U.S. patent application number 17/041143 was filed with the patent office on 2021-01-14 for method and a machine for producing a tissue web.
The applicant listed for this patent is VALMET AKTIEBOLAG. Invention is credited to Hans IVARSSON, Anders OTTOSSON.
Application Number | 20210010202 17/041143 |
Document ID | / |
Family ID | 1000005163476 |
Filed Date | 2021-01-14 |
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United States Patent
Application |
20210010202 |
Kind Code |
A1 |
IVARSSON; Hans ; et
al. |
January 14, 2021 |
METHOD AND A MACHINE FOR PRODUCING A TISSUE WEB
Abstract
The invention relates to a method of producing a structured
fibrous web of paper suitable for tissue products. The method
comprises forming a fibrous web and conveying the formed fibrous
web on a water receiving felt (5) to a dewatering nip. An endless
steel belt (11) with a smooth steel surface is passed through the
nip together with the fibrous web and the water receiving felt (5)
wherein the endless steel belt is heated by heaters (HE.sub.U,
HE.sub.L). After the dewatering nip, the fibrous web is conveyed by
the endless steel belt (11) to an endless textured fabric (12)
which is permeable to air and to which the web is transferred from
the endless steel belt (11) in a transfer nip. The textured fabric
(12) runs at a lower speed than the endless belt (11). After the
transfer to the textured fabric (12), the fibrous web is carried by
the textured fabric (12) to a drying cylinder (17). The transfer
nip is formed by two rolls of which one is a suction roll within
the loop of the textured fabric. The transfer nip has a length
which is 5 mm-40 mm. The endless steel belt (11) has a width that
exceeds the width of the textured fabric (12). The invention also
relates to a corresponding machine.
Inventors: |
IVARSSON; Hans; (Karlstad,
SE) ; OTTOSSON; Anders; (Karlstad, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
VALMET AKTIEBOLAG |
Sundsvall |
|
SE |
|
|
Family ID: |
1000005163476 |
Appl. No.: |
17/041143 |
Filed: |
April 15, 2019 |
PCT Filed: |
April 15, 2019 |
PCT NO: |
PCT/EP2019/059681 |
371 Date: |
September 24, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D21F 5/028 20130101;
D21F 11/145 20130101; D21F 7/12 20130101; D21H 27/002 20130101;
D21F 3/045 20130101 |
International
Class: |
D21F 3/04 20060101
D21F003/04; D21H 27/00 20060101 D21H027/00; D21F 7/12 20060101
D21F007/12; D21F 5/02 20060101 D21F005/02; D21F 11/14 20060101
D21F011/14 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 19, 2018 |
SE |
1850458-9 |
Claims
1-14. (canceled)
15. A method of producing a structured fibrous web of paper
suitable for tissue products, the method comprising the steps of:
forming a fibrous web and conveying the formed fibrous web on a
water receiving felt to a dewatering nip formed by a first press
unit and a second press unit and where an endless steel belt is
passed through the nip together with the fibrous web and the water
receiving felt, the endless steel belt having at least a steel
surface which contacts the fibrous web in the dewatering nip; after
the dewatering nip, conveying the fibrous web by the endless steel
belt to an endless textured fabric which is permeable to air and to
which the web is transferred from the endless belt, the textured
fabric running at a lower speed than the endless steel belt and a
speed difference in the range 2-25% (RT %), causing a wet creping
effect increasing the bulk of the fibrous web; and after the
transfer to the textured fabric, conveying the fibrous web by the
textured fabric to a drying cylinder, wherein: the endless steel
belt is heated by heating medium in a position at least partly
before the fibrous web is applied onto the endless steel belt; and
the web is transferred from the endless steel belt to the textured
fabric in a transfer nip, the transfer nip being formed between a
first transfer nip roll that lies within the loop of the endless
steel belt and a second transfer nip roll which is a suction roll
located within the loop of the textured fabric, the transfer nip
having a length in the machine direction which is in the range of 5
mm-40 mm.
16. The method according to claim 15, wherein the endless steel
belt has a speed that is 15% higher than the speed of the textured
fabric.
17. The method according to claim 16, wherein the endless steel
belt is heated on both sides of the belt.
18. The method according to claim 17, wherein the endless steel
belt is heated in a heating zone extending at least 50-70% of a
distance ranging from 1 to 7 meters.
19. The method according to claim 18, wherein the endless steel
belt runs in a loop over at least two rolls that deflects the
endless steel belt over at least 90 degrees of the circumference of
the rolls and that the rolls may have a function as a guide roll, a
press roll or a transfer nip roll and the total length of the
endless steel belt exceeding the total circumference of the rolls
by a factor above 2.
20. A machine for producing a structured fibrous web of paper
suitable for tissue products, the machine comprising: a forming
section that includes a first (3) and a second (5) forming fabric;
a dewatering nip (PN) defined by a first (8) and a second (9) press
unit, through which dewatering nip a water receiving felt (5) is
arranged to carry a fibrous web (W) formed in the forming section;
an endless steel belt (11) arranged to run in a loop over at least
two rolls (9,14) through the dewatering nip and arranged to pick up
the paper web and having at least one steel surface facing the
paper web that passes through the dewatering nip; at least one
heater (HE.sub.L HE.sub.U) arranged close to the endless steel belt
and heating the steel belt in a position at least partly before the
fibrous web is applied onto the endless steel belt; a textured
fabric (12) arranged to pick up the paper web from the endless
steel belt (11) at a point downstream of the dewatering nip; and a
drying cylinder (17) to which the textured fabric is arranged to
carry the paper web; and a transfer nip (TN) in which the paper web
is transferred from the endless steel belt to the textured fabric,
the transfer nip being formed between and by a first transfer nip
roll (14) located within the loop of the endless steel belt and a
second transfer nip roll (15) which is a suction roll that is
located within the loop of the textured fabric, the textured fabric
(12) driven by rolls (15,20) in the loop of the textured fabric
loop at a lower speed than the endless steel belt (11) driven by
rolls (9,14) in the loop of the endless steel belt (11) at a speed
difference in the range 2-25%; the transfer nip having a nip length
in the machine direction that is in the range of 5 mm-40 mm.
21. The machine according to claim 20, wherein the heater is a
heating box (HE.sub.U or HE.sub.L) arranged immediately close to a
surface of the endless steel belt and heated by steam.
22. The machine according to claim 21, wherein the endless steel
belt is heated on both sides of the endless steel belt by an upper
heater (HE.sub.U) and a lower heater (HE.sub.L).
23. The machine according to claim 20, wherein the endless steel
belt is heated by at least one steam heated roll supporting the
loop of the endless steel belt.
24. The machine according to claim 21, wherein the endless steel
belt is heated in a heating zone with heaters extending at least
50-70% of a distance ranging from 1 to 7 meters between two rolls
supporting the loop of the endless steel belt.
25. The machine according to claim 20, wherein the endless steel
belt (11) runs in a loop over at least two rolls (9,14) that
deflects the endless steel belt over at least 90 degrees of the
circumference of the rolls and that the rolls may have a function
as a guide roll, a press roll or a transfer nip roll and the total
length of the endless steel belt exceeding the total circumference
of the rolls by a factor above 2.
26. The machine according to claim 25, wherein the drying cylinder
is a Yankee drying cylinder to which the paper web is transferred
from the textured fabric in a second transfer nip formed between a
nip roll and the Yankee cylinder; and in which a doctor blade is
arranged to act on the Yankee cylinder.
27. The machine according to claim 25, wherein the drying cylinder
is a through air drying cylinder which is wrapped by the textured
fabric over a part of its circumference.
28. The machine according to claim 25, wherein the subsequent
drying of the fibrous web after transfer from the textured web
takes place on a sequence with at least one through air drying
cylinder and a final Yankee cylinder.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a method and a machine for
producing a structured fibrous web, in particular a tissue web. The
fibrous web produced may be used as, for example, kitchen towel,
toilet paper or facial tissue. In sharp contrast to production of
paper where the paper should have high density and printable
surface, the tissue paper should have high bulk and optimal
absorption using creping technique of the web during production of
the web.
PRIOR ART
[0002] A machine for manufacturing structured soft paper is
disclosed in U.S. Pat. No. 6,287,426. The machine disclosed in that
patent has a forming section with a head box and two forming
fabrics. The formed web is passed on a water receiving felt through
a dewatering nip. An impermeable belt is also passed through the
dewatering nip and the web is transferred to the impermeable belt.
The impermeable belt then conveys the fibrous web to a wire 22
which has a web-contacting side with a structure. A suction device
placed within the loop of the wire is used to pick up the web from
the impermeable belt and transfer it to the structured wire. The
web is then passed to a drying cylinder which may be a Yankee
dryer. When the web is passed from the impermeable belt to the
structured wire, a speed difference is used in order to achieve
structuring. This structuring is also called a wet creping effect
that increase bulk of the tissue web and is typical for tissue
paper machines. This wet creping in this position, made when the
web is not finally dried, cause a more permanent structuring of the
web than the final dry creping made on the web when the doctor
blade lift the web from the Yankee. This means that the wire moves
at a speed that is less than the speed of the impermeable belt.
Such a speed difference is sometimes referred to as "rush
transfer". It is stated in that document that the speed difference
can be 10-25%. While this machine may give a good result in terms
of bulk, the inventor of the present invention has discovered that
the paper web may sometimes be damaged. The inventor of the present
invention has found that it is difficult to operate such an
arrangement at speed differences larger than about 8%. When the
speed difference is larger than about 8%, sheet transfer is often
lost and the web is damaged. It is therefore an object of the
present invention to reduce the risk that the paper web is damaged,
even when the speed difference is larger than 8%.
[0003] U.S. Pat. No. 7,588,660, or 7,789,995 discloses another
machine for manufacturing structured soft paper. This technology
was developed by Georgia-Pacific Consumer products and is sold
under the name eTAD or TAD.TM.. In that patent, the formed web is
transferred to a felt and passed through a single-felted dewatering
nip in which the fibrous web is passed to a solid transfer roll
with a smooth surface that may be heated. From the transfer roll,
the web is passed through a nip to a creping fabric. Such an
arrangement requires that three rolls cooperate which is difficult
due to deflection of the rolls in the nips as well as possibilities
of controlling each individual nip individually. Moreover, the
creping wire may be subjected to wear as it contacts the transfer
roll.
[0004] Another machine for producing paper webs is disclosed in
U.S. Pat. No. 6,187,137, That document discloses how a wet web may
be transferred first from the forming section to a first transfer
fabric and from the first transfer fabric to a second transfer
fabric which may be adapted to impart texture and bulk to the web,
Transfer to the second transfer web may be done by means of rush
transfer whereafter the web may be transferred to a cylindrical
dryer.
[0005] Yet another machine is discussed in U.S. Pat. No. 5,830,321,
In that patent, rush transfer is discussed and the transfer takes
place when the fabrics involved pass over a vacuum shoe and a
deflection element respectively. Rush transfer is a frequently used
technique when producing tissue paper with high bulk as it
introduce a creping effect onto the web during transfer.
[0006] Finally, in U.S. Pat. No. 8,871,060 is disclosed another
improved machine for manufacturing structured soft paper where the
transfer between a first dewatering felt and the endless textured
fabric, i.e. creping fabric, takes place on an endless smooth belt
with a surface coating of polyurethane. This technology was
developed by Valmet AB and is sold under the name QRT.TM.. This
design avoid usage of three cooperating rolls as needed in the
machine as disclosed in U.S. Pat. No. 7,588,660.
[0007] In summary, above transfer systems in tissue machines
disclose the difficulties finding a system and design of the
transfer system between the dewatering felt and the subsequent
textured fabric, where the system must have a transfer surface with
a surface that both provide for proper adherence of the web after
the felt, and proper release of the web onto the subsequent
textured fabric. The operating window for such transfer system and
selection of surface material that both adhere and release the web
in opposite ends is very narrow, and must enable a proper rush
transfer to be established.
[0008] In paper production, where rush transfer is not sought for,
Valmet has developed the OptiPress.TM. Metal Belt, using a heated
metal belt for pre-calendering, obtaining excellent macro scale
smoothness, even topography after coating, better optical
properties and excellent macroscale topography with same or better
stiffness. The paper grades produced has some 60 to 70 g/m.sup.2
base paper. However, these product properties are not sought for
when producing tissue paper.
SUMMARY OF THE INVENTION
[0009] The invention relates to a method of producing a structured
fibrous web of paper suitable for tissue products. The method
comprising the steps of: [0010] forming a fibrous web and conveying
the formed fibrous web on a water receiving felt to a dewatering
nip formed by a first press unit and a second press unit and where
an endless steel belt is passed through the nip together with the
fibrous web and the water receiving felt, [0011] the endless steel
belt having at least a steel surface which contacts the fibrous web
in the dewatering nip wherein the endless steel belt is heated by
heating medium in a position at least partly before the fibrous web
is applied onto the the endless steel belt; [0012] after the
dewatering nip, conveying the fibrous web by the endless steel belt
to an endless textured fabric which is permeable to air and to
which the web is transferred from the endless belt. The textured
fabric running at a lower speed than the endless steel belt and a
speed difference in the range 2-25% (RT %). After the transfer to
the textured fabric, conveying the fibrous web by the textured
fabric to a drying cylinder, wherein the web is transferred from
the endless steel belt to the textured fabric in a transfer nip,
the transfer nip being formed between a first transfer nip roll
that lies within the loop of the endless steel belt and a second
transfer nip roll which is a suction roll located within the loop
of the textured fabric, the transfer nip having a length in the
machine direction which is in the range of 5 mm-40 mm.
[0013] These method steps enable an increase in speed difference
between the endless steel belt and the textured fabric as the
fibrous web may be subjected to improved web transfer while being
transported on the endless steel belt compared to using an endless
polyurethane belt. This improvement in web transfer may be used to
increase production capacity or increasing the bulk in the produced
tissue product. The risk of web breakage when transferring the
fibrous web from the endless steel belt onto the textured fabric
may alternatively be reduced if all other production factors are
equal, as the increased dewatering during transport on the endless
steel belt reduce the initial adhesive attraction between the
initially moisty fibrous web and the endless steel belt.
[0014] In a preferred method of operation could the endless steel
belt have a speed that is above 15%, i.e. in the range 15%-25%,
higher than the speed of the textured fabric, i.e. almost double
speed difference compared to using PUR-belts.
[0015] Heating the steel belt may increase the temperature of the
fibrous web by as much as 20.degree. C., reaching 4-6%-units higher
dryness already during transfer of the formed fibrous web from the
forming section to the texturing section, thus decreasing the
necessary effect in subsequent drying section after the texturing
section.
[0016] In a recommended way of operation is the endless steel belt
heated on both sides of the belt. This may reach higher temperature
in the steel belt and require less bulky heaters.
[0017] Further, in yet a preferred way of operation may the endless
steel belt be heated in a heating zone extending at least 50-70% of
a distance ranging from 1 to 7 meters. A heating effect distributed
over a longer stretch is needed in order to be able to heat the
endless steel belt that may travel at very high speed and therefore
has less retention time in the heating zone.
[0018] Finally, in a preferred way of operation is the endless
steel belt running in a loop over at least two rolls that deflects
the endless steel belt over at least 90 degrees of the
circumference of the rolls and that the rolls may have a function
as a guide roll, a press roll or a transfer nip roll and the total
length of the endless steel belt exceeding the total circumference
of the rolls by a factor above 2. This design will enable location
of above heaters in the loop of the endless steel belt and between
2 neighboring rolls supporting the loop.
[0019] The invention also relates to a machine for producing a
structured fibrous web of paper suitable for tissue products. The
machine comprising; [0020] a forming section that includes a first
(3) and a second (5) forming fabric; [0021] a dewatering nip (PN)
defined by a first (8) and a second (9) press unit, through which
dewatering nip a water receiving felt (5) is arranged to carry a
fibrous web (W) formed in the forming section; [0022] an endless
steel belt (11) arranged to run in a loop over at least two rolls
(9,14) through the dewatering nip and arranged to pick up the paper
web and having at least one steel surface facing the paper web that
passes through the dewatering nip; [0023] at least one heater
(HE.sub.L HE.sub.U) arranged close to the endless steel belt and
heating the steel belt in a position at least partly before the
fibrous web is applied onto the endless steel belt [0024] a
textured fabric (12) arranged to pick up the paper web from the
endless steel belt (11) at a point downstream of the dewatering
nip; and a drying cylinder (17) to which the textured fabric is
arranged to carry the paper web; and [0025] a transfer nip (TN) in
which the paper web is transferred from the endless steel belt to
the textured fabric, the transfer nip being formed between and by a
first transfer nip roll (14) located within the loop of the endless
steel belt and a second transfer nip roll (15) which is a suction
roll that is located within the loop of the textured fabric,
[0026] the textured fabric (12) driven by rolls (15,20) in the loop
of the textured fabric loop at a lower speed than the endless steel
belt (11) driven by rolls (9,14) in the loop of the endless steel
belt (11) at a speed difference in the range 2-25% (RT %); the
transfer nip having a nip length in the machine direction that is
in the range of 5 mm-40 mm.
[0027] The heater is preferably located close to the position
before the fibrous web is applied onto the endless steel belt
reducing heat from dissipating from the endless steel belt, making
most effect of the heating. The heater may thus be a roll that has
a surface exposed by the heater and located on the roll close to
the position before the fibrous web is applied onto the endless
steel belt running over the heated roll
[0028] In a further preferred embodiment of the inventive machine
is at least one heater a heating box (HE.sub.U or HE.sub.L)
arranged immediately close to a surface of the endless steel belt
and heated by steam. The box may have a heating surface exposed to
a part of the endless steel belt located at a short distance, i.e.
0.1-5 mm from said endless steel belt, or alternatively using low
friction guides on the edges of the box with no gap between the box
and the endless steel belt.
[0029] In a further preferred embodiment of the inventive machine
is the endless steel belt heated on both sides of the endless steel
belt by an upper heater (HE.sub.U) and a lower heater (HE.sub.L).
Heating from both sides enable a higher obtainable temperature
throughout the thickness of the steel belt.
[0030] In another embodiment is the endless steel belt heated by at
least one steam heated roll supporting the loop of the endless
steel belt. This could be done in addition to heating boxes.
[0031] In a further preferred embodiment of the inventive machine
is the endless steel belt heated in a heating zone with heaters
extending at least 50-70% of a distance ranging from 1 to 7 meters
between two rolls supporting the loop of the endless steel belt.
Arranging the heaters between two rolls enable an extended box
design of the heaters, preferably running in parallel with the
endless steel belt.
[0032] In still a further preferred embodiment of the inventive
machine the endless steel belt (11) runs in a loop over at least
two rolls (9,14) that deflects the endless steel belt over at least
90 degrees of the circumference of the rolls and that the rolls may
have a function as a guide roll, a press roll or a transfer nip
roll and the total length of the endless steel belt exceeding the
total circumference of the rolls by a factor above 2.
[0033] In another preferred embodiment of the inventive machine is
the drying cylinder a Yankee drying cylinder to which the paper web
is transferred from the textured fabric in a second transfer nip
formed between a nip roll and the Yankee cylinder; and in which a
doctor blade is arranged to act on the Yankee cylinder. This set up
enable a very compact Tissue machine where the forming section is
followed by the transfer by the endless steel belt to the textured
fabric, and with a Yankee cylinder immediately after the textured
fabric.
[0034] In an alternative preferred embodiment of the inventive
machine is the drying cylinder a through air drying cylinder which
is wrapped by the textured fabric over a part of its circumference.
Hence, the drying roll after the textured fabric may be any kind of
drying roll, either a single Yankee cylinder or alternatively a
through air drying cylinder.
[0035] In final alternative preferred embodiment of the inventive
machine may the subsequent drying of the fibrous web after transfer
from the textured web takes place on a sequence with at least one
through air drying cylinder and a final Yankee cylinder. Thus,
final drying may take place in a single step or in multiple steps
in 2.3 or more drying rolls of different design.
LIST OF DRAWINGS
[0036] In the following schematic drawings are details numbered
alike in figures, and details identified and numbered in one figure
may not be numbered in other figures in order to simplify
figures.
[0037] FIG. 1; shows a schematic side view of a first embodiment of
the invention with the inventive steel belt running over only 2
rolls, in which a Yankee drying cylinder is used for the structured
fabric web.
[0038] FIG. 2; shows an alternative schematic side view of a second
embodiment of the invention with the inventive steel belt running
over 4 rolls; in which a Yankee drying cylinder is used for the
structured fabric web finally collected on a reeling drum.
[0039] FIG. 3; shows a schematic side view of a third embodiment of
the invention with the inventive steel belt running over 3 rolls;
in which the drying cylinder is a through air drying cylinder.
[0040] FIG. 4; shows a schematic side view of a fourth embodiment
of the invention which is similar to FIG. 3 except for the
inventive steel belt running over 4 rolls.
[0041] FIG. 5; is a schematic representation of a fifth embodiment
of the invention which is similar to FIG. 4 but with the head box
feeding the pulp suspension onto the felt from below; and
finally
[0042] FIG. 6 is a schematic representation of yet another
embodiment in which the drying of the web takes place in a final
sequence of a first through air drying cylinder followed by a
Yankee.
DETAILED DESCRIPTION OF THE INVENTION
[0043] With reference to FIG. 1, a machine for producing a
structured fibrous web of paper is shown. The machine comprises a
forming section. The forming section has a head box 1 that is
arranged to inject stock into a gap between a first forming fabric
3 and a second forming fabric 5. Both forming fabrics 3, 5 may be
foraminous wires (i.e. wires that are permeable to water). However,
in advantageous embodiments, the first forming fabric 3 is a
foraminous wire while the second forming fabric 5 may be a water
receiving felt. It should be understood that in the context of this
patent application and any patent issuing therefrom, the term
"forming fabric" is used for any fabric used during forming of the
fibrous web. This could include both foraminous wires and
felts.
[0044] The reference numeral 2 designates a forming roll. In FIG.
1, it is shown how the first forming fabric 3 is arranged to run in
a loop guided by guide rolls 4, seen in FIG. 2. The second forming
fabric 5 is guided by guide rolls 6, seen in FIG. 2. The newly
formed fibrous web is carried on the outer surface of the felt 5 to
a dewatering nip PN (i.e. a press nip PN) formed between a first
press unit 8 and a second press unit 9. In the embodiment of FIG.
1, the felt that passes through the dewatering nip is identical to
one of the forming fabrics. It should be understood that
embodiments are conceivable in which the fibrous web is first
formed between two forming fabrics and then transferred to a felt
which is not used as a forming fabric. However, the overall design
of the machine becomes more compact when one of the forming fabrics
is the same felt that carries the web to the dewatering nip PN. The
press units 8, 9 will normally be formed by rolls such as for
example deflection controlled rolls. In the dewatering nip PN,
water is pressed out of the fibrous web such that the dry solids
content of the web increases. Dry solids content after the
dewatering nip PN may be in the range of 40%-50%. Optionally, a
suction roll 21, with a sector in the nip area connected to
suction/underpressure U, may also be arranged within the loop of
the second forming fabric 5 to dewater the felt and the newly
formed fibrous web by vacuum dewatering. An endless steel belt 11
is also arranged to pass through the dewatering nip PN together
with the felt 5 and the web W. The endless belt 11 forms a loop
running over two rolls 9 and 14. In all figures is the endless belt
slightly thicker than preceding forming felt and subsequent
textured fabric
[0045] According to the invention is at least the side of the
endless belt 11 that faces the paper web covered by a smooth steel
surface facing the fibrous web when the fibrous web and the endless
steel belt 11 pass through the dewatering nip. Hence, the steel
belt may be a homogenous steel belt but may also be a belt with a
base material covered by a thin steel sheet layer. As will be
described later the steel belt may be heated and increased weight
of the steel belt may retain the temperature better. In the prior
art design, such as that shown in U.S. Pat. No. 8,871,060, is a
polyurethane (PUR)-covered endless belt 11 used, but this design
does not enable heating and drying of the fibrous web, and the
PUR-covered endless belt increases the risk for the web break as
the fibrous web may stick to the PUR-covered endless belt 11.
Further, the PUR-covered endless belt may reduce the acceptable
speed difference (RT %) between the following structured fabric and
the PUR covered endless belt. The speed difference allowable
typically has been found at some 8-15 RT % when using PUR covered
endless belts.
[0046] The endless steel belt 11 is smoother than the felt 5.
Therefore, the fibrous web will adhere to the endless steel belt 11
after passage of the dewatering nip PN. After the dewatering nip
PN, the fibrous web is carried by the endless steel belt 11 to a
transfer nip TN downstream of the dewatering nip PN which transfer
nip TN is formed by a first transfer nip roll 14 located within the
loop of the endless steel belt 11 and a second transfer nip roll 15
which is a suction roll. A textured fabric 12 runs in a loop
through the transfer nip TN and the textured fabric 12 may be
guided by one or several guide rolls 23 (not shown in FIG. 1 but
seen in FIG. 2). The second transfer nip roll 15 is located within
the loop of the textured fabric 12. The textured fabric 12 is
arranged to pick the fibrous web from the endless belt 11 when the
fibrous web passes the transfer nip TN such that the fibrous web is
transferred to the textured fabric 12. The transfer is secured by
means of the second transfer nip roll 15 since this roll preferably
is a suction roll. The textured fabric 12 is air permeable such
that the second transfer nip roll 15 may draw air through the
textured fabric and cause the web to adhere to the textured fabric.
The air permeable textured fabric 12 may be a woven fabric such as
a forming wire or a through air drying fabric (TAD fabric).
[0047] The smooth surface of the endless steel belt 11 makes the
web adhere to the endless steel belt but the adhesive force is not
very strong, and the web can be picked up from the endless belt 11
without substantial risk of web breaks if the speed difference (RT
%) is kept within limits, especially if a suction roll 15 is used
in the transfer nip.
[0048] The inventors have seen that the adhesive adherence of the
moisty fibrous web on the smooth endless belt 11 works in the same
way when using a PUR-coated endless belt (like shown in U.S. Pat.
No. 8,871,060) as when using a smooth endless steel belt. The
moisty fibrous web is more or less sucked onto the surface due to
the water content press out on the surface of the fibrous web in
the press nip PN. As the fibrous web is carried along on the
surface of the PUR-coated endless belt only a minor evaporation of
water takes place, but if an endless steel belt is used a better
heat conductivity between steel belt and fibrous web may be
obtained. The thermal conductivity of stainless steel lies at 12-45
W/m K, while the thermal conductivity of PUR lies at a fraction
thereof at 0.20-0.45 W/m K, i.e. only about 1% of that of steel. In
fact, PUR is most frequently used as heat insulation material due
to the very low thermal conductivity.
[0049] It has also been seen that due to the usage of steel in the
endless belt 11, the endless steel belt may easily be heated to
such extent that the fibrous web may increase the temperature about
20.degree. C. using steam heated heaters on both sides of the steel
belt close to the surface of the steel belt. Heating the fibrous
web by 20.degree. C. could evaporate 20% more water, obtaining 6%
higher dryness units, as seen in heating steel belts in
pre-calendaring installations. The increased dewatering effect is
to great extent contributed to lowered viscosity of the water
content. The above figures on evaporation effect from a dual sided
heating arrangement using steam heated boxes has been proven in up
to date paper machines in calendaring, but application in a tissue
machine with more bulk in the fibrous web should likely result in
even better figures.
[0050] As to principles of heating the endless steel belt could
alternatively electrically heaters be used instead of steam boxes.
Complementary to the heating boxes HE.sub.U and HE.sub.L shown in
figures may also additional heating of the steel belt be obtained
from heated rolls, preferably steam heated rolls well known per se.
In FIG. 1 may roll 9 be steam heated but also roll 14. If steam
heated rolls could elevate the temperature of the fibrous web by
10-20.degree. C. could even the heaters HE.sub.U and HE.sub.L be
omitted.
[0051] The heating of the box heaters HE.sub.L and HE.sub.U may be
done with steam indicated with ST in FIG. 1, and the heating of the
rolls 9 and 14 in the loop of the endless steel belt 11 may also be
done by steam as indicated.
[0052] The effect of evaporation ahead of transfer to a textured
fabric between the press nip PN and the transfer nip TN is of
outmost importance when transferring the fibrous web onto the
textured fabric, as the adhesive effect onto the stainless steel
belt due to water content decreases during the travel between the
press nip PN and the transfer nip TN, and the fibrous web may be
transferred without breaking and at higher relative speed
difference which is of importance for obtaining a high bulk in the
web as the web is subjected to a creping effect.
[0053] The textured fabric has a texture, i.e. a three-dimensional
structure on at least the side facing the fibrous web. The textured
fabric 12 imparts a three-dimensional structure on the fibrous web
when the second transfer nip roll 15 (the suction roll) draws the
web by suction against the textured fabric 12. Thereby, the bulk of
the web is increased. To further increase the bulk of the web, the
transfer from the endless belt 11 to the textured fabric 12 is made
in the form of a rush transfer (RT %), i.e. there is a speed
difference between the textured fabric 12 and the endless belt 11.
Using a certain degree of speed difference helps sheet transfer if
the difference in speed is not too large. However, speed
differences above a certain limit can actually make sheet transfer
more difficult. The difference in speed may also improve bulk. When
the paper web is picked up by a textured fabric, the speed
difference may also contribute to improving the molding of the web
into the textured fabric, thereby further improving the bulk.
[0054] The endless steel belt 11 is preferably a belt with a smooth
surface and impermeable to water and air. An endless steel belt 11
with a textured surface (on the side facing the fibrous web W) and
which is impermeable to water and air is considered not quite as
advantageous but almost as good as a smooth and impermeable belt.
However, embodiments are also conceivable in which the endless
steel belt 11 has a limited permeability to air. The permeability
to air should not exceed 0.15 m/s (corresponding to 35 CFM) at a
pressure drop of 125 kPa between opposite sides of the belt. If the
endless steel belt 11 is permeable to air, a smooth belt is the
most preferred choice but a textured belt with a limited
permeability (not more than 0.15 m/s) can be considered.
[0055] The use of a smooth endless steel belt 11 is advantageous
for sheet transfer. Preferably is the steel belt 11 heated by
heaters HE.sub.U and HE.sub.L arranged on both sides of the endless
steel belt in a location between two rolls 9,14 where the steel
belt pass in a straight line. In FIG. 1 is an upper heater HE.sub.U
arranged above the steel belt 11 and a lower heater HE.sub.L
arranged below the steel belt. The heaters HE.sub.U and HE.sub.L
may be electrical heaters or steam box heaters.
[0056] In the dewatering nip PN, the surface of the fibrous web
will tend to adhere to the smooth surface and will follow the
endless steel belt 11 after the dewatering nip PN instead of
following the felt. However, as the web passes through the
dewatering nip PN and water is forced out of the web, and the
subsequent heating from the heated steel belt, the dry solids
content of the web increases. Compared to a web with low dry solids
content, a dryer web has less adherence to the surface of a
transfer fabric such as the endless steel belt 11. Therefore, when
the web W becomes successively dryer, due to continued heating from
the heated steel belt 11, it will become easier to transfer the web
W to a following fabric. Immediately after the dewatering nip PN,
the web tends to adhere relatively well to the endless steel belt
11. The inventor has observed that adherence of the fibrous web W
to the endless steel belt 11 decreases with time after passage of
the dewatering nip due to water evaporating from the fibrous web,
but if a PUR-coated belt is used, heating is not possible, and
hence the water content of the fibrous web decreases to a very
limited extent. But if a steel belt is heated could the water
content of the fibrous web decrease 2- to 3-fold more.
[0057] Without wishing to be bound by any particular theory, it is
believed by the inventor that a thin water film is present on the
endless steel belt 11 immediately after the nip and that this thin
water film creates adhesion between the endless steel belt 11 and
the fibrous web W. But if the steel belt is heated is the thin
water film directly affected and may increase in temperature and as
an effect will increase evaporation rate.
[0058] Regardless of whether this explanation is correct or not,
experience has showed the inventor that adhesion decreases
gradually after the dewatering nip PN when using an endless steel
belt, and especially if the steel belt is heated. For this reason,
the distance from the dewatering nip PN to the transfer nip TN, or
between the rolls wherein the heaters are arranged, should
preferably be at least 1 m to give the endless steel belt 11 time
to be heated when passing the heaters and to give the fibrous web
time to be heated by the steel belt. In some cases, the distance
may have to be larger, up to 7 m. It should be understood that the
distances mentioned are applicable to applications using a speed
which is in the normal range of speed for a tissue making machine.
In the last decade new tissue making machines may operate at a
speed of up to about 2000 m/minute. The heaters may thus be
producing a surface temperature of about 200-500.degree. C., or a
steam heated environment at about 200.degree. C., in order to heat
the endless steel belt sufficiently. The heaters HE.sub.U and
HE.sub.L may preferably extend some 50-90% of the total distance
between the rolls where the heaters are located.
[0059] The degree of adhesion of the fibrous web W to the endless
steel belt 11 is important. In and immediately after the dewatering
nip PN, the adhesion of the fibrous web W to the endless belt 11 is
relatively high such that the fibrous web follows the endless steel
belt 11 instead of following the water receiving felt 5. After the
dewatering nip PN, the adhesion of the fibrous web W to the endless
steel belt 11 decreases such that the fibrous web can easily be
picked up by the endless textured fabric 12, and preferably as
indicated in FIG. 1 using a transfer nip roll 15 (i.e. a suction
roll), with a sector in the nip connected to suction/under pressure
U.
[0060] In many realistic embodiments of the invention, the endless
steel belt 11 may run 2-25% faster than the textured fabric 12,
which may be compared with some 8%-15% faster than the textured
fabric 12 if a PUR coated endless belt is used (assuming all other
factors equal), before risks of web breaks becomes dominant. It is
highly desirable that the speed difference can be made large, as
this improves creping effect at the transfer and hence increased
bulk in the produced tissue paper. It has been seen that when the
length of the transfer zone is too long, this may cause damage to
the web in connection with rush transfer. The higher the speed
difference is, the greater the risk that the web be damaged. Since
a higher speed difference is desired to obtain higher bulk when
producing tissue webs, it is highly desirable that the speed
difference can be increased without simultaneously increasing the
risk that the web be damaged. The maximum length of the transfer
zone should not exceed 40 mm and preferably it should not exceed 30
mm. By using a transfer nip between two rolls 14, 15, it is
possible to ensure that the transfer nip can be kept short in the
machine direction. Suitably, the length of the transfer nip in the
machine direction is 5 mm-30 mm, preferably 15 mm-30 mm. For
example, it may be 25 mm. A nip length less than 5 mm is considered
impractical. The inventor has found that, when transfer is carried
out by means of only a suction shoe as in U.S. Pat. No. 6,287,426
or by means of only suction roll acting on one side of the web, the
transfer zone becomes extended and it becomes correspondingly more
difficult to achieve reliable web transfer without web damage,
especially when the speed difference is larger than 15%. A short
transfer zone can be achieved by means of a nip formed between two
rolls. Thereby, the transfer can be carried out even reliably and
without damage to the web even at speed differences exceeding
15%.
[0061] The textured fabric 12 may also risk being damaged in the
transfer nip in case its edges should contact the first transfer
nip roll 14. This problem is not so serious when there is no speed
difference. However, when a speed difference is used in the
transfer zone, the problem may become more significant. Damage to
the edges of the transfer fabric may also cause damage to the web.
To solve or at least reduce this problem, the width (i.e. the
extension in the cross machine direction) of the endless steel belt
11 can optionally be made larger than the width of the textured
fabric 12. In the same way, the width of the first transfer nip
roll 14 suitably exceeds the width of the textured fabric 12 such
that it can support the endless steel belt 11 over the entire width
of the endless steel belt 11. When the endless steel belt 11 has a
greater width than the textured fabric 12, the textured fabric 12
is protected by the endless steel belt 11. Preferably, also the
width of the first transfer nip roll 14 exceeds the width of the
second transfer nip roll 15 (the suction roll). The width of the
endless steel belt 11 may exceed the width of the textured fabric
by 10 mm-300 mm.
[0062] Preferably, the endless steel belt 11 is impermeable. If it
is not entirely impermeable, the permeability to air should
preferably not exceed 0.15 m/s measured at a pressure differential
of 125 kPa between the two opposite sides of the endless belt
11.
[0063] After the transfer nip TN, the web is carried by the
textured fabric 12 to a drying cylinder 17. In the embodiment of
FIG. 1, the drying cylinder 17 is a Yankee drying cylinder and the
web is transferred to the drying cylinder in a second transfer nip
formed by a nip roll 20 and the drying cylinder 17. The web W can
then be passed on the drying cylinder to a doctor blade 18 that
crepes the web W from the drying cylinder 17. The drying cylinder
17 is internally heated by for example steam. The drying cylinder
thereby causes water to evaporate from the web W. When the web W
has been separated from the surface of the drying cylinder 17, it
can be passed to a reel-up wherein a paper roll 24 is formed in a
reeling drum 25. The reference numeral 19 refers to a supporting
cylinder. Although the drying cylinder 17 must not necessarily be a
Yankee cylinder, it is preferred that the drying cylinder is a
Yankee cylinder from which the web is creped.
[0064] The linear load in the transfer nip is in the range of 0.5
kN/m-15 kN/m. This is a range which may be suitable for a lightly
loaded transfer nip in which the nip mainly serves to transfer the
web from the steel belt 11 to the textured fabric 12 while being
supported by the suction roll 15. The low load contributes to
protect the web from damage. However, that a certain load is
applied (as opposed to no load at all) is advantageous since it
ensures that a certain nip length can be defined such that the
transfer zone can be limited. Moreover, a certain linear load
improves stability in the nip which protects the web.
[0065] The second transfer nip roll 15 may suitably operate with an
internal under pressure in the range of 10 kPa-70 kPa. This is a
pressure range in which the web is reliably transferred, and which
helps the textured fabric 12 to give structure to the web. At the
same time, it is not excessively high which could lead to
unnecessarily high energy consumption.
[0066] In advantageous embodiments of the invention, the transfer
nip TN is located at a distance of 1 m-7 m from the dewatering nip
PN, preferably at a distance of 2 m-6 m.
[0067] The embodiment of FIG. 2 is substantially similar to the
embodiment of FIG. 1 except that the endless steel belt 11 is
running over 4 rolls (9-14-22-22) instead of only 2 rolls (9-14) as
shown in FIG. 1 As shown in FIG. 2 could optionally, a vacuum box
16 be arranged to act on the textured fabric 12 to further mold the
fibrous web into the surface of the textured fabric 12 at a point
between the transfer nip and the drying cylinder 17. The fibrous
web is molded into the surface of the textured fabric by means of
the vacuum (under pressure) in the vacuum box. Thereby, the
structuring of the web may be improved such that the bulk is
further increased. The vacuum box 16 may suitably operate at an
under pressure of 20 kPa-70 kPa. This is deemed to be a suitable
range for imparting further texture (three-dimensional structure)
to the web. For some cases, the upper limit of the under pressure
in the vacuum box 16 may be set to 60 KPa.
[0068] With reference to FIG. 3, a third embodiment of the
invention is shown. The embodiment of FIG. 3 is substantially
similar to the embodiment of FIG. 2 except that the drying cylinder
17 is formed by a through air drying cylinder (TAD cylinder). In
this embodiment, the textured fabric 12 is a through air drying
fabric (TAD fabric) and hot air is blown from the inner of the
cylinder 17 through the textured fabric 12. The textured fabric 12
wraps the fibrous web over a part of the circumference of the
drying cylinder 17. The wrap angle may suitably be in the range of
160.degree.-340.degree..
[0069] The embodiment of FIG. 4 is substantially similar to the
embodiment of FIG. 3 but the first press unit 8 is here formed by
an extended nip roll that may have an internal shoe 10 which is
looped by a flexible belt. In all embodiments of the present
invention, an extended nip roll having an internal shoe looped by a
flexible belt could be used. Such extended nip rolls (sometimes
also referred to as shoe press rolls) are disclosed in the prior
art, see for example U.S. Pat. Nos. 5,662,777, 6,083,352, 7,527,708
or EP 2085513. These documents disclose examples of extended nip
rolls (shoe rolls) that could be used as extended nip rolls in the
present invention. In the embodiment of FIG. 4, it is the first
press unit 8 that is an extended nip roll but it should be
understood that it could instead be the second press unit 9 that is
an extended nip roll. In the same way, an extended nip roll could
be used in the embodiment of FIG. 2 or FIG. 3. If one press unit 8,
9 is an extended nip roll, the other press unit 8, 9 could
optionally be a deflection controlled roll (a deflection
compensated roll) which is has a shell that is internally supported
by shoes or by one or several hydraulic chambers.
[0070] The embodiment of FIG. 5 is substantially similar to the
embodiment of FIG. 4 but here the forming section has been designed
differently and the drying cylinder 17 (which is also here a
through air drying cylinder) is placed in a high position (as
opposed to the lower position in FIG. 3).
[0071] In the embodiment of FIG. 6, the layout is similar to that
of FIG. 5 but in this is embodiment, the drying cylinder 17 which
is a through aft drying cylinder is followed by a second drying
cylinder 26 which is a Yankee drying cylinder. A nip roll 20 within
the loop of the textured fabric 12 forms a nip with the second
drying cylinder 26. In this nip, the web W is transferred to the
Yankee drying cylinder from which it is creped by a doctor blade
18.
[0072] In many embodiments, the dewatering nip is a nip using an
extended nip roll. In such embodiments, the linear load in the
dewatering nip may be in the range of 200 kN/m-1000 kN/m,
preferably 300 kN/m-800 kN/m. However, peak pressure in the
dewatering nip is more important than linear load. The peak
pressure is the highest pressure in the nip (the actual pressure
typically varies in the machine direction). Suitably, the peak
pressure may be in the range of 2 MPa-8 MPa. Preferably, the peak
pressure should be in the range of 4 MPa-7 MPa. Generally, a higher
linear load can be used when an extended nip roll is used such that
the dewatering nip is an extended nip (such as a nip formed between
a shoe press roll and a cylindrical counter roll). This is because
an extended nip roll makes it possible to distribute the linear
load over a larger nip area such that the peak pressure becomes
lower than in a nip between two conventional rolls. At a given nip
length, the average pressure is determined by the linear load. Peak
pressure is determined not just by the linear load and nip area but
also by the geometry of the nip which can determine pressure
distribution. The linear load, and thereby the pressure in the nip,
should be high enough to press out as much water as possible since
a high dry solids content before the drying cylinder reduces the
energy consumption for the drying cylinder (less water must be
evaporated). However, a high linear load with a correspondingly
high peak pressure may reduce the bulk of the fibrous web; the
caliper (thickness) of the web is reduced which is undesirable.
Tissue paper should preferably have a high bulk, i.e. a high
caliper also when the basis weight is low. In many realistic
embodiments, the linear load in the dewatering nip may be in the
range of 350 kN/m-700 kN/m when one of the press units 8, 9 is an
extended nip roll (depending on nip length). For example, the
linear load could be in the range of 400 kN/m-600 kN/m. The peak
pressure should not exceed 8 MPa since a higher peak pressure is
likely to cause significant reduction of bulk. If the dewatering
nip is a roll nip which does not include an extended nip roll, the
nip length will be shorter which may make it necessary to use a
smaller linear load. In many cases, it may be suitable to limit the
peak pressure to 7 MPa. At the same time, if the linear load and
the pressure is too low, dewatering will not be so effective.
Therefore, the pressure should be allowed to rise such that peak
pressure reaches at least 2 MPa and preferably to 4 MPa.
[0073] In all embodiments, the dewatering nip may be an extended
nip or a short roll nip.
[0074] The use of a short transfer nip which is 5 mm-40 mm reduces
the risk that the web is damaged during transfer to the textured
fabric. By using a steel belt that is wider than the textured
fabric, the textured fabric and especially the edges thereof is
also protected in the transfer nip and the risk of damage to the
textured fabric is reduced. Thereby, also the risk of damage to the
web in the transfer nip is reduced since a damaged textured fabric
could cause damage to the web, especially during transfer of the
web.
[0075] In those embodiments where the textured fabric is a through
air drying fabric (a TAD fabric), this fabric may be, for example,
such a fabric as is sold by Albany International under the name
Prolux 003 or under the name ProLux 005.
[0076] The invention is primarily intended for such tissue paper
grades that have a basis weight in the range of 10 g/m.sup.2-30
g/m.sup.2 but in some cases, it can be used also for papers with
even lower weight, e.g. down to 7 g/m.sup.2. Normally, it would be
used for papers with a basis weight in the range of 14 g/m.sup.2-28
g/m.sup.2. The indicated ranges for basis weight refer to the
weight of the ready-dried web, i.e. the basis weight of the paper
that is rolled to a paper roll on a reeling drum.
[0077] The endless steel belt 11 that is used should have smooth
surface, but the surface may have micro-scale depressions or
dimples.
[0078] A belt which is a suitable choice for the endless steel belt
11 is sold by Contibelt under the name CB 630 SGM, a cold rolled
stainless steel with martensitic structure. This steel quality has
good spring properties, high ductility and high strength as well as
very good weldability. By precipitation hardening, different levels
of tensile strength can be obtained, according to the customers'
individual requirements. The surface is mill finish according to 2B
of ASTM with a selected cold rolled temper finish. The surface is
smooth and clear as well as metallically clean. Alternative
qualities may be obtained from Sandvik in form of the 1650; and
1500SAF Qualities.
[0079] Embodiments are conceivable in which the fibrous web is
formed between two forming wires and subsequently conveyed from one
of the forming wires to the felt that passes through the dewatering
nip. However, it is preferable that the felt that passes through
the dewatering nip is also one of the fabrics used in the forming
section. Such a design makes the layout of the machine shorter and
simpler. Less space will be required for the machine.
[0080] The invention can be used for tissue applications where the
speed difference in rush transfer (the speed difference in the
transfer nip TN) is larger than 15% and preferably larger than 17%
which is almost double the speed difference obtainable when using
PUR coated transfer belt according to U.S. Pat. No. 8,871,060. This
improvement may be used in several approaches. In an existing
tissue machine could the speed of the forming section be increased
by some 17% while the speed of the transfer belt 11 is kept at
original speed, thus increasing the crep rate at transfer to the
textured fabric and by that increasing the bulk of the final tissue
web. Alternatively, the production capacity could be increased by
17% in the entire tissue machine (maintaining same bulk), or any
tradeoff between these extremes.
[0081] By using a transfer nip with a nip length which does not
exceed 40 mm for transferring the fibrous web to the textured
fabric 12, it is possible to achieve web transfer at higher speed
differences than 17%. However, the invention can also be applied to
such cases where the speed difference is lower than 17% in order to
reduce the risk that the web be damaged in the transfer nip TN.
There are cases where the invention may be useful even when the
speed difference is only 2%.
* * * * *