Shoe presses: dressing for success: to get all the benefits of new shoe presses, choose machine clothing that really "fits.".

Shoe presses have grown ever more popular since their introduction in the early 1980s, and papermakers are using them to produce more grades than ever before. While shoe presses were originally used mainly on machines framing woodfree grades, high-speed publication grades now represent the fastest-growing sector for shoe press installations. Virtually every new paper machine and press rebuild feature modern shoe press technology.

With longer press dwelltime, lower flow velocities, increased nip length, and lower maximum pressures (but higher linear three) compared to conventional roll presses, shoe presses significantly change the physics of pressing and drying. Benefits can include increased runnability and improved dryness--if mills know how adapt to the technology. That's where machine clothing can really make a difference. Clothing chosen to enhance shoe press performance can help mills maintain the stringent standards for sheet characteristics and high machine speed needed for profitability. Also, this clothing must work effectively with shoe press sleeves/belts. (Referring to the shoe press application, different manufacturers use the terms sleeve and belt to refer to the same device. The terms are used interchangeably in this article.)

It sounds promising, but for mills unaccustomed to the unique physics of shoe presses and their related press sleeves, putting the whole system together can seem daunting. Solutions! magazine contacted a few of the experts: suppliers experienced in providing working answers to papermakers' questions about what type of clothing really "fits" with a shoe press.

OPTIONS, OPTIONS

A great place to start is for papermakers to clearly define their desired outcomes. "In general, mill personnel should identify the priority and acceptable ranges of specific paper properties most impacted by the addition of a shoe press--for instance, bulk, surface properties (particularly smoothness), and marking," said Keith Kemp, press sales/service representative for Asten-Johnson, Charleston, South Carolina, LISA. "Then they should select belt and felt designs that can minimize the impact on these characteristics while maximizing dryness out of the press and maintaining life."

Kemp said that a natural first priority is to maximize dry content, since that's the reason most mills choose a shoe press in the first place. "We spend a lot of time fighting lower bulk or a rougher sheet, or learning the tricks for setting up draws to get a dryer product out of the press. While it is understandable that a shoe will give less bulk when making a dryer sheet, we tend to overestimate how wet the sheet will get and how much bulk we can get back by simply reducing shoe pressure," he advised. "Also, we usually forget that almost every shoe rebuild includes some form of single-tiering or blowbox work or both, which is one of the biggest changes to the operator."

Operators new to shoe presses must learn to re-think press capabilities. "The new opportunity offered by a shoe press is a tremendous increase in nip residence time. This means some of the limitations of traditional roll presses no longer apply," said AstenJohnson's Daniel Hedou, press product business leader. "For printing grades and lighter basis weights, the main mechanism for increasing water removal efficiency on roll presses was nip intensity. On shoe presses, the intensity is lower, which means we are not as limited by compaction and permeability as we used to be." Mills can run more closed fabrics to take advantage of increased pressure uniformity, due to finer weaves and belts, with less risk of compaction as a limiting factor.

Shoe presses can make finer fabrics an option even on heavier packaging grades, said Hedou. "Longer dwell times mean lower nip hydraulic pressures, thus less chance of crushing or hydraulically reducing the hydrogen bonding so critical to meeting quality requirements. This also allows papermakers the luxury of using finer press fabric structures to achieve improved sheet properties without compromising physical strength tests."

The variety of installations possible only adds to the factors each mill must consider. "Shoe presses can be found in a variety of press configurations--single felted open draw, three nips configuration, double felted with single or tandem shoe presses, to name a few," said Eric Arseneault, technical manager, graphic papers North America for Voith Fabrics, Raleigh, North Carolina, USA. "Each of these press arrangements has its own identity and will require specific methodology (sleeve, fabric, loading, conditioning, etc.) to optimize its efficiency."

Grade is another important consideration, Arseneault added--perhaps the most important of all. "For example, on woodfree grades, papermakers need a press fabric that will provide sufficient void volume and allow adequate construction rigidity to prevent excessive nip dewatering. Unbalanced dewatering on specific grades can lead to important sheet quality issues, such as two sidedness."

The emergence of shoe press systems has prompted press fabric manufacturers to re-think the foil owing principles of traditional press system design, said Arseneault:

* Surface properties/pressure uniformity

* Void volume/permeability

* Resilience/compressibility

* Start-up characteristics/dewatering balance

"Although these elements have always been very important in traditional roll pressing applications, an understanding of their interdependence and how they relate to shoe press sleeve properties is vital."

A QUANTUM LEAP 

Shoe press installations often mean fewer nips, calling for what Hedou calls "a quantum leap" in fabric uniformity. "Consecutive roll presses offered the benefit of compensating for previous presses' non-uniformity. Now, there may be fewer nips in the press which means each press must support the sheet more uniformly, not relying on the next one to press out wet sheet areas. This is particularly true in high speed machines that now operate with only two or even one nip."

Peter Slater, director of product application, process belts for Albany International, Albany, New York USA, agrees that the new technology requires a new approach to both clothing selection and some operating procedures. He recommends a "system" approach, optimizing press performance by designing the press fabric and shoe press belt to complement one another.

"The traditional function of the press fabric providing all the water handling as well as uniform pressure uniformity will result in a reduction in efficiency," said Slater. "Shifting the void volume function to the press belt allows the press fabric to optimize the pressure uniformity function and the belt to enhance efficient nip dewatering. The properly designed fabric/belt combination optimizes both dewatering and sheet properties."

Voith Fabric's Arseneault points out that a "system" approach is a definite necessity. "The process of designing and adapting press fabrics for shoe press systems becomes a multi-parameter equation," he said (see Figure 1.) "The building blocks of the system include the shoe press, the press sleeve and the press fabric. An in-depth understanding of each element and their interaction will lead to higher pressing efficiency and paper quality.

[FIGURE 1 OMITTED]

"The press sleeve is one of the fundamental differences between shoe pressing and roll pressing," he continued. "Sleeve properties can vary considerably under different operating ,conditions and also between manufacturers--such as whether they are grooved, blind drilled or smooth; their void volume; the open area and land area; and their hardness." Variances in loading conditions can cause even the same sleeve style to behave differently, due to the compressibility of the sleeve and the press fabric.

For shoe presses, said Slater, grooved belts are a good option. "Fifty percent of all belts used on shoe presses are now grooved. New manufacturing advances have allowed grooved belt void volume to be increased to 500 cc/sqm (compared with previously around 250 cc/sqm in older designs) and new urethane technology allows for longer life and void retention in grooved belts today.

"The save-all pans showers, and vacuum system also play a key role in the fabric selection," Slater continued. "The ability to handle water in the nip will determine how the void requirement is split between the press fabric and the belt. The lower peak pressure of the shoe press reduces the compression of the press fabric in the nip. This lower compression or 'working' of the fabric, which helps to keep fabrics clean in a roll press, is less effective in keeping the fabrics clean [with a shoe press]. Cleanliness is therefore more heavily dependent on proper design and the effectiveness of the shower and vacuum system."

While it is clear that water removal is the main issue, void volume may not be the top priority for every mill, according to Billy Summer. applications manager--pressing for Weavexx, Wake Forest, North Carolina, USA. "In high speed operations, felt compressibility and resiliency provide water removal performance and maintain that performance throughout life. Factors such as press fabric void volume and permeability become less significant in providing optimal water removal performance," he said. "The impact of press fabrics on sheet surface quality and profile uniformity become more significant. In high speed, closed draw arrangements, sheet rewet becomes a limiting factor in realizing the full water removal benefit of a shoe press operation, especially on lighter weight grades."

The felt's surface characteristics hold the key to controlling rewet, noted Arseneault. "Surface characteristics of modern felt constructions, which affect pressing uniformity, are constantly being enhanced and will likely be revolutionized over the next few years. Companies are investing significant R&D resources to understand and improve components known to affect start up and dewatering performance of fabrics used in shoe pressing applications."

Yet in many cases, said Arseneault, the operating methods of press fabrics designed for shoe press applications are not that different from the guidelines that apply to fabrics used in conventional roll pressing "Assuring optimum operation of the conditioning and cleaning system will reduce the potential risk of the fabric filling-in unevenly, which can cause sheet moisture profile issues," he said. "The conditioning and cleaning system should also be regularly monitored to avoid extreme running conditions. Poor suction box lubrication or very high hp shower pressures can deteriorate the surface characteristics of the fabric."

He also noted that, while the higher loading conditions of a shoe press may sometimes provide gains in sheet solids, "it can also adversely affect sheet quality and machine runnability. Selected fabric and sleeve styles are primary inputs in reaching optimal loading conditions."

TRACKING THE TRENDS

Advancements have continued as mills and suppliers work together to understand the mechanical relationships between shoe presses and machine clothing. Two industry trends in particular will fuel further advancements, predicts Bill Maddock, senior development engineer, press fabrics for Albany International. "The first is the demand across all grades for continued improvements in the printing properties of the sheet." he said. "The second is the growth of no-draw, straight-through shoe press sections, where the sheet does not come in contact with a smooth roll, but relies instead on the fabrics to provide smoothness, good two-sidedness and sheet transfer."

To address the quality challenge, structures featuring improved pressure uniformity will continue to be developed to provide both improved drying as well as better sheet quality. Novel methods such as non-woven components and polymeric membranes providing low flow resistance in conjunction with improved uniformity will become increasingly important, according to Maddock. "These structures will be developed for use closer to the sheet surface, and will replace some of the traditional bait structure," he said. "Taking this concept to its extreme, the use of completely non-porous belts in conventional press sections will become widespread. These belts provide no de-watering capacity ill the press nip but improve sheet smoothness by 20% to 30% in most cases."

Maddock added that, in no-draw presses, transfer belts are required to provide easy sheet release into the dryer section, even moisture profile and low rewet. "This technology is already proven but the next generation of belts under development will continue to enhance machine performance," he said. "Surface enhancements achieved through new materials and processes will provide optimum dryness while maintaining good sheet transfer characteristics."

AstenJohnson's Keith Kemp agrees that new materials will be key to continued refinements. "The use of advanced products in felt construction, both with and in place of traditional batts and bases, will allow papermakers to run a multi-layered product that will provide the surface and support structure needed to optimize sheet properties and allow maximum dewatering and sheet handling," he said.

An emphasis on sheet quality, fewer and preservation of functional properties will define further developments, said Billy Summer, Weavexx. "Suppliers will introduce new techniques and structures to inhibit the migration of water from the press fabric to the sheet at the transfer points. The development of techniques for enhancing surface planarity of the press fabric will take press fabrics to new levels," Summer said.

In addition, improvements in seamed fabric technology--including the use of more rigid materials--are likely to increase their use in shoe press applications, offering papermakers the same performance and safety benefits that seamed fabrics bring to conventional roll presses.

Bob Crook, product development manager for Voith Fabrics, predicts that the woven base fabric, as papermakers know it today, will be "extinct" within five years. "Its replacement will come from a range of new manufacturing processes that will laminate individual components--an array of yarns, an elastomeric membrane, an on-machine seam module, and/or a tri-axial filament composite fabric--into a complex finished press fabric that will function better in terms of uniform CD profiles, pressure uniformity to the sheet, inter- and intra-piece consistency, and 'steady-state' performance from startup to scheduled removal," he said.

BUILDING A BASE

The successful shoe press installations of recent years will ensure this technology's popularity into the future. Mills hoping to mirror that success, but lacking shoe press experience, should consider every resource for information, and make clothing decisions/accordingly.

As Crook noted, "the in-depth knowledge and synergy gained from sharing and developing core pressing technology with a machine builder, sleeve/roll cover provider, and press fabric supplier will provide unique insights to all three, and allow use of the new structures and materials to their utmost advantage."

About the author: Janice Bottiglieri is managing editor of Solutions! You may address email to: [e-mail]