Headbox of a paper/board machine by whose means the basis weight of the web can be regulated

The invention concerns a headbox for a paper/board machine by whose means the basis weight of the web can be regulated.

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.

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.

The invention will be described in the following with reference to some preferred embodiments of the invention illustrated in the figures in the accompanying drawings, the invention being, yet, not supposed to be confined to said embodiments alone.

FIG. 1 is an axonometric view of an equipment in accordance with the invention, being partly an illustration of principle.

FIG. 2A is a sectional view of a thickening unit employed in the equipment in accordance with the invention.

FIG. 2B shows a second embodiment of a thickening element in accordance with the invention.

FIG. 2C shows an embodiment of a headbox connected with the thickening element shown in FIG. 2 B.

FIG. 2D is a sectional view taken along the line II—II in FIG. 2 C.

FIG. 3A shows an embodiment of the invention in which the thickening elements have been fitted right in the vicinity of the intermediate chamber of the stilling chamber G and in which construction the stilling chamber O is provided with an overflow.

FIG. 3B shows a set of thickening elements in an area of width for use in the construction as shown in FIG. 3 A.

FIG. 4A shows an embodiment of the invention in which the thickening elements have been arranged right in the vicinity of the stock inlet header at a suitable distance from the stock inlet header, in which connection both there is time for a concentration profile to be formed and the pressure in the stock inlet header equalizes the flows so that the flow L 1 that is removed from the thickening element is compensated for by an additional-flow quantity passed from the stock inlet header.

FIG. 4B is a sectional view taken along the line I—I in FIG. 4 A.

FIG. 1 shows a headbox construction 10 in accordance with the invention. As is shown in the figure, the headbox 10 comprises a stock inlet header J 1 , which becomes narrower towards its end in the cross direction and from which header the stock is passed, in the embodiment shown in FIG. 1, through the tubes 11 a 1 , 11 a 2 . . . in the single-row tube bank 11 further to thickening elements 12 a 1 , 12 a 2 , 12 a 3 . . . . Each tube 11 a 1 , 11 a 2 . . . is connected with a thickening element 12 a 2 , 12 a 2 . . . . In the thickening element 12 a 1 , 12 a 2 . . . , which is illustrated in more detail in FIG. 2A, part of the stock flow is removed from the lateral areas of the stock flow into the ducts D 1 , D 2 . . . and further into a collecting header 13 as a flow proportion L 1 . Thus, from each thickening element 12 a 1 , 12 a 2 . . . , a duct D 1 , D 2 . . . of its own passes to the collecting header 13 . Each duct D 1 , D 2 . . . is provided with a valve V 1 , V 2 . . . , by whose means it is possible to regulate the flow L 1 to be removed from the thickening element 12 a 1 , 12 a 2 . . . . The flow L 1 is taken in the thickening element 12 a 1 , 12 a 2 . . . from the lateral areas of the chamber space 18 (in FIG. 2 A). Said removed proportion or component of the flow differs, in respect of its consistency, from the flow L 2 that is passed further, which flow L 2 is passed further expressly from the middle of the thickening element 12 a 1 , 12 a 2 . . . .

From the thickening elements 12 a 1 , 12 a 2 . . . , the tubes 14 a 1 , 14 a 2 , 14 a 3 . . . in the single-row tube bank pass into an intermediate chamber E, which is opened from the top into a stilling chamber G and from which intermediate chamber E the stock flow is passed into the tubes 15 a 1.1 , 15 a 1.2 in the turbulence generator 15 and further into the slice cone K and onto the forming wire H.

In order that the flow quantity should not be changed as a function of the removed flow L 1 , to the tube bank 14 , after the thickening elements 12 a 1 , 12 a 2 . . . , additional-flow ducts 16 a 1 , 16 a 2 , 16 a 3 . . . have been connected. They are connected to the tubes 14 a 1 , 14 a 2 . . . placed after the thickening elements 12 a 1 , 12 a 2 . . . . Their inlet ends communicate with the inlet header J 1 . Through the ducts 16 a 1 , 16 a 2 . . . , an additional flow L 3 is passed into connection with the flow L 2 coming from the thickening elements 12 a 1 , 12 a 2 . . . . The additional flow L 3 compensates for the flow quantity L 1 that has been removed at each thickening element 12 a 1 , 12 a 2 . . . , so that the flow quantity L 2 +L 3 is always unchanged, i.e. invariable, after each thickening element 12 a 1 , 12 a 2 . . . .

As is shown in the figure, further, from the collecting header 13 there is a duct 17 through the valve W to the wire pit F. From the wire pit F, there is a return duct e back to the inlet header J 1 of the headbox, however, so that by means of the flow passed from the return duct, new fresh stock is diluted for the headbox construction. The new fresh stock is passed into the wire pit F along the duct 50 , and white water is passed along the duct 51 into the wire pit F.

FIG. 2A is a sectional view of a thickening element 12 a 1 . The thickening element 12 a 1 comprises a chamber space 18 , to which a tube 11 a 1 of the tube bank 11 is connected from the inlet side. The tube 11 a 1 extends into the interior of the chamber space 18 similarly to a projection. Out of the chamber space 18 , there is an exhaust duct 14 a 1 . The flow L 1 , whose consistency differs from the consistency of the stock flow L 2 passing in the middle of the tube 11 a 1 , is transferred from the lateral areas 18 c of the thickening element 12 a 1 , 12 a 2 . . . out of the chamber space 18 as exhaust into the duct D 1 . The component L 2 of higher consistency taken from the middle of the thickening element 12 a 1 , 12 a 2 . . . is transferred further in the headbox and first into the tube 14 a 1 .

FIG. 2B illustrates a second preferred embodiment of a thickening element 12 a 1 , 12 a 2 . . . . In the embodiment shown in the figure, the thickening element 12 a 1 comprises a frame 18 a preferably a tubular frame, which confines a chamber space 18 in its interior. Through the chamber space 18 , a tube 11 a 1 is passed, which tube comprises flow openings f 1 , f 2 , f 3 . . . , which open into the chamber space 18 a . Through the flow openings f 1 , f 2 , f 3 . . . a flow L 1 with a lower concentration of fibers is passed, which flow L 1 is passed further into the duct D 1 and out of connection with the headbox. The embodiment of the invention shown in FIG. 2B is in the other respects similar to the embodiments shown in the preceding figures, the only difference being that the thickening element 12 a 1 comprises a perforated f 1 , f 2 . . . tube portion 11 a 1 , 11 a 2 . . . passing through the chamber space 18 a . When fitted in a headbox as shown in FIG. 1, a thickening element as shown in FIG. 2A or 2 B is placed so that its distance from the inlet header J 1 is larger than 5×diameter n of the tube 11 a 1 , preferably 10 . . . 20 times the diameter n of the tube 11 a 1 . In the embodiment of FIG. 2B, the headbox comprises a stock inlet header J 1 and from it, as is shown in the embodiment of FIG. 4A, flow tubes 11 a 1 , 11 a 2 passing into the intermediate chamber E and further from it, through the tubes 15 a 1.1 , 15 a 1.2 in the turbulence generator 15 , the flow is passed into the slice cone K and further onto the forming wire H 1 .

FIG. 2C shows an embodiment related to the thickening element 12 a shown in FIG. 2 B. In this illustration, the thickening elements 12 a 1 , 12 a 2 . . . are connected with flow tubes 11 a 1 , 11 a 2 . . . which open into the intermediate chamber E. From the intermediate chamber, the flow passes into the turbulence generator 15 and from it further through the slice cone onto the forming wire H.

FIG. 2D is a sectional view taken along the line II—II in FIG. 2 C. Thickening elements 12 a 1 , 12 a 2 . . . have been fitted in different positions of width of the headbox in view of regulation of the basis weight of the web across the web width as desired.

FIG. 3A shows an embodiment of the invention in which the thickening elements 12 a 1 , 12 a 2 . . . have been fitted right in the vicinity of the intermediate chamber E, into which intermediate chamber E further a stilling chamber G is opened. If the diameter of the tubes 11 a 1 , 11 a 2 . . . passing to the thickening elements 12 a 1 , 12 a 2 . . . is denoted with n, the length L of the tubes 11 a 1 , 11 a 2 . . . is larger than 5×n, and preferably (10 . . . 20)×n. When the tubes 11 a 1 , 11 a 2 . . . are formed sufficiently long, a consistency profile has time to be formed in the flow in the tube. In the embodiment shown in FIG. 3A, when the construction is provided with a stilling chamber G connected with the intermediate chamber E and when the thickening elements 12 a 1 , 12 a 2 . . . are fitted as close to the intermediate chamber E as possible, variations in the flow quantity of the differential flow L−L 1 =L 2 arising from the removed flow L 1 are compensated for by means of the overflow of the stilling chamber G. In this way, the flow quantity of the flow L 2 which is passed from the thickening elements 12 a 1 , 12 a 2 . . . further into the intermediate chamber E is kept unchanged, i.e. invariable.

FIG. 3B is a separate illustration showing thickening elements 12 a 1.1 , 12 a 1.2 and 12 a 1.3 placed one above the other at one position of width. The figure is a schematic illustration. From each of the thickening elements 12 a 1.1 , 12 a 1.2 and 12 a 1.3 placed one above the other, a flow L 1 is passed into the duct D 1 . Ducts or tubes 11 a 1.1 , 11 a 1.2 and 11 a 1.3 pass to the thickening elements 12 a 1.1 , 12 a 1.2 and 12 a 3 , and for the flows L 2 departing from the thickening elements, there are ducts or tubes 14 a 1.1 , 14 a 1.2 and 14 a 1.3 .

FIG. 4A shows an embodiment of the invention in which the thickening elements 12 a 1 , 12 a 2 have been fitted in the vicinity of the stock inlet header J 1 of the headbox so as to be connected with the flow tubes 11 a 1 , 11 a 2 . . . The length of the tubes 11 a 1 , 11 a 2 . . . is larger than 2−n, wherein n is the diameter of the tube 11 a 1 , 11 a 2 . Preferably, the length of the tube 11 a 1 , 11 a 2 is in the range (5 . . . 15)×n, i.e. 5 . . . 15 times the diameter of the tube 11 a 1 , which diameter is denoted with the letter n. When the stock that flows in the tube portions 11 a 1 , 11 a 2 is denoted with the reference arrow L, at the thickening element the flow L consists of the flow proportions L 1 +L 2 , in which the flow proportion L 1 flows along the walls of the tube 11 a 1 , 11 a 2 . . . , and the flow proportion L 2 flows in the middle of the tube 11 a 1 , 11 a 2 . . . The proportion flowing along the walls is passed through the thickening unit 12 a 1 , 12 a 2 . . . into the chamber 18 of the thickening unit and further away from the thickening unit 12 a 1 , 12 a 2 . . . The flow L 2 is passed further, and the flow L 2 , which is a differential flow, can be written as L 2 =L−L 1 . The fact that the flow L 2 remains invariable is permitted by the fact that the thickening unit is placed in the vicinity of the inlet header J 1 , in which case the pressure in the inlet header J 1 equalizes the flow quantities. The more flow L 1 is removed, the more is the flow L that is passed from the inlet header J 1 into the tube 11 a 1 , 11 a 2 . . . increased. Thus, the differential flow L 2 remains invariable after each thickening element 12 a 1 , 12 a 2 . . . .

As is shown in the figure, the flow L 2 is passed further into the intermediate chamber E and further through the tubes 15 a 1.1 , 15 a 1.2 . . . in the turbulence generator 15 into the slice cone K and further onto the forming wire H. As is shown in the figure, from the collecting header 13 , there is a duct 17 further through the valve W into the wire pit F. From the wire pit F, there is a return duct e passing back to the inlet header J 1 of the headbox, however, so that, by means of the flow passed from the return duct, the new fresh stock is diluted for the headbox construction. The new fresh stock is passed into the wire pit F along the duct 50 , and white water is passed along the duct 51 into the wire pit F.

FIG. 4B is a sectional view taken along the line I—I in FIG. 4 A. As is shown in the figure, the thickening elements 12 a 1 , 12 a 2 . . . 12 a n have been fitted at different positions of the width of the headbox, in which case, by means of the flows removed through the thickening elements, it is possible to regulate the basis weight of the web across the web width by regulating the concentration of the stock L 2 that is made to flow from the thickening elements further in the headbox. If the valve V 1 , V 2 . . . of the thickening unit 12 a 1 , 12 a 2 . . . is kept closed, no flow is removed through the thickening unit 12 a 1 , 12 a 2 . . . , and in such a case the flow L 1 =0, and the differential flow, i.e. the flow quantity that is made to flow forwards from the thickening unit, is equal to the flow L that enters into the thickening unit. In such a case, L 2 =L. However, in the construction in accordance with the present invention, the flow proportion L 2 that is made to flow from the thickening unit further in the headbox is invariable under all circumstances irrespective of the quantity of the flow L 1 that is removed.