Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A rotating element for a screeniny apparatus with a contour
surface
The present invention relates to a screening apparatus which
is intended primarily for screening and purification of pulp
and more specificall~ paper pulp. This screening apparatus
comprises a vessel, a cylindrical screen in the interior of -~
the vessel, a rotor which moves in the vicinity of the screen
surface, an inlet for the pulp to be screened, an outlet
for the reject and another outlet for the screened pulp,
which is called the accept.
In Canadian Patent 1~2l5l943~ a cylindrical screen plate is
described which has an inlet on one side for introducing
the unscreened pulp, and an outlet in the opposite side for
removing the reject portion. Means are provided for moving the
unscreened pulp along one first direction of flow. The screen
plate has grooves in the side of the inlet recessed in the
screen surface, the first direction of flow being essentially
transverse to the grooves. The grooves are formed of an up-
stream side plane, a downstream side plane and a bottom plane.
The bottom plane is essentially parallel to the envelope
surface of the screen plate. The grooves have perforations
in the bottom plane. The upstream side plane of the grooves,
as seen standing from the bottom of the grooves, is subs-
tantially perpendicular to the envelope surface and the down-
stream side plane of the grooves forms an angle of 60 - 5
against the envelope surface. According to a preferred embo-
diment, the angle between the downstream side plane of the
grooves and envelope surface of ~he screen plate is about 30.
This configuration is called a "contour profile".
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In the paper making process, pulp is produced by cooking
wood which separates the wood into fibers. Due to the different
properties of the wood even from the same tree, some of the
fibers do not separate and are dispersed as fiber bundles
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usually called debris, shives or slivers which form the re~eck.
There are also other impurities, such as bark, which must
be removed. The screen must separate the undesirable impurities
and debris called the rejects from the accept portion. In
order to avoid substantia] losses of fibres which could be
carried over together with the debris in the reject portion,
it is necessary to remove the impurities efficiently and
selectively.
It should be stressed that different applications have dif-
ferent requirements. In some application, it is necessary to
achieve a high content of long fibers, especially secondary
fibers, in the accepts because the long fibers ~ive strength
to the final product, for instance paper. In other applications,
on the other hand, the contrary is true. For instance, in
virgin or pulp mill fibers, it is desirable to concentrate
the long fibers in the reject for reject refining.
A great deal of work has been carried out in connection with
the screen plates and the rotors and it has been recognized
that means to create pulsations on the rotor will increase
the efficiency of the apparatus. U.S. Patent 3,363,759 and
U.S. patent 4,318,~05 describe drum rotors with a bumped
surface which provides pulsations. In U.S. Patent 4,318,805,
the bumps take the form of pins projecting from the rotor
with enlar~ed heads, the heads providing the pulses while
the pins offer little resistance to flow.
U.S. Patent 4,447,320 and U.S. Patent 4,2~0,537 describe
rotors which carry blades or vanes moving in the vicinity
of the screen which produce a large positive pulseO Other
patents describe other types of rotors, for instance U.S.
Patent 3,726,401 uses a rotor with bumps or protruberances
which produce about equal positive and negative pulses.
According to this patent, any form of bumps may be used pro-
vided it produces the desired pulses, the bumps and the
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depressions between them creating positi~e screening and
negative screen cleaning pressure pulses.
U.S. Patent 3,400,~20 describes a rotary member m~de up of
a plurality of separate segments joined together and forming
a selected undulating pattern which procluces about equal
positive and negative pulses.
One object of the present invention is to provide a rotating
element which increases the intensity of the pulses generated
near the openings, either orifices or slots within the screen
plate, for the purpose of creating the negative pulses which
are necessary to backwash the screen, and to prevent plugging,
thus increasing the flow oE the Eiber suspension through
the openings of the screen.
Another object is to provide a rotating element with very
high frequency pulses in addition to sufficient amplitude.
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Another object is to provide a rotating element which pro-
duces sharp and steep negative pulses, thus resulting in high
intensity.
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Still another object is to reduce the power requirements. -
Another object is to provide a rotor which permits to operate
with smaller orifices in the screen thus impro~ing the scree-
ning efficiency.
Still another object is to obtain an accept portion of con-
sistency as close as possible to the consistency of the pulp
introduced at the inlet.
Another object is to provide blade type segments to be used
instead of a rotor in certain applications in which the con-
tinous surface of a rotor is not desirable, for instance
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when ~ractionation of the long fibers should be avoided and a
larger inlet screening zone is preferable.
Still another object is to provide a rotor and blade type
segments which may be advantageously usea in conjunction
with the screen plate described and claimed in Canadian
Patent 1,215,943
It has now been found that a specific shape of the rotor
surface and the blade type segments as described hereinbelow,
is particularly advantageous in producing the higher intensity
pulses and in creating sufficient negative pulses so that
plugging of the screen is minimized.
The present invention will be illustrated in more detail
by reference to the accompanying drawings of which:
Fig. 1. illustrates a conventional pressure screen using
rotating hydrofoils for producing pulses;
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Fig. 2. illustrates the pulses which are produced by the
~¦ rotating hydrofoil screens of Fig. l;
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Figs. 3 and 3a illustrate the contour surface of the rotor
in accordance with the present invention;
Figs. 4 and 4a illustrate the nature of pulses which are
produced from a rotor or from the blade type segments
with a contour surface according to the present
invention, as shown in Figs. 3 and 5;
Fig. 5 illustrates the embodiment of the blade type segments
instead of a rotor, with a contour surface according
to the present invention;
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Fig. 6 shows the contour profile of the screen plate according
to Canadian Patent 1,~15,9~3;
Fig. 7 illustrates the intensity factor which is the ratio
of the change of amplitude over a unit of time with
a conventional rotating element;
Fig. 7a illustrates the intensity factor for a rotating element
according to the present invention;
Fig. 8 is a comparison of the pulse profile of conventional
rotor and -the rotor according to the present invention.
As shown in Fig. 2, the rotating hydrofoils produce both
negative and positive pulses. The symbol O pressure designates
the value o the pressure near the screen sur~ace or the
screening zone between the rotating foil and the screen cylin-
der. When the pulp pressure is greater than the reference O
point, the pressure has positive value, which is at maximum
at the point A and when it is less than the reference point,
the pressure has a negative value, the latter being at a
maximum at the point B.
The rotating hydrofoils of Fig. 1 known in the art produce
pulses illustrated in Fig. 2. As shown in the figure, the
leading surface or nose on the hydrofoil develops a positive
pulse with the maximum value or amplitude just about at the
minimum clearance point between the foil and the screen.
The positive pulse contributes to induce the flow in the
screening zone through the apertures within the screen because
the flow will always be from a higher pressure zone to a lower
pressure zone. However, after the minimum foil clearance
point a maximum negative pulse is developed due to the change
in slope of the foil blade from its minimum clearance position.
As shown in Figure 1, the angle of this slope for most foil
bla~es would be a maximum of approximately 30 or even less.
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After the maximum negatlve pu]se point -the pressure increases
again up -to the reference O point. The negative pulses induced
by the rotating hydrofoil cause a momentary reverse or back-
flow through the apertures within the screen thus dislodging
any build-up of fibers at the apertures. This self-induced
backwashing is greater, the greater is the negative pulse.
The foils or lobe-shaped surfaces or protrusions of rotors
and blade type segments known in the art have leading surfaces
curved towards the screen cylinder until the minimum rotor
clearance point is reached, followed by a trailing surface
curving away from the screen. At the minimum rotor clearance
point, the change in slope of the curved surface is O. ShortIy
after this point, the magnitude of the negative pulses starts
to increas~ to a maximum negative value after which the pulses
start decreasing again, thus repeating the cycle, as shown
in Fig. 2.
The crux of the present invention resides in providing a rotor
and blade type segments with grooves, the grooves having
a contour surface being formed of a first plane parallel to
the envelope surface, an inclined plane, an upper plane and
a side plane, the side plane being substantially perpendicular
to the first plane and the inclined plane of the grooves
forming a 30 - 60 angle against the first plane, the upper
plane being parallel to the first plane. According to a pre-
ferred embodiment this angle is about 45.
By reference to Figs. 3 and 3a, numeral 1 designates the first
plane and numeral 2 designates the inclined plane. Numeral 3
designates the upper plane and numeral 4 designates the side
plane perpendicular to the first plane. Numeral 6 designates
the envelope surface to the screen.
By reference to Fig. 5, the same numerals 1, 2, 3 and 4 are
used to designate the first plane, the inclined plane,
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the upper plane and the slde plane which is perpendicular
to the first plane.
As shown in Fig. 3a, starting from reference point A, the
leading surface has a first plane 1 parallel to the envelope
surface. It then slopes forming inclined plane 2 until it
reaches the upper plane 3. The length of the upper plane 3
should be between 1/4 and 3 inches. After the upper plane,
the groove comprises a side plane 4 perpendicular to the first
plane 1. The first plane 1 may vary from a length of O up to
3 inches.
This feature of the side plane 4 being substantially perpen-
dicular to the bottom plane is essential in order to produce
pulses which are more effective in keeping the screen from
plugging, as compared with rotors which have a curvecL surface.
Figure 4 illustrates the pulses from a rotating element having
the contour profile according to the present invention. A very
sharp negative pulse is produced by the 90 angle in the rotor
surface or in the blade type segments. This is due to the rapid
change in the flow path of the fluid coupled with a sernouilli
effect caused by the increasing velocity of the fluid within
the minimum rotor or blade type segment clearance point. This
severe or rapid change in pulsation makes the rotor or the
blade type segment more effective in back-flushing the screen
cylinder. The explanation for this effect is that the maximum
velocity and acceleration of the fluid which goes through
the apertures during back-flushing must be greater than with
conventional pulse producing rotating elements because the
change in pulses or amplitude with time is much greater with
a contour surface ~see Fig. 4a).
The term "intensity of the pulses" within the scope of the
present invention is intended to mean the change o~ amplitude
of the pulses per unit time. To state the matter in different
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words, the intensity is not merely the result of the amplitude
but also depends on the fast and sharp change. The intensity
factor, IF, is the ratio of the change of amplitude over the
unit of time Q am~ ude ~Fi
~ time
Figure 4 when compared with Figure 2 shows the superiority
of the rotating element of the present invention with the
contour surface with respect to conventional rotors or con-
ventional blade type elements. Further, the slope and leading
surfaces of the contour rotating elements produce positive
pulses which help in inducing the flow of the fibrous suspen-
sion through the apertures within the screen. The upper
surface 3 is needed just ahead of the side plane ~ to cause
the rapid change in pulsation.
Figure 5 illustrates the embodiment in which a large screening
zone with a substantial open area is desirable to allow inlet
stock to enter the full screening zone, thus minimizing
the concentration of the long fibers in the reject and mini-
mizing the consistency of the reject, so that more long
fibers are in the accept. In this embodiment, instead of a
rotor, blade type segments are used with a contour surfacè.
The contour surface of the rotating element, a rotor or blade
type segments according to the present invention, provides
high frequency pulsations. For instance, with a rotating
speed of 600 , it is possible to achieve a pulsation fre-
quency of 200 Hz and even higher. The high frequency is
advantageous because there is less tendency for the fibers
to plug the openings and due to the ~act that the pulses
also exhibit sufficient amplitude, back-flushing occurs.
The higher frequency permits the screen to handle stock of
higher concentration.
Comparison of Figures 7 and 7a shows the superiority in
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intensity factor achieved by the rotating element according
to the present invention as compared wlth a conventional
rotating element. In E'igure ~ the pulse profile of a conventio-
nal hydrofoil is shown in broken line and the pulse profile
of a rotor with the contour surface according to the present
invention is shown in solid line.
Other advantages of the rotor and blade type segments according
to the present in~ention are that it is possible to reduce
substantially the size of the apertures of the screen without
any appreciaible increase in power consumption, nor with any
substantial loss in long fibers and with good yield of the
accept portion. The consistency of the accept may be kept
essentially the same as the consistency of the stock at the
inl0t .
The rotor as shown in Fig. 3 and the blade type segment shown
in Fig~ 5 may be used with conventional screen plates and also
in conjunction with the screen plate having a contour profile
according to Canadian Patent 1,215,943 as shown in Fig. 6.
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