GB2374556A

GB2374556A - Disc Filtration Device

Info

Publication number: GB2374556A
Application number: GB0107010A
Authority: GB
Inventors: Terry Richard Heggs
Original assignee: BEDA TECHNOLOGY Ltd
Current assignee: BEDA TECHNOLOGY Ltd
Priority date: 2001-03-21
Filing date: 2001-03-21
Publication date: 2002-10-23
Also published as: GB0107010D0

Abstract

A disc filtration device comprises a filter chamber (1) containing a plurality of individual filter elements (4) each having substantially rigid filter membranes, being double frustoconical in shape, mounted to a common hollow member (5) extending coaxially through the filter elements such that the interior of each filter element is connected to the interior of the common hollow member. The filter element is preferably made from a material substantially inert to metal cutting fluids, for example stainless steel, and from two layers comprising a fine mesh (av. pore size 25 - 250 um) mounted on a coarse mesh (0.5 - 5mm) for support. Two conical filter membranes are joined together by means of circumferentially disposed fastening member to form a filter element. Spacing means (6) are optionally provided between neighbouring filter elements. The hollow member and filter elements may be rotated as a single unit. The device may be provided with means to detect blocking of the filter membranes (eg pressure sensor) and a means for backwashing the filter membranes.

Description

FILTRATION DEVICE The present invention relates to filtration devices for industrial process fluids. More particularly, but not exclusively, it relates to a filtration device for fluids used in metal cutting.

The majority of industrial machining processes on metal are carried out with the aid of fluids to cool and lubricate the machining head and the workpiece. While such machining processes may include drilling, planing, grinding, milling and the like, such fluids are customarily known as"metal cutting fluids". Metal cutting fluids are generally proprietary blends of water-soluble or water-miscible organic compounds, and are normally used as 3- 5% aqueous solutions, though higher concentrations of fluid may be used in particular circumstances. Indeed, in some cases, neat oil may be used.

It is usual for such metal cutting fluids to be recycled. However, the fluids flowing off the workpiece contain a significant quantity of debris, mainly metal particles, which must be

removed before the fluids can be re-used. A particularly suitable device for this purpose is that known as a disc filter. In this device, contaminated fluids are passed into a cylindrical vessel containing a stack of disc-shaped filter elements, each mounted to a coaxially extending pipe. Each filter element comprises a pair of circular filter cloths, connected by a circumferential rim and comprising a fine mesh of plastics material, such as nylon or polypropylene. The fluids pass through the filter cloths into the interior of each filter element, which is connected to the coaxial pipe. The filtered fluids are led off along the pipe while the debris is retained on the filter cloths.

When sufficient debris has built up on or in the filter cloths to cause blocking or blinding of the filter elements, it may be removed by passing a reverse flow of clean liquid in through the pipe and out through the filter cloths to dislodge the debris. This process is encouraged by spinning the stack of filter elements about its axis, urging the debris, including dense metal particles towards the outer walls of the cylindrical vessel. The liquid containing these metal particles is led out of the vessel for further treatment or disposal.

However, such disc filters suffer from a number of drawbacks. The service lifetimes of the filter cloths can be inconveniently short, particularly if fluids of greater than normal concentrations are used. A planned lifetime of two years can be shortened to six weeks in some circumstances It is believed that components of the metal cutting fluids may be attacking the material of the filter cloths, weakening them and leading to rapid failure of the mesh.

It is also a problem that however tautly the filter cloths are stretched, they retain a degree of flexibility. Thus, as the fluid flows inwardly through cloths on opposite faces of a filter

element, the cloths will tend to bow inwardly and ultimately may come into contact, reducing the effective filtration area of that filter element, before that area has been adequately blocked This leads to the requirement for more frequent back flushing to maintain effectiveness.

During cleaning, when fluid is flowing outwardly through the filter cloths, opposing filter cloths on corresponding faces of neighbouring filter elements may each bow outwardly and come into contact. This may interfere with the efficient removal of the metal particles and thereby lessen the time before the filter element blinds or is blocked once again.

In general operation, the relatively narrow gaps between neighbouring filter elements restrict access to the full surface of each filter cloth and even small build-ups of debris, in the form of a cake, can bridge between filter elements and further interfere with fluid flow through the device.

There is thus a need for an improved filtration device which can run without failure for longer periods, which can handle all types and concentrations of metal cutting fluids and which obviates other problems described above.

It is therefore an object of the present invention to provide a filtration device for metal cutting fluids comprising filtering means of improved robustness and durability, which is less vulnerable to blocking and blinding and which is easier to clean.

According to the present invention, there is provided a disc filtration device, comprising a plurality of individual filter elements each comprising substantially rigid filter surfaces of a

double frustocone, mounted to a common hollow member extending coaxially through said plurality of elements, with an interior of each said filter element connected to the interior of the common hollow member.

Each filter surface of each filter element may comprise a filter mesh of a material substantially inert to metal cutting fluids, preferably a metal such as stainless steel, optionally mounted to a coarse mesh, having significantly wider mesh spacing and with significantly larger diameter strands than said filter mesh.

Preferably, said coarse mesh comprises a material substantially inert to metal cutting fluid, preferably a metal, optionally stainless steel.

The filter mesh may have an average aperture size of between 25 and 2501lm, preferably between 38 and 1001lm.

The coarse mesh may have an average aperture size of between 0. 5 and 5mm, preferably 1- 2mm The two filter membranes of each element may be connected one to the other by means of a circumferentially disposed fastening member, or they may be spot-welded or peened together.

Spacing means may be provided between neighbouring individual filter elements.

The plurality of individual filter elements may be disposed generally coaxially within a substantially cylindrical vessel, with the common hollow member mounted to opposing circular faces of the vessel.

Preferably, the common hollow member is so mounted to the vessel that the plurality of individual filter elements and the common hollow member may be rotated as a single unit, with the common hollow member acting as a drivable axle.

The hollow member may be provided with a key to engage drivingly with a slot in each element.

The vessel may be provided with fluid inlet means and the common hollow member may be provided with fluid outlet means.

The filtration device may be provided with means, for example means to sense pressure differential, to detect blinding or blocking of the filter membranes.

The filtration device may be provided with inlet pumping means to pump fluid into the vessel via the fluid inlet means.

Additionally or alternatively, the filtration device may be provided with outlet pumping means to pump fluid which has passed through the filter membranes out of the common hollow member, via the fluid outlet means.

The outlet pumping means may be adapted to pump clean fluid into the common hollow member to provide a backflow of fluid through the filter membranes to remove cleaningly metal particles therefrom An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which :- Figure 1 is a schematic elevation of a device embodying the invention ; Figure 2 is a plan view of an individual filter element of the device of Figure 1 ; and Figure 3 is a cross-section of the filter element of Figure 2 taken along the line Ill-Ill Referring now to the drawings and to Figure 1 in particular, the filtration device comprises a cylindrical vessel 1, provided with a fluid inlet 2 and a drain 3 Within the vessel I are mounted a number of filter elements 4 (for clarity, only six filter elements are shown here, although other numbers are possible and a particularly favoured embodiment is provided with fifteen such filter elements). The filter elements 4 are configured as shallow basally-joined double frustocones and are each mounted to and surround a pipe 5 which extends coaxially through the vessel 1 Spacers 6, in the form of collars encircling the coaxial pipe 5, are disposed between neighbouring filter elements 4 to increase the separation between them The coaxial pipe 5 is mounted to the vessel I by two sealed bearings 7 and the filter elements 4, spacers 6 and pipe 5 are free to rotate, relative to the vessel 1, as a single body. The pipe 5 is provided with an axially extending drive key 23 to cooperate drivingly with a slot in each element 4. The pipe 5 extends through one end face of the vessel 1, as shown, and is connected to a fluid outlet 22. The portion of the pipe 5 external to the vessel 1 is also

connected to a motor (not shown) to drive rotationally the assembly of pipe 5, filter elements 4 and spacers 6.

Referring now to Figures 2 and 3, each filter element 4 comprises two frustoconical filter sections 8, connected one to another at an outer margin 9. Each filter section 8 is made up of an inner support layer 10 of stainless steel mesh of average aperture size two millimetre, and an outer filter layer 11 of stainless steel mesh of average aperture size in the region of 63 micrometre. Each filter section 8 is provided with central reinforcement rings 12, clamped around the inner support layer 10 and the outer filter layer 11 by means of rivets 13 (only four of which are shown for clarity), and attached to the pipe 5. The two filter sections 8 are held together at their outer margins 9 by means of a metal, such as stainless steel or aluminium, folded rim 14.

The pipe 5 is provided with a plurality of apertures 15 connecting an interior 16 thereof with an interior 17 of each filter element 4. Fluid can thus only enter or leave the interior 17 of the filter element 4 through the apertures 15 or through the filter sections 8.

In use, metal cutting fluid containing metal particles is passed into the vessel 1 through the fluid inlet 2. The drain 3 is closed at this stage. The fluid is passed through the filter sections 8 of each filter element 4 and the metal particles are retained on the outer filter mesh 11.

From the interior 17 of each filter element 4, the filtered fluid passes through the apertures 15 into the interior 16 of the pipe 5 and thereby to outlet 22 of the vessel 1.

After a period of time, which may depend on the level of contamination of the fluid, the metal particles will have blocked or blinded sufficient of the filter sections 8 for the filter

device to be losing effectiveness, as can be demonstrated by the pressure needed to pass the fluid through the device rising to a predetermined trigger level. At that time a cleaning step is performed. The fluid inlet line 2 is closed. Clean fluid is introduced to the interior 17 of each filter element 4 from the pipe 5, and the drain 3 is opened Fluid then passes outwardly

through the filter sections 8, removing the metal particles therefrom. Fluid entraining these z : l 9 particles is led off through the drain 3.

During the above cleaning step, the coaxial pipe 5 is rotated rapidly, spinning each filter 0 element 4 and thereby urging the metal particles thereon outwardly away from the filter elements 4 and towards the walls of the vessel 1. This significantly enhances the cleaning

effect of the fluid passing outwardly through the filter sections 8 0 Instead of monitoring the pressure differential, the cleaning step may alternatively be performed regularly after a predetermined running time.

Compared to a conventional filter device provided with filter elements comprising two parallel circular plastic mesh filter sections, the device described above has several significant advantages. The stainless steel filter meshes are not susceptible to attack by components of the metal cutting fluid and are thus far more durable. They are also more resistant to any sharp-edged metal particles which may be present in the fluid. The coarse mesh backing and supporting the filter mesh prevents the thinner filter mesh flexing and thus prevents the two filter sections of the filter element collapsing together, which would minimise the interior volume 17 of each filter element, and reduce the rate of possible flow through it..

The configuration of each filter unit 4 also increases the efficiency of the filtration process.

Due to the frustoconical shape of the filter sections 8, the gap 18 between a filter element 4 and a neighbouring filter element 19 is significantly wider adjacent an outer region 20 of the filter section 8 than adjacent an inner region 21 thereof An initial build-up of debris on said outer region 20 would not significantly impede fluid flow towards the inner region 21. The configuration of the gap 18 may even serve to funnel fluid towards the inner region 21. In any case, substantially the whole surface of each filter section 8 remains accessible to fluid and available to carry out filtration. The presence of the spacers 6 further improves this accessibility and makes it almost impossible for cakes of debris, building up on a filter section 8, to bridge to a neighbouring filter element 19.

The filtration device is thus easier to run, can be run for longer between cleaning steps and is easier to clean when cleaning becomes necessary.

Claims

CLAIMS 1. A disc filtration device comprising a plurality of individual filter elements each comprising substantially rigid filter surfaces of a double frustocone, mounted to a common hollow member extending coaxially through said plurality of elements, with an interior of each said filter element connected to the interior of the common hollow member.
2. A device as claimed in claim 1, wherein each filter surface of each filter element comprises a filter mesh of a material substantially inert to metal cutting fluids, preferably a metal such as stainless steel.
3. A device as claimed in claim 2, wherein the filter mesh has an average aperture size of between 25 and 250, um, preferably between 38 and loom.
4. A device as claimed in either claim 2 or claim 3, wherein the filter mesh is mounted to a coarse mesh, having significantly wider mesh spacing and with significantly larger diameter strands than said filter mesh.
5. A device as claimed in claim 4, wherein said coarse mesh comprises a material substantially inert to metal cutting fluid, preferably a metal such as stainless steel.
6. A device as claimed in either claim 4 or claim 5, wherein the coarse mesh has an average aperture size of between 0.5 and 5mm, preferably 1-2mm.

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7. A device as claimed in any one of the preceding claims, wherein the two filter membranes of each element are connected one to the other by means of a circumferentially disposed fastening member.
8. A device as claimed in any one of the preceding claims, wherein spacing means are provided between neighbouring individual filter elements.
9. A device as claimed in any one of the preceding claims, wherein the common hollow member is so mounted to a vessel that the plurality of individual filter elements and the common hollow member may be rotated as a single unit, with the common hollow member acting as a drivable axle.
10. A device as claimed in claim 9, wherein the hollow member is provided with a key to engage drivingly with a slot in each element.
11. A device as claimed in any one of the preceding claims, wherein the filtration device is provided with means to detect blinding or blocking of the filter membranes.
12. A device as claimed in claim 11, wherein the detection means comprise means to sense pressure differential.
13. A device as claimed in any one of the preceding claims, further comprising pumping means to pump fluid which has passed through the filter membranes out of the common hollow member and also adapted to pump clean fluid into the common

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hollow member to provide a backflow of fluid through the filter membranes to remove cleaningly metal particles therefrom.
14. A filtration device substantially as described herein with reference to the Figures of the accompanying drawings.