Abstract:
A method for removing dust-carrying air from a moving paper web is disclosed. The method includes the steps of: providing a moving web, the web having a first side and a second side, the web moving at a sufficient rate to produce a boundary layer of adjacent dust-carrying air; providing a NACA duct, the NACA duct having an intake opening and walls that diverge in increasing cross-sectional area to an exhaust opening having greater cross sectional area than the intake opening; and submerging the intake opening into the boundary layer to scavenge dust-carrying air from the boundary layer.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to systems and apparatus for dust and other particulate removal from the boundary layer of moving webs, including nonwoven and paper webs. 
       BACKGROUND OF THE INVENTION 
       [0002]    Paper machines, particularly machines making tissue paper such as toilet tissue, facial tissue, and paper towels, create substantial amounts of dust. Dust and other particulates gets carried in the boundary layer of a moving web but gets dislodged when the web is disturbed or changes directions. Dislodged dust that accumulates on the machinery can interfere with correct operation, lead to product quality problems in some circumstances, and can hinder or require maintenance. Additionally dust that is transferred into the air can also represent a fire hazard, and its inhalation can cause health problems for workers. 
         [0003]    Much effort has been directed to the development of dust hoods for vacuuming dust laden air from parts of such machines. However, such devices are themselves imperfect in operation and can require substantial power consumption as well as being the source of noise. 
         [0004]    One problem with methods involving vacuum applied to the web surface is that the vacuum, in addition to removing airborne fibers can partially dislodge fibers in the web, creating loose or loosened fibers which then can become airborne downstream from the vacuum area. 
         [0005]    There is thus a continuing need for a method and apparatus for removing dust in a power-efficient, environmentally friendly manner. 
       SUMMARY OF THE INVENTION 
       [0006]    A method for removing dust-carrying air from a moving paper web is disclosed. The method includes the steps of: 
         [0007]    providing a moving web, the web having a first side and a second side, the web moving at a sufficient rate to produce a boundary layer of adjacent dust-carrying air; 
         [0008]    providing a NACA duct, the NACA duct having an intake opening and walls that diverge in increasing cross-sectional area to an exhaust opening having greater cross sectional area than the intake opening; 
         [0009]    submerging the intake opening into the boundary layer to scavenge dust-carrying air from the boundary layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIGS. 1   a  and  1   b  are schematic representations of a typical NACA duct. 
           [0011]      FIG. 2  is a side view of one embodiment of an apparatus of the present invention. 
           [0012]      FIG. 3  is a top view of one embodiment of an apparatus of the present invention. 
           [0013]      FIG. 4  is a top view of one embodiment of an apparatus of the present invention. 
           [0014]      FIG. 5  is a side view of one embodiment of an apparatus of the present invention. 
           [0015]      FIG. 6  is a side view of one embodiment of an apparatus of the present invention. 
           [0016]      FIG. 7  is a perspective view of an embodiment of a NACA duct of the present invention. 
           [0017]      FIG. 8  is a side view of one embodiment of an apparatus of the present invention. 
           [0018]      FIG. 9  is a side view of one embodiment of an apparatus of the present invention. 
           [0019]      FIG. 10  is a side view of one embodiment of an apparatus of the present invention. 
           [0020]      FIG. 11  is a diagrammatic representation of a nested NACA duct arrangement. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    In a typical paper machine for making absorbent tissue, such as bath tissue, facial tissue, or paper towels there is a drying section typically in which the paper web is adhered to the surface of a rotating Yankee dryer and lead to a creping doctor blade. There, the web is creped off the Yankee dryer by the creping blade. The creped paper web can then be wound onto a reel, which is often referred to as a parent roll. At creping, and in other parts of the dry paper-making path, dust separates from the paper web. Part of this dust will be entrained in a boundary layer on each side of the creped web that can run forward at a velocity close to 25 m/s. This dust can become dislodged from the boundary layer and accumulate on the machinery. This accumulation can interfere with correct operation, lead to product quality problems, hinder maintenance, and may also present a fire hazard. Dust that is transferred into the air can also represent a fire hazard, and additionally can be breathed by workers. 
         [0022]    Similar problems with respect to dust and particulate creation and its removal are observed also in the converting of such paper webs, as well as in the manufacture and converting of other webs like nonwovens and other webs made of filaments. 
         [0023]    Accordingly, whereas the present invention can find beneficial application for removal of particulate-carrying air, including dust-laden air, on various web production and conversion applications, the invention will be described below primarily in its operates for catching and extracting at least a portion of the dust-laden air in a boundary layer of a moving paper web. Removal of particulate-carrying air, including dust-laden air, can be described as scavenging. 
         [0024]    The invention utilizes a NACA duct. NACA ducts are well known for the purpose of drawing off boundary layer air in moving vehicles without disrupting airflow otherwise. The design and construction of NACA ducts are well-known, for example, a description of NACA ducts can be found in the October 1945 National Advisory Committee for Aeronautics Advance Confidentiality Report #5i20 (NACA ACR No. 5i20) “An Experimental Investigation of NACA Submerged-Duct Entrances” by Charles W Frick, Wallace F. Davis, Lauros M. Randall, and Ernest A Mossman. This document is available on the interne as a downloadable web archive PDF file at http://naca.central.cranfield.ac.uk/report.php?NID=2176. 
         [0025]    Characteristic for a NACA-duct is an intake opening having a curved and divergent contour. The part of the intake opening which is submerged in the boundary layer can be configured as a ramp-like surface having an angle relative to an outer surface reference, such as, in the instant application, a moving web. There can be a sharp edge transition in between the outer surface reference and the inner ramp-like surface. A NACA duct contains as well an inlet profile adjacent the air intake. NACA duct functionality is based on the principle of generating rotating air vortices on the opening edges of the air intake, which help guide the boundary layer into the duct. 
         [0026]    In the present invention the term “NACA duct” includes NACA ducts having curvilinear-shaped intake opening sidewalls, including curvilinear-shaped according to the dimensions disclosed in the above-mentioned October 1945 National Advisory Committee for Aeronautics Advance Confidentiality Report. As used herein, the term NACA duct also includes ducts having substantially straight intake opening sidewalls. Ducts having substantially straight intake opening sidewalls can approximate NACA ducts having curvilinear-shaped intake opening sidewalls. In plan view, in a substantially straight walled version, the substantially straight sidewalls of a NACA duct form a trapezoidal shape, with opposite lengthwise sidewalls diverging from a relatively short upstream wall to a relatively long downstream wall. 
         [0027]      FIG. 1   a  shows a sectional view of a typical NACA air intake. An intake opening  4  extends down to a ramp-like inlet surface  6 . An airduct  1  joins the ramped inlet surface  6  with a profiled edge  8  and directs the air from the environment into this airduct. The airflow  3  passes the intake opening  4  and enters the airduct  1 , with only minimal disturbance of the airflow. 
         [0028]      FIG. 1   b . shows a top view of the opening  4 . The divergent opening contour  5  is apparent, where the ramped inlet surface  6  has typically the same contour. Vertical sidewalls  7  of the opening  1  defined by the contour of the opening  5  and the ramped inlet surface  6  are primarily perpendicular to the base surface  2 . The airflow  3  passes the opening  4  and enters by the formation of counter rotating vortices  9  in the airduct  1 . 
         [0029]    In an embodiment of the invention shown in  FIG. 2 , a system and apparatus  10  of the present invention includes a NACA duct  12  in operational proximity to a moving web  14 . NACA duct  12  is shown in cross-section to better indicate its operation. Moving web  14  has a boundary layer  16  on each side thereof, the boundary layer having a thickness related to the speed of the moving web by well known equations relating to the Reynolds number of air. For current processes on commercial paper machines, the boundary layer for a paper web running at about 700 m/min can be from about 1 mm to about 25 mm thick, i.e., the boundary layer can extend from 1 mm to about 25 mm perpendicularly from the surface of the web  14 . The boundary layer can be about 5 mm, 7 mm, 9 mm, 11 mm, 13 mm, 15 mm, 17 mm, 19 mm 21 mm or 23 mm thick. 
         [0030]    NACA ducts have an intake opening  18  (corresponding to intake opening  4  of  FIG. 1   a ) having walls that diverge in increasing cross-sectional area to an exhaust opening  20  having greater cross-sectional area than the intake opening. A smooth, rounded edge  22  allows a smooth transition of air passing the NACA duct, permitting some of the boundary layer to smoothly enter toward exhaust opening  20 , and some of the air to pass relatively undisturbed. As the boundary layer traverses the intake opening it is guided over the angularly oriented diverging walls to create rotating vortices directed away from the web. These rotating vortices carry dust-laden air to the exhaust opening. A NACA duct positioned for effective operation to effectively remove a portion of the air of a boundary layer of a moving web can be said to be disposed in operation relationship to the moving web. 
         [0031]    In an embodiment dust removal can be aided by a partial pressure, such as by vacuum, at the exhaust opening  20 . Vacuum can be supplied via known vacuum means, and can be balanced such that the mass balance of air entering the intake opening and air exiting the exhaust opening remains substantially equal. A vacuum generating apparatus can be situated relatively closely to exhaust opening, or exhaust can be effected via ductwork and/or manifolds such that the vacuum generating apparatus can be situated remotely and supply vacuum via the ductwork and/or manifolds. 
         [0032]    A NACA duct  12  is positioned in operational proximity to the moving web, which means the NACA duct is positioned in a non-contacting relationship to the paper web moving in a machine direction (MD), and that its inlet  18  is submerged in the dust-carrying boundary layer  16  with the narrowest portion of the intake opening being positioned upstream with respect to the MD. When positioned in operational proximity there is no direct contact with the moving web and no normal forces are applied to the web by the NACA duct, both conditions of which tend to produce more dust by virtue of disturbing fibers on the web. For example, normal forces applied by vacuum or shear forces from web-contacting components contacting a moving web can partially dislodge fibers that later become airborne, or fully dislodge fibers that are not removed upon separation from the web. Further, web-contacting portions of web handling equipment, including dust-removal equipment, disrupts the laminar flow of the boundary layer, causing additional dust-laden air to be directed out of the boundary layer. Dust from such re-directed dust-laden air can then settle on equipment or remain airborne as an environmental concern. 
         [0033]    Although  FIG. 2  shows a NACA duct on only one side of a moving web, a NACA duct can be placed on both sides of a moving web as shown in  FIGS. 8 and 9 , described in more detail below. In addition, as shown in  FIGS. 3 and 4 , a plurality of NACA ducts can be utilized. In the embodiment shown in  FIG. 3 , a series of closely spaced NACA ducts  12  can be disposed across a portion of the width of web  14 , and can be disposed substantially across the entire width of web  14 . 
         [0034]    Because the widest portion of the intake opening  18  of each NACA duct can be relatively narrow in a direction corresponding to the width, or cross direction (CD) of web  14 , in another embodiment, as shown in  FIG. 4 , a plurality of NACA ducts  12  can be staggered in CD-oriented rows of substantially side-by-side NACA ducts  12 , thereby increasing the area of total web boundary layer impacted by the NACA ducts. While two CD-oriented rows are shown in  FIG. 4 , in other embodiments, more than two CD-oriented rows can be employed as desired. In general, the size and spacing of NACA ducts  12  can be selected to ensure substantially 100% of the CD of the web  14  is covered by a NACA duct intake opening  18 . 
         [0035]    As shown in  FIG. 5 , in an embodiment, the NACA duct  12  can have on its upstream edge a converging plate  24  that can span in a width-wise dimension at least the width of intake opening  18 . Converging plate  24  can have sufficient length and can be angled sufficiently with respect to the plane of moving web  14  such that leading edge  22  can be outside of the boundary layer. In general angle θ can be from about 10° to about 50°. Converging plate  24  enhances the operation of the NACA duct by smoothly diverting more of the boundary layer into intake opening  18 . 
         [0036]    In an embodiment, the dust removal system and apparatus of the present invention can be utilized at a position of the web path in which the web is turning over a roller. A moving web going over a roller can be more stable, e.g., less prone to flutter, than a web spanning a free span. The added web stability imparted by a moving web in tension traversing a roller can be beneficially utilized by the NACA duct of the present invention by allowing the NACA duct to be placed closer to the web surface without inadvertently contacting the web surface. Additionally, the centrifugal forces imparted on the particles on the outer surface of the web will increase the effectiveness of this arrangement. As shown in  FIG. 6 , moving web  14  can move in a machine direction (MD) over a roller  26  such that the web path is changed. The change in web path can be from 10° to about 180°. A NACA duct  12  can have a shape such that the NACA ducts can conform substantially to the curvature of the web  12  around roller  26 . 
         [0037]    An embodiment of a NACA duct, specifically a NACA duct  12  as depicted in  FIG. 6 , is shown in  FIG. 7 .  FIG. 7  shows a NACA duct  12  from a perspective of looking at the web-facing surface. Three NACA ducts  12  are shown in a substantially side-by-side relationship.  FIG. 7  shows the convergence plate  24 , the diverging sidewalls of each intake opening  18 , as well as the exhaust openings  20 . Although  FIG. 7  shows a curved version of the NACA ducts  12  of the present invention, the same structure(s) is/are present in a flattened version, as depicted in  FIG. 2 . 
         [0038]    In an embodiment of the invention,  FIG. 8  shows an arrangement of two NACA ducts  12 , one on each side of a moving web  14 , the web  14  moving into a nip roll arrangement  30 . Nip roll arrangement  30  has two rolls,  32  and  34  between which web  14  traverses. Nip rolls  32  and  34  can be calendar rolls, emboss rolls, or any other of typical nip rolls used in web forming processes. The advantage of placing NACA ducts before a web enters the nip of nip rollers is that the dust-laden air in the boundary layer can be scavenged before the boundary layer is disrupted by the nip roll arrangement  30 . 
         [0039]    In another embodiment of the invention,  FIG. 9  shows an arrangement of two NACA ducts  12 , one on each side of a moving web  14 , the web  14  moving away from a nip roll arrangement  30 . Nip roll arrangement  30  has two rolls,  32  and  34  between which web  14  traverses. Nip rolls  32  and  34  can be calendar rolls, emboss rolls, or any other of typical nip rolls used in web forming or converting processes. These types of process typically liberate new dust from the web material which is then carried within the newly formed boundary layer after the nip. The advantage of placing NACA ducts after a web exits the nip of nip rollers is that this new dust that enters the boundary layer can be scavenged shortly after a new boundary layer forms after the nip roll arrangement  30 . 
         [0040]    In another embodiment of the invention,  FIG. 10  shows an arrangement of a NACA duct  12  in operative relationship to a first side of a moving web  14 . On the other, second, side of the moving web  14  is disposed a dimpled plate  36 , the dimpled plate being of sufficient size, design, and placement with respect to the web, as is known in the art, to ensure better controlled web handling. A dimpled plate on the opposite of web  14  from NACA duct  12  can stabilize the web, helping to prevent flutter and other web movement in an unsupported span, for example. 
         [0041]    In an embodiment, the size of a plurality of NACA ducts arranged generally in the CD web direction can be modified to get substantially full CD web coverage while utilizing a minimum length of total web coverage in the MD direction, LMD. By optimizing the sizes of the plurality of NACA ducts to minimize LMD, full web particulate collection can be utilized at any web span of greater length than LMD. As shown in the diagram of  FIG. 11 , it is believed that by disposing a plurality of primary NACA ducts  12   a  in an adjacent side-by-side relationship, and by placing a half-size secondary NACA duct  12   b  between each primary NACA duct  21   a  such that the leading edge of all the intake openings  18  lie substantially on the same CD-oriented line, coverage for particulate collection can be maximized. Such a staggered, nested relationship of NACA ducts can minimize the space requirements for full-web-width dust collection. 
         [0042]    As shown in  FIG. 11 , length Xa of the intake openings  18  of NACA ducts  12   a  can be twice the length Xb of the intake openings  18  of NACA duct  12   b  and the width Ya of the intake openings  18  of NACA ducts  12   a  can be twice the width Yb of the intake openings  18  of NACA duct  12   b . In the configuration shown and described, maximum nesting of NACA ducts can be achieved. In general, the length Xb of the intake openings  18  of NACA ducts  12   b  can be about 30% to 80% the length Xa of the intake openings  18  of NACA duct  12   a  and the width Yb of the intake openings  18  of NACA ducts  12   b  can be about 30% to 80% the width Ya of the intake openings  18  of NACA duct  12   a.    
         [0043]    The dimensions and values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as “40 mm” is intended to mean “about 40 mm.” 
         [0044]    Every document cited herein, including any cross referenced or related patent or application, is hereby incorporated herein by reference in its entirety unless expressly excluded or otherwise limited. The citation of any document is not an admission that it is prior art with respect to any invention disclosed or claimed herein or that it alone, or in any combination with any other reference or references, teaches, suggests or discloses any such invention. Further, to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern. 
         [0045]    While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.