GB1561978A - Filtration - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO FILTRATION (71) We, GOBLIN (B.V.C.) LIMITED, a British Company, of Leatherhead, Surrey, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to filtration, and more particularly relates to a filter unit containing one or more filter elements, a filter assembly formed from such filter units.

Recent legislation requires that equipment used by personnel in the course of their employment shall be made safe and without hazard to health. Various threshold limit values are clearly defined and these values indicate the safe limits of concentration of contaminants in the air such as toxic chemicals, nuisance dusts, or inert gas residuals to which personnel may be exposed.

Thus there is a need for filtration equipment adapted to filter such contaminants from the air.

Welding fumes, for example, are a hazard of the type falling within the scope of the legislation, and one difficulty experienced in connection with the filtration of welding fumes is that the nature of the fumes generated depends, to a large extent, upon the precise nature of the welding operation being performed, and upon the nature of the metal or metals being welded. Thus if a filter is provided suitable for use with one type of welding process, that filter may prove to be inadequate with an alternative welding process. Where many different types of welding process are carried out it may well prove necessary to provide several separate filtration units, each unit being intended for use with a respective one of the various types of welding process.

According to a first aspect of this invention there is provided a filter unit for use in a filter assembly of one or more such filter units, said filter unit comprising a canister having a first wall, a first aperture or set of apertures and a second aperture or set of apertures in said first wall, the said canister being adapted to be mounted in end to end relationship with adjacent such canisters with both said apertures or sets of apertures of one canister of an adjacent pair of such canisters in communication with the interior of the other canister of tbe said pair, said filter unit further comprising one or more filter elements, said filter element or elements being mounted within said firstmentioned canister so as to divide said first aperture or set of apertures from said second aperture or set of apertures such that in use of the filter unit a gaseous stream pa through the first-mentioned canister flows either through said first aperture or set of apertures or alternatively through said second aperture or set of apertures and is filtered by said filter element.

Conveniently the canister may be cylindrical and the base of the canister may comprise an upper peripheral portion and a depressed central portion, one of said apertures or sets of apertures being provided in said upper portion and the other of said apertures or sets of apertures being provided in said depressed portion.

-Advantageously each canister may be provided with an annular sealing ring mounted in an upper open end of the canister by means of a sealing member.

Conveniently each canister may be provided with location pegs provided in the upper portion of the base thereof for locating filter elements in position within the canister.

Conveniently the filter elements may be radial flow, annular, filter elements having upwardly and downwardly extending resilient flanges engaging with the canister and with a sealing ring mounted in the upper part of the canister, or alternatively the filter elements may be axial flow disc shaped or cylindrical filter elements provided with peripheral cylindrical sealing members having upwardly and downwardly extending flanges, ensuring axial air flow through.

the filter media, and in such an embodiment of the invention advantageously a plurality of filter elements are provided within the canister.

According to a further aspect of this invention there is provided a filter unit, for use in a filter assembly of one or more filter units, and comprising a canister adapted to be mounted in end to end relationship and in direct internal communication with adjacent such canisters so as to provide a continuous flow path therethrough in succession, the canister being capable of operatively receiving at least one filter element both when the or each said filter element is of the axial flow, disc or cvlindrical type and when the or each said filter element is of the radial flow annular type, the filter unit further comprising at least one such filter element so mounted within said firstmentioned canister in sealing engagement therewith and the filter unit being so constructed that whether the or each said filter element is of the axial flow, disc or cylindrical type or of the radial flow, annular, type, in use of the filter unit, a gaseous stream passing through the canister is caused to pass through the filter element or filter elements.

A filter assembly may be built up comprising a filter unit as previously defined and further comprising a lowermost canister which has an outlet duct in one side wall thereof, the outlet duct being connectable to a motor driven impeller. Conveniently the uppermost canister may be provided with a cap provided with an intake duct to which an intake hose may be secured.

In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described bv way of example with reference to the accompanying drawings in which: Figure 1 is a vertical cross-section through a first embodiment of a filter assembly in accordance with the invention and Figure 2 is a vertical cross-section through a second embodiment of a filter assembly in accordance with the invention.

A first embdiment of a filter assembly in accordance with the present invention comprises a trolley 1 on which is mounted means for generating a vacuum such as an electric motor and an associated impeller (not shown). Since the precise nature of the motor and the impeller do not form any part of this invention they will not be described in great detail.

Clams 2 are provided on the trolley 1 for securing to the trolley a filter unit comprising a cylindrical canister 3 formed of sheet metal containing a radial flow, annular filter element 4. The canister comnrises a cylindrical side wall 5 and a base, the base having a depressed central portion 6 and a slightly raised peripheral portion 7. The side wall 5 of the canister 3 extends to a point lower than the depressed central portion 6 of the base of the canister and a sealing ring 8 is provided located adjacent this portion 9 of the side wall which extends from the peripheral portion of the base of the canister 3 to the trolley. A first set of apertures 10 are formed in the base of the canister adjacent the sealing ring 8, and a second set of apertures 11 are formed adjacent the periphery of the depressed portion of the base 6.

Provided in one side wall of the canister 3 is an outlet duct 12 which is connected to the motor and impeller assembly. The outlet duct 12 may incorporate a valve (not shown) for the insertion of test probes to facilitate the detection of any toxic gases which have escaped filtration.

The anunular filter 4 within the canister 3 is provided with two end caps 13a, 13b having resilient axially extending flanges 14a, 14b. The arrangement is such that one axially extending flange 14b provided on the lower end cap 13b contacts the depressed portion 6 of the base of the canister inwardly of the apertures 11 formed in said depressed portion 6 and the other axially extending flange 14a provided on the upper end cap 13a of the filter contacts an annular sealing ring 15 which is mounted on the top of the side wall 5 of the canister 3 by means of a resilient sealing member 16.

Mounted on top of this first canister 3 is another canister 17 the lower portion of which is precisely the same as the lower portion of the first described canister there being a depressed central portion 61, a slightly raised peripheral portion 71 and a peripheral portion 91 which extends from the peripheral portion of the base of the canister 17, first and second sets of apertures 10l, 111 being also provided in the base of the canister 17. The canisters srp formed from sheet metal and are provided with snap locks or toggle clips 18 to secure the canisters together and the canisters 3, 17 are designed so that they may be mounted one on top of the other. The arrangement is such that if air flows through either the holes 101 provided adjacent the side wall of the second canister or the holes 111 provided in the depressed portion 6l of the base of the second canister 17, that air will flow through the annular filter 4 before passing through the outlet duct 12.

The arrangement is also such that if the filter 4 present in the lower canister 3 were inadvertently inverted it would not be possible to locate the second canister 17 in position since the downwardly protruding flange 14b adapted to engage with the lower part 6 of the base of the first canister 3 would engage with the lower part 61 of the base of the second canister 17 and would pre vent the canisters from being secured together.

The second canister 17 contains a stack of axial flow disc shaped filter elements 19 each of which has a cylindrical outer sealing member 20 provided with upwardly and downwardly extending flanges 21a, 21b. The downwardly extending flange 21b provided on the lowermost sealing member 20 engages with the upper portion 71 of the base of the second canister 17 radially inwardly of the apertures 101 provided in that portion of the base of the canister. The flanges 21a, 21b provided on the various filter elements 19 interengage and the upwardly directed flange 21a on the uppermost disc shaped filter element engages with a further annular sealing member 151 similar to the sealing member 15 provided in the first canister 3.

This annular sealing member 151 is mounted on the top of the top wall of the second canister by means of a resilient sealing member 161.

A third canister 23 is provided on top of the second canister and again it will be appreciated that air passing through any of the holes provided in the base of the third canister would be directed by means of the annular sealing ring 151 to flow through the filter elements 19 contained within the second canister 17 and would then flow out through the apertures provided in the depressed portion 61 of the base of the second canister.

The third canister 23 having a base similar to that of the first two canisters is provided with two radial flow, annular filter elements 24, 25 therein and is also provided with a cap 26 of sheet metal which has a central intake nozzle 27 provided therein.

The cap 26 is slightly dome shaped and a first annular filter 25 is provided within the third and uppermost canister 23, this filter being mounted above the depressed portion of the base of the third canister. The filter element 25 is provided with upwardly and downwardly directed flanges 28a, 28b of resilient material adapted to seal when the cap is placed in position, and the flanges 28b provided on the lowermost portion of the annular filter 25 engage the base of the canister 23 radially inwardly of the apertures provided in the lowermost portion of the base. A circular metal barrier 29 is provided which extends across the aperture through the annular filter 25 adjacent the base of the third canister 23 and this barrier 29 has three purposes. Firstly it prevents air directed into the cavitv contained within the centre of the annular filter 25 from flowing directlv through the anertures formed in the lowermost part of the base of the third canister. secondlv should the filter be in serted in the canister upside down the lowermost portion of the intake nozzle 27 will engage with the barrier thus preventing the cap 26 from being secured in position, and thirdly such barrier provides a heat proof collection tray, for hot particulate materials from the welding process.

A second annular filter 24 is provided which is located outside and surrounding the first cylindrical filter 25, and this second filter 24 has upwardly and downwardly extending flanges 30a, 30b which engage with an annular sealing ring similar to the annular sealing rings described hereinbefore and the peripheral part of the base of the third canister. The arrangement is such that air will flow through the intake 27, radially through the first annular filter 25, and then radially through the second annular filter 24 and will finally flow through the apertures provided in the uppermost part of the base of the third canister 23, downwardly into the second canister 17. This second filter 24 similarly is provided with a metal barrier as described in connection with the first filter 25 above. Thus, should the user find it convenient to dispense with the first filter then the metal barrier in the second filter will perform the function which would otherwise have been performed by the metal barrier 29 of the first filter 25.

It will be appreciated that all the canisters utilised in the particularly described filtration system have a very similar interconnecting means and may contain many different types of filter at will. Furthermore the canisters can be assembled in any desired order and the filter elements contained within the canister or canisters may be adopted selectively to filter many noxious or undesirable components from the air. Whilst three canisters have been described in use it will be appreciated that a stack of more than three canisters could be provided if so desired, or alternatively only one canister need be provided if the conditions require only one canister to be utilised. The canisters are provided with pegs 32 therein for locating the filters that are to be introduced into the canisters, the pegs being provided in the upper peripheral portion of the base of each canister. It will also be appreciated that standardising on the form of the canister will minimise manufacturing costs. Each canister 3, 17, 23 is provided with an aperture 34a, 34b, 34c to accommodate the outlet duct 12. The apertures which do not receive the outlet duct 12 may be sealed, if necessary, with a resilient cap 35.

A second embodiment of a filter assembly in accordance with the present invention also comprises a trolley having means mounted thereon for generating a vacuum.

As with the first embodiment the precise nature of the vacuum generating means will not be described in great detail. Secured to the trolley is a cylindrical canister 40 comprising a base portion 41, formed as an aluminium casting, a cylindrical side wall 42 formed of sheet metal and an upper portion 43 formed as an aluminium casting.

The base portion 41 comprises a lower cylindrical wall 44, the top edge of which is provided with a radially outwardly and upwardly extending wall portion 45 which terminates at an upper cylindrical wall portion 46 the diameter of which is greater than the diameter of said lower cylindrical wall 44. The lower end of the lower cylindrical wall 44 is provided with an annular flange 47 which extends radially outwardly and has a terminal downwardly depending portion. The lower cylindrical wall 44 is provided on its radially inner surface with a first annular shoulder 48 which is provided at intervals with vertical apertures 49 whicli extend from the upper portion of the shoulder through to the bottom of the base portion 41 itself. A second annular shoulder 50 is formed on the radially inner surface of the first shoulder 48 and the second shoulder 50 defines a large circular aperture 51 in the base of the canister 40. The second shoulder 50 is provided at intervals with recesses 52 in which bolts 53 which have been passed through holes in the trolley can be located to allow the cylindrical canister 40 to be secured to the trolley, the recesses 52 each being provided with a ledge 54 against which a nut 55 cooperating with one of the bolts may be located. A rubber sealing ring 56 is positioned adjacent the radially inner edge of the downwardly depending portion of the flange 47. The upper cylindrical wall portion 46 is provided on its inward surface with a circumferential groove 57 which engages with an outwardly protruding rib 58 provided adjacent the lower edge of the cylindrical side wall 42. The cylindrical side wall 42 is thus firmly retained in position with respect to the base portion 41. Aadjacent the upper edge of the cylindrical side wall 42 is provided a second outwardly protruding rib 59 which engages with a corresnonding groove 60 formed in a vertical cylindrical wall 61 forming part of the upper portion 43 of the canister. The upper portion of the canister further comprises an annular supporting flange 62 which extends radially inwardly from the top of the vertical cylindrical wall 61, and a rim 63 which extends upwardly from the outer wall nf the vertical cylindrical wall 61. The rim 63 is provided at intervals with radially outwardly protruding seating rings 64 against each of which may be located a locking cam 65 by means of a cam screw 66 which passes through the locking cam and through an aperture in the seating ring 64, the screw 66 being locked in position bv means of a nut 67 which bears against the lower surface of the seating ring 64. The function of these cams 65 will be described below.

Provided in the side wall 42 of the canister 40 is an aperture to which an outlet duct (not shown) is connected. The outlet duct may incorporate a valve for the insertion of test probes, for example, to facilitate the detection of any toxic gases which have escaped filtration. In a preferred embodiment of the invention an aperture for the outlet duct is only provided in the lowermost canister 40, although it will be appreciatde that corresponding apertures may be formed in the remaining canisters if necessary.

An annular filter element 69 located within canister 40 is provided with a lower disc shaped cap 70 and an upper annular cap 71, the diameter of the upper annular cap 71 being slightly greater than the diameter of the lower cap 70. The caps are of resilient material. The lower disc shaped cap 70 locates adjacent the radially inner wall of the first shoulder 48 of the base portion 41 of the canister 40 anct is sealinglv supported by the upper surface of the second shoulder 50 of the base portion 41, of the canister 40. The top surface of the upper annular cap 71 is provided with a resilient sealing member 72 which engages an annular sealing ring 73 which is itself mounted on the top of the annular sun porting flange 62 of the upper portion 43 by means of a resilient sealing ring 74.

Supported by the supporting flange 62 and mounted on top of the resilient sealing ring 74 of this first canister 40 is a second canister 40a which comprises a cylindrical side wall 42a having a greater height than the cylindrical side wall 42 of the first canister 40. In all other respects however, the second canister 40a is substantially identical with the first canister 40. The lower surface of the downwardly depending portion of the annular flange 47a formed at the bottom of the base portion 41a of the second canister locates above and seals against the resilient sealing ring 74 and the upper surface of said flange is provided with a number of grooves with which depending portions 65l of said locking cams 65 can engage, thereby to secure said second canister in position above the first canister.

The arrangement of the scond canister 40a on top of the first canister 40 is such that if air flows through either the large aperture 51a defined by the second shoulder 50a of the second canister, or the series of apertures 49a formed through the first shoulder 48a of the second canister 40a, that air will flow radially through the annular filter element 69 before passing to the outlet duct (not shown).

The arrangement is also such that if the filter element 69 present in the first canister 40 were inadvertently inverted, it would not be possible to locate the second canister 40a in position since in this event the upper annular cap 71 would sit above the upper surface of the first shoulder 48 and the lower disc shaped cap 70 would thus protrude above the level of the supporting flange 62 of the upper portion 43 of the first canister, thus preventing the sealing ring 74 from being correctly positioned with respect to the filter element 69.

The second canister 40a contains an axial flow, cylindrical filter element 75 which is provided with a cylindrical outer wall 76, a radially outwardly extending annular flange 77 being provided adjacent the top of the cylindrical outer wall 76. The cylindrical outer wall 76 of the filter element 75 sits above the second shoulder 50a of the base portion 41a of the second canister 40a and adjacent the radially inner surface of the first shoulder 48a of the base portion of the canister 40a The top surface of the cylindrical outer wall 76 is provided with a sealing member 72a which engages an annular sealing ring 73a which is itself mounted above the annular supporting flange 62a of the upper portion of the second canister by means of a resilient sealing ring 74a A third canister 40b is provided on top of the second canister 40a and it will be appreciated that the third canister is positioned and secured above the second canister in the same way as the second cansister 40a is positioned and secured on top of the first canister 40. Further it will be appreciated that the radially outwardly extending flange 77 on the cylindrical outer wall 76 of the filter element 75 prevents the filter element 75 being inadvertently inverted. The arrangement of the third canister 40h above the second canister 40a is such that air passing through any of the apertures 49b, 51b provided in the base of the third canister 40b would be directed by means of the annular sealing ring 73a to flow axially through the cylindrical filter element 75 contained within the second canister 40a. The air would then flow out through the aperture 51a defined by the second shoulder 50a of the second canister 40a The third canister 40b is again substantially similar to the first canister 40 and is provided with two concentric radial flow, annular filter elements 78, 79 therein. The third canister is also provided with a slightly dome shaped cap 80 formed as an aluminium casting which has a central intake nozzle 81 in the form of a rotating elbow provided therein.

The second shoulder 50b of the base portion 41 b of the third and uppermost canister 40b supports a circular tray 82 having a central circular portion and an outer depressed annular portion. The tray 82 closes off the aperture 51b defined by the second shoulder of the third canister.

The tray 82 forms part of a filter cage for containing a first annular filter element 78, which may be formed of paper for example.

The filter 78 is provided with a removable cap 83 which is held in position by three clips (not shown). The annular cap is provided with outstanding flanges which secure a replaceable seal 84 which is located on top of the cap 83. The sealing member 84 has a substantially rectangular cross-section and engages an annular sealing ring 73b to form a seal therewith. The sealing ring 73b is mounted above the inwardly extending flange 62b of the upper portion 43b of the third canister 40b by means of a resilient sealing ring 74b. The slightly dome shaped cap 80 locates against this sealing ring 74b and is secured in position by means of locking cams 65b. The annular cap 83 of the filter element 78 is removable to facilitate the replacement of the filter element.

A second annular filter element 79 is provided which is located inside the first annular filter element 78, and this second filter element 79 has a lower annular cap 85 and an upper annular cap 86. The lower annular cap rests on the central portion of the tray 82 and has a circular metal barrier 87 which extends across the aperture through the second annular filter element 79 adjacent the base of the filter element. The upper cap 86 is provided with a radially outwardly extending flange which contacts the upper surface of the resilient sealing member 84.

Thus air and noxious fumes for example flowing into the third canister 40b via the inlet nozzle 81, will have to pass radially through both the first and second filter elements 78, 79 before flowing to the second canister 40a through the apertures 49b in the first shoulder 48b of the third canister 40b.

Whilst this second embodiment of the invention has been described with reference to the use of three canisters containing various types of filter it is to be appreciated that a stack of more than three canisters could be provided if so desired, or alternatively only one canister need be provided if the conditions require only one canister to be used. The canisters may be provided with various types of filter therein depending upon the precise nature of any impurities or chemicals present in the gas to be filtered.

In utilising a filter assembly such as one of the embodiments described above with, for example, a welding process the number of canisters and the type of filter elements contained within the canisters would be selected having due regard to the type of welding process to be utilised and also having regard to the duration of the welding process. Thus if it is essential that the weld ing process proceeds continuously for a long period of time it may be necessary to provide a relatively large number of canisters so that the time that may elapse before a filter element is replaced or serviced may be extended, obviating the necessity to replace or service a filter element while the welding process continues. A nozzle mounted near the welding head would be connected by a conduit to the intake provided in the cap at the top of the stack of canisters and when the welding process starts the motor and the impeller would be activated to draw air into the filtration unit sweeping any noxious fumes generated during the welding process through the filter assembly.

The filter elements may be of the disposable, washable, or filament replaceable type, depending on the filtration medium used.

Whilst in the embodiments described herein all the filter units are positioned on the inlet side of such vacuum unit it may be convenient to position some filter units as described herein (or alternative filter units) on the outlet side of the vacuum unit in order more conveniently to interconnect with additional filter units or to provide a means of reducing the sound emitted by the vacuum unit.

WHAT WE CLAIM IS:- 1. A filter unit for use in a filter assembly of one or more such filter units, said filter unit comprising a canister having a first wall, a first aperture or set of apertures and a second aperture or set of apertures in said first wall, the said canister being adapted to be mounted in end to end relationship with adjacent such canisters with both said apertures or sets of apertures of one canister of an adjacent pair of such canisters in communication with the interior of the other canister of the said pair, said filter unit further comprising one or more filter elements, said filter element or elements being mounted within said firstmentioned canister so as to divide said first aperture or set of apertures from said second aperture or set of apertures such that in use of the filter unit a gaseous stream passing through the first-mentioned canister flows either through said first anerture or set of apertures or alternatively through said second aperture or set of apertures and is filtered by said filter element.

2. A filter unit according to claim 1 wherein said first-mentioned canister is cylindrical.

3. A filter unit according to claim 1 or 2 wherein said first wall of the first-mentioned canister forms the base thereof and comprises a peripheral portion and a depressed central portion. one of said apertures or sets of apertures being provided in said peripheral portion and the other of said apertures or sets of apertures being provided in said depressed portion.

4. A filter unit according to any one of claims 1 to 3 wherein said first-mentioned canister is provided with location pegs for locating one or more filter elements in position within the canister.

5. A filter unit according to claim 1 or claim 2 wherein said first wall of the firstmetnioned canister forms the base thereof and comprises a wall portion, a first annular shoulder for locating a filter element in position within the canister, and a second annular shoulder for supporting a filter element, one of said apertures or sets of apertures being provided in said first shoulder and one of sa

Claims (22)

**WARNING** start of CLMS field may overlap end of DESC **. ing process proceeds continuously for a long period of time it may be necessary to provide a relatively large number of canisters so that the time that may elapse before a filter element is replaced or serviced may be extended, obviating the necessity to replace or service a filter element while the welding process continues. A nozzle mounted near the welding head would be connected by a conduit to the intake provided in the cap at the top of the stack of canisters and when the welding process starts the motor and the impeller would be activated to draw air into the filtration unit sweeping any noxious fumes generated during the welding process through the filter assembly. The filter elements may be of the disposable, washable, or filament replaceable type, depending on the filtration medium used. Whilst in the embodiments described herein all the filter units are positioned on the inlet side of such vacuum unit it may be convenient to position some filter units as described herein (or alternative filter units) on the outlet side of the vacuum unit in order more conveniently to interconnect with additional filter units or to provide a means of reducing the sound emitted by the vacuum unit. WHAT WE CLAIM IS:-

1. A filter unit for use in a filter assembly of one or more such filter units, said filter unit comprising a canister having a first wall, a first aperture or set of apertures and a second aperture or set of apertures in said first wall, the said canister being adapted to be mounted in end to end relationship with adjacent such canisters with both said apertures or sets of apertures of one canister of an adjacent pair of such canisters in communication with the interior of the other canister of the said pair, said filter unit further comprising one or more filter elements, said filter element or elements being mounted within said firstmentioned canister so as to divide said first aperture or set of apertures from said second aperture or set of apertures such that in use of the filter unit a gaseous stream passing through the first-mentioned canister flows either through said first anerture or set of apertures or alternatively through said second aperture or set of apertures and is filtered by said filter element.

2. A filter unit according to claim 1 wherein said first-mentioned canister is cylindrical.

3. A filter unit according to claim 1 or 2 wherein said first wall of the first-mentioned canister forms the base thereof and comprises a peripheral portion and a depressed central portion. one of said apertures or sets of apertures being provided in said peripheral portion and the other of said apertures or sets of apertures being provided in said depressed portion.

4. A filter unit according to any one of claims 1 to 3 wherein said first-mentioned canister is provided with location pegs for locating one or more filter elements in position within the canister.

5. A filter unit according to claim 1 or claim 2 wherein said first wall of the firstmetnioned canister forms the base thereof and comprises a wall portion, a first annular shoulder for locating a filter element in position within the canister, and a second annular shoulder for supporting a filter element, one of said apertures or sets of apertures being provided in said first shoulder and one of said apertures or sets of apertures being provided radially inwardly of said second shoulder.

6. A filter unit according to claim 5 wherein said second shoulder defines a single circular aperture.

7. A filter unit according to any one of the preceding claims wherein said firstmentioned canister is provided with an annular sealing ring secured to an open end of the canister by means of a sealing member, for engagement with a filter element contained in the canister.

8. A filter unit according to any one of the preceding claims wherein said one or more filter elements are radial flow, annular filter elements each having upwardly and downwardly extending resilient flanges.

9. A filter unit according to any one of claims 1 to 7 wherein said one or more filter elements are axial flow, disc shaped filler elements provided with peripheral cylindrical sealing members having upwardly and downwardly extending flanges.

10. A filter unit according to any one of claims 1 to 7 wherein said one or more filter elements are radial flow, annular filter elements each having annular end caps, the end caps at opposite ends of each filter element being respectively provided with upwardly and downwardly extending flanges.

11. A filter unit according to claim 8, 9 or 10, as dependent on claim 7 wherein one said flange engages said first-mentioned canister and one said flange engages said sealing ring.

12. A filter unit according to any one of claims 1 to 7 wherein said one or more filter elements are cylindrical filter elements provided with peripheral cylindrical sealing members.

13. A filter unit according to claim 12 as dependent on claim 7 wherein one said peripheral sealing member engages said firstmentioned canister and one said peripheral sealing member engages said sealing ring.

14. A filter unit according to any one of claims 1 to 7 wherein said one or more

filter elements are radial flow, annular filter elements each having disc shaped or annular end caps.

15. A filter unit according to claim 14 as dependent on claim 7 wherein one said end cap engages said first-mentioned canister and one said end cap engages said sealing nng.

16. A filter unit, for use in a filter assembly of one or more filter units, and comprising a canister adapted to be mounted in end to end relationship and in direct internal communication with adjacent such canisters so as to provide a continuous flow path therethrough in succession, the canister being capable of operatively receiving at least one filter element both when the or each said filter element is of the axial flow, disc or cylindrical type and when the or each said filter element is of the radial flow annular type, the filter unlit further comprising at least one such filter element so mounted within said first-mentioned canister in sealing engagement therewith and the filter unit being so constructed that whether the or each said filter element is of the axial flow, disc or cylindrical type or of the radial flow, annular type, in use of the filter unit, a gaseous stream passing through the canister is caused to pass through the filter element of filter elements.

17. A filter assembly comprising a filter unit in accordance with any one of the preceding claims, and further comprising a lowermost canister which has an outlet duct in one side wall thereof, the outlet duct being adapted to be connected to a motor driven impeller.

18. An assembly according to claim 17 wherein an uppermost canister is provided with a cap provided with an intake duet adapted to be connected to an intake hose.

19. A filter unit substantially as herein described with reference to and as illustrated in Figure 1 of the accompanying drawings.

20. A filter unit substantially as herein described with reference to and as illustrated in Figure 2 of the accompanying drawings.

21. A filter assembly substantially as herein described with reference to and as illustrated in Figure 1 of the accompanying drawings.

22. A filter assembly substantially as herein described with reference to and as illustrated in Figure 2 of the accompanying drawings.