Pressing and drying developments in the past 12 months have been fast and furious in the paper world. Research work underway also suggests that this situation will continue for the foreseeable future

Shoe presses put a shine on several grades

In papermaking history, the year 2000 will more than likely be remembered for the startup and optimization of several new large-scale graphic paper machines in Europe - and more specifically in Germany. Over this year, Lang and Palm got used to the performances of their new PM 5 and PM 3 in Germany, while Haindl started up PM 3 at Augsburg and PM 9 at Schongau. Further south, the new PM 4 launched into action at Perlen's mill in Switzerland, while PM 2 was unwrapped at Soporcel's site in Portugal.

Boasting the latest gadgets in papermaking technology, most of the large new PMs are equipped with a shoe press or a double shoe press in some cases. As a result, this technology will once again be taking up a major part of the pressing and drying review, as we take a closer look at how the design is having an impact on various paper grades, not just the new graphic paper machines. On the drying side, the story is not as one-sided, with rapid development seen on a number of fronts, including infrared (IR) and air drying. The spotlight is also turned on the machine clothing suppliers which have had to come up with innovations of their own in order to keep up with the developments in the pressing and drying sections.

What's new with the shoe?

A new graphic paper machine design without a shoe press would be highly unlikely to make it off the drawing board these days, especially if the machine in question is a high speed, high capacity unit. On top of that, the shoe press is being used frequently in rebuild projects as a stand-alone, second, third or even fourth press. Of course, the unit was used in packaging grades for a number of years before moving onto graphic papers, and now it seems to be moving on again into the tissue arena.

The first tissue PM to start up with a shoe press was Celtech in the UK, where the mill invested in an Andritz TissueFlex PM. Another mill in the UK, Leicester Paper, has also ordered this equipment for startup in mid-2001.

The shoe press can also be installed as part of a rebuild on tissue machines and Andritz delivered this equipment to Metsä Tissue's Pauliström mill in Sweden for its PM 6 earlier this year.

Figure 2 - Shoe press benefits are also available to smaller producers

Although it may seem as if most of the action is in Europe for the moment, the second shoe press to start up on a tissue PM was actually in Brazil. Melhoramentos became the first paper producer in the Americas to start up a shoe press on a tissue machine in January 2000. The mill's PM 9 produces toilet tissue from 100% virgin fiber at a speed of 1,600 m/min. Albany International took part in the project, installing a new VentaBelt on the PM in the middle of this year. According to the supplier, the mill in Mogi das Cruzes, Sao Paulo, is also the first in the world to use a grooved belt in tissue production.

Kenneth Pulver, vice president of corporate communications at Albany International, explains that the grooved belt provides a gentler and more effective way of dewatering. He believes that producers are currently chasing three goals - improved productivity, dryness and quality. Using a shoe press in tissue production will help producers on their way to achieving these goals, Pulver says. "We will begin to see an explosion in tissue machines with shoe presses," he predicts.

Valmet has also carried out research in this area, but the supplier found that although there is a clear increase in bulk using a shoe press on tissue grades, other properties suffer as a result. "There is a price to pay for the bulk increase, and that price is lower tensile strength," according to Valmet. While acknowledging the benefits of higher dryness levels and bulk, Valmet also raises a questionmark over the deterioration of belts that the supplier saw during its trials with the shoe press. For example, wads became entangled on the in-going side of the SymBelt press which resulted in felts being burnt and the felt was also harder to keep open. The conclusion that Valmet has reached is that, for the moment, the shoe press technology may be more suitable for bathroom tissue products where tensile strength is less important.

According to the Finnish supplier, there is also room for improvement in the press sections used in graphic grades. The company recently saw its latest press design, the OptiPress I, go into action in the US. At the beginning of October, Willamette restarted uncoated fine paper machine #5 at its Johnsonburg mill in the US with a brand new OptiPress I unit. According to Kari Pantsu, marketing manager for paper machines at Valmet, the press section is unique because it has just one nip, two felts and a closed draw. "With the OptiPress, you would normally find two nips and four press felts, or three felts with a transfer belt," he explains. With one nip, the papermaker can eliminate two of the press felts on three felts which brings down costs, Pantsu adds. On top of that, the new press section is easier to run and operating costs are cheaper compared to existing units, according to Valmet.

Although this is the first installation of its kind, the supplier has had a busy year with its other shoe press units. Haindl's new PM 3 at Augsburg started up with a Valmet OptiPress unit, with a hard nip in the first position and a shoe press in the second. The supplier will be delivering a similar unit to Burgo's new PM 9 at Verzuolo, which will start up next year. The most recent order to hit Valmet's books is another double shoe press setup for Holmen's Hallsta mill. The 330,000 tonne/yr machine-finished magazine paper machine will start up in April 2002.

Size does not matter

While the benefits of shoe presses are well known for the huge new paper machines, Wolfgang Schuwerk at Voith Paper is keen to stress that the advantages are not merely limited to giant PMs. Voith Paper's equipment range also caters for the small and medium-sized PMs. "The application of shoe presses in small and medium size production machines, especially in rebuilds, generally requires compact rolls as well as compact press configurations," he explains. The smallest Voith Paper NipcoFlex press consists of rolls with a diameter of 1,100 mm and 1,050 mm. The maximum possible operating loads are 1,100 kN/m at a web width of 3 m or 700 kN/m at a web width of 5 m. Higher nip loads require slightly larger roll dimensions.

"The use of shoe presses in narrow and medium production machines, undoubtedly, requires very sophisticated and, in some cases, very individual press configurations," Schuwerk comments.

For example, in Voith Paper's case, the backing roll of the NipcoFlex press in narrow machines is generally designed without internal anti-deflection control. The flexible pressure shoe permits this without a reduction in the CD moisture profile quality. Special, position-stable Nipco-P backing rolls with internal anti-deflection control are used only for larger web widths and/or higher nip loads.

In addition to using the smallest possible roll diameters with reduced roll weights, the press configurations for small production machines must also be designed with the greatest possible functionality, Schuwerk explains. In September this year, Voith Paper rebuilt a machine in Finland where the overall length was just 4 m and the height was 3.8 m. The two-nip press section was overhauled using NipcoFlex technology, whereby a Combi NipcoFlex was installed using the same number of nips but with two additional dryers. According to Schuwerk, the benefits achieved include higher dryness with the same specific bulk as well as an improvement in roughness, two-sidedness and oil absorption (Figure 2). Improvements in dryness and runnability usually permit production increases between 10 and 25%. But on top of production improvements, costs are also an important factor for the smaller and medium-sized mills. As Schuwerk points out, "Taking production cost savings and improved runnability into account, the achievable additional overheads contribution of, for example, a 4.6 m wide Combi NipcoFlex press for SC papers, is approximately DM 10 million/yr."

In-depth interest

Apart from the machine suppliers, researchers also continue to focus considerable efforts on the press section and shoe press developments in particular. The Institute of Paper Science and Technology (IPST) in the US recently carried out a study into shoe pressing on low basis weight applications.

According to Fred Ahrens, professor and unit leader for water removal research, the expected water removal results were obtained. The researches found that shorter, higher pressure nips gave better water removal, at the expense of reducing bulk and increasing strength. For low basis weights, rewet is also a major concern. This concern may be supported by the water removal data, IPST explains, which indicate that slight changes in the rate of depressurization (ie, location of the peak pressure within the nip) appears to have a significant effect on outgoing solids. When the depressurization was steeper than average, higher solids were obtained than expected, and vice versa.

The Swedish Pulp and Paper Research Institute (STFI) has also been studying the impact of shoe presses, but this time on final paper properties. The research body has made a number of improvements to its pilot plant machine, called the EuroFEX, to carry out new trials. "This year we have a set of press shoes with different geometry which makes it possible to simulate both a roll press and a shoe press concept in the second and third press," Clas-Göran Hedström, manager for the pilot plant, explains.

In a recent study, the influence of the press nip geometry on final sheet properties was investigated comparing a roll press nip with a shoe press nip. A convex shoe was used to simulate a roll press nip with a shoe length of 25 mm and a concave shoe to simulate a shoe press nip with a nip length of 250 mm.

The experiments were performed for a wide range of linear load and roll surface temperatures, Hannes Vomhoff, senior reseach manager in Paper Process Technology, explains. Two-ply liner board was simulated as the experimental paper grade. The shoe pressing concept resulted in a lower final density of the paper at identical dryness levels. "With final sheet density being a very important paper property, these results give a strong indication of the potential of the shoe pressing technology for improving the final paper properties," he claims.

The EuroFEX pilot machine has also been used for STFI's research into impulse technology. Hedström explains the technique, saying, "Impulse technology can be regarded as an example of a strongly intensified process. After forming, the wet paper web is dewatered under pressure at high temperatures. A common picture of the course of events is that the water is partly vaporized and the steam formed assists in expelling the water from the web. This offers new possibilities of increasing the energy efficiency when paper is produced." Due to the transient character of the process, a sharp temperature gradient is formed in the thickness direction of the web. This gradient can be used to generate controlled thermal and chemical modifications of the paper web, in particular at the paper surface, Marie Bäckström, project manager, adds.

STFI is looking into a range of applications with impulse drying, for example using the unwinder to investigate the influence of the impulse process with a range of solids content or the effect of several impulse nips.

According to Professor Bo Norman at STFI, two consecutive impulse nips further improves dewatering and also permits impulse treatment of both paper sides. "Minimized two-sideness is important for many paper grades, in particular for printing grades," he adds.

STFI has also tested the printing properties of the paper produced by impulse technology. According to the institute, at a given print density, the amount of ink required was lower for the paper impulse-treated at 280°C, indicating that the paper surface in this sense had become denser.

Developments far from dried up

Moving on to drying, suppliers are also seeing the fruits of their research investments being borne out in the real world, while at the same time benefiting from the startup of new PMs. Over the past 12 months, Brunnschweiler has installed equipment on a number of the new paper machines across Europe. The Spanish supplier has delivered complete new air systems to Soporcel in Portugal, Saica and Papelera Guipuzcoana de Zicuñaga of Spain, Turkey's Karamursel mill and Fort James' Avigliano plant in Italy. On top of that, the company has carried out a number of rebuilds in order to boost production levels and cut energy consumption at various mills.

Meanwhile, for the drying division of ABB Fläkt Industri in Sweden, one of the biggest changes this year was in the company's name. The drying unit was turned into a new division of ABB Automation Systems and took the parent division's name.

On the order front, ABB Automation Systems was also kept busy this year, receiving a huge request from Burgo Ardennes in Belgium. Burgo will be upgrading the facilities at its Belgian mill over the next year with the aim of boosting the mill's pulp capacity by 100,000 tonnes/yr.

The company will replace an existing pulp dryer from the 1960s with a modern FCG Fläkt dryer. The work will take place during an 18 day shutdown in October next year. As a result, production capacity will rise to 700 air-dried tonnes/day. In addition to the dryer, the delivery will include a complete heat recovery system, ventilation of the bale handling area and complete mechanical and electrical installations.

Solar power

Although Solaronics launched its new Unidryer unit some two years ago, over the last 12 months the company has seen considerable interest in its drying system. Papermakers from both Europe and Asia are opting to install the Unidryer equipment, including UPM-Kymmene, International Paper's French mills, Burgo, Hankuk Paper and Sun Paper.

The Unidryer combines both infrared (IR) and flotation air drying techniques. According to the supplier, IR is excellent for energy transfer, while the second technique is known for its mass transfer and web stabilization properties. By combining the techniques, papermakers enjoy the advantages of both drying technologies, Solaronics believes. For example, higher efficiency, drying rates up to 230 kg/m2/hr, space savings and lower coat surface temperatures.

Valmet has spent the past year further developing its drying techniques for papermakers. The company's focus again seems to be on saving space and energy while at the same time boosting drying capacity. These factors appear to have been at the top of the supplier's agenda when the PowerFloat nozzle was designed. Used with the company's PowerDry units, the nozzle is the result of the integration of Valmet's traditional overpressure nozzle technology and the supplier's direct impingement technology. The new nozzle has 20% more drying capacity compared to conventional alternatives, according to Richard Solin, product line manager for air dryers at Valmet. The first air dryers equipped with the new nozzle launched into action in the third quarter of 2000 at a Finnish mill.

Heimbach launches its Matrix Forte

As machine speeds get faster and faster, drying systems typically have a hard time keeping up with the gear changes. "Although there are several other aspects of the papermaking process that have an influence on dryer section runnability, the air movements close to the surface of the web play a very important role, especially at higher machine speeds," Solin explains.

Air currents can cause a significant build-up of pressure in the nip areas causing the sheet to separate from the fabric. Valmet has been focusing on finding a solution to this problem and has come up with the new HiRun Blow Box. According to Solin, the vacuum induced by the blow box eliminates the pressure in the nip, preventing sheet detachment and also stabilizing the tail threading. Better sheet support naturally leads to fewer web breaks, he claims.

Other benefits include higher production speeds due to improved web stability, faster tail threading, fewer breaks due to reduced draws and savings in furnish costs, Solin explains.

Wringing out the research work

Along with the studies into pressing techniques, IPST has been working on new technologies for drying paper. For example, the institute is currently involved with a microwave drying project, working with Industrial Microwave Systems (IMS). The two teams have participated in the initial development of a microwave heating model for paper webs. According to the IPST, preliminary findings are encouraging and the priority now is to design and construct a laboratory-scale low power prototype to verify heating uniformity and to test the robustness to off-design operating conditions.

Another initiative at IPST is the laboratory scale Web Adhesion and Drying Simulator (WADS). This equipment supports experimental measurement of web adhesion (peeling tension) under controlled conditions, which simulate various dryer (or press) operating conditions. The WADS should be capable of providing data to make a quantitative map of the regions of greatest picking/sticking, which are a common problem for papermakers today.

Pressing and drying machine suppliers and research institutes have kept themselves occupied over the last 12 months, coming up with new products and carrying out installations across the globe. For their part, the papermakers have shown themselves to be more than willing to try out these new techniques, including the pioneer shoe presses on tissue machines, new drying developments and a single nip shoe press for graphic grades. With research galloping along at similar speeds to the larger paper machines these days, next year's review looks likely to be filled with another set of new products and developments for the pressing and drying section.

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