Press section audit, Janek Schiefer, Head of TASK, Heimbach

PAPERTECHNOLOGYINTERNATIONAL
Sheet transfer by pick-up roll
Correctly set end deckles are a pre-requisite for correct
functioning of the roll. Only then can the web be reliably and
successfully transported into the press section via the pick-up felt,
while the two edge trims run with the forming fabric through to the
pulper. The angle of the suction box must be adjusted so that the
pick-up zone is perfectly in contact with the forming fabric during
production (Fig. 3).
Figure 5: Suction zone setting suction press roll.
Figure 3: Adjustment of suction
zone to forming wire.
There is usually a doctor with a lubricating shower on the
back of the roll, which must be regularly checked and cleaned.
Some pick-up rolls have an internal HP oscillating shower to clean
the suction holes. Meanwhile, below the pick-up roll, a guide roll
manoeuvres the bottom felt to the paper web. The felts are partially
saturated with water prior to the nip, so that pressure builds up
rapidly upon nip entry. As a result of this squeezed-out water flows
freely into the open area of the rolls. At the exit point of the nip, part
of the water on the underside of the felt is now thrown off the roll
(Fig. 4a 4b).
Figure 4a: (Below) Good nip dewatering
Characteristics of the 1st press
Due to the substantial volumes of water present in this
position, and dependent of course on the type of paper being
produced, the felt design, roll cover, doctor and saveall must be
precisely positioned and matched.
Felt type selection, conditioning and vacuum settings on the
felt suction box and suction press roll influence both the dewatering
and the running behaviour of the paper web.
The suction press roll is usually equipped with a highperformance cover and has a second press nip with a hard centre
press roll. The suction box must be at the correct angle in the run
direction in order to cover both nips (Fig. 5 above).
A steam blow box is frequently located on the outside of the
suction press roll between the first and second nip to increase sheet
temperature. By using saturated steamwater, viscosity in the paper
web is reduced making dewatering more efficient in the subsequent
nips. When a zone-controlled steam box is incorporated, the cross
machine profile of the sheet can be regulated.
Correctly adjusted end deckles ensure
that no leakage occurs and that steam passes
through the felt into the suction box. Otherwise,
this could cause damage to both felt and roll
cover. Furthermore, precisely aligned deckle
positioning prevents the edges of the sheet
from being negatively affected
(Fig. 6 above right).
Figure 4b: (Below) Insufficient nip dewatering.
Practical Tip
Press felts perform best with an optimum
saturation level of around 50% water
content elative to felt weight. So for
example, if the felt weight is 1600 gsm
the water content before entry into the
nip should be around 800 gsm.
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