Introduction

It is useful to start with some definitions and to develop a basic understanding of

the mechanical set-up of a pressure screen before looking at the mostly mechanistic

and semi-empirical models that are used to describe the process phenomena

in a pressure screen.

Figure 6.1 shows the process streams around a screen. The pulp suspension

containing the impurities is fed with the feed stream QF at the pulp concentration

cF. The clean pulp passes through the apertures in the screen plate and is discharged

with the accept stream QA at the concentration cA. The reject stream QR is

rich in impurities. More in general, one fraction of a certain property is concentrated

in the reject stream QR, while the other fraction of different property leaves

the screen with the accept stream QA.

QF, cF QA, cA

QR, cR

Fig. 6.1 Streams around a screen.

In addition to the streams shown in Fig. 6.1, most industrial pressure screens

have an internal dilution by filtrate near the reject outlet. While dilution is often

needed from an operational perspective to avoid plugging of the screen’s reject

side, we can skip internal dilution during the present theoretical considerations

without losing insight.

The solid-solid separation with a pressure screen can be categorized by the type

of physical separation mechanism. In barrier separation, particles are rejected

from passage through an aperture because they are physically larger than the

aperture size in any orientation. In probability separation the particle can pass

through the aperture in at least one orientation and it depends on how the particle

approaches the aperture whether it passes, or not [1,2].

The distinction between barrier screening and probability screening is of utmost

importance for all screening applications. As for screening efficiency, probability

separation is apparently much more challenging than barrier separation.

The basic mechanical elements in control of separation in a simple pressure

screen are the screen and the rotor. In the most common design, the screen has

the shape of a cylindrical basket and is fixed in the screen housing. The apertures

in the screen basket have the form of either holes or slots and the screen surface

may be smooth or contoured.

6.2 Screening Theory 563

Pulp is fed axially to the screening zone. The accepted material passes through

the apertures in the screen basket, while the rejected material proceeds along the

inside of the screen basket towards the reject outlet. The rotor revolves inside the

screen basket. Over the years, a large number of different rotor designs has been

developed. Most of these are based on the closed bump rotor and open foil rotor

design principle (Figs. 6.2 and 6.3).

Reject

Accept

Reject

Accept

Reject

Accept

Rotor

Screen basket

Feed Feed Feed

Fig. 6.2 Rotor categories: closed rotor (left), open rotor (center), semi-open rotor (right).

Fig. 6.3 Examples of cross-sections of simple rotors:

left, bump rotor;center , step rotor; right, foil rotor.

Besides providing the tangential velocity near the screen and generating turbulence,

the most important task of the rotor is to keep the screen apertures clear.

This is accomplished by the regular backflush through the apertures caused by

the pumping action of the rotor’s pulsation elements.

6.2.2

Flow Regime

As the pulp suspension passes through the screening zone, the flow pattern near

the screen can be broken down into an axial flow vector from the feed side to the

reject side, a tangential flow vector induced by the rotor, and a radial flow vector

through the apertures to the accept side of the screen (Fig. 6.4).

The rotor plays a most essential part in influencing the flow regime. Its motion

fluidizes the pulp suspension, provides the tangential fluid velocity along the

screen plate, and backflushes the screen apertures. Fluidization suppresses the

particle–particle interactions and provides for quickly changing particle orienta-

564 6 Pulp Screening, Cleaning, and Fractionation

6.2 Screening Theory

v vtangential axial

vradial

Feed

Reject

Accept

Fig. 6.4 Flow vectors near the screen basket.

tion relative to the screen aperture, thus increasing the probability for acceptable

particles to pass. As the rotor element passes cyclically over the aperture, it generates

a backflush through the aperture every time it passes by. The backflush removes

particles trapped in the narrow screen apertures and thus keeps them clear.

While the screen performance is influenced by all the three flow vectors, the

radial accept flow through the apertures is most critical for continuous operation.

When the accept flow becomes limited or even completely disrupted by fibers

blocking the screen apertures, the situation is referred to as “blinding” or “plugging”

of the screen. Blinding leads to the formation of a fiber mat on the screen

surface, and it can affect only part of the screen or the total screen. In the latter

case, it may take several minutes for the screen to be blinded, but typically it takes

only a few seconds. The triggering factor of blinding – that is, the build-up of

fibers at the edge of a screen aperture – occurs very rapidly, within several thousands

of a second [3]. Consequently, very frequent backflush is required to avoid

plugging. The typical pulse frequency provoked by the rotor in a pressure screen

is above 30 Hz [4].

The aperture velocity, or passing velocity, v, is often regarded as a fundamental

design parameter for a pressure screen. v (m s–1) is calculated from the accept

flow rate QA (m3 s–1) and the open area of the screen basket, AO (m2):

v _

QA

AO _1_

It is important to realize that the true flow velocity through the screen apertures

is considerably higher than the passing velocity calculated from Eq. (1). On the

one hand, pressure pulsation as induced by the rotor action leads to a backflow

from the accept side to the feed side, thus both increasing the volume to be transferred

from the feed side to the accept side and reducing the time for this transfer.

On the other hand, fiber accumulation at the screen apertures reduces the open

area [5].

565

6 Pulp Screening, Cleaning, and Fractionation

6.2.3

Fiber Passage and Reject Thickening

The essential part of the screening operation occurs in the annular gap between

the rotor and the screen basket.

Both a mixed-flow model and a plug-flow model have been used to describe the

flow pattern in a pressure screen [1,6]. Here, we will focus on the plug-flow model

because it is more flexible and seems to describe the actual flow regime in a pressure

screen more accurately.

The plug-flow model assumes ideal radial mixing between the rotor and the

screen basket without backmixing in axial direction. Let us define a parameter

called passage ratio, P:

P _

cs

cz _2_

where cs is the solids concentration in the stream through a screen aperture (kg m–3)

and cz is the solids concentration of the stream just upstream of the aperture at a

position z [1].

The passage ratio is a characteristic parameter of the screening system, which is

influenced by many variables including screen plate and rotor design, screen operating

conditions and pulp grade. P is best determined individually for a given

screening application based on field measurements. A passage ratio of zero

means that all the solids are retained on the screen and will be rejected. At P = 1,

the concentrations in the accept and reject are equal to the feed concentration and

there is no separation.

z = 0

z = L

z

dQz, P·cz

Rotor

Screen basket

Qz, cz

Qz- dQz, cz - dcz

Annular volume element

Fig. 6.5 Flows and concentrations around an annular differential volume element.

Considering Fig. 6.5, the mass balance for pulp over the annular differential element

between the screen plate and the rotor gives:

Qz cz _ dQz P cz _ _Qz _ dQz_ _cz _ dcz_ _ 0 _3_

566

6.2 Screening Theory

where Qz stands for the total flow rate (m3 h–1) entering the element in axial direction

and dQz for the flow rate leaving in radial direction, respectively.

Equation (3) can be rewritten to give:

dcz

cz _ _P _ 1_

dQz

Qz _4_

In a first approach, it is assumed that the fiber passage ratio P is independent

of the flow rate and consistency. Then, Eq. (4) can be integrated using the overall

screen boundary conditions as per Fig. 6.1; that is, cz = cF and Qz = QF for z = 0, and

cz = cR and Qz = QR for z = L. L is the length of the screening zone.

cR

cF _

QR

QF _ __P_1_

_5_

Note that the concentrations may relate to the totality of pulp as well as to a

fraction only, for example, to shives. Then, different passage ratios will apply for

total pulp and shives. When the concentrations in Eq. (5) refer to total pulp concentrations,

the quotient cR/cF is defined as the reject thickening factor, T. QR/QF is

termed the volumetric reject ratio, Rv. With these definitions we obtain a relationship

between the thickening factor, total fiber passage ratio and volumetric reject

ratio:

T _ R_P_1_ v _6_

The mixed-flow model of pressure screening assumes a completely mixed volume

inside the screen. In the mixed-flow model, the feed entering at the pulp concentration

cF is immediately mixed into the volume inside the screen which is at

the reject concentration cR. All accept passes through the screen apertures at the

accept concentration cA. The overall mass balance, pulp mass balance and passage

ratio of this system are:

QF _ QA _ QR _ 0 _7_

QF cF _ QA cA _ QR cR _ 0 _8_

P _

cA

cR _9_

Equations (7) to (9) can be combined and rewritten using the definitions of the

thickening factor and volumetric reject ratio to give the expression for the thickening

factor in the mixed-flow model:

T _

1

P_1 _ Rv__Rv _10_

567

6 Pulp Screening, Cleaning, and Fractionation

While the mixed-flow model seems worth considering for screens with open

rotors, it has proven to be second to the plug-flow model at lower reject rates for

virtually any screen configuration. Figure 6.6 shows, graphically, the comparison

of the reject thickening behavior predicted by the mixed-flow and plug-flowmodels.

0.5

1.0

1.5

2.0

2.5

0% 20% 40% 60% 80% 100%

Reject thickening factor, T

Volumetric reject ratio, Rv

Plug-flow model

Mixed-flow model

Fig. 6.6 Reject thickening in a pressure screen predicted by the mixed-flow

and plug-flow models at a constant fiber passage ratio of P = 0.7.

By examining the fit between experimental data and the curves calculated from

the plug-flow model in Fig. 6.7, it can be seen that the plug-flow model describes

the thickening behavior of an industrial screen quite well. Note that reject thickening

increases dramatically at reject ratios smaller than 10%, and that a lower passage

ratio generally leads to higher thickening.

Earlier, it was assumed that the fiber passage ratio is constant along the screening

zone, which implies that the single fibers do not interact with each other. This

holds true only for very low consistencies and, to a certain degree, for fiber suspensions

under high shear forces in a turbulent environment. Figure 6.8 shows

that the fiber passage ratio in a commercial pressure screen is fairly constant over

the first two-thirds of the screen length, but then can fall significantly at the reject

end of the screen [4].

568

6.2 Screening Theory

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0% 10% 20% 30% 40% 50%

Reject thickening factor, T

Volumetric reject ratio, Rv

1.8 mm holes

0.4 mm slots

Fig. 6.7 Example of reject thickening as a function of the volumetric reject ratio.

Comparison of experimental data [6] with calculation results from the plug-flow

model; P = 0.72 for 0.4-mm slots, P = 0.55 for 1.8-mm holes.

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Fiber passage ratio, P

Normalised length of screening zone, z/L

Bump rotor

Step rotor

Fig. 6.8 Example of fiber passage ratio as a function of the screen length

and rotor geometry;smooth hole screen, eucalyptus pulp, Rv = 10% [4].

Figure 6.9 illustrates a typical consistency profile over the length of a pressure

screen. While the consistency of the accept remains fairly constant, the consistency

of the pulp flow passing along the screen increases disproportionately

towards the end of the screening zone. The profile explains why screens tend to

blind from the reject end.

569

6 Pulp Screening, Cleaning, and Fractionation

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.2 0.4 0.6 0.8 1.0

ed consistency, cz/cF

ed length of screening zone, z/L

Feed

Rejects

Accepts

Normaliz

Normaliz

Fig. 6.9 Example of consistencies as a function of the screen length;

smooth hole screen, bump rotor, eucalyptus pulp, Rv = 10% [4].

6.2.4

Selective Fiber Passage

The selective separation of the different types of solids contained in the feed

stream is of major importance for all contaminant removal and fractionation applications.

While the selectivity of barrier screening is essentially determined by

the chosen screen, the selective separation of particles is much more challenging

when screening is governed by the probability mechanism.

Several investigations have been made to evaluate the fiber length dependent

passage of fibers through pressure screen apertures (e.g., [7–10]). It has been

shown that the passage ratio can be approximated by the empirical equation

P _ e__l_k_b

_11_

where k is a size constant proportional to the size of the screen plate aperture and

l is the fiber length. k is to be determined experimentally for each screening application.

The second constant was found to be b = 0.8...1.1 for screen plates with

smooth holes, and b = 0.5 for contoured slotted screen plates. The different shapes

of the fiber passage ratio versus fiber length curves in Fig. 6.10 demonstrate the

divergent performance of holed and slotted screens reflected by b.

570

6.2 Screening Theory

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Fiber passage ratio,P

Fiber length [mm]

Slots

Holes

Fig. 6.10 Example of fiber passage ratio as a function of the fiber

length and screen type;smooth hole plate versus contoured slot plate,

bump rotor, softwood thermomechanical pulp (TMP) [8].

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Fiber passage ratio,P

Fiber length [mm]

Slots

Holes

Ideal

Fig. 6.11 Typical fiber passage ratio as a function of the fiber

length;comparison of ideal profile with typical profiles of

holed screen (b = 1) and slotted screen (b = 0.5) normalized

for a fiber passage ratio of 0.5 at 2-mm fiber length [8].

It is apparent from Fig. 6.11 that currently proven screening equipment is performing

far from ideally when it comes to fractionation. However, screening with

holed plates leads to better length-based fractionation because the holed screen

profile is closer to the ideal profile and the fiber passage ratio drops more quickly

571

6 Pulp Screening, Cleaning, and Fractionation

over the fiber lengths of main interest. Remember that P = 1 implies the distribution

of very short fibers between accept and reject according to the respective flow

rates. In contrast, very long fibers are selectively concentrated in the reject stream

as P approaches zero.

For a given combination of screen plate, rotor type and pulp furnish, the lengthbased

fiber passage ratio was shown to be independent of the reject ratio. While

for slotted screen plates the fiber passage ratio increases with the aperture velocity,

it is independent of the aperture velocity for holed screen plates. This behavior

marks another advantage of holed screens for fractionation, because it makes the

fractionation result independent of the production capacity [8]. Besides fractionation

for length, pressure screens separate fibers according to their coarseness

(weight per unit length) [11].

6.3

Screening Parameters

In this subsection we will review the parameters that affect the operation and

determine the performance of a screening system, and their qualitative influences

on screen capacity and screening efficiency.

These parameters include operating conditions, such as flow rates, feed consistency

or temperature. They also include equipment-specific parameters, such as

screen and rotor design or rotor tip velocity. In addition, it is necessary to observe

the furnish characteristics of both the pulp fibers and the contaminants.

Some of the above parameters can be adjusted, while some are intrinsic to a

special process step or piece of equipment. The chosen combination of adjustable

screening parameters depends on the individual requirements of the application,

and this is usually a compromise within performance limits and operating constraints,

because the optimization of single parameters often leads into opposite

directions.

Due to the complexity of the mechanisms involved and varying system performance

depending on the specific circumstances, the discussion of parameters

below will be often qualitative in nature.

6.3.1

Equipment Parameters

6.3.1.1 Screen Basket

The screen basket is fundamentally characterized by aperture size, aperture shape

and aperture spacing, as well as the character of its surface. There is a basic distinction

between perforated, or holed, screen plates and slotted screen plates.

Both types can be furnished with contours on the side of the screen surface which

faces the feed. Such contoured, or profiled, screens increase the turbulence near

the screen aperture and allow the screen to be operated at a higher capacity.

572

6.3Screening Parameters

Only profiled screens in combination with the wedge-wire design have made

today’s narrow screen slots practical and widely accepted. Slotted screens with a

slot width around 0.15 mm have become state of the art for applications targeted

at the removal of smaller contaminants. Wedge-wire screens consist of solid bars

placed aside each other, forming long slots over the complete length of the screen

basket, while machined slots are milled out of a solid screen basket. Wedge-wire

slotted screens have considerable capacity advantages over machine slotted

screens due to their larger open area.

Holed screens have been traditionally preferred for their high capacity and reliable

operation and easy control under varying conditions. Their robustness makes

them first choice for the removal of larger contaminants. The advantages of holed

screens for fractionation have been discussed above. Typical hole sizes are

4–10 mm for larger contaminant removal, and about 1 mm for fractionation.

The aperture size is the most critical design variable of a screen. Holes of small

diameter and slots of narrow width have advantages with regard to the screening

efficiency. Their size actually determines whether a particle is rejected on the principle

of barrier separation, or whether it is subject to probability separation. On

the other hand, smaller apertures mean lower capacity at a given screen surface

area.

Similarly, the profile depth of the screen surface causes divergent screen performance.

By tendency, the additional turbulence created by a higher contour provides

a greater capacity but reduces the screening efficiency. If in turn the aperture

size is reduced to regain lost efficiency, the capacity of the contoured screen

still remains higher [12].

It has been shown that slot spacing is important, and that longer fibers require

wider slot spacing than shorter fibers. If the slots are too close, then stapling of

fibers occurs as the two ends of individual fibers enter adjacent slots at the same

time. Similar conclusions have been drawn for holed screens.

Note that the performance of a screen will deteriorate over time if the pulp furnish

contains an abrasive material such as sand. Especially with heavily contoured

screens, wear will significantly decrease both the capacity and the screening efficiency.

6.3.1.2 Rotor

There is a variety of different rotors available, with special shapes and sophisticated

local arrangements of bumps or foils. All of these are deemed to have their

individual advantages regarding screen capacity, screening efficiency or power

consumption.

The characteristic shape of the pressure pulse generated by a rotor depends on

the design of the pulsation element, for example on the shape, length and angle

of incidence of the foil, or on the shape and length of the bump. The intensity of

the pulse is determined again by the rotor shape, as well as by the rotor tip velocity,

the clearance between the pulsation element and the screen basket, as well as

the pulp consistency and pulp furnish parameters.

573

6 Pulp Screening, Cleaning, and Fractionation

-3

-2

-1

0

1

2

0 10 20 30 40 50 60

Dynamic pressure [bar]

Time [10-3 s]

Fig. 6.12 Example of pressure pulse profile for a short foil rotor [5].

-3

-2

-1

0

1

2

0 10 20 30 40 50 60

Dynamic pressure [bar]

Time [10-3 s]

Fig. 6.13 Example of pressure pulse profile for a contoured-drum rotor (S-rotor) [5].

Figures 6.12 and 6.13 show typical pressure pulses caused by the movement of

a foil rotor and a step rotor, respectively. At a random point on the screen surface,

there is in general a positive pressure pulse upstream of the rotor element, and a

negative pressure pulse right after the smallest clearance between the rotor tip

and the screen basket has passed by. The negative pressure is responsible for the

backflush through the screen apertures.

It is evident that the profile of the pressure pulse is very different between rotors.

Short negative-pressure pulses, as created by bump rotors and rotors with short foils,

keep the backflush flow low. At the same time, they ensure comparatively low true

574

6.3Screening Parameters

aperture velocity and low overall screen resistance. Longer negative-pressure

pulses, as created by rotors with long foils and step rotors, reduce reject thickening

by intensified backflushing. Higher feed consistencies require longer negative-

pressure pulses to keep the consistency at the reject end of the screening zone

low enough to avoid blinding. Note that the screen capacity decreases with the

magnitude and duration of the negative pressure pulse.

The clearance between the pulsation element and the screen basket is quite different

between rotor designs. Common clearances are between 3 and 10 mm. Reducing

the clearance between the pulsation element and the screen basket leads

to some increase of the pressure pulse intensity [13,14].

6.3.2

Operating Parameters

6.3.2.1 Reject Rate

The reject rate is the most important operating parameter of a pressure screen. A

higher reject rate improves the screening efficiency and reduces the danger of

screen blinding caused by undue reject thickening (see Fig. 6.7; see also Figs. 6.23

and 6.24). The reject rate is also the only one parameter that really affects fractionation

efficiency (see Fig. 6.32).

However, there is an economic boundary on the reject rate, because large rejects

rates inflate subsequent screening stages and thus increase both investment and

operating costs. Typical volumetric reject rates for pressure screens range from

10% to 25%.

6.3.2.2 Accept Flow Rate

A pressure screen’s capacity is given by the accept flow rate, and is often expressed

in terms of the aperture velocity defined by Eq. (1). A higher aperture velocity

leads to a reduction in screening efficiency [15,16]. Figure 6.14 illustrates graphically

that a decreasing aperture velocity reduces the fiber passage ratio. This

means that reject thickening becomes more critical at lower aperture velocities,

and hence screen capacities. Note that the operation of a pressure screen below its

nominal capacity may soon lead to severe operating problems caused by reject

thickening (Fig. 6.15). The dependence of the fiber passage ratio on the screen

capacity of holed screens is less pronounced than that of slotted screens.

As mentioned in Section 6.2.2, the gross aperture velocity calculated from Eq. (1) is

a parameter of limited significance. Since the actual flow rate through the apertures

depends on many factors, the meaningfulness of the aperture velocity as a parameter

for evaluation of screen capacity or screening efficiency is restricted to systems

of similar mechanical design, pulp furnish and operating conditions.

For a given screen geometry, pulp furnish and pulp consistency, the pressure

drop across a screen is linearly related to the square of the accept flow rate, with

the slope determined by the hydraulic resistance of the screen plate [17].

575

6 Pulp Screening, Cleaning, and Fractionation

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5 6

Passage ratio, P

Aperture velocity [m/s]

Slotted plate

Holed plate

Fig. 6.14 Example of fiber passage ratio as a function of the aperture

velocity;experimenta l data, bleached softwood kraft pulp [6].

0.5

1.0

1.5

2.0

2.5

3.0

0% 10% 20% 30% 40% 50%

Reject thickening factor, T

Volumetric reject ratio, Rv

2.000 l min-1

3.000 l min-1

4.000 l min-1

Fig. 6.15 Example of reject thickening in a pressure screen as a

function of the accept flow rate;3000 L min–1 nominal screen capacity,

experimental data, bleached softwood kraft pulp [6].

Optimum levels of fractionation in slotted screens occur at low aperture velocities

where the passage ratio of long fibers remains low, but that of short fibers is

significant [18]. In contrast, the fractionation performance of holed screens is

widely independent of the aperture velocity at the hole sizes of interest for fractionation

[9].

576

6.3Screening Parameters

6.3.2.3 Feed Consistency

The feed consistency determines the amount of liquor that has to pass the screen

at a given pulp production capacity. Pressure screens can operate at feed consistencies

up to 4% or 5%. The latter figure represents hardwood pulp, which generally

allows higher feed consistencies than softwood pulp. The limiting factor

defining the feed consistency ceiling is reject thickening. A screen’s pulp capacity

increases with rising feed consistency, until a point is reached when it rapidly

decreases due to blinding caused by excessive reject thickening.

A screening system with a high feed consistency is more compact and requires

less electrical energy than a low-consistency system due to the reduced amounts

of liquor pumped around. It is, however, also more demanding to control because

it operates closer to the critical point of reject thickening. At higher consistency,

blinding is favored not only by the increasing population of fibers but also by a

reduced backflush through the screen apertures. It has been shown that the intensity

of the pressure pulse goes down considerably with increasing pulp feed consistency

[13].

The passage ratio decreases as feed consistency goes up [6]. While different

opinions exist about the effects of feed consistency on screening efficiency, it is

likely that the latter is not significantly affected by the feed consistency [15]. However,

a more dilute feed is clearly improving the fractionation efficiency [19].

Several designs of modern washing equipment require feed consistencies between

3% and 4%. If such a piece of equipment is installed downstream of the

screen, only higher-end feed consistencies can provide the needed levels of accept

consistency. If the accept consistency is not critical, a good compromise between

screening efficiency, operability and power consumption for standard screening

applications may be found in the feed consistency range of 2.5% to 3.5%, with softwood

furnish at the lower end and hardwood furnish at the higher end of the range.

6.3.2.4 Temperature

The operating temperature affects the behavior of both liquor and solids. On the

one hand, the pulp fibers become more flexible at higher temperatures (see also

Section 6.3.3.1), while on the other hand the liquor viscosity decreases with higher

temperatures, improving the turbulence in the screening zone. Both of these

effects cause the screen capacity to rise [12].

6.3.2.5 Rotor Tip Velocity

As described above, the rotor is responsible for creating turbulence, providing the

tangential speed of the pulp along the screen plate, and for backflushing the

screen by pulsation. A higher tip velocity means a higher turbulence and a more

intense pressure pulse at basically unchanged pressure pulse profile. The intensity

of the pressure pulse increases with the square of the rotor tip velocity [13]. The

recommended operating range of rotors is varying significantly between rotor designs

and equipment manufacturers. Common tip speeds are between 10 and 40 m s–1.

577

6 Pulp Screening, Cleaning, and Fractionation

Increasing the rotor tip speed improves the screen capacity and allows higher feed

consistencies, while increasing the power demand of the screen. The power requirement

was found to be proportional to the cylinder area and to the tip speed cubed [20].

Within the ranges of velocities recommended by rotor suppliers for their products,

the screening and fractionation efficiencies are not notably affected [10,12,15].

6.3.3

Furnish Parameters

6.3.3.1 Pulp Fibers

With respect to screening, pulp fibers are characterized by a number of physical

properties such as fiber length, fiber flexibility, freeness and disruptive shear

stress of the fiber network. Together with the consistency, these properties determine

the performance of the furnish in a pressure screen.

The influence of fiber flexibility on passage ratio is secondary to the influence of

fiber length. Flexibility plays no role as long as the fibers are shorter than the

width of the slot or the diameter of the hole. As the fibers become longer, however,

the flexible fibers’ passage ratios are higher than those of stiff fibers [2,18]. Note

that fiber stiffness is a function of the temperature, with fibers becoming more

flexible as the temperature rises. Figure 6.16 exemplifies the fiber passage ratio as

a function of the fiber length and hole size.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Fiber passage ratio

Fiber length [mm]

1.00 mm holes

1.75 mm holes

Fig. 6.16 Example of fiber passage ratio as a function of the

fiber length and hole size;smooth hole screen, bump rotor,

softwood thermomechanical pulp (TMP) [8].

Regarding the pulpwood raw material, a distinct difference can be observed between

the long softwood and the short hardwood fibers. The capacity of a given

slotted pressure screen with hardwood pulp is 20–30% higher than its capacity

with softwood pulp.

578

6.4 Centrifugal Cleaning Theory

6.3.3.2 Contaminants

The nature of a contaminant decides the preferred technical solution for its

removal. The most important contaminant parameters for screening are the contaminant

size and shape, and its deformability.

Apparently smaller contaminants require a smaller aperture size to be removed

efficiently. Irregularly shaped contaminant can pose a challenge to reasonable

screening, as do deformable contaminants or contaminants that break up under

shear stress.

A categorization of contaminants and selective ways for their removal are highlighted

in Section 6.7.

6.3.3.3 Entrained Air

A small or moderate air content usually has no effect on the separation of pulp in

pressure screening under typical industrial conditions [16].

6.4

Centrifugal Cleaning Theory

6.4.1