Note: Descriptions are shown in the official language in which they were submitted.
A METHOD AND A~ APPARATUS FOR TREATING FIBER SUSPENSION
The present invention relates to a method and an
apparatus for treating fiber suspension. The method in
accordance with the invention is especially suitable for
screening pulps of the pulp and paper industry and also
for thickening. The apparatus part of the invention
relates to a rotor construction of a power screen or a
thickener.
There are, in principle, two previously known types of
rotor arrangements, which both are commonly used and the
purpose of which is, as well known, to maintain the
filter surface clean, in other words to prevent the
generation of a fiber matting on the filter surface. An
example of one of the types can be appreciated for
example, from the U.S. patent 4193865 issued March 18,
1980, which discloses a rotor arrangement, in which a
rotatable rotor is arranged inside a cylindrical,
stationary filter cylinder, and which rotor comprises
blades located close to the surface of the filter
cylinder, which blades in the construction in accordance
with said patent form an angle with the axis of the
cylinder. The filter surface is subjected to pressure
pulses when the blades move and the pressure pulses
consequently open the openings on the surface. There are
also arrangements, in which the blades are located on
both sides of the filter cylinder. In such a case the
suspension being treated is introduced either to the
inside or the outside of the cylinder and the discharge
of the accept respectively takes place on the outside or
the inside of the cylinder.
An example of the second type is, for example, an arrangement
in accordance with US patent 3437204 issued April 8, 1969, in
which the rotor is a substantially cylindrical, closed piece,
the surface of which has almost hemispherical protrusions.
The pulp in this type of apparatus i5
A
introduced to the treating space between the rotor
cylinder and the filter cylinder outside the rotor
cylinder, whereby the purpose of the rotor protrusions,
so called bumps, is both to press the pulp against the
filter cylinder and to draw the matted pulp with the
trailing edge from the openings of the filter cylinder.
Because this type of construction has a highly thickening
effect on the pulp, three dilution water conduits have
been mounted in the arrangement in accordance with the
above mentioned patent at different levels in the filter
cylinder so as to satisfactorily carry out the screening
of the fiber suspension. A corresponding "bump rotor" is
disclosed also in US patent 3363759 issued January 16,
1968, in which the rotor is slightly conical for the
reason explained later on in this description.
Also other embodiments are known of the above mentioned
cylindrical rotor, in a connection of which different
kinds of protrusions on the filter cylinder side are
considered to be used within different specifications.
German patent application 3006482 published October 8, 1981
(J.M. Voith GmbH) discloses a knotter, which has on the
surface of a cylindrical rotor drum plough-shaped protrusions
made of plate material, the purpose of which is to create
strongly mixing forces in the pulp between the rotor and the
filter cylinder so that the fibers would penetrate the filter
cylinder as effectively as possible and the knots, chips and
the like were separated from the rest.
US patents 4188286 issued February 12, 1980 and 4202761 issued
May 13, 1980 illustrate a filter apparatus, which has a
rotatable cylindrical rotor inside the filter cylinder.
Protrusions have been arranged on the rotor surface on the
filter cylinder side, and the protrusions are V-shaped of
their axial cross-section in such a way that the one
rotational edge has an evenly
A P762EN/E8
. . .
rising front surface, a gurface parallel to the rlm of the
rotor and a back surface substantially perpendlcular agalnst
the rotor surface. These pro~rusions are arranged on the
surface of the rotor cylinder ~n a particular angle position
relative to the axial directlon so that all tha protrusions
of the rotor are at a slmilar position relatlve to the
axis of the rotor.
Accordlng to the US patent specifications pulp can be
introduced into this apparatus at either slde of the filter
cyllnder. If pulp ls lntroduced to the outside of the
fllter cyllnder and the accept ls discharged from the
inside of the filter cylinder, in other words from the
rotor side, the rotatlonal direction of the rotor is such
that the accept is sub~ected to a downwardly inclined
force component by the angle position of the protrusions
and that said inclined/rising ~urface of the protrusions
operates as a front ~urface. If, again, the pulp ls
lntroduced between the rotor and the filter cylinder, ln
other words the accept ls discharged from the outside of
the fllter cylinder, the rotational direction is opposite
relative to the prevlously descrlbed, the protruslons tend
to slow down the downwardly flowlng pulp and the surface
which ls perpendlcular relative to the surface of the
rotor cylinder operates as the front surface.
Practical industrial experiments have, however, proved
that the above described apparatuses do not operate
satisfactorily in all application conditions. For example,
the first mentioned blade rotor creates too strong pressure
pulses at the accept side of the filter cylinder, and is
not therefore suitable, for example, in head boxes of the
paper machines, since they should not have any fluctuation
of pressure. The apparatus also tends to dilute the accept,
and therefore the blade rotor ls not applicable ln
apparatuses, whlch require pulp of a constant consistency.
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Slnce the blades, (4 to 8 blades) ln the blade rotor are
spaced relatlvely far from each other, a fiber matting
always accumulates on the surface of the filter cyllnder
before the next blade scrapes lt off. Thus the use of the
fllter ls not effectlve. Addltlonally, said rotor type is
expenslve to manufacture due to accura~e shapes of the
blades and a careful finlshlng.
A substantlally cyllndrlcal rotor lllustrated as another
model, which has almost hemlspherlcal protruslons, operates
ln some applicatlons almost ideally, but, for example,
wlth the head box of a paper machlne lts operatlon can be
provlded wlth addltlonal precondltlons. Because the pulp
suspenslon flowlng to the head box should be homogeneous,
both ln conslstency and ln flber slze, the power screen
should not vary these values. However, thls klnd of a
"bump rotor" tends to dllute accept and addltlonally lt
causes fluctuatlon in the conslstency values. In the
performed experlments lt was dlscovered that a rotor of a
certain mentloned type diluted accept between -0.15 to-
0.45 ~ when the accept consistency was 3 %. Consequently,
the consistency varles absolutely countlng +/- 5 %, which
is too much when alming at a homogeneous and quallfled
flnal product. On the other hand, fractionation also takes
place in the screen comprising a "bump rotor~, in other
words the mutual relation between the fractlons of the
fiber suspension supplied to the filter cylinder changes
in the screen in such a way that the relation of the
fractions of the accept is no longer the same as that of
the orlginally supplied pulp. With a "bump rotor" the
extent of change of said fractionation was in the
experiments between 5 to 10 ~ according to the clearance
of the filter cylinder and the rotor. The corresponding
extent of change with a blade rotor was about 20 ~ and
therefore even a "bump rotorN is a considerable improvement
to the previous apparatuses.
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These defect~ of a filter apparatus provlded with a ~bump
rotor n described above have led to some improvement
attempts, such as the above mentioned guidance of the
dilution water to the filter surface and in another case
the slightly conical shape of the rotor. Both described
alternatives reflect the problem occurring in communica~lon
with the cylindrical rotor i.e. the uneverness in the use
of a filter cyllnder in its different zones. A fact is
that the flow through the filter cylinder is at its greatest
immediately after pulp has come into communicatlon with
the cylinder and the rotor. Consequently the pulp thickens
to some extent and when the pulp flows downwardly along the
surface of the filter cylinder, the amount of the suspension
flowing through the filter openlngs continuously diminishes.
Attempt~ have been made to prevent this by feedlng dilution
water to different levels of the fllter surface, whlch
results ln a sllghtly more effective operation of the
fllter cylinder, but which also results in the disadvantage
of a relatively high dilution of the accept. Another
possibility is to vary the clearance be~ween a filter
cylinder and a rotor, whereby a greater clearance in the
upper part of the filter apparatus enables a hlgher speed
of the downwardly flowing pulp, whereby the pulp fills the
clearance better and more homogeneously.
A respective operating method is appreciated from the
arrangement disclosed in the US-patent application 4188286,
in which the protrusions are inclined relative to the
axis of the filter cylinder. The main purpose of the
inclinatlon is to prevent the fiber or fiber flocs from
attaching to the front edge of the protrusion and from
being conveyed with it. A secondary purpose is to bring
the accept pulp in the treatment space between the rotor
and the filter cylinder sub~ect to a downward force
component, by which it ls possible to accelerate to some
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extent the operatlon of a filter apparatus, st least to
accelerate the dlscharge of the accept from the fllter.
The most developed embodlment ln the market at the moment
ls lllustrated by the method ln accordance wlth FI patent
77279 and by the arrangement developed to realize thls
method. The method ln accordance wlth sald patent ls
characterlzed ln that flber suspe~slon ls sub~ected to
axlal forces, the lntenslty and the dlrectlon of whlch
vary accordlng to the mutual axlal posltion between the
polnt of appllcation and the counter surface of the fllter
cyllnder, and which are utllized to change the axial speed
profile of the fiber suspension yet maintaining the
dlrectlon of the flow contlnuously towards the discharge
end.
The apparatus ln accordance wlth sald lnventlon is
characterlzed ln that at least one of the counter surfaces
of the fllter cylinder, belng agalnst the other, has at
least one bump or the llke, the dlrection of the front
surface of whlch varleg accordlng to the axlal positlon of
the bump and by whlch the pulp partlcle ln the space
between the counter surfaces ls sub~ected to an axlal force
component, the lntenslty of whlch varies ln functlon of
the axlal posltlon of the counter surfaces of the pulp
partlcles, and which changes the speed profile of the
flber suspenslon flowing between the comter surfaces.
Although the arrangement and the method in accordance with
the invention are superior over the prior art technique, it
is stlll possible to further develop the method described
in the patent specification and the technical arrangement
realizing it. Minute experiments have shown that all rotors,
whlch use any klnd of protrusions whether they are of
bladellke, hemlspherical, rectangular or any other type,
have beglnning from the peak of said protrusion towards
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the tralllng dlrectlon, pulp, whose conslstency and re~ect
content is hlgher than approxlmltely ln the screening
zone. Thls i8, of course, due to the fact that the pressure
stroke caused on the pulp by the bump has pressed acceptable
material through the fllter surface, whereby both llquid
and acceptable fiber material flows through the screen
surface. It has also been dlscovered in the experlments
that sald pulp whlch has a hlgher consistency and which
contains more re~ectable materlal tends to remain against
the screen surface regardless of the fact that the effect
of the bump on the rotor on said pulp portion stops. This,
of cource, weakens the capacity of the screener, because
fresh or less screened pulp must first penetrate said
layer of a higher con8igtency to pass the filter. When
thickeners are concerned the fiber mattlng accumulated on
the filter surface causes a situation where in order for
the flltrate to pass the opening of the fllter surface it
must also be pressed through said fiber matting.
The present inventio~ relates to a method and apparatus
for conveying the above mentloned thlcker and coarser pulp
portion from the fllter surface towards the surface of
the rotor so that the fresher pulp comes into dlrect
communication with the filter surface, whereby the defects
or disadvantages of the prior art apparatuses have been
eliminated or minimized.
The method in accordance with the present invention is
characterized in that the fraction concentrated ad~acent
to the filter surface and/or containing coarser material
is sub~ected to a force component directed away from the
filter surface, by means of which sald fraction is conveyed
away from the ad~acence of the filter surface.
An embodiment of the apparatus in accordance wlth the
present invention ls characterized in that at least one
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gulde plate i3 arranged to communlcate with the counter
member of the fllter surface, whlch gulde plate guldes the
coarser and/or thlcker suspension concentrated ad~acent to
the fllter surface, away from the ad~acence of the filter
surface.
Another embodlment of the apparatus ln accordance wlth the
present lnventlon ls characterlzed ln that the counter
surface to the fllter surface has at least one member,
whlch ls formed by a protruslon arranged on the counter
surface and a gulde plate extendlng from the level o~ the
counter surface hlgher than sald protruslon, whlch
protruslon and guide plate leave an opening therebetween,
through which the thicker and/or coarser fraction can flow
under the guide plate.
The method and apparatug in accordance wlth the present
invention are described more in detail below, by way of
example, with reference to the accompanying drawlngs, in
whlch:
Flg.l is a schematlc illustratlon of a screener in
accordance with FI Patent 77279;
Fig.2 is a schematlc side view of a protrusion of a
rotor described in the above mentioned FI patent
of a screener in accordance with Fig.l;
Flgs.3 _ and b are schematlc illustrations of a
protrusion of a rotor in accordance with a
preferred embodiment of the present inventlon
developed for the apparatus in accordance with
Fig.l; Fig. 3_ is a section on the line D-D of
Fig. 3b which is a section on line C-C of Fig.
3a and with the rotor surface omitted;
Figs.4 _ and _ are schematic illustrations of an auxlliary
apparatus for intensifying the operation of the
protrusion in accordance with the invention
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arranged into communlcatlon with an embodlment
ln accordance wlth Flg.3; and
Flgs.5 _ and b are schematlc sectional lllustrations of
two protruslon arrangements ln accordance with
a second embodiment of the lnvention to replace
the substantlally spherlcal calotte shaped
protrusions of a so called "bump~ rotor, wlth
Flg. 5c being a plan of the protrusions of
Flgs. 5_ and 5_ as such look the same ln plan
and wlth the rotor surface belng omitted.
Accordlng to Flg.l a fllter apparatus 1, ln accordance
with a preferred embodlment of the inventlon, comprises
the following members: an outer caslng 2, conduit 3 for
the lnflowlng pulp, condults 4 and 5 for the accept and
the reJect respectlvely, a statlonary fllter cylinder 6
and thereln a substantially cyllndrlcal or possibly conlcal
rotor 7 having a shaft 8 wlth drlve means 9. The filter
cylinder 6 may, ln princlple, be of any prevlously known
type, but the best results are achleved, if a contoured
filter cylinder ls used. Generally, the apparatus ln
accordance with the drawlng operates ln such a way that
the flber suspension is introduced through conduit 3 and
flows to the gap between the filter cylinder 6 and the
rotor 7, to the so called n treatment" space. The accept,
which has flowed through the openlngs of the filter
cylinder - when a thlckener is in question the filtrate
corresponds to the accept - is discharged through the
conduit 4 as far as to the lower end of the gap between the
filter cylinder 6 and the rotor 7 and the pulp, which flows
away from the gap is discharged through the reiect conduit
5. Fig.l also teacheg that members 10 on the surface of the
rotor 7, on the side faclng the f~lter cylinder 6 the
shape of which members 10 may vary, for example, in the way
described in FI patent 77279, according to in which zone
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the~ are located, in other words, ln whlch axlal part of
the rotor they are located.
Flg.2 ls a schematlc lllugtration of a member lO, the front
surface 11 of whlch may be either parallel to the dlrectlon
of the axl-~ of the cyllnder, lncllned to some directlon
relatlve to the axls or further dlvlded lnto portlons, the
incllnatlon of whlch portlons relative to the axls may be
chosen as deslred. Thls klnd of front surface 11, when
movlng relatlve to the pulp towards arrow B subJects the
pulp not only to a tangential force component, but also
almost always to an axial force component which pumps pulp
towards the center area of the cylinder and also always to
a pressure stroke towards the fllter surface, lntenslfylng
the pulp treatment, due to the effect of which the
acceptable fiber materlal and liquld are pressed through
the fllter and a zone of coarser, to some extent thlckened
materlal ls generated on the surface of the filter. The
front surface 11 of a member lO ln accordance wlth an
embodlment of FI patent 77279 ls substantially perpendlcular
agalnst the surface of rotor 7. The front surface 11 may,
of course, also be lncllned elther to one or the other
dlrectlon. The protruslon has a part 13 substantlally
parallel to the surface of rotor 7 and an inclined surface
14 descendlng from part 13 to the surface of the rotor 7.
The above mentioned pressure stroke, which intensifies
screening and also thickenlng, is generated exactly at the
front surface of the protrusion or slightly before it,
and on the other hand an underpressure zone has tended ~o
be created on the incllned surface of the trailing side
of the protrusion, which zone would draw said coarser and
thickened material away from the ad~acence of the filter
surface.
Flgs. 3_ and 3b illustrate a protrusion arrangement 20 of
the rotor in accordance with an embodiment of the present
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lnventlon, ln whlch arrangement a pressure pulse towardq
the filter surface ls generated by a protruslon 22, which,
ln princlple, may be of any shape. Slgnlflcant of the
protruslon arrangement 20 ig that a member 26 ha been
arranged at the tralllng slde of the front surface 24 of
the protruslon 22 whlch member extend~ closer to the lnner
surface of the fllter 6 than the protruslon 22. Member 26
ls further characterlzed by a gap 28 formed between the
member and the protruslon 22, through whlch the thlcker and
coarser pulp generated between the protruslon and the
fllter surface 18 allowed to pass to the space between
member 26 and the rotor surface, from where it is further
dlscharged to the ad~acence of the rotor surface, as arrows
A ln Flgs. 3_ and 3b illustrate. Consequently, an
underpressurlzed zone ls generated between the member and
the fllter surface, whlch zone ls fllled by fresher, less
treated pulp. The member 26 ls formed by a gulde plate 30
made of bent plate materlal in the embodiment of the flgure,
which plate 30 corresponds to the inclined tralllng surface
14 of the member lO ln the arrangement illustrated in
Flg.2 creatlng a corresponding underpressure stroke on the
fllter surface, and by a plate 32 located on the flow
inlet side of the protrusion 20 (most usually at the level
of the upper surface of the protrusion when the rotor is
vertically positioned). Plate 32 prevents the flow of the
fiber materlal, whlch has flowed under part 26 from flowing
to the upstream side of the protrusion, and on the other
hand, also the suspension flowing along the surface of the
rotor from flowing under the guide plate 30.
Fig.4 illustrates a protrusion 20 ~n accordance with Fig.3,
which communicates with a backwardly lnclined plate 40
arranged to the downstream side of it, the purpose of
which plate 40 ls to guide the unscreened or at least less
screened pulp, which has flowed along the surface of the
rotor, from under the part 26 to above the flowing pulp
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layer. In other wordg the ob~ectlve is to replace the
thickened pulp lncludlng reJect and flowlng accordlng to
arrow A by fregh pulp flowlng from the fllt~r surface
ad~acent to the rotor surface according to arrow E.
Flgs. 5_, _ and _ lllustrate how the pulp replacement ln
accordance wlth the lnvention may be reallzed when so
called bump rotors ln accordance wlth the prior art are
used. As known, the bump is formed elther by an exactly
hemlspherlcal or at least a sherical calotte-shaped
protrusion, which may either be a closed protrusion 50
(Fig.S_) attached to the ~urface of the rotor or ln some
cases possibly a protruslon 52 (Flg.5b) pressed on the
casing of the rotor from inside of the rotor. In that case
a scraper 54 may of cour~e be either a completely separate
bent protruslon made of metal or the llke material (Flg.5_)
- or a protruslon pressed more deeply from lnslde of the
surface of the rotor (Flg.Sb), whlch protruslon ls open
substantlally from the top and from the downstream slde as
lllustrated ln Figs.5b and 5c.
The operation corresponding to the prevlous embodlments may
be created by a so called blade, for example, in such a way
that both the outer and the lnner surfaces of the blade
are provided with guide plates arranged wlthin certaln
dlstances from each other, whlch gulde plates either "cut~
coarser flber material, thlckened ad~acent to the filter
surface, carry lt into the blade and remove it on the
tralling edge of the blade to the other side of the blade
or gulde ln a corresponding way fresher, less treated
suspension through the lnner space of the blade to the
fllter surface. Naturally, said flow passages of the
suspenslon lnslde the blade must be separated from each
other wlth lntermedlate walls, which may be elther
perpendicular against the blade surfaces or in a suitably
chosen angle relatlve to them. The guide plates are further
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characterized ln that they are located lntermlttently
relatlve to each other on the opposite surfaces of the
blade.
An alternatlve to the above described arrangement i8 to
arrange the guide plates above the blade surface wlth
lntermedlate members ln such a way that the flow ~f the
deslred pulp fractlon takeg place from between the blade
and the guide plate. By varylng the locatlon of these
gulde plates or the gulde plates mentloned ln the prevlous
alternatlve to different parts of the blade, the pulp
replacement process described already wlth the previous
embodlments ls carrled out.
A rotor provided with the protrusion or blade construction
in accordance wlth the invention is applicable to be
utlllzed both with smooth and grooved filter cylinders, so
the fllter cylinder may be either completely smooth or
grooved ln different ways, as described ln FI-patent 77279.
Thus by utllizlng thege arrangements ln accordance wlth
the lnventlon new rotors may be applled in older type of
filter cyllnders and vlce versa. Thls results in a fllter
cyllnder-rotor combinatlon operatlng better than the
prevlous screening and thickening arrangements.
The rotor arrangement ln accordance with the invention was
tested in the performed experiments in connection with
dlf~erent filter cyllnders and different rotors were
compared with each other. The filter cyllnders used in the
experlments were both smooth cylinders and cylinders made
of dlfferent plate contours. The test results were examined
and lt was dlscovered that the apparatus in accordance with
the lnventlon operates wlth all filter cyllnders more
effectlvely than the other rotors. In other words the most
preferred fllter embodlment accordlng to these tests was a
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fllter cyllnder, the grooves of whlch were formed
substantlally by the bottom surface parallel to the cyllnder
caslng, the lncllned slde ~urface on the upstream side
(the lnflow dlrectlon of the flow) relatlve to the bottom
surface and the slde gurface substantially perpendlcular
agalnst the caslng surface of the cyllnder on the downstream
slde relative to the bottom surface.
As ls clear from the above descrlptlon, the method and
apparatus ln accordance wlth the lnventlon have enabled
the ellmlnatlon or mlnlmlzatlon of the defects of the
methods and apparatugeg of the prlor art and at same tlme
lt has been posslble to galn conslderable increases ln the
maxlmum capacity. However, it must be noted that above
descrlption includes only a few most important embodiments
of our lnvention, whlch are by no means glven to restrict
the scope of invention from what ls deflned ln the
accompanylng clalms, whlch alone deflne the scope of
lnventlon.
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