GB1602399A

GB1602399A - Air cleaners

Info

Publication number: GB1602399A
Application number: GB21997/78A
Authority: GB
Original assignee: Donaldson Co Inc
Current assignee: Donaldson Co Inc
Priority date: 1977-05-25
Filing date: 1978-05-24
Publication date: 1981-11-11
Also published as: AU3565378A; FR2391761A1; JPS592531B2; SE421673B; DE2819198A1; JPS546684U; DE2819198C2; DE7813315U1; CA1118369A; JPS5434182A; SE7805286L; IT1094779B; FR2391761B1; AR214922A1; JPS5747054Y2; IT7823632A0; ES470146A1; AU509602B2; BR7803131A

Description

(54) IMPROVEMENTS IN OR RELATING TO AIR CLEANERS (71) We, DONALDSON COMPANY, INC.

a Corporation organised and existing under the laws of the State of Delaware, United States of America, of 1400 West 94th Street, Minneapolis, Minnesota 55431, United States of America, 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: Abstract of the Disclosure An improved cylindrical two-stage air cleaner in which the overall efficiency of the cleaner is improved by changing the configuration of filtering means therein from a cylindrical one to a conical one, and in which improved structure is included for sealing the cleaner to prevent passage of incoming air to the outlet without being cleaned, to automatically scrape particulate matter from the inside of the cleaner housing each time a principal filter is changed, and to facilitate the insertion and removal of a safety filter without tools.

This invention relates to two-stage air cleaners (that is, cleaners having at least two stages). In a preferred form it relates to twostage air cleaners which include a first, centrifugal cleaning stage (or precleaner), for removing larger particles, and a second, filtering, stage in which the filtering means includes an outer, principal filter enclosing an inner, "safety" filter.

In such cleaners the principal factor determining the pressure drop across the cleaner is the size of the outlet opening. Since this opening in the cleaners previously proposed, can be no larger than the size of the smallest component of the filtering means, the desired low pressure drop must frequently be achieved at the cost of an increased size of the cleaner as a whole. This disadvantage is further complicated by the fact that the effectiveness of the centrifugal cleaner stage is a function of the volume of air available outside the filtering means and near the collector for larger particles.

The present invention now provides air cleaners which obviate the disadvantage referred to above.

Accordingly there is provided a cleaner to remove particulate matter comprising larger particles and smaller particles from a stream of gas, the cleaner comprising: a housing containing a pleated filter for removing said smaller particles from the gas, said filter being of frusto-conical shape; an inlet in the housing for said stream of gas and communicating with the outer convex surface of the filter; an outlet for cleaned gas said outlet being disposed adjacent the larger diameter end of said filter and communicating with the interior of the filter; means upstream of the filter to provide said stream with vortical movement thereby to separate said larger particles from the stream by the resulting centrifugal force before the stream passes to the filter; means to collect the larger particles so separated; and sealing means to prevent passage of the gas to said outlet without passing through said filter.

The filtering means of the air cleaners of the invention preferably comprise a principal filter and disposed therein a safety filter.

Compared with the air cleaners of the prior art, the preferred air cleaners of the present invention improve both the pressure drop through the cleaner and the effectiveness of the precleaner, by changing the configuration of the principal and safety filters, making them conical, tapering away from the cleaner outlet opening. This can maximize both the available diameter of the outlet opening and the volume between the principal filter and the cleaner housing near the particle collector, for any given housing diameter. The arrangement of an improved seal between the principal filter and the end of the housing, to ensure that no air passes from the inlet to the outlet of the cleaner without having passed through at least the safety filter, also can function to clean and remove dirt from the housing wall when the filter is serviced. The invention also embodies improved structure by which insertion and removal of filtering means such as a safety filter can be simpler, quicker, and more certain, without the use of tools.

There is now described, by way of example and with reference to the accompanying drawings, two embodiments of two-stage air cleaners of the invention.

Brief Description of the Drawings In the drawings, Figure I is a view in elevation of an air cleaner including the invention, parts being broken away for clarity of illustration; Figure 2 is an end view of the same cleaner, parts also being broken away; Figures 3, 4, and 5 are enlarged fragmentary detailed views of portions of the invention; Figure 6 is a view in elevation of an air cleaner embodying the invention, parts being broken away for clarity of illustration; Figure 7 is an end view of the same cleaner seen from the left of Fig. 6, parts also being broken away; and Figure 8 is an exploded detailed view of a portion of the invention.

Description of the Preferred Embodiments In the drawings, an air cleaner 10 is shown to have a housing 11 which is generally cylindrical about an axis 12 extending between a closed end 13 and an open end 14. In normal use said axis is substantially vertical.

Housing 11 has a lateral opening 15 near closed end 13, to which is fitted the usual inlet connection member 16. Closed end 13 includes a permanent closure member 17 having an annular inward step 20 and an axial outlet opening 21 with a connection tube 22.

A support yoke 23 has a pair of legs 24 and 25 which diverge to be secured to the inside of tube 22 as shown at 26: the yoke extends along axis 12 and terminates in an axial externally threaded member 27, which serves as the outboard support for filtering means including a safety filter 30 and a principal filter 31.

Safety filter 30 comprises a hollow porous body 32 having a central axis of symmetry coincident with axis 12. Body 32 is - not cylindrical but frusto-conical: it tapers from a maximum diameter near closure member 17 to a minimuni diameter near member 27.

The larger end of filter 30 is secured in an annular mounting member 33 having an internal diameter to fit over the legs of yoke 23, and carrying a resilient ring 34 to seal against closure 17 around opening 21. The smaller end of body 30 is secured to a disk 35 which closes the end of the safety filter except for a central aperture 36 to pass member 27 and the shoulder of a wing nut 37 on member 27 which may be tightened to sealingly secure filter 30 coaxially in housing 11.

Principal filter 31 is also frusto-conical, tapering from a maximum diameter near closure 17 to a minimum diameter near member 27. Conveniently, the two filters may have the same apex angle. Filter 31 comprises a body 40 made up of pleated paper 41 between inner and outer protecting sheets 42 and 43 having large perforations to transmit air. By reason of the conical configuration the pleats, which lie along lines defined by the intersections with the conical surface of planes passing through axis 12, are more open near the larger end of the filter AT its smaller end body 40 is secured in a disk 44 which closes the filter end except for a central aperture 45 to pass member 27 and the shoulder of a sceond wing nut 46 on member 27. The larger end of body 40 is molded into an annular mounting member 50 having resilient portions 51 and 52 to engage closure 17 and step 20 respectively.

Member 50 also includes an outwardly directed lip 53 which resiliently engages the annular wall of housing 11 for a purpose presently to be described.

In order to prevent direct impingement of the air entering the cleaner at 15 on a porous filter surface, a frusto-conical baffle 55 surrounds the portion of body 40 near the larger end thereof. Baffle 55 fits on the end of body 40 before member 50 is molded on, and has lugs 56 to aid in its positioning and apertures 57 to pass the molding compound and ensure a good support. Baffle 55 has a smaller apex angle than body 40, so that air can move axially under the baffle from the right as seen in Fig. 1, and the effective cleaning area of the filter is not materially reduced.

At its end remote from member 50, baffle 55 is configured with a bayonet joint 60 to engage a mating joint 61 in a separately molded fin assembly 62 having a plurality of fins 63 extending angularly with respect to axis 12. The primary purpose of fins 63 is to cause air passing generally axially through them to have a vortical motion: this produces a centrifugal force by reason of which the larger airborne particles move outward to the inner surface of housing 11 and thereafter move helically therearound toward the open end thereof. Fins 63 also serve the secondary purpose of holding filter 31 approximately centered while disk 44 is moved to bring aperture 45 into alignment with member 27.

Tightening of a wing nut 46 then holds filter 31 coaxially positioned in housing 11.

The open end 14 of housing 11 is closed by removable end cap 70 having a bead 71 around its end. A second bead 72 is formed near the open end of housing 11, and a clamp ring 73 draws bead 72 against an O-ring 73a to hold the cap on the housing, the clamp ring being positioned axially along the housing by a further bead 74, and being tightened by a suitable fastener 75.

Cap 70 includes an annular partition 76, and the rim 77 of an inner cup 78 is sized to fit between an inner lip 80 of partition 76 and the outer rim 81 of disk 44 to prevent flow of air therebetween. A segmental opening 82 is formed in partition 76 near its outer rim to permit particles moving helically along the inside of housing 11 to pass into cup 70 and become trapped. Rim 77 of inner cup 78 is extended axially at 83 over but spaced from a portion of filter 31 to prevent any direct action of air flow through the small end of filter 31 on the movement of particles into trap 70.

Turning now to Figs. 6-8, an air cleaner 110 is shown to have a housing 111 which is generally cylindrical about an axis 112 extending between a closed end 113 and an open end 114. Housing 111 has a lateral opening 115 near closed end 113, to which is fitted the usual inlet connection member 116.

Closed end 113 includes a permanent closure member 117 having an annular inward step 120 and an axial opening 121 with a connection tube 112 fixed therein.

A support yoke 123 has a pair of legs 124 and 125 which diverge to be secured to the inside of tube 122 as shown at 126: the yoke extends along axis 112 and terminates in an axial member 127, externally threaded at its outer end 128, which serves as the outboard support for a safety filter 130 and a principal filter 131.

Safety filter 130 comprises a hollow porous body 132 having a central axis of symmetry coincident with axis 112. Body 132 is frustoconical, tapering from a maximum diameter near end closure 117 to a minimum diameter near member 127. The larger end of filter 130 is molded into a first annular resilient mounting member 133 having an internal diameter to fit over the legs of yoke 123. The radially inward concave surface of annular member 133 has a medial, outwardly directed circumferential groove 133a to removably engage a bead 1 22a at the inner end of tube 122; preferably the groove or the bead is rigid and the bead or groove, respectively, is flexible. A bevel 133b simplifies the positioning of the filter on bead 122a. The axially outer end of member 133 has a flattened surface 133c to sealingly engage end closure 117 around opening 121 when groove 1 33a and bead 1 22a are engaged, and has a radially outwardly extending lip 134. The smaller end of body 130 is molded into a disk 135 which closes the end of the safety filter except for a central aperture 136 to pass member 127, and which has an axially extending resilient lip 138 sealed securely against. 127 by a spring clamp 137.

Principal filter 131 is also frusto-conical, tapering from a maximum diameter near closure 117 to a minimum diameter near member 127: conveniently the two filters may have the same apex angle. Filter 131 comprises a body 140 made up of pleated paper 141 between inner and outer protective sheets 142 and 143 having large perforations to transmit air. By reason of the conical configuration the pleats, which lie along lines defined by the intersections with the conical surface of planes passing through axis 112, are more open near the larger end of the filter.

At its smaller end, body 140 is molded into a first mounting member 147 having first and second resilient portions 147a and 148: also molded into member 147 is an axially short frusto-conical shield 149 spaced radially from surface 142 and conveniently of the same apex angle. At its larger end body 140 is molded into a second annular mounting member 150 having a resilient portion 151 for engaging end closure 117 and further resilient portions 152a and 152b for engaging respectively annular step 120 of end closure 117 and lip 134 of mounting ring 133.

In order to prevent direct impihgement of the air entering the cleaner at 115 on a porous filter surface, a conical baffle 155 surrounds the portion of body 140 near the larger end thereof. Baffle 155 is molded into mounting member 150, and has a smaller apex angle than body 140, so that air can move axially under the baffle from the right, as seen in Fig. 1, and the effective cleaning area of the filter is not materially reduced.

At its end remote from member 150, baffle 155 is configured with a plurality of fins 163 extending angularly with respect to axis 112.

The primary purpose of fins 163 is to cause the air passing generally axially through them to have a vortical motion. This produces a centrifugal force by reason of which the larger air borne particles move outward to the inner surface of housing 111 and thereafter move helically therearound toward the open end thereof. Fins 163 also serve the secondary purpose of holding filter 131 approximately centered in the housing while the open end of the housing is closed.

This is accomplished by a removable closure member 170 having an outer annular groove 171 to engage a bead 172 at the end of housing 111, the joint being sealed by a gasket 173a. Closure member 170 has an annular groove 184, with a step 185, to sealingly engage portions 148 and 147a respectively of member 147. It also has a central aperture 186 to pass member 127 and the shoulder 187 of a wing nut 188.

Housing 111 is provided, near its open end, with a lateral outlet tube 190 for receiving particulate matter separated by centrifugal action. As is well known in the art, this tube is provided with an evacuator valve 192 which permits gravity discharge of particulate matter collecting in tube 190, by reason of vibration for example, without permitting significant air flow into housing 111.

Operation In use cleaner 10 of Figs. 1-5 is inserted in an air flow line so that air, bearing undesired particulate matter in a range of sizes, can enter the cleaner at 16 and leave it at 22. A safety filter 30 is inserted into the housing over yoke 23, so that its ring 34 seats against end closure 17, and its aperture 36 passes over member 27, and wing nut 37 is tightened. A principal filter 31 is slid into housing 11, lip 53 and fins 63 engaging the inside of the housing, until portions 51 and 52 of member 50 engage closure 17 and step 20, and so that its aperture 45 passes over member 27, and wing nut 46 is tightened.

Inner cup 78, which has been removed to permit the emptying of particulate matter out of cap 70, is reinserted into parition 76 in the end cap, and the cap is applied to housing 11, sealing the space at the end of the filtering means, and secured in place by ring 73 and fastener 75. Note that now there is no passage for air from inlet 16 to outlet 22 except through the filters, a double seal being provided at 77-8(8 1, and a quadruple seal being provided at 11/53, 52/20, 51/17, and 34/17.

The cleaner is now ready to be put into operation, which is done by drawing air through it. Air entering at 16 impinges on baffle 55, so the particles are not driven into any filter surface. The air flows to the right as seen in Fig. 1, being given a circumferential component of motion by fins 63 which causes the larger particles to pass outwardly to housing 11 and flow helically therearound until they reach partition 76 and pass through opening 82 into end cap 70. Air containing smaller particles passes radially through the axially central part of filter 31, and also flows axially under baffle 55 and cup lip 83 so that practically the entire surface of filter 31 is operative. After passing through filter 31 the air passes through filter 32 and thence through outlet tube 22.

When a new filter 31 is in place, little particulate matter reaches filter 30. As the period of use increases the pores in filter 31 become more and more clogged with particulate matter and the pressure drop in the cleaner mounts to an undesirable level.

When this occurs, it is necessary to interrupt the air flow through the cleaner, release clamp ring 73, remove end cap 70, remove wing nut 46, and replace filter 31. As the used filter is drawn from the housing lip 53 scrapes from the inner wall any particles which may have accumulated there, and draws them out of the housing. Filter 30 is inspected, and is usually found in satisfactory condition: if not, wing nut 37 is removed and filter 30 is replaced. Note that for the usual case, where filter 30 is satisfactory, the outlet tube 22 is at no time open to the ambient atmosphere but is protected by safety filter 30, hence the name of the latter.

Before reassembling the cleaner, inner cup 78 is removed from end cap 70, and the particulate matter which has collected there is emptied out. Reassembly is the reverse of disassembly, starting with the replacement of cup 78 in cap 70 and concluding with tightening of fastener 75 of clamp ring 73 and re-establishing air flow.

Cleaner 110 is inserted in an air flow line so that air, bearing undesired particulate matter in a range of sizes, can enter the cleaner at 116 and leave it at 122. A safety filter 130 is inserted into the housing over yoke 123 so that member 127 passes through aperture 136, and bevel 133b guides bead 122a into groove 133a. The filter is pressed firmly in place and clamp 137 is expanded, passed over lip 138, and allowed to contract.

A principal filter 131 is now slid into housing 111, fins 163 helping to guide the operation, until mounting member 150 prevents further insertion. End cap 170 is applied over member 127 and wing nut 188 is tightened.

Note that there is now no passage for air from inlet 116 to outlet 122 except through the filter, a triple seal being provided at 147a/185, 148/184, and 137/138 and a quintuple seal being provided at 152a/120, 151/117, 152b/134, 133c/177, and 133a/122a. The arrangement by which member 150 presses lip 134 against closure 117 is especially desirable, since it serves to maintain the seal 152b/134, while allowing the fit member groove 133a and bead 122a to be sufficiently relaxed to permit easy removal and replacement of safety filter 130.

The cleaner is now ready to be put into operation, which is done by drawing air through it. The air entering at 116 impinges on baffle 155, so the particles are not driven into any filter surface. The air flows to the right as seen in Fig. 6, being given a circumferential component of motion by fins 163 which causes the larger particles to pass outwardly to housing 111 and flow helically therearound until they pass through opening 189 into tube 190. Air containing smaller particles passes radially through the axially central part of filter 131 and also flows axially under baffle 155 and shield 149 so that practically the entire surface of filter 131 is operative. After passing through filter 131 the air passes through filter 130 and thence to outlet tube 122.

When a new filter 131 is in place, little particulate matter reaches 130. As the period of use increases the pores in filter 131 become more and more clogged with particulate matter and the pressure drop in the cleaner mounts to an undesirable level. When this occurs, it is necessary to interrupt the air flow to the cleaner, remove wing nut 188 and closure 170 and then remove filter 131 for replacement. Filter 130 is inspected, and is usually found in satisfactory condition: if not, clamp 137 is removed and filter 130 is replaced. Note that for the usual case, where filter 130 is satisfactory, the outlet tube 122 is at no time opened to the ambient atmosphere, but is protected by safety filter 130, hence the name of the latter.

Before reassembling the cleaner tube 190 is inspected and any material which may have been trapped therein is removed. Reassembly is the reverse of disassembly, starting with the replacement of the safety filter and concluding with the tightening of wing nut 188 and re-establishing air flow.

From the foregoing it will be clear the the invention comprises an improved air cleaner wherein minimization of pressure drop and maximization of precleaning efficiency, for any overall housing diameter, are accomplished by constructing the filter elements with a conical configuration rather than a cylindrical configuration, wherein there is provided improved sealing together with means for automatically cleaning the inner surface of the housing whenever a principal filter is changed, and wherein said improved sealing is combined with ready and convenient replacement of filters, the bead and groove principle being adaptable not simply for safety filters, but for general use in air cleaner construction.

Although the invention has been described with reference to cleaning a stream of air, it is applicable to the removal of particulate matter from streams of gases in general.

WHAT WE CLAIM IS: 1. A cleaner to remove particulate matter comprising larger particles and smaller particles from a stream of gas, the cleaner comprising: a housing containing a pleated filter for removing said smaller particles from the gas, said filter being of frusto-conical shape; an inlet in the housing for said stream of gas and communicating with the outer convex surface of the filter; an outlet for cleaned gas said outlet being disposed adjacent the larger diameter end of said filter and communicating with the interior of the filter; means upstream of the filter to provide said stream with a vortical movement thereby to separate said larger particles from the stream by the resulting centrifugal force before the stream passes to the filter; means to collect the larger particles so separated; and sealing means to prevent passage of the gas to said outlet without passing through said filter.

2. A cleaner for removing, from a stream of air, solid particles in a range of sizes, comprising, in combination: a housing, generally cylindrical about an axis extending between first and second ends thereof, having a lateral inlet opening near said first end, an axial outlet opening in said first end, and collecting means near said second end for receiving particulate matter; pleated filtering means in said housing, for removing smaller particles from the air, having the form of a frustum of a circular cone with its larger end directed toward said outlet opening and its smaller end closed and directed away from said outlet opening.

means upstream of the filtering means to provide said stream with a vortical movement thereby to separate from the stream solid particles of sizes in the upper part of said range of sizes; means supporting said filtering means coaxially in said housing; and sealing means preventingpassage of air through said housing from said inlet opening to said outlet opening except through said filtering means.

3. A cleaner according to Claim 2, in which said filtering means comprises a principal filter of porous paper pleated in alignment with lines defined on said frustrum by planes passing through the axis thereof, so that the pleats are more open at said larger end than at said smaller end.

4. A cleaner according to Claim 3, including baffle means in said housing outside filtering means for preventing direct impingement, on said principal filter, of the air entering said housing, and for imparting said air said vortical component towards said second end of said housing, to comprise with said collecting means a precleaning stage for said cleaner to remove larger particles from the air.

5. A cleaner according to claim 3 or 4, in which said filtering means further includes a safety filter comprising porous material in the form of a frustum of a circular cone having a larger end directed toward and at least as large as said outlet opening but smaller than said larger end of said principal filter, and a smaller end directed away from said outlet opening and smaller than the smaller end of said principal filter; and means supporting said safety filter coaxially within said principal filter.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. that practically the entire surface of filter 131 is operative. After passing through filter 131 the air passes through filter 130 and thence to outlet tube 122. When a new filter 131 is in place, little particulate matter reaches 130. As the period of use increases the pores in filter 131 become more and more clogged with particulate matter and the pressure drop in the cleaner mounts to an undesirable level. When this occurs, it is necessary to interrupt the air flow to the cleaner, remove wing nut 188 and closure 170 and then remove filter 131 for replacement. Filter 130 is inspected, and is usually found in satisfactory condition: if not, clamp 137 is removed and filter 130 is replaced. Note that for the usual case, where filter 130 is satisfactory, the outlet tube 122 is at no time opened to the ambient atmosphere, but is protected by safety filter 130, hence the name of the latter. Before reassembling the cleaner tube 190 is inspected and any material which may have been trapped therein is removed. Reassembly is the reverse of disassembly, starting with the replacement of the safety filter and concluding with the tightening of wing nut 188 and re-establishing air flow. From the foregoing it will be clear the the invention comprises an improved air cleaner wherein minimization of pressure drop and maximization of precleaning efficiency, for any overall housing diameter, are accomplished by constructing the filter elements with a conical configuration rather than a cylindrical configuration, wherein there is provided improved sealing together with means for automatically cleaning the inner surface of the housing whenever a principal filter is changed, and wherein said improved sealing is combined with ready and convenient replacement of filters, the bead and groove principle being adaptable not simply for safety filters, but for general use in air cleaner construction. Although the invention has been described with reference to cleaning a stream of air, it is applicable to the removal of particulate matter from streams of gases in general. WHAT WE CLAIM IS:

1. A cleaner to remove particulate matter comprising larger particles and smaller particles from a stream of gas, the cleaner comprising: a housing containing a pleated filter for removing said smaller particles from the gas, said filter being of frusto-conical shape; an inlet in the housing for said stream of gas and communicating with the outer convex surface of the filter; an outlet for cleaned gas said outlet being disposed adjacent the larger diameter end of said filter and communicating with the interior of the filter; means upstream of the filter to provide said stream with a vortical movement thereby to separate said larger particles from the stream by the resulting centrifugal force before the stream passes to the filter; means to collect the larger particles so separated; and sealing means to prevent passage of the gas to said outlet without passing through said filter.

6. A cleaner according to any of claims 3

to 5, in which said first end of said housing has an annular inward step, and said sealing means includes a resilient annulus carried by said larger end of said principal filter, and having first and second portions to engage said one end of said housing and said step severally.

7. A cleaner according to claim 6, in which said annulus includes a further outwardly extending annular lip for sealing engagement with the inner surface of said housing.

8. A cleaner according to any of claims 4 to 7, in which one end of said baffle means is molded into said sealing means at the larger end of said principal filter.

9. A cleaner according to any of claims 3 to 8, in which said sealing means comprises a resilient annulus engaging said first end of said housing and said larger end of said principal filter, and means including a disk occluding the smaller end of said principal filter.

10. A cleaner according to any of claims 2 to 9, in which said axis is vertical in use, and said collecting means includes a cap removable from the end of said housing remote from said outlet opening and extending axially below the end of said filtering means, and includes baffle means providing limited peripheral access to the space within said cap for passage of said solid particles, and preventing air movement across the smaller end of said filtering means.

11. A cleaner according to any of Claims 2 to 10, in which said lateral inlet opening is radial.

12. A cleaner according to claim 2, in which said filtering means comprises an outer principal filter, an inner safety filter, and means in said housing outside said principal filter for preventing direct impingement thereon of air entering said housing and for imparting to said air a vortical component toward said second end of said housing, said principal filter having the form of a frustum of a circular cone with its larger end directed toward and larger than said outlet opening and its smaller end closed and directed away from said outlet opening, and comprising porous paper pleated in alignment with lines defined on the frustum by planes passing through the axis thereof so that the pleats are more open at said larger end than at said smaller end.

13. A cleaner according to claim 2, in which said filtering means comprises a principal filter and a safety filter, said principal filter having the form of a frustum of a circular cone with its larger end directed toward and larger than said outlet opening and its smaller end closed and directed away from said outlet opening, and comprising porous paper pleated in alignment with lines defined on the frustum by planes passing through the axis thereof so that the pleats are more open at said large end than at said smaller end, and said safety filter comprising porous material in the form of a frustum of a circular cone having a larger end directed toward and at least as large as said outlet opening but smaller than said larger end of said principal filter, and a smaller end directed away from said outlet opening and smaller than the smaller end of said principal filter.

14. A cleaner according to Claim 5, in which the principal filter surrounds the safety filter and is axially removable from the housing, and means for preventing contamination of the safety filter, said means comprising a resilient lip carried by the inner end of said principal filter in radial engagement with the inner surface of said housing, so that when said principal filter is removed any material adhering to said inner wall is scraped therefrom and carried out of said housing as an incident to removing said principal filter.

15. A cleaner according to Claim 2, in which said sealing means comprises, in combination: a peripheral inward step in said first end of the housing; and a resilient annular sealing member mounted on the apposed end of said filtering means, said member having a stepped end, for engaging both said housing and said step therein, and an annular lip for peripherally engaging the inner surface of said housing, so that there are three sequential seals against passage of air from said housing past said filtering means, comprising said lip and the steps of said stepped end of said sealing member.

16. A cleaner according to Claim 5, in which said sealing means comprises, in combination: a peripheral inward step in said first end of said housing; a first resilient annular sealing member mounted on the end of said principal filter apposed to said first end of said housing, and having a stepped end, for engaging both said housing and said step therein, and an annular lip for peripherally engaging the inner surface of said housing: and a second resilient annular sealing member mounted on the end of said saTety filter apposed to said first end of said housing, for engaging said step therein, so that there are four sequential seals against passage of air from said housing past said filtered means.

17. A cleaner according to Claim 5, in which said sealing member comprises, in combination: a peripheral inward step in said first end of said housing; a first resilient annular sealing member mounted on the end of said principal filter apposed to said first end of said housing, and having a stepped end, for engaging both said housing and said step therein; and a second resilient annular sealing member mounted on the end of said safety filter apposed to said first end of said housing, said second sealing member having a first surface for engaging the stepped end of said first sealing member, and a second surface for engaging said step in said first end of said housing, so that there are three sequential seals against passage of air from said housing past said filtering means.

18. A cleaner according to Claim 2, in which said axial outlet opening comprises an axial aperture traversed by an outlet tube with a circumferential bead at its inner end, and said filtering means includes a resilient connecting annulus having an internal groove to engage said bead.

19. A cleaner according to Claim 18, in which said filtering means includes a centrally apertured resilient mounting member, and said means supporting filtering means comprises a mounting yoke having legs secured to the inside of said outlet tube, said yoke extending axially of said housing through said filtering means and including a shaft for passing outwardly through the aperture of said mounting member to support the filtering means coaxially in said housing.

20. A cleaner according to Claim 19, in which said mounting member includes an outwardly directed tubular extension, and the supporting means including clamping means for securing said extension frictionally to said shaft.

21. A cleaner according to Claim 6, in which said safety filter includes a resilient connecting annulus having an outward lip with a first surface for engaging said larger end of said principal filter, and a second surface for engaging said step in said end of said housing.

22. A cleaner according to Claim 18, in which one of said groove and said bead is rigid and the other is resilient.

23. A cleaner according to Claim 18 or 22, in which said bead and said groove are formed as parts of mutually telescoping tubular surfaces.

24. A cleaner according to any of Claims 1 to 23, substantially as described herein with reference to the accompanying drawings.

25. A cleaner substantially as described herein and substantially as shown in Figs. I to 5 of the accompanying drawings.

26. A cleaner substantially as described herein and substantially as shown in Figs. 6, 7 and 8 of the accompanying drawings.