NZ502908A

NZ502908A - Inline fluid mixing apparatus where the mixing nozzle for introducing a first fluid into an other is responsive to pressure of the other fluid

Info

Publication number: NZ502908A
Application number: NZ502908A
Authority: NZ
Inventors: Paul Nicholas Dalley
Original assignee: Paul Nicholas Dalley
Priority date: 1997-09-04
Filing date: 1998-09-02
Publication date: 2002-07-26
Also published as: CA2302291A1; ZA988046B; AUPO899297A0; WO1999011363A1

Abstract

Apparatus (10) such as an inline mixer for introducing a first fluid into a second fluid. The apparatus (10) comprises a body (25) defining a first space (45) for receiving the first fluid under pressure. The body (25) includes means (41, 42) providing an outlet opening (64) through which the first fluid can discharge into the second fluid, such means being responsive to the first fluid whereby the outlet opening (64) is established when the fluid pressure of the first fluid is at or exceeds a predetermined pressure and the outlet opening (64) is closed when the fluid pressure of the first fluid is below that predetermined pressure. Typically the means (42) providing the outlet opening (64) comprises first and second sections (41, 42) moveable relative to each other between a first condition in which the first and second sections are in spaced apart relationship to define the outlet opening (64) and a second condition in which the first and second sections are in sealing contact so as to close said outlet opening.

Description

Title Fluid Mixing Apparatus Technical Field The present invention relates to apparatus for introducing a first fluid into a 5 second fluid The invention also relates to a fluid mixer incorporating such apparatus The first and second fluids may each be any form of fluid such as a liquid, a gaseous fluid or a particulate fluid. Additionally, the first and second fluids may each be a single-phase fluid such as the liquid or gas, or a multiple-phase fluid 10 such as a slurry or a liquid entrained with a gas.

Background of the Invention The invention has been devised particularly, although not solely, for injecting and mixing gases or liquids into liquid or slurry streams One application for which the invention is particularly suitable is the introduction of oxygen into a slurry 15 used in a carbon-m-pulp gold recovery process One known system for introducing an entry fluid (the first fluid), such as a gas, into a primary fluid (the second fluid) such as a liquid or slurry stream, utilises a lance-type injector. Typically, a lance-type injector comprises an open-ended pipe through which the entry fluid is injected into the primary fluid Diffusers of 20 various designs are often fitted to the end of the lance-type injector in an effort to enhance mixing efficiency and prevent backflow of the primary fluid into the lance-type injector.

A disadvantage of lance-type injectors is that they can be inefficient and prone to blockages Indeed, the mixing efficiency of a lance-type injector is low and hence 25 utilisation of the injected entry fluid is also low. Consequently, there is a need to INTELLECTUAL PROPERTY OFFICE OF N.Z. - 9 JAN 2002 received inject surplus entry fluid to achieve the desired results, which consequently leads to wastage and an increase in process costs.

Another known system for injecting and entry fluid into a primary fluid utilises a high-shear-type injector. While high-shear-type injectors are generally more efficient than lance-type injectors, they do have a higher power utilisation. This higher power utilisation is disadvantageous both cost-wise and because of a tendency to accelerate wear which in turn increases maintenance costs.

The present invention seeks to provide a fluid mixing apparatus which overcomes or at least reduces at least some of the disadvantages referred to above with lance-type injectors and high-shear injectors, or alternatively at least provides a useful choice.

Summary of the Invention The invention provides apparatus for introducing a first fluid into a second fluid, the apparatus comprising a body defining a first space for receiving the first fluid under pressure, means providing an outlet opening in the body through which the first fluid can discharge into the second fluid, said means being responsive to the first fluid whereby said outlet opening is established when the fluid pressure of the first fluid is at or exceeds a predetermined pressure and said outlet opening is sealingly closed when the fluid pressure of the first fluid is below said predetermined pressure to inhibit entry of said second fluid.

With this arrangement, closing of the outlet opening when the pressure of the first fluid falls below said predetermined level seals the apparatus against contamination by the second fluid in the event that delivery of the first fluid to the apparatus at the prescribed fluid pressure is interrupted.

Said means providing an outlet opening may comprise first and second sections moveable relative to each other between a first condition in which the first and second sections are in spaced apart relationship to define said outlet opening WO 99/11363 PCT/AU98/00713 and a second condition in which the first and second sections are in sealing contact so as to close said outlet opening Preferably, the first section is stationery and the second section is movable with respect to the first section Biasing means may be provided for biasing the first and second sections into the second condition in which they are in sealing engagement, the arrangement being such that fluid pressure at the predetermined level counteracts said biasing means to cause the outlet opening to be established The outlet opening may comprise an opening extending about the periphery of 10 the body.

The body may comprise a first body portion defining the first space and having an aperture through which the first fluid can flow, and a second body portion associated with the aperture and moveable between open and closed conditions in relation to the aperture, wherein the second body portion sealingly closes the 15 aperture against fluid flow therethrough when in the closed condition and wherein the outlet opening is defined between the first and second body portions when the second body portion is in the open condition The first body portion may be provided with a seal face extending around the aperture and the second body portion may be provided with a complimentary seal 20 face, whereby the two seal faces are in sealing engagement when the second body portion is in the closed condition and whereby the two seal faces are in faced apart relationship to provide a gap therebetween defining the outlet opening when the second body portion is in the open condition.

The two seal faces may be of any suitable construction such as replaceable 25 ceramic sealing rings mounted on the first and second body portions. With this arrangement, the two sealing rings provide said first and second sections which when spaced apart define the outlet opening Printed from Mimosa WO 99/11363 PCT/AU98/00713 The biasing action of the biasing means may be adjustable to allow selective regulation of the predetermined fluid pressure at which the outlet opening is established.

The second body portion may be mounted on a shaft which is slidably mounted 5 on a support structure accommodated within the space within the first body portion, sliding movement of the shaft with respect to the support structure allowing movement of the second body portion between the open and closed conditions, and said biasing means acting between said support structure and the support shaft for biasing the second body portion into the closed condition The biasing means may comprise a compression spring, the spring tension of which can be selectively adjusted for varying the fluid pressure at which the outlet opening is caused to be established The body may be accommodated in a second space for containing the second fluid into which the first fluid is to be introduced The second space may be configured as a flow passage for flow of the second fluid in a flow direction, the outlet opening being disposed to introduce the first fluid into the second fluid in a direction transverse to the direction of fluid flow of the first fluid Preferably, the transverse direction at which the first fluid is introduced into the second fluid is generally normal to the direction of fluid flow of 20 the second fluid thereby to cause shearing of the flow of the first fluid entering the second fluid which assists in mixing of the two fluids Where the outlet opening is defined between said two seal faces, the plane of each of said seal faces is preferably normal to the direction of fluid flow of the second fluid.

The first body portion may be configured to have an external wall which tapers outwardly towards the outlet opening Printed from Mimosa INTELLECTUAL PROPERTY OFFICE OF N.Z. - 9 JAN 2002 received The flow passage for the second fluid may be defined by a passage wall which surrounds and is in spaced apart relationship to the external wall of the first body portion, whereby cooperation between the passage wall and the tapering external wall of the first body portion constricts flow of the second fluid in the direction towards the outlet opening and thereafter provides for rapid expansion of the mixed first and second fluids. This arrangement assists the mixing process between the two fluids.

A further constriction may be provided in the passage downstream of said body to further assist mixing of the two fluids.

Where the apparatus according to the invention is provided for introducing the first fluid at a relatively low pressure into the second fluid, a fluid dispersing means may be provided within the body through which the first fluid is dispersed when the outlet opening is open. The dispersing means may be in the form of a ring. This arrangement is particularly suitable for introducing a gaseous fluid at low pressure into another fluid.

The invention also provides a mixer for mixing first and second fluids, the mixer comprising a flow passage, an inlet through which the second fluid can be introduced into the flow passage to flow therealong in a flow direction, an outlet through which the first and second fluids can leave the flow passage after mixing thereof, a body accommodated in the flow passage and past which the second fluid can flow along the flow passage, the body defining a first space for receiving the first fluid under pressure, means providing an outlet opening in the body through which the first fluid can discharge into the flow passage for mixing with the second fluid, said means being responsive to the first fluid whereby said outlet opening is established when the pressure of the first fluid is at or exceeds a predetermined pressure and said outlet opening is sealingly closed when the pressure of the first fluid is below said predetermined pressure to inhibit entry of said second fluid.

INTELLECTUAL PROPERTY OFFICE OF N.Z. - 9 JAN 2002 received With this arrangement, closing of the outlet opening which the pressure of the first fluid falls below said predetermined level seals the body against contamination by the second fluid in the event that delivery of the first fluid to the body at the predetermined fluid pressure is interrupted.

Preferably, the outlet opening is configured to discharge the first fluid into the second fluid in a direction substantially normal to said flow direction.

Preferably, the body is located in the inner region of the flow passage and the outlet opening extends entirely around the periphery of the body.

The body may have an external wall which tapers outwardly in the flow direction and terminates adjacent the outlet opening. A constriction may be provided downstream of the body to further assist mixing of the two fluids.

Brief Description of the Drawings The invention will be better understood by reference to the following description of several specific embodiments thereof as shown in the accompanying drawings in which- Figure 1 is a schematic sectional side view of an in-line mixer according to a first embodiment, Figure 2 is an exploded, sectional side view of a mixing head forming part of the in-line mixer of Figure 1, Figure 3 is a schematic side view of a mixing head of a mixer according to a 10 second embodiment, Figure 4 is a schematic sectional side view of a mixer according to a third embodiment; and Figure 5 is a schematic exploded sectional view of apparatus according to a fourth embodiment Detailed Description of Preferred Embodiments The first embodiment, which is shown in Figures 1 to 2 of the accompanying drawings, is directed to an in-line mixer 10 for introducing oxygen under pressure into a slurry used in a carbon-in-pulp gold recovery process.

The in-line mixer 10 comprises a housing 13 of tubular construction The housing 20 13 has a cylindrical side wall 14 and mounting flanges 15 at the ends thereof for installation in a slurry flow line (not shown).

The housing 13 defines a flow passage 17 between an inlet end 19 through which slurry can enter the flow passage 17 and an outlet end 21 through which Printed from Mimosa WO 99/11363 PCT/AU98/00713 the slurry can leave the flow passage with oxygen mixed therein as will be explained later A mixing head 23 is mounted in the flow chamber 17 for delivering oxygen into the flow passage 17 The delivery head 23 comprises a body 25 which is 5 mounted on and integral with one end of a rigid delivery line 27 the other end of which is supported on a support plate 29 The delivery line 27 communicates with a source of oxygen under pressure by way of oxygen supply line 31 The mounting plate 29 is attached to a mounting flange 33 by way of mounting bolts 35 The mounting flange 33 surrounds an opening 37 formed in the cylindrical 10 side wall 14 of the housing 13, the opening 37 providing access to the interior of the housing for installation of the mixing head 23 and integral delivery line 27 The mixing head 23 is spaced from the outlet end 21 of the flow passage 17 so as to provide a mixing chamber 39 between the mixing head and the outlet end The body 25 of the mixing head 23 comprises a first body section 41 and a 15 second body section 42 The first body section 41 has a peripheral wall 43 which tapers outwardly in the direction of fluid flow through the flow passage 17 A first space 45 is defined in the region surrounded by the peripheral wall 43 and opens onto the flow passage 17 by way of an aperture 47 defined by the free end of the peripheral wall 43 A recess 49 is provided on the free end of the peripheral wall 20 43 to accommodate a first annular sealing ring 51 which presents a first sealing face 53 The second body section 42 is in the form of a circular plate 55 having a peripheral recess 57 which accommodates a second sealing ring 59 which presents a second seal face 61 The second seal face 61 is of complimentary 25 configuration to the first seal face 53 The circular plate 55 is moveable with respect to the first body section 41 between a closed condition in which the two seal faces 53, 61 are in sealing contact so as to close the aperture 47 and an open condition in which the two Printed from Mimosa WO 99/11363 PCT/AU98/00713 seal faces are in a spaced apart relationship to define a gap which provides an outlet opening 64 through which oxygen discharges from the inner space 45 within the body 23 into the flow passage 17 for mixing with the slurry. The size of the gap which provides the outlet opening 64 is regulated to control the size of 5 oxygen bubbles forming in the slurry Typically, the gap is about 0 3mm when the mixer is in normal operation The gap providing the outlet opening 64 extends entirely around the body so that oxygen is delivered to the slurry in a full 360° range. This ensures that oxygen is presented to the entire slurry stream The circular plate 55 is mounted on a support means 63 which regulates its movement between the open and closed conditions in response to the pressure of oxygen within the space 45 in the body 23. More particularly, the support means 63 is arranged to cause the circular plate 55 to assume the open condition and thereby establish the outlet opening 64 between the two seal faces 53, 61 15 when the oxygen pressure is at or exceeds a predetermined level The support means 63 is adapted to cause the circular plate 55 to move from the open condition to the closed condition and thereby bring the two seal faces 53, 61 into sealing engagement when the oxygen pressure falls below the predetermined level Closure of the circular plate 55 interrupts flow of oxygen from the mixing 20 head 23 into the slurry and also prevents entry of slurry into the head The support means 63 comprises a support shaft 65 slidably supported on a support structure 67 The support structure 67 is supported within the interior of body and incorporates ports 69 through which oxygen can flow.

A biasing means 71 in the form of a helical compression spring operates between 25 the support shaft 65 and the support structure 67 to bias the support shaft in an inward direction which corresponds to the closed condition of the circular plate 55 Means (not shown) are provided for adjusting the preload on the spring to regulate the fluid pressure at which the circular plate is caused to move from the closed condition to the open condition (le the predetermined fluid pressure).

Printed from Mimosa The circular plate 55 is attached to the outer end of the shaft 65 by way of a mounting bolt 73 which passes through a mounting aperture 75 within the plate and threadably engages in an axial hole (not shown) in the outer end of the shaft.

A locating pin 77 is attached to the shaft 65 by way of a lateral arm 79 The 5 locating pin 77 is slidingly received in one of the ports 69 and serves to guide sliding movement of the shaft 65 and restrain rotational movement thereof The tapered configuration of the body 25 cooperates with the inner face 81 of the cylindrical side wall 14 of the housing 11 to provide a constricted section 85 in the flow passage 17 about the outlet opening 64 through which oxygen is 10 introduced into the slurry. The constricted section 85 provides an area of lower fluid pressure in the slurry to assist the introduction of the oxygen The mixing chamber 39 which follows the constricted section 85 in the direction of fluid flow provides a zone for rapid expansion of the slurry which assists in the mixing action Because of the planar configuration of the two seal faces 53, 61 which establish the outlet opening 65 when the second body section 42 is in the open condition, the oxygen is introduced into the flow passage 17 at the restricted section 85 in a direction substantially normal to the direction of flow of the slurry. This causes shearing of the oxygen flow as it enters the slurry stream so forming bubbles in 20 the slurry stream which assists in the mixing process The mixing process is further assisted by an orifice plate 91 mounted on the housing 13 adjacent the outlet end 21. The orifice plate 91 incorporates a central orifice 93 which causes a further constriction of the fluid flow and thereafter provides for rapid expansion of the fluid flow to promote further mixing.

In operation, oxygen is delivered to the mixer head 25 under pressure. Provided that the pressure of the oxygen is at or above the predetermined level, the fluid pressure acts on the circular plate 55 causing it to move from the closed Printed from Mimosa condition into the open condition against the influence of the compression spring 71 When the circular plate 55 is in the open condition, the two seal faces 53 and 61 are in a spaced apart relationship to establish an outlet opening 64 therebetween through which oxygen can flow from the mixing head 23 into the 5 slurry stream flowing along the flow passage In the event of an interruption to the oxygen supply, which causes the oxygen pressure to drop below the predetermined level, the circular plate 55 is caused to move from the open condition into the closed position and so close the outlet opening As the oxygen stream enters the slurry stream in a direction normal to the 10 direction of slurry flow, the oxygen stream is sheared to create bubbles within the slurry stream, as mentioned previously Water or some other suitable liquid may be added to the oxygen stream prior to its entry into the slurry stream to enhance the shearing process and assist in creation of oxygen bubbles in the slurry.

In Figure 3 of the drawings, there is shown a mixing head 90 of a mixer according 15 to a second embodiment The mixing head 90 is similar to the mixing head 25 of the first embodiment, with the exception that it is fitted with a cowling 91 on the second body portion 42 The cowling 91 is configured to provide an extension to the constructed section 85 so as to control the rate of construction and subsequent expansion of the mixture of slurry and oxygen therein in the region 20 immediately beyond the outlet opening 64 The cowling 91 incorporates a wear-resistant shoulder 95 provided by a ceramic disc at the end of constriction which the cowling provides The third embodiment, which is shown in Figure 4 of the drawings, is directed to a mixer 100 having a housing 103 of elbow construction adapted to be installed in a 25 slurry flow line. The mixer 100 has a mixing head 105 of somewhat similar the constriction to the mixing head of the first embodiment, with the exception that it is formed in a sleeve 107 mounted on an end plate 109 attached to the housing .

The embodiment shown in Figure 5 of the drawings is directed to a mixing head 110 for introducing a gas such as oxygen at low pressure into another fluid such Printed from Mimosa as a pond of sewerage The mixing head 110 is of similar construction to the mixing head described in relation to the first embodiment, with the exception that there is provided a gas dispersing ring 111 through which the oxygen passes when the outlet opening is established The gas dispersing ring 111 allows the 5 introduction of gas (such as oxygen) at low pressure into the pond As with the first embodiment, if the gas supply is interrupted, the second body portion 42 is caused to move from the open condition to the closed condition thereby closing the outlet opening, and in doing so, retracting the gas dispersing ring 11 into the body 25 to seal it against contamination from the fluid contained within the pond.

In this embodiment, the outlet opening 64 is established between a tapering edge 113 on the first body section 41 and a flexible seal 115 on the second body section.

From the foregoing, it is evident that the present invention provides apparatus for mixing two fluids which is relatively simple in construction yet highly effective in 15 operation The various embodiments provide fluid mixing apparatus which overcomes some of the disadvantages of known lance-type injectors and high shear-type injectors, as described previously In particular, fluid mixing apparatus according to the embodiments result in higher entry fluid utilisation that lance-type injectors Consequently, such fluid mixing apparatus provide the 20 opportunity to reduce the consumption of the entry fluid injected into the primary liquid or slurry stream. Furthermore, fluid mixing apparatus according to the embodiments operate more effectively at lower pressures than high-shear-type injectors. Consequently, the fluid mixing apparatus consumes less power than high shear type injectors, thereby offering operating-cost savings without the loss 25 of mixing efficiency. Additionally, the construction of fluid mixing apparatus according to the embodiments minimises wear and blockages, and as a consequence a long service life, and low maintenance costs, can be expected.

It should be appreciated that the scope of the invention is not limited to the scope of the various embodiments described In particular, it should be understood that Printed from Mimosa the invention may have application in any suitable field where fluid mixing is required and that it is not limited in application to the embodiments described Other such applications include aeration of waste water streams, injection of air into biological oxidation reactors, and injection of carbon dioxide, chlorine, 5 ammonia, sulphur dioxide, hydrogen and hydrogen sulphide in chemical processing industries A still further application is the injection of oxygen or other oxidants to oxidise certain base metals during the hydrometallurgical processing of the base metals; for example, the oxidisation of Fe2+ to Fe3+ in the hydrometallurgical processing of copper concentrates or the oxidisation of cobalt 10 ahead of nickel solvent extraction.

Throughout this specification (including the claims if present), unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers Printed from Mimosa

Claims

f ' V _ . INTELLECTUAL PROPERTY OFFICE OF N.Z. - 14 The Claims Defining the Invention are as Follows - 9 JAN 2002 received

1. Apparatus for introducing a first fluid into a second fluid, the apparatus comprising a body defining a first space for receiving the first fluid under pressure, means providing an outlet opening in the body through which the 5 first fluid can discharge into the second fluid, said means being responsive to the first fluid whereby said outlet opening is established when the fluid pressure of the first fluid is at or exceeds a predetermined pressure and said outlet opening is sealingly closed when the fluid pressure of the first fluid is below said predetermined pressure to inhibit entry of said second fluid. 10

2. Apparatus according to claim 1 wherein said means providing an outlet opening comprises first and second sections moveable relative to each other between a first condition in which the first and second sections are in spaced apart relationship to define said outlet opening and a second condition in which the first and second sections are in sealing contact so as to close said 15 outlet opening.

3. Apparatus according to claim 2 wherein the first section is stationary and the second section is moveable with respect to the first section.

4. Apparatus according to claim 2 or 3 wherein a biasing means is provided for biasing the first and second sections into the second condition in which they 20 are in sealing engagement, the arrangement being such that fluid pressure at the predetermined level counteracts said biasing means to cause the outlet opening to be established.

5. Apparatus according to claims 2, 3 or 4 wherein the outlet opening comprises an opening extending about the periphery of the body. 25

6. Apparatus according to claim 5 wherein the opening extends entirely around the body. WO 99/11363 PCT/AU98/00713 -15-

7 Apparatus according to any one of the preceding claims wherein the body comprises a first body portion defining the first space and having an aperture through which the first fluid can flow, and a second body portion associated with the aperture and moveable between open and closed conditions in 5 relation to the aperture, wherein the second body portion sealingly closes the aperture against fluid flow therethrough when in the closed condition and wherein the outlet opening is defined between the first and second body portions when the second body portion is in the open condition.

8. Apparatus according to claim 7 wherein the first body portion is provided with a 10 seal face extending around the aperture and the second body portion is provided with a complimentary seal face, whereby the two seal faces are in sealing engagement when the second body portion is in the closed condition and whereby the two seal faces are in faced apart relationship to provide a gap therebetween defining the outlet opening when the second body portion is in 15 the open condition.

9 Apparatus according to claim 8 wherein each seal face is defined by a replaceable ceramic sealing ring mounted on the respective body portion.

10 Apparatus according to any one of claims 4 to 9 wherein the biasing means has a biasing action which is adjustable to allow selective regulation of the 20 predetermined fluid pressure at which the outlet opening is established.

11.Apparatus according to claim 10 wherein the biasing means comprises a compression spring, the spring tension of the compression spring being selectively adjustable for varying the fluid pressure at which the outlet opening is caused to be established 25

12 Apparatus according to any one of claims 7 to 10 wherein the second body portion is mounted on a shaft which is shdably mounted on a support structure accommodated within the space within the first body portion, sliding movement of the shaft with respect to the support structure allowing movement of the Printed from Mimosa WO 99/11363 PCT/AU98/00713 -16- second body portion between the open and closed conditions, and said biasing means acting between said support structure and the support shaft for biasing the second body portion into the closed condition.

13.Apparatus according to any one of the preceding claims wherein the body is 5 accommodated in a second space for containing the second fluid into which the first fluid is to be introduced.

14 Apparatus according to claim 13 wherein the second space is configured as a flow passage for flow of the second fluid in a flow direction, the outlet opening being disposed to introduce the first fluid into the second fluid in a direction 10 transverse to the direction of fluid flow of the first fluid

15 Apparatus according to claim 14 wherein the transverse direction at which the first fluid is introduced into the second fluid is generally normal to the direction of fluid flow of the second fluid thereby to cause shearing of the flow of the first fluid entering the second fluid 15

16 Apparatus according to any one of claims 8 to 15 wherein the plane of each of said seal faces is substantially normal to the direction of fluid flow of the second fluid

17.Apparatus according to any one of claims 7 to 17 wherein the first body portion is configured to have an external wall tapering outwardly towards the 20 outlet opening in said flow direction

18 Apparatus according to claim 17 wherein the flow passage for the second fluid is defined by a passage wall which surrounds and is in spaced apart relationship to the external wall of the first body portion, whereby cooperation between the passage wall and the tapering external wall of the first body 25 portion constricts flow of the second fluid in the direction towards the outlet opening and thereafter provides for rapid expansion of the mixed first and second fluids Printed from Mimosa INTELLECTUAL PROPERTY OFFICE OF N.Z. -17- - 9 JAN ?00? received

19. Apparatus according to claim 18 comprising a further constriction provided in the passage downstream of said body to further assist mixing of the two fluids.

20. Apparatus according to any one of the preceding claims further comprising a fluid dispersing means within the body through which the first fluid is dispersed when the outlet opening is open.

21. Apparatus according to claim 20 wherein the fluid dispersing means comprises a dispersing ring.

22. A mixer for mixing first and second fluids, the mixer comprising a flow passage, an inlet through which the second fluid can be introduced into the flow passage to flow therealong in a flow direction, an outlet through which the first and second fluids can leave the flow passage after mixing thereof, a body accommodated in the flow passage and past which the second fluid can flow along the flow passage, the body defining a first space for receiving the first fluid under pressure, means providing an outlet opening in the body through which the first fluid can discharge into the flow passage for mixing with the second fluid, said means being responsive to the first fluid whereby said outlet opening is established when the pressure of the first fluid is at or exceeds a predetermined pressure and said outlet opening is sealingly closed when the pressure of the first fluid is below said predetermined pressure to inhibit entry of said second fluid.

23.A mixer according to claim 22 wherein the outlet opening is configured to discharge the first fluid into the second fluid in a direction substantially normal to said flow direction

24. A mixer according to claim 22 or 23 wherein the body is located in the inner region of the flow passage and the outlet opening extends entirely around the periphery of the body. - 9 JAN ?00? received

25. A mixer according to any one of claims 22, 23 or 24 wherein the body has an external wall tapering outwardly in the flow direction towards the outlet opening.

26. A mixer according to claim 25 further comprising a constriction downstream of the body to further assist mixing of the two fluids.

27. Apparatus substantially as herein described with reference to the accompanying drawings.