SCREENING APPARATUS WITH REJECT FLOW RESTRICTION MEANS

Abstract: A screening apparatus (1) for separating fibre suspensions comprises a stationary tubular screen (9) for receiving a fibre suspension to be separated at one end of the screen, a rotor (5) coaxial with the tubular screen, and flow restriction means (21, 23, 25, 27) defining an annular reject passage (24) which is coaxial with the tubular screen and situated at the other end thereof for discharging a developed reject fraction of the fibre suspension from the interior of the screen. The flow restriction means comprise an annular member (27), preferably an elastic strip, which is expandable and retractable, respectively, in the radial direction of the annular reject passage (24) to decrease and increase, respectively, the cross-sectional area of the annular reject passage (24).

Description: Screening apparatus with reject flow restriction means 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, and a rotor journalled on the housing and situated in the central chamber coaxially with the tubular sreen. The apparatus further comprises flow restriction means defining an annular reject passage which is coaxial with the tubular screen and situated at the other end thereof for dis-charging a developed reject fraction of the fibre suspension from the central chamber, the cross-sectional area of the annular reject passage being adjustable by said flow restriction means. 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 problems, such as torque transmission between the rotor and the screen via the thickened reject fraction, which leads to 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. It has also been proved that the thickened reject fraction is insignificantly dissolved by non-thickened pulp suspension. As a consequence such non-thickened, easily flowing pulp suspension can pass thickened, viscous reject fraction and escape through the reject passage, which decreases the separation efficiency of the apparatus . U.S. Patent No. 4699324 discloses a screening apparatus of the present type, in which the reject passage is defined by an annular inner element mounted coaxially on the rotor and an annular outer element, which is mounted on the stationary screen and surrounds said inner element. Said annular elements comprise opposite grinding means, so that during operation, when the inner element moves along the outer element, said grinding means act on the passing reject fraction to effect reduction of shives and other components. According to this patent, the dimensions of the reject passage may be change by using a rotor, grinding elements and a screen which are all generally conical and axially displacing the conical rotor relative to the conical screen. The object of the present invention is to provide a screening apparatus, which solves the above problems of reject fraction thickening and in particular prevents non-thickened fibre suspension from passing thickened reject fraction. This object is achieved by an apparatus of the type described initially, which is characterized in that the flow restriction means comprise an annular member, which is expandable and retractable, respectively, in the radial direction of the annular reject passage to decrease and increase, respectively, the cross-sectional area of the annular reject passage. This is an advantageously simple and cheap solution to enable adjustment of the cross-sectional area of the annular reject passage, when required for the particular kind of fibre suspension to be separated. It has surprisingly been proved that an appropriate fine adjustment of the reject passage area gives rise to the advantageous consequence that non-thickened suspension instead of escaping past thickened reject fraction dilutes the latter, thereby facilitating the discharge of the reject fraction through the reject passage. According to a preferred embodiment of the invention, the flow restriction means comprise a stationary inner wall portion, which may be secured to the housing, and a stationary outer wall portion, which surrounds the inner wall portion and may be connected to the screen, the annular member, preferably an elastic ring, being secured to the inner wall portion. As an alternative, the annular member may be secured to the stationary outer wall portion. The flow restriction means may suitably comprise an annular flange provided on the stationary inner wall portion and forming an annular outwardly directed groove, and a resilient tubing positioned in the groove and connectable to a source of pressure for expanding and retracting the tubing, the elastic ring surrounding and abutting the tubing. By means of the source of pressure, a medium in the form of gas or liquid can be introduced into the tubing to expand the elastic strip to such an extent that the resulted reject passage area is suited for a specific fibre suspension to be treated. The invention is described in more detail in the following with reference to the accompanying drawing, in which Figure 1 is a longitudinal cross-section through a schematic screening apparatus according to a preferred embodiment of the invention, and Figure 2 shows an enlarged encircled detail in Fig. 1. In Fig. 1 there is shown a screening apparatus 1 of the inven- tion comprising a generally cylindrical housing 3 with an inlet 15 for a fibre suspension to be separated, and a cylindrical screen 9 secured to the housing 3 and dividing the interior of the housing 3 into a central chamber 13 for receiving the fibre suspension from the inlet 15 at one end of the cylindrical screen 9 and a circular outer accept chamber 18 for receiving an accept fraction of the fibre suspension, which has passed through the screen 9. A generally truncated conical rotor 5 provided with a number of circumferentially distributed periphe- ral wing elements 7 is journalled by bearings 11,12 on the housing 3 and positioned in the central chamber 13 coaxially with the cylindrical screen 9, so that the wing elements 7 sweep past the interior surface of the screen 9, when the rotor 5 is rotated by a drive motor (not shown) . The housing 3 is provided with an accept outlet 17 for discharging the accept fraction from chamber 13 and a reject outlet 19 for discharging the developed reject fraction from the interior of the screen 9 via a reject chamber 20 communicating with the central chamber 13. With reference to Fig. 2, a circular reject passage 24, which is coaxial with the screen 9 and situated at the other end thereof, is defined by flow restriction means comprising an inner wall portion 21 secured to the housing 3, an outer wall portion 23 connected to the screen 9 and surrounding the wall portion 21, a circular flange 25 provided on the inner wall portion 21 and forming a circular groove, a resilient tubing 29 positioned in the groove, and an elastic ring 27 of a suitable rubber grade surrounding and abutting the tubing 29. The dimension of the ring 27 is chosen such that the ring 27 is received by the groove and is in a prestressed state when the tubing 29 is collapsed. The tubing 29 is connectable to a source of pressure, not shown, for controllably expanding and retracting, respectively, the elastic strip 27 in the radial direction of the circular reject passage 24 to decrease and increase, respective- ly, the cross-sectional area of the reject passage 24. In the above described embodiment the radial width of the reject passage 24 is variable in the range of 2-25 mm, preferably 10- 20mm, by means of the tubing 29 and the elastic ring 27. In operation, the fibre suspension to be separated is supplied to the central chamber 13 through the inlet 15 and flows along the screen 9 while separating into an accept fraction, which passes through the screen 9 into the accept chamber 18 and discharges therefrom through the accept outlet 17, and a reject fraction, which passes through the reject passage 24 into the reject chamber 20 and discharges therefrom through the reject outlet 19. By pulsating action of the rotating wing elements 7 the screen 9 is prevented from being clogged by fibres and/or contaminants. Said source of pressure is controlled such that an appropriate cross-sectional area of the reject passage is achieved by expansion or retraction of the elastic strip 27, which can be verified by analysing samples of the produced reject fraction. Thus, if the reject samples contains too much fibres that should have been separated with the accept fraction, the strip 27 is gradually expanded until the amount of such fibres in the reject samples has satisfactorily decreased. On the other hand, if the reject flow is insignificant the strip 27 may have to be retracted to increase this flow, in order to avoid detrimental thickening of the reject fraction.

Claims: Claims 1. A. screening apparatus (1) for separating fibre suspensions, comprising a housing (3) , an inlet (15) to the interior of the housing for a fibre suspension to be separated, a stationary tubular screen (9) dividing the interior of the housing into a central chamber (13) for receiving the fibre suspension from the inlet at one end of the tubular screen and an outer accept chamber (18) for receiving an accept fraction of the fibre suspension, which has passed through the screen, a rotor (5) journalled on the housing and situated in the central chamber coaxially with the tubular sreen, and flow restriction means (21,23,25,27) defining an annular reject passage (24) which is coaxial with the tubular screen and situated at the other end thereof for discharging a developed reject fraction of the fibre suspension from the central chamber, the cross-sectional area of the annular reject passage (24) being adjustable by said flow restriction means, characterized in that the flow restriction means (21,23,25,27) comprise an annular member (27), which is expandable and retractable, respectively, in the radial direction of the annular reject passage (24) to decrease and increase, respectively, the cross-sectional area of the annular reject passage (24) . 2. The apparatus according to claim 1, wherein the flow restriction means (21,23,25,27) comprise a stationary inner wall portion (21) and a stationary outer wall portion (23) surrounding the inner wall portion, the annular member (27) being secured to the inner wall portion. 3. An apparatus according to claim 2, wherein the outer wall portion (23) is connected to the screen (9) . 4. An apparatus according to claim 2 or 3 , wherein the inner wall portion (21) is secured to the housing (3) . 5. An apparatus according to any of claims 2-4, wherein the annular member comprises an elastic ring (27) . 6. An apparatus according to claim 5, wherein the flow restriction means (21,23,25,27) comprise an annular flange (25) provided on the stationary inner wall portion (21) and forming an annular outwardly directed groove, and a resilient tubing (29) positioned in the groove and connectable to a source of pressure for expanding and retracting the tubing, the elastic ring (27) surrounding and abutting the tubing. 7. An apparatus according to claim 6, wherein the elastic ring (27) is received by said groove and is in a prestressed state when the tubing (29) is collapsed. 8. An apparatus according to claim 6 or 7, wherein the radial width of the reject passage (24) is variable in the range of 2- 25 mm, preferably 10-20mm, by means of the tubing (29) and the elastic ring (27) .

Inventors: BERGDAHL ANDERS, FORSLUND KJELL, SVENSSON LENNART, WIKSTROEM BJOERN, BERGDAHL, ANDERS, FORSLUND, KJELL, SVENSSON, LENNART, WIKSTROEM, BJOERN
Applicants: BERGDAHL ANDERS, FORSLUND KJELL, SUNDS DEFIBRATOR IND AB, SVENSSON LENNART, WIKSTROEM BJOERN, BERGDAHL, ANDERS, FORSLUND, KJELL, SUNDS DEFIBRATOR INDUSTRIES AB, SVENSSON, LENNART, WIKSTROEM, BJOERN
Application year: 1998
Main classification: D21D5
IPC codes: D21D5