.
'l
Ill SLOTTED PRESSURE SCREENS
Slot width and consistency need to be balanced
BY C.M. VlTORl
tial positive pulse generated by the leading edge of the foil will increase the passing velocity. The negative pulse generated by the trailing edge of the foil to
back-flush the screen plate, results in
0
II
Slot width
VER THE PAST DECADE there has
been a significant quality
improvement in secondary fibre
stock preparation systems by the use of
fine slotted pressure screens. Further
advances in screen plate designs over
the past few years motivated a trend
towards the use of progressively narrower slot widths. Slot widths as narrow
as 0.15 mm (0.006 in.) now provide substantial improvements in stock quality,
particularly with respect to stickies
removal. As suppliers and paper mills
strive to further advance stickies
removal efficiency, new attention has
been focused on fine slotted screens.
Recently, particular attention has
been directed towards the velocity at
which stock passes through the screen
slots, or passing velocity as it has been
referred to. It is believed that as passing
velocity increases, the tendency of conformable stickies to be extruded
through the screen slots is also
increased resulting in reduced stickies
removal efficiency [ 1,2].
Passing velocity may at first be
thought of as a process design characteristic. As flow through the screen is
increased, it follows that the velocity at
which the stock passes through the slots
must also increase. However, passing
velocity is greatly affected by the
hydraulic actions of the rotating element which is designed to align contaminants for ease of removal and backflush the screen plate [ 3 ] .
In the case of foil-type blades, the ini-
1441
4
stock being pulled back through the
screen plate, creating a negative velocity.
The complex hydraulic and mechanical interactions between the foil elements, screen plate, and the stock are
II
Open area
0.15 mm (0.006 in)
0.20 mm (0.008 in)
0.25 mm (0.010 in)
8,968 mm sq (1 3.9 in sq)
1 1,923 mm sq (1 8.48 in sq)
14,903 mm sq (23.1 in sq)
1550
P
A
:::
R C
C N
L T
E
I
BIN 2
BIN 1
BIN 4
BIN 3
BIN 5
I
E 100
F
F
I
C
% I
E
N
C
Y
0
1
2
3
4
5
VELOCITY (mls)
PULP & PAPER CANADA 94:12 (1993) 109
not completely understood; therefore,
passing velocity may be considered a
theoretical or calculated average value
based only on gross flow rates through
the measured open area of the screen
cylinder.
A research program was designed to
develop a method to compare stickies
removal efficiency between various slot
widths at various passing velocities. Particular attention was paid to efficiency
based on particle sizes rather than just
overall efficiency. When analyzing stickies removal, overall efficiency may be
misleading if the furnish contains substantial quantities of large or small particles which tend to weight results. It was
theorized that different ranges of particle sizes would be removed with different efficiencies as the passing velocity
changed; therefore, it would be important to determine efficiency by particle
size range.
system was shut down and a screen cylinder with a different slot width was
installed. The order in which each screen
cylinder was tested was also random.
Once testing was complete for the
three different screen cylinders the feed
stream was then diluted to 0.75% consistency and the tests were repeated.
Note from the screen operating data
listed in Table 11, that approximately
Slot width
Stock
velocity
0.15mm
1 m/s
2 m/s
3 m/s
4 m/s
5 m/s
1 m/s
2 m/s
3 m/s
4 m/s
5 m/s
1 m/s
2 m/s
3 m/s
4 m/s
5 m/s
0.20mm
OETHOD
Furnish The furnish consisted of 100%
overissue newsprint collected from a
local publisher and contaminated by
adding 4% by weight shredded pressure
sensitive adhesive address labels. The
furnish was pulped at 6% consistency
and ambient temperature for 20 minutes.
Pressure screen: A Black Clawson Model
12 centrifugal pressure screen having
four foil-shaped rotating elements on
the inlet side of the screen cylinder was
used for screening stickies from the
newsprint furnish. Contoured surface
slotted screen cylinders with three different slot widths, 0.15 mm (0.006 in.),
0.20 mm (0.008 in.), and 0.25 mm
(0.010 in.) were compared. The screen
cylinders are listed in Table I along with
their corresponding open areas.
Theoretical stock velocities of 1, 2, 3,
4, and 5 metres per second (m/s) were
compared at 0.75% and 1.25% screen
feed consistency through each screen
cylinder. Flow rates about the screen
were derived based upon the desired
velocity, screen cylinder open area, and
a 15% volumetric reject rate from the
screen.
The screen runs were done first at
1.25% feed consistency in a closed loop
by setting the proper feed, accept, and
reject flow rates for the desired velocity
through the slots based upon the
installed screen cylinder’s slot size. Samples from the accept stream were then
taken at each of the five stock velocities.
The order of setting the flow rates for
the different velocities was random. Feed
stream samples were taken at the first,
third, and fifth velocity settings. Reject
stream samples were taken at the third
velocity setting. Once sampling for a
given screen cylinder was completed, the
110
25 mm
equal capacity is obtained under the following conditions; 5 m/s with the 0.15mm slot, 4 m/s with the 0.20-mm slot,
and 3 m/s with the 0.25-mm slot. This
information helps in comparing efficiency at equal capacity between the
three slot widths.
Sample preparation: Observation sheets
weighing 1 gram were prepared from all
feed and accept samples in a Williams
Feed flow
L/min.
Accept flow
L/min.
632
1264
1896
2528
31 60
a40
1681
2525
3365
4205
1048
2104
31 57
4209
5257
Reject flow
L/min.
537
1075
1612
21 50
2687
71 5
1431
2146
2861
3577
893
1790
2684
3577
4470
95
1 a9
284
378
473
125
250
378
503
628
155
314
473
632
787
----
0
0.15 mm Slot
0 0.20 mm Slot
y
01
0
2
1
4
3
5
VELOCITY ( d o )
----
0
F
..-_..
A 0.25 mm Slot
F
I
%C
I
E
N
C
PULP & PAPER CANADA 94:12 (1993)
0.15 mm Slot
0 0.20 mm Slot
0
0
hm
m
v
-
0
0
-*--
4 T442
.I
200-mm x 200-mm sheet mold for image
analysis. Great care was taken when
removing the sheets from the mold so
that the stickies would not be deformed.
Very light pressure was used to couch
the sheets onto blotter paper to remove
them from the sheet mold. The sheets
were then air dried instead of the common practice of using heat to dry. This
prevented any thermal deformation or
bleeding of the stickies within the observation sheets prior to image analysis.
Image analysisequipment: A Cambridge
Quantimet 520 Image Analysis System
with macroscopic detection capabilities
was used to count and classify the stickies particles by size.
Procedure: Observation sheets from the
screen feed and accept samples were
analyzed by image analysis and compared to determine screening efficiency
by particle size. Six sheets from each
feed sample and eight sheets from each
accept sample provided sufficient particle counts to be considered significant.
As discussed, the feed stream samples
were taken from the first, third, and fifth
velocity settings for each screen cylinder
test. Two observation sheets from each
of these feed samples were grouped to
form a set of six sheets representing the
feed stream for each screen cylinder
test.
Counting bins for particle size based
upon particle area were established to
take into account the fact that different
ranges of particle sizes were removed by
each screen cylinder. The counting bins
listed in Table I11 provided sufficient
coverage of particle size ranges to compare efficiency based upon particle size
between the three screen cylinder slot
widths. Data were also compiled from
the total black area (total area of the
stickies) measured in the samples to
compare efficiency on the basis of
reduction of black area.
Efficiencies were determined by
comparing the number of stickies in
each bin in the feed sample observation
sheets per area measured to the number
of stickies in each bin of the accept
stream observation sheets per area measured. Calculations were then made
according to TAPPI Technical Information Sheet TIS 003-4 to determine cleanliness efficiency, E,.
0
1
4
3
2
5
VELOCITY ( m k )
-
0 0.15 mm Slot
- - - - - 0 0.20 mm slot
I
E loo
01
___c_
I
0
3
VELOCITY (mls)
2
4
5
1
%loo
E
......... A
t3
F
F
F--r
I
C
I
.- % -.
0
0.25 mm Slot
0
---D
p-“=
.?gA
E
I
0
3
2
4
5
VELOCITY (mls)
0ESULTS
Furnish: Pulping of the newsprint and
address labels resulted in a gray furnish
containing conformable black sticky
particles of various sizes and mostly nonround shapes. When viewed in observation sheets, the black stickies contrast
sharply against the gray sheet, resulting
in excellent detection by computerized
image analysis. Figure 1 shows the average distribution of stickies particles in
T443
4
Bin number
1
2
3
4
5
6
7
8
9
1
0
Particle
0.018- 0.071- 0.159- 0.283- 0.442- 0.636- 0.866- 1.130- 1.431- 1.767-
Area
0.071 0.159 0.283 0.442 0.636
0.866 1.130 1.431 1.767 2.000
I
I
PULP & PAPER CANADA 94:12 (1993)
111
.. .
the furnish. Particle count bins 6
through 10 (particles 0.636 mm sq. and
larger) did not contain large enough
quantities of stickies to be considered
significant.
Efficiency/size 0.75% feed consistency:
At 0.75% feed consistency particles in
bins 1 through 3 were not removed with
any significant efficiency; however, particles in bins 4 and 5 were removed with
measurable efficiency. Figures 2 and 3
show the difference in efficiencies
between the three slot widths used.
A slight trend of decreasing efficiency as velocity increases is evident
from the data. The rate of decrease in
efficiency is more significant on the
larger particles in bin 5. Most significantly, the efficiency is always greater
with the smaller slot widths even when
compared at equal capacity. The efficiency of removal of particles in bin 4 is
only slightly greater with the 0.20-mm
slot than with the 0.25-mm slot; however,
the efficiency decreases only slightly as
the velocity increases resulting in equal
or greater efficiency with the smaller
slot width.
1.25% feed consistency: Only particles
in bin number 1 were too small to be
removed with measurable efficiency at
1.25% feed consistency, indicating
increased efficiency at the higher consistency. Figures 4 through 7 illustrate the
differences in efficiency amongst the
three slot widths used for removal of
particles in bins 2 through 5. Again, the
data indicates a slight trend of decreasing efficiency as the passing velocity is
increased. This trend tends to increase
as the particle sizes become larger; however, a smaller decrease in efficiency as
velocity increases is evident with the
0.15-mm slot than with the 0.20-mm or
0.25."
slots.
When compared at approximately
equal capacities, again the efficiency is
greater for the smaller slot widths except
between the 0.20-mm and 0.25-mm slots
on particles counted in bin number 5.
The efficiency of removing the particles
in bin 5 is approximately equal between
the two slot widths and decreases significantly as the velocity increases. At 5 m/s
velocity through the 0.20-mm slot and at
4 m/s through the 0.25-mm slot, capacities are approximately equal (points
labelled A and B respectively in Fig. 7).
Equal efficiency is obtained with the
wider slot in this case; however, the 0.15mm slot still produced higher efficiency
than both the other slot widths even
when comparing this slot at 5 m/s to the
other slots down to 1 m/s velocity.
A comparison of the efficiency data
for particles in bins 4 and 5 between
0.75% consistency and 1.25% consistency indicates that efficiency is substantially higher at the higher consistency.
Efficiency/area: Efficiency as deter112
F
0
I
----.
c l
I
I
0
0.20mm Slot
- - .- - - - -
F loo
0.15 mm Slot
A 0.25 mm Slot
0
€l
0
1
2
0
3
VELOCITY (m/s)
mined from the total black area in the
samples was greatly affected by the lack
of efficient removal of the large quantity
of particles in bins 1 through 3 at 0.75%
consistency and bin 1 at 1.25%. In fact
95% of the particles are in bins 1
through 3. The results from the 0.75%
consistency runs are therefore not
reported. The data from the 1.25% consistency run shown in Fig. 8 indicate
these efficiencies are also heavily
weighted by the large quantity of stickies
from bin 1 that were not removed efficiently. The total black area data suffer
from excess variance and do not appear
to offer a suitable method of showing
trends of reduced efficiency as velocity
increases.
OONCLUSlON
Image analysis seems to provide a
more informative method to evaluate
the effects of screening system designs
on efficiency. The technique removes
the potential of efficiency data being
weighted by too many large or small particles in the samples as can happen
0
4
5
when efficiency is evaluated by gravimetric means that measure only total efficiency.
The data indicate that the most productive path to stickies removal efficiency is using a combination of the narrowest screen slot width and highest
feed stock consistency practical for that
slot width. Although increased passing
velocities through the screen slot tend
to reduce stickies removal efficiency,
higher efficiencies are obtainable by
using narrower slot widths at equal
capacities even though the velocity may
be much higher in the case of the narrower slot width.
OEFERENCES
1. LEVIS, S. Removal of Contaminants From Recycled Fibre, High/Low Density Screening. Tech '91
Recycling & Deinking of Newsprint and Fine Papers
Course, Technical Section, CPPA, Toronto, ON.
February 1991.
2. HEISE, 0. Screening of Foreign Material and
Stickies - A Critical Review. TAPPI Papermaker's
Conference, Seattle, WA, April 1991.
3. MCCARTHY,C.E. Various Factors Affect Pressure
Screen Operation and Capacity, Pulp &f Paper,
September 1988.
Abstract: Stickies removal efficiency is a primary concern in designing slotted screening systems for recycled paper fibres. A research program compared screening efficiencies based on
slot widths and velocities through the slots. Specific attention was paid to the effects of consistencies, velocities, and slot widths on removal of particular particle sizes rather than overall efficiency. These tests were conducted under pilot plant conditions where particle sizes and stock
velocities could be more precisely controlled.
Reference: VITORI, C.M. Stock velocity and stickies removal efficiency in slotted pressure
screens. Pulp Paper Con 94(12): T441-444 (December 1993). Paper presented at the 1st
Research Forum on Recycling of the Technical Section, CPPA at Toronto, ON, on October 29
to 31, 1991. Not to be reproduced without permission. Manuscript received June 9, 1991.
Revised manuscript approved for publication by the Review Panel March 17, 1993.
Keywords: STICKIES, STOCK FLOW, SCREENS, RECLAIMED FIBRES, SCREENING.
PULP & PAPER CANADA 94:12 (1993)
4 T444