Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DEWATERING PRESS
Backqround of the Invention
The field of the invention relates to mechanisms
for extracting water from a web of material, and more
particularly from a fibrous web formed in a papermaking
machine.
During the papermaking process, a web is formed
by depositing a fibrous slurry on a forming wire. A large
amount of water is drained from the slurry during this
process, after which the newly-formed web proceeds to a
press section. The press section includes a series of press
nips. The web finaLly proceeds to a drying section includ-
ing heated dryer drums where the water content is reduced
to a desirable level.
~n view of the high cost of energy, it is desirable
to remove as much water as possible from the web prior to
its entering the drying section. The dryer drums in this
section are often heated by steam and costs can be sub-
stantial if a large amount of water needs to be removed.
The use of extended press nips has been found to
be advantageous over the use of nips formed by pairs of
adjacent rollers. By extending the time the web is
subjected to pressure in the nip, a yreater amount of water
can be removed. This fact has been recognized by those
skilled in the art, and several patents have been granted
in the area. These patents include Re. 30,268, 4,201,624,
4,229,253 and 4,229,254.
~ n using e~tended press nips to dewater a fibrous
web, the web has typically been sandwiched between two
moisture-absorbing felts and a belt. The felts are trained
around a cylindrical press roll with the web between them
while the belt is arranged for applying pressure to the
felts and roll. A pressure shoe exerts pressure on the
belt in the press area.
A problem has been encountered during the dewater-
ing of webs in extended nips. It has been found that a
bulge develops in the belt ahead of the nip. Irhis
problem is recognized in U.S. Patent Nos. 4,229,2S3 and
4,229,254, and certain belt constructions are suggested
for overcoming the problem.
Summary of the Invention
The invention is directed to an apparatus for
dewatering a fibrous web including an extended press nip
provided hy a roll and a specifically designed belt which is
forced toward the roll by a pressure shoe for applying
pressure to the felt and fibrous web in the nip. The belt
comprises a base fabric which is impregnated with a thermo-
plastic or thermosettin~ polymeric material. The base
fabric has a coated and irnpregnated side which would
operate against the pressure shoe and a second side facing
the felt and web. This second side contains voids into
which water or other liquid could be transferred during
passage of the felt and web through the nip. A significant
advantage of this belt construction is that it can be made
in any length since it does not require a mandrel during
the manufacture thereof. A base fabric may be made endless
using conventional fabric -technology and then coated with
the material.
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The resulting structure is both light in weight
and suf-ficiently stable to operate under paper machine con-
ditions. It has sufficient abrasion resistance to resist
any wear that might take place in the ex-tended nip apparatus.
Unlike materials which are built up in the manner of a rubber
tire and can flow and/or delaminate, the invention provides
a belt which will maintain its integrity~ It also solves
the problem of bulging near the press nips.
Brief Description of the Drawin~s
Figure 1 is a side elevational view of an extended
press nip according to the invention,
Figure 2 is a partially sectional front view of
the press nip shown in Figure 1,
Figure 3 is a sectional side elevational view of
a belt employed in the invention, and
Figure 4 is a sectional side elevation view of an
alternate embodiment of belt employed in the inven-tion.
Detailed description of the Invention
An extended press nip is provided by the invention
for dewatering a travelling web of material. The nip 10 is
defined by a cylindrical press roll 12, a pressure shoe 14
having an arcuate surface facing the press roll, and a belt
16 arranged such that it bears against the surface of the
press roll. The arcuate surface of the pressure shoe has
about the same radlus of curvature as the press roll. The
distance between the press roll and the pressure shoe may be
adjusted by means of conventional hydraulic or mechanical
apparatus (not shown) connec-ted to a rod 18 pivotally secured
to the shoe 14. The rod may be also be actuated to apply the
desired pressure to the shoe. It will be appreciated that
the pressure shoe and press roll described above and shown
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in Figures 1-2 are conventional and that other arrangements
may be utilized in accordance with the invention.
One embodiment of the belt 16 in accordance with
the invention is shown in detail in Figure 3. The belt 16
cornprises a two layer woven monofilament base fabric 20
with multifilament or spun stuffer yarns 21 which is
impregnated with a polymeric material 22. Thermosetting
resins such as polyurethanes have been found to be suitable
impregnating materials. Thermoplastic polymers such as
polypropylene are also acceptable.
The base fabric 20 is sufficien-tly open to allow
impregnation to eliminate -the possibility of undesirable
voids forming in the final fabric on the pressure shoe side.
These voids are undesirable because they would allow the
lubrication used between ~he belt and shoe to pass through
the belt and contaminate the felt and fibrous web. The
stuffer yarns 21 provide a barrier of sufficiently low perme-
ability to prevent passage of the resin during the coating
and impregnating process to the side with voids. It is
endless in final construction and uniform in thickness. The
fabric must also be made to have sufficient stability under
paper machine conditions. In other words, it must have
length stability, width stability, and guidability.
The thermoplastic resin or thermosetting resin used
should be substantially one hundred percent solid composition
to avoid the formation of bubbles during the curing process
of the resin in the belt structure which could cause voids
on the coated side.
Side 17 of the belt can be grounded smooth for
contact wi-th the pressure shoe 1~. The opposite side contains
voids, as a result of the weave and absence of impregnation,
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into which liquid can be transferred during the passage
of the web, felt and belt throu~h the nip.
A further example of the first embodiment is shown
in Figure 4. The components are similar to -those in Fiyure 3.
The components of Figure ~ are numbered as those of Figure 3
with a prime following. ~elt 161 of Fi~ure 4 is a multi
layer structure with a barrier layer provided by stuffer
yarns 211. These stuffer yarns 211, can be multifilament
or spun. The belt is coated and impregnated as described
previously to provide a smooth impervious surface 171 and
a surface with voids 181 on the remaining side. The use of
belt 161 is the same as previously described for belt 16.
A second embodiment of this invention uses a
different method of introducing the polymeric material into
the belt structure. In this second method polymeric material
is pressed into the surface. The depth of penetration is
controlled by the pressing force used and the temperature
applied. Either thermoset or thermoplastic polyrners can be
used. Thermoplastic polymers are especially ad~antageous
for this method since their viscosity in melted form can be
controlled by temperature. By controlling the viscosity the
depth of penetration can be controlled and the necessity of
a barrier in the fabric structure is eliminated for polymers
of the right viscosity.
In this me~thod the belt 16 is positioned around a
pair of rollers. One of the rollers is an oil heated cylinder.
Polymeric material in sheet form, of the proper caliper, is
placed between the belt and the heated cylinder and allowed
to rotate around the cylinder. The cylinder temperature is
maintained at the level desired to melt the polymeric mate-
riaL to the proper viscosity~ It is then forced into the belt
by -the tension of the belt around -the cylinder and roller.
~n outside pressure roll can be used to create additional
pressure if needed. The sheet of polymeric material is
trimmed so no overlap occu~s in the material pressed into the
belt.
Belts manufactured in accordance with the invention
have been found to have many desirable characteristics. They
move easily over the pressure shoe and are capable of trans-
mitting pressure from the shoe to the web and press roll.Sufficient flexibility is obtained, and the belts have proven
to be unaffected by lubricant applied prior to entering the
press nip.
In comparison with belts currently known to the art,
the invention provides a belt which is relatively thin and
light in weight. Thick belts have the disadvantage of tending
to flow while within the nip. A 24'6" X 170" belt impregnated
with a thermosetting resin in accordance with the invention
weighs about two hurldred pounds. A similar size belt having
bulge-resistant characteristics and having a structure defined
in the above mentioned patents weighs about twelve hundred
pounds. Unlike the heavier belts, the invention does not
require a reinforcing structureO
The manufacture of the belt according to the inven-
tion may be accomplished economically and without the need formandrels or au-toclaves which limit the size of other belts.
A belt of any length can accordingly be produced. A web may
be needled into the woven base if desired. If the base struc-
ture has not been woven endless it is joined endless using
conventional joining techniques applicable to forming fabrics
in the paper industry~
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A belt made in accordance with this invention may be
utilized with the apparatus shown in Figure 1. llhe belt 1~ is
positioned between the pressure shoe and the pressure roller~
The smooth coated side 17 of the belt is engaged by the shoe.
A fibrous web 24 carried between first and second felts 26 and
27 respectively is introduced into the press nip 10. The side
of the belt 17 engaging the shoe 14 is lubricated by lubricat-
ing means 28 positioned ahead of the nip.
The belt 16 is easily repaired should a hole or
other surface irregularity develop therein. The damaged por-
tion is cleaned with a solvent and a suitable amount of
coating is applied with a blade~ A heat gun is employed to
cure the surface which can then be sanded.
Because of the excellent fle~ibility characteristics
of the base structure and the fact -that the coating layers
can be kept to a minimu~, the surface of the belt will have
less tendency to fail due to bending fatigue. This is due
to the fact that, because of the low caliper, the surface
plane of the coated surface is at a minimum distance from the
neutral axis of bending. This reduces the percentage of
elongation and compression at the surface plane during bending.
It will be appreciated that the belt utilized in
the invention may include a base fabric made from various
polymeric materials having the necessary properties for
application in paper~aking machines. Materials other than
polyurethane and polypropylene may also be employed as the
coating material~ Also the weave of the belt may be altered
although a multilayered belt is desirable for creation of
the voids.
