HEAD BOX FOR A PAPER-MAKING MACHINE

Abstract: A paper machine head box includes perforated plates (2, 5) being used as a distributor and a turbulence generator and having flow ducts (3, 6), of which the ducts or channels at the edges are of different size than the remaining flow ducts. According to a preferred embodiment, slides (11), which are adjustably displaceable sideways, are provided at the ends of slice ducts at both edges of the perforated plates in order to enable the flow to be regulated. The slides and the perforated plates define the outermost flow ducts (16) of the turbulence generator and the distributor.

Description: Technical Field. The present invention relates to a head box for a paper-­ making machine which head box includes a turbulence generator and/or a distributor comprising a perforated plate having a large number of flow ducts in rows extending across the head box. Prior Art Because of the retarding friction caused by the side walls of the head box, less pulp flows through the slice of the head box onto the edges of the wire than onto the center. This results in a decrease of grammage at the edges of the web and in quite wide trimmings. To avoid the problem, Finnish Patent Specification No.30095 for example suggests that pulp be added to the edges of the wire from outside. This, however, usually creates an edge-wave as a flow fed-in from a side, even if guided by the design of the nozzle, is never precisely parallel with the head box flow. Disclosure of Invention An object of the present invention is to provide a head box construction which eliminates or minimizes the above disadvantages. The head box according to the invention is substantially characterized by the feature that the size of the flow duct at the edges is different from the size of the remaining ducts in the perforated plate. According to the present invention there is provided a head box for a paper making machine including a turbulence generator and/or a distributor comprising a perforated plate having a plurality of flow ducts in rows extending through the head box, characterized in that the size of the flow ducts in at least one edge area is different from the size of the remaining flow ducts in the perforated plate. According to a preferred embodiment of the invention both sides of the perforated plate are provided with slides which are sidewardly displaceable and form with the perforated plates the laterally outermost flow ducts of the perforated plate and/or the turbulence generator. Whilst the need to adjust the cross sectional area of the flow ducts is in general small, approximately 0.5 to 2 %, the apparatus according to the inventions does allow a remarkably wider adjustment range. Brief Descriptions of Drawings: The invention will be described further, by way of example, with reference to the drawings, in which :- Fig. 1 is a longitudinal cross section through a paper making machine head box according to the invention; Fig. 2 is a section along line A - A of Fig. 1; Fig. 3 is a section along line B - B of Fig. 1; Fig. 4 is a section along line C - C of Fig. 1; Fig. 5 is a section along a line C - C of a modified embodiment; and Fig. 6 is a section along line D - D of Fig. 1. Modes of Carrying out the Invention. In the drawings there is illustrated a cross header 1 of a head box for a paper making machine and by means of which pulp is introduced into the head box of Fig. 1. From the cross header 1 pulp flows to an apertured block or perforated plate 2 extending through the head box and having several superimposed stepped or offset rows of flow ducts 3 extending in the machine direction. Perforated plate 2 is used as a distributor for spreading the pulp supplied into the head box evenly over the whole width of the machine. Flow ducts 3 lead to an expansion chamber 4. An apertured block or perforated plate 5,which is used as a turbulence generator and has flow ducts 6 of appro­ priate design, is disposed in the flow direction downstream of the expansion chamber 4. Flow ducts 6 lead to a slice duct 7 from which pulp is discharged as a spray through a slice 10 onto a wire 9 running over a breast roll 8. As is illustrated in Figs. 2 to 6, both edges of the perfora­ ted plates 2 and 5 are provided with slides 11 and 12 which can be adjustably displaced sideways by means of screws 14 in a side wall 13 of the head box. The slides 11, 12 are displaceable towards and away from plates 2, 5. The slides 11, 12 are displaceably disposed in recesses 15 in the side walls 13. The distributor (2), the turbulence generator 5 and the slides 11, 12 disposed at their edges together with the outermost flow ducts 16 and 17 of the turbulence generator constitute a control device. The edges of the perforated plates 2 and 5 are provided with projections 18 and 19 and grooves 20 and 21 defined by the projections between them; the slides 11 and 12 are provided with gliding projections 22 and 23 disposed in a corresponding position between the grooves so that the thus formed inter-disposed castellated portions define flow ducts 16 and 17 between the perforated plates and the slides. The width H1, H2 and H3 and thus the cross-section area of the flow ducts and further the volume of pulp flowing through the ducts 16, 17 can be controlled by moving the slides 11, 12 sideways. In the alternative embodiment illustrated in Fig. 5, the bottom 24 of the grooves 21 between the projections 19 of the perforated plate 5 is curved. Also the end sur­ faces 25 in the projections 23 of the slide 12 are curved so as to form flow ducts 17 with two rounded sides. Industrial Applicability: The apparatus according to the invention allows accurate adjustment of the edge flow in the head box and permits the provision of an even flow of pulp through the slice onto the whole width of the wire of the paper making machine. Adjustment of the edge flow does not cause transverse flows since the flow in the edge ducts is parallel with the main flow. Usually the flow ducts are wider at the edges than the other ducts, but in special cases it may be advantageous to design them smaller. The invention is not limited to the embodiments presented here as examples only, but it can be modified within the scope of protection defined by the patent claims. Thus, for instance, the slides 11 and 12 can be provided as one part or they can be composed of parts which are disposed one on top of the other and are movable in relation to each other.

Claims: 1. A head box for a paper making machine including a turbulence generator and/or a distributor comprising a perforated plate (2, 5) having a plurality of flow ducts (3, 6) extending through the head box, characterized in that the size of the flow ducts in at least one edge area is different from the size of the remaining flow ducts in the perforated plate. 2. A head box as claimed in claim 1, characterized in that slides (11, 12) are provided in both edges of the perforated plates and said slides (11, 12) are movable sidewardly and form with the perforated plates the outermost flow ducts (16, 17) of the distributor and/or the turbulence generator. 3. A head box as claimed in claim 2, characterized in that the edges of the perforated plates (2, 5) are provided with projections (18, 19) and grooves (20, 21) are defined between said projections, and corresponding gliding projections (22, 23) of the slides (11, 12) are disposed in said grooves so as to define flow ducts (16, 17) between the perforated plates and the slides, the width (H) of the ducts being adjustable. 4. A head box as claimed in claim 2 or 3, characterized in that the slides (11, 12) are movable with screws 14 disposed in side walls (13) of the head box. 5. A head box as claimed in claim 2, 3 or 4, characterized in that the cross section of the outermost flow ducts (16, 17) is square. 6. A head box as claimed in claim 2, 3 or 4, characterized in that two sides of the outermost flow ducts (16, 19) are rounded. 7. A head box as claimed in any of claims 1 to 6, characterized in that the slides (11, 12) are slidingly disposed in recesses (15) in the side walls (13) of the head box.

Inventors: EVALAHTI, OSMO, Evälahti, Osmo
Applicants: VALMET-AHLSTROM INC.
Application year: 1986
Main classification: D21F1
IPC codes: D21F1