Abstrict
In a rotating filter a filter cloth (18) is arranged outside a
filter skeleton (34, 36). The filter cloth is provided with at least
one bending resistant element (37, 38) extending through a pocket
(49) arranged in or on the filter cloth (18) to restrain bulging
of the filter cloth due to forces perpendicular to the plane of
the filter cloth. The filter sector skeleton (34, 35, 36, 45) is
provided with radially outer support and guide means (43, 44) and
radially inner support and guide means (45, 46). The bending resistant
element (37, 38) is at its ends adapted for detachable engagement
with said means such that the ends are undisplaceable perpendicularly
to the support and guide means (43, 44, 45, 46). Preferably, at
least one end of the bending resistant element is displaceable in
the circumferential direction of the filter sector.
Claims
We claim:
1. A filter cloth device for rotating filters, said device comprising:
a filter sector skeleton (34, 35, 36, 45) able to be carried by
a filter rotor (15);
a filter cloth, said filter cloth (18) being arranged outside said
filter sector skeleton;
at least one bending resistant element (37-42) extending through
a pocket (49, 50) connected to the filter cloth (18) to restrain
bulging of the filter cloth due to forces perpendicular to the plane
of the filter cloth, said filter sector skeleton (34, 35, 36, 45)
having radially outer support and guide means (43, 44) and radially
inner support and guide means (45, 46; 73, 74) parallel thereto,
said bending resistant element (37-42) at its ends being detachably
engageable with said support and guide means such that the ends
are undisplaceably perpendicularly fixed to the support and guide
means (43-47; 73, 74) only by compression.
2. A filter cloth device according to claim 1, characterized in
that at least one end of the bending resistant element (37-42) is
displaceable in the peripheral direction (A) of the filter rotor
along one of the support and guide means (43-46).
3. A filter cloth device according to claim 2, characterized in
that the radially outer support and guide means (43, 44, 73, 74)
and the radially inner support and guide means (45, 46) have the
shape of concentric circular arcs.
4. A filter cloth device according to claim 1, characterized in
that the radially outer support and guide means (43, 44; 73, 74)
and the radially inner support and guide means (45, 46) have the
shape of concentric circular arcs.
5. A filter cloth device according to claim 1, characterized in
that the filter cloth (18) at its radially inner edge is provided
with engagement means (51, 52; 58) adapted to cooperate with fastening
means (53, 54; 56) provided at the radially inner end of the filter
sector skeleton.
6. A filter sector device according to claim 1, characterized in
that the filter sector skeleton at its radially outer periphery
(36) is provided with a tightening device (63, 64, 65) adapted to
tighten the filter cloth (18) radially outwardly upon actuation.
7. A filter cloth device according to claim 1, characterized in
that at least one of the parts (34, 35) defining the filter sector
angle of the filter sector skeleton is provided with a tightening
device adapted to tighten the filter cloth (18) in the circumferential
direction upon actuation.
8. A filter cloth device according to claim 7, characterized in
that the tightening device includes a part (65) unfixed to the filter
sector skeleton and positioned between the filter cloth (18) and
the filter sector skeleton and being actuatable from the interior
of the filter sector.
9. A filter cloth device according to claim 1, characterized in
that the tightening device includes a part (65) unfixed to the filter
sector skeleton and positioned between the filter cloth (18) and
the filter sector skeleton and being actuatable from the interior
of the filter sector.
10. A filter cloth device for rotating filters, said device comprising:
a filter sector skeleton (34, 35, 36, 45) able to be carried by
a filter rotor (15);
a filter cloth, said filter cloth (18) being arranged outside said
filter sector skeleton;
at least one bending resistant element (37-42) extending through
a pocket (49, 50) connected to the filter cloth (18) to restrain
bulging of the filter cloth due to forces perpendicular to the plane
of the filter cloth, said filter sector skeleton (34, 35, 36, 45)
having radially outer support and guide means (43, 44) and radially
inner support and guide means (45, 46; 73, 74) parallel thereto,
said bending resistant element (37-42) at its ends being detachably
engageable with said support and guide means wherein the ends have
slots for slidingly mounting onto said support and guide means (43-47;
73, 74).
Description The present invention concerns a filter cloth device for rotating
filters, said filter cloth being arranged outside a filter sector
skeleton adapted to be carried by a filter rotor, and said filter
cloth being provided with at least one bending resistant element
extending through a pocket arranged in or on the filter cloth to
restrain bulging of the filter cloth due to forces perpendicular
to the plane of the filter cloth.
For filtering or screening in process industry there exist different
kinds of rotating filters to separate solid particles from liquids,
such as cellulose suspensions within the paper industry, either
to thicken a suspension or to clean a liquid from particles. In
a common kind of such filters a plurality of filter discs are mounted
on a filter rotor, each filter disc comprising a plurality of annularly
arranged filter sectors. Each filter sector comprises a skeleton
outside which is arranged a screen cloth. The filter rotor is arranged
partly submerged in a vessel. Screening can take place either from
the vessel, which then includes suspension or the like, to the inner
of each filter sector, as is the case in the filter according to
for instance SE-B-7406315 (corresponding to U.S. Pat. No. 4,056,473),
or, if the vessel is adapted to receive filtrate, from the inner
of the filter sectors to the vessel, as is the case in the filter
according to SE-A-9000557-0. In the latter kind of filter, which
is used particularly when screening so called fines, i.e., very
small particles, it is a problem that the screen cloth is subjected
to large stresses due to internal pressure when a filter sector
raises above the filtrate surface in the vessel and a large amount
of unfiltered liquid is still present within the filter sector.
Then the filter cloth tends to bulge out from the filter sector
skeleton and to be stretched. Supporting ribs on the outside of
the filter cloth attached to the filter skeleton, as suggested in
FR-A-1 108 828, is one possible but practically unuseful solution
to this problem, since the filter cloth from time to time must be
changed. Another problem in this kind of filter is that spraying
with water under high pressure takes place from the outside in order
to remove particles deposited on the inside of the filter cloth,
which leads to inward bulging of the filter cloth, which thereby
may rupture. To some extent this problem may be remedied by ribs
on the inside of the filter cloth attached to the filter skeleton,
but still it can not be avoided that the filter cloth slides against
these ribs and is subject to wear. Thus, a filter cloth must be
permanently kept so tight that it cannot bulge in any direction.
However, too much convexly curved surfaces which could facilitate
keeping the cloth tightened result in a filter sector taking up
too much room.
U.S. Pat. No. 4,578,192 describes a device for supporting a filter
cloth against bulging when compressed air is introduced into a filter
sector to remove a filter cake deposited on the outside of the filter
cloth. This known device, referred to in the preamble, includes
a U-shaped hair pin like clip having two bending resistant elements
in the shape of extended legs integrated in the filter cloth and
connected by a spacing means, the length of which corresponds to
the thickness of the filter sector. In one embodiment, the two legs
are introduced radially from the outside into pockets in the filter
cloth on both sides of the filter sector. With necessity this must
be done with both legs at the same time and in parallel, which in
turn necessitates that the radially outer openings of the pockets
are located exactly opposite to each other and that the pockets
have the same radial direction. Further, the known device ensures
adequate support against outwardly directed pressure only in the
end where the spacing means keep the two legs together.
The object of the present invention is to provide a solution to
the problems stated such that the filter cloth is supported against
pressure influence in both directions but still is simple to change
and such that the bending resistant elements provide support to
the filter cloth at both its ends. Further, the stiff elements shall
enable the filter cloth to move in the circumferencial direction.
This has been accomplished in that the invention has been given
the characteristics stated in the appended claims.
The invention will be described hereinafter reference being made
to the accompanying drawings, wherein
FIG. 1 is an axial section through an embodiment of a disc filter,
FIG. 2 is a cross section through the filter of FIG. 1,
FIG. 3 is a partly broken up side view of a filter sector according
to the present invention,
FIG. 4 is a side view at an enlarged scale of the filter sector
of FIG. 3 having the filter cloth entirely removed,
FIG. 5 is a view from underneath of the filter sector of FIG. 4
having a filter cloth mounted thereon,
FIG. 6 is a radial section at an enlarged scale through the filter
sector of FIG. 5,
FIG. 7 is a section similar to that according to FIG. 6 but through
another embodiment of the filter sector,
FIG. 8 shows a radially outer support and guide rib according to
another embodiment of the invention,
FIG. 9 is a section taken after line IX--IX of FIG. 8,
FIG. 10 shows a detail of a radial section through a filter sector,
and
FIGS. 11, 12 and 13 show different possibilities to provide the
filter cloth with channels for the radial rods.
The disc filter shown in FIGS. 1 and 2 is of the kind which is
the subject of SE-A-9000557-0 and is utilized herein merely as an
example of a filter at which the device according to the present
invention is applicable. In a machine stand 1 having a forward wall
2 and a backward wall 3 is arranged a filtrate vessel 4. Portions
of the walls 2 and 3 also constitute the forward and backward walls
of the vessel. By means of bearings 5 a shaft 6, in a non-shown
manner being rotatably driveable in the direction of arrow A, is
journalled on a bracket attached to the backward wall 3. On the
inner end of this shaft extending through the backward wall 3 is
non-rotatably mounted a hollow shaft or drum 7 having a hexagonal
cross-section and having an end wall 8 carried by the shaft 6 and
tightly closing the end of the drum facing the backward wall 3.
The end of the drum 7 facing the wall 2 is open and is terminated
by a circular ring 9 slide-sealingly contacted by a likewise annular
lip seal 10 carried by an annular cylinder 11 arranged at the wall
2. More precisely, the cylinder 11 is mounted at the inside of a
door 12 tightly closing the forward wall 2. The drum is tight per
se, apart form its open end which, however, is sealed by means of
the sealing arrangement described above.
In the flat limiting surfaces 7' of the drum 7 are openings 13
through which the interior of filter sectors 14 mounted on the outside
of the drum communicates with the interior of the drum. In the example
shown the drum 7 is provided with twelve filter sectors 14 arranged
in the shape of two filter discs 15 and 16. Each filter sector 14
has substantially the shape of a circular sector having a truncated
apex and comprising a skeleton 17 on which is arranged a filter
cloth 18. Radially outwardly the filter sector is closed, whereas
it radially inwardly has an opening 19 the internal shape of which
corresponds to the shape of the openings 13 of the drum 7.
A collecting trough 20 extending into the drum 7 somewhat beyond
the inner filter disc 16 is rigidly and liquid-tightly mounted onto
the inside of the door 12. The bottom 21 of the collecting trough
is somewhat inclining from the inner end towards the door 12 where
it merges with an outlet 22 for solid particles. Also rigidly and
liquid-tightly mounted at the door 12 is an inlet tube 23 for the
liquid to be filtered. The inlet tube 23 extends into the drum at
least to the inner filter disc 16. The inlet tube 23 has openings
or tube pieces 24 at its periphery located opposite to the filter
discs 15 and 16, respectively, to direct the incoming fluid towards
the openings 13 in the drum 7 and, thereby, towards the interior
of the filter sectors of the filter discs.
A filtrate outlet 25 extends from the vessel 4 through the wall
2.
On the machine stand is mounted a spray device 26 serving to direct
jets of water via an inlet 27, an axially directed conduit 28 and
conduits 29 connected thereto and extending along the entire radial
extension of the filter discs 15 and 16 and having nozzles directed
towards the filter sectors, towards the filter cloth 18 of the filter
sectors for the purpose of cleaning it from particles clogging the
openings of the filter cloth, but also for knocking off larger particles
deposited on the inside of the filter cloth.
The function of the disc filter according to the present invention
is the following. The liquid or suspension 30 to be filtered is
introduced through inlet tube 23 and its openings 24 into the interior
of the liquid tight drum 7. The normal operational level of the
liquid 30 is indicated at L.sub.1 (FIG. 2) and is always below the
upper edge of the collecting trough 20. The liquid will now flow
into the filter sectors located below the liquid level L.sub.1 through
the openings 13 in the drum 7 and the openings 19 in the filter
sectors, and, under filtration through the filter cloth 18 thereof,
out to the vessel 4 and therefrom to the outlet 25. By controlling
the normal operational level L.sub.2 of the filtrate 31 in the vessel
4 such that it is always below the level L.sub.1, the filtering
process takes place solely depending on the static pressure difference
between the levels L.sub.1 and L.sub.2. Naturally, some filtration
of liquid remaining in a filter sector takes place during the time
it raises above the liquid level L.sub.2. When the edge of a filter
sector leading in the direction of rotation is in a position approximately
two o'clock, i.e., where the spray conduit 29 is located in FIG.
2, the spray device 26 sprays the outside of the filter cloth 18.
Particles hereby released from the filter cloth fall, or, follow
the water down through the interior of the filter sector and are
collected in the collecting trough 21, from where they are discharged
through the outlet 22.
Due to the fact that the difference between the levels L.sub.1
and L.sub.2, as is shown in FIGS. 1 and 2, may be rather great,
the filter cloth is exposed to a considerable internal pressure
during a great part of the filtering process which tends to bulge
the filter cloth outwards. On the other hand, during the above-mentioned
spraying from the outside the filter cloth is exposed to substantial
inwardly directed pressures. Particularly the first mentioned pressure,
but also the alterations of pressure directions, result in that
the filter cloth may be stretched and eventually rupture due to
fatigue.
Therefore, according to the present invention, the filter cloth
is provided with at least one and preferably a plurality of bulge
restricting stiffeners in the shape of rails, ribs, profiles or
other bending resistant (rigid) elements extending through pockets
in or on the filter cloth along a substantial portion of the radial
extension of the filter cloth, preferably in the radial direction
of a filter sector. The stiffening elements should be as narrow
as possible in order to as little as possible restrict the operative
filtering area of the filter cloth. The stiffening elements are
preferably arranged at the inside of the filter cloth and their
ends are supported against portions of the filter sector skeleton
in the shape of support and guide means, particularly against displacement
perpendicularly to the plane of the filter cloth, whereas, in certain
cases, freedom of displacement is preferred in the plane of the
filter cloth.
Some embodiments of stiffening elements and their cooperation with
the filter cloth 18 and the filter skeleton 17 will be described
hereinafter, as well as some arrangements to otherwise keep the
filter cloth in a tightened condition to a filter sector.
In the example shown the filter skeleton 17 includes a fastening
flange 32 having an internal opening forming the opening 19 of the
filter sector. The fastening flange 32 is flat and provided with
mounting holes 33 for its mounting on one of the flat surfaces 7'
of the filter drum. From locations close to the ends of the fastening
flange two mutually diverging straight ribs 34 and 35 extend, the
relative angle of which define the sector angle and which in their
radially outer ends are connected by an arcuately curved rib 36
having its centre in the centre of the sector and, consequently,
in the centre of the filter rotor. In other embodiments the rib
36 may be straight i.e., when the outer circumference of the filter
disc describes a polygon. According to the conventional art, a filter
sector skeleton often further includes a plurality of radially and/or
otherwise directed ribs which are fixedly attached to the fastening
flanges 32, the curved rib 36 and possible the straight ribs 34
and 35 in order to serve as internal support for a filter cloth.
In the embodiment of a filter sector according to the present invention
shown in FIGS. 3, 4 and 5 there are instead arranged three circumferentially
displaceable radially directed ribs on each side of the filter sector
of which the three ribs 37, 38 and 39 located on one side of the
filter sector are seen in FIG. 4 and only one of the three ribs
40, 41 and 42 on the other side of the sector, viz., the middle
rib 41 is seen in FIG. 5.
The displaceability in the circumferential direction is achieved
in that the radially outer and inner ends of the ribs are supported
by and guided along radially outer arcuately curved support and
guide rails 43 and 44, and radially inner likewise arcuately curved
support and guide rails 45 and 46, respectively, which are concentrical
to the former ones. Preferably, the arcuate shapes have their common
centre in the rotational centre of the filter rotor or drum 7. In
this case, the ribs 36-42 have equal length, whereby mixing-up between
the ribs is avoided at mounting. In such embodiments where the rib
36 is straight the radially outer and the radially inner support
and guide rails are instead parallel, the ribs 36-42 having different
lengths. The outer guide ribs 43 and 44, which are mutually substantially
parallel, are fixedly arranged on the radially inner side of the
curved rail 36, whereas the inner guide rails 45 and 46 are fixedly
arranged on the fastening flange 32 on either sides of its opening
and, thus, adapted in their longitudinal extension to the oval shape
of the opening shown. Of course, other shapes of the opening are
possible, such as rectangular. For co-operation with the guide rails
the ribs are provided in their ends with slots 47 and 48 shown in
FIG. 6 which are somewhat wider than the thickness of the guide
rails and engage over the outer guide rails 43 and 44 and the inner
guide rails 45 and 46, respectively. Hereby the ends of the ribs
are displaceable along a respective guide rail, whereas they are
restricted from displacement perpendicularly thereto. The outer
guide rails here comprise the flanges of an U-section, the web of
which is attached to the curved rib 36.
In order to keep the filter cloth 18 to the ribs 37-42 not only
against external but also internal pressure, the ribs are integrated
in the filter cloth which here is to be understood such that movement
of the filter cloth perpendicular to its plane is substantially
immediately transferred to the ribs. One form of such integration
(not shown in FIGS. 3, 4 and 5) is shown in FIG. 6 where the ribs
extend through pockets or tubular channels 49,50 arranged on the
inside of the filter cloth. These channels may be formed in the
cloth material or be separate channels which in a suitable manner
are connected to the cloth. The internal dimension of the channels
is here adapted to the outer dimension of the ribs such that the
movement of the cloth in the plane of the cloth is transferred to
the ribs if the movement takes place in the circumferential direction,
the ends of the ribs then being free to slide along the guide rails
43-46, whereas the cloth with its channels may slide in the radial
direction along the ribs which are unmovably kept to the guide rails.
In a known manner the filter cloth 18 has the shape of a bag which
is formed after the form of the filter sector and at some edge is
provided with a zip fastener to close the bag after it has been
pulled over the filter sector. Such a zip fastener (not shown) may
be arranged along some of the edges of the bag, for instance the
arcuate edge located over the curved rib 36, which suitably has
an inverted U-section with softly rounded transitions between the
web 36' of the profile and its flanges 35". To keep the open
end of the filter cloth downwards, i.e., against the fastening flange
32, the free lower edges of the cloth are provided with fastening
strips 51,52 which for instance are sewn to the cloth such that
an upwardly turned portion of the respective fastening strip may
be brought from underneath to engagement with a hook-like fastening
profile 53,54 attached to a respective inner guide rail 45,46. After
the fastening strips 51,52 have been introduced into the hook-like
fastening profiles the zip fastener is closed and the filter cloth
18 is shrinked by hot air treatment to tighten the filter cloth.
An alternative of the arrangement shown in FIG. 6 is shown in FIG.
7. Here the outer ends of the ribs are undisplaceable by being introduced
into sleeve-like holders 55 attached to the curved ribs 36, whereas
the inner ends are displaceable in the circumferential direction
in a similar manner as according to FIG. 6. In this instance, however,
the respective inner guide rail 45' is integrated in a holding device
56 comprising a profile attached to the fastening flange 32. This
profile has a partly closed cross-section having an upwardly directed
opening 57 through which the filter cloth extends. The filter cloth
is arranged around a body 58 having for instance circular cross-section
of a somewhat greater width than the opening 57, whereby the filter
cloth is prevented from passing upwards through the opening 57.
At mounting of the filter cloth the body 58 is introduced together
with the filter cloth 18 through an open end of the holding device
56.
When one rib end is fixedly arranged and the other displaceable
the channels may have an increasing cross-section in the direction
towards the displaceable rib end.
The ribs 37-42 shown have a circular cross-section. Other cross-sectional
shapes are also conceivable such as T-shaped, whereby there is also
a possibility to utilize the flanges of the T-profile to glue or
sew the ribs to the filter cloth.
FIGS. 8 and 9 show an embodiment where the curved rib 36 and the
radially outer support and guide rails 43 and 44 are integrated
in a U-section 70. By plastic working the flanges 71 and 72 thereof
are shaped with a number of flat portions 73, 74, respectively,
depressed into the interior of the U-section and serving as guides
for the radially outer end of one rib 37-42 each. By the inwardly
directed portions 75, 76, respectively, of the flanges which connect
the flanges to the flat portions 73, 74, respectively, and which
have a length in the peripheral direction exceeding the thickness
of the ribs, the displaceability of the ends of the ribs is limited.
Hereby is further obtained well defined locations for the ends of
the ribs which facilitates the mounting. Corresponding limiting
guides may be arranged on the radially inner support and guide rails.
In a case where the filter cloth cannot be tightened by shrinkage,
for instance if it is made of metal, the arrangement shown in FIG.
10 is proposed for tightening the cloth in the radial direction.
A nut 63 is welded to the inside of the U-shaped section the flanges
of which form the outer guide rails 43 and 44. Through a nut 63
and non-shown holes in the web of said section and the web 36' of
a curved rib 36 a screw 64 may be threaded thereby pressing against
the inside of the web of a U-shaped section 65 which is loosely
arranged between the curved rib 36 and a filter cloth 18. Suitably,
a plurality of nuts 63 and associated screws 64 are arranged along
the curved rib 36. For tightening the filter cloth in the circumferential
direction similar arrangements (not shown) may be located along
the straight ribs 34 and 35.
Some different possibilities to provide the pockets or channels
49, 50 are shown in FIGS. 11, 12 and 13. In FIG. 11a piece of filter
cloth or other suitable material is shaped to a profile 66 which,
by means of outwardly turned flanges 67, is sewn or glued to the
filter cloth 18. In FIG. 12, in addition, a reinforcing layer 68
of filter cloth or, for instance, a plastic strip is sewn through.
As an alternative, in FIG. 13 a layer 69 of filter cloth or other
suitable material having a greater extension than the flanges 67
of the profile 66 is attached between the profile and the filter
cloth 18.
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