This invention relates to a multi-stage fluid filter for filtering entrained particulate matter from a moving fluid stream. In particular, the application discloses a multi-stage fluid filter wherein enhanced filtration efficiency is achieved by sequentially redefining filtration zones as entrained particulate matter accumulates on filter media in certain of the filtration chambers.

Prior art patents disclose the concept of forming a filter medium of enhanced filtration capacity by using a bare filter surface and accumulating on the upstream side of the filter surface a layer of the particulate matter being filtered. It has been learned that such accumulated particulate matter forms a particularly efficient filter of that particular substance. However, the difficulty with such structures is that as the layer of particulate matter accumulates on the filter surface, the resistance to passage of fluid through the accumulating filter media increases as the filtration efficiency increases. At some point, the energy required to force the fluid through the accumulated layer of particulate matter is so great as to render the filter uneconomical both from a capacity and energy consumption standpoint. Prior art patents relating to the above concept (such as EP-A-0026069) utilize an endless drum having a filter screen thereon through which fluid is passed. Particulate matter in the fluid accumulates on the upstream side of the drum. The interior and/or exterior of the drum is defined into a prefilter zone in which the filter layer is accumulated to its optimum thickness. Air passing downstream of the filter layer in this zone is recirculated back through the filter to prevent the introduction of less than optimally

filtered fluid into the environment. Once the filter layer has reached its optimum thickness, it is passed into a second zone where fluid is passed through the layer of particulate matter for its primary filtration step. Fluid downstream in this zone is exited into the environment. A third doffing zone strips the accumulated particulate matter from the surface of the drum whereupon the bare drum surface moves back into the first filter zone. Such filters represent a large improvement over earlier filters which caused substantial variations in filtration efficiency and energy consumption because of the necessity to periodically strip the layer of particulate matter from the filtration surface. However, the rotating drum-type of filter must be very carefully sealed on its ends and at the line of separation between the various zones. Perfect sealing under such circumstances is difficult since the drum is rotating and a seal must be created against the underside of the rotating drum which is a porous surface.

EP-A-244938 discloses a multistage fluid filter having a plurality of filtration chambers arranged in line, and inflow and outflow conduits selectively communicable with selected chambers via doors in first and second ends of the chambers. Each chamber has opposed ends, each of which has two doors, to close openings which communicate with the inflow or outflow conduits. Sequencing means, comprising for example solenoids and related hardware is employed to control the fluid flow route within the filter. At any given moment a selected first filtration chamber may be acting as first filter stage for accumulating particulate matter on the filtration medium thereof and a selected second filtration chamber may be acting as a second filter stage having a previously accumulated layer of particulate matter on the filtration medium thereof. The apparatus of EP-A-244938

represents a good solution to the sealing problem described above but is a somewhat cumbersome apparatus requiring a relatively complicated control system.

According to a first aspect of the present invention there is provided a multi-stage fluid filter for filtering entrained particulate matter from a moving fluid stream, within which filter particulate matter may undergo primary and secondary filtration stages, the secondary filtration stage being through a filter medium which in use carries a layer of accumulated particulate matter, the first filtration stage being through a filter medium on which in use a layer of particulate matter accumulates, for use subsequently in a said second filtration stage, the filter comprising: first and second filtration chambers, each filtration chamber having a filter medium positioned therein from an upstream side to a downstream side thereof; an inlet into the fluid filter; an outlet from the fluid filter; first conduit means leading from said inlet; second conduit means leading from the first conduit means to the first chamber on the upstream side relative to the filter medium thereof; third conduit means leading from the first conduit means to the second chamber on the upstream side relative to the filter medium thereof; a node at which the first, second and third conduit means meet; valve means associated with the node and having a first control position in which it permits fluid to flow from the first conduit means to the second conduit means, and a second control position in which it permits fluid to flow from the first conduit means to the third conduit means;

and means for directing fluid which has flowed through the first or second chamber in a primary filtration stage, to the second or first chamber for a secondary filtration stage, and then to the outlet.

Suitably the conduit means for directing fluid which has flowed through the first or second chamber in a primary filtration stage, to the second or first chamber for a secondary filtration stage, and then to the outlet, comprises a fourth conduit means which meets the first, second and third conduit means at the node, and through which fluid may be conveyed toward the chamber providing the secondary filtration stage, wherein said first control position of the valve means permits fluid to flow from the first conduit means to the second conduit means and, simultaneously, from the fourth conduit means to the third conduit means, and wherein said second control position of the valve means permits fluid to flow from the first conduit mean to the third conduit means and, simultaneously, from the fourth conduit means to the second conduit means.

According to a second aspect of the present invention there is provided a multi-stage fluid filter for filtering entrained particulate matter from a moving fluid stream, in which filter particulate matter may undergo primary and secondary filtration stages, the secondary filtration stage being through a filter medium which in use carries a layer of accumulated particulate matter, the first filtration stage being through a filter medium on which in use a layer of particulate matter accumulates, for use subsequently in a said second filtration stage, the filter comprising:

first and second filtration chambers, each filtration chamber having a filter medium positioned therein from an upstream side to a downstream side thereof; an inlet into the fluid filter; an outlet from the fluid filter; conduit means "one" leading to said outlet; conduit means "two" leading from the first chamber on the downstream side relative to the filter medium thereof, to the conduit means "one"; conduit means "three" leading from the second chamber on the downstream side relative to the filter medium thereof, to conduit means "one"; a node at which conduit means "one", "two" and "three" meet; valve means associated with that node and having a first control position which permits fluid to flow from conduit means "three" to conduit means "one", and a second control position in which it permits fluid to flow from conduit means "two" to conduit means "one"; and means for directing fluid which has entered the filter through the inlet, selectably through the first or second chamber in a primary filtration stage, then to the second or first chamber in a secondary filtration stage, then to said node at which conduits "one", "two" and "three" meet.

Suitably the means for directing fluid which has entered the filter through the inlet selectably through the first or second chamber in a primary filtration stage, then to the second or first chamber in a secondary filtration stage, then to said node at which conduits "one", "two" and "three" meet, comprises conduit means "four" which meets conduit means "one", "two" and "three" at the node, and through which fluid may be conveyed from the chamber providing the primary filtration stage, wherein said first control position of the valve means

permits fluid to flow from conduit means "two" to conduit means "four" and, simultaneously, from conduit means "three" to conduit means "one", and wherein said second control position of the valve means permits fluid to flow from conduit means "two" to conduit means "one" and, simultaneously from conduit means "three" to conduit means "four".

Preferably, both the two previous aspects may be used together, whereby the filter has two nodes, one adjacent the inlet and one adjacent the outlet. Preferably, both nodes are four-branch nodes and said fourth conduit means corresponds to said conduit means "four". Said fourth conduit means thus extends between the nodes to provide the route along which fluid passes, between its primary filtration stage and its secondary filtration stage.

Therefore, according to a third aspect of the present invention there is provided a multi-stage fluid filter for filtering entrained particulate matter from a moving fluid stream, the filter comprising: first and second filtration chambers, each filtration chamber having a filter medium positioned therein from an upstream side to a downstream side thereof; an inlet into the fluid filter; an outlet from the fluid filter; first conduit means leading from said inlet; second conduit means leading from the first conduit means to the first chamber on the upstream side relative to the filter medium thereof; third conduit means leading from the first conduit means to the second chamber on the upstream side relative to the filter medium thereof; fourth conduit means;

a first node at which the first, second, third and fourth conduit means meet; first valve means associated with the first node and having a first control position in which it permits fluid to flow from the first conduit means to the second conduit means, and, simultaneously, from the fourth conduit means to the third conduit means, and a second control position on which it permits fluid to flow from the first conduit means to the third conduit means, and, simultaneously, from the fourth conduit means to the second conduit means; fifth conduit means (corresponding to conduit means "one" of the second aspect of the present invention) leading to said outlet; sixth conduit means (corresponding to conduit means "two" of the second aspect of the present invention) leading from the first chamber on the downstream side relative to the filter medium thereof, to the fifth conduit means; seventh conduit means (corresponding to conduit means "three" of the second aspect of the present invention) leading from the second chamber on the downstream side relative to the filter medium thereof, to the fifth conduit means; a second node at which the fourth, fifth, sixth and seventh conduit means meet; and second valve means associated with the second node and having a first control position which permits fluid to flow from the sixth conduit means to the fourth conduit means, and, simultaneously, from the seventh conduit means to the fifth conduit means, and a second control position in which it permits fluid to flow from the sixth conduit means to the fifth conduit means, and, simultaneously, from the seventh conduit means to the fourth conduit means.

Thus, when both valve means are in their first control positions, fluid to be filtered passes from the first conduit means into the second conduit means, through the first chamber and into the sixth conduit means. At the second node it is diverted by the second valve means into the fourth conduit means. It then passes back to the first node, where the first valve means diverts it into the second conduit means. It thus enters the second chamber where it undergoes the second filtration stage and leaves the second chamber along the seventh conduit means. At the second node the second valve means diverts it into the fifth conduit means, leading to the outlet.

When both valve means are in their second control positions, the converse of the previous paragraph applies, fluid undergoing its first filter stage in the second chamber, then being conveyed to the first chamber, for the second filter stage.

In certain embodiments the two valve means may be so arranged that they must both be in their first control positions, or both be in their second control positions. This may be achieved by mechanical or electro-mechanical interlinking means. In such embodiments two stage filtration always occurs, and cleaning is effected whilst the filter is not operational.

In other embodiments it is desirable to effect cleaning of one chamber during operation, the other chamber being the sole filtering chamber during the cleaning phase. The first chamber is taken out of filtering operation when the first valve means is in its second control position and when the second valve means is in its first control position. Conversely, the second chamber is taken out of filtering operation when the first

valve means is in its first control position and when the second valve means is in its second control position.

The aspects of the present invention presently being described, employing one or more nodes, and one or more valves associated therewith, may be applied to systems, as described above, having three or more chambers, with provision for any one chamber being cleaned whilst the others provide continuous two-stage filtration. However it may also be applied, with particular benefit, to "tandem" systems in which two-stage filtration is the normal operative condition, and cleaning is carried out either by filter shut-down or by taking one chamber out of filtration operation, whereby one-stage filtration takes place whilst that chamber is being cleaned.

Preferably a filter medium comprises a rigid filter body, of a material inert to the fluid undergoing filtration, and to the particulate matter, for example a stainless steel or plastics mesh, and can support a relatively thick matt of particulate layer, if necessary. Thus, the filter medium preferably does not comprise a flexible paper, card or textile filter body.

Suitably, a valve means associated with a node is a butterfly valve, having a valve plate movable about a medial axis. The first and second control positions of a said butterfly valve are suitably the respective diagonal positions of the valve plate, across the respective node.

Desirably, in all applications other than low stringency applications, a said valve means provides an effective seal, without fluid leakage across the valve, whereby only the fluid flow modes defined above are permitted. Sealing may be provided by mechanical contact,

for example by means of elastomeric or other polymeric sealing means along the contact regions of the valve member of the valve means, and its seating. However, in particularly harsh environments abrasion, leading to seal deterioration, may take place therebetween. A preferred valve means for use in relation to the present invention may maintain a seal by emission of a gas in the region in which the valve member and its seating are juxtaposed. Such a valve means is not only useful in the context of the embodiments described above, having a node and valve means associated therewith, but is thought to be novel and may be of wider application.

Therefore in accordance with a further invention there is provided a valve means having a valve member and a seating therefor, the valve means having passage means for the emission of a gas in the region in which the valve member and the seating are juxtaposed, whereby a seal may be substantially maintained therebetween.

Whilst the passage means could be such as to emit gas from the seating, for example being provided in housing means which defines the seating, the passage means is preferably such as to emit gas from the valve member.

Preferably, in a valve means in accordance with this aspect, the valve member and its seating do not mechanically contact each other, but have a gap therebetween, and gas is emitted in the region of this gap, to substantially maintain the seal. In other embodiments the valve member and its seating can mechanically contact each other, but only lightly, so that the seal is effectively maintained by both the mechanical contact and by the gas emission. In other embodiments the valve member and its seating can mechanically contact each

other so that they alone provide a seal, the valve means having passage means as described above, arranged to become operative, to provide a seal, only when the mechanical seal fails. For example the valve means may be constructed so that the outlet(s) of the passage means are normally blocked by the mechanical seal, so that if the mechanical seal is good gas cannot egress therefrom, but if the mechanical seal fails, gas can egress therefrom, to maintain the seal. In such embodiments, means may be provided for sensing that condition, for example by detection means arranged to detect flow of the gas upstream of, or in, or downstream of, the passage means, whereby repair of the mechanical seal may be effected, when convenient.

The valve means could be one of a number of types. For example it could be a rotary valve, a gate valve, a slide valve or a butterfly valve. Preferably, the valve means is a butterfly valve.

Considering now a filter in accordance with earlier aspects, having one or more nodes and one or more valve means associated therewith, when a valve means is a butterfly valve, as is preferred, a preferred butterfly valve for use in relation to such aspects maintains a seal not by mechanical contact between the edges of the valve means and the node, but by emission of a gas, as described above, preferably from the edges of the valve plate. Such a valve is not only useful in the context of the filter embodiments described above, but is thought to be novel and can be expected to be of wider application.

Therefore, in accordance with a further invention there is provided a butterfly valve having:

a valve plate movable about a medial axis, and having edges past which a fluid whose flow is being controlled by the valve plate is substantially unable to pass, in use, when the valve plate is in an operative position; means defined within the valve plate for the passage of a gas; and gas outlet means associated with said edges, said gas outlet means communicating with said means defined within the valve plate for the passage of a gas.

In operation, the gas used is conveniently air, and is fed through the valve plate to the gas outlet means at a sufficient rate to maintain an effective non-mechanical seal. There is a small gap between the edges of the valve plate and the housing therefor. This gap is preferably no more than 4mm in width, preferably no more than 2mm. The rate at which the gas needs to be fed to achieve this will depend on the flow conditions of the fluid, the flow of which is being controlled by the valve.

Preferably, the axis of the valve plate, about which axis the valve plate may turn, is provided with conduit means which communicates with said means defined within the valve plate for the passage of a gas. In use, the gas may be fed to the valve plate through such conduit means.

In accordance with a further invention there is provided a method of filtering particulate matter and an undesired gaseous material from a moving fluid stream in a multi-stage fluid filter in which particulate matter may undergo primary and secondary filtration stages, the secondary filtration stage being through a filter medium which carries a layer of accumulated particulate matter, the first filtration stage being through a filter medium on which a layer of particulate matter is accumulating.

for use subsequently in a said second filtration stage, wherein the layer of accumulated particulate matter which is employed in the second filtration stage incorporates a particulate reactant material which reacts with the undesired gaseous material.

Thus, in the method of this embodiment the filter will offer a matt or cake of accumulated particulate matter to the fluid stream at all times.

Suitably, the particulate reactant material is fed into the fluid to be filtered such that it accumulates with the accumulating layer of particulate matter on the first filter medium. It may be fed into the fluid continuously, whereby it is distributed evenly throughout the layer which accumulates, or it may be fed into the fluid discontinuously, in one or more batches, so that the layer comprises a stratum or strata preferentially rich in the reactant material.

Preferably a filter medium comprises a rigid filter body, of a material inert to the fluid undergoing filtration and to the particulate material, for example a stainless steel or plastics mesh, and can support a relatively thick matt of particulate layer, if necessary. Thus, the filter medium preferably does not comprise a flexible paper, card or textile filter body.

The particulate reactant material may have similar mesh/size characteristics to said particulate matter to be filtered. Thus, the mean diameter of the particulate reactant material may be within 25% of the mean diameter of said particulate matter to be filtered. In alternative embodiments the particulate reactant material may be of considerably smaller mesh/size than said particulate

matter to be filtered, so as to locate in interstices therebetween in the layer which accumulates.

The particulate reactant material may react with said gaseous material without producing a different gaseous material. Alternatively it may react with said gaseous material to produce a different gaseous material, which is less undesirable.

One example of the method of this aspect is the incorporation of particulate sodium bicarbonate in a filter medium of a filter used to filter exhaust from a power station, to reduce sulphur dioxide emission.

Another example of the method of this aspect is the incorporation of particulate activated charcoal in a filtration medium of a filter, to reduce the emission of malodorous vapours.

A said fluid which may be filtered by a filter as described herein is preferably a gas. Most commonly it is air.

It will be appreciated that the method of this aspect may be used in conjunction with any other method or any apparatus described herein. Any such combination constitutes a further aspect of the present invention. Also, the method of this aspect may be used with other multi-stage apparatus or methods, for example prior art apparatus or methods, such as those described in EP-A- 0026069, FR-A-2200040 and EP-A-244938.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:

Figures 1 to 4 are schematic views of a filter, employing butterfly valves at nodes between conduits, in sequential stages of operation;

Figure 5 shows a filter having mechanically interlinked butterfly valves;

Figure 6 shows a butterfly valve which may be used in the embodiment of Figures 1 to 4, in cross-section;

Figure 7 shows the butterfly valve of Figure 5, in position at a node between conduits;

Figure 8 shows, schematically, a layer of particulate matter which acts as a filter medium, containing an even distribution of a chemical decontaminant material;

and Figure 9 shows, schematically, layer of particulate matter which acts as a filter medium, containing a layer of particulate matter with a discrete stratum rich in a chemical decontaminant material.

Figures 1 to 4, show a filter with first and second stationary filter chambers 100 and 102. The fluid stream to be filtered is controlled by two butterfly valves 104, 106, set at respective nodes 108, 110 between conduits.

Referring to Figures 1 to 4 in greater detail, the stream, typically air, to be filtered enters a first conduit 112 through an inlet 114, flow being caused either by upstream pressurisation or downstream suction. The conduit leads to the node 108, a four-branch node on the inlet side of the filter. A second conduit 116 from the node 108 communicates with the upstream side of the chamber 100, and a third conduit 118 from the node 108

communicates with the upstream side of the chamber 102. A fourth conduit 120 from the node 108 communicates with the second node 110, a four-branch node on the outlet side of the filter. A fifth conduit 121 leads from the second node 110 to an outlet 128. A sixth conduit 122, leading to the node 110, communicates with the downstream side of the chamber 100, and a seventh conduit 124 leads from the node 110 to the downstream side of the chamber 102. The conduits 112, 116, 118 and 120 of node 108 are orthogonally arranged, as are the conduits 120, 121, 122 and 124 of node 110.

Each butterfly valve 104, 106 has two operative positions, across the two diagonals of the respective node 108, 110. The two operative positions for butterfly valve 104 can be seen in Figures l and 3. The two working positions for butterfly valve 106 can be seen in Figures 2 and 4. Moreover each valve is movable independently of the other in this embodiment, so that there are four possible combinations of valve positions. These four combinations are shown in Figures 1 to 4.

Referring now to Figure l, the butterfly valve 104 on the inlet side of the filter is arranged so that air to be filtered flows from first conduit 112, leading from inlet 114, into conduit 116, leading to the upstream side of chamber 100. Chamber 100 is thus defined as a first filter stage, represented by Roman Numeral I. Air which has passed through filter mesh 130 of the chamber 100, and through the layer of particulate matter building up on the filter mesh 130, leaves chamber 100 along conduit 122, to the second node 110. The butterfly valve 106 of node 110 is arranged so that air flows along conduit 120 back to first node 108, where butterfly valve 104 diverts it to conduit 118, leading to chamber 102, which is thus defined

as a second filter stage, represented by Roman Numeral II. Filtered air leaves the chamber 102 along conduit 124, and at node 110 is diverted by butterfly valve 106 into conduit 121, leading to filter outlet 128. Referring now to Figure 2, at the point during the filtration process when the layer of particulate matter on the filtration medium 132 in the filtration chamber 102 (filter stage II) reaches a thickness and/or density where air passage is reduced by more than a given amount, filtration chamber 102 is redefined as a cleaning stage (filter stage III) by switching over butterfly valve 106, whereby air, having been filtered in chamber 100, is diverted to conduit 121 leading to filter outlet 128, without undergoing a second filtration stage. In many environments this is arranged to occur when the pressure drop across the filter is about 4" (102mm) water gauge. The chamber 102 may be cleaned by opening a lower cleaning door (not shown) , above a removable container (not shown) . The layer of particulate matter is removed from the filtration medium 132 by means of a mechanical shaker, or mechanical rapping, or by a compressed air pulse, leaving the bare stainless steel wire mesh filter surface.

Referring now to Figure 3, the chamber 102 has been brought back into operation and two-stage filtration is once again taking place. In many environments the pressure drop across the filter is now about 2" (51mm) water gauge. Relative to Figure 2, the butterfly valve 104 has been switched over, so that the chamber 102 is defined as the first stage filter, in which the layer of particulate matter is built. By virtue of the position of the butterfly valve 106 the chamber 100 is now defined as the second stage filter.

Referring now to Figure 4, this shows the chamber 100 in its cleaning stage, the butterfly valve 106 having been switched over to divert air filtered in chamber 102 into conduit 121 leading to outlet 128.

It will be observed that air flow in the embodiment of Figures 1 to 4 is controlled by two moving parts, the butterfly valves. (In contrast, in the prior specification EP-A-244938, the corresponding two-chamber embodiment shown in Figures 12 to 15 requires eight moving parts, the doors to the chambers) . The position of the butterfly valves may be controlled in standard manner, for example by means of electric solenoids or pneumatic cylinders, or by simple manual means. The position of valves may be altered, automatically or manually, in response to sensed pressure conditions, or following lapse of a given period of time, or at the start of a new shift.

The embodiment of Figures 1 to 4 permits continuous filtration. For most of the time two-stage filtration occurs, and only for short periods of time, when one chamber is being cleaned, does one-stage filtration occur.

However there are circumstances in which continuous filtration is not needed, for example because of regular machine shut-down, and in which cleaning does not need to be effected during operation. In such circumstances, the butterfly valves are only required to adopt two combinations of positions, those shown in Figures 1 and 3, that is, the positions in which they are parallel to each other. To this end, the valves may be electrically or mechanically interlinked, so that only those positions are possible.

Some particulate materials, for example hygroscopic materials, may require removal by means of water, steam or

an organic solvent. It may be desirable to ensure that the cleaned chamber is dry before bringing it back into operation, by waiting a sufficient time for a cleaning liquid to evaporate, under natural evaporation or assisted evaporation. Assisted evaporation may involve an air flow and/or the application of heat, for example by providing warm air. For example, after a cleaning phase (Figure 4) , the system could be switched to its previous phase (Figure 3) , for drying the cleaned chamber with the air which has been filtered by the other chamber, and then switched over to two-stage filtration (Figure 1) . Alternatively or additionally, evaporation can be assisted by passing a warm gas through a chamber to be dried via an input conduit and an output conduit additional to those already described, and shown in Figures 1 to 4.

Conveniently, a gas can be warmed, in order to assist evaporation, by external heating of a conduit upstream of the chamber to be dried.

Figure 5 shows a filter similar to that shown in Figures 1 to 4, but in which the butterfly valves shown in hidden detail as 140 and 142 can only adopt the parallel configurations shown in Figures 1 and 3. To this end they are mechanically interlinked by a push rod 144 which has a central gripping handle 146. At its top end the push rod is pivoted to one end of a lever 148, the other end of which is secured to the butterfly valve 140 about its medial pivot axis 150, for common movement therewith. At its lower end the push rod is pivoted to one end of a lever 152, the other end of which is secured to the butterfly valve 142 about its medial pivot axis 154, for common movement therewith.

With the embodiment of Figure 5, at the end of a working shift and if a pressure gauge (not shown) shows it to be necessary, the chamber which was acting as the second filter stage (II) is cleaned, following removal of a lower collection container 156 or 158. These are connected to the lower regions of the respective chambers by means of quick release clamps, and can be removed for cleaning whenever necessary. After cleaning, the push rod is pushed to its other working position, so that the cleaned chamber next acts as the first stage filter, and the other chamber acts as the second stage filter.

Figures 6 and 7 show a butterfly valve which may be used in the embodiments of Figures 1 to 5, but which may also find application in other, unrelated, fields. With reference to Figure 6, the butterfly valve is located at a node 162 between the four conduits, arranged orthogonally. The valve plate 160 contains air passages 163, 164, fed by a common feed conduit 165 which is co- axial with the medial pivot axis of the valve. The narrow air passages extend to all edges of the valve plate 160, evenness of flow and of rate of emission of air, to all sides of the valve plate being assisted by means of distribution baffles 166, 167 indicated in dotted line in Figure 7.

It will be observed in Figures 6 and 7 that the edges of the valve plate 160 are located adjacent to, but do not touch, the internal walls, namely quadrant walls 167A and side walls 168, of the node. There is a gap therebetween of Vie" (1.6mm) . However despite not touching these walls, a good seal is obtained by means of air flow through the narrow air passages fed by the feed conduit 165. The air leaves the narrow air passages at the edges of the valve plate, closely adjacent to the internal walls of the node.

and is forced to both sides of the plane of the valve plate, by the adjacent walls 167A, 168 of the node, as indicated by the arrows in Figure 6. The air flow is sufficient to prevent passage past the valve of the air whose flow is being controlled by the valve.

Figures 8 and 9 relate to the entrainment, within the matt or cake 170 of particulate matter built up on a rigid stainless steel woven filter mesh 172, of a particulate chemical reactant/decontaminant material able to react with an undesirable gas carried by a fluid stream being filtered. In this embodiment the reactant material is sodium bicarbonate, and the undesirable gas is sulphur dioxide (S0 ₂ ) , carried by a flue gas stream. The fluid which passes through the filter medium is of reduced sulphur dioxide content, because of the presence of sodium bicarbonate.

In the embodiment of Figure 8 the sodium bicarbonate, in particulate form, ground to substantially correspond in fineness to the particulate matter to be filtered, is steadily fed into the stream to be filtered so that the matt contains an even distribution of sodium bicarbonate. In the embodiment of Figure 9 it is fed into the stream to be filtered over a relatively short period shortly after the matt has started to build up, to form a discrete stratum 174 which is rich in sodium bicarbonate.

It will be appreciated that the use of a reactant material in the formation of a matt which is to act as a filter medium can be used with any of the embodiments described or defined herein, in which there are at least two filtration chambers arranged in series with means for selectively redefining the filter stages thereof; and also

in any other apparatus in which the matt itself is intended to act as a filter medium.

It should be noted that various aspects and embodiments referred to herein, in the Claims and elsewhere, may be independent of each other, but may also be used advantageously in combination, if desired. Further aspects of the invention are constituted by combinations of aspects or embodiments described or defined herein.