Superior Tools for Adjustable Screening Process
Kati Lindroos, Minna Puro
Metso Paper Valkeakoski Oy, Finland
ABSTRACT
Between 2004 and 2006, extensive rotor foil and screen basket studies with flow simulations, laboratory studies and
pilot and mill-scale trials were carried out to find new ways of controlling the pressure screen throughput conditions
in order to better meet the requirements of different screening applications.
As a base for the foil study, the pressure pulses created by different low consistency foil shapes were compared to
determine the influence of the pulse shape created by the foil on the screening performance. The basket study
focused on the influence of the flow resistance of a screen basket panel during production flow and suction pulse
modes on the capacity and quality of the screening.
The work commenced with flow simulations using a commercial CFD code to gain a deeper understanding of the
basic flow patterns and particle separation phenomena inside the screen. The simulation results were then verified
with the trials carried out with an industrial scale screen with a 0.9 m² screening area.
Based on the results of the simulations and trial measurements, Metso has created a new foil, called ProFoil; and a
new screen basket wire type, called Nimax.
Optimal screening results by combining the new screen basket wire and new foil design can be summed up as
follows:
¾
¾
¾
¾
¾
10 – 30% energy saving
10 – 20% less thickening
20 – 30% more capacity
excellent pulp quality
excellent runnability
Mill scale results prove that a foil shape with a gentle pressure gradient combined with a sufficiently strong
pulsation and screen basket wire shape with optimized flow conditions deliver high separation efficiency, improved
capacity, reduced energy consumption and less fiber loss.
INTRODUCTION
The operational demands for the separation processes in deinked pulp line have intensified significantly in recent
years. The raw material contains more than double the amount of macro-stickies experienced ten years ago.
However, accept pulp quality has to be at the same level as previously. In addition, a detailed consideration of the
economical aspects of the screening and the actions to reduce the costs are increasingly important.
Deinked pulp screening is nowadays typically performed with #0.15 mm slots. As a result of the constantly
decreasing quality of incoming pulp, #0.12 mm slots are inevitably required for the screening in the future.
However, a conventional combination of robust foil design and #0.12 mm narrow slot screen basket has resulted in
capacity problems and a strong reject thickening, which restricts the runnability of the screen and leads to substantial
fiber losses. The energy consumption of the screen has also been at an unnecessary high level due to relatively high
foil speeds and a very strong co-rotation of the pulp caused by the conventional heavy foil designs.
The combination of optimized foil geometry together with a recycled fiber specified screen basket allows a low level
of thickening and minor fiber losses. Through use of the narrowest possible slot, excellent pulp quality will be
achieved. Higher slot velocities can be used without blockage problems, which means that a higher capacity can be
reached without adding another screen, or that an even smaller screen can be used. By lowering foil speeds and
reducing unnecessary co-rotation, energy consumption will be at a reasonable level.
The above mentioned issues were the basis for this extensive foil and screen basket study, for which minimized fiber
loss and energy consumption combined with increased capacity and screening efficiency were the major targets.
NUMERICAL STUDY
A great amount of foil shape [1,2,3,4,5] and screen basket geometry [6,7] studies has been done during the past
decades. However, despite the extensive investigations, the ideal shape of the pressure pulse and, accordingly, the
foil and the wire profile has remained undefined in precise engineering terms. Also, thorough insight into the flow
patterns over the screening surface and wires is needed to fully understand the principles of particle separation in
each screening position. Deep understanding of the effect of both critical elements in screening; foil and wire shape
is needed in order to reach the best screening performance for each application.
Work was started with flow simulation using modern computational tool; CFD simulation which was utilized to
provide more information for profound analysis than only the pressure pulses created by foil. In addition to the
pressure pulses, the radial and tangential velocities around the foil are also investigated and compared in this
numerical study. Since measuring the exact velocity field at the screen basket wire area through an opaque
suspension is difficult, CFD simulation was used as a way to ‘see’ inside the machine.
Simulation methods
The flow field inside the pressure screen was modeled using a commercial CFD code Fluent (version 6.1). At the
start, three different foil shapes were chosen for this comparison, see Table 1. In the screen basket wire study, three
wire geometries were compared (Table 2).
Table 1. The basic data of the simulated foils.
Foil 1
Foil 2
foil width [mm]
80
87
foil thickness [mm]
24
25
foil tail height [mm]
15
23
foil gap [mm]
2-4
2-4
foil speed [m/s]
12-16
12-16
foil frequency [Hz]
31
31
Foil 3
125
35
25
2-4
12-16
31
h
t
w
w
Table 2. The basic data of the simulated wires.
Wire 1
Wire 2
profile width [mm]
2.5
2.3
profile height [mm]
1.2
0.8
panel thickness [mm]
5.0
6.0
slot width [mm]
0.12
0.12
Wire 3
2.5
0.8
6.0
0.12
t
h
The foil simulations were carried out in a 2D rotationally periodical co-ordinate frame with a screen basket diameter
of 1300 mm. A control volume method with approximately 60,000 grid points was used to discretisize the flow
domain. The flow geometry consisted of three separate zones: a rotating reference frame around the foil, a stationary
basket frame in the middle of the flow domain and a stationary accept frame in the outer part of the flow domain, as
shown on the left-hand side of Figure 1.
The geometry used in the micro-scale screen wire simulations is shown on the right hand side of the Figure 1.
Stationary 2D flow domain consisted of three wires next to each other with periodic boundaries on both sides of the
model. Foil-induced velocity field was set as a boundary condition on the feed side of the screen and fixed tangential
velocity was set on the accept side of the screen. Approximately 25,000 grid points was used to discretisize the flow
domain.
accept flow
stationary accept frame
porous media
basket frame
fixed tang.
velocity
periodic
boundaries
rotating frame
periodic boundaries
feed flow
Figure 1. The flow geometry and boundary conditions used in the foil and wire simulations.
Boundary conditions
The flow field information gained from foil simulations was utilized when setting the boundary conditions in the
wire simulations, and vice versa. In the foil simulations, the basket frame was modeled as a porous media with a
certain pressure drop over it, meaning that the detailed slot geometry was not included in the model. The pressure
drop over the basket in the radial direction is characteristic of the actual slot geometry and was set to 10-20 kPa
based on the detailed wire simulations. In the tangential direction, the pressure drop was set 1,000 times higher than
in the radial direction since there cannot be any tangential flow in the basket area.
There was no inlet or outlet in the foil model. In other words, the rotating foil was the only driving force causing the
flow inside the screen. In a real screening process there is, of course, a pressurized feed inducing the net throughput
over the screen basket, and the function of the foil is mainly to keep the basket surface clean. However, conclusions
can be drawn from the relative differences between the modeled pulses and flow throughputs even though the
pressure level itself is somewhat different from the actual screening conditions.
In the wire simulation model, radial and tangential velocity values between 1-10 m/s were used as boundary
conditions on the feed side of the screen basket to describe the flow field induced by a passing foil. The tangential
velocity on the accept side was fixed at 0.5 m/s, describing the continuous collection of the flow around the screen
housing.
In the foil simulations, a low-Re k-epsilon turbulence model with standard wall functions was used in turbulence
modeling. The basket area was simulated as a laminar zone since the Reynolds number in the area of narrow slots is
very low. The basket was treated as a 100% open flow zone in the model, so there were no walls and, accordingly,
no wall-induced turbulence in that area either.
In the wire simulations, a low-Re k-epsilon turbulence model with enhanced wall treatment was used in turbulence
modeling. The mesh was strongly refined close to the walls and particularly at the slot area with a maximum y+ < 4.
Since we were concentrating on low-consistency screening, the material properties of pure water were used
throughout most of the simulation set. The effect of increasing consistency on pressure pulse was tested in one case
by increasing the viscosity of the fluid up to 100 times higher than water viscosity.
RESULTS
Simulation results of foil study
The effect of the foil shape, foil speed, foil gap and fluid viscosity on the form of the pressure pulse, radial velocity
pulse and tangential velocity pulse were studied during the foil simulations. Detailed simulation results for chosen
conventional foils 1–3 (Table 1) are presented in a previously published paper [8].
After comparing the flow fields and different pulses of numerous conventional foil shapes, a new foil shape for lowconsistency screening was designed [8]. An optimal shape of the foil is an important criterion for an adjustable
screening process. Having an appropriate tangential co-rotation level in a particular application is a key issue from
both a capacity and quality viewpoint.
In deinked pulp screening applications, the quality, i.e. the removal of soft and elastic stickies, is proposed to be
mostly controlled by slot dimensions and an appropriate slot speed rather than by a very high level of tangential corotation over the basket surface [8]. The process consistency is low, typically from 0.5% to 1.5%, so high
fluidization energy to break the fiber flocks is not needed. Accordingly, a considerably lower level of co-rotation can
be utilized in DIP screening to reduce the energy consumption and to increase the capacity of the screening process
with the same quality standard.
The main idea of new foil design was to obtain a remarkably stronger suction pulse with the same positive pressure
peak level than in the case of a conventional low-consistency foil. Another target for foil development was to reduce
the co-rotation level and thereby achieve lower energy consumption at all rotation speeds. These demands are well
fulfilled with the thin and streamlined design of this new foil.
The radial and tangential velocity fields of new foil and a conventional foil* are illustrated in Figures 2−3. The color
scale slides from black to white such that black corresponds to the velocities lower than -1 m/s and white
corresponds to the velocities higher than +1 m/s. Based on Figures 2−3, it can be concluded that the radial suction
pulse of new foil is much stronger and the tangential co-rotation at a lower level than in case of a conventional lowconsistency foil.
Figure 2. Radial velocity fields of new foil (left) and conventional foil (right).
* conventional foil represents traditional, heavy foil design of low consistency foils 1–3 listed in Table 1
Figure 3. Tangential velocity fields of the new foil (left) and conventional foil (right).
Conventional foil
150
100
100
50
50
0
0
-50
-50
P [kPa]
P [kPa]
New foil
150
-100
-100
-150
-150
-200
-200
12 m/s
-250
16 m/s
-300
18 m/s
-350
Time [ms]
12 m/s
-250
16 m/s
-300
18 m/s
-350
Time [ms]
Figure 4. Simulated pressure pulses of the new foil and conventional foil at different foil speeds.
It has been noticed in many measurements that the conventional foils react rather weakly with changes in foil speed,
which makes it difficult to adjust the capacity and the quality specifically to the desired level. In contrast, the new
foil provides a good response to increasing foil velocity, as shown in Figure 4. By using this new kind of foil in
deinked pulp screening, an optimum compromise between highest possible quality and capacity with lowest possible
energy consumption can be achieved for a specific process.
Simulation results of screen basket wire study
In addition to the shape of the rotating foil, the design of the screen basket wire has a significant effect on both
screening capacity and quality. Increasing the height of the wire profile tends to increase the back-flow area and
turbulence on the basket surface (see Figure 5), which is thought to be a beneficial phenomenon from a fluidization
and capacity point of view. On the other hand, a too large back flow vortex with unnecessarily high turbulent kinetic
energy, especially in low consistency screening applications, has a negative effect on quality.
The shape of the accept channel right after the narrow slot is highly critical to the efficiency of the suction pulse and
therefore to the cleaning effect of the foil. The accept channel should be as smooth and streamlined as possible with
a moderate opening angle to avoid the flow separation and unwanted vortex formation after the slot, see Figure 5.
The opening angle should be at just the correct level to achieve the maximum capacity without any quality
problems. Too high an opening angle creates a flow separation vortex, which is very likely an origin for the
stringing phenomena in the accept side of the screen. On the other hand, too low an opening angle leads to a higher
pressure drop over the basket and thus limits capacity.
foil rotation direction
Figure 5. Streamlines around Wire 1 (left) and Wire 2 (right) in forward flow phase.
Sim ulated pressure drop
Measured pressure drop
w ire 1
w ire 2
w ire 2
w ire 3
w ire 3
dp [kPa]
dp [kPa]
w ire 1
Slot speed [m/s]
Slot speed [m/s]
Figure 6. Simulated (left) and measured (right) pressure difference over wires 1-3.
The correlation between wire measurements and simulation results is rather good, as can be seen in Figure 6, where
simulated and measured pressure drops over the screen basket are compared qualitatively. Wire 1 corresponds to the
first generation wire design with higher fractionation effect, while wire 2 represents the next generation wire design
with a lower thickening tendency and higher capacity. Wire 3 with the lowest pressure drop curve in Figure 6 is a
totally new innovation with extremely high capacity and excellent runnability properties. This wire type is still in the
development stage and will be launched to market later after a detailed testing procedure.
Pilot results of foil study
The operation of the new foil was verified in screening trials with 30% OMG and 70% ONP using an industrialscale Metso screen with a 0.9 m² screening area. A conventional foil design was used as a reference. In these trials,
conventional foil represents generally low consistency foils 1−3 (see Table 1) with heavy design and a strong
fractionation tendency.
Foil speed was kept constant (16 m/s) to facilitate the comparison of the foils, although the recommended foil speed
for new foil is much lower due to its extremely efficient suction pulse. #0.12 mm and #0.15 mm screen baskets with
medium profile heights and a pulp consistency range from 1.1% to 1.3% were used for both foil types. Three
different mass reject rates, with constant slot speed of 1.2 m/s, were used.
The total screening performance with the new foil and with conventional foil was evaluated comparing capacity,
energy consumption, reject thickening and values for sticky removal efficiency.
Capacity tests
The trials were started with capacity tests at a 15 % volumetric reject rate. In this capacity test, the flow through the
screen was increased until the plugging point was reached. The pressure difference and motor load values of the
screen were recorded. The results of the capacity run are presented in Figure 7.
As shown in Figure 7, the new foil clearly helps the flow on the screen basket surface. With the reference foil and
#0.12 mm screen basket, the plugging occurred at a 0.8 m/s slot speed, which is an unacceptably low value. In DIP
screening, the maximum slot speed in the primary stage should be close to 1.2 m/s to yield the desired capacity flow
through the screen. With #0.15 mm screen basket, the difference was not so remarkable, although the new foil was
much easier to operate.
#0.15 mm basket
#0.12 mm basket
45
45
35
30
25
20
#0.15 +ref foil
15
0,0
0,2
0,4
0,6
0,8
10
1,0 1,2
40
Pressure difference [kPa]
Pressure difference [kPa]
40
#0.15+new foil
35
30
25
1
20
#0.12+ref foil
15
#0.12+new foil
10
1,4
1,6
1,8
2,0
2,2
0,0
0,2
0,4
0,6
0,8
Slot speed [m/s]
1,0
1,2
1,4
1,6
1,8
2,0
2,2
Slot speed [m/s]
Figure 7. Capacity results.
Energy consumption
The energy consumption of the screen depends mainly on the foil speed and such process parameters as reject rate,
consistency and reject thickening. However, the shape of the foil itself also has a considerable influence on the total
energy consumption, as shown in Figure 8.
#0.12 mm basket
30
30
28
28
26
24
22
20
18
#0.15+ref foil
Motor load [kW]
Motor load [kW]
#0.15 mm basket
26
24
22
20
16
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2
Slot speed [m/s]
Figure 8. Energy consumption results.
#0.12+ref foil
18
#0.15+new foil
0,0
0,2
0,4
0,6
0,8
16
1,0 1,2
#0.12+new foil
1,4
Slot speed [m/s]
1,6
1,8
2,0
2,2
Because of thin and streamlined shape of new foil, the energy consumption was approximately 10% less than in the
case of the reference foil with a #0.12 mm screen basket at the same foil speed (16 m/s) in these trials. Depending on
the specific process conditions, the saving in energy consumption attained by changing the conventional foil to new
foil can be even 30%, as reported in previous customer projects.
Quality
Consistency and thickening
In DIP screening, a strong reject thickening is certainly not a desired property due to high fiber losses and energy
consumption. As discussed earlier, the reject thickening with conventional foil designs appears mostly due to the
high tangential co-rotation level between the foils, which is not needed to achieve quality accepts in DIP screening.
High reject thickening also leads to higher energy consumption. When the new foil design was considered, a
controlled thickening phenomenon inside the screen was the first target.
Figure 9 shows that the thickening effect was so strong with the reference foil and narrow slots (#0.12 mm) that the
screen was not runnable (not in operation window) at low reject rates. An ultra-narrow slot typically heavily
dominates the screening. This means that foil design must provide enough strong suction pulse to keep the screen
basket surface clean.
#0.12 mm basket
2,0
1,8
1,8
1,6
1,6
Thickening factor
Thickening factor
#0.15 mm basket
2,0
1,4
1,2
1,0
#0.15 + ref foil
0,8
1,4
1,2
1,0
#0.12+ref foil
0,8
#0.15+new foil
#0.12+new foil
0,6
0,6
0
5
10
15
20
25
30
Mass reject rate [%]
35
40
45
50
0
5
10
15
20
25
30
35
40
45
50
Mass reject rate [%]
Figure 9. Thickening results.
With wider slots, the thickening factor was around 10% to 20% higher with the reference foil than with new foil. It
can be concluded that as a result of the lower tangential co-rotation level, the new foil fulfilled the target of
controlled thickening regardless of the slot size and reject rate.
Sticky removal efficiency
When designing a new foil, the greatest advantages of the foil shape were considered to be the improved runnability
and reduced energy consumption as presented in earlier mill-scale results. With regard to the quality and the
cleanliness of the pulp, the target was set at the same level as with a conventional foil with equal process conditions.
Figure 10 presents the sticky reduction (INGEDE 4 method, Somerville #0.10 mm screen plate) presented as
computed average Q- index values with #0.12 mm and #0.15 mm baskets. It can be concluded that the quality with
the new foil and #0.15 mm screen basket is at the same level as measured with conventional foils. When the slot size
is decreased to #0.12 mm, the quality is excellent with the new foil. The operation of the screen with a #0.12 mm
slot and conventional foil design is difficult or impossible due to too week suction pulse, thickening problems and
unstable flow through the screen basket, which may lead to even lower quality than with the new foil design.
#0.12 mm basket
100
90
90
Sticky reduction STRE [%]
Sticky reduction STRE [%]
#0.15 mm basket
100
80
70
60
50
40
30
#0.15+ref foil
20
#0.15+new foil
10
Q-index 0.993
0
80
70
60
50
40
30
#0.12+ref foil
20
#0.12+ new foil
10
Q-index 0.997
0
0
5
10
15
20
25
30
35
Mass reject rate [%]
40
45
0
5
10
15
20
25
30
35
40
45
Mass reject rate [%]
Figure 10. Screening efficiency results.
It can be concluded that the new foil gives the equal pulp quality compared to reference foil. A great advantage is
that new foil can be run with low reject rates with #0.15 mm slot and especially with ultra-narrow #0.12 mm slot,
when with reference foils reject rates are very high due to strong fractionation tendency.
It should be kept in mind that in these trials the foil velocity was higher than the actual design velocity for new foil,
which adversely affects the quality.
Pilot results of screen basket wire study
The second step was to identify the best design of the screen basket wire in combination with the new foil. This
operation was verified in screening trials with 30% OMG and 70% ONP using a screen with a 0.9 m² screening area.
Results of three different wire shapes with a narrow #0.12 mm slot and medium profile are presented in this paper.
Several other wires were also studied but those results are confidential and not reported publicly. The foil speed was
15 m/s based on earlier foil studies. Three different mass reject rates with a constant slot speed of 1.2 m/s and feed
consistency 1.2% were used.
The screening performance with the new foil and screen basket was evaluated through capacity, energy
consumption, reject thickening and values for sticky removal efficiency.
Capacity tests
The trials were started with capacity tests at 15 % volumetric reject rate. Flow through the screen was increased until
the plugging point was reached. The pressure difference and motor load values of the screen were recorded.
As shown in Figure 11, the new wire geometry has superior runnability compared to the reference wires A and B,
which are commercial versions of simulated wire types 1–2 (see Table 2). Although the #0.12 mm ultra-narrow slot
is, in general, complicated to operate, the new wire design gives 20% more capacity with smooth operation when
compared to conventional designs.
#0.12 mm baskets + new foil
Pressure difference [kPa]
35
30
25
20
wire A
15
wire B
new wire
10
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2
Slot speed [m/s]
Figure 11. Capacity results.
Energy consumption
Previously, energy consumption has not been a major issue in screen basket design itself. However, nowadays
energy saving is a critical element in worldwide sustainable development, so even minor savings are beneficial to
consider when discussing about the total energy consumption of the screen room. Based on the motor load
measurements shown in Figure 12, it can be concluded that the new wire design saves approximately 10% of energy
when compared to the reference wire designs. This result comes through the optimized flow conditions around the
wire.
#0.12 mm baskets + new foil
30
wire A
wire B
Motor load [kW]
25
new wire
20
15
10
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
Slot speed [m/s]
Figure 12. Energy consumption.
1,6
1,8
2,0
2,2
Quality
Consistency and thickening
As discussed earlier, strong thickening leads to high fiber losses and energy consumption and for this reason, it is
not a desired property in DIP screening.
The new foil gave a very low thickening tendency due to a minor co-rotation effect and strong suction pulse. This
new wire gives an equal result through optimal flow conditions on the screen basket surface.
#0.12 mm baskets + new foil
2,5
Thickening factor
2,0
1,5
1,0
wire A
wire B
new wire
0,5
0
5
10
15
20
25
30
35
40
45
50
Mass reject rate [%]
Figure 13. Thickening results.
As shown in Figure 13, the thickening factor with the new wire is on a good level with reject rates of more than
15%, while conventional wires have a strong thickening tendency even at high reject rates.
Sticky removal efficiency
The quality target of the new wire design was set at the same level as for the reference wires with equal process
conditions, as it was set in the case of new foil development.
Figure 14 presents the sticky reduction efficiency (INGEDE 4 method, Somerville #0.10 mm screen plate) at
different mass reject rates. It can be concluded that the quality with the new wire was at the same level as for
conventional wires in these trials. In later mill trials, it has been reported that quality will be improved even further
with properly selected foil and wire.
#0.12 mm baskets + new foil
100
Sticky reduction STRE [%]
90
80
70
60
50
40
30
wire A
wire B
new wire
Q-index 0.964
20
10
0
0
5
10
15
20
25
30
35
40
45
50
Mass reject rate [%]
Figure 14. Screening efficiency results.
CONCLUSIONS
Minimizing fiber losses and energy consumption concurrently with increased capacity and screening efficiency is
the major issue in deinked pulp screening today. There are two critical tools in screening to achieve this goal: the
screen basket design and the foil design.
In this extensive long-term study, flow simulation was found to be a good tool together with further laboratory and
mill-scale studies and it corresponds well with real-life measurements.
By combining the new screen basket wire with the new foil design, optimal screening results can be summed up as
follows:
¾
¾
¾
¾
¾
10 – 30% energy saving
10 – 20% less thickening
20 – 30% more capacity
excellent pulp quality
excellent runnability
Results prove that a foil shape with gentle pressure gradient combined with sufficiently strong pulsation and screen
basket wire shape with optimized flow conditions deliver high separation efficiency, improved capacity, reduced
energy consumption and less fiber loss.
REFERENCES
1.
Julien Saint Amand, F. and Perrin, B., TAPPI 1999 Pulping Conference Proceedings, TAPPI PRESS, Atlanta,
941 (1999).
2.
Karvinen, R. and Halonen, L., Paperi ja Puu (2):80 (1984).
3.
Pinon, V., Gooding, R.W. and Olson, J.A., TAPPI J. 2(10):9 (2003).
4.
Marko, J.J. and LaRiviere, C.J., TAPPI 1999 Papermakers Conference Proceedings, TAPPI PRESS, Atlanta,
1477 (1999).
5.
Yu, C., Crossley, B. and Silveri, L., TAPPI J. 77(9):125 (1994).
6.
Gooding, R.W., “Flow resistance of screen plate apertures“, PhD thesis, University of British Colombia (1996).
7.
Julien Saint Amand, F., 9th CTP Recycled Fibres Forum, CTP/Grenoble, February 7 & 8 (2007).
8.
Lindroos, K. and Puro, M, "ProFoil – Superior tool for adjustable screening process", 12th PTS-CTP Deinking
Symposium, Leipzig, February 25-27 (2006).