Compact wet end systems for superior papermakingJ. L. Cutts

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aper machine designs have historical-ly included large volume wet end pro-cesses as a necessary part of an effi-cient and productive papermakingfacility. The conventional large vol-

ume processes, as shown in Figure 1, provide mix-ing and blending of thick stock components andcontrol of entrained air from the forming sectionwhite waters.

When designed properly, these systems initial-ly perform to an acceptable level. However, asproduction rates, operating speeds or gradestructures change, performance can deteriorate.Results can include poor stock blending andincreased levels of entrained air in the headboxwhite waters, which can negatively impactmachine efficiency, productivity and quality.

Other problems associated with large volumewet end systems include:• Low operating efficiencies with frequent gradechange• Poor attenuation of low frequency variations• Accelerated bacteria and scale growth• Process variability from compression ofentrained air

Compact wet end systems, as shown in Figure2, have been developed to address these operat-ing issues and to improve wet end performanceover what is available with conventional systems.

DEVELOPMENTThe principle of compact wet end systems wasfirst introduced to the paper industry by PaulOlof Meinander at the TAPPI Papermakers Con-ference in 1993 [1]. As entrepreneur andfounder of POM Technology, Meinanderreceived mixed reviews of his compact wet endsystem concept. However, Meinander proceededwith his development work and in 1997 the firstfull scale system was placed in service at MD Papi-er, Albbruck, Germany [2]. Currently, 25 systemshave been delivered worldwide.

Compact wet end systems focus on reducingthe volumes of conventional wet end processes toprovide significant improvements in overallpaper machine performance. System volumereductions based on the POM principle utilizecompact stock mixing systems to replace conven-tional blend and machine chest and dry stockflow control technology to eliminate the stuff

box. Centrifugal deaeration, cleaner systemsbased on the flexible cascade concept and pres-surized white water distribution systems minimizeshort circulation volumes by eliminating the wirepit, silo and vacuum deaeration system. Total wetend volume reductions of up to 90% have beenimplemented with compact wet end systems.

Meinander’s development and progress withcompact wet end systems has not gone unnoticedin the paper industry. Today, all major manufac-turers of paper machinery equipment and systemsoffer compact wet end systems as an effective toolfor improving paper machine performance.

COMPONENTS AND PRINCIPLESOF OPERATIONContrary to historical beliefs, large volume stockchest, machine silos and wire pits are not manda-tory requirements for an efficient high speed,high quality paper machine operation. In manycases these large volume systems contribute toinstability, variability and overall reduced effi-ciencies in the papermaking process.

Compact wet end systems employing provencomponents and process applications have signif-icantly reduced process volumes and exceed theperformance of conventional wet end systems.The principles employed apply to both the thickstock and white water systems as described below.Thick stock systemMixing and blending of single and multiple thickstock components, including associated wet end addi-tives, is often inefficient with large volume systems.Poor agitation, flow channeling and air introductioncan contribute to poor wet end performance.Compact stock mixerA compact wet end system solution for a poormixing and blending problem, which maximizessystem volume reductions, is the compact stockmixer [3] as shown below in Figure 3.

The compact stock mixer will replace theblend and machine chest and provides significantimprovements in thick stock system stability.

Multiple furnish components and appropriatewet end additives can be successfully mixed andblended in this unit. One to three minutes reten-tion is provided to suit process needs related tochemistry, color or other process requirements.The compact stock mixer is located as close to the

J. L. CUTTSPOM Technology AmericasBirmingham, ALJohn.Cutts@pomta.com

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Abstract: Compact Wet End Systems have contributed to improving paper machine operatingefficiencies by up to 10%. With Compact Wet End Systems, wire pits, silos and mechanical deaera-tion systems are eliminated as well as thick stock blend and machine chests. Wet end process vol-ume reductions of up to 90% have been implemented. This paper will discuss the principles andoperation of Compact Wet End Systems. Specific applications will be reviewed and discussed.

headbox as practical, on the operation floor, to minimize processvolumes. Where frequent grade change is an issue, the compactstock mixer will allow plug-flow operation for fast and efficientchanges. Some color operations utilize the plug-flow opportuni-ty for “on-the-fly” changes not possible with conventional systems.Dry stock flow controlTo insure thick stock flow stability from the compact stock mix-er, dry stock flow control technologies are incorporated. Here,consistency and flow measurements are used to control the thickstock pump speed for accurate dry fiber flow to the machine.Process stability is such that the stuff box can be eliminated fordirect fiber delivery to the fan or cleaner pump suction.

The compact stock mixer operates at 4% to 5% consistency andeliminates one consistency control point to the machine. Withincreased consistency, excess water from the machine in reduced.White water and thin stock systemThe short circulation loop of the paper machine wet end processmust efficiently remove entrained air for recirculation to theheadbox. The large volume wire pits and silos are designed withvery low flow velocities to provide retention time needed foratmospheric deaeration. Often these units are ineffective inreducing the entrained air content of the white waters to accept-able levels. For many high quality, fine and newsprint grades,

mechanical systems must be used to achieve the required levelof deaeration.

In additional to being inefficient deaerators, wire pits andsilos are breeding grounds for bacteria growth with their lowflow velocity designs. To maintain control, expensive biocides,slimicides and defoamers are required including regular systemboilouts. The large volume systems also contribute to low fre-quency variations that are difficult if not impossible to effective-ly manage. These variations can contribute to significant MDbasis weight swings on the paper machine.

Compact wet end systems address these problems and effectsignificant volume reductions using centrifugal deaeration,pressurized white water distribution systems and selective reuseof white water. See Figure 2.Centrifugal deaerationWhite water deaeration in a compact wet end system is accomplishedthrough a centrifugal deaerator [4] shown in Figure 4.

Forming section white waters are deaerated through this unitimmediately on exiting the forming section allowing the wire pitto be eliminated. The deaerated quality of the white water fromthe centrifugal deaerator is sufficient to replace mechanicaldeaerators, further reducing system volumes and providing sig-nificant energy reductions. For paper machines currently oper-ating without deaerated white waters, compact wet end systemswith the centrifugal deaerator will provide the valued benefits ofimproved formation and quality.Pressurized white water distribution The virtual “air free” white water is delivered by the centrifugaldeaerator, under pressure, to the enclosed white water distribu-tion system. This pressurized header transfers the deaeratedwhite water to the respective fan and cleaner pumps and othersystem white water users. A stable pressure is maintained on thetotal system by means of an elevated overflow head tank incor-porated at the top of the distribution header.White water segregationRich and lean waters are processed separately by individual cen-trifugal deaerators and delivered under pressure to the distribu-tion header. Rich waters enter the header such that they are con-sumed first and lean waters last. This insures lean water only flowsto excess contributing significantly to reduced system losses.

The white water header is a closed, air free hydraulic system

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FIG. 1. Conventional wet end system. FIG. 2. POM compact wet end systems.

FIG. 4. Centrifugal deaerator.

FIG. 3. Compact stock mixer.

with higher flow velocities than a conventional white water system.Therefore, bacteria growth and scale build-up are minimized.Flexible cleaner arrangementAs an additional volume reduction opportunity, compact wetend systems can incorporate a cleaner arrangement that reducesequipment sizing and energy cost while further reducing systemvolume. This system is shown in Figure 5.

The primary cleaners of the flexible cascade arrangement aresized for minimum headbox flow or optimum cleaning efficien-cy. Here, secondary accepts are recirculated in a full cascadearrangement. As headbox flow increases, secondary accepts feedforward automatically for an open cascade operation to satisfythe higher flow requirement. When flow requirements exceedthe combined primary and secondary accepts flow, additionalwhite water dilution flow is drawn from the distribution header.

Cleaners of the “Flexible Cascade” arrangement operate at opti-mum flow or cleaning efficiency throughout a broader range of theheadbox flow. Equipment selections are minimized, horsepowerrequirements are reduced and control and piping systems are sim-plified.Compact seal pitThe flat box or vacuum leg seal pit is often overlooked as an areaof the papermaking process requiring improvements orupgrade. For compact wet end systems, it provides anotheropportunity to reduce system volume and improve papermachine performance.

Typical, seal pit operations are dirty, turbulent, large volumechest that can generate vacuum variations at the forming section.In addition, slime buildup can be difficult to manage leading toincreased frequency of required thermal, mechanical and/orchemical cleaning. With the compact wet end system approach,a compact, pre-fabricated seal pit is provided. See Figure 6.

This unit allows each vacuum leg to be sealed individuallyand operators can view the performance of each table elementand make adjustments as may be necessary. The individual sealchambers can be easily fitted with a “V-notch weir” to allow directand simple flow measure and sampling from each vacuumdevice. Figure 7, is an operating compact seal pit unit.

RESULTSAfter the first compact wet end system startup in 1997, it becamequickly apparent that performance and additional opportunitiesfor improvement would be much greater than anticipated. Ben-efits that have been reported to date include:• 60% reduction in grade change time• 75% reduction in wet end breaks• 50% reduction in wash-up frequency• 25% reduction in energy consumption• 20% improvement in sheet formation• 99.5+% “air-free” white water• 10% operating efficiency improvement

A discussion of these improvements and opportunities [5]include:Reduced grade change timePaper machines incorporating compact wet end systems havereduced grade change time on average by 60%. Reduced pro-cess volumes allow thick stock and white water systems to purgequickly, minimizing off-quality production and transition time.

The improvements in change time apply to simple basisweight changes or complete color and/or wet end chemistrychanges. One installation reported simple basis weight changeswere improved by 65% where a color producer reduced grade-to-grade change time by greater than 80%. Many specialtymachines find downtime for wash-up between grade changes areno longer required and major production changes are nowmade on the fly.Reduced wet end breaksCompact wet end systems have disproved the age-old assump-tion that large volume processes are required for maintaining astable system. Whether in thick stock or white water systems,large volume systems can generate low frequency variations thatare difficult if not impossible to attenuate. These variations usu-ally contribute to significant MD weight swings.

With the minimum retention time in a compact stock mixerand the higher flow velocities of the hydraulic white water dis-tribution system, low frequency variations are avoided. The ele-vated overflow head tank further enhances the white water sys-tem stability with the optimum in pressure control forminimizing variability.

Stable white water systems are the key to a stable wet end. One

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FIG. 5. Flexible cascade cleaner arrangement. FIG. 6. Compact seal pit.

FIG. 7. Operating compact seal pit.

compact wet end system installation report-ed an 80% reduction in wet end breaksdue to improved stability. Other users noteMD variation reduced significantly withtwo sigma reductions of up to 50%.Reduced wash-up frequencyWhite water system cleanliness is affectedby flow velocities and entrained air con-tent. Wire pits and silos maintain low flowrates by design, allowing stagnant areas toexist which support bacteria growth. Air inthe white waters further enhances bacteriagrowth requiring use of biocides, slimicidesand defoamers for control.

Compact wet end systems removeentrained air immediately on exiting theforming section; therefore air is not intro-duced to the downstream system. Also,high flow velocities in the hydraulic whitewater system prevent stagnate areas anddirt accumulation. Each of these con-tributes to improved system cleanlinesswith a compact wet end system.

The improved cleanliness reduces boilout frequency and chemical additionsneeded for bacteria and slime control.The boil out frequency on one machineimproved by 50%; 6 week cycles for systemboil outs was increased to 12 weeks with acompact wet end system. Chemical usagewas also reduced with defoamer con-sumption down by 50% and biocidereductions of 75%. Boil out chemicalrequirements are less with the reducedvolumes and boil outs are more effective. Reduced energy consumptionMany components of conventional wetend systems can be eliminated with a com-pact system. Blend and machine chests,white water wire pits, silos, vacuum deaer-ation systems and water storage tankshave been deleted.

The elimination of these componentshas resulted in energy savings of up to25%. Combined with these savings is theincreased efficiencies gained from thepressurized white water distribution sys-tem. Here, the distribution system sup-plies white water to all system users athigher suction pressures for increasedpump performance and improved pump-ing stability.Improved sheet formationRemoving air from the short circulationwhite waters increases drainage in theforming section. With increased drainage,lower headbox consistencies can beobtained. Lower headbox consistencieswill improve formation and thereforequality. All compact wet end system users

report improved formation and in onecase slice flows were increased by 20%.Air-free white waterThe management of entrained air in the wetend white waters is critical with compact wetend systems. Centrifugal deaeration hasproven to be effective in removing entrainedair from the short circulation waters. The vir-tually air free white waters produced by thesecentrifugal deaeration units rival moreexpensive vacuum systems.

One compact wet end installationreplaced their vacuum deaeration systemwith centrifugal deaeration for equal to bet-ter results. Other installations reportentrained air levels as low as 0.1%.Improved operating efficiencyEach of the above results from currentlyoperating compact wet end systemsimprove paper machine operating effi-ciency. Combined, they can significantlyreduce bottom line cost. To date, compactwet end system users have reported over-all efficiency improvements of 2.5% up tomore than 10% as compared to earlieroperation with conventional systems.

CONCLUSIONOf the many compact wet end systems inoperation to day, no installation has elect-ed to go back to the old (conventional)way of operating the wet end of theirpaper machine. Once the magnitude ofthe benefits has been realized, the oper-ating units only look forward to addition-al opportunities.

Many different grades employ compactwet end systems including coating basestock, specialty fine papers, decorativelaminates, core stock, photographicpaper, recycled linerboard and medium.

Compact wet end systems improvepaper machine wet end operation as com-

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Reference: Paper presented at the 88th Annual Meeting in Montreal, QC, Canada, January 28-31, 2002. Not to be reproduced without permission of PAPTAC. Manuscript received November16, 2001. Revised manuscript approved for publication by the Review Panel on June 6, 2002

Keywords: PAPER MACHINES, WET ENDS, MACHINE DESIGN, PERFORMANCE EVALU-ATION,

Résumé:Les systèmes compacts pour la partie humide ont permis d’améliorer de jusqu’à 10 % le gain derendement de la machine. Grâce aux systèmes compacts pour la partie humide, les fosses soustoile, les silos et les systèmes de désaération mécanique sont éliminés, ainsi que les cuviers demachine et de mélange de la pâte épaisse. Une réduction de jusqu’à 90 % du volume à la partiehumide a été obtenue. La présente communication discute des principes et de l’exploitation dessystèmes compacts à la partie humide. Des applications particulières sont évaluées et font l’objetde discussion.

pared to conventional large volume sys-tems. Utilizing compact stock mixing,white water centrifugal deaeration andpressurized white water distribution sys-tems, wet end processes are extremelyresponsive to operational demands whilemaintaining stable operations.

With deaeration of white waters imme-diately off the forming section, systemsoperate much cleaner than conventionalones and pulsations from air in the pro-cess are avoided. The pressurizedhydraulic white water distribution systemprovides segregation of rich verses leanwhite waters for 100% recycle of richwaters. Only the leanest water flows toexcess significantly reducing system losesto sewer.

Compact wet end systems have been suc-cessfully applied for new greenfield installa-tions as well as cost-effective alternatives toconventional upgrade options. When con-sidering your next upgrade or a new installa-tion, compact wet end systems should not beoverlooked. This proven, reduced volumeapproach has established itself as the futurein paper machine wet end technology.

LITERATURE1. Meinander, P.O. An Approach To Just On TimePapermaking. TAPPI Paperrmakers Conference,CITY, PROV (MONTH, 1993).2. Meinander, P.O., Olsson, L-H. Compact, Airless WetEnd Systems. TAPPI Proceedings, TAPPI Press: 1213-1218 (MONTH, YEAR).3. Meinander, P.O. Apparatus and Process for FeedingStock to a Paper Machine. PCT patent applicationPCT/FI96/00052 (January 25,1996).4. Helle, T-M., Meinander, P.O., Paulauro, H. Removalof entrained air from white water by application ofcentrifugal force. Paper and Timber 80(5): 379-382(MONTH, YEAR).5. Meinander, P.O., Cutts, J., Kontkanen, M., Nykanen,R. Experience Confirms the Validity of Compact WetEnd Systems. TAPPI Papermakers Conference, CITY,PROV (MONTH, YEAR).