Classifying – separating – and assorting solids – Sifting – Elements
Reexamination Certificate
2000-07-31
2002-07-30
Nguyen, Tuan N. (Department: 3653)
Classifying, separating, and assorting solids
Sifting
Elements
C209S399000, C209S411000, C210S498000, C029S896620
Reexamination Certificate
active
06425486
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
The present application is the national stage under 35 U.S.C. 371 of PCT/SE99/00122, filed Jan. 29, 1999.
This invention relates to a cylindrical screen for separating undesired particles from a fibre suspension, namely a screen which has an inner side and an outer side and which is provided with a plurality of circumferentially extending projections on the outer side of the screen, grooves formed on the inner side of the screen and extending in a direction having an axial component, and screening passages in the form of slots disposed in the area of each groove and diverging from the inner side of the screen towards the outer side thereof.
A cylindrical screen of this kind may form part of a pressure screening apparatus of the kind used for cleaning of papermaking pulp. Such a pressure cleaning apparatus conventionally comprises a housing in which the screen is accommodated as a stationary part, and a rotor which is rotatably mounted coaxially inside the screen. A fibre suspension to be freed from undesired particles is fed into an annular space between the rotor and the surrounding cylindrical screen, and the greater part of the fibre suspension is forced through the screening passages of the screen, while the greater part of the undesired particles continues along the annular space and is discharged axially therefrom. Normally, the fibre suspension to be cleaned is fed into the annular space at one axial end of the screen and the undesired particles are discharged at the other axial end of the screen. Within the housing, the screen is surrounded by a compartment in which the cleaned fibre suspension, the accept part, is collected and conveyed to an outlet. At the said other axial end the housing has an outlet for the undesired particles, the reject part.
As stated initially, the screening passages, commonly in the form of elongate slots, diverge from the inner side of the screen towards the outer side thereof. The screening passages may be given that form in connection with cutting them in a planar plate which is then formed into a cylindrical screen by bending. Upon bending of the planar plate into cylindrical shape, the width of the slots is reduced at the inner side of the screen. The more the screening passages diverge, the lesser the risk of the fibres in the fibre suspension becoming stuck in the screening passages and therefore the greater the accept flow through the screen. In order that the best possible cleaning of certain kinds of fibre suspension, such as papermaking pulp containing recycled fibre, may be achieved, the width of the screening passages should not exceed about 0.1 mm at the inner side of the screen.
WO93/04797 discloses a cylindrical screen manufactured as follows: One side of a planar plate is formed with a large number of parallel V-grooves, and backing ribs extending transversely to the grooves are welded to the raised portions separating the grooves. The other side of the plate is also formed with grooves, which are located opposite to the grooves on the first side of the plate. In the bottom of each groove elongate screening passages are formed by milling from the said other side of the plate such that the plate is divided into a multiplicity of bar-like elements held together by the backing ribs. Then the plate and the backing ribs are bent to form a cylindrical screen with the backing ribs extending circumferentially on the outer side of the screen. The bending operation results in a reduction of the width of the screening passages at the inner side of the screen.
The screen disclosed in WO93/04797 has the following disadvantages:
Firstly, it is not possible to provide screening passages which diverge adequately from the inner side of the screen towards the outer side thereof, because the screening passages are milled from the said other side of the plate so that the screening passages have to have parallel longitudinal walls or even have to be wider at the said second side than at the first side. only when the plate is bent to cylindrical shape will the screening passages diverge slightly from the inner side of the screen towards the outer side thereof.
Secondly, it is very difficult to make the screening passages uniform. The welding of the backing ribs to the raised portions separating the grooves results in stresses in the welds. When the screening passages are milled subsequent to the welding, the stresses will cause variations of the width of the screening passages along the length thereof. Moreover, the welds have to be exactly alike in order that they may produce a uniform bending resistance. Even very small differences between the welds will result in the width of the screening passages varying still more as a result of the bending of the plate into cylindrical shape. Uniform screening passages are a prerequisite for a proper separation of undesired particles from fibre suspensions. Because the screening openings are only one or a few tenths of a millimetre wide, dimensional variations as small as 0.01 mm will have a negative impact on the quality of the separation.
Thirdly, when using a plate of a thickness that is economically acceptable having regard to the screen diameter in each particular case, it is very difficult to mill screening passages narrower than 0.3 mm, and it accordingly is difficult to obtain, as a result of the bending of the plate, screening passages that are only 0.1 mm wide at the inner side of the screen.
The object of the present invention is to provide a cylindrical screen of the kind indicated initially and having uniform screen passages which diverge adequately from the inner side of the screen towards the outer side thereof and which can readily be made with a width of only about 0.1 mm. This object is achieved in accordance with the invention by producing the aforesaid projections, grooves and slots in a screen of the kind defined by working of a substantially planar screen plate bent into cylindrical shape subsequent to the working.
By producing the projections, grooves and slots of the cylindrical screen by working of a planar plate, welding operations in the initial steps of the manufacture of the screen are avoided. Thereby it is possible to achieve uniformity in the width of the screen passages. If the projections are formed by milling, for example, a high dimensional accuracy is achieved, and the projections will be uniform and have a uniform bending resistance. As a result, the screening passages will remain uniform after the bending of the plate. Alternatively, the projections may be formed by rolling of the planar plate. Using a modern production method, such as laser beam machining, makes it possible to shape the screen passages such that they diverge even in the planar plate, that is, before the bending operation is carried out, and the width of their narrow end can be made as small as about 0.22 mm with an accuracy of about 0.01 mm. Owing to the projections, it will be possible to reduce the width of the screening passages at the inner side of the screen to about 0.1 mm as a result of bending the plate into cylindrical shape.
In one embodiment of the invention, the grooves and slots are produced by material-removing working of the screen plate.
In another embodiment of the invention the projections are also produced by material-removing working of the screen plate.
In the above-mentioned embodiments of the invention, it is not necessary for every projection to extend throughout the circumference of the screen. This is so because it is also possible to reduce the width of the screen passages using projections which extend only partly around the screen but overlap one another circumferentially.
However, in a preferred embodiment of the invention, at least one and preferably all of the projections extend throughout the circumference of the screen.
According to a further embodiment of the invention each projection has an outer face and the slots extend between adjacent projections, and the cross-section of the projections as viewed in a longitudinal section
Andersson Roine
Backman Jan
Rundqvist Lars-Göran
Browdy and Neimark
GL& V Sweden AB
Nguyen Tuan N.
LandOfFree
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