WO2001017638A2

WO2001017638A2 - Constant arc contour hydrocyclone cleaner

Info

Publication number: WO2001017638A2
Application number: PCT/US2000/023609
Authority: WO
Inventors: Steven E. Slattery
Original assignee: Kadant Black Clawson, Inc.
Priority date: 1999-09-09
Filing date: 2000-08-28
Publication date: 2001-03-15
Also published as: WO2001017638A3

Abstract

A forward hydrocyclone (10) for the centrifugal separation of heavy contaminants contained in a suspension of papermakers fibers is formed with an elongated hollow arcuate body (12) of circular cross-section which converges with constant curvature from a first inlet end (16) to a second apex end (28).

Description

CONSTANT ARC CONTOUR HYDRQCYCLONE CLEANER

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to hydrocyclone cleaners used for cleaning a suspension of papermakers' fibers, otherwise commonly known as papermakers' stock, and more particularly to a hydrocyclone cleaner having an interior profile having constant curvature from its base to its apex.

2. Discussion of Background and Related Art

The terms "forward cleaning" and "reverse cleaning" have become well understood in the art of cleaning papermakers' stock, and relate primarily to the manner in which a cyclone-type centrifugal cleaner is operated. Examples of cyclone-type cleaners connected and used as forward cleaners, in which the accepts are removed at the base of the cone while the rejects are removed from the apex, are shown in Samson et al, U. S. Patent No. 2,377,524 issued June 5, 1945 and Grundelius et al, U. S. Patent No. 3,486,619 issued December 30, 1969. In a reverse cleaner system, a cyclone-type cleaner is operated in such a manner that the accepts are removed from the apex of the cone, while the lighter rejects are taken out at the base, as shown in Braun, U. S. Patent No. 3,912.579 issued October 14, 1975, and in Braun et al, U.S. Patent No. 3,557,956 issued January 26, 1971.

Hydrocyclone cleaners have been used for many years in the preparation of papermakers' stock and particularly for cleaning such stock by removing undesirable heavier and lighter weight components from the water base. Until the advent of the teachings of Braun as disclosed in U.S. Patent 3,912,579, such hydrocyclone cleaners were operated in what was then considered the "conventional" manner, in which the heavier contaminants were extracted from a bottom or apex outlet, while the "good" fibers and water absent the heavier contaminants were extracted from a top outlet in the base of the cone. Flow conditions within the hydrocyclone have been studied and reported in the patent literature, and two patents stand out by reason of detailed analysis of the hydrocyclone and attempts to arrive at optimum diameters and the optimum taper or slope of the cone walls. These include Samson et al. U.S. Patent 2,377,524 and Tomlinson U.S. Patent 3,096,275.

Samson et al. proposed a considerably longer cone length than had previously been used, and recommended cone length-to-base diameter ratios of about 11 to 1 (cone diameter at base 3", length of cone 33", page 4, column 2, lines 12-15), up to about 15 to 1 (page 5 col. 2 lines 30-50). The latter example provided an included cone angle of about 3.6 degrees.

Since the heavy material which is to be separated flows to the outside of the vortex and alongside the inside sloping surfaces of the cone, too great a cone angle will provide resistance to the separation of the heavier components, since a steep cone angle will resist the flow of these heavier components to the apex outlet. In spite of the use of shallow cone angles, Samson et al. did not have particularly good success since they lost between 20% and 33% of the dry weight of the good fibers through the bottom rejects opening along with the heavy contaminants (page 3, col. 2, lines 15-30, and page 5, col. 1, lines 29-51). It is likely that this high loss of good fiber was, at least in part, due to the fact that the good papermakers' fibers also have a specific gravity which is greater than water and tend also to be carried to the wall of the hydrocyclone and then become separated with the heavier contaminants. Tomlinson 3,096,275 re-studied the problem of defining the efficient size and shape of a hydrocyclone and, in his Example 13, Table HI, he even tested a unit having the measurements "selected by Samson and Croup" (col. 12, lines 24-33). He recommended the use of large diameter cones with a head section of from 7" to 12" in diameter (column 4 lines 38-45) and he increased the included angle to between 10° and 18°. As a result, Tomlinson was able to control the discharge from the rejects outlet to a very small flow where it contained only 1.31% of the total solids applied (column 4, line 73). By use of a large diameter cone and a steep wall angle, he obtained higher internal shear to which he attributed the high rate of retention of good fibers (column 7, lines 55-75).

The typical forward centrifugal cleaner has a cylindrical body with a tangential feed inlet and a cylindrical overflow outlet extending within the body. The cylindrical body length is typically two to six times the body diameter, and the overflow length projects inside the body a distance of approximately one body diameter. A straight conical section then typically follows the cylindrical body section with a total included angle typically in the range of four to sixteen degrees. The cone is truncated to a specific diameter to allow for an underflow stream.

The tangential feed forces the pulp slurry to rotate inside the cleaner. In order for the cleaner to separate the slurry into light and heavy fractions, the cylindrical section provides the necessary residence time for centrifugal force to work on the rotating fluid so that the light fraction can migrate toward the central axis and the heavy fraction can travel to the inner wall. At the core of the cleaner is a central zone wherein the pressure and centrifugal force are lower. The light fraction tends to become drawn into this central zone and flow in a direction contrary to the downward flow of the slurry, to be removed through the accepts overflow outlet.

The heavy fraction continues to travel down the outside wall of the cone to be rejected through the underflow. Along the entire length of the conical section there will be a "reverse" flow of the light fraction back to the central axis, with half of the reversal typically occurring within the first 30 percent of the cone length. The steeper the cone angle, the higher the influence on the lighter fraction of the slurry to travel toward the overflow. This results in a high thickening factor (that is, the ratio of the consistency of the underflow to the consistency of the feed). The steeper the cone angle, the greater the thickening. In many instances, an included cone angle of just four degrees results in a high thickening factor that limits the runability of the cleaner.

While the cylindrical/conical cleaner combination has been in use for many years, there are many limitations to its effectiveness. First, the operation of the cylindrical section is inherently unstable, since the feed flow has a tendency to "short circuit" to the overflow. Second, the flow is stable in the conical section with the constant cone angle urging the light fraction to flow inwardly toward the overflow. This maintains a sharp boundary layer between the heavy fraction and the light fraction of the slurry. This would seem to be desirable, except that too much of the desirable good fiber remains in the heavy fraction which continues its descent down to and out the underflow. To alleviate this condition, the cone angle can be made shallower, but this adds dramatically to the length of the cleaner. For example, if the cone angle is cut in half, the cone length essentially doubles, making it extremely costly to produce and difficult to install.

Moreover, at the point where the cylindrical section and the conical section intersect, the slurry abruptly changes course, which causes turbulence and some remixing of the light and heavy fractions. The interface at the cylindrical/conical section boundary thus causes a reversal of the outward flow of the slurry at the core so that some of the heavy fractions flow toward, and may become entrained in the accepts flow through the overflow outlet. The heavy fraction thus forced back toward the central axis by the straight conical inner wall may become caught up in the reverse flow.

Thus, there is a need to prevent slurry from short-circuiting to the overflow outlet. Also, there is a need to prevent turbulence that causes remixing of the light and heavy fraction and allows some of the heavy fraction to exit through the overflow. Further, there is a need to promote an amount of mixing sufficient to separate desirable fibers from the heavy fraction.

SUMMARY OF THE INVENTION In the broadest sense, the invention is a forward hydrocyclone paper stock cleaner comprising a tangential inlet near its base and an elongated hollow body which is arcuate over a substantial part of its length. The inner wall of the cleaner defines a constant arc contour having a constant radius. That is, the effective included angle of the cleaner changes slowly and gradually from zero degrees at the tangential feed to a maximum cone angle at the underflow. The base of the conical body terminates in a cap mounting a cylindrical vortex fmder. The arcuate section and the vortex finder define underflow (rejects) and overflow (accepts) outlets such that the area of the underflow outlet is approximately equal to, or slightly larger than, the area of the overflow outlet. The underflow and overflow outlets are positioned at the ends of opposed passages with the lower passage diverging toward the interior of the body. In a particularly preferred form, the tangential inlet has an area approximately the same as the area of the overflow (accepts) outlet.

The cleaner of the invention differs significantly from the typical cylindrical/conical cleaner, which has a single abrupt cone angle change from zero degrees for a length equal to about two to eight body diameters, to a fixed cone angle which is maintained all the way through to the underflow outlet. The constant arc centrifugal cleaner cone angle changes linearly as a function of the distance traveled by the slurry down the inner wall. This avoids severe mixing caused by abrupt angle changes while reducing boundary layer separation that tends to keep desirable fibers in the heavy fraction. Also, the effective cone angle at the underflow outlet can be very close to the cone angle for a similarly sized cylindrical/conical cleaner.

Another factor contributing to the low fiber content of the rejects is the increased stability in the flow due to the gradually increasing included angle of the inside wall, which reduces mixing between annular layers of the fluid once the particles have begun to separate radially. Smooth interior surfaces likewise promote the stability of the flow and help to offset power losses due to the increased inner surface area of the cleaner.

From the centripetal acceleration equation, (CA = tangential velocity squared divided by the radial distance from the central axis) the fluid continually is accelerated during its movement through the cleaner, because the body diameter is decreasing. The particles with a specific gravity lower than the fluid medium, which is generally water (1.0 specific gravity), move radially inwardly toward the axial center of the hydrocyclone. The contaminants that are removed in the forward cleaner have a specific gravity greater than the fluid medium and move radially outwardly toward the inside wall. Since the entire flow starts adjacent to the inside wall, the lighter materials must be provided a certain period of time to move to the axial center. This necessary time is determined by cleaner diameter and the relationship of the contaminant's physical characteristics in relation to the fluid and the fiber consistency. Internal mixing interrupts the movement of these particles to the axial center. Mixing also disrupts the movement of the contaminants to the inside wall. When the mixing from an unstable flow disrupts the movement of contaminants toward, or the maintaining of the position of the contaminants at the inside wall, then the contaminants can be hydraulically dragged into the accepts stream.

In a forward cleaner, the fiber is desired in the accepts overflow stream while the contaminants are desired in the rejects underflow stream. Since the fiber and contaminant move in radially opposite directions, a greater retention time per radial distance traveled and an increased rotational flow stability encourage the fiber to be accepted and the contaminants to be rejected. The converging arcuate section results in continual acceleration of the rotational rate, thereby increasing flow stability, while keeping the overall length within reasonable limits. In contrast, Samson et al., with a length-to-diameter ratio of about 15 to 1, showed a high loss rate of good fibers. Also, the examples of Tomlinson would seem to show that further decreases in the included angle would be counterproductive.

A further important consideration in the design of the cleaner according to this invention is that it should have an arcuate shape throughout a substantial portion of its length.

Applications and systems including forward cleaners are described in co-pending U.S. Patent Application entitled "Forward or Reverse Hydrocyclone

Systems and Methods," Serial No. 09/011,032, filed February 2, 1998, the disclosure of which is incorporated herein by reference.

It is accordingly an important object of the invention to provide a forward-type hydrocyclone cleaner for papermakers' stock in which the major portion of the body of the cleaner is arcuate and has an inside wall contour with constant curvature converging from the base toward the apex.

Another object of the invention is to provide a forward flow cleaner in which a sufficiently long dwell time is provided under conditions of relatively high stability of flow to permit optimum separation of heavy contaminants from good fibers.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an enlarged sectional diagrammatic and partially broken away view of the cyclonic cleaner of the invention;

Fig. 2 is a sectional diagrammatic view of the cyclonic cleaner of the invention showing the dimensional relationship of various parts of the cleaner; and Fig. 3 is a schematic diagram of the cyclonic cleaner of the invention illustrating the relationship of the body inner profile to various physical dimensions of the cleaner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring initially to Figs. 1 and 2, a centrifugal cleaner 10 includes an elongated hollow body 12 having a tangential inlet 14 near its base 16. The body is arcuate over substantially all of its operational length although a small portion at the base end may be substantially cylindrical. The body may be composed, for example, of a thermoplastic or fhermoset polymer or from a ceramic or the like. At present, polyurethane thermosetting polymer is prefered for use. The base 16 of the arcuate body 12 is coupled to an end cap 18 likewise composed of a material similar to the body and defines a substantially planar annular surface 20 surrounding a cylindrical vortex finder 22. As shown, the vortex finder 22 extends into the cleaner a distance L which is about equal to the diameter D at the base end 16.

The elongated arcuate body 12 and the vortex finder 22 respectively define overflow and underflow outlets 24, 26. The areas of the tangential inlet 14 and the overflow outlet 24 are approximately the same, while the area of the underflow outlet 26 is substantially smaller than the area of the overflow outlet 24. The overflow and underflow outlets 24, 26 are positioned at the opposed base 16 and apex 28 ends of the body 12.

Referring to Figs. 2 and 3, the constant arc contour cleaner 10 can be described as having a wall represented by an arc of a circle rotated about the cleaner centerline 32 such that the curve defined by the inner wall 34 is extremely prolate. The center of rotation 36 of the arc is located substantially off axis from the cleaner centerline 32. This cleaner centerline 32 represents the longitudinal axis of the cleaner. This differs from a conical cleaner, which is essentially a non-parallel line rotated about the cleaner centerline.

The internal contour of the inner wall 34 is described by the following equation with reference to Fig. 3:

where y = the radial distance from the cleaner central axis to the inner wall; x = the distance from the base to a point on the central axis;

R_st = the radius of the inner wall at the base; and R = the radius of the arc.

In Fig. 3, R_nd is the radius of the inner wall at the underflow outlet 26.

In use, water entraining paper pulp fibers and contaminant particles are injected tangentially through the tangential inlet 14 into the interior of the arcuate body 12. The fluid within the arcuate body 12 forms two annularly-arranged flow domains each rotating in the same direction: an outer flow domain near the inner wall of the arcuate body 12 spiraling toward the underflow outlet 26 and an inner flow domain spiraling toward the overflow outlet 24 between the outer flow domain and defining a central air core. As is well-known in the art, the forces acting on the fluid and on the relatively high specific gravity contaminant particles move the heavier weight contaminant particles toward the outer flow domain and hence toward the underflow outlet 26, which constitutes the rejects. The forces acting on the relatively lower specific gravity fibers move the fibers toward the inner flow domain and hence toward the overflow outlet 24, which constitutes the accepts outlet.

It is accordingly apparent that a hydrocyclone is provided herein for centrifugally separating heavy rejects entrained in a papermaker's fiber containing slurry. The hydrocyclone comprises a hollow elongated body portion with the base located at one longitudinal end of the body and with the rejects opening located at the apex at the other longitudinal or lengthwise end of the body. The body defines a cyclonic separation chamber having a longitudinally extending axis 32 (see Fig. 2) extending therethrough. The body has a generally circular cross-section when viewed transverse to this longitudinal axis and, at least along a portion of the body between the base and the apex, the walls of the cleaner are generally arcuately shaped and have a constant curvature relative to a point existing on a line which line extends perpendicularly through to the longitudinal axis 32. This can be clearly seen in Fig. 3 wherein the axis 32 is specified and wherein the line y represents the line passing perpendicularly through the longitudinal axis 32. The wall 34, as shown, is of a constant curvature relative to the point 36 located on the line y.

Stated somewhat differently when viewed in a plane parallel to the longitudinal axis of the hydrocyclone, the wall exhibits an arcuate line of constant curvature relative to the longitudinal axis.

While the form of apparatus herein described constitutes a preferred embodiment of this invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims. What is claimed is:

Claims

-CLAΓMS-

1. A hydrocyclone for centrifugally separating heavy rejects entrained in a papermaker's fiber containing slurry, said hydrocyclone comprising a hollow elongated body portion having a base located at one longitudinal end of said body and an apex at the other longitudinal end of said body, said body defining a cyclonic separation chamber and having a longitudinally extending axis extending through said chamber, said body having a generally circular cross section transverse to said longitudinal axis and having, at least along a portion of said body between said base and said apex, generally arcuately shaped walls having a constant curvature relative to a point existing on a line perpendicularly extending through said longitudinal axis.

2. Hydrocyclone as recited in claim 1 wherein said generally arcuately shaped walls extend along the entire distance of said body from said base to said apex.

3. Hydrocyclone as recited in claim 1 further including a tangential feed opening proximate said base, a rejects outlet at said apex, and an accepts outlet proximate said base.

4. A hydrocyclone for centrifugally separating heavy rejects entrained in a suspension of papermaker's fibers comprising: a hollow arcuate body of circular cross-section having an internal profile converging with constant curvature from a first end toward a second end a tangential inlet positioned near said first end; a vortex finder extending through said first end into the hollow conical body, the vortex finder defining an overflow outlet; and an underflow outlet positioned at said second end.

5. A hydrocyclone as recited in claim 2 wherein said hollow conical body includes a cap extending across said first end and forming said vortex finder.

6. A hydrocyclone as recited in claim 2 wherein the area of said tangential inlet is approximately equal to the area of said overflow outlet.

7. A hydrocyclone cleaner for papermaker's stock comprising: a hollow elongated body having an arcuate configuration over a substantial part of its length and defining a base, an apex and an axis extending between the base and the apex, said hollow body converging from said base toward said apex; a cap closing said base; a vortex finder mounted in said cap, the vortex finder defining an overflow outlet aligned with said axis and converging in a direction opposite said apex; a tangential inlet through the hollow conical body adjacent said base; and an underflow outlet at said apex; said hollow arcuate body defining an inner profile having constant curvature from said base to said apex.

8. A cyclonic cleaner comprising: a hollow body at least a substantial portion of which is arcuate defining a base, an apex and an axis extending between said base and said apex, the body portion converging from said base toward said apex; a cap closing said base of said hollow body; a vortex finder mounted by said cap and extending into said body; an overflow outlet defined in the vortex finder in alignment with the axis; a tangential inlet through the body near said base; an underflow outlet at said apex; an area of the overflow outlet being approximately equal to an area of said inlet, and the area of said underflow outlet being substantially less than the area of said overflow outlet; and said body defining an inner contour having constant curvature from said base toward said apex.