Paper making and fiber liberation – Processes and products – Running or indefinite length work forming and/or treating...
Reexamination Certificate
1998-08-27
2001-04-10
Hastings, Karen M. (Department: 1731)
Paper making and fiber liberation
Processes and products
Running or indefinite length work forming and/or treating...
C162S346000, C162S353000
Reexamination Certificate
active
06214169
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to a sheet formation system for a fourdrinier machine for producing, e.g., paper or cardboard, and to a method for reducing edge waves in a sheet formation system.
2. Discussion of Background Information
Sheet formation systems for fourdrinier machines include an endless screen (sieve) for formatting paper or cardboard webs that is guided or runs over an open, flat table surface. Pulp heavily diluted with water, i.e., pulp suspension, is added to a headbox that produces a pulp suspension jet through a nozzle that extends along the width of the machine and that directs the pulp suspension jet onto the endless screen. Subsequently, fluid is drained through the screen and a pulp web is produced. The pulp web is then guided through the machine and is processed into a finished paper web or cardboard web.
After an impact point of the pulp suspension jet upon the screen, the pulp suspension at the edges of the screen tends to run off laterally. For this reason, in the area of the edges of the screen, i.e., after the nozzle of the headbox, so called deckle plates (or a deckle pan assembly) are attached to prevent both a lateral flow of the pulp suspension and a thinning of the produced paper or cardboard web in this area.
Deckle plates of the type generally discussed above are known from, e.g., Patent Application DE 43 34 641 A1, the disclosure of which is expressly incorporated by reference herein in its entirety. This document discloses as its object preventing permanent accumulation of fiber layers that form in the area of the edges of a flowing pulp suspension. The accumulation of fiber layers results from drying and these dried layers may flake into the fresh pulp suspension. For this purpose, the noted document discloses providing spray nozzles in the area of the deckle plate to cover the deckle plate surfaces with a flowing aqueous film, so as to prevent drying-on of pulp accumulations. The deckle plate itself is comprised of a flat strip or deckle edge which is strengthened in its directional stability by hollow sections aligned in a machine direction.
A major problem with the known deckle plate is that, in the area of the edge, secondary flows develop from the pulp suspension flowing past the boundary layers, and these secondary flows disadvantageously produce wavy edges in the pulp suspension that run toward the machine center.
SUMMARY OF THE INVENTION
The present invention provides a sheet formation system of a fourdrinier machine that includes deckle plates for substantially reducing or substantially preventing the formation of wavy edges in the pulp suspension.
Further, the present invention provides a method for reducing wavy edges in a sheet formation system of a fourdrinier paper or cardboard machine in an area in which the pulp suspension is applied.
Accordingly, the present invention provides deckle plates (a deckle pan assembly) that are positioned adjacent to an endless screen so that a distance between the deckle plates and a centerline of the screen is adjustable in a machine direction over a considerable part of a length of the shields. Further, the present invention provides a method that includes continuously widening a flat side of the pulp suspension in an area of the edges of sheet formation.
The inventors of the present invention discovered that the problem with edges in the sheet formation system is a problem at an interface between the headbox and the screen section of the web producing machine. The problem arises from the control of the jet opening, the associated reflected wave, and the limits set for the sheet size during sheet formation. While developments in the headbox area have resulted in improvements in the cross-directional profile of the web, these improvements, e.g., sectioning of the headbox to control pulp density, may more negatively effect the web edge than the previous systems.
The causes of wavy edges or reflected waves may have various origins. Generally, wavy edges are caused by edge effects that arise in the form of curved secondary flows that move toward an upper part of the deckle plate. Because of the large wetted areas, the flow in the edge area may be considerably retarded and may, due to continuity conditions, occupy a larger space along the wall. Thus, the slower edge flow substantially blocks the desired flow of the main flow moving substantially along the length of the deckle plate. As a result, the jet is deflected locally toward the machine center, which has the appearance of a reflected wave.
This problem may particularly occur with slow-running machines having large nozzle openings. In this manner, the reflected waves have ample time to expand to the fourdrinier in the web production area. While the various effects are slightly weaker with fast machines, in principle, the problems exist here also.
Secondary flows may occur as the result of various effects. For structural reasons, the orifice of the nozzle may be smaller than the space provided between the deckle plates. As a result, an opening between the orifice and the deckle plate is created. Ideally, this opening should be approximately 0.15 mm, but should not exceed approximately 0.25 mm. A groove in the orifice seal cannot be extended to the tip of the opening, thus, an automatic seal in this area, which is caused by fibers, particularly in faster machines, is made more difficult because of the prevailing high pressure. Further, in addition to the secondary flow at the deckle plate, problems, e.g., splashing, contamination of the screen section, and braid formation, may occur. These problems can result in considerable interference, particularly if the opening is too large, e.g., as a result of an orifice position not centered in the middle of the machine.
Another possibility of creating secondary flows may be through wall friction. That is, as a result of wall friction in the headbox and at the deckle plate, a boundary layer may be produced that has a flow speed slower than that of the central main flow. Without the deckle plates, the deposited pulp suspension would tend to laterally expand and to produce a larger volume required by the slower speed of flow. Because this expansion is prevented by the deckle plate limits, an upwardly curved secondary flow develops at the deckle plate.
Another manner in which secondary flows are created is that the jet, at a jet impact point, receives a generally strong impulse in the transverse direction as a result of change in flow direction due to lateral expansion. Thus, the transverse impulse results in an additional, curved secondary flow at the deckle plate, the effect of which largely depends on an impact angle between the screen section and the headbox position.
Secondary flows may also arise out of screen joggling, in which speed components are impressed upon the pulp suspension in the cross machine direction. Due to a defined sheet size limits, resulting impulses are reflected toward a center of the web as a wavy edge.
Accordingly, the inventors of the present invention have found that reflection waves or the wavy edges can be reduced by providing adequate space for lateral expansion of the secondary flow.
While adequate space may be provided with the known, non-flexible deckle plates such that, depending upon a specific requirement, an opening may be set between the screen and the deckle plate. In this manner, a substantial portion of the impulse may be laterally “released” to substantially reduce the wavy edge. However, in this area it is also necessary to avoid build-up of pulp through the spray pipes. A solution to this consequential problem is described in the above-noted document DE 43 34 641 A1.
It is also possible to provide a space through the deckle plate contour for the undesired secondary flow so as not to restrict the main flow or not to divert the secondary flow toward the center of the machine. This can be provided, e.g., with an adjustable deckle plate that is able to influence the flow shortly after it is ejected from
Dietz Thomas
Fenkl Konstantin
Ruf Wolfgang
Greenblum & Bernstein P.L.C.
Hastings Karen M.
Voith Sulzer Papiermaschinen GmbH
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