Abstract:
A vacuum drum washer including: a cylindrical drum including a screen and deck defining an exterior cylindrical surface; a plurality of outer filtrate channels disposed inward of the screen, the outer filtrate channels extending along a longitudinal axis of the drum and substantially an entire length of the drum; an radial array of filtrate end conduits extending radially inward from the outer filtrate channels towards a rotational axis of the drum, the radial filtrate conduits have an inlet positioned at a first end of the drum and draining filtrate from the outer filtrate channels; a filtrate chamber at the first end of the drum and receiving filtrate discharged from the end conduits, and an array of radial filtrate drainage conduits coupled to receive filtrate from the filtrate channels, the drainage conduits each having an inlet proximate to the filtrate channels, the inlets are arranged between a center of the drum and a second end of the drum, and the radial drainage conduits directing filtrate to the filtrate chamber.

Description:
BACKGROUND OF THE INVENTION 
     The field of the invention is rotary drum vacuum washers, e.g., filters, used in the pulp and papermaking industry to form a mat of wood pulp and cleanse the mat of filtrate. In particular, the invention relates to the filtrate drainage systems for vacuum drum washers. 
     Vacuum washer drums remove pulping liquors and other liquids from pulp. A vacuum washer has a large rotating cylindrical drum that sits partially in a vat of pulp and liquor. The following references the drum as it rotates in a clockwise direction. As the drum surface rotates through the vat, e.g., 3:00 to 9:00 drum positions, a pulp mat forms on the wire screen surface of the drum. The screen prevents pulp from flowing into drainage passages in the drum. A suction is applied to the drum surface through the drainage passages. The suction pulls the liquor through the wire screen on the drum surface and causes a pulp mat to form on the surface. The suction draws the wash liquid through the mat and into the drainage passages. As the drum surface with pulp mat rotates up and out of the vat from the 9:00 to 12:00 position, water is sprayed on the pulp mat to remove cooking liquor from the pulp. The water and liquor (but not pulp fibers) pass through the wire screen and flow into the drainage passages. The water and liquor in the drainage passages is referred to as “filtrate”. The washed pulp mat is removed from the drum surface, at about the 2:00 to 3:00 drum position, before the drum surface rotates down into the vat. The drum surface rotates back into the vat to pickup another pulp mat. 
     The drainage passages are internal to the drum and typically include channels immediately behind the wire screen surface and deck extending along the entire length of the cylindrical wire screen surface. The channels conventionally drain into radial passages at the end of the drum (“end draining drum”) or into a conical array of drain tubes extending from a center annular drain behind the wire screen and deck (“annular center draining drum”). The drain tubes of the annular center draining drums extend from the drum surface at the center of the drum to an end of the drum. The conical array of drainage tubes discharge through an annular disc tube sheet at an end of the drum and into a V-trunnion that caps the tube sheet. The tube sheet and V-trunnion have relatively large diameters, e.g., 50 inches to 60 inches (127 cm to 152 cm), to accommodate a large number of drainage tubes, e.g., 30 to 36 tubes, that each have a relatively large diameter of, for example, 6 inches (15 cm). 
     The radial end drain tends to be inexpensive to manufacture and maintain, as compared to the center draining drum. The radial end drain has difficulty in draining filtrate from the far end of long drums, such as where the drum length exceeds 20 feet (6 meters). The annular center drain is typically used for longer drums, e.g., longer than 20 feet (6 meters), but is expensive to manufacture and maintain. The annular center drain is expensive, in part, because the V-trunnion is a large device having intricate drain passages that direct filtrate from each of the tubes to an axial drain. There is a long felt need for a less expensive filtrate drainage system for vacuum washers having long drums. 
     BRIEF SUMMARY OF THE INVENTION 
     A novel drainage system for a vacuum drum washer has been developed that includes an end-draining drum and a reduced size annular drain that is offset from center towards a far end of the drum. The reduced sized annular drain has relatively small diameter drain tubes that discharge through a small diameter tube sheet. A V-trunnion is unnecessary because the small tube sheet is suitable to operate with a cylindrical trunnion. As the radial passages and drain tubes rotate through the radial position where substantially no filtrate flows, e.g., 1:00 to 5:00 positions, a novel valve seal blocks suction for both the radial end drain passages and drain tubes that is typically used with end-draining drums. The novel drainage system is suitable for drums having a length greater than 20 feet (6 meters). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the vacuum drum washer is described in detail with reference to the accompanying drawings which include: 
         FIG. 1  is a cross-sectional end view of a conventional vacuum drum washer assembly. 
         FIG. 2  is perspective view of a conventional end-draining vacuum washer drum. 
         FIG. 3  is a perspective view of a conventional center draining vacuum washer drum. 
         FIG. 4  is a cross-sectional side view of a vacuum washer drum having an end drain and an annular drain offset from the center of the drum. 
         FIG. 5  is a perspective view of the valve seat and tube sheet of the vacuum washer drum shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a conventional end-drain rotary drum vacuum filter  10  that includes a rotary drum  12  in a vat  14  of pulp slurry. The drum is partially submerged in a pulp slurry vat vessel, such as up to the horizontal centerline of the drum. As the outer drum surface rotates clockwise through the slurry (3:00 to 9:00 positions), a pulp mat  16  forms on the outer face  17  of the drum. To promote mat formation, suction is applied to the drum porous outer surface  17 , e.g. a screened and wire or corrugated deck surface. The porosity of the screen surface  17  is sufficiently fine to retain fibers on the surface and pass primarily filtrate, e.g., cooking liquor and water, into the channels  18  behind the porous surface. The channels  18  are arranged in a longitudinal array behind the screen and extend the length of the drum. The channels drain into radial channels  20  at one end of the drum or, alternately, tubes extending from a center annular drain. The radial channels or tubes lead to a central filtrate chamber  28 . As the surface  17  of the drum travels up and out of the vat (corresponding to the 9:00 to 12:00 rotational positions of the drum), the pulp mat  16  on the surface is washed with a liquid spray  22 , e.g., wash water, that cleans the pulp mat of chemical pulping liquor. Suction draws the water and liquor from the pulp mat into the channels  18  behind the drum surface  17 . The channels drain to the radial end channels  20  which drain into a filtrate chamber  28  that is typically at one end of the drum and coaxial to the drum. As the drum surface passes over the top rotational position (12:00 to 1:00), the wash water spray is stopped. As the drum rotates towards the 2:00 position, the suction stops, but water continues to drain through the pulp and into the channels and radial drain passages. Air also starts to enter the channels and ribs because of the stoppage of wash water. 
     The concentrated pulp is generally referred to as a pulp cake. As the drum rotates through to the 2:00 to 3:00 position, a scraper  24  removes the pulp mat from the drum surface. The pulp cake is collected in a chamber  26  for further processing. Vacuum washers typically receive a low consistency pulp slurry (1.0-1.5% pulp by weight) in the vat vessel. The pulp is thickened on the drum surface as the drum surface rises out of the vat to about a 10% consistency. The pulp is further thickened to a discharge consistency from the drum of 12% or greater. After the cake is removed, the drain channels  18  and ribs (e.g., radial drain passages) are typically filled with air. As the drum surface (now scraped clean of the pulp mat) rotates past the 3:00 position, the surface renters the vat  14 . Suction is reapplied to the channels and ribs after the surface is submerged into the vat. A pulp mat  16  begins to form again on the drum surface  17 . The formation of a pulp mat, water cleaning of the mat, and scraping of the map off the drum is a continuous process that occurs as the drum rotates. 
     The motive force for the suction on the drum surface is a vacuum created in the drain passages as the extracted filtrate drops approximately 30 feet (ft.) to 40 ft. (10 to 13 meters) from the rotary drum vacuum washer  10  to a filtrate tank (below the washer). The pipe through which the filtrate passes is known as a drop leg  32  ( FIG. 2 ). 
       FIG. 2  shows an exemplary prior art end drain vacuum drum  19 . The radial end drain channels  20 , e.g., ribs, are each separated by channel walls  21 . The filtrate chamber  28  in the drum  12  is coupled to a cylindrical trunnion conduit  34  that rotates with the drum. The trunnion conduit  34  is typically driven through a worm gear  36  and a matching drive worm gear collar  37  to rotate the drum. The elbow  30  and down leg  32  conduits are stationary. An inlet end of the elbow is coupled to the outlet of the rotating trunnion conduit.  FIG. 2  is an exploded view of the trunnion conduit and elbow and down leg. In practice, the outlet of the trunnion conduit is rotatably coupled to the inlet to the elbow conduit  30  and the elbow and down leg  32  conduits are connected. 
     The center shaft supports a valve seal  40  that includes a generally arc shaped section that extends from about the 1:00 position to the 5:00 position relative to the rotation of the drum. The outer face of the valve seal is positioned in the filtrate chamber  28  and juxtaposed against the drainage outlets for the ribs  20  (as the ribs pass through the 1:00 position to the 5:00 position). The drainage outlets of the ribs open to the filtrate chamber  28 . 
     A center shaft  38  extends from the elbow into the trunnion conduit  34 . The center shaft is of a relatively small diameter as compared to the inner diameter of the filtrate passage in the elbow and down leg. The center shaft  38  is hollow to allow gases in the filtrate to vent into an aperture in the valve seal  40  and into the shaft and avoid entering the filtrate passage in the elbow  30  and down leg  32 . 
     The valve seal  40  blocks the outlets of the ribs  20  in the drum as the ribs rotate through the 1:00 to 5:00 positions. The arc width of a conventional valve seal is typically about 120 degrees which corresponds to rotating the drum through the 1:00 to 5:00 positions. The ribs are prevented by the valve seal from draining to the filtrate chamber  28  and into the trunnion conduit. As the ribs rotate from 1:00 to 5:00, filtrate and gases, e.g., air, in the ribs are intended to remain in the ribs. The valve seal  40  prevents most of the gases in the ribs from flowing into the filtrate chamber  28  and to the trunnion conduit  34 , elbow conduit  30  and down leg conduit  32 . 
     The valve seal  40  also prevents suction from being applied to the ribs as the ribs pass from the 1:00 to 5:00 positions. Suction is neither needed nor desired as the surface  17  of the drum passes from the 1:00 to 5:00 positions because gravity holds the pulp mat  16  on the surface until the scraper  24  ( FIG. 1 ) removes the pulp cake  16  at about the 2:00 to 3:00 position. Suction if applied from the 1:00 to 5:00 positions would draw air into the channels and ribs and impede removal of the pulp mat. 
     The valve seal  40  does not block the application of suction to the ribs or the drainage of filtrate from the ribs as the ribs rotate clockwise from the 5:00 position to the 1:00 position. As the ribs move through the vat, suction (applied through the ribs by the down leg) draws a pulp slurry onto the drum face screen and pulls filtrate through the screen and into the channels, ribs and to the filtrate chamber  28 . Similarly, as the ribs move up out of the vat to the top drum position (3:00 to 12:00), the suction draws filtrate, including the wash water, through the screen and into the channels, ribs and filtrate chamber. The flow of filtrate into the ribs moving from the 5:00 position to the 1:00 position is sufficient to create a substantial suction as the filtrate flows into the elbow conduit  30  and down leg conduit  32 . Substantial amounts of air are prevented from entering the elbow and down leg because the channels and ribs are substantially filled with liquid filtrate as the channels are submerged in the vat and pass under the water spray, which occurs as the drum moves from the 5:00 position to the 1:00 position. After the channels rotate past the water spray (at about the 12:00 to 1:00 position), the outlets to the ribs are blocked by the valve seal to prevent gas from entering the filtrate chamber and trunnion conduit. 
       FIG. 3  is a perspective view of the end and side of a conventional center drain vacuum drum washer  50 . The drum includes a pulp mat  16  [not labeled], a porous cylindrical surface  52 , a deck  54  supporting the surface  52 , a cylindrical drum support surface  56  and longitudinal channel bars  58  supported by the support surface  56  and in turn supporting the deck  54 . The longitudinal filtrate channels  18  are defined by the channel bars  58  and are formed between the deck  54  and the support surface  56 . At the longitudinal center (C) of the drum is an annular center drain  60  which includes an annular channel beam  62  attached to the support surface  56 . The support surface has an annular opening for the channel beam. The channel beam has an open face that receives filtrate from the filtrate channels  18 . The channel beam  62  is segmented by dams  64 . Each segment of the channel beam drains into a drain tube  66 . The drain tubes are typically about 6 inches (15 cm) in diameter. The drain tubes  66  are arranged in a conical array that extends from the channel beam  62  to an annular tube sheet  68  at one end of the drum. 
     The tube sheet  68  has openings for each of the drain tubes. A conventional tube sheet  68  is typically 50 to 60 inches (127 cm to 152 cm) in diameter. The large diameter of the tube sheet  68  is necessary to accommodate the ends of the drain tubes  66 . The tube sheet must have sufficient surface area to provide an outlet to each of the drain tubes. The tube sheet has an opening for each of the drain tubes. The large number of drain tubes and their relatively large diameter, e.g., 6 inches, cause the tube sheet to have a relatively large diameter. 
     Because of the large diameter of the tube sheet, a V-trunnion is conventionally used in center drain drums rather than the cylindrical trunnion used in radial drain drums. The V-trunnion  70  covers the tube sheet and provides a corresponding filtrate passages for each of the outlets in the tube sheet for the drain tubes. The filtrate passages in the V-trunnion each have an inlet corresponding to an outlet on the tube sheet. To correspond to the outlets on the tube sheet, the inlet diameter of the V-trunnion must be as large as the diameter of the tube sheet. Because of its relatively large inlet diameter, and the need for internal passages corresponding to each drain tube, conventional V-trunnions are expensive to manufacture and maintain. The filtrate passages in the V-trunnion conduct the filtrate flow from each drain tube towards an internal filtrate chamber and to an outlet  71  of the V-trunnion. A stationary conical valve seal is arranged in the V-trunnion to block outlets of the filtrate passages in the V-trunnion as those passages move from the 1:00 to 5:00 positions. 
     The V-trunnion  70  is mounted to the end of the drum, is coaxial to the drum and covers tube sheet  68 . The V-trunnion rotates with the drum and is mounted on a bearing  72 . A worm gear  74  on the outlet to the trunnion coupled to a drive motor (not shown) to turn the vacuum drum washer  50 . 
       FIG. 4  is a cross-sectional diagram of a novel vacuum washer drum  80  for washing and concentrating pulp. The drum includes an end drain  82  and an annular drain  84 . Drain tubes  85  are arranged in a conical array in the interior of the drum and extend from the annular drain  86  to a filtrate chamber  28 . The annular drain is offset from the longitudinal center (C) towards an end  79  of the drum opposite to the radial drain  82 . The annular drain  90  for the drain tubes  85  may be offset form center (C) such that it is in the last one third or one fifth of the drum length. For example, if the length (L) of the drum is between 22 feet to 32 feet (7.7 meters to 9.8 meters), the distance between the annular drain  84  and the end  86  of the drum may be 4 feet to 12 feet (1.2 meters to 3.7 meters). 
     The drum  80  is generally conventional except for its combined end drain  82  and annular  86  drain with drain tubes  85 , a small diameter tube sheet, and a novel valve seat. As does the drum shown in  FIG. 3 , the drum  80  picks up a pulp mat  90  as it rotates through a vat and the mat is sprayed with water and the mat is removed as the drum rotates through the 3:00 position and down into the vat. The drum includes a porous cylindrical screening surface  92 , that may include a cylindrical wire screen or deck, and channel bars  93  supported by a cylindrical support surface  94 . The screening surface  92  is supported by the channel bars. Filtrate flows through the filtrate channels  18  between the channel bars  93  and in the annular gap between the screening surface  92  and the support surface  94 . 
     The drain end  76  of the drum  80  includes the end drain  82 , a filtrate chamber  28  coaxial to the rotation axis of the drum, a cylindrical trunnion conduit  34 , a trunnion bearing unit  77 , an elbow joint  30  and a drop leg  32  that extends down, e.g., 30 to 40 ft (10-13 meters) to a sealed filtrate collection chamber. The trunnion bearing unit may include a worm and bull gear that are coupled to a motor that turns the drum. Alternatively, an electric motor and drive gear unit  78  may be attached to the opposite end  79  of the drum to turn the drum. Generally, the drive unit  78  is on just one end of the drum. 
     The annular drain  86  may include an annular channel attached to the cylindrical support surface  94 . The annular drain  86  may be similar in structure (but not position) to the annular center drain shown in  FIG. 3 . The inner support surface  94  has an annular slot opening for the channel beam  84  such that filtrate flowing along the longitudinal channels  18  flows into the annular filtrate drain  86 . The channel beam has an open face that receives filtrate from the filtrate channels  18 . The upper rim of the filtrate drain  86  is at or below the inner support solid surface  94  for the filtrate channels  18 . The annular filtrate drains include dams (see  64  of  FIG. 3 ). The dams block filtrate from flowing annularly around the channel of the filtrate drain and seeping out through the pulp mat and back into the vat (rather than into the drain tubes). Each segment of the channel beam between opposite dams has a drain  86  coupled to a corresponding drain tube  85 . 
     The longitudinal channels direct filtrate along the length of the drum to either the end drain ribs  82  or the annular drain  86 . Other than longitudinal channel bars, flow guides may not be needed in the longitudinal channels to direct filtrate to the end drain or to the annular drain. The filtrate should naturally flow to the end ribs and annular drain that offers the least resistance to the filtrate in the longitudinal channels  18 . Presumably, most of the filtrate flows towards the ribs at the end of the drum. The filtrate near the opposite end of the drum will flow to the annular drain  86 . 
     The lateral distance (LD) between the far end of the drum and the annular drain  86  can be selected such that the volume of filtrate expected to flow into the drain  86  can be accommodated by the small diameter drain tubes  85 . Further, the total cross-sectional area of all of the drain tubes can be divided by the total volume of filtrate that passes through the drum in a single revolution. The resulting fraction, which should be less than one half, can be used to estimate the distance from the far end of the drum at which the annular drain  96  should be positioned. 
     The drain tubes  85 , e.g., conduits, are typically about 2, 2½ or 3 inches (5 cm, 6.3 cm or 7.6 cm) in diameter and are substantially small in diameter than a conventional drain tube. The drain tubes  85  are arranged in a conical array such that each tube extends from its corresponding filtrate inlet  86  to an annular tube sheet  92  at one end of the filtrate chamber. The tubes may be arranged in a symmetrical radial array about the axis of the drum. 
     The tube sheet  92  defines one end of the filtrate chamber  28 . The tube sheet is attached to the drum and includes an outlet apertures for each of the drain tubes. The apertures are arranged annularly to correspond to the annular arrangement of the drain tubes  85 . The apertures in the tube sheet for each drain tube is at an angular position corresponding to the angular position of the inlet  86  to the tube. Filtrate flowing through the drain tubes discharges through the tube sheet into the filtrate chamber  28 . Filtrate also flows into the end drain  82  from the outer edge of the cylinder and at the discharge of the gap between the screening surface  93  and the support surface  94 . The end drain comprises an annular array of radial channels  20  ( FIG. 2 ) each separated by a radial dam  21  ( FIG. 2 ) extending substantially from the drum axis to the cylindrical support surface  94 . 
     The filtrate chamber  28  has a bottom semi-cylindrical wall  88  to direct filtrate into the trunnion conduit  34  and to prevent filtrate from flowing into the vat. 
     The filtrate flows from the filtrate chamber, through the trunnion conduit and elbow  30  and down into the down leg  32 . The downward flow of the filtrate creates a suction in the end drain, drain tubes and in the filtrate channels. The suction draws the pulp slurry onto the screening surface while the drum surface is in the pulp slurry vat and draws water and cooking liquor through the pulp mat as the drum surface is raised and subjected to the water spray wash. Suction is not applied to the ribs and tube as they rotate from about the 1:00 position to about the 5:00 position which is while there is no water spray (and thus a lack of liquid to support a continual flow of filtrate through the channels  18 ) and while the pulp mat is removed from the cylinder surface. A valve seal in the filtrate chamber stops suction. 
       FIG. 5  is an exploded view of a valve seal  100 , a tube sheet  92  and radial channels  102  (which are show exposed but would in practice be confined between opposite walls of the end drain). One wall  103  of the end drain is in the same plane as the tube sheet  92  and an opposite wall (not shown) is forward of the tube sheet. The channels walls  104  between the drain channels, e.g., ribs, have a radial inner edge near the perimeter of the tube sheet. 
     To block the suction as the drum rotates from the 1:00 to 5:00 positions, a valve seal  100  is applied to the outlet ends of the radial channels  102  of the end drain as the channels pass from the 1:00 position to the 5:00 position. The valve seal includes a curved plate  106  that is positioned adjacent the outlet of the radial channels  102 . The valve seal plate  106  may include an aperture(s)  108  that allow gases and filtrate in the end drain channels  102  to drain as the channels pass over the plate. The aperture  108  is an inlet to a gas and filtrate drain that extends through the valve seal and through a support shaft  110 . The plate may be doubled-walled to provide a closed passage for the aperture or include a pipe behind the plate that directs gas and foam into the shaft  110 . 
     The valve seal also includes a pie-shaped plate  109  that faces and is adjacent the tube sheet  92 . The pie-shaped plate blocks the outlets to the drain tubes as the tubes pass from the 1:00 to 5:00 positions. The pie-shaped plate may include aperture(s) to allow foam and gas from the drain tubes (as they pass from 1:00 to 5:00) to discharge into the valve and into the shaft  110 . The pie-shaped plate may be doubled-wall to provide a closed passage for foam or gas or have a pipe behind the plate for foam and gas to flow to the shaft  110 . 
     The valve seal is stationary and is supported by the shaft  110  extending from the elbow conduit  30  ( FIG. 4 ) and through the trunnion conduit. The support shaft is hollow to allow gas and filtrate from the drain tubes and end drain channels to exhaust from the drum without being drawn into the filtrate flowing down into the down leg  32 . The support shaft may be offset from the rotational axis of the drum and positioned down below the axis to facilitate the drainage of gases and foam from the drain tubes and end drain channels. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.