Fundamental Advances in Pulp Screening Technology · 41st International Meeting of Slovene Paper...

41st International Meeting of Slovene Paper Industry 2014 Bled, Slovenia, November 19th – 20th, 2014

Hamelin, Jokerinne and Gooding Fundamental Advances in Pulp Screening Technology

Fundamental Advances in Pulp Screening Technology Mathieu Hamelin, Erkki Jokerinne and Robert Gooding Aikawa Fiber Technologies

Abstract

Pulp screening is an essential process in the manufacture of high-quality paper and board products. Screen performance is determined by the screen rotor and cylinder. Both components have been the subject of technical innovation that has led to: reduced power consumption, cleaner accept pulp, increased capacity, and reduced fibre loss. In the case of the rotor, small-scale turbulence and three-dimensional flow structures have been determined to be of at least equal significance to the backflushing effect of the rotor elements. A comprehensive model of the rotor action has been developed to embrace both pulsation and non-pulsation effects. Two novel rotor designs that follow from this model are discussed in the context of mill trials. One is based on a solid-core rotor that has waveform features on the leading edge of the each rotor element. With these features, the rotor is able to achieve higher capacities, or to operate at lower speeds to save energy, or to operate with smaller slots for increased accept cleanliness. The second rotor is a foil-type design that has a thicker foil to increase wake turbulence. Swept foils and supports ensure that strings and other debris do not accumulate on this rotor. The screen cylinder has also benefited from recent innovation. In particular, a novel manufacturing technique has increased both the precision and accuracy of the slot widths which is of critical importance in allowing small slots to be used. Moreover, even for the same slot width, increased precision can provide a 5-point increase in debris removal efficiency. Advanced technology is thus seen to have a powerful impact on screen performance in a wide range of mill applications.



Introduction

Pulp screening is an essential operation in the production of high-value pulp and paper products, and screens are present in virtually all pulp and paper mills. Pulp screens have screen slots as narrow as 0.10 mm and thus are able to remove a high percentage of contaminants that would otherwise reduce paper appearance, strength, and surface quality. Pulp and paper producers have become increasingly reliant on screening because of its reliability, efficiency and low-cost.

Screening is used in all segments of the pulp and paper industry and its importance is increasing because of the increasingly stringent demands for high-quality paper and board products. The challenge of providing increasingly-higher levels of cleanliness is compounded by the increasing quantity and variety of contaminants in recycled paper furnishes.

The basic parameters used to assess pulp screening are: Capacity – expressed either in terms of a volumetric or a mass-based flows, i.e.

either l/min or t/day; Runnability – which is a subjective quantity reflecting the ability of screens to

operate reliably even during variations in feed consistency and furnish quality; Efficiency – the degree of contaminant removal; Power – as consumed by the pulp screen rotor; Fiber loss – the amount of desirable fiber rejected by the final stage of the screen

system.

The two essential performance components in a pulp screen are the screen cylinder and the screen rotor. The screen cylinder has either holes or slots. “Accept” pulp flows through these apertures and leaves the screen through the accept port, while the oversize contaminants and reject pulp do not pass and exit from the reject port of the screen. The screen rotor backflushes the apertures and clears them of blockages. It also establishes the flow conditions adjacent the screen cylinder surface that supports the screening action.

Screen apertures are intrinsic to overall performance, with aperture size being the primary variable. Narrow slots provide the highest levels of contaminant removal, but also tend to reduce capacity, illustrating a typical trade-off made in optimizing the screen configuration for a particular application. The screening action that blocks the passage of contaminants from the accept pulp can be divided into two fundamental mechanisms: “Barrier screening” prevents the passage of oversize contaminants when they cannot fit through the apertures regardless of their orientation. “Probability screening”, on the other hand, restricts the passage of contaminants that could pass through the apertures if presented in a certain way, but that tend not to pass because their size, shape or stiffness makes it difficult for the contaminants to follow the flow streamlines through the apertures.

Small slots have been made practical through the use of contours on the feed-side of the screen cylinder surface. Contours came into more widespread use in the 1980s and are believed to work by: 1) streamlining the flow through the slot, 2) inducing turbulence to disperse fiber flocs and any fibers that have accumulated at the slot entry, and 3) reducing the potential for fibers to become immobilized at the slot entry. Like slot width, contour height is specified according to the particular application, i.e. considering the feed pulp consistency, pulp character, and the nature of the contaminants.

Improved rotor technology also has the potential to increase screen capacity for a given slot size, or conversely to make smaller apertures possible without a loss of capacity. Power is an additional consideration with rotor technology. The trade-off in some cases then becomes one of power versus capacity (or versus minimum aperture size). There has been great diversity in rotor designs since pressure screens were developed in the 1960s. Four common designs are shown in Figure 1, though well over one hundred rotor designs have been used commercially. Rotors are generally classified as either “open” or “closed” designs. Open rotors have foils, and pulp passes on both sides of these foils. Closed rotors have a

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material that has accumulated with the aperture entry [3]. Smaller clearances between the cylinder and rotor, and increased rotor speeds will increase this effect [4].

Fluid Activity Large and small-scale turbulence and other forms of fluid “activity” are also important in the removal of incipient fibre accumulations within the apertures. Fibres may have become immobilized at the slot entry by a flow bifurcation that leads to fibre “trapping” [5]. High-frequency flow variability at the aperture entry will destabilize the balance of forces inherent to fibre trapping and prevent a significant accumulation of fibres. The bump rotor (Figure 1a) is an example of a rotor that relies more on fluid activity than outright pulsations / flow reversals given that the “land area” of each bump facing the cylinder is relatively small. The arrangement of elements and their shapes has been used to develop fluid activity, as seen in the modified bump rotor (Figure 1c).

Flow Field Development The “screening zone” of pulp screen is defined as the annular space between the rotor and screen cylinder. If the rotor did not turn, flow would enter one end of the screening zone and pass axially through the screening zone, with the axial velocity (i.e. the velocity upstream of an aperture) decreasing steadily as flow as drawn off through the screen cylinder. The screen rotor, however, induces a largely circumferential flow within the annular screening zone. Thus flow upstream of an aperture is relatively uniform through the length of the screening zone, even as the accept flow is drawn off, because the upstream flow is a vector sum which is dominated by the circumferential flow component. The rotor thus provides an easily-controlled, relatively uniform , high-speed flow field ahead of the apertures which is critical to the preferential passage of fibres to contaminants [6].

Reject Zone Pressurization While the screen rotor provides a relatively uniform flow field through the screening zone, the nature of screening flows causes water to flow more easily through apertures than fibres. Pulp consistency thus increases axially as the flow spirals towards the reject end of the screening zone [7]. Higher consistencies will lead to the increased accumulation of fibres within the apertures between backflush pulses. Anecdotal evidence suggests that most of the accept flow passes through the cylinder in the first third of the screening zone. Thus, overall capacity is limited by not making full use of the remaining two-thirds of the cylinder. The AFT GHC™ Rotor solves this problem by using angled rotor elements to pressurize the reject-end of the screening zone, which leads to increased flow through the latter two-thirds of the cylinder apertures and, in effect, a more balanced flow and increased overall capacity [8]. Alternatively, instead of increased capacity, a smaller slot can be used for a higher degree of contaminant removal. In a third approach using the GHC Rotor technology, rotor speed can be reduced without a loss of capacity. Reduced rotor speed is also supported by the use of an optimized element cross-section in the GHC Rotor design [9]. Power savings in excess of 30% were found relative to some of the older rotor designs shown in Figure 1.

Active-Pulse Rotor Technology: AFT GHC2™ Rotor

Many rotors have been developed to accentuate the strength of the pressure pulsation for increased backflushing, or to streamline the rotor elements for reduced power consumption. Certain other rotors have attempted to induce “fluid action” or wake turbulence to reduce any fibre accumulations with the apertures between pulsations, as discussed above. The use of angled elements to pressurize the reject-end of the screening zone and thus to enhance screen performance has proved to be successful, as shown in the GHC Rotor [8].

The AFT GHC2 Rotor was developed to combine all three of the rotor mechanisms to further enhance pulp screen performance. As with the original GHC Rotor, the GHC2 element has a cross-section that has been optimized to provide a strong suction pulse with minimal fluid drag (i.e. minimal power consumption). Similarly, the angled elements pressurize the reject end of the screening zone to balance the accept flow and maximize capacity. What is distinctive with the GHC2 design, however, is the presence of a waveform on the leading edge of the rotor elements, as shown in Figure 2. The streamtubes shown in Figure 2a were generated using computational fluid mechanics, and show how the flow over the element is

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20th, 2014

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reject rate was 25%. An original equipment manufacturer (OEM) rotor was operated in a parallel primary screen with a comparable screen cylinder. The study showed:

14% less power was consumed by the AFT GHC2 Rotor relative to the OEM rotor at the same rotor tip speed of 18 m/s;

the full mill production could be handled by a single screen equiped with a GHC2 Rotor operating at a tip speed of 18 m/s, enabling the second screen to be shut down and thus saving 43% in energy costs as well as additional maintenance costs;

stickies removal increased by 8 percentage points for the screen equipped with the AFT GHC2 Rotor relative to the OEM rotor at the same nominal mill operating conditions (including equivalent mass reject rates).

GHC2 Applications

The GHC2 has been installed in a range of mill applications including OCC, kraft and deink pulp mills including screens as large as the M1600 Centrisorter (~1.2 m diameter). Some selected installations are shown in Table 1.

Table 1. Selected GHC2 installations.

Country Furnish Position Screen Model Principal Benefits

Canada SWK Primary IR 210 22% energy savings

Finland HWK Primary Ahlrstrom M1600 33% energy savings

Finland SWK Secondary Ahlstrom M800 Reduced dP by 12 kPa

Germany DIP Primary Voith MSS 10/06 43% energy savings

USA SWK Primary IR 212 Improved runnability

Canada OCC Primary KBC PS30 Increased capacity by 15%

Finland SWK Primary Ahlstrom F6R 20% energy savings Increased efficiency1

Finland SWK Secondary Ahlstrom F4 27% energy savings Increased efficiency2

Finland SWK Tertiary Ahlstrom F2 9% energy savings

1 Reduced slot width from 0.27 mm to 0.20 mm. 2 Reduced slot width from 0.30 mm to 0.20 mm

Some general guidelines ave been developed from mill studies and pilot plant work to understand how the GHC2 benefits mill operations. Table 2 summarizes these benefits relative to the performance of AFT’s GHC Rotor under two possible scenarios: one where the mill simply substitutes the rotor at the same operating speed, and the other where the rotor tip speed is decreased by 2 m/s to take advantage of the GHC2 Rotor’s more effective rotor action. Thus one can, for example, obtain an ~30% power savings from a lower tip speed, or substantially increase debris removal possible by operating with a smaller slot, as two of the many possible strategies available by using a GHC2 Rotor with PowerEdge technology.

41st Intern

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20th, 2014

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ced slot precses use welndard deviaed manufacnd, and novons. The AF

are is takenre shown inn them defithe highest

ed slot widtll tend to pluity of undeross of caparough the s

cylinders an in slot widwires themsThe methodcture will al

s in the suppe tools. Appty. In const

oduce additthe notchesty is introdu

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ng of Slovene P

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e double-endre consistencompetitor effective ro

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cision signifds, glue, rivation of slotcturing techvel wire-ringFT MacroFl

n in selectin Figure 9, wning the slot level of deth variabilityug with pulprsized slots acity. More ocreen cylind

re typically dth exists, hselves are tys used to inso lead to sport rings isplying a layeructions whtional variabs and installuced when tn after the cn erode thes, with both

Roto

Paper Industry

ded suppornt rotor-cylinr rotor. A mootor pulse a

Technology:

ficantly benveting or rolt widths is tyniques, with

g locking teclow2 cylinde

g the approwith their shot. Slot widthebris removay will lead top, causing awill lead to oversized sder and into

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AFT EPX (UV

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y 2014

Fundame

rt for the scrnder clearanore consisteand better sc

: AFT Macro

efits screenlling to connypically in thh state-of-thchniques haer (Figure 8

opriate slot whape defininh is typicallyal while stillo more undea reduction a larger-tha

slots increaso the accep

with all the snd it comes med by a roewires in th

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V400)

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learance (mm)

Bled

mental Advanc

D

reen foil in tnce than theent clearanccreen runna

oFlow2

n performannect the wirhe range of he-art laser ave reduced8) is an exam

width and cng the contoy specified l providing aersized andof screen can-ideal slose the posst pulp.

slots havingfrom a ranglling procese support rion. For exa

perfect whetas “metal b

ded to supping and slot

e screening nd support sthe mill procnot only wi

uced screen

ltifoil

d, Slovenia, Nov

ces in Pulp Sc

DOUBLE-ENSUPPOR

the AFT EPe single-endce leads to ability.

nce. Traditioes to the suf 10 to 15 mcutting, cyl

d the standample of this

contour typeour and the as small asadequate sc

d oversized capacity – oot being speibilities for c

g the same wge of sourcess and can vings during mple, the pther it is basbonding”) wort rings, tht width. Som“mat” is flat

structure is cess, contader slots bu

n efficiency.

SINGL

ovember 19th – 2

Screening Tec

NDED RT

PX Rotor ledded suppora more con

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microns. Morinders madard deviatios.

e. Some indminimum s

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LE-ENDED S(CANTILEV

20th, 2014

chnology

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SUPPORT VER)

41st Intern

Hamelin,

Figure 8

Figure 9

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Table 3number

national Meetin

n, Jokerinne a

8. The suppinter

9. Closimagrightappldefin

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cellent accue range of see cylinders

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AFT Macroport rings horior (feed-si

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of the meane the practicn). A measud the mean150 mm (m

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Paper Industry

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Fundame

dgewire cyliwedge wirescylinder is a

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ochure claimmitations in

Bled

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nder is shows that definet the right.

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ms and actutraditional m

d, Slovenia, Nov

ces in Pulp Sc

wn on the lee the slots.

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ual measuremanufacturi

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ovember 19th – 2

Screening Tec

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ements mading process

20th, 2014

chnology

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41st Intern

Hamelin,

based o15 mm. deviatiocutting, MacroF

Figure 1

It is selfoversizeremovaprecisio[10]. Theach mo

C

national Meetin

n, Jokerinne a

on the use oSome inex

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Flow2 have

10. Slot (m* (m* conc

Table 3.

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ng of Slovene P

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Paper Industry

ue or rollingommodity-gadvanced m-round, andriability in s

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y 2014

Fundame

g, will commrade cylind

manufacturind novel wireslot width to

ree cylinderwhile Cylinde suffer fromprecision a

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10 mm

12 mm

12 mm

-

variability areen acceptissue is the

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Bled

mental Advanc

monly lead toers have ev

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m poor accure shown in

r some indu

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and more ovts and a dece quantitativthe subject taminant anysis assum

d, Slovenia, Nov

ces in Pulp Sc

o standard ven higher les, with stag technique

deviation va

linder B is c= -15 mm; sracy and prn the figure

strial scree

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8 mm

15 – 20 m

10 – 16 m

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15 mm

20 mm

versize slotcrease in cove relationsof a study b

nd fibre sizemed:

ovember 19th – 2

Screening Tec

deviations olevels of state-of-the-ar

es, as seen lues below

close to the s = 15 mm) recision. Thon the right

n cylinders

andard n

mm

mm

ts will lead tontaminant hip betweenby Gooding distribution

20th, 2014

chnology

of about andard rt laser in 10 mm.

ideal and C e t.

.

to more

n slot et al.

ns were

41st Intern

Hamelin,

ContamEfficiencdistributprecisiostandarremovaa typicaefficiencMacroFimprove

An expeto confirprecisioand in p

Figure 1

Tests wUnivershardwooThe feespecks

An AFTconfigur

national Meetin

n, Jokerinne a

Slot velVery smwidth wThe “pluchangeThe likethe amoContamdetermiFibre paupstreawith fibe

minants and cy and constions over a

on is shown rd deviationl efficiency

al wedgewirecy, to 86%,

Flow2 with aement of 5 p

erimental strm the theo

on, were tesparticular th

11. A thcontsize

were made usity of Britishod and soft

ed consistenthat had pa

T EP™ Rotorations were

ng of Slovene P

and Gooding

ocity is equmall slots wiwas chosen tug-flow mods in debris l

elihood of coount of flow

minant remoned by a siassage is dm velocity (er length an

slots both hsistency cha

all particle-sin Figure 1

. A low-endof 75%. Ree cylinder ccomes from

a 7 micron spercentage

tudy, using oretical analsted to see he thickening

heoretical astaminant re

e distribution

using a Beloh Columbiatwood kraft ncy to the passed throug

or was operae created fr

Paper Industry

ual for all sloll plug, whicto be as smdel” [7] appllevels, consontaminantsthrough thaval is determple competermined b(driven by thnd slot width

have a ranganges (e.g.

size classes1, which sh cylinder wi

educing the circa 2010) m the use ostandard de

points in ef

a small induysis. Two show improvg factor and

ssessment emoval efficn has a mea

oit MR-8 pu. The furnispulps with aulp screen wgh 0.3 mm

ated at a tiprom a single

y 2014

Fundame

ots; flow throch follows o

mall as posslies to flow sistency, fibs passing that slot and tmined only arison of coby probabilihe rotor) is h.

ge of contam thickening

s and slot whows the incith a standastandard dincreases ef a state-of-viation. Thufficiency by

ustrial pulp screen cylindved slot precd contamina

of slot preciency for a an of 175

ulp screen ash for the triaa length-wewas 1%. Pu(300 mm) w

p speed of 1e cylinder th

Bled

mental Advanc

ough each on the assumsible to achithough the

bre length dhrough a pathus to the wby barrier s

ontaminant ty screeninthe same in

minant sizesfactor) are

idths. The bcrease in efard deviationeviation to

efficiency to-the-art cylinus one seesincreasing

screen in ader configucision wouldant removal

ision (i.e. strange of slom and stan

at the Pulp aal was a 50

eighted averulp was seewide openin

12 m/s in thhat had a 0.

d, Slovenia, Nov

ces in Pulp Sc

slot is propomption that eve the grescreen, whistribution, earticular slotwidth of thescreening. Psize to slot g. Since slon all slots, p

s (x) and slocalculated

benefit of imfficiency witn of 20 micr15 microns 81%. A furnder, such s the potentslot precisi

a pilot plant rations, whd affect screl efficiency.

tandard devot widths. Tdard deviat

and Paper C0-50 blend orage fiber leeded with blngs.

hese trials. T15 mm nom

ovember 19th – 2

Screening Tec

ortional to itthe nomina

eatest efficieich predictsetc. t is proporti

e particular sPassage is width.

ot velocity apassage var

ot widths (wby integrati

mproved sloth reduced rons has a (representa

rther increasas the AFT tial for an on.

setting, waich varied ineen perform

viation) on he contamition of 40 m

Centre of Thof reslushedength of 1.4lack polyeth

Two cylindeminal slot w

20th, 2014

chnology

ts width. al slot ency. s axial

onal to slot. thus

and the ries only

w). ing the t

debris ative of se in

s made n slot mance,

nant m.

he d market 43 mm. hylene

er idth, 3.2

41st Intern

Hamelin,

mm wiredeviatiohammethe widtdeviatioaveragetrial was

For eacsamplesconsistehandshe

Consistin standmeasurartificialthis studusing a

where Ethe pass

As expewas fouthe benconsiste

Figure 1

The findwith adv

national Meetin

n, Jokerinne a

e width andon of 12 mmring a wedgth of the we

on of the sloe slot width,s then run w

ch trial, the ss were colleency and coeets and us

ent with thedard deviatiorements of c contaminady was to m“least-squa

E is efficiencsage ratios

ected, the sund to have efit of the iment with the

12. Theorigeffic

dings providvanced scre

ng of Slovene P

and Gooding

d 0.9 mm com. Followingge into a spedged slot, aot width dist, the wire shwith the mod

slot velocityected at fouontaminant sing image a

eoretical eston. The impcontaminan

ants seededmake measuared” metho

cy, RM and for contam

creen cylindthe higher

mproved slo theoretical

e impact of iinal cylinde

ciency than

de clear andeen cylinde

Paper Industry

ontour heighg a first trial,ecific numband reducinribution thuhape and todified cylind

y was maintr volumetrictesting. Coanalysis to

timates, thicpact on scret levels are into the pu

urements atod and a cu

RV are masminants and

der configucontaminan

ot precision estimates s

mproved sler (s = 12

the modifie

d quantitativr constructio

Origina

y 2014

Fundame

ht. The initia, the slot widber of slots cng slot widths became 2

otal open arder.

tained at 1.0c reject rationtaminant mdetermine t

ckening faceen efficien difficult to o

ulp. To deal t a series ofrve form ba

ss and volumpulp [7].

ration with tnt removal eis seen to b

shown in Fi

ot precisionm) having a

ed (s = 22 m

ve evidenceon, such as

al Cylinder

Modified C

Bled

mental Advanc

al slot widthdth distributcausing theh in the two 21 mm. Thisrea remaine

0 m/s. Feedos (20%, 30measuremethe number

ctor was notcy is shownobtain – evewith the va

f reject ratioased on the

metric rejec

the lower amefficiency. Abe 8 percenigure 11.

n on debris about 8 percmm) cylinde

e of the valus the AFT M

Cylinder

d, Slovenia, Nov

ces in Pulp Sc

h distributiontion was ma

e wires to deadjacent sl

s approach ed exactly th

d, accept an0%, 40%, 5ents were mr and size o

t much affecn in Figure 1en in pilot p

ariability, theos and to fit

plug-flow m

ct ratios, and

mount of sloAt a typical rntage points

removal effcentage po

er.

ue of improvMacroFlow2

ovember 19th – 2

Screening Tec

n had a stanade worse beform, increlots. The staensured th

he same. A

nd reject pu0%) for

made by preof contamina

cted by the 12. Accuratplant settinge approach a curve to

model:

d PC and PP

ot width varreject ratio s, which is

ficiency, witints higher

ved slot pre, providing

20th, 2014

chnology

ndard by easing andard at the second

lp

paring ants.

change e

gs with taken in the data

P are

riability of 25%,

th the

ecision, an



improvement in contaminant removal efficiency of about 5 percentage points without compromising capacity or any other screen performance parameter.

Conclusions

The AFT GHC2 Rotor, EPX Rotor and MacroFlow2 cylinder represent examples of how advanced technology can be used to enhance screen operation. The GHC2 Rotor is of general use. It can be used to provide reduced power, increased contaminant removal and higher capacity. The EPX Rotor is designed to enable good runnability with highly-contaminated furnishes that require a foil-type rotor. The MacroFlow2 cylinder provides more uniform slot widths, which benefits contaminant removal efficiency without compromising capacity. This equipment reflects the ongoing application of theoretical tools, plus practical mill knowledge, to develop technology that improves pulp screen systems.

References

1 Karvinen, R., Halonen, L., The effect of various factors on pressure pulsation of a screen. Paperi ja Puu 66(7):80–83 (1984).

2 Feng, M., Gonzalez, J., Olson, J.A., Ollivier-Gooch, C., Gooding, R.W., Numerical simulation and experimental measurement of pressure pulses produced by a pulp screen foil rotor. J. Fluids Eng. 127(2):347-357 (2005).

3 Martinez, D.M., Gooding, R.W., Roberts, N., A force balance model of pulp screen capacity. Tappi J. 82(4):181–187 (1999).

4 Pinon, V., Gooding, R.W., Olson, J.A., Measurements of pressure pulses from a solid core screen rotor. Tappi J. 2(10):9–12 (2003).

5 Salem, H.J., Gooding, R.W., Martinez, D.M, Olson, J.A., Some fundamental aspects of pulp screen capacity. Proc. 15th Fundamental Research Symposium, Cambridge, England (2013).

6 Gooding, R.W., The passage of fibres through slots in pulp screening. M.A.Sc. thesis. The University of British Columbia, Canada (1986).

7 Gooding, R.W., Kerekes, R.J., Consistency changes caused by pulp screening. Tappi J. 75(10) :109-118 (1992).

8 Konola, A., Poikolainen, I., Kovasin, K., Karppinen, J., Gooding, R., Reduced power consumption in softwood kraft screening at Botnia Aanekoski, Paperi ja puu 91(3):27-32 (2009).

9 Luukkonen, A., Delfel, S., Olson, J.A., Ollivier-Gooch, C., Pfleuger, C., A computational fluid dynamic simulation of the pressure pulses produced by a solid core pulp screen rotor. Appita J. 61:6 (2008).

10 Gooding, R., Olson, J., Hayart, C., Labbe, F., Enhanced pulp screen performance from increased slot precision and accuracy, ATIP 66(3) (2012).

Fundamental Advances in Pulp Screening Technology · 41st International Meeting of Slovene Paper...

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