Liquid purification or separation – Filter – Supported – shaped or superimposed formed mediums
Reexamination Certificate
1999-08-19
2002-07-30
Savage, Matthew O. (Department: 1723)
Liquid purification or separation
Filter
Supported, shaped or superimposed formed mediums
C029S896620, C029S897150, C209S395000
Reexamination Certificate
active
06426003
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention refers to a screening device and a method of manufacture thereof that are particularly suitable for screen cylinders or screen plates which screen, filter, fractionate, or sort cellulose pulp suspensions (that is in the pulp and paper making industry), or other similar suspensions. The present invention thereby refers to screening devices, such as screen cylinders or bended or flat screening elements, for screening, filtrating, fractioning or sorting pulp suspensions in pulp and paper making industry or other similar suspensions. The present invention more particularly refers to screening devices of the type comprising a plurality of filter wires positioned at a small spacing parallel to each other, the plurality of filter wires forming a screening surface facing the pulp suspension to be screened and adjacent wires forming screening openings therebetween allowing an accept portion of the pulp suspension to flow therethrough. EP 0 316 570 suggests such a screening device in which the filter wires are fixed by welding, on the downstream side of the wires, to transversely extending slots in solid support elements, support rings or support bars. The screening devices may have various forms, e.g. be flat, bended, cylindrical or conical.
In known screening devices of this type the support elements, which form supports for the filter wires, are formed of solid bars, mainly rectangular or round in cross section and most typically positioned perpendicular to the filter wires.
The filter wires are generally fastened to the supporting bars by a welding process which gives rise to a number of disadvantages such as variability distortion, thermal stresses and burrs. The heat induced by the welding often cause distortion of the wires and changes in the screening opening width between adjacent wires. It is therefore difficult to get completely uniform screening openings, which means that the efficiency of the screen suffers. Today, when the desired width of screening openings may be as small as 0.1 mm, only minimal distortions are acceptable.
The thermal stresses and the burrs may also lead to failure in operation due to the loading on the screening device in the user's process. Such loading may be either in the form of a constant load or a cyclic loading giving rise to failure by fatigue. Burrs may also catch fibers in the suspension, leading to gradual clogging of the screen or filter, or the formation of so called “strings” which are very detrimental in the user's process.
It has also been suggested, e.g. in U.S. Pat. Nos. 5,090,721 and 5,094,360, to connect filter wires of a certain “key” cross section into recesses, in the support bar, having the same “key” form. By means of bending the supporting bars into rings, the filter wires are clamped into position. This design, thereby, requires the manufacturing of a number of relatively complicated and therefore expensive recesses. Further, it can only be adapted to circular screens and screens, where the flow is from the inside to the outside of the circular screen.
In another known screening device the filter wires are fastened by looping them around support bars. Such a screen construction is strong, but the looping areas around the support bars is locally closing the openings and thereby reducing throughput of the screen. Also the looped areas tend to have cavities and uneven spots which are facing the suspension potentially causing fiber hang-up.
The above difficulties tend to result in poor quality of screening or mechanical weaknesses or to high manufacturing costs, it is therefore the object of the present invention to minimize the above mentioned drawbacks and provide an improved screening device and an improved method of manufacturing such device.
It is thereby also an object of the present invention to provide an easily manufactured and assembled screening device without thermally induced distortion of filter wires.
It is also an object of the present invention to provide an improved strong screening device with accurate and consistent screening openings, i.e. screening slots.
It is thereby further an object of the present invention to provide an improved method of manufacturing a screening device, so that uniform screening openings, i.e. good tolerances, are provided, whereby slots with very small widths may be manufactured.
It is further an object of the present invention to provide an improved screening device with minimum of burrs or other protruding elements causing accumulation of fibers on upstream side surfaces of support rods.
According to one aspect of the present invention a screening device is provided comprising the following components: A plurality of filter wires each comprising a first section and a second section opposite the first section, the second section having a base portion. At least one longitudinal one longitudinal support element having first and second side surfaces, a plurality of slots in the first side surface, and a cavity in the second side surface in open communication with the slots. And, the second sections received by the slots so that the base portions extend through the slots into the cavity, and the wires are supported by the at least one support element so that the wires are substantially parallel to each other and define screening openings therebetween adjacent the first sections thereof.
The wires are preferably fixed to the support element(s) by local deformation of the base portions in the cavities, such as by effecting an increased material thickness portion of the base portion formed by reciprocation of a tool, by bending over a flap of the base portion, or by forming a weld between the base portion and the supporting element substantially within the cavity. Typically the second sections and base portions have a smaller cross-sectional area than the first sections. Alternatively the support element could be deformed within the cavity.
Typically the wires define screening slots having a slot width of less than 0.5 mm. The at least one support element may typically comprise a bar or a support ring; for example the at least one support element comprises a plurality of support rings, and the wires define a screen cylinder as the screening device. In the preferred embodiment the slots and wires are substantially transverse to the support rings, and the screen cylinder is positioned in a pressure screen so that the first sections of the wires and the first surfaces of the support rings impact slurry flowing through the screening slots.
The support element may be substantially U, L, or V-shaped in cross-section. Preferably the support element has a total height H, and the slots have a depth h
2
, wherein h
2
equals (0.25-0.5) H; and the filter wires have a height h, and h
2
equals (0.3-0.9) h.
According to another aspect of the present invention a method of manufacturing a screening device is provided comprising: (a) Providing a plurality of filter wires each having opposite first and second sections, and a base portion at the second section; and at least one longitudinal support element having first and second side surfaces, a plurality of slots in the first side surface, and a cavity in the second side surface in open communication with the slots. (b) Inserting the wires in the slots so that the base portions thereof extend into the cavity and the wires are substantially parallel to each other and define screening openings between the first sections thereof. And, (c) fixing the wires to the at least one support element substantially within the cavities.
Thereby a preferred screening device according to the present invention, comprising a plurality of filter wires supported by at least one longitudinal support element is provided, in which a plurality of supporting slots or recesses are made through the upstream side surface of the support element and the filter wires are fixed to the slots. The longitudinal direction of the supporting slots or recesses thereby form an angle, typically an angle of 90°, with the longi
Jurvanen Jari
May Jonathan
Stephenson John
Wood Tim
CAE Screenplates Oy
Nixon & Vanderhye P.C.
Savage Matthew O.
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