Abstract:
A device is provided for dewatering a fabric in which the fabric is used in a papermaking process that exposes it to water before it is directed to the fabric dewatering device. The fabric dewatering device includes leading and trailing guide rolls about which the fabric is partially wrapped to form a loop portion. The loop portion is suspended between the leading and trailing guide rolls and includes a trough portion with a relatively small diameter that is positioned below the axes of the guide rolls. Running the fabric through the trough portion at high speeds exposes the water trapped therein to high centrifugal forces. The centrifugal forces expel the water from the fabric. The geometry and positioning of the trough portion is maintained either through a difference in fabric speed maintained by the leading and trailing nips or by the use of a rider roll positioned within the loop. A non-contact sensor in proximity to the trough portion senses the position of the trough portion and provides feedback to a controller which controls the motion of the rolls.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of U.S. application Ser. No. 09/736,511, filed Dec. 13, 2000, which is hereby incorporated herein in its entirety by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to fabric dewatering devices, and more specifically to the dewatering of continuously moving fabrics used in papermaking machines, wherein the fabric is guided through a small-radius turn which causes water to be expelled from the fabric via centrifugal force.  
         BACKGROUND OF THE INVENTION  
         [0003]    Permeable fabrics, or belts, are often used in paper machines for supporting a paper web during the papermaking process. After the web is separated from the fabric, the fabric typically undergoes a partial or full-width cleaning or washing process. During the cleaning process, the fabric is exposed to water which the fabric tends to retain when it returns to receive a new portion of the paper web. Papermaking applications that involve a process of thermal drying of the paper web supported on a drying fabric are sensitive to the quantity of residual water retained by the fabric, since retained water may rewet the paper web. Thus, such drying fabrics must be treated to remove all, or at least a major part of, the residual water after cleaning. Other applications may also require fabrics to be at least partially dewatered to prevent the water from slinging onto other parts of the machinery or other parts of the paper web while the fabric is returning from the cleaning process to receive a new portion of the paper web.  
           [0004]    Tissue paper production requires special types of fabrics to achieve a final product with a high bulk. Very often TAD fabrics, or other types of tissue-making fabrics or belts, are used for the manufacture of textured tissue or web. This requires a special textured structure of the fabric itself and, consequently, a definite fabric thickness. Thick, structured fabrics are especially prone to water absorption during washing and retention of that water in the deeper parts of the fabric structure.  
           [0005]    Pressing is a conventional means of dewatering fabrics that tends to be effective for non-woven fabrics such as felt. However, pressing is not as effective for simple woven fabrics such as those used for forming or TAD applications. Such woven fabrics are prone to retain water due to their thickness and less compressible structure.  
           [0006]    A roll press for squeezing water from a papermaking felt is disclosed in Great Britain Patent No. 1,273,827 (&#39;827). The press has two press rolls with parallel axes and an intermediate roller which is of substantially smaller diameter than that of either press roll. The intermediate roll is located between the two press rolls so as to form two press nips with the respective press rolls. The intermediate roll is arranged offset to one side of the common axial plane of the two press rolls and is movable toward this common axial plane. The felt runs into the first nip from the side of the common axial plane remote from the intermediate roll and leaves the second nip towards the remote side. Tension in the felt draws the intermediate roll against the press rolls with sufficient linear pressure to compress the felt so as to squeeze water from the felt.  
           [0007]    Vacuum pans are well known in the art for dewatering fabrics and consist of a collection pan connected to a vacuum source and in proximity to the travelling fabric. The vacuum source exerts a suction pressure on the fabric, drawing water out of the fabric and into the pan. Another well-known method is to use an air knife that blows air out a narrow slot and through the fabric, thus blowing water out of the fabric and into a collection pan. U.S. Pat. No. 4,116,762 to Gardiner (&#39;762) teaches the use of a hollow, foraminous cylinder over which a felt is passed. The cylinder allows air flow through to the travelling felt to drive water out of the felt. For fabrics that are very permeable, methods such as the ones described above involving blowing or sucking air through the fabric require a very large air flow and flow velocity, and hence consume a great deal of energy.  
           [0008]    Centrifugal force has been used to aid in the dewatering of fabrics by running the felt over a curved surface with a small radius at high speeds. The &#39;827 and &#39;762 patents use centrifugal force to aid dewatering to a certain extent. As another example, U.S. Pat. No. 6,153,056 to Schiel (&#39;056) discloses a draining device that drains water by circulating a press felt loop about a short region of convex curvature on a guide roll. The centrifugal force displaces water out of the belt and into a collecting device.  
           [0009]    Whenever a wet moving fabric changes direction, by passing around a roll or foil for instance, there is a tendency to throw off water. The magnitude of this tendency depends both on the angular velocity and duration for which this is maintained. A small radius and a large wrap angle will tend to maximize the water removal tendency. In the case of a lead roll, a small radius is difficult to achieve, especially when combined with a large wrap due to problems with roll deflection and critical speeds. Therefore, in the case of a lead roll a small radius is not practical, although a large wrap presents no difficulty. In the case of a foil or stationary element, a small radius can readily be achieved but the wrap must be severely limited in order to avoid fabric wear.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention meets these and other needs, and is characterized by a fabric dewatering device and method in which a fabric to be dewatered is passed over a leading guide roll and a trailing guide roll. The leading guide roll is rotatable about its axis and the trailing guide roll is rotatable about its axis and is parallel to the axis of the leading guide roll. The leading and trailing guide rolls are spaced apart such that the fabric passes over a portion of a circumference of the leading guide roll and then over a portion of the circumference of the trailing guide roll, in the same rotational direction about both rolls. The fabric is wrapped about both rolls so as to form a fabric loop between the leading guide roll and the trailing guide roll. This fabric loop includes a trough portion spaced to one side of the plane defined by the axes of the guide rolls.  
           [0011]    A control device controls passage of the fabric through the fabric dewatering device so as to maintain the position of the trough portion of the fabric loop. In one embodiment, the control device includes a drive connected to each of the rolls and operable to rotate each roll about an axis thereof in the same rotation direction. A sensor is used for detecting a position of the trough portion of the fabric loop and is connected to a controller. The controller and the drive control the rotational speed for each roll so as to maintain the position of the trough portion spaced to one side of the axes of the rolls such that the fabric loop has a radius of curvature sufficiently small to cause water to be expelled from the fabric by centrifugal force.  
           [0012]    In another embodiment the control device includes a rider roll inserted within the trough portion so as to maintain the position and geometry of the fabric loop. The rider roll preferably has a diameter in the range of 50 mm to 100 mm. Smaller diameters result in greater centrifugal forces, but decrease the dwell time while larger diameters increase the dwell time of the fabric. Advantageously, the fabric has a wrap angle around the rider roll of about 200° to 300°. Tension in the fabric draws the rider roll against the guide rolls but there is no attempt to compress the fabric to squeeze water from the fabric. Rather, dewatering is accomplished primarily by centrifugal forces on the fabric passing about the rider roll. Dewatering can also be aided, in some embodiments, by making the rider roll permeable, and forcing or drawing air through the rider roll and the fabric wrapped thereabout.  
           [0013]    In one embodiment, maintenance of the trough portion below the axes of the guide rolls is facilitated by way of a lead nip and a trailing nip. The lead nip is formed between a first surface and the leading guide roll while the trailing nip is formed between a second surface and the trailing guide roll. The fabric passes through the lead nip upstream of the trough portion and then through the trailing nip downstream of the trough portion. The first and second surfaces can be portions of a single top roll or two separate top rolls. Tension in the fabric is relieved in the lead nip such that the fabric loop is essentially free of tension in the machine direction.  
           [0014]    In another embodiment, the two nips are formed by a single top roll that is deformable. The top roll is pressed with a greater force against the trailing guide roll than against the leading guide roll, whereby the trailing nip has a greater indentation than the lead nip. Any of the three rolls can be driven. The loop length can be regulated by controlling the indentation of the trailing nip. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:  
         [0016]    [0016]FIG. 1 is a schematic side view of parts of the fabric dewatering device showing a first embodiment which includes a top roll which forms a leading nip and a trailing nip.  
         [0017]    [0017]FIG. 2 is a schematic side view of parts of the fabric dewatering device showing a second embodiment which includes a top lead roll forming a lead nip and a top trailing roll forming a trailing nip.  
         [0018]    [0018]FIG. 3 is a schematic side view of parts of the fabric dewatering device showing a third embodiment which includes a rider roll inserted in a trough portion of the fabric.  
         [0019]    [0019]FIG. 4 is a schematic side view of parts of the fabric dewatering device as shown in FIG. 3 further comprising an air plenum positioned above the rider roll.  
         [0020]    [0020]FIG. 5 is a schematic side view of parts of the fabric dewatering device showing a fourth embodiment which includes a shower pipe suspended above a rider roll. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.  
         [0022]    [0022]FIGS. 1 through 5 schematically depict several embodiments of a fabric dewatering device  10  for fabrics used in a papermaking process. FIG. 1 is a schematic depiction of a first embodiment of the fabric dewatering device  10  showing a continuous fabric  16  passing through the device. The fabric passes through a lead nip  11  formed between a top roll  13  and a leading guide roll  12 . The fabric then passes through a trailing nip  15  formed between the top roll  13  and a trailing guide roll  14 . Between the lead nip  11  and the trailing nip  15  the fabric  16  forms a loop  20  with a trough portion  17  that has a small radius relative to that of the leading and trailing guide rolls  12  and  14 . Movement of the fabric  16  with a relatively high linear speed induces a centrifugal force that throws water out from the fabric when it is forced to swing through the relatively small radius of the fabric loop  20  and trough portion  17 . This water is captured and prevented from falling on electrical parts, or the paper web being manufactured, by a collection pan  21 .  
         [0023]    The leading guide roll  12  and the trailing guide roll  14  are preferably two parallel cylindrical rolls of equal diameter and with hard outer surfaces. The rolls  12  and  14  are rotatable about their respective long axes and are supported in a suitable frame (not shown) of a papermaking machine wherever dewatering a fabric belt or web is desired. The rolls  12  and  14  extend in a cross-machine direction, transverse to the direction of travel of the continuous fabric  16 , and are the same length, or longer, than the transverse width of the fabric.  
         [0024]    The top roll  13  is a rotatable cylindrical roll and has a covering  18  of rubber or other deformable material on its outside surface. Unlike the leading and trailing guide rolls  12  and  14 , the top roll  13  has an adjustable center axis  19 . Adjustments in the position of the center axis  19  can be either manual or by way of an automatic control in response to an electrical or mechanical signal. Preferably, a non-contact sensor  30  detects the position of the trough portion  17  and sends a signal to a controller  31 , which is connected to an actuator (not shown) operable to move the roll  13  toward and away from the guide rolls  12  and  14 .  
         [0025]    Adjusting the position of top roll  13  in relation to the guide rolls  12  and  14  varies the amount of indentation of the deformable cover  18  at the leading and trailing nips  11  and  15 , which regulates the length of loop  20  and hence the radius of the trough portion  17 . Preferably, the minimum indentation required to grip the fabric  16  is used at the lead nip  11 , while a greater indentation is used at the trailing nip  15  so that the fabric speed entering the trailing nip is reduced. Because the length of the fabric  16  will increase as tension is applied, the surface speed of the fabric at the entrance of the trailing nip  15  will always be slower than the surface speed of the fabric at the entrance of the lead nip  11 . The fabric  16  runs at constant tension and speed outside of the fabric dewatering device  10 , but between the nips  11  and  15 , the loop  20  runs at nearly zero tension and a lower surface speed. The fabric speed differential between the leading and trailing nips  11  and  15  depends on the modulus of the fabric  16 . A lower modulus results in more fabric stretch at the trailing nip  15 , and hence a greater speed differential between the nips.  
         [0026]    In other embodiments, it is possible to construct the top roll  13  of a range of materials. For instance, the top roll  13  could be constructed entirely of deformable material rather than a deformable cover  18  on a hard roll. Various deformable materials can be used. Rubber is a suitable material due to its relatively low modulus of elasticity, high durability and excellent friction characteristics.  
         [0027]    Any of the three rolls  12 ,  13  and  14  can be driven by a conventional drive system such as an electric motor operably attached to the axes of the rolls through a speed reducer. FIG. 1 depicts a drive  32  coupled with the top roll  13 . Driving the leading guide roll  12 , the top roll  13 , or both, maintains tension on the fabric as it travels through the lead nip  11  on the upstream end of the dewatering device  10 . Driving the trailing guide roll  14 , the top roll  13 , or both, restrains the downstream flow of the fabric as it travels through the trailing nip  15 . Driving any one of the rolls  12 ,  13  and  14  will result in rotation of all three rolls due to the contact forces present in both of the nips  11  and  15 .  
         [0028]    The actual dewatering process is best illustrated by describing the path of the fabric  16  as it travels into and through the fabric dewatering device  10 . Upstream of the fabric dewatering device  10 , the fabric  16  supports a paper web during the manufacturing process. In some applications, the paper web may be a textured tissue paper that is dried on a through-air-drier (TAD) fabric. These fabrics have a special textured structure and a definite thickness. After the fabric  16  is separated from the paper web, it is cleaned of any fibers or other contaminates that adhered to it during the papermaking process. It is cleaned using a conventional washing technique that typically involves spraying water onto the fabric. Before the fabric returns to pick up more of the paper web, the fabric  16  is drawn at a fixed speed into the fabric dewatering device  10  by the tension in the lead nip  11 .  
         [0029]    Due to the decrease in the speed of travel of the fabric  16  between the lead nip  11  and the trailing nip  15 , little or no tension is present in the loop  20  of the fabric  16 . There is clearance between the two guide rolls  12  and  14  in the machine direction. This permits the fabric  16  to form loop  20  by extending downward over a portion of the circumference of the leading guide roll  12 . The bending stiffness of the fabric  16  causes the formation of the trough portion  17  below the plane formed by the axes of the guide rolls  12  and  14 . The loop  20  is completed as the fabric  16  extends up a portion of the trailing guide roll  14  and into the trailing nip  15 .  
         [0030]    Water is expelled from the soaked fabric  16  because the relatively high linear speed swinging through a small radius trough portion  17  induces large centrifugal forces. Preferably, the diameter of trough portion  17  will be in the range of 50 mm to 100 mm. At a fabric speed of 15 meters per second (m/s), the water experiences a force of over 450 times gravity for the 100 mm diameter trough portion  17 . Halving the diameter of the trough portion  17  to 50 mm increases this to 900 times gravity. Because of the orientation of the loop  20  suspended between the guide rolls, the water tends to be expelled generally downward and laterally outward. As it is flung out of the fabric  16 , the water is captured in the collecting pan  21  disposed below and surrounding the trough portion  17 , where it flows away through a drain (not shown).  
         [0031]    After the traveling fabric  16  exits the dewatering device  10  through the trailing nip  15 , its water load has been reduced. Depending upon the tolerance of the papermaking process for the remaining water in the web  16 , it can either be immediately returned to pick up more of the paper web, or it can be sent to another drying apparatus. Vacuum and forced air drying apparatuses are usually expensive to operate when water loads are high. However, the water load is greatly reduced when the fabric  16  has been pretreated by the dewatering device  10 , making a serial use of drying apparatuses an effective strategy for dewatering fabric. The fabric can be passed sequentially through two or more dewatering devices  10 , if desired.  
         [0032]    A second embodiment of the fabric dewatering device is schematically depicted in FIG. 2. The second embodiment replaces the top roll  13  of the first embodiment with a top leading roll  22  and a top trailing roll  23 . The top leading roll  22  and the leading guide roll  12  form the lead nip  11 . The top trailing roll  23  and the trailing guide roll  14  form the trailing nip  15 . Of the two rolls forming each nip  11  and  15 , one of them is driven by a drive assembly (not shown). The top leading roll  22  and the top trailing roll  23  can be run at different speeds so as to allow the formation of the loop  20 . A sensor system  31  as described in the first embodiment can be used for detecting the position of the trough portion  17  to control the speed of the two top rolls  22  and  23 . An advantage of the use of two top rolls over one is that it eliminates the need for the deformable cover  18  and allows freer air access through the gap between the top lead and trailing rolls  22  and  23 .  
         [0033]    A third embodiment (shown in FIG. 3) eliminates the top rolls and includes a rider roll  24  that is inserted within the trough portion  17  of the fabric  16  to control the position and geometry of the trough portion. The diameter of the rider roll  24  is preferably between 50 mm and 100 mm. The ends of the rider roll  24  need not be supported, but some form of restraint of the rider roll  24  in the cross-machine direction is required. Preferably, the roll ends are shaped into blunt cones (not shown) which are arranged to rub against plastic strips (not shown). Alternatively, light arms and bearings can support the ends of the rider roll  24 . The fabric  16  and guide rolls  12  and  14  provide the restraint required to prevent the rider roll  24  from whirling. This allows operation at higher rotational speeds of about 6000 rpm for the 50 mm diameter rider roll. The rider roll  24  may also be of disk type or other segmented construction, with or without a fixed or revolving shaft.  
         [0034]    Increasing the dwell length and time that the fabric passes through the small-radius path increases the dewatering effect of the centrifugal forces for a given level of centrifugal force. The dwell time can be increased by increasing the angle of wrap of the fabric about the rider roll  24 . The wrap is preferably on the order of 290°, which corresponds to a dwell length of about 250 mm and a dwell time of 16 ms at fabric  16  travel speeds of 15 m/s for a rider roll  24  diameter of 100 mm. The centrifugal forces exerted in this case are on the order of 450 times the force of gravity (g). Operation at the same fabric speed with a 50 mm diameter rider roll  24  would double the centrifugal forces from 450 g to 900 g, but would halve the dwell length and time to 125 mm and 8 ms. Wrap angles of 200° to 300° are suitable.  
         [0035]    The third embodiment can also include a midfeather deflector  27 , which is a plate structure positioned between the leading guide roll  12  and the trailing guide roll  14 . The plate structure of the deflector  27  prevents water flung from the portion of the fabric upstream of the trough portion  17  from rewetting the exiting portion of the fabric web  16  downstream of the trough portion  17 .  
         [0036]    [0036]FIG. 4 depicts a fourth embodiment, which is similar to the third embodiment, except that the rider roll  24  is permeable and air is discharged through the permeable rider roll  24  so as to pass through the fabric  16 . Air flow can be generated using an air knife (not shown) or an air supply plenum  25  with the air flow directed toward the permeable rider roll  24  and the trough portion  17 . Air flow can also be generated using a vacuum source (not shown) attached to the collecting pan  21  that would draw air through the permeable rider roll  24  and the trough portion  17 . Preferably, a vacuum seal  26  on the collecting pan  21  seals against the guide rolls to prevent leakage of air between the guide rolls  12  and  14  and the collecting pan. Using the air knife and the vacuum source together is also a possibility if additional air flow is desired through the loop portion  20 .  
         [0037]    [0037]FIG. 5 schematically depicts a fifth embodiment comprising a fabric cleaning device  10 ′ that includes the use of a flooded nip and/or scarfing shower to clean and dewater the fabric  16 . The device  10 ′ includes a permeable rider roll  24  and a shower pipe  29 . The rider roll  24  is of larger diameter than in the previously described embodiments to allow clearance for the shower pipe  29  which is positioned above the rider roll. Water from the shower pipe  29  floods a nip  33  between the lead guide roll  12  and the rider roll  24  so as to clean the fabric  16  as it passes around the rider roll  24 . One or more dewatering devices (of any of the previously described embodiments) could be arranged in series with the cleaning device  10 ′ to cleanse and/or dewater the fabric  16  continuously, thus forming a cleaning and dewatering system. The large centrifugal forces in the cleaning device  10 ′ can increase cleaning efficiency, and the device  10 ′ can have a more compact arrangement than a conventional flooded nip device.  
         [0038]    Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.