Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE: A DRYER FELT DEVICE
The present invention relates to a dryer felt device for supporting a
web which extends through a single tier drying section of a paper machine.
More specifically, the present invention relates to a looped felt which
extends around a plurality of dryer cylinders of a single tier drying section.
In the papermaking art, a pressed web is transferred to a drying
section of a paper machine so that water remaining in the web is driven
therefrom by passing the web around a plurality of dryer cylinders disposed
in a single tier.
More specifically, with the advent of the BeIChamp type drying
section, drying speeds have greatly increased.
More specifically, U.S. Patent No. 4,934,067 assigned to Beloit
Technologies, Inc., disclosed a BeIChamp type dryer of the single tier type
and envisioned that drying speeds of 10,000 feet per minute could be
attainable. Already machine speeds of 8,000 feet per minute are being
attained on a single tier pilot machine.
However, in the aforementioned arrangement even when a vacuum
roll is disposed in close proximity with adjacent drying cylinders, there
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exists a tendency for the sheet to blow away from the supporting dryer felt
or fabric when the web is moving from the dryer towards the vacuum roll.
The present invention overcomes the aforementioned problem of
sheet blowing by the provision of a looped dryer felt having a high
permeability. The high permeability tends to permit the vacuum within the
vacuum roll to generate a negative pressure on the opposite side of the felt
run relative to the web during transit of the web from the dryer to the
vacuum roll.
Therefore, it is the primary objective of the present invention to
provide a dryer felt device which overcomes the aforementioned problems
associated with the prior art arrangements and which make a considerable
contribution to the art of drying a web.
The present invention relates to a dryer felt device for supporting a
web which extends through a single tier drying section of a paper machine.
The device includes a looped felt which has a permeability which is greater
than 90 cfm (cubic feet per minute).
More specifically, the present invention includes a dryer felt having a
permeability which is greater than 150 cfm and preferably at least 600
cfm.
Many modifications and variations of the present invention will be
readily apparent to those skilled in the art by consideration of the detailed
description contained hereinafter taken in conjunction with the annexed
drawings. However, such modifications and variations fall within the spirit
2 5 and scope of the present invention as defined by the appended claims.
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Fig. 1 is a side elevational view partially in section of a drying
cylinder and an adjacent vacuum roll showing the web blowing away from
the drying fabric.
Fig. 2 is a side elevational view similar to that shown in Fig. 1 but
with the vacuum roll disposed in close proximity to the adjacent dryer.
Fig. 3 is a similar view to that shown in Fig. 2 but includes a dryer
fabric which has a high permeability according to the present invention.
Fig. 4 is a side elevational view of a first dryer section of a single tier
1 o drying section of a paper machine according to the present invention.
Similar reference characters refer to similar parts throughout the
various views of the drawings.
Fig. 1 is a side elevation view of part of a prior art single tier drying
15 section generally designated 10 having a drying cylinder 12 and an adjacent
vacuum roll 14. A dryer fabric or felt 16 extends between the drying
cylinder 12 and the vacuum roll 14 and the web W is shown being blow
away from the supporting fabric 16 during transit between the drying
cylinder 12 and the vacuum roll 14.
20 Fig. 2 is a similar view to that shown in Fig. 1 but it shows the
vacuum roll 14 disposed in close proximity relative to the dryer 12.
Fig. 3 is a similar view to that shown in Fig. 1 but shows the
vacuum roll 14A disposed in closer proximity to the adjacent drying
cylinder 12A and a felt having a high permeability according to the present
25 invention.
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Fig. 4 is a side elevation view of a first dryer section 18 of a drying
section of a papermaking machine.
More specifically, as shown in Fig. 3, a dryer felt device 16A
supports a web WA which extends through a single tier drying section 18
of a papermaking machine. The device 16A includes a looped felt 16A
having a permeability which is greater than 90 cfm.
In one embodiment of the present inventio:~, the permeability of the
dryer felt is at least 150 cfm and in a preferred embodiment of the present
invention the permeability is at least 600 cfm.
1o In the prior art BeIChamp type dryer sections, the dryer felt 16 has
typically had a permeability of less than 90 cfm. The aforementioned low
permeability fabrics tend to clog easily and they are difficult to clean.
Additionally, another problem with the use of low permeability
fabrics or felts is that the application of an effective vacuum at the paper
surface as the paper passes over the vacuum roll is reduced.
Also, low permeability or closed fabrics are more likely to lower the
drying rate of the paper web.
The present invention includes the employment of an open fabric or
dryer felt having a permeability of at least 90 cfm and usually greater than
2 0 150 cfm. Additionally, trials on a pilot single tier BeIChamp type dryer
section have shown that an open fabric having a high permeability of 600
cfm can be used without blowing the sheet in the pocket areas.
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Furthermore, such open fabrics are easier to clean and there is the
potential for increasing the drying rate.
An additional advantage of using a high permeability fabric is that the
cost of production thereof may be reduced.
Clearly, the advantages of using a high permeability fabric include
improved machine runnability since the fabric stays open and is easy to
clean thereby increasing the drying capacity of the drying section.
The present invention also envisages use of the aforementioned high
permeability fabric which is capable of withstanding high temperatures
such as is present in those machines incorporating an air cap for blowing
heated air through the aforementioned fabric onto the web disposed
between the fabric and the dryer cylinder.
The advantage of the aforementioned air cap type drying section is
that the length of the dryer section can be reduced thus resulting in capital
savings.
Furthermore, the dryer capacity is variable which may be important
for future multigrade machines.
Also, drying on both sides of the sheet using air caps provides better
curl control and the potential for eliminating any bottom felted sections.
The felt according to the present invention is preferably of the type
having an open structure and having a permeability of 600 cfm while being
of a lower caliper.
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The aforementioned fabric according to the present invention may be
utilized in a single tier drying section having vacuum rolls which may be of
the stationary type or pivoting vacuum rolls.
Also, the fabric according to the present invention may be utilized in
a single tier drying section having passive vacuum boxes which generate a
negative pressure due to their proximity to adjacent vacuum rolls.
During trials using a high permeability fabric according to the present
invention, the web ran well in all pockets at 3,000 feet per minute. Also at
3,500 feet per minute ;he web ran well in pockets incorporating passive
to vacuum boxes of the aforementioned type.
As shown in Fig. 4, vacuum boxes 20 and 22 were disposed in the
pockets between the second and third dryers 24 and 26 and the third dryer
26 and fourth dryer 28, respectively.
The vacuum box 22 in the third pocket had felt seals of a simple
design due to the fixed geometry. The center shaft and seals in the second
and third vacuum rolls 30 and 32 respectively were rotated
counterclockwise 4 inches and 2.5 inches respectively, to provide more
vacuum in the box 22. The trial plan included comparing a conventional
BeIChamp type fabric having a permeability of 90 cfm with an open fabric
2 0 having a permeability of 600 cfm. The trial included the following
criteria.
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~ Run conventional BeIChamp 90 cfm fabric.
~ Draw: 50 (2"d press) - 60 (sample felt) - 10 ( 1 St press) fpm
~ Pocket/vacuum Gap bet~,rveen dryer Vacuum in the roll
roll # and VFR
2 (passive box) 2" 10" water column
3 (passive box with
felt seals) 3" 10" water column
4 2" 10" water column
5 3" 10" water column
6-7 0.5" 10" water column
~ Machine speed 3000 fpm or higher
~ Change: following conditions 30 min. prior to shut down
Draw: 40 (2"d press) - 45 (sample felt) - 10 (1 St press) fpm
Lower VFR 6 and 7 to 3"
~ Run open (600 cfm) fabric
~ Run same conditions as above
~ In case of threading or runnability problems:
Increase the vacuum in the rolls
Move the VFRs (4 - 7) closer to the dryer while threading/running
2 o Reduce machine speed
From the above trials, it became apparent that conventional
BeIChamp fabrics having a permeability of 90 cfm were causing serious
sheet blowing problems without the vacuum boxes at 3,000 feet per
minute. However, as shown in Fig. 1, as the gap between the dryer 12
2 5 and the vacuum roll 14 increased, the sheet flutter and blowing problem
became worse.
Even for the closed gap arrangement shown in Fig. 2, having a gap
of .5 inches between the dryer 12 and the vacuum roll 14, the sheet W
became slightly separated from the fabric 16 and had a tendency to follow
30 the dryer 12. However, the sheet in the second and third pocket ran very
well and stuck to the fabric 16A due to the passive vacuum boxes 20 and
22 even when the vacuum in such boxes 20 and 22 was very low. The
vacuum in the second and third roll 30 and 32 was varied to observe the
effect of low vacuum in the boxes on runnability. Even for zero vacuum in
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the rolls 30 and 32 there was some negative pressure in the boxes 20 and
22 which was enough to hold the sheet WA against the fabric 16A. Such
was thought to be due to the foil effect of the cross machine seal. The
vacuum levels in the rolls 30 and 32 and the boxes 20 and 22 are given in
Table 1. The vacuum in the third pocket box 22 was higher than the
second pocket box 20 due to better sealing. The vacuum level in the boxes
20 and 22 had no effect on runnability. Therefore, center shaft and seal
rotation are not necessary to obtain more vacuum in the boxes. The
change in draw had no significant effect on runnability.
For the open fabric 16A, the sheet WA ran better than with the
closed fabric at 3,000 feet per minute machine speed. Threading with
vacuum rolls in an open position that is with a three inch gap between the
dryer and the vacuum roll, was as good as threading with vacuum rolls in
the closed position that is the .5 inch gap. The sheet WA separated from
the fabric 16A in pockets 34, 36, 38 and 40 by approximately 3/8 inch but
was stable and without flutter. Accordingly, it became apparent that the
openness, that is the permeability or cfm value, had a major influence on
sheet flutter causing the reduction thereof. The sheet ran very well in the
second and third pocket and sheet marking was reduced by decreasing the
vacuum level and felt tension.
Table 1
Vacuum in the Rolls and Boxes
Pocket Vacuum in Vacuum in the
the roll, in. box, in. of
of water water column
column
Bel-Champ Fabric Open fabricBel-Champ fabricOpen fabric
2 9.8 10.3 0.74 0.47
3 3 0.27 0.18
0 1.2 0.03 0.08
3 10.2 10 1.25 0.68
3 1.51 0.47 0.15
0 0.06
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To study the effect of machine speed, 30 minutes prior to shut
down, the speed was increased to 3,500 feet per minute.
The sheet blowing and flutter increased significantly in non-passive
vacuum box pockets with increase in speed. Runnability in the passive
vacuum box pockets was not affected by speed.
Accordingly, it was concluded that the use of open fabrics in existing
BeIChamp or single tier designs will greatly increase the speed at which
such dryer sections can be operated.
