Patent Application
20120318298
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.
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.
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.
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.
There is thus a continuing need for a method and apparatus for removing dust in a power-efficient, environmentally friendly manner.
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;
submerging the intake opening into the boundary layer to scavenge dust-carrying air from the boundary layer.
a and 1b are schematic representations of a typical NACA duct.
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.
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.
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.
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.
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.
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.
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.
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.
In an embodiment of the invention shown in
NACA ducts have an intake opening 18 (corresponding to intake opening 4 of
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.
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.
Although
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
As shown in
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
An embodiment of a NACA duct, specifically a NACA duct 12 as depicted in
In an embodiment of the invention,
In another embodiment of the invention,
In another embodiment of the invention,
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
As shown in
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.”
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.
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.
