Note: Descriptions are shown in the official language in which they were submitted.
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BKS 137 P2 -1-
APPARATUS FOR SCREENIN~ PAPER FIBER STOCK
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Backqround of the Invention
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Paper mills have for many years made extensive
use, for the cleaning of paper making stock, of screening
apparatus embodying a cylindrical perforated sc~eening
member defining inlet and accepts chambers on the opposite
sides thereof in a closed housing and including a rotor
member which operates in one of the chambers to keep the
screening perforations open and free from solid material
tending to cling to ~he screening surface. Commonly, the
stock or furnish is delivered to the inlet chamber
adjacent one end of the screening cylinder, and the
material rejected by the screening cylinder is collected
and discharged from its opposite end.
The assignee of this inven~ion has manufactured
; and sold many such screens in accordance with a series of
U.S. patents, commencing with Staege U.S. Patent No.
2,347,716, and followed by Martindale U.5. Patent No.
2,835,173; Seifert U.S. Patent Nos. 3,849,302 and
4,105,543 and Chupka-Seifert U.S~ Patent No. 4,155,841 and
4,383,918. Starting with the construction shown in the
Martindale patent, all such screens manufactured and svld
by applicant's assignee have been characterized by a rotor
comprising bars or vanes of airfoil section moving in
closely spaced but non-contacting relation with the
surface of the screening cylinder for the purpose of
creating alternating positive and negative pressure waves
effective on the perforations in the screening cylinder to
prevent plugging thereof.
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BKS 137 P2 -2-
The art has experimented widely with detailed
variations in screen~ of the above type, including
variations in the vane shape and other forms of rotor, and
also in the size, configuration, and spacing of the
5 perforations in the screening cylinder. Thus in the era
of the Staege patent in the mid-1940's, the screening
cylinder was fabricated from steel plate with multiple
uniformly cylindrical drilled perforations. When this
drilled plate was rolled into a cylinder, a natural result
10 Of the rolling operation was to effect some constriction
and expansion, respectively, of the inner and outer ends
of the drilled holes which were on the inlet (inside3 and
accepts (outside) sur~aces of the resulting screenin~
cylinder. This led in due course to the practice of
relieving the accepts (discharge) end of each cylindrical
perforation by a conical bore or countersunk portion to
minimize the possibility of plugging.
In more recent years, the trade has been offered
pressure screens yenerally of the above type wherein the
20 perforations in the screening cylinder are elongated slots
rather than round holes, typical such constructions being
shown in Lamort U.S. Patent No. 3,617,008, Holz U.S.
Patent No. 3,581,903, and the above-noted Seifert '302 and
Chupka-Seifert patents. Both Lamort and ~olz show slotted
25 screening cylinders wherein the slots have parallel sided
portions on the inlet side of the cylinder~ but on the
accepts side, each slot has widely diverging side walls.
Similarly in Chupka-Seifert, the screening cylinder is
fabricated from wire of triangular section with the base
30 of the triangle on the inlet side of the cylinder.
BKS 137 P2 -3-
This practice of providing relief on the accepts
side of the perforations in the screening cylinder vf a
pressure screen was thus followed consistently in the
paper industry since it was first introduced. The art
5 demonstrated a conviction that this practice was necessary
to minimize the possibility that the perforations would be
plugged by the fiber an~/or contaminant particles unless
their minimum dimension is at their inlet ends. Indeed,
this conviction became so firm that all of the claims of
the Holz patent include limitations to a "boat-shaped"
configuration of the screening slots on the accepts side
of the screening cylinder.
Another patent issued to the assignee of this
invention, Lehman 4,276,159, disclosed the discovery that
the prior art had been wrong in its conviction that the
screening perforations in the screening cylinder of a
pressure screen needed to be relieved on the accepts side
of the cylinder. More specifically, that patent disclosed
that not only will such screens operate without plugging
when the screening perforations are not relieved on the
accepts side of the cylinder, but most unexpectedly,
significantly improved results are obtainable when the
perforations are relieved on the inlet side of the
cylinder, e.g. in the same manner and to the same extent
has had commonly been done on the accepts side of the
cylinders of the prior art~
A recent modification of the Lehman patent screen
is disclosed in Lampenius U.S. Patent No. 4,529,520,
wherein the inlet side of a screen cylinder is provided
with axially extending grooves of trapezoidal section~ and
BKS 137 P2 -4-
cylindrical screening holes are drilled in the bottoms of
these grooves. The featured claimed in the Lampenius
patent is that the downstream side of each groove $s at a
relatively oblique angle to the flow direction o~ the
5 stock therepast, preferably an angle of approximately 30,
and in this respect, ~ampenius closely followed the
teaching in Fredriksson Canadian Patent No. 972,322,
wherein screening perforations are shown on the upstream
side of grooves or are provided with a countersunk relief
~ on their downstream sides. The emphasis in both of those
patents is on the desirability of reducing the target area
of the inlet side of each screening perforation which is
"seen" by the fibers in the flow therepast.
Summary of the Invention
In the screen of the Lehman patent, the screening
perforations - both slots and cylindrical holes - are
shown as arranged with their portion of minimum diameter
at the surface of the accepts side of the screening
cylinder. This portion of each hole or slot is of
2~ relatively small or short axial extent, and the remalnder
of the hole or slot is expanded in diameter or width
toward the inlPt side of the cylinder and reached its
maximum dimensions at the inlet surface of the cylinder.
In accordance with the present invention, it has
~5 been discovered that unexpectedly significant advantages
in operation are achieved with screening members employing
cylindrical screening holes if the portion of each hole
facing the inlet side of the cylinder is contoured to a
substantially larger diameter than ~hat of the minimum
diameter portion, while keeping its axial dimension
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sKS 137 P2 ~5-
relakively small or short, e.g. by countersinking to a
maximum diame-ter two or more times that of the minimum
diameter~ This countersunk portion, however, preierably has
an angle of taper which is relatively small with respect to
the surrounding surface of the screening member, e.y. 22.5%,
so that each of the countersunk portions is correspondingly
shallow.
In a preferred embodiment of the invention, the
screening holes are arranged in rows bo-th axially and
circumferentially of the screening member, and -thus also in
rows extending spirally around the cylinder. The depth and
angle of the countersunk portions of -the holes are such that
adjacent portions overlap or closely approach each other in
one or more of those directions so as to cover by far the
major fraction of the surface of the cylinder and to provide
multiple relatively sharp ridges along their overlapping
portions. The remainder of each hole may be uniformly
cylindrical, but preferably it will include a relatively
short cylindrical portion of minimum diameter adjacent the
countersunk inlet portion, while the rest of the hole will be
tapered outwardly at a small angle to the centerline of the
portion of minimum diameter, to provide each hole in axial
section with an outline like that of a short-stemmecl wine
glass.
Brief Description o~ the Drawin~s
Fig. 1 is a perspective view, partly broken away, o
pre~sure screening appara-tus embodying the inven-tion;
Fig. 2 is an enlarged fragmentary view of the inlet
side of the screening cylinder in the screen of Fig. 1 prior
to formation of the cylinder from a flat metal sheet;
Fig 3 is a fragmentary section on the line
3--3 of Fig. 2; and
Fig. 4 is a fllrther enlarged fragmentary section on
'~ -the line 4--4 o Fig. 2.
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BKS 137 P2 -6-
Description of the_Preferred Embodiments
The screening apparatus shown in Fig. 1 is
constructed generally in accordance with 5eifert U.S,
Patent No. 4,105,543, with certain exceptions in
5 accordance with the invention. It comprises a main
housing 10 on a base 11, and in the upper end of the
housing is an inlet chamber 12 having a tangential inlet
port 13 to which the furnish is supplied under peessure as
is customary with such screening apparatus. ~ cylindrical
1~ screening member 15 divides the interior of the housing
below chamber 12 into a central supply chamber 16 and an
accepts chamber 17 having an outlet port 18.
The bottom wall 20 of the supply chamber 16
includes a trough 21 leading to a discharge port 2~
15 provided with a control valve assembly 23 which can be
preset to provide a desired continual bleed of reject-rich
stock. Heavy particles which settle into the trough 21
drop therefrom to the heavy trash collection box 24 by way
of manually controlled valve 25 for intermittent removal.
A rotor 30 is suppor~ed on a drive shaft 31 in
the center of the supply chamber 16 and is driven through
suitable gearing or belts by a motor 33 also moun~ed on
the base 11. Vanes or bars 35 are mounted on the rotor 30
by support rod~ 36, and ad~ustable connections 37 between
25 the inner ends of rods 36 and rotor 30 provide for
positioning the vanes 35 in properly spaced relation with
the inner surface of screening member 15, in accordance
with standard practices.
The vanes 35 extend the full length of the
30 screening surface of screen member 15, and they are
BKS 137 P2 -7-
helically curved and so arranged that the upper end of
each vane is spaced forwardly of the lower end in the
direction of rotation of the rotor, shown as clockwise.
Two vanes 35 are shown, but other numbers can be used, and
5 in general ~ greater number, e.g., four, may make possible
improved operation at higher consistencies.
The screening cylinder 15 is provided with
multiple screening holes 40 of circular section, which for
ease of fabrication should be arranged in rows extending
10 both circumferentially and axially of the cylinder, and
which in practice will cover substantially the entire
surface of the cylinder. These holes are preferably
drilled or otherwise fabricated in flat metal plate, which
is then rolled to cylindrical form and has its abutting
15 edges welded together. Typical plate stock which has been
found satisfactory for the practice of the invention is
stainless steel 0.188 inch in thickness.
Referring specifically to the enlarged view in
Fig. 4, each hole 40 is composed of three concentric
20 portions, an internal cylindrical portion 41 of
predetermined minimum diameter, an inlet portion 42, and a
discharge portion 43. The inlet portion 42 has a diameter
which increases rapidly from a minimum, where it coincides
with the diameter of the cylindrical portion 41, to a
25 maximum at the surface 44 which is substantially greater
than its minimum diameter. The por ion 41 of minimum
diameter may extend all the way to the discharge side 45,
as in the Lehman patent, but preferably the discharge
portion 43 is slightly outwardly tapered to reduce the
30 axial length of the cylindrical portion 41, satisfactory
results having been obtained with the portion 43 tapering
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BKS 137 P2 -8-
outwardly at an angle of 7 to the centerline of
cylindrical portion 41.
It has been found important to the success of the
invention that the axial dimension of each inlet portion
5 42 be substantially less than its maximum diameter at the
inlet side 44 of the screening member, so that it will be
relatively shallow but will have a relatively extended
outer periphery. This condition can readily be
established by forming the inlet portion 42 by
10 conventional countersinking with a relatively flat taper
angle, as illustrated in Fig. 4~ and excellent results
have been obtained with an angle of taper of 22.5 with
respect to the surface of the plate.
As shown, the axial dimensions of the ~hree
15 individual portions 41-43 are such that the combined axial
dimensions of the portions 41 and 42 total less than that
of the discharge portion 43. For example, highly
satisfactory resu~ts have been obtained with this combined
axial length in the range of 0.040 to 0.060 inch in steel
20 plate 0.188 inch in thickness, so tha~ the discharge
portion 43 is two or more times the axial length of the
combined portions 41 and 42.
The diameter and depth of each inlet portion 42
are specially related to the size of the associated
25 cylindrical portion 41 and the spacing of adjacent holes
40. For preferred results in the practice of the
invention, the relative spacing of adjacent holes and the
maximum dimension of their inlet portions 42 should be
such that the combined areas of the ~ountersunk faces will
3~ constitute a large major fraction of the total surface 44
BKS 137 P2 -9-
of the screening member. A result of this condition will be
that adjacent inlet portions will so closely approach or
overlap each other in at least one direction as to define
multiple relatively sharp ridges 4~ o:E arcuate contour over
the entire surface 44. Also, as it is apparent from Fiy, 2,
even if the maximum diameter of the inlet portions 42 is such
that none touch each other, they will still project axially
and circumferentially of the cylinder into the spaces between
adjacent inlet portions in the rows on either side thereof
and thus overlap such ad~acent inlet portions without
actually intersectin~.
For example, in one embodiment of the invention, the
minimum diameter for each hole 40 was .050 inch, and adjacent
holes were spaced .125 inch on centers circumferentially of
the screen member and .272 inch axially thereofO Then with
each inlet portion .020 inch in maximum depth, the maximum
radius (r) was .0733 inch, which gave a maximum overlap of
.022 inch circumferentially of the cylinder and .010 inch
spirally of the cylinder. This combination of dimensions
resulted in having the total of the countersunk areas
(~r2 times the total number n of holes) except 80% o~ -the
surface 44, with the remaining plane areas beiny
discontinuous and of individually small size.
In another example, the holes 40 had a minimum
diameter of .062 inch and their spacing circumferentially and
axially of the screen member was respectively ~250 inch and
.219 inch. With each inle-t portion .030 inch deep at its
inner periphery, the outer cliameter of each inlet portion was
0.224 inch, which provided a maximum overlap of .102 inch
spirally of the screening member and a maximum overlap of
.005 inch circumferentially of the screening member. The
combined total of the countersunk areas (~r2n) would
theoretically exceed the area of surface 45, except for the
overlaps.
r~
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BKS 137 P2 -10-
In another example, the holes 40 had a minimum
diameter o~ .125 inch and their spacing circumferentially
and axially of the screen member was respectively .250
inch and .375 inch~ With each inlet portion .030 inch
5 deep at its inner periphery, the outer diameter of each
inlet portion was .27 inch, which provided an overlap of
.020 inch between adjacent inlet portions axially of the
screening member and .045 inch spirally of the screening
member, and a ridge 46 having a minimum width of .10 inch
10 between adjacent inlet portions axially of the screening
member. ~he total of the countersunk areas (~r2n) would
again theoretically exceed the area of surface 45, except
for the overlaps.
Analysis of these examples will show that in each
15 case, the countersunk areas will cover by far the majority
of the surface of the inlet side of the screening member,
and will provide a great multiplicity of relatively sharp
arcuate edges or ridges. Testing of a number of such
examples indicates that the combination of the resulting
20 irregular surface of the screenîng member with the
hydraulic action of the vanes or foils on the rotor
results in patterns of irregular eddies adjacent the inlet
to each of the screening holes which greatly promote
efficient screening action.
~5 While the forms of apparatus herein described
constitute preferred embodiments of this invention, it is
to be understood that the inventon is not limited to these
precise forms of apparatus, and that changes may be made
therein without departing from the scope of the invention
30 which is defined in the appended claimsO
What is claimed is:
