Abstract:
A screw press inlet section comprises a housing defining a radial inlet opening leading to an axially extending chamber through which a screw is rotatably mounted for conveying and dewatering a solid-liquid mixture fed into the chamber through the inlet opening. The housing has an end wall extending in a plane normal to the screw and to which a perforated plate is integrated to provide the additional surface available for drainage at the inlet section. A pulsator is provided within the housing to generate hydraulic pulses against the perforated plate so as to prevent plugging thereof.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a screw press for extracting liquids from solid-liquid mixtures and, more particularly, addresses the issue of hydraulic capacity of a screw press inlet section. 
     2. Description of the Prior Art 
     It is widely known to use a screw press to extract liquids from a solid-liquid mixture, such as a pulp suspension. 
     Conventional screw presses typically include a perforated cylinder having axially spaced-apart inlet and outlet ends, and a screw rotatable within the perforated cylinder to compress and dewater a solid-liquid mixture as it is conveyed thereby from the inlet end to the outlet end of the perforated cylinder. The liquid is forced to drain across a perforated cylindrical screen surface extending axially between the inlet and outlet ends of the housing. 
     Although such conventional screw presses are effective, it has been found that it would be beneficial to improve the drainage capacities of the inlet end thereof and, thus, lift the restriction on the total admittable feed flow to the presses. 
     SUMMARY OF THE INVENTION 
     It is therefore an aim of the present invention to improve the dewatering performances in an inlet area of a screw press. 
     It is also an aim of the present invention to increase the surface available for drainage at an inlet end of a screw press. 
     Therefore, in accordance with the present invention, there is provided a screw press inlet section comprising a housing defining an axially extending chamber having a longitudinal axis and a radial inlet opening for receiving an incoming solid-liquid mixture. The chamber has an outboard end wall defining a plurality of liquid flow passages for allowing the same to act as a drainage surface. 
     In accordance with a further general aspect of the present invention, there is provided a screw press for extracting liquids from a solid-liquid mixture, comprising a housing having longitudinally spaced-apart inlet and outlet sections, and a pressing section between said inlet and outlet sections. A rotatable feed and compression screw is mounted within said housing for conveying the solid-liquid mixture from the inlet section to the outlet section while compressing and dewatering the liquid-solid mixture such that liquid is discharged from said housing. The inlet section has an outboard end wall defining a plurality of liquid flow passages for liquid to drain therethrough in a direction opposite to a general traveling direction of the solid-liquid mixture within the screw press. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, and in which: 
     FIG. 1 is a longitudinal cross-sectional view of a screw press in accordance with a first embodiment of the present invention; 
     FIG. 2 is an enlarged cross-sectional view of an inlet section of the screw press illustrated in FIG. 1; 
     FIG. 3 is a cross-sectional view taken along line  3 — 3  in FIG. 2; 
     FIG. 4 is a top plan view of a baffle in accordance with a second embodiment of the present invention; and 
     FIG. 5 is front elevational view of the baffle illustrated in FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now referring to the drawings, and in particular to FIG. 1, a screw press embodying the elements of the present invention and generally designated by numeral  10  will be described. 
     The screw press  10  generally includes an intermediate pressing section  12  including a number of cylindrical screens  14   a,    14   b  and  14   c  mounted in an end-to-end relationship and supported between axially spaced-apart inlet and outlet housing sections  16  and  18 , which are, in turn, mounted on a rigid base frame  20 . The inlet housing section  16 , the intermediate pressing section  12  and the outlet housing section  18  form an elongated cylindrical cage or housing concentrically surrounding a rotatable screw member  22  operable to compress and dewater a solid-liquid mixture, such as a light consistency cellulosic pulp suspension, as the same is conveyed thereby from the inlet housing section  16  to the outlet housing section  18 . 
     The solid-liquid mixture is fed to the screw press  10  through a radial inlet opening  24  defined in a top surface of the inlet housing  16 . The inlet opening  24  starts axially at the beginning of the screw member  22 , that is, next to an inner surface  26  of an outboard end wall  28  of the inlet housing section  16  and extends over an appropriate length of the screw member  22  to ensure proper distribution of the incoming solid-liquid mixture in the inlet housing section  16 . 
     The inlet housing section  16  is provided with a bottom semi-cylindrical screen plate  30  extending axially in continuity with the cylindrical screens  14   a,    14   b  and  14   c  to form therewith a uniform drainage surface for the solid-liquid mixture to be processed. The semi-cylindrical screen plate  30  is typically made of a screen plate shaped into a half cylinder and is welded into a lower portion of the inlet housing section  16 . The cylindrical screens  14   a,    14   b  and  14   c  are typically each made of a thick stainless steel plate that is rolled into a cylinder, and seam welded. The axially opposed ends of the cylindrical screens  14   a,    14   b  and  14   c  are provided with flange connections  29  for allowing the same to be removably connected together and supported between the inlet and outlet housing sections  16  and  18 . The cylindrical screen  14   c  is provided in the form of two halves  31   a  and  31   b  interconnected along respective longitudinal lateral flanges  33 . 
     The screw member  22  is supported at an inlet end thereof by a heavy duty spherical roller bearing  32  mounted within a supporting fixture  34  secured to an outer surface  36  of the outboard end wall  28  of the inlet housing section  16 . Likewise, the screw member  22  is supported at an outlet end thereof by an outlet bearing  38  mounted to an end wall  40  of the outlet housing section  18 . The outlet end of the screw member  22  is drivingly connected to a coaxial drive shaft (not shown). 
     The screw member  22  has a continuous flight  44  extending helically around a smooth outer surface  46  of a screw shaft core  48  from an inlet end thereof to a location generally corresponding to a downstream end of the cylindrical screen  14   c  of the intermediate pressing section  12 . The diameter of the outer smooth surface  46  of the screw shaft core  48  gradually increases in a direction from the inlet housing section  16  to the outlet housing section  18 , while the pitch of the screw flight  44  gradually decreases in that same direction. As a result, the volume between adjacent turns of screw flight  44  and the cylindrical screens  14   a,    14   b  and  14   c  decreases progressively towards the outlet end of the screw member  22 , thereby gradually increasing the pressure on the solid-liquid mixture so as to force liquid to drain through the drainage surface formed by the semi-cylindrical screen plate  30 , and the cylindrical screens  14   a,    14   b  and  14   c.  The screen holes in the semi-cylindrical screen plate  30 , and the cylindrical screens  14   a,    14   b  and  14   c  are preferably conical with the smaller openings on the inner side of the screen plate  30  and of the cylindrical screens  14   a,    14   b  and  14   c  to prevent accumulation of fibers and plugging of the screen holes. The liquid draining off through the semi-cylindrical screen plate  30  and the cylindrical cylinders  14   a,    14   b  and  14   c  is collected in a trough  50  defined in the base frame  20  and drained off from there to an appropriate location via a drain  52 . 
     The axial portion of the screw member  22  which extends through the outlet housing section  18  has shredder arms  54  for breaking up the separated solid phase of the processed material, which could be in the form of a cake, before the same is discharged from the screw press  10  through a radial discharge opening  56  defined in a bottom surface of the outlet housing section  18 . 
     In contrast to conventional screw press inlet sections which are provided with a solid non-perforated impermeable end wall, the outboard end wall  28  of the inlet housing section  16  includes a perforated plate  58  in the form of an annular disc mounted about the screw member  22  upstream of the helical screw flight  44  thereof. The perforated plate  58  provides extra drainage surface at the inlet end of the press screw  10 , thereby advantageously increasing production capacity by increasing the total admittable feed flow to the press, reducing feed pressure for a same flow as compared to a conventional screw press, and allowing for increased consistency of the dewatered material at the outlet housing section  18 . 
     As shown in FIGS. 2 and 3, the perforated plate  58  is secured to the supporting fixture  34  housing the bearing  32 . The supporting fixture  34  defines an annular chamber  60  for receiving the liquid drained across the perforated plate  58 . The liquid received into the annular chamber  60  is directed back to the screw press drain  52  via an evacuation pipe assembly  62  provided at the bottom of the annular chamber  60 . 
     As shown in FIG. 2, a water passage  64  extends through the supporting fixture  34  to direct a jet of water from a source of pressurized water (not shown) to a location comprised between a pair of axially spaced-apart annular seals  66  and  68  mounted about the screw shaft core  48 . The jet of water and the annular seals  66  and  68  cooperate to prevent the liquid flowing into the inlet housing section  16  from flowing to the bearing  32 . A third annular seal  70  is mounted about the screw shaft core  48  adjacent the bearing  32  as an additional liquid barrier. 
     The perforated plate  58  includes a plurality of round holes  72  distributed thereon between a pair of imaginary concentric circles extending around the screw shaft core  48 . The last row of holes is provided on the outer circle which has a diameter which is slightly less than that of the imaginary envelope described by the screw flight  44  when the screw member  22  is rotated. The holes  72  are step drilled with the nominal size perforations executed on an inner side  74  of the plate  58 . The holes  72  are then enlarged (using the existing holes as a pilot) from an outer side  76  of the plate  58  but stop short of the inner side  74  thereof such that a short length (about a third of the original depth) of the nominal size of the hole remains. The resulting hole geometry prevents blocking and ensures positive flow from the inner side  74  of the perforated plate  58 . 
     It is contemplated to manufacture the screen plate  58  with either one of the following open areas: 17.3%, 22.7%, 27.5% or 28.8%. However, other specs could be used as well. Also, the round perforations or holes  72  in the screen plate  58  could be replaced by slots. 
     As shown in FIGS. 2 and 3, a pair of diametrically opposed baffles  78  and  80  can be securely mounted to the screw shaft core  48  in front of the perforated plate  58  to keep the incoming solid-liquid mixture from building up on the screen plate  58  by creating gentle hydraulic pulses as the screw member  22  rotates. Therefore, the baffles  78  and  80  act as a pulsator to direct waves of incoming material against the screen plate  58 , thereby preventing the holes  72  from becoming plugged which would obviously impede the dewatering action of the screen plate  58 . The baffles  78  and  80  are particularly useful in the case of pulp suspension. 
     The strength of the hydraulic pulses generated by the rotation of the baffles  78  and  80  can be adjusted by letting the baffles  78  and  80  come more or less in proximity of the inner surface  74  of the screen plate  58 . Satisfactory results have been obtained by positioning the baffles  78  and  80  at a distance of 0 to 2 mm away from the inner surface  74  of the screen plate  58 . 
     As shown in FIGS. 2 and 3, each baffle  78  and  80  is provided in the form of a fin  82  extending outwardly from one end of a curved base  84  adapted to be secured to the smooth outer surface  46  of the screw shaft core  48 , such as by bolting. The fin  82  includes a curved rib  86  and a fin-shaped web  88  extending centrally from a concave trailing side  90  of the curved rib  86 . 
     FIGS. 4 and 5 show another possible construction of a baffle  92 . The baffle  92  includes a straight L-shaped blade  94  extending from a curved base  96  adapted to be bolted to the screw shaft core  48 . The baffle  92  is secured to the screw shaft core  48  with blade surface  98  facing the screen plate  58 . 
     It is noted that the number of baffles required to prevent plugging of the screen plate  58  can vary depending on the substance to be processed. In some instances, the action of the screw flight  44  can be sufficient and, thus, no baffle needs to be added to the screw member  22 . 
     It is also pointed out that the baffles  78 ,  80  and  92  do not necessarily have to be mounted to the screw shaft core  48  but could rather form part of another rotating structure mounted within the inlet housing section  16 .