US3713541A

US3713541A - Screening machine with slotted screen

Info

Publication number: US3713541A
Application number: US00141639A
Authority: US
Inventors: G Nelson
Original assignee: Bird Machine Co Inc
Current assignee: Bird Machine Co Inc
Priority date: 1971-05-10
Filing date: 1971-05-10
Publication date: 1973-01-30
Anticipated expiration: 1990-01-30

Abstract

A machine for screening liquid fiber suspension, having a screen with elongated slots disposed along the flow path of the suspension which changes in direction along its length, is made more efficient and plug-free by arranging the slots in at least two areas of the flow path with the long aces of the slots in one area at an angle to the long axes of the slots of the other area such that the slots of said areas have their long axes at substantially a uniform angle, preferably approximately a right angle, to the direction of flow of suspension over them despite difference in said flow direction in the respective areas.

Description

limited States Patent [191 Nelson 1 Jan. 30, 1973 54] SCREENING MACHINE WITH SLOTTED SCREEN g Primary Examiner.lohn Adee [75] Inventor: George L. Nelson, Wcstwood, Mass. AlmmeyEdgar Kent [73] Assignee: Bird Machine Compang South 7211- [57] ABSTRACT p0 ass A machine for screening liquid fiber suspension, hav- Flledl y 10,1971 ing a screen with elongated slots disposed along the App]. No.1 141,639

References Cited UNITED STATES PATENTS 7/1899 Smith ..209/273 1/1968 Clarke-Bonner ..209/273 flow path of the suspension which changes in direction along its length, is made more efficient and plug-free by arranging the slots in at least two areas of the flow path with the long aces of the slots in one area at an angle to the long axes of the slots of the other area such that the slots of said areas have their long axes at substantially a uniform angle, preferably approximately a right angle, to the direction of flow of suspension over them despite difference in said flow direction in the respective areas.

5 Claims, 3 Drawing Figures SCREENING MACHINE WITH SLOTTED SCREEN This invention relates to the screening of fluids comprising liquid suspensions of fibrous material and in particular relates to machines for screening such suspensions through a slotted screen to separate shives, dirt and other foreign matter from the acceptable fiber.

For screening many liquid-fiber suspensions, particularly those containing relatively long fibers which should not be rejected, screens with elongated slots should be better suited than round hole screens. However, prior art screening devices with slotted screen plates have suffered from serious limitations which have made them less than satisfactory or even unacceptable in many cases. In fine screening applications in which narrow slots are required, excessive screen area has been needed to provide the requisite screening efficiency and capacity. In addition, the slots become plugged too frequently. Furthermore, the slotted screens have not performed as well as theoretically they should, especially at the high consistencies which are becoming more common in pulp and paper manufacture.

Accordingly, it is a principal object of the invention to provide slotted screen apparatus which screens more efficiently than existing such devices.

Another object of the invention is to provide such screening apparatus which is less subject to plugging and easier to maintain than existing devices.

A still further object is to provide such screening apparatus which is capable of screening a wide range of liquid-fiber suspension consistencies with nearly equal efficiency.

I have found that in slotted screen machines for screening liquid fiber suspensions it is important in obtaining uniform maximum efficiency, capacity and freedom from plugging that the slots be arranged uniformly at substantially the same angle to the direction of flow of the suspension past each slot and that generally the selected uniform angle should be approximately a right angle to the direction of flow. In screens of the type in which this invention is concerned, the slots have been cut with their long axes parallel to each other and usually parallel to the screen plate axis. Since the flow pattern in this type of screen changes rather drastically as the flow of the suspension proceeds over the plate from inlet to outlet, the result has been poor performance because, I have found, of the lack of uniformity in angularity of slots to the direction of flow over them.

According to the present invention a screening machine with slotted screen plate is provided in which the angularity of the long axes of the slots to the screen axis is not uniform but is varied in general conformity with change in the angle of flow of the suspension over the screen as it proceeds from inlet to outlet. This change in angularity of the slots is such as to maintain an approximate uniformity of angular relation of the slots to the direction of flow of the suspension over them.

In typical pressure screening machines for pulp and paper making fiber, which have a tubular screen and spiral flow of suspension thereover, the flow of suspension across the screen plate changes in angularity to the screen axis from a rather steep-angled spiral at the inlet end of the order of 45 to the screen axis, to nearly a circular flow about the screen axis adjacent the rejects outlet. In a preferred embodiment of the invention applied to such a screening machine the slots of the group adjacent the inlet end are arranged with their long axes at substantially a 45 angle to the screen axis while the slots of the group adjacent the rejects outlet are arranged with their axes substantially parallel to the screen axis. Intermediate these two groups there may be provided one or more groups of slots with their long axes at an angle to the screen axis intermediate 45 and 0 conforming to the flow angle of the suspension to the screen axis in which the group of slots is located. Preferably the machine includes rotatable pulsing means for providing positive and negative pulses to the screen and this pulsing means is desirably a drum having portions of its surface displaced radially of the drum with respect to other portions to provide the pulsing.

Other objects, features, and advantages will become apparent from the following description of a preferred embodiment of the invention, taken together with the attached drawing thereof, in which:

FIG. 1 is a side view, partially in vertical cross-section and partially broken away, of a preferred screening apparatus according to the present invention;

FIG. 2 is a horizontal cross-sectional view of the screening apparatus of FIG. 1, taken on line 2-2 thereof;

FIG. 3 is a perspective view of a cylindrical screen plate constructed in accordance with the invention.

Referring to the screening apparatus shown in FIG. 1, the apparatus has a vertically arranged pressure casing 10 having a removable pressure dome 12. An annular inlet gutter 14 is defined in the upper region of casing 10 and an inlet conduit 16 is arranged to introduce pulp to be screened into the inlet gutter 14. A gutter trap 18 communicates with inlet gutter 14 for removing heavy debris thrown to the periphery by centrifugal force.

The inlet gutter 14 communicates radially over a baffle 20 with the central part of the screening apparatus. Circumferential, apertured tubular screen member 22 is located below baffle 20 and has wall 24 spaced inwardly relative to casing 10 in the manner to define an annular accepts chamber 26 outside of screen member 22. A tangential accepts conduit 28 arranged to remove fluid under substantial pressure is connected to accepts chamber 26 and has its initial portion extending the full height of screen member 22.

Below screen member 22 is arranged an annular rejects gutter 30 in communication with the inside of the cylindrical screen member 22. A rejects conduit 32 communicating with rejects gutter 30, is arranged to remove fluid under substantial pressure and at a relatively low rate (e.g., not over 20 percent, usually 10 percent or less, of inlet flow), and is provided with valve 34 which serves to regulate the flow therethrough.

It will be observed that casing 10 is slightly scrollshaped in horizontal cross-section (FIG. 2), and screen member 22 is arranged therein so that accepts chamber 26 increases in radial width gradually abut its circumference, all the way along its extent until it discharges to accepts conduit 28.

In this embodiment the pulsing means is rotary drum 36 which has irregular surfaced wall 38 and cylindrical shape, and has a length at least equal to that of screen member 22 and a diameter substantially the same, though with clearance, and is arranged to rotate about screen member axis 40 on shaft 42, the screen member and drum defining the opposite sides of the screening compartment. Shaft 42 extends into and below bearing pedestal 44 and carries pulley 46 which is driven by a belt 48, or series of belts, driven by an electric motor 50 which should usually be capable of turning drum 36 to develop a surface speed of at least 5,000 f.p.m. when the casing is full of fluid. It will be recognized that other forms of pulsing means, such as hydrofoils and paddles, are suitable for use with the present invention.

Referring to FIG. 3, screen member 22 has two groups, generally designated 52 and 54, of slots formed therein. The slots in group 52 are formed in the upper half of member 22 at about a 45 angle to screen member axis 40 and may be, for example, 1% inch long, 0.016 inch wide and spaced to define generally rectangular slotted sections 56. The slots of group 52 are inclined to axis 40 forwardly in the direction of flow about the axis (clockwise in the figures) from their ends nearest the outlet end of the screening compartment, the angle of such inclination placing their long axes at approximately a right angle to the direction of flow of the suspension past them. The slots in group 54 are formed in the lower half of member 24 with their long axes similarly inclined at about a angle from member axis 40, which places their axes about at a right angle to the direction of flow past them. The slots in group 54 may, for example, be 1% inch long, 0.016 inch wide and spaced to define generally rectangular slotted sections 60.

Groups

52 and 54 are arranged in the manner to align the respective axes of the slots 58 and 62 as close as is practical in the direction perpendicular to the direction of stock flow at the location of the particular slots when the screen apparatus is operating in order to increase the efficiency of the apparatus.

In operation motor 50 is started and drum 36 rotates. Stock under pressure enters inlet gutter 14 through conduit 16, where it flows tangentially and overflows baffle 20, then passes spirally downwardly between drum 36 and screen member 22 where it is pulsed by the rotating irregular wall 38 of drum 36. The rejects portion of the flow is screened out by member 22 and continues to pass downwardly into rejects gutter 30 and through rejects conduit 32. The accepts portion flows through the slots in member 22 into accepts chamber 26 and issues through accepts conduit 28. As is known in the art, the smaller the width of the slots, the better is the removal of smaller debris from the stock flow. However, when narrower slots are used, the frequency of screen clogging increases substantially for a given speed of drum rotation or the drum speed must be increased considerably to maintain the same low incidence of clogging.

The direction of stock flow in the screening compartment changes in angle as the stock progresses toward the rejects outlet. The flow is spiral about the axis 40 induced by tangential overflow of the suspension from gutter 14 into the screening compartment, at an angle to the axis which progressively increases as the suspension flows toward the outlet end. This change in angle of flow is caused primarily by the low capacity of the rejects outlet from the screening compartment as compared with the capacity of the inlet, the escape of suspension through the screen progressively reducing the axial velocity of flow in the screening compartment with consequent increase in flow angle therein to the compartment axis.

The angularity of the flow path at various points along the axis can be measured by instruments or determined by observation of flow streamers. However, it is more easily calculated with sufficient accuracy for purposes of the present invention, using basically a wellknown formula for fluid flow, in a confining body such as a pipe or the screening compartment, Q A V, where Q is the quantity of flow past the selected point, A is the cross-sectional area of the confining body at that point and V is the average velocity of the flow at that point (see, e.g., Dodge and Thompson, Fluid Mechanics, 1937, pages 74-75). Q is known, being the inlet flow less loss of suspension through the screen, which is closely proportional to the distance the suspension has traveled along the screen axis. A is also known (in the screen illustrated it is a constant). Consequently, the formula transposed as V= Q/A is used to determine the velocity of flow at the inlet (tangential) and the axial component of velocity at selected points along the axis in the screening compartment.

The angle of flow (angle of velocity) at any point is then determined as the resultant or vector of the axial and tangential components of velocity at that point (e.g., dividing the axial component by the tangential component gives the cotangent of the flow angle from the axis). The tangential component of velocity is increased above the inlet velocity by acceleration of the rotor proportionately as the suspension flows in contact with the rotor, which increase may total approximately percent. Therefore, the tangential component of velocity at the selected point may be taken as the inlet velocity plus the acceleration thereof up to that point.

For example, with a 2,000 g.p.m. inlet flow and 10 percent thereof rejects outlet flow in apparatus of the design shown, the angle of suspension flow in the screening compartment was so calculated for various points along the screen to yield the following results:

location axial tangential angle of along component component velocity (flow) screen of of velocity from axial plate velocity inlet end velocity 14.15 ft/sec l0 ft/sec 35 k 10.97 ft/sec l2 ft/sec 48 1% 7.78 ft/sec l4 ft/sec 61 34 4.60 ft/sec 16.5 ft/sec 74 outlet end 1.412 ft/sec 20 ft/sec 86 1. In a screening machine for liquid-fiber suspension having a tubular screen with elongated slots, said screen defining a wall of an annular screening compartment surrounding an axis, an inlet to said compartment adjacent one end of said screen, an outlet adjacent the other end of said screen for suspension rejected by said screen and a separate outlet for suspension passing through said screen, said outlet for rejected suspension having a low capacity relative to said inlet, said inlet arranged to feed suspension into said compartment in a flow which spirals about said axis toward said outlet, the angle of said flow to said axis increasing progressively and substantially as the flow proceeds along said axis, the improvement wherein a first multiplicity of said screen slots adjacent the inlet end of said compartment are arranged substantially parallel to one another and at a substantial angle to a second multiplicity of said screen slots arranged substantially parallel to one another adjacent its outlet end, and said first slots are inclined to said axis at a substantially greater angle than said second slots, said first slots having their ends nearest the inlet further in the direction of flow about said axis than their ends nearest the outlet from said compartment.

2. A screening machine according to claim 1 wherein said angle of said first slots to said axis is about 45.

3. A screening machine according to claim 1 wherein said second slots are substantially parallel to said axis.

4. A screening machine according to claim 1 which includes a pulsing means spaced opposite said screen and rotatable about said compartment axis.

5. A screening machine according to claim 4 wherein said pulsing means has a substantially continuous surface provided with portions displaced from other portions to produce pulses.

UNITED STATES PATENT crncn QERTEWCATE @E QQRREQHGN Patent No. 2 .71? Jill-1 Dated January Q0 n 10']? Invent r( Gecr e L on It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The second reference cited change the patent number from "336,759" to --3,363,759-- and change the inventor from "Clarke-Bonner" to --C1arke-Pounder--. Y

The abstract, line 6, change "aces" to --axes-.-.

Col. A, line. 5 of the table, second column, delete "velocity" before l. i .15 ft/sec".

Col. '4, line 6 of the table, first column, change "1/2" to --l/4-.

Col. 4, last line of'the table, second column, change 1. 412 ft/sec" to --l. ll5 ft/seo. v

Signed and sealed this 26th day of June 1973.

(SEAL) Attest;

EDWARD M.PLETci1ER,JR, f ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM F'O-1 USCOMM-DC 60376-P69 U.S. GOVERNMENT PRINTING OFFICE 1969 0-366-334

Claims

1. In a screening machine for liquid-fiber suspension having a tubular screen with elongated slots, said screen defining a wall of an annular screening compartment surrounding an axis, an inlet to said compartment adjacent one end of said screen, an outlet adjacent the other end of said screen for suspension rejected by said screen and a separate outlet for suspension passing through said screen, said outlet for rejected suspension having a low capacity relative to said inlet, said inlet arranged to feed suspension into said compartment in a flow which spirals about said axis toward said outlet, the angle of said flow to said axis increasing progressively and substantially as the flow proceeds along said axis, the improvement wherein a first multiplicity of said screen slots adjacent the inlet end of said compartment are arranged substantially parallel to one another and at a substantial angle to a second multiplicity of said screen slots arranged substantially parallel to one another adjacent its outlet end, and said first slots are inclined to said axis at a substantially greater angle than said second slots, said first slots having their ends nearest the inlet further in the direction of flow about said axis tHan their ends nearest the outlet from said compartment.

2. A screening machine according to claim 1 wherein said angle of said first slots to said axis is about 45*.