With changing technology, growing customer demands, and increasing supplier consolidation, mills now face a variety of challenges relating to paper machine clothing. To determine the clothing issues that currently concern mills, Pulp & Paper asked papermakers from a variety of mills to submit anonymous questions that they would like answered by clothing suppliers. These papermakers came from mills that produce grades ranging from linerboard to uncoated freesheet, which is reflected in the questions submitted.
Many of the questions were associated with clothing design, especially as it relates to new technology such as the shoe press. In addition, the papermakers were curious about operating parameters that would allow optimum performance and paper characteristics. Still others were concerned about the impacts of service levels and supplier consolidation on pricing and product availability. These questions were randomly assigned to clothing suppliers, who provided the following answers.
What direction is the design for Fourdrinier fabric or wet press fabric taking to achieve smoothness and good visual appearance, while still providing the required fiber support?
Response: To achieve improved smoothness while maintaining the other necessary properties of forming fabrics (drainage, stability), the trend has been towards finer fabrics with increased numbers of sheet side yarns. Newer, finer mesh triple-layer fabric designs offer one way to achieve higher support point density. With more support points on the sheet side of the fabric, less indentation of the fiber mat on the forming fabric takes place, yielding a smoother surface.
Controlled drainage to limit fiber mat indentation and improve fines retention is a key parameter in achieving the desired smoothness, and much work is being done to balance void volume with required drainage. One result of this work is a family of forming fabrics having air permeability in the 150 cfm to 250 cfm range, which is normally considered low for forming fabrics in the U.S. Of course, each application must be looked at individually, since there are many other factors to consider in achieving the desired outcome.
Improving sheet smoothness from the press felt standpoint requires a coordinated engineering approach for both the base and batt construction. First, we optimize the base by maximizing pressure distribution and contact points with micro-fine monofilaments or our exclusive formed (flat pressing surface) composites structures. These steps can improve the overall available contact area of the base structure by approximately 50% compared to conventional woven textile structures, while still maintaining excellent water handling capabilities, resiliency, and low flow resistance.
Fine surface batt, such as three denier fiber, is well known to increase surface contact area at the sheet interface. Unfortunately, these small diameter fibers are also very weak, and tend to fill rapidly. New approaches, such as using flat shaped fiber rather than round fibers, are showing advantages with some grades for improving smoothness. Our proprietary new surface treatment may be able to further increase micro-scale surface contact while providing wear and filling resistance. Finally, for suitable applications, we will soon be able to offer a belt-like surface with a smoothness comparable to a plain rubber roll.
–Bob Crook, press fabrics product development manager, and John Craft, forming fabrics technical manager, Voith Fabrics
How can you determine the vacuum level on the uhle boxes for a certain wet felt cfm in order to achieve optimum water removal?
Response: Optimum water removal has to be measured by the monitoring of both nip and uhle box dewatering levels. There are several companies that provide hardware and software to measure, monitor, and control press section dewatering for optimum machine performance.
Uhle box dewatering strategy is not a simple topic, and it is one that the industry has struggled with for many years. Surprisingly, the predictive work for uhle box dewatering goes back into the 1970s and has been updated and confirmed in the early 1990s to include machine speeds over 6,000 fpm. For constant volume systems (or, liquid ring pumps), DeCrosta1 described the relevant conditions as follows:
1) Moisture ratio before uhle box
(lb water/lb fabric)
2) Moisture ratio after uhle box
(lb water/lb fabric)
3) Machine speed (effect on uhle box dwell time)
4) Total slot width
5) The acfm/in. of machine width, airflow from the vacuum pump
6) New fabric air permeability
(cfm/ft2, at 1/2 in. water)
The result of these interactions is usually described by a table of values of estimated air flow required to remove shower water, or to reduce fabric moisture ratio from common ingoing to common outgoing levels. Companies that are engaged in designing press section vacuum dewatering systems are very familiar with these tables.
Naturally, every input to the press section that has a potential to change any or all of the conditions above will have an effect on uhle box dewatering. Shower water addition rates are the easiest to control and optimize for best overall performance–fabric cleanability and overall fabric dewatering.
Please note that constant vacuum systems require an entirely different set of control parameters and differ in key inputs and effects.
–Steve Sassaman, vice president & general manager, U.S. Press Fabrics, Albany International Corp.
Response: Optimum water removal with a wet press fabric is achieved by having a dwell time across the uhle box slot in the range of 2 milliseconds to 4 milliseconds and an air flow through the felt sufficient to remove the water from the felt.
–Pieter S. Diehl, vice president of technical design, Orr Felt Co.
Felts are not only loading water, but are increasingly involved in power transmission. In some shoe press designs, over 700 hp is transmitted through the felts. What are the limits and how can they be overcome?
Response: In today’s most modern shoe press applications, press fabrics are used in closed draw configurations that involve multiple nips, multiple drive points, and transmission of drive to non-driven press section elements.
Paper machine clothing used in shoe press applications must provide acceptable sheet properties while sustaining the rigors of very destructive forces (tensile, compression, hydraulic, thermal, and frictional forces). The unit of work stated in the question — horsepower–estimates shoe press fabric operational work. (The general definition of horsepower, which is hp=550 (ft-lb/sec), is made up of a mass component, displacement component, and time.)
The limits of the ability for press fabrics to meet the requirements of water removal, drive transmission, and durability are reflected by the materials and the structures used in press fabric design. The main properties that quantify press fabrics "fit for use" in shoe press applications fall in the following areas:
1.) Operating fabric mass (ounce weight)
2.) Usable in-nip void volume
3.) Flow resistance (water transfer rate from sheet to fabric or fabrics)
The above criteria assumes that the fabric’s structure well exceeds the tensile requirements for a press, which is generally greater than or equal to 500 pli, and that the felt structure delivers acceptable sheet qualities and runnability (sheet control, dimensional stability, etc.).
The ideal shoe press fabric would embody minimal mass, maximized in-nip void volume, and minimized flow resistance to water (in-nip water removal) and air (at uhle box dewatering). To meet increasing demands for sheet print quality, press fabric designs have become more massive and dense to maximize sheet pressure uniformity. These fabric designs subsequently sacrifice usable void volume and flow resistance, which ultimately reduce the effective fabric life at the press.
Today, materials technology and newer design concepts (contoured or flat fibers and monofilaments, and engineered or matrix fabrics, and laminated base fabric structures) offer new approaches to reduced weight press fabric designs while maintaining (or even enhancing) void volume and flow resistance. Faster felt break-in time, compaction resistance, water removal capacity, fiber retention, bulk improvement, smoothness, and printability enhancements are but a few of the attributes associated with this new generation of paper machine clothing. Fabrics with these attributes are offered by some suppliers in both endless and seamed designs–each presenting its own set of specific benefits and clothing manufacturing challenges.
–Joe DiCesare, product applications for press fabrics, AstenJohnson Corp.
Response: We have supplied endless designs on many shoe presses and don’t feel we have reached the limits of power transmission yet. However, through the use of multiple layer felts, I feel sure we can keep abreast of future demands.
–Pieter S. Diehl, vice president of technical design, Orr Felt Co.
How do suppliers design dryer felts for minimum sheet shrinkage?
Response: While restrained drying is primarily achieved with unirun or single-tier dryer section configurations, dryer fabric design also plays a role. The primary function of a dryer fabric is to maintain and/or improve the sheet’s contact with the dryer. The dryer-to-sheet contact is often related to the contact between sheet and dryer fabric. More specifically, the greater the contact between sheet and dryer fabric, the greater the contact between sheet and dryer can. Certainly, this contact is improved as dryer fabric tension is increased in the section. Most single-tier and many unirun sections are operated at very high tensions (often approaching 20 pli to 25 pli). Materials selection, weave structure, seam type, and fabric finishing procedures are all selected and engineered to meet the rigors of elevated operating tensions.
The greatest single dryer fabric contribution to restrained drying (or minimized shrinkage) is the contact area provided by the dryer fabric. Contact area is determined by the weave structure and the geometries of the machine direction (MD) and cross machine direction (CMD) components utilized to construct the dryer fabrics.
Today, most dryer fabrics are constructed using "shaped" MD yarns. These flat yarns are rectangular in cross section, and can even be described as "ribbon-like" in some cases. The flat yarn surfaces provide a degree of contact superior to the contact provided by round MD monofilament yarns that were previously used to construct dryer fabrics. The contact is further increased utilizing the weave structure now known widely as "warp float" (Figure 1). The warp float structures present a knuckle-free sheet-side surface wherein the MD yarn extends over several CMD yarns. This smooth surface almost doubles the sheet-to-fabric contact available with the standard weave structures having the knuckled surface.
Two recent dryer fabric developments have expanded the contact provided by the warp float structure. Use of hollow CMD monofilaments allows for an elongated, "flattened" float. This enhanced float, in turn, provides greater contact. The second development includes a dryer fabric structure that employs very wide MD yarns woven in a stacked pairing. The resulting surface provides contact area almost twice that of the warp float weave structures.
FIGURE 1: The warp float weave structures shown in these AstenJohnson fabric designs present a knuckle-free sheet-side surface and high sheet-to-fabric contact. Due to ease of cleaning, they are especially applicable for recycled grades.
Both of these developments produce a surface that is near planar in appearance and contact area. These robust designs can also enhance ease of cleaning for high contaminant grades that require high levels of secondary/recycled fiber.
–T. Payton Crosby, product applications for dryer fabrics, AstenJohnson Corp.
Response: Dryer fabric design, as it pertains to sheet shrinkage, centers upon the designer’s knowledge of the following areas: the characteristics of the designated paper grade, the fabric and sheet path as they pertain to the machine builder’s configuration of the dryer section, the drying rate of the section where the design is applied and how it affects the paper grade that is produced, and the type and performance capability of any runnability components installed in the designated section (for example, blow boxes, transfer boxes, or vacuum rolls).
From this information, the designer examines the fabric characteristics that best address the paper grade and machine hardware issues, such as: contact area, frequency and distance between contact points, fabric permeability, and fabric surface characteristics.
Recommending the correct materials for the machine and cross machine direction structural components allows the designer to maximize the amount of sheet restraint applied to the paper sheet as it is dried, while still controlling the drying rate. The designer’s goal is to set the sheet without inducing a fabric mark or edge quality defects so that when it first enters an unrestrained draw area, there will be very little residual shrinkage left in the sheet to narrow the width of the sheet delivered to the reel. The key to obtaining this result is to provide cross machine sheet restraint from the shrink forces induced by the drying of the sheet. Once the sheet is set, there is very little cross machine shrinkage left to narrow the sheet in an open draw.
–DWight Payne, technical manager, Voith Fabrics
Service plays a larger role now in the total product of fabric suppliers. What role does the level of service play in pricing?
Response: Just as service is playing a larger role in the total product offered, it is playing a larger role in pricing. Clothing suppliers must factor in the cost of service when determining their prices. If clothing suppliers’ margins get squeezed by demand for lower pricing from paper companies, the cost of providing service becomes a critical issue in the value/benefit equation for buyer and supplier.
This makes it critical that clothing suppliers fully understand the mill’s requirement for service before preparing pricing programs. Also, clothing suppliers must clearly understand their actual costs for their people, equipment, and travel involved in providing service. Likewise, paper companies need to understand their service requirements and the value of the service when selecting suppliers and negotiating prices.
Service to cover product warranty is part of the base price of fabrics. The cost for routine, diagnostic, and consulting services all need to be factored into pricing programs. The mill’s supplier evaluation process must balance traditional price-based sourcing strategies with product performance, mill service and technical support, and value adding problem solving that meets their end customer’s needs. The evolution to true partnership or alliance-based relationships is gaining momentum as consolidation continues in the paper industry and its supply chain, which spans from forest or curbside to end user marketplace.
–Charlie Abraham, customer service, AstenJohnson Corp.
Response: Service has become a larger and larger part of the total cost of supplying clothing to the paper industry. So far, we have been able to absorb this cost due to the volume of business done at the mill. However, I can see, in the not too distant future, where service costs will have to be shared with the paper mill or where another type of pricing adjustment will have to be made to cover the service cost.
–Pieter S. Diehl, vice president of technical design, Orr Felt Co.
Membrane technology is evolving very rapidly, but what are its limits? Can seamed felts have a membrane layer?
Response: The incorporation of membrane technology, although not new, is still very much in its exploratory stages. However, it is hard to describe some of the present offerings as truly membrane technology. Part of the definition of a membrane is a "thin layer," and some of the present layers have been relatively thick, making it hard to get good total felt integrity and delamination or shedding resistance. Two of the most important properties that a well-engineered membrane can impart to the felt are improved pressure transfer uniformity and reduced rewetting.
If differences in pressure support limit the quality and sheet dryness of the paper, then a membrane like our fiber reinforced polymermatrix can be incorporated in the felt. By nature of its structural formation, this product compensates for the felt base fabric knuckle points and levels the specific load variations that the hard pressure points would otherwise transfer to the sheet.
In addition, the polymermatrix temporarily absorbs the moisture transferred during pressing and prevents its backward migration to the paper sheet on the exit side of the press nip. An added benefit of this fiber reinforced polymermatrix is that its engineered structure balances the elasticity in the membrane to provide excellent pressing, while improving the stability of the felt by controlling MD and CMD stability of the felt base. Most importantly, this polymermatrix, unlike many membranes, adds very little thickness to the felt structure. It can also easily be incorporated into seamed felts.
–Tom Gulya, vice president of technology, Weavexx Inc.
Response: The term "membrane technology" is used for so many different kinds of press felts that it is really loosing its meaning. Some companies are using the term "membrane felt" for totally different kinds of products.
For example, in some cases, the term is used for the felts that have no unique features other than some differently built fiber (batt) lay-out in the felt compared with the normal stratified or straight forward fiber lay-out. On the other hand, the term refers to some special layers in the base structure itself. We have a new felt design that could be called our membrane design because of its special base construction on the paper side. It has been successful in demanding positions, including shoe press applications.
So, combining all of the different kinds of membrane technologies, I would say that there is no real limit for its applications. Also, many different kinds of membrane technologies could be combined with seamed felts.
–Jukka Lehto, product manager/pressing, Tamfelt Corp.
What is the direction of clothing design? For example, is it the design itself (weave), the materials it is made up of, or the coating of the materials, such as where each strand is treated with a coating?
Response: Paper machine clothing has dramatically improved in the last decade, and the improvements have come from many of the items described in the question. The direction for press fabrics in the future will come from providing solutions to meet the needs of our customers, namely:
1) Higher sheet dryness
2) Better sheet surface and less two-sidedness
3) Steady-state pressing behavior
4) Improved durability in an ever-more hostile papermaking environment, including improved resistance to abrasion, compaction, plugging, chemical attack, and high temperature conditions
Each of the preceding solutions has areas of active development. We have emphasized proprietary raw material development that is designed to improve chemical resistance, wear resistance, and to provide enhanced sheet contact. We have also created an entirely new class of fabric structures called multiaxial press fabrics that exhibit a high level of uniformity and steady-state pressing behavior. Our first move into advanced materials technology - using porous polymeric layers — has already produced benefits in sheet smoothness and printability on both white and brown paper grades, along with higher sheet dryness on demanding positions such as shoe presses on fast HSPG machines. We expect many more offerings using advanced materials technology in the near future.
–Steve Sassaman, vice president & general manager, U.S. Press Fabrics, Albany International
Response: The use of triple layer forming fabrics has increased steadily. The biggest advantages of triple layer fabrics have been stable running and longer life compared to double layer or extra weft added fabrics. However, lower dry content of the web after forming section at high speeds (over 4,500 ft/min) is going to limit the use of existing triple layer fabrics on new high-speed paper machines. Therefore, new weaves need to come to the market. These will utilize multi-layer structures for rigidity. Other requirements are improved retention and clean running.
Rigidity of forming fabrics can also be influenced by proper choice of yarn materials. Running tension of forming fabrics will go up when the speed of a paper machine goes up. Forming fabric manufacturers are in search of higher modulus yarns.
Anti-soil forming fabrics have been an issue for 25 years and there is still no real anti-soil yarn material or coating for forming fabrics available. In many cases, the development of the deinking process has solved the major problem of keeping forming fabrics clean. Forming fabric protection chemicals have also been used successfully in some cases. However, there will be new efforts by forming fabric manufacturers to help keep fabrics clean both by using anti-soil yarns and by coating fabrics.
Future needs for press felts. In the future, press felts should be able to operate under conditions even more demanding than today, such as:
• Speeds up to and over 2,000 m/min
• Higher loads in roll presses–up to 250 kN/m
• Higher loads in shoe presses–up to 1,200 kN/m
• Higher amounts of recycled stock (DIP) in all printing paper grades
• Even higher requirements for paper surfaces
To reach good runnability and overall performance, together with required paper quality and long enough felt life, the press felts should be as follows:
• As fine paper side base fabric as possible to pre vent all possible base fabric marking
• As incompressible base fabric as possible to pre vent compaction and to retain required water removal capacity through the felt life
• As fine batt in paper side as required for paper surface quality
• As little batt in the felt as possible to minimize compaction and plugging
Development of dryer section concepts. For tomorrow’s high-speed dryer sections, the key word is runnability. However, we must understand the meaning of the word runnability in a wider sense. It not only refers to the number of breaks in the dryer section, but also to sheet support, space requirements, costs, maintenance, broke handling, drying efficiency, paper quality, tail threading, and more.
In today’s single felted dryer sections, the dryer fabric has an important and versatile job. In order to perform its main role of carrying the web through a dryer group without causing web damage, the fabric, its construction, and its material should meet various challenging demands. The paper side surface of the fabric must be smooth, but still porous, and the fabric must not mark the paper web. It must be capable of supporting the sheet and pressing it properly against the dryer cylinder to give uniform heating of the paper web. The fabric must not carry too much air with it in order to prevent harmful pressure in the fabric nips and to minimize sheet flutter. Also, the permeability of the fabric has to be correct. Permeability that is too low reduces evaporation through the fabric and the supporting vacuums do not work properly. However, a fabric that is too open carries too much air with it.
Even in the dirtiest conditions the fabric should be kept clean. Further, practically no CD shrinkage or MD stretch is allowed in the fabric loop, although the fabric must act as a driver for the freely rotating cylinders and rolls in its group. Moreover, the high temperature and humidity in the hood create very demanding conditions. For instance, in some hoods, the temperature of the air can be more that 300º C.
In the future, paper machine speed will continue to increase and new drying methods will be developed. There will be new tasks and requirements for the dryer fabrics. To increase speed, the aerodynamic properties of the fabric must be enhanced and the speed difference in the unirun group between the top and bottom cylinders will be more critical. Also, because of the dryer fabric driven groups and higher tensions, the fabrics must be more stable. New drying techniques, such as impingement drying, call for more hydrolysis resistant dryer fabrics.
New drying techniques, such as impingement drying, call for more hydrolysis resistant dryer fabrics, such as the one shown here in a finishing process at Tamfelt’s Tampere, Finland, mill.
Also, new, more resistant and more expensive materials must be used, for example PEEK (polyether—etherketone), which has a higher melting temperature than other materials used today, as well as excellent hydrolysis resistance.
Our latest development in the area of hydrolysis resistant dryer fabrics has been designed for demanding impingement dryer sections. It is a PPS monofilament dryer fabric suitable for fast running paper machines where lower fabric permeability is required for runnability reasons.
–Riitta Kilpeläinen, forming fabrics product manager, Jukka Lehto, press fabrics product manager, Juhani Saari, dryer fabrics product manager, Tamfelt Corp.
What are the criteria for determining the cfm, void volume, etc., for the Fourdrinier wires of your paper machine(s)? How do you determine the dryer felt cfm of your paper machine?
Response: The most important criteria for determining the drainage and water handling properties of a forming fabric are paper grade, speed, dewatering mechanism, and cleaning/conditioning equipment. Although cfm and void volume are some common terms used to characterize forming fabric water handling, they tend to be very specific to the type of structure, and all fabrics are not created equal.
For example, 450 cfm in one fabric structure does not mean the same as it does in another fabric structure. It is important to recognize that even a very low cfm fabric can quickly drain all of the water a modern headbox can deliver. In reality, it is how the initial fiber mat forms in the first instant of headbox impingement on the fabric that sets the drainage and sheet characteristics for the rest of the paper machine.
Similarly, the water carrying capacity of the fabric is more influenced by the drainage portal and shape than it is by the absolute void volume of the fabric. Flow characteristics of the fabric are engineered by the design and applications engineer to balance the required sheet properties with the former’s drainage mechanism and the ability to condition and return the fabric to the headbox slice opening just as clean and open as it was in the previous revolution.
Dryers. Dryer felt permeability (cfm) is based on a number of criteria. Most significant to the selection of a dryer fabric is the machine speed, paper grade, air handling/air flow of the section, incoming sheet moisture, and the ability to control sheet flutter in the section at high speeds.
–Tom Gulya, vice president of technology, Weavexx Inc.
Felt design for shoe press technology has so far been limited to existing products. Excluding manufacturing issues, what would the ideal shoe press felt look like for a third press? First press? Single shoe press?
Response: It is unfair to say that today’s shoe felt is "the existing product." As shoe presses, and especially shoe press applications have evolved, the felts for shoe pressing have also had to evolve. If anything, this technical evolution has spilled over to improve the felts used in conventional roll pressing. Typical felt weights for brown paper shoe presses start at 1,700 gsm with three and four layer bases and go up to more than 2,100 gsm.
Negative experience taught that felts for single shoe pressing lightweight sheets, such as newsprint and fine papers, needed to be much lighter in weight and thin, in order to facilitate nip dewatering. This required development of better-laminated felt designs in the 1,200 gsm to 1,700 gsm range. New felt structures had to incorporate lightweight, thin base fabrics with a smoother top cloth, low flow resistance, and significant load hysteresis to facilitate fast nip dewatering, nip impulse resistance, and minimized rewetting.
–Tom Gulya, vice president of technology, Weavexx Inc.
How do suppliers see design capabilities (R&D resources) changing with consolidation of paper machine clothing suppliers? Will clothing choices be limited or will there be more variety available from the remaining suppliers?
Response: The combining of Scapa Paper Machine Clothing/Appleton Mills into the Voith Fabrics Div. of Voith Group has expanded our R&D capabilities, since both companies brought a tradition of product development to the union. Moreover, our Voith Sulzer Paper Technology Div. provides added paper machine and papermaking expertise to our team. The Voith family also has three pilot paper machines available to us for developmental work.
While the existing range of products will be consolidated, for Voith Fabrics’ customers, our enhanced R&D capabilities mean more new products on the way at shorter inception-to-market intervals.
A goal of consolidating any industry is to improve value for the customer and supplier. If these goals are realized, there will be adequate funds available for product development, thus fostering more innovation. Conversely, if reducing the supplier base does not improve returns, R&D is hindered.
–Bill Jeffrey, corporate account manager, Voith Fabrics
Nylon is a strong material, but with low pli and chlorine limitations. Are there any substitutes available on the horizon?
Response: Despite some perceived shortcomings, polyamides (Nylon is DuPont’s trade name) still have significant performance benefits over other common polymers. There is, however, a continuing revolution in materials technology that we expect will lead to another level of breakthrough materials for paper machine clothing. Operating environments like high temperature shoe presses or impulse pressing will require improved raw materials for press fabrics. Through technology partnerships at our research company, we can receive and perform quick prototype testing of new materials, and can consider how to best modify these materials for use in paper machine clothing.
–Steve Sassaman, vice president & general manager. U.S. Press Fabrics, Albany International
REFERENCES
1.) DeCrosta, Edward, "Suction Pipe Felt Conditioning," Albany International Felt Facts, May/June 1975.
