WO1995007813A1

WO1995007813A1 - End plate for multi-tube assembly

Info

Publication number: WO1995007813A1
Application number: PCT/GB1994/002018
Authority: WO
Inventors: Stephen David Bode
Original assignee: W.L. Gore & Associates (Uk) Limited
Priority date: 1993-09-17
Filing date: 1994-09-16
Publication date: 1995-03-23
Also published as: GB2282200A; AU7621394A; GB9418689D0; GB9319245D0

Abstract

An end plate (30, 33) for a multi-tube assembly, such as a cross-flow filter, comprises a first layer of material (22) defining an array of apertures (26), a plurality of tubes (12), an end of each tube (12) being positioned to have its lumen aligned with a respective aperture (26), and a second layer of material (24) overlying the first layer (22) and being bonded thereto, the ends of the tubes (12) being embedded and retained within the second layer (24).

Description

ENDPLATE FOR MULTI-TUBE ASSEMBLY FIELD OF THE INVENTION

The present invention relates to an endplate construction for a multi-tube assembly, particularly for a crossflow filter. BACKGROUND OF THE INVENTION

A crossflow filter comprises a bundle of parallel porous tubes, each porous tube wall acting as a filter membrane. Liquid to be filtered is passed through the lumens of the tubes. Filtrate passes through the tube walls and is collected in the extra-tubular space, leaving a more concentrated liquid within the tubes. Thus, a liquid having a higher concentration of substance to be filtered out (e.g. solid particles) leaves the downstream end of the tubes, whilst a filtrate which has passed through the porous tube walls is collected in the extra-tubular space. Since the liquid to be filtered is flowing through the tubes (generally at right angles to the direction of filtrate flow) it acts to prevent build up of filtered solid on the inside of the tube walls.

In order to maximise the filter membrane surface area, a plurality of filter tubes, for example 50 to 500 tubes, typically make up the bundle. These tubes must be fitted into an endplate, which forms part of a manifold for directing the bulk flow of liquid through the tube bundle. The sealing of the individual tubes into the endplate requires careful consideration. Firstly, the tubes should be sealed into the endplate so that leakage between the tubes and the endplate does not occur. Secondly, the tubes should be uniformly spaced so as to maximise filter capacity and avoid dead zones. Thirdly, the restriction to flow should be minimised. Fourthly, the construction should be strong enough to withstand normal usage; which is a particular problem when the tubes are closely spaced.

Several known methods of sealing the tubes into the endplate are known. Our earlier British patent specification GB2229240 discloses belling the ends of the tubes, and fitting the belled ends in place in drilled apertures in the endplate by means of ferrules. However, this arrangement requires extensive drilling to be carried out and is expensive to produce.

GB-A-1190425 discloses a plastic harness plug assembly which is formed in a connector mould provided with a plurality of stepped upwardly extending mandrels. The ends of conduits to be bonded to the plug are positioned on the mandrels before the mould is filled with suitable liquid plastics material, to submerge the ends of the conduits. On curing the plastic material integrally interconnects and forms the connector to the conduits. The assembly may then be removed from the mould. The resulting assembly includes integral extensions defining fluid flow passages and a body portion in which the conduit ends are located. The disclosed arrangement would not possess the necessary rigidity to form an endplate as, in one direction, only the body portion forms a continuous layer with the spaced extensions providing little stiffness.

It is an object of the present invention to address these requirements.

SUMMARY OF THE INVENTION

In one aspect, the invention provides an endplate for a multi-tube assembly which comprises

- a first layer of a first material having an array of apertures therethrough,

- a series of tubes, an end of each tube being positioned to have its lumen aligned with a respective aperture; and

- a second layer of a castable second material overlying the first layer and attached thereto, the ends of the tubes being embedded and retained within the second layer.

It is found that the first layer gives strength and rigidity to the assembly, which allows a close spacing of the membrane tubes. This is because the apertures through the first layer are generally only the size of the internal diameter of the tubes, rather than the external diameter as in conventional constructions. The second layer of material gives a good seal between the tubes, and bonds to the first layer. Endplates for a cross-flow filter constructed without such a first layer have been found to lack the necessary strength, the endplate bending and ultimately leaking under elevated pressure test conditions.

The first layer may be formed in any suitable way such as to provide an array of apertures therethrough, for example by casting liquid material around mould pins and allowing the liquid to set. It may also be formed by, machining or drilling material in the solid state.

Generally, the apertures are of substantially circular cross-section and of substantially the same diameter as the internal diameter of the tube lumens, so as to minimise flow resistance. However, by using pins whose cross-section varies along their length the apertures may have a different cross-section to the tube lumens, possibly slightly bigger to provide a lead-in for the liquid flow in use. Generally, the tubes have an internal diameter in the range 0.5 to 8mm, particularly 2 to 6mm; and an external diameter 0.7 to 12mm, typically 4 to 8mm. In the case of a crossflow filter, the tubes are preferably formed of a porous material of pore size chosen for the particular filtration application being undertaken. The tubes may be formed of any suitable filter membrane material, such as polytetrafluoroethylene (PTFE) , polypropylene, polysulphone or a ceramic material. However, the particular endplate construction is not limited to use with porous tubes, and may be employed also for non-porous tubes, such as in heat exchanger constructions.

Preferably, an end of each tube abuts the first layer.

Preferably also, the first and second layers are formed of the same material or of compatible materials, such that when the second layer is cast on to the first layer, a good bond is formed between the layers. To assist bonding between the layers, the surface of the first layer may be roughened to provide a key, for example by mechanical abrasion or chemical etching. The first and second materials are preferably a curable resin (such as an epoxy resin, a polyurethane resin, an acrylic resin, or a polyester resin) or other castable material, such as a ceramic silica-based resin or a thermoplastic or thermosetting material. The material may be selected from known potting compounds. However, the curable resin should in general have a viscosity in the liquid state which provides good flow properties so as to allow the liquid to penetrate between the pins. The materials in the solid state should be dimensionally stable, withstand thermal cycling and have good strength.

In an alternative embodiment, the first and second layers may be formed, for example cast, in a single step and are thus integral. This may be achieved, for example, by locating the tubes on moulding pins prior to casting the first and second materials around the mould pins and tube ends.

The invention also relates to a process for the production of an endplate for a multi-tube assembly. which comprises;

- providing an array of upstanding pins;

- providing a first layer of a first material over the pin array such that the pins project through apertures in the first layer;

- providing a plurality of tubes;

- locating an end of each tube over a respective projecting pin;

- applying a second layer of a castable second material in a liquid state over the first layer;

- allowing the second material to set around the tube ends so as to retain the tube ends within the second layer; and

- removing the pins from the endplate. In a preferred embodiment, the castable second material in liquid state is applied over the solid first layer around the projecting pins prior to fitting the ends of the tubes over the pins, by pressing the tube ends downwards through the liquid castable material. The castable material is then allowed to solidify and complete the casting process, prior to removal of the pins from the endplate.

However in an alternative embodiment, where the tubes are spaced far enough apart to allow good flow of liquid castable material between the tubes and the liquid has a suitably low viscosity, the tubes may be first fitted onto the pins and then the second layer poured around the tube ends. In a still further embodiment, a castable first layer may be provided, and the first and second layers may be formed in a single operation.

Generally, the pins are mounted on a base for ease of handling.

The invention also relates to a crossflow filter comprising at least one endplate as described above. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way of example only with reference to the drawings wherein:

Figure 1 is a schematic cross-section of a mould for casting an endplate of a cross-flow filter;

Figure 2 shows introduction of a first layer of resin;

Figure 3 shows the ends of tubes which have been pushed over pins in the mould, and been embedded in a second layer of resin;

Figure 4 shows a finished endplate; and Figure 5 shows schematically in cross-section a completed crossflow filter assembly. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The mould 2 shown in Figure 1 comprises a removable base section 4 having an array of pins 6,7,8 fitted into holes 10 drilled in the base. Collectively the pins form an upstanding array of parallel pins spaced according to the desired spacing of the filter tubes. Only three pins are shown for the sake of clarity. In this example of a typical cross-flow filter, the porous tubes 12, of outside diameter 6mm and inside diameter 4mm, are arranged in a substantially hexagonal pattern. 352 tubes are arranged within an approximate hexagon which is 144mm (in the direction of 12 parallel lines of tubes) measured across one pair of opposed flats of the hexagon and 151mm (in the direction of 11 parallel lines of tubes) across another opposed pair of flats. The pins are arranged accordingly. The mould also includes an outer portion 14 into which the base 4 is fitted. The outer mould portion includes steps 15, 16 which act to define a first zone 18 for receiving a first layer of castable resin, a second zone 19 to receive a second layer of resin, and an upper zone 20.

Figure 2 shows the first stage in the process of producing an endplate. First, the surfaces of the mould are sprayed with a mould release agent (e.g. a silicone, such as that available under the trademark Ambersil from Ambersil Ltd.). Alternatively, the mould surfaces could be precoated with a release polymer coating, such as a polytetrafluoroethylene/perfluoroalkoxy composition available from W.L. Gore & Associates under the trademark Fluoroshield. An epoxy resin liquid formulation is then made up. In a preferred embodiment the resin is made up of 93-94% by weight Stycast (trademark - available from Grace N.V. , Belgium) and 6-7% by weight of Catalyst 9 (also from Grace N.V.), which allows curing at room temperature. If necessary, the mould and/or the resin can be heated, e.g. up to 60°C, in order to reduce its viscosity and assist mould filling. A first layer 22 of the resin is poured into the first zone 18 up to the level of the step 15 and allowed to set around the pins 6,7,8. Setting typically takes 4-5hrs at room temperature. The outside diameter of each pin is substantially equal to the inside diameter of the porous filter tubes (4mm in this example) .

The upper surface of the set epoxy resin is then scored with a scalpel to provide a mechanical key, care being taken to avoid damaging the pins.

Figure 3 shows the production of the second layer. First, a second layer 24, usually of the same liquid potting resin prepared as described above, is poured into the mould so that it penetrates between the pins 6,7,8 without leaving any voids. Before the resin has set, the free ends of porous filter tubes 12 are pressed over the pins (which extend about 20mm above step 16) and down through the resin until the free ends bottom on the top of the previously set first layer 22. When the tubes are formed of polytetrafluoroethylene (PTFE) , the ends of the PTFE tubes have been previously etched to ensure good bonding to the potting resin, for example by dipping in TETRA-ETCH (trademark of W.L.Gore & Associates) to etch the PTFE surface, followed by washing in water at 80°C and degreasing. Filling the mould with resin prior to introduction of the ends of the filter tubes helps ensure that no voids are left in the second layer of resin notwithstanding the close spacing of the filter tubes. After the tubes have been inserted, a split ring 25 is placed in the mould so as to sit on the step 16 within the zone 20. The split ring is in two semicircular halves to allow it to be fitted around the bundle of tubes leaving a free central region containing the tubes into which excess epoxy resin is displaced. The lower inner circular edge 27 of the split ring is radiused to relieve stresses in the finished endplate. The split ring has a flat lower face 29 so as to provide a flat sealable surface in the finished endplate. The second layer of resin 24 is then allowed to set. After setting, the completed endplate is removed from the mould, by first removing the split ring 25, then the annular mould portion 14, and finally removing the base portion 4 and pins. To avoid damage to the endplate the mould is disassembled using screw jacks (not shown) .

Figure 4 shows the finished endplate, apertures 26 being left in the endplate when the pins are removed. These apertures have the same internal diameter as the internal diameter of the filter tubes, so that there is minimal resistance to flow of liquid to be filtered in the tubes. Nevertheless, despite the close spacing of the filter tubes, the layer 22 of the endplate is sufficiently strong to give good rigidity and strength at elevated pressures and temperatures. The endplate is dimensionally stable, has good strength and can withstand thermal cycling. Figure 5 shows a completed filter arrangement. The bundle of filter tubes is contained within a cylindrical housing 32 (e.g. of stainless steel) having end flanges 34, 43. A hexagonal mesh cage 36 is provided around the filter tubes to prevent movement of the tubes, so as to relieve strain on the bonds between the filter tubes and the endplates.

The filter is produced as follows. Firstly, an endplate 30 of diameter similar to the diameter of the flanged portion 34 is cast onto the right hand end of the filter bundle (viewed as in the figure) in the manner described above. Secondly, a smaller endplate 33 of a diameter less than the internal diameter of the housing 32 is cast on the other end of the filter bundle. This allows the filter bundle to be inserted into the housing, with the smaller end 33 being inserted first.

The smaller endplate 33 is cast in place in the manner described above, but using a correspondingly smaller outer mould portion. A similar split ring as before is employed but has a hexagonal central aperture, which becomes filled with resin so as to cast a hexagonal boss 39 onto the inner end of the endplate. The hexagonal boss is to locate the mesh cage 36. The thickness of the boss is the same or less than the thickness of the flange 43. The outer mould portion and the split ring are then removed, leaving the base 4 and pins in place on the smaller endplate 33. The cage 36 is fitted around the tube bundle and over the hexagonal boss 39. The mesh cage is split longitudinally into two halves, which are fitted around each side of the bundle and bolted together. A rubber strip 37 bonded to the inside of the mesh cage is provided at the opposite end to the hexagonal boss to locate the cage around the other end of the tubes 12. The cast small endplate 33 is then roughened with a file around its circumference and degreased, preparatory to casting more resin around it, so as to build up a larger endplate of the same size as endplate 30.

The tube bundle and cage assembly is then inserted into the housing 32, small endplate 33 first, until the larger cast endplate 30 abuts the housing flange 34. A gasket (not shown) is interposed between the endplate and the flange.

The housing 32 is then moved to a vertical orientation with the larger endplate 30 uppermost. In order to prevent resin from adhering to the lower housing flange 43, a top-hat section gasket (not shown) is provided on the face of the lowermost flange. The larger mould portion 14 (as used above) is fitted around the lower endplate 33, so that the housing flange 43 sits on the step 16 of the mould. The mould has been sprayed with mould release as before. The outline of the mould is shown in dotted lines. The flanges 34, 43 have bolt holes 35 to be used for bolting the endplates in position. Further potting resin, preferably of the same type, is poured through the bolt holes in lower flange 43, filling up the remaining space in the mould; and penetrates some way up the inside of the vertical housing to the level of the top of the hexagonal boss 39. The resin is allowed to set. The mould parts and pins are then removed as before. Thus a further enlarged annular section 38 becomes cast around the smaller endplate 33, and provides a further portion 40 extending up inside the housing 32 and around the hexagonal boss. This serves to securely locate the mesh cage 36 in position.

The thickness of the boss 39 and further portion 40 are the same or less than the thickness of the flange 43 and this helps minimise stresses in the finished endplate. The endplates are then bolted to the flanges, by drilling holes through the endplates. Thereafter the entire assembly is placed in an oven to post-cure, at 65°C for lhr followed by 95°C for 4hrs. Following post- curing bolts are passed through the endplates and through the holes 35 in the housing flanges. Manifolds may be provided on either end of the crossflow filter in known manner for connecting to liquid flows.

The present invention allows endplates of good mechanical properties to be produced, despite the close spacing of the filter tubes, thereby providing a compact arrangement of high filtration capacity.

It will be clear to those of skill in the art that the above described embodiment is merely exemplary of the present invention, and that various modifications and improvements may be made thereto without departing from the scope of the invention.

Claims

1. An endplate for a multi-tube assembly which comprises

- a first layer of a first material having an array of apertures therethrough,

2. An endplate according to claim 1 wherein the apertures in the first layer have an internal diameter substantially the same as the internal diameter of the tube lumens.

3. An endplate according to any preceding claim wherein the internal diameter of the tube lumens is 2 to 6mm.

4. An endplate according to any preceding claim wherein the tubes are formed of porous polytetrafluoroethylene.

5. An endplate according to any preceding claim wherein the materials of the first layer and the second layer are the same.

6. An endplate according to claim 5 wherein the material of the first and second layers is a curable resin.

7. An endplate according to any preceding claim wherein said end of each tube abuts the first layer.

8. An endplate according to any preceding claim wherein the first and second layers are integral.

9. A process for the production of an endplate for a multi-tube assembly, which comprises

- providing an array of upstanding pins;

- providing a plurality of tubes;

- locating an end of each tube over a respective projecting pin; - applying a second layer of a castable second material in a liquid state over the first layer;

- allowing the castable material to set around the tube ends so as to retain the tube ends within the second layer; and - removing the pins from the endplate.

10. A process according to claim 9 wherein the first material is castable and is cast around the pins and allowed to set prior to application of the second material.

11. A process according to claim 10 wherein the first layer is roughened to provide a key for the castable material of the second layer, prior to casting the second material thereon.

12. A process according to any of claims 9 to 11 wherein the second material is applied onto the first layer prior to pushing the tube ends over the pins and into the second material.

13. A crossflow filter which comprises an endplate according to any of claims 1 to 8.