Note: Descriptions are shown in the official language in which they were submitted.
~3~155~
1 60412-1907
This invention relates to the rotor in a pressure
knotter.
In the papermaklng process, wood knots and foreign
material such as stones or pebbles must be separated from the
fluidized pulp. This separation is generally accomplished by
passing the fluid pulp through a cylindrical screen whose
apertures are sized to reject unwanted solids. During this
separation operation, the oversized solids such as knots and
pebbles plug the screen apertures. It is known in the prior art
to produce periodic hydrodynamic pulses in the direction opposite
to the flow of the fluidized pulp through the screen so as to
clear the screen. A typical prior art configuration is shown in
Fig. 1. In this configuration, hydrofoils 10 carried on a rotor
12 sweep past a screen 1~ to clear the screen 14. The hydrofoils
10 are subject to wear at the portion of the hydrofoil near the
screen resulting in part from the action of small solid particles
which pass through the screen. Because of many commercially
available screens the hydrofoils 10 and rotor 12 form a unitary
structure, upon wear, the whole rotor/hydrofoil assembly has to be
replaced. In other commerclally available screening apparatus,
the hydrofoil alone is replaced even though only a portion of the
hydrofoil experiences wear. Other rotor-blade configurations are
shown in United States Patents 4,200,537; 3,363,759 and 3,680,696
and Canadlan Patent No. 1,136,092.
Summary of the Invention
The present invention provides a screen apparatus for
removing wood knots and other foreign material from a pressurized
~tream of fluidized pulp, said apparatus comprising: a
.
1321551
2 60412-1907
cylindrical screen for receiving the pressurized stream of
fluidized pulp, the screen havlng openings sized to reject foreign
material travelling in the pressurized stream, and there being a
pressure differential across the stream; a drum disposed
concentrically within the screen, the drum carrying one or more
blades in close proximity to the screen, for clearing the openings
of foreign material, each blade having a first portion and a
second portion, the first portion having an inclined leading face,
the face being configured so that at its downstream end it is
closer to the screen than at its upstream end, with the result
that fluid i5 forced outwardly from the drum to the screen, the
inclined leading face having an axial extent at both its upstream
and downstream ends, with the result that fluid is forced to
travel over the blades and thus is forced outwardly from the drum
to the screen, the second portion having a substantially radially
extending leading face positioned downstream of the inclined
leading face of the first portion, with that leading face
extending radially toward the screen beyond the radially outermost
end of the inclined leading face, the second portion of the blade
being positioned so that it is subjected to the most wear of any
portion of the blade, and the second portion being removably
afflxed so that lt can be replaced when worn.
In a preferred embodiment, the thickness of the second
blade portion is equal to or greater than the diameter of holes in
the screen. Further, the length of the second portian extending
beyond the first portion is greater than the diameter of holes in
the screen. It is also preferred that the length of the
projection of the inclined face be greater than the length of the
132t551
2a 60412~1907
radlally extending face of the first portion. It ls al80
preferred that fluid flowing between the rotor and the screen has
an axial velocity greater than or equal to four feet per second.
The annulus between screen and rotor is sized to maintain this
relationship.
The second blade portion which extends to the proximity
of the screen is subjected to the most wear. Slnce it is
removably affixed to the first blade portion, it can be removed
and replaced without the necessity of replacing the whole rotor
assembly or whole hydrofoil as known in the prior art. This
invention thus results in lower operating costs of the screening
apparatus. That the thickness of the second portion of
1321551
the blade is equal to or greater than the diameter of
holes in the screen assures effective hydrodynamic
pulses for keeping the apertures unclogged. Because the
length of the second blade portion which extends beyond
the first blade portion is greater than the diameter of
the holes, there is a reduced likelihood of jamming as a
result of solid matter passing through the screen.
Brief Description of the Drawinq
Fig. 1 is a perspective view of a prior art
pressure knotter;
Fig. 2 is a perspective view of the pressure
knotter screening apparatus of the present invention;
Fig. 3 is a perspective view of the rotor of
the screening apparatus; and
Fig. 4 is a cross-sectional view of the rotor
of Fig. 3.
DescriPtion of the Preferred Embodiment
Pressure knotter apparatus 20 shown in Fig. 2
includes a~ inlet 22 for receiving fluid pulp having a
pulp concentration in the range 2-6~. The fluid pulp
travels upwardly and passes inwardly through a screer.
24. Fluid pulp which passes through the screen 24 exits
through an accepts exit 26. Dilution water enters the
apparatus 20 through an inlet 28. Knots, pebbles and
other solid material too large to pass through the holes
in the screen 24 exit through a rejects exit 30.~
With reference now to Figs. 2, 3, and 4, a
rotor assembly 32 is disposed concentrically with
respect to the screen 24. The rotor assembly 32
generates outwardly directed hydrodynamic forces tending
to unclog the screen 24. In particular, the rotor
assembly 32 includes a plurality of blades 34 which
sweep across the screen 24 in close proximity to it to
generate hydrodynamic flow radially outwardly. As shown
\
1 32 1 55 1
- 4 -
in Fig. 3, the blades 34 are disposed on the rotor
assembly 32 in a staggered fashion so that
circumferential bands or rows of blades overlap one
another~ This arrangement insures that the entire
surface area of the screen 24 is swept by the blades
34. The blades 34 include a first portion 36 having an
inclined face 38 and a substantially radially extending
face 40. A second portion 42 of the blade 34 is
removably affixed to the radially extending face 40 of
the first portion 36. The second portion 42 may be
affixed to the first portion 36 by machine screws 44.
As will be appreciated by those skilled in the art, the
second portion 42 will receive the most wear because of
its motion in close proximity to the screen 24. Because
the second portion 42 is removable, it can be replaced
as needed after a period of use.
It is preferred that the thickness W of the
second portion 42 be greater than or equal to the
diameter of the holes in the screen 24. By keeping the
thickness W of the pulsemaking second portion 42 greater
than the hole diameter assures that the maximum positive
pulse is developed for unclogging the screen 24. If the
blade were narrower than the hole diameter, there will
be a negative pulse developed on the trailing side of
the blade as the blade passes across the hole diameter.
lt is also preferred that the length X of the seçsnd
portion 42 of the blade 34 which extends beyond the
first portion 36 be greater than the diameter of holes
in the screen 24. In this case, there is less
likelihood that solid particles passing through the
screen 24 will become lodged between the blade 34 and
the screen 24~ It is furthermore preferred that the
length B be greater than or equal to the dimension A
shown in Fig. 4. This relationship restricts the
., , ' . .
.
-
.
1 321 551
-- 5 --
steepness of the angle of the inclined face 38. If the
length B were less than the dimension A, the resultant
angle would be quite steep~ A steep angle would have a
higher drag coefficient and would in turn require more
power for rotation of the rotor assembly 32. By keeping
the angle less steep, the fluid undergoes a more gradual
acceleration which in turn requires lower horsepower for
operation. It is also preferred that the fluid pulp
travelling upwardly in the space between the screen 24
lQ and the drum S0 of the rotor assembly 32 be greater than
or equal to four feet per second. Maintaining this
velocity will minimize the buildup of pulp process in
the area between the blades and screen. Such a buildup
would increase the power required to rotate the rotor.
Furthermore, keeping the velocities up will encourage
the process to move axially toward the accepts exit 26
and will minimize the chance of the process settling out.
As shown in Fig. 3, the blades 34 are
substantially aligned with an axis of rotation of the
rotor assembly 32. The blades 34, however, may be
disposed on the drum 50 making an angle with the axis of
rotation, if desired.
The rotor assembly of the present invention is
highly effective in unclogging the screen 24. Because
the second portion 42 of the blade 34 is removable,
substantial reductions in operating costs are ach~evable
since the whole rotor assembly 32 need not be replaced.
It is recognized that modifications and
variations of the present invention will occur to those
skilled in the art and it is intended that all such
modifications and variations be included within the
scope of the appended claims.
