Dewatering press and process

Details Dewatering press and process Dewatering press and process

1997-12-04

2001-05-08

Chin, Peter (Department: 1731)

Paper making and fiber liberation

Processes and products

Running or indefinite length work forming and/or treating...

C162S198000, C162S358300

Reexamination Certificate

active

06228221

ABSTRACT:

CROSS-REFERENCE OF RELATED APPLICATION
The present invention claims the priority under 35 U.S.C. §119 of German Patent Application No. 196 50 396.5 filed on Dec. 5, 1996, the disclosure of which is expressly incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dewatering press of a machine for producing a fibrous web, e.g., a web of paper or cardboard. The dewatering press may include at least one extended nip, lengthened in the web travel direction, formed between a rotating or revolving flexible press belt of a shoe press unit and a counterpart face. The flexible press belt can be pressed against the counterpart face by at least one press shoe associated with the shoe press unit. A length of the extended nip may be determined at least substantially by a length of the press shoe length, measured in the web travel direction, of the shoe press unit. The fibrous web may be guided through the extended nip with at least one dewatering belt through the lengthened nip.
2. Discussion of Background Information
Dewatering presses similar in general to the device described have been in use for some time, and have been discussed in, e.g., German patent DE-PS 22 62 201, German patent DE-PS 34 10 172, and European Patent EP 258 169. In devices of this type, the result of the dewatering has been improved by utilizing a long dwell time of the fibrous web in the extended nip. However, because the extended nips utilize increasing pressure pressing surfaces, increasing contact pressure forces are required to ensure a certain or reliable pressing power. Thus, the prior art devices require substantial technological effort and, accordingly, considerable costs as well.
SUMMARY OF THE INVENTION
According to the present invention, the above-described extended nip press utilizes a dwell time t
w
of the fibrous web in the extended nip of less than 12 ms. As a result of the recognition that within a dwell time of approximately 12 ms, and in particular even approximately 6 ms, most of the possible dewatering performance is achieved, the present invention makes it possible to limit the length of the extended nip and, therefore, limit the length of the press shoe in the web travel direction. In general, dwell time t
w
is determined by the extended nip length and the speed of the fibrous web.
Because a minimal pressing power (pressure) must be available for dewatering operation, i.e., to press the water out of the fibrous web and into the dewatering belt, optimizing the press shoe length enables a minimization of the contact-pressure force that must be brought to bear by the shoe press unit.
For effective dewatering in general, the present invention utilizes a minimum pressing power (pressure) established in accordance with the dry content of the fibrous web. That is, the minimum pressing power in the extended nip is determined or set to be (T
abs
×100)
3
N/m
2
, where T
abs
represents absolute dry content at the beginning of the extended nip. For example, for an initial dry content of 20%, T
abs
=0.2 and a pressing power of more than 8000 N/m
2
is required. In an alternative, a minimum pressing power may be preestablished to be 500 KN/m
2
.
However, to achieve acceptable dewatering performance in the nip, a minimal dwell time must naturally also be ensured. In accordance with the present invention, the minimal dwell time is approximately 2 ms, and preferably approximately 4 ms.
According to the features of the present invention, there are a number of ways in which the extended nip may be formed. Further, one or more dewatering belts may be guided through the nip with the web. For example, the extended nip may be formed by a flexible press belt of a shoe press unit being pressed against a cylindrical roll, and, under some circumstances, with controlled sagging. In another example, the press shoe of the shoe press unit may have a concave contact-pressure face. In a further example, the extended nip may be formed by two shoe press units, in which the contact-pressure faces are preferably formed as flat.
In addition, the press shoe of one or more shoe press units may include a plurality of smaller press shoes axially disposed and/or positioned side by side in the web travel direction. Further, at least some of the smaller press shoes may be provided to be independently pressable with respect to one another.
For ascertaining the nip length and thus the dwell time, in particular the portion or portions of the nip in which the pressing power is greater than (T
abs
×100)
3
N/m
2
and in particular greater than 500 KN/m
2
are determinative.
The present invention is directed to a process of dewatering a fibrous web in a dewatering press that includes a counterpart face, a shoe press unit having a revolving flexible press belt and at least one press shoe such that the at least one press shoe is arranged to press the flexible press belt against the counterpart face, at least one nip formed by the flexible press belt being pressed against the counterpart face having a length extending in a web travel direction and being at least substantially similar to a length of the at least one press shoe in the web travel direction, and a device that guides the fibrous web and at least one dewatering belt being guided through the extended nip. The process may include pressing the fibrous web within the extended nip, determining a dwell time of the fibrous web within the extended nip, and maintaining the dwell time at less than approximately 12 ms.
In accordance with a further feature of the present invention, the process may further include maintaining the dwell time at less than approximately 6 ms. Further, the process may include maintaining the dwell time at greater than approximately 2 ms, and preferably, maintaining the dwell time at greater than approximately 4 ms.
In accordance with still another feature of the present invention, the determining of the dwell time may include establishing a minimum pressure, determining a length of the extended nip in accordance with one of a portion and a plurality of portions of the extended nip in which greater than the minimum pressure power is exerted, determining a speed of the fibrous web through the extended nip, and calculating the dwell time from the determined length of the extended nip divided by the determined speed of the fibrous web. Further, the establishing of the minimum power pressure may include selecting the minimum power pressure of approximately 500 KN/m
2
. Alternatively, the establishing of the minimum pressure may include selecting the minimum pressure in accordance with an absolute dry content of the fibrous web preceding the extended nip. In this regard, the selecting of the minimum pressure may be selected in accordance with the formula P
min
=(T
abs
×100)
3
N/m
2
, where P
min
represents the minimum pressure and T
abs
represents the absolute dry content of the fibrous web at an inlet of the extended nip.
In accordance with a further feature of the present invention, the speed of the fibrous web may be between approximately 600 and 1500 m/min.
In accordance with a still further feature of the present invention, the length of the extended nip may be between approximately 25 and 35 cm.
In accordance with yet another feature of the present invention, the absolute dry content of the fibrous web may be between approximately 0.20 and 0.60 at an inlet of the extended nip.
The present invention is also directed to a dewatering press of a machine for producing a fibrous web. The dewatering press may include a counterpart face and a shoe press unit that includes a revolving flexible press belt and at least one press shoe. The at least one press shoe may be arranged to press the flexible press belt against the counterpart face. At least one nip may be formed by the flexible press belt being pressed against the counterpart face and may have a length extended in a web travel direction. The extended nip length in the web travel direction may be at leas

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