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Bel champ 15 july 2020

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My opinion of Bel-Champ dryer section

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Bel champ 15 july 2020

  1. 1. Pele Oy CD Shrinkage Profile and Dryer Section Layout Pekka Komulainen July, 2020
  2. 2. Pele Oy Preface I would like to comment one very technical question about paper machine dryer sections. About 30 years ago Beloit designed new layouts for dryer section by names Bel-Run and Bel-Champ. They were well accepted. Everybody said that especially Bel-Champ is a revolutionary design. There are several running examples all over the world. I have not seen any written critics. However, when Beloit presented Bel-Champ to me in 1995, I immediately found several problems in that design and recommended Valmet to a big Asian customer. Today there is no Beloit anymore, so I can write down my opinions. Please feel free to comment. Helsinki, July 2020 Pekka Komulainen Seasoned Papermaking Professional 2
  4. 4. Pele Oy Web dimension changes from wire to reel ◼ From wire to reel paper shrinks and elongates. These dimension changes are not constant in CD direction. ◼ Web edges tend to shrink more in CD and elongate more in MD than the middle of the web. 4 Wet paper after wire Dry paper at reel (longer and narrower)
  5. 5. Pele Oy 5 Typical CD shrinkage profile ◼ Higher original grammage at the edges due to drying shrinkage. Must be compensated closing the slice at edges. ◼ Moisture expansion depends on the amount of drying shrinkage. Shrinkage and moisture expansion are always higher at web edges. Edge compared to the middle part tends to have: • Higher weight • Higher caliper • Higher roughness • Higher porosity • Lower dimensional stability and higher curl • Lower web tension (longer web) Shrinkage, % Picture:Valmet
  6. 6. Pele Oy Wrinkles at web edges ◼ Edge shrinkage is the main reason for edge wrinkles. 6 Picture: L&W
  7. 7. Pele Oy 7 Effect of drying concept on shrinkage ◼ In a single tier drying section the average shrinkage is lower but the increase of the shrinkage at the edges is very sharp. -2.0 -1.0 1.0 2.00 6.0 5.0 4.0 3.0 2.0 1.0 0 Position to the center [m] Shrinkage[%] Picture: Modified from Voith Paper
  8. 8. Pele Oy CD shrinkage profile for different free web lengths ◼ Shrinkage profiles at varying length and constant width. Shorter free draw, such as in Bel-Champ, means that the shrinkage gradient at edges is very high. This means problems for paper wrinkling, cockling and curl. 8 Lab trial of Wahlström et Lif
  9. 9. Pele Oy Lab trials of Wahlström et Lif ◼ Same web width W but decreasing web length L. ◼ Shorter free length decreases shrinkage in the middle but not at the edges. ◼ Bel-Champ has shortest free draw – highest gradient at edges. ◼ This means for copy paper problems for edge rolls e.g. diagonal curl. 9
  10. 10. Pele Oy Simulated CD shrinkage and web draw ◼ With short web draw it is possible to prevent shrinkage in the middle but not much at the edges. ◼ With shorter draw (Bel-Champ) shrinkage difference between middle and edges will be high. ◼ This has influence on all paper properties and causes cockling, curl and wrinkles at edges. 10 Problems to edge rolls and sheets Picture: Markku Parola et al., VTT Finland
  11. 11. Pele Oy Free draw and web tension profile ◼ With shorter free draw (e.g. Bel-Champ) web tension at edges is very low compared to the middle. 11 Picture: Markku Parola et al. VTT Finland
  12. 12. Pele Oy Shrinkage profile and vac-roll suction ◼ The effect of the vac-roll vacuum is highest in the middle. ◼ The CD profile of the web is more even with lower vacuum and higher average shrinkage. ◼ Bel-Champ requires high vacuum due to the low diameter vac-roll and high centrifugal forces. ◼ In addition, smaller diameter vac-roll can bend more, and high fabric tensions are not possible. 12 Picture: Wahlström
  13. 13. Pele Oy Valmet patent for Soininen et al. filed in 1972 ◼ A Valmet patent was granted to Soininen et al. already in 1975. This was principally very similar design as Bel-Champ patented for Champion international in 1991. ◼ The principle was that there is one common fabric for each dryer group and between dryers there is a small diameter suction roll. 13
  14. 14. Pele Oy Patent for Champion International, filed 1988 ◼ Patent of Judson Hannigan was granted in 1991. 14
  15. 15. Pele Oy Beloit Bel-Champ dryer section from 1993 ◼ Bel-Champ is very, very challenging dryer section. The web time on vac- roll is very short. Shrinkage is small in the middle (may be 2%) and about half meter on each edge shrinks up to 8%. ◼ There are practically no means to change this fault without big investments. 15 Bel-Champ Inverted group
  16. 16. Pele Oy Bel-Champ dryer section example ◼ These pictures are of a paper machine with 1000 mpm speed and 3.4 m width. ◼ Here the dryers are Ø1522 mm and vac-rolls Ø513 mm. The diameter ratio is about 3. Wide Bel-Champ is 1830/610 mm = 3.0 (exactly same ratio, why?). ◼ Modern Valmet machines have dryers Ø1830 mm and vac-rolls Ø1500 mm (ratio 1.22 is much smaller). 16
  17. 17. Pele Oy Two-tier dryer and active restraint 17 Picture: Jeffrey H. Pulkowski, Beloit, 1990
  18. 18. Pele Oy Bel-Champ dryer diameters ◼ In a Bel-Champ dryer length of free shrinking per cylinder is only 880 mm (16%), vacuum restraint 1175 mm (21%) and under fabric 3530 mm (63%). This means that main shrinking is at the edges (may be 0.5 m) and in the middle the shrinking is only about 2%. 18 Picture: Jeffrey H. Pulkowski, Beloit, 1990
  19. 19. Pele Oy Beloit patent WO1996023103A1 ◼ In 1995 Beloit patented an improvement for Bel-Champ to increase the free evaporation length and to shorten the total length of the dryer section. ◼ This solution seems to be very complicated compared to a Ø1500 mm vac-rolls. No references (?). 19 Patent: WO1996023103A1
  20. 20. Pele Oy Heat flux as a function of paper temperature ◼ In this case there is air dryer on top of a cylinder. It can be seen how much heat flux depends on paper temperature. This means that after every cylinder the web should be cooled on top of a large diameter vac-roll. ◼ In a Bel-Champ system there is not enough time for cooling – low efficiency. 20 Picture: Janne Keränen
  21. 21. Pele Oy Web temperature in standard single tier drying ◼ Web cools from 87 to 72 °C when it is over the fabric. This long time is important with Ø1500 mm vac-roll. Heat flux depends on temperature difference. ◼ Bel-Champ dryer has very short free evaporation distance. Paper temperature stays higher and drying efficiency is lower. 21
  22. 22. Pele Oy Bel-Champ example ◼ Bel-Champ dryer has lower average shrinkage but at the edges the gradient is extremely high. This causes problems with copy paper curl. 22 Picture: Jeffrey H. Pulkowski, Beloit, 1990
  23. 23. Pele Oy Drying shrinkage and vac-roll vacuum ◼ Total shrinkage is lower with higher vacuum in the vac-rolls. ◼ Vac-roll vacuum is needed to compensate centrifugal force. Comparing 1500/610 mm vac- rolls gives factor about 2.5. ◼ Theoretically vacuum in a Bel- Champ should be 150% higher than in a more conventional dryer. This increases energy consumption. ◼ High vacuum reduces shrinkage but increases gradient at the web edge. 23 Reel
  24. 24. Pele Oy Summary of Bel-Champ dryer section ◼ The main purpose of the Bel-Champ patent was to reduce shrinkage, especially at the edges so much that web defects at edges would be eliminated. ◼ With Bel-Champ the shrinkage in the middle was almost eliminated, but it was impossible to reduce the shrinkage near web edges. Therefore the shrinkage gradient at edges is high and higher than with other designs. ◼ High shrinkage gradient causes problems in cockling, curl, wrinkles, runnability and roll winding. Copy paper machines have more broke with edge sheets. Biggest problems are with never-dried pulps due to the high shrinkage potential. ◼ Bel-Champ web and fabric temperatures are high. This reduces heat flux and paper strength. Dryer section must be longer even if the web path is short. ◼ High fabric temperature also reduces lifetime of the fabric. ◼ Giacomozzi and Joutsimo: The tensile stiffness index, Scott bond, and elongation were reduced, whereas the bulk, opacity, air permeability, and light scattering values increased at higher drying temperatures. ◼ High web temperature softens all hydrophobic substances such as sizes and pitch. This increases stickies problem. 24
  25. 25. Pele Oy Selected literature 25 ◼ Soininen et al. United States Patent 3,868,780, March 4, 1975 ◼ Pulkowski, J.H. Operating Results with the Bel-Champ Single-Tier Dryer, TAPPI 1990 Engineering Conference Proceedings ◼ Päivi H. Viitaharju and K. Kaarlo J. Niskanen Dried-in shrinkage profiles of paper webs Vol. 76, No. 8 (Aug. 1993), Tappi Journal p. 129 ◼ Torbjörn Wahlström and Jan O Lif DRYER SECTION SIMULATOR FOR LABORATORY INVESTIGATIONS OF SHRINKAGE PROFILE 2003 Int. Paper physics Conference, Victoria, British Columbia, Sept. 7-11, 2003, pp. 169- 174 ◼ Dante E. Giacomozzi and Olli Joutsimo Drying Temperature and Hornification of Industrial Never-Dried Pinus radiata Pulps. 1. Strength, Optical, and Water Holding Properties BioRes. 10(3) 2017, 5791-5808
  • OzkanAydn

    Aug. 29, 2020

My opinion of Bel-Champ dryer section


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