Abstract:
An apparatus for distributing fiber from a carding machine to an airlay wherein the apparatus comprises a system of ducts or conduits for controlling air flow having a curved top wall and a curved bottom wall that define a converging passageway and the distance between the walls is defined by an exponential equation.

Description:
This application claims priority upon provisional application No. 60/091262, filed Jun. 30, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to airlay fiber handling equipment such as an airlay web former and more particularly to controlling the air stream into the web former. 
     BACKGROUND OF THE INVENTION 
     In the airlay web forming process in use by E.I. du Pont de Nemours and Company (DuPont) in the manufacture of spunlaced fabrics sold under the trademark Sontara®, fiber is carried by a relatively fast-moving air stream to a screen conveyor forming a web of randomly arranged fibers. The commercial process is disclosed and described in U.S. Pat. No. 3,797,074 to Zafiroglu. 
     Upon investigation, it has been hypothesized that the air flow which carries the fiber to the screen conveyor is subject to eddies, vortices and other indicators of turbulence at the peripheral sides of the web which is undesirable. In accordance with Zafiroglu, the air that is used to carry the fiber is introduced through a system of large conduits and fans. Prior to receiving the fiber, the air flow is directed through screens and straighteners to provide a uniform flow substantially free of large-scale turbulence and vortices. Thereafter, the large volume, relatively slow-moving air flow is accelerated through a converging section or nozzle into a reduced cross sectional area conduit which is substantially flat and wide to be suited for laying down a wide web. It is believed that the acceleration nozzle of Zafiroglu creates, or allows the creation of the vortices and turbulence at the peripheral sides of the web which is believed responsible for certain defects. 
     U.S. Pat. 5,564,630 to Giles et al (assigned to DuPont) is directed to an improved nozzle over that of Zafiroglu by providing smoothly curving, low angle peripheral walls. The nozzle is particularly helpful in reducing edge defects which can result from vortices and turbulence. Regardless, considerable need remains for improvement of web properties. 
     U.S. patent application Ser. No. 08/760,119 now abandoned (also assigned to DuPont) is directed to combining the advantages of feeding carded fibers to an airlay. The air stream in that patent application is controlled by use of fans and a series of filters and air straighteners to create a laminar air flow. However, such an arrangement may present disadvantages in terms of space requirements and by making the maintenance of the cards especially difficult. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide means for controlling the air stream into an airlay web former arrangement which substantially reduces the defects of the web and overcomes the drawbacks of existent arrangements as described above. 
     These objects of the invention are accomplished by a device for directing air flow comprising a first conduit with an inlet and an outlet and having an upper curved wall surface and a lower curved wall surface wherein the bottom curved wall surface has a greater degree of curvature than does the upper curved wall surface so that the distance between the curved wall surfaces generally decreases and thereby substantially changes the direction of air flow from the inlet to the outlet. These objects of the invention are further accomplished by a device comprising the first conduit and a second conduit of similar configuration to the first conduit and wherein the first conduit and second conduit combine at their outlet areas to form a third single conduit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more easily understood by a detailed explanation of the invention including drawings. Accordingly, drawings which are particularly suited for explaining the invention are attached herewith; however, it should be understood that such drawings are for explanation only and are not necessarily to scale. The drawings are briefly described as follows: 
     FIG. 1 is a generally schematic view of an existent device in which a carding machine feeds fiber to an airlay. 
     FIG. 2 is a view similar to FIG. 1 showing the air controlling device of the invention. 
     FIG. 3 is a representation of a portion of the device shown in FIG. 2 superimposed on Cartesian coordinates. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, the invention will be described in greater detail so as to explain the contribution to the art and its application in the industry. Referring specifically to FIG. 1, the fiber handling system of an existing embodiment is generally referred to by the number  10  and may be more easily understood as having an airlay portion generally indicated by disperser rolls  50  and an air duct  70  and a carding machine portion generally indicated by main carding rolls  40 . The existing embodiment transports fiber through the carding machine portion and then through the airlay portion. It is well known that cards typically have worker and/or stripper rolls associated with the main carding roll as well as other secondary carding rolls. However, for the sake of simplicity such detail is omitted here. 
     Referring again to FIG. 1, the disperser roll  50  carries the fiber from the main carding roll  40  to an air duct  70 . In the air duct  70 , an air stream is made to pass over the surface of the disperser roll  50  in a generally tangential relationship to receive the fiber being doffed from the disperser roll  50 . The fiber is quite likely to doff from the disperser roll  50  without the presence of the air stream creating a cloud of individualized fiber; however, it is preferred to provide the individualized fiber into an air stream where it may be more easily handled. It is preferred that the air stream be generally free of turbulence so as to allow the fiber to be dispersed throughout the air stream. Eddies, vortices and other turbulence tend to disturb the distribution of the fiber in the air duct  70  which causes undesirable consequences depending on the use that will be made with the fiber in the air stream. The webs produced under such conditions typically exhibit splotchiness and non-uniformities caused by the fiber following the path of the vortices and eddies and not laying down properly. 
     The fiber can be laid onto a web on a screen conveyor belt  80  at the base of the air duct  70 . The screen conveyor belt  80  is carried by a series of rollers including rollers  82  and  83 . Below the screen conveyor  80  a vacuum duct (not shown) can be positioned to pull air in the air duct  70  down through the screen conveyor belt  80  to pin the fiber thereon and remove it from the system. 
     As shown in FIG. 1, the cards may be generally enclosed by card covers  11  and the airstream is drawn from the atmosphere around the covers. Here a system is depicted where two separate card systems feed fiber into a common air duct  70 . Air is drawn into the air duct  70  by the action of the doffing rolls  50 , but such air does not behave in a uniform manner. Because the atmospheric air is not controlled in any fashion, the air does not easily form into a uniform, laminar air stream. By placing a grid over the web former and attaching strings to the grid it was found that during operation of the web former that the strings exhibited violent, random movements indicative of turbulent air flow. Further, a videotape was made of the fibers as they were subjected to the turbulence and the video showed that the fibers moved back and forth across the web area which would cause undesirable streaks. 
     In view of the need to control the air going into the air duct  70  and to address some of the physical limitations that are associated with the area around the cards that would prevent control of the air, the subject invention was developed. As depicted in FIG. 2 an air controlling device  200  was developed. The device  200  is depicted as comprising two identical passages  210  that transport air and would typically change the air flow from a substantially horizontal direction to a substantially vertical direction. It should be noted however that the air flow is to be controlled so as to achieve a laminar flow without any indicator of turbulence and is not limited to changing the air flow from vertical to horizontal or some other change in orientation. Each passageway is defined by an outer upper surface  220  and an inner lower surface  230 . Both surfaces have a curvature such that the distance between them decreases in the direction of the air flow. Although the system is described in terms of two passageways, it should be understood that either a single passage or a plurality of passages can be used consistent with the desired throughput to the airlay and amount of control of the air stream. 
     The air enters the device  200  at a relatively large inlet  205  and where a vertical screen  206  is located which provides a pressure drop to slow and to straighten the incoming air. The screen is oriented in a generally vertical direction because a horizontal screen would collect stray fiber and debris which could cause defects in the web. Even using a vertical screen it is desirable that the air enters screen  206  at a relatively low speed because at high speeds airborne particles and debris may collect on the screen even with the vertical orientation. Typically, it is preferred that the air stream speed at the screen be less than about 2 meters per second. The air proceeds through passage  210  and exits at a small end outlet  215  (small relative to inlet  205 ). 
     In one embodiment the device  200  is adapted to fit onto the top and between an existing pair of cards by replacing all or part of the card covers  11  as generally depicted in FIG.  2 . Instead of air entering the air duct  70  in a random fashion, the device  200  provides a specifically curved path that causes two separate air streams from two ducts  210  to join and form one airstream in a single duct  240  having a controlled laminar flow. By laminar, it is meant that the air stream substantially travels uniformly in one direction without any eddies, vortices or other indicators of turbulence. To further ensure that that the air flow becomes laminar an extension wall  216  may be added at the juncture of the upper curved surfaces  220 . 
     In FIG. 3 the dimension d 1  of the large end inlet  205  is shown as superimposed on a y-axis and the dimension a 1  of the small end outlet  215  is shown superimposed on an x-axis. The distance D is shown as the distance between upper curved surface  220  and lower curved surface  230  as a function of angle θ. It can be generally stated that the curvature of the upper surface  220 , the curvature of the lower surface  230  and the distance D between the curved surfaces can be expressed by the following mathematical equations:        Upper                 Curved                 Surface                 x   =                  (       a   1     +     c   1       )        cos                 θ                 y   =                  (       b   1     +     d   1       )        sin                 θ                                where                   π   2       ≤   θ   &lt;   π                   Lower                 Curved                 Surface                 x   =                  [       c   1     -         4        (       d   1     -     a   1       )         π   2              (     θ   -   π     )     2         ]        cos                 θ                 y   =                  [       b   1     +     d   1     -     a   1     -         4        (       d   1     -     a   1       )         π   2              (     θ   -   π     )     2         ]        sin                 θ                                where                   π   2       ≤   θ   &lt;   π                         D   =                      4        (       d   1     -     a   1       )         π   2              (     θ   -   π     )     2       +     a   1                                  where                   π   2       ≤   θ   ≤   π                                  
     The distance D is expressed by the exponential equation above at a power of two, but D could also be expressed by a cubic equation or any other equation that would provide the desired laminar flow to the air transported through the device. The equation for the upper curved surface as presented above defines an ellipse and was chosen primarily because of ease of formation in manufacture. However, the upper curved surface can be expressed by any twice continuously differentiable surface that is concave down. 
     The device&#39;s ability to distribute air as desired was evaluated by use of modeling software available from Fluent Inc. (Lebanon, NH). It was found from the modeling software that the curved surfaces of the subject invention provided laminar flow with virtually no formation of eddies and vortices. Such a condition would be expected to provide uniform webs.