Abstract:
A particle size classifier in which feed material composed of product and overs moves downwardly through a separator unit by the action of negative pressures created by a first air blower associated with the collection of product and a second air blower associated with the collection of overs. As the feed material moves downwardly, the product and the overs are separated with the product passing from within a screen unit to outside the screen unit and the screen unit preventing passage of the overs. Blowback air, applied to the screen unit from the outside cleans the cylindrical screen unit to permit continued passage of product through the screen unit.

Description:
TECHNICAL FIELD 
     The present invention relates, in general, to the handling of materials and, in particular, to the separation of desired powder product (i.e. “fines”) from oversized, undesired components (i.e. “overs”) in feed material containing both. 
     BACKGROUND OF THE INVENTION 
     There are many types of equipment which handle bulk material in which particles of many different sizes are present and which must separate out desired size particles of from undesired, oversized particles. Generally, such equipment that is commercially available for separating out desired size particles fails to meet one or more of the following desired features: 
     (1) clean product separation, namely a distinct separation of desired size particles from undesired, oversized particles 
     (2) high screening efficiency, namely the separation of large percentages of the desired size particles 
     (3) high throughput capacity 
     (4) relatively dust free surrounding environment 
     (5) minimum material build-up on screens that affects separation, particularly static build up of material in the winter 
     SUMMARY OF THE INVENTION 
     A particle size classifier, constructed in accordance with the present invention, includes a housing and means for supplying feed material composed of product and overs. This particle size classifier also includes a vertically disposed separator unit mounted within the housing and which has a cylindrical screen unit through which product passes from within the cylindrical screen unit to space outside the cylindrical screen unit within the housing and within which overs are retained. The separator unit also has a plurality of horizontally disposed perforated circular plates fixed to and within the cylindrical screen unit at spaced intervals along the length of the cylindrical screen unit and through which feed material passes downwardly. A particle size classifier, constructed in accordance with the present invention, further includes a vertically disposed center pipe extending through the plurality of circular plates and to which the plurality of circular plates are fixed. This center pipe has at an upper end means for conducting feed material from the feed material supply means to the center pipe and from within the center pipe to within the cylindrical screen unit and at a lower end means for conducting overs from within the cylindrical screen unit to the center pipe and from within the center pipe to a container for overs. Also included in a particle size classifier constructed in accordance with the present invention is a blowback air unit mounted within the housing spaced from the cylindrical screen unit and extending parallel to the cylindrical screen unit for supplying blowback air into the cylindrical screen unit. This particle size classifier further includes means for imparting rotary movement to the center pipe and means for conducting product from the space outside the cylindrical screen unit within the housing to a container for product. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is vertical schematic view, partially in section, of a particle size classifier constructed in accordance with the present invention. 
     FIG. 2 is a vertical sectional view of a separator unit that is a part of a particle size classifier unit constructed in accordance with the present invention. 
     FIG. 3 is a cross-sectional view of a cylindrical screen unit that is a part of a particle size classifier constructed in accordance with the present invention. 
     FIG. 3A is a side view of a portion of the FIG. 3 cylindrical screen unit. 
     FIG. 4 is a plan view of a circular plate that is a part of a particle size classifier constructed in accordance with the present invention. 
     FIG. 5 is a side view of a conical member that is part of a particle size classifier constructed in accordance with the present invention. 
     FIG. 6A is a first side view of a blowback air pipe that is a part of a particle size classifier constructed in accordance with the present invention. 
     FIG. 6B is a second side view of the FIG. 6A blowback air pipe disposed 90° from the side view of FIG.  6 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a particle size classifier, constructed in accordance with the present invention, includes a housing  10  and means for supplying feed material composed of product and overs. Such means can be a feeder hopper  12  that contains desired particles (i.e. “product”) which are to be separated from other, larger size particles (i.e. “overs”) and an inlet funnel  14 . 
     A particle size classifier, constructed in accordance with the present invention, also includes a vertically disposed separator unit  16  mounted within housing  10 . Separator unit  16  has a cylindrical screen unit  18  through which product passes from within the cylindrical screen unit to space  20  outside the cylindrical screen unit within the housing and within which overs are retained. As shown most clearly in FIGS. 3 and 3A, cylindrical screen unit  18  includes a separation screen  18   a  having a mesh size which permits product to pass from within the cylindrical screen unit to space  20  outside the cylindrical screen unit and prevents overs from passing from within the cylindrical screen unit to the space outside the cylindrical screen unit. For separation of extremely fine product, cylindrical screen unit  18  preferably also includes a support screen  18   b  to which separation screen  18   a  is attached by welding for example. Support screen  18   b  has a mesh size larger than the mesh size of separation screen  18   a.    
     Separator unit  16  also has a plurality of horizontally disposed perforated circular plates  22  fixed to and within cylindrical screen unit  18  at spaced intervals along the length of the cylindrical screen unit and through which feed material passes downwardly. As shown in FIG. 4, each circular plate  22  has a plurality of openings  22   a  at the periphery thereof through which feed material passes downwardly. 
     Separator unit  16  further includes a plurality of conical members  24 , shown in FIG. 5, each extending from the inner periphery of an upper circular plate  22  downwardly and radially outwardly to the next lower circular plate  22 . Each circular plate  22  has a second plurality of openings  22   b  radially inward of a circle at which a conical member  24  meets a circular plate. 
     A particle size classifier, constructed in accordance with the present invention, further includes a vertically disposed center pipe  26  extending through the plurality of circular plates  22  and to which the plurality of circular plates are fixed. The upper end of center pipe  26  preferably is connected to funnel  14  of the feed material supply means outside housing  10 . Center pipe  26  has at an upper end means for conducting feed material from funnel  14  of the feed material supply means to the center pipe and from within the center pipe to within cylindrical screen unit  18 . For the embodiment of the invention being described, center pipe  26  is hollow throughout the length thereof and feed material is conducted at the upper end of the center pipe through first and second diametrically opposed openings  28  (only one being shown in FIGS. 1 and 2) in the center pipe at a point within cylindrical screen unit  18  to the cylindrical screen unit. A blank  30  in center pipe  26  below first and second openings  28  blocks downward movement of feed material in the center pipe. 
     The lower end of center pipe  26  preferably is connected to an overs container  32  outside housing  10 . Overs container  32  is in the form of a filter and is known as a “baghouse.” Center pipe  26  has at a lower end means for conducting overs from within cylindrical screen unit  18  to the center pipe and from within the center pipe to container  32  for overs. The means for conducting overs to overs container  32  include a first suction source in the form of a blower  34  for drawing overs from within center pipe  26 . Overs are conducted at the lower end of center pipe  26  into the center pipe through third and fourth diametrically opposed openings  36  (only one being shown in FIGS. 1 and 2) in the center pipe at a point within cylindrical screen  18 . Periodically (e.g. every few seconds), air at a pressure greater than the pressure drawn by blower  34 , is injected into baghouse  32  at the top of the baghouse to clean filters  32   a  in the baghouse causing overs caught in the filters to drop to the bottom of the baghouse from where the overs can be removed through a valve  37 . 
     A particle size classifier, constructed in accordance with the present invention, further includes a blowback air unit  38  mounted within housing  10  spaced from cylindrical screen unit  18 . Blowback air unit  38  extends parallel to cylindrical screen unit  18  and supplies blowback air into the cylindrical screen unit. As shown in FIGS. 6A and 6B, blowback air unit  38  includes a vertically disposed pipe  38   a  and an inlet channel  38   b . Pipe  38   a  has a slit  38   c  extending along the length thereof that can be adjustable and through which blowback air, introduced through inlet channel  38   b , is conducted to cylindrical screen unit  18 . Blowback air is supplied from a blower  40 . 
     A particle size classifier, constructed in accordance with the present invention, further includes means for imparting rotary movement to center pipe  26 . For the embodiment of the invention being described, such means include a pair of bearings  42  by which center pipe  26  is mounted and a motor  44  coupled to the center pipe. As rotary movement is imparted to center pipe  26 , cylindrical screen unit  18  also rotates because the center pipe is attached to circular plates  22  of the cylindrical screen unit. 
     A particle size classifier, constructed in accordance with the present invention, further includes means for conducting product (i.e. fines) from space  20  outside cylindrical screen unit  18  within housing  10  to a product container  46  (i.e. a “baghouse”) in the form of a filter. Such means include a second suction source in the form of a blower  48  for drawing product from space  20  outside cylindrical screen unit  18  within housing  10  through an outlet  50  to baghouse  46 . Periodically (e.g. every few seconds), air at a pressure greater than the pressure drawn by blower  48 , is injected into baghouse  46  at the top of the baghouse to clean filters  46   a  in the baghouse causing product caught in the filters to drop to the bottom of the baghouse from where the product can be removed through a valve  52 . 
     In operation, feed material composed of product and overs is supplied from feeder hopper  12  through funnel  14  into center pipe  26 . The feed material passes through openings  28  in the center pipe into cylindrical screen unit  18 . The centrifugal force imparted to the feed material by the rotating conical members  24  causes product and overs to be urged radially outward with some portion of the product passing through cylindrical screen unit  18  to space  20 , while overs and the remaining portion of the product do not pass through the cylindrical screen unit. A portion of the product passing into space  20  drops downwardly due to the negative pressure created by blowers  48  and gravity. Blowback air, introduced through blowback air unit  38 , cleans cylindrical screen unit  18  of product and overs. The overs and that portion of the product which remains inside cylindrical screen unit  18  or returns to within the cylindrical screen unit by the action of the blowback air are urged radially outward by conical members  24  and fall downwardly through openings  22   a  in circular plates  22  due to the negative pressure created by blower  48  and gravity. Openings  22   b  in circular plates  22  are provided to permit product getting beneath conical members  24  to pass through to the next lower conical member. 
     Upon reaching the bottom of separator unit  16 , product is drawn through outlet  50  to baghouse  46  by the negative pressure created by blower  48  and overs are drawn from center pipe  26  to baghouse  32  by the negative pressure created by blower  34 . Product cleaned off filters  46   a  in baghouse  46  is removed from this baghouse through valve  52  and overs cleaned off filters  32   a  in baghouse  32  are removed from this baghouse through valve  37 . 
     While in the foregoing there have been described preferred embodiments of the present invention, it should be understood by those skilled in the art that various modifications and changes can be made without departing from the true spirit and scope of the present invention.