Abstract:
A centrifugal pellet dryer screen ( 540 ) especially adapted for drying polymer pellets and micropellets includes an exterior or outer support screen ( 542 ), an inner screen ( 546 ) of an irregular or rough surface, and an optional middle screen(s) ( 544 ) sandwiched between the outer support screen and the inner screen. The screen layers are in intimate contact and the multi-layer screen assembly produces drier pellets and micropellets exiting the dryer. Plugging of the dryer screens and banding of the pellets or micropellets are significantly reduced.

Description:
[0001]    This patent application is a continuation-in-part of U.S. patent application Ser. No. 11/017,216 filed Dec. 21, 2004, owned by the same assignee as the instant application. 
     
    
     BACKGROUND OF INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to screens incorporated into a centrifugal pellet dryer for drying pellets produced by an underwater, strand, water ring or similar pelletizer that enter the dryer as a water and pellet slurry. More specifically, the present invention relates to centrifugal pellet dryers and dryer screens particularly useful for drying polymer pellets and micropellets. 
         [0004]    The dryer screens of this invention include an exterior or outer support screen or plate, an optional middle screen or screens, and an inner screen. The outer support plate, middle and inner screens are in intimate contact. The screens are supported in a centrifugal pellet dryer and function in a manner similar to that disclosed in U.S. Pat. Nos. 4,447,325 (May 8, 1984), 5,265,347 (Nov. 30, 1993), 6,237,244 (May 29, 2001), and 6,739,457 (May 25, 2004) having common ownership with this application and which are expressly incorporated herein by reference as if fully set forth. 
         [0005]    2. Description of Prior Art 
         [0006]    Much reference has been made to the use of screens, particularly multiple layer screens, in filtration of underground oil, purification of coal, for use in vibratory equipment, and especially for fines removal. Arrangement of the screen layers has been used to facilitate cleaning of the screens, to entrap fines recoverably especially from middle layers of the screens, and to control particulate levels in the filtrate or fluid passing through the screens. 
         [0007]    Prior art centrifugal pellet dryers utilizing a mesh type screen or perforated plate type screen operate effectively when the pellets being dried have a diameter substantially greater than micropellets. Typical prior art screens are self-supporting single sheets generally in the form of a cylindrical screen sheet or plate with either circular holes or slotted holes. The screen sheet or plate is typically perforated in a flat condition and then rolled into the cylindrical screen shape. 
         [0008]    One typical embodiment of a prior art dryer screen having round holes 0.075 inches (1.9 mm) in diameter produced a 50% open area, while still remaining self-supporting. Efforts to form smaller holes by punching the sheet metal resulted in the punches which form the holes breaking off. The smallest diameter hole that can be successfully punched is generally in the range of 0.062 inch ( 1/16 inch) but the use of such small punches reduces the open area to well below 50%. Such known prior art screens also tend to plug up and essentially form a smooth internal surface with very little drag on the pellets engaging the interior of the screen. The smooth surface causes the pellets to move or band in a circular path rather than moving axially upwardly and radially under the action of the inclined blades of the driving rotor in the dryer. 
         [0009]    As used herein throughout this specification, the term “open area” is defined as that area of the screen which is open for water, moisture, or air to flow therethrough. 
         [0010]    There are known prior art screens for use in drying polymer micropellets produced in pelletizers including underwater, water ring, strand, or hot face, for example. Polymer micropellets are very small thermoplastic or other polymer pellets, having a diameter or outside dimension typically less than 0.050 inches (1.3 mm). In known screens for drying such micropellets, the sheet or plate is formed into a cylindrical shape with the holes formed therein, such as by laser cutting or the like. Laser-perforating the holes, however, results in a very smooth interior surface, thus exacerbating the problem of the pellets simply rotating around the interior of the screen without moving upwardly therein, and thereby increasing the tendency of the screen holes or perforations to become plugged by pellets. 
         [0011]    When round holes are used in the prior art polymer micropellet screens, such as in a 22 gauge screen, the holes preferably are approximately 0.40 mm in diameter which produces a retained open area of only about 8.5%. When slotted holes are used, the 22 gauge screen is formed with slots that are typically 0.40 mm in height and 4 mm in length which provides approximately a 14% open area. However, screens with slotted holes tend to crack or tear during use in the centrifugal dryer. 
         [0012]    Drying polymer micropellets in centrifugal dryers has become very difficult using known prior art screens. Because polymer micropellets have a tendency to band around the inner surface of the cylindrical screen, especially when the inner surface is smooth or otherwise not interrupted, the micropellets simply circulate around the inside of the screen, plugging the screen holes, and do not move axially upward with rotation of the dryer rotor. The micropellets move up only through the forced introduction of more micropellets into the dryer inlet. As a result, centrifugal pellet dryers with prior art screens have heretofore been generally ineffective in drying polymer micropellets. Hence, there is a need for a centrifugal dryer screen which will overcome the banding and plugging problems and provide for effective drying of polymer micropellets in a centrifugal dryer. 
         [0013]    The additional prior U.S. patents and U.S. published patent applications which may be pertinent to the present invention are as follows: 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 U.S. Patents 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Re. 28,470 
                 Jul. 8, 1975 
               
               
                   
                 4,126,560 
                 Nov. 21, 1978 
               
               
                   
                 4,290,889 
                 Sep. 22, 1981 
               
               
                   
                 4,293,414 
                 Oct. 6, 1981 
               
               
                   
                 4,295,918 
                 Oct. 20, 1981 
               
               
                   
                 5,076,875 
                 Dec. 31, 1991 
               
               
                   
                 5,145,729 
                 Sep. 8, 1992 
               
               
                   
                 5,182,008 
                 Jan. 26, 1993 
               
               
                   
                 5,411,084 
                 May 2, 1995 
               
               
                   
                 5,915,566 
                 Jun. 29, 1999 
               
               
                   
                 6,510,947 
                 Jan. 29, 2003 
               
               
                   
                 6,514,408 
                 Feb. 4, 2003 
               
               
                   
                 6,573,314 
                 Jun. 3, 2003 
               
               
                   
                 6,715,300 
                 Apr. 6, 2004 
               
               
                   
                 6,894,109 
                 May 17, 2005 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 U.S. Published Patent Applications 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 20040044111 
                 Mar. 4, 2004 
               
               
                   
                 20050126779 
                 Jun. 16, 2005 
               
               
                   
                   
               
             
          
         
       
     
       SUMMARY OF THE INVENTION 
       [0014]    The dryer screen constructed in accordance with the present invention comprises a dryer screen of two or more layers including an outer cylindrical support screen and an inner screen having irregular surfaces. A middle screen or screens can be sandwiched between the inner screen and the outer support screen depending upon the application. 
         [0015]    The outer support screen is typically a foraminous membrane formed from plastic, wire-reinforced plastic, or sheet metal which has been molded or forged, pierced or perforated by punching, laser-cutting or the like to form the holes therein which may be round, square, rectangular, triangular, hexagonal, octagonal or similarly suitable effective geometry. 
         [0016]    Alternatively, the outer foraminous membrane may be a structural assembly of plastic, wire-reinforced plastic, or metal wires, bars or rods which may be round, square, rectangular, triangular, wedge-shaped, hexagonal, or of similar multi-dimensional geometry. These components are interwoven or adhered together in a grid-like manner with similar or different geometries as delineated above to generate the screen structure by thermal bonding, chemical bonding, resistance welding, sintering, diffusion bonding, or by any suitably similar assembly techniques known to those skilled in the art. 
         [0017]    Preferably, the thickness of the outer support screen is between 18 gauge (about 0.05 inches) and 22 gauge (about 0.0312 inches), and most preferably about 20 gauge (0.0375 inches). Stainless steel sheet material has been found most suitable for the present invention. Preferably, the holes or openings are round perforations having a hole size of at least about 0.075 inches in diameter. The open area of the outer support screen should be at least about 30%, and preferably about 50%, or more. 
         [0018]    The inner screen and optional middle screen or screens may have the structure and be made by any of the techniques described above for the outer support screen. The individual screens may be similar or different in structure and composition and may be the same or different in percent open area, i.e., the portion of the screen through which fluid, air, and smaller diameter materials may pass unobstructedly. The open area geometries of the individual screens may be oriented laterally, longitudinally, or rotationally relative to the other screen layers. 
         [0019]    Preferably, the inner screen and optional middle screen or screens are a woven wire screen which may be in a square, rectangular, plain, Dutch or similar weave. While the warp and weft wire diameters may differ dimensionally and compositionally, the inner and middle screen or screens are preferably a plain square or rectangular weave screen wherein the warp and weft wires are of the same size and made of the same material. The percent open area is preferably 30% or greater. Most preferably, the inner screen and optional middle screen or screens are 30 mesh grade  304  or grade  316  stainless steel, wherein the warp and weft wires are of a size to allow at least 30% open area and, most preferably, at least 50% open area, or more. 
         [0020]    The adjacent screens are in intimate contact and may remain unbound or may have their surfaces bonded together, preferably bonded together. Bonding of the surfaces may be achieved by chemical or thermal adhesion, locally by spot welding or brazing, resistance welded, or preferably they may be diffusion bonded or sintered at all adjacent contact points throughout their surface areas. This attaching mechanism reduces the tendency of the inner screen and/or middle screens to slip or wrinkle with respect to the outer supporting screen or plate during use in the operation of the centrifugal dryer. 
         [0021]    It has been surprisingly found that the multi-layer dryer screens of the present invention can have very small inner screen openings that will retain the small polymer micropellets within the screen enclosure. At the same time, the multi-layer dryer screens of the present invention provide a high percentage of open area to allow water, air, and/or fines to pass out of the dryer screen at a higher rate. Typically, the open area is of the multilayer dryer screens in accordance with the present invention should have an open area of about 30%, or more. 
         [0022]    It has also been found that irregular surfaces on the middle screen or screens and particularly on the inner screen cause the pellets to bounce radially inwardly in a random fashion when impacting against the inner surface of the screen. This random inward movement or bouncing of the pellets allows the rotating inclined blades on the rotor to more effectively elevate the pellets and to more effectively direct the pellets outwardly for continued impacting engagement with the irregular surfaces of the inner screen. This recirculation of the pellets radially inwardly and outwardly in relation to the screen produces a more effective removal of surface water or moisture from the pellets and discharge of such moisture through the screen, while retaining the pellets or micropellets interiorly of the screen and moving the pellets axially upwardly within the screen. 
         [0023]    Additionally, it has been found that the orientation of the irregularities of the middle screen or screens and particularly of the inner screen facilitates the sweeping action of the rotor and lifter blades to aid in moving the pellets and especially micropellets from the surface of the screens. This movement of the pellets leads to a reduction in banding of the pellets and clogging of the screens which otherwise occurs by entrapment of the pellets physically within the screen or by action of the water and pellet slurry against the pellets on the irregular screen surface. 
         [0024]    In addition, the multi-layer dryer screens of the present invention result in drier polymer micropellets, i.e. less surface moisture, upon exiting the centrifugal dryer. While intending not to be bound by any theoretical explanation, it is believed that the drier micropellets exiting the dryer are a direct result of the irregular surface of the dryer inner screen, which produces a more effective removal of the surface water or moisture from the pellets, and the high percentage of screen open area in the neighborhood of 30%, as stated above. The high percentage of open area permits a greater volume of air to flow into the top of the dryer and/or the pellet discharge outlet, and then through the screen. This increased air flow further assists in removing the surface water or moisture from the pellets as they rise inside the screen and air flows through the pellets entrained therein. 
         [0025]    Accordingly, it is an object of the present invention to provide a screen assembly for a centrifugal pellet dryer especially useful for drying polymer pellets and micropellets which includes an outer support screen or plate combined with at least one inner screen; the inner screen has openings appropriate to the diameter of the polymer pellets to be retained within the interior of the screen while enabling passage of surface water or moisture and fines from the pellets out through the screen during rotation of the dryer rotor. 
         [0026]    A further object of the present invention is to provide a multi-layer dryer screen in accordance with the preceding object which dries the pellets, especially polymer micropellets, to a lower percentage of moisture when they are discharged from the centrifugal dryer. 
         [0027]    Another object of the present invention is to provide a polymer pellet and micropellet screen for a centrifugal dryer in which the inner screen is provided with an irregular, rough interior surface to cause random radial movement of the micropellets for more effective removal of moisture, and to eliminate the tendency of polymer pellets, especially micropellets, to band and move in generally a circular path around an otherwise smooth surface on the interior screen surface. 
         [0028]    Still a further object of the present invention is to provide a dryer screen in accordance with the preceding object in which the plugging of the screen openings is substantially reduced as a result of the irregular, rough interior surface of the inner screen and is further enhanced by the orientation of open area geometry of the inner screen. 
         [0029]    Still another object of the present invention is to provide a dryer screen in accordance with the preceding objects in which the screen includes a plurality of screens having a high percent open area to provide maximum air flow from the top of the dryer and/or pellet discharge opening through the pellets and screen for a more effective drying of the pellets during their movement upwardly within the screen enclosure. 
         [0030]    Yet a further object of the present invention is to provide a multi-layered dryer screen in which the screen layers are diffusion bonded or sintered at all contact points throughout their full surface areas to reduce the tendency of the inner screen to slip or wrinkle during operation of the centrifugal dryer. 
         [0031]    Yet another object of the present invention is to provide a multi-layered dryer screen in which the inner surface of the inner screen is irregular, rough, undulated, or provided with ridges and valleys to reduce the tendency of pellets, especially polymer micropellets, to band along the inner surface in a circular path rather than moving axially upwardly and radially inwardly and outwardly by the inclined blades on the dryer rotor. 
         [0032]    A final object of this invention to be specifically enumerated herein, is to provide a multi-layered dryer screen in accordance with the preceding objects which will conform to conventional forms of manufacture, be of simple construction and easy to use so as to provide a centrifugal pellet dryer screen that will be economically feasible, strong and long lasting, and relatively trouble free for installation and use. 
         [0033]    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 made to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  is a schematic side-elevational view of one type of existing centrifugal pellet dryer, similar to FIG. 6 in U.S. Pat. No. 6,237,244, illustrating one application of a dryer screen according to the present invention associated with the operational components of the dryer. 
           [0035]      FIG. 2  is a side elevational view of the centrifugal pellet dryer of  FIG. 1  and similar to FIG. 3 in U.S. Pat. No. 6,237,244, illustrating the dryer screen according to the present invention relationally associated with the lifted operational components of the dryer. 
           [0036]      FIG. 3  is a vertical sectional view, on an enlarged scale, illustrating specific structural details of another type of existing centrifugal pellet dryer, similar to FIG. 3 in U.S. Pat. No. 5,265,347, for a dryer screen according to the present invention. 
           [0037]      FIG. 4  is a perspective view of a hinged construction, similar to FIG. 7 in U.S. Pat. No. 5,265,347, used operationally in association with the dryer shown in  FIG. 3  for a dryer screen according to the present invention. 
           [0038]      FIG. 5  is a schematic elevational view of another type of existing centrifugal pellet dryer, similar to FIG. 1 in U.S. Pat. No. 6,739,457, illustrating a sectional cylindrical dryer screen according to the present invention associated with the operational components of the dryer. 
           [0039]      FIG. 6  is an elevational view of a dewatering screen similar to FIG. 1 of U.S. Pat. No. 4,447,325, optionally used with dryers included in this invention, such as illustrated in  FIG. 5 , which dewatering screen can be made in accordance with the present invention. 
           [0040]      FIG. 7  is a transverse sectional view taken substantially through a plane indicated by sectional, line B-B in  FIG. 6 . 
           [0041]      FIG. 8  is a plan view of one of the screen sections shown in  FIG. 5 , made in accordance with the present invention and illustrating the outside surface of the screen in a flat condition prior to it being formed into a cylindrical screen section and showing the mounted deflector bars. 
           [0042]      FIG. 9  is an edge view of the screen section illustrated in  FIG. 8 . 
           [0043]      FIG. 10  is a sectional view, on an enlarged scale, taken along section line A-A on  FIG. 8 , illustrating the structure on one of the deflector strips and its mating mounting strip including their association with the screen and the fastening structure for securing the strips to the screen. 
           [0044]      FIG. 11  is a plan view of one of the screen sections shown in  FIG. 5 , made in accordance with the present invention and illustrating the outside surface of the screen in a flat condition prior to it being formed into a cylindrical screen section which does not use deflector bars. 
           [0045]      FIG. 12  is an edge view of the screen section illustrated in  FIG. 11 . 
           [0046]      FIG. 13   a ,  13   b ,  13   c ,  13   d ,  13   e , and  13   f  are schematic views of exemplary configurations of various three layer screen constructions made in accordance with the present invention. 
           [0047]      FIGS. 14   a ,  14   b ,  14   c , and  14   d  are schematic views of exemplary configurations of various two layer screen constructions made in accordance with the present invention. 
           [0048]      FIG. 15  is a fragmental elevational view of a segment of a 3-layer micropellet dryer screen constructed in accordance with the present invention and viewed from the outer support screen. 
           [0049]      FIG. 16  is a sectional view taken along line C-C of  FIG. 15  showing an outer support screen, a middle wire mesh screen, and an inner wire mesh screen of different mesh size, more particularly a larger mesh size or smaller opening size, as shown. 
           [0050]      FIG. 17  is a sectional view similar to  FIG. 16 , but illustrating a 2-layer screen having an outer support screen and an inner wire mesh screen. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0051]    Although several preferred 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 preferred 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. 
         [0052]    Referring specifically to  FIG. 1  of the drawings, the polymer pellet and micropellet dryer screen of the present invention is generally designated by reference numeral  10 .  FIG. 1  discloses the association of the pellet screen  10  within one typical centrifugal pellet dryer generally designated by reference numeral  12  that is disclosed in detail in U.S. Pat. No. 6,237,244. The centrifugal pellet dryer  12  includes a water and pellet slurry inlet  14  at its lower end and a dried pellet outlet  16  at its upper end. Inlet  14  communicates with the interior of the screen  10  near its lower end and the outlet  16  communicates with the screen  10  near its upper end. A rotor generally designated by reference numeral  18  is rotatably positioned within the screen  10  and is driven by a motor  20  drivingly connected to the rotor, shown at its upper end in the configuration of  FIG. 1 . The rotor  18  includes inclined blades  21  rotating within the screen  10  to move the water and pellet slurry upwardly and to move the pellets and water radially outwardly into impacting engagement with the interior of the screen  10 . The impact against the interior of the screen  10  causes water to be discharged outwardly of the screen into a housing generally designated by reference numeral  22  for downward gravity discharge, such as into a water tank generally designated by reference numeral  24 . 
         [0053]      FIG. 2  illustrates additional details including the lifting and swivel operations of the dryer illustrated in  FIG. 1 . The support tube  28  facilitates raising the housing  22  directionally as indicated by the arrow  32  for removal of the screen  10  from around the rotor  18 . The structure swivels by rotation of the movable telescopic tube  30  as indicated by the directional arrow  34 . Orifice  26  facilitates discharge of the water from the water tank as required. The details of the structure and operating mechanisms follow that disclosed in the aforementioned U.S. Pat. No. 6,237,244. 
         [0054]    The screen  10  as shown in  FIG. 2  in this application of the current invention is a self-supported cylindrical structure which is delineated by a peripheral wall, at both the lower end  38  and upper end  39  of the screen  10 . Optionally, one or more structural supports  40  are attached to each of the peripheral end walls  38  and  39 , respectively. Affixed to the upper peripheral wall  39  or optionally attached to one structural support  40  is an upwardly extending hook  36  which is oriented diametrically and opens peripherally outward to attach to the support structure mechanistically analogous to that described in U.S. Pat. No. 6,237,244 referenced previously. 
         [0055]    Referring next to  FIG. 3 , the polymer pellet and micropellet dryer screen generally designated by reference numeral  110  is another embodiment of the present invention. The centrifugal pellet dryer associated with screen  110  of this embodiment is generally designated by reference numeral  112  and is disclosed in detail in U.S. Pat. No. 5,265,347. The centrifugal pellet dryer  112  includes a simulated water and pellet slurry inlet  114  at its lower end and a dried pellet outlet  116  at its upper end. Inlet  114  communicates with the interior of the screen  110  near its lower end and the outlet  116  communicates with the screen  110  near its upper end. A rotor  118  is rotatably positioned within the screen  110  and is driven by a motor, not shown, drivingly connected to the rotor via a belt affixed to the pulley  119 . The rotor  118  includes inclined blades  121  rotating within the screen  110  to move the water and pellet slurry in a manner similar to that previously described for  FIG. 1 . Water which has been removed from the pellets flows out of the housing  122  through the drain pipe  123  directionally as indicated by arrow  125 . 
         [0056]    As shown in  FIG. 4 , the screen  110  in this embodiment of the current invention is self-supporting and consists of two semi-cylindrical structures  141  and  142  hingedly connected by a vertical hinge  144 . The screen  110  is supported with peripheral edge frame members  148  and a central belt frame member  150  which maintain its shape and configuration. Once placed in the centrifugal pellet dryer shown in  FIG. 3 , the semi-cylindrical elements  141  and  142  are connected to each other utilizing quick-acting latches  152 . Construction details are similar to those described in the aforementioned U.S. Pat. No. 5,265,347. 
         [0057]    A further embodiment of the present invention finds application to dryers incorporating screens as exemplified in  FIG. 5  and similar in design to FIG. 1 of the previously cited U.S. Pat. No. 6,739,457. Multiple screen sections within the centrifugal pellet dryer in this embodiment are designated by reference numeral  210 . The centrifugal pellet dryer  212  includes a water and pellet slurry inlet  214  at the upper end of an auxiliary dewatering device generally designated as  300 , and a dried pellet outlet, not shown, at the upper end of the dryer. Inlet  214  communicates with the interior of the dewatering screen or screens  310  which further communicate with the feed screen  510  oriented at an angle to convey the pellets with significantly reduced water content into the lower end of the base screen  410  of the centrifugal dryer. The excess water removed through the dewatering screen(s)  310  and feed screen  510  passes through outlet  226 . Details of the design follow the aforementioned U.S. Pat. No. 4,447,325 and are further illustrated in  FIGS. 6 and 7  of the accompanying drawings, which are similar to  FIGS. 2 and 3 , respectively, of U.S. Pat. No. 4,447,325. A flange  315  (see  FIG. 6 ) connects the dewatering screen  310  directly to the feed screen  510 . Details of the dewatering screen  310  as viewed across the B-B line are reflected in  FIG. 7  wherein the screen end angles  338  and  339  are joined by the flange connector  360 . 
         [0058]    The significantly dewatered pellets feed into the lower portion of the screen section  410  and through screen sections  210 , which may be the same or different dimensions than is  410 , and subsequently to the outlet, not shown, which communicates with the uppermost screen section  210  near its upper end. A rotor  218  is rotatably positioned within the screen sections  210  and  410  and is driven by a motor, not shown, drivingly connected to the rotor via a belt affixed to the pulley, also not shown. The rotor  218  includes inclined blades  221  rotating within the screen sections  210  and  410  to move the water and pellet slurry in a manner as previously described with respect to  FIG. 1 . Water which has been removed from the pellets flows out of the housing  222  through the drain pipe  226 . 
         [0059]    The screens  210  and  410  are fixed into position by a clamp or clamps  260  which affix the screen end angles,  238  and  239 , to each other. The screen sections are held in place, vertically aligned and interconnected by ring supports  237 . One, two, three, or more screen sections may be fixedly attached vertically as necessitated by production rates and product moisture level reduction specifications. 
         [0060]      FIG. 8  illustrates an alternate screen assembly  265  wherein the screens are supported by deflector bars  294  held in place by bolt assemblies  290  affixed diametrically to solid support structures  286  across the width of the screen assembly  265 . Structural screen assembly supports  284  traverse the screen along its length similarly. These supports  284  and  286  segmentalize the screen area  282  in approximately equal proportions. Details of the construction of this screen are delineated in the aforementioned U.S. Pat. No. 6,739,457. An edge view,  FIG. 9 , and a detailed bolt assembly,  FIG. 10 , are shown diagramatically for the screen assembly  265 . 
         [0061]    Alternatively, screens  210 ,  310 , and/or  410  may have an overall construction as illustrated in  FIGS. 11 and 12 . The screen construction shown in  FIGS. 11 and 12  differs from that shown in  FIGS. 8-10  in that the  FIGS. 11 and 12  construction does not contain the deflector bars  294 , and consequently no bolt assemblies  290 , nor do they require the structural screen assembly supports  286  across the width of the screen. 
         [0062]    A port screen  610  is similarly affixed in place and is positioned nearest the effluent opening, not shown, in  FIG. 5  and described in detail in the aforementioned U.S. Pat. No. 6,739,457. Alternatively, a port screen may be at the base of the screen chamber, below screen section  410  in  FIG. 5 , and positionally not indicated but included herein by way of reference. 
         [0063]    The cylindrical screens  10 , hinged screens  110 , screen panels  210  and  410 , dewatering screens  310 , feed screens  510 , and port screens  610  are all dryer screen embodiments which can be made according to the present invention. Compositionally and structurally, they may be the same or different from other screen structures in a particular dryer assembly. 
         [0064]    In accordance with the present invention, the dryer screens are compositionally two or more layers functionally consisting of an outer support screen and an inner screen which accomplishes the effective drying of the pellets and micropellets. Additionally one or more screen layers may be sandwiched between the outer support screen and the inner screen depending upon the particular application. Exemplary embodiments of the present invention are shown in  FIGS. 13   a  through  13   f  for three-layer screens, and in  FIGS. 14   a  through  14   d  for two-layer screens. 
         [0065]    The three-layer dryer screen assemblies illustrated in  FIG. 13   a  through  13   f  are generally designated by reference numeral  450   a  through  450   f , respectively. They include an outer support screen, identified by  452   a  through  452   f , respectively, which provides structural support to the screen assembly. The outer support screens  452   a - 452   f  may be composed of molded plastic or wire-reinforced plastic and compositionally may be polyethylene, polypropylene, polyester, polyamide or nylon, polyvinyl chloride), polyurethane, or similarly inert material which capably maintains its structural integrity under chemical and physical conditions anticipated in the operation of centrifugal pellet dryers. Preferably the outer support screens  452   a - 452   f  are a metal plate of suitable thickness to maintain the structural integrity of the overall screen assembly  450  and flexible enough to be contoured, exemplarily cylindrically, to fit tightly and positionally in the appropriate centrifugal pellet dryer. The metal plate is preferably 18 gauge to 22 gauge and most preferably 20 gauge in thickness. The metal may compositionally be aluminum, copper, steel, stainless steel, nickel steel alloy, or similarly non-reactive material inert to the components of the drying process. Preferably the metal is stainless steel and most preferably is Grade 304 or Grade 316 stainless steel as necessitated environmentally by the chemical processes undergoing the drying operation. 
         [0066]    The metal plate may be pierced, punched, perforated, or slotted to form openings which may be round, oval, square, rectangular, triangular, polygonal, or other dimensionally equivalent structure to provide open areas for separation and subsequent drying. Preferably the openings are round perforations and geometrically staggered to provide the maximum open area while retaining the structural integrity of the outer support screen. The round perforations are preferably at least 0.075 inches in diameter and are positionally staggered to provide an open area of at least 30%. More preferred is an open area geometric orientation such that the effective open area is 40 percent or more. Most preferred are round perforations having a diameter of at least 0.1875 inches which are positionally staggered to achieve an open area of 50 percent or more. 
         [0067]    Alternatively, the outer support screen may be an assembled structure or screen composed of wires, rods, or bars, stacked angularly or orthogonally, or interwoven, and welded, brazed, resistance welded or otherwise permanently adhered in position. The wires, rods, or bars may be plastic or wire-reinforced plastic compositionally similar to the molded plastic described above for outer support screens  452   a - 452   f  or may be metal, similarly and compositionally delineated above also for the outer support screens  452   a - 452   f , and may be geometrically round, oval, square, rectangular, triangular or wedge-shaped, polygonal or structurally similar. The wires, rods, or bars across the width or warp of the screen may be the same or different dimensionally as the wires, rods, or bars longitudinally contained as the weft, shute, or otherwise known to those skilled in the art. 
         [0068]    Preferably the wires, rods, or bars are a minimum of 0.020 inches in the narrowest dimension, more preferably are at least 0.030 inches in the narrowest dimension, and most preferably are about 0.047 inches in the narrowest dimension. Open areas are dimensionally dependent on the proximal placement of adjacent structural elements and are positionally placed so as to maintain a percent open area of at least about 30 percent, more preferably above about 40 percent, and most preferably about 50 percent or greater. 
         [0069]      FIGS. 13   a - f  show perforated outer plates identified as  452   a - 452   d , a slotted or pierced outer plate identified as  452   f  and a structural assembly of resistance-welded round rods and wedge-shaped rods identified as  452   e . Outer support screens  482   a - d  are similarly illustrated for 2-layer screens  480   a - d  of the present invention in  FIGS. 14   a - d  wherein the outer support screen is the leftmost element in the drawing views. Outer support screens  482   a  and  482   b  are in the form of perforated plates, screen  482   c  is a slotted screen and screen  482   d  is a pierced screen. 
         [0070]    The optional middle screen or screens and the inner screen are structurally similar to that described herein for the outer support screen. Dimensionally and compositionally the screens in the respective layers may be similar or different. The percent open area of the respective screens may be similar or different wherein lesser percent open area will reduce the effective open area of the screen and the least percent open area will be the most restrictive and therefore the delimiting percent open area for the screen assembly. The middle screens are identified in  FIGS. 13   a - f  by numerals  454   a - f , respectively. They include, by way of example, woven wire screens  454   a ,  454   e  and  454   f , slotted screen  454   b  resistance welded bar screen  454   c  and pierced screen  454   d . Exemplary inner screens are similarly illustrated in  FIGS. 13 and 14  by numerals  456   a - f  and  484   a - d , respectively. The orientation of any screen relative to other layers of the assembly as well as the dimensions and structural composition of the screens may be similar or different as shown by way of example in  FIG. 13   f  with a reduced mesh size for inner screen  456   f  relative to middle screen  454   f  in  FIG. 13   e  with the angle of orientation of inner screen  456   e  rotated relative to middle screen  454   e.    
         [0071]    Preferably the middle screen  454   a - f  is a woven wire screen which may be in a square, rectangular, plain, Dutch or similar weave wherein the warp and weft wire diameters may be the same or different dimensionally or compositionally. More preferably the middle layer is a plain square or rectangular weave wire screen wherein the warp and weft wires are similar compositionally and dimensionally and the percent open area is 30 percent or greater. Even more preferably the middle layer screen is 30 mesh grade  304  or grade  316  stainless steel wherein the warp and weft wires are of a size to allow at least 30 percent open area and most preferably are 50 percent open area or more. Multiple middle screens are included within the embodiments of the current invention and may be similar or different than another middle layer screen structurally and compositionally. 
         [0072]    The inner screen  456   a - f  and  484   a - e  preferably is a woven wire screen which may be in a square, rectangular, plain, Dutch or similar weave wherein the warp and weft wire diameters may be the same or different dimensionally or compositionally. More preferably the inner screen is a plain square or rectangular weave wire screen wherein the warp and weft wires are similar compositionally and dimensionally and the open area is 30 percent or greater. Even more preferably, the inner layer screen is plain square or rectangular 30 mesh or larger mesh grade  304  or grade  316  stainless steel wherein the warp and weft wires are of a size to allow at least 30 percent open area and most preferably are 50 percent open area or more. Still more preferred is an inner screen of a plain square or rectangular weave of 50 mesh or greater mesh, with a percent open area of 50 percent or greater oriented similarly to the middle screen orientation, when a middle screen is present in the structural assembly. Most preferred is a rectangular weave of 50 mesh or greater mesh where the warp and weft are compositionally and structurally similar, allowing an open area of 50 percent or greater. As is readily apparent to those skilled in the art, the higher the mesh the smaller the diameter of the pellet, and preferably the micropellet, which will be retained by the screen and ultimately dried through the drying process. 
         [0073]    Essentially, the pellet and micropellet screens of the present invention, such as screens  10 ,  110 ,  210 ,  310 ,  410 ,  510  and  610 , can be utilized in combination with the components of the centrifugal dryers as described above and shown in the aforementioned U.S. patents or in combination with any other centrifugal pellet dryer which can accommodate the screen of the present invention and serve to dry pellets, especially polymer micropellets. 
         [0074]      FIG. 15  schematically illustrates a three layer screen  540  in accordance with the present invention as viewed through the perforated outer support screen  542  to the plain square woven middle screen  544  to the larger mesh (smaller openings) plain square woven inner screen  546 .  FIG. 16  shows this structure in cross-section at line C-C in  FIG. 15 . A comparable two layer screen  560  is illustrated in  FIG. 17  with a plain square woven inner screen  562  bonded to perforated outer screen  564 . 
         [0075]    The component layers of the multi-layer screens of the present invention are in intimate contact and may be bonded together. Preferably the individual layers are thermally bonded, chemically bonded, soldered, spot-welded, brazed, resistance welded, diffusion bonded or sintered. The preferred configurations of the screen are most preferably diffusion bonded or sintered at all contact points between each of the component screens. The screens may be rolled, drawn, calendered or otherwise compressionally altered as is understood by those skilled in the art. Preferably the screens of the preferred embodiments are calendered. 
         [0076]    The multi-layer dryer screens of the present invention have been specifically described for 3-layer and 2-layer embodiments. It is believed that the middle layer in the 3-layer embodiment actually increases the open area of the dryer screen and serves as a drainage field for the water escaping through openings of the inner screen, thus providing for more rapid removal of the water and moisture during the drying operation. In addition, those skilled in the art will readily appreciate that 3-layers and 2-layers are preferred for the multi-layer screens of the present invention, but additional layers beyond three may be possible, such as four layers, or more if desired. 
         [0077]    While the centrifugal pellet dryer screens of the present invention have been described as especially useful for drying polymer micropellets, the dryer screen of the present invention can be useful in drying other size and type pellets particularly where the pellets being dried have a tendency to band and circulate around the screen, rather than moving axially up the screen towards the dryer exit, or otherwise tend to plug the screen holes. Exemplary of the materials for which the dryer screens of the present invention can be useful are filled or unfilled polyethylene (PE) including low density (LDPE), linear low density (LLDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), ultrahigh molecular weight polyethylene (UHMWPE), polypropylene including amorphous and crystalline, acrylonitrile-butadiene-styrene (ABS), styrene-acrylonitrile (SAN), polystyrene, polyesters, polyamides or nylon, polycarbonates, polyacrylics, polyacetals, polyurethanes, expandable polystyrene (EPS), expandable polyethylene (EPE) and expandable polypropylene (EPP), thermoplastics including elastomers and thermoplastic rubbers in general regardless of how they are made. 
         [0078]    The foregoing is considered as illustrative only of the principals of the invention. Further, numerous modifications and changes will readily occur to those skilled in the art. Therefore, it is not intended to limit the invention to the exact construction and operation shown and described, and all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.