Abstract:
An embossed screen having raised deflector elements on the internal surface is provided for a centrifugal pellet dryer. The deflector screen is formed as an integral structure, eliminating the need for separate deflector components secured by fastening elements along with the associated risks such as loosening or separation of the strip and contaminate build-up between the strip and the screen. The embossed deflector screen effectively deflects the pellets back toward the rotor where the pellets are reengaged with rotor energy, resulting in increased dryer efficiency and flow rate. The embossed deflector screen also enhances the overall structural strength of the screen, reduces manufacturing costs and prevents pellet entrapment that can lead to contamination in future runs.

Description:
This application is entitled to and hereby claims the priority of U.S. Provisional application Ser. No. 60/924,627 filed May 23, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a centrifugal pellet dryer of the type which utilizes a bladed lift rotor conveying moisture laden plastic pellets or other solid particles upwardly within a cylindrical screen. The centrifugal force imparted to the particles by rotation of the lift rotor causes the particles to engage the interior surface of the screen, and moisture on the particles is discharged through the screen in a manner well known in the art. More specifically, the present invention relates to a product flow modifying deflector associated with the internal surface of the cylindrical screen. 
     2. Description of the Related Art 
     Centrifugal pellet dryers are well known in the art for separating water or moisture from plastic pellets and other solid particles, such as a slurry of water and plastic pellets produced by underwater pelletizers. Centrifugal pellet dryers of the prior art include a vertically disposed outer housing, a cylindrical screen oriented in the housing and a driven bladed rotor positioned centrally inside the screen. The rotor moves water laden pellets or other solid particles upwardly within the screen with centrifugal forces imparted to the particles by radial air flow from the rotor (see  FIG. 1 ) causing the particles to move radially outwardly into engagement with the screen for discharge of water through the screen. The dried particles are discharged from the upper end of the screen and housing, and water is discharged from the lower end of the housing. 
     Centrifugal pellet dryers of this type are disclosed in U.S. Pat. Nos. 7,171,762, 7,024,794, 6,807,748, and 6,237,244, commonly owned by the assignee of this application. In the operation of such dryers, the pellets or other particles being moved vertically and radially by the bladed rotor engage the cylindrical screen with substantial velocity and usually bounce off the screen back toward the rotor for imparting further vertical and centrifugal forces to the particles as they are moved upwardly inside the screen. This is depicted by the “good” flow characteristic illustrated in  FIG. 2 . As further shown in  FIG. 3 , the “best” flow of both product and air occurs when the radial air flow from the rotor does not just push the pellets but actually flows around them. 
     However, conventional centrifugal dryers used in the market today all have a common problem relating to the air flow created by the normal rotation of the rotor. The combination of rotor blade geometry and other physical factors creates an air flow that can greatly affect the flow of the product through the dryer as it bounces between the rotor and the screen. 
     Furthermore, with the advent of newer plastic materials which form softer pellets, or pellets with flat or lentoid geometries, and the making of very small pellets, or so-called micropellets, using underwater pelletizers, difficulties have been encountered in conveying and subsequently drying such pellets in known centrifugal dryers. In addition, known centrifugal dryers have encountered difficulty in conveying and subsequently drying ground flake plastic materials which are formed from recycled soda bottles, milk containers and the like, as well as certain other plastic particles such as ground battery casings. 
     More specifically, and as depicted by the “poor” flow characteristics in  FIG. 2 , softer and smaller pellets, pellets with flat or lentoid geometries, and plastic flakes, as well as certain other plastic and similar particles, tend to collect and circulate in the clearance band “X” (see  FIG. 2 ) between the outer edges of the rotor blades and the inner surface of the screen. Rather than bouncing around in the manner of harder and larger pellets or particles, these particles become trapped against the screen by the air flow. This undesirable circular flow and resultant entrapment of the softer and smaller pellets, pellets with flat or lentoid geometries, and plastic flakes and particles along the inner surface of the screen is sometimes referred to as “banding”. This banding reduces product flow through the rotor area of the dryer and increases power requirements for maintaining rotational speed of the rotor. Further, it has been found that banding also reduces the efficiency of moisture separation from the solid particles, can cause high amperage requirements within the dryer, and reduces overall efficiency of the centrifugal dryer. These problems often result in fines and fiber-like “hair” production (often referred to as angel hair in the industry). 
     The problem of banding is particularly evident with pellets having a flat or lentoid geometry as the relatively large planar surface area of this shape most naturally causes the pellets to adhere to the inner surface of the screen and, because of the associated low profile of such pellets, makes them difficult to dislodge. As illustrated by the “worst” flow in  FIG. 3 , the larger the product&#39;s surface area in one dimension, or the more flake-like or lentoid the product, the greater the opportunity for the outward air flow of the rotor to trap the product against the screen. This phenomenon greatly reduces the necessary bounce required to reengage the product with the outward and upward action of the dryer rotor. 
     One solution for overcoming this problem of banding is set forth in U.S. Pat. No. 6,739,457 (“the &#39;457 patent”), which is commonly owned by the assignee of this invention. The disclosure of the &#39;457 patent is hereby expressly incorporated herein by reference as if fully set forth in its entirety. 
     In the &#39;457 patent, deflector strips are fastened to the inside of the dryer screen using multiple fasteners fitted within countersunk holes machined within the strips. This method of fastening results in the deflector strips being relatively expensive to manufacture and also necessitates that the screen also be provided with dedicated holes which can create undesirable stress concentrations within the screen. In addition, should the fasteners become loosened, either through vibration, aging or other cause, there is the risk that the deflector strips could extend into the moving rotor with resulting damage. Further, any spacing between the deflector strip and the screen may collect portions of the pellets or other foreign matter, particularly with pellets having a flat or lentoid geometry, thus leading to possible contamination in future product runs. 
     SUMMARY OF THE INVENTION 
     The present invention is used with a centrifugal pellet dryer of the vertical type having a vertical cylindrical screen associated with a vertical housing and a bladed rotor oriented inside the cylindrical screen for conveying a slurry of water and polymer resin particles upwardly in the dryer. Centrifugal forces imparted to the solid particles by the rotor cause the particles to impact the screen to discharge water outwardly through the screen, while dried particles are discharged from an upper end of the dryer and water is discharged from the lower end of the housing in a manner well known in this art. Cylindrical screens for centrifugal pellet dryers are typically made from several screen sections which are vertically aligned and interconnected together. 
     In order to overcome the problems of such centrifugal dryers when separating water from soft and/or small pellets or plastic flakes, and certain other plastic particles with difficult to convey geometries such as lentoid-shaped pellets, as well as the problems associated with the fastened-on deflector strips of the &#39;457 patent, the inside of the cylindrical screen is provided with one or more embossed regions, each of which effectively forms an integral deflector protruding from the inside surface of the screen. As with the fastened-on deflector strips of the &#39;457 patent, the embossed screen of the present invention disrupts the circular flow of the particles to improve particle flow through the rotor area of the dryer by aiding in the rotor&#39;s vertical lift of the particles and by eliminating particle banding. Unlike the prior art, however, the embossed deflector screen eliminates the risks of contamination and of a loose deflector strip extending into the moving rotor, while also reducing manufacturing costs. In addition, because the embossed regions are preferably integrated into a non-perforated area of the screen, the embossed regions actually strengthen the overall screen structure. 
     It is therefore an object of the present invention to provide a deflector element on the interior of a cylindrical screen of a centrifugal pellet dryer in the form of one or more elongated deflector strips which are formed by embossing a perforated or non-perforated area of the screen. 
     Another object of the present invention is to form one or more deflector elements or regions through embossing at locations that are circumferentially spaced around the interior surface of the screen with the number of deflector elements or regions being varied depending upon the diameter of the screen, with there preferably being one to four deflector elements or regions in most cases. 
     A further object of the present invention is to integrally form one or more deflector elements in the form of vertical or acutely angled elongated deflector strips on the interior surface of the cylindrical screen in a cylindrical pellet dryer in accordance with the preceding objects such that the embossed strips enhance the overall strength of the screen. 
     A still further of the present invention is to form one or more embossed deflector elements in the form of elongated strips on the dryer screen having smoothly ramped edges formed integrally with the screen which prevent pellets from being lodged against the strips and ensure redirection of the pellets back into the rotor where the pellets are reengaged with rotor energy for reenergized upward movement. 
     Yet another object of the present invention is an embossed deflector screen that provides a retrofitable solution to the known problems of flat and lentoid-shaped products becoming trapped against the screen in a centrifugal pellet dryer. 
     A further object of the present invention is an embossed deflector screen that allows a centrifugal pellet dryer of a given size to run higher product flow rates which expands the scope of production achievable without obtaining a larger dryer. 
     A still further object of the present invention is to provide a deflector element or region for the cylindrical screen of centrifugal pellet dryers in accordance with the preceding objects and which will conform to conventional forms of manufacture, be of simple construction and easy to use so as to provide a deflector screen that will be economically feasible, long lasting and relatively trouble free in operation. 
     These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  generally depicts the radial air flow of a conventional rotor in a centrifugal pellet dryer. 
         FIG. 2  illustrates the effects of air flow from the rotor of  FIG. 1  and the resulting flow characteristics of various different shaped pellets. 
         FIG. 3  is a further illustration of best and worst air and pellet flow characteristics associated with the various shaped pellets of  FIG. 2 . 
         FIG. 4  is a schematic elevational view of a prior art centrifugal pellet dryer illustrating a sectional cylindrical screen and bladed lift rotor assembly associated with a dryer housing. 
         FIG. 5  is a perspective view of one of the dryer sections of  FIG. 4 , having two deflector strips mounted on the interior surface with fastening elements in accordance with the prior art. 
         FIG. 6  is a schematic partial sectional view of the screen section and one of the conventional deflector strip shown in  FIG. 5 . 
         FIG. 7  is a plan view of a screen section with two embossed deflector elements each in the form of an elongated vertical strip in accordance with the present invention. 
         FIG. 8  is a photograph of the center embossed deflector strip of  FIG. 7 . 
         FIG. 9  is a photograph of the end embossed deflector strip on  FIG. 7 . 
         FIG. 10  is a schematic partial sectional view of the screen section and one of the embossed deflector strips shown in  FIG. 7 . 
         FIG. 11  is an enlarged photograph of a portion of an embossed deflector strip formed in a non-perforated area of a screen section according to the present invention, adjacent to which a portion of a deflector strip according to the &#39;457 patent is shown placed onto the screen section for comparison. 
         FIG. 12  is a plan view of a screen section with two embossed deflector elements in the form of vertical and angled deflector strips in accordance with the present invention. 
         FIG. 13  is a perspective view of the screen section of  FIG. 12 . 
         FIGS. 14A-14D  are illustrations of alternative configurations for the embossed regions of the embossed deflector screen according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In describing preferred embodiments of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
     Although only certain embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, in describing the various embodiments, specific terminology will be resorted to for the sake of clarity. It is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
     Referring to  FIG. 4 , a conventional centrifugal pellet dryer of the vertical type is generally designated by reference numeral  10  and includes a dryer housing  12  having a sectional screen  14  mounted vertically therein. The sectional screen  14  is shown having four approximately equal screen sections  15  aligned vertically and interconnected at  17 . The screen  14  encloses and is concentric to a bladed rotor, generally designated by reference numeral  16 , which includes inclined blades  18 . The blades  18  include outer edges adjacent the interior surface of the screen sections  15  supported in a manner well known in the art. 
     The dryer  10  includes an inlet  20  for receiving a slurry of water and pellets from an underwater pelletizer, or other type water slurry containing solid particles, such as plastic flakes, from recycled soda bottles, milk containers, etc., or other solid plastic particles such as ground battery casings. The inlet  20  typically, although not in all centrifugal dryers, discharges the slurry into a dewaterer  22  for initial separation of water from the pellets or other solid particles for discharge of water through an outlet  24  and discharge of moisture laden particles into the bottom section of the sectional screen  14 . The solid particles move upwardly through the screen sections  15  by the action of the rotor  16  to an outlet  26  at the upper end of upper screen section  15  in the direction indicated by the arrow  126 . The rotor imparts lift and centrifugal forces to the particles to impact the particles against the screen for separating water from the particles with the separated water passing through the screen into the housing and out through outlet  24  in a manner well known in the art as exemplified by the previously mentioned prior patents. 
     Each of the screen sections  15  includes a plate  28 , typically of stainless steel with 20 or 18 gauge thickness and 0.075 inch diameter holes  30  punched therethrough from the surface facing inwardly of screen section  15 . Other hole shapes and diameters such as 0.038 inch, 0.085 inch, 0.0625 inch are also commonly used. As shown in  FIG. 5 , the holes  30  have staggered centers and are oriented in discrete areas  32  thereby defining intersecting solid sections  34  and  36 . Each of the screen sections  15  is initially formed as a flat plate  28 , which is retained in a cylindrical configuration by connecting outwardly extending vertical side edge flanges  38  and  40  on the respective vertical solid edges the screen section. Further description of the connecting mechanisms is set forth in the &#39;457 patent. 
     The two deflector strips shown in  FIG. 5 , and generally designated by reference numeral  70 , are mounted on the inside surface of the cylindrical screen section  15 . As can be seen in  FIGS. 5 and 6 , the deflector strips  70  are attached by bolts which protrude through matching holes in the cylindrical screen section  15  on which appropriate locking nuts  74  can be installed to affix each deflector strip  70  in place on the inside of the screen section. Once in place, the deflector strip  70  redirects pellet flow as indicated by the solid arrowed line  75  in  FIG. 6 , when the rotor  16  turns as indicated by the inner arrow  77 . 
     Attachment of the deflector strips as shown in  FIGS. 5 and 6  is subject to loosening of the fastening elements  74  and possible separation of the deflector strip  70  as previously described. In addition, because the deflector strips are a separate component, the interface between the strips and the screen also provides an area in which pellets, particularly those having a low profile and a flat or lentoid shape, may become lodged or trapped. As with bolted on deflector strips  70 , the area  69  of the screen just in front of the bevel (see  FIG. 6 ), as viewed with respect to the direction of rotor rotation, is subject to greater wear and resulting erosion. 
     In view of the foregoing, a screen section  115  in accordance with the present invention, having an embossed region in the form of a vertical or substantially vertical embossed deflector strip generally designated by the reference numeral  170  is shown in  FIG. 7 . The embossed deflector strip  170  is typically formed in a non-perforated solid section  136  of the screen that runs between the discrete areas  32  having holes therein of the plate, generally designated by the reference numeral  128 . The screen section  115  may be provided with only a single embossed deflector or multiple deflectors such as, for example, the two embossed deflectors  170  shown in the center and end portions of the screen section  115  of  FIG. 7 . These center and end portion embossed deflectors are depicted photographically in  FIGS. 8 and 9 , respectively. 
     Alternatively, the embossed deflector strip could be formed in the perforated areas  32 , although this is not preferred as structural strength may be impacted. As a further alternative, if produced at an acutely angled orientation relative to the vertical, the embossed deflector strips may be made to extend across or into portions of both the perforated and non-perforated sections of the screen. 
     As illustrated in  FIG. 10 , and like the conventional deflector strip in  FIG. 6 , the embossed deflector strip  170  effectively redirects pellet flow as indicated by the solid arrow  171  when the rotor  16  turns in the direction indicated by the inner arrow  173 . Unlike the deflector strip of the &#39;457 patent, however, the embossed deflector strip eliminates the risk of loosened fasteners as well as loose and/or detached deflector strips. 
     Forming the deflector regions by embossing, whether the regions are embodied in strips or other configurations, also creates a smoothly ramped edge that offers no opportunity for pellet entrapment between the strip and the screen, thus eliminating the associated risk of contamination in future runs. Particularly when used with flat or lentoid shaped pellets, the smooth continuous embossed edge is more effective at ensuring consistent pellet redirection off the screen and into the rotor than earlier designs. Once redirected, the rotor can then impart continual energy to the pellets which facilitates their upward movement and overall efficiency of the dryer by decreasing the tendency for the rotor to act as an auger when pellets are the type that resist current methods of centrifugal drying. In addition, the embossed deflector strips actually reduce screen wear in the area of the screen just in front of the embossed strip. By providing increased wear resistance in the area of the screen where the pellets are deflected, an area that had previously been subject to wear and resulting screen erosion, the embossed deflector strips increase the life of the screen. 
     Further, as can be seen by the photograph of  FIG. 11 , the embossed deflector strip  170  strengthens the solid section  136  of the screen section  115  against bending and flexing. The deflector strip  70  of the &#39;457 patent, by contrast, with its bolted fasteners which necessitate that dedicated holes be located in the perforated or non-perforated areas of the screen, can create stress concentrations within the screen such that the addition of the strip  70  may not offer additional screen structure integrity. 
     The number, angle and spatial relation of the embossed deflector regions may vary depending on the diameter of the screen sections  115  and the particular application of the dryer. Usually one to four embossed deflector strips  170  are adequate in most screen sections up to about 64 inches in diameter; greater numbers of deflector strips may, of course, be included as desired. Also in dryers having multiple screen sections  115 , the lowest screen section  116  (see  FIG. 4 ), where the water and solid particle slurry enter the screen, may be constructed without deflector strips as the pellets have a lot of energy upon entry from the feed chute. In the upper sections having the deflector strips, the strips are preferably aligned vertically, although such alignment is not always necessary. 
     The embossed deflector strips can be implemented with conventional unitary screens of screen sections, as of stainless steel with 20 or 18 gauge thickness and holes having commonly used diameters of 0.038 inch, 0.075, 0.085 inch, or 0.0625 inch. The screens or screen sections can also be made with lasered holes or by other methods of manufacture as would be understood by persons of ordinary skill in the art. Multi-layered screens such as those set forth in co-pending application, Ser. No. 11/017,216, which is commonly owned by the assignee of this application, can also be modified to include the embossed deflector strips in accordance with the present invention. 
     While in a preferred embodiment the embossed deflector strips are vertical or substantially vertical, the embossed deflector strips may be formed at an acute angle relative to the vertical so as to lean away, moving from the bottom of the screen plate  128  to the top thereof, from the direction of the rotor. Such an angled embossed deflector strip  270 , as illustrated in  FIGS. 12 and 13 , may be used to create a more upward trajectory in the movement of the pellets as they impact against and are redirected toward the rotor by the upwardly inclined edges of the embossed deflectors. Vertical and angled deflector strips may also be combined within the same screen as shown in  FIG. 12  in which the arrow  175  indicates rotor direction. 
     The embossed deflector strips  170  of the present invention are typically U-shaped in cross section (see  FIG. 10 ) and protrude inwardly, relative to the non-embossed inner surface of the screen section  115 , by about 0.10 inches to about 0.25 inches, and most preferably about 0.14 inches, and have a width of about 0.25 inches to about 0.80 inches, and most preferably about 0.62 inches. Other dimensions can, of course, be provided and, unlike the mounted deflector strips, do not impact the overall weight of the screen section. For example, in the arrangement of  FIGS. 4-6 , adding a deflector strip having a larger thickness or larger width will add to the weight of the screen section and place greater demands upon the fastening elements in larger dryer applications. With the embossed deflector regions of the present invention, however, the dimensions of the embossed strip simply alter the percentage of the solid section  136  that projects inwardly but do not change the overall weight of the screen section  115 . 
     The length of the embossed deflector strips  170  depends upon the height of the cylindrical screen section  115 , or cylindrical screen if one piece, and are preferably of a length so as to leave a space of about one inch from the top and bottom ends of the deflector strip to the upper and lower edges of the screen section (or screen) so as to not interfere with sealing, although this spacing can be varied as desired. 
     While the embossed deflector strips  170  are preferably continuous raised strips, they could be discontinuous and of smaller length so as to facilitate the required wrapping of the screen around the support rings to obtain the cylindrical configuration. Continuous raised strips are preferred because they tend to provide a continuous length of deflection and to offer a great degree of added stiffening to the cylindrical screen or screen section. As such, it is possible that the embossed strips may allow for a thinner screen plate  128 . 
     If the embossed strips  170  are discontinuous, they might preferably be arranged in a vertical staggered array from adjacent the bottom edge of the screen plate  128  to adjacent the top edge of the screen plate  128 . In such a manner, banding solid particles which miss one raised strip in a circumferential pass around the clearance band would encounter another raised strip in its path. This staggered arrangement could be similarly embodied with angled embossed deflector strips. 
     The embossed regions can also be produced in shapes other than elongated strips. Without being limited thereto, examples of alternate embossed region configurations are shown by the horizontally and vertically staggered rectangles  201  shown in  FIG. 14A , the vertically spaced circles  203  shown in  FIG. 14B , the vertically spaced arrowheads  205  shown in  FIG. 14C  and the vertically spaced rectangles  207  in sloped alignment shown in  FIG. 14D . 
     Whatever the specific configuration of the embossed regions, the embossed deflector screen in accordance with the present invention increases the overall efficiency of the pellet dryer and results in considerable savings in terms of capital investment. More particularly, for a dryer of a given size, greater product flow rates can be achieved with the embossed deflector screen than with conventional dryer screens. As a result, a smaller dryer can be used to effectively meet production requirements that would otherwise have required the purchase of a larger dryer. By avoiding the need for this purchase, cost savings on the order of tens and even hundreds of thousands of dollars can be realized. 
     The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and, accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.