Note: Descriptions are shown in the official language in which they were submitted.
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IMPRINTING PRESS SECTION FELT FOR
PAPER PULP, METHOD OF IMPRINTING AND
DRYING PAPER PULP, AND PULP ~ ; l HAVING
MACHINE DIRECTION GROOVES AND RIDGES FORMED THEREIN
5 VVITH ENHANCED DRYING AND STRENGTH CHARACTERISTICS
RELATED APPLICATION
This application claims the benefit of prior U.S. provisional
10 application Serial No. 60/037,009, filed on January 31, 1997.
BACKGROUND OF THE INVENTION
This invention relates to press section felts and more particularly to a
specialized felt, the process of using the same and the process and product which
15 results from its use in the form of an improved paper pulp sheet. The felt isparticularly intrn~lecl for use in the press section of a paper pulp producing m~rhine
in which the press section delivers a web in the form of a sheet of pulp to an air
pleS~U~'e type dryer.
Paper pulp mills commonly use fourdrinier m~rhines for forming
20 mechanical or chemical wood pulp in the form of a relatively heavyweight web on a
wire, which web is then run through a press section in which press section feltsremove a substantial portion of the water content. The wet but dewatered web from
the press section is delivered to an air dryer in which the web is m~int~ined in an
airborne condition as it passes through the dryer stages. Commonly, such dryers are
25 known in the paper industry as Flakt dryers.
The dryer delivers the web as a flat sheet which is then cut into
convenient sections or slabs and stacked, as distinguished from being wound on aroll. These stacks of pulp sheets are later used in the m~nllf~rture of paper and paper
products as a source of wood pulp cellulose which may then be repulped in a
30 hydrapulper and optionally mixed with reclaimed fibers, as well known in the art.
The drying appaldLus that m~int~in~ the pulp web in an airborne
condition and therefore in generally non-contacting relation with impervious objects
differs substantially from conventional paper web dryers that use a series of heated
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dryer rolls also known as can dryers. In the airborne type dryer, referred herein by its
tr~ n~me the Flakt dryer, the web from the press section is contacted only by heated
air and is dried simultaneously from both sides. Such dryers are disclosed in U.S.
patent numbers 3,321,165 and 4,212,113 incorporated herein by reference. In such5 dryers, upper and low blow boxes are arranged in facing relation and support such
web along a path of travel in closely spaced but non-contacting relation with such
boxes. A characteristic of the Flakt dryer is that it does not rely on heated
contacting surfaces nor does it use dryer felts which m~int~in and support the web.
Rather, the web is supported, during drying passes, by the air itself.
Conventionally, the web or sheet of pulp delivered from the press
section to the dryer is a sheet which has smooth top and bottom s~ ces, since the
final press is either between the face surface of two felts supported in a nip between a
pair of rollers, or against the face surface of one felt (usually carried on a grooved
suction roller) and the rubber face surface of an opposing roll defining a pressure nip.
In either case, commonly this press section at the nip is run at 1,000 pli (pounds per
linear inch) or higher. The resulting web entering the air dryer therefore has surfaces
which normally are flat and smooth corresponding to the support surfaces in the final
press section stage. When a break occurs within the dryer, which is bound to happen
from time to time, the dryer must be opened up and the broke removed by hand.
Since the broke is relatively heavy and has not dried sufficiently to have adequate
strength, it often breaks into small pieces as it is removed, resulting in tedious and
time consuming removal and clean up.
SUMMARY OF THE INVENTION
One aspect of the invention consists of a specially modified press
section felt in which special m~hine direction or longitudinally extending yarns are
bound to the open or face surface of the felt by needling a lightweight non-woven
batting into the felt and into and through an array of special machine direction yarns.
These yarns are substantially larger than any of the yarns in the base material and
therefore are formed of relatively heavyweight, and preferably, a twist material. The
yarns are woven as a separate fabric by using lightweight fill yarns primarily for the
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purpose of providing some structure in orientation to the machine direction yarns, to
permit h~ndling and laying ofthe fabric for needling.
The batting which is used for the purpose of providing adherence to
and protection for these face or surface oriented m~chine direction special yarns must
5 be sufficiently heavy in weight so as to provide firm adherence to the base batting
and base fabric and yet must be sufficiently light so that the special m~hine
direction yarns will imprint into the newly formed web, in the press section, thereby
to form an imprinted pattern on one or both sides of the web. The imprinted pattern,
as described below in further detail, provides a pulp web with m~rhine direction10 extentling alt~rn~ting compression grooves or valleys and ridges, at least on one
surface, but in a press section using two opposed felts, on both surfaces. The
increased surface area which results substantially enhances the rate of drying of the
pulp web in the Flakt dryer.
This increased rate of drying is due not only to the increased surface
15 area but is also believed to be due to the airflow interrupting and diverting effect of
the valleys and the ridges. Since the air from the air jets necessarily impinges the
surface and flows laterally, any decrease in the boundary layer condition or anydisruption which creates greater turbulence at the surface, will necess~rily andinherently increase the rate of moisture transfer from the web into the air. Therefore,
20 a significant portion of the increased rate of drying is believed to be due to the
turbulence inducing effect which the alternating ridges and compression grooves
have on the airflow which results in a reduction of the boundary layer and a
corresponding increase in the rate of moisture removal.
A further and unexpected result of the formation of a sheet having
25 machine direction ridges and depressions (compression grooves), as described, is the
fact that this sheet has been observed to have substantially increased early strength.
In the case of a web break within the dryer, it has been observed that the broke can
be removed by h:~ntllin~ and pulling, using conventional techniques, in much larger
pieces than before, thus substantially and significantly reducing the clean up time and
30 the down time by reason of the break. It is believed that this substantial strength
increase is due not only to the compression zones at the grooves, that are believed to
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add to the tensile strength to the sheet, but also due to the greater extent of dryness
within the web itself which directly and positively impacts upon the tensile strength.
The web which has been treated by and imprinted by the felt of this
invention in the high pressure nip of a press also has, transversely, regions of5 relatively high compaction or density of fibers at the in(lent~tion portion of the ribs
separated by regions of somewhat lower density. While this variation in density
would be undesirable in most fini~he~l products, it does not adversely affect the
usefulness of the sheets thus made since they are intentle~ to be repulped when the
pulp product is eventually lltili7.~-1 However, it is believed that these high density
10 m~hine direction portions also contribute significantly to the tensile strength of the
web in the partially dried condition and contribute to its ease of removal in larger
sheet sections in the process of cleaning up a web brake.
Therefore, another aspect of the invention is the provision of an
improved web or sheet in formation and in drying, within the dryer, which has an15 improved and accelerated characteristic of drying and which has greater strength, in
the drier, compared to a plain web of the same weight. A further advantage of this
sheet is that it has a high slide angle when stacked with other sheets and therefore is
easier to collate and stack into large bundles for subsequent use.
Another hllpol ~ll aspect of the invention is the provision of a press
20 section felt for use in the dewatering of newly formed webs of pulp at pulp mills and
the like which felt has an hnp~ g surface formed of machine direction extentlingyarns that have been integrally bonded to the felt by nee-lling and which are covered
by a thin scrim or layer of non-woven batting. The batting is interlocked with the
underlying structure of the felt including the felt batting and backing, preferably by
25 nee~ling from both sides. The felt provides an hllplilllillg or m~rking surface for
forming machine direction extPn-lin~ ridges and valleys on the web for enhancing the
drying characteristics of the web, and the skength of the web in the dryer.
A further important and unexpected advantage of the invention
relating to the felt is that field tests on such felt has indicated that it has superior
30 water removal characteristics and durability. The special machine direction yarns
form intermediate drainage channels which have been found to remain open and
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relatively immllne from compaction over extended use. Felts have been removed
after ru~ning approximately 60 and 80 days respectively and in other cases over 140
days which should at that time show substantial compaction and filling but rather
show substantial openness and have m~int~ined high drainage with minim~l wear
5 characteristics on the surface, thereby leading to a reasonable expectation of longer
on-machine performance as compared to conventional smooth surface press section
felts. This increase in felt life and observed openness was not expected and it is
believed to be due to the ability of the felt to m~int~in a high degree of openness
between the machine direction hllp~ ling yarns by reason of the concentration ofthe
10 nip loading in the exact region of the individual special m~rking yarns and therefore
a reduction of the loading in the spaces between the yarns.
In the m~nllf~ture of the improved press section felt of this invention,
a conventional duplex weave base fabric will have a face batting and a back batting
needled into the woven construction in conventional manner. One or more face
15 battings may be applied. The base fabric may be woven endless or it may be
provided with a pin seam. Such a duplex weave with a pin seam is shown in U.S.
patent 4,938,269, for example.
While the fabric is in the needle loom, a second fabric is applied and
positioned over the face surface. This fabric includes heavy and relatively widely
20 spaced yarns which will be machine direction yarns in the finished felt and light fill
yarns in the cross machine direction in the finished yarn, with the heavy m~hinedirection yarns having a ratio to the base fabric warp yarns of 1 to 2 or higher, and
typically may be spaced apart with a density of 8 to 10 to an inch or less. The weight
of the heavy yarns, which will lie in the m~hine direction in the finished felt, may
25 be around 1,100 grains per 100 yards. This second fabric is then needled into the
base fabric using a lightweight non-woven batting in which the added batting is of a
relatively low weight such as a 3/4 oz./sq. ft. batting, which is then needled in place
through the face. Additional batting, if desired, may be needled through the back-by
turning the felt over in the needle loom. The felt will be fini~he~l in the conventional
30 manner with the auxiliary heavy yarns finished in close relation to the ultimate face
of the fabric so that, in the press section, these yarns will make a physical impression
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into the pulp sheet or web, while the tension in the felt will be carried by theconventional woven base structure. The completed fabric may be about 4 to 6 oz./sq.
ft. in weight, but the most important aspect of the felt is that the warp yarns in the
added fabric are sufficiently close to the surface of the batting as to make a
5 substantial impression along the length of the web on the face surface. A twisted
yarn is pler~lled for these m~chine direction m~rkin~ yarns in order to permit needle
penetration through the yarn so that the body of the yarn may be adequately
interlocked with the face batting to assure the integrity of the felt.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a cross-sectional view through the improved press section felt
of this invention;
Fig. 2 is a perspective partially diagrammatic and partially broken
away view showing the arrangement of the layers of woven material making up the
15 felt of this invention;
Fig. 3 is a diagrammatic cross-sectional view of the disassociated
elements of a felt looking at the m~hine-direction end with the cross machine
direction represented by the arrow, showing the base fabric and the supplementalfabric, and diagrammatically illustrating the non-woven battings in a condition prior
20 to nee-lling, at their respective positions;
Fig. 4 is a representation of a high plt;S~UIe carbon impression made at
the face of a specimen of the felt made in accordance with this invention; and
Fig. 5 represents one surface of a pulp sheet which has been run
through a press section using felts made in accordance with this invention, showing
25 the impressed surface configuration including the longitudinal zones of relatively
high compression separated by zones of lower compression;
Fig. 6 is an enlarged sectional view through the sheet of Fig. 5
looking generally along the line 6--6 of Fig. 5; and
Fig. 7 diagrammatically illustrates the principal components of a sheet
30 forming pulp mill including a fourdrinier machine, a press section, a sheet dryer and
stacker, to which the present invention is applied.
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DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to Figs. 1-3, Fig. 1 is a sectional view through a press
section felt made according to this invention while Fig. 2 diagrammatically
represents certain layers of material which go into the felt but does not show the
unwoven material as being needled in place and does not show the top and bottom
batting. Fig. 3 shows the relative position of the principal components of the felt of
this invention. The felt has a conventional woven base fabric 10 which may be a
duplex woven base, as well known in the art. A simplified illustration of the base
fabric 10 is shown in Fig. 1 for the purpose of illustrating the manner of making the
press section felt of this invention.
Next, the base fabric 10 has needled into it from the face surface as
well from the back surface, battings 12 and 13 of non-woven m~teri~l The batting13 is applied to the roll side and the batting 12 is applied to the open or face side of
the base fabric 10. A portion of the batting 12 will remain above the woven basefabric 10 and will tend to isolate the woven base from the special overlying base
fabric which is to be applied by nee-1ling.
This overlying fabric is illustrated generally at 15 in Figs. 1 and 2 as
including heavy machine direction extçn-lin~ spaced apart yarns 20, held together by
a relatively lightweight cross machine yarn 22. This sheet side fabric 15 is then
applied at the face side of the felt, while still in the needle loom. Of course, the felt
may be woven endless or it may be woven as a pin seam felt.
The fabric 15 has heavy m~ ine direction yarns 20 which typically
have a spacing of about twice that of the machine direction yarns 1 Oa in the base as
shown in Fig. 3. A typical density which has proved to be effective is 8 to the inch.
Then, on top of the fabric 20 is placed a further batting layer 23. The
batting layer 23 is then needled into the composite felt from both the top and from
the back side. For example, in a 5.38 oz./sq. ft. felt, the supplemental batting 23
which is added for the purpose of fixing and attaching the yarns of the fabric 15 was
a 3/4 oz./sq. ft. batting, while the face batting 12 which had previously been applied
was a 1 1/4 oz./sq. ft. batting and the base or roll side batting was about 3/4 oz./sq.
ft.
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After the application of the fabric 15 in position over the face of the
needled batting 12, the composite structure is again needled from both the face side
and from the back side. Fig. 1 illustrates that a minimum of the unwoven or batting
fibers will extend above the face side or upper side of the felt as seen at 25, sufficient
5 to interlock in place and hold the heavy yarns 20 in a locked condition with respect
to the base fabric.
The heavy yarns 20 become the m~rkinE yarns. Typically, these yarns
comprise a twisted plied multi-filament that provide the necessary hardness to carry
the loading in the nip, and to provide the necessary compaction to the web of pulp
10 m~teri~l In a plefelled example, the yarns 20 consist of three bundle groups each of
which consists of two twisted together bundles of fibers each individual bundle
having a total denier of about 840, thereby providing a denier of 6 x 840, or a weight
of about 1,100 grains per 100 yards. It is within the scope of this invention to use a
heavier or a lighter weight yarn, or to use a spun yarn, a plied multi-fil~ment yarn, as
15 disclosed, or even a monof;l~ment yarn, as the yarns 20, as long as they can be
adequately attached to the face side of the fabric.
The fill yarn 22 may be woven as a plain weave and its purpose is to
provide integrity to the fabric 15 and to hold the heavy machine direction m~rkinE
yarns 20 in place prior to and during the beginning of the nee~llinE operation. Thus,
20 a lightweight monofilament may be used, such as an 11 mil monofil~ment having a
weight of 100 grains per 100 yards. After nee-llinE, the felt may be conventionally
finished.
Fig. 4 represents the image made by a carbon impression taken at the
face or sheet surface of a felt in accordance with this invention, loaded at 1,000 psi.
25 The ribbed pattern which is left by the yarns 20 is evident, as represented by the lines
20a in Fig. 4.
Fig. 5 is a view of one surface of a dryed sheet 40 of paper pulp which
has been pressed, as a web, in a press section at 1,100 pli at 600 ft./min. between
opposed pairs of pressed section felts in accordance with this invention. The dry
30 basics weight of this sheet was 600 lbs./1,000 sq. ft. The opposite side of the sheet is
similarly formed as a series of ridges and depressions as shown in Fig. 6,
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g
corresponding to those imparted to the sheet on the surface as illustrated in Fig. 5
without regard to alignment between the opposed arrays of hllpl;~ g yarns. Thus,the sheet 40, as illustrated, has one or both of its surfaces heavily in~l~nted and
compressed longitudinally or in the m~ine direction to form grooves, valleys or
5 compressed zones 42, corresponding to the positions of the yarns 20 in the press
section, separated by wider regions or ridges 44 of cellulose m~t~ri~l which has been
compressed to a lesser degree. The mean caliper of the sheet was about 1/16 inch.
The valleys or zones 42 were compressed by about 16% to 25% of the total caliperthickness of the sheet, after drying.
This sheet, in a partially dryed state, has been found to have high
tensile strength, particularly when pulled in a direction parallel to the longitudinal
direction, i.e. parallel to the ridges 44 and this strength, over that of a conventionally
pressed shape, is believed to be due to the zones 42 of high compaction which, even
in a partially dryed condition enhance the tensile strength of the partially dryed sheet.
15 For this reason, it has been surprisingly found that broke within the drier can be
cleaned out at a fraction of the time required to clean out broke formed by a
conventionally pressed web of the same weight, thus providing substantial savings in
down time.
A further advantage of the ribbed pattern is its ability to increase the
20 drying rate, even when the ribbed pattern is formed on one side only. It is believed
that this increased rate of drying is due not only to the increased surface area which
has been exposed by the impressed pattern, but also due to the ridges 44 formingturbulence inducing projections which promote mixing within the boundary layer
region and thus promote a decrease in the thickness of the boundary layer, during air
25 drying.
Field tests have indicated that the press section felt according to this
invention has a capability of running longer than the conventional felt, as measured
by the degree of compaction and wear which occurs in use. Generally, press section
felts must be replaced when the compaction reaches the point where the felt no
30 longer adequately drains as measured by an increase in web moisture at the dryer
end. To compensate, the speed of the line must be reduced. Substantially less
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compaction has been seen in felts made in accordance with this invention that have
been removed for routine replacement. It is believed that this may be the result of a
selective distribution of the nip loading, as illustrated by the carbon print in Fig. 4, to
the regions of the m~rking yarns 20, thereby forming intermediate regions of
5 substantially lower compression. Lower compression over a substantial area of the
felt translates into a correspondingly longer service life for the felt.
Referring to Fig. 7, which only diagrammatically illustrates some of
the components of a pulp mill, a sheet forming device may be a fourdrinier wire 80
in which a headbox applies a slurry or suspension of the pulp on the wire 80 for10 drainage.
The off running end of the fourdrinier applies the formed and partially
dried sheet 82 to one of a series of typical press sections, in which water is extracted
by running the sheet 82 between pairs of pressure rolls such as the rolls 84 and 85.
One or more of these rolls is covered by a felt made according to this invention15 which carries a substantial quantity of the wetness of the sheet 82 away from the
pressure nip defined between the pairs of rolls. Press sections may include only a
single felt over a suction or grooved roll or may include a pair of felts, and typically
two or three or more separate press sections are used. Typically, the felt of this
invention will be used on the last of the press sections in order that the i~~p ~ i made
20 by the felts on the sheet 82 go into the air dryer 100 although the felts of this
invention may be advantageously used for the felts in any one of the press sections.
As illustrated in Fig. 7, an upper felt 86 goes over the upper press roll and a lower
felt 88 forms a nip with the felt 86 at a lower press roll. Typically, pressures in the
press section may run at from 500 to 1,400 PLI pressure loading typically at speeds
25 in the range of 200 to 600 feet per minute.
The sheet 82 leaving the air dryer or driers is typically cut and stacked
as shown at 110 for binding and shipment to paper plants or newsprint plants. The
sheets in the stack 110, where an upper and lower felts are used in the final press
section according to this invention, will be formed with inrlente~l and colllplessed
30 longitudinally çxtenlling compression zones 42 and separated by wider regions 44 of
cellulose material which has been compressed to a lesser degree, as described above
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in connection with Figs. 5 and 6. The surfaces thus configured tend to hold the
sheets in a stack better than the conventional smooth sheets in that the groovedsurfaces provide greater friction and a higher slip angle. Tests have shown thatoverall production can be increased up to 5% by reason of the increase in drying rate
5 of the sheets configured as shown in Figs. 5 and 6 as compared to conventionally
formed smooth sheets of the same weight.
While the forms of appaldlus, process and product herein described
constitute ~l~relled embo~1iment~ of the invention, it is to be understood that the
invention is not limited to these precise forms of apparatus, process and product, and
10 that changes may be made therein without departing from the scope of the invention
which is defined in the appended claims.
What is claimed is:
