CA2137546A1

CA2137546A1 - Fluid diffuser

Info

Publication number: CA2137546A1
Application number: CA 2137546
Authority: CA
Inventors: William Allison
Original assignee: Individual
Current assignee: LANMARK (WATER) Ltd
Priority date: 1992-06-09
Filing date: 1993-06-04
Publication date: 1993-12-23
Also published as: GB9311736D0; WO1993025484A1; EP0649389A1; GB2267655A; AU4341993A

Abstract

A diffuser for use in submerged conditions comprises an elongate material strip made from two separate sheets (1, 2) of synthetic plastics material which are separately perforated in two lateral zones (1a, 1b and 2a, 2b) and then heat sealed together at their longitudinal edges (3, 4) and at intermediate positions (5, 6) to either side of a steel cable (7) positioned between the sheets (1 and 2). The steel cable (7) is arranged to act as a negative buoyancy aid. When air is supplied to the tubes (8, 9) they distend into an inflated condition and air passes into the surrounding medium through the tube perforations. Various other forms of diffusers are also disclosed, including one of circular design.

Description

W093~2~4 213 7 5 ~ ~ PCT/GB93/01196 FLUID DIFF~SER

This invention relates to a diffuser, for example an aerator, for delivering air or another liquid or gaseous fluid in finely divided form to a surrounding medium.
More particularly, the invention relates to a fluid diffuser for use in submerged conditions, for example for treating sewage or polluted lakes or waterways with air.

It has been previously proposed in PCT application No. GB 9l/01092 to provide a tubular diffuser made by perforating a stretched elastomeric membrane or a heat-shrink material which is relaxed or shrunk after perforation so as to reduce the pore size. The tubes are extruded in the required diameter from respective dies which are expensive to manufacture. Subsequent perforation of the_ tube may require a preliminary stretching operation in the case of a tube made of elastomeric material. The alternative heat-shrink material is expensive and requires an additional processing stage after perforation when the tube is heat-shrunk. In either case, perforation of a material tube by needling or laser operation is difficult to achieve satisfactorily at high speeds.
The prior diffuser also has the disadvantage that when used in submerged conditions, for example for aerating sewage or polluted waterways, the requisite negative buoyancy aid, which may be a metal filament in the form of a steel cable, can most easily be associated with the tubular diffuser by accommodating it within the bore of the tube thereby occluding a significant proportion of the volume of the tube which would otherwise be available for the passage of fluid.

2137S~ G
W093/2~U~ ~ PCT/GB93/01196 It is an object of the present invention to provide a fluid diffuser for use in submerged conditions, in which the aforesaid disadvantages are obviated or mitigated.

According to a firs~ aspect of the present invention, there is provided a fluid diffuser for use in submerged conditions, comprising a flexible member having opposed walls defining a perforated fluid delivery tube, and a negative buoyancy aid held captive by said walls in spaced relationship with said tube.

Said flexible member may be made from a flexible material, for example a heat-sealable synthetic plastics material. Preferred heat-sealable synthetic plastics materials include polyvinyl chloride, polyethylene and ethylene vinyl acetate.

Preferably, the perforated fluid delivery tube is arranged to have a circular cross-section when a fluid is flowing therein. Preferably, the perforated fluid delivery tube is arranged to collapse, suitably into a substantially flat state, when a fluid is not flowing therein. Preferably, the fluid delivery tube is in a relaxed state when collapsed.
Said flexible member is preferably elongate. More preferably, said flexible member is in the form of a flexible material strip.

Preferably, said opposed walls of said flexible member are made of the same material. Preferably, the opposed walls are of the same thickness. Preferably, each of said opposed walls is of perforate construction. Thus, preferably, opposing parts of said fluid delivery tube are perforated.

W093/2~U~ 2 1 3 7 5 4 6 PCT/GB93/01196 The opposed walls of the fluid delivery tube may be made from a single sheet of material and/or may be made from a flattened tube. Preferably, the opposed walls are made from separate shee~s of material. Preferably, inwardly facing surfaces of said separate sheets are joined to one another, for example, by heat-sealing, in order to define said perforated fluid delivery tube.
Preferably, said separate sheets are perforated to form longitudinal zones of perforations at corresponding locations before being joined at opposite sides of said zones to form said tube.

Preferably, the perforations of said perforated fluid delivery tube are arranged to be self-closing when not lS subject to internal pressure. This may be achieved by non-ablative perforation of an appropriate material.
Mechanical needling__involving piercing without material removal results in the formation of perforations which are closed when the tube is not in a relaxed condition, i.e.
when it is not subject to tension as a result cf internal fluid pressure above a predetermined threshold level.

The performance of a diffuser having self-closing perforations as described above may be improved by adapting the diffuser such that the proximity of the perforations and the elasticity of the tube material are such that internal operating pressures cause asynchronous opening and closing of the perforations.

Preferably, perforations of the fluid delivery tube taper inwardly towards the centre of the tube. The perforations may be trapezoidal or, preferably, frusto-conical in shape.

W093/2~4 ` ` PCT/GB93/01196 When the fluid diffuser is used in submerged conditions, said perforations may be arranged to produce bubbles of less than 4 ~m in diameter. Said perforations are preferably arranged to produce bubbles less than 2 ~m in diameter. Said perforations are preferably equally spaced.

Said negative buoyancy aid is preferably held captive between two opposing portions or said flexible member.
When the flexible member is elongate, said negative buoyancy aid is preferably elongate and is held captive between elongate portions of said flexible member. Said elongate portions are preferably defined by portions of said flexible member which are joined to one another, for example, by heat-sealing. When said flexible member is made from separate sheets of material, preferably inwardly facing surfaces of separate sheets are joined to one another in order tO define said elongate portions.

Said negatlve buoyancy aid is preferably held captive by said walls along substantially the whole extent of the flexible member. Said negative buoyancy aid is preferably enclosed, suitably along substantially its whole extent, by said flexible member.
Preferably, the perforated fluid delivery tube and the negative buoyancy aid are movable relative to one another. For example, where the flexible member is elongate, the fluid delivery tube and nègative buoyancy aid may pivot relative to one another about an elongate axis defined between them. Said elongate axis may be defined by portions of the flexible member which are joined to one another, as described above.

W093/2~4 213 7 5 ~ ~ PCT/GB93/01196 Said negative buoyancy aid is preferably flexible.
Said negative buoyancy aid preferably comprises a metal filament, for example, a steel cable or flattened strip.

The fluid diffuser may include buoyancy adjustment means for altering the buoyancy of the diffuser. Said buoyancy adjustment means may be provided if it may be desirable to vary the submerged depth of the fluid diffuser. Said buoyancy adjustment means may comprise a further, unperforated tube which is inflatable to a variable extent so as to alter the buoyancy of the diffuser. Opposed walls of said flexible member preferably define the walls of the unperforated tube.

The provision of a buoyancy adjustment means as described may be particularly useful in the case of a submergible boom as_described in U.K. Patent Application No. 9116072 (relating to a submergible boom for controlling oil spillages) because the boom may be positioned at any required depth depending upon the weather conditions or it may be positioned at a suitable depth below an oil spillage.

The fluid diffuser preferably further includes tethering means for tethering the diffuser in position in use. Where the negative buoyancy aid is in the form of a metal filament, said metal filament may extend beyond the ends of the flexible member and be equipped with end fittings for tethering the diffuser in a position of use.
The fluid diffuser preferably includes means for removably securing a fluid delivery line.

The diffuser described above is suitably at least 2 metres in length. Preferably, the diffuser is at least 5 21375~6 W093/2~U~ PCT/GB93/01196 metres in length. The diffuser may be greater than lO
metres in length.

The diffuser is preferably sufficiently flexible such that is may be stored in a rolled up state; for example, it may be stored on a reel in the manner of a hose.

According to a second aspect of the present invention, there is provided a fluid diffuser for use in submerged conditions, comprising a perforated fluid delivery tube with tube perforations which are self-closing when not subject to internal pressure, characterised in that the proximity of the perforations and the elasticity of the tube material are such that internal operating pressures cause asynchronous opening and closing of the perforations.

Preferably, adjacent perforations exhibit ~
asynchronous opening and closing. Non-adjacent perforations may exhibit synchronous opening and closing.

The fluid diffuser of the second aspect may include any feature of the fluid diffuser of the first aspect.

According to a third aspect of the present invention, there is provided a method of diffusing a fluid into a surrounding medium for example air into sewage, the method comprising placing a diffuser according to the first or second aspects into said medium and introducing the fluid into the diffuser at a pressure such that the fluid is forced through the perforations of the fluid delivery tube into the surrounding medium.

In the method, a part of the diffuser, for example a negative buoyancy aid thereof (when provided) may rest W093/2~4 213 7 S 4 ~ PCT/GB93/01196 upon the bottom of a container, for example, a river or sewage tank, which contains the medium. Suitably, in the method, the diffuser is not supported by any external support means.

According tO a fourth aspect of the present invention, there s provided the use of a fluid diffuser according to the first or second aspects for diffusing a fluid into a surrounding medium.
According to a fifth aspect of the present invention, - there is provided a method of manufacturing a fluid diffuser according to the first aspect, the method comprising forming a perforated fluid delivery tube using a flexible member and arranging said flexible member so as to captivate a negative buoyancy aid in spaced relationship with sa_d tube.

The flexible member is preferably formed from two sheets of material. Preferably, the two sheets of material are arranged both to form said perforated fluid delivery tube and to captivate said negative buoyancy aid.

The method preferably includes the step of perforating the flexible member before formation of the tube and/or before captivation of said negative buoyancy aid. The perforations are preferably formed when the material of the flexible member is held taut, but is unstretched.
The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:

2 1 3 7 5 1 ~
W093/2~ PCT/GB93/01196 Fig. l is a side view of a perforating machine in use;
Fig. 2 is a plan view corresponding to Fig. l;
Fig. 3 is a side view of a machine for making one S embodiment of fluid diffuser;
Fig. 4 is a section on line IV-IV of Fig. 3;
Figs. 5 and 6 are diagrammatic views showing this embodiment of the fluid diffuser in the inflated (fluid delivery) and non-inflated conditions respectively;
Fig. 7 shows the same fluid diffuser when submerged;
Figs. 8 to lO show different designs of the fluid diffuser;
Fig. ll shows the supply (air inlet) end of one embodiment of fluid diffuser in accordance with the lS invention;
Figs. 12, 13 and 14 show alternative designs for the other end of the ~luid diffuser;
Fig. 15 shows an alternative embodiment of the fluid diffuser in the in-use condition;
Fig. 16 shows a further embodiment with an alternative negative buoyancy aid;
Fig. 17 shows an alternative design of fluid diffuser made from a perforated tube;
Fig. 18 shows the perforated tube from which the fluid diffuser of Fig. 17 is made;
Fig. l9 shows an alternative design of fluid diffuser in which the fluid delivery tube is in the form of concentric rings;
Fig. 20 is a section of Fig. l9 showing the diffuser in use; and Fig. 21 illustrates in diagrammatic form one mode of operation of perforations of a fluid diffuser.

Although it is to be understood that the fluid diffuser of the invention may be used for delivering any 21375~6 ~~ 3; ~
W093J2~4 PCT/GB93/01196 g fluid (whether gas or liquid) to any (preferably denser) fluid in which it is submerged, the normal requirement for a fluid diffuser is to deliver air into a liquid, for example water or sewage, and the following description will assume that the diffuser is used for delivering air into water.

Referring firstly to Figs. 5, 6 and 7, the illustrated embodiment of fluid diffuser comprises an elongate material strip (of which only a short section is shown) made from two separate sheets l, 2 of synthetic plastics material, for example plasticised polyvinyl chloride, low or medium density polyethylene, or ethylene vinyl acetate which are separately perforated in two lateral zones la, lb and 2a, 2b (as described in greater detail below) and then heat-sealed together at their longitudinal edges 3J 4 and at intermediate positions 5, 6 to either side of a steel cable 7 positioned between the sheets l and 2. The flexible material strip so formed thus has opposed walls formed by the sheets l, 2 incorporating fluid delivery tubes 8, 9 with perforations provided by the coinciding perforated zones la, 2a and lb, 2b. The steel cable 7 provides a negative buoyancy aid which is trapped and sealed in position by the heat-sealed portions 5, 6. In the relaxed out-of-use condition shown in Fig. 6, the strip material is flat and can readily be stored on a supply reel. When air is supplied to the tubes 8, 9, they distend into the inflated condition shown in Fig. S and air passes into the surrounding medium through the tube perforations. In Fig. 7, the fluid diffuser is shown submerged in water lO with the encased steel cable 7 resting on a bottom surface and the air delivery tubes 8, 9, floating upwardly with the heat-sealed portions 5, 6 acting as hinges. The tubes 8, 9 are subjected to a pressure of up to 4 psi say l to 4 psi in W093/2~U~ 213759 i ` ~ PCT/GB93/01196 excess of the ambient pressure. This pressure differential is sufficient to open the pores in the tube 8, 9 thereby releasing numerous fine air bubbies generally indicated at 10.

Figures 1 to 4 show how the fluid diffuser of Figs.
to 7 may be ~anufactured. Figs. 1 and 2 show a perforating machine for perforating a strip 1 or 2 of synthetic plastics material intended to form the walls of the diffuser strip. The machine comprises pay-off and take-up wheels 11, 12 for the plastics strip material 1, 2 which is trained between pairs of tensioning rollers 13, 14 between which are mounted on a commcn shaft twin needle wheels 15, 16. The tensioned strip 1,2 runs around the underside of the needle wheels lS, 16 which rotate at high speed to perforate the strip 1, 2 during its travel between the reels 11, 12 so as to provide the two perforation zones la, 2a, and lb, 2b mentioned above.
Each needle wheel is provided on its periphery with numerous fine radially directed needles designed to pierce the strip material 1, 2 without any ablation, i.e. wi.hout removing any material. By appropriate choice of material, the perforations produced by the fine needles will be small, downwardly tapered (as seen in Fig. 1) and self-closing in the sense that when the material is in therelaxed condition, or subject to a tension below a threshold value, the holes close automatically by virtue of the resiliency of the surrounding material. This self-closing or self-healing feature is known in other applications and is therefore not further described.

Figs. 3 and 4 show a machine fcr making a flexible material strip intended as a fluid diffuser. Two delivery rolls 17, 18 (which may be take-off rolls 12 of the machine of Figs 1 and 2) are arranged to deliver W093/2~ 2 1 3 7 5 4 6 PCT/GB93/0l196 -perforated strip 1 with perforation tracks la, lb and perforated strlp 2 with correspondingly positioned perforation tracks 2a, 2b to pairs of edge guide rollers 19, 20 and 21, 22 with the two pairs of each set 19, 20 and 21, 22 being spaced apart transversely for contact with opposite longitudinal edges of the travelling composite strip. Intermediate the guide roll of pairs 19, 20 and 21, 22 are two pairs of heat-sealing rollers 23, 24 which as seen in Fig. 4 likewise contact the opposite longitudinal edges of the travelling material strips so as to heat-seal these edges. A further pay-off roll 25 supplies steel cable 7 centrally between the material strips 1, 2. A pair of peripherally grooved heat-sealing rollers 26 in line with the heat-sealing roller pairs 23, 24 engages the cable 7 through the strips 1, 2 to ensure accurate positioning of the cable 7 and at the same time nips the strips 1, 2_together at either side of the cable 7 and heat-seals Ihe nipped portions so as to sealingly encase the cable 7 and form the flanking portions 5, 6.
After heat-sealing, the finished material strip is wound onto a take-up roll 27.

It will be appreciated that alternative means may be provided for interconnecting the perforated strips 1, 2.
For example, ultrasonic welding may be employed;
alternatively, the strips may be interconnected by adhesive at the appropriate locations. Furthermore, although it is desirable to manufacture the diffuser in the way described using separately perforated strips 1, 2, this is not essential and it is possible, for example, to provide the perforation tracks on a single strip which is then folded along the longitudinal centre lone and heat-sealed or otherwise connected along only one longitudinal edge, the other eage seal being provided by the fold. In this case, the perforations for the strip sections W093/2~ i t ~ PCT/GB93/01196 destined to become the opposed walls of the diffuser are preferably made from opposite sides so that the perforations taper Lnwardly with respect to the air delivery tubes 8, 9.

Alternative diffuser designs are readily possible using similar machinery. Thus the embodiment of Fig. 8 comprises three air delivery tubes 38, 39 and 40 with two interposed cables 37 and 37' held captive by flanking sealed portions 35, 36 and 35', 36'. In the embodiment of Fig. 9, there are three cable 47, 47', and 47" trapped in position by flanking portions 45, 46 and, in the case of the central cable 47', by narrower flanking portions 45', 46'. In this case, there are again two air delivery tubes 48, 49. In the e~bodiment of Fig. lO, there is a single perforated air tube 58 with flanking cables 57, 57' each of which is held in position by flanking sealed portions 55, 56 and 55', 56', the seals 55 and 56' coinciding with the longitudinal edge seals.
In the embodiment of Fig. 15, there is a central perforated air tube 68 with a trapped cable 67 to one side thereof and an unperforated air tube 69 to the other side thereof. The unperforated air tube 69 can be inflated to a required extent to vary the buoyancy characteristics of the diffuser of Fig. 15. The embodiment of Fig. 16 is similar to the embodiment of Fig. 9 except that the cables are replaced by flat metal strips 77, 77' and 77"
positioned between the perforated air tubes 78, 79.
The embodiment of Fig. 17 is similar to the embodiment of Figs. 5 and 6 except that instead of being made from two strips which are heat-sealed together it is made from a flat, perforated tube as shown in Fig. 18.
3S Such a construction is acceptable if tube perforation s ~ ~
W093~2~W4 213 7 ~ ~ 6 PCT/GB93J01196 poses no problems, for example if only one side of the - tube requires to be perforated or if a single perforation operation involving piercing of both tube layers simultaneously is acceptable.

Fig. ll shows a possible design for the air supply end of the embodiment of Figs. S and 6. End fittings 27, 28 seal the ends of the air tubes 8, 9 and provide a connector for air supply pipes only one of which is shown at 29. The cable 7 extends from the end of the material strip and has an eye connector 7' for tethering the diffuser in a position of use. The opposi~e end of the diffuser may have a similar design as shown in Fig. 13 with an eye connector 7" but in this case the end fittings 27', 28' are simple stoppers and do not have the pipe connector extensions. An alternative design for the remote end of the d~ffuser is shown in Fig. 12 in which the stoppers 27', 28' can be omitted because the end of the flexible strip is flattened and heat-sealed. A
further variation is shown in Fig. 14 in which the cable stops short of the end of the diffuser and the heat-sealed end portion is provided with two eyes for use in tethering together the diffuser.

The diffusers so far described are all of elongate construction and can be made to a wide range of designs depending upon the application of the diffuser. The number and weight of the metal filaments providing the negative buoyancy aids may be varied as may the number and size of the air delivery tubes. Additional features such as buoyancy tubes may be provided as described. In one example of use, a diffuser of appropriate length is run across a polluted river and tethered on both banks using the end fittings. The negative buoyancy aid ensures that W093/2~4 213 7 ~ 4 ~ PCT/GB93/01196 the diffuser strip is held down onto the river bed even when air is supplied to the diffuser tube or tubes.

An alternative design shown in Figs. l9 and 20 is of circular design with concentric air chambers and no negative buoyancy aid. If appropriate, the negative buoyancy aid may also be omitted from the other embodiments. In Figs. 19 and 20 the flexible material strip of the other embodiments is replaced by a flexible material sheet. A negative buoyancy aid may be incorporated in the design of Fig. l9 and 20 by trapping a metal cable or strip between the opposed sheets between the concentric air chambers.

For each of the embodiments of diffuser described above, the proximity of the perforations and the elasticity of the tube material may be selected such that internal operating pressures cause asynchronous opening and closing of the perforations. The perforations of the diffuser may not open simultaneously as might be expected when a predetermined internal threshold pressur~ is achieved. The desired effect may only be obtained if the perforations are closely spaced in material of suitable elasticity such that the lateral pressure exerted by air passing through one perforation exerts pressure on the land around that perforation thereby preventing closely adjacent pores from opening. Once the air, in the form of a bubble, has cleared the membrane the lateral pressure ceases thereby permitting adjacent pores to open and reversing the lateral pressure to close the first perforation. The phenomenon described does not exist when the land between the perforations is not stressed by the passage of air through the perforations, whether by reason of the distance between the perforations, lack of elasticity in the membrane, excessive pressure drop across W093/2~ 213 7 5 4 G ^ ~ PCT/GB93/01196 the membrane, or otherwise. The phenomenon is also dependent upon ~erforations which are self-closing or self-healing when not subject to internai pressure.
Depending upon the internal pressure, the perforations may open and close, i.e. oscillate, on a regular cycle with no perforation ever ~eing fully open or closed. The cycle time or oscillating rhythm can be changed by varying the internal pressure. In order to ensure that all pores are simultaneously energised when pressure is applied to the fluid diffuser, s,ress within the system must be evenly distributed. The perforations should therefore be distributed with equal spacing in a regular pattern over the entire surface of the diffuser tube or at least within the perforation zones described with reference to the drawings. The material from which the diffuser tube is made should also be of consistent quality and thickness.
Additionally, ll the_perforations are preferably axially and geometrically aligned with the central axis of the- ~ -tube when pressurised into its cylindrical form.
The closely spaced perforations are interactive as described above .~ith no pore ever being fully open or fully closed along its entire length. At a particular instant, the internal end section of a perforation may be opened to admit a small parcel of air. This causes the stress in the adjacent wall material or land to rise compressively. '~.aterial rebound closes the perforation behind the air parcel so forcing the parcel further up the perforation repeating the stress and rebound process until the air parcel is ejected as a small bubble. The frequency of the stress/rebound process determines the size of the air parcel and hence the size of the bubbles that are released into the surrounding fluid in use. The action is similar to peristalsis in the human intestine 2137~ 46 ` ~-W093/2~ J PCT/GB93/01196 except that muscular contraction of the intestine is replaced by elastic rebound of the tube material.

Pressure variations cause the action of the perforations to change. It is therefore possible to specify and produce bubble fluxes of different bubble sizes and distribution by varying the qualities of the membrane, the piercing pattern, depth and size of needle provided that eaual pressure is applied under working conditions which is achieved by the diffuser being cylindrical in use. The perforations do not close - i m~ iately when the pressure drops but allow air to escape from within until the tube collapses and flattens.
The perforations therefore act as valves preventing flow of external fluid into the diffuser tube. If the internal air pressure is totally removed the external pressure will cause the tube to flatten completely, i.e. it becomes a layflat tube, and contact of the internal faces of the tube provides a further seal for the perforations.
The asynchronous opening and closing of perforations may be obtained using a diffuser tube made of synthetic plastics material, for example plasticised polyvinyl chloride, polyethylene or ethylene vinyl acetate. In an example, the Shore hardness ranges from 65 to 85 and the density of the perforations is 200 to 350 perforation per square inch. The thickness of the diffuser tube material varies depending on various factors, for example the hardness of the material, the design operating pressure and the size of the needles used. A typical wall thickness for a 3 inches diameter tube would be 0.015 to 0.020 inches with perforations obtained by using needles of diameter 0.028 inches with a tip tapered at an angle of 22.5. Only the tip penetrates the membrane and not the needle shank. A preferred range of needle diameters is W093/2SU~ ~ PCT/GB93/01196 0.020 inches to 0.032 inches shank diameter for all tube diameters.

- Fig. 21 illustrates the peristaltic action of the perforations as described above. In each of the four diagrams a portion of the tube wall is shown in section through three perforations with the insert of the tube below the wall and the outside of the tube above the wall.
Figs 21a and 21b show successive stages in bubble formation using a diffuser tube operating with a relatively low internal pressure and Figs. 21c and 21d - show corresponding stages when a relatively high internal pressure prevails. It will be noted that the number of air parcels or bubbles reduces and their size increases as the pressure increases. Thus a low operating pressure produces a large number of small bubbles and a high operating pressure ~roduces a smaller number of large bubbles.

20In a modification, the pores are not necessarily the same size as long as mixed pore sizes are maintained in equally spaced regular patterns. For example, with a twin pore size regular pattern the same complex stress patterns mentioned above give rise to the regular oscillating pore cycle. However, tWO different bubble sizes are produced, one relatively larger than the other. This strategy may be useful in producing both relatively large bubbles for agitation purposes as well as smaller diffuser bubbles.
The ratio of large bubble producing perforations to small bubble producing perforations is optimised to give the desired effect, preferably with a preponderance of small diffusion bubbles.

The needle piercing may be adjusted so that incomplete perforation of the diffuser tube material is 21375`46 W093/2~4 PCT/GB93/01196 obtained. Most of the wall is pierced to leave a thin membrane at the inner end of the perforation. When the diffuser is first pressurised the thin sealing wall or membrane at the bottom of each perforation is ruptured to form a flap extending inwardly into the perforation. This flap contro~s air entering the perforation and acts as a non-return valve which assists in the prevention of fluid inflow when the diffuser is unpressurised.

Preferably, the diffuser tube material is transparent to facilitate inspection of perforation formation and the diffuser tube bore.

The following modifications may also be applied to lS any of the fluid diffusers described herein.

If a long diffuser follows an undulating path, for example because it rests on the uneven bottom of a lake or river bed, the differing heads of water above the diffuser as considered along its length might cause malfunction because air would tend to be emitted from the diffuser in the region subjected to the lowest water head. This problem can be overcome by altering the pressures in the perforated tube so as to change the buoyancy of the diffuser and position it at any desired level above the undulating bed.

The diffuser tube may have a dual wall construction.
A relatively thin layer of more resilient material can be bonded to a relatively thick layer of less resilient material. Typically, the thick layer or substrate would be above 0.012 inches thick and the resilient thin layer would be O.OOl to 0.002 inches thick. The bonded laminate would be pierced as a single strip. When fabricated into a tube the more resilient thin layer perforations would W093/2~ ~13 7 S 4 6 PCT/GB93/01196 open up more than the thick layer perforations under pressure. Thus the thin layer perforation~ govern the size of bubbles released. A very fine low pressure diffuser would result. This would be achieved, whilst still retaining the low stretch characteristics of the substrate which are useful in avoiding pore opening in hydraulically stressful conditions.

Ultra-fine pores capable of working at very low ;0 internal pressures can also be achieved by lining pores of a pierced, very low elastic membrane. This can be achieved by dipping piercing needles in an appropriate solution of resilient polymer material immediately prior to piercing. 1he impressed needles would leave a coating of resilient polymer on the walls of the pores. The thickness of the coating could be a few microns. A light air blow immediately following the piercing/lining procedure would clear excess solution from the pores and dry out the resilient polymer coating the open pore mode.
The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the reatures disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

W093/2~4 213 7 S ~ 6 PCT/GB93/01196 Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series or equivalen~ or similar features.

The invention is not restricted to the details of the foregoing em~odiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A fluid diffuser for use in submerged conditions, comprising a perforated fluid delivery tube with tube perforations which are self-closing when not subject to internal pressure, characterised in that the material in which the tube perforations are defined has a Shore Hardness in the range 65 to 85 and in that adjacent perforations are spaced so that internal operating pressure causes said adjacent perforations to interact with one another and thereby to open and close asynchronously.

2. A diffuser according to Claim 1, wherein the density of the perforations in an area of the fluid delivery tube is 200 to 350 perforations per square inch (31 to 54 perforations per square cm).

3. A diffuser according to Claim 1 or Claim 2, wherein the perforations are each axially aligned with the central axis of the tube when the tube is in a cylindrical form.

4. A diffuser according to any of Claims 1 to 3, wherein the wall thickness of the tube is in the range 0.015 to 0.
020 inches (0.038 to 0.051 cm).

5. A diffuser according to any preceding claim, comprising a flexible member having opposed walls defining said perforated fluid delivery tube.

6. A diffuser according to Claim 5, wherein a negative buoyancy aid is held captive by said walls in spaced relationship with said tube.

7. A diffuser according to Claim 5 or Claim 6, wherein said flexible member is made from a heat-sealable synthetic plastics material.

8. A diffuser according to any of Claims 5 to 7, wherein said flexible member is elongate.

9. A diffuser according to any of Claims 5 to 8, wherein said opposed walls of said flexible member are made of the same material.

10. A diffuser according to any of Claims 5 to 9, wherein each of said opposed walls is of perforate construction.

11. A diffuser according to any of Claims 5 to 10, wherein said opposed walls are made from separate sheets of material.

12. A diffuser according to any of Claims 6 to 11, wherein said negative buoyancy aid is held captive between elongate portions of said flexible member.

13. A diffuser according to Claim 12, wherein said elongate portions are defined by portions of said flexible member which are joined to one another.

14. A diffuser according to any of Claims 6 to 13, wherein the perforated fluid delivery tube and the negative buoyancy aid are movable relative to one another.

15. A diffuser according to any of Claims 6 to 14, wherein said negative buoyancy aid comprises a metal filament.

16. A diffuser according to any of Claims 6 to 15, further including buoyancy adjustment means for altering the buoyancy of the diffuser.

17. A diffuser according to Claim 16, wherein said buoyancy adjustment means comprises a further, unperforated tube which is inflatable to a variable extent so as to alter the buoyancy of the diffuser.

18. A diffuser according to any of the preceding claims, further including tethering means for tethering the diffuser in position.

19. A diffuser according to any of the preceding claims, the diffuser being of at least 2 metres in length.

20. A diffuser according to any of the preceding claims, the diffuser being sufficiently flexible such that it may be stored in a rolled up state.

21. A method of diffusing a fluid into a surrounding medium for example air into sewage, the method comprising placing a diffuser according to any of Claims 1 to 20 into said medium and introducing the fluid into the diffuser at a pressure such that the fluid is forced through the perforations of the fluid delivery tube into the surrounding medium.

22. Use of a fluid diffuser according to any of Claims 1 to 20 for diffusing a fluid into a surrounding medium.