Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A NEEDLED PRESS FELT
BACKGROUND OF THE INVENTION
Field of the Invention
The subiect invention concerns a needled press felt which is
5 designed for use in the press section of a papermaking machine. The batt
part of the felt compr;ses at least two different kinds of fibers, of
which a first kind of fiber is of a considerably finer-diameter size
than the other kinds of fibers. The batt part may be needled into a base
fabric.
Brief Description of the Prior Art
A papermaking machine comprises three different sections, viz. the
forming section, the press section and the dryer section. In the forming
section the stock suspension is discharged onto a forming fabric or
between two forming fabrics. Most of the water contained in the stock
15 suspension is removed by gravity drainage or by suction, passing through
the forming fabric, whereby a continuous paper sheet is formed. The
formed sheet is carried to the press section and in this section more
water is removed from the fibrous web when the latter is squeezed
between press rollers. Finally, the sheet is dried in the dryer section
20 by being applied against heated cylinders, the moisture being removed
through evaporation.
From a dewatering point of view the most important section is the
press section. Economically, it is considerably more advantageous to
rernove the water by using squeezing methods than to drive it off by
25 using heat. For this reason, one aims at achieving a fibrous web having
maximum dry contents when leaving the press section in order to reduce
the energy consumption in the drier section.
In the press section of the papermaking machine the paper web is
made to travel together with one or several press felts through a number
of press nips. In the press nips, water is squeezed from the paper web
and into or through the press felt. Normally, the press felt structure
contains one soft and comparatively compressible surface layer.
Underneath the surface layer is positioned another layer of a more
incompressible nature, which layer is designed to retain most of its
volume, also when the felt is exposed to compressive pressure. This part
of the felt serves the purpose of absorbing an optimum amount of water
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from the paper web when the web and the felt are compressed in the press
nip, of retaining an optimum amount of the squeezed-out water following
the press nip and of carrying the squeezed-out water away from the press
nip and discharging it before the felt is reintroduced into the pressnip
Immediately following the press nip, when the felt and the paper
web expand, the risk of rewetting of the paper web is considerable. To
counteract such rewetting risks the surface layer of the felt preferably
should contain as fine-dimension fibers as possible, whereby the pore
size of the layer is minimized. As a result, thé higher adhesive
properties of the finer-d;mension capillaries improve the
waterretainment ability of the felt.
The surface structure of the paper is considered largely to depend
on the nature of the pressing operation and therefore dependent on the
surface evenness of the press felt. The pressure distribution felt/paper
web is considered to be decisive in determining the paper quality and
the dewatering results. The side of the press felt which faces the paper
web therefore must be as even as possible and contain as fine-dimension
pores as possible. This aspect, too, speaks in favour of using
fine-dimension fibers in the surface layer.
US 3 392 079 proposes the possibility of needling a finer-fiber
layer onto the surface which is intended to abut against the paper web.
US 3 928 699 describes a felt for a papermaking machine, which felt
comprises two fibrous layers of which the layer intended to face the
paper web contains fibers which predominantly have a diameter size of
0.027 mm or less, whereas an underlying layer contains fibers having a
diameter which is at least 1.75 times larger than that of the fibers of
the surface layer. In accordance with one embodiment shown in this
publication, fibers of a fineness of 0.012 mm have been used for the
surface layer. Thus, it is previously known to use a press felt
comprising at least two layers of fibers having different fineness of
which the layer facing the paper web contains finer-dimension fibers
than the layer below.
Among experts in the field the importance of the running-in period
of press felts is generally recognized. The running-in period is the
period immediately following the mounting of a new press felt on the
papermaking machine and this period may last from a couple of hours of
operation up to several weeks. During this period it often becomes
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necessary to lower the speed of production and several other types of
operational problems may occur. During the running-;n period the felt,
when passing through the press nip, is permanently compressed, and this
permanent compression is at its highest when the machine ;s started up
but it is reduced successively, as the felt is being compressed. During
the running-in period the pores of the felts become clogged by fibers
and fillers emanating from the paper sheet.
In addition, when the felt is used, its surface structure
gradually becomes more even and consequently the running-in of the felt
attributes to improving the paper quality. Attempts have been made to
simulate the course of events of the running-in period during the
manufacture of the felt and to deliver felts which already when leaving
the manufacturing plant possess the qualities which the felt normally
exhibits only after the running-in period.
However, this has proved to be difficult. Pre-compression effected
during the manufacture affects only to a-limited degree the extent of
compression of the felt. Probably this is due to an exist;ng interaction
between compression and clogging which cannot be imitated outside the
papermaking machine.
US 4 482 601 describes a method of mixing a temporary material
; into a textile batt which is then incorporated into the felt structure.
After strong pre-compression to reduce the volume of the felt and to
increase the density thereof the temporary material is removed in order
to form voids in the felt and give the felt a sufficient degree of
openness. This technique has proved not to be useful in practice,
probably because the voids which are formed during the running-in period
are permanently compressed and clogged by material emanating from the
paper suspension.
Although it is possible to design the felt in such a manner that
it will have optimum usefulness at the start-up the permanent
compression and clogging will, however, produce a felt which exhibits
such a degree of impermeability that operational problems of a practical
nature will arise. For instance, the web cannot be removed very easilY
from an impermeable felt after the press nip. Another problem
encountered with felts of reduced permeability is one that is connected
with the air currents ahead of the press nip. The air which enters
between the felt and the web, when these two elements are brought into
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contact with each other, must be removed and in case this air
cannot ba removed through the felt because the latter is too dense
the paper web tends to wrinkle or break. A felt which is designed
for optimum operation already from the start ~herefore as a rule
must be replaced prematurely after having been in operation for a
short time only because of insufficient permeability and
consequently the felt economy will be unsatisfactory.
Instead, prior-art solutions comprise designing felts
having coarser surface fibers than what is desirable for the
purpose of allowing permanent compression and clogging during the
running-in period and still retain sufficient permeability for the
continued operation of the felt.
SUMMARY OF THE INVENTION
The invention comprises a needled press felt for use in
the press section of a papermaking machine, which felt comprises:
a first and at least one other kind of fiber in the
batt part of the felt,
said first kind of fiber constituting at least part
of the surface layer, which in position of use of said
press felt, facing the paper web, is of considerably
finer dimension than any said other kind of fiber and
having a diameter of 0.010 mm or less and having low
resistance to being essentially removed during the
service of the felt on the papermaking machine, and
any said other kind of fiber being essentially
resistant during the removal of said first kind of fiber.
The removal of the first kind of fiber may be effected
successively or periodically. The first kind of fiber may be
fibers which are less abrasion resistant fibers and are
successively worn off from the surface layer of the batt part or
else the first kind of fiber may be fibers which could be dissolved
in suitable chemicals, causing these fibers to degrade. In certain
machine constructions, the press felt travels in abutment against
a hot cylinder. Machine constructions also exist wherein the felt
passes across steam chests. In felts intended for such machine
constructions, the first kind of fiber preferably are fibers which
are degraded by heat whereas the rest of the fibers essentially are
thermal-resistant fibers.
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The subject invention has for its purpose to eliminate the
operational problems in the press felts during their running-in period
and to provide a felt having optimum efficiency during its entire
serviceable life. Because the surface layer of the felt consists at
least partly of fibers having a diameter of 0.010 mm or less, this
surface layer will form a system of very minute pores. The permeability
thus is adjusted to ensure that optimum operation efficiency exists
already at the start-up. The minute pores retain water inside the felt
without causing rewetting in conjunction with the exparlsion which occurs
lo following the press nip. At the same time the extremely fine fibers
result in a felt surface which provides for optimum distribution of the
pressure against the paper web and thus improved paper quality and
improved dewatering effects already from the start.
If a felt according to prior-art techniques were to be used
it would, when exposed repeatedly to compression, become permanently
compressed. In addition, the slurry in the form of fibers and fillers
would clog the very minute pores and cause such a reduction of the
permeability that the felt could no longer be used but would have to be
replaced after having been in operation for only a very brief period.
In addition to being an extremely fine-diameter fiber the first
kind of fiber has been chosen to ensure that it will disappear
continuously or discontinuously through abrasion or degradation. The
unevitable effects of permanent compression and clogging are compensated
for by these fiber losses, and during its entire running-in period the
felt will achieve a useful permeability balance.
The felt may function with optimum results form the start and it
reaches its normal degree of compression and clogging without reduction
of the permeability because of the simultaneous removal of fibers form
the batt with resulting opening-up of the felt.
According to the prior art, the fiber material in a press felt
usually consist of polyamide fibers which are capable of resisting the
abrasion occurring upon the repeated passages through the press nip.
Tests have shown that if e.g. polyester fibers are used in the surface
layer, these fibers will wear out within a very short time. For this
reason, these fibers are very suitable for use as the first kind of
fiber in accordance with the invention.
