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CD Shrinkage Profile and Dryer
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
Helsinki, July 2020
Seasoned Papermaking Professional
BEL-CHAMP DRYER SECTION COMPARED
TO LARGE VAC-ROLL DRYERS
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.
Wet paper after wire
Dry paper at reel (longer and narrower)
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)
Wrinkles at web edges
◼ Edge shrinkage is the main reason for edge wrinkles.
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
Position to the center [m]
Picture: Modified from Voith Paper
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.
Lab trial of Wahlström et Lif
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
◼ Bel-Champ has shortest
free draw – highest
gradient at edges.
◼ This means for copy
paper problems for edge
rolls e.g. diagonal curl.
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.
Problems to edge
rolls and sheets
Picture: Markku Parola et al., VTT Finland
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.
Picture: Markku Parola et al.
Shrinkage profile and vac-roll suction
◼ The effect of the vac-roll
vacuum is highest in the
◼ 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
◼ In addition, smaller diameter
vac-roll can bend more, and
high fabric tensions are not
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.
Patent for Champion International, filed 1988
◼ Patent of Judson Hannigan was granted in 1991.
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.
Bel-Champ Inverted group
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).
Two-tier dryer and active restraint
Picture: Jeffrey H. Pulkowski, Beloit, 1990
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%.
Picture: Jeffrey H. Pulkowski,
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 (?).
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.
Picture: Janne Keränen
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.
◼ Bel-Champ dryer has lower average shrinkage but at the edges the
gradient is extremely high. This causes problems with copy paper curl.
Picture: Jeffrey H. Pulkowski,
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
◼ High vacuum reduces
shrinkage but increases
gradient at the web edge.
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.
◼ 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
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