A METHOD FOR FORMING A WEB FROM AEROSUSPENSION OF FIBROUS MATERIAL

Abstract: A process and an apparatus for making paper by an aerodynamic method. An aerosuspension is fed onto a moving forming wire positioned at an angle relative to the aerosuspension flow. Then, an air component is removed by means of a suction device located on the opposite side of the wire. The aerosuspension feed and air removal are performed in a direct-flow channel in which the value of a suction force is set to increase in the direction of the forming wire motion so that the suction force applied to the fibrous material in the surface layer of the web being formed is approximately the same along the forming wire in the web formation zone. As a result, a low fiber concentration in the aerosuspension is maintained up to the moment the suspension reaches the forming wire followed by an effective air removal featuring no vortexes in the vicinity of wire, the fibers are reliably held on the wire, and the possibility of shear of the fibers falling onto the wire is excluded. The above factors, combined, enable production of paper with a high degree of uniformity at a high production rate.

Description: A METHOD FOR FORMING A WEB FROM AEROSUSPENSION OF FIBROUS MATERIAL FIELD OF THE INVENTION The present invention relates to making paper by an aerodynamic method, and more particularly, to a method of forming a web from aerosuspension of fibrous material on a forming wire. BACKGROUND OF THE INVENTION A method for forming a web from aerosuspension of fibrous material is known, in which the aerosuspension is fed onto a moving wire positioned perpendicular to the direction of the aerosuspension flow and an air component of the aerosuspension is removed by means of a suction device, the so-called suction box, installed on the opposite side of the wire. This method also provides for redistribution of the sucked airflow across the formed web width, which permits production of a web profile of various configurations. USSR Author's Certificate No. 1172975, 4 D 21 H 5/26, published 08/15/85, Information Bulletin No. 30. This method, however, is unable to produce a web of high uniformity at high production rates, since the aerosuspension flow shears the web being formed due to the interaction of the flow pressure and high velocity of moving wire. Generating a high vacuum in the suction box to enhance pulling of the web to the wire could prevent the web shear. However, this is difficult and costly. According to another conventional method for forming a web from aerosuspension of fibrous material, the aerosuspension is fed onto a wire positioned at an angle relative to the aerosuspension flow and an air component of the aerosuspension is removed by means of a suction device located on the opposite side of the wire, with the airflow sucked off from the opposite side of the wire being normal to its surface. USSR Author's Certificate No. 746015, 2 D 21 H 5/26, published 07/07/80, Information Bulletin No. 25 - prototype. This method represents the closest one to the invention claimed. In this method, the wire positioned at an angle relative to the direction of the aerosuspension flew decreases the web velocity component that is normal to the aerosuspension flow direction. This allows achievement of an increase in the web velocity without causing web shear. In addition, a removal of a certain part of the air component of the aerosuspension is also provided as the aerosuspension passes through a feeding channel before reaching the forming wire. Thus, one of the main problems of such methods, namely removal of a large volume of air from aerosuspension in order to provide a high production rate, is solved. This solution, however, results in a portion of fibers (1 - 5%) being carried away by the sucked air when the aerosuspension passes through the feeding channel. Besides, the concentration of fibers in the aerosuspension grows as the latter moves through the channel, which leads to fiber conglomeration and, consequently, to non-uniformity of the formed web. SUMMARY OF THE INVENTION The present invention provides an effective method for forming a substantially uniform web from an aerosuspension of a fibrous material. According to the present invention, a method of forming a web from an aerosuspension of fibrous material comprises feeding the aerosuspension to a moving wire positioned at an angle relative to the aerosuspension flow, and removing the air component of the aerosuspension by means of a suction device located on the opposite side of a wire, wherein the aerosuspension is fed and its air component is removed in a direct-flow channel. The claimed method provides for removal of air from the aerosuspension only after the aerosuspension reaches the wire, thus maintaining a low concentration of fibers prior to the specified moment, which makes the formed web highly uniform. The most favorable regime for an effective air removal that excludes occurrence of turbulent airflow in the vicinity of the wire, the so-called sub-wire vortexes, is created by feeding the aerosuspension and removing its air component in a direct-flow channel. However, the implementation of the claimed method can give rise to the following problem. The suction force F (Fig. 1) applied to layer-forming fibers (point "A" in Fig. 1) starts featuring two components F n , which is normal to the wire surface, and F t , which extends along the wire surface, The F n component presses the fibers falling onto the wire against the wire, thus holding the formed layer on the wire. The F, component produces a shearing effect upon the fibers that may shear them along the wire surface and produce a "dune" at this spot, when this component reaches the value exceeding the force of fibers friction against the wire. These considerations are true in relation to the entire surface layer of the formed web. So, concurrent problems arise from holding fibers on the forming wire by a vacuum generated by a suction device from its opposite side, and preventing the shearing of fibers along the wire surface under the action of the F t component. Solving this problem gives rise to a new problem: if the suction force is uniformly distributed across the suction channel, this being the simplest option from the standpoint of its technical realization, a non-uniform aerodynamic resistance to the sucked air occurs due to the growth in the formed web thickness. The above phenomenon leads to the following: if the suction force is set at a value adequate for holding the fibers on the wire surface at the beginning of the formation zone without their shear along the wire surface, the above value may turn out to be inadequate for holding the surface layer of the fibers at the end of the formation zone, since the suction force at the formed layer surface will decrease because of the aerodynamic resistance of the latter. And vice versa, if the suction force is set at a value adequate for holding the fibers on the wire surface at the end of the formation zone (with the above aerodynamic resistance of the formed layer taken into account), this value may turn out to be sufficient for generating a value of the F t component that will start shearing of fibers along the wire surface, thus creating the above-mentioned non-uniformity ("dunes") in the formed layer. To solve this problem, the claimed method additionally provides for setting a value of the suction force that increases in the direction of the forming wire motion so that the suction force applied to the fibrous material on the surface layer of the formed web is approximately the same along the forming wire in the web formation zone. So, maintaining a low fiber concentration in the aerosuspension until it reaches for the forming wire followed by an effective air removal is the technical result achieved through the application of the above newly introduced features, which ensures reliable holding of the fibers on the forming wire in the process of web formation and excludes a possibility of shearing of the formed web, thus, producing paper with a high degree of uniformity at a thigh produce rate. BRIEF DESCRIPTION OF DRAWINGS The Figure illustrates an arrangement for forming a web according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION Referring to the Figure, the aerosuspension is fed and its air component is removed in a direct-flow channel comprising a feeding section 1 , through which the prepared aerosuspension is fed, and a suction section, represented by a suction box 2 which contains a suction device, for example, an exhaust fan of an adequate capacity (not shown). The aerosuspension is fed onto an endless moving belt, or wire, 3 set at an angle to the aerosuspension flow direction. As wire 3 moves, layer 4 of a formed web 5 becomes gradually thicker, being held on the surface of wire 3 by a vacuum generated by the suction box 2. At the outlet of channel 1 , an endless wire 6 moving at the same speed as the wire 3 picks up formed web 5 and transports it to pressing and drying equipment (not shown) by holding the web between both wires (i.e., wire 3 and wire 6). Means for modifying the value of suction force along the direction of moving wire 3 are also shown in Fig. 1. A section of suction box 2 adjacent to the wire 3 is divided by partitions 7 erected consecutively one after another along a direction of the movement of the wire 3 and across its entire width. The partitions 7 form, together with the walls of the suction box 2, separate air ducts 8, the aerodynamic resistance of which is determined, to a large extent, by their length. Partitions 7 are equipped with turning plates 9 that can also modify the aerodynamic resistance of ducts 8. This provides the ability to set and maintain a maximum value of the aerodynamic resistance in a portion of that duct 8 which is adjacent to the point where the wire 3 enters the feeding section 1 , and a minimal value of the aerodynamic resistance in that duct 8 which is adjacent to the point where the wire 3 exits the feeding section 1. The specific value of suction force, as well as the number of ducts 8 and the ratio of their aerodynamic resistance, all being dependent on a number of technological parameters, such as width and length of the layer being formed, velocity of the forming wire, type of fibrous material, moisture content of fibers, etc., are selected individually for each specific case. While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims: WHAT IS CLAIMED: 1. A method for forming a web from an aerosuspension of fibrous material comprising the steps: (a) feeding the aerosuspension of fibrous material onto a moving forming wire placed at an angle relative to a direction of the aerosuspension flow; and (b) applying a fluid pressure differential to the forming wire in a forming zone, thereby removing an air component of the aerosuspension, wherein the aerosuspension is fed and its air component is removed in a direct-flow channel. 2. The method of claim 1 , wherein the step applying a fluid pressure differential further includes a step of applying the fluid pressure differential at an increasing level along the direction of the forming wire motion so that the fluid pressure differential applied to the fibrous material is approximately the same along the forming wire within the forming zone. 3. An apparatus for forming a web from an aerosuspension of fibrous material, the apparatus comprising: a forming belt continuously moving in a machine direction; a direct-flow channel comprising a feeding section and a removing section, said feeding and removing sections being separated from one another by said forming belt, and said feeding section being configured for directing a flow of aerosuspension to said forming belt; and a vacuum apparatus configured to apply a fluid pressure differential to said forming belt, thereby removing an air component of the aerosuspension into the removing section of said direct-flow channel. 4. The apparatus of Claim 3, further comprising a plurality of partitions disposed in the removing section of said direct-flow channel, said partitions being consecutively spaced from a first partition to a last partition relative to said machine direction, to form a plurality of air ducts between mutually adjacent partitions and between the partitions and walls of the removing section adjacent thereto, said plurality of air ducts being consecutively spaced in said machine direction from a first air duct to a last air duct, each of the air ducts being structured to maintain an air resistance. 5. The apparatus of Claim 4, wherein the air resistance of the said air ducts consecutively decreases from the first air duct to the last air duct. 6. The apparatus of Claim 4, wherein each partition from said plurality of partitions has a first end adjacent to said forming belt and a second end opposite to said first end, and a length between said first and second ends, said length decreasing consecutively from said first partition to said last partition. 7. The apparatus of Claim 4, wherein at ieast some of said partitions have turning plates articulably connected to the second ends of said partitions. 8. An apparatus for forming a web from an aerosuspension of fibrous material comprising: a forming belt continuously moving in a machine direction; a direct-flow channel comprising a feeding section and a removing section, said feeding and removing sections being separated from one another by said forming belt; said feeding section being configured for directing a flow of aerosuspension to said forming belt; said removing section comprising a plurality of air ducts consecutively spaced therein in said machine direction from a first air duct to a last air duct, each of said plurality of air ducts having an air resistance that increases from said first air duct to said last air duct; and a vacuum apparatus structured to apply a fluid pressure differential to said forming belt, thereby removing an air component of the aerosuspension into the removing section of said direct-flow channel. 9. The apparatus of Claim 8, further comprising means for controlling the air resistance of each individual air duct. 0. A method for forming a web from aerosuspension of fibrous material comprising the steps: feeding the aerosuspension of fibrous material onto a moving forming wire placed at an angle relative to the aerosuspension flow, and removing an air component of the aerosuspension by means of a suction device located on the opposite side of the wire, wherein the aerosuspension is fed and its air component is removed in a direct-flow channel.

Inventors: DROBOSYUK VIKTOR MIKHAILOVICH, DROBOSYUK, VIKTOR MIKHAILOVICH
Applicants: DROBOSYUK VIKTOR MIKHAILOVICH, PRAGMATIC VISION INC, DROBOSYUK, VIKTOR MIKHAILOVICH, PRAGMATIC VISION, INC.
Application year: 1999
Main classification: D21F9
IPC codes: B27N3, D21F1, D21F9, D21F11, D21H27