Yankee Dryer a Bottleneck? Shoe Calender the Answer
By Pulp & Paper Canada
By Pulp & Paper Canada
The development of an innovation in board making was set out by Dr Rdiger Kurtz, Voith’s manager for paper process technology. He highlighted work carried out in the company’s pilot plant. Stora Enso’s technical mill manger at the Baienfurt mill…
The development of an innovation in board making was set out by Dr Rdiger Kurtz, Voith’s manager for paper process technology. He highlighted work carried out in the company’s pilot plant. Stora Enso’s technical mill manger at the Baienfurt mill in Southern Germany, Helmut Endler, described the first commercial application of this technology. ‘Many current board machines have a Yankee cylinder installed in the dryer section. The Yankee provides the potential to maintain bulk while preserving surface treatment but with the restriction of a narrow operational window. It is also a significant restricting influence on speed and on productivity.’
This bottleneck has been overcome in some of the latest board machines by installing a soft nip calender after the dryer section. Pushing the idea of gentle, bulk-saving calendering action to its limits leads to an extended nip calender, a so-called shoe calender.
The principle of shoe press technology has been well known in the industry since the Extended Nip Press (ENP) was introduced in the press section by Beloit in the 1980s. Voith Paper successfully started developing its own shoe calendering technology in the early 1990s. Basic research was completed at that time and several patents were granted but, due to market demands in the second half of the decade focus was concentrated on the Janus calender. In 2000 work was resumed and the NipcoFlex pilot calender has been in operation since January 2002.
On the pilot calender, this new roll is typically installed in the bottom position. The heated roll is located on top so that the top side of the sheet is calendered.
In the pilot calender, this roll is heated from inside and, additionally, by an external inductive heating unit. This permits higher temperatures of the heated roll and compensates for the high energy transfer from the heated roll to the web during shoe calendering caused by the prolonged dwell time in the nip, compared to conventional roll calendering. It also helps to stabilize temperatures quickly, making trials more efficient.
The first stack can be equipped with hard or soft nips so that pre-calendering or profiling can be undertaken on-line. The system is completed by moisturizing units (steam or water).
The pilot calender can also be configured in reverse order, with the new calender in the first stack, thus permitting on-line post-calendering.
It can be operated at speeds up to 1500 m/min, with linear loads of up to 1200 N/mm and roll surface temperatures up to approximately 260C, the latter depending on grade, basis weight and speed. Nip lengths vary between 40 and 250 mm. The shorter nips are mainly used in graphic grades and the longer nips for board grades.
A very large number of trials have been conducted since January 2002, mainly focusing on folding boxboard and white lined chipboard. They covered a wide range of basis weights from folding boxboard at 370 g/m2 down to LWC paper grades at 47.4 g/m2. To save time and costs and to utilize the most recent methods in trial planning, evaluation and presentation, a process optimization software package was used.
Every trial showed that the correlation of surface properties to bulk or bending stiffness can be improved. A comparison was made for folding boxboard produced conventionally using a Yankee dryer with board treated with the NipcoFlex pilot calender after pre-drying and before coating. Trials were operated with different nip lengths and at varying calender settings, at speeds 40% above the current production speed with a Yankee dryer.
The surface micro roughness of the board, before and after coating, was measured as PPS S10, vs. bulk. With the new calender, the same level of PPS was obtained as with a Yankee dryer but with up to 4 % more bulk and at higher speeds.
Results from white lined chipboard, evaluated with process optimization software showed that load and temperature have a similarly high effect on bulk, whereas the effect of steam is much lower. Linear load and temperature also have a significant impact on surface micro roughness (PPS S10). Steam can be used to support surface smoothing.
The effects of linear load and steam level off at high settings. The process optimization software can also be used to derive an operational window. Data was derived from trials with a conventional soft nip calender compared to a NipcoFlex calender. In this final stage of data evaluation, threshold values can be defined for every parameter.
Combining these requirements proved that calender settings were all met or exceeded. In this case, no operational window remained for the board treated with a soft nip calender: requirements for surface properties were met, but bulk was too low. With this calender, all requirements were fulfilled in the required range.
It has been shown that with increasing temperatures the linear load can be reduced. The increase of bending resistance with increasing temperatures is surprising, but it was observed in numerous trials. It is most likely due to a hornification effect caused by high temperatures which increases bending resistance of the board.
Trials that Voith has conducted over the last two years have shown that best results are obtained at high temperatures and with the application of steam (temperature and moisture gradient calendering), thus permitting a reduction in load.
However, ‘optimum temperature’ does not necessarily equate to ‘maximum temperature’. The appropriate level has to be found, considering issues of economy and technology. The nip length is also a crucial factor in obtaining optimum results for a particular grade.
Due to the elongated nip, their influences increase, enhanced by the high temperature level. This allows significant reductions in line load, i.e. pressure, which in turn leads to the desired gains in bulk.
Baienfurt BM3 calender
As a result of these trials it was agreed with Stora Enso that one of these calenders should be installed on BM3 at the Baienfurt mill during its major rebuild that took place in January this year. The calender has been installed following the final dryer sections just after the Yankee cylinder. Endler said that the configuration of BM3 after the rebuild has allowed the sheet to be re-routed, bypassing the Yankee cylinder altogether. “For the time being we have retained the Yankee so that for some grades still required by customers, we can still run the sheet in the earlier configuration. Eventually we expect to be able to remove the Yankee altogether.”
The shoe calender at Baienfurt operates with a surface temperature of 2500C, with a nip width varying from 110 mm to 250 mm and a maximum line load of 500 kN/m. At the same time as the shoe calender was installed, the press section was changed from a four nip press to a three nip press with a NipcoFlex shoe press which has increased the dry content out of the press section by 2%.
The conclusion ascertained from the trials and the Baienfurt installation is that this new technology provides a very interesting alternative for the production of paper or board. It can be installed in new machines and it is an attractive, economically and technically feasible option for rebuilds.
In many cases machine speeds are limited by the Yankee dryer. It has become clear that at the same level of surface properties, more bulk is obtained with the new technology than with a Yankee dryer or a soft-nip calender. These savings can be traded for fibre savings and for cost reductions. At the Vienna conference it was asserted that machine speeds and production capacity could be raised by 15-20%, by by-passing the Yankee and incorporating the shoe calender. It was said that apart from the ease with which this technology could be retrofitted to existing board machines, in the future there would be a saving in capital investment in a new machine of 15-20% or even more. One expert was even heard to say that this could add plenty of capacity without the need to build new board machines.
Operational conditions and nip lengths, balancing the influences and dwell t
imes of temperature and load are crucial for successful shoe calendering. The shoe calender installed at Stora Enso Baienfurt, on the folding boxboard machine BM3, started up at the beginning of February this year following a 25-day rebuild. The mill reports itself more than satisfied with initial results and has added further commercial production experience with shoe calender operations.
Stora Enso’s Board of Directors approved the _70 million investment program for the Baienfurt board mill in May 2002. The mill, located close to Lake Constance, Heidenheim, is part of the Consumer Boards Division of Stora Enso Packaging Boards.
Stora Enso is today an integrated paper, packaging and forest products company producing publication and fine papers, packaging boards and timber products — business areas in which the it is a global market leader. The Baienfurt mill was originally founded in 1871 and today produces about 210,000 tpy of fully coated folding boxboard and 120,000 tpy of bleached groundwood pulp. The mill site of 325.000 m2 is large enough for further significant investment. In 2003, the turnover was about _160 m
The NipcoFlex calender consists of a heated roll acting against a concave shoe. The shoe is covered by a specially-patented sleeve. The paper or board web is calendered between the sleeve and the heated roll. The following parameters are especially important for shoe calendering:
The sleeve and its properties: structure, material, surface roughness, and hardness;
The shoe itself: nip length, MD-load profile and lubrication system between shoe and sleeve;
The heated roll: Shoe calendering is especially effective at temperatures well above 200C.
Oil-based heating systems cannot be used at this temperature level. Instead, Voith Paper furnishes the calender with rolls heated by induction.
The shoe and the sleeve are mounted in the roll. The design of this roll was adapted from the well-established press, as well as the basic design of the sleeves.
Beside these parameters, others, well known in every calendering technology, are also used in shoe calendering, namely, pressure, temperature and moisture. The principle of shoe calendering is based upon the effects of moisture and temperature gradients.
Voith Paper’s future R&D on extended nip calendering will also consider light-weight graphic papers.