Pushing performance limits with dryer fabrics: make your paper machine work at its full potential with help from dryer fabrics.
With machine speeds increasing, dryer fabric runnability becomes paramount. Solutions! asked several machine-clothing supplier representatives about some very critical fabric design and operating issues that mills should consider for higher speed machines.
Going to higher speeds affects many machine and fabric parameters, according to the AstenJohnson Dryer Specialists Team, Charleston, South Carolina, USA. As machine speeds increase, more air is carried by dryers, rolls, and fabrics, leading to increased sheet flutter and potential for sheet breaks. To combat this, mills need fabrics of lower permeability, with smooth surfaces for lower boundary layer air, they said.
According to Steve Cole, directorvalue creation management, Weavexx, Wake Forest, North Carolina, boundary layer air is particularly important above a machine speed of 4000 ft/min. "It is imperative that the fabric have a surface topography that is very smooth to avoid carrying excessive air," Cole said. "Permeability control is equally as important. With sheet control devices employed (blow boxes and transfer boxes), applying fabrics carefully controlled to a permeability of 80-120 f[t.sup.3]/min is key to optimizing the effectiveness of these units and keeping the sheet tight against the fabric throughout the run." To maintain high permeability, mills should be assiduous about keeping the fabric free of contaminants.
According to Scott Carson, vice president-marketing and applications, North American dryer fabrics, Albany International Corp., the fastest machines rely on single run configurations for some or all their sections. "The best single-run dryer fabrics optimize runnability while providing stable, steady-state performance," he said. "Runnability at high speeds is dominated by fabric aerodynamics. A key concern is the ability of a fabric to handle air within a single run section to minimize sheet disturbance.
"Not all dryer sections can take advantage of thin neutral line fabrics. On machines without vacuum rolls or grooved rolls in their single run sections, directional void volume becomes the dominating factor to improve runnability," Carson continued. "Fabrics with machine direction backside channels are available on these positions to guide the air around the bottom cans, reduce pocket air pressure, and increase sheet stability."
Other parameters--neutral-line, caliper, and seam design--are important to consider when choosing fabrics for high-speed machines. Blake Tholen, sales manager-U.S. Southeast, Voith Fabrics Inc., noted an area often ignored during the selection process. "From a runnability aspect, an important but overlooked consideration is selecting the proper 'add-ons' to ensure the proper installation and care-free operation of the fabric. Lead-on harnesses, zipper-assists, and durable edge treatments are key additions, particularly on the highest speed machines that have challenging installation setups or abrasive guide paddles. The money saved on skipping these extras can be lost many times over in failures due to missed seam loops or edge raveling," Tholen warned.
Some research appears to show that significant improvements in drying capacity are possible by operating dryer fabrics at tensions higher than traditionally used. Mills should be careful about raising tensions beyond the traditional limits of 12-14 pounds per linear inch (PLI) because of potential problems caused by increased mechanical forces.
Tholen noted that the effective tension range has increased as paper machines run faster. "Today it is not unusual to find running tensions at 20-24 PLI," he said. "Care is necessary never to run a dryer fabric at tensions that exceed the roll journal ratings on the machine. Exceeding these tension ratings can result in a cataclysmic failure of the rolls or dryer cans.
"Once a papermaker has determined that he can safely increase dryer fabric tensions, more care should be taken during running and while shut down," Tholen continued. "While running, having a good sheet-break/knockdown system is important to prevent massive pileups in the dryer section. When the machine is down, slacking off the tension is important to prevent roll and dryer can deflection, particularly on the weaker single-tier vacuum roll systems."
Weavexx's Cole said research shows that very little additional drying takes place past 12-14 PLI, although some machines routinely operate above those levels. "Some machines see improvement in sheet width stability and profile, but caution is necessary since few machine rolls and bearings can withstand the mechanical forces at excessive tensions. Rolls will deflect and cause the dryer fabric to develop a centerahead distortion pattern or 'smile'. Minor distortions can be rectified through a negative crown roll. In severe cases, these put enormous stress on the fabric and can cause narrowing and seam problems."
Potential drying benefits are not the only factor sparking a move to higher tensions. Another item has been the increase in the number of silent drive applications that run higher tensions and the need to restrain the sheet to help reduce sheet cockles according to Ian Lang, team leader, advanced product development-drying, AstenJohnson. "Previously accepted 'rules of thumb' put 12 PLI as the upper limit for fabric tension from a drying capacity perspective. We find that important drying capacity increases occur at higher fabric tension. In general, drying capacity increases with increasing fabric tension and gradually levels off at 18-21 PLI."
According to Lang, grade and steam temperature determine the upper limit of fabric tension for maximum drying capacity. "In general, higher basis weight sheets will benefit more from drying with higher fabric tension compared with lightweight sheets. Likewise, a higher drying temperature increases the upper limit to fabric tension."
Although citing the benefits of higher tension, including a more uniform CD shrinkage profile resulting from the increased sheet restraint higher tension gives, Lang also offered a caveat. "Dryer fabrics are designed to withstand tensions well in excess of what can be run on the paper machine. The papermaker must ensure that the paper machine components are designed for the intended loads. Bearings and rolls must be sized for the intended loads, and rolls must be properly dimensioned to minimize deflections. Roll deflection at higher tensions can cause dryer fabric bowing that will result in narrowing. The paper-maker should be aware of the limitations of the paper machine and know his drying requirements to maximize the benefits of increased fabric tension," he said.
Contamination problems will affect dryer fabric performance and ultimately paper machine performance if not prevented or treated. Stickies and pitch are among the most challenging contaminants. Oil, grease, and latex can also be problematic. "A good wet-end chemistry strategy and cleaning program can help to minimize the deposits in the dryer section. Most machines require some level of dryer fabric cleaning. A combination of high pressure steam showers and chemicals will usually keep the dryer fabrics' permeability and surfaces clean enough to achieve economic life," said Cole.
The most challenging contaminants come from 100% recycled paper machines, added Voith Fabrics' Tholen. "A veritable cornucopia of contaminants precipitate as the wet sheet of paper is brought up to evaporation temperatures," he said. "The papermaker can reduce the effects of contamination by choosing the right dryer fabric. Is it contaminant release? What are the physical properties of the fabric that might attract and retain contaminants? After selecting the proper dryer fabric, mills can obtain additional help by adding a cleaning shower."
Albany International's Scott Carson agreed. "Typically, the most challenging types of contamination occur when 100% recycle brown furnishes are first heated above 75[degrees]C. The right approach to prevent operational problems is to take a total system view by combining proper fabric design, high-pressure showering, chemical cleaning, or all these factors. It is critical to use contamination release dryer fabrics that incorporate a release additive in the filaments to prevent strong adhesion of contaminants." Carson recommends dryer fabrics with both flat machine direction and flat crossdirection yarns to reduce the open area between yarn crossovers where contaminants can become trapped.
While no dryer fabric is truly "contaminant resistant," fabric design can improve the management of these contaminants, according to the AstenJohnson Dryer Specialists Team. Low void volume dryer fabrics made of all monofilament construction--no spun or cabled yarns to trap contaminants--provide the best results. Fabrics made with components containing contaminant release additives or surfaces structured to prevent contaminant particles from becoming large provide additional benefit, they said.
LIVE LONG AND PROSPER
Some mills experience extended dryer section life beyond traditional norms by relying on fabric properties and operating techniques that prevent damage. "Damage has a range of root causes, but generally comes down to properly maintaining auxiliary equipment such as guide, stretch, and tensioning mechanisms," said Carson. "Machine alignment is also important to ensure steady-state, predictable performance."
"Fabric design can also have a key impact on life," he continued. "Seams should use 100% of the machine direction filaments as load-bearing members. This adds strength and durability at the weakest point in the fabric. If hydrolysis--moist heat degradation--is a problem, temperature resistant designs can be used," Carson said.
"Depending on the paper grade and position application, we can extend fabric life through the selection of polymer type, yarn structure, and fabric structure," noted Tholen. "For a high-humidity application that degrades conventional PET yarns via hydrolysis or dry-heat degradation, we can substitute yarns that are resistant or even impervious to conventional hydrolytic conditions." Because these super-resistant yarns are expensive, "blended fabrics" with high-temperature yarns alternating in the machine direction or only on the edge help balance costs.
According to Weavexx's Cole, improvements in monofilaments have paved the way for extended life so that fabric at many positions routinely operate a year or more. "Historically, hydrolysis has been the major reason for early removal, but modern polymers have resolved much of that problem," he said. "It is imperative that proper maintenance be done to ensure roll and dryer can surfaces are not corroded or pitted, and that guide systems work properly to prevent edge damage."
Further developments in dryer fabrics may be slowed by the commercial challenges inherent in today's difficult market environment, according to Cole. "Due to the long lives, inventory carry, and low market pricing issues, serious future dryer fabric development may not resume until the ROI recovers some-what."
Mills should avoid focusing exclusively on fabric life if savings are far outweighed by losses in machine runnability, Carson noted. "For example, if there is no cleaning system, fabric permeability can drop, possibly resulting in lower drying rates, lower productivity, and higher costs. There have been cases where a mill saved more money by running its dryer fabrics at half their normal life."
The cornerstone of any effort to extend fabric life is the supplier to mill relationship, according to Jim Heaton, dryer product manager for AstenJohnson. "It is important for the supplier to have a complete understanding of the papermaker's objectives and of the machine conditions that are limiting life," he said. "For instance, smooth-surfaced polyester fabrics with low profile seams counter abrasion problems, while oil degradation on the edges may call for a fabric with edges of a different material from the body of the fabric. When the papermaker and supplier work closely together, fabric design can be optimized to meet the needs of a particular position," he concluded.
* Make the most of shutdowns with machine clothing checks," by Janice Bottiglieri, Solutions! August 2003, page 35.
* TAPPI Virtual Seminar-Paper Machine Troubleshooting: Tools to Maximize Productivity and Runnability. Learn more at http://www.tappi.org/index.asp?pi d=27484.
* For a listing of TAPPI technical papers about dryer fabrics, go to www.tappi.org and search for "dryer fabrics" in All Technical Papers.
RELATED ARTICLE: IN THIS ARTICLE, YOU WILL LEARN:
* The critical issues in fabric design that affect machine runnability.
* Whether higher tensions really improve drying capacity.
* How to extend the life of your dryer fabrics.
JANICE BOTTIGLIERI, Senior Editor
About the author: Janice Bottiglieri is senior editor of Solutions! and editor of TAPPI JOURNAL. Contact her at email@example.com.
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|Title Annotation:||Machine Clothing|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Nov 1, 2003|
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