Solid material comminution or disintegration – Apparatus – Including means applying fluid to material
Reexamination Certificate
1999-10-13
2001-01-09
Rosenbaum, Mark (Department: 3725)
Solid material comminution or disintegration
Apparatus
Including means applying fluid to material
C209S273000, C241S074000
Reexamination Certificate
active
06170769
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a screening apparatus for separating fiber suspensions, comprising a housing, an inlet to the interior of the housing for a fiber suspension to be separated, a stationary tubular screen dividing the interior of the housing into a central chamber for receiving the fiber suspension from the inlet at one end of the tubular screen and an outer accept chamber for receiving an accept fraction of the fiber suspension, which has passed through the screen, and a rotor journalled on the housing and situated in the central chamber coaxially with the tubular screen. More particularly, the present invention comprises flow restriction means defining an annular reject passage which is coaxial with the tubular screen and situated at the other end thereof for discharging a developed reject fraction of the fiber suspension from the central chamber, the cross-sectional area of the annular reject passage being adjustable by said flow restriction means.
SUMMARY OF THE INVENTION
Apparatus of the above-disclosed type is generally used for separating paper pulp suspensions, such as for fractionating fibers or for separating contaminants and other undesirable particles, such as incompletely treated fibers. 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 fiber 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 only a relatively small flow of the reject fraction is permitted to escape through the reject passage. Too large a thickening of the reject fraction, on the other hand, gives rise to problems, such as torque transmission between the rotor and the screen in view of the thickened reject fraction, which leads to an increased energy consumption for rotating the rotor. The rotor can possibly also become affixed to the screen, which requires a costly break in production for manual cleaning of the rotor and screen. The thickened reject fraction can 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 established 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. Pat. No. 4,699,324 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 that inner element. These annular elements comprise opposite grinding means, so that during operation, when the inner element moves along the outer element, this 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 changed 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 thus to provide a screening apparatus, which solves the above problems of reject fraction thickening, and in particular which prevents non-thickened fiber suspension from passing thickened reject fraction.
SUMMARY OF THE INVENTION
In accordance with the present invention, these and other objects have now been realized by the invention of a screening apparatus for separating fiber suspensions comprising a housing including an interior and an inlet for the fiber suspension, a stationary tubular screen having an inlet end adjacent to the inlet and an outlet end, and dividing the interior of the housing into a central chamber for receiving the fiber suspension from the inlet and an outer accept chamber for receiving an accept fraction of the fiber suspension which has passed through the stationary tubular screen, a rotor journalled on the housing for rotation within the central chamber coaxially with the stationary tubular screen, a reject passage for receiving a reject portion of the fiber suspension at the outlet end of the stationary tubular screen, and flow restriction means defining an annular reject passage having a radial width and a cross-sectional area at the outlet end of the stationary tubular screen for discharging the reject portion of the fiber suspension from the central chamber to the reject passage, the flow restriction means comprising an expandable and contractable annular member for decreasing and increasing the cross-sectional area of the annular reject passage. In a preferred embodiment, the flow restriction means comprises a stationary inner wall portion and a stationary outer wall portion surrounding the stationary inner wall portion, the annular member being attached to the stationary inner wall portion.
In accordance with one embodiment of the screening apparatus of the present invention, the stationary outer wall portion is connected to the stationary tubular screen.
In accordance with another embodiment of the screening apparatus of the present invention, the stationary inner wall portion is connected to the housing.
In accordance with another embodiment of the screening apparatus of the present invention, the annular member comprises an elastic ring. In a preferred embodiment, the apparatus includes an annular flange attached to the stationary inner wall portion, the annular flange providing an outwardly facing annular groove, resilient tubing disposed in the annular groove, the resilient tubing being connectable to a source of pressure whereby the resilient tubing can be extended and contracted, and an elastic ring surrounding the resilient tubing. In a preferred embodiment, the elastic ring is received by the annular groove and is in a prestressed state when the resilient tubing is contracted.
In accordance with one embodiment of the screening apparatus of the present invention, the radial width of the annular reject passage can vary between about 2 and 25 mm by means of the expansion and contraction of the resilient tubing, and preferably between about 10 and 20 mm.
The object of the present invention is achieved by an apparatus of the type described above, which includes flow restriction means which 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 type of fiber suspension to be separated. It has surprisingly been shown that an appropriate fine adjustment of the area of the reject passage gives rise to the advantage 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 present 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 oute
Bergdahl Anders
Forslund Kjell
Svensson Lennart
Wikström Björn
Lerner David Littenberg Krumholz & Mentlik LLP
Rosenbaum Mark
Valmet Fibertech Aktiebolag
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