Abstract:
A centrifuge for separating solid-liquid mixtures including: a rotating bowl having a head wall with at least one drain opening for clarified liquid, said bowl having a rotational axis; a weir plate fixed to the head wall of the bowl and rotating with bowl, wherein the weir plate is aligned with the at least one drain opening; a choke plate coupled to and rotating with the rotating bowl, the choke plate having surface axially aligned with the drain opening or the weir plate, wherein said choke plate is movable axially with respect to the head wall, and a gap having between the drain opening or the weir plate, wherein the gap has a radially inward inlet receiving the clarified liquid from the bowl and a radially outward outlet for discharging the clarified liquid from the bowl.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/222,616, filed Jul. 2, 2009, which is incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to a solid bowl centrifuge for separating solids from a liquid, for different applications, such as in a waste water treatment facility. In particular, the invention relates to a weir and choke plate assembly for a solid bowl centrifuge. 
         [0003]    Conventional solid bowl centrifuges typically are embodied as helical conveyor centrifuges having a screw within a bowl. The screw and bowl are coaxial and rotate independently of each other. The screw rotates to transport solids in a liquid solids mixture to a solids discharge end of the centrifuge. The rotations of the screw conveyor and the bowl apply a centrifugal force causing the liquid and solids in the bowl to form an annular ring having an outer surface against the wall of the bowl and an inner cylindrical ring surface (the “pond level”) that surrounds a gas filled void at the center of the centrifuge. 
         [0004]    Solids in the rotating bowl tend to settle radially outward on the bowl wall and are moved by the screw to the solids discharge end of the centrifuge. The clarified liquid, also referred to as centrate, is radially inward and flows to a liquid discharge at a head wall which is at an end of the centrifuge opposite to the solids discharge end. 
         [0005]    Weir assemblies are attached to the head wall of the centrifuge. The weir assembly typically includes a passage for the discharge of liquid centrate from the bowl of the centrifuge. The conventional weir assembly is adjustable to regulate the level of the liquid in the bowl. For example, German Patent Publication DE 1 183 023 discloses a weir assembly having two concentric ring-disks forming a V-shaped drain opening that is radially adjusted during operation of the centrifuge to regulate the liquid level in the centrifuge bowl. Other adjustable weir assemblies and weir aperture geometries are disclosed in DE 1 452 260. Similarly, DE 39 21 327 discloses weir elements for weir assemblies formed of flaps, slides and wedges arranged adjacent drain openings in the head wall of a centrifuge. These weir elements rotate with the bowl of the centrifuge and are adjusted radially by a set collar. The pond level of the liquid centrate in the centrifuge bowl, as well as the quantity of the centrate being drained, is regulated by adjusting the radial position of the set collar. 
         [0006]    DE 102004019368 discloses a solid bowl centrifuge with an adjustable weir system having adjacent weir plates in which the inner weir plate rotates with bowl and the outer weir plate is fixed. The outer weir plate does move in a small set-wise rotational movement that allows for an adjustment of the effective gap between the outer and inner weir plates. The adjusting mechanism for the outer weir plate is eccentrically mounted with respect to the rotating centrifuge. The centrate flows through the gap between weir plates. 
         [0007]    DE 43 20 265 discloses a solid bowl centrifuge with an adjustable weir having a non-rotating choke plate, also referred to as a throttle plate. The choke plate is axially displaced, arranged outside of the bowl, and is adjacent a rotating liquid drain openings in the bowl. The choke plate is in a plane parallel to the drainage cross sections for the liquid and of the liquid pond level in the bowl of the centrifuge. As the gap between the choke plate and drain openings decreases, the liquid drainage flow resistance increases and the liquid pond level in the centrifuge increases by extending radially inward during centrifuge operation. 
         [0008]    DE 102 03 652.7 discloses a weir discharge with a rotating weir plate and a non-rotating choke plate that creates a liquid centrate discharge opening in which at least one nozzle is assigned to an outlet for discharging clarified liquid from the drum. Energy may be saved depending on the relative angle of the nozzles. Another energy savings concept for a weir is disclosed by WO2004035221. 
         [0009]    U.S. Patent Application Publication US2004/0058796 discloses a weir discharge system where the centrate is directed outwards through a non-rotating annular cup with at least one opening that is connected to the centrifuge housing. The position of the annular cup can be adjusted during operation and thereby the centrate flow and liquid pond level are controlled. 
         [0010]    DE 37 28 901 C1 discloses a centrate discharge system having inner and outer rotating weir plates between which is a fixed gap opening. The inner weir plate has a larger inner diameter than does the outer weir plate. The fixed gap between the weir plates creates a flow path for the centrate. After the gap is closed by a flange, the liquid level rises until the centrate flows over the edge of the outer weir. 
         [0011]    U.S. Pat. No. 5,169,377 discloses a weir discharge system in which the liquid outflow is regulated by an annular discharge gap between rotating ring-weir plates. The outer ring weir plate moves axially to change the size of the gap between the ring and a circular discharge opening. 
         [0012]    A weir and choke plate assembly should provide one or more of an easy adjustment of the liquid pond level in the centrifuge, a relatively low torsion moment to drive the assembly, a gas seal to isolate the gas filled void in the centrifuge from ambient atmosphere, and a decanting function in which the liquid level in the centrifuge is periodically raised to a radially inward drain opening. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0013]    A solid bowl centrifuge has been developed having a variably adjustable weir choke and plate assembly. By axially adjusting the choke plate, the liquid pond level in the centrifuge bowl may be regulated in an operationally reliable manner. The centrate discharges from the centrifuge head in a radially outward direction through a radial gap between a weir plate and a choke plate. The opening of the gap is directed radially outwards. 
         [0014]    The weir and choke plates rotate at the same rotational speed as the bowl. Because the plates rotate together, the total torsion moment applied to the plates is lower as compared to the torsion moments applied to a weir plate and choke plate that rotate at different speeds. Due to the lower torsion moment, reduced energy is required to drive a centrifuge having weir and choke plates that rotate at the same speeds. 
         [0015]    The adjusting device for varying the level of the liquid in the centrifuge bowl comprises an open radial gap defined by a distance between the parallel opposing ring faces of the weir plate and choke plate. The width can be varied between total closure up to a distance where the centrate does not contact the choke plate. 
         [0016]    The gap between the weir and choke plates creates a flow resistance that increases as the axial distance of the gap decreases. As the gap closes and the flow resistance increases, the pressure of the liquid increases to raise the level of the liquid in the bowl. As the gap increases, the level of the liquid in the bowl drops until the flow is dictated by a natural crest height over the weir plate. 
         [0017]    The weir and choke plate assembly disclosed herein may allow for a periodic separate discharge of accumulated top layered fractions of the centrate without significant interference in the operation of the centrifuge. The choke plate may include apertures to allow an outflow of the centrate if the regular flow through the radial gap is too low or blocked. The flow through the apertures in the choke plate and the flow through the gap can be combined or separated in the discharge casing. 
         [0018]    Moreover, the top layer of the liquid in the centrifuge may be decanted periodically by reducing the radial gap to raise the liquid level and eventually an overflow of the centrate through the apertures in the choke plate. 
         [0019]    A centrifuge has been developed for separating solid-liquid mixtures comprising: a rotating bowl having a head wall with at least one drain opening for clarified liquid, said bowl having a rotational axis; a weir plate fixed to the head wall of the bowl and rotating with bowl, wherein the weir plate is aligned with the at least one drain opening; a choke plate coupled to and rotating with the rotating bowl, the choke plate having surface axially aligned with the drain opening or the weir plate, wherein said choke plate is movable axially with respect to the head wall, and a gap between the drain opening or the weir plate, wherein the gap has a radially inward inlet receiving the clarified liquid from the bowl and a radially outward outlet for discharging the clarified liquid from the bowl. 
         [0020]    A method for clarifying liquid in a liquid and solid mixture has been developed using a solid bowl centrifuge having a rotating bowl, a choke plate and a weir plate, the method comprising: feeding the liquid and solid mixture into the bowl; forming the liquid and solid mixture in the bowl into an annulus having an inner annular liquid surface by rotating the bowl; draining clarified liquid from the liquid and solid mixing by draining a radially inward portion of the annulus through an opening in the head wall and over a radially inward edge of the weir plate fixed to and rotating with the head wall, and forming a gap between the choke plate and the head wall or weir plate, wherein the gap extends in a generally radial direction and includes a radially inward inlet to receive the clarified liquid and a radially outward outlet to discharge the clarified liquid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a side view of a solid bowl helical conveyor centrifuge, shown in partial cross-section. 
           [0022]      FIG. 2  is a perspective view of the outer surface of a head wall of a bowl of the centrifuge, weir plates attached to the head wall and weir plates attached to a collar. 
           [0023]      FIG. 3  is a cross-sectional view of an upper-half of head wall of the bowl, choke plate, weir plate and associated mechanism for axially moving the weir plate. 
           [0024]      FIG. 4  is a cross-sectional view of a first weir and choke plate assembly having an upper-half of the choke plate outside of the head wall and weir plate showing their relative relationship and the centrate gap between the plates. 
           [0025]      FIG. 5  is a part perspective and part cross-sectional view of an alternative weir and choke plate assembly in which the choke plate is inside of the head wall. 
           [0026]      FIG. 6  is a front view of a choke plate for the weir and choke plate assembly shown in  FIG. 5 . 
           [0027]      FIG. 7  shows another embodiment of the weir and choke plate assembly shown in  FIGS. 5 and 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]      FIG. 1  is a side view of a conventional solid bowl centrifuge  10 . A rotating inner screw assembly  14  transports solids along the length of a bowl  12 . The bowl  12  rotates to centrifugally displace a liquid solid mixture  13  radially outward against the bowl that surrounds the screw. The bowl  12  may be a drum, basket or other generally annular container and they are collectively referred to herein as a “bowl.” A housing  11  of the centrifuge encloses and supports the bowl  12  and screw  14  assembly. 
         [0029]    A mixture of solids and liquid  13  is typically held in a holding pool and enters an axial inlet  34  of a feed pipe  32  that discharges the mixture to a distributor  38  at a center portion of the bowl. The distributor  38  discharges the liquid and solids mixture  13  in a central portion of the channel  20 . The liquid and solids mixture flows through the feed pipe  32  and to a generally conical distributor  38  that rotates with the screw body  16 . The distributor has radially outward openings through which the liquid and solids mixture pass through the screw body  16  and enter a center portion of the channel  20 . 
         [0030]    Once in the rotating bowl, the mixture  13  forms an annular ring shape bounded an inside surface of the rotating bowl. The solids tend to settle radially outward against inside surface the rotating bowl. The radially inward portion of the mixture  13  is clarified liquid, which is referred to as centrate. 
         [0031]    A screw assembly  14  coaxial to the bowl and within the bowl moves the solids to a solids discharge casing  22  at one end of the bowl. The opposite end of the centrifuge includes a head wall  60  of the centrifuge. The screw assembly  14  has a cylindrical screw body  16  and a screw blade  18  forming a helix around the screw body. Turning the screw blade  18  moves the solids to the end of the bowl having the discharge casing  22 . 
         [0032]    The liquid solid mixture  13  forms an annulus in the bowl with a cylindrical inner liquid surface  15  facing a gas filled annular void  21  between the mixture and a cylindrical body of the screw. The annular liquid surface level  15  is referred to as the “pond level”. The pond level  15 , in a radial direction, is generally uniform in the channel. An annular channel  20  between the inside surface of the bowl  12  and a cylindrical screw body  16  defines a passage for the liquid and solids mixture  13  in the centrifuge  10 . A gas filled void  21  forms in a region of the channel between the pond level of the mixture  13  and the outer surface of the cylindrical screw body  16 . 
         [0033]    The screw body  16  and bowl  12  are separately rotatably driven by, for example, a motor  24  and a generator  26 , respectively. Rotation of the bowl imparts centrifugal forces that cause the liquid and solids mixture to move radially outward in the channel  20  and form an annular ring in the channel  20 . The liquid passes through openings and over edges in the screw blade  18  to ensure that the pond level  15  remains uniform through the channel. 
         [0034]    The bowl and optionally, the screw body may taper  27  radially inward between the distributor  38  and the discharge casing  22 . As the solids material move along the tapered  27  portion of the bowl, the solids are moved radially inward and beyond the liquid surface level in the channel. Once the solids have moved in the channel beyond the liquid surface level, the solids are separated from the liquid and can be discharged through the discharge casing  22 . 
         [0035]    A novel assembly of weir plates and choke plates has been developed for a solid bowl helical conveyor centrifuge.  FIG. 2  is a perspective view of the outer surface of a head wall  60  of a bowl of the centrifuge, weir plates  28  attached to the head wall and choke plates  44  attached to a collar  76 .  FIG. 3  shows, in cross-section, a side view of an end portion of the solid bowl helical conveyor centrifuge and particularly shows a portion of the head wall  60 , centrate discharge casing  36 , the weir plate  28  and the choke plate  44 . The choke plate moves axially with respect to the rotational axis  52  of the screw and bowl.  FIG. 4  shows in cross-section a side view of the weir plate  28  and choke plate  44 . A centrate discharge casing  36  provides a housing over the outside of the head wall  60  and for the weir and choke plate assembly. 
         [0036]    As shown in  FIG. 2 , the weir plates  28  may be attached, e.g., bolted, to the outer surface of the head wall. The weir plates  28  may include a flat bracket  29  and a U-shaped channel  31  which forms a short flow passage  43  ( FIG. 3 ) axially between the head wall and the choke plate. The U-shaped channel  31  may be welded to the bracket  29  of the weir plate  28 . A head wall plate  49  extends axially from the outer surface of the bowl head wall  60  and faces in a radial direction an open end of the U-shaped channel  31  of the weir plate  28 . 
         [0037]    The weir plates  28  are mounted to the head wall  60  adjacent and partially covering drain openings  50  in the wall. The drain openings  50  are generally arranged in an annular array on the head wall. Each drain opening may be at different angular positions on the headwall. All of the drain openings may be at common radial distances from the axis of the head wall. 
         [0038]    Each weir plate  28  covers a radially outer portion of a drain opening  50  to define a radially outer edge of a centrate flow passage  43  through the opening  50  in the head wall. The choke plates  44  are each aligned with and adjacent one of the weir plates. In each weir plate, the U-shaped channel  31  has an axial end  58  ( FIG. 4 ) opposite to a flat surface  56  on the corresponding choke plate  44 . 
         [0039]    Centrate flows  51  radially through a gap  54  between the axial end  58  of the U-shaped channel  31  of the weir plate and the flat surface  56  on the choke plate. These surfaces of the weir and choke plates forming the gap  54  may extend radially for a sufficient distance, e.g., 1 mm to 25 mm, to form a radially extending centrate flow passage  51  through the gap  54 . The radial length of the gap  54  is sufficient to cause the centrate to flow  51  radially through the gap. 
         [0040]    The desired pond level  15  in the bowl is indicated by the dotted line  53  shown on the choke plate in  FIG. 2 . The actual pond level of the centrate liquid  15  is shown in  FIG. 3 . The centrate in the bowl is radially outward of the pond level  15 . From the pond, centrate liquid flows  51  through the drain opening  50  and the U-shaped channel  31  towards the choke plate  44  and turns radially outward to flow out a gap  54  between the axial end  58  of the U-shaped channel  31  and a face  56  of the choke plate  44 . 
         [0041]    A gas filled void  21  in the bowl is radially inward of the pond level  15 . Gases may escape through a gap between the head wall plate and U-shaped channel  31  of the weir plate  28 . 
         [0042]    The radial position of the weir plates  28  on the head wall may be adjusted by means of parallel and generally radial slots  46  in the weir plate bracket  29 . These slots receive the bolts holding the weir plate to the head wall. Each weir plate bracket  29  may be marked with gradations  47  that are aligned with a reference circle  48  marked on the head wall. By aligning the proper gradation marking  47  to the reference circle  48  for each of the weir plates, the radial position of each of the weir plates on the head wall may be precisely positioned at a uniform radial distance from the axis of the bowl. 
         [0043]    As shown in  FIGS. 3 and 4 , the discharge clarified liquid, e.g., “centrate”, flows through the channel  20  in the bowl towards the head wall  60  and through drain openings  50  arranged annularly in the head wall  60 . These openings  50  are preferably arranged in a circle on the head wall, wherein the circle of openings is centered on the rotational axis of the bowl. 
         [0044]    The gap (G)  54  between the weir plate  28  and choke plate  44  defines a passage for the centrate flowing to the discharge casing  36 . The flow  51  of centrate is generally axially as the centrate moves through the channel  20  and into the opening  50  of the head wall. Because of centrifugal force, the flow  51  quickly turns radially outward as the centrate flows over the edge of the channel  31  on the weir plate  44  and enters the gap  54  between the weir plate and the choke plate. The centrate flows  51  radial outward through the gap  54  and into the centrate discharge casing  36 . 
         [0045]    The choke plates  44  are mounted on the shaft  64  ( FIG. 3 ) of the bowl or screw conveyor. The choke plate includes a collar  77  that engages the shaft. An upper surface of the collar supports a ball bearing assembly  66 , which provides an engagement between the choke plate and a non-rotating axial adjustment mechanism  75 . This adjustment mechanism  75  is supported by a pillow box bearing  70  mounted on the shaft  64 . The choke plate adjustment mechanism includes a turning wheel  72  for manual or automated adjustment of the gap  54 . The turning wheel  72  causes a helically threaded ring  74  of the adjustment mechanism to move the ball bearing assembly  66  axially and thereby axially move the choke plate. The choke plate may include labyrinth seals  78  that engage the axial adjustment mechanism  75  and the head wall  60 . A sealing gasket may extend annularly in the labyrinth seal  102 . 
         [0046]    By adjustment of the turning wheel  72 , the width of the gap  54  may be varied between total closure in which substantially no centrate flows out through the weir plate to a gap width in which the centrate does not fill the gap and thus does not impinge on the choke plate. 
         [0047]    The choke plates  44  arranged adjacent to the outside of the head wall may include an annular array of discharge openings  45  positioned radially inwardly of the gap  54 . These openings  45  provide centrate discharge in the event the gap becomes clogged or the gap unduly restricts the discharge of centrate. If the pond level  15  increases radially inward because of excessive liquid and solid mixture  13  in the centrifuge, the discharge openings  45  allow the centrate to flow into the centrate discharge casing. Instead of openings  45  in the choke plates, lowering the U-shaped side walls of channel  31 , also allow the centrate to discharge into the centrate discharge casing. 
         [0048]    The centrifuge may be operated in a decanting mode. In this mode, the gap  54  is narrowed by axially advancing the choke plate towards the weir plate or, towards the head wall if the choke plate(s) is inside of the head wall. With the gap narrowed or closed, the pond level  15  in the centrifuge rises radially inward. With the gap narrowed or closed, the centrate flows through optional openings  45  in the choke plate or overflows the side walls of the U-shaped channels  31  that extend from the choke plate. The centrate flows through the openings  45  or over the channel side walls and into the centrate discharge casing  36 . By allowing the pond level to rise, the decanting mode provides greater separation of solids from the liquid and the resulting centrate may have less solids than the centrate that would have otherwise flowed through the gap  54 . 
         [0049]    The decanting mode may be performed periodically or a regular cycle or when the operator of the centrifuge desires to reduce the solids content in the centrate. The decanting mode may also be performed when the operator of the centrifuge desires to reduce the floating solids or foam in the centrifuge which, of course, results in a periodically higher solid or foam content in the centrate which may be treated differently downstream of the centrate casing. 
         [0050]      FIG. 5  is a part perspective and part cross-sectional view of an alternative weir and choke plate assembly  80  in which the choke plates  92  are inside of the head wall  84  of the bowl  86  of a solid bowl helical conveyor centrifuge.  FIG. 6  is a front view of a choke plate collar  88  which supports arms  90  that are attached to the choke plates  82 . The arms extend through openings  93  in the head wall  84 . The weir and choke plate assembly  80  functions to control the pond liquid surface level  15 , control the flow of centrate out of the bowl, and seal the gas in the gas filled void  21  in the bowl from ambient air outside of the centrifuge. Gas sealing is helpful to prevent or minimize oxygenation of the liquid solid mixture  13  in the bowl. The choke plate  92  is preferably an annular plate or an annular array of plates having an inside diameter  89  that is slightly greater than an outside diameter of the hub  104  for the screw conveyor. A sealing ring  91  provides a seal between the inner rim of the choke plate  92  and the hub  104  of the screw conveyor. If the choke plate does not serve as a gas seal the sealing ring  91  may be omitted and the choke plate  92  may be equipped with openings  106  (illustrated by dotted lines in the choke plate  92  in  FIG. 7 ) that serve the same purpose as the openings  45  in the choke plate shown in  FIG. 4 . 
         [0051]    The choke plate  92  may be an annular plate forming a ring or an annular array of plates each aligned with one of the openings  93  in the head wall. The choke plate(s) has a front surface  92  that conforms to an inside surface of the head wall  84 . The openings  93  allow centrate to flow from the bowl to a discharge casing or channel. The choke plate  92  is attached, e.g., bolted, to an arm  90  extending axially between the plate and the choke plate collar. The arm  92  extends through the opening  93  in the head wall. The choke weir and choke plate assembly  80  includes an annular array of choke plates  82  each adjacent one of the openings  93 . Each choke plate is attached by an arm  90  to the choke plate collar in a centrate casing (see  36  in  FIGS. 1 and 3 ). 
         [0052]    The choke plate  92  may be advanced axially (see arrow  95 ) to define the width of a gap  96  between the front surface  92  of the choke plate and the inside surface of the head wall at the rim of the opening  92  in the head wall. The gap  96  has a radial length of preferably 1 mm to 25 mm which corresponds to the overlap between the front surface of the choke plate and the inside surface of the head wall. Adjacent the radial gap  96  is an axial gap  103  between the outer rim of the choke plate and the inner wall of the bowl. The gaps  96  and  103  form a restriction to the centrate flowing (see arrows in  FIG. 7 ) from the bowl to the centrate discharge casing  36 . 
         [0053]    The choke plate  92  is advanced axially by an operator moving the choke plate collar  88  axially with respect to the shaft of the bowl or screw conveyor. The choke plate  92  may attach to shaft with a pillow box bearing ( 70  in  FIG. 3 ) and may be moved axially by an axial adjustment mechanism ( 75  in  FIG. 3 ). The mechanisms for axially moving the choke plate shown in  FIG. 3  may be also applied to the choke plate  82 . 
         [0054]    The weir plates  94  mounted to the head wall  84  may be generally rectangular plates having an inside surface conforming to an outer surface of the head wall adjacent to an opening  93  in the head wall. The weir plates  94  may be bolted to the head wall  84  and adjusted radially with respect to the head wall in a manner similar to the weir plate bracket  29  shown in  FIG. 2 . 
         [0055]      FIG. 7  shows the embodiment of the weir and choke plate assembly shown in  FIGS. 5 and 6  in which the assembly substantially isolates the gases in the void in the bowl from the ambient air outside of the centrifuge. The annular choke plate  82  generally prevents ambient air from mixing with the gases in the void in the bowl. A seal  91  between the inner rim of the choke plate and the hub  104  of the screw conveyor prevents gas passage between the void  21  and ambient air. 
         [0056]    The solid bowl centrifuges disclosed herein have a rotating bowl having an end region with drain openings for clarified liquid. The drain openings are aligned with a weir plate and choke plate assembly that provides an adjustable radial gap for varying a level of the liquid in the centrifuge bowl during operation of the centrifuge. The weir and choke plate assembly has opposing parallel plates rotating together with the centrifuge bowl. 
         [0057]    The choke plate may be arranged inside the bowl to face an inner surface of the head wall or outside the bowl to face an outer edge of a weir plate. The choke plate may include radially inward openings through which centrate may flow during a decanting function. 
         [0058]    The choke plate may alternatively be used to seal the gas filled void in the centrifuge against the out atmosphere. The centrate exits solely through the radial gap. The centrate in the gap forms an effective gas seal between the gas filled void  21  in the centrifuge and ambient air. The gas filled void is radially inward of the liquid annular ring formed by the spinning bowl. 
         [0059]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.