Calendering has seen some dramatic improvements in recent years as the competition to provide the best technology has heated up

by Roger Grant

Another year,another development

The three main suppliers have conveniently staggered development of their latest generation of calenders. In last November's review, we looked in detail at Valmet's OptiLoad calender, while the year before it was Voith Sulzer's Janus Concept that came under the microscope. This year, the covers have finally come off Küsters Beloit's Prosoft calender, offering the first chance to get a really close look at the technology on offer.

One of the most striking features on the new Küsters Beloit calender is that all of the rolls are driven, in contrast to a single driven roll on a conventional calender. This fundamental difference improves several aspects of the calendering performance, because the driving forces are no longer transmitted through the roll covers and paper web.

In addition, by using variable camber rolls in both the top and bottom positions, the diameter of the intermediate rolls can be reduced. This increases the compressive stresses and also reduces roll weight. The resulting benefits are reduced horizontal deflection of the rolls and a more even paper profile (including under the roll ends), together with longer roll cover life and higher paper tensile strength.

To make paper grades of average smoothness, the three-roll version is probably most suitable. It has polymer-covered rolls in the top and bottom positions and a heated Peritherm roll in between. For more demanding grades, such as rotogravure, SC-A and LWC grades, additional intermediate rolls can be added. As an alternative to using two stacks in tandem, the stacks can instead be placed vertically one above the other. While that may sound like an old-fashioned calender stack, separating the two small stacks both physically and in terms of machine control avoids the numerous problems that used to be commonplace in such systems.

The first Prosoft unit to be installed is an in-line calender at a French paper mill, which is being used to produce label and packaging papers. During the rebuild at the French mill, three rolls were added to the existing five-roll stack. The paper grades on the machine are now being calendered to supercalender quality at speeds greater than 800 m/min. If installed off-line, two Prosoft installations can equal the calendering ability of three supercalenders.

Valmet's first OptiLoad calender started up at Enso Fine Paper's Oulu mill in April 1997 and is being used to finish double-coated woodfree paper. Both this installation and UPM-Kymmene's new LWC PM at Rauma (PM 4), have two 12-roll off-line calenders. Design values go up to 450 kN/m linear load and 1,450 m/min speed. (1 kN/m = 1.02 kg/cm = 5.71 lb/in).

The calenders run with the same linear load across all nips. In practice, this is about 30% lower than loads in a conventional supercalender, even though the machine speed is almost twice as fast. Two in-line OptiLoad units are on order and both are six-roll units. One is destined for newsprint production at Enso Publication Papers' Varkaus mill, while the other will form part of a rebuild at Irving Paper's St John mill, where grades similar to SC-B will be produced. At last count in September, four OptiLoad calenders were running and 11 were on order for grades that included improved newsprint, catalog, SC-A and film-coated LWC with a design speed of 2,000 m/min.

OptiLoad trials point the way to improving quality. On an unfilled newsprint grade, a six-roll unit produced a PPS (S10) smoothness of 2.5 mm at a linear load of 350 kN/m. Raising the filler content to 20% gave the same smoothness at only 50 kN/m, while the 350 kN/m linear load produced a PPS smoothness of 15 mm. On SC-A rotogravure, an eight-roll unit raised gloss levels to 50% Hunter at a running speed of 1,500 m/min. This was achieved by fully relieving the weight of all intermediate rolls, while no relief gave only 46% gloss.

Shoes again

Last year, PPI reported that Valmet had a slightly unusual type of calender running at a Swedish mill. The unit was producing high quality board and the calender had a nip lengthened by means of a belt. We can now reveal that the mill was Korsnäs and the grade was coated liquid packaging board. This development - now named the OptiDwell Shoe - is based on SymBelt shoe press technology and uses an elastic belt to vary nip length over a range of 25-275 mm. This is up to 10 times the nip length of a conventional soft calender and permits low pressures that conserve caliper (and thus bulk and stiffness), while improving gloss and smoothness.

Print uniformity is improved because the modulus of the belt is significantly lower than that of most soft calender covers. This allows more uniform contact and pressure distribution in the nip. Additional flexibility comes from the ability to control dwell time and pressure independently. The shoe roll mates with a heated thermo roll.

In fact, the first supplier to publicize its use of belt and shoe calendering technology was Uddevalla Mekaniska Verkstad (UMV) back in 1994. It now has two Supersoft research units running (one with a large US company) and mill installations are a possibility.

Voith Sulzer has made further developments on its Janus Concept technology and has called the outcome Janus MK2. As Figure 1 shows, the calender has been lowered into the basement and occupies a 45 reclining position. This configuration allows a less tortuous paper web run, reduces vibration, cuts down on the likelihood of heat pockets and improves accessibility for maintenance. The resulting calendering effect, between matte and gloss, can be chosen by varying the nip sequence between one stock of six rolls and two stacks of five rolls. The first installation will take place at the Gebruder Lang mill in Ettringen, Germany.

Stora Port Hawkesbury in Nova Scotia has started up a Janus unit to produce a range of papers, including the company's new SC-A+ (SC-A Plus) and SC-A++ grades. Teething difficulties reported with the calender are presumably the explanation for the fact that no quality values have yet been made available.

Internal induction

The Japanese supplier, Tokuden, has been manufacturing thermo rolls since 1963, but the company has only recently moved into the paper industry. The group's specialty is induction heated rolls, whose shell temperature is raised as it rotates around a stationary internal induction coil with an applied AC voltage. Multiple gun-drilled holes along the shell contain a "thermal medium", which gives out latent host as it condenses (and then revaporizes, according to the heat pipe principle). This latent heat boosts the induction-generated heat going to the shell and also assists accurate temperature profile control. Such internal heating results in host losses being essentially eliminated and a supporting host supply is also unnecessary. The system also boasts low maintenance levels, because journal temperature is only about half that of the shell surface. Rolls such as these are operating on soft, gloss and temperature gradient calenders, supercalenders and converting operations (eg embossing), within the paper industry.

The supercalender continues to be improved. Valmet's ControStack is used in new and rebuilt machines to compensate for roll load overhang. Roll deadweight can be reduced by up to 30%, offering a more uniform load profile at lower line loadings with less filled roll regrinding. There is also the ControSet, which is a spindle system for automating roll changes.

Improvements in soft roll cover materials have also played an important part in the calendering revolution. Du Pont has had over 70 supercalender rolls on trial in Europe filled with its meta-aramid Nomex material. Experience at UPM-Kymmene's Rauma mill points to a roll life of at least 10 years, compared to 1-1.5 years for conventional cotton or wool-filled rolls. This improvement comes from a combination of factors, including the decrease in regrind frequency (from every 150 hours to every 1,000 hours) and the reduced thickness of the material that needs to be removed (0.5 mm versus 2.0-2.5 mm). The mill was set to install one Nomex roll in the top position of each of its seven supercalenders early last year and then evaluate multiple position operation.

The world's first soft calender with no roller bearings has been started up at Holmen Paper's Braviken mill. Instead of roller bearings, sliding bearings are used for both the variable camber SymCDS roll and the thermo roll. Valmet sees sliding bearings as the answer to several unsolved problems. Not only do they allow zero or low loading of closed frame soft calenders without bearing failure, but they also reduce barring problems (through better grinding accuracy) and are suitable for speeds exceeding today's fastest machines.

An interesting future prospect is a calender that will save 85% of the energy used conventionally, the saving normally being as electrical energy. This novel approach incorporates a roll with a shell made of fiberglass reinforced plastic (FRP) rather than cast iron, while the calendering effect is achieved by hydraulic elements without the use of heat. The development is being subsidized by the European Union DG XVII's Thermie energy-saving program.