Abstract:
A centrifugal separator provides for the discharge of solids by either an axial-motion scraper or a piston/extrusion assembly. The axial-motion scraper is used with hard-packed or friable solids, and includes an integral feed liquid accelerator and feed holes. The piston/extrusion assembly is used with pasty solids, and includes a piston extending into a separator bowl and having openings permitting fluid communication across the piston. After high-speed separation is complete, a centrate valve closes one end of the bowl, and the piston is moved axially in the bowl by an actuator. Accumulated solids are scraped from the sides of the bowl and extruded out of the piston openings for discharge from the bowl. A bowl suspension employs a spherical mounting structure and a short, stiff spindle. A spherical portion of a bearing housing is mounted in a spherical mounting region at one end of the separator, with a cylindrical portion of the bearing housing extending along the rotational axis. A bearing and the spindle of the separator bowl are mounted within the cylindrical portion of the bearing housing. The suspension is retained by a stiff resilient ring and retaining member secured to the separator in compressive contact with the spherical portion of the bearing housing.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 10/412,943, now U.S. Pat. No. 6,776,752 filed on Apr. 14, 2003 entitled, AUTOMATIC TUBE BOWL CENTRIFUGE FOR CENTRIFUGAL SEPARATION OF LIQUIDS AND SOLIDS WITH SOLIDS DISCHARGE USING A SCRAPER, and also claims priority under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 60/372,153 filed Apr. 12, 2002, the whole of which are hereby incorporated by reference herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to centrifuges and in particular to a centrifuge enabling automatic discharge of solids that accumulate during separation. 
     Many different types of centrifugal separators are known for separating heterogeneous mixtures into components based on specific gravity. A heterogeneous mixture, which may also be referred to as feed material or feed liquid, is injected into a rotating bowl of the separator. The bowl rotates at high speeds and forces particles of the mixture, having a higher specific gravity, to separate from the liquid by sedimentation. As a result, a dense solids cake compresses tightly against the surface of the bowl, and the clarified liquid, or “centrate”, forms radially inward from the solids cake. The bowl may rotate at speeds sufficient to produce forces 20,000 times greater than gravity to separate the solids from the centrate. 
     The solids accumulate along the wall of the bowl, and the centrate is drained off. Once it is determined that a desired amount of the solids has been accumulated, the separator is placed in a discharge mode. In one such discharge mode, a scraper blade extending the length of the rotating bowl is placed in a scraping position against the separator wall and the bowl is rotated at a low scraping speed. Then, a radial-motion scraper scrapes the solids from the sides of the bowl, and they fall toward a solids collecting outlet. However, such a radial-motion scraper does not effectively remove wet or sticky solids which may have a consistency like that of peanut butter. In such instances, the sticky solids remain stuck on the scraper blades or fall from the wall and then reattach to the blades before reaching the collecting outlet. As a result, the solids recovery yield is reduced and the remaining solids undesirably contaminate the separator. 
     An additional important consideration in the design of centrifugal separators is to minimize vibration and other ill effects of operation at high rotational speeds. The separator bowl and its mounting structure form a mechanical unit having inherent resonant or “critical” speeds which are preferably avoided during operation. An additional consideration is potential for axial movement of the separator bowl, for example in the presence of imbalance or the motion of liquid axial waves in the bowl, which can result in unstable operation. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a centrifugal separator is disclosed that includes features addressing the shortcomings of existing centrifugal separators, especially shortcomings associated with solids recovery and mechanical instability. 
     In one aspect, the disclosed centrifugal separator provides for automatic discharge of solids by means of either an axial-motion scraper or a piston/extrusion assembly with exchangeable parts, having variable speed operation for greater versatility. The axial-motion scraper is used with hard-packed or friable solids, and includes an integral feed liquid accelerator and feed holes. The scraper blades flex outwardly under high centrifugal force to lock the scraper in place against the bowl. This provides a rigid or fixed end condition for the lower end of the scraper shaft to allow for high critical speed of the shaft. The scraper provides less surface area for solids to stick to, and can be used in conjunction with relatively long separator bowls. 
     The piston/extrusion assembly is used for pasty, sticky solids that can be extruded. A centrate valve at the top of the bowl is used to enable the centrate (separated liquid) to be discharged during a feed mode of operation, and then to close off the top of the bowl for a solids discharge mode of operation. The assembly further includes a piston that sits at the bottom of the bowl during the feed mode of operation. The piston has an integral feed accelerator and feed holes through which the feed liquid passes. These holes also provide exit paths for the solids during the extrusion that takes place in the solids discharge mode of operation. The piston/extrusion assembly can be used with sticky solids that other existing centrifuges cannot discharge efficiently, and provides for nearly complete removal of the solids, which is desirable for example when the solids contain valuable materials. 
     In another aspect, the disclosed centrifugal separator includes a separator bowl suspension that employs a short, stiff spindle and a spherically mounted bearing housing. Conceptually, the arrangement is analogous to a vertical rotating beam with a simply supported upper end. This arrangement has a very high critical speed as compared to existing centrifuges. It is possible to achieve a critical speed greater than the highest operating speed, so that the critical speed is not encountered during operation. The spherically mounted bearing housing restrains axial motion of the separator bowl and provides for stable operation at higher speeds than prior mounting arrangements. In one embodiment, the bearing housing comprises a semi-spherical portion having an upper semi-hemispherical portion and a lower semi-hemispherical portion, and a cylindrical portion. 
     In yet another aspect, the disclosed centrifugal separator employs a half-ball-shaped solids discharge valve at the bottom of the case. The discharge valve incorporates respective passages for the feed liquid and for residual liquid being drained from the bowl. The valve rotates between a closed position in which the bottom of the case is closed except for the openings to and from the feed liquid and residual liquid passages, and an open position in which solids being discharged from the separator bowl are able to fall out of the bottom of the case. This arrangement is generally more compact than prior art arrangements for discharge valves, and can be used in sanitary and/or clean-in-place applications. 
     Other aspects, features, and advantages of the present invention will be apparent from the Detailed Description Of The Invention that follows. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood by reference to the following Detailed Description Of The Invention in conjunction with the Drawings, of which: 
         FIG. 1  is a section view of a centrifuge having a first construction in accordance with the present invention; 
         FIG. 2  is a detailed section view of a lower portion of a separator bowl in the centrifuge of  FIG. 1 ; 
         FIG. 3  is a section view of the centrifuge of  FIG. 1  illustrating operation in feed mode; 
         FIG. 4  is a section view of the centrifuge of  FIG. 1  illustrating operation in residual liquid drain mode; 
         FIG. 5  is a section view of the centrifuge of  FIG. 1  illustrating operation in solids discharge mode; 
         FIG. 6  is a detailed section view of a lower part of the centrifuge of  FIG. 5 , as viewed from a point to the left in  FIG. 5 ; 
         FIG. 7  is a detailed section view of an upper bowl portion of the centrifuge of  FIG. 5 ; 
         FIG. 8  is a section view of a centrifuge having a second construction in accordance with the present invention; 
         FIG. 9  is a top perspective view of a scraper in the centrifuge of  FIG. 8 ; 
         FIG. 10  is a bottom perspective view of the scraper of  FIG. 9 ; 
         FIG. 11  is side sectional view of the scraper of  FIG. 9 ; 
         FIG. 12  is a section view of the centrifuge of  FIG. 8  illustrating operation in feed mode; 
         FIG. 13  is a detailed section view of a lower part of the centrifuge of  FIG. 12 ; 
         FIG. 14  is a section view of the centrifuge of  FIG. 8  illustrating operation in drain mode; 
         FIG. 15  is a section view of the centrifuge of  FIG. 8  illustrating operation in solids discharge mode; 
         FIG. 16  is a detailed section view of a bowl suspension structure in the centrifuges of  FIGS. 1 and 8 ; and 
         FIG. 17  is a detailed section view of an alternative bowl suspension structure suitable for use in the centrifuges of  FIGS. 1 and 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a centrifugal separator in vertical section, with a middle portion removed so as to illustrate a horizontal section as well. The centrifugal separator includes a cylindrical separator bowl  10  mounted in a central region  11  of a separator housing  13 . The separator bowl  10  is preferably a tubular type bowl having a relatively small diameter D and a length L such that the ratio of L/D is approximately 5/1 or greater. Mounted within the separator bowl  10  is a piston assembly consisting of a piston head  12  connected to a piston shaft  14 . 
     A variable speed drive motor  16  is connected to a drive pulley of a spherically mounted bearing and spindle assembly  18 . The connection is made by a drive belt  20  at a collar-like extension  21  of the upper end of the separator housing  13 . The drive motor  16  is controllably operated to rotate the separator bowl  10  at desired speeds for separating the feed liquid. A piston shaft clutch  22  is mounted in a crosshead  24  of a piston actuator which includes two piston actuator plungers  26  mounted in respective piston actuator cylinders  28 . Each piston actuator plunger  26  is operatively connected to the piston shaft  14  via the crosshead  24  and the piston shaft clutch  22  for raising and lowering the piston assembly within the separator bowl  10  in response to compressed air or hydraulic fluid introduced at piston actuator ports  29 . In a discharge mode of operation, the piston shaft clutch  22  is engaged for holding the piston shaft  14  while the piston actuator is raised so that the edges of the piston head  12  scrape solids from the walls of the separator bowl  10 . In other operating modes, the piston shaft clutch  22  is disengaged so that the piston assembly simply rotates with the separator bowl  10  and does not move axially. In these operating modes, a lock ring  31  prevents the piston assembly from falling out of the bottom opening of the separator bowl  10 . 
     Also shown in  FIG. 1  are a centrate case  30 , centrate outlet port  32 , centrate valve  34  and centrate valve actuator  36 , all of which are involved in removing the centrate, or clarified liquid, from the centrifugal separator during operation, as described in more detail below. A solids valve  38  is mounted in a lower end region  39  of the separator housing  13 , below an inward-facing flange  41 . The solids valve  38  incorporates both a feed liquid passage  40  in communication with a feed liquid port  42 , as well as a residual liquid drain passage  44  in communication with a residual liquid drain port  46 . A solids valve seal  48  is disposed on a lower surface of the flange  41 . Additional structural and functional details of the solids valve  38  are described below. 
       FIG. 2  shows the area of the piston head  12  in detail. The central area  43  of the piston head  12  has an inverted cone-shaped cross section, with openings  45  arranged around the perimeter. In a feed mode of operation, as described below, feed liquid from the feed liquid passage  40  enters the cavity beneath the central area  43 , as indicated at  47 , and is directed out of the openings  45  toward the inner surface of the separator bowl  10 . Due to rotation of the piston head  22  in this operating mode, the openings  45  serve to accelerate the feed liquid and distribute it around the bottom of the separator bowl  10 . 
     A feed mode of operation of the centrifugal separator is described with reference to  FIG. 3 . The piston shaft clutch  22  is disengaged so that the piston shaft  14  is free to rotate at high speed with the separator bowl  10  under the influence of the drive motor  16 . The solids valve  38  is in a closed position in which its outer upper surface rests against the solids valve seal  48 . The solids valve seal  48  is pneumatically or hydraulically inflatable by a solids valve actuator  50  via an inflating passage  53 . In the feed mode, the seal  48  is maintained in an inflated state. 
     The feed liquid is introduced through the feed liquid port  42 . The feed liquid flows from the feed liquid port  42  into the feed liquid passage  40 , and upon reaching the end of the feed liquid passage  40  continues in a stream  55  toward the bottom of the piston head  12 . As described above, the piston head  12  includes structure that operates to accelerate the feed liquid and direct it toward the inner wall of the bowl  10  as it rotates. Due to the centrifugal force, the liquid flows up the inner surface of the separator bowl  10  forming a pool surface  52 . As shown, the centrate valve  34  is open, so that any overflow liquid decants over a weir  54  as clarified liquid (centrate) at the top of the separator bowl  10 . The centrate then flows into the centrate case  30  and out of the centrate outlet port  32  as shown at  58 . As the liquid flows through the separator bowl  10 , it is clarified of entrained solid particles by the high centrifugal force acting upon the liquid. The solids are forced to settle on the inside wall of the separator bowl  10  and collect as a compressed solids cake  56  as a result of the centrifugal force. 
     When the separator bowl  10  has been determined to be sufficiently full of solids, for example by sensing the turbidity of the centrate, the centrifugal separator is placed in a bowl drain mode which is depicted in  FIG. 4 . The feed liquid is shut off and the driver motor  16  electronically brakes the separator bowl  10  to a full stop. The residual liquid in the separator bowl  10  drains down through the openings in the piston head  12  onto a shaped upper surface of the solids valve  38 , which channels the residual liquid into the liquid drain passage  44 . The residual liquid then exits via the liquid drain port  46  as shown at  60 . The separator bowl  10  may be rotated again to further separate liquid from the solids, depending on the application. 
     When the separator bowl  10  has been completely drained of residual liquid, the centrifugal separator enters a “piston” mode in which the accumulated solids are forced out of the separator bowl  10 . The piston mode is illustrated in  FIGS. 5 and 6 . The solids valve seal  48  is deflated and the upper offset portion  61  of the solids valve  38  is rotated away from the opening defined by the inner edge of the flange  41 . The piston shaft clutch  22  engages the piston shaft  14 , and the centrate valve  34  is closed by action of the centrate valve actuator  36 . Then, by action of the piston actuator including plungers  26  and cylinders  28 , the crosshead  24  is slowly raised, and with it the piston shaft  14  and piston head  12 . As the piston head  12  is drawn upward, the accumulated solids are scraped away from the inner surface of the separator bowl  10  and eventually fill the compressed space  62  above the piston head  12 . Further raising of the piston head  12  results in pressure on the enclosed solids, forcing them to be extruded downward through the openings in the piston head  12 . The extruded solids fall downward through the open bottom of the separator bowl  10  and past the open solids valve  38 , as indicated at  64 . This extruding action continues until the piston head  12  has been raised to its maximum height, at which point substantially all of the accumulated solids have been removed. At this point, the components including piston head  12 , centrate valve  34  and solids valve  38  are returned to their respective positions as shown in  FIG. 1  for the next feed/drain/piston cycle. At this point, a cleaning operation may also be performed in preparation for the next operational cycle. 
       FIG. 7  shows the area of the centrate valve  34  during the piston mode of operation in greater detail. The centrate valve  34  is normally held open by return springs  66  and  68 . Under the action of compressed air or hydraulic fluid  70 , the centrate valve actuator  36  is raised, bringing the centrate valve  34  to a closed position. As the piston head  12  is raised by action of the piston actuator, the soft solids are extruded through openings  70  of the piston head, as indicated at  64 . As shown, several seals including piston shaft seal  72 , piston head seal  74 , and centrate valve seal  76  provide for fluid-tight sealing of the upper part of the bowl  10  in the piston mode, such that the solids are forced only through the piston openings. 
       FIG. 8  shows a centrifugal separator similar in many respects to the centrifugal separator of  FIGS. 1–7 . The primary difference is the use of a scraper having a scraper shaft  78  and scraper head  80  instead of a piston. Also, the centrifugal separator of  FIG. 9  does not include the centrate valve  34  and associated apparatus found in the centrifugal separator of  FIGS. 1–7 . The centrifugal separator of  FIG. 8  employs a helical scraping action on the inner surface of the bowl  10  rather than an extruding action, and can generally be used with accumulated solids that are relatively dense and rigid. 
       FIGS. 9–11  show different views of the scraper head  80 . Four scraper arms  82  extend from a central body portion  84 , which includes a number of radially directed feed accelerator holes  90 . Alternative embodiments may use fewer or more scraper arms  82 . Each scraper arm  82  has a forward surface  86  with an edge portion  88  that is in close contact with the inner surface of the separator bowl  10 . The forward surface  86  may be integral with the rest of the arm  82  or may be part of a separate hard material that is attached to the arm  82 , such as by welding or brazing. Also shown in  FIGS. 9–11  are skirt portions  89  extending downwardly below the arms  82 . The function of the skirt portions  89  is described below. 
       FIG. 12  shows the centrifugal separator of  FIG. 8  in a feed mode of operation, which is substantially the same as the feed mode of operation of the centrifugal separator of  FIGS. 1–7 .  FIG. 13  shows the area of the scraper head  80  in detail during the feed mode of operation. The scraper head  80  is located at the lower end of the bowl  10 , and rotates with the bowl  10  at high speed. The skirt portions  89  of the scraper head  80  extend into a lower opening of the bowl  10 , and during the high-speed rotation actually flex slightly outward in response to the centrifugal forces to urge against a lower rim  91  of the bowl  10 . By this action, unwanted vibration of the scraper assembly is reduced. 
     During the feed mode of operation, the feed liquid stream  55  is accelerated radially by action of the scraper head  80  rotating with the separator bowl  10 . Specifically, the feed liquid stream  55  hits the underside  93  of the body portion  84  of the scraper head  80  (see  FIGS. 10 and 11 ) and is directed outwardly to the inner surface of the separator bowl  10  through the holes  90 . The solids  56  accumulate near the inner surface of the separator bowl  10  as the centrate flows up the inner surface of the separator bowl  10  and eventually out of centrate port outlet  32  as described above with reference to  FIG. 3 . 
       FIG. 14  illustrates the drain mode of operation of the centrifugal separator of  FIG. 8 . Again, operation is similar to the drain mode of operation of the centrifugal separator of  FIGS. 1–7 . 
       FIG. 15  shows a scrape mode of operation of the centrifugal separator of  FIG. 8 . The solids valve seal  48  is deflated and the solids valve  38  is rotated away from the bottom of the separator bowl  10 , as shown in  FIG. 6 . The scraper clutch  22  is engaged to prevent the scraper shaft  78  from rotating and to lift the scraper shaft  78  as the scraper actuator is lifted. The motor  16  rotates the bowl at a slow speed as the scraper head  80  is slowly raised. This causes the packed solids to be scraped away along a helical path on the inner surface of the bowl  10 . This action continues until the scraper head  80  reaches the top of the bowl  10 , at which point it is slowly lowered, scraping away any residual solids as it does so. When this scraping cycle is complete, the solids valve  38  closes again and the solids valve seal  48  is re-inflated, enabling the next feed/drain/scrape cycle to commence. 
     Optionally, cleaning and/or rinsing fluid may be introduced through the same fluid feed pathway, with operation of the drive motor  16  enabling complete distribution of the cleaning and/or rinsing fluid. A scrape mode of operation, as discussed above, may then be entered to further clean the interior of the separator bowl  10 . 
       FIG. 16  shows the area of the spindle and bearing assembly of the centrifugal separator of  FIGS. 1 and 8 . A bearing housing has a semi-spherical portion  96  and a short cylindrical spindle portion  98 . In the embodiment shown by  FIG. 16 , the semi-spherical portion  96  comprises an upper semi-hemispherical portion and a lower semi-hemispherical portion. Mounted within the spindle portion  98  are a bearing  100  and an extended spindle or hub  102  of the separator bowl  10 . A driven pulley  104  engaged by the drive belt  20  (which extends through a lateral opening  105  of the spherical portion  96  of the bearing housing) attached to the hub  102 . The spherical portion  96  rests against mating surfaces of seats  106 . A clearance adjustment nut  108  is used to retain the seats  106  while providing for a desired amount of clearance between the seats  106  and the bearing housing. As seen in  FIG. 16 , the seats  106  each have an arched surface generally conformed to an outer surface of the semi-spherical portion  96 . The arched surfaces of the seats are in substantially compressive contact with and substantially surround or conform to the upper and lower semi-hemispherical portions of the portion  96 . The seats  106 , therefore, substantially stabilize the portion  96  within the separator housing. A damping rubber support ring is secured to the top of the spherical portion  96 . The support ring  107  and a swing-damping rubber ring are retained by a ring compression adjustment nut  112 . A bearing housing anti-rotation pin  114  prevents the bearing housing from rotating. The pin extends through an enlarged opening  115  in the housing  13 . 
     The structure depicted in  FIG. 16  provides a “simple support” for the rotating spindle  102  and cylindrical separator bowl  10 . This simple support permits a limited amount of outward swiveling of the spindle  102  as it rotates about the central vertical axis of the separator at high speed during operation. This helps to reduce vibration associated with the natural frequency of the rotating apparatus, providing for smoother operation and longer life. It will be noted that the anti-rotation pin  114  is fixed with respect to the bearing housing can move within the opening  115  relative to the separator, and therefore does not interfere with this swiveling action. 
       FIG. 17  shows an alternative scheme for mounting a bearing and spindle assembly  18 ′. The bearing housing has a cylindrical upper portion  96 ′ with notches for receiving two rubber isolation rings  116 . As seen in  FIG. 17 , the isolating rings  116  are positioned near the ends of the upper cylindrical portion  96 ′ and disposed within the mounting region of the separator housing. The assembly is held in place by a ring compression adjustment nut  112 ′. In alternative embodiments, the nut  112  or  112 ′ may be replaced by other structure, including a bolted-on ring or disk. 
     It will be apparent to those skilled in the art that modifications to and variations of the disclosed methods and apparatus are possible without departing from the inventive concepts disclosed herein, and therefore the invention should not be viewed as limited except to the full scope and spirit of the appended claims.