US2754007A

US2754007A - Shielding shoe for filters

Info

Publication number: US2754007A
Application number: US236883A
Authority: US
Inventors: Leffler Nils Gustav
Original assignee: Individual
Current assignee: Individual
Priority date: 1945-08-14
Filing date: 1951-07-16
Publication date: 1956-07-10
Anticipated expiration: 1973-07-10

Description

y 10, 1956 N. G. LEFFLER 2,754,007

SHIELDING SHOE FOR FILTERS Filed July 16, 1951 2 Sheets-Sheet 1 I! Q 3 JNI/ENTOR 1V1 ls Gusiay Lefifiler M, m #m ATTORNEYS y 10, 1956 N. G. LEFFLER snmnmc SHOE FOR FILTERS 2 Sheets-Sheet 2 Filed July 16, 1951 [NVENTOR Nils Gusial/Leffler C d/221mm, Xflawum ATTURZVEYJ I E ha I United States Patent '0 SHlELDING SHOE FOR FILTERS Nils Gustav Lefller, Sundsvall, Sweden Application July 16, 1951, Serial No. 236,883

Claims priority, application Sweden August 14, 1945 13 Claims. (Cl. 210201) The present invention relates to filters of the type embodying a movable perforated filter member adapted to pick up on its one side solids from a suspension and to discharge suspension liquid at its other side after passage through the openings thereof under the influence of a differential pressure. between said sides. Such a filter is usually, but not always, formed as a rotary cylindrical drum the peripheral surface of which is perforated and provided with a fine mesh wire fabric for picking up the solids of the suspension in which the drum is partially submerged, and wherein the differential pressure is produced either by evacuating the interior of the drum or subjecting the outer surface thereof to a superatmospheric pressure. The invention is especially adapted for embodiment in rotary drum filters for picking up solids, such as fibres, powders and granules, from suspensions in liquids and is more particularly concerned with an improved construction of a shielding shoe for such filters. application is a continuation-in-part of my previous application Ser. No. 689,314, filed August 9, 1946, now Patent No. 2,561,282, dated July 17, 1951, which was a further development of the invention I have disclosed in my application Ser. No. 652,464, filed March 6, 1946, now Patent No. 2,560,581, also dated July 17, 1951.

As in the case of my previous inventions above referred to, the present invention has also for its primary object to provide a shielding shoe of the kind defined which will secure a sufilciently good seading around that portion of the wire fabric where the taking off takes place, and further to provide a shielding shoe which will close off the take off portion or zone effectively in filter drums even if the drum is not perfectly circular. An especial object of the present invention is to provide a shielding shoe which will be more or less independent of variations in said difference in pressure.

The invention will now be described more in detail with reference to the accompanying drawings illustrating various embodiments by which the invention may be realized in connection with vacuum drums for paper making but it is to be understood that the invention is not limited to the features so shown but has to be taken in its broadest scope.

In the drawings:

Fig. 1 illustrates in cross sectional view the invention adopted in a shielding shoe comprising a movable band travelling with the filter member,

Fig. 2 is a view of the shoe illustrated in Fig. I viewed from the right hand side, the filter member and part of the band being removed,

Fig. 3 is a cross sectional view of a portion of the filter drum at the upper roller of the shoe, taken along the line 3-3: in Fig. 1,

Fig. 4 is a fragmentary cross sectional view of a modified form of the shoe having plunger balancing means,

Fig. 5 is a cross sectional view, similar to that of Fig. 1, of a shielding shoe of the type illustrated in Fig. 5 of my Patent No. 2,560,581 provided with pressure balancing means according to the present invention,

This I 2,754,007 Patented July 10, 1956 Fig. 6 is a view of the shoe illustrated in Fig. 5 viewed as Fig. 2, from the right hand side, the filter member being removed and parts of the sealing portion of the shoe broken away and Fig. 7 is a fragmentary cross sectional view of another modified form of shielding shoe having a movable band like that of Figs. l-3 as well as plunger or piston balancing means of the character illustrated in Fig. 4.

Referring first to Figs. 1 to 3, the filter drum comprises a cylindric perforated support or shell 1 on the outer surface of which the wire mesh fabric, not shown in the drawings, is applied for taking up the solids suspended in a slurry of, for instance, cellulose fibres to form therefrom the fibre cake 4. The support or filter member 1 is mounted in conventional manner in disc-like ends 2 being rotatably journalled in bearings, not shown in the drawings. At the outside of the filtering member the doctor 3 is arranged in the usual way for removing the cake 4 deposited on the filter. The drum is considered to rotate in the direction of the arrow at the upper end of Fig. 1 and to be partly submerged in the fibre slurry 5. The interior of the drum may be evacuated in any of the ways well known to the art so as to maintain a subatmospheric pressure within the drum.

The shielding shoe is positioned at the place of discharge of the filter, that is at the zone at which the fibre cake is removed from the drum by the doctor knife and downwards to the level of the slurry 5. The function of the shoe being to seal olf the communication between the interior of the drum, the filtrate space or chamber 33, and the outer atmosphere, partly to facilitate the removal of the cake by shielding olf the portion of the filter member at the removal zone from the pressure existing in the interior of the drum and partly to prevent air from getting access to the interior of the drum through the portion of the filter member made naked at the doctor, the shoe extends beyond this place of discharge at either side in the peripheral direction. The shoe is in the axial direction substantially coextensive with the filtering member or with the foraminous portion thereof. In the drawings the shoe is submerged in the filtrate below the level 6 but this is not at all necessary for its satisfactory function. In filter drums where the filtrate is discharged at a low point in the drum the level of the filtrate is preferably below the lowermost portion of the shoe.

In the embodiment shown in Figs. 1-3 the invention is applied to a shielding shoe comprising a rigid stationary shoe member 12 and a movable sealing shoe member in theform of an endless flexible band 7 of rubber or steel or the like running over rollers 8 at either peripheral end of the. shoe, said band being tightened by means of a tightening roller 9 suspended in an arm 10 and pressed by a spring 11, and being adapted to contact and travelv with the perforated shell or screening plate 1 along the portion thereof to be sealed at the taking off part of the drum. The screening drum being submerged in the fibre suspension 5, the vacuum prevailing in the interior of the drum will cause the suspension liquid to penetrate the filter plate 1 so as to fill the interior of the drum up to a givenlevel 6 determined by the continual removal of filtrate liquid in conventional manner, not illustrated. The fibre material deposits on the plate 1 forming a thin fibre cake 4 successively increasing in thickness until the filter plate upon rotation of the drum will ultimately emerge from the suspension. 5 and continue its passage in the surrounding atmosphere where washing liquid orother treating liquid may be added over limited axial (longitudinal) sections of the drum and, due to the vacuum created in the interior of the drum, be sucked through the fibre cake. At least duringthe latter part of the rotationof the drum in the atmosphere abovetthe fibre suspensiomairwill be sucked through the cake so that the latter is more or less 3 dried. At the position near above the fibre suspension the fibre cake is ultimately removed from the screen by means of doctor 3.

The stationary shoe member 12 is securely mounted in the interior of thedrum in conventional manner as on the shaft of the drum or on a bracket attached to stationary end Walls of the drum, as the case may be, not illustrated. The stationary shoe member 12 has a rigid central wall 13 extending along the length of the drum and on a projecting portion 15 (best seen in Fig. 2), at each end of the stationary shoe member 12 a U-shaped fork piece 14 is pivoted by means of a shaft pin 16 carrying the fork leg 21, extending by a portion 23 opposite shaft 16. The legs 21 and extension portions 23 are unitary, rigid parts of the fork piece 14. The projection 15 of the stationary shoe member 12 projects into a recess 22 of the leg 21 and provides a stationary support for the axis of shaft 16. The roller 8 has a shaft 24 journalled in the portion 23 of leg 21. The fork 14 thus constitutes a pivotally mounted supporting means for roller 8 of the movable sealing shoe member, and the pivotal axis of shaft 16 on which said supporting means is mounted is so located that certain portions of said means, i. e., the legs 21 and extension portions 23 of fork 14, lie on one side of said axis while the cross bar or bottom portion of the fork lies on the opposite side. A spring 25 presses the roller towards the filter plate 1 so as to render contact between band 7 and the plate. The cross bar or bottom portion of the fork 14 extends along the roller 8 and is in substantially liquidtight contact therewith. The bar is sealed against the stationary central wall 13 by means of a sealing bar 17 fitted in a groove 18 in the fork 14. The bottom portion of the groove 18 communicates with a pressure chamber 19, which lies between wall 13 and band 7, through a plurality of small channels formed in the fork, whereby the pressure in chamber 19 will press bar 17 against wall 13. Bar 17 may also be fitted in a groove in wall 13 similarly to the construction illustrated in Fig. 4.

The cross bars or bottom portions of forks 14 thus form closing members for the spaces between rollers 8 and wall 13 which would otherwise provide passages between the filtrate chamber 33 and the pressure chamber 19 which is sealed off or secluded from the rest of the filtrate chamber by said wall, the rollers 8 and the forks 14. Each fork 14 also acts as a counterbalancing member for the associated roller 8 since, as is evident from Figs. 1 and 2, the roller 8 and the cross bar of fork 14 lie on opposite sides of the axis of fork pivot shaft 16.

In this manner both of the rollers 8 are so mounted as to press with a given power band 7 against the filter plate 1 at the ends of the shoe. The intermediate portion is pressed against the plate by means of a pressure medium, preferably water, introduced at a preferably constant pressure into the pressure chamber 19 through pipe 26, which is passed through the end wall of the drum in any conventional manner. The pressure chamber 19 is sealed against the filtrate chamber 33 of the drum crosswise thereof by the rollers 8, the longitudinal cross bars of forks 14, the sealing bars 17 and the stationary central wall 13 as well as the packings 27 sealing against the ends of the rollers. In the axial direction of the drum the sealing is secured by means of side walls .of the stationary wall and sealing bars 28 engaging the filter plate 1 out side the perforations thereof. The bars 28 are suspended in cars 39 on wall 13 via links 29 and are pressed against the plate 1 by springs 31. The central and side walls of stationary shoe member 12, forks 14, rollers 8 and the associated

sealing elements

17, 27 and 28 thus form an enclosure for pressure chamber 19, the open side of which faces and is substantially coextensive in area with the portion of the filter member 1 which it is desired to .seal off from the filtrate space 33, and is closed by t e band 7.

By appropriately choosing the dimensions of the .surface. of the fork 14 exposed to. the pressure chamber 19 in relation to the surface of roller 8 facing the pressure chamber and the moment levers of these pressure surfaces relative to the pivoting shaft 16 it is possible wholly or partly to eliminate the effect of the difference between the pressures in the filtrate chamber 33 and the pressure chamber 19, the latter being accommodated to and being dependent on the pressure outside the filter drum, usually the atmospheric pressure. If the pressure surfaces and moment levers are so chosen as to render the total pressures exerted on those portions of said surfaces lying on opposite sides of the axis of shaft 16 substantially equal, the fork and roller assembly will be balanced and the pressure of roller 8 against band 7 will be dependent upon spring 25. By making the pressure surface of fork 14 or the moment lever thereof or both greater than those of the roller the influence of the fork will dominate over that of the roller causing the latter perpetually to be pressed towards the filter plate 1 by a pressure dependent on the pressure head in the pressure chamber 19 over that in the filtrate chamber 33. In this case the spring 25 may, if desired, be dispensed with.

In Fig. 4 a modified structure is illustrated which may with advantage be used in shoes having no travelling band. This embodiment is in many respects similar to that illustrated in Figs. 1 to 3 and therefore substantially only structural features different from those described will now be particularly described.

The central wall 13 of the stationary shoe member of Fig. 4 is provided with a pair of piston devices (one at each end of the wall) each comprising an opening 40 in said wall, which may with advantage be cylindric, and a pressure actuated plunger or piston 41 movable therein and sealed against the wall of the opening by packing 42. The plungers 41 thus form closing members for the passages provided by openings 40 between the filtrate chamber 33 and chamber 19, and the latter is secluded from the rest of the filtrate chamber by wall 13, rollers 8 and plungers 41. Each plunger is secured to a rod 43 journalled in bearing 44 and carrying at the end projecting into the filter chamber 33, a pressure transmitting lever 45 by means of pin 46. Lever 45 is pivotally supported on the stationary portion of the shoe, by means of a pin 47 and has a projection 48 in which roller 8 is pivoted by means of shaft 49, the projection being so arranged as to form with the body of arm 45 a double armed lever pivoted about the stationary axis of pin 47 and constituting supporting means for the movable sealing shoe member represented by roller 8. The projection 48 is pressed by a spring 25 in a direction to cause roller 8 to engage the inner surface of the perforated shell or plate 1 of the filter drum with a certain pressure, while an inwardly directed force on lever 45 resulting from the differential pressure on'plunger 41 causes a corresponding movement of roller 8 in an outward direction to force the roller against the inner surface of the drum. The roller 8 is sealed against the stationary wall 13 by means of a sealing bar 17 housed in a groove 18 in the wall 13 and, in cooperation with the wall 13 and the roller at the opposite end of said wall, shields off the portion of the drum shell at the Zone where the fibre cake is removed. The groove communicates with the pressure chamber 19 by channels 2.6 similarly to the embodiment of Figs. 1-3 so that bar 17 is subjected to the pressure of chamber 19 and consequently the pressure outside plate 1, as chamber 19 communicates freely with the exterior of the drum through the perforations of plate 1. By adequately proportioning the surface-50 of roller 8 exposed to chamber 19 in relation to the surface of plunger 41 exposed to chamber 19 and the effective moment levers of these pressure surfaces relative to the pivoting pin 47 i. e., the relative lengths of

lever arms

45 and 48, the plunger may act as a counterbalancing member for the associated roller and the engaging pressure of roiler 8 against filter plate 1 can be made wholly or partly independent of the var a ons in the differential pressure between the filtrate chamber 33 and the pressure chamber 19. If desired, the influence of the plunger can be made to dominate so that the pressure head in chamber 19 will provide a positive engaging pressure of roller 8 against plate 1. In this case spring 25 may be dispensed with.

In Figs. 5 and 6 the invention is illustrated as embodied in a structure of the type illustrated in Fig. 5 of my earlier Patent No. 2,560,581. The filter drum represented by the foraminous cylindric plate 1 is submerged in the fibre suspension 5 and the fibres are picked up on the plate as described above forming a fibre cake 4 which is removed by doctor 3 at a place somewhat above the level of suspension 5. The naked portion of plate 1 between doctor 3 and suspension 5 is sealed by the shielding shoe having a stationary shoe member 12 supported in the interior of the drum. The stationary shoe member 12 has a rigid central support plate or wall 60 on which a movable sealing shoe member or shield 61 is suspended in cars 62 by pivotally mounted supporting means comprising links or arms 63 secured by pins 76 to a shaft 64 journalled in the ears for rotation about a stationary axis and pins 65 journalled in the shield. At either end of the shield 61 springs 25 are provided between supports 66 in the shield and 67 adjustably mounted in the stationary plate 60 by means of bolt 68. These springs warrant a given engagement pressure of shield 61 against the filter plate 1. The-shield has a recess 71 situated just opposite the place where the fibre cake 4 is to be removed and a pipe 72 connected to air pipe 73 by a flexible conduit 74 opens in this recess, these means being provided for supplying pressure air below the cake to facilitate its removal.

An arm 75 is also secured to the shaft 64 by a pin 76 so that arm 75 and links 63 are rigidly connected to each other and form the equivalent of a double-armed lever. The other end of arm 75 is rotatably secured to the piston rod 77 of the piston 78, movable in the piston cylinder '79, which is tightened along the wall of the cylinder by a packing 80. The piston rod 77 is journalled in a bearing 81 in the bottom of cylinder 79. The space within cylinder 79 constitutes a chamber which is sealed ofi or secluded from the remainder of the filtrate space 33 by the surrounding portion of plate 60, piston 78 and packing 80, the piston 78 forming a closing member for the end of said chamber which would otherwise provide a passage between it and said filtrate space. Although cylinder 79 is illustrated as having a circular cross section this is not essential but the shape may be different, such as rectangular. The cylinder chamber communicates through a pipe 82 with any convenient source of pressure medium, such as the surrounding atmosphere. The

pipes

73 and 82 extend outside the drum preferably through the trunnion of the drum in a manner not shown but obvious to those skilled in the art.

When in operation of the drum a vacuum prevails in the interior 33 thereof the atmospheric pressure has a tendency to press shield 61 inwards against the pressure of springs 25 as the atmosphere has access to a great portion of the outer surface of the shield through the perforations of the filter plate 1 whereas the inner surface of the shield is exposed to the vacuum inside the drum. On the other hand, in case pipe 82 leads to the atmosphere, atmospheric pressure prevails also in cylinder 79 and imparts to piston 78a tendency to move outwards of the cylinder, thus rotating arm 75 and link 63 secured thereto so as to press shield 61 outwards. By properly proportioning the surfaces of shield 61 and piston 78 exposed to the differential pressure of the atmospheric pressure and the vacuum of the drum and the lengths of link 63 and arm 75, the piston may act as a counterbalancing member for the shield and the pressure of the atmosphere over the pressure in the interior of the drum can be balanced so that the influence of variations in the vacuum in the drum upon the engagement pressureof shield 61 will be reduced or eliminated. By increasing the surface of piston 78 or the length of arm or both the influence of the piston may be dominating so that a positive engagement pressure will be obtained by the influence of the pressure in cylinder 79. In this case springs 25 can be made weaker or be entirely dispensed with.

In addition to or supplementary to the balancing efle'ct obtained by adequately dimensioning the pressure surface of piston 78 and the length of arm 75 the present object can be obtained by using a medium for piston cylinder 79 of superatmospheric pressure.

The modified structure illustrated in Fig. 7 is substantially the same as that of Fig. 4 with the addition of an endless flexible band 7 running over rollers 8 and a pipe 26 for supplying fluid under pressure to chamber 19, which added elements are counterparts of those bearing; the same reference characters in Figs. 1-3. The construction and mode of operation of the shielding shoe of Fig. 7 will be obvious from the foregoing description of the devices of Figs. 1-3 and 4.

As used in the foregoing specification and appendedt claims, the word pressure will be understood to meant fluid pressure, except Where it is evident from the context: that mechanical pressure is intended.

I claim:

1. A shielding shoe for filters of the type embodying a movable perforated filter member adapted to pick up: on its one side solids from a suspension and to discharge: suspension liquid at its other side after passage through the openings thereof under the influence of a differential pressure between said sides, said shoe being substantially coextensive with the width of the perforated portion of said filter member and comprising a rigid stationary shoe member, supporting means pivotally mounted on said stationary shoe member for movement about a stationary axis so located that portions of said supporting means lie on opposite sides of said axis, a roller rotatably supported by said supporting means with the roller axis offset from said stationary axis, an endless flexible member of impervious material movable about said roller and along with and in part in contact with said movable filter member, said stationary shoe member including walls cooperating with said supporting means and said roller to form a chamber having an open side facing said movable filter member, means including the part of said endless member in contact with said movable filter member for closing the open side of said chamber and rendering the latter substantially liquidtight, and means for supplying a fluid under pressure to said chamber, the surfaces of.said roller and said supporting means exposed to the pressure insaid chamber being so dimensioned and disposed relative to the stationary pivotal axis of said supporting means as to render the total pressures exerted on those portions of said surfaces lying on opposite sides of said axis substantially equal.

2. A shielding shoe for filters of the type embodying a movable perforated filter member adapted to pick up on its one side solids from a suspension and to discharge suspension liquid at its other-side after passage through the openings thereof under the influence of a differential pressure between said sides, said shoe being substantially coextensive with the width of the perforated portion of said filter member and comprising a rigid stationary shoe member, supporting means pivotally mounted on said stationary shoe member, a roller rotatably supported by said supporting means, and an endless flexible member ofimpervious material movable about said roller and along with and in part in contact with said movable filter member, said stationary shoe member being so constructed and arranged as to form in cooperation with said supporting means and said roller the walls of a chamber having an opening facing said movable filter member, means including the part of said endless member in contact with said filter member for closing said zms goo z opening and forming with said stationary shoe member a substantially liquid tight chamber, said supporting means comprising a U-shaped member having two legs and a bottom portion, means for journaling said roller in said legs, and means for pivotally connecting said legs to said stationary shoe member for movement about a stationary axis olfset from the axis of said roller.

3. A shielding shoe for filters of the type embodying a movable perforated filter member adapted to pick up onits one side solids froma suspension and to discharge suspension liquid at its other side after passage through the openings thereof'uuder the influence of a differential pressure between said sides, said shoe being substantially coextensive with the width of the perforated portion of said filter member and comprising a rigid stationary shoe member, at least one roller adapted to be supported on said stationary shoe member, an endless flexible member of impervious material movable about said roller and along with and in part in contact with said movable filter member, said stationary shoe member being so constructed and arranged as to form in cooperation with said roller the walls of a chamber having an opening facing said movable filter member, means including the part of said endless member in contact with said filter member for closing said opening and forming with said stationary shoe member a substantially liquid tight chamber, means for supplying a fluid under pressure to said chamber, a lever at each end of said roller pivoted on said stationary shoe member for movement about a stationary axis offset from the axis of said roller, said roller being journaled in one arm of each of said levers, a pair of piston devices in said stationary member responsive to the pressure in said chamber, and means connecting the pistons of said piston, devices to the other arms of said levers at points ofiset from said stationary axis.

4. A filter for extracting fibres or the like in the form of a cake from a fibre pulp mixture, comprising a rotary drum adapted to be covered over a portion of itscircumference by the pulp mixture and to maintain in its interior a pressure lower than that which prevails outside the drum, said drumhaving a perforated shell, a shielding shoein the interior of said drum including a sealing shoe member adjustable towards and away from the inner surface of said perforated shell and cooperating with a portion of said inner surface of' said shell at a zone at which the fibre cake is removed from the outer face of said shell so as to shield otf said portion of the shellat said zone from the pressure in the interior of said drum, said shielding shoe having a wall with. an opening therein, a pressure actuated member closing said opening. and capable of movement relatively to said wall under the influence of the pressure difference betweenthe pressure exerted against the side of said wall facing said drum and the pressure in the interior of said drum, and a pressure transmitting device supported in the interior of said drum and operatively connected to said pressure actuatedmember and to said sealing shoe member in such manner that said device is causedto move under theinfiuence of an inwardly directed force resulting, from the differential pressure on said pressure actuated member and thereby to cause a corresponding movement of, said sealing shoe member inthe outward direction to force said sealing shoe member against the inner face of said drum.

5. A filter for extracting fibres or the like in the form of-a cake from a fibre pulp mixture, comprising a rotary drum adapted to be covered over a portion of its circumference by thepulp mixture and to maintain in its inte-rior a pressure lower than that which prevails outside thedrum, said drum having a perforated-shell, a shielding shoe in the interior of saiddrum including a sealing shoe member adjustable towards and away from the inner surface of'said perforated shell at a zone at which the fibrecake is removed from the outer 'faceof said shell so-as tto shield oif said portion of the shell at-saidzone from the pressure in theinterior of said drum, said shielding shoe having a wall withan opening therein, a pressure actuated member closing; said opening and capable of movement relatively tosaid wall under the influence of the pressure difference between the pressure exerted against the side of said wall facing said drum and the pressure in the interior of said drum, and a pressure transmitting device comprising a double-armed lever pivotally supported in the interior of said drum for movement about a stationary axis, means pivotally connecting one arm of said lever to said pressure actuated member, and means pivotally connecting the other arm of said lever to said'sealing shoe member, whereby inward movement of said first mentioned arm of said lever under the action ofan inwarddirected force produced by the differential pressure on said pressure actuated member results in outward movement of said last mentioned arm of said lever producing an outward directed force acting upon said sealing shoe member.

6. A filter as claimedin claim 5 in which the areas of said pressure actuated member and said sealing shoe member subject to said pressure difference are so related and said arms of said lever have such relative lengths that the product of, the area of said pressure actuated member and thelength of the lever arm connected to said member is greater than the product of the area of said sealing shoe member and the length of the lever arm connected to said sealing shoe member.

7. A shielding shoe for filters of the type embodying a movable perforated filter member adapted to pick up on its one side solids from a suspension and to discharge suspension liquid at its other side after passage through the openings thereof under the influence of a differential pressurebetween said sides, said shoe comprising a rigid stationary shoe member, supporting means pivotally mounted on said stationary shoe memher, for movement, about a stationary axis, said stationary hoemember including a central wall substantially coex tensive. with the width of said filter member and side walls extending in the direction of movement of said filter, member, asealing shoe member at at least one of the ends of'said' central wall movable towards and away from said filter member, sealing means between said sealing shoe member'and said central wall, said sealing shoe member being supported by said supporting means, and means including said central wall, side walls, supporting means, sealing meansand sealing shoe member forming an enclosure having one side substantially co extensive with a portion of said filter member, said sealing shoe, member and said supporting means having surfaces lying on opposite sides of the stationary pivotal axis ofsaid' supporting means exposed to the pressure in said enclosure, said, surfaces being so dimensioned and disposed relative to said axis as to render the total pressures exerted on those portions of said surfaces lying on opposite sides of. said axis substantially equal.

8. A shielding shoe for'filters of the type embodying a movable perforated filter member adapted to pick up on its one side solids from a suspension and to discharge suspension liquid at" its other side after passage through the openings thereof under the influence of a differential pressure between said sides, said shoe comprising a rigid stationary shoe member, supporting means pivotally mounted on said stationary shoe member for movement about a stationary axis, said stationary shoe member including a central wall substantially coextensive with the width of said'filtermember and side walls extending in the direction of movement of said filter member, a sealingshoe member at at least one of the ends of said central wall movabletowards and away from said filter member, sealing means between said sealing shoe member and said central wall, said sealing shoe member being supported by saidsupporting means, and

means ineluding said central "wall, side walls, supporting means, sealing means and sealing shoe member forming an enclosure having one side substantially coextensive with a portion of said filter member, said sealing shoe member and said supporting means having surfaces 1ying on opposite sides of the stationary pivotal axis of said supporting means exposed to the differential pres sure between the atmosphere outside said walls and the pressure in said enclosure, said surfaces being so dimensioned and disposed relative to said axis as to render the total pressures exerted on those portions of said surfaces lying on opposite sides of said axis substantially equal.

9. A shielding shoe for filters of the type embodying a movable perforated filter member adapted to pick up on its one side solids from a suspension and to discharge suspension liquid at its other side after passage through the openings thereof under the influence of a differential pressure between said sides, said shoe being substantially coextensive with the width of the perforated portion of said filter member and comprising a rigid stationary shoe member, a lever pivoted on said stationary shoe member for movement about a stationary axis, a sealing shoe member adapted to cover the portion of said filter member to be sealed, said sealing shoe member being pivotally supported on one arm of said lever, a piston chamber in said stationary member, a piston in said piston chamber responsive to the differential pressure between said chamber and the atmosphere surrounding said stationary shoe member, and means connecting said piston to the other arm of said lever.

10. A shielding shoe for filters of the type embodying a movable perforated filter member adapted to pick up on its one side solids from a suspension and to discharge suspension liquid at its other side after passage through the openings thereof under the influence of a differential pressure between said sides, said shoe comprising a rigid stationary shoe member, a lever pivoted on said stationary shoe member for movement about a stationary axis, said stationary shoe member including a central wall substantially coextensive with the width of said filter member and side walls extending in the direction of movement of said filter member, a sealing shoe member at at least one of the ends of said central wall movable towards and away from said filter member, sealing means between said sealing shoe member and said central wall, said sealing shoe member being pivotally secured to one arm of said lever, said central wall, side walls, sealing means and sealing shoe member forming together with a portion of said filter member an enclosure, a piston device in said central wall responsive to the pressure in said enclosure, and means connecting the piston of said piston device to the other arm of said lever.

11. A shielding shoe for filters of the type embodying a movable perforated filter member adapted to pick up on its one side solids from a suspension and to discharge suspension liquid to a filtrate space at its other side after passage through the openings thereof under the influence of a differential pressure between said sides, said shoe being substantially coextensive with the width of the perforated portion of said filter member and comprising a rigid stationary shoe member, supporting means pivotally mounted on and sealed against said stationary shoe member for movement about a stationary axis, a sealing shoe member movable towards and away from said filter member, said sealing shoe member being supported by said supporting means, means for sealing off a portion of said filtrate space from the remainder thereof to form a chamber wherein a different pressure prevails from that prevailing in the remainder of said filtrate space, said sealing shoe member and said supporting means having surfaces against which said different pressures are exerted and said surfaces being so disposed with respect to the pivotal axis of said supporting means that the differential pressure exerted on the surfaces of said supporting means tends to move said sealing shoe member in a direction relative to said filter member opposite to that in which said sealing shoe member tends to be moved by the differential pressure exerted on the surfaces thereof.

12. A shielding shoe 'for filters of the type embodying a movable perforated filter member adapted to pick up on its one side solids from a suspension and to discharge the suspension liquid to a filtrate space at its other side after passage through the openings thereof under the influence of a differential pressure between said sides, said shoe being substantially coextensive with the width of the perforated portion of said filter member and comprising a rigid stationary shoe member, supporting means pivotally mounted on said stationary shoe member for movement about a stationary axis, a sealing shoe member movable towards and away from said filter member, a wall formed at least in part by said sealing shoe member for sealing off a portion of said filter member from said filtrate space and separating said filtrate space from another space wherein a different pressure prevails from that prevailing in said filtrate space, said sealing shoe member having one of its sides exposed to said filtrate space and its opposite side exposed to said other space, means in said filtrate space enclosing a space secluded from the rest of said filtrate space and providing a passage between said filtrate space and said secluded space, the pressure prevailing in said secluded space being the same as that prevailing in said other space, and a closing member sealed aaginst said stationary shoe member and movable relative thereto in said passage under the influence of the differential pressure between said filtrate space and said secluded space, said closing member and said sealing shoe member being supported by said supporting means in such mutual relationship to the stationary pivotal axis of said supporting means that the rotary movement of said supporting means induced by the movement of said closing member in said passage in a direction from said filtrate space towards said secluded space is opposite to the rotary movement of said supporting means induced by the movement of said sealing shoe member in the direction towards said filter member.

13. A shielding shoe for filters of the type embodying a movable perforated filter member adapted to pick up on its one side solids from a suspension and to discharge the suspension liquid to a filtrate space at its other side after passage through the openings thereof under the influence of a differential pressure between said sides, said shoe being substantially coextensive with the width of the perforated portion of said filter member and comprising a rigid stationary shoe member, a sealing shoe member having one surface exposed to the pressure prevailing in said filtrate space and a second surface exposed to a different pressure, means for supporting said sealing shoe member on said stationary shoe member for movement towards and away from said filter member, said supporting means being pivotally mounted on said stationary shoe member for movement about a stationary axis, and a counterbalancing member movable relative to and sealed against said stationary shoe member and having one surface exposed to the pressure prevailing in said filtrate space and a second surface exposed to the same pressure as that to which the second surface of said sealing shoe member is exposed, said sealing shoe member, supporting means and counterbalancing member being so interconnected and disposed with respect to the stationary pivotal axis of said supporting means that the differential pressures exerted on said sealing shoe member and said counterbalancing member tend to move said supporting means in opposite directions about said pivotal axis.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Elling May 10, 1921 Berry May 10,. 1927 Haug Mar. 8, 1932 Hillel et a1. Sept. 8, 1936 Cuno et a]; Apr. 20, I937 Euahs May 19, 1942 12 Street July 21, 1942 Vitalius et' ali Dec. 19, 1950 Lefiier- July- 17, 1951 Leffier July 17, 1951 FOREIGN PATENTS Germany Nov. 3, 1921 Great Britain Dec. 24', 1931