Classifying – separating – and assorting solids – Sifting – With liquid treatment
Reexamination Certificate
1999-10-13
2001-09-18
Walsh, Donald P. (Department: 3653)
Classifying, separating, and assorting solids
Sifting
With liquid treatment
C209S724000, C209S725000, C209S731000, C209S283000, C209S284000
Reexamination Certificate
active
06290067
ABSTRACT:
The present invention relates to a screening apparatus for separating fibre suspensions, comprising a housing, an inlet to the interior of the housing for a fibre suspension to be separated, a stationary tubular screen dividing the interior of the housing into a central chamber for receiving the fibre suspension from the inlet at one end of the tubular screen and an outer accept chamber for receiving an accept fraction of the fibre suspension, which has passed through the screen, a rotor journalled on the housing and situated in the central chamber coaxially with the tubular screen, and a reject chamber for receiving a developed reject fraction from the other end of the screen. The apparatus further comprises dilution liquid supply means for supplying dilution liquid the central chamber.
Such an apparatus is generally used for separating paper pulp suspensions, for instance for fractionating fibres or for separating contaminants and other undesirable particles, such as incompletely treated fibres. A well-known problem in connection with the separation of a pulp suspension by this type of apparatus is that the pulp suspension in the central chamber has a higher fibre concentration relatively close to the reject passage than relatively close to the end of the tubular screen where the pulp suspension to be separated enters the screen. This is due to the fact that the liquid, usually water, in the pulp suspension easily separates through the screen immediately after having entered the tubular screen. As a consequence the developed reject fraction is thickened in the vicinity of the reject passage. This thickening of the reject fraction is further increased if just a relatively small flow of the reject fraction is allowed to escape through the reject passage. A too large thickening of the reject fraction gives rise to torque transmission between the rotor and the screen via the thickened reject fraction, which results in an increased energy consumption for rotating the rotor. The rotor might even get stucked to the screen, which requires a costly production break for manual cleaning of the rotor and the screen. The thickened reject fraction might also be difficult to discharge through the usually narrow reject passage and further through a valve situated outside of the apparatus.
U.S. Pat. No. 4,749,474 discloses an apparatus of the above described type in which the dilution liquid supply means is adapted to pump dilution liquid from a reject chamber of the apparatus into the central chamber against the flow of developed reject fraction, to counteract the thickening of the suspension along the screen. A disadvantage of this known apparatus is that the apparatus produces a final reject fraction that has an unsatisfactorily low consistency, because the reject fraction is mixed with a significant volume of dilution liquid in said reject chamber.
WO 93/23609 discloses another apparatus of this type in which the dilution liquid supply means comprise a complex pipe system arranged on the rotor for rotating therewith while flushing jets of dilution liquid through pipe openings against the screen. However, it has been proved that such dilution liquid openings on the rotor often are clogged with fibres with the consequence that the operation of the apparatus has to be interrupted for cleansing of said openings and removal of thickened reject fraction deposited on the screen.
The object of the present invention is to provide a screening apparatus with reliable dilution liquid supply means, which insignificantly affect the consistency of the final reject fraction.
This object is achieved by an apparatus of the type described initially, which is characterized in that the dilution liquid supply means comprise a rotationally symmetrical element of the rotor and an opposite stationary annular wall portion, said element and wall portion defining an annular passage for the dilution liquid. Since said rotor element moves along the stationary annular wall portion during operation of the screening apparatus, any particles present in the annular passage will be subjected to shearing actions by the rotating rotor element, which significantly reduces the risk of the annular passage being clogged with fibres or contaminants.
To further reduce said clogging risk, the rotationally symmetrical element may be provided with at least one protrusion, which extends in the annular passage but is spaced from the stationary wall portion. During each revolution of the rotor the protrusion will push away any particles present in the annular passage. As an alternative, or in combination with said protrusion, the axial width of the annular passage may vary in the circumferential direction, so that the liquid present in the annular passage will be subjected to a pulsating action by the rotor element during operation.
The annular passage suitably opens into the tubular screen, preferably at a distance from said other end of the screen which is about 5-50% of the axial length of the screen. As a result, the dilution liquid can directly flush the desired location of the screen, so that no excessive volume of dilution liquid is supplied to the central chamber.
According to a preferred embodiment of the invention, the rotor comprises a circumferential wall and a radial wall connected to an end of the circumferential wall, the rotationally symmetrical element forming part of said radial wall. The dilution liquid supply means comprises a stationary circumferential wall in flush with the circumferential wall of the rotor and connected to the stationary annular wall portion, the stationary circumferential wall and annular wall portion defining a dilution liquid chamber in fluid communication with the annular passage. To provide favourable flow conditions in the central chamber, the circumferential walls of the rotor and the liquid supply chamber taper in the direction toward said one end of the screen. As a result, the above preferred embodiment comprises a simple inexpensive and reliable dilution liquid supply means.
REFERENCES:
patent: 3713536 (1973-01-01), Hooper
patent: 4267035 (1981-05-01), Martin
patent: 4749474 (1988-06-01), Young
patent: 4851111 (1989-07-01), Martin et al.
patent: 5096127 (1992-03-01), Young
patent: 5925249 (1999-07-01), Fredriksson
patent: 0 649 940 A1 (1995-04-01), None
patent: 507 905 C2 (1997-04-01), None
patent: 93/23609 A1 (1993-11-01), None
European Patent Application (Publication No: 0 649 940 A1); Application No.: 94307610; Publication Date: Apr. 26, 1995; Applicant: Ishikawajima).
Bergdahl Anders
Forslund Kjell
Svensson Lennart
Wikström Björn
Lerner David Littenberg Krumholz & Mentlik LLP
Rodriguez Joseph
Valmet Fibertech Aktiebolag
Walsh Donald P.
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