Liquid purification or separation – Filter – Movable medium
Reexamination Certificate
1998-07-22
2002-04-30
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Filter
Movable medium
C210S402000, 49
Reexamination Certificate
active
06378705
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The invention refers to an apparatus for filtering plasticized thermoplastic synthetic plastic material, comprising a housing with at least one inflow channel for the material to be filtered, which channel leads this material to the inflow side of at least one filter element, whereby the filter element has fine filtering holes for the filtrate and from the outflow side of the filter element to at least one outflow channel for the filtrate leads out of the housing, and whereby at the inflow side of the filter element scrapers moved relative to the filter element are disposed their scraping edges brushing over the filter element; push the waste residues across the surface of the filter element to at least one discharge channel, and whereby, in the direction in which the waste residues are being carried off, there is a non-perforated zone adjoining the zone of the filter element with filtering holes, which non-perforated zone is also brushed over by the scrapers.
The invention also refers to a filter element for an apparatus of this kind.
An apparatus of this kind is known (WO 93/15819). There, the non-perforated zone adjoining the hollow cylindrical perforated filter area is executed as a solid cylinder to increase the resistance to torsional strain of the filter element rotating around its longitudinal axis.
The object of the invention is to improve an apparatus for filtering plasticized thermoplastics synthetic plastic masses of the aforementioned kind by further increasing its service life and filtering efficiency. The invention solves this task by having grooves present in the surface of the non-perforated zone of the filter element facing the scrapers, which grooves are placed in a direction which intersects the direction of the scraping edges scraping over these grooves such that the thickened material on the non-perforated zone is transported to an even greater extent toward the discharge channel.
The invention is based on the following findings: in most of the currently known filter apparatuses, the scrapers glide over the perforated area of the filter, pushing the impurities separated out from the melt being filtered in front of them and toward the discharge channel. This residual material thickens increasingly as it moves toward the discharge channel and the usually solid impurities have an abrasive effect on the filter area. For this reason, there is extreme wear in the filter element in the discharge channel area which leads to the removal of the material of the filter element so that the filter element in this area is ultimately unable to withstand the pressures being exerted against it anymore and breaks. The danger of this happening increases with the pressure exerted by the synthetic plastic material fed in to be filtered, which pressure can reach
300
bar and more in modern filter devices. The wear of the filter element is increased even more by the holes in the filter area acting more or less as abutments for the residue material being transported by the scrapers, because granular constituents in the residue material fit positively into these holes and enter them.
Moreover, investigations have also shown that the concentration of solid constituents in the material the scrapers push along in front of them is extremely high right in front of the scrapers, i.e. this material is thickened to a high degree. However, immediately behind the scrapers, a clean zone emerges into which zone the pressurized material being filtered flows. When such still substantially unfiltered constituents of the melt enter the area of the discharge channel, relatively pure material is drawn off there. The material conveyed off in the discharge channel, usually by means of a discharge screw, is therefore highly non-homogeneous in its composition, because there is a constant changeup between portions coming from the areas behind the scrapers with portions coming from in front of it. Those portions from in front of the scrapers usually contain highly thickened material with a relatively high proportion of impurities whereas those from behind the scrapers are relatively high in plastics and the share of impurities is quite low.
By means of the measure taken in accordance with the invention, the scrapers now do not scrape across an area of the filter element perforated throughout but rather the non-perforated zone, with its grooves, adjoining the discharge channel create what are more or less abutments for the circulating solid material so that this material is transported to a greater extent toward the waste discharge channel. As to say, the grooves, in effect, promote conveyance. Thereby in an advantageous manner the effect is achieved that the impurities collect in the area adjoining the waste discharge channel, pushing away the plastics material, that is the useful constituents of the melt, so that the useful constituents once again arrive in the perforated area of the filter and are filtered through the holes there. As a result, the material found in the discharge channel becomes more homogenous than that in the known design initially described—and also relative to other common types of filters whose filter surface is brushed over by scrapers—and, surprisingly, less plastic that is lost, reaches the discharge channel, than was expected. One would expect, namely, that the non-perforated zone would result in a worse filtration of the material, however, in reality just the contrary is the case.
Moreover, the grinding effect becomes less on the impurity constituents of the thickened mass transported by the scrapers in comparison to a perforated zone of the filter since the grooves in the design in accordance with the invention have a less anchoring effect on the solid constituents of the residual material than the holes do.
The invention is applicable both to filter elements which are longitudinally symmetrical, that is more or less hollow cylinders or filter elements shaped like the envelopes of a cone, e.g. such as those in accordance with the aforementioned WO 93/15819, as well as, to particular advantage, to disk-shaped filter elements, particularly those shaped as circular disks, in which the discharge channel for the contaminants is positioned centrally. The reason for this latter feature is that the thickening of the residual material becomes especially great there. In any case, the filter element can be impinged either from within or from without by the material being filtered, i.e. the material can flow through the filter element either from in to out or from out to in.
The invention is likewise non dependent on whether it is the filter element which is moved (as with the aforementioned WO) or the scrapers or both elements.
In accordance with a preferred embodiment of the invention, the non-perforated zone of the filter element has a greater wall thickness than the perforated zone. The advantage of this approach is that the grooves in the non-perforated zone can be deep enough to be able to fulfill their desired conveyance function to a great extent. In a hollow cylindrical filter element, as the document mentioned initially indicates, the non-perforated zone does not by any means have to be solidly executed, in fact, it can be in the shape of a hollow cylinder, just as the perforated filter zone is. The hollow space created thereby not only reduces the weight and the material required, it can also be used to accomodate other parts of the apparatus.
The ratio of the length of the non-perforated zone, as measured in the direction in which the residues are removed, to that of the perforated zone measured in the same direction is advisably from 0.5:1 to 1:1, this ratio will depend on the prevailing circumstances and the type of synthetic plastic material usually being processed.
In accordance with a preferred embodiment of the invention, the axes of the holes in the perforated zone are inclined in the direction of the removal of the residues, preferably at an angle of 60 to 80° to the surface of the filter element. When the filter element is di
Bacher Helmut
Schulz Helmuth
Wendelin Georg
Fortuna Ana
Salter & Michaelson
Ward Richard W.
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