With respect to the prior art, reference is made to the solutions in EP 634,523 and in DE 44 35 860 and DE 44 16 909. From said publications, on the whole, the use of dilution liquid is known in connection with a multi-layer headbox in order to regulate the basis weight of a multi-layer web across the web width.

When multi-layer paper is produced, one of the main requirements is purity of the layers. In the present patent application, a novel construction of a multi-layer headbox is described, which is suitable both for paper machines and for board machines. In accordance with the invention, it has been realized to construct the multi-layer headbox so that it comprises separate inlet headers of their own for each stock and, after them, a common distribution plate, through which the dilution liquid is distributed to the desired portions of the web width and, preferably at the same time, into all of the layers in view of regulating the basis weight of the web across the web width. Further, preferably the same amount of dilution liquid, favourably dilution water, is passed into each layer.

Since, in the construction of a multi-layer headbox in accordance with the invention, for regulation of the dilution liquid, in each zone of regulation across the width, one valve only is needed, the solution becomes less expensive and simpler, compared with the prior-art solutions. In the solution in accordance with the invention, the layers can be made stable, in which connection even partial mixing together of layers of different consistencies is prevented. Thus, in the solution in accordance with the present invention of ours, the layers remain in equal proportions, which is important when the individual layers are thin, which is the case with printing papers. The

optimal field of application of the invention is a 3-layer headbox in the manufacture of printing papers.

Further, in accordance with the invention, the construction of the headbox is such that the constructional portions corresponding to its individual layers are equal to one another both in respect of the numbers of pipes and in respect of the cross-sectional shapes of the pipes, in which case the flow velocities become equal in each layer.

Thus, since the flow velocities in each layer are substantially equal, no impurity of layers, arising from differences in flow, occurs. Further, in the construction in accordance with the invention, between the layers, in the slice cone long aprons are used which become narrower towards their ends and extend substantially over the entire length of the slice cone. Further, in each layer, between the horizontal rows of pipes, intermediate aprons are used which become narrower towards their ends.

In a multi-layer headbox in accordance with the invention, preferably a what is called single-stock system is employed. The stock is passed from one common tank and branched into branch ducts, in which the fillers/admixtures required by each layer are added to the basic stock.

In a multi-layer headbox in accordance with the invention, there are at least two layers, preferably three. There may also be more than three layers.

The multi-layer headbox in accordance with the invention is mainly 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 draw- ings, the invention being, however, not supposed to be confined to said embodiments alone.

Figure 1A is a longitudinal sectional view of a multi-layer headbox in accordance with the invention.

Figure 1B is a sectional view of the construction shown in Fig. 1A viewed from the top and taken along the line I-I in Fig. 1A.

Figure 2A is an illustration on a larger scale of the supply of dilution liquid into the multi-layer headbox in accordance with the invention into connection with the distribution plate common of its different layers.

Figure 2B is a sectional view taken along the line Il-Il in Fig. 1A. As is shown in the figure, the dilution flow is distributed from the narrowing duct at the same time into all the flow pipe components that form the different layers in the multi-layer headbox in each vertical row of pipes.

Figure 3 illustrates the stock system employed in the multi-layer headbox in accord- ance with the invention.

Figure 4A shows a second preferred embodiment of the invention.

Figure 4B is a sectional view taken along the line Ill-Ill in Fig. 4A.

Figure 5A shows an embodiment of the invention in which the dilution liquid is passed at the same time through the same valve into one surface layer and into the middle layer in a three-layer headbox.

Figure 5B is a sectional view taken along the line IV-IV in Fig. 5A.

Fig. 1A is a longitudinal sectional view of a multi-layer headbox 10 in accordance with the invention for a paper/board machine. The multi-layer headbox shown in the figure comprises three stock inlet headers: the headers J1 ,J2,J3 for the stocks M1,M2 and M3. Each stock M1,M2,M3 is fitted to be passed from the inlet headers J1,J2 and J3 so that the stocks are not mixed with each other but that three separate web layers are formed. As is shown in Fig. 1A, the dilution liquid, preferably dilution water, is passed out of its header T through the valves V1 and/or V2 and/or V3..

connected with the header into the ducts D1 and/or D2 and/or D3 connected with the valves selectively. In this way, by regulating the valves V1 and/or V2 and/or V3....

it is possible to regulate the flow of dilution liquid, preferably dilution water, across the web width. The concentration of the dilution liquid, preferably dilution water, differs from the average concentration of the stock suspension. In this way the basis weight of the web can be regulated in the desired mode across the web width.

This has been achieved so that the stocks M1,M2,M3 are made to flow out of their inlet headers J1,J2, J3 into the tube bank 11 placed after the inlet headers, into the individual tubes 11a1.1, 11a1.2...;11a21, 11a2.2... inthe tube bank and further, out of said tubes in the tube bank 11 into intermediate chambers E1,E2,E3 related to the stocks M1,M2 and M3, and out of the intermediate chambers El,E2,E3 into the <BR> <BR> <BR> turbulence generator 12, into its pipes 12a1.1, 12a1.2...;12a2.1;12a2.2..., and further into the slice cone 13, which comprises aprons 14a1 , 14a2 which extend substantially over the entire length of the slice cone 13. Further, in the slice cone 13, between the two pipes in the vertical row in the slice-cone block or layer provided for each stock M1,M2,M3, an intermediate apron 15a1,15a2,15a3 has been fitted, which is substan- tially shorter than the aprons 14a1,14a2 that are placed between the blocks and divide the blocks and that extend substantially over the entire length of the slice cone 13 and become narrower towards their ends.

In the headbox in accordance with the invention, by means of the inlet plate 160, the stocks M1,M2 and M3 are passed out of their headers J1,J2 and J3, in the way indicated by the arrows L2',L2" and L"', into the flow pipes 16a1.1,16a1.2...; 16a2 16a22... in the inlet plate 160. From the plate 160 the flow is passed into the ducts provided with corresponding numerals in the distribution plate 16, and in said distribution plate 16 the dilution liquid L1 is divided into each pipe in the vertical row of pipes so that the dilution liquid L1 is divided uniformly into all of the stocks M1,M2 and M3. From the distribution plate 16 the combined stock flow and dilution <BR> <BR> water flow is passed into the tubes 16a1.1, 16a1.2...; 16a2.1,16a2.2... in the tube bank 11. From the tubes in the tube bank the stocks M1,M2 and M3 and the dilution flow L1 combined with them are passed into the intermediate chamber compartments

E1,E2 and E3. The intermediate chambers E1,E2 and E3 are separated from one another in the vertical direction by means of horizontal walls g1 and g2 extending across the width of the headbox. From the intermediate chamber compartment E1 the stock is passed into the pipes 12a11,12a12...; 12a2.1,12a2.2 ; 12a3 1v12a3 2* in the turbulence generator 12. From the middle compartment E2 the stock is passed <BR> <BR> <BR> <BR> into the middle pipes 12a1 3,12a1 4; 12a2 3,12a2 4. . . in the turbulence generator 12, and from the compartment E3 the stock is passed into the pipes 12a1 5,12a1 6; 12a2 5, 12a2 6 in the turbulence generator 12. From the turbulence generator 12 the stocks M1,M2 and M3 and the dilution waters added to them are passed further into the spaces between the main aprons 14a1,14a2 in the slice cone 13 and further onto the forming wire H. In a headbox in accordance with the invention, it is further possible to use an adjustable top slice bar K at the end of the slice cone 13.

As is shown in the figure, the inlet headers J1,J2,J3 form a construction enclosed by a unified frame R. Between the inlet headers J1 and J2 there is a partition wall C1 passing horizontally in the figure between the frame R and the inlet plate 160.

Similarly, between the inlet headers J2 and J3 there is a horizontal partition wall C2 passing between the frame R and the inlet plate 160.

The figure illustrates a headbox by whose means it is possible to manufacture three- layer paper. It is obvious that the invention is not supposed to be confined to the embodiment described above only, but the headbox may also consist of a construc- tion unit that manufactures two-layer paper and comprises two inlet headers J1 and J2 only. In such a case the distribution plate 16 divides the dilution liquid into the stocks M1 and M2. It is obvious that the invention is also suitable for use in multi- layer headboxes in which there are more than three inlet headers J1,J2,J3,J4... for more than three different stocks M1,M2,M3,M4...

Fig. 1B shows the construction of Fig. 1A viewed from above. As is shown in Fig.

1B, the dilution liquid is distributed through the valves V1,V2.. VN (in Fig. 2B) into the multi-layer headbox in accordance with the invention into different points of width of the headbox into the vertical ducts 17a1,17a2... and further into the tubes

16a1 1,16al 2 ; 16a2 1,16a2 2... in view of regulation of the basis weight of the web. The dilution liquid can be dilution water, for example fibrous or clear water or water taken from the wire as retention. As a rule, the concentration of the dilution liquid differs from the concentrations of the stocks M1,M2 and M3.

In accordance with the invention, the dilution liquid (arrow L1) is distributed through a valve V1,V2... (in Fig. 2B) placed at each point of width into connection with the multi-layer headbox 10 to a certain point of width into all the layers in the multi- layer headbox 10, i.e. both into connection with the stock M1 shown in the figure and into connection with the stock M2 and into connection with the stock M3 to regulate the basis weight of the multi-layer web to be formed across the web width.

Fig. 2A is an illustration on a larger scale of the supply of the dilution liquid into a multi-layer headbox in accordance with the invention. As is shown in Fig. 2A, a separate distribution plate 16 is employed, which is fitted after the inlet headers J1,J2 and J3 in the stock flow direction L2 ahead of the tube bank 11. In front of the distribution plate 16 there is an inlet plate 160. Said separate distribution plate 16, which is fitted in connection with the tube bank 11, comprises flow ducts 17a1, 17a2... narrowing in the direction of flow of the dilution liquid. The dilution liquid L1 coming from the flow duct D1 and from the valve V1; from the flow duct D2 and from the valve V2, etc. flows into the connected narrowing duct 17a1,17a2..., from which the flow is distributed further, at the same time, into all of the tubes 16a1 1 <BR> <BR> <BR> 16a12...; 16a2 1,16a2 2. . . in the vertical line of tubes connected with each particular valve V1,V2... In this way, through the duct 17a1,17a2... connected to the outlet side of one valve V1,V2... at the distribution plate 16, the dilution water is distrib- uted into all the stocks M1,M2 and M3 in the multi-layer headbox.

Fig. 2A shows the duct 17a1 connected with the valve V1 partly in section, which duct becomes narrower towards its end. By means of said narrowing shape, the flow can be distributed in the desired way and preferably uniformly into all the flow tubes 16al 1,16a1.2 and 16razz in said vertical row of tubes. Similarly, through the valve V2, the dilution liquid is distributed into the tubes 16a2 16razz and 16a2 3 etc.

Fig. 2B is a sectional view taken along the line Il-Il in Fig. 1A. In the way shown <BR> <BR> <BR> in Fig. 2B, the branch ducts 18a1 1,18al 2,18a1 3... are opened into the narrowing duct 17a1. In the way shown in Fig. 2B, the branch duct 18a1 1 distributes the flow L1 from the narrowing duct 17a1 into the tube 16all of the stock M1, and similarly the branch duct 18razz distributes the flow L1 of dilution liquid from the narrowing duct 17a1 into the tube 16razz of the stock M2, and similarly the branch duct 18a1.3 distributes the flow from the narrowing duct 17a1, from its lower end, into the tube 16a1.3 of the stock M3.

As is shown in Fig. 2B, the arrow L1 illustrates the introduction of the dilution flow from the valve V1 into the narrowing duct 17awl, and the arrows illustrate the <BR> <BR> distribution of the dilution flow into connection with the flows L2',L2" and L2"' "' that come from the inlet headers J1,J2,J3 of the stocks M1,M2,M3.

The headbox 10 is a construction in which the pipe systems and the headbox con- structions related to the treatment of each stock M1,M2,M3 are similar to one another, for example, in respect of their pipe numbers and pipe sizes. In this way differences between the flow velocities in the different layers are excluded, which differences would further result in impurity of layers. For example, the number of tubes provided for the stock M1 in the tube bank 11 is equal to the number and size of tubes provided for the middle stock M2, and similarly the number and size of the tubes for the stock M3 of the other surface layer are equal to those of the preceding layer. Similarly, the constructions provided for the stocks M1,M2 and M3 are similar to one another at the intermediate chamber and the turbulence generator. The aprons 14a1,14a2 in the slice duct extend preferably over the entire length of the slice duct and become evenly narrower towards their ends. Further, in each layer in the slice duct 13, the headbox in accordance with the invention comprises intermediate aprons 15a1, 15 a2, 15a3, which are substantially shorter than the main aprons 14a1,14a2.

Fig. 3 illustrates the single-stock system connected with the multi-layer headbox in accordance with the invention. The stocks M1,M2 and M3 to be passed into the inlet headers J1J2 and J3 have been brought from the same stock tank F placed in

connection with the multi-layer headbox. The stocks M1,M2 and M3 have been formed out of one common basic stock M by to the basic stock M adding the admixtures/fillers required by the layers and, thus, by each individual stock M1,M2 and M3. Thus, the basic stock M is passed from the stock tank F by means of a pump P and branched into the branch lines e1,e2,e3,e4, e5, after which the fillers and/or retention agents are added at the points b1 and b2 in compliance with the requirements of each stock M1,M2andM3.

Figs. 4A and 4B illustrate an embodiment of the invention in which the dilution liquid is passed from its header T through the set of valves V1, V2... into the ducts D1, D2... and to the desired locations along the width of the headbox, always depending on the regulation setting of the valve V1, V2... The dilution liquid, preferably dilution water, is passed at the same time, for example, in the way illustrated in Fig. 4A, through the duct D1 into connection with the stock flows L2' and L2"' passed from the stock inlet headers J1 and J3. Thus, as is shown in the figure, for example, the dilution flow L1 is passed through the duct D1 out of its header T, at the same time, through one regulation valve (the valve V1 as shown in the figure), into at least two stock layers, in the way shown in the figure, into connection with the stocks M1 and M3 that form the surface layers in the three-layer headbox. The stock M2 is passed out of its inlet header J2 as the flow L2" so that, in the embodiment shown in the figure, no dilution liquid L1 is passed into connec- tion with said middle flow L2".

Thus, as is shown in Fig. 4A, the dilution liquid L1 is first passed into the duct D1 out of its header T, and after that through the duct 16 into the narrowing duct 17awl and from it through the branch ducts 18a1 and 18razz into the stock flows L2' and L2"' of the stocks M1 and M3 flowing in the ducts 16a1 1 and 16a1 3.

Thus, it is an essential feature of the embodiment of Figs. 4A and 4B that the dilution liquid L1 is passed, being regulated by one regulation valve V1 and/or V2 ... and/or V3 ... and/or Vn, into the duct D1 and/or D2... connected with said valve, and said dilution liquid is passed out of its header T, at the same time, into

at least two stock layers and, in the embodiment shown in the figure, into connection with the stock flows L2' and L2"' that form the surface layers M1 and M3 in the three-layer headbox. The stock flows L2' and L2"' are passed into the ducts 16a1 1 and 16razz out of their inlet headers J1 and J3, and in said ducts 16all and 16a1:3 the dilution liquid L1 is passed, in the way shown in the figure, out of the narrowing duct 17a1 into connection with said flows L2, and L2',. The combined flow of stock flow and dilution liquid is passed further into the tube bank 11 and further in the headbox construction.

Figs. 5A and 5B show an embodiment of the invention which is in other respects similar to the embodiments shown in the preceding figures, but in which the dilution liquid L1 is passed into connection with the flow L2' of the stock M1 that forms a surface layer and into connection with the flow L2" of the stock M2 that forms the middle layer. In this embodiment, no dilution flow L1 is passed into connection with the stock flow L"'. Also in this embodiment, there is a set of valves V1, V2...

fitted across the width of the headbox, in which case the dilution flow L1 is passed from the header T of the dilution liquid, preferably dilution water, in the desired way through the valve V1 and/or V2 and/or V3... into the connected duct D1 and/or D2... and further into the narrowing duct 17a1 and/or 17a2, and from said duct the flow is branched at the same time into connection with the flows L2' and L2" of the stocks M1 and M2 passed from the stock inlet headers J1 and J2.