US2727441A - Pulp screens - Google Patents

Description

H. G. CRAM PULP SCREENS Dec. 20, 1955 2 Sheets-Sheet 1 Filed Aug. 9, 1951 |I|Y N r r w .u N m s 3 Mm o 0 o o o v INVENTOR.

HERVEY 6. CRAM 244% ATTORNEY H. G- CRAM PULP SCREENS Dec. 20, 1955 2 Sheets-Sheet 2 Filed Aug. 9, 1951 00000000000000 OOOOOOOOOOOOOOO .oooaaoouoouuoo uoooooocucaoc 00000000000000 oooouc aooo oouoo aaocao INVENTOR.

Heaven Ga. CRAM 0. W A-r-roauav United States Patent 0 PULP SCREENS Hervey G. Cram, Appleton, Wis assignor to {Fombined :Locks Paper Company, Combined Locks, fl/i3 carporation of Delaware Application August 9, 1951, Serial No. 241,090

2 Claims. (Cl. 92-30) This invention relates to improvements in screens, particularly to screens for pulp or paper stock and the like.

A screen of this type is described in U. S. Patent No. 2,480,119 for Disc Pulp Screen, issued August 30, 1949, to Hervey G. Cram, and in U. S. Patent No. 2,679,193 for Disc Screen, issued May 25, 1954-, to Hervey G. Cram. While the screen described in the former patent operates satisfactorily, it does not have high enoughefii- 'ciency to meet the present requirements of the industry. The screen described in the latterpatent has a higher efiiciency ,due to the use of a specially designed'dished screen. However, in both of these ,prior screens the incoming pulp travelling radially over-,the upstream face o'fthe'screens will gather enough circumferential speed to materially lessen the push-pull action of the runners. Also'the radial movement of the pulp across the upstream face of the screens is unrestricted and consequently the-percentage of loss ofgood stock in rejected pulp is undesirably 'L high.

The-object of this invention, therefore, is to provide a-screen-forpaper pulp and the like which has a higher overall 'efi-lciency than heretofore obtained by such screens.

Thisobject is obtained by placing radially extending bailies-on'the upstream surface of thescreen whichslow down the'circumferential speed of thepulp imparted by the runners. This keeps the difieren'tial speed between the blades and'pulp high and produces greater back and forth pressure and Velocity through thescreen .with .a consequential increase in the volume of accepted stock. Toe-wall ofthe screen chamber adjacentthe upstream face of 'the screen is provided with circumferentially eX- tending'screenwardly projecting ba'files coaxially arranged w-i'th respectto the axis of rotation of therunner blades. Several ofthese bafiles at proper spacing may be .used, the outer-one encircling the periphery of the runner blades. The e'ifect of these baffles is to slow down the radial speed of'the pulp-andto make'the radial-path of the :pulp tortuous and thereby avoid the passage of nnscreened pulp radially outwardly with the rejected pulptbefore it has had opportunity to pass through the screen. The efiect'ofboth battles is tospeed up production and cutdown theloss of-good stockin the rejected material, thus increasing the overall efficiency of the screen.

.Thenovel features, which are considered characteristic..0f .theinvention, .are set. forth with particularityin the appended claims. ,The invention itself, however, both as to its organization and itsmethod of operation, .together with additional objects and advantages thereof, will best be understood from the following description of'a'specific embodiment when read in connection with the'accompanying drawings, in which:

Fig.l-is a view in side elevation of a screen embodying the present invention, with-parts-being broken-away and in section for'the sakeof illustration;

-Figfi2 is a sectional View takenon line 2 2-of Fig. 1;

2,727,441 Patented D c- .20, 19.5.5

Fig. 3 is a sectional view taken on line 33 of vFig. 1;

Figs. 4 and 5 are enlarged fragmentary .plan views showing the smaller screen size area immediately adjacent the radialbatfles; and

Figs. 6 and 7 are fragmentary enlarged sectional views taken on line 6-6 of Fig. 3 and diametrically illustrating the movement given to the pulp as the runners pass the radial baffies.

The screen viewed in the drawings and herein described is especially adapted for the screening of stock or pulp in the process of making paper. -Its primary function is to remove shives (oversize piecesof wood) from the smaller fibers and fiberbundles. It may be used either for primary screening or secondary screening. When used for secondary screening the rejected pulp flows from the primary screen to the intake of a similarly-designed screen. However, the screen herein describedmay be used for other purposes than the screening of pulp as it is very easy to install a screen plate with screen size mesh or perforation size consistent with the material to be screened and the final results required.

The casing for the screen consists ,of .an .annularly shaped base in with an open end closed by .acoverll. A screen frame or spider i3 is held in ,place between the base and cover by the clamping action thereof. The base is held in a substantially vertic al,plane.by,a support 12 which is suitably secured tothefloor or other supporting structure. Located centrally of the :closed end or" the'base 10 is a bearing 14 whichrotatably supports the end of a horizontally positione'd shaft {161.89 that it extends withinthe casing with its axis substantially normal to the central pla e of such casing. 'Ijhe bearing includes apacldng gland jto preyentdeakageof material from the casing. Theclosedend Dime/base 1.6 has four inletsid angularly, equally spacedabout and as close to the bearing 14 as is structurallyposs e. An inlet manifoldiil for conyeyingpulp to be screened to the inlets is secured in suitablamanner to thebase it) and encircles such shaft. Thisinsures that,the..,i ncoming pulp will be evenlydistributed (astovolume and pressure head) about the central part of the ,screen plate. A trough 22 for collecting rejected pulp extends about vthe inner peripheryof the base ,110 andfhasa bleed-oil opening 2% .at'its top. The headof the incoming pulp plus the centrifugal forcefimparted by the upstreamrunners 7% cause therejectedpulp torise 1d flow out through said opening. At the lower end I, of the base it) may have aplean-out opening 26 e xtending from side to side thereof and connectedwiththe trough 22. While this clean out isclosed by covers 28, under some operating conditions the covers 2 3,may b e removed and desired pipe coupled therflto; for.. exampl e, the dischargefor rejected material might befrom'this lower opening. An elbow Ell is connected tosuch opening and has an upper support 32 on which is-QT' l ie'd atrolley beam 3d. A discharge pipe 36 iscoupled to the-elbow 3% for carrying rejected pulp to a secondary screening operation or other disposal. The base (1,0,thi1s provides the support for the screen, the inlet pipes,-,the discharge pipe, the power shaft, and the trolley beam. The open end of the base 10 has a coyer locating flange 29 which projects from a shoulder 31, =such,shoulder forming a rest for the screen frame 13.

The cover 11 has-a peripheral 'rlange38. which'ffits wit in the flange 29 and rests against a flange onthe screen frame 13. When the cover is so seated thescreenj frame is held in place and thecasingclosed. The closed end "40 of the cover. 11 has an outlet '42 at its upper end'from-which the accepted pulp is discharged from the downstream side of the casing. A downwardly directed conduit44'1's formed on'the'end 40 andconnected with such joutlet 42. The bottom of-theconduit"44=is'furnishedwitha coupling flange 46 by which the conduit may be connected to *an outlet pipe. In order to secure the cover to the base for quick and easy removal, the base is provided with a plurality of angularly spaced exterior lugs 50 fin which are swingably mounted clamping bolts 52. These bolts fit into appropriate slots in angularly spaced exterior lugs 54 on the cover 11 and nuts 56 apply clamping pressure. An overhead trolley 58 of well-known design rides along the trolley beam 34 and the cover 11 is adjustably connected to such trolley by a threaded shaft 60. As the shaft is turned the cover may be slightly raised or lowered.

The screen frame 13 is of spider-like formation and has an outer ring 61 terminating in a laterally extending flange 6 2. This flange rests on the shoulder 31 and is held thereagainst by the flange 38 when the cover 11 is clamped in place on the base. Legs 63 terminate in a hub 65 having a central bearing 64 for the reduced end of shaft 16.

To disassemble the screen and remove the screen frame 13, it is only necessary to loosen the nuts 56 and swing the bolts 52 out of engagement with the lugs 54 and slide the cover 11 away from the base 10 while supported on the trolley beam 34. The downstream runner is then taken off by removing the lock nut. The screen plate 13 can then be withdrawn from the base 10. A demountable trolley (not shown) may be used to temporarily support the screen plate.

The screen plate 66 of this embodiment consists of a flat metal disk with perforations 68. The screen plate rests against the upstream face of the frame or spider 13 and is held thereagainst by screws or the like threaded into the legs of the spider. Thus the screen is adequately supported against undue deflection. When the screen is used for primary screening purposes it is recommended that the diameter of the perforations be approximately .050". However, if the screen is being used for secondary screening operation it is recommended that the perforations be reduced in size to approximately .040" diameter. These dimensions may be changed consistent with the initial character of the pulp to be screened and with the characteristics required of the acceptable pulp. It is not necessary that the screen be a perforated plate. It may consist of suitable wire mesh or be a metal plate with slots instead of perforations.

The incoming pulp is directed by the inlets 18 against the upstream side of the plate in substantially a ring-like mass surrounding the center of the screen plate. This is accomplished by the curved oblong sectional shape of such inlets and their equal angular spacing. A better and more even distribution of pulp to the screen plate results. i In order to keep the screen plate from clogging, the downstream pressure is made pulsating by the presence of upstream runners 70 mounted by a hub 73 on the shaft 16 and downstream runners 78 also mounted on such shaft. These runners are shaped like those described in the aforesaid patent and as they are rotated alongthe surface of the screen plate the pressure is increased along the leading edge and decreased along the following edge. This results in a downstream-upstream motion acting to clear the perforations in the plate. Depending upon the character of the pulp being screened, either two or four of such runners may be used on the upstream side equally angularly spaced. Both downstream and upstream runners are rotated in a counterclockwise direction as viewed in Fig. 3 looking downstream. With the screen plate herein described, it is recommended that the drive shaft 16 be rotated at approximately 195 R. P. M.

Under the constant rotation of the runners 70 a circumferential movement as well as a radial movement will be-imparted to the incoming pulp within the upstream chamber. This decreases the relative speed between the runners 70 and the pulp. As a result the back and forth or downstream-upstream movement is perceptibly lessened. To prevent this and intensify such cross movement, and to keep the perforations from clogging, there is secured to the upstream face of the screen plate 66 a plurality of radially extending baffles 72. These baflles consist of right angle sections with the base resting against the upstream surface of the screen plate 66 directly over the leg sections 63 to which they are secured by screws 74. The placing of the baflles directly over the legs of the spider eliminates any additional loss of effective screen area. The bafiles 72 resist the circumferential movement of the pulp and over a narrow area immediately adjacent the runner-approaching face of such baflles the pulp has imparted to it a whirling radially spiralling motion diagrammatically shown in Figs. 6 and 7 by the curved arrows. This causes a concentration of rejectable material along such narrow area, and due to the increased intensity of the screening action at such area the quality of the acceptable pulp has a tendency to decrease; that is, unacceptable material would pass through the screen at this area. To avoid this the screen plate along such narrow areas, indicated generally at 76 in Figs. 4 and 5, has smaller perforations or is smaller in mesh than elsewhere. The efficiency of the screen as to volume of pulp passing through is thus increased without decrease in quality of acceptable pulp. The smaller perforation or finer mesh may be accomplished by a plastic or an electrolyte coating placed on the plate or the screen coextensive with sucharea.

Such coating cuts down the initial size of the perforations or makes the mesh of the screen smaller. The downstream runners 78 are mounted on a hub 80 keyed to the end of shaft 16 and held thereon by a nut 82. These runners have sloping leading and following edges substantially identical with those of the upstream runners 70. The downstream runners coact with the leg sections 63 in the same way as the upstream runners coact with the radial baflles 72 to increase the intensity of the screenmg action. The intensified action also takes place along the same narrow area as that hereto described for the upstream runners. As the leading edges of both runners create oppositely acting pressures the downstream runners are angularly spaced with respect to the upstream runners about forty-five degrees. Thus the upstreamdownstream motion effected by the upstream runners will not be offset by the downstream runners. In some conditions it has been found desirable to use more downstream runners than upstream runners. For example a combinatron of two upstream runners and four downstream runners both operating at approximately R. P. M. has been found satisfactory. Other combinations will be found satisfactory depending again upon the initial condition of the stock being screened as well as the characteristics desired as acceptable stock. The function in any case is to get an intense pulsation from side to side of the screen to dislodge from the upstream surface any of the rejected material clogging the perforations of the screen plate.

If the radial movement (both from natural flow and centrifugal force) of the incoming stock is not restrained, the amount of acceptable pulp passing out with the rejected pulp will increase and lower the overall efliciency. This may be avoided or lessened a practical amount by placing on the inner face of the base 10 a plurality of circumferentially extending baffles 84. These baflles project toward theupstream surface of the screen plate 66, the two inner baifies terminating close to the upstream runners 70 and the outer circumferential baflie surrounding the run ners 70 and extending to within approximately one-quarter inch of the screen plate 66 to form a slot through which rejected pulp passes to the trough 22. The flow rate of the rejected pulp is controlled by the size of such slot and/or a valve (not shown) in the discharge pipe 36. The regulation of the rejection flow rate has an effect on the rate of the radial movement across the screen plate and thus indirectly controls the amount of acceptable ma terial lost in the rejected material. All the battles are held by brackets and screws, indicated at 86, and may be readily replaced. These circumferential batfles cause the radial path of the incoming pulp to be tortuous. It is directed first toward and then away from the screen plate 66. This prevents any of the incoming pulp from passing directly to the bleed-off 24 without having first been submitted to the screening action of the plate 66. As a result the amount of acceptable material passing out with the rejected material is considerably lessened and the overall efficiency of the screen increased.

A considerable number of tests have been made on the screen described herein to establish the relative importance of the various features heretofore described in the obtaining of highest possible efiiciency in screen operation. It was established that ground wood pulp of a consistency at the inlets 18 of approximately 0.4% gave the best results in decreasing the shive count. These tests also substantiated that the screen plate sizes, the runner shape and speed, and the baffies heretofore described, all contributed to the increase in the overall efficiency of the screen. The tests also showed that by using a bleed-off outlet 24 with a diameter of approximately three inches and the one-quarter inch slot along the outer circumferential bafiles, the eflective removal of shives is increased and loss of acceptable pulp decreased. By placing such bleed-off outlet above the inlets 18 and the outlet 42, the control of rate of flow of rejected material would be more easily controlled and such rejected material could be conveyed by gravity to a secondary screen on the same level.

Although only one embodiment of the invention is shown and described herein, it will be understood that this application is intended to cover such changes or modifications as come within the spirit of the invention or scope of the following claims.

I claim:

1. In a screen for pulp, a casing forming an upstream chamber, said chamber having a trough extending annularly about the inner periphery of said chamber for the collection of rejected pulp, an inlet for pulp in said chamber, a restricted bleed-off outlet in the wall of said trough at the highest point thereof above said inlet, a screen at one side of said chamber having its periphery terminating at said trough and to one side thereof, runners rotatably mounted in said chamber, means for rotating said runners to impart a circumferential and radial movement to pulp entering said inlet, and a circumferentially extending bafile closing said trough except for a restricted slot-like opening running around said chamber on the downstream side of said runners, and leading to said trough.

2. In a screen for pulp having a flat perforated screen plate, runners rotatably mounted adjacent the upstream side of said plate, means for rotating said runners, radially extending baffles positioned on the upstream surface of said plate and extending toward said runners, and narrow areas on said screen plate adjacent the runner-approaching side of said bafiles having perforations of smaller size than those perforations in the rest of the area of said screen plate.

References Cited in the file of this patent UNITED STATES PATENTS 607,312 Westwood July 12, 1898 943,908 Bailey et a1 Dec. 21, 1909 1,585,817 Bailey et a1. May 25, 1926 1,679,371 Pease Aug. 7, 1928 1,864,818 Hang June 28, 1932 2,230,647 Knight Feb. 4, 1941 2,310,587 MacNeill Feb. 9, 1943 2,480,119 Cram Aug. 30, 1949

Claims (1)

1. IN A SCREEN FOR PULP, A CASING FORMING AN UPSTREAM CHAMBER, SAID CHAMBER HAVING A TROUGH EXTENDING ANNULARLY ABOUT THE INNER PERIPHERY OF SAID CHAMBER FOR THE COLLECTION OF REJECTED PULP, AN INLET FOR PULP IN SAID CHAMBER, A RESTRICTED BLEED-OFF OUTLET IN THE WALL OF SAID TROUGH AT THE HIGHEST POINT THEREOF ABOVE SAID INLET, A SCREEN AT ONE SIDE OF SAID CHAMBER HAVING ITS PERIPHERY TERMINATING AT SAID TROUGH AND TO ONE SIDE THEREOF, RUNNERS ROTATABLY MOUNTED IN SAID CHAMBER, MEANS FOR ROTATING SAID RUNNERS TO IMPART A CIRCUMFERENTIAL AND RADIAL MOVEMENT TO PULP ENTERING SAID INLET, AND A CIRCUMFERENTIALLY EXTENDING BAFFLE CLOSING SAID TROUGH EXCEPT FOR A RESTRICTED SLOT-LIKE OPENING RUNNING AROUND SAID CHAMBER ON THE DOWNSTREAM SIDE OF SAID RUNNERS, AND LEADING TO SAID TROUGH.

Images