As paper machines get even faster, suppliers are matching the speed increases with a rapid rate of new technical developments

By Rhiannon James

Squeezing more out of pressing and drying

Statistics such as these spell big opportunities for shoe press suppliers in the future, especially with their rapidly growing use in the graphic paper sector. Since the first Voith Sulzer installation in Switzerland, all suppliers have seen shoe press sales for graphic papers go through the roof, with more and more papermakers opting to install the technology on new machines or as part of rebuilds.

Among papermakers’ demands for the press section of graphic paper machines are high dryness levels, bulk-protecting dewatering with good structure and surface properties, optimum two-sidedness, as well as high runnability and maximum availability. But another factor that suppliers must take into account when designing their presses is increasing PM speed. And, according to the manufacturers, this is where the shoe press really outstrips previous designs.

As Schuwerk explains, “ trend is toward high speed machines and the shoe press is the only tool that will support this trend. Papermaking is comparable to a Formula 1 race. You want to run as fast as you can without too many pit stops. The shoe press allows you to do this.“

As a result of the speed increases, the shoe press also allows producers to boost production levels. A Beloit spokesman explains, “ depends on the grade, but the shoe press can increase dryness from 10-25% before entering the dryer section. Most people use this saving to increase production rather than to lower steam consumption in the drying section. The ROI is paid back in the production increase.“

Voith Sulzer's new shoe press uses a total closed draw concept

Doubling up

Shoe presses used on graphic paper machines have evolved considerably over the last few years. First came the standard shoe press and this was swiftly followed by the addition of a second shoe press into the equation. The latest vogue among suppliers is for an upgraded version of the two shoe press design such as Voith’s Tandem NipcoFlex (a step up from the DuoNipcoflex), the OptiPress from Valmet (formerly the Symbelt II) or Beloit’s ENP-C. This time though, rather than adding something to existing models, suppliers have opted to eliminate the center roll in their latest designs.

Valmet’s OptiPress design has proved popular among graphic paper producers so far. Both Haindl in Germany and Italy’s Burgo have chosen this particular shoe press for their latest giant lightweight coated (LWC) machines. Added to that, the OptiPress has been chosen by newsprint producers such as Norske Skog and Nanping in China. Norske Skog started up its new machine at Golbey in France at the beginning of this year, complete with a new OptiPress design.

Meanwhile, Voith Sulzer has just seen its latest offering for graphic paper grades go into commercial operation. Both Papierfabrik Lang and Papierfabrik Palm recently started up Tandem NipcoFlex presses on their new machines in Germany. With two stand-alone shoe presses and double felts, the new concept offers a closed sheet run from the wire section through both presses to the dryer section. According to the supplier, the larger the difference between web strength and web tension, the more reliable the web run becomes and the risk of breaks is reduced. Maximum security against breaks is achieved by reducing the web tension and completely avoiding open draws.

Previous models such as the NipcoFlex can still be used at 1,400 m/min as the third or fourth press. But at higher speeds a closed web run through the press section with an integrated shoe press is essential, according to Voith Sulzer. The Tandem NipcoFlex’s total closed draw principle makes it possible to achieve production speeds in excess of 1,800 m/min. In trials, speeds of up to 2,200 m/min have already been achieved, according to the supplier.

At the same time, Beloit has been busy developing its ENP shoe press design. The company’s latest offering is the ENP-C, with either double or single felted presses. Here again, the C stands for “” The Dual DF (double felted) ENP press eliminates the center roll and all the issues associated with the draw and the release characteristics of the center roll cover. Beloit believes that the low draw has a positive effect on machine runnability, sheet edge shrinkage and the number of press breaks. It is also possible to replace the bottom felt in the last press position with a transfer belt which has a smooth, non-permeable surface built on a filament base. The idea is that the belt supports the sheet in a “ draw” condition between presses or from the press to the first dryer. The web is under complete control and re-wetting from the felt is eliminated.

A Beloit spokesman explains that the shoe press belt is one area that the company is working on at the moment. “ are working on increasing the wear life of this belt. They usually have a working life of 40-80 million nip cycles. We are working with the machine clothing industry to improve belt and shoe press designs,” he says.

The company is not only working on designs though, Beloit also has its sights firmly set on extending its user base too. As one company spokesman explains, “, for all grades except tissue, the shoe press is considered common practice. Few projects in the printing/writing and board sectors today are without a shoe press.“

Going forward, the suppliers all share the belief that the shoe press is worth investing both time and money in. “ shoe press will be with us for many years to come,” was the prediction of one Beloit engineer. Voith Sulzer shares this optimism and continues to pump money into new developments. “ have something in the background to surprise the market, but there are still some trials to be run. We are working on improving the dryness, runnability and reducing the maintenance and installation costs.” Papermakers will have to be patient though, as Voith Sulzer is keeping its new design under close wraps at the moment.

Acting on impulse

In the meantime, companies can catch a glimpse of what the future may hold for pressing and drying techniques by looking at the latest developments in impulse drying. This basically involves subjecting the paper web to very high temperatures in the press nip in order to drive moisture out of the web.

At the end of last year, the Institute of Paper Science and Technology (IPST) and Beloit brought the technique one step closer to the commercial market when they successfully produced 161 g/m‰ linerboard using impulse drying on a 1.0 m wide pilot machine. This was the first time that impulse-dried linerboard had been formed, pressed, dried and reeled in one continuous operation.

The tests showed that impulse drying produced significant increases in press dryness and cross directional (CD) ring crush, compared to single-felted wet pressing. The impulse-dried liner was also considerably smoother, according to professor David Orloff at IPST. “ key finding was that impulse drying significantly reduced the roughness of the linerboard and that impulse-dried liner would not need to be calendered,” he explains.

IPST has since followed this experiment by converting the linerboard into boxes at the Stone Container box plant in the USA at Keokuk, Iowa. In this conversion process, the flexo-folder-gluer operation proceeded smoothly. There were no problems encountered with die cutting or score cracking either. The impulse-dried liner was compared to the single-felted wet pressed control as well as a commercial sample and it was found that the new technique increases ECT and box compression strength by up to 10%. According to the IPST team, impulse drying could be used to reduce fiber usage by about 10%, while maintaining strength properties.

Since the 1998 trials, IPST and Beloit have been sharing this data with linerboard manufacturers and evaluating the commercial impact that the impulse drying process will have on overall manufacturing costs (capital, operating, and furnish costs). These evaluations have included lab scale investigations, pilot trials, and economic studies. From these efforts, researchers hope to identify an appropriate candidate for the first commercial installation of impulse drying. “ believe that the ideal first application would be on a narrow machine that will not require roll crown control and where the attributes of impulse drying will have the greatest economic impact,” Orloff comments.

As Orloff goes on to explain, these attributes present an opportunity to reduce basis weight, improve printability, increase productivity through the potential for speed increases, reduce downtime due to press breaks and increase first dryer section temperatures.

The IPST is not alone in its efforts to introduce impulse drying though. The STFI research organization in Sweden is also involved in this area and is in the process of installing equipment to enable it to carry out further research. As Bo Norman at STFI explains, “ is just being installed to allow the running of two consecutive impulse technology nips acting on both product sides. Impulse treated paper has been rolled and evaluated for different product characteristics.“

The Swedish researchers believe there is still a lot of work to be done before commercial applications will be seen though.

Beloit's Dual Double Felted ENP gets rid of the center roll cluster

Dried out

Moving along the machine, this year has also been an important in terms of drying developments and, in particular, for Valmet. The first milestone for the supplier was the startup of the second commercial installation of a Condebelt application at Dong Il’s Ansan mill in South Korea. This was swiftly followed by the startup of Valmet’s first OptiDry installation at UPM-Kymmene’s Nordland Papier unit in Germany. On top of that, the company also saw the first installation of its new TurnDry system come on stream at Burgo’s Avezzano mill in Italy.

At Dong Il, Valmet installed a high Z-pressure Condebelt system on the mill’s testliner PM 1. Using the new technique, the web travels between a steam-heated upper belt and a water-cooled lower steel belt instead of conventional steam-heated cylinders. Following the rebuild, Dong Il has been able to run at a maximum production speed of 700 m/min with improved strength and humidity resistance, according to the supplier.

At Nordland Papier, Valmet installed what it claims to be the world’s first impingement drying unit on a commercial PM. With the OptiDry system, air is not only used to transport the evaporated water from the dryer section and to ventilate the area close to the sheet, it is also used to dry the sheet.

Bjorn Johansson, the manager of marketing services at Valmet, outlined the main principle of the technique. “ air with a temperature of up to 350ºC is blown at high velocity (80-120 m/s) against the sheet by means of an impingement hood. In this way, an evaporation rate of well over 100 kg water/m‰h (per hood area) can be achieved, depending on impingement velocity, temperature, sheet dryness and furnish, etc. Considerably higher evaporation rates have even been observed,” he explains. “ into dryer section length, this means that impingement drying is two to three times more efficient than conventional dryer sections per unit length.“

Optimal drying at UPM-Kymmene's Nordland Papier mill

Nordland’s PM 1 has a trim width of 4.7 m and a capacity of 180,000 tons/yr of 100-300 g/m‰ of fine paper and coating base stock. The company found that the primary dryer section had become a bottleneck in the process and decided to look into raising its capacity. One possibility was to add a shoe press to the PM to achieve a higher dry content after the press section, but it became clear that this solution alone would not be enough. As a result, Valmet and Nordland Papier came up with the idea of trying out the OptiDry impingement drying unit.

The unit was installed in the first dryer fabric group comprising two impingement hoods placed on a large diameter vacuum roll - a so-called OptiDry Roll. This vacuum roll is positioned at half height in the PM cellar. In order to keep the web well attached to the surface, the roll is grooved, drilled and connected to a vacuum system. With the 3,600 mm diameter roll, a very large impingement area between the web and the hoods is created.

As the OptiDry cylinder was installed at a lower level, the preceding and following dryer cylinders had to be leveled with it. All the top position cylinders were lowered so that they were just above the top edge of the former bottom cylinders. The former bottom cylinders were substituted by two smaller 1,000 mm diameter VacRolls. As a result, it was possible to retain all the “” cylinders in direct contact with the web. This means that the mill is also able to completely utilize the old primary dryer group capacity. The web run in the impingement drying unit is arranged so that it runs on top of the drying fabric, which is covering the vacuum roll. The bottom side of the web is therefore in direct contact with the hot impingement air, which maximizes the effect of the impingement drying.

The hoods are equipped with integrated air systems. This means that all the equipment necessary for moving and heating the impingement air is located inside the hoods themselves, saving a lot of valuable space around the machine.

According to Valmet, the results at Nordland have been very encouraging so far. Production has increased by 15-20%, depending on paper grade, and the maximum speed has jumped from 720 m/min to 870 m/min. On top of that, both energy and drying costs have dropped.

This year, Valmet also saw its first TurnDry unit go into action on PM 2 at Burgo’s Avezzano mill in Italy. As part of the rebuild, Valmet installed an online coating SymSizer station and a TurnDry air dryer on PM 2. As the product manager at Valmet, Richard Solin, explains, “ is very suitable for upgrades because of its high evaporation capacity and the fact that it fits easily into small spaces. But it is also being used on new machines, for example, the Burgo and Haindl PMs.“

The new dryer has been developed for coating machines and incorporates an air turn and air dryer. It can simultaneously turn and dry the web without contact after the coater. The air turn part of the concept blows heated air against the web, building up an air pad that allows the web to turn without any contact. The counter-side of the air turn section functions as an air dryer and is used to both blow and recirculate air. The upper dryer also recirculates its own air along with the impingement air coming from the air turn. At the same time, the lower air dryer section works as a conventional air dryer.

According to Solin, “ main advantage is the improvement in runnability. The paper web is very stable because there are no free draws. Compared to infrared units, there is also little need for maintenance with the TurnDry, which cuts costs. The system also has good energy efficiency with its recirculation system.“

On top of that, with no free draws between the drying elements, the heat and humidity loads in the machine hall are greatly reduced, Solin adds.

Fan club

Other suppliers have also been busy over the last 12 months. ABB Fläkt, for example, has just started using a new technique at Södra’s Mönsterås mill that will allow the company to increase pulp drying capacity. As Berndt Kaltin, deputy division manager for drying, explains, “ air fans have always been fixed speed fans and the impingement velocity has been constant. Now we can increase the evaporation rate and impingement velocity by increasing the fan speed using variable frequency drives (VFD).“

At Mönsterås, the company has installed a giant 3.3 MW frequency converter which supplies power to all the mill’s 141 circulation fans. As soon as the required drying capacity exceeds the maximum capacity of the open steam valve, the steam valve is set in the fully open position and the control of the outlet pulp moisture is taken over by the speed of the circulation fans. This means that the power requirement is maintained at the original level until the required drying capacity exceeds that of the fully open steam valve. With the new heat recovery unit and VFD system, Södra will be able to boost the capacity of its existing pulp dryer by 15%.

Another technique developed by ABB Fläkt has been used to increase drying capacity without extending the drying section - namely, the gas heated paper dryer (GHPD). According to Kaltin, “ gas heated paper dryer (GHPD) breaks with the old traditional way of paper drying which uses steam cylinders. The main advantage is the increase in evaporation rates.“

ABB Fläkt's gas heated paper dryer provides an alternative source of energy

When designing the equipment, the company concentrated on mills that wanted to extend their drying capacity, but within a limited space. In terms of dimension, the GHPD is similar in size to the steam cylinders. But the company claims that the gas fired infrared system can deliver up to five times the drying rate of conventional cylinders. Willamette in the USA and Australia’s Visy have already chosen to try out the new technique and the supplier hopes to confirm another GHPD installation order shortly.

Although steam dominates the industry at present, the future could lie with these alternative sources of energy. When papermakers look at upgrading drying equipment, the main consideration is more often than not cost-related. But even with these alternative energy sources, there are a lot of variables to take into account. For example, gas infrared drying costs are linked to gas prices which fluctuate, and electric infrared drying can be cost prohibitive unless the mill has its own generation plant.

As Johansson at Valmet points out, future electricity costs are far from certain. “ the time being, competition in the electricity industry is very hard and you can get cheap electricity. But with the ongoing consolidation of the industry, we do not know what the outcome will be,” he says.

Under control

While it is important for producers to ponder the energy costs involved in splashing out on new drying equipment or a rebuild, they must not forget the next step along the line either. A number of suppliers have launched products over the last 12 months designed to monitor and control drying equipment.

One of these companies is Johnson, which has recently extended its TechDry Optimization system. As part of the package, the company offers dryer section surveys, steam system design, rotary joint and syphon systems, as well as installation services and dryer monitoring. An additional feature that the company has developed is an online performance monitoring system.

The Backtender is a software package using input from the DCS and other sources to optimize press and dryer section performance. The system compares actual performances to benchmarks and alerts an operator when the parameters fall out of range, giving advice on how to solve the problem. The company has also launched its Advanced Dryer Drainage System Controls to take complete control of the steam and condensate system. According to Johnson, dryer steam pressures typically follow a grade specific evaporation curve to ensure optimum product quality and high efficiency. These curves are stored in the system and can be selected when needed. On top of that, the system is self-diagnosing to prevent any problems that may cause dryer drainage difficulties.

Meanwhile, the newly-formed Voith Sulzer Automation group has also launched a new moisture measurement system for the drying section. The company was formed on 1 October 1999 through the linkup of Impact Systems and the Voith Sulzer controls group.

According to the supplier, even moisture profile at the pickup of the dryer section is essential for PM runnability. With measurement at the end of a dryer section, it is difficult to determine the origin of uneven moisture profile and work out whether it is being caused by the press or the dryer section. Such control strategies are often incomplete or ineffective, according to Voith Sulzer Automation. For this reason, the supplier decided to measure moisture levels immediately after the press section.

The new scanner can be installed after the first or second cylinder of the dryer section and can take measurements against the free sheet, or against the sheet on a vacuum roll. The system offers an effective solution to improve CD control of the early web. The supplier claims that moisture measurement after the press section (shortly after the entrance of the web into the pre-dryer section) is genuine and unaffected by the felts and other influences of the dryer section. On grades as light as 40 g/m‰, the supplier claims that accuracy is as good as ± 0.5 %.

Keeping pace

As PM speeds increase and papermakers place increasingly tough demands on pressing and drying designs, suppliers are showing every sign that they are able to keep up with this rapid pace of development. Whether your interest lies in upgrading an existing press, getting more out of your dryer section, or improving your control over this equipment, then it could be worth your while having another closer look at the latest developments in this fast moving area.

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