Headbox of a paper/board machine by whose means the basis...

Details Headbox of a paper/board machine by whose means the basis... Headbox of a paper/board machine by whose means the basis...

2000-08-11

2002-08-13

Hastings, Karen M. (Department: 1731)

Paper making and fiber liberation

Apparatus

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

C162S258000, C162S259000

Reexamination Certificate

active

06432275

ABSTRACT:

FIELD OF THE INVENTION
The invention concerns a headbox for a paper/board machine by whose means the basis weight of the web can be regulated.
BACKGROUND OF THE INVENTION
From the prior art, regulation of the basis weight of a web (paper/board web) across the web width is known so that a dilution fluid, favourably dilution water, is passed to the desired area of width of the headbox. Thus, the dilution water is passed to the desired area of width into connection with the stock flow passed from the stock inlet header. The dilution water is favourably passed from an inlet header of its own through separate flow ducts, which flow ducts comprise a valve that regulates the flow. By means of regulation of the valve, the dilution fluid can be passed to the desired areas of width of the web and as the desired quantities.
From the U.S. Pat. No. 4,888,094 of Weisshuhn, a method is known for regulation of a flow, in which method a certain flow is passed out of the desired area of the width of the headbox. Further, in respect of the prior art, reference is made to the publication DE 42 34 940.
OBJECT AND SUMMARY OF THE INVENTION
In the present patent application, a method of an entirely novel type is suggested for regulation of the basis weight of a web, which method is based on removal of a flow component whose concentration differs from the average stock concentration out of the stock flow. In accordance with the invention, it has been realized to employ a separate thickening element, which comprises a chamber space, into which chamber an inlet tube is opened and out of which chamber an outlet duct departs. Through a central discharge duct, the main flow is passed further, and the proportion whose concentration differs from the average concentration of the stock suspension and which contains a smaller amount of fibers and whose basis weight is lower than the average basis weight of the stock is passed first out of the side walls of the tube that passes into the chamber space of the thickening element and after that away from the side face of the chamber space through a duct which opens at said face. In the inlet tube that passes into the thickening element, the flow tends to be differentiated so that the proportion with a higher concentration of solid matter flows in the middle of the tube, and the proportion with a lower concentration of fibers flows in the lateral areas of the tube. Said lateral proportion is passed further into the chamber space of the thickening element and away from the chamber space through the duct placed at the side wall of the chamber space. When the flow is passed into the thickening element, the total flow is denoted with L. At the thickening element, when the flow enters into the thickening element, the flow L consists of a flow proportion L
2
flowing in the middle of the inlet tube and passed forwards, and of a flow proportion L
1
flowing along the faces of the tube and to be removed from the headbox. The middle flow L
2
contains a higher concentration of solid matter than the flow proportion L
1
to be removed does. The flow L
2
is passed forwards in the headbox, and the flow L
1
is removed through the thickening element.
Thus, the central flow L
2
with a higher concentration of fibers is passed forwards, and the flow L
1
with a lower concentration of fibers, from the walls of the tube, is removed through the thickening element. The flow L
2
which is passed from the thickening element forwards in the headbox is a what is called differential flow and equal to =L−L
1
. The flow quantity (liters per minute) of the flow L is always invariable. As the flow quantity of the flow proportion L
1
removed from the headbox can be regulated by means of the valves V
1
, V
2
, it is further possible to regulate the concentration of the central flow L
2
. Thus, by means of said flow L
2
, it is further possible to regulate the basis weight of the web as desired across the web width. In order that the quantity of the flow L
2
should remain invariable, the proportion L
1
to be removed must be compensated for by a corresponding increase in the flow introduced into the thickening element.
In a preferred embodiment of the invention, the thickening element is fitted in the vicinity of the stock inlet header of the headbox. In said embodiment, when the flow proportion L
1
(X
1
liters per minute) is removed, the flow quantity of the flow passed from the inlet header is increased with an equal amount X
1
liters per minute, and, thus, the differential flow L−L
1
=L
2
is invariable. Said compensation for the flow quantity (X
1
liters per minute) that has been passed away takes place, in an embodiment of the invention, so that a separate duct is passed from the stock inlet header to the outlet side of the thickening element. Thus, through said duct it is possible to pass a compensation flow L
3
into connection with the flow L
2
. The sum flow L
3
+L
2
is always as desired and unchanged, i.e. invariable. Herein, when flows are spoken of (e.g. sum flow and differential flow), what is meant is flow quantity per unit of time, for example liters per minute.
The headbox in accordance with the invention for regulation of the basis weight of the web in a paper/board machine is characterized in what is stated in the patent claims.


REFERENCES:
patent: 4888094 (1989-12-01), Weisshuhn et al.
patent: 5490905 (1996-02-01), Huovila et al.
patent: 5885420 (1999-03-01), Heinzmann et al.
patent: 5944957 (1999-08-01), Fagerlund et al.
patent: 4234940 (1993-02-01), None

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