Not your father's press section.

For papermakers, change is a "pressing" concern: emerging technologies guarantee that tomorrow's press section will be considerably different than today's. Solutions! asked a few industry experts for their thoughts on what's happening in this dynamic area of the machine, and here are their answers--"hot off the presses," of course.

RADAR BLIP: SINGLE-NIP

Since the first installation of a single-nip shoe press (in 2000, on PM5 at the mill in Johnsonburg, Pennsylvania, USA owned at the time by Willamette Industries) the industry has been keeping its collective eye on the promising new technology. Since that time, two other installations have been made--a second by Metso, which did the Johnsonburg rebuild, and one by Voith--and sources report that others are scheduled.

"The single-nip shoe press is here to stay," said John Norman, application manager-packaging and Billy Summer, application manager-printing and writing for Weavexx, Youngsville, North Carolina, USA. "While installations to date have been primarily for machines making copy paper, the single-nip shoe press will not be limited to this market. With its lower water load content and its price-driven characteristics, the mechanical pulp grade market would be an attractive option for the single-nip press arrangement. Pilot machine development will pave the way in the near future for this market."

Norman and Summer note that existing single-nip installations can achieve post press solids at a level that competes with multi-nip press arrangements that have a shoe press. "There are conventional shoe press machines making packaging grades today that can achieve 50% press solids out of a 1st press shoe arrangement. The single-nip concept, however, still requires some development if the concept is to expand into these grades. With extremely high water loads and flow controlled dewatering requirements, current single nip arrangements are less viable until further developments are achieved."

[ILLUSTRATION OMITTED]

Georg Kleiser, product manager-pressing for Voith Paper, Heidenheim, Germany, predicts that his company's Single NipcoFlex technology will, to some extent, replace the Tandem NipcoFlex technology in the production of woodfree uncoated paper. "Dryness values of 54% at a machine speed of 1450 m/min were achieved with this technology. The typical application for such grades will be rebuilds and new machines with moderate machine speed (up to 1500 m/min); a prerequisite is good dewaterability of the furnish," Kleiser said. "Woodfree coated paper demands on surface quality are a limitation for the Single NipcoFlex technology. However, with further development of fabric designs, the application in this area will become feasible."

Currently, the most promising applications for single nip shoe presses are on grades for which two-sided smoothness is important, and in reducing draw, said Peter Slater, sales and application director, process belts for Albany International, Albany, New York, USA. "So far, single nip shoe presses are not really replacing tandem shoe press applications; they are a complimentary press option, applied to date on mid-weight range machines. On lighter weights or on profile sensitive grades, dual press applications with a transfer belt in the second press are favored, due to profile and dryness considerations."

According to Frank Swietlik, senior product manager for Metso Paper, Norcross, Georgia, USA, the basic advantages in a single nip press are its simplicity and lack of an open draw. While Metso has not yet replaced any tandem presses with a single nip press, ongoing pilot trials are expected to expand the use of single nip presses. "Pilot machine data as well as data in commercial installations are limiting the application of the single nip press to uncoated wood free sheets. The limitation is related to sheet quality, mainly sheet roughness and CD moisture profiles. In addition, felt technology is improving to aid in solving some of the quality issues."

Because machine clothing plays such a vital role in press section performance, it is a critical consideration for mills exploring single-nip shoe press technology. As press section machinery evolves, press fabric design will need to keep pace for installations to achieve success. "Having only one nip puts a significant demand on the press fabric to provide not only the required press efficiencies but also uniform moisture profiles through life. Proper conditioning of the press clothing is essential to consistent performance," said Norman and Summer.

Does the single nip shoe press have the potential to work in any grade that a tandem can work? Yes--including heavier weight packaging grades, said Keith Kemp, shoe press business manager, AstenJohnson, which currently supplies every single nip shoe press in operation. The company has compared the dewatering capabilities of single nip shoe press configurations versus comparable tandem presses, and has performed simulations and other research as well. "Tandems continue to have some benefits such as longer fabric lives, more tolerance to clothing design and operational flexibility; but the additional costs and space requirements will dictate that most new machines and rebuilds will be single shoes," Kemp predicted. "The only real limitation to single shoes at this time is grades that require the sheet surface texture that has only been achievable with a single felted hard nip. With continued equipment and fabric technology development, we believe this hurdle will also be overcome."

[ILLUSTRATION OMITTED]

Metso's Swietlik agreed. "To date, there have been several single nip shoe presses installed in commercial applications and one common area of challenge has been the press felts copying the felt surface topography to the paper," he said. "Felts originally developed for standard roll or multi-nip shoe presses do not give the sheet surface characteristics, felt life or rewet performance required for high quality printing and writing papers when used in a single nip shoe press. Felt suppliers have recently developed more suitable felt designs for single nip application."

ON TRACK: MULTI-AXIAL

Single-nip shoe presses aren't the only thing fostering continued development of new press fabric technologies. One of the hottest developments in the press fabric arena has been the emerging popularity of multi-axial fabrics.

"The multi-axial felt concept bypasses conventional weave looms by producing base cloths on narrow, high speed looms; these base cloths are subsequently spiraled at a low angle to form the wider base fabrics required for paper machines," explains Eric Diamond, technical director, pressing, Weavexx. "This concept offers some of the advantages of the traditional lamination process at a lower manufacturing cost but has shown significant limitations in hysteresis and dimensional stability." Diamond added that his company's research favors full-width manufacturing techniques as the ideal method of producing base fabrics; other companies have taken a different approach.

"The development of multi-axial technology has been one of our top priorities," reported AstenJohnson's Keith Kemp. "By using this modern platform, we make a more consistent structure with materials that would not have been possible on traditional looms, Additionally, stronger product stability has made it possible to meet demands that didn't even exist five years ago, such as for single nip shoes, and has also allowed us to incorporate surfaces and engineer layered densities that would not have worked in the past." He noted that no existing single shoe press applications run with standard loom bases.

As the latest step in the evolution of laminated technology, multi-axial fabrics are designed to answer a host of papermaker demands including enhanced water removal, energy efficiency, economy of operation and sheet quality, says Mike Riffle, application specialist-graphics for Voith Fabrics. Riffle said that Voith Fabrics pioneered non-woven axial systems in 1994; its recent tri-axial Vector line represents more than 55% of the company's total press fabric production.

"The components used in this type of design results in a structure that offers many benefits," explained Riffle. "Fine top base structures allow for maximum pressing uniformity; the underlying base structures are built with water handling as a goal. With this concept, the fabric supplier is able to adjust the batt-to-base ratio for optimal performance. Uniform pressing, high void volume, compaction resistance and steady state permeability promote good sheet finish and high dewatering from day one until time of removal."

To create its multi-axial products, Voith Fabrics uses a system of longitudinally arranged yarn networks oriented in multiple directions, rather than weaving a base system composed of machine direction and cross machine direction yarns (see Figure 1). "The advantage of this multi-axial type of structure is that the yarn systems can be tailored to either meet the demands of a 'fine' sheet side pressing surface or provide the void space necessary for high water removal," said Riffle.

He also noted that seamed fabric technology, which has been in use for more than two decades, has seen important developments as well. "The early seams had a place on older machines where installation was difficult. Evolutions in seamed technology have enabled seamed designs to be run on some of the highest speed machines here and in Europe."

BATCH VS. CONTINUOUS CLEANING

Everyone agrees that proper cleaning is vital to press fabric performance and longevity--but when it comes to performance and economics, which is best: batch cleaning or continuous cleaning? According to each of our experts, the clean winner is ... both.

"A combination of both continuous and batch cleaning is considered the optimal solution for effective press fabric cleaning," said Weavexx's Norman and Summer. "Careful monitoring of the operating characteristics of the press fabric is the best guide as to the level and frequency of the cleaning required.

"The most efficient and cost effect cleaning procedure remains the continuous mechanical conditioning; this process uses high pressure showers with clean water at stock temperature and pH to dislodge contaminants and remove them through displacement over suction zones. TAPPI recommendations are available for dewatering times, optimal pressures and angles and required vacuum levels." (Editor's note: See Additional Resources box at beginning of this article.)

Careful monitoring of effective cleaning, both mechanical and chemical, is also important, they noted. "It is insufficient to rely solely on the water perm tests to determine effective cleaning and/or the requirements for additional cleaning. Water loads as measured by traditional scan pros, caliper measurements, press consistencies and most importantly total water flow out of the press and suction zones are other factors that must be monitored."

Batch washing helps papermakers target specific contaminants with chemicals that might not be compatible with the paper making chemistry, they noted. "An effective batch cleaning process will eliminate an important quantity of the contaminants. This can be an effective method to prolong the life of press fabrics; this will however not mechanically alter the hysteresis of the press fabric. Batch cleaning should always be used in combination with a continuous mechanical cleaning program and, if necessary, a continuous chemical cleaning program."

David Buchanan, business development manager-board and packaging, Voith Fabrics, agrees that a mill-specific combination of batch and continuous cleaning will work best--but that any cleaning system must be properly maintained to be efficient. "With cost being the one of the biggest drivers in today's mill environment, cleaning of press fabrics can be an area where savvy managers will look to cut out cost of chemicals. The existing fabric cleaning equipment can be very effective in achieving the desired results in the press section. The proper location and angle of the cleaning showers are critical to the performance of such conditioning tools.

"Another often overlooked factor in effectiveness of normal conditioning equipment is temperature," he continued. "The most effective traditional cleaning method is to maintain shower temperature that is 10 degrees hotter than the stock entering the press. This hot water cleaning is very effective in removing most contaminants from press clothing," said Buchanan. He noted that temperature control plus proper distance from, and angle to, the fabric provide very effective cleaning--and is cost effective as well.

"Each machine, grade, system chemistry, and process systems are unique and every factor must be taken into account," said AstenJohnson's Keith Kemp. "There is no doubt that batch applications applied from the inside of the fabric as well as prior to a fabric roll and the uhle boxes is the most cost effective; however, the step change to fabric permeabilities is sometimes significant to operational factors, and most machines are unable to do this on the run. For those than can handle it on the run, even this method loses its effectiveness if the water is removed in the nip and not at the uhle boxes. Continuous cleaning, especially applied with the high pressure showers, can be effective in heavy nip dewatering applications, but it is not as efficient in uhle dewatering applications and typically costs the most to administer. Continuous, whether by needle showers or a dedicated fan shower, does have the benefit of a smoother impact. This leads most paper mills to utilize some combination of the two. This has the potential to balance the costs and smooth out the bumps of the batch applications."

IN THIS ARTICLE YOU WILL LEARN:

* Whether single-nip shoe presses are replacing tandems.

* How multi-axial press fabrics are designed.

* Which is better: continuous or batch cleaning.

ADDITIONAL RESOURCES:

* "Press section fabric selection and performance improvement," D. Reese, Solutions! October 2004. To access this article, enter Product Code 04OCTSO42 into the search field on www.tappi.org.

* TAPPI standards and technical information papers (TIPs) dealing with the press section are available at a discount to TAPPI members--visit www.tappi.org and click "Standards" in the yellow menu box.

JAN BOTTIGLIERI, SENIOR EDITOR

ABOUT THE AUTHOR

Janice Bottiglieri is senior editor for Solutions! and editor of TAPPI JOURNAL. Jan can be reached at [e-mail] or by phone at [phone].

[ILLUSTRATION OMITTED]