Basket centrifuge with material removing means

Abstract

A basket centrifuge is provided with an improved arrangement of radially extending accelerator vanes to improve flow orientation, reduce turbulance, and in general improve the effectiveness of the centrifuge apparatus when concentrating sludge. The improved arrangement of accelerator vanes is located in an end portion of the centrifuge bowl, and extends outwardly from a point located between the feed outlet means and the overflow lip. The basket centrifuge may also be provided with a knife blade for removing solids which have accumulated against the inner peripheral wall of the centrifuge bowl, the knife blade cooperating with the accelerator vanes so that it can be moved outwardly to contact the peripheral wall.

Claims

1. A centrifuge for separating first and second materials from a mixture thereof in feed delivered thereto, wherein said first material has a higher specific gravity than said second material, said centrifuge having a bowl adapted to rotate about an axis, said bowl comprising an imperforate peripheral wall formed about said axis and an annular end wall extending inwardly of said bowl at one end thereof from said peripheral wall toward said axis, the inner edge of said annular end wall providing an overflow lip, said bowl at the end thereof opposite said one end having an end portion extending for an axial distance of not more than onethird the total axial length of said bowl, feed means for conducting said feed to said bowl, said feed means having an outlet disposed in said end portion for discharging feed outwardly from said outlet toward said peripheral wall, removing means movably mounted within said bowl for removing first material accumulated at said peripheral wall, a plurality of from 12 to 90 radially extending accelerator vanes, each vane being wholly disposed in said end portion of the bowl, the inner end of each vane being positioned a radial distance from said axis in the range of between the radial distance of said outlet from said axis and the radial distance from said axis of a point located one-half inch farther from said axis than said overflow lip, the outer end of each vane being disposed from said axis at least a radial distance equal to the sum of the radial distance of said overflow lip from said axis and one-third the radial distance from said overflow lip to said peripheral wall, and means for moving said removing means between a first position disposed closer to said axis than said overflow lip and a second position disposed farther from said axis than said overflow lip, said removing means in said second position being engageable with first material accumulated at said peripheral wall for removing first material from said bowl. 2. An improved centrifuge according to claim 1 wherein said removing means comprises a knife blade disposed inwardly from said overflow lip and mounted for outward movement toward said peripheral wall, said bowl having an opening disposed in the opposite end thereof for the passage of said removed first material. 3. An improved centrifuge according to claim 2 wherein said knife blade has a notch disposed therein which cooperates with said acceleration vanes as the blade is moved outwardly, so as to allow the cutting edge of the blade to move outwardly beyond the inner edges of the vanes, said bowl having an opening disposed in the opposite end thereof for the passage of said removed first material. 4. An improved centrifuge according to claim 1 wherein each of said vanes is angularly spaced from 5* to 20* apart, and wherein each of said vanes extends axially from said feed outlet toward said overflow lip a distance ranging between 0.5 inch and 2.5 inches. 5. An improved centrifuge according to claim 4 wherein said centrifuge is adapted to produce a force of between 1,200 and 1, 800 g at the peripheral wall of the bowl. 6. An improved centrifuge according to claim 4 wherein said removing means comprises a knife blade disposed inwardly from said overflow lip and mounted for outward movement toward said peripheral wall, said bowl having an opening disposed in the opposite end thereof for passage of said removed first material. 7. An improved centrifuge according to claim 6 wherein said knife blade has a notch disposed therein cooperating with said accelerator vanes as the blade is moved outwardly, so as to allow the cutting edge of the blade to move outwardly beyond the inner edges of the accelerator vanes. 8. An improved centrifuge according to claim 5 wherein said removing means comprises a knife blade disposed inwardly from said overflow lip and mounted for outward movement toward said peripheral wall, said bowl having an opening disposed in the opposite end thereof for passage of said removed first material, each of said vanes extending axially to said opposite end of said bowl. 9. An improved centrifuge according to claim 1 wherein each of said vanes is angularly spaced from 5* to 20* apart, said centrifuge being adapted to produce a force of between 1200 and 1800 g at the peripheral wall of the bowl, each of said vanes extending axially from said feed outlet toward said overflow lip a distance ranging between 1.0 inch and 2.0 inches. 10. An improved centrifuge according to claim 9 wherein said removing means comprises a knife blade disposed inwardly from said overflow lip and mounted for outward movement toward said peripheral wall, said bowl having an opening disposed in the opposite end thereof for the passage of said removed first material. 11. A centrifuge according to claim 1 wherein said bowl, between said vanes and said end wall, is free of structure connected to said peripheral wall.
United States Patent [1 1 Keith, Jr. et al. [54] BASKET CENTRIFUGE WITH MATERIAL REMOVING MEANS [75] Inventors: Frederick Walker Keith, Jr., Gladwyne; Robert Edward Krause, Philadelphia, both of Pa. [73] Assignee: Pennwalt Corporation, Philadelphia, [22] Filed: Feb. 26, 1971 [21] Appl. No.: 119,247 [56] References Cited UNITED STATES PATENTS 11/1941 Lieberman ..233/27X 8/1916 Christianson ..233/2 [111 3,734,398 51 May 22, 1973 FOREIGN PATENTS OR APPLICATIONS 1,133,906 1l/1956 France ..233/3 Primary Eiraminer-George H. Krizmanich Attorney-Edward A. Sager [57] ABSTRACT A basket centrifuge is provided with an improved arrangement of radially extending accelerator vanes to improve flow orientation, reduce turbulance, and in general improve the effectiveness of the centrifuge apparatus when concentrating sludge. The improved atrangement of accelerator vanes is located in an end portion of the centrifuge bowl, and extends outwardly from a point located between the feed outlet means and the overflow lip. The basket centrifuge may also be provided with a knife blade for removing solids which have accumulated against the inner peripheral wall of the centrifuge bowl, the knife blade cooperating with the accelerator vanes so that it can be moved outwardly to contact the peripheral wall. 11 Claims, 9 Drawing Figures Patented May 22, 1973 3,734,398 4 Sheets-Sheet 1 ;/7 INVENTOR. Frederick W. Keith Fig 2 BY Robert E. Krouse ATTORFH Y. % SOLIDS RECOVERY Patented May 22, 1973 4 Sheets-Sheet 2 |6 1 I I 3211 l 260. Fig 3 IO I5 3O 4O EFFLUENT RATE (gpm) INVENTOR. Fig. 5 Frederick W. Keith Rqberf E. Krause ATTORNEY Patented May 22, 1973 SOLIDS RECOVERY 4 Sheets-Sheet 5 G\ K 9S N 80 I 7O 50 IO 20 3O 4O EFFLUENT RATE (gpm) INVENTOR. Frederick W. Keith BY Robert E. Krouse ATTORNEY. MOVING LAYER THICKNESS .5 AT BREAKUn.) Patented May 22, 1973 4 Sheets-Sheet 4 Fig 8 IO 15 2O EFFLUENT RATE (gpm) INVENTOR. Frederick W. Keith Robert E. Krouse ATTORNEY. BASKET CENTRIFUGE WITH MATERIAL REMOVING MEANS BACKGROUND OF THE INVENTION Basket centrifuges at the present are generally provided with two to six radial accelerator vanes located within the centrifuge bowl for the purpose of accelerating feed being introduced into the separation chamber within the bowl, these acceleration vanes extending nearly from the top to the bottom of the centrifuge bowl; utilization of such centrifuges has not always produced the results desired, particularly when attempting to separate alum sludge from water which has been treated in water treatment plants. It is common in water works plants to add an agent such as aluminum sulfate or chloride to purify the water. Flocs are formed in the water, and in general are very fragile, and great care must be taken so that the flocs are not broken. If the water is to be separated in a centrifugal separator, care must be taken to see that the separation process is carried out as gently as possible. The effluent or separated water is not always of the desired clarity, and the cake concentration of the accumulated alum sludge is not always satisfactory when utilizing this particular accelerator arrangement. Also, in utilizing a centrifuge of this type having accelerator vanes which extend substantially the full length of the centrifuge bowl, there is a problem of heel" accumulating in the bowl. This heel," which must be expected even from the softest sludges, is a firm accumulation of solids which remains along the entire peripheral wall of the centrifuge bowl and does not show plastic flow axially after the removal of the liquid and softer separated products from the interior of the bowl. The centrifuge cannot be provided with a knife blade to reduce this accumulation of heel because the acceleration vanes extend almost the entire length of the centrifuge bowl. While other accelerator vane arrangements are also in use, they are not particularly effective. Eliminating the vertical accelerator vanes completely, as is also done in many present basket centrifuges, allows removal of the heel by a knife, but also produces inferior separation results by not delivering effluent of the clarity obtained in a centrifuge bowl having acceleration vanes; neither is the cake concentration of the accumulated alum sludge as high as that desired. It being desired that the incoming feed be accelerated to the same angular velocity as the bowl, or that the tangential velocity of the feed by the same as the tangential velocity of the bowl at any particular point, and also that the removal of heel be facilitated, a more suitable arrangement of accelerator vanes is necessary. SUMMARY OF THE INVENTION According to the present invention, a basket centrifuge having an imperforate wall is provided with a plurality of radially extending accelerator vanes, at least one edge of each vane being axially located between the outlet endof the feed introducing means and the annular effluent discharge lip, and the inner edge of each vane being radially located between the feed outlet means and a point located one-half inch outward of the overflow lip of the centrifuge bowl. Also, each accelerator vane is wholly located within an end portion of the bowl not more than one third the total height of the centrifuge bowl, and preferably not more than onequarter to one-fifth the total height. A scraper blade is mounted inwardly of the overflow lip to rotate about a vertical axis so that the blade can be moved outwardly toward the outer peripheral wall of the bowl when it is desired to remove solids which have accumulated against the wall. In one modification the accelerator vanes do not extend completely to the bottom of the centrifuge bowl, but are spaced upwardly therefrom, and the blade for removing the accumulated solids is appropriately notched so that it can move around the arrangement of accelerator vanes when being brought into contact with the accumulated solids. The solids which are removed by the knife blade are discharged through an opening in the bottom of the centrifuge bowl, and are conveyed away by appropriate means. In another modification, a centrifuge is provided with only a skimmer tube for removing both the separated liquid and the accumulated solids. This arrangement is entirely satisfactory where the accumulated solids are softer and capable of flowing axially on the bowl wall to and through the skimmer apparatus; the knife blade being eliminated, each of the accelerator vanes are preferably extended completely to the bottom of the centrifuge bowl. Tests have shown that this arrangement of accelerator vanes in a basket centrifuge greatly increases the effectiveness and efficiency of the separation process, and the clarity of the effluent is much greater than with present arrangements; also, when it is desired, a knife blade may be provided in combination with a skimmer apparatus which allows the interior or peripheral wall of the centrifuge bowl to be cleaned or scraped after one or more cycles, thus conventionally discharging the accumulated heel which remains in the bowl. A knife blade may be provided only in cases where the accumulated solids within the centrifuge bowl are not capable of being removed by a skimmer apparatus, or it may be provided in all cases merely to discharge the accumulation of heel within the centrifuge bowl. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of a basket centrifuge embodying the invention, showing both a knife blade and a skimmer for removing separated materials; FIG. 2 is a transverse sectional view through line 2-2 of FIG. 1 showing an arrangement of accelerator vanes in the centrifuge bowl; FIG. 3 is a longitudinal sectional view of a portion of a basket centrifuge embodying the present invention, the centrifuge utilizing only a skimmer for removing the separated materials and where each of the accelerator vanes extend completely to the bottom of the centrifuge bowl; FIG. 4 is a transverse sectional view through line 44 of FIG. 3 showing a top view of the arrangement of accelerator vanes; FIG. 5 is a graph of the effluent rate plotted against solids recovery for several variants of the preferred arrangement of accelerator vanes; FIG. 6 is a graph of effluent rate plotted against solids recovery which compares the present invention with the prior art; FIG. 7A illustrates a modification in which the accelerator vanes extend completely to the bottom of the centrifuge bowl, combined with the scraper blade; FIG. 78 illustrates a modification in which the accelerator vanes do not extend completely to the outer peripheral wall of the centrifuge bowl; FIG. 8 is a graph of the effluent rate plotted against moving layer thickness which compares several variants of the present invention. Referring more specifically to the drawings, an arrangement of accelerator vanes embodying the present invention is indicated in FIG. 1, and the centrifuge apparatus is generally designated by the numeral 10. Although not shown in the accompanying drawing, the centrifuge apparatus 10 is comprised of a base having a plurality of upright standards to which the suspension means are attached, the suspension means supporting the centrifuge bowl housing 12. The centrifuge bowl 14 is mounted on a shaft which passes through the housing 12, the shaft having mounted on it lower end a pulley, which pulley is connected via a belt to a driving motor. The centrifuge bowl is adapted to rotate about its vertical axis of rotation so as to produce forces of between 900 and 2,000 G, but preferably in the range between 1,200 to 1,800 G. Feed slurry is introduced into the bowl 14 through feed tube 16, which tube is stationarily mounted over a bowl. This tube is provided with a solenoid operated valve 18, the valve controlling the passage of feed into the centrifuge apparatus. The feed slurry exits the tube 16, and enters a fee cone 20, the feed descending to the lower end of the cone from where it is discharged outwardly toward the outer peripheral wall of the centrifuge bowl. it can be seen that as the centrifuge bowl 14 is rotating and with it the feed cone 2.0, feed slurry entering the cone will be thrown outwardly toward the interior surface of the cone and will descend within the cone until reaching the feed outlet means 2!; feed outlet means 21 is located adjacent to the bottom of the centrifuge bowl 14 and are adapted to discharge the incoming feed outwardly toward the outer peripheral wall of the centrifuge bowl 14. in certain situations it may be desirable to replace the cone Zil with a single feed pipe or a feed receiving cup, the feed receiving cup having one or a plurality of feed conduits which extend longitudinally to the bottom of the centrifuge bowl, each conduit having a feed discharge outlet which is adapted to discharge the feed slurry radially outwardly toward the outer peripheral wall of the centrifuge bowl. Connected to the lower end of the centrifuge housing 12 is an effluent outlet conduit 22. This conduit leads to an effluent tank (not shown) from which the product effluent is later withdrawn. For removing the sludge or accumulated solids in the cyclical operations of the centrifuge, there is provided a skimming device or means 24. This includes a skimmer tube which presents an open mouth 26 facing against the rotation of the bowl. The skimmer is mounted on the housing in a plurality of bosses 28 in which the skimmer device is longitudinally movable as in the past, the basic operation of the skimmer being similar to other Skimmers. As the skimmer tube is moved outwardly, that is, deeper into the bowl, a combination of the velocity of the liquid passing into the stationary skimmer tube, and the centrifugal pressure head of the liquid in which the skimmer tube is submerged, causes the liquid to be pumped into and through the skimmer tube. Means for driving a skimmer device are indicated schematically at 30. These means may comprise a pair of pneumatic piston-cylinder assemblies which drive the skimmer alternately forward and back respectively. The drive means 30 is carefully arranged so that at one extreme of travel, the skimmer is disposed with its mouth inward out of the liquid in the centrifuge bowl, and inward of the overflow lip of the centrifuge bowl. As can be seen, the bowl 14 includes an outer peripheral wall 32 against which solids are sedimented from incoming feed. The feed which is introduced into the interior of the centrifuge bowl is directed towards the outer peripheral wall, and due to the differences of the specific gravities of the materials comprising the feed, it is separated into a heavy phase and alight phase, usually a liquid and solid or semi-solid, the heavy phase being located nearest to the outer peripheral wall. The inwardly disposed or light phase material will accumulate until reaching the radius of the discharge lip 34 at which time the liquid will flow over the lip and into the centrifuge housing 12, from where it is discharged via the effluent outlet conduit 22. The overflow lip 34 is provided by the upper end wall of the centrifuge bowl. As stated above at one extreme of travel, the skimmer is disposed with its mouth 26 located inwardly of the liquid discharge lip 34. At the other extreme end of its travel, the skimmer tube is located so that its mouth 26 has an extreme of one-sixteenth to one-fourth inch from the peripheral wall 32 so that it can remove sludge or accumulated solids from virtually the entire peripheral wall of the bowl. Also mounted to the housing 12 of the centrifuge ap paratus is a knife assembly 36, the knife assembly including a motor 38 utilized for rotating a shaft 40 upon which is mounted knife blade 42, which knife blade can also be utilized for removing accumulated solids from the peripheral wall 32 of the centrifuge bowl; the knife blade 42 is utilized subsequent to, or instead of skimming the effluent or liquid from the interior of the bowl. Solids removed by the knife blade are discharged through an opening 44- disposed in the bottom of the centrifuge bowl, and then through an opening 46 in the centrifuge housing 12, these openings being ever opened to permit free flow of the removed solids to a bin, screw conveyor, or conveyor belt disposed therebelow. Attention is now directed to an essential portion of the invention. Mounted in the lower portion of the separation chamber d7 of the centrifuge apparatus is an accelerator wheel designated generally by the numeral 48; the centrifuge wheel 48 is so designated because the arrangement resembles a wheel, i.e., it is comprised of an outer rim S0, and an inner rim 52, with a plurality of radially extending accelerator vanes 54, the vanes extending between the outer and inner rims. The angu lar spacing between vanes is 4 to 30, and preferably 5 to 20. As can be seen in FIG. 1, knife blade 42 has a notch 56 disposed therein, the notch extending in wardly from the outer edge of the cutting blade and being dimensioned so as to cooperate with the accelerator wheel 48 and allow the knife blade 42 to be moved radially outwardly to remove the accumulated solids or sludge from the peripheral wall 32. It is noted that dimension a is less than dimension b, thus insuring that liquid effluent is discharged over the lip 34, and not out the bottom opening 44; solids are discharged out opening 44 upon being removed from the inner surface of peripheral wall 32 by knife blade 42. It is also noted that accelerator vanes 54 are entirely located in an end portion of the centrifuge bowl 14 which is opposite the effluent overflow discharge lip 34, the vertical dimension of this end portion being represented by the letter e. Dimension e is not more than c/3 where c is the total vertical height of the bowl; preferably e ranges between c/5 and c/4. Where the entire wheel accelerator 48 is located above feed outlet 21, the height or width d of each vane is preferably 0.5 to 2.5 inches, the various dimensions being discussed in more detail below. In applications in which the feed to be separated is comprised of a flowable heavy phase material, the knife blade illustrated in FIGS. 1 and 2 may be eliminated and a skimmer device may be utilized for removing both the light and heavy phases from the interior of the bowl. This arrangement is illustrated in FIGS. 3 and 4. In such cases each accelerator vane of the accelerator wheel may then be extended to the bottom of the centrifuge bowl. As shown in FIGS. 3 and 4, it is not necessary to have an outer and/or inner rim, as each individual accelerator vane may be mounted directly to the interior of the centrifuge bowl if desired. This applies to the accelerator vane arrangement of FIGS. 1 and 2 also; it may also be desirable, with respect to the modifications shown in FIGS. 1 through 4, to leave a gap or space between the outer peripheral wall of the centrifuge bowl and the accelerator vanes. As will be more fully discussed below, the most important aspects of the present invention is the location of the edges of each vane, the dimensions of the vane, and the total number of vanes within the centrifuge bowl. In a series of tests run at a water treatment plant, it was found that water treatment alum sludge can unquestionably be concentrated satisfactorily and economically in a basket centrifuge with the present accelerator arrangement. The present arrangement or invention gives good clarity without coagulating aides, and the whole skimmable cake, excluding heel, produced concentrations which were 5 to times the feed concentrations. Buildup of heel is relatively slow except in periods of high raw water turbidity; relatively infrequent knifing will handle the accumulation of solids or sludge within the centrifuge bowl. In the event it is calculated that the buildup of heel will be relatively high, the arrangement shown in FIGS. 1 and 2 may be utilized; however, in the event the buildup of heel will be relatively low or the accumulation of solids will be of a flowable consistency, then the knife blade may be eliminated and the arrangement illustrated in FIGS. 3 and 4 may be utilized instead. The test centrifuge used was a basket centrifuge which was 18 inches in height, and inches in diameter; the test centrifuge had a 4% inch deep lip ring, i.e., the overflow lip 34 or 34a was located 4% inches inwardly from the outer peripheral wall 32 of the centrifuge. Various arrangements were tested and compared and comparisons of the preferred arrangement with the arrangements set forth in the prior art and other tested arrangements are given herein. Basket centrifuges of the prior art generally contain no accelerator vanes, or are provided with two to six accelerator vanes which extend almost the entire length of the centrifuge bowl, i.e., from near the top to the bottom, leaving only sufficient space at the top of the bowl for the skimmer device to be operated m. The accelerator vanes may also be arranged so as not to extend completely to the outer peripheral wall of the bowl (FIG. 78). It is only necessary that the vanes extend outwardly from the locus of overflow lip 340 a distance equal to at least a/3, a being the distance from the overflow lip to the inner surface of the outer peripheral wall of the centrifuge bowl (see FIG. 1). In any of the accelerator arrangements discussed in the present application, the inner end of each accelerator vane may be located anywhere between the feed outlet 21 (see FIG. 1) and a point located one-half inch outwardly of overflow lip 34; and, as stated above, the outer end of each vane may be located outwardly from overflow lip 34 anywhere beyond a distance equal to a/3. The accelerator vanes may also extend completely to the bottom of the centrifuge bowl when used in conjunction with a scraper or knife blade (FIG. 7A). In such an arrangement the bottom 33b of the bowl is inclined so as to minimize solids accumulations between the vanes by directing the liquid-solids feed in an upward direction. After the solids have accumulated along the inner surface of the peripheral wall 32b, the scraper blade is moved outwardly to contact the wall and discharge the solids through a bottom opening in the bowl as in FIG. 1. Referring to FIG. 5, a graph is shown in which eFfluent rate is plotted against solids recovery; several variants of the wheel accelerator were tested, and the curves shown in FIG. 5 correspond to some of these variants. Curve A is representative of a wheel accelerator having thirty accelerator vanes, each vane being 1 inch high as measured in the direction of the vertical axis of rotation of the centrifuge, and extending radially inwardly from the wall 32 approximately 4% inches (one-fourth inch inward of the overflow lip), the lower edge of the accelerator wheel being located 1% inches above the bottom of the centrifuge bowl. Curve A resulted from high turbidity test conditions. Tests seemed to indicate that the silt content of the alum sludge under high turbidity conditions did improve the sedimentation characteristic (as well as the compaction qualities). Curve B is representative of the same wheel accelerator corresponding to Curve A, but utilized under low turbidity conditions. When the number of accelerator vanes on the wheel accelerator was reduced from 30 to 16, this resulted in a decrease in recovery of about 6 percent under the same conditions. Raising the bottom of the wheel accelerator (with 1 inch high vanes) from 1% inches off the bottom of the centrifuge bowl to 2% inches, did not change the efficiency of operation. Curve C is representative of a wheel accelerator utilized under low turbidity conditions having a total of thirty accelerator vanes, each of the vanes extending from the bottom of the centrifuge bowl 2% inches upwardly, and each of the accelerator vanes having a 4% inches immersion, the innermost edge of each accelerator vane being located approximately one-fourth inch inward from the overflow lip of the test centrifuge. The results as indicated in FIG. 5 indicate that better orientation of the flow, reduction of turbulence, and presumably increased use of the bowl height, occurs when acceleration is provided immediately adjacent to the bottom of the centrifuge bowl; it also appears that the additional guidance provided by extending the accelerator vanes inch to 2.5 inches as a broad range, and 1.0 inch to 2.0 inches as a specific range above feed outlet 21 improves the performance relative to the narrower vanes. Guidance by the accelerator vanes at some distance from the bottom of the bowl is apparently sufficient to reduce turbulence markedly without the assistance of vanes located near the bottom of the centrifuge bowl. However, with the more difficult low turbidity sludge, the 2.5 inch accelerator vanes gave somewhat better performance even than the 1 inch high vanes on high turbidity feed. Other data on conventional secondary activated sewage sludges also show the 2.5 inch accelerator vane to operate with appreciably better efficiency than the 1 inch high vane. Generally, increasing the number of accelerator vanes increases the efficiency of separation without limit in the range tested. Operation definitely improved in going from 0 to 4, to 13 to 16, to 26 to 30 vanes. Tests also indicate that acceleration vanes located 6.5 inches or more up from the bottom of the centrifuge bowl and- /or addition of further vanes higher in the centrifuge bowl does not affect clarification. In comparing the present invention with accelerator arrangements of the prior art, reference may be made to FIG. 6. Curve D, for low turbidity, the bottom curve, is representative of a basket centrifuge having no arrangement of accelerator vanes whatsoever. Curve E, for low turbidity, located immediately above Curve D, represents a basket centrifuge having what is referred to as the standard accelerator vane arrangement, i.e., four vertical acceleration vanes which extend substantially the entire height of the centrifuge bowl, leaving only sufficient space at the top of the bowl to utilize the skimmer device. The present invention is represented by Curve F for high turbidity, and Curve G for low turbidity which as can be seen, are superior to either of the basket centrifuges utilized in the prior art. Curve F is representative of a basket centrifuge having a wheel accelerator in which the accelerator vanes are 1 inch high, the bottom of the accelerator wheel being located 1% inches from the bottom of the centrifuge bowl, thus placing the upper or top edge of each of the accelerator vanes 2% inches from the bottom of the bowl; again each of the accelerator vanes has a total immersion of 4% inches, each vane extending one-fourth inch inwardly from the radial location of the overflow lip 34 or 34a, to the outer peripheral wall 32 or 32a of the centrifuge bowl. Curve G, indicated by the dotted line, is a profile for the same wheel accelerator characterized or represented by Curve F, but under conditions of low turbidity. Most of the test runs involved a bowl speed corresponding to 1,600 g at the peripheral wall of the bowl, but runs were made at 1,300 g, and at 1,900 g. In general, a higher bowl speed proved beneficial to clarification, and additional acceleration at this general speed level does not increase feed deaggregation appreciably. Thus, the improvements are at least in proportion to that expected by centrifuge theory; by theory, 1,600 g should give about 20 percent better capacity than 1,300 g at the same clarity. The basic concept of all feed liquid moving through the centrifuge bowl in a layer adjacent to the air-liquid interface has been previously developed by the present inventors. Evidence for the same type of flow was found in the present alum sludge test; values for layer thickness were determined under a wide range of operating conditions by noting the interface position of the settled cake with the skimmer device after clarity decreased. As the feed stream is introduced into the separation chamber near the bottom of the bowl, the feed moves along the air-liquid interface, forming a nearly constant thickness layer, solids sedimenting outwardly from the moving layer under centrifugal acceleration toward the outer peripheral wall. Solids recovery is almost constant for continual feeding until a certain point is reached after which there is a rapid decline in the solids recovery. This is best explained by the moving layer phenomenon. In general, an effluent of nearly constant clarity is obtained through most of the operating cycle followed by a sudden increase in solids carry over at the break point, and increasingly dirty effluent for continued operation. A nearly stagnant layer is present between the outer peripheral wall of the centrifuge bowl and the moving liquid layer, the nearly stagnant layer consisting initially of water or feed medium which is gradually replaced by deposited cake until the total cake accumulation reaches the interface between the moving and nearly stagnant layers. This gives rise to a nearly constant degree of clarification for a given feed rate until the cake intrudes on the moving layer so the effluent subsequently carries over increasingly large quantities of solids from the cake. It is at this point that the accumulated solids should be removed by the skimmer or scraper blade, during which time the feed input should be stopped. This is due to the fact that the inner portion of the cake consists of the least compacted material which is highly sensitive to interfacial scrubbing action or turbulence. The thickness of the moving layer increases with increasing feed rate. There was considerable variation in thickness at a given flow rate for the various arrangements of accelerator vanes which were tested, thus indicating that various arrangements of accelerator vanes to affect turbulence in the nearly stagnant layer, and probably change conditions at the interface of the layer. Extending the moving layer thickness outwardly from the overflow lip much beyond a locus located one-third the distance a (see FIG. 1) such as by operating at high volumetric feed rate, reduces the cake holding capacity of the centrifuge. Vanes should extend through the liquid layer. I Because a thinner moving layer for a given throughput rate results in a greater cake thickness, and consequently a longer cycle and better compaction, comparison of the accelerator vane arrangement on the basis of cake thickness permitted must be made. Referring to FIG. 8, a graph is shown in which efi'luent rate is plotted against moving layer thickness; four curves illustrate the characteristics of various arrangements for acceleration devices. The curves, reading from left to right, correspond to going from a thicker moving layer to a thinner one for the same liquid throughput rate. Three of the curves represents a variant of an accelerator wheel, while the fourth represents the standard four vane bowl, each of tests being conducted at 1,600 g. Curve L is representative of a basket centrifuge having four vertical accelerator vanes which extend nearly the entire height of the centrifuge bowl, each vane being fully immersed, each vane extending from onefourth inch of the overflow lip to the outer peripheral wall of the bowl. Curve M represents a wheel accelerator having vanes which are each 1.0 inch in height extending radially from one-fourth inch inward of the liquid surface to the outer peripheral wall, and the bottom of the accelerator wheel being located 1.5 inches up from the bottom of the centrifuge bowl. Curve N represents a wheel accelerator having vanes which are each 2.0 inches in height, extending from one-fourth inch above the liquid surface to the outer peripheral wall, the accelerator wheel being located 0.5 inch up from the bottom of the centrifuge bowl. Curve represents a wheel accelerator in which each individual accelerator vane is 2.5 inches in height, extending from one fourth inch above the liquid surface to the outer peripheral wall, there being no space or gap between the accelerator wheel and the bottom of the centrifuge bowl. The percent increase in cake capacity over the base (a standard four vane bowl) of each of the accelerator arrangements represented by curves M, N and O is quite significant, and is set forth in the Table 1 below. Because of the relative thinness of the moving liquid layer, as soon as the feed enters the pond, most of the flow is axial rather than radial. The process value of the several acceleration devices and accelerator wheels were compared on two bases; solids recovery versus throughput capacity, and liquid layer thickness and cake thickness versus throughput. A summary of these process comparisons is given in the following table. The different accelerator arrangements are listed in order of decreasing recoveries for low turbidities; then the difference in recovery is listed for each relative to recovery in the standard basket centrifuge bowl having four accelerator vanes which extend substantially the entire height of the centrifuge bowl, and the full depth of the annular pond. The wheel accelerator is outstanding on this basis. Liquid layer thickness and cake thickness are also indicated for each of the accelerator arrangements. The best modifications are the wheel accelerator. This table also lists the results of an accelerator vane arrangement known or termed herein as a bottom accelerator. The bottom accelerator comprises a plurality of radially extending vanes which extend from a radius less than that of the inside of feed cone 20 to a radius beneath the liquid surface; the vanes extend axially from the bottom of the centrifuge bowl to the bottom of the cone, or in other words the height of the accelerator vanes is equal to the height of the feed discharge openings 21. As can be seen from the table, none of the bottom accelerator arrangements which also correspond to prior art performed as well as the preferred wheel accelerator. Similarly, the centrifuge bowl with no acceleration device performs poorly. ticular size, and the entire arrangement is located at a particular place within a given centrifuge bowl. The wheel accelerator? does not necessarily require an outer and inner rim as illustrated in FIGS. 1 and 2. What we claim: 1. A centrifuge for separating first and second materials from a mixture thereof in feed delivered thereto, wherein said first material has a higher specific gravity than said second material, said centrifuge having a bowl adapted to rotate about an axis, said bowl comprising an imperforate peripheral wall formed about said axis and an annular end wall extending inwardly of said bowl at one end thereof from said peripheral wall toward said axis, the inner edge of said annular end wall providing an overflow lip, said bowl at the end thereof opposite said one end having an end portion extending for an axial distance of not more than one-third the total axial length ofsaid bowl, feed means for conducting said feed to said bowl, said feed means having an outlet disposed in said end portion for discharging feed outwardly from said outlet toward said peripheral wall, removing means movably mounted within said bowl for removing first material accumulated at said peripheral wall, a plurality of from 12 to radially extending accelerator vanes, each vane being wholly disposed in said end portion of the bowl, the inner end of each vane being positioned a radial distance from said axis in the range of between the radial distance of said outlet from said axis and the radial distance from said axis of a point located one-half inch farther from said axis than said overflow lip, the outer end of each vane being disposed from said axis at least a radial distance equal to the sum of the radial distance of said overflow lip from said axis and one-third the radial distance from said overflow lip to said peripheral wall, and means for moving said removing means between a first position disposed closer to said axis than said overflow lip and a second position disposed farther from said axis than said overflow lip, said removing means in said second position being engageable with first material accumulated at said peripheral wall for removing first material from said bowl. 2. An improved centrifuge according to claim 1 wherein said removing means comprises a knife blade disposed inwardly from said overflow lip and mounted for outward movement toward said peripheral wall, said bowl having an opening disposed in the opposite end thereof for the passage of said removed first material. 3. An improved centrifuge according to claim 2 wherein said knife blade has a notch disposed therein which cooperates with said acceleration vanes as the TABLE I.-COMPARISON OF ACCELERATION DEVICES Recovery (percent) Percent Liquid Cake increase Height Immer- Gap to At 20 Relative layer thickness in cake Number k) sion (i bo n) K-pm to base (in.) (in.) capacity Aceel'r common name 2.5 4. 5 0 100 +12 0.8 3. 7 25 Wheel and extns. 2.0 4.5 0. 5 J7 +9 1. l 3.4 18 D0. 1. 0 4. 5 1. 5 +7 1. 5 3.0 5 Wheel. 1.0 4. 5 2.6 05 +7 1.5 3. 0 5 D0. 1. 1 2. 0 0 90 +2 1. 2 3. 3 15 Bottom. 15 4.5 0 88 Base 1.6 2.0 Base Standard (full vane). 0 78 10 1.6 2.0 0 No-vane. ' At low turbidities 12 JIU). Base is 4 full-vane bowl. In 30X18 at 1,600 g. with 4.5" lip. N0'rE.Irnmersion is defined as the distance a vane extends radially outwardly from the surface of the pond (the material retained within the bowl). What has been referred to as a wheel accelerator herein merely refers to an arrangement of radially extending accelerator vanes; each of the vanes is of a parblade is moved outwardly, so as to allow the cutting edge of the blade to move outwardly beyond the inner edges of the vanes, said bowl having an opening disposed in the opposite end thereof for the passage of said removed first material. 4. An improved centrifuge according to claim 1 wherein each of said vanes is a'ngulariy spaced from 5 to apart, and wherein each of said vanes extends axially from said feed outlet toward said overflow lip a distance ranging between 0.5 inch and 2.5 inches. 5. An improved centrifuge according to claim 4 wherein said centrifuge is adapted to produce a force of between 1,200 and 1,800 g at the peripheral wall of the bowl. 6. An improved centrifuge according to claim 4 wherein said removing means comprises a knife blade disposed inwardly from said overflow lip and mounted for outward movement toward said peripheral wall, said bowl having an opening disposed in the opposite end thereof for passage of said removed first material. 7. An improved centrifuge according to claim 6 wherein said knife blade has a notch disposed therein cooperating with said accelerator vanes as the blade is moved outwardly, so as to allow the cutting edge of the blade to move outwardly beyond the inner edges of the accelerator vanes. 8. An improved centrifuge according to claim 5 wherein said removing means comprises a knife blade disposed inwardly from said overflow lip and mounted for outward movement toward said peripheral wall, said bowl having an opening disposed in the opposite end thereof for passage of said removed first material, each of said vanes extending axially to said opposite end of said bowl. 9. An improved centrifuge according to claim 1 wherein each of said vanes is angularly spaced from 5 to 20 apart, said centrifuge being adapted to produce a force of between l200 and 1800 g at the peripheral wall of the bowl, each of said vanes extending axially from said feed outlet toward said overflow lip a dis tance ranging between 1.0 inch and 2.0 inches. 10. An improved centrifuge accordingto claim 9 wherein said removing means comprises a knife blade disposed inwardly from said overflow lip and mounted for outward movement toward said peripheral wall, said how] having an opening disposed in the opposite end thereof for the passage of said removed first material. II. A centrifuge according to claim 1 wherein said bowl, between said vanes and said end wall, is free of structure connected to said peripheral wall.

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