February 2008
A PPI Special Report
Current market conditions in the paper industry are forcing papermakers to continually look for improvements in both sheet quality and machine efficiency, and these objectives often tie in directly with paper machine clothing. Be it a graphics papers producer looking for formation, smoothness, opacity and profile improvement or a board and packaging manufacturer looking for test values at reduced basis weights, sheet quality is paramount in today's competitive marketplace. At the same time, machine efficiencies are critical to profitability. Everyone is looking for ways to make more with less. Improved uptime, reduced wet end breaks and higher couch solids resulting in reduced draws are a few of the benefits papermakers are realizing with the latest in forming fabric technology.
VOITH FABRICS
Finer and thinner forming fabric designs, using smaller diameter monofilaments, have the potential to meet these stringent demands, but the tradeoff is usually wear potential/life and fabric stability. Today's papermaker demands the benefits offered by a finer fabric papermaking surface. However, they cannot accept shorter fabric life or poor sheet profiles that are common with thinner fabrics. In fact, they expect a reduced "cost-in-use" with their paper machine clothing – longer fabric life, improved uptime, reduced breaks and improved sheet profiles.
R&D efforts are leading to new fabric designs that aim to meet all of these demands. The recent development of the warp exchange binding technology, patented by Voith Paper Fabrics, combines a fine papermaking surface needed for sheet properties and retention with a wear surface durable enough to meet demanding paper machine applications. The triple-layer warp exchange design has fabric calipers as much as 15% lower than conventional sheet-supporting binder (SSB) triple-layer designs. Thinner calipers are critical for improved drainage and couch solids, reduced draws, low water carry/rewet, low fiber carry back, reduction of wet end breaks and reduced drive loads.
The warp exchange allows for a broad range of application flexibility. By offering a thinner caliper without the need for a smaller wear weft diameter, the desired benefits are being achieved without sacrificing fabric life. If fabric life improvement is the customer's main objective, Voith can make a warp exchange product with a bigger wear yarn diameter at the same caliper as an SSB triple-layer with a smaller wear yarn. The product can be moved in whatever direction is needed.
The warp exchange technology is available for all paper grades, commercially known as PrintForm EX for graphics papers, MultiForm V and MultiForm EXP for board and packaging applications, and TissueForm V for tissue and towel.
Case study
A newsprint mill in Canada operating a 6.12 m-wide SymFormer at 730 m/min was running a standard double-layer extra support (150 mesh) on its base position. The papermaker's primary objectives in trialing a PrintForm EX were to reduce polymer usage, improve formation and improve machine speeds.
The warp exchange construction of the PrintForm EX provides advantages for improving sheet characteristics over the double-layer extra support design, while reducing fabric caliper with the same bottom wear diameter so fabric life is not sacrificed.
The initial trial PrintForm EX ran extremely well for just over four weeks before being damaged off the machine.
The following improvements were noted:
- A 20% reduction in drainage aid
- Improved first pass retention numbers
- Improved couch solids
- Improved formation numbers
- The ability to maintain or increase machine speed across the grade range.
The customer has re-ordered one PrintForm EX base fabric that is now awaiting installation.
HEIMBACH
In recent times, the forming fabric industry has seen the widespread adoption of SSB technology in all paper grades from tissue to packaging, and in all forming section types. Over half of all forming fabrics delivered in western Europe in 2007 were SSB.
In all forming fabric designs to date, the monofilament in the warp (machine direction: MD) has been made of polyester, which is a hydrophobic polymer - meaning it has little or no affinity with the water molecule - to ensure acceptable dimensional stability characteristics in wet conditions.
The current trend in SSB forming fabric designs is for finer and finer designs (to increase support properties) that require finer and finer monofilaments. Polyester is now at the limits of acceptability in terms of its elastic properties in these fine diameters. The need for a step change in monofilament properties, rather than systematic evolution, has seen new materials be introduced.
Stabilon from Heimbach is a new generation polymer designed to give a 30% increase in the elastic modulus for the same diameter. Heimbach has begun using Stabilon as a standard material in primobond XF.
Case study
The main issue for the customer was when running light grades - coating color penetration through the web at the first coating station was problematic. The topside of the sheet was coated first, so any strike through caused an increase in sheet break frequency and coater downtime because of the higher incidence of wash-ups. The main trial objective was therefore to reduce this strike through.
Immediately after startup, the visual formation was significantly improved, with dryness and machine runnability at normal levels. Heimbach analyzed the sheet quality using its Paper/Print Quality Index (PQI) image analysis technique, and found that sheet uniformity had improved dramatically.
The PQI is a measure of sheet density, attained by capturing light transmitted through a sheet of paper. The results are used to calculate and assign numbers to sheet density uniformity, which is used to predict final print quality for print gloss uniformity, mottle, strike through and set off. Higher PQI and lower floc and void indices indicate better sheet uniformity.
The smoothness of the sheet was also significantly improved. Together with the improvements in sheet uniformity, this should have a positive influence on coater efficiency. The normal PPS value on the topside improved from 2.5-2.6m to 2.3-2.4m.
Lifetime
Heimbach plans to launch a new design in early 2008 that also contains Stabilo as the MD component. The supplier's research indicates that this product will increase fiber support but not at the expense of fabric density, drainage capacity or fabric stability. It expects product lifetime to at least match that of a super fine SSB.
Heimbach has also looked to improve the life potential of its forming fabric range – increasingly more important with the move to finer and finer monofilaments. Some two-three years ago Heimbach introduced Duralon, a monofilament combining polyester's dimensional stability benefits with the benefits of polyamide, namely abrasion resistance.
The polymer was specifically developed for use in forming fabrics. The following case study looks at the application of this material in the packaging sector:
In 2005, the customer embarked on a structured approach to improve the lifetimes in all positions. At that time the standard designs were 16 Shaft 2½ Layers and Triple Weft specifications. Through a systematic engineered approach Heimbach has introduced SSB designs containing Duralon in each position with dramatic effects on fabric life time.
The average lifetimes have increased in each position, leading to a potential reduction in annual expenditure of 23%. Heimbach has also estimated that there will be 20 fabric changes/yr instead of 25/yr with the previous designs.
TAMFELT
Tamfelt launched its HiSpeed forming fabric at the beginning of 2007. Designed to combine the high fiber support and excellent stability of SSB fabrics with the thinness and efficient water removal of the double layer (DL) fabrics, HiSpeed clothing has been successfully run on gap and hybrid formers, on newsprint, LWC, and SC papers.
Although SSB fabrics offer several advantages over DL fabrics, they also have their drawbacks: traditional SSB fabrics are thicker than DL fabrics, increasing water carry and causing splashing at high speeds. The dry content after the forming section also tends to be lower than with DL fabrics, particularly with high-speed paper machines.
In Tamfelt's patented HiSpeed structure, all the MD yarns are thin. On the wear side, the warp density is higher than on paper side, allowing a thin and stable fabric structure. The HiSpeed caliper is closer to double layer (DL) fabrics than to traditional SSB fabrics. With a lower warp density on the paper side, the number of crosswise yarns on the paper side can be increased, thus improving fiber support while keeping the paper side structure open and allowing efficient water removal. The wear side has high MD yarn density, together with a manufacturing process aiming at high stability for optimal dimensional stability and excellent diagonal stability.
Case study 1
A high-speed newsprint machine with a vertical gap former ran a HiSpeed fabric on the inner position together with a standard DL fabric on the outer position. The HiSpeed ran for the targeted time and the dimensional stability was excellent throughout the fabric life. Steam consumption was low and runnability was excellent. Oil absorption two-sidedness was clearly less than with earlier fabric pairs.
Case study 2
A customer making high-quality SC-A paper had marking problems with DL fabrics. The mill had tried a traditional SSB fabric in the inner position, which is more sensitive to marking. However, the dry content was then found to be noticeably lower than with DL fabrics, and machine runnability was poorer.
The customer initially installed a HiSpeed fabric in the inner position. Marking was clearly reduced from the first paper roll. After the new fabric had been running for a few weeks, the mill also installed a HiSpeed fabric in the outer position and marking reduced further (Fig. 2).
With HiSpeed fabrics, the mill's paper was found to be denser, with less two-sidedness. Machine runnability also received special praise. Today, the machine runs HiSpeed as a standard fabric in both positions.
An essential factor in paper machine runnability is edge trimming: if this does not work, there will be napping problems in pick-up and increased sheet breaks.
Edge trimming showers are normally slightly slanted towards the fabric edge. In a HiSpeed fabric, there are less MD yarns on the paper side than in a traditional SSB fabric. This explains why the distance between the MD yarns is approximately 35% higher in HiSpeed than in traditional SSB fabrics, and the water has more open surface to penetrate into the fabric structure.
HUYCK.WANGNER
The greatest potential savings for today's papermakers lie in higher machine speeds combined with fewer breaks and improved runnability. Real-world tests conducted by Huyck.Wangner show that one fewer break per day results in an increase in paper production totaling 14.4 tonnes/day on a paper machine producing graphic paper (80 g/m2, 1,200 m/min, 5 m wide). Increasing the speed of the same paper machine from 1,200 to 1,220 m/min increases the total daily output by some 11.4 tonnes. In both cases, several million euros can be added to the bottom line at the end of the year based on the typical market price of paper.
When designing new generations of forming fabrics for the wet end, development focuses on drainage capacity, dry content, sheet formation and runnability. Better retention properties are also being specified because of rising raw material costs and increasing ash content. For forming fabrics, this means on the one hand providing higher fiber support (FSI) and on the other a larger open surface on the paper side (SOA) - clearly conflicting objectives for conventional SSB designs. Furthermore, rising paper machine speeds and high dry content (runnability, energy costs) require ever shorter drainage times and demand better drainage performance from the forming fabric. At the same time, paper quality (e.g. printability) must continuously be improved. To achieve this, the forming fabric must gently control drainage at the former. Huyck.Wangner has responded to this challenge by designing forming fabrics that it believes can give the best trade off between fiber support, open surface and fabric caliper. Having the right balance between the retention properties and drainage capacity is decisive when it comes to optimum sheet formation, particularly for SC paper types.
The latest generation Selectra/Vortexx and Compressor forming fabric families were developed with exactly these specifications in mind - namely, to provide very high fiber support combined with a large open surface beyond the limits of existing SSB designs. The result is an expanded paper machine operating window. Field studies of Selectra's performance show that despite higher paper machine speeds, dry content is improved by 1-1.5%, which translates into improved runnability (fewer web breaks). It should also be noted that 1% higher dry content at the end of the press section translates on average into a production improvement of approximately 4%.
An integrated approach
The typical approach to date has been to optimize machine sections independently of one another. Although this resulted in added value in the respective machine section, it was no guarantee that the value added would carry over to the end of the paper machine clothing chain. Higher dry content at the end of the fabric section is of limited use if no dry content improvement is achieved at the end of the press section.
Optimally tuned forming fabrics make it possible to improve the dryness of the paper web at the entrance to the press section by approximately 1-2%. This means that a much lower volume of water needs to be drained (up to 25% less water at the front presses). Given these conditions, modifying the press felts to suit is unavoidable if the advantages in the fabric section are to be converted to improved machine performance.
Huyck.Wangner's clothing specialists have determined that the improvement potential can only be optimally tapped when the boundaries between segments are eliminated and an integrated approach is taken to the overall process, a concept the supplier dubs "Total PMC Optimization".
Papermakers often focus on increasing the dry content at the end of the press section when they want to increase capacity and cut power consumption. One thing specialists can do to optimize the results is to analyze all factors affecting the clothing elements and their individual performance, right back to the beginning of the dewatering process. Results show that sheet dryness values after the press section only increase significantly when the runnability of the forming fabric is synchronized with that of the press felt.
Case study
The drainage in the fabric section of a newsprint machine (45-55 g/m2, 8.60 m wide, 1,700-1,812 m/min.) was optimized in two stages. The objective was higher dry content and improved runnability. At startup, a double-layer fabric was in the top position and an Optiply 1:1 in the bottom position. For the analysis, the top wire on the top position was replaced by a Selectra 2:1 fabric and the bottom position stayed the same.
Results showed that drainage at the Hivac box rose 1.2%, almost doubling the amount of water removed, while at the couch roll or former drainage was 4% higher. The second step in the optimization process was to replace the Optiply on the bottom position with a Selectra 3:1. The results were even better. The dry content at the pickup was now 19.6%, exactly 2.3% higher than with the initial clothing, a substantial improvement.
However, measurements at the press section revealed that this advantage had not been transferred from the wet end to the dry end. The improvement in dry content to 19.6% after the wire section resulted in a reduction in water volume of 22.3% at the first press. The paper machine in this case study was therefore processing exactly 500 L water/min less at the press. The logical next step would be to adjust the applied felt to the new operating conditions with the objective of passing at least 1% of the dry content improvement after the wire section to the dry end.
An integrated approach to aligning the individual clothing elements with the requirements of the respective paper machine causes the potential for optimization to spiral upwards. This in turn sets in motion a cycle that further improves the potential to achieve the targeted capacity and efficiency improvements.
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