Note: Descriptions are shown in the official language in which they were submitted.
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PRESS
TECHNICAL FIELD
The present invention relates to a press in a machine for the production of a
continuous web
of cellulosic fibrous material, comprising
(a) a press device;
(b) a rotatable counter roll having a cylindrical roll body forming a press
nip with the press
device;
(c) an endless clothing extending through the press nip for carrying the web
through the
press nip, said clothing having the ability to receive and carry away liquid
from the
web; and
(d) devices for heating the web in the press nip, said heating devices
including
(da) a movable heat transferring device for transferring heat to the web in
the press nip;
(db) at least one heating apparatus for heating the heat transferring device.
PROBLEM PRESENTATION AND PRIOR ART
Pressing of a wet fibrous web in a press at an elevated temperature and at a
high pressure
has been found to result in a number of favorable effects, for instance highly
increased
dewatering rate. The technique, which is called impulse technique, impulse
pressing or
impulse drying, is described in SE-7803672-0, equivalent to US-4,324,613. The
top
pressures that the method described therein uses lie within the interval 3 - 8
MPa and the
surface temperatures between about 150 °C to about 350 °C at a
cylinder in a press nip of
conventional type. In this case, "conventional press nip" means a press nip,
in which two
rolls with cylindrical cross-section are co-operating against each other under
pressure.
However, because of the short press nip, the dwell time for the paper web in
this type of
press nip is only some few milliseconds, and this dwell time is too short for
it to derive
advantage from the favorable effects of pressing at high temperature under
simultaneously
high pressure. Therefore, it has also been proposed to utilize impulse drying
in a heated
shoe press, wherein the press nip is extended to about 20 to 30 cm, so that
the dwell time
for the heat treatment becomes considerably longer.
Consequently, at impulse drying, high temperature and high pressure are
combined under a
short time period in order to give very high thermal flows to the web. A heat
transferring
tool in the form of the mantle surface of a steel or cast iron roll, which
transfers heat to the
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fibrous web, achieves top thermal flows within the interval 2 - 8 MW/m2, which
results in
very high dewatering rates. The mechanism that is the cause of these high
dewatering rates
with accompanying high dry solids contents is not yet entirely clear. One
theory that has
been submitted is that the vapor, which develops near the surface of the heat
transferring
S device closest to the fibrous web, through its expansion helps to extrude
substantially all
remaining water in the fibrous web into the felt that is in contact with the
fibrous web.
Another theory is that the reduced viscosity of the water, caused by the high
temperature, in
combination with the fact that water of high temperature (> 100 °C)
evaporates fast at a
rapidly reduced pressure, which occurs when the web leaves the press nip and
the pressure
returns to normal atmospheric pressure, results in that high dry solids
contents in the paper
web is obtained.
In said SE 7803672-0 a burner supplies heat to one of the rolls in a press,
which press has
two rotating rolls. The heat is supplied onto the mantle surface of the roll
immediately
before the press nip. The patent states that the roll can have a surface layer
with low
capacity of heat transmission so that the surface layer thereby can keep a
high temperature.
The patent US-4,738,752 describes an extended, heated press nip, in which the
fibrous web
encounters a hot surface, which is formed of a rotatable press roll or a metal
belt, that runs
in a loop around a plurality of guide rolls. The press roll and the belt are
heated by a heating
apparatus. T'he press roll may have a first coaxial layer and a second coaxial
layer, which
second layer extends around the first layer and has a coefficient of thermal
conductivity that
is larger than that of the first the layer. The first layer may be of
ceramics, while the second,
external layer consists of metal and has a thickness of 0.0127 - 1.27 cm. The
layers are in
intimate contact with each other and together they constitute a uniform body.
When a roll of the described, known types is heated from the outside, one
problem is that
the external layer will become warmer than the layer or the layers that is
located inside of
the external layer, which is firmly connected with the next inner layer. The
external layer
will therefore expand more than the inner layer so that tensions will occur
between the two
layers. If the roll is homogeneous the same differences in expansion and
tensions between
the external and the internal sections, which merge into each other without a
determined
boundary, will arise. In order to reduce the risk for damages in the roll from
said difference
in tensions the initial heating of the roll must be done slowly. Another
problem is that it is
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difficult to keep the geometrical shape of the roll in the cross machine
direction because of
the difficulties in maintaining the same temperature along the mantle surface
of the roll and
on the roll heads. Since the mantle surface and the heads can not expand
freely and
independently of each other, large tensions occur between them and the mantle
surface
becomes curved outwards or inwards across the machine direction. Since also
the internal
layer or part of the roll will absorb some of the heat energy supplied, the
heating costs
become high.
One disadvantage of using a metal belt as a heat-transferring device is that
it must be
arranged in a loop between at least two rolls, which constitutes a
configuration that is space
requiring. In order to clean the surface of the belt with a doctor blade, a
counter roll must be
arranged inside of the belt loop opposite the doctor blade. Another
disadvantage is that the
belt usually can not be coated with layers in order to achieve certain
properties of release
and certain thermal conductivities.
In production of soft paper, of lower basis weight and which for instance is
used for making
household paper, paper towels and other hygiene products, it is a common wish
that the
bulk, i.e. the relationship between the volume and the weight of the paper,
shall be
substantially higher than for other papers, as paper with high bulk has a
desirable
combination of softness and high power of absorption. In conventional
production of paper,
a coherent fibrous web is formed on a wire by dewatering a pulp suspension
with initially
very high water content. The moist web runs through a press section comprising
one or
more presses, each with at least one press nip, in which additional water is
pressed out of
the web. However, in a conventional wet press a certain re-wetting of the web
always
occurs at the outlet of the press nip and moreover the fibers are pressed
together in a
disadvantageous way at said pressing, the desired properties of the paper
obtained by the
pressing do not remain in full, but a relatively flat and compact soft paper
web with lower
bulk than desired is obtained after the web has dried in a drying section. In
the
manufacturing of paper, board and cardboard, usually a drying section is used
which has a
large number of alternating drying cylinders and guide rolls, around which the
moist web
runs. This large number of cylinders and rolls gives the paper machine a
disadvantageous
length.
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In the manufacture of soft paper, usually a Yankee dryer has hitherto been
used for the
main drying of the web. The conditions at an Yankee dryer differ from those at
a
conventional wet press, for instance the dewatering of the formed fibrous web
is not merely
done by pressing but largely by thermal drying of the web.
However, certain attempts have been made to manufacture soft paper with a bulk
that is
substantially increased from the one obtained earlier. For instance, in the
patent US-A-
3,806,406 is described a procedure and a device for forming a high bulk soft
paper web.
The wet web is transported on a felt through a press nip between a press roll
and a heated
Yankee dryer. Since the compressing of the fibrous web consequently results in
a for soft
paper unwanted low bulls, it has been tried in the Yankee dryer described in
US-A-3,
806,406 to reduce the surfaces of the fibrous web that are exposed to pressure
in the press.
The Yankee dryer has a mantle surface with alternately raised and lowered
surface portions,
which constitute a relief pattern for placing against the web. Substantially,
only the parts of
the web that are in contact with the raised surface portions are compressed in
the press nip,
while the parts in between are left relatively unaffected. On the whole, the
entire drying of
the web is done thermally by supplying heat from the heated Yankee dryer,
while the soft
paper web is maintained in a fixed position relative to the said surface
portions. If required,
a certain pre-drying or after-drying of the web can be done as well by through-
air drying.
Thereby, the web obtains a certain embossing pattern that consists of
compressed and non-
compressed portions, which equals the current pattern of the mantle surface.
Pattern
imprinting of a moist fibrous web and drying of the imprinted fibrous web
without
destroying the procured structure gives a high bulk and, simultaneously, the
structure
remains even after the fibrous web has absorbed water.
However, the Yankee dryer has some essential disadvantages. It has a large
diameter, why
it is very bulky. In using a Yankee dryer, it is hard to achieve the very high
temperatures
that are being exploited in impulse drying. This is due to that the Yankee
dryer is heated
from its interior by steam, which consequently leads to that the entire Yankee
dryer must be
heated up, with high energy costs as a consequence. Despite the fact that the
steam inside
the drum can have a high temperature, the external mantle surface of the drum
reaches only
about 95 °C - 100 °C. The relatively low temperature partly
leads to that the web must be
in contact with the Yankee dryer under a longer time period, which is the
reason for its big
diameter, partly to that the Yankee dryer often must be completed with either
pre-drying or
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after-drying, occasionally both. Some of the problems mentioned above
concerning the
differences in temperature between different parts of a heated roll and-the
unfavorable
tensions which then occur, may be considered to be valid also for the heated
Yankee dryer.
5 OBJECT AND FEATURES OF THE INVENTION
The object of the present invention is to achieve a press, which has at least
one press nip
with an improved heat transferring device, which press at least substantially
reduces the
problems mentioned above so that the favorable effects of both the impulse
drying and of
the pattern imprinting, which have been stated above; can be utilized in a
better way than
possible earlier.
The press according to the invention is characterized in that
said heat transferring device includes
- a heatable sleeve, which is an integral part of the counter roll and
encloses the roll
body;
- said sleeve having an external mantle surface for cooperation with the press
device;
and
- a plurality of imprinting elements arranged in a pattern being provided on
said
external mantle surface of the sleeve for a generating in the fibrous web a
pattern
imprint corresponding to the pattern of the imprinting elements.
LIST OF FIGURES
The invention will be described in more detail hereinafter with reference to
the drawings.
Fig. 1 is a perspective view of parts of a press comprising a press with a
counter roll and
a shoe press roll, which counter roll has a rigid sleeve which has a diameter
that is
shown highly exaggerated, part of a pattern being shown on the external mantle
surface of the sleeve.
Fig. 2 is a cross sectional view of the shoe press according to fig. 1 in
which a fibrous
web runs through a press nip which is formed by the counter roll and the shoe
press roll.
Fig. 3 is a cross sectional view of the shoe press according to fig. 2, but
with an
alternative run of the fibrous web, by which the web is arranged to follow a
longer
section around the sleeve after the press nip.
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Figs. 4 and 4a are enlarged scale views of the pattern according to a first
embodiment.
Figs. 5 and Sa are enlarged scale views of the pattern according to a second
embodiment.
Figs. 6 and 6a are enlarged scale views of the pattern according to a third
embodiment.
Figs. 7 and 7a are enlarged scale views of the pattern according to a fourth
embodiment.
S Fig. 8 is a perspective view of a press with a counter roll and a shoe press
roll similar to
the view shown in figure l, but with the counter roll having a flexible sleeve
according to a second embodiment, part of a pattern being shown on the
flexible
sleeve.
Fig. 9 is an enlarged scale view of the pattern according to fig. 8 according
to a first
embodiment.
Fig. 10 is an enlarged scale view of the pattern according to fig. 8 according
to a second
embodiment.
DETAILED EMBODIMENT DESCRIPTION
In figures l and 2, there are schematically shown parts of a press 1, which is
part of a press
section in a machine for the production of a continuous web 2 of cellulosic
fibrous material.
The invention is particularly but not exclusively applicable to paper machines
for the
production of paper with low basis weight, i.e. soft paper. However, the paper
machine can
be designed for the production of any other paper grade. The press 1 comprises
a rotatably
journalled counter roll 3; a cooperating press device 5 for forming a press
nip 4, and a
clothing 6 that is in direct contact with the fibrous web 2 and which
preferably has the
ability to receive liquid in fluid state and gaseous state from the web 2 and
to carry it out of
the press nip 4. The counter roll 3 comprises a rugged cylindrical roll body 7
which has
stub shafts 8, see figure 1, for rotatable journalling of the counter roll 3
in bearing houses,
not shown. The roll body 7, which is arranged rotatably around an axis of
rotation 9, has a
mantle surface with a circular cross-section. In the embodiment shown, the
press 1 is
constituted of a shoe press with an extended press nip 4 and the press device
5 of a shoe
press roll, comprising a press shoe 10 and an impermeable flexible jacket 11,
which jacket
11 is fixed at rotatable peripheral roll head parts 12. The press shoe 10 has
a concave
surface 13, see figures 2 or 3, for generating pressure under cooperation with
the convex
roll body 7 of the counter roll 3 and for achieving the extended the press nip
4. The jacket
1 I runs through the press nip 4 in sliding contact with the concave surface
13 of the press
shoe 10. Alternately, the flexible jacket 11 may run in a loop around a
plurality of rolls (not
shown) and, consequently, in which loop the press shoe 10 is mounted.
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The clothing 6 of the shoe press 1 is arranged to run in an endless loop
around a plurality of
guide rolls (not shown) and through the extended press nip 4. The fibrous web
2 is
supported on the clothing 6 up to the press nip 4 and is then separated from
said clothing 6
after the press nip 4. Instead it bears on and is carried by the counter roll
3 around a
predetermined section of the mantle surface 14 of the counter roll 3.
Alternatively, the web
runs in an open draw before the press nip, i.e. unsupported of the clothing.
The web may
also be separated from the counter roll 3 directly after the press nip 4, so
that the web runs
entirely free from any support.
In figures 2 and 3 there are shown two alternative runs for the fibrous web 2
after the press
nip 4. In the web run shown in figure 2, the fibrous web 2 is arranged to bear
on and to
enclose only a small surface section of the mantle surface 14 of the counter
roll 3. In the
web run shown in figure 3, the fibrous web 2 is arranged to follow and to be
in contact with
the mantle surface 14 of the counter roll 3 over a substantially longer
section, i.e. over a
considerably larger surface section than in the embodiment shown in figure 2.
By arranging
the fibrous web 2 to contact a greater proportion of the mantle surface, more
heat can be
transferred to the web, and therefore a higher dry solids content can be
achieved in the web.
The surface section over which the fibrous web 2 is arranged to follow,
(including the nip
length) has a length in the direction of circulation, determined by its sector
having the angle
a, see the figures 2 and 3, which preferably is within the interval
(10° to 300°), depending
on the after-drying that is needed in order to achieve the desired dry solids
content.
The clothing 6 usually consists of a press felt, but also a permeable process
belt that has
through going openings and possibly recesses located between these can be
used, like an
impermeable belt with recesses of suitable depth, shape and orientation. Said
recesses exist
on the side that is facing the fibrous web 2. Said openings and/or recesses
are arranged to
receive and transfer liquid from the fibrous web 2 out of the press nip 4. The
openings
and/or the recesses also give the web 2 an impression in the form of an
imprinted pattern,
which imprint increases the bulk of the web 2. Instead of a permeable or
impermeable belt,
an open or closed wire that has a sealing layer that is turned from the web 2
can be used.
The press shown, comprising a shoe press 1, is the preferred embodiment, but
the invention
also comprises other types of press devices, as for instance a press
comprising a
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conventional, cylindrical press roll against which the counter roll is
arranged and with
which it is cooperating.
Further, the press 1 comprises a movable device 15 for continuous transfer of
heat to the
fibrous web 2, when this passes through the press nip 4, and a heating
apparatus 16 for
heating said heat transferring device 15 from its exterior to the desired
operating
temperature at startup and then for a continual or continuous heating during
operation in
order to maintain this operating temperature. The heating apparatus 16, which
can be fixed
or movable, e.g. oscillating, is arranged at a predetermined location or
within a
predetermined interval upstream of the press nip 4 in order to heat the
circumference of the
heat transferring device 15, e.g. around 30° - 330° ahead of the
inlet of the press nip 4, i.e.
also near the outlet of the press nip 4 for more-efficient utilization of the
space around the
counter roll 3, which is possible because the heat loss from the heat
transferring device 15
before the contact with the moist web 2 is comparatively small.
According to the invention, the counter roll 3 comprises a heatable sleeve,
which forms said
heat-transferring device 15. The sleeve 15 encloses at least the entire axial
portion of the
cylindrical roll body 7 active in the press nip 4 and is so arranged that it
can expand freely
in relation to the roll body 7 at said heating before the press nip 4 without
causing
unfavorable tensions in the sleeve 15. The sleeve 15 has an interior surface
17, which is
arranged to be brought in contact-with the roll body 7 within the zone of the
press nip 4,
and an exterior surface, that constitutes the mantle surface 14 of the counter
roll 3
mentioned above, around which the fibrous web 2 runs corned by the surface.
The sleeve
15 is free from mechanical, adherence or any other such permanent, fixing
connections,
which would block a free expansion of the sleeve 15 in the axial and/or radial
direction in
relation to the cylindrical roll body 7 outside the press nip 4, i.e. where
heating occurs, and
which connections on the contrary would give rise to unfavorable tensions in
the sleeve 15.
Flat annular rings 18 are coaxially mounted at the end surfaces of the
cylindrical roll body
7, which rings 18 have a somewhat larger outer diameter than the cylindrical
roll body 7 to
form axial end stops 19 for the sleeve 15 during the passage of the sleeve 15
through the
press nip 4. In the embodiment shown, the rings 18 constitute annular flanges
(see figure 1),
of which one is arranged to run in a groove 20, that is arranged on the inside
17 of the
sleeve 15, parallel with the end face of the sleeve 15. Even during operation,
the sleeve 15,
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which is expanded axially by the heat, is free from contact with one of or
both of these end
stops 19 in order to avoid also axial compressive stresses in the sleeve 15 in
the zone of the
press nip 4, so that the straightness of the cylindrical mantle surface 14 can
be ensured.
Consequently, the sleeve 15 is completely free from permanent locking
connections in all
directions in order to allow the sleeve 15 to expand axially as well as
radially in a stress
free manner in relation to the cylindrical roll body 7 during heating. During
operation, the
roll body 7 is arranged to press and hold the sleeve 15 at a place of contact
21 within the
zone of the press nip 4 at the press shoe 10 for generating pressure in
cooperation with the
roll body 7 to form a friction force in said place of contact 21 between the
roll body 7 and
the sleeve 15 large enough to ensure that the sleeve 15 will be rotated by the
roll body 7
and at the same speed at said place of contact 21 (see figures 2 and 3).
The sleeve 15 may be a rigid sleeve that is stable in shape, (see especially
figures 1-3), or
alternatively as a flexible sleeve 15, that consequently is not stable in
shape (see especially
figure 8).
In figures 1-3 the sleeve 15 is schematically illustrated as a cylindrical,
rigid, sleeve 15 that
is stable in shape and has an exterior 14 that constitutes the rigid mantle
surface of the
sleeve 15 and an interior 17 that has a constant, circular cross-section. When
the sleeve 15
is heated to its desired operating temperature, the interior surface 17 has a
larger diameter
than the outer diameter of the cylindrical roll body 7, as shown in the
drawings. However,
the diameter of the rigid sleeve 15 is shown highly exaggerated in relation to
the roll body
7. Consequently, the sleeve 15 shown in figures 1-3 has a certain wall
thickness, measured
between the inner side 17 and the mantle surface 14, in every cross-section
arbitrary
between parallel end faces of the sleeve 15. The wall thickness of the sleeve
15 is selected
after the specific use with varying demands for the properties of the sleeve
15. For instance,
the wall thickness is optimized for heating the loose sleeve 15 as quickly as
possible and for
keeping the straightness of the mantle surface 14 across the machine. The wall
thickness of
the sleeve 15 is large enough to make the sleeve 15 rigid and stable in shape,
i.e. is at least
self supporting, so that, resting on a flat supporting surface, it will
maintain its radius on all
sides and will not collapse to oval shape. Preferably, it is so rigid and so
stable in shape
that it will not be considerably deformed by normal tensions at assembling and
during
operation. The rigid sleeve 15 has a su~cient wall thickness in order to
resist the Iarge
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tensions in the press nip 4, which can arise from the differences in radius
between the
different construction elements.
During operation the rigid sleeve 15 is arranged to rotate around its own axis
of rotation,
5 i.e. its central axis 22, which is situated eccentrically in relation to the
axis of rotation or
central axis 9 of the roll body 7. The eccentricity, i.e. the distance between
these axes 9 and
22, equals half of the difference between the outer diameter of the roll body
7 and the inner
diameter of the sleeve 15, i.e. the difference in radius lengths. When the
press shoe 10 and
the counter roll 3 are displaced vertically away from each other by a distance
corresponding
10 to this radius difference, the sleeve 15 will hang on the roll body 7 and
its central axis 22
will be situated beneath the central axis 9 of the roll body 7. In this
condition the sleeve 15
is entirely loose in relation to the roll body 7 for a simplified change of
the sleeve 15 in that
a necessary space for the axial removal of the sleeve 15 thus exists between
the press
device 5 and the roll body 7.
It is possible to size the inner diameter of the sleeve 15 in relation to the
outer diameter of
the roll body 7 such that at room temperature the sleeve forms a tight or
interference fit
about the roll body, yet when heated up to its operating temperature the
sleeve expands
sufficiently to become loose about the roll body. In this situation, removal
of the sleeve
must be performed while the sleeve is heated sufficiently to enable it to be
slid axially off
the roll body; similarly, installation of a new sleeve would require heating
of the sleeve to
expand it sufficiently to allow it to be slid axially onto the roll body.
The concave surface 13 of the press shoe 10 has a radius that is adjusted to
the radius of the
rigid, cylinder-shaped sleeve 15. The sleeve 15 is so rigid and stable that it
self will exert a
press function by the action of the roll body 7, i.e. the pressure from the
roll body 7 at the
shorter place of contact 21, compared to length of the press nip 4, is
propagated forward
and backward, seen in the direction of rotation, through the sleeve 15 to the
press nip 4, so
that the length of the press nip 4 becomes about equal to the width of the
press shoe 10.
When a section of the rigid sleeve 15 passes the press shoe 10, a minor change
in the shape
of the passing section will occur caused by differences in length between the
inner radius of
the sleeve 15 and the outer radius of the roll body 7. These minor changes in
shape,
however, will disappear as soon as the pressure is relieved after the press
nip 4, and some
harmful tensions do not occur in the rigid sleeve 15 because it is thick
enough to resist this
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load. Therefore, the rigid sleeve 15 still fulfills the requirement for being
"stable in shape"
as it is defined in this description.
During operation when the sleeve is heated to its operating temperature, the
difference in
diameter between the outer diameter of the roll body 7 and the inner diameter
of the sleeve
is so determined that a free space 23 is formed at that side of the roll body
7 that is
opposed to the press nip 4. Thanks to this free space 23 inside the sleeve 15
only minor heat
quantities will be consumed of the roll body 7. During operation, this air
containing space
23 has the shape of a crescent. Only in the zone of the press nip 4 within the
surface of the
10 place of contact 21 a transfer of heat from the sleeve 15 to the roll body
7 occurs, but the
heat quantities that are consumed here yet are small, since the contact
surface here
constitutes a diminutive portion of the peripheral surface and the fact that
the heat is
transported from one surface to another. In general, the temperature on the
hot mantle
surface 14 of the sleeve 15 at the inlet to the press nip 4 may be within the
interval 150 °C -
15 400 °C.
Because the rigid, cylinder-shaped sleeve 15 remains round during operation, a
constant
distance between the sleeve 15 and the heating apparatus 16 is maintained with
constant
operating temperature.
Generally, the wall thickness of the rigid sleeve 15 is 5 - 100 mm, preferably
15 - 40 mm.
Preferably, at room temperature the sleeve fits closely or with an
interference fit about the
roll body, and when heated to its operating temperature the sleeve expands to
have a greater
inside diameter than the outside diameter of the roll body. The difference
between the
internal diameter of the rigid sleeve 15 and the external diameter of the roll
body 7 when
the sleeve is heated during operation generally is 0.01 - 100 mm, preferably
0.5 - 10 mm.
The less the wall thickness of the sleeve 15 is, the faster a high surface
temperature is
achieved and this without high tensions occurring, because the mass which must
be heated
in the radial direction decreases in degree accordingly.
In the embodiment with a flexible sleeve 15 (see figure 8) this can be a
deformable
cylinder, which will yield when exposed to an external load, i.e. the sleeve
15 will deform
at the passage through the press nip 4, but will substantially recapture its
not self
supporting cross sectional shape thereafter when the external load is released
or is moved
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along the mantle surface 14 during the rotation of the sleeve 15. Generally,
the flexible
sleeve 15 has a wall thickness within the interval 0.4 - 5.0 mm, preferably
0.8 - 2.3 mm.
The difference between the internal circumference of the flexible sleeve 15
and the external
circumference of the roll body 7 during operation in general is within the
interval of about 0
100w mm, preferably about 0.1 - l Ow mm. Since the sleeve 15 is flexible, a
cleaning
doctor may be arranged at a suitable place so that the sleeve 15 will be
deflected against the
roll body 7, which will function as a counter member. Furthermore, the
flexible sleeve 15 is
free from any torque from frictional forces, and the sleeve handling is
facilitated
considerably by using the flexible embodiment of the sleeve 15.
The rigid or flexible sleeve 15 described above, which is impermeable, i.e.
impervious, can
be replaced by another embodiment of sleeve, namely a permeable, rigid or
flexible sleeve
15. For instance, this will facilitate the release of the fibrous web from the
sleeve 15 and
will simplify the cleaning of said sleeve 15. The permeable sleeve 15 may
consist of a
metallic fabric formed into a cylinder, which e.g. may be of bronze. However,
the
permeable sleeve 15 can consist of any form of suitable permeable elements,
that can be
formed into a cylinder, including plate-shaped elements, which are attached to
each other,
end part to end part with a seam, for instance a certain length of metal
plate, in which a
pattern of through going elongated slots, openings or holes 35 are made to
form said
cylindrical, pervious sleeve 15. The elongated slots may be so arranged that
they by
stretching of the plate in its plane expands to form a net of openings, for
instance elliptical
ones, after which the end parts of the plate are attached to each other by a
welded joint.
With a permeable sleeve 15 the desire is to achieve a reduction of the risk of
delamination,
especially for paper webs of medium and high basis weights, and not to cause
the web to
get stuck on the mantle surface 14 of the sleeve 15. In addition, the
releasing may be
facilitated, if desired, by the provision of spray nozzles (not shown) for
blowing air
powerfully, directly or through the sleeve, against the fibrous web at a
position downstream
of the web releasing point. Furthemore, with a permeable imprinted sleeve 15
it is
normally easy to loosen the fibrous web 2 unharmed from the sleeve 15 and to
clean the
sleeve 15 from remaining fibers. In order to facilitate the release of the web
2 from the
mantle surface 14, air-knives (not shown) may be provided at suitable
locations at the outlet
from the press nip 4. It is known, however, that the web 2 loosens easily from
the mantle
surface 14 of the sleeve 15 at temperatures over 200 °C.
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13
If desired, the air jet may consist of or cooperate with the hot gases from a
gas burner, or
the like. All fibers stuck to the sleeve will immediately be burnt to ashes
and will be carried
away by the jet of gas. Furthermore, the heating of the fabric by the hot
gases decreases the
need of electromagnetic heating of the sleeve.
The roll body 7, which is enclosed by the permeable sleeve 15, may (in a not
shown
embodiment) consist of a roll that has a first blow zone at which the web is
detached from
the sleeve, and a second blow zone, which is arranged downstream of said first
blow zone
in the direction of rotation of the sleeve, for cleaning of the permeable
sleeve. If desired,
also a suction zone may be arranged at the nip of the impulse press for
removing vapor and
for reduction of the risk of delamination. For instance, the roll with at
least one blow zone
may have a suction zone situated in the press nip.
Furthermore and according to the present invention the sleeve 15 comprises a
plurality of
imprinting elements 24, which are arranged on, in or through the external, web
carrying
mantle surface 14 of the sleeve 15 (see figures 1 or 8). The imprinting
elements 24 are
arranged in a predetermined pattern, preferably a relief pattern 25, for
placing against the
fibrous web 2 so that a pattern imprint, i.e. imprinting, is generated in the
fibrous web 2
according to the imprinting elements 24 when said fibrous web 2 runs through
the press nip
4 together with the clothing 6. The imprinting causes positive and remaining
changes in the
properties of the web 2, inter alia the bulk of the web 2 increases with 50 -
150 %,
compared to conventional pressing with a smooth counter roll.
The imprinting elements 24 (see figures 4 - 7a and 9 - 10) are formed by
recesses in the
mantle surface 14 of the sleeve 15, which recesses preferably are arranged to
constitute, in
relation to each other, raised or lowered surface portions 26, 27 (see the
figures 4 - 7a). The
imprinting elements 24 may be uniformly distributed, but all, some or one
specific
imprinting element 24 may also be so arranged, separate or in groups at a
desired spacing
from one another, that they together constitute any other predetermined
pattern 25, for
instance in the form of structure, symbols, figures, or common decors, such as
grids, digits,
letters, trademarks, names or other depicting. All, some, or one specific
imprinting element
24 may be of elliptical, circular, rectangular, square or other polygon shape,
e.g. triangular,
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14
or may be of any another imaginable geometrical shape, which is considered
reasonable for
obtaining the desired pattern 25 in the fibrous web 2 to be produced.
In the embodiment with raised or lowered surface portions 26, 27 (see figures
4 - 7a), the
raised surface portions 26, which are arranged between said lowered surface
portions 27,
may for instance have flat, mufti-faceted, tapered, arched or other convex
cross-sectional
shapes or parts. Accordingly, the lowered surface portions 27 comprise flat,
mufti-faceted
or curved, hollowed out or in another way concave formed finite cross-
sectional shapes or
parts.
The imprinting elements 24 may also, instead of the surface portions 27, which
are lowered
finitely in the radial direction in the mantle surface 14 of the sleeve 15,
comprise a great
plurality of through going, drilled or in any other suitable way produced
holes 35, which
extend through the wall of the sleeve 1 S to form a desired permeable pattern
25 (see figures
9 and 10), or a combination of both raised and lowered surface portions 26, 27
and through
holes 35 (not shown). The imprinting elements 24 are so arranged in size and
number that
the raised surface portions 26, that are in pressing contact with the fibrous
web 2, together
constitute an essential portion of the total mantle surface 14, preferably
between 20 - 50
of the mantle surface 14.
Furthermore, the imprinting elements 24 may be arranged so that they form
continuous
grooves with even spacing across the mantle surface 14, bands of several
continuous
grooves arranged near each other, which bands of grooves thus are mutually
separated by a
proportionately greater distance, or intermittent and finite groups of
elements in the mantle
surface 14, which imprinting elements 24 are arranged in the direction of
circulation of the
sleeve 15, with an angle to the direction of rotation of between 10° to
80° diagonally from
one or both of the end surfaces of the sleeve 15 to one or both of the opposed
end surfaces,
or substantially across the direction of circulation. The grooves, bands of
grooves or the
groups may be straight or waved, e.g. sine or zigzag shaped, and the distance
between them
can vary, or between two adjacent and mutually parallel grooves, bands or
groups be fixed,
from 1 to 5 mm, while the grooves have a width between about 0.5 to 2.0 mm and
a depth
of about 0.1 to 2.0 mm. The imprinting elements 24 may for instance be
comprised of
radially finite channels, through going notches or other finite recesses or
through going
recesses which are formed with the aid of mechanical, laser or chemical
working of the
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mantle surface by for instance milling, boring, turning, electrochemical
pickling or etching
at least a short distance down in the mantle surface of the sleeve 15 from its
outside 14.
Seen in a cross-section of the sleeve 15, the difference in altitude, i.e. the
difference in
5 radius, between the raised and the lowered surface portions 26, 27 lies
within the interval
0.1 - 2.0 mm, while the width of the raised surface portions 26 suitably lies
within the
interval 0.5 - 5 mm. At the passage of the web 2 through the press nip 4
substantially only
the parts of the web, which are in contact with the raised surface portions
26, will become
compressed in the press nip 4, while the parts of the web in between are left
relatively
10 unaffected.
In the figures 4 - 7a and 9 - 10 described below, there are especially shown a
number of
different embodiments of patterns 25 for the mantle surface 14 of a sleeve 15,
which sleeve
15 can be either rigid or flexible. In the embodiments shown in figures 4 - 7
the sleeve 1 S is
15 impermeable, but according to what is mentioned above it may also be
designed as a
permeable sleeve 15, for instance through its construction, for instance
comprising a net or
fabric structure, or by the arrangement of said through holes 35 (see the
figures 9 and 10).
Consequently, the invention is in no way limited to the especially shown
embodiments, but
any other configuration of imprinting elements 24 for imprinting of
predetermined patterns
25 in the fibrous web 2 according to what is mentioned above falls within the
inventive
concept.
In figures 4 and 4a there is schematically shown, in view and cross-section, a
detail of a
pattern 25 according to a first embodiment, which pattern 25 comprises
imprinting
elements 24 in the form of a plurality of straight and mutually parallel
grooves 28, which
divide the mantle surface of the sleeve in elongated, alternately raised and
lowered surface
portions 26, 27. The surface portions 26, 27 have, seen in a cross-section
through the
sleeve, a flat bottom surface 29 and a flat top surface 30, respectively,
between which
convergent wall surfaces 31, 32 extend from the bottom surface 29 towards the
top surface
30. The corners between the surfaces 29, 31 and 30, and also 29, 32 and 30,
are shown in
figure 4a as being entirely straight, but they may also be designed with a
small radius for
rounding said corners. In the cross-sectional view shown in figure 4a, the
width and the
difference in altitude of the two surface portions 26, 27 are illustrated with
the markings a,
b and c, which widths and differences in altitude are within the interval
mentioned above.
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16
The grooves 28 in the shown embodiment extend in the direction of rotation of
the sleeve
around its entire circumference in endless rings, but in another embodiment
(not shown)
they may instead have a certain determined angle to the end face of the
sleeve.
In figures 5 and Sa there is schematically shown, in view and cross-section, a
detail of a
pattern 25 according to another embodiment, which pattern 25 comprises
imprinting
elements 24 similar to the grooves 28 that are shown in the first embodiment,
but which
differ in that they have wall surfaces 31, 32, which are mutually parallel,
and which extend
perpendicularly between a bottom surface 29 and a top surface 30.
In figures 6 and 6a there is schematically shown, in view and cross-section, a
detail of a
pattern 25 according to another embodiment, which pattern 25 comprises
imprinting
elements 24 in the form of grooves 28 similar to the ones that are shown in
the first and the
second embodiment above, but which differs in having a roundish bottom surface
29 (see
figure 6a) and flat top surface 30 at the raised the surface portions 26 thus
formed between
the grooves 28.
In figures 7 and 7a there is schematically shown, in view and cross-section, a
detail of a
pattern 25 according to another embodiment, which pattern 25 comprises
imprinting
elements 24, which consists of two groups of grooves 33, 34, which each
comprise a
plurality of grooves 28, which are mutually parallel within the respective
group, which
groups of grooves 33, 34 extend in a first group 33 from the first to the
second end surface
of the sleeve, and in a second group 34 from the second end surface of the
sleeve to the
first, at which the two groups of grooves 33, 34 cross each other at an angle
of between 10°
to 170°. Both groups of grooves 33, 34 have a concave roundish bottom
surface 29. The
two groups of grooves may also, in an embodiment not shown, have different
angles to the
sleeve end surface in order to avoid that the same bands in the clothing are
compressed turn
after turn.
In figure 9 there is schematically shown in perspective view a detail
according to a first
embodiment of a through going pattern 25 of a permeable sleeve 15, which
pattern 25
comprises imprinting elements 24, which consist of substantially circular and
through
going holes 35 extending in the thickness direction of the sleeve 15. The
holes are arranged
at a predetermined mutual spacing, successively in line and at a certain
determined angle to
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17
the direction of rotation of the sleeve 15, going diagonally from one end
surface of the
sleeve 15 to the other. The width of the holes 35, the angle, the mutual
spacing between the
holes in the line plus the spacing between said lines lie within the interval
mentioned.
In figure 10 there is schematically shown in perspective view a detail
according to a second
embodiment of a through going pattern 25 of a permeable sleeve 15, which
pattern 25
comprises imprinting elements 24, similar to the holes 35 that are shown in
the first the
embodiment, but which differ in that they have a rectangular shape.
The imprinted sleeve 15 may be homogeneous, i.e. be represented by one and the
same
material straight through without any layers, or consist of two or several
uniform and
coherent layers of different materials, of which layers at least one functions
as a carrier for
one or all of the other layers. Like the rigid sleeve 15, the flexible one may
consist of two
or more layers.
Consequently, when the rigid sleeve 15 consists of several layers, one of the
layers is a
supporting layer with the default function to be a stabilizing carrier with a
sufficient wall
thickness to provide the supporting layer with e.g. a surface layer with
desired properties.
The exterior surface of the supporting layer may be provided with a surface
layer of a
suitable material in order to obtain the desired release ability and/or a
desired capacity of
heat transmission of the mantle surface 14 of the cylinder-shaped sleeve 15. A
special
capacity of heat transmission of the mantle surface 14 may be desirable in
order to control
the supply of heat to the fibrous web 2 and to prevent delamination of the web
2.
Delamination is the phenomenon when the fibrous web 2 bursts at exiting the
press nip 4,
because the steam pressure inside the web 2 is greater then what the bonding
forces in the
web 2 can withstand.
The cylindrical roll body 7 may be provided with a surface layer of one or
several suitable
materials on its mantle surface in order to increase the resistance to wear
and/or increase
the corrosion-resistance and/or to achieve an insulation so that the heat,
which is being
transferred from the sleeve to the cylindrical roll body 7, is additionally
reduced within the
press nip 4. Alternately or furthermore, the sleeve 15 may have such a surface
layer (not
shown) on its interior 17 side.
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18
As suitable materials, in order to obtain a surface layer with the desired
release ability and
desired capacity of heat transmission, aluminum oxide, chromium oxide and
zirconium
oxide based ceramics can be used. Suitable materials in order to increase the
resistance to
wear are carbides of wolfram and/or chrome, and chrome steel. Suitable
materials in order
to increase the corrosion resistance are alloys based on one or more of
nickel, chrome, and
cobalt. Suitable insulation materials are zirconium oxide based ceramics. As
suitable
materials for a homogeneous sleeve (i.e. without layers) and for the
supporting layer of a
sleeve consisting of at least two layers we can mention nodular-iron, cast
steels and
weldable high-strength structural steels.
Any suitable heating apparatus 16 may be used, although an induction heater
presently is
preferred. Examples of other heating apparatus 16 are electric heaters,
infrared heaters,
laser heaters and gas burners.
The roll body 7 of the press 1 may be a so-called solid press roll, which has
been modified
according to the invention, but a deflection compensated roll may also be
used. Since the
contact surface between the hot sleeve 15 and the mantle body of the
deflection
compensated roll is small, only a little heat is transferred to the roll.
Enough heat can be
removed with the oil in the roll in order to enforce a reasonable temperature
and thereby a
high enough viscosity in the oil to achieve a satisfying function of the roll.
The temperature of the web 2 can be increased before the press nip 4 by
arranging one or
more steam boxes in the felt loop and/or near the free side of the web 2.
Preheating of the
wet web 2 gives a lower viscosity of the water, this facilitates the pressing
of water out of
the fibrous web at the press nip 4. Additional drying devices may also be
arranged after the
press nip 4, as for instance one or more through-air drying machines, for more
high-grade
drying of the web 2.
In the embodiment with a loose, rigid sleeve 15, the advantage is obtained
that the mantle
surface may be coated to enhance the release properties, the heat transfer to
the paper, and
to reduce unwanted thermal flow radially inwards of the sleeve 15. By making
the wall
thickness of the rigid sleeve 15 large enough, the mantle surface 14 of the
sleeve 15, which
is in contact with the web 2, may be treated like a conventional press roll
surface in order to
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19
be cleaned, when it is necessary, with a cleaning device 36, for instance a
doctor blade, a
metal brush or by spray nozzles for air blowing. In the embodiment with a
loose, flexible
sleeve 15, a heat-transferring device is achieved which is easy assembled. In
the
embodiment with a loose, permeable sleeve 15, the you achieve the advantages
of
minimizing the risk of delamination of certain paper grades, since a suction
zone may be
provided at the counter roll for removal of the steam, and also a release of
the web from the
sleeve is facilitated since through-blowing of air is possible, and cleansing
is made possible
with the aid of a cleaning device 36 in the form of a burner for burning off
sticky fibers.
Furthermore, the handling of the sleeve is made easier.
With the invention described above, a press section is achieved comprising one
or more
presses 1 for impulse drying with press nips 4 arranged in succession, which
press section
is power saving and has a stable geometry in the press nip 4, which press nip
4 comprises a
counter roll 3 with a heat transferring device 15 in the form of a loose,
imprinted sleeve 15
for faster and more efficient heating than with conventional rolls since less
mass is heated.
Furthermore, no problems with tensions in the counter roll 3 occur, wherefore
its
straightness is maintained. A fibrous web 2 which has been dried rapidly at a
highly
increased temperature in conjunction with imprinting of a structured pattern
25 in the web,
which is done in the present invention, keeps its structure after the drying
even after
absorption of liquid, whereby also the advantageous product characteristics
achieved by the
imprinting, such as high bulk, great softness, favorable stretchability and
high liquid
absorption are maintained.
By the fast drying in the impulse press 1 and by the lack of a long
conventional drying
section and/or a large Yankee dryer the advantages of a high paper production
and a lower
energy cost is achieved. In production of soft paper, a softer paper with more
bulk is
obtained by means of the imprinted sleeve 15.
Through the imprinting elements 24, i.e. the through going holes and/or the
finite recesses
in question, outlets and space, respectively, for the steam are created in the
press nip 4. This
results in a reduction of the steam pressure and, thereby, also a reduction of
the risk of
delamination in the web 2. However, this is mostly for applicable primarily to
other papers
than soft paper, since soft paper lets the steam through. The main task of the
imprinting
elements 24 is, however, to structure the web 2 in accordance with the used
pattern on the
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mantle surface 14, whereby some additional favorable effects are achieved. The
steam that
penetrates through the web 2 is considered to give the web 2 more advantageous
release
properties against the sleeve 15. The total contact surface of the fibrous web
2, which is
heated by the sleeve 15, increases substantially, so that the drying is
considerably faster,
5 while the total contact surface, which is exposed to pressure from the
sleeve 15, instead is
considerably reduced down to 20 - 50 % of the total web surface 2 ahead of the
press nip 4.
In addition, the steam allows the soft paper web 2 to expand in the recesses
of the pattern
25, so that a product is achieved that is structured and thereby fluffier,
i.e. has more bulk.
At the depression of the web 2 into the cavities, i.e. the lowered surface
portions 27 and the
10 through going holes 35, the web 2 is protected from sticking by the steam
film, which is
formed against the heating roll, i.e. the sleeve 15, which is an hitherto
unknown effect.
Normally the drying section constitutes an essential length of the
conventional paper
machine, why an advantageous reduction in the length of the paper machine also
is
15 achieved when the total length of the press and the drying sections is
substantially reduced
compared to earlier designs.
It is realized that the number of, the size, the material and the shape of the
elements and
details which are comprised in the press, for instance the roll body 7, the
sleeve 15 and the
20 imprinting elements 24, are to be adapted to the press device or devices 5
and to the fibrous
web 2 to be imprinted and dried. The press device 5 may thus be comprised of
one of
several different types of press devices 5, for instance a conventional press
roll, an open or
a closed shoe press roll, or any another suitable press roll. The size of the
sleeve 15, i.e. its
outer diameter, is usually over about 1.0 meter. The sleeve 15 may be
manufactured by one
or more of the methods sintering, casting, cold pressing, milling and welding.
