Note: Descriptions are shown in the official language in which they were submitted.
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Draw-free web transfer in a press section
The invention relates to a device and a method for the
draw-free web transfer of a web of fibrous material,
preferably a paper web, in the press section of a paper
machine.
Reference is made to the German Utility Model
G 92 06 340.
In the case of previously known web transfers in press
sections, it has always been attempted to separate the press
felt and the paper web as far as possible directly after the
press nip, in order to avoid rewetting of the paper web. For
this reason, the press felt is separated from the paper web
as rapidly as possible adjacent to the press nip, as a result
of which, however, tensions and ~YpAn~ions in the paper web
are produced which have a negative effect on the quality of
the paper produced.
In the case of a design corresponding to the document
EP 0 364 114 Al, therefore, in a single-felt press unit, the
guidance of the web of fibrous material together with the
press felt is carried out on the mating roll as far as a
pick-up roll which forms a nip with the mating roll. There,
the pick-up of the web of fibrous material and, furthermore
the transfer to a second felt are carried out, said second
felt being designed, however, as a transfer felt from which
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the web is led away in a free draw for further treatment.
It is therefore the object of the invention to describe
a method for guiding the paper web and a device in which
expansion of the paper web is avoided, the dryness content of
the paper web not being negatively influenced at the same
time, however. That is to say, therefore, the additional
transfer felt which is normally present between two presses
is made superfluous, but in spite of this the web of fibrous
material is still intended to come into contact with both its
sides consecutively with a smooth press roll in each case and
still intended to be transferred without a free web draw from
the first to the second felt.
The achievement of this object, according to the
invention, is characterized by the characterizing features of
claim 1 and of claim 18. Advantageous refinements are
reproduced in the subclaims.
The achievement of the object according to claims 1 and
18 is based on the knowledge that the final dryness content
downstream of a second press nip, which is designed as a long
press nip, virtually does not change if the input dryness
content in this second press nip is slightly different - that
is to say with or without previous rewetting of the web of
fibrous material by the first press felt. A separate transfer
felt can thus be dispensed with, so that the first press felt
virtually represents the transfer felt at the same time. This
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therefore means, presupposing that the following press nip is
formed by a long nip press, that the immediate separation of
the press felt from the web of fibrous material at the press
nip lying upstream can be dispensed with and the press felt
can be used as transfer felt.
Accordingly, the achievement of the object is based on
the knowledge that a high final dryness content because of
the utilization of a high-capacity press unit, for example a
long nip press, as second press nip can also be achieved even
if rewetting of the web of fibrous material takes place on
the first felt downstream of the first press nip. This
results from the long common run of the paper web and the
first felt downstream of the first press nip. This long
common run is necessary in order to transfer the web of
fibrous material to the second felt by means of the first
felt, that is to say dispensing with a separate transfer felt
and an open draw in the web of fibrous material.
In press sections having a press unit which comprises a
rotating body which forms a ring, preferably a roll, and a
mating element - which can be described as a roll or by a
press belt running over a shoe -, which together form a press
nip through which at least one endless felt belt (first felt
belt), together with the web of fibrous material, is guided,
for this purpose there is assigned to the rotating body
forming a ring, downstream of the press nip in the running
- 21587~8
direction of the web of fibrous material, a pick-up roll
which forms with said body a further second nip at which the
web of fibrous material is transferred with the first felt
belt to the pick-up roll. The web of fibrous material with
the first felt belt thus wraps around, downstream of the
press nip, part of the circumference of the rotating body
forming a ring, that is to say the web of fibrous material is
guided with the first felt belt along part of the
circumference of the rotating body forming a ring. The
guiding of the first felt belt is carried out in this case
along part of the circumference of a rotating body forming a
ring, the pick-up roll and further felt guiding elements. As
a result of guiding in this manner, the web of fibrous
material is further supported by the felt belt adjacent to
the press nip, which proves to be advantageous above all in
the case of a design of the rotating body forming a ring as a
granite roll. The complicated transfer, carried out manually,
of the web of fibrous material during the start up phase by
means of blast air in the free draw to the next press unit
can be omitted. The necessary pivoting of a press roll in the
subsequent press unit can be dispensed with.
In addition, according to the invention, the web of
fibrous material can be picked up from the first felt belt by
means of a second felt belt and fed with the latter through a
third press nip which is designed as a high-capacity
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dewatering nip. As a result of the provision and feeding of
the web of fibrous material with the second felt belt through
a high-capacity dewatering gap, the final dryness content
downstream of the latter remains virtually unchanged if the
input dryness content in this third press nip is slightly
different - that is to say with or without previous rewetting
of the web of fibrous material by the first press felt.
The rotating body forming a ring can be designed, for
example, as a roll or granite roll. The first mating element
can likewise be designed as a smooth roll or as a shoe roll,
according to the requirements of use.
The invention allows the use exclusively of
conventionally proven components, while similar arrangements
according to DE 40 26 021, for example, must use either
smooth press belts contacted by the web or smooth press
covers contacted by the paper, in each case with the high-
risk direct web transfer, or must use a paper suction and
blow roll. In this case there is the risk that the web of
fibrous material gets wound around the felt guide rolls and
"wraps them up".
In addition, by means of the arrangement and web
guidance according to the invention, there is the possibility
that the problems, occurring in press units of press sections
adjacent to nips in whose formation a granite roll is
involved, with the pick-up of the web of fibrous material
- 215 8~ 28
from the smooth surface and the transfer to subsequent press
units can be avoided, especially in the start up phase. Here,
it was previously necessary to realize the transfer by means
of additional aids such as, for example, blast air and the
opening of the subsequent press nip by pivoting away one of
the elements forming the nip.
In the case of the use of shoe press units, where the
guiding of the web of fibrous material is essentially
determined by the guiding of the felt belts, the problems of
rewetting of the web of fibrous material after passing
through the press nip and the damage occurring to the web of
fibrous material during its transport away with the felt on
the shoe side as a result of the drawing of the felt over the
edge of the shoe was previously especially relevant. This
disadvantage and the wear on the felt belts and the loading
of the web of fibrous material with respect to conventional
web guiding after the press nip can be virtually avoided,
since felt and web of fibrous material are no longer drawn
over the shoe, especially the exit radius of the shoe, at the
exit from the press nip when the web of fibrous material is
being carried away by means of the felt on the shoe side.
First and second press unit can be designed as single-
felt or double-felt press units.
In the case of designing the first press unit as a
double-felt press unit, for example, the web of fibrous
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-- 7 --
material is guided between two felt belts, the first felt
belt and a further third felt belt, through the press nip and
is subsequently guided between both felts with the latter
along part of the circumference of the rotating body forming
a ring, that is to say the guiding of the felt belts is
carried out in such a way that the first felt belt contacts
the mating element at least directly, the rotating body
forming a ring at least over part of its circumference
adjacent to the press nip as far as to the nip directly, and
downstream of the nip wraps around the pick-up roll once more
on part of its circumference, while the third felt belt wraps
directly around the rotating body forming a ring over part of
its circumference in the press nip and adjacent to the press
nip. The separation of the two felt belts from each other is
carried out at the pick-up roll which is arranged in the
running direction of the web of fibrous material downstream
of the press nip and in relation to the rotating body forming
a ring in such a way that said pick-up roll and said body
form a nip with each other. In this arrangement, the
separation can be carried out directly in the region of the
nip or on the circumference of the pick-up roll adjacent to
the nip. In the first case, the pick-up roll is preferably
designed as a suction roll. The web of fibrous material is in
this case sucked on to the first felt belt. The second felt
belt is guided in such a way that it is separated from the
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-- 8 --
web of fibrous material in the free draw and from the first
felt belt in the nip.
In the second case, the pick-up roll can be designed as
a full-cover roll. The pick-up roll is wrapped around over
part of its circumference by the first felt belt. The guiding
of the third felt belt is then carried out in such a way that
the third felt belt likewise wraps around the pick-up roll
over part of its circumference downstream of the nip, the
wrap angle of the third felt on the pick-up roll being
smaller than the wrap angle of the first felt belt in
relation to the pick-up roll. Because of the guidance of the
felt belt, the third felt belt is drawn off from the web of
fibrous material in the free draw and is preferably
ventilated on both sides, while the web of fibrous material
remains on the first felt belt. The web of fibrous material
and the felt belt on the shoe side, that is to say the felt
belt which wraps directly around the mating element at least
over part of its circumference, are not drawn over the exit
radius of the shoe in this case, as a result of which damage
to the web of fibrous material as a result of pressure/draw
loading is avoided.
Press units of this type can be arranged in press
sections of various designs, for example in press sections of
machines for the production of tissue, the second press unit
being able to be designed as a single-felt press unit. A
21~872g
design of the second press unit as a double-felt press unit
is likewise conceivable. For the purposes of closed web
guidance within the press section, in this case the web of
fibrous material is transferred from the first felt belt to a
second felt belt which is guided with the web of fibrous
material through the press nip of the second press unit. The
transfer can in this case be carried out either exactly on
the pick-up roll or on a further felt guiding element of the
first felt belt. For this purpose it is necessary that the
second felt belt likewise wraps around the pick-up roll or
the felt guiding element in the regions of the pick-up roll
or of the felt guiding element which are wrapped around by
the first felt belt. The felt guiding element is preferably
likewise constructed as a blow roll.
In the case of the transfer on the pick-up roll, the
latter preferably has a suction zone directed toward the
rotating body forming a ring and a blowing zone directed
toward the second felt belt. The blowing zone can extend over
the entire width transversely to the running direction of the
web of fibrous material or can be constructed only in the
edge regions of the roll.
From an economic standpoint, the pick-ups are preferably
designed in accordance with the patent application P 44 40
948.6, which is not a prepublished document, with a variably
adjustable suction zone width across the machine width.
2158728
-- 10 --
In a further aspect of the invention, it is conceivable
to realize a draw-free web guidance between two press units,
the pick-up of the first felt belt and the web of fibrous
material being carried out adjacent to the first press nip by
means of a suction device in the form of a rigid suction box.
The guiding of a web of fibrous material, according to
the invention, is explained hereinafter with reference to
figures, in which:
Figure 1 shows a guiding of a web of fibrous material,
according to the invention, between the first press
nip and a long nip press;
Figure 2 shows a device like figure 1, but with an
additional felt guide roll;
Figure 3 shows a guiding of a web of fibrous material,
according to the invention, downstream of a double-
felt shoe press unit with separation in the press
nip of the mating element, using a simple pick-up
roll;
Figure 4 shows a guiding of a web of fibrous material,
according to the invention, downstream of a double-
felt shoe press unit with separation at a suction
roll;
Figure 5 snows a guiding of a web of fibrous material,
according to the invention, downstream of a double-
21c~8~28
felt shoe press unit with separation on a suctionblower roll and subsequent transfer of the web of
fibrous material on to a further felt belt;
igure 6 shows a guiding of a web of fibrous material,
according to the invention, downstream of a press
unit with a granite roll;
igure 7 shows a draw-free web transfer from a press nip to
a long nip press, having a felt guide roll which is
variable in its width to which suction is applied;
igure 8 shows a felt guide roll corresponding to Figure 7;
igure 9a shows a draw-free web transfer from a press nip to
a subsequent long nip press with the aid of a
suction box;
igure 9b shows a view in cross section through the contact
pressure strips of the suction box, with a spray
pipe as cleaning device;
igure 9c shows a longitudinal section through the suction
box according to the invention.
Figure 1 shows a section of a press section having draw-
free web transfer from a press nip I of a press unit 1, via a
press nip II to a third press nip III, which is designed as a
high-capacity dewatering nip and, as in the case shown, is
formed by a long nip press 3. The first press nip I is formed
by a rotating body 4 forming a ring and a first mating roll
5, in the case shown a pair of rolls, comprising a central
- 21S8~28
roll 4.1 and a profiled roll 5.1. An endless first felt belt,
here the press felt F1, runs over a felt guide roll into the
press nip I on the side of the profiled roll 5.1, while at
the same time on the side of the central roll 4.1 the web of
fibrous material B which is to be dewatered runs in. Adjacent
to the press nip I, the pressed web of fibrous material B,
resting on the central roll 4.1, together with the press felt
F1 lying thereon, wraps around the central roll 4.1 by an
angle ~, until it is finally picked up by a pick-up device 6
in the form of a pick-up roll, together with the felt F1,
from the central roll 4.1. In this case the pick-up roll is
represented by a suction roll 6.1 which, in this example, has
two suction zones, the suction zone 7 which is first in the
running direction of the web of fibrous material being
equipped with a higher vacuum than the second suction zone 8.
The pick-up roll forms the second press nip II with the
central roll 4.1.
The suction roll 6.1 leads the press felt F1, together
with the web of fibrous material B which remains thereon, to
a second opposite suction roll 9 which is designed as a felt
guide roll for a second felt F2 which leads through the long
nip press 3. The felt F2 takes over the web of fibrous
material B, supported by the vacuum of the suction guide roll
9, and guides the web of fibrous material B into the press
nip III of the long nip press, which is formed by a profiled
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roll 10.1 and a shoe roll 11.1, comprising a rotating jacket
which can be pressed against the profiled roll by means of a
shoe.
In the case of a design corresponding to Figure 1, the
pick-up roll 6.1 also serves for transferring the web of
fibrous material on the part of the first felt F1.
Figure 2 shows a press section similar to Figure 1. The
corresponding reference symbols have been used. The
difference consists in the fact that the transfer device in
this case comprises a first roll 6.2 to which suction is
applied and a subsequent simple felt guide roll 12, the
transfer of the paper web on to the felt F2, which is guided
over the suction guide roll 2, being carried out between the
roll 6.2 and the roll 12. According to the invention, the
roll 6.1 shown in Figure 1 is of pivotable design, so that
the paper web can, for example in the case of a break of the
web of fibrous material, be guided along the roll surface of
the roll 4.1 to a doctor 13 and lands in the broke pit. It is
likewise possible, in the case of Figure 2, to pivot the two
rolls 6.2 and 12 of the pick-up and transfer device away from
the roll 4.1 coupled to each other.
In Figures 1 and 2, the pick-up roll is also a component
of the transfer device for transferring the web of fibrous
material to the second felt F2.
Figure 3 illustrates the further possibility of closed
215872~
web guidance according to the invention in a section of a
press section. A press unit 1, including a press element
designed here as a shoe roll 5.2, forms with a rotating body
forming a ring, here a roll 4.2, an extended press nip I
through which a web of fibrous material B is led. The web of
fibrous material B is guided between two endless felt belts,
a first felt F1 and a further felt F3, through the press nip
I and is there dewatered under the action of pressure. After
the exit from the press nip I, the web of fibrous material B
is fed between the first felt F1 and the felt F3 over part of
the circumference 15 of the roll 4.2, according to the
invention. For this purpose, the felt F3 is guided in such a
way that it wraps around the roll 4.2 over part of its
circumference 15 and around at least one felt guiding
element, not shown here. The first felt F1 wraps around the
circumference 16 of the press element 5.2, at least in the
region of the press nip I, and after the exit from the press
nip I is guided together with the felt F3 and the web of
fibrous material B over part of the circumference 15 of the
roll 4.2 and then wraps around a suction roll 6.3 over part
of its circumference 17 and around further felt guiding
elements, not shown here. The suction roll 6.3 is arranged
opposite the roll 4.2 in such a manner that the two form a
nip II with each other. At the exit from the nip II, because
of the suction effect of the suction roll 6.3, the web of
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- 15 -
fibrous material B is separated from the felt F3 and is
guided together with the felt Fl along part of the
circumference 17 of the suction roll 6.3 and is transferred
at a felt guiding roll 18 to a further endless felt belt F2.
The felt guide roll 18 is primarily wrapped around by the
felt belt F2 and is preferably designed as a suction roll
which sucks on the web of fibrous material B. The felt F2
serves to guide the web of fibrous material B through a third
and extended press nip III of a second press unit 3, which is
formed by a second shoe roll 10.2 and a second mating element
11.2. The guiding of the web of fibrous material is carried
out in this region of the press section without a free draw,
that is to say the web of fibrous material is supported
during its passage through the press section in each case by
at least one felt.
The arrangement, shown in the figure, of the pick-up
roll 6.3 and of the roll 18 for the purposes of transferring
the web of fibrous material B from the felt Fl on to the felt
belt F2, however, generally has the effect, during transfer,
of an elongation of the web of fibrous material - shown here
as a broken line - as a result of which, in the region
between the felt guide roll 18 and the press nip III, a
separation of the web from the felt belt F2 can result, for
which reason the transfer of the web of fibrous material from
the felt Fl on to the felt F2 should preferably be carried
2158728
out as described in the following figures.
Figure 4 likewise illustrates a section of a press
section of a machine for the production of endless webs of
fibrous material, having two press units, as described in
Figure 3, for which reason identical reference numbers are
used for identical elements. The web of fibrous material B is
also guided here through the extended press nip I of the
press unit 1 between the felt F3 and the felt F2. After the
exit from the press nip I, felt F1 and felt F3 with the web
of fibrous material lying between them are guided together on
part of the circumference 15 of the roll 4.2. The further
guidance of the web of fibrous material B with the felt Fl is
carriéd out by picking up by means of the suction roll 6.3 in
the nip II in a manner similar to Figure 3. The web of
fibrous material B is guided with the felt F1 over a further
second roll 19 (felt guiding element of the felt Fl), which
is wrapped around over part of its circumference 20 by the
felt Fl and is designed as a blow roll. The transfer of the
web of fibrous material B to the endless felt belt F2, which
is guided with the web of fibrous material B through the
extended press nip III to a second press unit 3, is carried
out on this blow roll 19. The felt belt F2 does not only
touch but wraps around the blow roll 19 indirectly over part
of its circumference 20.
- 21S87 28
- 17 -
Figure 4 shows a possibility for guiding the web of
fibrous material, in which the felt F3 is guided downstream
of the extended press nip I over part of the circumference of
the roll 4.2 and along part of the circumference 17 of the
pick-up roll 6.3, which forms a nip II with the roll 4.2. The
web of fibrous material B is likewise guided with the felt
Fl. However, the separation of felt Fl and F3 is carried out
in this design only downstream of the nip II on the
circumference 17 of the pick-up roll 6.3. Because of its
guiding by means of felt guiding elements, not shown here,
the felt F3 is drawn off from the felt Fl and ventilated on
both sides downstream of the nip II in the free draw in the
wrap-around region of the felt Fl with the pick-up roll 6.3.
Separation of the felts Fl and F3 in the nip II is
likewise conceivable but not shown here.
Figure S illustrates the possibility of guiding the web
of fibrous material downstream of a first press unit 1,
designed as a shoe press unit, with a supporting element
acting in the direction of gravity in the installed position
and with an adjoining second press unit 3, the transfer of
the web of fibrous material B on to the felt belt F2 being
carried out after the separation of the felts Fl and F3 on a
pick-up roll 6.4, which is designed as a suction blow roll.
The shoe press unit comprises a rotating body forming a ring,
for example in the form of a roll 4.3, which forms with a
- 21~2~
- 18 -
mating element 5.3 a first press nip I through which the web
of fibrous material B is led between a first endless felt F3,
which wraps directly around the ring over part of its
circumference 21, and a felt belt Fl, which wraps directly
around or contacts the mating element 5.3 over part of its
circumference. The separation of the two felt belts F1 and F3
is carried out in the nip II, which is formed by the pick-up
roll 6.4 with the roll 4.3. The web of fibrous material B is
in this case sucked on to the felt belt F1, for which reason
the pick-up roll 6.4 has a suction part 22 directed toward
the roll 4.3. The transfer of the web of fibrous material B
from the felt belt F1 to a felt belt F2, which is guided
together with the web of fibrous material B through a further
high-capacity dewatering press nip III of a second press unit
3, is likewise carried out on the pick-up roll 6.4. For this
purpose, the latter is partly wrapped around by the felt belt
F2 in the wrap-around region of the first felt belt F1 with
the pick-up roll. The pick-up roll 6.4 has in this region a
blast zone 23 which is preferably provided only in the end
regions of the pick-up roll 6.4.
Figure 6 illustrates a possibility for carrying off the
web of fibrous material directly downstream of the press nip
of a single-felt press unit 1 which comprises two rolls, one
roll being designed as a granite roll 4.4. After passing
through a press nip 24, the web of fibrous material is guided
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-- 19 --
to a double-felt press device, which is formed by a roll 25
and a mating element 26, along the circumference of the
mating element downstream of the press nip 24, with a felt
band 28 which wraps around the mating element over part of
its circumference 27. For this purpose, the web of fibrous
material B is already separated at the exit of the press nip
24 from a felt belt 30, which directly partly wraps around
the roll 25 over part of its circumference 31 and is guided
together with the felt belt 28 through the press nip 24. The
roll 26 is designed as a suction press roll for this purpose.
The guidance of the individual felt belts - felt belt 28 and
felt belt 30 iS carried out in such a way that the felt belt
28 is already separated from the felt belt 30 at the exit
from the press nip 24 and, because of the design of the
mating element 26 as a suction press roll, the web of fibrous
material B is sucked on. The web of fibrous material B is
guided with the felt belt 28 through a further press nip 29
which is formed by the roll 26 with a granite roll 4.4. After
completing the dewatering process in the press nip 29, the
web of fibrous material B is guided on its own further along
the surface 32 of the granite roll 4.4 and led together with
a felt belt F1 through a press nip I which is formed by the
granite roll 4. 4 and a further press roll 5.4. The felt Fl
and the web of fibrous material B, after passing through the
press nip I, are guided together along the surface 32 of the
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- 20 -
granite roll 4.4 and through a second press nip II which is
formed by the granite roll 4.4 with a pick-up roll 6.5,
designed as a suction blow roll, in order then to be
transferred to a felt belt F2 which wraps indirectly around
the pick-up roll 6.5 over part of its circumference and is
guided together with the web of fibrous material B to a
further press unit 3. The pick-up roll 6.5 is constructed
here as described in Figure 5. Said roll includes a suction
region 33, directed toward the granite roll 4.4, and a blast
region 34 which is preferably constructed only in the end
regions of the roll and is directed toward the felt belt F2.
The sections from the press sections which are cited in
the figures are only examples for a possible arrangement of
press units. The significant fact here is that the transfer
of the web of fibrous material on to a felt belt after
passing through a press nip is essentially carried out in
such a way that the web of fibrous material, after the exit
from the press nip, is guided with the felt over part of the
circumference of one of the press elements and passes through
a nip which is formed with this press element by a pick-up
roll which is at the same time guiding element of the felt.
According to the invention, it is essential that,
following the draw-free web transfer from a previous press
nip to a subsequent press nip, the subsequent press nip is
designed as a high-capacity dewatering nip, for example in
21~8728
the form of a long nip press, since because of the properties
of the long nip press the rewetting of the web of fibrous
material as a result of the continuous contact with the felt
downstream of the first press nip is compensated. On the
other hand, it is of course possible according to the
invention to design the previous or one of the previous press
nips as a long nip press.
The possibilities of the transfer of the web of fibrous
material for further transport to further press units can be
used for the passage in press units of various designs; the
press unit arrangements cited in the figures are therefore
not compulsory.
Figure 7 illustrates a draw-free web guidance in a press
section, in which the design of the pick-up roll on a first
press unit is carried out in such a way that the latter is
especially suitable for the problems of the start-up process,
that is to say when the production of a paper machine begins
to start up following a preceding standstill and a narrow
strip of the still wet paper web is cut out from somewhere
upstream of the drying section or at the beginning of the
drying section (so-called tail) and this tail is initially
fed through the entire drying section. The main part of the
still wet paper web is, in contrast, diverted away at the
point of separation into a pit underneath the paper machine
in order to be converted, by means of stirring with the
- 215~7~8
addition of water, once more into a stock suspension which is
fed back once more to the stock circulation. This process is
described in EP 0 584 492 Al. The so-called tail can be
generated here by being already separated by a water jet on
the paper machine wire from the remaining paper web just
produced. However, said tail can also be cut out at a
subsequent location, viewed in the machine direction, by
cutting out from the already consolidated paper web. The
separation of tail and remaining web is carried out, for
example, at a press roll, for example at the central roll of
a roll press having three press nips. At this location, the
main part of the web - as mentioned above - is diverted into
a catching pit, while the tail is transferred on to a suction
roll. The suction roll can be equipped with two suction
zones. The first of these two suction zones is located at one
end of the suction roll, specifically where the tail is
guided. This first suction zone is as narrow as the tail
itself. The second suction zone extends over the remaining
part of the suction roll width. In this arrangement, the
first suction zone has vacuum applied to it first, in order
to transfer the tail from the previous press roll to said
suction roll. As soon as the tail has passed through the
entire paper machine, the tip cutter is moved transversely to
the running direction of the paper web, specifically in the
direction in which the tail is continuously broadened, until
215~2~
the complete web width is reached. Vacuum must already be
applied to the second suction zone at the beginning of this
process, in order to seize the broadening web and to transfer
it to the suction roll. However, in this process the
following problem occurs: already from the first instant of
making the web wider (= beginning of the movement of the tip
cutter transversely to the running direction), the complete
vacuum must be applied to the second suction zone. At this
time, however, the perforations of the second suction zone
are not yet covered by the web. This means that immense
quantities of air must be sucked through the perforations,
with the result that the energy consumption for applying the
vacuum is very large. Accordingly, the vacuum pump must also
be dimensioned to be correspondingly large.
Figure 7 shows a section from a paper machine, prefer-
ably a section from a wire section and a subsequent press
section, having a suction roll picking up the web of fibrous
material at the first press unit and having an adjustable
suction zone width corresponding to the increase of the tail
width.
The wire section 40 comprises a wire 41. The wire
section 40 has a row of built-in parts, for example a suction
means 42, deflection rolls 43 and 44. A water jet nozzle 45
directs a water jet against the paper web which is in the
course of production, in order to cut out herefrom a narrow
21~872~
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strip, the so-called tail, at the beginning of starting up
the paper machine. The wire section is followed by a press
section. The latter has a plurality of press felts, for
example the felt F1, which wraps around a pick-up roll 46,
and which picks up the paper web (or the tail) from the wire
41. The pick-up roll 46 is followed by a first press unit 1
having press rolls 4.5 and 5.5. The first press unit 1 is
followed by a second press unit 3 having the press rolls 47
and 48. Both the press units are designed as long nip presses
and the press rolls 5.5 and 48 are designed as shoe rolls.
As already explained, in the case of conventional
installations there is the following problem: if the tail is
being fed, it must be picked up at the suction roll. The
suction roll in the case of conventional installations is
therefore divided into a first suction zone for the tail and
into a second suction zone for the main part of the paper
web. At the beginning of broadening the paper web, the second
suction zone is still not covered, but vacuum must already be
applied to this second zone, energy being lost - as mentioned
above - or great quantities of air having to be sucked
through the perforations of the second zone.
Hence, it is to be recommended to configure in
accordance with the invention all those suction rolls in
which there is a risk that the said disadvantages occur
during feeding of the paper web, in the figure this is in
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particular the pick-up roll 6.6.
Furthermore, the following combination of the definitive
elements can be seen from Figure 7: the water jet nozzle 45
has a drive 54 assigned to it. Likewise, a drive 55 is
assigned to the adjustment device for varying the sucked
width in the suction roll 6.6. A switch 56 is provided which
serves for the purpose of driving the two drives 54 and 55
simultaneously. The drives 54 and 55 are connected to each
other via electric lines 57, 58. Furthermore, an actuating
device 59 for varying the control signal in line 58 relative
to the control signal in line 57 is provided. The result
achieved is that the two drives 54 and 55 have one and the
same speed.
Finally, a delay unit 60 is provided. This has the
effect, if required, that the drive 55 starts later than the
drive 54, to be specific by the time interval ~t.
The value of ~t depends on the machine speed, and in
addition on the distance which is covered by the web between
the water jet nozzle 45 and the suction roll 6.6.
The suction roll 6.6 is shown in Figure 8. Said roll
has, on the front side, a vacuum connection 61 and a sealing
plate 62, perpendicular to its axis, in its internal space.
The sealing plate 62 can be moved in the longitudinal
direction of the suction roll by means of a threaded spindle
63 which can be rotated about its own axis. During the moving
- 2 15 ~
- 26 -
of the sealing plate 62 in the figure from left to right, the
roll width X to which vacuum is applied is increased and the
roll width Y to which suction is not applied is decreased to
the same extent. This is carried out once more at the same
time and to the same extent as the water jet nozzle moves
transversely to the paper web in accordance with Figure 7.
Figures 9a to 9c likewise illustrate a draw-free web
transfer from a first press nip I to a further press nip III
which is designed as a long nip press, in this case a
stationary suction box 67 being provided to support the web
guidance and picking up the felt F1 from the cover of the
central roll 4.6 of the first press unit 1. The stationary
suction box 67, as shown in Figures 9b and 9c, is subdivided
into a plurality of suction zones Z1, Z2 and Z3, at least one
of the contact pressure elements 68-71, according to the
invention, being suspended resiliently on the felt.
Furthermore, Figure 9b shows a possible arrangement of a
spray pipe S for cleaning the felt or for separating the
paper web from the felt F1 if a transfer of the paper web
into the broke pit is desired.
According to the invention it is essential that,
adjacent to the draw-free web transfer between two press
units, the second press unit is designed as a long nip press
since, because of the properties of the long nip press, the
rewetting of the web as a result of the continuous contact of
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- 27 -
the felt downstream of the first press nip is compensated. On
the other hand, it is of course possible according to the
invention to design the preceding or one of the preceding
press nips as a long nip press.
In conclusion, it is established that the advantages of
the invention described are:
- closed web guidance and thus avoidance of any extension
of the web;
- no paper draw for separating the web from a smooth press
roll and, as a result, no expansion of the paper web, no
loss of strength and the maintenance of a greater
elasticity of the paper web;
- reduction of web breaks in continuous operation;
- high dryness content;
- high dryness content even in the case of rewetting after
passing through the first press nip.
