Mills Upgrade Supercalenders with Janus Technology
By Franz Kayser and Michael Gwosdz-Kaupmann, Voith Sulzer Paper Technology
When the Janus Concept in calendering was introduced in 1995-96, even the greatest optimist could not have foreseen that this new technology would replace the traditional and well known supercalender in such a short time. No one would have anticipated also that the concept would set a new standard and clear some long-standing boundaries associated with calendering technology.
Today, more than a dozen Janus calenders are in operation, processing a diverse range of grades including wood-containing and woodfree papers, coated and uncoated papers, and gravure papers—both online and offline. The Janus technology has also contributed to the fact that a wastepaper-containing, uncoated paper, that in former times would have been similar in quality to newsprint, can now be finished to a degree comparable to classic supercalendered gravure papers.
One of the advantages of the Janus Concept is inherent in its modular design system. This permits, for example, optimum adaptation to a variety of different tasks, with the result that a six-roll Janus calender can process gravure papers at the end of a paper machine while another Janus, based on two five-roll stacks, does the same to woodfree qualities in a coating machine.
Currently, several 2 x 7-roll Janus calenders are processing woodfree specialty papers in the U.S., and several 11-roll calenders are producing a quality spectrum form matte finish to standard and even high gloss. Figure 1 depicts various Janus calender lay-outs.
Modifications:
- Lever position with OLC cylinders
- Marun air stream rolls with JanuTec coating
- Water heating/cooling system
- Rotary joints for heating rolls
- MicroFlow steam moisturizers
- Bearing housing for top and bottom S roll
- Bottom hydraulic cylinder
- Auxiliary drives for guide rolls
- Guide roll brackets
- Control system
These variation and adaptability features of the Janus technology are particularly applicable to existing calender upgrade projects. The following upgrades at a Finnish uncoated gravure paper mill and a German natural gravure paper mill illustrate the use and value of these features.
Finnish mill project
The Finnish project was initiated when the mill decided to modify its more than 25-year-old Eck supercalender so that, together with modernization to the paper machine itself, natural gravure paper could be produced that would meet the highest quality standards in today's marketplaces.
Elements necessary to convert the mill's supercalender to a Junus are illustrated in Figure 2. Compensation of the overhanging loads formed the basis for this conversion, creating a geometrically straight nip.
The previously sliding calender bearing housings were mounted on swivel levers, which were fitted with hydraulic cylinders and used to eliminate the detrimental effect from the overhanging loads. Existing anti-friction bearings and bearing housings of the supercalender were re-used.
The extremely complicated mechanical and electric/electronic means that were necessary when changing filled rolls with different diameters became obsolete. This facility provided the basis for replacing the filled rolls with plastic rolls. The filled roll shafts, however, were converted into a Marun roll provided with a JanuTec cover, and were continued to be used.
Some investigation was necessary in relation to the process heat needed with the Junus technology. In this particular case, extensive laboratory tests in conjunction with the mill and calender manufacturers enabled a process to be worked out where a 100oC surface temperature of the heating roll proved to be sufficient.
In this project, the restriction of surface temperature to 100oC had the advantage that the old heating rolls of the supercalender need not be replaced, leaving only the procurement of a new, more effective heating installation.
The surface moisturizing by steam necessary for natural paper was modified to the latest state-of-the-art, and to also allow the future addition of new micro-steam moisturizers. The result of this is illustrated in Figure 3. A can be seen in this figure, very ambitious quality targets can be achieved with the Junus technology, even with a modified supercalender.
Apart from the elements that have a direct influence on paper quality, some well-know problems can be eradicated by this type of modification. For example, the integration of a rope feeding system not only simplifies the threading itself to a considerable degree, thus avoiding frequent complaints (i.e. marking of rolls during threading), but also eliminates the problematic nip guards.
In addition to the traditional surface parameters of gloss and smoothness, Figure 3 also shows so-called blackening as a quality target. This is a parameter that, to date, does not appear to be controllable in any way by the classic calendering process. Blackening or black calendering means that pulp fibers collapse locally at cross-over points, thus becoming transparent and, in regard to the calendered paper, appear to have been blackened.
Before Janus, this knowledge was at best "shady." It has been researched in the development stage of Janus to the extent that only a blackening index of 45 can be promised. However, this number is quite meaningless to most people. The higher the index, the blacker the paper surface appears. Supercalender paper has a normal index value of 55 or more.
In direct comparison of two paper samples, a "trained eye" can detect an increased blackening of two index points. On a purely subjective level, all paper with an index number below 48 is classified as being free of any blackening. As an illustration of blackening, Figure 4 shows paper processed on a supercalender (top) versus that processed on a Junus calender (bottom). The difference in blackening is very apparent in this illustration.
German mill project
The German mill was calendering natural gravure papers on three Wartsila supercalenders installed in 1987. At a paper machine speed of 1,200 m/min, the supercalenders were just able to keep up with production capacity. To provide some reserve capacity for the supercalenders and to allow for the paper machine speed to be increased to 1,400 m/min, the mill considered fitting the three calenders with a zero speed flying splice system at the unwind.
Discussions during the project phase explored the possibility of taking one of these calenders out of operation after the two remaining units were rebuilt to the Janus Concept. Figure 5 shows that the two remaining calenders, after the rebuild to Janus technology, and without the planned flying splice retrofit, would have a speed reserve of 100 m/min even at an unrealistically high 100% availability of the paper machine.
In addition, the heating rolls were also changed. A higher calendering temperature of 140oC was used, and the new rolls were also fitted with smoothness and gloss promoting Sume Cal surface plating. With the higher calendering speed of almost 1,000 m/min, higher technical demands were placed on the reel. In this regard, Sensomat Plus-E units were installed in the winder system.
The calender rebuilt project is depicted in Figure 6, the top illustration showing before the rebuild and the bottom illustration showing the configuration after the rebuild.
Although the drives in the 1987 supercalenders were not designed for conditions normally associated with Janus technology performance and speed, they did not have to be replaced. The power requirement of the JunuTec plastic rolls was much lower than with the old filled rolls, and the speed ratio could be adapted by modifying the diameter of the new drive roll.
Franz Kayser and Michael Gwosdz-Kaupmann are with the finishing division of Voith Sulzer Paper Technology in Krefeld, Germany,
Edited by Ken Patrick