The TissueFlex™ shoe press is proven successful in all commercial installations. Let's have a closer look at its advantages compared to other press concepts.
Jörg Baubock (Andritz AG)
Bulk, softness and handfeel have become increasingly important for both tissue producers and machine suppliers, who are put under pressure by the market trend towards higher tissue qualities. Therefore, these characteristics are top priority objects of investigation for the R&D teams on both sides. In the tissue production process, pressing basically influences the sheet properties, but also has an effect on machine efficiency and energy consumption.
TISSUE PRESSING. Normally paper pressing takes place between two rolls. In the case of tissue pressing, one roll is the Yankee cylinder. The sheet is compressed at the inlet of the press, reaching the maximum compression in the narrowest point of the press, i.e. in the nip. The sheet gets thinner, with water flowing out of the sheet and into the felt, which is running through the nip together with the sheet. The sheet has its maximum press dryness in the nip. After the nip the sheet is decompressed, the pressure gradient reverses and a certain amount of water is sucked back from the felt into the sheet. This so-called rewetting is an important factor in tissue pressing and influences the post-press dryness substantially. Due to the decompressing the thickness of the sheet recovers to a certain extent. The parameters of the pressing process influence the post-press dryness on the one hand, and bulk, structural softness and strength on the other.
A HIGH DRYNESS AFTER THE PRESS IS A BASIC REQUIREMENT. The pressing parameters which influence the post-press dryness and the sheet properties are line force, nip length and nip pressure curve over the nip length. Due to the low basis weight of tissue paper the pressing is structure controlled, not flow controlled. This means that the post-press dryness basically depends on line force and rewetting and only very little on the machine speed. For a high post-press dryness - which is demanded for a lower energy consumption in tissue drying - a high line force is necessary with a conventional press. This affects bulk and structural softness.
BULK AND SOFTNESS DEPEND ON THE MAXIMUM NIP PRESSURE. The lower it is, the higher is the potential bulk generation. At a given line force the maximum nip pressure is a function of the nip length. For a suction pressure roll, the nip length is given by roll diameter and hardness of the rubber cover. Low hardness and large diameter elongate the nip, resulting in improved bulk and softness.
However, rewetting at the outlet of the press increases as a consequence of the longer nip. This means the dryness after the press decreases.
TISSUEFLEX™ IS THE SOLUTION. Voith and Andritz have developed the TissueFlex™ press - a shoe press directly against the Yankee - in order to combine the advantages of improved bulk and softness with a higher post-press dryness. The nip of a shoe press is much longer than that of a pressure roll. Furthermore the pressure curve over the nip length can be influenced by the shape of the shoe. Figure 1 shows the press profiles of a conventional suction press and the TissueFlex™. As the nip of the shoe press is longer, the maximum nip pressure at a given line force is much lower. Reduced compression of the sheet leads to an increase of bulk and softness.
The steep decrease of the nip pressure curve after the nip reduces rewetting in comparison to the curve of a conventional suction pressure roll.
TISSUEFLEX™ HAS PROVEN SUCCESSFUL. The TissueFlex™ concept is well proven in the industry. Eight TissueFlex™ presses are in operation in commercial tissue machines around the world, most of them as upgrades of existing machines. In the last months, Andritz received another two TissueFlex™ orders, one of which will be part of a completely new Andritz PrimeLine™ tissue machine.
The main advantage of TissueFlex™ is the production of a sheet with improved structural softness and handfeel, as proven at all installations so far. A number of tissue mills, which have upgraded their machines with a TissueFlex™ press, were able to enter the market with new products of higher quality (i.e. softness and handfeel). There is however further potential of improving the properties of the final tissue product. Let's have a closer look at the most important advantages out of commercial TissueFlex™ installations.
BULK INCREASE OR FIBER SAVINGS. Several tissue machines with TissueFlex™ have been investigated, with the aim of defining the effective improvement of the paper quality, achievable with the shoe press, in comparison to the situation of a conventional machine before the upgrade. The higher bulk achieved with the TissueFlex™ press allows the reduction of the basis weight, this means less fibre is needed. The sheet properties, however, remain practically the same as those for the two pressure roll configuration before the modernization. Strength losses are negligible, while fibre savings of 5 - 7% were possible. On one TissueFlex™ machine a fibre reduction of as much as 10% was determined. Tab. 1 shows a comparison of results for bathroom tissue and towel, measured before and after the TissueFlex™ upgrade. Thanks to the increase in bulk a reduction in basis weight was achievable. After the upgrade with TissueFlex™, one tissue mill is in the position to sell 200 sheet toilet rolls having the same diameter as 220 sheet rolls produced before.
Another mill can distribute 50 napkins converted from TissueFlex™ paper in the same package size that previously contained 60 napkins.
Optimizations of felts and press sleeves - which are still ongoing - have enhanced the increase in post press dryness with TissueFlex™ compared to the situation before with a suction pressure roll. Thus, theoretical production losses due to the rebuild from two presses to a single press could be largely compensated.
HIGHEST OPERATIONAL FLEXIBILITY. The first installation of the full tissue machine concept with TissueFlex™ was started up successfully more than a year ago, at LPC Group in Leicester/UK. The tissue quality produced met the expectations. The outcome is high-quality bathroom tissue with excellent softness and handfeel, produced at a relatively low line force with reduced machine speed.
The quality of the tissue paper can compete with a single recreped sheet. Bulk figures of more than 10 cm3/g in a 19 g/m2 sheet are achieved. On the same machine (15 ft. Yankee diameter) standard tissue grades can be produced at line forces up to 170 kN/m and machine speeds up to 1750 m/min. In this case, the energy consumption of the machine is very low due to the high post-press dry content. In order to be able to apply line forces of that scale, a specially designed Yankee has been installed. The so-called T-rib Yankee - the shell of which is reinforced through the trapezoidal shape of the ribs (Figure 2) - allows line forces up to 200 kN/m, depending on the Yankee parameters. This results in 3-4 percentage points more dryness after the press, which means 10 - 12% more production or 15% less energy consumption. In addition to the effect of the higher post-press dryness, the T-rib Yankee shows approx.
5% more evaporation than a conventionally ribbed Yankee, due to the larger heat transfer area. By that, there is the opportunity of either reducing the steam consumption or increasing the production. Applying a lower steam pressure is therefore possible without production losses. Two T-rib Yankees are running very successfully so far. Together with the T-rib Yankee, the TissueFlex™ press gives the papermaker the highest operational flexibility with regard to product quality. A Large product range, from super-soft products to standard tissue papers, can be produced on the same machine.
TISSUE PRESSING IN TRIAL. Tissue machine suppliers have developed press concepts and machine configurations with the aim of improving the tissue quality in terms of bulk and softness. Different considerations around this matter have already been reported in previous issues of the Perini Journal. To learn more about the performance of concepts, Voith and Andritz have investigated different press configurations at their pilot plant in Sao Paulo, Brazil. It is a completely functional production machine, however only 1 m wide, with multi-layer headbox, CrescentFormer (double-wire configuration possible), Yankee (3600 mm diameter), high-efficiency hood and reel. It is available for customer trials and also extensively used for Voith's and Andritz' own research and development. Three press configurations have been investigated at the pilot plant: A conventional suction pressure roll, TissueFlex™ and a concept with a suction turning roll ahead of a blind-drilled rubber covered press roll against the Yankee (see Figure 3). The furnish was typical for bathroom tissue (50% hardwood, 50% softwood) in a 2-layer headbox configuration. The basis weight at reel was 17 g/m2 and the machine speed 1000 m/min. The measurements comprised bulk of the creped sheet, post-press dryness and handfeel.
BULK VS. LINE FORCE. The relation bulk-Line force (see Graph 1) showed the highest results for the TissueFlex™ configuration and the lowest for the single suction pressure roll. The third configuration with the plain pressure roll ranged in between. The bulk achieved with the TissueFlex™ at a line force of 170 kN/m was the same as that with the suction pressure roll at 70 kN/m. Reducing the line force, increases bulk considerably. The highest strength was achieved with the single suction pressure roll, while the strength values of TissueFlex™ were 5% lower, and those of the concept with the plain pressure roll were 10%lower.
HANDFEEL VS. LINE FORCE. Handfeel was determined by comparison to standard samples and evaluated on a scale with a maximum of 120 points. Graph 2 shows that the handfeel achieved with the TissueFlex™ was the best, worst was the handfeel of the paper produced with the suction pressure roll concept. As expected the handfeel improves with a higher line force, due to smoothing effects, which is contrary to the bulk development and the development of structural softness.
POST-PRESS DRYNESS VS. LINE FORCE. The relation between dryness after the press and line force is shown in Graph 3. The suction pressure roll and the TissueFlex™ give the same post-press dryness for a line force up to 9O kN/m, the result of the plain pressure roll is much worse. With this configuration the rewetting effect is much more severe as there is no suction in the press itself.
Another reason for the low dry content with the plain pressure roll is the gentle decrease of the pressure curve after the nip (see Figure 1). The difference is reflected in 34 percentage points of loss in dry content after the press. In terms of production loss and energy consumption, this means 12-15 % less production or 15-20% more energy consumption.
BULK VS. POST-PRESS DRYNESS. Operated at 170 kN/m, the TissueFlex™ gave practically the same bulk as the plain pressure roll concept at 90 kN/m (Graph 1) but 8 percentage points more post-press dryness (Graph 4). This graphs shows again the clear advantage of TissueFlex™ in comparison to the other two press concepts.
CONCLUSIONS. The following conclusions have been drawn from the experiences made with the running commercial TissueFlex™ machines and the pilot trials. The trial results met the expectations based on the theoretical considerations on tissue pressing made at the beginning of this article. In the trials, the TissueFlex™ press showed the best results for bulk, handfeel and post-press dryness compared to the other two press concepts investigated. Especially with regard to the relation between bulk and post-press dryness, the configuration with a plain pressure roll ranged far behind TissueFlex™. The main advantage of TissueFlex™ - the production of a sheet with improved structural softness and handfeel - has been proven on all commercial installations around the world. The comparison of the tissue quality before and after an upgrade with TissueFlex^™ shows the enormous improvement potential given by the shoe press.
Together with a T-rib Yankee, the TissueFlex™ press guarantees highest operational flexibility with regard to product quality. The trial results and the experiences from the commercial machines show that the TissueFlex™ is the most advanced press solution today.