Liquid purification or separation – With repair or assembling means
Reexamination Certificate
2001-08-07
2003-11-11
Lithgow, Thomas M. (Department: 1724)
Liquid purification or separation
With repair or assembling means
C210S323200, C210S333010, C210S411000, C210S457000, C210S483000, C210S497010
Reexamination Certificate
active
06645379
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a filter element comprising a support element to which a flexible filter medium is applied. The invention further relates to a filter comprising a housing in which a filter element of the foregoing type is installed. The invention additionally relates to a method of backwashing a filter of the aforedescribed type.
Filter elements of the foregoing type are known. For instance, DE 198 33 381 A1 discloses a filter element comprising a helical spring as a support element to which a filter foil is mounted. This filter foil contains fine slots that allow the fluid to be filtered to flow through and retain the particles to be filtered out. As an alternative to this foil, screen netting may also be used for filtering.
These filters are used, for example, as backwash filters. Backwashing serves to clean the influent side of the filter medium when it becomes dirty and the openings through which the fluid passes become clogged. The filter medium on the support element is subject to mechanical loads due to the backwash pulses as well as resulting from any pulsating fluid flow. Over time, the alternating loads applied to the filter medium by the fluid to be filtered cause mechanical fatigue in the filter medium, which can lead to failure. The filter foil may tear, so that the purity of the filtrate is compromised.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide a filter element comprising a support element and a filter medium mounted thereto, which is reliable in operation and has a long service life.
This object and other objects are attained in accordance with the present invention by providing a filter element comprising a cylindrical support element which is permeable to a fluid to be filtered, and a filter medium fixed to the support element which retains oversize particles contained in the fluid to be filtered, wherein the filter medium is fixed to the support element in such a way that relative movement between the filter medium and support element due to fluid flow through the filter element is prevented.
In accordance with a further aspect of the invention, the objects are achieved by providing a filter comprising an openable housing having an inlet for a fluid to be filtered and an outlet for filtered fluid, and a filter element disposed in the housing between the inlet and the outlet, the filter element comprising a cylindrical support element which is permeable to a fluid to be filtered, and a filter medium fixed to the support element which retains oversize particles contained in the fluid to be filtered, wherein the filter medium is fixed to the support element in such a way that relative movement between the filter medium and support element due to fluid flow through the filter element is prevented.
In accordance with another aspect of the invention, the objects are achieved by providing a method of backwashing a filter according to the invention comprising opening a discharge valve at a sludge outlet so that fluid from the filter flows in a reverse direction through the filter element and cleans the filter element, whereby sludge removed by backwashing from the filter element is discharged from the sludge outlet together with a portion of fluid, and thereafter closing the discharge valve so that fluid in the filter flows in a normal filtering direction, wherein the entire backwashing process takes less than 2 seconds per filter element.
The filter element according to the invention is characterized in that the filter medium is fixed to the support element, particularly a support tube, in such a way that any relative movement between filter foil and support element is prevented. This is true even if the fluid flow results in pulsating stress on the filter medium. It has been shown that mechanical loading of the filter medium is primarily produced by relative movements on the support element or by vibrations of parts of the filter medium excited by pulsations in the fluid to be filtered. A pulsating load can also be generated by backwashing with previously filtered fluid in order to clean the filter element. Critical loading of the filter medium is also caused by deformation due to indentations or buckling of unsupported partial areas of the filter medium. This may occur when the pressure difference on the filter medium is reversed, e.g., during backwashing of the corresponding filter. If the relative movement between filter medium and support element is prevented, however, the filter medium is simultaneously stabilized by the support element to the point where vibrations of the filter medium are also effectively prevented.
The flexible filter medium is disposed around the support element. To this end, the support element preferably is cylindrical to assure uniform bending of the filter medium. However, the cross section of the support element may also deviate from the circular form. Elliptical cross sections, for example, are also feasible. Screen mats made, for example, from woven wires may be used as the filter medium. Another option is to use slotted or perforated foils. These foils are extremely thin, and the slots or holes are suitable for allowing the fluid to be filtered pass through while the oversize particles in the fluid are retained. The size of the holes or slots directly influences the size of the particles being retained. Flow through the filter medium from the outside toward the inside is just as feasible as from the inside toward the outside. Backwashing is achieved by reversing the flow direction of the fluid.
In accordance with one specific embodiment of the invention, the filter medium contacts at least 3% of the support area provided by the support element without any play between the filter medium and the contacted areas of the support element. Contact occurs in different support regions, which are uniformly distributed over the entire support area. Tolerances between the support regions result, for example, from production inaccuracies of the support element. In the major part of the remaining support area, a radial clearance tolerance of a maximum of 0.2 mm is permitted with respect to the lateral surface formed by the support element. This assures that due to tolerances between support element and filter medium the support regions are randomly distributed over the support area and that the respective support regions are close enough to one another that any vibration of the filter medium between the individual support regions is effectively prevented.
The support area provided by the support element is smaller than the lateral surface area of the support element. The lateral surface area is provided with openings to allow the filtered fluid to pass from the clean side of the foil on the exterior of the support tube to the interior or vice versa. Thus the support area is formed by the segments between the openings. A support tube may, for example, comprise a helical spring. Another possibility is to use a perforated metal sheet that is bent into a tube. It is particularly advantageous to use so-called slot or gap tubes. They comprise a helically coiled strand having a, for example, triangular cross section. If desired, the support tube may be provided with axial supports to impart greater stability. The support area may advantageously be minimized to the edge of the triangular cross section of the strand so that the largest possible portion of the lateral surface is not covered by the support element, but is available directly for filtering.
It has been found that even if the permissible tolerance of 0.2 mm between filter medium and support element is exceeded over a maximum of 15% of the support area, a uniform distribution of the support regions on the support area is assured. The actual clearance tolerance may be exceeded, for instance, due to a fold in the filter foil created when the foil is bent around the support element. Such deformations, however, have an increased intrinsic rigidity due to their geometry, which also helps prevent vibrations in the filter mediu
Gohle Peter
Poh Ralf
Roesgen Andre
Stahl Holger
Thalmann Christian
Crowell & Moring LLP
Filterwerk Mann & Hummel GmbH
Lithgow Thomas M.
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