GB2155799A - A centrifugal pulp screening device - Google Patents

Abstract

A centrifugal pulp screen device includes a perforate wall cylindrical screen 10, an outer enclosing housing 12, a centrally located stock supply conduit 16 and means for distributing the stock including an annular distribution slice opening 30 having a fixed lower lip 24 and movable upper lip with the slice discharging against fixed angular vanes 36 which direct the stock in a spiralling centrifugal flow along the inner surface of the screen. The volume of flow is controlled by adjustment of the slice opening 30 and the velocity is controlled by a variable delivery supply pump, the velocity effecting the spiralling centrifugal action by impact of the stock against the vanes 36. <IMAGE>

Description

SPECIFICATION A centrifugal pulp screening device The present invention relates to screening devices for screening papermaking pulp fibres and more particularly to an improved vertical centrifugal screen for pulp stock.

In the screening of pulp fibres, it is desirable to separate the fibres into their individual discrete form. The stock as fed to screens normally contains varying amounts of splinters and fibre bundles, and it is the primary purpose of the screening operation to remove such undesirable coarse material so that a uniformly clean stock is available for the papermaking machine. While the problem of obtaining clean stock exists for all types of fibres, it is especially troublesome with groundwood fibres.Other types of screening devices in dude horizontal and vertical screens, although the present invention pertains particularly to a vertical centrifugal screen wherein the stock is fed to the centre of the screen, and the accepts, or acceptable fibres, pass through openings in the screen to be collected in a housing surrounding the screen, and the rejects, including splinters, fibre bundles, sheaves, particles of dirt and bark, are retained within the screen and pass out through a reject opening in the housing.

In vertical screens of the type to which the present improvements apply, it is important that continuous operation be achieved without plugging or impeding the continual screening flow of stock through the screen. It is also important to the device that a regulatable control of flow be achieved with a structure of uncomplicated design which is relatively easy to control. Centrifugal screens of the type described herein are usable in other industries, but the present invention finds particular advantage in the papermaking industry and will be described in connection with the screening of paper pulp. The present invention presents an improvement over screens heretofore used in providing a lower cost per unit of volume processed and requiring less space for installation.It also provides a structure which is more convenient for operation, particularly with respect to adjustment of the jet opening which controls the rate of flow of stock through the screening mechanism.

It is accordingly an object of the present invention to provide an improved structure for a centrifugal screen for screening paper pulp.

A further object of the invention is to provide an improved screen which will increase the screening flow through it and will provide an improved resultant product, and which achieves an improved centrifugal effect without having to drive the screen or other parts in rotation.

According to the invention there is provided a centrifugal pulp screening device comprising in combination: a perforate wall cylindrical screen; an outer enclosing housing for containing fibre accepts passing through the screen from stock within the screen; a concentric centrally located stock supply conduit having an annular discharge at the upper end of the screen with a lower slice lip; an upper annular slice lip defining a radially facing annular slice opening with the lower lip for the discharge of stock to the interior of the screen; means at the lower end of the housing for withdrawing reject material not passing through the screen openings; and means for adjustably moving said upper lip for controlling the size of the slice opening and the centrifugal movement of stock through the centre of the screen.

The invention also provides a centrifugal pulp screening device comprising: a perforate wall cylindrical screen; an outer enclosing housing for containing fibre accepts passing through the screen from stock within the screen; an annular slice opening for stock discharge at the upper end of the screen; and means inducing a spiral flow of the stock along the inner surface of the screen as it flows from the upper to the lower end thereof.

The invention further provides a method of screening pulp stock in a device which has a cylindrical screen, an annular outwardly directed slice X opening for discharging stock at one end of the screen, and angled spiralling deflector vanes positioned at the end of the screen to be impacted by stock flowing out through the slice, the method including the step of varying the size of the slice opening to control the volume of flow against the vanes.

The following is a detailed description of an embodiment of the invention, reference being made to the accompanying drawings, in which; Figure 1 is a somewhat schematic view, shown in vertical section, of a centrifugal screening device according to the invention, Figure 2 is an enlarged more detailed vertical sectional view of the head end of the structure, and Figure 3 is a somewhat schematic horizontal section taken substantially along line Ill-Ill of Figure 2.

As shown in Figure 1, a fractionating or scalping screen mechanism is provided having a cylindrical rotatable screen 10 with screening perforations or openings 11 thereon. Stock flows from the upper end of the screen toward the lower end and is given a controlled centrifugal flow. The accepts or suitable fibres pass through the opening while the dirt, fibre bundles and sheaves and other objectionable material remain within the screen, passing down toward the lower end.

The rotatable cylindrical screen 10 is enclosed in a housing 12 so as to provide an annular space 19 between the screen and housing where the accepts flow downwardly and out of the housing through a fines fraction conduit at a base 13 of the housing.

An annular stationary shell 15a in the base of the housing provides a continuation of the annular space 18 for the fines which space communicates with the outlet 20. A seal 23 is provided between the lower end of the cylindrical screen 10 so that it can be set on its annular support base 15a.

In the base is a support pipe 15 leading from both radial directions to a centre tee 16a supporting the central inlet tube 16. The tube is concentric with the screen 10 and supports the screen at its upper end. Stock is supplied through fitting 14 and 17 at the sides of the housing and flows inwardly and up through the centre of the tube 16 to be distributed by an annular distributor head 18. As the stock is distributed at the upper end, it flows downwardly through the centre of the cylindrical screen 10, and the rejects flow as indicated by the arrowed lines 21 to a coarse fraction outlet shown schematically by the arrow 22. The fine fraction flows through the screen and down as indicated by the arrows 19a.

For obtaining the improved movement of the stock and improved fractionating screening effect, the stock is discharged as an umbrella jet at the slice opening 30 and turned in a spiralling centrifugal action along the inner surface of the screen.

As shown in Figure 1, but primarily as shown in Figure 2, at the upper end of the structure the stock flows upwardly through the centre tube 16 and out through radial openings 26 into a cap structure 28 mounted at the upper end of the tube 16. The cap structure is an upwardly facing cupshaped annular unit having radial openings 27 through which the stock flows from the tube 16, and the stock exits radially outwardly over an annular lower lip slice 29. The lower slice lip coacts with an upper slice lip 35 to provide a circumferential slice opening 30. As the stock flows radially outwardly through the slice opening 30, it impinges upon a series of circumferentially spaced spiralling deflector vanes 36, Figures 2 and 3.

These vanes are supported on a circumferential outer flange 45 which is part of the vertically movable head 18, the position of which determines the size of the slice opening 3. The vanes 36 are rigidly supported and located so that the stock impinges thereon and they have a shape to direct or deflect the stock in a spiral flow path down along the inner surface of the cylindrical screen 10. An expansible connection 37 is located between the screen 10 and the flange member 45 to allow for vertical movement of the head 18 relative to the screen 10.

The stock is delivered to the mechanism by a controllable pressure or output delivery pump 46, Figure 1, and the radial velocity of the stock exiting through the slice opening 30 is controlled by varying the pressure of stock delivered to the mechanism which varies the pressure of the stock in the let chamber 28a of the cup-shaped member 28.

The volume is controlled by varying the size of the slice opening 30 which is varied by the position of the upper slice lip 35. The slice lip can be moved up and down by vertically positioning the head 18 by turning a handwheel 41. The head includes the head member 33 which supports the annular flange 45, and which slides vertically against the cap 31. The cap 31 has a smooth cylindrical outer surface 32 to receive O-ring seals 34 carried on the head member 33.

The head member 33 includes a flanged collar 42 which moves freely vertically on a vertical post 43 threaded at 44 into the top of the cap 31. The post 43 is threaded at its upper end to receive the rotatable handwheel 41. The handwheel bears down on a cup 40 which presses down on an annular ring 39 that bears on the top of a pneumatic actuator 38. The actuator 38 is air inflated and may be a continuous annular doughnut or a plurality of in flatable members that apply a downward biasing force against the head member 33, holding the flange 42 of the collar 42a against the top of the ring 39. As the handwheel 41 is tightened, a downward pressure is applied by the cup 40 and ring 39 against the pneumatic backing 38 to urge the head member 33 downwardly to urge the upper slice lip 35 downwardly to tend to close the slice opening.

The pneumatic backing accommodates sudden surges in liquid pressure to temporarily force the head upwardly to open the slice opening 30. The pneumatic actuator 38 can be collapsed by deflation for quickly opening the slice opening for un blocking. Also, the pressure in the actuator 38 can be varied to control the slice opening size and can be regulated in relation to the pressure of the stock being delivered by the pump 46. The volume of stock delivered into the screening chamber is varied by varying the size of the slice opening.

In operation, the stock is delivered to the screening mechanism by the pump 46 through the inlets 14 and 17 and flows up through the central tube 16. At the top end of the tube, the stock flows through the radial openings 26 and 27 and out through the annular slice lip 30. The stock impinges on the spiralling deflector vanes 36 and is sent downwardly along the inner surface of the screen in a spiral path, which improves the screening effect. Increased velocity and increased centrifugal action is achieved by increasing the delivery pressure of the pump 46. Increased flow volume is achieved by opening the slice opening 30 to a greater size.

The structure is simple in arrangement, requiring few elements and requiring a minimum amount of machining. The structure utilises standard structures for components such as the vertical tube 16 and simple castings for the head, requiring a minimum amount of machining such as the cylindrical surface 32 for the cap, and the slice lip surfaces 29 and 35. The structure is made so that it can be readily pulled apart by pulling up the head 18 with the screen, and the housing is constructed at its upper end to permit this action.

Claims (15)

1. A centrifugal pulp screening device comprising in combination: a perforate wall cylindrical screen; an outer enclosing housing for containing fibre accepts passing through the screen from stock within the screen; a concentric centrally located stock supply conduit having an annular discharge at the upper end of the screen with a lower slice lip; an upper annular slice lip defining a radially facing annular slice opening with the lower lip for the discharge of stock to the interior of the screen; means at the lower end of the housing for withdrawing reject material not passing out through the screen openings; and means for adjusting moving said upper lip for controlling the size of the slice opening and the centrifugal move ment of stock through the centre of the screen.

2. A centrifugal pulp screening device according to claim 1, including means for directing the flow of stock from the annular slice opening in a spiral direction within the screen.

3. A centrifugal pulp screening device according to claim 2, wherein said directing means includes deflector vanes positioned at the upper end of the screen on the inner surface thereof, which vanes are angled to induce a spiral flow of stock along the inner surface of the scren as the stock emits from the slice opening and engages the vanes.

4. A centrifugal pulp screening device according to any of claims 1 to 3, wherein said stock supply conduit extends concentrically from the lower to the upper end of the screen and radial discharge openings lead from the conduit to an annular space in communication with the slice opening.

5. A centrifugal pulp screening device according to any of claims 1 to 4, including a pneumatic actuator biasing the upper slice lip with adjustment means for limiting the movement of the upper slice lip to control the size opening.

6. A centrifugal pulp screening device according to any of claims 1 to 4, including a stock distribution head at the upper end of the screen receiving stock from the conduit and including a lower cup-shaped slice lip having an annular central head; an upper slice member slidably located on the head and having an upper lip coacting with the lower lip to provide a continuous annular radially opening slice opening; said member being biased toward open position by a pneumatic actuator; and a threaded bolt extending upwardly from the head with an adjustment wheel for controlling the size of the slice opening.

7. A centrifugal pulp screening device according to any of claims 1 to 6, including a plurality of circumferentially spaced spiralling deflector vanes mounted at the upper end on the inner surface of the screen positioned opposite the annular slice opening for spiralling the flow of stock emitted from the slice opening.

8. A centrifugal pulp screening device comprising; a perforate wall cylindrical screen; an outer enclosing housing for containing fibre accepts passing through the screen from stock within the screen; an annular slice opening for stock discharge at the upper end of the screen; and means including a spiral flow of the stock along the inner surface of the screen as it flows from the upper to the lower thereof.

9. A centrifugal pulp screening device according to claim 8, wherein said spiral flow inducing means comprise angled spiral vanes at one end of the screen; said annular slice opening directing stock flow radially outwardly against the vanes for inducing a centrifugal screening flow along the the inner surface of the screen.

10. A centrifugal pulp screening device according to claim 8, or claim 9, including means for controlling the size of the slice opening to control the volume of flow against the vanes.

11. A centrifugal pulp screening device according to any of claims 8 to 10, including means for controlling the pressure of stock delivery to the slice opening to controllably change the velocity of flow against the vanes and thereby change the centrifugal spiralling action of the stock along the screen.

12. A centrifugal pulp screening device according to any of claims 8 to 11, wherein the annular slice opening at the upper end of the screen is defined by a fixed slice lip and a movable slice lip; and wherein there is provided a power actuator for the movable slice lip including pneumatic means biasing the slice lip toward opening position and a screw threaded device limiting the movement of the movable slice lip in opposition to the pneumatic actuator.

13. A method of screening pulp stock in a device which has a cylindrical screen, an annular outwardly directed slice opening for discharging stock at one end of the screen, and angled spiralling deflector vanes positioned at the end of the screen to be impacted by stock flowing out through the slice, the method including the step of varying the size of the slice opening to control the volume of flow against the vanes.

14. The method of screening pulp stock according to claim 13, including the further step of controlling the pressure of flow through the slice opening to controllably affect the centrifugal spiralling action induced in the stock as it impacts the vanes and flows along the inner surface of the screen.

15. A centrifugal pulp screening device substantially as hereinbefore described with reference to the accompanying drawings.