FINISHING/CONVERTING

New units overcome speed limitations, uptime issues of supercalenders when used on-machine or as rebuild of existing supercalenders

By Harold M. Cody, Editorial Director

Multi-Roll Calenders Make Super-Calender Quality On-Machine Option

Calenders have come a long way since the days when steel-rolled machine calender stacks were mainly used to remove lumps and even up the caliper profile. Soft calendering, which developed rapidly in the late 1980s and early 1990s, is now a standard technology for many paper and board grades (see September 1997 Pulp & Paper soft calendering review). Lightweight coated and soft-nip uncoated mechanical papers that used to require supercalendering are now produced in mills pushing the limits of soft calenders. However, the newest multi-roll calenders are even more innovative, and provide capabilities to papermakers previously limited to slow, high maintenance off-machine supercalenders.

There are two commercial multi-roll calenders available: Valmet’ OptiLoad and Voith Sulzer Paper Technology’ Janus calender. Both units are targeted at new installations on- and off-machine, as well as for rebuilds of existing supercalenders. Voith’ system was commercialized first, starting with the October 1996 startup of two units at KNP Leykam’ Maastricht, Netherlands, mill. Valmet’ first two off-machine OptiLoad units were brought online in April 1997 on the huge 354-in.-wide (9-m) coated woodfree line at Metsa-Serla’ Oulu, Finland, mill. Most installations are off-machine, but the long-term potential of these units appears to include wider on-machine application. Two Janus calenders are operating on-machine, at Lang Paper in Germany on SC grades, and for an off-machine coater in Japan. Valmet’ first on-machine units will start up in 1998; the first in Canada, followed by one in Finland.

In off-machine applications, cost savings compared to supercalendering is a key selling point. For example, in new or rebuild situations, two supercalenders could be replaced by one multi-roll unit. It would require less staffing and the new cover materials provide longer life and, thus, less downtime. According to Frank Ferlet at Voith Sulzer, the new Stora Forest SCA machine to start up soon in Nova Scotia would have required four supercalenders according to design data, but only two off-machine Janus calenders were ordered for the 374-in.-wide, 4921 ft/min (9.5-m, 1500 m/min) machine.

An advantage when located on-machine is that no additional staff is required vs the staff needed to run a supercalender, and they can often be installed in existing space on-machine. This avoids the cost of moving reels, winders, etc. associated with installing two or more soft-calenders. For example, energy use (for equal sheet finishing) on the Janus is stated to be much less than soft calendering and modestly less than supercalendering. The main advantage vs soft calendering is much lower heating energy requirements.

In addition to on-machine use, a large amount of the business for these units is expected to come from either replacing or upgrading off-machine supercalenders. For example, if a mill has three supers, parts from these could be used to make two multi-roll calenders. Steps in rebuilding vary depending on how far a mill wants to go. A rebuild can incorporate the existing frames (which may be strengthened), part of the unwind/rewind, and replace the cotton rolls with synthetic covers. Building out from the shaft and then applying synthetic covers can reuse the existing filled roll shafts. Steel rolls could be replaced with new heated rolls to raise temperatures up to 300° using mill process steam in order to provide additional smoothing capability. One capability from Voith is to heat the top roll on off-machine applications using its Calcoil system, vs a standard super where the sheet enters cold.

Is online Operation the Future? The inclusion of a multi-roll calender on-machine, while seeming to be the way of the future for many commodity grades such as uncoated mechanical, raises the question that is probably of most concern to papermakers: How will the uses of this equipment on-machine impact operating efficiency? All contacts interviewed mentioned this as a critical concern. Specific issues of concern include cover life, threading at PM speed, motor maintenance issues (since the rolls are driven), and, of course, paper quality.

The Lang mill (the only on-machine unit) reported that the unit was providing better quality than the previous soft calender, and producing SC-gravure quality grades based on a high deinked fiber content. The best quality achieved to date has been a 1.3-µ PPS-10S smoothness. Sheet threading in this, and all on-machine units, is much more difficult, but no specific machine operating efficiency data have been reported. The Janus at Lang has a combination of vacuum belts before and after the calender and ropes within the stack. Roll life has been about eight weeks between regrinds, according to the mill. The sheet’ moisture profile is handled differently since there is no moistening and conditioning prior to supercalendering, as is traditionally done on SC grades; this raises the dried papers moisture of 2% to 3% up to 8% to 9%. At Lang, the sheet is dried to 5% moisture and then water sprays raise it to 7% before calendering. The CD-caliper profile is controlled using top and bottom Nipcorrect rolls.

One of the major challenges of online operation, according to various contacts, is to operate these units (either Janus or OptiLoad) at the design speed of new machines. As noted by Martti Tuomisto of Valmet, when running at 1,500 m/min or even higher, the question of the impact on operating efficiency really comes to bear as it pertains to the ability to thread the sheet. He noted that the company has done extensive R&ampD work on a pilot plant unit to simulate the Irving Paper design in order to address this issue.

Valmet’ design differs from Voith’ in the way the rolls are loaded. In the OptiLoad, the same nip load can be used in all nips from the top to the bottom of the stack. In conventional multi-nip stacks (i.e., supercalenders) most of the work is done in the bottom nips, because the weight of the rolls themselves means most of the nip force is on the bottom, with only light loading on the top. In the OptiLoad, however, the dead load impact of the stack is relieved by hydraulically compensating for the weight of the intermediate rolls. Additional loading can then be applied by hydraulically loading the cylinders. As a result, each nip can do the same amount of work, or can be loaded the same. In the Janus calender, there is an increase in loading from top to bottom, which Voith believes is the preferred method. The Janus can also provide a hot variable crown roll which allows caliper profiling. This will be installed on the Stora Forest SCA units. Observers appear mixed on whether either design has an advantage.

In soft calenders used to produce high-quality grades, the range of temperature required has led to the use of oil as the heating medium in the steel roll. The oil is heated, often by use of natural gas, and then is pumped through the hot roll, which contacts the covered roll (which has crown control capabilities). In contrast, the use of a multi-roll approach means that lower temperatures can be used. Therefore, mill process steam, rather than the more complex and expensive oil heating system, can supply heat. Also, energy savings can be considerable. In a multi-roll calender, the use of smaller diameter rolls with less surface area for heat loss, in conjunction with the fact that the heated rolls work against two opposing rolls vs one in a conventional singe nip calender, saves energy.

Details on the covers used, whether it is grind cycle or resistance to marking, is probably the key technical question asked by operating personnel. The advent of new synthetic or polymer roll covers has been one of the major developments making these new systems possible. Each manufacturer provides their own soft roll covers, but details are considered confidential. Valmet jointly developed their Dura line of covers with Yamauchi. Voith Sulzer’ JanuTec covers are provided via an arrangement with KRC Rolls (Scapa-Kern in Europe), a Scapa Group Company. Valmet is adding capacity to resurface the rolls at its Appleton, Wis., facility by the end of 1998. Both companies tout their covers as a key aspect of the new systems. Valmet’ is reported to be thicker, harder, and smoother, thus providing more life to a roll. Voith’ covers are reported thinner, which they state provides better heat dissipation.

Other calendering Developments. Valmet is also developing the concept of ‘ calendering.’ Currently, this technology is mainly geared for board and kraft paper grades. One unit has been running on a commercial board machine producing liquid packaging grades since 1994. At this point, the technology is still under development, but the company said a couple of options could be used: one using a belt running between two rolls, or another using a shoe-type system.

Beloit Corp. currently does not offer a multi-roll system of this type, although they report that a new system called Polysoft is under development and will be introduced shortly. Beloit’ approach will be mainly geared to off-machine operations, according to the company, although it will be available for online use. Little in the way of detail on the new system is currently available. Beloit, however, does view developments somewhat differently, as they believe that an off-machine location may be preferable. For example, they state that a 1% drop in machine efficiency caused by the incorporation of on-machine multi-roll calendering could outweigh the potential savings.

That’ not to say that soft calendering is obsolete. Using very high temperatures due to the use of heated oil in the hot roll, soft calenders are used to produce grades ranging from newsprint to uncoated mechanical to fully on-machine finished coated. MacMillan Bloedel uses a film coater and two single-nip Valmet calenders to produce LWC at their mill in Alberni, B.C. A recent startup is the 8-m-wide, 1,200 m/min, No. 4 lightweight coated machine at SCA’ Ortviken, Sweden, mill. PM 4 uses on-machine Voith Speedcoaters (two separate stations for top and bottom) and two single-nip calenders. However, the new units offer capabilities that will actually result in competition for existing lines of both supercalenders and soft calenders.

References

1. CPPA Annual meeting, Jan. 1998, p. A229-A235.