Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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. PATENT APPLICATION
TITLE: AN APPARATUS FOR REDUCING THE WATER CONTENT OF A
FORMED WEB
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to an apparatus and method
for reducing the water content of a formed web. More
particularly, the present invention relates to an apparatus
for inhibiting delamination of a pressed web during passage of
the web through ~ heated elongate pressing section.
INFORMATION DISCLOSURE STATEMENT
In a typical papermaking process, a slurry of paper stock
is ejected onto a moving fourdrinier screen where the slurry
is initially dewatered in order to provide a formed web which
is subsequently removed from the fourdrinier screen. The
formed web is then pressed between counter-rotating rolls for
removing a further quantity of water from the formed web. The
pressed web is then guided around a plurality of drying
cylinders until the desired water content of the dried web is
achieved.
Because of the enormous thermal input requirement of a
typical drying section, various attempts have been made in
order to increase the amount of water removed from the formed
web in the press section.
Notably, the extended nip press has greatly increased the
amount of water removed from the formed web during the
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pressing operation. Essentially, the extended nip press
includes a backing roll and an elongate pressing shoe which
cooperates with the backing roll to define therebetween an
elongate pressing section. An endless looped blanket extends
through the pressing section such that when the formed web is
disposed between the blanket and the backing roll, the
residence time of the web within the pressing section is
increased and a correspondingly greater quantity of water is
remo~ed from the web. Consequently, less water is present in
the pressed web entering the dryer section, thereby reducing
the thermal input requirements of the dryer section.
More recently, as taught in U.S. Patent No. 4,738,752 to
Busker et al, it was discovered that by heating the backing
roll to an elevated temperature, even greater quantities of
water could be removed from the web during the passage of the
web through the elongate pressing section. The theory behind
the so-called heated extended nip press process was that water
vapor generated within the elongate pressing section by the
heated backing roll urged water in the liquid phase disposed
within the web away from the web.
However, although the heated extended nip exhibited
greatly increased water removing capabilities, a problem was
experienced in certain paper grades in that the web exiting
the extended pressing section tended to delaminate.
Accordingly, the present invention seeks to overcome the
aforementioned problem associated with known high temperature
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extended pressing techniques and provides an apparatus for
reducing the water content of a formed web that makes a
considerable contribution to the web pressing art.
More particularly, the backing roll of the heated
extended nip press of the aforementioned U.S. Patent No.
4,738,752 is heated to a surface temperature in excess of 250
Fahrenheit. The high surface temperature rapidly heats the
web as it passes through the extended nip, and the paper
fibers are softened thereby. The aforementioned softening of
the paper fibers greatly enhances the water removing
capabilities of the press section. Such softening also
improves sheet property development.
In prior heated extended nip pressing techniques, the web
enters the heated elongate pressing section with an in-going
web solids content of between 30 to 45 percent by weight.
However, there are several problems with entering a high
temperature press nip at such low solids content. First, the
high heat transfer rates and low sheet permeabilities result
in delamination of the sheet due to excessive vapor flashing
at the end of the extended nip.
Second, the combination of high temperature and moisture
results in increased tack and adhesion of the sheet to the hot
surface of the backing roll.
According to the present invention, the aforementioned
problems are overcome by increasing the dryness of the web at
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the nip entrance by initially wet pressing or drying, or both
pressing and drying, the web prior to guiding the web into the
heated extended pressing section.
By entering the heated extended pressing section at a
higher dryness level, the amount of free water is reduced and
the volume of vapor flashed at the end of the nip is reduced.
Also, with less free water in the pores of the web, there is
less resistance to the flow of flashing vapor as such flashing
vapor exits the web.
The reduction in volume of the aforementioned flashing
vapor and the reduction in flow resistance to the vapor
exiting the sheet reduces the potential for delamination.
Additionally, it has been shown that moisture acts as a
plasticizer and can increase the tackiness of the paper fibers
at high temperatures. Also, the free water in the sheet
facilitates bonding of such fibers to the hot surface of the
backing roll just as it facilitates bonding between adjacent
fibers within the sheet. Accordingly, the problem of fiber
picking and web adhesion can be greatly reduced by entering
the heated extended pressing section at a higher solids
content.
Accordingly, a main object of the present invention is
the provision of an apparatus which inhibits delamination of a
pressed web when such web exits a heated extended pressing
section.
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Another object of the present invention is the provision
of an apparatus which reduces fiber picking and web adhesion
to a backing roll when such web exits from a heated extended
pressing sectlon.
Another object of the present invention is the provision
of an apparatus which provides the maximum effectiveness of
the heated extended pressing process while reducing
runnability problems to a minimum.
Other objects and advantages of the present invention
will be readily apparent to those skilled in the art by a
consideration of the detailed description and drawings
contained hereinafter.
SUMMARY OF THE INVENTION
The present invention relates to an apparatus and method
for reducing the water content of a formed web. The apparatus
includes a press means for pressing the formed web such that a
first portion of water is removed from the formed web during
passage of the formed web through the press means. A dryer
means is disposed downstream relative to the press means for
drying the pressed web such that a second portion of water is
removed from the pressed web during passage of the pressed web
through the dryer means. A further press means is disposed
downstream relative to the dryer means for further pressing
the dried web such that a third portion of water is removed
from the dried web during passage of the dried web through the
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further press means. The further press means includes a
backing roll and heating means for heating the backing roll.
Additionally, the press means includes an elongate press
member which cooperates with the backing roll for defini~g
therebetween an elongate pressing section for the passage
therethrough of the dried web such that during passage of the
dried web through the pressing section, the dried web is
subjected to increased pressure and temperature for an
extended period so that water vapor is generated within the
pressing section for urging the third portion of water in the
liquid phase away from the web during passage of the web
through the pressing section for inhibiting delamination of
the further pressed web.
In a more specific embodiment of the present invention,
the press means includes a press roll and a further press roll
which cooperates with the press roll for defining therebetween
a press nip for removing the first portion of water from the
formed web.
In an alternative embodiment of the present invention,
the press means includes a pressing roll and an elongate
pressing shoe which cooperates with the pressing roll for
defining therebetween an elongate press section for removing
the first portion of water from the formed web.
In either embodiment of the present invention, the dryer
means includes a plurality of rotatable dryer drums with the
pressed web extending around at least a portion of the heated
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periph~eral surface of the dryer drums for removing the second
portion of water from the pressed web.
The press means and the dryer means permit the dried web
between the dryer means and the further press means to attain
a solids content of more than 45 percent by weight such that
the web entering the further press means has a solids content
of more than 45 percent by weight.
The present invention also includes a method for removing
water from a formed web. The method includes the steps of:
pressing the formed web such that a first portion of
water is removed from the web during passage of the
formed web through a press;
drying the pressed web downstream relative to the press
such that a second portion of water is removed from the
pressed web during passage of the pressed web through the
dryer;
further pressing the dried web such that a third portion
of water is removed from the dried web during passage of
the dried web through a further press; and
the further pressing step including subjecting the dried
web to increased pressure and temperature for an extended
period so that water vapor is generated within a pressing
section defined between a heated backing roll and an
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elongate press member such that the water vapor urges the
third portion of water in the liquid phase away from the
web during passage of the web through the pressing
section for inhibiting delamination of the further
pressed web.
Many modifications and variations of the present
invention will be readily apparent to those skilled in the art
by a consideration of the detailed description contained
hereinafter taken in conjunction with the annexed drawings.
However, such variations and modifications fall within the
spirit and scope of the present invention as defined by the
appended claims. Included in such modifications would be the
provision of a hydrostatic instead of a hydrodynamic shoe in
the further press.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a side-elevational view of an apparatus
according to a first embodiment of the present invention;
Figure 2 is a side-elevational view of an alternative
embodiment of the present invention showing the press means as
an extended nip press;
Figure 3 is a graph showing the drying rate to percentage
of solids entering the heated extended pressing section for
newsprint;
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~ igure 4 is a graph for newsprint showing sheet and felt
weight changes to solids entering the heated extended pressing
section; and
Figure 5 is a graph showing for newsprint the percentage
of solids in the web exiting the heated extended pressing
section to the percentage of solids entering the same.
Similar reference characters refer to similar parts
throughout the various embodiments of the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
Figure 1 is a side-elevational view of an apparatus
generally designated 10 for reducing the water content of a
formed web W. The apparatus 10 includes press means generally
designated 12 for pressing the formed web W such that a first
portion of water is removed from the formed web W during
passage of the formed web W through the press means 12.
Dryer means generally designated 14 are disposed
downstream relative to the press means 12 for drying the
pressed web PW such that a second portion of water is removed
from the pressed web PW during passage of the pressed web PW
through the dryer means 14.
A further press means generally designated 16 is disposed
downstream relative to the dryer means 14 for further pressing
the dried web DW such that a third portion of water is removed
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from the dried web DW during passage of the dried web DW
through the further press means 16.
The further press means 16 includes a backing roll 18 and
heating means 20 for heating the backing roll 18. An elongate
press member 22 cooperates with the backing roll 18 for
defining therebetween an elongate pressing section generally
designated 24 for the passage therethrough of the dried web DW
such that during passage of the dried web DW through the
pressing section 24, the dried web DW is subjected to
increased pressure and temperature for an extended period so
that water vapor is generated within the pressing section 24
for urging the third portion of water in the liquid phase away
from the web during passage of the web through the pressing
section 24 for inhibiting delamination of the further pressed
web FPW.
In the embodiment shown in Figure 1, the press means 12
includes a press roll 26 and a further press roll 28 which
cooperates with the press roll 26 for defining therebetween a
press nip 30 for removing the first portion of water from the
formed web W.
Figure 2 shows an alternative embodiment of the present
invention in which a press means generally designated 12A
includes a pressing roll 26A and an elongate pressing shoe 32.
The pressing roll 26A cooperates with the pressing shoe 32 for
defining therebetween an elongate press section 34 for
removing the first portion of water from the formed web W.
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In both embodiments of the invention, the dryer means
14,14A includes a plurality of rotatable dryer drums 36,36A
and 38,38A respectively. The pressed web PW extends around at
least a portion of the heated peripheral surface 40,42 of the
dryer drums 36 and 38 for removing the second portion of water
from the pressed web PW.
In both embodiments of the present invention, as shown in
Figures 1 and 2, the dried web DW and DWA enter the pressing
section 24 and 24A respectively with a solids content of more
than 45 percent by weight.
