Note: Descriptions are shown in the official language in which they were submitted.
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Apparatus for filtering suspensions
The present invention relates to an apparatus for filtering
suspensions, such as fibre pulp suspensions.
EP-Bl-515492 discloses such an apparatus, which includes at
least one annular vertical hollow filter disc having two opposed
side walls of filter material and forming a central chamber.
Means is provided for rotating the annular filter disc about a
horizontal axis extending centrally through the filter disc.
There are partition walls extending between the opposed side
walls of the filter disc and positioned to divide the interior
of the filter disc into a number of peripheral chambers disposed
in series around the filter disc. Each peripheral chamber
communicates with the central chamber via an outlet. The filter
disc is adapted to be partly submerged in a body of a suspension
to be filtered, such that the central chamber is at least partly
submerged in said body of suspension, whereby a fine fraction of
the suspension is forced by hydrostatic pressure through the
side walls of filter material into the peripheral chambers and
further into the central chamber via the outlets of the
peripheral chambers. Means is provided to maintain a pool of
fine fraction in the central chamber. The partition walls are
arranged such that each peripheral chamber is at least partly
above the pool of fine fraction at some point of the revolution
of the filter disc at the upwardly moving part of the filter
disc, while the outlet of the peripheral chamber opens into the
pool of tine fraction.
The peripheral chambers of this known apparatus function as
droplegs when they move upwardly, because at this stage of
revolution the created mats of coarse particles covering the
filter wall portions of the peripheral chambers are more or less
liquid impervious. This has the advantage that the coarse
fraction of the suspension is dewatered efficiently. This type
of filter disc with integrated droplegs also eliminates the need
for space demanding external droplegs.
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However, a drawback to the apparatus of EP-B1-515492 is that the
rotational speed of the disc is limited, because of the fact
that each upwardly moving peripheral chamber has to be emptied
before it reaches a knock-off zone, in which the mat is sprayed
off the filter walls of the peripheral chamber. In the case of
filtering a fibre pulp suspension of the kind which forms very
tight fibre mats, so that air can only pass very slowly through
the mat into the peripheral chambers, the rotational speed of
the disc has to be particularly low which reduces the capacity.
The object of the present invention is to improve the filtering
capacity of the type of apparatus disclosed in EP-B1-515492.
This object is- fulfilled by a filtering apparatus of the
integrated dropleg type defined above, which is characterized
by venting means effective to supply air into each peripheral
chamber at the upwardly moving part of the filter disc, when the
peripheral chamber is at least partly above the body of
suspension during rotation of the filter disc, in order to speed
up the emptying of the peripheral chamber.
Said supply of air into the peripheral chambers by the venting
means can make the emptying of the peripheral chambers about two
to three times faster, as compared with the known filtering
apparatus. Therefore the rotational speed of the filter disc can
be significantly increased to improve the filtering capacity of
the apparatus.
The venting means is preferably adapted to supply the peripheral
chambers with air from the central chamber. As an alternative,
however, the venting means may comprise valves to the respective
peripheral chambers adapted to open in a predetermined zone,
through which the peripheral chambers pass during rotation of
the filter disc, to allow air from outside the apparatus to
enter the peripheral chambers.
The venting means may form venting passages extending from the
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central chamber into the respective peripheral chambers and
preferably opening in the peripheral chambers proximate to the
radially outermost and leading ends of the peripheral chambers,
as seen in the direction of rotation of the filter disc. Each
venting passage is inoperable as long as it does not communicate
with the air above the pool of fine fraction in the central
chamber. As soon as each peripheral chamber has moved upwardly
to a position in which air can enter the venting passage
associated to the peripheral chamber, the venting passage is
operable to supply air into its peripheral chamber.
The venting passage of each peripheral chamber may open into the
central chamber in advance of the outlet of the peripheral
chamber, as seen in the direction of rotation of the filter
disc, Which makes the function of the venting means particularly
reliable.
In the most simplified embodiment of the apparatus of the
invention, the venting means comprises pipes extending from the
central chamber into the respective peripheral chambers to
positions proximate to the leading ends of the peripheral
chambers, as seen in the direction of rotation of the filter
disc, whereby the interior of the pipes constitutes said venting
passages.
The apparatus disclosed in EP-B1-515492 has means for separating
the developed fine fraction into cloudy and clear fine
fractions. Reference is also made to EP-H1-515502 disclosing
said separating means in more detail. Said fine fraction
separating means includes stationary rigid partition wall means
extending axially in the central chamber and dividing the latter
into at least two axial channels for receiving a cloudy fine
fraction and a clear fine fraction, respectively. The central
chamber is defined by a cylindrical wall and there is fine
fraction discharge means for discharging said cloudy and clear
fine fractions separately from the axial channels. The apparatus
of the present invention may be designed likewise, if desired to
obtain cloudy and clear fine fractions. However, it has been
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proven in practice that the increased rotational speed of the
filter disc made possible by the above defined venting means
results in an increased volume of mixed cloudy and clear fine
fractions. To reduce said volume to at least some extent the
partition wall means may be displaced forwards in the direction
of rotation of the disc, provided that the resulted reduced flow
of clear fine fraction from the apparatus is acceptable.
To allow such a fine fraction separating means to be incor-
porated in the apparatus of the invention without being forced
to restrict the rotational speed of the disc, so that the
optimal flow of clear fine fraction can be obtained, the
stationary partition wall means and the cylindrical wall define
first and second distribution chambers. The first distribution
chamber neighbours the axial channel for cloudy fine fraction
and is adapted to receive relatively clear fine fraction flowing
through the outlets of the peripheral chambers passing the first
distribution chamber during rotation of the filter disc and to
distribute said received relatively clear fine fraction to the
axial channel f or cloudy fine fraction. The second distribution
chamber neighbours the axial channel for clear fraction and is
adapted to receive clear fine fraction flowing through the
outlets of the peripheral chambers passing the second distri-
bution chamber during rotation of the filter disc and to
distribute said received clear fine fraction to the axial
channel for clear fine fraction.
Since the pressures in the two distribution chambers are due to
the heads between the body of suspension and the respective
distribution chambers, the relatively clear fine fraction is
forced from the first distribution chamber into the axial
channel containing cloudy fine fraction. whereas substantially
all of the clear fine fraction flowing into the second distri-
bution chamber is forced into the axial channel containing clear
fine fraction. Only a minor amount of clear fine fraction will
pass from the second distribution chamber into the first distri-
bution chamber, because of the small pressure difference between
the two distribution chambers. Thus, the separation of cloudy
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and clear fractions can be made sharply, which enables a higher
rotational speed of the filter disc, and consequently a larger
flow of clear fine filtrate.
The invention is explained below in more detail by way of
example with reference to the accompanying drawings, in which
Figure 1 is a side view of an apparatus of the invention,
Figure 2 is a sectional view along line II-II of Figure 1,
Figure 3 is a part of a section along line III-III of Figure 2,
Figure 4 is an enlarged view of a detail shown in Figure 1,
Figure 5 is an enlarged vertical cross-section of an upwardly
moving part of a filter disc for the apparatus of the invention,
Figure 6 is a part of an enlarged sectional view along line VI-VI
of Figure 2,
Figure 7 is a part of an enlarged sectional view along line VII-
VII of Figure 2,
Figure 8 is an enlarged detail of the embodiment shown in
Figure 6,
Figure 9 is a modification of the embodiment shown in Figure 8,
and
'Figure 10 is a part of a longitudinal section along line X-X of
Figure 9.
In Figures 1 and 2 there is shown an apparatus of the invention
comprising a container 1 and a hollow rotor shaft 2, which
forms a central chamber 3 with a cylindrical wall 4 which is
journalled on the container 1, so that the shaft 2 is rotatable
about a horizontal axis 5. Three annular vertical hollow filter
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discs 6 are mounted on the shaft 2 spaced from one another and
are coaxial with the horizonta l axis 5. A drive motor 7 is
connected to the shaft 2 and adapted to revolve the discs 6 in
counter-clockwise direction, as shown in Fig. 1. An inlet device
8 is arranged to supply a suspension to be filtered to the
container 1 at the downwardly moving side of the filter discs 6
and an outlet device 9 is located at the upwardly moving side of
the filter discs 6 for receiving mats or cakes of coarse
particles which are removed from the filter discs 6 by shower
means 10.
Each filter disc 6 comprises two opposed side walls 11 of filter
material. Partition walls 12 extend between the opposed side
walls 11 and divide the interior of the filter disc into a
plurality of peripheral chambers 13 (the number of chambers 13
may suitably be chosen from sixteen to twenty), the peripheral
chambers 13 being disposed in series around the filter disc 6.
Each peripheral chamber 13 communicates with the central chamber
3 via an outlet 14 in the cylindrical wall 4.
In the central chamber 3 there is a means for separating
developed fine fraction into cloudy and clear fine fractions.
This separation means comprises a stationary rigid partition
wall means 15 dividing the central chamber 3 into an axial
channel 16 for receiving cloudy fine fraction and an axial
channel 3a for receiving clear fine fraction. The channel 16
extends axially along the central chamber 3 and out of the
latter to an outlet device 17. Clear fine fraction is able to
collect in channel 3a at the lower part of the central chamber 3
and to form a pool 18 of fine fraction therein. The level 18a of
the surface of the pool 18 is determined by properly
dimensioning the central chamber 3. As an alternative the level
18s may be determined by an adjustable overflow. The clear fine
fraction collected in the axial channel 3a is discharged to an
outlet device 19. The partition wall means 15 is turnable about
said horizontal axis 5 and is connected to the container 1 by
adjustment means 20 for adjusting the position of the partition
wall means 15 in the central chamber 3.
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Figure 5 shows the design of the peripheral chambers 13 in more
detail on the upwardly moving part of the filter disc 6. Each
chamber 13 is defined by two straight radially extending
partition walls 12a and 12b, which at their radially innermost
ends adjoin two backwardly extending slightly curved partition
walls 12a' and 12b', as seen in the direcion of rotation. The
radially innermost ends of the curved partition walls 12a', 12b'
abut the cylindrical wall 4 at opposite sides of the outlet 14
of the peripheral chamber 13, so that the outlet 14 extends from
the end of the lower curved partition wall 12b' but is spaced
from the end of the upper curved partition wall 12a'.
Accordingly, there is a circumferential wall portion 4a of the
cylindrical wall 4 between the outlet 14 and the end of the
upper curved partition wall 12a'.
A venting means in the farm of a tube 21 extends from the
central chamber 3 into each peripheral chamber 13 to a position
proximate to the leading end 22 and the radially outermost part
of the peripheral chamber 13. The interior of the tube 21
constitutes a passage 23 for supplying air from the central
chamber 3 to the peripheral chamber 13. The tube 21 preferably
extends through the wall portion 4a, but may alternatively have
its lower end 21a directed to the outlet 14, suitably at the
leading part of the outlet 14, as indicated in Fig. 5.
The stationary partition wall means 15 comprises three straight
axially extending walls 24, 25, 25 having three longitudinal
edges close to but spaced from the cylindrical wall 4 of the
rotor shaft 2, at the downwardly moving part of the cylindrical
wall 4, see Figs. 6 and 8. The walls 24, 25, 26 and the
cylindrical wall 4 define a first distribution chamber 27
neighbouring the axial channel 16 for cloudy fine fraction, and
a second distribution chamber 28 neighbouring the axial channel
3a for clear fine fraction. The free ends of the straight walls
24, 25, 26 are provided with axially extending plates 29a,29b,
29c, which are adjustable in the radial direction of the rotor
shaft 2 for adjusting the gap between the cylindrical wall 4 and
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the respective straight walls 24, 25, 26, see Fig. 8. In the
longitudinal direction, the distribution chambers 27, 28 are
defined by two gable wails 30, 3i situated at opposite sides of
the three filter discs 8, see Figs. 3 and 7. The gable walls
30, 31 are provided with adjustable sealing plates 32, 33.
Figs. 9 and 10 show a modification of the distribution chambers,
wherein an additional chamber 34 is provided between the distri-
bution chambers 27 and 28. In this case the partition wall means
15 shall be displaced somewhat upwardly by the adjustment means
20 so that the distribution chamber 27 receives a relatively
clear fine fraction that is less clear than the corresponding
relatively clear fine fraction obtainable in the embodiments of
Figs. 1 to 8. In other words the distribution chamber 27 shall
receive a fine fraction that could also be characterized as
relatively cloudy. The distribution chambers 27,28,34 are
defined longitudinally by two additional gable walls 35, 36
provided with sealing plates 37, 38. The additional gable walls
35, 36 encompass the three filter discs 6 and are situated a
distance from the gable walls 30,31, respectively. As a result
fine fraction, that may be relatively cloudy, escaping from
chamber 27 past sealing plates 37, 38 can pass into the axial
channel 16, as is indicated by arrows in Fig. 10, whereby
relatively cloudy fine fraction is substantially prevented from
escaping to the axial channel 3a via the sealing plates 32 and
33.
In operation, a suspension to be filtered is supplied to the
container 1 through the inlet device 8, so that the container is
filled with suspension normally to a level above said horizontal
axis 5. The filter discs 6 are rotated by the drive motor 7, so
that the peripheral chambers 13 are submerged one after the
other into the suspension. With reference to Fig. 6, when a
peripheral chamber 13 has moved downwardly to the position
denoted by capital A, a cloudy fine fraction is forced by
hydrostatic pressure through the filter material into chamber 13
while a mat of coarse particles is developed on the filter
material. In the position denoted by capital B chamber 13 is
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filled with cloudy fine fraction, which begins to flow through
the outlet 14 to the channel I6. now the mat on the filter
material already is thick and tight enough to serve as a filter
medium itself, whereby a clear fine fractior. is forced into the
chamber 13 and is mixed with the cloudy fine fraction therein.
In the position denoted by capi~ai C chamber 13 contains cloudy
fine fraction near the outlet 14, clear fine fraction near the
filter material and relatively clear fine fraction there-
between. ~n the position denoted by capital D all cloudy fine
fraction should have left chamber 13, otherwise the adjustment
means 20 is operated to adjust the position of the distribution
chambers 27, 28 somewhat downwardly. In Fig. 6 angle a
illustrates the sector of the central chamber 3 into which
cloudy fine fraction is received. As the outlet 14 of chamber 13
passes the distribution chamber 27 the latter receives
relatively clear fine fraction. In the position denoted by
capital E clear fine fraction flows into the distribution
chamber 28.
Since the pressures in the distribution chambers 27, 28 are
substantially the same but stronger than the surroundings in the
central chamber 3, the relatively clear fine fraction flowing
into the distribution chamber 27 continues to flow through the
gap between the cylindrical wall 4 and the plate 29a into the
axial channel 16 for cloudy fine fraction, whereas the clear
fine fraction flowing into the distribution chamber 28 continues
to flow through the gaps between the cylindrical wall 4 and the
plates 29c,32 and 33 into the axial channel 3a for clear fine
fraction. Only an insignificant flow of clear fine fraction
occurs from distribution chamber 28 to distribution chamber 27,
because of the minor pressure difference between the two distri-
bution chambers 27, 28.
With reference to Fig. 5, when a peripheral chamber 13 on the
upwardly moving part of the filter disc 6 is in the position
denoted by the capital F, the filter material of the chamber 13
is covered with a tight mat of coarse particles which is
substantially liquid impervious. Since the outlet 14 of the
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chamber 13 opens into the pool 18 o~f fine fraction and the
Iiauid content of the chamber I3 is above the surface ieve= 18a
of . the pool, a negative pressure is created in the chamber 13.
As the chamber 13 moves further upwardly from position F and up
above the sarface level of the suspension in the container 1
outside of the filter disc 6, said negative pressure in the
chamber 13 increases. When the chamber 13 has reached the
position denoted by the capital G, air is able to ente:: the pipe
21 from the central chamber 3 and be sucked by said negative
pressure through the pipe 21 to the upper part of the chamber
13. As a result, the chamber 13 is emptied rapidly. When the
chambe: 13 has moved from the position G to a zone in which the
shower 10 is effective to knock off the high consistency mat of
coarse particles from the filter material, so that the mat rolls
off the filter material and passes into the outlet device 9, the
chamber 13 is totally empty.
The arrangement of the venting means and the fine fraction
distribution chambers according to the invention makes it
possible in some applications to increase the rotationa2 speed
of the filter discs by 100% or more, when filtering fibre pulp
suspensions. Thus, the capacity of the apparatus of the
invention is substantially increased.
