US2899069A

US2899069A - Device for screening pulp

Info

Publication number: US2899069A
Authority: US
Publication date: 1959-08-11
Anticipated expiration: 1976-08-11

Description

Aug 11, 1959 A. G. SANDISON DEVIQCEIYFOR SCREENING PULP Filed April 18, 1957 2 Sheets-Sheet 1 FIG. 1

INVENTOR A. G. SANDISON er: 7%: 4 ATTORNEYS fl- 11, 19 59 A. G. SANDISON 2,899,069

DEVICE FOR SCREENING- PULP Filed April 18, 1957 v 2 Sheets-Sheet 2 INVENTOR A. G. SANDISON er: 7&1 ATTORNEYS 2,899,069 DEV'ICEFOR SCREENING PULP Alexander Greswolde' sandisou, Owen Sound, Ontario,

Canada,,assi gngr to {The William Kennedy. & Sons Limited, Owen Sound, Ontario, Canada, a corporation Application Ami-18,1957, Serial No.- 653,553 6 cums. C1, 210 -405) This invention relates to devices for" screening pulp of the type usually referred to as centrifugal screens and, in particular, hasto dowithmeans for'applyin'g the pulp 'stoclt to the screen element.

In screening devices of this type the unscreened stock is impelled and applied with considerable tangential velocity to the interior ofa cylindrical screen element, the 'velocity being sufficiently high to maintain the stock against the screen element by the action of centrifugal force. It is commonpractice to impart the necessary tangentialvelocityto the stock by means of a rotor revolying Within the space defined by the screen element, the rotor'usu'ally' being constructed with substantially radially extending yanes.

To obtain satisfactory output from screens of this type it is necessary to employ a relatively high tangential velocity, velocities of theio rderof lO feet per second being commonly used. Considerable power is necessaryto impart 'such'velocities to the 'stock, and the actual power required naturally dependson the efficiency of'theimpelling" device, when consideredas a means for imparting energyof tangential elo'city'td thestQck. The efiiciency considered may be more closely "defined 'as the ratio of the energy of tangentialfldw' imparted tothe-s'tock per ;i"secdn d to the ehergyper second supplied tothe-impeller.

It can be shown that wherea simple rotorof conventior'ial construction iser'nployed to deliver the'stock i directly to the screen'surface the theoretical limiting maximum value of the"efiiciency "so 'defin'ed is fifty percent, owing to unavoidable high shock'l'oss'es amounting to one 1 half of the eriergy'supplied. 'This has been confirmed in practice, the actual efficiencies obtained being below 'fifty percent. Moreover, it "has been usual to regulate the fiow of stock to the screen"by"means ofa throttling valve in the'supply line sothat no effective'use is made of the potential eiiergyfavailable from the pressure of supply.

An object of the present invention is to provide means whereby tangentialvelocity may be imparted to stock in acentrifiigal screen without incurring high shock losses, 1 thereby effecting a 'savingin the power required.

Anothefobject'"ofthe invention is'to provide aconss'trllcticin' whreby the'pote'ntial'energy of the supply pressure may,in considerable measure, be utilized in imparting tangential elocity to the 'stock,'thereby making' possible' a further s aving inthe power required.

A preferied embodiment ofthe invention is'described and disclosed'here'in in detailwith'referen'ce to the accompanying-drawings inwhichlike reference numerals refer tolike parts in the'various views and'inwhich:

Figure 1"is'a peispective view of a pulp screening device embod in thepresent invention with portions thereof cut awayto show the interior construction;

Figure 2'is a viewtaken along line 22 of Figure lyand a, ,7

Figure 3 is a section taken along line"3 3 of Figure 1. Referring now to Figure l the pulp screening device -embodying the present invention will be seen tobe of relatively conventional construction with respect to the extent of; the vanes l8 is suchthat the outer edges 18a 2,899,069 Patented Aug. 11, 1959 "in ajor portions thereof. The device is provided with an outer casing 10 to each end of which is secured an end wall 11. Ashaft l zisjournalled in bearings 13 and 14 and is provided, at one end, with a pulley 15 by which it may be driven through the belt 16. A sleeve 17 surrounds and is secured to the shaft 12 between the end walls 11 and, extending radially outwardly from the sleeve 17, are a series ofIvanes 18 which serve to maintain the ';centrifugal velocity vof the stock within the screen in a manner which has been roughly outlined above.

Within the outer ,casing 10, concentric therewith and spaced therefromis a screen element 19 which is held in position by means ofmounting rings 20 carried by the screen and flanges 21 carried by the end walls 11. The screen 19 is concentric with the'shaft'12 and the radial of the vanes approach the'interior'surface of the screen element19 so as to act upon the stock which is applied thereto as a whirling shell under the action of centrifugal force. I

It will be {observed that the radially extending vanes 18 have portions cut -away adjacent one end wall Use as to'clear apipe' 22 which extends into the interior of the device to" deliver 'dilutingwater to the stock Within the device bymeans ofholes23 as can be seen in Figure 1.

The invention with which the present disclosure is principally concerned is'er'nbodied in the construction adjacent the endw'all ll'remote from the diluting water pipe "22. 30

-se"parating the centrifugalimpeller from the radial vanes,

'It is appreciated that the sleeve 17 and the vanes 18 also constitute a sort of centrifugal impeller but for the purposes"of"the'present description and the claims it is "-to'be understood-that when the words centrifugal impeller are used it is intended to mean that impeller defined by'the wall 25"and the vanes '24. p

The er'id "wall' 1 1' is formed with a'central aperture 26 through which -the'shaft 12 passes, the edges of the opening 26 being "extend ed in adirectio'n axially of the shaft to define -an elbow 27 which," at its free end, is fitted with a"fla'nge"2 8' to whicha'source of unscreened pulp may be 'eorineaed m'suppI this"materia1'to'-the interior of the impeller.

It will thus be s'een that iffiuid pulpis supplied to the elbow "27-"by ineans 'ofthe inlet 29 defined'by the flange 28, the fluid will enter the centrifugal impeller and be impelled radially outwardly by means of the vanes 24 when the shaft-12 is rotated'by means of pulley 1S and belt' l6. Thera'dialextent of the vanes 24 of the centrifugal impeller isnot as great as the radial extent of the vanes 18 of the rotor within the body of the screening The wall '25, however, extends radially outwardly further than the tips24a of the vanes 24 to define :an annular pocket surrounding the periphery of the vanes lfl. Within this pocket are mounted a series of guide vanes"30 which are pivoted to the end plate 11 by means of' shaft 31 which lie in a direction transverse to the chord of each guide vane. Each of the shafts 3l passcompletely through bushings in the end Wall 11 wand, .exter nally ofthe end wallll are fitted with crank arms.:32- each of which is 'keyed to its shaft 31 and each 1 ofnwhich is'.providedwitha stub shaft 33 extending in thesameTdirectionfrorn the crank arm -32 as the shaft 31 extends from theguide vane 30.

A,notched. -1;ing 34r is mounted externallyof the end wallll and c ncentric with the, shaft 12 and elb0w' 27,

, each notch 35'receiving1one stub shaft 33 of one crank arm 32. The stub shafts 33 are mounted in blocks 36 3 which are a sliding fit in the notches 35 and which are a pivoting fit about the stub shafts 33. The internal diameter 37 of the ring 34 is a sliding fit around the portion of the elbow 27 immediately adjacent the end wall 11 and the ring 34 serves to link all the crank arms 32 together. I i

A radially extending arm 38 is provided on the ring 34 and, at its radially outer extremity, is pivotally secured to a block 39 by means of a pin 40. The block 39 is secured to a rotatable threaded shaft 41 which is in threaded engagement with a bushing 42 pivotally mounted on a bracket 43. A hand wheel 44 is fitted and keyed to the end of shaft 41 remote from block 39. Accordingly, it will be seen that by rotating the hand wheel 44 the shaft 41 can be moved in a direction parallel to its own axis relative to the bracket 43 and can thereby cause the rotation of the ring 34 relative to the end wall 11. This will cause the angular displacement of the crank arms 32 and will result in a similar angular displacement of the shafts 31 and will, as a result, vary the angular position of the guide vanes 30 relative to the tips of the impeller blades 24.

Referring now to Figure 3 it will be seen that the guide vanes 30, by virtue of their configuration define a series of convergent outlet nozzles for the centrifugal impeller comprising the vanes 24. The dimension shown by the arrow A is larger than the dimension shown by the arrows B and, as a result, fluid which flows between adjacent guide vanes 30 in the direction of the arrow C will experience a drop in pressure and an increase in velocity. The degree of convergence of the nozzles defined by the guide vanes 30 can, of course, be varied by rotating the hand wheel 44 in one direction or another to increase or decrease the distance represented by the arrow B.

It will also be seen that the impeller vanes 24 of the centrifugal impeller are provided with a curvature to provide a slight forward discharge towards the inlets of the convergent nozzles defined by the guide vanes.

Having now described the construction of the preferred embodiment the method of operation will be set forth.

Assuming that the belt 16 is in engagement with an operating prime mover (not shown) and that the shaft and the centrifugal impeller are rotating at operating speed, pulp may be delivered to the inlet 29 defined by the flange 28 of the elbow 27. This will deliver fluid pulp to the centrifugal impeller and the pulp will be impelled radially outwardly by means of the vanes 24. The curvature of the vanes 24 will discharge the pulp with both a radial and a tangential velocity into the inlets 45 of the convergent outlet nozzles defined by the guide vanes 30. As the fluid passes in the direction of the arrow C (Figure 3) its velocity will be increased due to the convergence between the adjacent faces of adjacent guide vanes and its direction will be changed so that when it leaves the convergent nozzles the direction of flow will be substantially tangential to the screen element 19. The impeller diameter, as measured across the tips of the vanes 24 should be so related to the diameter measured across the tips of the vanes 18 that the tangential velocity of the stock issuing from the nozzles defined by the vanes 30 will be closely matched to the peripheral speed of the vanes 18 of the rotor within the body of the screening device. As a result, the stock will be applied to the screen 19 substantially tangential thereto and at a speed which will minimize the shock experienced by the stock when it leaves the guide vanes and is acted upon by the vanes 18. The quantity of flow of the pulp through the screening device can be varied at will by rotating the hand wheel.

Once the stock has been applied to the screen 19 through the nozzles defined by the guide vanes 30 its tangentlal velocity is maintained by the @Qfion of the vanes 18 which are also rotated by the shaft 12. That portion of the stock which passes through the screen 19 will enter the space 46 between the external surface of the screen element 19 and the internal surface of the outer casing 10. From here the screened stock will pass outwardly through an opening 47 in the casing which will be connected to a conduit delivering the screened pulp to a storage vessel. The fibres of the pulp which are too coarse to pass through the screen element 19 will be discharged from the screen element through an exhaust port 43 which will also be connected to a conduit to deliver the rejected pulp to a suitable storage vessel. In order to maintain the water content of the stock under the action of the screening device at a sufficient level to promote elficient screening, a pipe 22 has been fitted in end wall 11 remote from the inlet 29, the pipe being provided with openings 23 through which diluting water may be admitted to the interior of the screening device.

A construction of a pulp screening device having a centrifugal impeller associated therewith has been described which enables the unscreened pulp to be applied to the screen in a substantially tangential direction without encountering high shock losses. In addition, it will be appreciated that the pressure of the pulp supply will assist in imparting a velocity to the stock as it passes through the centrifugal impeller. Obviously, a high head of pressure on the stock entering through the inlet 29 will add to the pressure in the space between the vanes 24 and the vanes 30 and this pressure energy will, in turn, be converted to tangential velocity energy as the stock passes through the convergent nozzles defined by the guide vanes 30.

While a preferred embodiment of the invention has been described in detail with reference to the accompanying drawings, this is intended to be illustrative rather than limiting and minor modifications may be made in the construction and arrangement of parts without departing from the spirit of the invention or the scope of the appended claims.

What I claim as my invention is:

1. In combination with a device for screening pulp, the device including a screen element of the shape of a surface of revolution; a centrifugal impeller mounted for rotation within one end of the screen element about an axis coaxial with the screen element, means to deliver pulp to the impeller and a series of circumferentially spaced guide vanes positioned about the periphery of the impeller to define convergent outlet passages for the pulp acted on by the impeller, the vanes being directed so that the pulp passing through the outlet passages will be applied to the screen element in a direction substantially tangential to the screen element.

2. In combination with a device for screening pulp, the device including a screen element of the shape of a surface of revolution, a centrifugal impeller mounted for rotation within one end of the screen element about an axis coaxial with the screen element, means to deliver pulp to the impeller and a series of circumferentially spaced guide vanes adjustably mounted on a member stationary relative to the impeller and positioned about the periphery of the impeller to define convergent outlet passages for the pulp acted on by the impeller, the vanes being directed so that pulp passing through the outlet passages will be applied to the screen element in a direction substantially tangential to the screen element.

3. In combination with a device for screening pulp, the device including a screen element of the shape of a surface of revolution, a centrifugal impeller mounted for rotation within one end of the screen element about an axis coaxial with the screen element, means to deliver pulp to the impeller and a series of circumferentially spaced guide vanes adjustably mounted on a member statipnary relative to the impeller and positioned about the periphery of the impeller to define adjustable convergent outlet passages for the pulp acted on by the impeller, the vanes being directed so that pulp passing through the outlet passages will be applied to the screen element in a direction substantially tangential to the screen element.

4. In combination with a device for Screening pulp, the device including a screen element of the shape of a surface of revolution, a centrifugal impeller mounted for rotation within one end of the screen element about an axis coaxial with the screen element, means to deliver pulp to the impeller and a series of circumferentially spaced guide vanes adjustably mounted on a member stationary relative to the impeller by means of a pivot normal to the chord of the vane and parallel to its axis, the vanes being positioned about the periphery of the impeller to define adjustable, convergent outlet passages for the pulp acted on by the impeller, the vanes being directed so that pulp passing through the outlet passages will be applied to the screen element in a direction substantially tangential to the screen element.

5. In combination With a device for screening pulp, the device including a screen element of the shape of a surface of revolution, a centrifugal impeller mounted for rotation within one end of the screen element about an axis coaxial with the screen element, means to deliver pulp to the impeller, and a series of circumferentially spaced guide vanes adjustably mounted about the periphery of the impeller by means of pivot pins which are normal to the chords of the vanes and parallel to their axes, each pivot pin being journalled in a member stationary relative to the impeller and having a crank arm on the end thereof remote from the vane, means linking each crank arm with all the other crank arms to enable all the vanes to be adjusted in unison, the guide vanes defining adjustable convergent outlet passages for the pulp acted on by the impeller, the vanes being directed so that pulp passing through the outlet passages will be applied to the screen element at a high velocity and in a direction substantially tangential to the screen element.

6. A device as claimed in claim 5 in which the means linking each crank arm With all the other crank arms includes a ring having a series of radially extending slots in its outer periphery, each slot receiving one crank arm and means to adjust the angular position of the ring relative to the member stationary with respect to the impeller.

References Cited in the tile of this patent UNITED STATES PATENTS 990,284 Miller Apr. 25, 1911 2,341,974 Browne Feb. 15, 1944 2,663,424 Bynum Dec. 22, 1953