Filters for dewatering suspensions
US 3125514 A
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Description (OCR text may contain errors)
March 17, 1964 H. FRYKHULT 3,125,514
' FILTERS FOR DEWATERING SUSPENSIONS Filed March 23. 1960 2 Sheets-Sheet 1 March 17, 1964 R. H. FRYKHULT 3,125,514
FILTERS FOR DEWATERING SUSPENSIONS Filed March 23. 1960 2 Sheets-Sheet 2 United States Patent 3 125,514 FILTERS FOR DEWATERING SUSPENSIONS Rune Helmer Frykhult, Sundsbruk, near Sundsvall,
Sweden, assignor to Sunds Verkstader Aktiebolag,
Sundshruk, Sweden Filed Mar. 23, 1960, Ser. No. 17,136 Claims priority, application Sweden Mar. 24, 1959 3 Claims. ((1210-396) The present invention relates to improvement in filtering devices for dewateiing liquid-containing mixtures of substances, preferably fibrous pulp suspensions, comprising a screening drum intended to rotate partially immersed in the mixture of substances and consisting of an outer, perforated shell and an inner imperforate shell between which shells there are provided substantially longitudinally extending partitions dividing the inter-space between the shells into cells extending substantially axially of the drum, said cells communicating with the interior of the drum through a number of discharge ducts connected to the cells and curving backwards in the direction of rotation.
The essential feature of the improvements consists therein that all discharge ducts of the drum are arranged either in one single group, or in several groups which are separated from each other by a zone of the drum which is free from discharge ducts, and that in each such group all discharge ducts are located in one and the same plane extending perpendicularly to the shaft of the drum. Furthermore, the discharge ducts of the drum are located within the effective operating area of the drum and they are of an extension in the axial direction which is less than the operating width of the drum.
These and other features of the invention will appear from the following description of three different embodiments, applied to filtering devices and in the first place intended for the treatment of fibrous pulp suspensions. Reference is made to the figures of the accompanying drawings, in which FIGS. 1 to 3 illustrate longitudinal sections through the three difierent embodiments,
FIGS. 4 and 5 are sections on the lines IVIV and VV, respectively, of FIGS. 1 to 3.
In the embodiment according to FIG. 1 (in combination with FIGS. 4 and 5) the reference numeral 1 designates a central shaft, mounted in bearings 2, 3 and supporting the rotary screening drum. The trough belonging to the filtering device is designated by 4 and the inlet for the pulp suspension into the trough by 5. The filtrate outlet is designated by 6. In the trough 4 there is, furthermore, provided an adjustable damming device 7 for the filtrate, and between the screening drum and the trough there is provided a ring 8 sealing against the pulp suspension. The screening drum is, on one hand, equipped with an outer perforated shell 9, the outside of which is provided with a Wire cloth, and, on the other hand with an inner imperforate shell 16, tapering conically towards a number of filtrate discharge ducts described below. The two end walls of the drum are denoted by 11 and 12. Part of the end wall 11 at the same time constitutes one end Wall of the inner imperforate shell 10, the other end wall 13 of which is located at some distance from the other end wall 1?. of the drum. From the interspace between the end Walls 12 and 13 there extend a number of filtrate discharge ducts 14 on a level with the surface of the inner shell of the drum, said discharge ducts 14, as will be seen from FIG. 4, curving backwards in the direction of rotation of the screening drum, said direction of rotation being indicated by an arrow. All discharge ducts 14 are located in one and the same plane extending perpendicularly to the shaft 1 of the screening drum, and each discharge duct extends 3,1255% Patented rial. 17, was
from its respective cell of a number of cells 15 located in the interspace between the outer perforated shell 9 of the drum and the inner imperforate shell 1d of the drum, said cells extending in the axial direction of the drum and along the whole inner periphery of the drum. The individual cells 15 are separated by radially, or substantially radially, directed walls 16. In that zone of the drum where the discharge ducts 14 are grouped, those parts of the walls 17 of all discharge ducts 14 which are located next to the periphery of the screening drum, are formed in such a manner that, either directly or by means of extensions, they merge each into, or are contiguous to, its respective cell wall 16, in the way disclosed in FIG. 4. At some suitable place along each continuous surface of flow thus constituted by a duct wall, an extension thereof, if any, and the contiguous cell wall there is provided a curvature, the angle a of which is less than said angle being directed rearwardly when seen in the direction of rotation of the screening drum. Said curvature, which thus forms a pocket, directed rearwardly in the direction of rotation, may be provided either on the portion constituting the duct wall 17 itself (or an extension thereof, if any) or at the transition between the duct wall 17 and/or an extension thereof, if any, and the cell wall 16 or, finally, on the cell wall 16 itself. In the last-mentioned case said pocket may be located even quite close to the periphery of the screening drum. The duct walls 17 extend substantially parallel to each other excepting those parts which are located next to the periphery of the screening drum, said last-mentioned parts diverging from each other. The layer 18 formed on the screening drum is removed from the drum via a takeoff roller 19.
The device operates in the following manner:
Upon rotation of the screening drum, the major part of which is immersed in the fibrous pulp suspension 2% in the trough 4, there will, on the outside of the drum, continuously form from the suspension a layer 18 which by the drum is lifted out of the trough and conveyed to the take-off roller 19, where the layer is removed from the drum in the form of a continuous web. The reason why such formation of a layer takes place on the drum is the existence of a hydrostatic overpressure prevailing on the outside of the drum and generated as a result of the difference in levels existing between the higher level 21 of the pulp suspension in the trough and the lower level 22 of the filtrate in the cells, and, respectively, the likewise lower level 2.3 of the filtrate within the drum. As a consequence the filtrate is pressed through the wire cloth and the perforated shell 9 into the drum, the separated fibers then assembling on the Wire cloth. On the ascending side of the screening drum the filtrate flows from the cells 15 into the ducts 14. Due to the fact that the openings of the ducts, at the beginning of the ascending side of the drum, are located below the level 23 for the filtrate within the drum, air cannot, on this part of the drum, enter the cells 15 via the ducts 14, and, consequently, the filtrate will produce a successively increasing vacuum in the cells 15. As a consequence of this vacuum, liquid will be sucked out of the layer 18 formed on the drum and also air will be sucked through said layer. This will result in the layer becoming concentrated. Furthermore, the vacuum, even when the cells are positioned on the ascending side of the drum below the level 21 of the pulp suspension in the trough, will cause a further formation of layer on the screening drum. When the opening of the ducts 14larrive above the level 23 of the filtrate within the drum, air will penetrate, via the ducts 14, into the cells 1'5 and the vacuum will cease sufiiciently rapidly to permit a reliable take-off of the layer from the drum. The filtrate will then flow out of cells and ducts.
The embodiment described above in the first place is intended for use in cases where the operating width of the filtering device and/or the speed of rotation thereof are moderate.
in case of greater operating widths and/ or higher speeds of rotation an embodiment according to FIG. 2 is suitable. From the device according to FIG. 1 the device disclosed in FIG. 2 differs substantially in that the discharge ducts M are not positioned at one end of the screening drum but have a difierent axial position, e.g. nearer the middle of the drum, in the manner shown in the figure. The inner, imperforate shell it then tapers conically from both end walls towards the ducts 14. Furthermore, in said figure the drum shaft, correspondingly to FIG. 1, is designated by 1, the outer perforated shell of the drum by 9, the cell walls by 16, the Walls of he ducts by 17, the pulp suspension in the trough by and the level of the filtrate within the drum by 23.
in case of exceptionally great operating widths and an exceptionally high speed of rotation the device may preferably be embodied in accordance with the disclosure of FIG. 3, i.e. the discharge ducts 14 being arranged in two groups, one at each end of the drum. The inner shell 10 of the drum then tapers conically from the middle of the drum towards both ends and the groups of ducts provided there. Also in this figure the shaft of the drum is designated by l, the outer perforated shell of the drum by 9, the cell walls by 16, the walls of the ducts by 17, the suspension in the trough by 2G, and the level of the filtrate in the drum by 23.
A filtering device arranged in accordance with the invention can be operated at a high speed, resulting in a great capacity and a high content of dry substance being obtained. The reasons why the filter can be operated with a good result at a higher speed than has been possible in connection with filtering devices of the types heretofore in use, are inter alia the fact that the inner shell tapers conically all the way to the inlets of the ducts, by which a rapid, axial flow of filtrate through the cells to the inlets of the ducts is promoted, and furthermore that said duct inlets are funnel-shaped, resulting in no reduction of the speed of the fiow of the filtrate occurring at the transition from the cells to the ducts. Even if, in spite of the very favourable conditions of flow in the new filtering device, it should happen that, at very high speeds, there should not be sufiicient time for the filtrate to become completely discharged from cells and ducts, the screen will function all the same. This is due to the fact that the radially, or substantially radially, directed cell walls-also in the zone of the filtering device in which the ducts are grouped-prevent the filtrate, after the cells having passed their highest point during the rotation of the drum, from being turned out of the cells and, via the perforated outer shell, thrown out on to the layer assembled on the drum. Such turning out of the filtrate will, however, take place in such a case in previously known filter constructions in which the angle corresponding to the angle a. in FIG. 4, is 180.
As will ap ear from FIG. 4 the walls of the ducts are of an extension in the peripheral direction, which is at least 60-", preferably 1l0120. This feature contributes to a high vacuum being obtained. It is of advantage also to have the height of the ducts (i.e. the interior radial distance between the duct walls extending parallel to each other) less than, or in any case not essentially larger than, the average height of the cells.
Having now described my invention, What I claim as new and desire to secure by Letters Patent is:
1. A filtering device for liquid suspensions comprising a trough adapted to contain liquid suspension, means for supplying liquid suspension to said trough, a screening drum mounted in said trough to rotate on a substantially horizontal axis partially immersed in said liquid suspension, means for rotating said drum, means for discharging filtrate from at least one interior space of said drum and means for separating solid material from the outer surface of said drum, said drum comprising an outer shell having a pervious, substantially cylindrical side Wall and a wall at each end thereof which is perpendicular to said axis, at least one of said end walls having an opening therein, an inner shell having an impervious side wall of circular cross section spaced apart from the cylindrical side Wall of said outer shell and an impervious wall at each end thereof perpendicular to said axis, at least one of said end walls of said inner shell being spaced a substantial distance from said end wall of said outer shell having an opening therein, a plurality of partitions extending axially and radially between said side walls of said outer and inner shells and spaced circumferentially from each other, said partitions dividing the space between said side walls into a plurality of axially extending cells, said cells extending substantially the full length of said outer shell, at least one group of circumferentially spaced apart curved walls forming at least one group of ducts, the ducts of such group equalling in number the number of cells, said spaced apart curved walls being tightly connected with said partitions forming said cells, each of said curved walls extending axially the entire distance between said spaced end Walls of said inner and outer shells and inwardly from said partitions to said interior space, said ducts being axially alined and contiguous with each other thereby occupying the entire circumference of said interior space, the width of each duct being several times its height, said ducts curving backward with respect to the direction of rotation of said drum, the distance between said spaced apart end walls of said inner and outer shells being substantially less than the axial length of said drum and said curved Walls being substantially parallel to each other adjacent said interior space and diverging outwardly adjacent to said partitions.
2. A filtering device for liquid suspensions as defined in claim 1 in which each end wall of said outer shell has an opening therein and each of said impervious end walls of said inner shell is spaced a substantial distance from the adjacent end wall of said outer shell and there is a group of discharge ducts in each of the spaces between said impervious end walls of said inner shell and the adjacent end walls of said outer shell.
3. A filtering device for liquid suspensions comprising a trough adapted to contain liquid suspension, means for supplying liquid suspension to said trough, a screening drum mounted in said trough to rotate on a substantially horizontal axis partially immersed in said liquid suspension, means for rotating said drum, means for discharging filtrate from an interior space of said drum and means for separating solid material from the outer surface of said drum, said drum comprising an outer shell having a pervious, substantially cylindrical side wall, an impervious end wall perpendicular to said axis at one end thereof and an end wall having an opening therein and perpendicular to said axis at the other end thereof, a first inner shell of substantially circular cross section positioned with one of its ends in contact with said impervious end wall of said outer shell, said first inner shell having an impervious side wall spaced from the side wall of said outer shell and an impervious inner end wall perpendicular to said axis and at a substantial distance from both end Walls of said outer shell, a second inner shell of substantially circular cross section positioned with one of its ends against the end wall of said outer shell having an opening therein, said second inner shell having an impervious side wall spaced from the side wall of said outer shell and an inner end wall perpendicular to said axis and spaced a substantial distance from the impervious inner end Wall of said first inner shell and having an opening therein, a plurality of circumferentially spaced apart partitions extending axially and radially between the side walls of said outer shell and said inner shells and dividing the spaces therebetween into a plurality of axially extending cells, said cells extending substantially the full length of the outer shell and communicating with the space between the inner end walls of said first and second inner shells, a plurality of circumferentially spaced apart curved walls forming a group of ducts, said ducts in said group equalling in number the number of cells, said spaced apart curved walls being tightly connected with the partitions forming the cells, each of said curved walls extending axially from the inner end wall of said first inner shell to the inner end wall of said second inner shell and inwardly from said partitions into said interior space, said ducts being axially alined and contiguous with each other thereby occupying the entire circumference of said interior space, each of said ducts connecting one of said cells directly with said interior space, the width of each References Cited in the file of this patent UNITED STATES PATENTS 2,537,414 Lindblad Jan. 9, 1951 2,899,068 King et al. Aug. 11, 1959 FOREIGN PATENTS 162,005 Sweden Jan. 28, 1958