Considering the constant growth in energy costs, all efforts are made to reduce energy consumption by
enhancing mechanical dewatering. Tissue producers can benefit directly from reduced energy costs by raising the level of dry content after the pressing process and thus enhancing the efficiency of the complete Yankee machine.
Pressure and suction pressure rolls are designed to dewater the paper web as efficiently and gently as possible. Yet the roll covers are used in very difficult operating conditions, creating certain challenges
defining the optimal nip geometry of the roll covers, primarily due to the extreme diameter/length ratio of the Yankee cylinder, the related dynamic deformation of the cylinder under internal steam pressure, temperature and linear loads. Low roll diameters and high operating speeds witnessed in modern tissue machines result in high linear loads and nip frequencies.
As a consequence, the roll covers are exposed to extreme thermodynamic loads. Roll covers used in the manufacturing of tissue products thus have to be designed to ensure maximum service life, stability and consistent quality considering elasticity, resilience and even CMD (cross machine direction) profile.
Suction pressure rolls in Yankee machines do the lion’s share of the work when it comes to the mechanical dewatering of the tissue web. As a result, they are pivotal to energy savings and thus directly affect the overall performance of the production system. Thus, a few key features are important when selecting a roll cover.
First: high water storage capacity, achieved with optimised surface designs with high open surfaces and high void volumes. Second: sufficient nip widths for gentle dewatering. And third: abrasion resistance for longer service life and stable operating conditions. SchäferRolls has developed the tisQ.X suction pressure roll cover especially for fastrunning tissue machines designed to achieve ultimate dewatering performance and energy efficiency.
tisQ.X covers are designed to deliver maximum resistance to moisture, temperature and wear. They are also suited to maximum linear loads and speeds. The polyurethane compound developed for tisQ.X suction pressure rolls offers unparalleled mechanical, dynamic and chemical stability.
tisQ.X: a considerable improvement to the machine efficiency in the manufacturing of tissue
Thanks to the outstanding resilience of the functional layer, the roll cover is suitable for all surface designs (blind and suction holes, grooving) without compromising the stability of the cover surface. All of this allows for variable surface designs, which come into their own in terms of increased void volumes coupled with short dewatering paths.
Ultimately, this results in highly efficient dewatering – creating extra potential to improve dry content and operating speeds, and thus increase machine efficiency (Figure 1 and 2).
Through the use of a tisQ.X suction pressure roll cover with optimised surface design, the machine and dewatering performance of the PM10 were increased significantly at WEPA Leuna GmbH. The PM 10 in Leuna produces mainly toilet paper (70%) and kitchen towel paper (30%) at a speed of 1,800 to 1,950 m/min on a work width of 5,500 mm. The weight range is between 15.4 and 18 g/m².
At WEPA Leuna, the aim was to achieve an increase in dry content through improved mechanical dewatering with the suction pressure roll, in order to achieve additional energy savings. As there were no problems in the previously used blind and suction drilled standard rubber covers with regard to cross profile irregularities or lifetime, additional potential could be achieved by increasing the dewatering performance by optimising the surface design by applying a groove instead of the previous blind drill hole.
Due to its excellent mechanical surface stability, the tisQ.X polyurethane suction pressure roll cover provides the best prerequisites for this. To determine the final surface design, SchäferRolls Application Engineering performed a complete machine analysis on site, including a surface impression of the design of the rubber suction pressure roll in use (Figure 3).
Based on the results of the application engineering analysis, with the support of the SchäferRolls SurfaceOptimizer, a software for the configuration and planning of surface design and dewatering performance of roll covers, a new surface design was determined.
In comparison to the rubber suction pressure roll cover in use with blind and suction holes, it was shown
to the customer that with the tisQ.X cover, in a first step, with a conservative groove design, an increase of from 29.8 to 39.3% in the open surface can be achieved. With this measure, the free void volume is enlarged by approx. 15%, i.e. from 1063 ml/m² to 1223 ml/m² (Figure 4).
With the changeover to a tisQ.X polyurethane cover, the customer very clearly saw the opportunity of increasing the dewatering performance of the suction pressure roll without major effort and thereby achieving savings with respect to energy costs.
After the suction pressure roll with the tisQ.X cover was installed in the PM10 at WEPA Leuna, a dry content increase of 1% oven dryness was determined from 42 to 43% compared to the previously used rubber cover. Even after a running time of a respectable 413 days, the roll cover still makes a good impression, and it is not necessary to grind the surface (Figure 5).
Prospectively, the roll will be installed again in the machine at the end of the year. At present, the sister roll is in use, which also received a tisQ.X cover, in view of the success. “As the target running time of 12 months has been exceeded, an interval of 18 months is now being aimed for”, says Dipl.-Ing. Lars Helge Peters, Mill Manager at WEPA Leuna. Therefore, tisQ.X suction pressure roll covers are making a valuable contribution to the reduction of energy and operating costs and are thereby making a considerable improvement to the machine efficiency in the manufacturing of tissue.