Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND DEVICE FOR PULP WEB FORMATION
The present invention relates to a method as set forth in the preamble of
claim 1 for forming pulp web.
According to such a method, the pulp drying process, and more specifically
the web formation of pulp stock, can be implemented in a preferred fashion.
The invention relates also to an apparatus as set forth in the preamble of
claim 8.
The publication US 3823062 discloses a twin-wire web formation apparatus,
comprising a wedge-shaped gap constituted by two wires, the pulp being
delivered therein for web formation. It has been stated that the gap
constituted by the wires be created by means of drainage boxes, supporting
the wires and boundary to the wire with a water permeable surface.
According to the publication, the web formation is regulated by measuring
and throttling the quantity of percolated water flowing out of the drainage
boxes. This presents the method very schematically and it involves a number
of shortcomings, particularly if the equivalent method is applied to pulp web
formation. It has been discovered that this prior art method provides a very
difficult way of controlling the formation of a web and its coherence
downstream of a the formation section, especially with a thick web, which is
usually the case with pulp. This solution has its limitations also in terms of
its
dewatering capacity, it is not capabie of reaching a dry content substantially
higher than about 12%, since the pressure would increase at the downstream
end of a wedge-shaped section to such a high level that the pulp would
discharge from the gap portion unmanageably, which causes the wire
sections to come apart, resulting in the web losing control of its coherence
and the failure of web formation.
There are prior known pulp drying mechanisms, comprising a Fourdrinier-
wire former section, wherein a head box is used for supplying stock on top of
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the Fourdrinier wire. The stock is dewatered over the wire section, whereby
the dry content downstream of the wire section is typically about 20%. One
problem with a Fourdrinier wire regarding pulp drying is one-sided
dewatering. At a given point, the so-called wet wire shall have developed
against itself a layer of web, which impairs the discharge of water from the
web. In the prior art, this problem has been addressed by fitting the wire
section with so-called underpressurized dewatering devices, such as so-
called vacuum foil boxes or vacuum flatboxes, whereby the dewatering is
intensified by increasing a pressure difference which brings about or
encourages the separation of water. In this type and the like former sections,
the vacuum system is an element unavoidable in terms of operation.
However, the vacuum system in particular constitutes a significant part of the
equipment investment costs as well as operating expenses as regards the
forming section of a pulp drying machine.
The Finnish publication print 61218 discloses a pulp drying machine, said
machine comprising a wet or wedge-shaped section, a register section with
a number of twin rolls, a forward press section, as well as a main press
section for applying compression to a material being dried. This publication
suggests that the wet section be provided with vacuum boxes for absorbing
moisture through the wires. Even in this type of solution, the vacuum system
brings along the above-mentioned drawbacks.
The publication WO 97/42374 describes a sort of wet lap machine, which in
turn comprises a variety of sub-assemblies, which thus makes it highly
complicated in itself.
Another problem in the prior known solutions is generally the efficiency of
dewatering. The available solutions are not capable of dewatering pulp at a
desired efficiency. In particular, the vacuum boxes designed for a powerful
underpressure lead to the compaction of web surface layers, as well as to the
clogging of wires, and hence to impaired dewatering.
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It is an object of the present invention to eliminate the prior art drawbacks
and to provide a totally novel type of solution for pulp web formation and
drying, wherein the prior art disadvantages are minimized.
5!n order to accomplish the objects of the invention, a method of the
invention
for forming pulp web, said method comprising feeding pulp from a head box
into a wedge-shaped converging space defined by two wires or the like, said
wires or the like being supported on chamber units, each including at least
one water-receiving chamber, in which method the pulp has its dry content
increased in the converging space to the value of at least about 10% to
about 12%, is characterized in that the method comprises, in the
substantially immediate vicinity of the converging space and/or its
downstream end section, subjecting the pulp to an independently adjustable
compression force perpendicular (z direction) thereto.
An apparatus of the invention for pulp web formation, said apparatus
comprising a wedge-shaped converging space defined by two wires or the
like, said space being supplied with pulp at a first dry content from a head
box, and said wires or the like being supported on chamber units, each
including at least one water-receiving chamber, is characterized in that the
converging space has its downstream section or its substantially immediate
vicinity provided with compaction members for subjecting the pulp between
the wires or the like to a compression in the perpendicular direction (z
direction).
According to one preferred embodiment of the invention, the compaction
members are designed as a plane press.
The invention will now be described in more detail with reference to the
accompanying drawings, wherein:
Fig. 1 shows schematically an apparatus of the invention for pulp web
formation,
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Fig. 2 shows schematically one modified way of implementing the
invention in the apparatus of fig. 1,
Fig. 3 shows schematically a compaction member for a pulp web
forming apparatus of the invention in a side view,
Fig. 4 shows the compaction member of fig. 3 in a front end view, and
Fig. 5 shows alternative design for a compaction member.
Fig. 1 shows schematically an apparatus 1 of the invention for forming a web
W. The apparatus comprises a headbox 2, which is used, according to a pulp
drying process of the invention, for supplying pulp at a dry content of at
least
about 1,5% into a converging wedge-shaped space 5 defined between two
endless wire loops, such as an upper wire 3 and a lower wire 4. The space 5
is formed by adapting the wires 3, 4 to travel around spaced-apart reverse
rolls 50 and 51, respectively, set at the upstream end of the space 5, and
thence in a relatively converging fashion for defining a wedge-shaped space.
In engagement with the inside surface of the wires 3 and 4 (away from the
pulp flow) are mounted chamber units 6 and 7, respectively, which are
provided with a number of dewatering chambers 8, the flow rate, and
secondarily also the pressure, of water to be discharged from pulp
therethrough being regulable by means of valves 22 and which, as the
process is going on, are substantially full of water removed from the pulp.
Hence, the water discharging from pulp is guided to mix with already
removed water in the chambers 8, and further into discharge channels. From
the dewatering chambers extend water discharge channels for carrying water
removed from the web to a further process. Thus, in the solution of the
invention, the flow of water discharging from a pulp web is regulated as
desired for this way controlling the dewatering process. Hence, the discharge
channels are provided with the valves 22 for regulating the flow rate by
variable throttling. It is desirable that the flow rate for each chamber be
maintained at a preset value. For the sake of clarity, the figure only depicts
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the channels from the lower chambers, but it is obvious that the upper
chambers are also provided with respective channels, or, alternatively, the
respective chambers in an upper and lower tier are connected to a single,
common channel. In the illustrated solution, pulp is delivered at such a rate
5 and pressure between the wires 3, 4 while the same are running that said
space 5 carries a positive pressure. This overpressure results in the
percolation of pulp water through the wires into the dewatering chambers B.
What is essential is that there, is a pressure difference between the
dewatering chambers and the space 5. According to one preferred
embodiment of the invention, a pressure difference between the space 5 and
the dewatering chambers 8 is measured continuously and this measuring
value is used in process control. Dewatering from=the chambers 8 is
regulated in such a way that the speed ratio within the area of each chamber
is preferably about 0,9-1,1. The speed ratio refers to a flow of liquid
occurring
between the chambers parallel to the web divided by the wire advancing
speed. The chambers 8 are otherwise substantially impervious, except that
the surfaces in engagement with the wires 3, 4 are such that the water to be
removed from pulp is able to migrate into the chambers 8. There are
preferably several chambers 8 arranged sequentially in such a way that the
surface in engagement with the wires is nevertheless substantially
continuous. The separate chamber units 6, 7 commence substantially directly
downstream of the reverse rolls 50, 51 for the top wire 3 and bottom wire 4,
and extend a certain distance to define the wedge-shaped converging space
5, the upper and lower wire 3, 4 passing thereth rough. Preferably, the
chamber units 6, 7 are substantially symmetrical and fixedly supported in a
suitable position for providing a preferred wedge shape. The converging
space 5 defined by the chamber units 6, 7 is provided in its downstream
section, or in the immediate vicinity thereof, with a compaction member 9 for
adjusting a relative distance between the upper wire 3 and the lower wire 4
or the like as desired. In fig. 1, the compaction members are depicted to
comprise a plane press 9, having its top element 31, 43 articulated, while its
bottom element 30 is solid. During its passage through the space 5, the pulp
delivers water in such a way that the pulp has a dry content of at least 12%
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before the plane press 9, and the plane press is used for removing more
water from the pulp in such a way that, after the plane press 9, its dry
content
is about 18%, preferably about 20%. The dry content can also be increased
to about 18-20% as early as within the converging space 5 and the plane
press 9 can thus be used merely to substantially maintain the coherence of
pulp emerging from the outlet of the space 5. The pulp emerging from the
wedge-shaped space comes out of the space 5 at a high pressure and,
without the compaction members of the invention, that would occur
uncontrollably and the wires 3, 4 would spread apart, resulting in
disintegration of the already formed pulp web. If necessary, water is still
removed according to the invention downstream of the plane press 9 while
the web W remains supported between the two wires 3, 4, for example by
compressing the aggregate between two rolls 10, 11. This is followed by
guiding the upper wire 3 and the lower wire 4 away from each other and the
web W is carried further, in accordance with the invention, on the top surface
of the lower wire 4, the web having a dry content of roughly 28%. Thus,
according to the invention, the web travels in a fully supported condition. If
necessary, the lower wire has its bottom surface fitted with a so-called
transfer vacuum box for securing a passage of the web along with the lower
wire. The web W is carried further in a supported condition on the top
surface of the lower wire 4 through a press nip N, comprising an upper felt
14, i.e. the web W travels with its top side against the felt and its bottom
side
against the lower wire 4 in the nip N between rolls 12, 13. The nip is
preferably constituted by a so-called shoe press.
Particularly, a pulp web can be formed and effectively dewatered by means
of a comprehensive process, wherein pulp is delivered into the converging
wedge-shaped space 5 defined between two endless wire loops, such as the
upper wire 3 and the lower wire 4, having a plurality of dewatering chambers
8 in communication with the wedge-shaped space, the flow rate of water to
be removed from the pulp therethrough being regulable, wherein the water
flow discharging from the pulp web is adjusted as desired for thus controlling
the dewatering process, wherein the converging space 5 defined by the
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chamber units 6, 7 is provided in its downstream section, or in the immediate
vicinity thereof, with the compaction member 9 for adjusting a relative
distance between the upper wire 3 and the lower wire 4 or the like as
desired, wherein the web W is subsequently advanced further, according to
the invention, on the top surface of the lower wire 4 in a supported condition
through the press nip N, comprising the upper felt 14, i.e. the web travels
through the press with its top side against the felt and its bottom side
against
the lower wire 4 in the nip N between the rolls 12, 13, said nip being
especially preferably constituted by a so-called shoe press for providing a
sufficient nip pressure and a long action time.
In this process, the web W is dried in such a way that its dry content
downstream of the nip N is roughly 40%. The above-described method of the
invention can be used in a pulp drying machine for the effective and reliable
formation of the web W. This solution provides a simplified former section
and the web W is comprehensively supported until it has a dry content of
roughly 40%, at which point it has a sufficient strength to withstand without
breaking a first unsupported passage to the subsequent process phase. In
the method of the invention, the'pulp web has also a considerable thickness
and, thus, according to the invention, the passage of the lower wire 4, to the
extent that it supports the web W, is adapted to extend in a substantially
straight line.
The invention provides major benefits, which include e.g.:
- pulp web formation can be performed substantially more
effectively than with the prior known apparatus,
- the web former section of a pulp making machine can be
accommodated in a substantially smaller space than the prior
known solutions, which may lead to the savings of up to several
million marks in investment costs,
- the passage of a web is substantially more reliable by virtue of
a supported run, which reduces downtimes and increases the
overall efficiency of a machine.
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Fig. 2 shows schematically an apparatus 1 which differs from the
embodiment of fig. 1 principally in that the plane press 9 is designed to be
adjustable regarding both its top element and its bottom element. In addition,
fig. 2 depicts an adjustment and control unit 21, which is connected to
appropriate measuring devices transmitting measuring data and which in tum
transmits drive commands to various appropriate elements, as shown by way
of example with lines 15-20. According to the invention, such data contains
some or all of the following: headbox supply consistency, headbox pressure,
pressure in space 5, pressure in chambers 8, speed of wires, compression
force/pressure of plane press, water flow rate through valves 22. These or
some preselected ones of these quantities are forwarded to the
adjustment/control unit, wherein drive commands are executed in a pre-
desired manner on the basis of these for controlling a drying process. The
drive may preferably include the following controls: control of valves 22,
loading of plane press, speed of wires, control for headbox supply pump,
control of headbox consistency, etc. According to the invention, the plane
press 9 may be provided with a loadcell and/or a pressure measuring sensor
for measuring and adjusting the compression force. In communication with
the chambers 8 is fitted a gauge for measuring of pressure from the
chambers and, if desired, also a pressure transmitter for measuring pressure
in the space 5 and/or for determining a pressure difference between the
chamber 8 and the space 5, whereby it is possible to use e.g. the valve 22
for effecting the adjustment of a pressure difference between the chamber
and the space 5. The web W is also measured for its dry content at a
suitable point of the drying process, making it possible on the basis of
information received therefrom to execute drive commands, if necessary, by
means of the adjustment/control unit. Furthermore, according to the
invention, the pressure can be regulated as desired in the space 5, as well
as by means of the plane press 9 and the press rolls 10, 11 and 12, 13 for
adjusting the dry content and dewatering as desired at various points. Since,
according to the invention, the discharge of water from the chamber 8 is
controlled comprehensively by observing all operating parameters of a drying
machine, explicitly on the basis of information originating from the process,
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and not just the water flow rate out of the chamber, the drying/dewatering
process can be driven in such a way that it proceeds as desired and
continuously in an optimal fashion.
Figs. 3 and 4 depict one embodiment for the plane press 9, including a solid
bottom element 30, and a top element 31 whose position relative to the
bottom element 30 is adjustable. The top element 31 is articulated at 32 to
the downstream portion of a space 5. The pulp is delivered between the
wires from the space 5 along a slot 40. The element 31 is secured to upright
struts 35 linked to each other at the bottom part thereof by a connecting
member 37, which is in turn fastened through the intermediary of cushion
elements or bellows 36 to the solid bottom element 30 of the plane press 9.
According to the invention, a web to be delivered out of the space 5 can be
guided forward in a controlled fashion, the top and bottom elements of the
plane press being adjustable regarding the relative position thereof, as well
as relative to the position of the chamber units. Furthermore, according to
the
invention, the loading of the plane press is effected by means of implements
which are capable of complying to suitable degree with possible fluctuations
in web thickness. Thus, the generation of excessive pressure in the plane
press is avoided in possible malfunction incidents. In a normal condition, the
plane press is loaded with a desired force by means of regulable adjusting
rods 33 or the like, with a certain force against these rods 33, for
maintaining
a certain stable setting. Consequently, the relative position between the top
element 31 and the bottom element 30 can be mechanically defined by
means of adjustable implements. The adjusting rods or the like are preferably
provided with actuators for setting the same while the machine is running, if,
for example, the plane press is in a position which must be reset because of
a change in production. The loading can be implemented preferably by
means of the cushions 36 using a compressible medium, e.g. air, whereby an
occasional increase in web thickness is compensated for by the
compressibility of the medium. After the malfunction has passed, the distance
returns automatically back to what it was. Thus, by virtue of the elasticity
of
the cushion elements 36, this solution is capable in a very simple manner of
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eliminating malfunctions possibly occurring in pulp, causing fluctuations in
thickness. It is also possible to provide a different type of distance
control,
the plane press having its position adjusted by means of pressure and
distance measurement.
5
The above-described compaction member is a plane press, but the invention
is conceivable also be using a compaction member, which comprises a nip
press constituted by two rolls, the. nip thereof being located in the
immediate
vicinity of the outlet of a converging space. This can be implemented, e.g. as
10 shown in fig. 5, by designing a tail-end section 61 of the chamber units to
extend to the proximity of the nip constituted by rolls 61, 62.
