Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ROTARY PULP SCREEN OF THE HORI'~ONTAL PRESSURE
TYPE HAVING PULP STOCK FE;E~D ~T DIFFERENT
AXIAL POSITIONS ON THE SCREEN
The present invention relates to rotary screening
of pulp stock and more particularly to improvements in both
the method and the device for screening pulp stock in
pressure screens.
The screening of a wood pulp slurry is necessary
in the production of paper to remove large fibers, shives
and other rejects. Over the years equipment and processes
to screen the pulp have improved. Initially pulp stock was
passed through a vibrating screen, an improvement o~ this
method was a cylindrical pulsating method where foils were
rotated inside a screen to produce a pulsating effect on a
mat of fibers aajacent 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. The screens may be vertical or horizontal, in the
case of a horizontal screen,a series of impeller blades
rotate inside the screen, and pulp slurry is fed in at the
inlet end of the screen. As the slurry passes through the
screen, the fihers are formed into a mat between the screen
plate and the impeller blades, the mat rotates due to the
movement of the impeller blades and at the same time has an
axial movement along the screen so that the reject particles
in the pulp stock are held in the mat and conveyed to the
exit end of the screen where they are removedvia a reject
chamber. The mat should retain substantially the same
thic~ness throughout the length of the screen, the rotating
impeller blades preventing the mat from becoming too thick and
causing fibers to plug holes in the screen plate.
As aisclosed in U;S. patent 3,713,536 and U.S.
patent 4,268,381, which ~oth refer to vertical screens,
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it is shown that a pulp stock flow through the pulp screen-
ing device follows an approxi~ately parabolic curve. To these
ends the main shape of the impeller has been arrangcd so
the distance between the screen and the impeller face at
the inlet end of the screen is considerably more than the
distance between the screen and the impeller face at the
exit end of the screen. The aim of both these prior art
designs is to provide a streamline flow of pulp stock
through the screening device and also to ensure that the
full length of the screen is used. If the mat of fibers
along the screen varies in thickness, plugging of the screen
can occur and some screen areas do not screen fibers. This
results in the screening device working inefficiently.
Inasmuch as the impeller blades rotate at a constant
speed, it is assumed that-the mat between the blades and
the face of the screen also rotates at a substantially
constant speed. However, in the past it has not always
been possible to ensure that the thickness of the mat and
the axial speed of the puIp stock in the mat of fibers
adjacent the screen is constant for the full length of the
screen. It is, therefore, a purpose of the present invention
to direct fresh pulp stock to the screen plate at points
axial~y distant from the inlet end of the pulp screen and
thus utilize the full length of the pulp screen. This
introduction of fresh pulp stock has the effect of in-
creasing the rate of acceptance of stock-through the screen
plate at the different axial points, since the highest rate
of flow in existing puIp screens is at the inlet where the
stock is fresh. It also has the effect of acting as dilution
thereby decreasing the need for internal dilution water at
the different axial points. The resulting machine maintains
a substantially constant velacity for the pulp stock in the
axial direction along the face of the screen. In addition
to this, dilution water is added downstream of the last
point of application of pulp stock to the screen to dilute
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the remaining puIp stock and ensure a reason~bly constant
velocity of puIp stock,' including rejects, along the face of
the screen do~nstream of the axial points of pulp stock
applications.
The arrangement of dividing the pulp stock flow
into parts,'together with the locations where the parts are
directed towaras the screen, and the quantity of dilution
water mixed with the puIp stock downstream o these screen
locations, are all variables that can be tuned to suit the
geometry of a particuIar pulp screening device. The highest
rate of pulp stock flow always occurs at the inlet end of
present day puIp screens, so by having different points on
the screen where-fresh puIp is applied,increases the capacity
at those points and consequently increases the capacity of
the whole puIp screen.
At the inlet end, the consistency of the pulp
stock that has passed through the screen is similar to the
consistency of the fresh puIp stock fed to the screen,
whereas further along the screen there is a considerable con-
sistency drop. The effect of bringing in fresh pulp stock
at different points along the screen is to reduce the over-
all consistency drop between the pulp stock and the screened
puIp stock along the screen. A reduction of this consistency
drop is beneficial to the process.
The introduction of fresh pulp stock at different
axial points along the screen, coupled with the maintenance
of.a sùbstantially constant axial velocity of the pulp stock
along the screen, has the effect of making the puIp screen
unit more stable. It has been observed in field trials that
the puIp screen unit of the present invention is more
difficuIt'to plug than existing pulp screens, and further-
more, the screen unplugs with only minor corrective action,
as compared to existing types of puIp screens. --
It has been found that this arrangement of screen-
5 ing pulp stock increases the efficiency of the screen in-
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as much as less horsepower is required to rotate theimpeller for a similar flow of pulp stock through a known
type of screening device. Alternatively, an increased
flow of pulp stock may be screened for the same horsepower
requirement using a known type of screening device.
Several horizontal pressure type rotary pulp
screens exist today and it has been found that by the
introduction of a new rotar having baffles to introduce
lo fresh pulp stock at different points along the screen, the
efficiency of existing horizontal pulp screens is
increased and horsepower requirements are reduced.
The present invention provides in a rotary pulp
screening device of the horizontal pressure type,
including a cylindrical housing having means defining an
inlet chamber and a screening chamber with a disc ring
dividing the inlet chamber from the screening chamber,
horizontal cylindrical screen within the screening
chamber, rotary impeller mounted for rotation about a
central horizontal axis within the screen, the impeller
having a body with an inlet end and an outlet end adjacent
the downstream end of the cylindrical screen, the body
having a shape with circular axial cross section from the
inlet end to the outlet end, whose diameter increases from
the inlet end to the outlet end, thus leaving a larger
annular space at the inlet end, and means defining an
annular inlet between the disc ring and the body of the
impeller, means for rotating the impeller, impeller blades
radiating from at least a portion of the body of the
impeller and extending to within a short distance from the
screen for the length of the screen, and means defining a
pulp discharge outlet from the screening chamber, the
improvement comprising at least one substantially frusto-
conical shaped baffle with an inlet end and an outlet end
of the baffle being disposed in parallel planes, the
baffle being disposed in the annular space between the
body of the
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impeller and the screen extending downstream from the
annular inlet, the baffle being joined exteriorly to the
impeller blades and having a shape to conform to the shape
of the body of the impeller to form an annular axial
cross sectional opening between the baffle and the body
of the impeller with the opening having a substantially
equal vertical cross sectional area from the inlet end of
the baffle-to the outlet end of the baffle, the baffle
dividing the flow of pulp stock entering the annular inlet
so that a portion of the pulp stock entering the annular
inlet passes along the exterior surface of the baffle-and
a portion of the pulp stock entering the annular inlet passes
between the baffle and the body of the impeller,-and whereby
the substantially equal vertical cross sectional area from
lS the inlet end to the outlet end of the baffle maintains a
substantially constant axial velocity of the pulp stock
passing between the baffle and the body of the impeller.
In another embodiment the annular axial cross
sectional opening between the baffle and the body of the
impeller has no impeller blades therein.
In an embodiment of the invention, a frusto-conical
inlet ring may be provided in the inlet chamber extending
upstream from the disc ring, with the smallest diameter at
the entry to the inlet ring, and the rotary impeller has a
cone portion formed to the paraboloidal segment shaped body
to form an approximate paraboloid shape. The axial length
of the baffle is preferably about 20% of the length of the
: cylindrical screen and dilution water is preferably applied
to the cylindrical screen downstream from the baffle. In
another embodiment the impeller has a paraboloidal segment
shaped body, which may be formed from a plurality of frusto-
: conical segments. In a still further embodiment the impeller
: has a frusto-conical shaped body.
;~ In yet a further embodiment, two baffles are pro-
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vided one following the other, positioned such that the
flow ofpulp stock entering the annuIar inlet is dividied
into three parts, a first part passing through a first
annular space between the inlet end of the first baffle
and the disc ring, a second part passing throuah a second
annuIar space between the outlet end of the first baffle
and the inlet end of the second baffle, and a third part
passing through a third annular space between the outlet
end of the second baffle and the surface of the body of the
impeller. In yet a further embodiment, three baffles may
be provided, the embodiment is particularly useful in long
screens. Again the length of each baffle is preferably about
- 20~ of the length of the cylindrical screen. A still
further embodiment provides for a plurality of baffles,
extending downstream one after the other.
In a still further embodiment there is provided
a rotary impeller adapted to rotate inside a cylindrical
screen of a pulp screening device of the horizontal pressure
type, the impeller comprising a body having a shape with
circular axial cross section from inlet end to outlet end
whose diameter increases from the inlet end to the outlet
end, impeller blades radiating from at least a portion of
the body of the impeller and extending to within a short
distance from the screen for the length of the screen, at
least one substantially frusto-conical shaped baffle with an
inlet end and an outlet end of the baffle being disposed
in parallel planes, the baffle being disposed in the annular
space between the body of the impeller and the screen extending
downstream from the annular inlet, the baffle being joined
exteriorly to the impeller blades and having a shape to
conform to the shape of the body of the impeller to form
an annuIar axial cross sectional opening between the baffle
and the body of the impeller with the opening having a
substantially equal vertical cross sectional area from the
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inlet end of the baffle to the outlet end of the baffle,
the baffle dividing the flow of pulp stock entering the
annular inlet so that a portion of the pulp stock entering
the annular inlet passes along the exterior surface of the
baffle and a portion of the pulp stock entering the
annular inlet passes between the baffle and the body of
the impeller, and whereby the substantially equal vertical
cross sectional area from the inlet end to the outlet end
of the baffle maintains a substantially constant axial
velocity of the pulp stock passing between the baffle and
the body of the impeller.
There is also provided in a process of screening
a pulp stock in a rotary pulp screening device of the
horizontal pressure type including, a cylindrical housing
having an inlet chamber and a screening chamber with a
disc ring divlding the inlet chamber from the screening
chamber, horizontal cylindrical screen within the
screening chamber, rotary impeller mounted for rotation
about a central horizontal axis within the screen, the
impeller having a body with an inlet end and an outlet end
adjacent the downstream end of the cylindrical screen, the
body having a shape with circular axial cross section from
the inlet end to the outlet end whose diameter increases
from the inlet end to the outlet end, thus leaving a
larger annular space at the inlet end representing an
annular inlet between the disc ring and the body of the
impeller, means for rotating the impeller, impeller blades
radiating from at least a portion of the bady of the
impeller and extending to within a short distance from the
screen for the length of the screen, and pulp discharge
outlet from the screening chamber, wherein pulp stock
entering the annular inlet is rotated by the impeller
blades, flows along the screen and mixes with dilution
water, the pulp stock passing through the screen for
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the f~ll length of the screen and exiting through the
puIp discharge, the improvement comprising the steps of:
dividing the flow of pulp stock entering the annular chamber
into a plurality of parts before the pulp stock is rotated
by the impeller blades using at least one frusto-conical
shaped baffle concentric with the body of the impeller and
extending downstream from the annular inlet such that an
annuIar axial cross sectional opening between the baffle
and the body of the impeller is formed with the opening
lO. having a substantially equal cross sectional area from an
inlet end to an outlet end of the baffle, directing each of
the parts to different locations along the length of the
screen such that the velocity of puIp stock along the screen
- is su~stantially constant, and adding dilution water, if
necessary, to the puIp stock at further downstream locations
on the screen after the locations where the parts are
directed such that the velocity of the pulp stock along the
screen remains substantially constant.
In yet a further embodiment, the flow of pulp
20. stock is divided into a plurality of equal parts, the first
part directed at the inlet end of the screen and the remaining
parts directed at distances apart representing about 20% of
the length of the screen.
In drawings which illustrate the embodiments of
the invention,
Figure l is a side elevational section of one
embodiment of a-horizontal rotary pulp screening device
of the present invention with two baffles.
Figure 2 is an end view of the screening device
shown in Figure 1.
Figure 3 is an end view of the impeller and rotor
blades for the screening device shown in Figure l.
Figùre 4 is an elevational view of one pair of
impeller blades at line 4-4 of Figure 3.
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Figs. 5A to 5G are sectional views taken at line
5-5 of Fig. 4 showing different embodiments of impeller
blades.
Fig. 6 is a partial side elevational section of
another embodiment of a rotary pulp screening device having
one ba~fle.
Fig. 7isapartial side elevational section of yet
a further embodiment of a rotary pulp screening device
having three baffles.
Fig. 8 is a partial side elevational section of a
rotary pulp screening device having a frusto-conical shaped
impeller body and a single baffle.
One embodiment of a pulp screening device 10 is
shown in Fig. 1 having a generally cylindrical housing 11
lS with an end cover 12 joined to the cylindrical housing 11
at inlet flange 13. The cylindrical housing 11 has an out-
let flange 14 which is joined to an end impeller mounting
flange 15.
In the cylindrical housing 11, spaced in from the
inlet flange 13, is a disc ring 16, which divides the
housing into an inlet chamber 17, upstream of the disc ring
16, and a screening chamber 18, downstream of the disc ring
16. A tubular inlet pipe 19, having a flange 20, at the end
thereof provides entry to the inlet chamber 17. A conical
inlet ring 21 has a flange 22, at its largest diameter which
is attached to the disc ring 16. The flange 22, overlaps
the disc ring 16, so that puIp stock passing into the inlet
chamber 17, moves along the conical side of the inlet ring
21, and passes over a small diameter lip 23, flowing through
the conical inlet ring 21, into the screening chamber 18.
A hori~ontal cylindrical screen 24, is mounted
axially in the screening chamber 18, and extends to an out-
put screen flange 25 in the chamber 18, attached to the
housing 11, from the disc ring 16. An accept chamber 26,
is provided in the screening chamber 18, outside the screen
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24. An accept outlet 27, is provided tangential to the
accept chamber 2~, in the cylind~ical housing 11, and
allows the screened accept fibers to leave the screening
device 10. A flangc 28, at the end of the outlet 27, pro-
vides a connection to discharge ducts.
A rotary impeller 2 9, is positioned axially within
the screen 24. In the embodiment shown, the rotary impeller
29,in the screening area`is shaped approximately in the form
of a paraboloid segment. The paraboloid segment is formed
from a series of truncated cones joined together and has a
curved nose cone 30,at the inlet end so the overall shape
of the impeller is paraboloidal. The impeller 29, is made
in this manner for ease of construction but the approximate
paraboloid shape provides streamline flow to the screen. In
lS the embodiment shown, the nose cone 30, extends upstream
into the conical inlet ring 21, of the inlet chamber 17.
In other embodiments the nose cone 30, may be omitted. The
shape of the impeller may be ~ormed from only two truncated
cones with different slopes as disclosed in United States
Patent 3,713,536, dated January 30, 1973.
The present invention can be applied to existing
rotary screens by retrofitting a new impeller in an existing
screen. The retrofit impeller may have a frusto-conical
shape rather than a paraboloid shape or two different
truncated cQneS with different shapes.
The rotary impeller 29, is mounted on a rotating
axial drive shaft 31, which rotates in a bearing assembly 32,
abou~ the axis of the cylindrical screening device 10. The
drive shaft 31, is connected to a V-belt drive 33, connected
to an electric motor 34, mounted on top of the screening
device 10. Fig. 2 shows the inlet end vïew of the assembly.
A number of impeller blades 35, are equi-spacea
about the rotary impeller 29, radiating from at least a
portion of the body of the impeller 29. The blades 35, ex-
tend to within a short distance from the screen 24, for the
full length of the screen 24, leaving an annular space 36,between the tips of the blades 35, and the screen 24. De-
tails of the arrangement of rotor blades 35,-and i~peller 29,
are shown in more detail in Figs. 3 - 5. An annular inlet
40, is formed àt the entrance to the screening chamber 18,
at the disc ring 16,,where the pulp stock passes from the
inlet ring 21, into the screening chamber 18. A first
baffle 41, having a substantially frusto-conical shape is
located concentric with the surface of the impeller 29,
extending downstream from the annuIar inlet 40. The first
baffle 41, is shown substantially parallel with the surface
of the impeller 29, for the length of the first baffle 41.
It is not essential for the baffle 41, to be parallel to
the surface of the impeller 29, however, it is necessary
for the annuIar axial cross sectional opening between the
baffle 41, and the surface of the impeller 29, to have a
substantially similar cross sectional,area for the full
length of the baffle 41. By keeping this cross sectional
area constant for the length of the baffle, the velocity
of,the puIp stock passing between the baffle 41, and the
surface of the impeller 29, remains about the same. The
baffles are described as being frusto-conically shaped,
however, they may be flat or curved, dependent upon the
shape of the impeller surface and are designed to provide
constant velocity to the pulp stock flowing therein.
A second frusto-conical shaped baffle 42,-also concent.ric
with the surface of the impeller 29, is positioned down-
stream from the first baffle 41, leaving an aperture 43,
between the outlet end of the first baffle 41, and the inlet
end of the second baffle 42. Like the first baffle 41, the
second baffle 42, is shown as being substantially parallel
to the surface of the impeller 29, for the length of the
second baffle 42. As in the case ofthe first.. baffle 41,
the cross sectional area between the second baffle 42, and
the surface of the impeller 29, is substantially the same
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for the length of the second baffle 42. The annular space
bet~een both baffles 41 and 42, and the surface of the
impeller 29, may have no impeller blades 35, therein, and
therefore is open. However, the impeller blades 35, radiate
outward from the baffles 41 and 42, to the annular space 36,
so that the tips of the blades 35, extend for the full length
of the screen. In a preferred embodiment the annular space
between the impeller blades 41 and 42, and the surface of the
impeller 29, is open, that is to say no impeller blades
extend through this space so that initially when pulp stock
enters this area it is not immediately rotated by the action
of the impeller blades'35.
The positioning of the two frusto-conical baffles
41 and 42, divides the flow of puIp stock`entering the
puIp inlet'`40, into separate parts, two baffles provide
three parts of puIp stock'flow directed at different
locations along the pulp screen 24. In the embodiment shown
the position of the first baffle 41, in the annular inlet 40,
is such that the pulp stock flow is divided into two parts,
a first part which flows outside the first baffle 41, be-
tween the first baffle 41, and the inlet ring 22, into the
impeller blades'35, where it is immediately rotated by the
impeller blades. Much of this first part of the pulp stock
passes through the first portlon of the screen 24, however,a
rotating mat of fibers is formed, and there'is an axial flow
of pulp stock horizontally along the-screen. The second part
of the puIp stock'flow entering the annular inlet 40, flows
along the surface of the impeller 29, and in the preferred
embodiment is not subjected to substantial rotation. The
first part and the second part of puIp stock flow may be
about equal, or the second part may be twice that of the
first part, depending upon the arrangement of baffles. The
second part is then divided into two separate and sub-
stantially equàl parts at the outlet end of the first baffle
41. The first of these two equal parts`exits through aper-
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ture 43, and is inullediately rotated by ~he impeller blades.
Much of the pulp stock in this second part joins the rotating
mat of fibers and passes through the screen, however, there
is an axial horizontal movemen~ of the pulp stock in the mat
of fibers along the surface of the screen. This movement
is at substantially the same speed as the movement of the
first part of the puIp stock' entering at the first portion
of the screen 24. The last part of the pulp stock exits
through aperture 44, `at the out`let end of the second baffle
42, between the second baffle 42, and the surface of the
impeller 29, and as in the case of,the other parts of pulp
is immediately rotated ~y the impeller blades 35, and joins
the rotating mat of fibers. Some of the pulp stock passes
through the screen and there is a movement of the pulp stock
in the mat of fibers axially along the screen at the same
speed as the other two parts of puIp. The length of each
of the baffles 41 and 42, is preferably substantially the
same, each baffIe has a length of approximately 20~ of the
total length of the cylindrical screen 24.
Thus, in the embodiment shown puIp stock entering
the annul'ar inlet 4~, is in effect divided, and three parts
of pulp are delivered to the impeller blades 35, and conse-
quently to the pulp screen ~4, at different locations along
the puIp screen 24. The length of the baffles and the
position of the baffles is determined primarily by the flow
characteristics entering the annuIar inlet 40. Other con-
siderations include the type of screen and the type of pulp
stock being screened. The criteria is to ensure that the
axial movement of pulp stock moving along the screen has
substantially the same velocity from the inlet end to the
outlet end of the screen.
Whereas two baffles are shown in Fig. l, small
pulp screening devices may have only one baffle therein,
long puIp screening devices may have three or more baffles.
Again the criteria being that the axial velocity of the pulp
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stock'along the screen is arranged to b~ substantially
the same from the inlet end to the outlet end of the screen.
Different types of impeller blades'35, may be
incorporated with rotary puIp screen of the present in-
vention. One type of impeller blade is illustrated in Figs.3, 4 and 5A,' and other types are illustrated in Figs. 5~ to
5G. The selection of impeller blades is made dependent
upon a number of factors such as the type of puIp stock
being screened, the desired properties of the screened pulp,
and the capacity,of,the pulp stock through the rotary pulp
screen. These listed ~actors are by no means limiting,
neither do they represent the only factors.
The impeller blades 35, shown in Figs. 3, 4 and
5A'have a first leading blade 50, and a second trailing
bla~e 51, which are spaced apart with a gap 52 therebetween
the blades 50 and 51, to allow dilution water to be sprayed
directly from the body of the impeller 29, downstream
from the second baffle 42, and have a plurality of holes
53, in the gap 52, between the bla~es 50 and 51, to allow
dilution water to be sprayed directly onto the pulp screen
24, Thus, dilut'ion water is added at locations on the pulp
screen 24, downstream from where the parts of pulp'stock
are fed onto the screen. Sufficient dilution water is
applied such that the axial velocity of the mat of fibers
moving along the face of the screen remains the same or
substantially the same for the ful'l length of the screen 24.
In Fig. 5B, the leading blade 50, extends to with-
in a short distance from the screen, and the second trailing
blade 51, does not extend so far. Figs. 5C, 5D, and 5E
illustrate an impeller blade assembly with a cap 55, across
the ends of the leading blade 50, and the trailing blade
51, and an exit aperture 56, in the trailing blade 51, for
the dilution water. Figs. 5F and 5G show a single trailing
blade 51, and leading blade 50 respectively without a
second blade. The dilution water is fed to the screen
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airectly from the hole 53, in the body of the impeller 29.
In all emb~diments shown, different flows of dilution water are
provided along the screen to ensure that the axial velocity of
pulp stock is substantially constant along the screen.
A first water inlet duct 57, shown in Fig. 1 leads to a
first annular chamber 58, having openings 59, at the sides and
the periphery allowing dilution water to pass into that portion
of the interior of the impeller body that is adjacent the second
baffle 42. Adiaphragm plate 60, extending across the body of
the impeller 29, prevents dilution water ~-ssing into the front of the
impeller body. A second diaphragm plate 60A, extends across
the body of the impeller downstream of the first annular chamber
58 to contain the dilution water within the first annular
chamber 58.
A second water inlet duct 61, leads to a second annular
chamber 62 downstream of the second aiaphragm plate 60A. The
second annular chamber 62 has peripheral openings 63, allowing
dilution water to pass into another portion of the interior of
the impeller body downstream and adjacent to the portion
containing the first dilution water. The two dilution water
systems allow different flows along the screen 24.
~ n one embodiment a reject chamber 65 is provided at
the end of the screen 24, and an impeller blade 66, extendsinto
the reject chamber 65, to continually sweep the chamber. -In
another embodiment there is virtually no reject chamber. A
reject outlet 67, is provided so that fiber rejects, shives, etc.
which pass along the screen and do not pass through the screen,
exit from the reject chamber at the end of the screen.
By introducing fresh pulp stock at points along the
screen, a higher rate of flow is achiev~d for the rotary pulp
screening device of the present invention. Furthermore, it has
been found that the power requirement torotate the impeller is
reduced. This drop in po~er reguirement is believed to be due to
the fact that there is a more even use of the pulp screen for its
full length, the mat is believed to be substantially the same
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thickness ~rom the inlet e~d to the outlet end of the screen,
and fibers do not clog up any one area of the screen but flow
through the screen for its full length. The majority of the
pulp stock does not pass through the first portion of the screen
as in most existing pulp screens of the horizontal pressure
type. This eomplete use of the screen combined with the stream-
line flow through the aperture between the impeller and the
screen improves the utilization of the pulp screening device.
As the pulp stock moves to the screen surface the impeller blades
rotate the pulp stock and a mat of pulp fibers is formedbetween
the edges of the blades and the screen. This mat rotates relative
to the screen and also has an axial movement which is at a sub-
stantially constant velocity horizontally along the screen to
the reject chamber 65. Due to this rotationaland axial movement
of the mat, the thickness of the mat remains about the same for
the screen length and a shearing force occurs between one side
of the mat and the tips of the impeller blades which prevents
the holes in the cylinarical screen from plugging.The acoept pulp
fibers pass through the fiber mat which is formed by the shives
and other reject fibers, together with the accept fibers, ana then
pass through the screen of the accept chamber 26. This axial
movement of the mat is substantially constant along the face of
the screen, and in order to retain its speed after the second `
baffle 42, the dilution water ensures that the mat continues its
rotation and axial movement. By having separate dilution water
supplies, one supply may be at a higher pressure or higher flow
than the other supplies to ensure the axial velocity of the mat
is substantially the same along the length of the screen. The
rejects in the mat then pass into the reject chamber 65, where
they are ejected through the outlet 67. The accept or screened
pulp stock passes out of the accept chamber 26, through outlet 27.
Fig. 6 shows another embodiment of the pulp screening
device with only one baffle 41. Fig. 7 shows a pulp screening
device with three baffles, a fi~rs`t baffle 41,
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second ~af~le ~2, ~nd a third baf~le 70. The sp~cing between
the three baffles is such that the pulp stock flo~ entering
inside the first ba~fle 41, is divided into three substan-
tially equal parts when e~iting bet~een the first baffle 41,
and second baffle 42, between the second baffle 42, and the
third baffle 70, an~ a,Eter the third baffle 70. Fig. 8
shows an impeller 29, which has a frusto-conical shape, and
only one slope for the length of the impeller 29. A single
baffle 41, is illustrated. This type of impeller would
likely be provided to be installed in older types of pulp
screening devices.
It will be apparent to those s~illed in the art
that changes and variations may be made to the pressure pulp
screening device of the present application without departing
from the scope of the present invention which is limited only
by the following claims.
