Liquid purification or separation – Filter – Movable medium
Reexamination Certificate
2004-02-09
2004-10-26
Prince, Fred G. (Department: 1724)
Liquid purification or separation
Filter
Movable medium
C210S393000, C210S396000, C210S402000
Reexamination Certificate
active
06808076
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquids filtration equipment and, more specifically, to a drum filter employing a woven mat as the filtering medium.
BACKGROUND OF THE INVENTION
Devices for removing particulate materials suspended in liquids are already known and widely employed, said devices comprising a rigid cylindrical drum having a perforated circumferential wall extending between two substantially closed end portions, said perforated wall being entirely covered by the filtering mat. Said drum is immersed in the liquid to be filtered, and a pressure differential is established, with the pressure inside the drum being lower than on the outside. The liquid with the suspended solids flows through said filtering mat, said solids being collected at its outer surface, while the clean filtered liquid inside the drum is removed and conveyed to the intended utilization. The size of the particulate material being removed depends on the mesh of the filtering mat, being determined by the conditions of the liquid as well as by the application, such as water treatment for domestic and industrial use, papermaking, steel mills, irrigation, etc., said mesh usually ranging between 2 millimeters and 1 micrometer. The pressure differential between the drum's outside and inside is a function of the flow rate, the mesh size and the amount of cake that is collected on the outside surface of the filtering mat; ranging usually between 0.05 and 2 bar.
Actually, as the amount of solids retained by the filtering mat increases, the flow of liquid passing through will decrease and a greater pressure difference will be needed to effect the flow of liquid. The removal of this material—usually called “cake”—is one of the main problems in the design of such filters. In some of the known systems, the normal filtering operation is periodically shut down and scraping elements are activated in order to detach the cake from the outside surface of said filtering mat. This is undesirable, as such down time may not be tolerated in some industrial applications that require a continuous flow of filtered liquid.
Another known system, described in U.S. Pat. No. 5,558,042, comprises a cylindrical drum with a perforated circumferential wall over which is placed a flexible and porous flexible layer which supports a resilient filter screen. The cleaning apparatus for removing the cake collected on the outside of said screen is shown in
FIG. 1
, in which R indicates the drum's direction of rotation. This cleaning device employs a rigid plate
98
having its face
98
a
in sliding contact with the resilient filter screen
96
. This plate is provided with a narrow slot
100
equal in length to the cylinder generatrix, connected to a vacuum device through a conduit
104
, which has a flexible portion to allow the application of a spring's force to hold the wear surface
98
in continuous contact with the outer surface of the filter screen. The apparatus comprises associated cleaning means provided by one or more nozzles
124
connected via a water conduit
126
to a pressurised water supply, said nozzles generating a fan-shaped spray of water to dislodge the particulate material remaining on the mesh filter, said particulate material being added to the unfiltered water. This device has several drawbacks, such as the wearing down of plate
98
, due to the fact that the cake removal depends on the vacuum head that can only be maintained by pressing said plate against the mesh filter surface. This abrasion of said plate requires its periodical substitution, with the attendant shutdown of the filtering operation. Additional disadvantages of said apparatus comprise the fact that it requires a high pressure pump for the water spray, as well as a vacuum pump for the cake removal through conduit
104
. Moreover, the sediment-carrying water from the water spray action is added to the unfiltered water, so the sediment is retained again by the mesh filter.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a rotary drum filter capable of continuous operation that dispenses with the need to press the edges of the suction slot against the surface of the filtering mat.
It is another object of the invention to provide a system in which the removal of the cake from the filtering mat takes advantage of the pressure of the force pump that propels the liquid, thus obviating the need of additional pumps for suction or removal of said cake.
It is a further object of the invention to provide a filtering device in which the filtering mat rests directly upon the outside wall of the rigid cylindrical drum frame, doing away with the need for an intermediate porous flexible support layer.
According to a first aspect of the present invention, there is provided a filtering system consisting of a pressure vessel containing the cylindrical drum frame to which the filtering mat is attached, and a mat cleaning device comprising a scanner plate composed of a rigid plate parallel to the drum axis fixedly attached to the inside wall of said vessel, having a first surface facing said drum and a second surface facing said inside wall of said vessel, the opening that allows the flow of the flushing liquid between said surfaces being provided by a slit substantially equal in length to the cylinder generatrix, said slit forming a channel between the substantially flat first surface and a second surface, the latter being conformed in a way that forms a trough substantially as long as said slit, said trough communicating with an outlet for draining the flushing liquid that passes through said slit.
According to a further feature of the invention, the pressure in said trough is lower than the pressure of the filtered liquid collected inside the drum, which is lower than the pressure head between the containing vessel wall and the outside of the filtering drum assembly.
According to yet another feature of the invention, the cake is removed by the concurrent effect of two liquid streams, the first one flowing straight through the filtering mat, from the inside to the outside of the filtering drum, and the second tangential to said mat consisting of two converging streams that flow from the edges of the scanner toward said slit, the total flushing liquid flux through said slit being the sum of said first and second currents.
According to a further feature of the invention, the cake is loosened by the action of one or two brushes extending alongside the edges of the scanner plate having the bristles substantially perpendicular to the scanner's first surface, said bristles having their length dimensioned so as to slidingly touch the filtering mat surface.
According to a further feature of the invention, the cylindrical drum frame wall is provided with a plurality of through-holes to allow the flow of the filtered liquid, the outside of said wall consisting of a multitude of grooves parallel to the cylinder generatrix, said filtering mat resting upon the ridges separating said grooves, the latter conveying the liquid uniformly over the drum surface in the regions between said through-holes, resulting in a greater filtering area comprising substantially the whole surface of the filtering mat.
According to a further feature of the invention, the drum supporting and rotating means allow for lateral oscillations, thus accommodating any eventual drum eccentricity.
REFERENCES:
patent: 3617556 (1971-11-01), Cole et al.
patent: 4154588 (1979-05-01), Herndon, Jr.
patent: 5558042 (1996-09-01), Bradley et al.
patent: 6123841 (2000-09-01), Gotoh
patent: 6579458 (2003-06-01), Mickelat et al.
patent: 6616840 (2003-09-01), Boele
patent: 2257332 (2000-06-01), None
patent: 2 302 818 (1997-02-01), None
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