FEATURE: Pressing and Drying
Role of shoe design
in long nip pressing
By Petter Honkalampi
Petter Honkalampi’s presentation on the role of shoe design was chosen as the best presentation of the Paper Session at the ABTCP-Zellcheming Conference held at São Paulo, Brazil in October 2007. The assessment was made by congress attendees and an evaluation committee, together with the chairmen of the technical sessions.
Shoe pressing provides an excellent opportunity to achieve good dryness levels and paper quality over a wide speed range. The design of the shoe in long nip pressing plays an important role by dividing the nip pressure over the length of the shoe, so that a given linear load is applied in the most economical way.
For copy paper, one of the most important characteristics influenced by pressing is paper thickness. Sufficient paper thickness, or stiffness, is required to ensure good sheet runnability in copying machines, and to ensure that the paper sheet feels rigid, easy to handle and read.
As pressing controls thickness, this can, in principle, be used to increase machine line profitability by saving raw material costs, or just by getting a better price for the paper or increasing productivity.
This article shows the effects of pressing on copy paper bulk and dry content in a two-nip shoe press concept, and especially the role of the 1st press shoe length. The results shown are based on pilot machine tests, and are compared to ones from production machines.
The results indicate that the 1st press affects bulk more than dry content. This information already provides machine builders with tools for constructing press configurations that are more profitable for papermakers and that pay more attention to raw material or energy savings.
This information can also be used to control machine runnability by improving dry content, wet strength, or the density of the paper according to grade-specific requirements. The shoe press technique itself creates tools for reaching higher production speeds, and the knowledge of shoe design gives additional value to this.
Tests with two different shoe lengths
Tests were carried out by using a Metso pilot paper machine and pilot calender.
Two different shoe lengths were applied in the 1st press to study the effect of shoe length on paper quality. The 2nd press shoe was kept unchanged during the test. Furnishes were typically copy paper furnishes based on eucalyptus and acacia, with 20 % long fiber and 20 % filler. The forming section concept was a gap former.
For these tests, the linear load and 1st press shoe length were chosen as the variables. The length of the longer shoe was about 10 inches, and the shorter shoe was about 30 % shorter. Otherwise, the shoe designs were similar. With this relationship, an approximately 50 % higher linear load was needed with the longer shoe to reach the same average nip pressure level (see Figure 2) as with the shorter shoe.
1st press loading affects bulk
It is quite reasonable to conclude that the 1st press loading has more effect on bulk than on dry content after the press section. The paper is actually compacted first in the 1st press nip, and then the 1st press determines how the compressed sheet structure will begin to develop.
During forming, the compacting forces applied to the paper are on a scale of 1-100 kPa, but in pressing the range is 1-10 MPa. If the paper is already too heavily compressed in the 1st press, it is difficult to maintain good paper thickness because pressing in the subsequent nips makes the sheet even more dense.
The effect of the change in the 1st press linear load on dry content after the press section is small. In wet pressing, each press attempts to balance the moisture changes in the paper (and in the felt) so that only a part of the change in dry content created before the nip is left after the nip [2].
Shoe pressing balances the nip incoming moisture changes even more than roll pressing. Shoe pressing has a longer dwell time, which makes the pressing more compression-controlled and, of course, enables higher dry content than roll pressing.
2nd press controls dry content and strength
The 2nd press shoe nip controls the dry content entering the dryer section and also the wet strength of the paper. These two factors are the key to reaching sufficient runnability in the dryer section and during paper finishing.
The 2nd press shoe just has to be long enough to handle the larger amount of water when the 1st press felts are new. This means that the dry content after the 1st press is lower until the felts have been started. Also the 2nd press shoe has to enable a high enough nip pressure in order to reach good dry content.
In this study the 2nd press is single felted (felt on the top side of the paper, transfer belt on the bottom side), which means that it makes the top side of the paper somewhat rougher than the bottom side. This means that, in the one-nip soft calender, the thermo roll has to press against the top side of the paper. As the 1st press is double felted, this naturally creates similar surfaces on both sides of the paper. Depending on the felt designs, this symmetry can be controlled.
The correlation of the nip dwell time or just the shoe length on the paper bulk is interesting. Figure 3 shows the difference in bulk as the 1st press shoe is changed. There is a clear trend indicating that a shorter shoe in the 1st shoe press enables higher bulk. This can already be seen in uncalendered base paper (see the starting points in the curves, top right corner, in Figure 3).
It seems that the 1st press determines how the structure of the paper will be compressed in subsequent pressing nips or in calendering. About the same difference in bulk (at the same roughness level) that was found from base paper can be found from the calendered results.
In Figure 4 the maximum pressure values are collected from the measured nip pressure curves (1st press) as linear load is changed (1st press from 200-1000kN/m, 2nd press 800-1000 kN/m). This result shows that, if the maximum pressure is increased, the bulk will decrease [1]. This is logical behavior because the nip dwell time in shoe pressing is long enough to compact the sheet for as long as the pressure is increased.
One can also discover that, with the same pressure level, the shorter shoe creates higher bulk. In this case, it means that also the linear load (and the press impulse) is lower. It should be noted that the normal level of the peak pressure with shoe presses varies between 3-6 MPa when paper bulk is a desired property.
Better paper, energy savingsv
An even more interesting feature can be found when the bulk vs. dry content correlation is examined (see Figure 5). These points have been estimated from the calendered results (with a calendered roughness of 200 ml/min in Bendtsen units), similarly to Figure 3, while the dry content values are measured after the press section. Now the 1st press loading varies between 200 kN/m and 800 kN/m and the 2nd press has a constant 1000 kN/m linear load.
The result in Figure 5 can be studied from two perspectives: either considering the bulk saving with the same dry content, or the potential to reach a higher dry content with the same paper bulk.
The first perspective would mean optimization of paper production through the use of less expensive raw material or, alternatively, production of better paper quality. The second perspective could mean energy savings at the dryer section or an increase in machine speed and production. In addition, the correlation lines will cross each other with low 1st press loadings (at lower dry content and higher bulk in Figure 5).
This means that, when the linear load or nip pressure is low enough, the effect of shoe length on bulk will vanish. On the other hand, when linear load is increased, pressing with a shorter shoe maintains the paper bulk better.
Fig 6 shows the well-known behavior with respect to bulk and dry content [1,3]. As discussed earlier, a longer 1st press shoe will decrease bulk at the same dry content. This means that optimization of the shoe length offers a tool for quality optimization. Figure 6 also shows that the furnish itself plays a key role in this optimization.
Careful pilot tests give reliable results
The correlation of pilot paper bulk results with ones from production machines has been studied (Figure 7). The results prove that, even though pilot machine facilities are quite different from production machines, the pilot paper making process is still similar enough to make reliable conclusions.
Further optimization with impingement drying
In addition to the press section, there are other production line positions that have an influence on bulk, for example sizing, calendering, reeling and winding. However, the influence of the 1st press on bulk is the most significant.
Development of the entire machine line, or press section and dryer section together, introduces many possibilities to optimize profitability. An example of this is the combination of closed draw transfer from press section to dryer section using the OptiDry Twin impingement drying technique [4]. In this solution, press section operation could be optimized with respect to bulk, while runnability at the dryer section is controlled through the clever use of impingement drying.
References
[1] Halme P, “Quest for bulk in a fine paper machine”, ABTCP Oct 2006
[2] Honkalampi P, Kataja M, “Dry content analysis in wet pressing: sensitivity to pressing variables”, NPPRJ (Nord. Pulp & Paper Res.J. ) vol. 17, no3/2002
[3] Honkalampi P, Jaakkola J, “Single nip shoe press impact to paper quality”, PAPTAC Jan 2005
[4] Juppi K, “Improved runnability and drying capacity through new technology”, Metso Paper Technology Days, June 2006
Shoe design test
| Pressing concept | Close draw press section with two shoe presses |
| Felts | 1st press double felted 2nd press single felted with transfer belt in the bottom position |
| Nip loading | 1st press 200 – 1000 kN/m 2nd press 600 – 1200 kN/m |
| Testing variable | Linear load and 1st press shoe |
| Furnish | Copy Paper: eucalyptus and acacia with 20 % long fiber and 20 % filler |
| Forming concept | Gap former |
| Calender | One-nip soft calender, with a heated (thermo) roll against the top side of the paper |
Figure 1. Principle configuration of pilot press concept for pressing tests – two shoe presses.
 |
 |
 |
 |
 |
 |
For more information, please contact
Petter Honkalampi
Manager, Product Development
Paper & Board Business Line
Jyväskylä, Finland
e-mail petter.honkalampi@metso.com
