Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 0223430~ 1998-04-02
FIELD OF THE INVENTION
The present invention relates to a method of rel,ofilling a papermaking
apparatus having a conventional pressing felt with a through-air-dryer unit without the
expenses involved in increasing the length of the papermaking apparatus. The present
invention further relates to the use of elevated applied pressure differential to the drying
section of a through-air-dryer to efficiently remove moisture using a limited size through-
air-drying unit. Finally, the present invention relates to a method of making a web using
an applied pressure differential across the web and fabric from greater than about 1 to
about 15 inches of Hg.
BACKGROUND
In consumer paper products, such as facial tissue, paper towels, and toilet
tissue, the industry has seen a shift away from products made by conventional wet-
pressing (CWP) to those made by the more recent through-air-drying (TAD) technology.
Conventional felted wet press (CWP) processes are significantly more energy efficient
than processes such as through-air-drying (TAD), since they remove excess moisture
by mechanically pressing it from the web. Final drying of the web, after pressing, is
obtained while the web is on a heated Yankee drying cylinder which is maintained at
the proper drying temperature. CWP processes conserve energy because they do not
require heating and moving large quantities of air as are required by TAD processes,
and also because they remove a greater fraction of the water in the web mechanically,
rather than via evaporation.
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In contrast to CWP processes, the TAD process uses the passage of heated air
through the wet fibrous web after it is formed on a wire and transferred to a permeable
carrier to dry the web. The result is no overall compaction of the web during drying.
The lack of overall compaction, such as would occur when the web is pressed while on
a felt and against the drying cylinder when it is transferred thereto, reduces the
opportunity for interfiber bonding to occur, and allows the finished product to have
greater bulk and absorbent capacity than can be achieved in a wet press process.
Because of the consumer perceived softness of these products, and their greater ability
to absorb liquids than webs formed in wet press processes, the products formed by the
newer processes enjoy an advantage in consumer acceptance. The present invention
takes advantage of this newer TAD technology by ret,urilling existing conventional wet
press (CWP) paper machines.
CWP systems include three basic sections, a forming section, a pressing
section, and a drying section. One conventional CWP system is described with
reference to Figure 1; however, variations and alternatives will be understood by the
skilled artisan. In a conventional wet press process and apparatus 10, a furnish is fed
from silo 50 through conduits 40, 41 to headbox chambers 20, 20'. A web W is formed
on a forming wire 12, supported by rolls 18, 19, from a liquid slurry of pulp, water and
other chemicals. Materials removed from the web through the forming fabric when
wrapped on forming roll 15 are returned to silo 50, from saveall 22 through conduit 24.
The web is then transferred to a moving felt or fabric 14, supported by roll 11. The web
is then pressed by suction press roll 16 against the surface of a rotating Yankee dryer
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cylinder 26 which is heated to cause the paper to substantially dry on the cylinder
surface. The moisture within the web as it is laid on the Yankee surface causes the
web to transfer to the surface. Liquid adhesive may be applied to the surface of the
dryer to provide substantial adherence of the web to the creping surface. The web is
then creped from the surface with a creping blade 27. The creped web is then
optionally passed between calender rollers (not shown) and rolled up on reel 28 prior to
further converting operations, for example, embossing. The action of the creping blade
on the paper is known to cause a portion of the interfiber bonds within the paper to be
broken up by the mechanical action of the blade against the web as it is being driven
into the blade.
TAD systems, like conventional CWP systems, can include three sections, in this
case a forming section, a predrying section and a drying section. Unlike CWP systems,
TAD systems can forego a drying section and do complete drying of the web in the
predrying section. One example of a through-air-drying paper machine having three
sections is set forth in Figure 2. As depicted, this through-air-drying line includes a
forming section 40, a predrying section 42, and optionally a drying section 44. The
forrning section 40 of the through-air-drying machine parallels that of the conventional
paper machine. The forming section 40 includes a headbox 46 and a forming wire 48.
The former can be any conventional former such as a twin wire former, a crescent
former, a suction breast roll former and the like. The fibrous slurry is fed from the
headbox 46 to the forming wire 48 to form a nascent web. The nascent web is
transferred from the forming wire 48 to the predrying section 42. The predrying section
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includes a drying fabric 50 for supporting the wet web during passage over vacuum
dewatering box 51 and during the passage of hot air through the web and the fabric to
further dewater and dry the web. This section, as depicted in Figure 2, includes two
TAD cylinders 52 for supporting the fabric and web during drying. The web, if
sufficiently dry, can be removed directly from the fabric to a take up reel 54. However,
as depicted the web proceeds to an additional drying section 44 that, like the CWP
system, includes a Yankee dryer 56. If a Yankee dryer 56 is used, the web is creped
using a creping blade 58 and then rolled up on a take up reel 54.
A variety of TAD processes and apparatus are described in the patent literature.
One example can be found in U.S. Patent No. 3,303,576 to Sisson. In that process a
sheet initially at 20% dryness was reduced to a dryness of 50% on a four foot TAD
cylinder using a flow of hot air that is maintained using a pressure differential across the
web, fabric and cylindrical roll of about 5 to 25 inches of water. This pressure
differential was believed sufficient to achieve a reasonable flow of heated air through
the cylinder, web and fabric. However, the operating speed was only about 1200 fpm.
In this disclosed process, dryness was increased to 80% using a second drying cylinder
and hot air.
An alternative TAD configuration is set forth in U.S. patent No. 3,447,247, which
discloses a drying system where the drying air is in the form of high speed jets of small
diameters for directing heated air onto the sheet. Instead of allowing the air to follow
the path of least resistance through the fiber materials, the jets force the heated air
through the sheet across its entire surface. This provides more uniform drying of the
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sheet. Moreover, the speed of the jets may reduce side leaks and lessen the need for
seals. This patent discloses air jets with speeds of up to 40 m/sec which are much
higher than in conventional through-air dryers; however, the total pressure differential
across the cylinder according to this disclosure is still only about 30 inches of water or
less, and, in fact is progressively reduced. This total pressure differential comprises
that needed across the nozzle to create the jet and that across the web and fabric. The
use of concentrated, high-speed air jets can cause disruption of the web making the
process unpredictable.
Products made by TAD technology have consistently exhibited superior
absorbency and handfeel, often at lighter weights than corresponding conventional wet-
press products. These TAD products cannot be manufactured using the conventional
apparatus found on the existing wet-press paper machines. The excessive cost
associated with installing new TAD paper machines makes the introduction of new TAD
lines cost prohibitive as a replacement for existing and functioning wet-press paper
machines.
Retrofitting existing CWP machines has always been a possibilit,v for achieving
the product advantages of TAD paper. Unfortunately, retlurllling has not been available
at reasonable costs due to limited space between the forming section and the Yankee
dryer cylinder of conventional wet-press machines. The space between the forming
section and the Yankee dryer, conventionally used for the press-felt, is generally too
small to allow introduction of either a single standard TAD drying cylinder often having a
diameter of at least about 16 ft, or of multiple TAD cylinders with diameters of about six
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to seven feet as would be required to maintain machine productivity.
Sufficient space between the forming section and the Yankee has been
necessary to accommodate one or more large diameter TAD cylinders. Conventionally,
these cylinders are on the order of 16 feet in diameter for a single cylinder and could be
the same or smaller for multiple cylinder installations. The size of conventional TAD
cylinders stems from a number of processing characteristics including but not limited to
the amount of residence time on the dryer that is needed to dry the web at conventional
differential pressures and machine speeds to an appropriate dryness before removing it
from the TAD fabric. These TAD dryers are conventionally operated at modest air
pressure differentials across the web and supporting fabric, on the order of 0.5 to 0.75
in. Hg, thereby, among other advantages, minimizing the electrical energy needed to
drive the hot air through the web. This results in a relatively small cost in the production
of the paper product, but TAD energy costs are still above a typical conventional wet
press process expressed on the basis of dollars per ton.
The cost associated with moving one or both of the forming section and Yankee
dryer section has also heretofore made rel~ofilling cost prohibitive. The use of one
smaller cylinder and conventional pressure differentials (typically ~ 1 in. Hg) leads to
unacceptably low production capacities. Some machines have multiple TAD cylinders
of six to seven foot diameters, but these are used at conventional pressure differential
and thus, due to the number required, rel,~ritli"g such multiple cylinders into an existing
machine is cost prohibitive.
As discussed below, the use of high-intensity through-air-drying according to the
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present invention allows satisfactory drying using a much smaller cylinder, thus, making
ret,ufilLi"g of conventional wet-press machines a viable alternative.
SUMMARY OF THE INVENTION
Further advantages of the invention will be set forth in part in the description
which follows and in part will be apparent from the description. The advantages of the
invention may be reali7ed and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
To achieve the foregoing advantages and in accordance with the purpose of the
invention, as embodied and broadly described herein, there is disclosed:
An apparatus for forming a paper web comprising: a forming means for forming a
nascent web; a non-compactive dewatering means for removing liquid water from the
nascent web where the means does not include passage of air at elevated
temperatures through the sheet; an impression fabric for supporting the nascent web
during drying; means for creali~g a pressure differential of from about 1.5 in Hg to
about 15 in of Hg between the first side of the impression fabric and the second side of
the web on the impression fabric; and means for removing the dried web.
There is further disclQsed:
A method of rel,orilling a conventional wet press paper line including a forming
section, a felted dewatering section and a drying section comprising: removing the
felted dewatering section of a paper line and replacing that section with a through-air-
drying cylinder sized to use all available space between the forming section and the
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drying section; where the through-air-drying cylinder has associated with it, means for
creating a pressure differential of from at least about 1.5 in. Hg to about 15 in. Hg.
There is still further disclosed:
A method of making a fibrous web comprising: providing fibers in an appropriate
liquid dispersion to a forming structure; forming an embryonic web; dewatering the
embryonic web to a solids content of at least about 20% by removal of liquid water;
passing heated air through the web wherein the pressure differential across the web is
between about 1.5 and 15 in of Hg.
Finally, there is disclosed:
A method of making a cellulose web comprising: providing fibers in an
appropriate solution to a forming structure to forrn an embryonic web; dewatering the
embryonic web to a solids content of greater than 30% by removal of liquid water from
the web; thereafter passing heated air through the web, where the pressure differential
across the web during passage of the hot air is between about 1 in. Hg and 15 in. Hg.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illust~tes a conventional wet press processing apparatus.
Figure 2 illustrates a conventional through-air-drying (TAD) apparatus.
Figure 3 is a graphical representation of the residence time requirement of TAD
processes as a function of pressure differential across a TAD fabric and web.
Figure 4 is a graphical representation of the energy cost associated with TAD
processes as a function of the pressure differential across the TAD fabric and web.
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Figure 5 is a graphical representation of the relationship between residence time
requirement of TAD processes and through-air-dryer energy cost.
Figure 6 illustrates a preferred retrofit arrangement for a conventional wet-press
paper line using both high vacuum assisted dewatering and high-intensity through-air-
drylng.
Figure 7 is a graphical representation of the drying rate based uponeffectiveness, as a function of air inlet velocity.
DETAILED DESCRIPTION
In a preferred embodiment of the present invention a higher than conventional
pressure differential is applied across the paper web during through-air-drying in order
to achieve higher than normal drying rates per unit area, so that the required size of the
TAD equipment will be reduced to the point that its cost and size facilitate a low-cost
rebuild of an existing CWP machine to a TAD process. The improvements according to
the present invention are not, however, limited to ret,ufilli,,g and are expected to be of
benefit for new and /or existing TAD paper machines, as well.
Figure 3 illusl-ates the effect of the pressure differential on the residence time
required to dry a web via the through-air-dryer method. This figure illustrates the
improvements that can be realized in one preferred embodiment of the present
invention where high pressure differential (vacuum) through-air-drying is used to reduce
drying time by a factor of at least about two to six.
Figure 4 illusl,ates the effect on through-air-drying energy cost that can be
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expeded when high vacuum through-air-drying is used.
Figure 5 illustrates the tradeoff between the increased cost for the energy to
assist the TAD process through higher pressure differential and the residence time
required to dry the web.
The TAD process of the present invention retains essentially the same steps as
conventional TAD processes:
1) formation of a nascent web;
2) transferring the nascent web to an impression fabric;
3) vacuum or any other suitable dewatering of the web;
4) through-air-drying of the web;
S) transferring the web to a Yankee dryer (optional);
6) creping the web (optional); and
7) transporting the dry web to a reel.
In the case of rel,of,lling CWP machines for TAD use, it would be evident to the skilled
artisan that art recognized changes can be effected to any of the retained apparatus,
particularly for improving process emciency. In most retrofit instances, the Yankee
dryer would be retained.
A nascent web is formed from a fibrous slurry in a liquid dispersion. The fibers
can be natural fibers, artificial fibers, or mixtures thereof. The fibers are preferably
selected from softwood, hardwood, chemical pulp obtained from softwood and/or
hardwood by treatment with sulfate or sulfite moieties, mechanical pulp obtained by
mechanical treal"~ent of softwood and/or hardwood, recycle fiber, refined fiber and the
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like. The slurry can contain appropriate modifying chemicals including but not limited to
dry- strength agents, wet-strength agents, softeners, debonders, surfactants,
defoamers, scavengers, retention aids, dyes or colorants, release agents, and pH
control agents.
The present invention contemplates the use of any known headbox configuration
for feeding the fibrous slurry to the forming wire. Accordingly, a single feed nozzle or
multiple feed nozzles may be used leading to a homogeneous or stratified product. The
forming means may be any art recognized configuration including but not limited to a
twin wire former, a crescent former, a suction breast roll former, and the like.
Dewatering of the web can take place on the forming fabric, on an impression
fabric, and/or on an intermediate carrier fabric. According to the present invention, the
dewatering means can be selected from any art recognized system including one or
more of: steam preheating of the web, followed by vacuum dewatering; capillary
dewatering; and vacuum dewatering augmented with hot, moist air. Capillary
dewatering is described in U.S. Patent Nos. 4,556, 450 and 5,274,930, as well as PCT
Application No. WO 96/16305, each of which are incorporated herein by reference in
their entirety. Vacuum dewatering augmented with hot, moist air is described in PCT
Application No. WO 96/29467 to Marchal et al., which is incorporated herein by
reference in its entirety. The present invention may use any known non-compactive
dewatering technology thus allowing the dewatering system to be selected from the
most cost-effective dewatering technologies available at the time. Furthermore, it
allows for dewatering to be applied up to any physically reasonable solids level, likely in
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the 20% to 40% range, prior to starting the TAD step. The preferred solids content after
dewatering is at least 20%, more preferably at least 25%, still more preferably at least
30% and in some instances as much as 40% solids may be achieved in the dewatering
step.
The nascent web can be transferred from the forming structure to a carrier fabric
which may also be a TAD impression fabric. During transfer of the paper web from the
forming fabric or a carrier fabric to a TAD fabric, differential speed of the two fabrics
(with the forming fabric speed exceeding the TAD fabric speed) can create conditions
which impart properties to the web similar to creping. The effect of this differential
fabric speed has been referred to as fabric/fabric creping. In a preferred embodiment of
the present invention, the fabric/fabric crepe is carried out at 0% to 30%, more
preferably 5% to 15%, most preferably 7%-10% speed differential (using the TAD fabric
speed as the base).
After dewatering, the web is passed through a predryer means where hot air is
passed through both the web and the impression fabric to cause substantial drying bf
the web. For paper lines already conta-ining a TAD drying section, the present
technology can be used to modify the existing TAD lines through the addition of an
increased pressure differential across the web and TAD fabric, and thus increase
productivity.
TAD systems operate in two basic configurations. When a TAD system is
operated in the inside/out mode, hot air is pushed out of the TAD cylinder and through
the web and fabric sandwich into a collector. In this mode, the impression fabric
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tension must be in force balance with the pressure differential across the fabric and
web. Thus, the pressure differential and/or cylinder diameter must be limited, due to
limitations on allowable fabric tension (fabric strength).
When a TAD system is operated in the outside/in mode, hot air is directed from a
supply hood through the web and fabric sandwich into the TAD cylinder. This is the
preferred mode for implementing the present invention, due to the fabric support
provided by the TAD cylinder, which permits greater pressure differentials to be utilized.
In this configuration, the air must be removed from inside the cylinder. The TAD
cylinder should be selected considering the maximum allowable air velocity inside the
cylinder necessary to ensure sufficiently uniform cross-directional distribution of air flow
through the web and thus concomitant drying uniformity. The average air velocity
through the web tends to increase with increasing pressure differential across the
web/fabric sandwich, for a given web basis weight and furnish.
The predryer section can include a drying cylinder for supporting the fabric and
web during the hot air drying. Such a cylinder can be any known configuration
including but not limited to honeycomb type TAD cylinders, drilled rolls, and the like. In
one preferred embodiment, the drying roll is a perforated drying drum, more preferably
a drilled roll including internal seals to reduce air inrill,dlion that never p~sses through
the web. The cylinder may have an open area from about 30 to 97% of the area
theoretically available. The cylinder preferably has an open area in the range of 60 to
80% of the area theoretically available. The total pressure dirrerenlial used with the
present invention will have to be adjusted upward at lower percent open area to retain
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the same drying effect per unit time. This total pressure differential comprises at least
the pressure drop across the web, fabric and cylinder shell.
The TAD drying cylinders of the present invention must be capable of
withstanding a pressure differential across the web, fabric and cylinder of greater than
about 1 in. Hg to about 15 in. of Hg, more preferably about 2 in. Hg to about 10 in. Hg,
still more preferably, about 2 in. Hg to about 5 in. Hg. The preferred pressure
differential across the web, fabric and cylinder may be affected by the % open area
available on the TAD cylinder selected and thus, in one embodiment of the present
invention, the pressure differential across the web, fabric and cylinder is preferably
between about 2 in. Hg and about 3 in. Hg. In another embodiment of the present
invention, the total pressure differential across the web, fabric, and cylinder is preferably
between about 4 in. Hg and about 5 in. Hg.
The through-air-drying step, which would likely begin after the sheet has been
dewatered to 20% to 40% solids and end when the sheet has reached 50% to 80%
solids, if the system includes a Yankee dryer, will be done using an applied pressure
differential across the web and fabric in the range of greater than about 1 in. Hg to
about 15 in. Hg. Hot air is forced through the sheet at a rate many times that in a
conventional TAD dryer which typically employs pressure differential across the web,
fabric and cylinder of 0.5 to 0.75 in. Hg.
The differential pressure described above affects the air velocity, which affects
the drying rate of the paper. Figure 7 is a graphical representation of the effect of air
velocity on the drying rate at inlet temperatures of 200 to 220~C dry bulb and 60~C wet
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bulb. The graph provides a means for considering the effectiveness or effficiency of the
system.
In the ideal case, 100% effectiveness, all of the air leaving the moist web is at
the wet bulb temperature of the supply air and is saturated with water vapor. The
quantity of saturated air leaving the web, and the quantity of hot unsaturated air
entering the web, per unit time and per unit web are, are directly proportional to the air
inlet velocity. The drying rate for this ideal situation is again represented by the 100%
effectiveness curve of Figure 7.
A variety of variables can affect this ideal condition resulting in an effectiveness
somewhere below 100%. These variables include but are not limited to finite air
velocity, basis weight, finite residenee time, and air channeling. Thus, in practice, the
air leaving the web may not be fully-saturated.
The average air inlet velocity used during conventional TAD does not exceed
about 4.0 m/sec, and is probably closer to about 2.5 or 3.0 m/sec. With reference to
Figure 7, it can be seen that for conventional TAD, the drying rates would be less than
140 Ib/hr-ft2. In pradice, most conventional TAD systems operate in the area of 60
b/hr-ft2.
The average air velocity through the web for the present invention is preferably
greater than about 5 m/s, more pleferal)ly from about 5 m/s to about 20 m/s, most
preferably from about 7 m/s to about 15 m/s. These air velocities allow the drying rates
achieved by the present invention to be about 140 Ib/hr-ft2, more preferably between
140 and 400 Ib/hr-ft2 and most preferably 140 to 250 Ib/hr-ft2.
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The inlet temperature for TAD dryers can range from .e.g., 120 to 300~C. In this
range of temperature, the required size for a high-pressure-differential dryer will be
several times less than for a conventional TAD dryer, for equal production rate and inlet
temperature. Average diameters for the TAD dryer are less than about 14 feet,
preferably less than about 12 feet, more preferably less than about 10 feet, still more
preferably less than about 8 feet, and most preferably between about 5 and 8 feet.
With smaller TAD cylinders, the necessary space and related installed cost will
be far less, facilitating low-cost rebuilds of CWP machines to TAD capability. The
energy cost associated with operating the hot air circuit at a high pressure differential
will see a marginal increase. However, given the potential reduction in installed cost,
the marginal increase will not make retlofitling in this manner cost prohibitive. An
additional benefit of a rebuild in accordance with the present invention is that greater
production rates are possible, for a given TAD cylinder diameter, due to the high-
intensity drying.
The web can then be transferred to another carrier fabric or may be pressed to
the surface of a rotating Yankee dryer cylinder. Liquid adhesive can be sprayed on the
surface of the Yankee to adhere the web to the Yankee. The web can then creped
from the surface with a creping blade. The web is preferably creped with a crepe ratio
of 0% to about 20%, more preferably from about 5% to about 15% and most preferably
about 10%. The creped web can be passed between calendering rolls and rolled up
prior to further converting operations.
As an alternative to adhering the web to the Yankee and creping it from that
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surface, the web may in some embodiments be dried to about 95% solids on the
impression fabric, removed directly from the impression fabric and rolled up.
The web produced according to the present invention has a basis weight of from
10 to 80 g/m2, more preferably 10 to 50 g/m2.
One preferred embodiment of the present invention is set forth in Figure 6. A
paper slurry in headbox 70 is fed to a forming wire 72 where a nascent web is formed.
The nascent web is transferred to impression fabric 74 and subjected to high-vacuum
dewatering. The web passes steam box 76 used to preheat the web; shaping box 78 to
conform the web to the fabric and to remove liquid water from the web; and dewatering
box 80 used to remove additional liquid water from the web. Some liquid water is
removed from the web and some shaping of the web occurs during transfer of the web ~
from the forming fabric.
In this preferred embodiment, the dewatering box 80 maintains a pressure
differential across the web of about 10 in. Hg. In this preferred embodiment, the
shaping box 78 maintains a pressure differential across the web of about 15 in. Hg.
The vacuum for the shaping box 78 is preferably supplied from a liquid ring pump. In
still a more preferred embodiment, hot moist air is also supplied to the web at the
shaping box 78. In this embodiment, burners are integrated with the supply boxes 82
opposite the shaping 78 and dewatering 80 boxes. Moist air exhausted from the
dewatering vacuum box circuit is recirculated to feed the shaping-box circuit,
constituting a counter-current process.
The dewatered web at a solids content of about 35%, then passes to a TAD
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cylinder 84. In a preferred embodiment, the cylinder is about 6.5 feet in diameter and is
run with a differential pressure of about 4 in. Hg across the cylinder shell, fabric and
web. The inlet temperature is about 230~C and the air velocity entering the web is in
the range of about 5 to about 20 m/sec. The web is dried to a solids content of about
60% and then transferred from the impression fabric 74 to the Yankee cylinder 86. The
web is dried at a temperature of about 90~C and then creped from the Yankee cylinder
at a solids content of about 95%.
Other embodiments of the invention will be apparent to those skilled in the art
from consideration of the specification and practice of the invention disclosed herein. It
is intended that the specification and examples be considered as exemplary only, with
the true scope and spirit of the invention being indicated by the following claims.
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