Abstract:
A deflector associated with the internal surface of a cylindrical screen in a conventional centrifugal pellet dryer. In a preferred form, the deflector is one or more vertical deflector strips mounted in circumferentially spaced relation on the interior surface of the dryer cylindrical screen by bolting to mating mounting strips on the outside of the screen. The deflector strips disrupt the circular flow of the particles along the interior surface of the screen. The edge of the deflector strip facing the direction of rotational movement of the rotor and circulating particles is preferably inclined to deflect the particles back toward the rotor. In another form, the elongated deflector strip has generally an angle iron shape with one flange clamped between the outwardly extending vertical side edge flanges which are bolted together to form the cylindrical screen and the other flange extending at an oblique angle into the clearance band to deflect the circulating particles back toward the rotor. The elongated deflector strip results in more efficient operation of the rotor and increasing production rates while decreasing power requirements for operation of the dryer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    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 with the centrifugal force imparted to the particles by rotation of the lift rotor causing the particles to engage the interior surface of the screen, and moisture on the particles to be discharged through the screen in a manner well known in the art. More specifically, the present invention relates to a deflector associated with the internal surface of the cylindrical screen.  
           [0003]    2. Description of the Prior Art  
           [0004]    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 in the screen. The rotor moves water laden pellets or other solid particles upwardly within the screen with centrifugal forces imparted to the particles by the rotor 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. 3,458,045; 4,565,015; 4,896,435; 5,265,347 and 6,138,375, 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.  
           [0005]    However, with the advent of newer plastic materials which form softer pellets, and the making of very small pellets, or so-called micropellets, on underwater pelletizers, difficulties have been encountered in drying such pellets in known centrifugal dryers. Further, known centrifugal dryers have encountered difficulty in 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, softer and smaller pellets and plastic flakes, as well as certain other plastic and similar particles, tend to collect and circulate in the clearance band 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. This circular flow of the softer and smaller pellets and plastic flakes and particles along the inner surface of the screen is sometimes referred to as “banding” and 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 surge requirements within the dryer, and reduces overall efficiency of the centrifugal dryer.  
           [0006]    Accordingly, there is a need to improve existing centrifugal dryers in order to overcome this problem of banding when separating solid particles and water slurries, especially with soft and/or small pellets and certain plastic flakes and particles.  
         SUMMARY OF THE INVENTION  
         [0007]    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 in the cylindrical screen for conveying a slurry of water and polymer resin particles upwardly in the screen. 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 the dried particles are discharged from an upper end of the screen 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.  
           [0008]    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, the inside of the cylindrical screen is provided with a deflector in the form of one or more vertical elongated strips provided on the inside surface of the screen. The edge or side of the strip which faces the direction of rotation of the blades on the rotor, and thus the solid particles being circulated by those blades, is preferably inclined from a leading edge at the screen to a trailing edge so that particles moving in a rotational direction or circulating along the surface of the screen are deflected back toward the blades on the rotor. The vertical strip disrupts the circular flow of the particles, particularly softer and smaller pellets, plastic flakes, and certain other plastic particles, and directs the solid particles radially inward toward the rotor blades. This disruption of the circular flow improves particle flow through the rotor area of the dryer by aiding in the rotor&#39;s vertical lift of the particles and by eliminating particles banding.  
           [0009]    Further, the deflector of the present invention reduces the power required for rotating the rotor at a desired speed and eliminates shutdown of the dryer due to high amperage surges required to drive the rotor when the plastic particles are banding along the inner surface of the screen, which banding increases the load on the bladed rotor. In addition, the preferred deflector structure of the present invention also assists in stiffening the cylindrical configuration of the screen.  
           [0010]    Preferably in accordance with the present invention, all of the cylindrical screen sections of the centrifugal dryer, except for the lowermost screen section, are provided with deflector strips mounted on or extending into the interior surface of each screen section. The deflector strips are preferably aligned vertically if mounted in more than one screen section. Further, one to four deflector strips are preferred for each section depending upon the size of the screen, with the deflector strips circumferentially spaced around the interior surface of the screen section when two or more are utilized. Small diameter cylindrical screens may require only a single deflector strip, whereas larger diameter cylindrical screens may require three or four, or more, circumferentially spaced deflector strips.  
           [0011]    The deflector strips are disposed in the clearance band or annular space between the outer edges of the rotor blades and the interior surface of the screen. Usually, the outer edges of the rotor blades are spaced from the screen a distance of about ⅜ inch to about ½ inch, and the deflector strips therefore have a thickness, or project, less than this clearance space between the screen and blade edges. A preferred thickness for the strips has been found to be about 0.10 inches to about 0.25 inches, and most preferably about 0.14 inches. A preferred width for the deflector strips has been found to be between about 0.50 inches and about 0.80 inches, but more in one embodiment. In the preferred embodiment, the preferred width is about 0.62 inches. The edge of each strip facing the direction of rotation of the bladed rotor is preferably beveled or inclined from a leading edge at the screen surface to a trailing edge at the inner surface of the strip to form an inclined surface. In one form, the inclined surface is preferably angled at about 45°, although other angles could be adopted. In another form, the inclined surface is preferably angled at about 70°. While not preferred, the deflector strips of the present invention could present a leading edge without any bevel or incline.  
           [0012]    The deflection of the solid particles toward the rotor is especially significant when the particles being dried have a tendency to collect on and move along the inner surface of the screen rather than bouncing back toward the rotor as occurs when drying harder or large pellets and particles. The deflector strip interrupts the circular flow of the particles around the clearance band inside the screen, and the inclined surface deflects the particles back toward the rotor. The deflector strips therefore eliminate the tendency of the particles to flow in a band around the inner circumference of the screen, and cause the particles to move radially back toward the rotor blades thereby increasing product flow and efficiency of moisture separation and reducing power requirements and high amperage surge requirements which sometimes can result in temporary shutdown of the dryer.  
           [0013]    The deflector strips can be attached to the cylindrical screen or sections by any convenient attaching mechanism. One preferred mechanism includes a mating mounting strip having approximately the same strip size and shape, which mounts to the exterior surface of the cylindrical screen or section. The deflector strip on the inside of the cylindrical screen and the mounting strip on the outside of the cylindrical screen can then be bolted together through the wall of the cylindrical screen. The bolt head is preferably positioned on the deflector strip side, and counter sunk to present a smooth surface toward the rotor. The mating interconnected deflector strips and mounting strips also serve to stiffen the cylindrical screen or screen sections. While such a mechanism is preferred for mounting the deflector strips vertically within the cylindrical screens, other mechanical and other type mechanisms will readily occur to those skilled in the art.  
           [0014]    In another form of deflector strip in accordance with the present invention, the elongated deflector strip is in the general form of an angle iron with the angle between the two flanges being approximately 110°, rather than the standard 90°, or perpendicular, relationship. In this form, one of the flanges of the angle iron is clamped between the outwardly extending vertical side edge flanges which are bolted together in forming the cylindrical screen or screen section. In addition to being clamped, the flange is also preferably welded to one of the side edge flanges. The other flange of the deflector strip extends into the clearance band or annular space between the outer edges of the rotor blades and the interior surface of the screen to present an oblique inclined surface to the advancing particles circulating in the annular space. The angle Of the inclined surface is on the order of preferably 160° from a plane tangent to the cylindrical screen or section inner surface.  
           [0015]    It is therefore an object of the present invention to provide a deflector on the interior of a cylindrical screen of a centrifugal pellet dryer in the form of one or more vertical elongated deflector strips which deflect the solid particles moving along the interior surface of the screen back toward the rotor conveying the moisture laden particles upwardly in the dryer screen in order to increase particle flow and more efficiently separate moisture from the particles.  
           [0016]    Another object of the present invention is to provide one or more vertical deflector strips circumferentially spaced around the interior surface of the screen with the number of deflector strips being varied depending upon the diameter of the screen, with there preferably being one to four deflector strips.  
           [0017]    A further object of the present invention is to position one or more vertical deflector strips on the interior surface of a cylindrical screen in a centrifugal pellet dryer such that the edge or side of the strip facing the direction of rotation of the rotor is inclined with the inclined surface having a leading edge adjacent the screen in advance of the trailing edge adjacent the path of the rotor blades to form a surface which deflects the pellets or other solid particles toward the rotor as the particles move along the interior surface of the screen.  
           [0018]    Still another object of the present invention is to position one or more vertical deflector strips on the interior surface of the cylindrical screen in a cylindrical pellet dryer in accordance with the preceding objects in which the inclined leading surface on each deflector strip interrupts the tendency of certain type plastic pellets and other particles to collect and circulate in the clearance band along the inner surface of the screen, thereby more efficiently enabling separation of moisture from the particles by causing them to move back toward the rotor for engagement with the rotor blades.  
           [0019]    Still a further object of the present invention is to mount one or more deflector strips on the interior surface of the cylindrical screen in a cylindrical pellet dryer by means of a mounting strip having approximately the same strip size and shape which mounts to the exterior surface of the cylindrical screen opposite the deflector strip which then can be affixed together through the wall of the cylindrical screen by an appropriate bolt or other fastener connection.  
           [0020]    Yet another object of the present invention is to provide a deflector strip in the general form of an angle iron which can be clamped between the outwardly extending vertical side edge flanges, and preferably welded to one flange, which are bolted together to form the cylindrical&#39;screen so as to position the other flange on the interior surface of the cylindrical screen to present an oblique inclined angle to the solid particles which collect and circulate in the clearance band along the inner surface of the screen.  
           [0021]    Yet a further object of this invention to be specifically enumerated herein is to provide a deflector 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 device that will be economically feasible, long lasting and relatively trouble free in operation.  
           [0022]    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  
       [0023]    [0023]FIG. 1 is a schematic elevational view of a centrifugal pellet dryer illustrating a sectional cylindrical screen and bladed left rotor assembly associated with a dryer housing.  
         [0024]    [0024]FIG. 2 is a plan view of one of the screen sections shown in FIG. 1, illustrating the outside surface of the screen in a flat condition prior to it being formed into a cylindrical screen section with four deflector strips mounted on the inside surface and showing the four mounting strips on the outside surface, in accordance with a preferred embodiment of the present invention.  
         [0025]    [0025]FIG. 3 is an edge view of the screen section illustrated in FIG. 2.  
         [0026]    [0026]FIG. 4 is a sectional view, on an enlarged scale, taken along section line  4 - 4  on FIG. 2, illustrating the structure of 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.  
         [0027]    [0027]FIG. 5 is a perspective view of another cylindrical screen section for a centrifugal pellet dryer having two deflector strips in accordance with the present invention mounted on the interior surface, but using a different mounting mechanism.  
         [0028]    [0028]FIG. 6 is a sectional view, on an enlarged scale, taken along section line  6 - 6  on FIG. 5.  
         [0029]    [0029]FIG. 7 is a perspective view of another type deflector strip in accordance with the present invention for assembly between the outwardly extending vertical side edge flanges of the cylindrical screen.  
         [0030]    [0030]FIG. 8 is a partial cross-sectional view illustrating the deflector strip of FIG. 7 mounted in position within the outwardly extending vertical side edge flanges. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]    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.  
         [0032]    Referring to FIG. 1, 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.  
         [0033]    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  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 rotor  16  to an outlet  26  at the upper end of upper screen section  15 . 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.  
         [0034]    [0034]FIG. 2 illustrates the outside of one screen section  15  which includes a stainless steel plate  28  of 20 gauge thickness with 0.075 inch diameter holes  30  punched therethrough from the surface facing inwardly of screen section  15 . The holes  30  have staggered centers and are oriented in discrete areas  32  thereby defining intersecting solid sections  34  and  36 . The plate  28  which is formed when in a flat condition is retained in a cylindrical configuration by connecting outwardly extending vertical side edge flanges  38  and  40  on the respective vertical solid edge sections  60  and  62 .  
         [0035]    As illustrated in FIG. 3, the side edge flange  38  includes an attaching flange  39  overlying and secured to edge section  60 , such as by spot welding on 3 inch centers, with the outer edge of edge section  60  spaced inwardly about ½ inch from the juncture between flanges  38  and  39 . The other side edge flange  40  includes an attaching flange  41 , preferably spot welded to edge section  62 . As shown, the edge section  62  is wider than the edge section  60 . Also, the juncture between flanges  40  and  41  is spaced inwardly about ½ inch from the outer edge of edge section  62 . This spacing allows the free side edge of edge section  62  to underlie the portion of attaching flange  39  outwardly of side edge section  60  when the screen section  15  is formed into a cylindrical configuration and flanges  38  and  40  are joined. A preassembled stud  42  on the flange  38  includes slots to enable limited movement of the stud  42  for connection with the flange  40  by extending through apertures therein and receiving anchoring nuts. The joined edges of the side edge sections  60  and  62  provide a continuous interior surface to the screen section  15  when formed into a cylindrical configuration.  
         [0036]    The deflector strips of the present invention are preferably continuous elongated metal strips, generally designated by reference numeral  44 . The strips  44  are preferably made from stainless steel or other material similar to the screen plate  28 . As shown in FIGS. 2 sand 3, the strips  44  are mounted on the intermediate solid sections  36  and side edge section  62  against the inner surface of the screen plate  28 . Associated with each deflector strip  44  is a continuous vertical mounting strip  48  positioned against the outer surface of the screen plate  28 . The deflector strip  44  and mounting strip  48  have approximately the same size and are positioned facing each other with the screen plate  28  sandwiched therebetween. A bolt  50  having a countersunk tapered head  52  extends from the inner surface  46  of the deflector strip  44  beyond the outer surface of the outer strip  48 . A retaining lock nut  53  retains the strips  44  and  48  in assembled relation on the screen solid sections  36  and  62 .  
         [0037]    When assembling the deflector strips  44  and mounting strips  48  on the screen plate  28 , the mounting strip  48  is first preferably spot welded onto the outside surface of the screen plate  28  with the holes for receiving bolt  50  in proper alignment. The deflector strips  44  are then positioned in alignment on the inside surface of the screen plate  28 , the bolts  50  inserted through the openings, and the retaining lock nuts  53  threaded thereon.  
         [0038]    The edge surface of the deflector strips  44  is preferably inclined at  54 . The inclined surface  54  includes a leading edge along the inner surface of the screen plate  28  and a trailing edge along the strip inner surface  46  adjacent the rotor  16 . The surface  54  is opposed to the direction of rotation of the rotor  16  thereby defining a wedge shape or cam surface facing the direction of rotation and causing pellets or other solid particles moving along the inner surface of the screen sections  15  to be deflected inwardly toward the rotor  16 .  
         [0039]    The inclined surfaces  54  on the vertical deflector strips  44  will deflect pellets and other solid particles that tend to collect and move along the inner surface of the screen sections  15  back toward the rotor for engagement by the rotor blades and prevent the particles from circulating around the inner surface of the screen and beyond the reach of the rotor blades. The rotor then moves the particles outwardly again to impact against the screen to more effectively separate moisture from the particles. The inclined edge surfaces  54  of the deflector strips  44  is especially effective in preventing softer and/or smaller pellets and plastic flakes and particles from collecting and moving in a circular path in the clearance band along the inner surface of the screen sections. The elimination of banding of these particles as they move in the clearance band of the screen sections increases particles flow, more effectively removes moisture from the pellets, reduces power requirements and eliminates high amperage surge requirements.  
         [0040]    Turning now to FIGS. 5 and 6, another embodiment of the deflector strip of the present invention is disclosed and generally designated by reference numeral  70 . In this embodiment, two deflector strips  70  are mounted on the inside surface of the cylindrical screen section  15 , and a different mounting mechanism is disclosed. In particular, machine threaded bolts  74  are welded to the inside surface of the deflector strips  70  at appropriately spaced locations. The threaded bolts  74  protrude through matching holes in the cylindrical screen section  15  on which appropriate locking nuts  76  can be installed to affix the deflector strip  70  in place on the screen section. The beveled or inclined leading edge is shown at  78 .  
         [0041]    The number and spatial relation of the deflector strips  44  and  70  may vary depending on the diameter of the screen sections  15 . Usually one to four deflector strips  44  or  70  are adequate in most screen sections up to about 64 inches in diameter. Also in dryers having multiple screen sections  15 , the lowest screen section  15 , where the water and solid particle slurry enter the screen, may be constructed without deflector strips. In the upper sections having the deflector strips, the strips are preferably aligned vertically, although such alignment is not always necessary.  
         [0042]    The deflector strips  44  and  70  of the present invention have a thickness preferably about 0.10 inches to about 0.25 inches, and most preferably about 0.14 inches, and a width of about 0.50 inches to about 0.80 inches, and most preferably about 0.62 inches. The beveled or inclined leading edge  54  is preferably angled at about 45°, although any angle which deflects the pellets or other solid particles banding around the inner surface of the screen back towards the rotor blades could be used in accordance with the present invention. In some applications it may be possible to eliminate the bevel or inclined leading edge, although this configuration is not preferred.  
         [0043]    Another embodiment of the elongated deflector strip of the present invention is disclosed in FIGS. 7 and 8 and generally designated by reference numeral  80 . As shown, the deflector strip  80  is generally in the form of an elongated section of angle iron with flanges  82  and  84  extending from interconnection  86 . The flanges  84  and  86  are interconnected at an obtuse angle, rather than the normal perpendicular relationship of angle iron. The flange  82  is clamped between the outwardly extending vertical side edge flanges  88  and  90  which are attached together by bolts  92  when forming the cylindrical screen or screen section  94 . Before the screen or screen section  94  is formed and side edge flanges  88  and  90  attached together, flange  82  of angle iron  80  is preferably welded to side edge flange  90 . The flange  82  of deflector strip  80  has appropriately spaced holes  96  aligned with the holes in flanges  88  and  90  to receive the bolts  92  therethrough.  
         [0044]    When welded and clamped in position, as seen generally in FIG. 8 (a slight spacing is shown for illustration purposes), the elongated deflector strip flange  84  extends into the clearance band or annular space, indicated by reference numeral  98 , between the outer edges of the rotor blades and the interior surface of the screen or screen section  94 . The flange  84  has an inclined leading edge or surface  100  which deflects the pellets or other solid particles banding around the inner surface of the screen or screen section in annular space  98 , back towards the rotor blades. The angle α of the inclined surface  100  is preferably about 70°, although smaller and larger angles could be used, as much as + or − 10 ° or more. It will be noted that the side edge sections  102  and  104  are adjusted to accommodate the deflector strip  80  extending therethrough (compare FIG. 8 to FIG. 3).  
         [0045]    In its preferred form the deflector strip  80  is thicker and wider, and therefore more sturdy, than the deflector strips  44  and  70 . The deflector strip  80  has special application in centrifugal pellet dryers where the particles being dried are large and/or heavy, thus contributing to potentially heavier wear and strip breakage, for example when drying slurries containing ground battery casing particles. In this configuration, the deflector strip  80  will not dislodge completely from the interior of the cylindrical screen For screen section  94 , rather only parts of flange  84  might tear away under particularly heavy particle flow around annular space  98 . The elongated deflector strip  84  preferably has a thickness of about 0.19 inches, and each of the flanges  82  and  84  are preferably about 1.00 inch in width. While this embodiment of the present invention is not preferred, the deflector strip  80  has beneficial characteristics in certain centrifugal pellet dryer applications, as mentioned above.  
         [0046]    The length of the deflector strips  44 ,  70  and  80  depends upon the height of the cylindrical screen section  15 , 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), although this spacing can be varied as desired.  
         [0047]    While the deflector strips  44  and  70 , and mounting strips  48 , are preferably continuous metal strips, they could be discontinuous and of smaller length. Continuous metal strips are preferred because they tend to stiffen the cylindrical screen or screen section, thus perhaps allowing for a thinner screen plate  28 . If the strips  44  and  48  are discontinuous, they might preferably be mounted in a vertical staggered array from adjacent the bottom edge of the screen plate  28  to adjacent the top edge of the screen plate  28 . In such a manner, banding solid particles which miss one metal strip in a circumferential pass around the clearance band would encounter another metal strip in its path.  
         [0048]    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.