Patent Publication Number: US-RE45408-E

Title: Low pressure dryer

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This patent application is entitled to the benefit of the filing date of provisional U.S. patent application Ser. No. 60/059,579 filed Sep. 19, 1997 in the name of Stephen B. Maguire entitled “Low Pressure Granular Material Dryer”, under 35 USC 120. 
     CROSS-REFERENCE TO RELATED FILINGS  
     This patent application is entitled to the benefit of the filing date of provisional U.S. patent application 60/059,579 filed 19 Sep. 1997 in the name of Stephen B. Maguire entitled “Low Pressure Granular Material Dryer”, under 35 USC 119(e). More than one application has been filed for reissue of U.S. Pat. No. 6,154,980, issued 5 Dec. 2000. The reissue applications are application Ser. No. a division of 10/309,777, filed 4 Dec. 2002, and this application Ser. No. 11/474,257 filed 22 Jun. 2006, as a division of application Ser. No. 10/309,777. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to drying granular or powdery material, preferably granular resin material, prior to processing thereof into intermediate or finished products, preferably by extrusion or molding. 
     2. Description of the Prior Art 
     Plastic resins are initially granular materials and are produced in pellets. These pellets are processed by extrusion or other means in which the granular resin pellets are heated until the pellets melt and may then be molded or extruded into a desired shape. Typically granular resins melt at elevated temperatures, for example from 300-400° F., which is well above the boiling point of water. 
     Many granular resins have affinity for moisture. These hydroscopic resins absorb moisture and cannot be properly processed by molding or extrusion until dried. If processed before dry, moisture in the plastic boils at or approaching the high plastic molding or extrusion process temperatures, leaving bubbles and perhaps other imperfections in the finished product. Hence, hydroscopic granular resins must be dried prior to molding or extrusion. 
     Some granular resin materials are extremely hydroscopic and become unprocessable by molding or extrusion in ten minutes or less after exiting a dryer, due to the rapid absorption of moisture by the granular resin material. 
     It is known to dry granular resin material by placing the granular resin material pellets on large shallow trays to a depth of one or two inches, and putting those trays into ovens for several hours. With this approach to granular resin material drying, resin temperatures of 150-180° F., but no higher, can be used since many granular resin materials begin to soften at 200-210° F. 
     During the drying process, the granular resin material cannot be permitted to soften, since it becomes unmanageable. Once granular resin material begins to soften, at temperatures above the boiling point of water, the granular resin material pellets stick together in lumps or even melt into useless masses of solid plastic, making it impossible to further process the resin material into a useful article. 
     SUMMARY OF THE INVENTION 
     In one of its aspects, this invention provides a low pressure granular or powdery granular material dryer. The dryer preferably includes a rotatable preferably vertical shaft, a plurality of preferably vertically-oriented, open-ended preferably cylindrical hoppers which are preferably equiangularly positioned and rotatable about a vertical axis, which is preferably defined by the shaft, serially among material filling and healing, vacuum drying and dispensing positions. 
     The dryer preferably further includes a pin extending vertically and radially displaced from the axis, a preferably triangular preferably horizontal plate rotatably receiving the pin proximate the center of the plate, a preferably horizontal link pivotally connecting said shaft and the plate, and a plurality of preferably pneumatic piston-cylinder combinations equiangularly operatively connected to the plate for rotating the shaft by sequentially moving the plate relative to the shaft thereby to move the hoppers among the filling and heating, vacuum drying and dispensing positions. 
     The dryer yet preferably includes preferably pneumatic piston-cylinder actuated means for sealing the cylindrical hoppers at the vacuum drying station. 
     In another of its aspects, this invention provides a hopper for use in a low pressure granular resin material or powdery material dryer where the hopper includes a preferably vertically-oriented preferably cylindrical shell having open ends with the shell preferably adapted to be sealingly closed by selectably contacting top and bottom plates thereagainst, thereby enabling vacuum to be drawn within the shell when desired. The hopper further preferably includes a funnel within the cylindrical shell and located proximate the shell bottom. The hopper further preferably includes an internal material flow control plate in the form of a dump flap located within the shell beneath the funnel. The dump flap is preferably pivotally connected to the shell for movement about the connection point away from a downwardly opening discharge orifice of the funnel, thereby to selectably release granular resin material from the hopper. 
     In yet a further aspect of the invention, top and bottom plates preferably selectably seal the cylindrical shell thereby allowing vacuum to be drawn therewithin. Pneumatic piston-cylinder means may be provided for urging the top and bottom plates into sealing contact with the shell. 
     The shell is desirably adapted to selectably dispense granular or powdery material stored therewithin at a dispense position, when the shell is at that position. The dispense position is preferably removed from the vacuum drying position. 
     The hopper is further preferably adapted to effectuate material dispensing upon contact by an upwardly moving rod of a pneumatic piston-cylinder combination, thereby permitting downward flow from the funnel of material with the material thereby flowing out of the cylindrical shell. 
     In yet another of its aspects, this invention provides a method for continuously drying granular or powdery material preparatory to mixing, molding, extruding or other processing of that material. The method preferably includes supplying granular or powdery material to a vertically-oriented cylindrical shell at a fill and heat position and heating the material within the shell by introduction of heated air into the cylindrical shell while at the fill and heat position. 
     The method yet further preferably includes moving the vertically-oriented cylindrical shell through an arc about a vertical axis outboard of the shell periphery to a vacuum drying position and sealing open ends of the shell at such position. 
     The method still yet further preferably includes drawing a preselected level of vacuum within the sealed shell for a time sufficient to evaporate moisture from the heated material within the shell to a desired degree of dryness. 
     The method even yet further preferably includes bringing the shell to a material discharge position at which the bottom of the shell is open and then discharging the dried material from the cylindrical shell responsively to action of a preferably pneumatic piston-cylinder combination inserting a rod into the shell interior from below to move a material discharge gate proximate the bottom of the shell. 
     The method preferably still yet even further includes moving the shell through an arc about the vertical axis to the fill and heat position and sequentially repeating the steps of supplying material to the shell, heating the material within the shell, moving the shell to the vacuum drawing position, drawing a sufficient level of vacuum within the shell to evaporate moisture from the material within the shell and moving the shell to a discharge position, for so long as the material is to be continuously dried. 
     In yet another of its aspects, this invention provides a method for continuously supplying dried granular resin material for processing from a supply of material which is excessively moist where the method preferably includes substantially simultaneously performing the steps of heating a portion of the moist granular resin material to a selected temperature at which the moisture evaporates from the granular resin material at a preselected level of vacuum, drawing and maintaining the preselected vacuum for a second portion of the granular resin material which has been heated to the selected temperature for time sufficient to cause the moisture to evaporate therefrom and result in the second portion of granular resin material being at the preselected dryness and supplying to granular resin material processing equipment a third portion of the granular resin material which was dried to the preselected dryness by evaporation in the preselected level of vacuum after having been heated to the selected temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front elevation of part of the low pressure granular or powdery material dryer showing a cylindrical hopper portion of the dryer at a material filling and heating position prior to supply of heated air to the hopper interior, in accordance with the preferred embodiment of the invention. 
         FIG. 2  is a plan view of the supply plenum portion of the low pressure dryer at the heating and filling station, taken at arrows  2 - 2  in  FIG. 1 . 
         FIG. 3  is a front elevation of part of the low pressure dryer showing a hopper portion of the dryer at the material filling and heating position, as illustrated generally in  FIG. 1 , configured for supply of heated air to the hopper. 
         FIG. 4  is a partially sectioned, schematic elevation of a vertically-oriented open ended cylindrical hopper, forming a part of the low pressure dryer, showing the hopper at a vacuum drying position with the hopper open so that pressure within the hopper is ambient. 
         FIG. 5  is a partially sectioned, schematic elevation of the vertically-oriented open ended generally cylindrical hopper illustrated in  FIG. 4 , with top and bottom plates sealing the hopper, thereby allowing vacuum to be drawn within the hopper and further illustrating the hopper connected to a vacuum pump. 
         FIG. 6  is a broken front schematic elevation of the lower interior of a vertically-oriented open ended generally cylindrical hopper as illustrated generally in  FIGS. 4 and 5 , showing two material discharge funnels within the hopper, with the hopper illustrated at the material dispensing position. 
         FIG. 7  is a broken front schematic elevation of the lower interior of a vertically-oriented open ended generally cylindrical hopper as illustrated in  FIG. 6 , at the same material dispensing position illustrated in  FIG. 6 , illustrating the material dispensing piston-cylinder combination actuated, thereby actuating a discharge flap beneath the discharge funnels within the hopper permitting material flow out of the hopper. 
         FIG. 8  is a broken schematic side elevation of the lower interior of a vertically-oriented open ended generally cylindrical hopper as shown in  FIGS. 6 and 7 , taken looking from the right in  FIG. 7  illustrating the material dispensing piston-cylinder combination actuated, thereby moving a discharge flap beneath the material discharge funnels depicted in dotted lines within the hopper, to dispense material from the hopper. 
         FIG. 9  is a top view of low pressure dryer illustrated in  FIGS. 1 through 8 . 
         FIG. 10  is a front elevation the low pressure dryer illustrated in  FIGS. 1 through 9 . 
         FIG. 11  is a top view, similar to  FIG. 9 , schematically illustrating a portion of the low pressure dryer. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT AND BEST MODE KNOWN FOR PRACTICING THE INVENTION 
     Referring to the drawings in general and to  FIGS. 9 ,  10  and  11  in particular, a low pressure granular material dryer manifesting aspects of the invention is designated generally  10  and includes a plurality of cylindrical hoppers, preferably three, each of which has been designated generally  12 . Each hopper  12  preferably includes a cylindrical shell  14  and is preferably substantially vertically-oriented with the axis of the cylinder extending substantially vertically in order to be rotatable preferably unitarily with the other hoppers about a substantially vertical axis defined by a preferably vertical shaft  24 . 
     Dryer  10  includes a frame, designated generally  22 , on and within which vertical shaft  24  is rotatably mounted for rotation relative to frame  22 , the details of which are discussed below. Cylindrical hoppers  12  rotating unitarily with vertical shaft  24  preferably move serially among a material fill and heat position designated generally  100 , a material vacuum drying position designated generally  102  and a material dispensing position designated generally  104 . Hoppers  12  move when and as required among fill and heat position  100 , vacuum drying position  102  and dispensing position  104 . The three hoppers  12  start and stop together as required; they do not move continuously in a merry-go-round fashion among positions  100 ,  102  and  104 . 
     Referring principally to  FIGS. 9 and 10 , frame  22  is formed of a plurality of vertically and horizontally extending preferably angle iron members which collectively define what appears as the edges of a rectangular parallelepiped. As visible in  FIG. 10 , frame  22  includes preferably four substantially vertical members  160 , only two of which are visible in  FIG. 10 ; the remaining two substantially vertical members  160  are hidden behind the two members  160  visible in  FIG. 10 . 
     Frame  22  further includes four upper substantially horizontally extending members  162  which collectively define the outer periphery of a square in geometrical terms; the four upper substantially horizontally extending members  162  are visible in  FIG. 9 ; not all of members  162  are visible in  FIG. 10 . 
     Frame  22  further yet preferably includes four lower horizontally extending members  164 , one of which is visible in  FIG. 10 . The remaining lower members  164  lie immediately under the corresponding upper horizontally extending members  162  illustrated in  FIG. 9 . The four lower horizontally extending members  164  define the base of frame  22  for contacting a floor or other weight supporting structure on which dryer  10  rests. 
     At least one and preferably a plurality of suspension members  166  extend laterally across the upper end of dryer  10 , between selected upper horizontal members  162 . One of such suspension members  166  is illustrated in  FIG. 10 . A hopper top sealing piston-cylinder combination designated generally  44 , serving to seal the top of a hopper  12  at the vacuum drying position, is supported by one of horizontally extending suspension members  166  as illustrated in  FIG. 10 . Similarly, a hopper upper closure piston-cylinder combination  98  located at material fill and heat position  100 , which piston-cylinder combination is used to close an upper end of a cylindrical hopper  12  at the fill and heat position  100 , is supported by one of horizontally extending suspension members  166  as also illustrated in  FIG. 10 . 
     First, second and third driving rotation piston-cylinder combinations  34 ,  36 ,  38  are preferably pivotally connected to selected ones of upper horizontal members  162  of frame  22  as illustrated in  FIG. 10 . In the case of first driving rotation piston-cylinder combination  34 , a triangular or cantilever extension may be provided from the proximate upper horizontal member  162  where the triangular extension has been designated generally  182  in  FIG. 9 . Connections of driving rotation piston-cylinder combinations  34 ,  36  and  38  to frame  22  are denoted as pivotal connections  180  in the drawings. 
     Connection of generally triangular plate  28  with vertically-oriented shaft  24  is effectuated by means of a pin connector  168  which is vertically-oriented and resides rotatably slidably within an aperture formed at the center of horizontal central portion  30  of generally triangular plate  28 . Pin connector  168  fits rotatably not only within triangular plate  28  but also fits rotatably within an aperture in one end of a plate-pin connection arm  116  best shown in  FIG. 9 . While plate-pin connection arm preferably lies under triangular plate  28  as is apparent from  FIG. 10 , plate-pin connection arm  116  has been illustrated in solid lines in  FIG. 9  to facilitate understanding. 
     Plate-pin connection arm  116  is fixedly connected to vertical shaft  24  at the upper end thereof. 
     In this arrangement movement of triangular plate  28 , as effectuated by any of first, second or third driving rotation piston-cylinder combinations  34 ,  36  or  38 , results in pin connector  168  translating such motion to plate-pin connection arm  116 . Rotation of plate-pin connection arm  116 , being fixedly connected to shaft  24 , results in shaft  24  rotating. As shaft  24  rotates, it carries hoppers  16  among the fill and heat position  100 , vacuum drying position  102  and material dispense position  104 . Hoppers  16  move this way unitarily with shaft  24  as a result of hoppers  16  being fixedly connected to shaft  24  by cantilever connecting rods  110 , illustrated in phantom lines in  FIG. 10 . 
     Shaft  24  is journaled in suitable bearings mounted on upper and lower shaft suspension plates  112  to define upper and lower shaft bearing assemblies  114  as indicated in  FIG. 10 . Upper shaft suspension plate  112  is connected to a horizontally extending suspension member  166  by suitable nut and bolt combinations which have not been numbered but are clearly visible in  FIG. 10 ; lower shaft suspension plate  112  is connected to a lower horizontal member  164  as indicated generally in  FIG. 10 , again by suitable nut and bolt combinations which have not been numbered in the drawings. 
     Referring to  FIGS. 1 through 3  illustrating the fill and heat position  100  of lower pressure dryer  10 , at fill and heat position  100  a moist material supply hopper  64  has a supply of moist granular or powdery material, which is to be dried resident therein. A butterfly valve at the bottom of hopper  64  is within a conduit  144  and is operated by a piston-cylinder combination  146  as clearly visible in  FIGS. 1 and 3 . 
     Conduit  144  includes a telescoping portion  148  for connecting moist material supply hopper  64  with a hopper top sealing plate  150  at fill and heat position  100 . Positioned around the outer periphery of hopper top sealing plate  150  is an annular lip  152 . An aperture  154  is within hopper top sealing plate  150  and facilitates communication between telescoping portion  148  of conduit  144  and the interior of a hopper  14  when at the fill and heat position. 
     Still referring to  FIGS. 1 and 3 , a hopper  14  is illustrated in position as a result of having been rotated to that position by rotation of vertical shaft  24  in response to first, second and third driving rotation piston-cylinder combinations  34 ,  36  and  38 . 
     At material fill and heating position  100 , a blower  76  facilitates recirculation of heated air through material  74  resident within hopper  14  to heat material  74 . Blower  76  has an intake aperture  78  and an exhaust aperture  80 . Exhaust aperture  80  connects to conduit  156  within which there are a plurality of heater elements  82  to heat air exhausted from blower  76  prior to flow through material within hopper  12 . Conduit  156  includes a telescoping portion  158  connecting with and exhausting into a supply plenum designated generally  86  via a plenum inlet  90  which is visible in both  FIGS. 1 and 2 . 
     Plenum  86  includes an outlet screen  88  mounted at the upper end thereof, as illustrated generally in  FIG. 2 . Outlet screen  92  has a plurality of apertures  184  formed therein with apertures  184  concentrated towards the central portion of outlet screen  92  as illustrated in  FIG. 2 . Apertures  184  serve to concentrate upward flow of the heating air about the central portion or central axis of hopper  14  which is desirable since this is where the majority of the material is concentrated to the shape of dispensing funnels  94 ,  96 . A preferably silicon annular gasket  88  on plenum  86  provides tight sealing between supply plenum  86  and the open bottom of hopper  12  at the material fill and heat position illustrated in  FIGS. 1 and 3 . 
     A pneumatic piston-cylinder combination  106  is mounted on a suitable cross-member, not illustrated in the drawings but forming a part of frame  22 . When actuated, piston-cylinder combination  106  serves to close the bottom of hopper  12  in the fill and heat position by moving supply plenum  86  vertically upwardly, from the position illustrated in  FIG. 1  to the position illustrated in  FIG. 3 , thereby effectuating a tight seal between outlet plenum  86  and hopper  12  to facilitate passage of heated air through granular or powdery material in hopper  14 . 
     Heated air, having passed through granular or powdery material within hopper  14 , exhausts from hopper  14  via telescoping portion  148  of conduit  144 . A butterfly valve  66  having closed conduit  144  so that heated air passing through telescoping portion  148  of conduit  144  cannot escape through supply hopper  64 , causes the heated, moist air to flow into heated air recirculator  70  at heated recirculation intake  72 . A thermocouple  68  positioned at heated air recirculation intake  72  senses temperature of heated air leaving hopper  14 . A second thermocouple  84  is positioned proximate the outlet of the heated air supplied by blower  76 , after the heated air has passed along heating elements  82 . When the temperature sensed by thermocouples  68  and  84  are substantially equal, this is indicative of the granular or powdery material within hopper  14  having reached the desired temperature, namely the selected temperature of the air entering into supply plenum  86  after having been heated by heating elements  82 . 
     During material heating at the fill and heat position, hopper top sealing plate  150  is lowered into position against the upper extremity of hopper  14  by action of a pneumatic piston-cylinder combination  98  which is connected to a suitable cross-member extending across the top of frame  22 . 
     Referring to  FIGS. 4 through 8  in particular, each preferably cylindrical hopper  12  preferably includes a cylindrical shell designated generally  14 . Each cylindrical shell  14  is preferably defined by an inner cylindrical tube referred to as a vacuum tube and designated  52  in the drawings and a concentric outer cylindrical tube referred to as an insulation tube and designated  54  in the drawings. Annular space between tubes  52  and  54 , which space is designated generally  55  in the drawings, is preferably filled with thermal insulation to minimize heat transfer and heat loss out of cylindrical shell  14 . 
     A pair of downwardly opening material dispensing funnels designated  94  and  96  respectively are secured within each cylindrical shell  14  of cylindrical hopper  12  proximate the bottom of hopper  12 . The higher of the two material dispensing funnels is referred to as an upper material dispensing funnel and is designated  94  in the drawings. The lower of the two material dispensing funnels is referred to as the lower material dispensing funnel and is designated generally  96  in the drawings. Material dispensing funnels  94  and  96  are preferably fixedly secured, by suitable sheet metal screws or other fastening means, to a lower portion of vacuum tube  52  at the positions generally indicated in the drawings. 
     Material dispensing funnels  94  and  96  preferably share a common funnel angle such that the sloped sides of the respective funnels are essentially parallel one with another. The sloped surface or side of upper material dispensing funnel is designated generally  122  in the drawings while the sloped side of lower dispensing funnel  96  is designated generally  124  in the drawings. 
     As further apparent from the drawings, particularly  FIGS. 6 through 8 , upper dispensing funnel  94  is configured as an extremely truncated cone such that the downwardly dispensing opening of upper material dispensing funnel  94 , which is designated  126  in the drawings, is substantially larger than a corresponding downwardly dispensing opening  128  of lower material dispensing funnel  96 . This results from lower dispensing funnel  96  being less truncated in the vertical direction than upper dispensing funnel  94 , as is illustrated in the drawings. 
     Use of two dispensing funnels such as dispensing funnels  94 ,  96  facilitates circulation of heated drying air around material in hopper  14  at filling and heating position  100  and further facilitates drying of the material in hopper  14  when the hopper is at vacuum drying position  102 . 
     Each hopper  14  preferably further includes a dump flap designated generally  20  located below downwardly dispensing opening  128  of lower funnel  96 . Dump flap  20  is pivotally connected to vacuum tube  52  by suitable screw connections which are illustrated in the drawings, particularly in  FIGS. 4 ,  5  and  8 , and are numbered  140  and  170  respectively. 
     Dump flap  20  includes a central portion  172  which is generally planar in configuration as illustrated in the drawings, particularly  FIGS. 6 ,  7  and  8 , and has a weight  130  located at one side thereof, offset from the point of pivotal connection between dump flap  20  and dump actuator  62 , which point of pivotal connection is denoted  132  in the drawings, and also offset from the pivotal connection of dump flap  20  with vacuum tube  52  of hopper shell  14  as effectuated by screw-nut connection  170  and offset from pivotal connection  140  of pivoting arm  134  to the interior surface of vacuum tube  52 . Weight  130  helps to cause dump flap  20  to return to the position illustrated in  FIG. 6  in response to gravitational force after material dispensing piston-cylinder combination  108  has been deactuated. 
     Dump actuator  62  engages a generally vertical air  134  forming a part of dump flap  20 . Dump actuator  62  includes a vertically movable arm  136 , also illustrated in  FIG. 7 . Vertically movable arm  136  is mounted for sliding, vertical movement along the interior surface of vacuum tube  52  of horizontal shell  14 . The extent of vertical movement of vertically movable arm  136  is controlled by a pin  174  illustrated in  FIG. 7 , which is preferably mounted fixedly to and extending radially inwardly from the interior of vacuum tube  52 . A vertical slot  176 , similarly visible in  FIG. 7 , in vertically movable arm  136  receives pin  174 . Interference between pin  174  and the ends of slot  176  limits vertical travel of movable arm  136 . 
     Movement of arm  136  upwardly in  FIGS. 6 ,  7  and  8  results from actuation of material dispensing piston-cylinder combination  108 , which is preferably a pneumatically powered piston-cylinder combination. When piston-cylinder combination  108  is actuated, a piston rod  178  extending from piston-cylinder combination  108  contacts a horizontal tabular extension portion of vertically movable arm  136 . This horizontal tabular extension of vertically movable arm  136  is designated  138  and is shown in  FIG. 8 . There tabular extension  138  is illustrated in solid lines in the “at rest” or unactuated position and in dotted lines in the position assumed by tabular extension  138 , and hence vertically movable arm  136 , when material dispensing pneumatic piston-cylinder combination  108  has been actuated and the piston rod associated therewith extends therefrom. 
     Actuation of material dispensing piston-cylinder combination  108  moves vertically movable arm  136  upwardly, to the position illustrated in solid lines in  FIG. 8 ; the movement of arm  136  is from the position illustrated in  FIG. 6  to the position illustrated in  FIG. 7 . 
     Vertically movable arm  136  is pivotally connected to an arm  134  portion of dump flap  20 . 
     Arm  134  connects the horizontal part of dump flap  20  to the inside of vacuum tube  52  via a pivotal connection identified as  140  in  FIGS. 6 ,  7  and  8 . Arm  134  is pivotally connected not only to the interior vacuum tube  52  at connection  140  but is also pivotally connected to vertically movable arm  136  at a pivotal connection  132 . As a result, upward movement of vertically movable arm  136  causes pivotal movement of pivoting arm  134  about pivotal connection  140 . Since pivotal connections  140  and  170  are horizontally aligned along a common axis, pivotal movement of arm  134  about this axis moves the horizontal part of dump flap  20  away from the dispensing aperture of lower funnel  96  thereby permitting granular or powdery material contained within hopper  12  to float downwardly outwardly therefrom when dump flap  20  is in the position illustrated in  FIG. 7 . 
     Once preferably pneumatic hopper dispensing piston-cylinder combination  108  has been deactuated, gravitational force acting with weight  130  tends to rotate dump flap  20  back to the horizontal, hopper closed, position illustrated in  FIGS. 6 and 8 . This causes vertically movable arm  136  to drop downwardly, from the position illustrated in  FIG. 7  to the position illustrated in  FIG. 6 . This further causes arm  134  to rotate counterclockwise from the position illustrated in  FIG. 7  to the position illustrated in  FIG. 6 , about pivotal connection point  140 . This returns dump flap  20  to the horizontal position illustrated in  FIG. 6  where granular material in hopper  12  cannot flow outwardly downwardly therefrom through the open bottom of hopper  12 . 
     The horizontal portion  172  of dump flap  20  is positioned sufficiently close to and sufficiently overlaps downwardly dispensing opening  128  of lower funnel  96  about the periphery of dispensing opening  128  that the angle of repose of any granular or powdery material within hopper  12  is sufficient to prevent downward flow of material through the gap between horizontal portion  172  of dump flap  20  and dispensing opening  128  of lower funnel  96 . 
     Material dispensing piston-cylinder combination  108  is preferably mounted either on a portion of frame  22  below dryer  10  or on some other stable member such as the floor of an installation where dryer  10  may be used. In either case, material dispensing piston-cylinder combination  108  is stationary in the sense that piston-cylinder combination  108  does not rotate with hoppers  12  as they are moved among fill and heat position  100 , vacuum drying position  102  and material dispense position  104 ; hopper dispensing piston-cylinder combination  108  remains at material dispense position  104 . 
     As apparent from  FIG. 8 , dump flap  20  includes two arms  134 ,  134 A. Arm  134 A which is located at the side of dump flap  20  remote from material dispensing piston-cylinder combination  108  is pivotally connected directly to vacuum tube  52 , preferably by screw-nut combination  170  as illustrated in  FIGS. 4 and 5 , for pivotal movement as dump flap  20  is actuated. 
     In  FIGS. 4 and 5  one of hoppers  12  is illustrated at vacuum drying position  102 .  FIG. 4  illustrates hopper  12  at vacuum drying position  102  prior to movement of hopper top and bottom vacuum sealing plates  40 ,  42  into position to seal cylindrical shell  14  so that a vacuum may be drawn therewithin. 
     Hopper top and bottom vacuum sealing plates  40 ,  42  are preferably respectively connected to unnumbered piston rod extensions which are connected to and are parts of hopper top and bottom sealing piston-cylinder combinations  44 ,  46  respectively. Piston-cylinder combinations  44 ,  46  are preferably pneumatically actuated; the cylinder portions thereof are preferably fixedly connected to horizontally extending cross-members of frame  22  as indicated generally in  FIGS. 4 and 5 . 
     Hopper top and bottom vacuum sealing plates  40 ,  42  are most preferably of dome-like shape, as illustrated in  FIG. 4 , and have upper and lower vacuum sealing gaskets  58 ,  60  positioned running circumferentially around the unnumbered preferably circular lips of preferably dome-like hopper top and bottom vacuum sealing plates  40 ,  42  respectively. 
     When a hopper  12  is located at vacuum drying position as illustrated in  FIG. 4 , pneumatic actuation of respective hopper top and bottom sealing piston-cylinder combinations  44 ,  46  respectively causes respective dome-like hopper top and bottom vacuum sealing plates  40 ,  42  to move vertically towards cylindrical hopper shell  14 . Arrows A in  FIG. 4  denote the vertical movement of hopper top and bottom vacuum sealing plates  40 ,  42  respectively. 
     When hopper cylindrical shell  14  is located at vacuum drying position  102 , actuation of respective piston-cylinder combinations  44 ,  46  moves top and bottom sealing plates  40 ,  42  downwardly and upwardly respectively to effectuate an airtight, vacuum maintaining seal between the preferably circular periphery of top and bottom sealing plates  40 ,  42 , where vacuum gaskets  58  and  60  are preferably located and the preferably circular circumferential top and bottom edges of vacuum tube  52 . The hopper top and bottom vacuum sealing plates  40 ,  42  in this position, with gaskets  58 ,  60  in sealing connection with the circumferential circular top and bottom edges of vacuum tube  52 , as illustrated in  FIG. 5 . 
     Top vacuum sealing plate  40  preferably includes a fitting, not numbered in the drawings, selectably connectingly receiving a preferably flexible vacuum line  50  which is preferably connected to a vacuum pump depicted schematically in  FIG. 5  and designated  48 . When hopper top and bottom vacuum sealing plates  40 ,  42  have been engaged with cylindrical shell  14  as illustrated in  FIG. 5  and vacuum pump  48  is actuated, vacuum is drawn within hopper  12  at this vacuum drying position. As pressure drops within hopper  12  at this vacuum drying position, moisture rapidly evaporates from granular resin material within hopper  12 . 
     Once moisture has been evaporated from resin material within hopper  12  when located at vacuum drying position  102  and the resin material has reached a desired degree of dryness, hopper top and bottom sealing piston-cylinder combinations  44 ,  46  are permitted to return to their default positions illustrated in  FIG. 4 . This retracts hopper top and bottom vacuum sealing plates  40 ,  42  away from and out of contact with cylindrical shell  14 , thereby permitting air once again to enter cylindrical shell  14  and permitting cylindrical shell  14 , having the now-dried granular resin material therewithin, to be moved to the material dispensing position. 
     The time during which vacuum is drawn within hopper  12  while located at vacuum drying position  102  may be adjusted by microprocessor control means connected to and associated with the low pressure granular material dryer. Similarly, the level of vacuum drawn in hopper  12  at vacuum drying position  102  may be adjusted. Furthermore, air withdrawn from hopper  12  by vacuum pump  48  may be monitored for moisture content and vacuum pump  48  may be halted once the desired low level of moisture of the material within hopper  12  has been attained. The microprocessor control means controls operation of the low pressure dryer, including operation of the pneumatic piston-cylinder combinations, the blower, the vacuum pump, etc. 
     Referring to  FIGS. 9 ,  10  and  11 , plate-pin connection arm  116  is rotatably connected to a generally horizontal plate  28  by pin connector  168 . Pin connector facilitates rotation of plate  28  respecting plate-pin connection arm  116  and hence respecting pin-like extension  26  and vertical shaft  24 . 
     Plate  28  includes a horizontal central portion  30  and downwardly projecting lips  32  extending from the periphery of plate  28 . 
     Three preferably pneumatically actuated piston-cylinder combinations  34 ,  36  and  38  are designated respectively first, second and third piston-cylinder combinations and are pivotally connected to frame  22 , specifically to upper horizontally extending member  162  of frame  22 , as generally illustrated respecting second and third piston-cylinder combinations  36 ,  38  in  FIG. 9 . The pivotal connections are designated  180  in  FIG. 9 . 
     To facilitate rotation of plate  28  about an axis defined by vertical shall  24 , first, second and third piston-cylinder combinations  34 ,  36 ,  38  are actuated as needed. Each piston-cylinder combination  34 ,  36 ,  38  has a piston rod extension which fits loosely within a respective aperture formed in a respective portion of a downwardly projecting lip  32 , with the piston rods being retained in position within those apertures by nuts threaded on the piston rod extremities as illustrated generally in  FIGS. 9 and 11 . 
     With this arrangement, as piston-cylinder combinations  34 ,  36 ,  38  are actuated to move their associated piston rods, from extended positions in which the piston rods of piston-cylinder combinations  36 ,  38  are illustrated in  FIG. 11  to the retracted position in which the piston rod extension of piston-cylinder combination  34  is illustrated in  FIG. 11 . As a result plate  28  and hence, vertical shaft  24  and cylindrical hoppers  12  attached thereto rotate about the axis of vertical shaft  24 , thereby moving hoppers  12  serially among the material fill and heat, vacuum drying and material dispense positions  100 ,  102 ,  104  respectively as illustrated in  FIGS. 9 and 11 . 
     For example, referring to  FIG. 9 , upon actuation of first driving rotation piston-cylinder combination  34  to extend the piston shaft therefrom forwardly out of the retracted position illustrated in  FIG. 9  and actuation of third driving rotation piston-cylinder combination  38  to cause the piston shaft associated therewith to retract to within piston-cylinder combination  38 , plate  28  rotates counterclockwise as considering  FIG. 9 , in the direction indicated by arrow A, with such rotation of plate  28  being about pin connector  168  and as illustrated in  FIG. 11  and indicated by arrow B. 
     As plate  28  rotates about pin connector  168  in the direction indicated by arrow A, plate  28  together with pin connector  168  rotate with horizontally extending plate-pin connection arm  116  pivotally about the axis defined by vertical shaft  24  thereby rotating shaft  24 . This rotation results from plate-pin connection arm  116  being fixedly connected to shaft  24 . Hence, as first, second and third driving rotation piston-cylinder combinations  34 ,  36  and  38  respectively are actuated in a sequential manner, plate  28  rotates about pin connector  168  and plate  28 , pin connector  168  and plate-pin connection arm  116  all rotate about the vertical axis defined by shaft  24  thereby to rotate shaft  24 . 
     The vertically-oriented cylindrical sides of hopper shells  14  defined by vacuum tubes  52  and insulation tubes  54  are connected to shaft  24  for rotation therewith by cantilever connecting rods  110  as best Illustrated in  FIG. 10 . Each cylindrical shell  14  of a cylindrical hopper  12  may be removable from its associated cantilever connecting rods  110  if desired; preferably two cantilever connecting rods  110  are provided for each hopper  12 , with one rod  110  connecting hopper  12  to vertical shaft  24  at positions relatively close to but removed from the vertical extremities of hoppers  12 , as illustrated in  FIG. 10 . 
       FIG. 9  has been drawn without depiction of moist material supply hopper  64 , exhaust plenum  142  and the structure associated therewith, to enhance drawing clarity. Similarly, hopper dispensing piston-cylinder  108  has been depicted in  FIG. 9  even though it is to be understood that such piston-cylinder combination would not be visible in the view from above dryer  10  since when a hopper  12  is at material dispense position  104 , piston-cylinder combination  108  is blocked from view from above. 
     Arrow B in  FIG. 11  depicts the preferred direction of rotation of vertical shaft  24  and hoppers  12  so as to move hoppers  12  serially from the material fill and heat position  100  to material vacuum drying position  102 , then to material dispense position  104  and then to material fill and heat position  100 , where this cycle may repeat. 
     At the material vacuum drying position, the heated material is preferably subjected to a vacuum of about 27.5 millimeters of mercury or greater. This lowers the evaporation point or boiling point of water to only 120° F., thereby causing the moisture within the heated material to evaporate and be drawn off through the vacuum pump drawing vacuum within hopper  12  at the vacuum drying position  102 . Once the vacuum drying process is sufficiently complete, piston-cylinder combinations  44 ,  46  retract hopper top and bottom sealing plates  40 ,  42  so that hopper  12  may move from the vacuum drying position to the material dispense position. 
     Blower  70  is preferably a one horsepower blower. Preferably two heater elements  82  are utilized, as illustrated in the drawings. Air flow through supply plenum  86  is preferably restricted to 4.5 ounces of pressure. 
     As depicted schematically in the drawings by line  74  indicating the angle of repose of within hopper  12 , an air space is permitted to remain within hopper  12  to accommodate material spillage during movement of hoppers  12  and cycling of the drying process. 
     The material fill and heat and vacuum drying functions may each take approximately twenty minutes. Accordingly, in one hour, all three hoppers  12  preferably cycle through material fill and heat position  100 , material vacuum drying position  102  and material dispense position  104 . If each hopper  12  is approximately 10 inches in diameter and 24 inches high, each hopper  12  will hold about one cubic foot of granular resin material, which is about thirty-five pounds of granular resin material. With such configuration, dryer  10  embodying the invention can provide about 100 pounds per hour of dried granular resin material for subsequent processing by plastic injection molding or extrusion equipment. 
     As is apparent from the drawings, hoppers  12  are preferably provided equally spaced around vertical shaft  24  with hoppers  12  120° apart.