METHOD FOR FORMING WEB FROM AEROSUSPENSION OF FIBROUS MATERIAL

Abstract: A process for forming a web by aerodynamic method comprises feeding an aerosuspension onto a moving forming wire positioned at an inclination angle alpha relative to the aerosuspension flow and removing an air component of the above aerosuspension by means of a suction device located on a backside of the wire, with the incline angle alpha being selected by the equation arcctg(Vw/Vf + 2ROOT eta ) < alpha < arcctg(Vw/Vf - 2ROOT eta ), where: Vw - velocity of forming wire; Vf - air filtration rate through a layer of formed web and wire provided by a suction device at a steady stationary airflow from the aerosuspension into the suction device. As a result, a possible shear of the formed web caused by the aerodynamic pressure of aerosuspension flow is prevented, which permits production of paper having a high degree of uniformity.

Description: METHOD FOR FORMING WEB FROM AEROSUSPENSION OF FIBROUS MATERIAL FIELD OF THE INVENTION The 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 Formation of a web as a fibrous layer on a continuous moving wire by feeding aerosuspension of cellulose, or any other fibers prepared beforehand, is one of the main technological operations in the paper- making process. The web, thus formed, is then pressed and dried. Methods for forming a web from aerosuspension of fibrous material are known (see, USSR Author's Certificate No. 1172975, 4 D 21 H 5/26, published 08/15/85, Information Bulletin No. 30). According to such a method, 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 suction device is generally referred to as a suction box, and 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. 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, although such a procedure is being difficult and costly. According to another method for forming a web from aerosuspension of fibrous material (USSR Author's Certificate No. 746015, 2 D 21 H 5/26, published 07/07/80, Information Bulletin No. 25), the aerosuspension is fed onto a moving 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. In this option, the wire positioned at an angle to the direction of aerosuspension flow decreases the web velocity component that is normal to the aerosuspension flow direction. This allows the wire velocity to be increased without causing the above shear of the formed web by the aerosuspension flow. An extra removal of a certain part of the air component of the aerosuspension when the latter passes through a feeding channel before reaching the forming wire is also provided for. Thus, one of the main problems of such methods, namely removal of a large volume of air from the aerosuspension, is solved. This solution, however, causes a portion of the 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 may lead to fiber conglomeration and, consequently, to non-uniformity of the formed layer. The main capacity limitation of the above method is related to the fact that the fibers deposit on the forming wire at a low velocity of the aerosuspension flow. To increase the rate of web formation, the velocity of aerosuspension flow fed to the wire must be dramatically increased, which, however, raises the aerodynamic pressure exerted on the web under formation by the aerosuspension flow. The above aerodynamic pressure of the aerosuspension flow may shear the layer of fibers formed on the wire, the shear being either coincident or opposing to the direction of wire motion, depending on the ratio of wire and flow velocities and the wire inclination angle relative to the aerosuspension flow. As a result, non- uniformities occur in the formed layer that lower the quality of produced paper. According to another method for forming a web from aerosuspension of fibrous material (US Patent No. 4375448, 3 B 29 J 5/00, published 03/01/83 - prototype), the aerosuspension is fed onto the forming 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. This method is believed to be the closest one to the present invention. In this method the angle of the wire incline relative to the aerosuspension flow is selected based on the required thickness of the layer to be formed and the capacity of a fiber- feeding device. However, a high rate forming entailing a substantial rise in aerodynamic pressure of the aerosuspension flow upon the web being formed requires a selection of a velocity ratio of wire motion and the aerosuspension flow, as well as of the angle of wire inclination in relation to the aerosuspension flow direction while taking into consideration a possible web shear under the aerodynamic pressure of the aerosuspension flow. Otherwise, the density of the formed web will not be uniform. SUMMARY OF THE INVENTION According to the invention, a method for forming a web from an aerosuspension of fibrous material, the method comprising the following steps: feeding the aerosuspension onto a moving forming wire positioned at an angle 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 an inclination angle ^ formed between the forming wire and a direction of the aerosuspension flow is selected according to the equation: where: V. velocity of forming wire motion; V f - rate of air filtration through a layer of formed web and wire provided by the suction device at a steady stationary airflow from the aerosuspension into the suction device; ^ - coefficient of sliding friction of fibers against the forming wire. BRIEF DESCRIPTION OF THE DRAWING The Figure illustrates structural elements of a forming unit of a paper- making machine and a process of web formation from aerosuspension, according to an embodiment of the present invention DETAILED DESCRIPTION OF THE INVENTION Referring to the Figure, an aerosuspension is fed in a continuous flow through a feeding channel 1 onto a moving wire 2. As the wire 2 moves, a layer of fibers 3 is being formed on the wire 2 and transported by the wire 2 for further processing. An air component of the aerosuspension is filtered through the layer of fibers 3 and openings in the wire 2, and is sucked through a suction box 4 located on a backside side of the wire 2. The drawing also shows an inclination angle ^ of a direction of motion of the forming wire 2 relative to the direction of aerosuspension flow, a velocity of the aerosuspension V a , and a velocity of the forming wire V . Force F P generated by an aerodynamic pressure of the aerosuspension flow acts upon the layer of fibers 3 being formed, with a F force component " perpendicular to the plane of wire 2, and a force component F ' parallel to the plane of the wire 2,. The force component F F n presses the layer of fibers 3 to the wire 2, and the force component ' produces a shearing effect upon the layer 3. To prevent the shear of the fibrous layer 3 formed on the wire 2, the following condition must be met: F ' < r 'ι F " , (2) where ^ j s the coefficient of sliding friction of fibers against the forming wire. The component F ' of a force vector F P of the aerodynamic pressure of the aerosuspension flow is proportional to a squared difference between the wire velocity V w and the aerosuspension flow velocity component V ' parallel to the wire plane: F, = 0,5 - C„ - S Pa - (V, - V w f where: c w is the coefficient of aerodynamic resistance of fibers in layer 3; t is the cross-section area of fibers; Pa is the air density. F F The component " of the force vector P of the aerodynamic pressure of the aerosuspension flow is proportional to a squared velocity component of aerosuspension flow V " perpendicular to the wire plane: F n =0,'5-C w -S i.- rpa -V n (4) S o i-nce V l t -V n a -cos and . V "„=V„ a sma t ,h,e i .nequal .i.t.y ( / 2), wi .t.h. (3) and (4) taken into account, can be transformed as: ctga-[η < -x- < ctg + [η (5) The velocity component v " of aerosuspension flow is equal to the V rate of air filtration J_ through a layer of formed web 3 and wire 2 at a steady stationary airflow from the- aerosuspension into the suction device. The filtration rate v f is defined by the suction device's capacity and also depends on the air-tightness of forming unit design. With the equality of velocities V " and _ Vf_ taken into consideration, expression (5) transforms into a ratio (1) used for selecting angle depending on given technological parameters: filtration rate - J determined by the capacity of the suction device and the degree of airtightness of the forming unit; wire velocity - V w dependent on the capacity and parameters of the layer being formed; coefficient of sliding friction of fibers against the forming wire - f A proper selection of the incline angle -^ within the specified range prevents a possibility of web shearing caused by the aerodynamic pressure of aerosuspension. Implementing the invention claimed will permit realization of a highly productive technology for making paper with a high degree of uniformity. The above result achieved through use of the method claimed has been experimentally verified. Samples of paper were made on an aerodynamic forming device at V V rv different ratios of * / and ^,, with the samples uniformity examined. Parameters V _ and V f were modified to verify the claimed ratio, since the design of the above device provided for setting the angle -^ at 20° or 60 ° only. The coefficient of sliding friction of fibers against the forming wire ^ was experimentally determined to be 0.2. The above parameters were set within such a value range that the angle ^ (equal to 20° or 60°) was either within the range provided for by the ratio (1), or was larger or smaller than the values of this range. The results of the experiment are summarized in the table below. The uniformity of the formed web was evaluated by weighing. A formed sample of web was pressed on a roller press at the rate of 0.03 m/s with a roller pressing force of 50 kg/cm. A pressed sample of 10x10 cm 2 size was dried at 105-C and cut into pieces of 2x2 cm 2 that were weighed on analytical balance featuring the error of less than ±0.0005. A degree of non-uniformity of the formed web was determined as percentage of root- mean-square deviation in weight of samples vs. their average weight: n - x where X ι; - weight of / ' -th piece; X ^ - average weight of pieces; ^- number of weighed pieces (^j^ ^). The experimental results obtained fully confirmed the above result achieved by applying the method claimed. Shearing of the formed web along the wire is prevented by setting the wire at angle -^ selected within the claimed range, which permits production of paper of a high degree of uniformity. 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, the method comprising the following steps: (a) feeding the aerosuspension onto a first side of a moving forming wire positioned at an inclination angle -^ relative to the aerosuspension flow, and (b) removing an air component of the aerosuspension by means of a suction device located proximate to a second side of said forming wire which is opposite to said first side, wherein said inclination angle -^ formed between the forming wire and a direction of the aerosuspension flow is selected according to an equation: V where: - velocity of the forming wire; V f - air filtration rate through a layer of formed web provided by a suction device at a steady stationary airflow from the aerosuspension into the suction device; ^ - sliding friction coefficient of fibers against the forming wire. 2. An apparatus for forming a web from an aerosuspension of fibrous material comprising: a forming wire for transporting the aerosuspension of fibrous materials thereon; a suction device for removing an air component of the aerosuspension of fibrous materials being transported by said forming wire; and a feeding channel for directing said aerosuspension of fibrous materials onto said forming wire, said feeding channel being inclined at an angle -^ relative to said forming wire; wherein J^L is selected according to an equation: - j Γη) V where: - velocity of the forming wire; V f - air filtration rate through a layer of formed web provided by a suction device at a steady stationary airflow from the aerosuspension into the suction device; *1 - sliding friction coefficient of fibers against the forming wire.

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, D21H27