Note: Descriptions are shown in the official language in which they were submitted.
CA 02436449 2003-08-05 (ley~~OCIC~t Fto. 188253 (9687-4 CA)
SLOTTED SCREEN FOR DIGESTER
Field of the invention
[0001] The invention relates to digesters for wood-chips, and especially to a
slotted screen
for use in such a digester.
Background of the invention
A digester is a vessel in which wood chips are treated with heat, liquid, and
chemicals in
order to break down the chips into a pulp that can be further processed into
paper,
paperboard, or the like. The digester is typically an upright cylinder v~~ith
a sonically tapering
or domed top end. A batch of wood chips may be loaded into the digester and
then pulped, or
wood chips may be continuously loaded in at the top end and pulp continuously
removed at
the bottom. The cooking liquor is continuously pumped into and out c~f the
digester, so that its
temperature and chemical content can be maintained.
[0002] In order to allow free circulation of the cooking liquor into and out
of the digester,
while keeping the chips within the digester, perforated metal screen pliates
are provided,
spaced a short distance inward from the walls of the digester. These screen
plates define and
separate a central volume, containing the wood chips, and an annular space,
acting as a
manifold by means of which the liquor can be distributed and collected. The
screen plates are
typically made of stainless steel, and the perforations in the screen plates
are typically
circular holes.
[0003] For practical reasons, the screen typically consists of a circle of
from 16 to 24 panels,
each individually mounted on and removable from supports within the digester.
The panels
are usually curved so that they together form a cylinder or a truncated cone,
depending on
where in the digester they are to be installed. For example, in the conical
top of a digester
there may be 20 panels, each about 60" (i.5 m) long, 6.5" (165 mm) wide at the
narrow end,
and 20" (500 mm) wide at the wide end, forming a cone about 40" (1 m) in
diameter at the
narrow end and about 125" (3.25 ) in diameter~at the wide end. The panels may
instead be
rectangular or trapezoidal, forming a mufti-sided prism or pyramid. These
panels are easier to
fabricate and, especially if the number of panels in the circle is large, form
an adequate
approximation to a cone or cylinder for many purposes.
CA 02436449 2003-08-05
[0004) As an alternative to holes, it has been proposed to use screen plates
with straight slots
angled obliquely. Examples of such screen plates are shown in U.S. Patents No.
6,039,841
(Hernesniemi) and 6,165,323 (Shearer), both assigned to Ahlsfa~om.
Summary of the invention
[0005] The present invention in one aspect provides a digester screen with
curved slots. The
slots have a nominal width of from. 3 mm (0.12") to 10 mm (0.4"), preferably
around S mm
(0.2"). The slots are mast preferably substantially circular arcs with a
radius of curvature of
from 1.5" (38 mm) to 3" (75 mm), preferably around 1.5". The slots rnay be
formed with
both sides in curves of equal diameter. In that case, the centers of cun~ature
of the two sides
are preferably separated by the nominal width of the slots. The slots are then
of their nominal
width at the center, and slightly narrower at the ends.
[0006) The slots may have semicircular ends, and may have a length of from 1"
(25 mm) to
3" (75 mm) preferably around 1.5" (38 mm) between imaginary parallel lines
tangent to the
ends of the slot. Each slot is preferably oriented approximately square on to
the lengthwise
direction of the digester, with its concave side towards the top (the narrow
end of a conical
screen). The exact alignment is not critical, and for ease of manufacture all
the slots in one
panel of a conical screen may be aligned perpendicular to the ~aenter line of
the panel. The
slots are preferably arranged in lengthwise rows, with a pitch spacing of from
3/s" (9.5 mm) to
3/4" ( 19 mm), preferably around 0.447" ( 11.3 5 mm), and a gap of from '/s"
(3 mm) to '/z" ( 12. S
mm), preferably around '/g", between rows. At the sides of a tapered panel for
a conical
screen, some of the slots will be truncated. Those slots are preferably
arranged with their
inner ends in the position dictated by the regular array of slots., and their
outer ends
terminating in a semicircular end wherever they meet the edge of the slotted
part of the screen
plate.
[0007] The present invention in another aspect provides a digester screen with
slots that open
from a nominal width towards both faces of the screen plate. The slots
preferably have their
nominal width for the middle half of the thickness of the screen plate, and
widen with a half
angle of taper of from S° to 30°, preferably from 10° to
20°, especially around 15°, for
preferably around a quarter of the thickness of the sheet on each side. Thus,
if the sheet is'/4"
(6.35 mm) thick, the slot will have a parallel-sided section of the nominal
width for'/" (3.2
mm) and two tapered sections each 1/16" (1.6 mm) deep.
2
CA 02436449 2003-08-05
brief description of the drawings
[0008] For purposes of illustrating the invention, the drawings show one or
more forms in
which the invention can be embodied. 'The invention is not, however, limited
to the precise
forms shown unless such limitation is expressly made in a claim. In the
drawings:
[0009] Figure 1 shows a schematic; elevational view of a digester
[0010] Figure 2 shows a developed view of a screen plate for the cotucal part
of a digester.
]0011] Figure 3 is a view similar to Figure 2, showing the frame on which the
screen plate is
mounted.
[0012] Figure 4 is a section along the line 4-4 in 1?'figure 3.
(0013] Figure 5 shows an enlarged developed view of one of the screen sections
of the
screen plate shown in Figure 2.
]0014] Figure 6 is a further enlarged detail of Figure 5.
]0015] Figure 7 is a section along the line 7-7 in Figure 6.
[0016] Figure 8 is a view similar to Figure 7 of an alternaxive form of
screen.
Detailed description of the drawings
]0017] Referring to the drawings, and initially to Figure 1, one form of
digester indicated
generally by the reference numeral 10 has a generally cylindrical body 12,
mounted in an
upright position, with conical top and bottom ends 14 and 16. 'The body 12 is
typically about
12 to 16 feet (3.5 to 5 meters) in diameter, and the digester is typically
about 40 to 70 feet (12
to 20 meters] high.
[0018] An inlet 18 for wood chips to be digested is provided at the center of
the top end 14.
An outlet 20 for digested wood chips, which also serves as an inlet f~r liquor
during the
cooking cycle, is provided at the center of the bottom end 16. Further inlets
22 and outlets 24
for cooking liquor are provided i~ the top end 14 and the cylindrical body 12,
respectively. In
use, the liquor inlets and outlets are connected by appropriate pipe-work to
pumps, heaters,
and other apparatus for maintaining the temperature, pressure, flow :rate, and
chemical
3
CA 02436449 2003-08-05
composition of the circulating liquor. All of that apparatus may be
conventional and, in the
interests of conciseness, is not further shown or described here.
(OOI9] In order to prevent the wood chips from entering the liquor circulation
system, the
inlets and outlets 22 and 24 are covered by perforated metal screens 26 and
28, respectively.
The screen 26 is generally conical, conforming to the conical shape ofthe top
end 14 of the
digester 10. The screen 28 is generally cylindrical, conforming to the
cylindrical shape of the
body 12 of the digester. In order to promote an even distribution of liquor
around the digester
10, the screens 26 and 28 are spaced inward from the walls of the digester 10,
forming an
annular plenum 30.
(0020] For practical reasons, each of the screens 26 and 28 preferably
consists of a number,
typically I8 to 24, of screen panels 32 and 34, respectively. The panels 32 of
the screen 26
are trapezoidal, each forming an identical wedge of the conical. screen. 26.
As will be
explained in more detail below, the panels 32 are preferably curved, so that
they form sectors
of a frustoconical screen surface. Instead, the panels 32 may be flat, so that
they in fact form
an I8 to 24 sided truncated pyramid. Similarly, each of the parcels 34 of the
screen 28 is
preferably a sector of a cylinder, but the panels 34 may instead be flat:, so
that the screen 28 is
an 18 to 24 sided prism.
[0021] In operation of the digester I0, wood chips are loaded in through the
top inlet 18
Liquor may also be introduced through the top inlet at this time. Once the
chips have been
loaded, the digester is closed, and the cooking cycle begins. The process of
digesting wood
chips is well known and, in the interests of conciseness, is not further
described here. During
the cooking cycle, hot liquor is typically introduced into the digester
through the inlets 20 and
22 and removed through the outlets 24. Idowever, the liquid flow pattern may
be reversed or
otherwise varied. In particular, the flow pattern is preferably sequenced so
that the top inlets
22 serve as outlets, and the body outlets 24 serve as inlets, for some part of
the cooking cycle.
[0022] Referring now to Figures 2 to 4, one form of screen panel 32 for the
conical top 14 of
a digester 10 comprises a screen plate indicated generally by the reference
numeral 40. In
use, the panel is shaped as a sector of a truncated cone, but in the interests
of clarity is shown
in the drawings developed onto a plane. The screen plate 40 consists
essentially of six slotted
sections 42, 44, 46 separated by unperforated areas 48, 50, 52 and with
unperforated borders
4
CA 02436449 2003-08-05
54, 56, 58 for mounting on a frame indicated generally by the reference
numeral 60. The
screen plate consists of a sheet of'/~" (6.35 mm) thick stainless steel,
preferably duplex 2205.
For ease of design and fabrication, the slotted sections are trapezoidal. To
adapt the
trapezoidal shape of the slotted area to the conical shape of the finished
panel, the borders 54
and 56 at the ends of the screen plate are broad, and have curved outer ends.
[~023] The frame 60 comprises longitudinal edge members 62;, a longitudinal
center member
64, and cross members 66, which define between them spaces corresponding to
the slotted
areas of the screen plate 40. The frame members rest against the wall of the
digester, ensuring
the location of and support for the screen plate, but have parts cut away, as
shown at 70 in
Figure 4, to allow liquor to circulate through them as desired. ~.s may be
best seen from
Figure 4, the frame members 62, 64, 66 are wider perpendicular to the screen
plate than
parallel to the screen plate, so that they define a substantial space betu7een
the outside of the
screen plate 40 and the inside of the digester wall within which liquor can
circulate. It will be
appreciated that, by providing some frame members with cutouts 70 and some
without
cutouts, the annular space between the screen plates 40 and the digester wall
may be
subdivided into distinct sections as desired. These sections may then be used
as manifolds for
inlets and outlets for cooking liquoa~ in accordance with the design of the
particular digester.
(0024] At each end of the panel, the frame comprises a plate 72 that in use
lies flat against
the wall of the digester, and is clamped down to hold the panel in place. The
arrangements for
clamping the panel may be conventional and, in the interests of conciseness,
are not further
described here. At the end of each clamping plate 72 nearer the: middle of the
panel, an
angled fairing plate 74 connects the clamping plate to the level of the
radially inner edges of
the frame members 62, 64, 66. The fairing plates 74 follow the curve of the
clamping plates
72. When the screen panel is assembled, the broad end borders of the screen
plate 40 cover
the space between each fairing plate 74 and the nearest frame cross member 66.
The
unperforated parts of the screen plate 40 may be welded to the frame members
62, 64, 66, and
74.
[0025] Referring now especially to Figures 5 to 7, a typical one 42 of the
slotted areas of the
screen plate 40 has numerous curved slots 80, arranged in rows 82 separated by
unslotted
lands 84. The slots are arranged with their end-to-end extent pf;rpendicular
to the axial
centerline of the screen plate 40, anal with their concave sides facing
towards the narrow end
CA 02436449 2003-08-05
of the screen plate. The rows 82 and the lands 84 run parallel to the axial
centerline of the
screen plate 40: As a result, the outermost row 82 is cut off obliquely by the
outer
unperforated border 58. The slots 80 in that row, to the extent that they are
within the slotted
area, follow the regular pattern of slots. The slots in alternate rows 82 are
offset axially from
one another by half the slot pitch.
(0026] Each slot 80 is an arc of a circle of approximately 1.5" (38 mm) radius
and 0.197" (5
mm) wide, with semicircular ends 96. Each slot has a chord length (measured
along a straight
line between notional parallel tangents to the two ends) of approximately 1.5"
(38 mm). As
shown in Figure 6, if the slots are cut out, it may be preferred tc~ form the
sides 88, 90 as arcs
of equal radius with offset centers. 'The centers 92, 94 of the arcs 88, 90
are then 0.197" apart,
and the slots are then 0.197" wide in the middle, and slightly narrower
towards the ends. The
pitch spacing of the slots 80 along each row 82 is then approximately 0.447"
(11.35 mm).
The lands 84 separating the rows oI° slots are approximately 0.125" (3
mm) wide between the
notional lines tangent to the ends ofthe slots 80 on either side.
[0027] The size of the slots is determined by the size of the chips that need
to be handled in a
particular application. For fine chips, for example, slots with a radius of 1"
to 1.25" (25 to 30
mm) and a width of 0.125" to 0.156" (3 to 4 mm) may be appropriate. The
preferred shape of
the slots is substantially an arc of a circle because the curve prevents a
thin, flat chip from
passing through by aligning itself with the slot. If the slot were of a more
complicated shape
it would be the maximum radius that would determine the largeat chip that
could pass
through. Thus, circular arc slots allow a less delicate construction than.
other shapes for a
given chip size.
[0028] The number of slots is determined by the required liquor flow rate
through the screen
26 and the acceptable pressure drop across the screen, taking into account
both the power
consumption of the pumps that must generate the necessary pressure and the
mechanical
strength of the components that must support that pressure. A l~~rge safety
margin, typically a
safety factor of 2 or 2r/2, is included in the design, to allow for the fact
that there may be
significant obstruction of the slots by chips being processed.
(0029] Referring now especially to Figure 7, each slot has the nominal width
of 0.197" only
through the middle half of the thickness of the metal sheet forming the;
screen plate 40.
6
CA 02436449 2003-08-05
Towards both faces of the screen plate, the slot is widened by sloping
surfaces 96 with a half
angle of taper of 15° over approximately a quarter of the thickness of
the sheet. The slots 80
are thus approximately 0.23" (5.8 mm) wide where they open out through the
surface of the
sheet. The slots may be formed by water jet cutting, followed by machining to
shape.
[0030 Surprisingly, it has been found that the curved slots and the slots
widening towards
both faces of the screen plate, and especially the combination of slots that
both are curved
and widen towards both faces of the screen plate, substantially reduces
clogging of the screen
by wood chips being digested. Without wishing to be limited to any particular
theory, it is
believed that wood chips lodge less readily in the curved slots than in
straight slots or circular
holes. It is believed that the widening shape of the slots allows any chips or
pulp that have
accumulated to be more easily dislodged by the wiping action of chips moving
past the slot,
especially in a continuous digester where there is a constant downward
movement of chips. It
is believed that the narrowing taper of the upstream ends of the slots
accelerates the liquor
flowing through the slots, while the widening taper of the downstream ends of
the slots
makes any trapped material easier to dislodge. The combination of these
effects, which
requires slots that are tapered at both sides of the plate and that are back-
washed by serving
as inlet screens for part of the digester cycle, enhances the w<~shing action
of the liquor flow
to dislodge chips in the downstream ends of the slots.
[0031 It is found that if chips are allowed to remain lodged in the slots for
a prolonged
period of time, they tend to "bake on" to the screen, and must be either
mechanically or
chemically removed. It may therefore be preferred to adopt a flow sequence
that includes
comparatively frequent short periods of back-washing. Further, because the
design of the
slots 80 may permit an accumulation of larger chips on the inside of the
screen 32 or 34,
frequent back-washing may be preferred in order to keep the proportion of the
slot area that is
obstructed, and thus the pressure drop and flow impedance across the screen 32
or 34, within
the designed operating range of the digester. In a batch digester, the normal
sequence of
operation typically uses the screens 32 and 34 as inlets and outlets at
different parts of the
batch cycle. This results in the screens being back-washed several times a day
in normal use,
which is found sufficient to keep them clear. In a continuous digester, the
liquor flow through
the screens is typically continuously outward. In such a digester, back-
washing normally
needs to be effected specifically by intermittently reversing the liquor flow.
Those skilled in
7
CA 02436449 2003-08-05
the art will understand how this may be done without unduly disrupting the
continuous
digestion process.
(0032] It will also be appreciated that the liquor used in the digester is re-
circulated as much
as possible, and the re-circulated liquor may carry with it solid bodies
barely small enough to
pass through the slots. Such solid bodies may lodge in the upstream ends of
slots that are
serving as inlets for the re-circulated liquor into the digester, which will
consequently also
benefit from back-washing by being used as outlets.
[0033] The shape of the slots also presents a lower resistance to liquor #low
than
conventional circular holes. This means that, for a given flow resistance, a
lower proportion
of the screen plate is needed for the open area of the slots than. would be
needed in the case of
circular holes, and a higher proportion can be the metal between the slots. As
a result, screen
plates according to the present invention can be made considerably stronger
than equivalent
conventional screen plates.
[0034] Referring now to Figure 8, in an alternative embodiment of the screen
the slots 80
may have sloping surfaces 96 only at one face of the screen plate 40, and may
have parallel
sides extending out to the other face of the screen plate. In this embodiment
the sloping
surfaces 96 may have a half angle of taper of 15° and extend over up to
half the thickness of
the screen plate 40. Thus, if the screen plate 40 is'/a" (6.3 mm) thick and
the slots have a
nominal width of 0.197" (5 mm), the width between the tapered surfaces at the
surface of the
plate may be 0.264" (6.7 mm). In this embodiment, the screen plate 40 is
positioned with the
widened ends of the slots 80 at the face that is exposed to wood chips. If the
screen panel in
question is being used as an outlet for liquor from the digester, the wood
chips may then be
dislodged from the slots by back-washing.
(0035] Although a panel 32 of the screen 26 has been shown in, and
described.with reference
to, Figures 2 to 8, it will readily be seen how the present invention may be
applied to the
panels 34 of the screen 28.
(0036] Although the invention has been described and illustrated wiith
reference to
exemplary embodiments thereof, it will be understood by those skilled in the
art that various
changes, omissions, and additions may be made thereto, without departing from
the spirit and
scope of the invention as recited in the attached claims. For example:,
although the digester 1 O
8
CA 02436449 2003-08-05
shown in Figure l has been described as operating in a batch pB-ocess, the
screens opthe
present invention may equally be applied to a continuous-flow digester. In a
continuous-flow
digester, the constant movement of the chips past the screens 2~5 and 28 may
assist in
promptly dislodging chips that become lodged in the mouths oi'the slots 80.
9
