Paper making and fiber liberation – Apparatus – Automatic control
Reexamination Certificate
2000-05-26
2002-04-09
Hastings, Karen M. (Department: 1731)
Paper making and fiber liberation
Apparatus
Automatic control
C162S259000, C162S336000, C162S343000
Reexamination Certificate
active
06368462
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a headbox for a paper or board making machine.
BACKGROUND OF THE INVENTION
The prior art discloses various headbox constructions in which the board or paper stock is pumped into a flow header that tapers toward the inlet end of the headbox and is then passed from the flow header into the inlet pipe manifold and, therefrom, onwards via a mixing chamber into the pipes of a turbulence generator and finally into the tapered slice. From the slice, the stock is passed onto the forming wire.
In headboxes of the type described above dilution water is passed form a dilution water flow header into dilution water channels and further via control valves to different points distributed over the cross-machine width of the headbox so as to control the basis weight profile of the formed web over the entire web width.
The dilution water is preferably taken from the drainage white water which passed from the wire section via the closed circulation system of the paper/boardmaking machine and thus has a low fiber and solids content. It has been found that the consistency of the dilution white water changes in the dilution water flow header so that the consistency is higher at the inlet side than at the bypass circuit side. This phenomena results in errors in controlling the basis weight profile by the weight basis control system because the same settings of the dilution water valve give different basis weight control responses at the inlet and outlet sides.
The cause of the above-mentioned error occurs because in a conventional dilution flow header arrangement the flow rate of dilution water circulation through the header is measured from only one side of the header. Thus, because the consistency of the dilution water varies along the length flow header, accurate control of basis weight profile cannot be attained.
OBJECTS AND SUMMARY OF THE INVENTION
The present application teaches a flow header construction in which the outlet flow of dilution water from the flow header has a constant consistency over the entire cross-machine length of the header. Toward this end, the dilution water flow header according to the present invention is provided with such a construction so that it is capable of disturbing the flow of the dilution water into the flow header and uniformly mixing the same. In order to reach this goal, one embodiment of the present invention uses a restriction or the like in front of the flow header to adjust the flow pattern of the dilution liquid to thereby insure that the consistency of the dilution liquid is consistent.
In another embodiment of the invention the inlet channel to the flow header is provided with area of increased diameter relative the remainder of the inlet channel. The are of increased diameter causes a pressure loss in the dilution liquid flow and equalizes the overall flow pattern, whereby the flow can be made to pass from end to end of the dilution water flow header with a substantially constant consistency over the entire cross section of the header. As a result, the header can keep the consistency of the dilution water flowing into the flow chamber outlet channels substantially equal in all running situations.
According to another aspect of the invention, the dilution water flow header is constructed so that the flow in the header is brought into a vorticose movement. The vorticose movement mixes the flow so that a zone of higher consistency does not develop at the rear wall of the flow header. The vorticose movement is achieved by passing the dilution water from the flow header into the dilution water outlet channels in a tangential fashion. By virtue of the vorticose flow pattern of the dilution water in the flow header, there is no risk of having a low-consistency zone formed at the rear wall of the flow header.
The present invention is further characterized by the use of “flow mixers” located at the inlet side of the header for equalizing the cross-sectional consistency profile and by utilizing a vorticose flow pattern for preventing the consistency profile from redeveloping its previous unequalized pattern.
The headbox construction according to the invention is further characterized in that the dilution water flow header includes integrated therein means for equalizing the flow pattern of the dilution water, whereby the dilution water flow has a substantially equal consistency over the entire cross-machine length of the flow header and in cross sections taken in planes perpendicular to the longitudinal axis of the flow header, thus equalizing the quality of dilution water in all the flow header outlet channels.
REFERENCES:
patent: 5510005 (1996-04-01), Gingerich
patent: 5549792 (1996-08-01), Begemann et al.
patent: 5707495 (1998-01-01), Heinzmann et al.
patent: 5853545 (1998-12-01), Haraldsson et al.
patent: 6136152 (2000-10-01), Begemann
Lumiala Juhana
Poikolainen Antti
Steinberg & Raskin, P.C.
Valmet Corporation
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