Aeration Device for Filtration System

Abstract

An aeration device configured to be fitted to a membrane filtration module having membranes mounted therein. The aeration device comprises a sleeve configured to at least partially surround the membrane filtration module. The sleeve has one end adapted to engage with the membrane filtration module and another end adapted to engage with a filtrate collection conduit or manifold.

Description

TECHNICAL FIELD

Aspects and embodiments disclosed herein relate to membrane filtration systems, and more particularly to those systems employing porous or permeable membranes located in a pressurised shell or a tank or cell open to atmosphere and an aeration device for use with such membrane filtration systems.

BACKGROUND

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

Membrane filtration systems typically can be operated in a single or double-ended manner. That is, filtrate can be withdrawn from one or both ends of the membranes, particularly when the membranes are in the form of tubes or fibres. Double-ended systems are typically more efficient in that more permeate can be withdrawn from the membranes within a set period due to reduced pressure drop along the length of the membrane.

Similarly, the efficiency of the membrane filtration systems is typically dependent on the surface area of the membrane exposed to liquid to be filtered. In the case of filtration systems using bundles of membranes tubes or fibres, the surface area of the system may be increased by a number of methods including increasing the packing density of the tubes or fibres and/or by increasing the length of the tubes or fibres extending between their end supports.

The success of a membrane filtration process largely depends on employing an effective and efficient membrane cleaning method. Porous membrane filtration systems require regular backwashing of the membranes to maintain filtration efficiency and flux while reducing transmembrane pressure (TMP) which rises as the membrane pores become clogged with impurities. Typically, during the backwash cycle the impurities are forced out of the membrane pores and/or scoured from the membrane surfaces into the feed tank or cell by one or more of pressurised gas, gas bubbles, liquid, or a mixture thereof. The liquid containing impurities and deposits from the membranes is then drained or flushed from the tank.

Minimising the footprint of filtration systems is desirable in terms of space eventually occupied by the filtration plant. Compact systems have lower cost, less waste volume, lesser impact on the environment and are more acceptable to the market.

SUMMARY

Aspects and embodiments disclosed herein seek to overcome one or more of the above mentioned problems of the prior art, provide one or more of the advantages outlined above or at least provide a useful alternative.

According to one embodiment, there is provided an aeration device configured to be fitted to a membrane filtration module having membranes mounted therein. The aeration device comprises a sleeve configured to at least partially surround the membrane filtration module, the sleeve having one end adapted to engage with the membrane filtration module and another end adapted to engage with a filtrate collection conduit or manifold. The sleeve comprises an outer wall and an inner wall spaced therefrom. The inner wall and the outer wall downwardly extend co-axially from a joining portion. The outer wall and the inner wall define a chamber therebetween having an open lower end at a distal end of the inner wall and a closed upper end at the joining portion where the inner wall and the outer wall of the sleeve are joined. A gas inlet is configured to communicate gas from a source of gas to the chamber. One or more aeration openings are defined in the inner wall of the sleeve and are configured to provide fluid communication between the chamber and the membranes of the membrane filtration module. One or more drain openings are defined in the outer wall of the sleeve and are configured to provide fluid communication between the membranes of the membrane filtration module and an outside of the sleeve.

In some embodiments, the aeration device is engaged to the filtrate collection conduit or manifold by a threading engagement between complimentary screw threads provided on the respective filtrate collection conduit or manifold and the one end of the outer wall of the sleeve.

In some embodiments, the inner wall extends downwardly part way along a length of the outer wall of the sleeve.

In some embodiments, a first set of portions of the inner wall extend downwardly along the length of the outer wall of the sleeve to a greater extent than a second set of portions of the inner wall extend downwardly along the length of the outer wall of the sleeve.

In some embodiments, the first set of portions of the inner wall extend downwardly along the length of the outer wall and terminate below an upper extent of the one or more drain openings.

In some embodiments, the second set of portions of the inner wall extend downwardly along the length of the outer wall and terminate above the upper extent of the one or more drain openings.

In some embodiments, a plurality of aeration openings are provided in the inner wall of the sleeve, the aeration openings being circumferentially spaced from one another around the periphery of the inner wall of the sleeve.

In some embodiments, a first set of the plurality of aeration openings have lower ends disposed below an upper extent of the one or more drain openings.

In some embodiments, a second set of the plurality of aeration openings have lower ends disposed above an upper extent of the one or more drain openings.

In some embodiments, the plurality of aeration openings are formed as vertically extending slots.

In some embodiments, the slots are open at their lower ends and taper inwardly towards upper closed ends of the slots.

In some embodiments, the taper is stepped with an initial taper along part of a lower portion of the slot then an inward step of reduced width and a further taper from the step to the upper end of the slot.

In some embodiments, the gas inlet is provided at the joining portion of the sleeve.

In some embodiments, the one or more drain openings include a plurality of circumferentially spaced drain openings defined in the outer wall of the sleeve.

According to another embodiment, there is provided a membrane filtration system having a plurality of vertically extending membrane filtration modules mounted in a rack, each membrane filtration module including membranes in fluid communication with upper and lower filtrate collection manifolds, wherein each membrane module is provided with an aeration device as described above and the aeration device is engaged with the lower filtration collection manifold.

BRIEF DESCRIPTION OF DRAWINGS

Aspects and embodiments will now be described, by way of example only, with reference to the accompanying drawings. The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labelled in every drawing. In the drawings:

FIG. 1 is a top perspective view of the aeration device according to one embodiment;

FIG. 2 is a sectional side elevation view of the aeration device of FIG. 1;

FIG. 3 is a side elevation view of the aeration device of FIG. 1 when fitted to a membrane filtration module;

FIG. 4 shows a top perspective view of a membrane filtration module rack according to one embodiment;

FIG. 5 shows a front elevation view of a portion of a filtrate manifold according to one embodiment;

FIG. 6 shows a front elevation view of the lower portion of two membrane filtration modules connected to the filtrate manifold according to one embodiment;

FIG. 7 shows a section side elevation view of the lower portion of one membrane filtration module connected to the filtrate manifold according to one embodiment;

FIG. 8 is a sectional side elevation view of the aeration device of FIG. 1 when fitted to a membrane filtration module showing a plenum mode of operation of the device;

FIG. 9 is a sectional side elevation view of the aeration device of FIG. 1 when fitted to a membrane filtration module showing an aeration mode of operation of the device; and

FIG. 10 is a sectional side elevation view of the aeration device of FIG. 1 when fitted to a membrane filtration module showing a drain mode of operation of the device.

DETAILED DESCRIPTION

Aspects and embodiments disclosed herein are not limited to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. Aspects and embodiments disclosed herein are capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.

In filtration systems employing gas bubble scouring of the membranes it has been found advantageous to confine the bubbles as much as possible in the region of the membranes to assist with the scouring process.

Referring to the Figures, FIG. 1 shows one embodiment of an aeration device 5 according to one aspect. In this embodiment, the aeration device 5 is in the form of a generally cylindrical sleeve 6, however, it will be appreciated the shape of the sleeve is not narrowly critical and sleeves having any suitable cross-sectional configuration may be used. For example, sleeves with elliptical, square, rectangular, hexagonal, or any multi-sided cross-section could be used given one function of the sleeve is to retain aerating bubbles within a membrane filtration module 7. Further, it is not essential that the sleeve be solid or completely surrounds the circumference of the membrane filtration module 7.

The sleeve 6 has a stepped reduced-diameter portion 8 at an upper end 9 and attachment means in the form an internal threaded portion 10 at its lower end 11. The internal threaded portion 10 is formed on an inner side 12 of the lower end 11 of the sleeve 6. The form of attachment is again not narrowly critical and any suitable form of attachment may be used, for example, a circlip arrangement or a bayonet-type fitting.

As best shown in FIG. 2, the sleeve 6 below the stepped reduced-diameter portion 8 has an inner wall 13 radially spaced from and extending co-axially with an outer wall 14 of the sleeve. The inner wall 13 extends at least part way along the axial length of the lower portion 15 of the outer wall 14 of the sleeve 6. The inner wall 13 and the outer wall 14 define a chamber 16 therebetween having an open lower end 17 at a distal end 18 of the inner wall 13 and a closed upper end 19 where the inner wall 13 and the outer wall 14 of the sleeve 6 are joined. Chamber 16 may be an annular chamber internal of outer wall 14 and coaxially surrounding inner wall 13.

A gas connection inlet 20 is provided along an outer side 20′ of the stepped reduced-diameter portion 8 in the form of an axially extending hollow rib 21. The hollow portion of the rib 21 defines a gas communication channel 22 which at its open lower end 23 fluidly communicates with an upper end 24 of chamber 16 formed between the inner wall 13 and the outer wall 14 of the sleeve 6. The open upper end 25 of the gas communication channel 22 is adapted to be connected to a source of pressurised gas, for example, air, though any suitable gas may be used. In some circumstances a cleaning gas such as chlorine may be used as or included in the pressurized gas.

Although the gas connection inlet 20 shown in this embodiment is one preferred configuration, it will be appreciated that any suitable inlet arrangement may be used to deliver gas to the chamber 16. For example, a simple tube connected to an opening in the chamber 16 may be provided with a connector which sealingly communicates gas from a gas source to the chamber. A variety of gas connectors may be used, for example, screw threaded, clipped or flexible push-on tubing.

The inner wall 13 of the sleeve 6 is provided with one or more aeration openings 26 defined in the inner wall 13 of the sleeve 6 for communicating gas from the chamber 16 to an inner side 27 of the inner wall 13. The aeration openings 26 are configured to provide fluid communication between the chamber 16 and membranes 34 of the membrane filtration module 7. The openings 26 are located below the upper end 19 of the chamber 16. In this embodiment, a plurality of openings 26 is circumferentially spaced around the periphery of the inner wall 13. It will be appreciated that any form of suitable opening may be used to provide fluid communication of gas between the chamber 16 and the inner side 27 of the inner wall 13.

In some embodiments, the openings 26 in the inner wall 13 are formed by open-ended upwardly extending slots. The slots 26 are tapered in width from their lower open ends 28 to their closed upper ends 29. In some embodiments, the taper is stepped, with an initial taper along part of the lower portion of the slot then an inward step (not shown) of reduced width and a further taper from the step to the upper end of the slots 29. It will be appreciated that other forms of opening may be used, for example, a group of vertically spaced slots or holes. The size of the slots or holes may be configured to reduce along the vertical extent of the group of the slots or holes to achieve a similar function to the tapering of a single slot.

The outer wall 14 of the sleeve 6 is provided with one or more drain openings 31. The one or more drain openings are defined in the outer wall 14 of the sleeve 6 and are configured to provide fluid communication between the membranes 34 of the membrane filtration module 7 and an outside of the sleeve 6. In some embodiments, the drain opening or openings 31 are located below the lower ends of the slots 26 formed in the inner wall 13. Further, in some embodiments, the drain openings 31 comprise a plurality of circumferentially spaced openings in the outer wall 14. The size and shape of the drain openings is not narrowly critical, though it is preferred they be configured to provide effective draining of liquid therethrough. In some embodiments, the drain openings 31 are each located at the same distance from the bottom of the sleeve 6 and are identical in size and shape. In other embodiments one or more of the drain openings 31 may differ in one or more of size, shape, and/or distance from the bottom of the sleeve 6 than one or more other of the drain openings 31.

As best shown in FIG. 2, in this embodiment, the inner wall 13 has a stepped downward extent. Portions 30 of the inner wall 13 extend further downward along the length of the outer wall 14 than portions 30′ of the inner wall 13. Portions 30 of the inner wall 13 extend to below upper portions of the drain openings 31 in the outer wall 14 and terminate below an upper extent of one or more or each of the drain openings 31. Portions 30′ of the inner wall 13 having the upwardly extending slots 26 only extend downward to above the drain openings 31 in the outer wall 14 and terminate above the upper extent of one or more or each of the drain openings 31. The upwardly extending slots 26 defined between the portions 30 of the inner wall 13 have lower ends 28 located above the upper extents of the drain openings 31. The upwardly extending slots 26 defined adjacent the portions 30′ of the inner wall 13 have lower ends 28 located below the upper extents of the drain openings 31.

FIG. 3 shows the aeration device 5 fitted to the lower end of the membrane filtration module 7. The lower portion 11 of the sleeve 6 extends beyond a lower potting head 32 of the membrane filtration module 7 and extends upwardly along the outer extent of the module 7. In this embodiment, the upper end 9 of the sleeve 6 engages with a fluid retention sleeve 43 which extends at least part way along the length of the module 7 to retain fluid, including gas bubbles and liquid, around the membranes 34. It will be appreciated the use of a fluid retention sleeve 43 is not narrowly critical to the operation of the device and is merely preferable in some implementations. In some embodiments, a perforated screen structure 45 may be provided which surrounds the membranes 34 and may be fitted between the membranes 34 and the fluid retention sleeve 43. The perforated screen structure 45 may be provided in addition to or as an alternative to the fluid retention sleeve 43. When used, this screen structure 45 may serve a number of purposes including preventing damage to the membranes 34 during handling of the sub-modules and supporting the spaced upper and lower potting heads 32. The internal threaded portion 10 at the lower portion of the sleeve 6 engages with a complimentary threaded portion 44 of a lower filtrate collection conduit or manifold 33. Both the aeration openings 26 and the drain openings 31 in the sleeve 6 are in fluid communication with membranes 34 mounted in the membrane filtration module 7. The aeration openings 26 provide fluid communication between the membranes 34 and the chamber 16. The drain openings 31 provide fluid communication between the membranes 34 and an area outside of the filtration module, for example, a tank, vessel, or other structure in which the membrane filtration module 7 may be mounted.

Referring to FIGS. 4 to 7, one embodiment is shown where the membrane filtration modules 7 are vertically mounted in a rack module array 35.

FIG. 4 shows one embodiment of a rack mounted membrane filtration module array 35. The modules 7 are arranged in groups comprising pairs of modules vertically extending between a corresponding pair of lower filtrate collection manifolds 33A and 33B that extend along the length of the rack and an upper clover headpiece 36, which receives the upper ends 37 of two pairs of membrane filtration modules 7. The upper end 38 of each clover headpiece 36 is in sealing fluid communication with an upper filtrate collection manifold 39 which extends along the length of the rack array 35 generally parallel to the lower filtrate collection manifolds 33A, 33B. FIG. 4 shows three rows of the module groups forming a rack module array 35, though it will be appreciated the arrangement of modules and the number in a group and the number of rows of modules is non-critical and may be configured to meet the requirements for any particular installation.

The membrane module 7 may comprise a plurality of membranes 34 extending between spaced upper and lower potting heads 32. Further, the membranes 34 may comprise permeable, hollow fibre membranes. The permeable hollow fibre membranes may be arranged in bundles extending between the potting heads 32. The membranes 34 may be open at one or both ends to allow removal of filtrate therefrom. The membrane filtration modules 7 are typically, in use, located in a tank or vessel 46 open to atmosphere and filtrate is withdrawn by applying a vacuum or negative pressure to the lumens of the membranes 34.

At least one of each of the lower filtrate collection manifolds 33A, 33B is fluidly coupled to a double-elbowed tee-piece 40 having upwardly extending filtrate transfer conduit 41, which is fluidly coupled to a corresponding upper filtrate collection manifold 39 to fluidly communicate filtrate between the lower filtrate collection manifold 33A, 33B and the upper filtrate collection manifold 39. It will be appreciated that the double-elbowed tee-pieces 40 can be provided at one or both ends of the lower filtrate collection manifolds 33A, 33B dependent on system requirements. Similarly, filtrate can be removed from either or both the upper and lower filtrate collection manifolds, though it is typically removed from the upper filtrate collection manifolds 39.

FIG. 5 shows a detailed view of the lower connection between a lower filtrate manifold 33A and the base of each membrane filtration module 7. The lower filtrate manifold 33A is provided with upwardly extending tubular opening 42 having an externally threaded portion 44 for engagement with the internal threaded portion 10 of the aeration device sleeve 6 of the membrane filtration module 7. It will be appreciated that this embodiment of the connection is not narrowly critical and any suitable form of fluid connection may be used. For example, a clip type or push-fit engagement with sealing members such as O-rings or the like may be used. Further, the sleeve 6 could be directly fitted into the side of the manifold without the use of the tubular opening or have a fitting which clamps to the outside wall of the filtrate collection manifold about an opening therein. In one embodiment, a saddle type fitting (not shown) sits astride the tubular filtrate manifold adjacent an opening therein. The saddle type fitting having an upwardly extending tubular conduit with an externally threaded portion adapted to be engaged with the internal threaded portion 10 of the sleeve 6. The saddle of the fitting may be fixed to the lower filtrate manifold by clamping, adhesive or any other suitable means.

FIGS. 6 and 7 show side elevation views of the base of membrane module 7 and the aeration device sleeve 6. In this embodiment, the internally threaded portion 9 of the lower portion of the aeration device sleeve 6 engages with the external threaded portion 44 of the tubular opening 42 of the lower filtrate manifold 33A. As best shown on FIG. 7, when the aeration device sleeve 6 is fully engaged with the lower filtrate manifold 33A, the membranes 34 supported in the lower potting head 32 are in fluid communication with the lower filtrate manifold 33A. The lower potting 32 is sealingly engaged with the lower filtrate manifold 33A to provide fluid communication of filtrate passing out of the open ends of membranes 34 supported in the lower potting head 32. In this embodiment, the sealing engagement is provided by a set of sealing members 52, for example, O-rings or the like, positioned between the outer surface 53 of the potting head 32 and the inner surface 54 of the tubular opening 42. It will be appreciated the type of sealing engagement is not narrowly critical and any suitable form of fluid sealing between the potting head 32 and the lower filtrate manifold 33A may be used to enable the filtrate collected from the membranes to be transferred to the lower filtrate manifold 33A without contamination by liquid 47 to be filtered.

The operation of one embodiment of the aeration device will now be described with reference to FIGS. 8 to 10 of the drawings.

FIG. 8 shows the aeration device 5 mounted to a membrane filtration module 7 and suspended generally vertically in a vessel or tank 46 containing liquid 47 to be filtered. Initially, the aeration device 5 operates in plenum mode where gas is initially accumulated in an upper plenum region 48 of the chamber 16. Gas is fed into the chamber 16 from a source of pressurised gas (not shown), for example, by a pipe or tube connected to the gas connection inlet 20 and accumulated in the plenum region 48 of the chamber 16 above the upper end of the aeration openings 26.

As the amount of gas within the chamber 16 increases, the aeration device 5 enters the aeration mode, as best shown in FIG. 9, where the gas in the chamber 16 passes through the aeration openings, in this embodiment slots 26, as gas bubbles 51 and into the membrane filtration module 7 to aerate the membranes 34 and clean the surface of the membranes 34. The tapered width of the aeration slots 26 serves to self regulate the gas bubble flow along the length of each slot 26 to provide a generally uniform flow of gas bubbles 51 into the membranes 34.

As best shown in FIG. 10, once the aeration process has been completed, the flow of gas to the chamber 16 is stopped or suspended and waste liquid 49 containing the fowling substances and impurities dislodged during the aeration process is removed, drained, or pumped from the vessel or tank 46 in which the membrane filtration module 7 is located. As the waste liquid is removed from the vessel or tank 46, liquid remaining in the membrane filtration module 7 drains out through the gap 50 between the distal end 18 of the inner wall 13 of the sleeve 6 and the base of the sleeve and then through the drain openings 31 in the outer wall 14 of the sleeve 6. Liquid may also drain from the membrane filtration module 7 through the aeration openings 26 in the inner wall 13 and then through the drain openings 31 in the outer wall 14 of the sleeve 6.

To provide for initiating and suspending flow of gas to the aeration device and filling/draining of the vessel or tank, in different embodiments, a controller (not shown) may monitor parameters from various sensors within the membrane filtration system. The controller may be embodied in any of numerous forms. The monitoring computer or controller may receive feedback from sensors, for example, pressure, trans-membrane pressure, temperature, pH, chemical concentration, or liquid level sensors in the feed tank, the aeration device, or in the feed supply piping, permeate piping or other piping associated with the filtration system. In some embodiments the monitoring computer or controller may produce an output for an operator, and in other embodiments, automatically adjusts processing parameters for the filtration system, based on the feedback from these sensors. For example, a rate of flow of gas to one or more membrane filtration modules 7, and/or one or more aeration devices 5 may be adjusted by the controller.

In one example, a computerized controller for embodiments of the system disclosed herein is implemented using one or more computer systems (not shown). The computer system may be, for example, a general-purpose computer such as those based on an Intel PENTIUM° or Core™ processor, a Motorola PowerPC® processor, a Sun UltraSPARC® processor, a Hewlett-Packard PA-RISC® processor, or any other type of processor or combinations thereof. Alternatively, the computer system may include specially-programmed, special-purpose hardware, for example, an application-specific integrated circuit (ASIC) or controllers intended specifically for wastewater processing equipment.

The computer system can include one or more processors typically connected to one or more memory devices, which can comprise, for example, any one or more of a disk drive memory, a flash memory device, a RAM memory device, or other device for storing data. The memory is typically used for storing programs and data during operation of the controller and/or computer system. For example, the memory may be used for storing historical data relating to measured parameters from any of various sensors over a period of time, as well as current sensor measurement data. Software, including programming code that implements embodiments disclosed herein, can be stored on a computer readable and/or writeable non-volatile recording medium such as a hard drive or a flash memory, and then copied into memory wherein it can then be executed by a processor. Such programming code may be written in any of a plurality of programming languages, for example, Java, Visual Basic, C, C#, or C++, Fortran, Pascal, Eiffel, Basic, COBAL, or any of a variety of combinations thereof.

Having thus described several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope defined by the appended claims. Accordingly, the foregoing description and drawings are by way of example only.

Claims

1.-15. (canceled)

16. A method of facilitating treating water in an aeration device configured to be fitted to a membrane filtration module having membranes mounted therein, comprising:

installing a sleeve configured to at least partially surround the membrane filtration module, the sleeve having one end adapted to engage with the membrane filtration module and another end adapted to engage with a filtrate collection conduit or manifold, the sleeve comprising: an outer wall and an inner wall spaced therefrom, the inner wall and the outer wall downwardly extending co-axially from a joining portion, the outer wall and the inner wall defining a chamber therebetween having an open lower end at a distal end of the inner wall and a closed upper end at the joining portion where the inner wall and the outer wall of the sleeve are joined; a gas inlet configured to communicate gas from a source of gas to the chamber; one or more aeration openings defined in the inner wall of the sleeve and configured to provide fluid communication between the chamber and the membranes of the membrane filtration module; and one or more drain openings defined in the outer wall of the sleeve and configured to provide fluid communication between the membranes of the membrane filtration module and an outside of the sleeve.

17. The method of claim 16, wherein the aeration device is engaged to the filtrate collection conduit or manifold by a threading engagement between complimentary screw threads provided on the respective filtrate collection conduit or manifold and the one end of the outer wall of the sleeve.

18. The method of claim 16, wherein the inner wall extends downwardly part way along a length of the outer wall of the sleeve.

19. The method of claim 18, wherein a first set of portions of the inner wall extend downwardly along the length of the outer wall of the sleeve to a greater extent than a second set of portions of the inner wall extend downwardly along the length of the outer wall of the sleeve.

20. The method of claim 19, wherein the first set of portions of the inner wall extend downwardly along the length of the outer wall and terminate below an upper extent of the one or more drain openings.

21. The method of claim 20, wherein the second set of portions of the inner wall extend downwardly along the length of the outer wall and terminate above the upper extent of the one or more drain openings.

22. The method of claim 16, wherein a plurality of aeration openings are provided in the inner wall of the sleeve, the aeration openings being circumferentially spaced from one another around the periphery of the inner wall of the sleeve.

23. The method of claim 22, wherein a first set of the plurality of aeration openings have lower ends disposed below an upper extent of the one or more drain openings.

24. The method of claim 23, wherein a second set of the plurality of aeration openings have lower ends disposed above an upper extent of the one or more drain openings.

25. The method of claim 22, wherein the plurality of aeration openings are formed as vertically extending slots.

26. The method of claim 25, wherein the slots are open at their lower ends and taper inwardly towards upper closed ends of the slots.

27. The method of claim 26, wherein the taper is stepped with an initial taper along part of a lower portion of the slot then an inward step of reduced width and a further taper from the step to the upper end of the slot.

28. The method of claim 16, wherein the gas inlet is provided at the joining portion of the sleeve.

29. The method of claim 16, wherein the one or more drain openings include a plurality of circumferentially spaced drain openings defined in the outer wall of the sleeve.