Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2~08598
TITLE: ROTARY 8CREENING DEVICE
FIELD OF THE I~YEM~ION
The present invention relates to an apparatus for
removing impurities in pulp stock, more particularly to a
novel rotary screening device that uses power more
efficiently.
BACRGROUND OF THE INVENTION
The screening of wood pulp slurry is necessary in the
production of paper to remove large fibers, stones and
other rejects. Over the years, eguipment and processes to
screen pulp have improved. Initially, pulp stock was
passed through a vibrating screen, an improvement of this
méthod was a cylindrical pulsating method where foils were
rotated inside a screen to produce a pulsating effect on
a mat of fibers adjacent the screen. The present method,
and the one that is used in most modern paper mills, is
the rotating method where fibers are screened in a
cylindrical screen, generally a vertical screen. A series
of blades rotate inside the screen and pulp slurry is fed
to the top of the screen. As the slurry passes through
the screen, the fibers are formed into a mat between the
screen basket and the impeller blades. The mat rotates
due to the movement of the impeller blades and at the same
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time has an axial movement downwards so that the reject
particles in the pulp stock are held in the mat and
conveyed to the bottom of the screen where they are
removed in a reject chamber.
The United States patents 4,642,189 issued on
February 10, 1987 discloses a rotary pressure screen of
this type. The screen features an improved rotor
construction allowing to increase the efficiency of the
machine. More specifically, the upper section of the
rotor has a parabolic shape and carries a frusto-conical
baffle. The baffle and the upper section of the rotor
define therebetween a first pulp stock passageway that has
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a constant cross-sectional shape from top to bottom. A
second passageway is defined between the outer surface of
the frusto-conical baffle and the screen basket. Pulp
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stock to be screened discharged on top of the rotor is
split in two parts that flow through respective
passageways and are directed at different axial locations
of the screen. As a result of this arrangement, the
screen surface is utilized more efficiently. ~ ~;
The present invention is an improvement of this prior
art rotary screening device. More particularly, the
present inventor has made the surprising discovery that by
re-configuring the pulp stock flow splitting baffle a
significant gain in efficiency can be achieved.
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As embodied and broadly described herein, the
invention provides a rotary pulp screening device of the
vertical pressure type, comprising:
- a housing defining an upper inlet chamber and a
lower screening chamber in fluid communication;
- a generally cylindrical screen mounted in said
lower screening chamber;
~ a rotor mounted for rotation about a generally
vertical axis within said screen, said rotor having an
upper section that tapers toward a top end of said rotor;
- a plurality of blades mounted to said rotor and
radially extending therefrom to within a short distance
from said screen, said blades constitute means for
creating localized pressure pulsations in the pulp stock
that act against said screen in order to dislodge
impurities obturating apertures of said screen;
- a pulp flow splitting baffle having a generally
circular configuration mounted to said rotor and
encircling said upper section, said baffle and said rotor
defining therebetween a first pulp f low passage having a
gradually decreasing cross-sectional shape in a direction
of pulp flow through said passage, said first pulp flow
passage having an outlet opening through which pulp flow
is discharged against said screen located at a point
intermediate said top end and a bottom end of said rotor,
said baffle and said screen defining therebetween a second
pulp flow passage having a cross-sectional area
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substantially smaller than a cross-sectional area at an
inlet end of said first passage, whereby pulp stock
discharged in said inlet chamber enters said screening
chamber and is split in two substantially unequal parts,
a largest one of said parts flowing downward through said
first passage toward said outlet opening, a sm~llest one
of said parts flowing downward through said second passage
along said screen and merging with pulp stock egressing
from said outlet opening.
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The dramatic gain in efficiency observed with the
screening device in accordance with the invention is a
result of splitting the incoming flow of pulp stock to be
screened in two substantially unequal parts, the large
part flowing between the baffle and the upper section of
the rotor and being discharged against the screen at an
intermediate axial location thereof, while the smaller
part flows between the outer surface of the baffle and the
screen basket.
In a most preferred embodiment of the invention, the
baffle is generally cylindrical maintaining from top to
bottom a constant distance with the screen basket. The
baffle is secured to the rotor by the intermediary of the
blades provided to clear the screen openings. The blades
are in the form of elongated vertical bars bolted to the
lower cylindrical portion of the rotor. The upper part of
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the blades that extend at the level of the upper section
of the rotor define a cage in which the baffle is received
and supported. Since the blades extend over the outer
surface of the baffle, there cleaning action is effective
over the entire surface of the screen. This reduces the
possibility of screen clogging and further proves the
efficiency of the machine.
BRIEF DE8CRIPTION OF THE DRA~ING8
- Figure 1 is a schematical vertical cross-sectional
view of a screening device of the pressure type in
accordance with the invention;
15- Figure 2 is a top plan view of the screening device
shown in Figure l; and
- Figure 3 is a graphical representation of test data
illustrating the efficiency of the screening device in
20accordance with the invention comparatively to a prior art
machine.
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DE8CRIPTION OF A PREFERRED EMBODIMENT
,
25One embodiment of a pulp screening device 10 is shown
in Figures 1 and 2 having a generally cylindrical housing
11 with a top cover 12 joined to the cylindrical housing -
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11 at flange 13. In the cylindrical housing 11, spaced
down from the top flange 13, is a disc ring 17 which
divides the housing into an upper inlet chamber 18 above
the disc ring 17, and a lower screening chamber 19 below
the disc ring 17. A tubular inlet pipe 20 provides entry
to the inlet chamber 18. Thus, pulp stock admitted in the
inlet chamber 18 flows over the disc ring 17 and in the
screening chamber 19.
A vertical cylindrical screen 25 is mounted axially
in the lower chamber 19, and extends for the full height
of the chamber 19. An annular accept chamber 26 which
receives accepts completely surrounds the cylindrical
screen 25. An outlet 27 at the bottom of the accept
chamber 26 in the cylindrical housing 11, outside the
screen 25, allows the screen accept fibers to leave the
screening device 10.
A rotor 30 is positioned axially within the screen
25. In the embodiment shown, the rotor has an upper
upwardly tapering portion 32 that is shaped approximately
in the form of a paraboloid segment, and a lower
cylindrical section 34. The paraboloid segment is formed
from a series of truncated cones joined together and has
a curved nose cone 36 on the top so the overall shape of
the portion 20 is paraboloidal. The rotor 30 is mounted
on a rotating axial shaft 38, which rotates in a bearing
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assembly . The drive shaft 34 has a V-belt pulley 41
mounted thereon for connection by means of V-belts to an
electric drive motor (not shown in the drawings).
Five blades 40 are equi-spaced about the rotor 30.
Each blade 40 is a vertically extending bar bolted to the
cylindrical section 34 of the rotor in a parallel
relationship with the axis of rotation of the rotor 30.
The blade 40 extends for the full height of the screen 25.
The blades 40 form at the level of the paraboloidal upper
segment of the rotor 30 a cage in which is fitted a pulp
stock flow splitting baffle 42. The baffle is cylindrical
in shape and has upper and lower edges lying in parallel
horizontal planes. It is bolted to the blades 40 that
support it on the rotor 30. The baffle 42 is positioned
near the inlet end of the screening chamber 19 and
provides two distinct passageways for the incoming flow of
pulp stock. The first passageway 44 is defined between
the baffle 42 and the paraboloidal segment of the rotor
30. It will be noted that this passageway has a gradually
decreasing cross-sectional shape in the direction of flow
of pulp stock therein which has the effect of gradually
increasing the velocity of the pulp stock as the latter
travels downwardly in the screening chamber 19. The
passageway 44 terminates with an annular outlet 46 defined
between the lower edge of the baffle 42 and the upper edge
of the lcwer cylindrical segment 34 of the rotor.
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A portion of the pulp stock entering the screening
chamber 19 also flows through a second passageway 46
defined between the outer surface of the baffle 42 and the
screen 25. The passage 46 has a uniform cross-sectional
shape along its length as a result of the constant gap
between the baffle 42 and the screen 25. It will be noted
that the passage 46 is significantly smaller than the
passage 44, particularly than the inlet end of the passage
44.
In operation, pulp stock delivered to the inlet
chamber 18 passes over the disc ring 17 and flows in the
screening chamber 19. The pulp stock is divided in two
parts by the baffle 42. The predominant part flows in the
passage 44 and it is gradually accelerated as it advances
downwards. The pulp stock in the passageway 44 exits
through the annular outlet 46 located at an intermediate
point between the axial extremities of the screen 25.
Only at this point the pulp stock is in contact with the
screen and it is subjected to a screening action while
traveling downwards along the lower cylindrical portion 34
of the rotor.
The smaller part of the pulp stock delivered through
the inlet of the screening shamber 19 flows through the
small annular passage 46. Immediately upon entering this
passage, the pulp stock is subjected to screening action.
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As it moves downwards along the screen 25, the pulp stock
gradually thickens because water is removed at a high rate
through the screen apertures. When the thickened pulp
stock reaches the annular outlet 46, it is then diluted
with the incoming main pulp flow stream and the screening
action is then continued along the bottom portion of the
screen 25.
As it is well known to those skilled in the art, the
purpose of the blades 40 is to unplug the screen
apertures. The rotation of the blades 40 causes local
pressure pulsations in the pulp stock that act against the
screen and are capable of dislodging large fibers or other
debris obturating the screen apertures. By providing
blades that extend vertically over the lower cylindrical
section 34 of the rotor and over the entire axial length
of- the baffle 42, enables to provide a cleaning action
over the full surface of the screeb 25. As a result, the
screen basket 25 can be maintained cleaner for a more
efficient operation.
In a most preferred embodiment, the screen has an
inside diameter of 29.375 inches. The cylindrical baffles
defines with the screen 25 a gap of 1.125 inches measured
at some point between two adjacent blades. The height of
the rotor is of 40.250 inches, the axial dimension of the
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baffle 42 is of 14 inches, and the axial dimension of the
annular outlet 46 is of 2.8125 inches.
Comparative studies conducted with prior art rotary
pulp screening devices have demonstrated the superiority
in terms of efficiency of the present rotor design. Test
runs conducted with a rotary screen constructed in
accordance with the teaching of the above-identified U.S.
patent 4,642,189 and a system in accordance with the
invention, have shown significant gains in efficiency in
favour of the invention. The test data is represented
graphically in Figure 3. It will be noted that for a
given horse power value the screening device in accordance
with the invention has a significantly higher pulp stock
screening capacity than the prior art design.
It should be noted that the above description is not
intended to be limiting as variations are possible without
departing from the spirit of the invention. The scope of
the invention is defined in the appended claims and their
equivalents.
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