Pressure screening apparatus for screening a paper stock...

Details Pressure screening apparatus for screening a paper stock... Pressure screening apparatus for screening a paper stock...

2000-03-16

2001-11-06

Lithgow, Thomas M. (Department: 1724)

Liquid purification or separation

Filter

With residue removing means or agitation of liquid

C210S394000, C210S396000, C210S397000, C209S250000, C209S273000, C209S306000, C209S304000, C162S055000, C162S251000

Reexamination Certificate

active

06311850

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a pressure screening apparatus for screening a stock suspension and to a screen clearer.
2. Discussion of Background Information
Pressure screening apparatuses are typically used in treating paper stock suspensions, in order to process the stock suspension in a wet screening. For this purpose, the pressure screening apparatus contains a screen element which includes a number of apertures. The fibers contained in the suspension are allowed to pass through these apertures, while the undesired solid components of the suspension are prevented from doing so and are rejected and conducted out of the screening apparatus. This device may also be use for separating various grades of fiber components, i.e., separating the shorter fibers from longer fibers. Such screening apparatuses typically use round holes or slits as the grading apertures. In most cases, pressure screening apparatuses of the type under consideration here also utilize screen clearers which have clearing surfaces that are moved past the screen. This conventional design is intended to prevent clogging of the screen apertures.
A screen clearer is known from WO 98/53135 which utilizes blade elements to clear the screen. These blade elements have a hydrodynamic profile which extends over the entire length of the screen element. This design also utilizes a revolving screen. Owing to their movement relative to the surrounding suspension, the blade elements exert a pressure impulse in front of the screen and a suction impulse behind the screen. This design causes part of the suspension which was rejected at the screen or part of the suspension which has already passed the screen as accepted stock, to be sucked back. As a result of this design, the screen apertures are kept clear or are cleared. In order to effectively process suspensions having different solid contents (“consistency”) in different zones of the screen element, the cross sections of the blade elements are shaped differently. However, the ability of this design to efficiently screen the stock suspension is very limited.
U.S. Pat. No. 3,586,172 discloses a pressure screening apparatus whose rotor is divided into four different zones. After a first zone, which serves only to accelerate the material, the suspension reaches a grading zone in which the rotor is equipped with elevations which are intended to fluidize the stock suspension and prevent the formation of fiber flocks (“dusters”). The subsequent grading zone is similar to the first zone and has depressions in the rotor which are intended to ensure that the flow speed is maintained and thus to prevent clogs (“plugging”). However, the clearing effect of such depressions in this design is poor.
In EP 0 289 020, it is suggested to prevent or to reduce the too rapid deckering of the residue by utilizing a rotor which accelerates the axial transport along the revolving screen. For this purpose, the rotor has a number of projections (“bulges”) with oblique surfaces which produce axial flow components. Depending on the axial position of the projection in question, axial impulses of different strengths are thus produced.
SUMMARY OF THE INVENTION
The invention therefore provides a pressure screening apparatus which utilizes a special screen clearer acting on the entire screen element. The design promotes a good grading effect and is particularly robust or sturdy so as to withstand forces which are encountered in continuous use. It also enables a high throughput, and is moreover suitable for suspensions whose consistency is above 2% as well.
The invention includes a screen clearer which is provided with a number of clearing elements that differ depending on their position. While the actual shape of the clearing elements used is known per se from prior art, their combination in the manner shown is unknown and results in considerable advantages. The invention provides that in the course of the transport flow of the stock suspension, the suspension travels along the screen element. Further, the screen clearing is carried out with increasing clearing effect as the suspension travels within the screening apparatus. The suspension is acted upon by various clearing element configurations, until the portions of the suspension which did not pass through the screen element leave the apparatus via the reject outflow. In order to provide a grading effect on the suspension, it should be noted that both the size and shape of the grading apertures in the screen element as well as the shape of the clearing elements which keep the screen element clear are important. Moreover, the clearing elements produce pressure and suction impulses at the screen apertures, with the suction impulses serving to accomplish the clearing, i.e., keeping the screen clear.
The invention uses various combinations of clearing elements in which distinctions can be drawn between several types of these clearing elements whose construction is fundamentally different. Thus, one type of clearing element is in the form of hydrodynamically effective blades around which the suspension flows. These can be moved relative to the suspension in the intermediate space between the rotor and the screen element. As a result of this design, pressure and suction phenomena specifically occurs which promotes throughput efficiency. Another type of clearing element utilizes mounted elevations which are essentially in the form of spherical segments or “bumps”. These are disposed on a rotor body. Another clearing element design utilizes mounted elevations which have a more or less pronounced wedge shape. Still another clearing element design uses the rotor body itself which has a particular shape. This shape may have an oval configuration or may have the form or shape of a rounded polygon, e.g., a triangle or a lobed rotor.
The effect that the various clearing elements have on grading characteristics and the throughput of the suspension is described for example in the scientific article by R. Rienecker: “Sortierung—ein Werkzeug zur Sticky-Entfernung: Maschinen [Grading—A Tool for Sticky Removal: Machines]” (Wochenblatt für Papierfabrikation [Paper Manufacturing Weekly], Nos. 17 and 18, 1997, pp. 787 to 793, 855 to 859), the disclosure of which is expressly incorporated by reference in its entirety. These possible configurations described based on cleared revolving screens also exist, of course, for screen clearers intended to keep flat screens clear.
With the rotor of the pressure screening apparatus according to the invention, the different clearing elements can be arranged such that the screen clearing effect in different clearing zones is increased during the course of the transport flow. Initially, the paper stock suspension is relatively thin immediately after entry into the pressure screening apparatus and has a high content of accepted stock and fillers. Thus, the demands on the screen clearing are lower at this point than during the later course. As a general rule, a decreased clearing effect serves to improve the grading quality, i.e., the separation efficiency. Thus, more fibers reach the accepted stock. In contrast, if too much of the accepted stock and fillers were to flow through the screen in this first zone, they would not be present as wet and support material in the subsequent grading zones. By utilizing the invention, the grading effect can be evened out so that a favorable mixture of fibers and accepted stock is present in larger areas of the screening apparatus. This is important for “saving” the long fibers, which would otherwise be too readily rejected permanently, i.e., would reach the reject outflow of the pressure screening apparatus.
The tendency to force substances through in the transport direction can also be increased with the invention. For example, the separation boundary between the rejected and accepted portions at a screen element can be shifted as the clearing effect increases. As a result, portions which are held back when the force-through tendency

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