BRUSSELS, Sept 1, 2014 (PPI Magazine) - With the ever-increasing demands for tissue machine performance, efficiency, and reduced operating costs, today's world-class tissue makers demand the highest performance from their roll covers and paper machine clothing (PMC) products.
Traditional venting in tissue suction rolls was a limiting factor. Roll covers for these positions are designed to generate the proper nip intensity at the maximum load. Traditionally, these covers are specified in the 30 to 45 P&J range since bulk and conformity to the Yankee are key design criteria. Cover compounds must possess good abrasion resistance, crack resistance and hardness stability (to prevent hardening from operating against the hot Yankee dryer).
Traditionally, the use of venting on these applications was limited by a number of variables. In suction pressure roll positions, venting consisted of suction-through holes only. The through-hole diameter is limited by hole diameter in the shell so the total open area typically ranges from 16 to 19%. Adding blind drilled holes to the suction pressure roll was not often used. To fit blind drilled holes in the suction pattern, relatively small and ineffective holes would be required (0.078-in. or 0.085-in. diameter). Blind drilled holes would also require the felt to take on more moisture. This is not desired when the suction pressure roll is followed by a pressure roll position, or if the felt conditioning system is not capable of handling the additional moisture.
Since soft covers historically will deform under pressure, the use of grooves was restricted due to the fact that groove height is reduced and the walls will bulge inward under pressure. The narrowing of the groove will restrict the flow of water resulting in the remaining void volume becoming ineffective. The narrowing of the grooves also creates the potential for felt "pinching" in the nip. Conventional industry guidelines only utilized grooves on roll covers harder than 12 P&J. At 40 P&J, studies have shown that the void volume contributed by the grooves is one half of the initial available void volume.
Overcoming traditional limitations
Advancements in roll cover and press felt technology have opened the door to increase the use of venting in tissue applications. Softer polyurethane covers possessing higher modulus materials offer greater abrasion resistance, better hardness stability and stable hysteretic characteristics. These stable physical properties of the polyurethane covers now make it possible to utilize newer venting patterns to allow increased nip dewatering and decreased energy consumption. Slightly harder 20-30 P&J polyurethane covers develop similar nip intensities as 30 to 45 P&J rubber covers.
Advancements in rubber compounds have also generated more opportunity to utilize venting in softer covers. Superior abrasion resistance and low hysteretic characteristics allow 30 to 45 P&J rubber covers to run longer. Rubber covers possess greater chemical resistance and stronger bond strength. The softer material offers optimal conformity to the Yankee surface for improved sheet qualities and drying.
The advancements in polyurethane cover materials now make it possible for grooves to withstand the conditions in the suction pressure roll nip. Adding grooves to a suction pressure roll can improve the nip dewatering in the press. The grooves increase the open area and available void volume in the cover. Void volume generated by grooves is also assisted by vacuum since the grooves intersect the suction through holes. Increased nip dewatering will generate both a drier sheet and felt exiting the nip. The drier sheet requires less external energy to completely dry the sheet and the drier felt now can offer additional void volume in the 2nd press (pressure roll nip).
Xerium has improved rubber cover technology in recent years which utilizes grooves in suction pressure roll applications. By implementing engineered grooving technology, "EnerVent", soft rubber covers can also offer increased nip dewatering. The EnerVent grooves are geometrically incapable of closing.These grooves have excellent groove wall stability and are less affected by groove narrowing in the nip. The result is more effective void volume and greater nip dewatering. The soft rubber covers still maintain excellent bonding strength, chemical resistance and conformity to the Yankee surface. Case studies have documented 10+% increases in production and substantial reduction in energy usage, including 41% reduction in fuel usage to the hood. Tissue makers report hundreds of thousands of dollars per year in energy savings after adopting EnerVent technology.
The importance of forming fabric technology
For modern tissue machines, a significant barrier for increased production and improved sheet quality is often the forming zone. Its overall ability to rapidly drain water while simultaneously controlling fiber orientation and fiber distribution in the sheet can dictate the amount and quality of tons produced. A critical component of the tissue forming zone is the forming fabric which has three major requirements:
Very high fiber support (FSI) and uniform cross machine direction (CMD) oriented sheet side: Since tissue sheets are of very low basis weights, and higher operating speeds require extremely high fiber retention, uniform fiber distribution and improved sheet transfer, achieving these objectives requires the forming fabric surface to have a fine CMD oriented surface to support-and form-the mainly MD-oriented fibers.
Straight-through drainage for rapid water removal and no water carry: As production rates increase, drainage times decrease. The sheet side of the forming fabric should determine the amount and the orientation of drainage resistance, while the machine side should be open to allow water to exit easily during sheet formation, and, to minimize shower water flow resistance on the return run.
Structural stability and wear resistance: Although the paper side of a tissue forming fabric contributes to the structural stability, the machine side of the fabric is primarily responsible for fabric stability and useful life. The goal is to deliver the above-mentioned properties, while increasing the economic life of the forming fabric.
Forming fabric design must balance the need for fiber retention, drainage and life. This inspired Xerium developers to create Formsoft, a concept tissue forming fabric specifically engineered to provide high-speed drainage over a short forming length, while retaining high fiber support. It features a symmetrically woven design that ensures dimensional stability and uniformity for the life of the fabric, Fig. 1.The top side improves the sheet's tensile strength, formation and hand feel, while the bottom structure provides extended life capacity.
Compared with conventional triple layer designs, Formsoft provides significantly higher FSI (+6% @ the same Perm), and up to 10% lower fabric caliper. It also provides measurable improvements in production rates, CD basis weight profile, tensile strength (MD and CMD), softness, and formation, Fig. 2.
Tissue felt technology is key for efficiency, quality
Since the rapid adoption of Crescent-former machines in recent years (where the tissue sheet is formed between the forming fabric and the tissue felt), demands placed on tissue felt technology have increased significantly. Modern tissue felts require very high fiber anchorage with uniform surface characteristics. In addition, the modern tissue felt can play an enormous role in overall machine productivity and energy consumption.
To provide the highest machine efficiency right from startup, Xerium developed Huyspeed based on a specially engineered yarn. The resulting construction is characterized by high elasticity in the nip direction providing increased dewatering at the suction press roll. As a consequence, reduced steam consumption at the Yankee cylinder and lower hood temperatures resulting in lower energy consumption.
The Huyspeed product family covers the broad application needs of tissue felts. With both the (1+1) laminated version Huyspeed GU and the special duplex version Huyspeed BD, with its very fine top surface, Xerium has developed a concept designed for market needs. By providing increased drainage at a lower caliper, the third design of the product family, Huyspeed BE, performs with top results on high performance tissue machines. As a result, Huyspeed is now considered a benchmark on tissue machines using waste paper and intensive high pressure needle shower. Due to its increased drainage, Huyspeed BE enables not only reduced cleaning but also lower pressures on needle showers. On balance this leads to valuable savings in water consumption. Beyond that the gentle mode of operation delivers improved felt lifecycles. The major advantage of Huyspeed BE is its wide operation window. In addition, the Huyspeed HC has a uniquely designed surface structure for increased contact, reduced rewet, and superior sheet handling on a variety of Crescent formers.
Xerium's R&D focus in the tissue segment will continue to deliver innovations our customers demand. These three product families have shown impressive results enabling world-class tissue machines to run faster and with greater economic efficiency.
Oliver Baumann, global VP technology PMC/IT; Robert Marchhart, global product manager PMC/engineered fabrics); Glen A. Harvey, director of rolls engineering technology North America); Steve Cole, director - global marketing, Xerium.
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