Classifying – separating – and assorting solids – Sifting – With liquid treatment
Patent
1991-08-05
1993-11-09
Bollinger, David H.
Classifying, separating, and assorting solids
Sifting
With liquid treatment
209397, 210415, 210498, B07B 104, B07B 149
Patent
active
052595129
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a screen for pressure sorters for fiber suspensions comprising a rotor, in particular, adjacent to an inlet side of the screen, for generating positive and negative pressure thrusts in the fiber suspension, the screen having a shape which is rotationally symmetrical in relation to a screen axis.
BACKGROUND OF THE INVENTION
In the processing of fiber suspensions, as a rule, several such pressure sorters are connected one behind the other, with the screen openings of the pressure sorters connected one behind the other becoming smaller and smaller, i.e., the screen openings of the pressure sorters following a first pressure sorter are always smaller than the screen openings of the preceding pressure sorter. In this way, impurities and clumps of fibers are separated in dependence upon their size stepwise from the usable fibers as the so-called accepted material of the preceding pressure sorter is fed to each pressure sorter (the accepted material is that part of the fiber suspension which has passed through the screen openings of a pressure sorter).
The paper industry demands apparatus with greater and greater capacity; for pressure sorters this means that not only a higher and higher specific throughput capacity is called for (amount of fiber suspension passing through a screen surface of a certain size per time unit) but that a certain sorting fineness is to be attainable with fewer and fewer pressure sorters connected one behind the other, which is only achievable with relatively fine screen openings, which does, however, conflict with the increasing of the throughput capacity.
Therefore, to increase the throughput capacity not only new rotor shapes were developed, but the circumferential speed of the rotor regions adjacent to the inlet side of the screen was also increased in order to generate relatively high positive and negative pressure thrusts and turbulences in the fiber suspension - clogging of the screen openings by impurities contained in the fiber suspension is to be prevented by the pressure thrusts (backwashing effects occur at the screen openings) and the turbulences are to prevent the fibers from forming at the inlet side of the screen a kind of fibrous mat which reduces the throughput capacity or prevents passage of usable fibers through the screen openings altogether. The measures for increasing the throughput capacity (increasing the rotor speed and the rotor circumferential speed as well as increasing the pressure difference occurring at the screen) do, however, result in the forces acting on the screen becoming greater and greater, and they often cause rapid destruction of the screen, in particular, as a result of cracks occurring in the screen plate. To achieve higher service lives, i.e., to obtain more resistant screens, one has, therefore, increased the wall thickness of the screens; as a result of this measure, the flow channels formed by the screen openings are, however, also increased in length, a consequence which negatively affects the throughput capacity of the pressure sorter. Moreover, the demand for more and more efficient apparatus has not only resulted in a pressure sorter having to process greater and greater fiber suspension quantities per hour, but in it also having to process fiber suspensions with a higher substance density (fiber component per volume unit) and, above all, with fiber suspensions of relatively high substance density, comparatively long screen opening channels lead to high pressure losses between the inlet side and the outlet side of the screen which, in turn, result in rapid clogging of the screen openings.
A wide variety of suggestions has already been made for the design of the screen wall of such screens:
In U.S. Pat. No. 3,581,903 of the applicant, it was suggested that in a screen with slit-shaped screen opening channels a boat-shaped recess be milled in the outlet side of the screen wall for each screen opening and such a screen be installed in a pressure sorter in which the rotor rotates adjacent
REFERENCES:
patent: 3581903 (1971-06-01), Holz
patent: 4676903 (1987-06-01), Lampenius et al.
patent: 4717471 (1988-01-01), Winkler
patent: 4842722 (1989-06-01), Holz
patent: 4898665 (1990-02-01), Lamort
patent: 4950402 (1990-08-01), Frejborg
patent: 4986900 (1991-01-01), Mason
patent: 5000842 (1991-03-01), Ljokkoi
Bollinger David H.
Hermann Finckh Maschinenfabrik GmbH & Co.
Nguyen Tuan N.
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