Paper making and fiber liberation – Apparatus – Automatic control
Reexamination Certificate
1998-08-19
2002-04-09
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Apparatus
Automatic control
C162S263000, C162S259000, C162S252000, C700S128000
Reexamination Certificate
active
06368461
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device and a method to control or regulate the basis weight of a paper or cardboard web in the manufacturing process.
2. Discussion of Background Information
An essential quality factor of a paper or cardboard web is the uniformity of the basis weight of the manufactured web. In the manufacturing process of such a web, several interfering factors may arise that can adversely affect or influence the uniformity of the basis weight both across the machine width and in the longitudinal direction. These interfering factors include, e.g., temperature variations, pressure variations, finishing tolerances, and also include, e.g., errors in the operation or adjustment of the machine in the manufacturing process. To substantially eliminate these negative interfering factors and to obtain a uniform production of the paper or cardboard web, devices and methods for controlling and regulating the basis weight, particularly the cross-direction profile of the basis weight, are used.
Patent document DE 35 35 849 discloses changing the width of an outflow opening of a headbox at certain points along the web width so that the throughput of the material suspension is locally altered. However, if the throughput of the material suspension changes locally at an identical concentration over the machine width, the quantity of solids at this point in the web relative to the web width is influenced, which can change the basis weight at this point on this web strip.
In patent application DE 40 19 593, a device and method for regulating the basis-weight cross-direction profile of the web is disclosed. In accordance with this document, when the basis-weight cross-direction profile of the paper or cardboard web deviates at a particular point in the width of the web, the concentration of the material suspension flow should be adjusted or modified at this point. In order to perform this, the above-noted document discloses a headbox to be sectioned at least partially over the machine width with each section to be provided with an individually adjustable concentration via controlled section flows. The individual adjustment of the concentration of a respective section flow occurs through a regulation of the feed stream conditions of two individual flows of constant but different concentrations. Due to the different solid matter content of the section streams, a change of the basis weight results at the corresponding points in the web width.
In the control method utilized above, the basis-weight cross-direction profile of the paper or cardboard web is measured at the end of the paper or cardboard machine and the aperture at the headbox or the feed stream conditions of the individual streams of different concentration are regulated for respective sections via a control loop.
Such a control method with measuring basis-weight cross-direction profile is also known, for example, from patent application DE 40 05 281 or DE 42 38 037. In the above-mentioned documents, the basis-weight cross-direction profile of the paper or cardboard web is measured at the end of the paper or cardboard machine and the necessary adjustment of actuators is performed via a process control system for sectional regulation of the basis-weight.
Also known, e.g., from the disclosure patent document DE 20 19 975, the disclosure of which is expressly incorporated by reference herein in its entirety, is the technique of determining the actual basis-weight cross-direction profile and the actual basis-weight longitudinal profile on a running web with the aid of two sensors. The sensors measure a diagonal profile and a longitudinal profile, and utilize this information to control the web or regulate the basis-weight of a web.
The above-noted control methods substantially control the cross-direction profiles of the paper or cardboard web. It is also known to utilize, during the manufacturing process of a paper or cardboard web, a control loop that measures the basis-weight of the finished paper or cardboard web, which is generally the mean value of the measured cross-direction profile. Based upon the measured basis weight, the material amount supplied to the screen loop is regulated and the basis-weight of the produced paper or cardboard web is adjusted.
Moreover, in the above-noted prior art, the known regulation of the basis weight has a relatively sluggish behavior with long reaction times.
SUMMARY OF THE INVENTION
In accordance with the present invention, known paper or cardboard machines and methods can be enhanced to provide a regulation of the basis weight with a shorter reaction time than the prior art devices. In this manner, short wavelength variations in the manufactured paper or cardboard web can be controlled relative to the running time of the paper or cardboard web through the paper or cardboard machine.
The present invention is directed to a paper or cardboard machine that regulates the basis weight of a produced paper or cardboard web during the manufacturing process. The paper or cardboard machine includes at least one headbox for a machine-wide distribution of a material suspension either onto a screen or between two screens, at least one material suspension supply coupled to the headbox with at least one concentration, and at least one initial control loop (I). The at least one initial control loop (I) includes at least one actuator for adjusting the at least one concentration of the at least one supplied material suspension, at least one device for determining a basis weight of the finished web, at least one concentration modifier for modifying the at least one concentration of the at least one material suspension supplied to the headbox, and at least one device for adjusting the at least one actuator for adjusting the at least one concentration of the at least one material suspension supplied. The paper or cardboard machine also includes at least one second control loop (II) for further regulating the concentration of the material suspension supplied to the headbox.
Accordingly, the present invention improves the prior art paper or cardboard machine with regulation of the basis weight of a paper or cardboard web in the manufacturing process by utilizing at least a second control loop for regulating the concentration of the material suspension supplied to the headbox.
A further feature of the paper or cardboard machine of the present invention is that the second control loop includes at least one device for either the direct or indirect determination of the basis weight of the resulting web.
In accordance with another embodiment of the present invention, the device for either the direct or indirect determination of the basis weight of the resulting web is positioned in a wet area of the paper or cardboard machine. This positioning may be utilized to obtain measurement directly behind the headbox jet, e.g., with a radiometric sensor. Further, the ability to measure the height of the material suspension on an extended screen is possible. This measurement may advantageously occur, e.g., preferably behind a first roll and prior to a first draining unit so as to substantially avoid influences that arise through the draining unit. However, if it is necessary, e.g., for reasons of space requirements or other prevailing conditions, measuring may be alternatively performed after the first draining unit. Further, even in a double-screen environment, the basis weight of the resulting web can be determined in the wet section, e.g., with the radiometric measurement.
In another embodiment of the present invention, the headbox can have two material supplies provided with different material concentrations, e.g., C
L
and C
H
. These material supplies may be formed so that at least one of the two material supplies is divided at the headbox into section streams that are to be individually controlled. In this regard, reference is made to the above-noted prior art documents, which disclose various embodiments of headboxes that are controllable for consistency.
In
Lehleiter Klaus
Loser Hans
Ruf Wolfgang
Fortuna Jose
Voith Sulzer Papiermaschinen GmbH
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