The steady increase in tissue production is, today, leading to a slight overcapacity, especially in the Central European area. That’s why more and more mills are focused on boosting their paper quality in terms of handfeel, softness and bulk, in order to be able to better sell all their production on the market.
Michael Pichler (Andritz AG):
A valuable technology in improving the above three features of the tissue roll can be found in the use of the TissueFlexTM press.
Ten TissueFlexTM presses are in operation as of today, and three more are currently in manufacturing.
As described in the course of previous technology updates, the main benefits of the TissueFlexTM are:
• lower maximum peak pressure (higher bulk);
• faster pressure drop after the max peak pressure (less rewetting);
• improved softness and handfeel.
Previously, the only thing which could be done in order to reach the desired paper quality, was to lower the line force directly at the press. Press loadings between 70 and 100 kN/m or between 70 and 170 kN/m, in connection with a reinforced Yankee (T-rib), could be used.
In order to increase the flexibility of the TissueFlexTM press further, and to allow the operator to adjust the after-press dryness/paper caliper to needs, Andritz AG and Voith Paper together developed the MD shoe movement.
WITH THIS NEWLY DEVELOPED TOOL, IT IS NOW POSSIBLE TO:
• attain, with just one press, the same after-press dryness as that of a conventional two-press machine, without the need for a special reinforced Yankee. So by using the TissueFlexTM with the MD shoe movement in combination with a conventional Yankee, after-press dryness values of 45% can be reached;
• attain even higher after-press dryness values at new applications, in connection with a reinforced Yankee;
• change the press characteristic for high bulk grades within minutes.
HOW DOES THE MD SHOE MOVEMENT WORK? With the MD shoe movement, the relative position between the press shoe (movable part), the Yankee and pistons which are loading the shoe (fixed parts) changes. The limits for the movement are +/- 15 mm and result in different nip pressure curves (see Figure 1). Figure 2 shows in detail how the nip pressure curves are changed just by moving the shoe, while keeping the line force of 100 kN/m unchanged.
To better highlight this difference, the nip pressure curve of a conventional suction pressure roll is shown in the background, while the green curve shows the nip pressure for the shoe in its original position. By using the MD shoe movement, just the nip pressure curves can now be changed, at the operator’s request.
THE RED CURVE SHOWS THE NIP PRESSURE FOR A SHOE MOVEMENT OF 15 MM AGAINST THE FELT RUNNING DIRECTION. By moving the shoe to this position, the max. peak pressure increases to 39 bar while the total nip length decreases. Operating a tissue machine with this shoe position leads to very high after-press dryness values and a slightly reduced paper caliper, due to the correlation between bulk and max. nip pressure. That means that this position is ideal for products which are optimised in the direction of best energy saving and highest output.
The blue curve again shows the nip pressure for a 15-mm shoe movement, but this time in the felt running direction. As can be seen, the max. peak pressure is further removed, compared to a fixed shoe, which results in an additional bulk gain. Running with this shoe position is ideal for SuperSoft paper grades, where bulk and softness are most important.
TABLE 3 SHOWS A COMPARISON BETWEEN AFTER-PRESS DRYNESS AND BULK INCREASE FOR THE DIFFERENT SHOE POSITIONS. The basis for the comparison is a conventional suction pressure roll configuration.
The “+” in front of the shoe position means movement in the felt running direction and a “-“ indicates movement against felt running direction.
These values are achievable without using a reinforced Yankee.
Using the most advanced combination TissueFlexTM, with the MD shoe movement together with a reinforced Yankee will boost the max. allowable nip load to 170 kN/m and will result in the nip pressure curves shown in Figure 4. Running this configuration can increase max. peak pressure to 59 bar and create very high after-press dryness values.
THE SHOE CAN BE MOVED FROM THE OUTSIDE OF THE TISSUEFLEX™ PRESS WITHIN SECONDS JUST BY TURNING A HANDWHEEL. Due to safety reasons, it is only possible to move it when the nip is open. Several guiding systems inside the roll ensure that there is no misalignment possible during the position change.
During operation, the shoe is secured by air clamping devices inside the roll as well as by an additional mechanical lock on the outside.
Two TissueFlexTM presses with the MD shoe movement have been in operation for 6 months, and three more are in manufacturing. All the nip pressure curves in Figures 2 and 4 as well as the data in Table 3 are measured at these two applications, under normal operating conditions. Further trials done so far regarding bulk and after-press dryness have also shown that moving the shoe to get more bulk is the much more economic solution rather than just lowering the line force at the TissueFlexTM.
CONCLUSION: with the development and implementation of the MD shoe movement, Andritz AG and Voith are now in a position to offer a press which can be perfectly adjusted for each product within minutes. For economy products, the same after-press dryness values as those of a conventional two-press machine can be reached, while by just changing the shoe position, SuperSoft grades with the highest possible bulk and softness can be produced. Again the switchover is possible from the outside in less than 5 minutes.
In combination with a reinforced Yankee (T-rib) it is possible to increase the flexibility even further due to the fact that press loads of 170 kN/m are possible. This will lead to after-press dryness values of more than 45%.•