pulsations. Depending upon their specific geometries, these may offer lower specific power consumption than cage rotors; and, because the bumps and depressions are distributed over the rotor surface, the pressure pulsations are distributed about the screen plate surface and do not concentrate alternating stresses along the aperture pattern.
This invention is an improvement to the hydrodynamic device shown in Young et al U.S. Pat. No. 5,497,886. The 5,497,886 patent discloses a hydrodynamic device for generating negative pressure excursions in a pulp slurry during fine screening, including a plurality of half-foil or lifting members disposed on a substantially cylindrical outer surface of a rotor, the rotor being mounted within and co-axial with a substantially cylindrical screen having a circumferentially continuous apertured zone to define an annular screening chamber between the rotor and the screen, the half-foil members being so arrayed as to sweep across the full axial extent of the apertured zone on the screen with each revolution of the rotor.
In one embodiment of the 5,497,886 patent, as shown in
One of the objects of this invention is to create a rotor that can keep a screen working properly without the creation of pressure pulses that can waste energy and create uneven flow of pulp through the screen.
Another of the objects of this invention is to create a rotor that keep a screen working properly while using a third less energy at the same rotor speed than traditional rotors, the rotor speed being measured at the part or tip of the rotor closest to the screen surface. This is accomplished by putting the same amount of energy into the flow across the screen surface, but by doing so with a mixing vortex without a significant pressure pulse.
This invention provides a rotor that creates vortices that create flow across the screen surface that keep the apertures open, while at the same essentially eliminating pressure pulses. The vortices created by the rotor of this invention cover a greater extent of the surface of the rotor than the pulses created by traditional rotors, and this further increases the cleaning ability of the rotor.
By placing the vane structures on the rotor with a slight overlap between successive vane structures, the direction of flow of the mixing vortices can be changed with each successive group of vane members. This produces the result of improving the removal of obstructive material from the screen because of the reversal of the cleaning flow across the screen.
The vane member height is such that the top of the vane is close to the screen surface, but with the necessary clearance for avoiding any collisions with the screen surface. If the top of the vane is too far from the screen surface, then vortices will not be as strong and will not keep the screen clear.
More particularly, the preferred embodiment of this invention provides a rotor adapted for use in a hydrodynamic device comprising a screen having a circumferentially continuous apertured zone, the rotor having a longitudinal axis of rotation and including an outer surface adjacent the screen surface. The rotor also includes a plurality of sets of radially extending, relative to the rotor axis, adjacent but spaced apart vane members attached to the rotor surface, each set including at least one, but typically four adjacent vane members with equally spaced apart and parallel trailing edges ending in a common plane. In other embodiments, the trailing edges may be located in different planes. The two outermost vane members have leading edges forward of the two most inside vane members, with the leading edges forward of the two most inside vane members being angled inward toward its next adjacent vane. The circumferential length of the two innermost vane members is about half of the circumferential length of the two outermost vane members. Each of the outermost vane members is angled relative to the circumferential direction of travel of the rotor by each of the outermost vane members having a leading edge that is angled relative to its trailing edge. Each of the vane members has a leading edge forming an acute angle relative to the surface of the rotor and has a top edge with the same radius of curvature as the screen surface. The rotor is mounted within and co-axial with the screen to define a screening chamber between the rotor surface and the screen surface, and the plurality of sets of vane members are spaced out throughout the screening chamber in the direction of the rotor axis. The rotor outer surface does not have any longitudinally extending lifting bodies that produce pressure pulses throughout the screening chamber.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.
Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of the construction and the arrangements of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways, particularly with respect to construction of various forms of single or multiple vanes and/or fences that may be used to generate vortices. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. Use of “including” and “comprising” and variations thereof as used herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Use of “consisting of” and variations thereof as used herein is meant to encompass only the items listed thereafter and equivalents thereof. Further, it is to be understood that such terms as “forward”, “rearward”, “left”, “right”, “upward” and “downward”, etc., are words of convenience and are not to be construed as limiting terms.
Referring to
Rotor 36 has a closed top and a generally cylindrical surface 44, and the screen 40 has apertures 42 through which accepted fiber along with pulp liquor has a normal outflow. More particularly, the screen 40 is a cylindrical screen having a circumferentially continuous apertured zone. The annular space between rotor 36 and screen 40 defines a screening chamber 48, while the space outboard of the screen 40 contains accepts chamber 52 which is drained by accepts discharge 56. Below accepts chamber 52 and screening chamber 48 is rejects chamber 60 which empties through rejects discharge 64. Rotor 36 is rotated by a shaft, which extends through a sealed center column 68 and which is driven by a prime mover (not shown) through drive pulley 72.
Mounted on the surface 44 of the rotor 36 is a set 80 of four vane members. The rotor 36 is mounted within and co-axial with the screen 40, and the plurality of sets 80 of vane members are spaced out throughout the screening chamber in the direction of the rotor axis. The rotor outer surface 44 does not have any longitudinally extending lifting bodies that produce pressure pulses throughout the screening chamber 48. Or, in other words, the vane members extend radially directly from the rotor outer surface 44.
As shown in
As shown in
More particularly, as shown in
The two outermost vane members 90 and 92 have leading edges 94 and 96, respectively, forward of the two most inside vane members 100 and 104, with the leading edges forward of the two most inside vane members 100 and 104 being angled toward its next adjacent vane member. The height of the two innermost vane members 100 and 104 is about the height of the two outermost vane members 90 and 92. In other words, each of the outermost vane members 90 and 92 is angled relative to the circumferential direction of travel of the rotor 36 by each of the outermost vane members 90 and 92 leading edges being angled relative to its respective trailing edge. Each of the vane members 90, 92, 100, and 104 has a leading edge 108, as shown in
An angled vane member as defined by this invention is an at least partial radially extending structure on a rotor, the structure being angled in the rotor circumferential direction, the structure creating a positive pressure on the upstream side of the structure and a negative pressure on the downstream side of the structure without including any appreciable pulp lifting body. The pressure difference across the vane creates a free vortex that keeps the screen surface clear.
A pulp lifting body as defined herein is an at least partially circumferential extending rotor structure that turns fluid flow into some physical force on the structure thereby creating a pressure pulse.
The described embodiment is for fiber concentrations of 1% to 6% and will operate at rotational velocities such that the speed of the vane members, as measured at the closed point to the screen cylinder surface, can be 10-35 meters/second.
For fiber concentrations of 0.25-1.5% being processed immediately prior to a paper machine, a different hub design (not shown) is required due to the volume of fiber suspension required per time unit. This embodiment would be a central rotor shaft having radially extended arms supporting relatively thin flat plates on which would be mounted vane members as describe above. Coverage of the screen cylinder surface would also be as noted above. In other words, each of the sets of vane members can be attached to separate curved pieces forming a curved surface coextensive with the set of vane members, with each curved surface piece being supported above the rotor surface, would could constitute a central rod, by an arm extending between the curved surface piece and the central rod.
Various other features and advantages of the invention will be apparent from the following Claims.
