US2958461A

US2958461A - Centrifuge machine

Info

Publication number: US2958461A
Application number: US794464A
Authority: US
Inventors: Sr Albert Peltzer
Original assignee: Dorr Oliver Inc
Current assignee: Dorr Oliver Inc
Priority date: 1959-02-19
Filing date: 1959-02-19
Publication date: 1960-11-01
Anticipated expiration: 1977-11-01

Description

Nov. 1, 1960 A. PELTZER, sR

CENTRIFUGE MACHINE 3 Sheets-Sheet 2 Original Filed July 17 1951 FlIE FIE

INVENTOR. A/ber/ pe/fzer; .5

BY M

Nov. 1, 1960 A. PELTZER, SR

CENTRIFUGE MACHINE Original Filed July 17, 1951 I5 Sheets-Sheet 3 ATTOEIVEY5 CENTRIFUGE MACHINE Albert Peltzer, Sn, San Francisco, Calif., assignor, by mesne assignments, to Don-Oliver, Incorporated, Stamford, Conn, a corporation of Delaware Continuation of abandoned application Ser. No. 237,236, July 17, 1951. This application Feb. 19, 1959, Ser. No. 794,464

13 Claims. (Cl. 23314) My present invention relates generally to centrifuges of the type adapted for the continuous separation and discharge of solid components from a fluid feed material containing suspended solids. My invention also relates to centrifugal separating methods which may be carried out by such machines, and the present application is a continuation of my copending application Serial No. 237,236 filed July 17, 1951, and now abandoned, which in turn is a continuation-in-part of my then copending application Serial No. 100,488, filed June 28, 1949, now abandoned.

More particularly my invention relates to continuous return circuit centrifuges in which part of the separated underflow is continuously returned into the centrifuge rotor for redischarge through the underflow nozzles with provision for the efficient recovery of solubles of the feed liquid, as well as for the separation of solids. The first presently known proposals for centrifuges providing for return of underflow are disclosed in Swedish Patent No. 25,442 of 1907 and in the corresponding British Patent No. 10,314 of 1908. In each of these patents, one embodiment is shown with relatively large tapering return tubes discharging at the rotor nozzle entries, and in another embodiment such tubes are shown discharging between the nozzles. No effort was made to recover solubles in the feed liquid, and the stated object of these patents was the provision of centrifugal machines for separating solids from liquids provided with means for preventing obstruction of the discharge nozzles. These structures are such that mixing of feed liquid with the returned underflow necessarily results in causing substantial loss of solubles as well as inefficient separation of solids. The sole control mentioned, but with no disclosure of method, is the maintenance of the total amount of discharged solids and return fluid nearly constant, independently of variations in the amounts of solids separated. Tests have shown that large tapering return feed tubes clog readily, particularly with heavy underflow return feed. No settling zone is disclosed between the discs and bowl periphery, and the general arrangement is very inefiicient, although the patents were owned by the oldest highly skilled manufacturer of continuous non-return centrifuges. If any such machines were built, they were not put into general commercial use, and the owner until recently has manufactured only non-return types of continuous centrifuges.

The first commercially successful return type centrifuges providing for fairly eflicient recovery of solubles and separation of solids appeared on the market years later. They were manufactured under U.S. Patents 1,847,751; 1,923,454 and 1,945,786. In these machines fairly successful control of the important function of separating the original feed liquid from the discharging solids to efiect soluble recovery was provided. US. Patent No. 1,847,751 illustrates the earliest such commercial machine utilizing return circuit tubes which dis- United States Patent G ice tubes were improved upon at an early date by a cone shaped return impeller provided with impeller vanes extending from the lower conical surface of the impeller to the lower conical bowl section and which distribute the return feed around the discharge periphery of the bowl as illustrated for example in US. Patent No. 1,945,- 786. These and later tubeless centrifugal machines such as shown in U.S. Patents 2,013,668 and 2,060,239 utilize a rotor which has provision for introducing fluid feed material into the separating chamber, an overflow lip for discharging the centrifugally separated overflow, and underflow discharge nozzles mounted upon the peripheral margin of the rotor. The rotor is also provided with an impeller which has an axially disposed opening and which is connected to discharge material into the outer part of the centrifuge chamber through an annular space of substantially uniform cross-sectional flow area which distributes the returned underflow into a so-called racing chamber forming the outer part of the separating chamber. The housing which surrounds the rotor is provided with a volute or like means for collecting the centrifugally separated underflow. The flow connection from this volute serves to deliver a large portion of the underflow back into the centrifuge chamber through the axial impeller opening. Provision is made for introducing wash liquid into the return circuit to displace substantially all of the liquid in the feed into the overflow with the solubles contained therein. This is a highly important operation termed soluble blocking in the art and to improve the efficiency thereof is a primary object of my present invention.

Under certain operating conditions and more noticeably in prior machines of smaller sizes, there has been a tendency towards dispositing of solids between the underflow discharge nozzles. After periods of operation, deposits of underflow solids have been found in the outer part of the separating chamber in regions intermediate the discharge nozzles. These deposits frequently attain the proportions of tetrahedrons with one of the triangular corners parallel with the center of rotation which may extend inwardly to the separator discs. These difiiculties are also present more noticeably in a recently introduced return type commercial centrifuge machine using a series of tubes for continuously returning underflow material. In this latter machine the return circuit tubes extend outwardly into the lower part of the separating chamber and discharge into pockets formed at the inlet ends of the underflow discharge nozzles in an effort to prevent clogging of the nozzles. During operation, underflow solids also accumulate in the outer part of the separating chamber of this machine in regions intermediate the inlet ends of the underflow nozzles, tending to form substantial tetrahedrons ext-ending for a substantial distance into the separating chamber. In both types of machines such accumulations tend to limit separating capacity, and frequent clogging of the underflow nozzles occurs which is attributed to sloughing off of fragments of such deposits. In addition to these difliculties the latter tube type machine is subject to clogging of the tubes which deliver the underflow material into the separating chamber.

The range of commercial uses for continuous return centrifuges is in part dependent upon their capacity. The separating capacity of such prior art machines for a given fluid feed material and desired efliciency of sepa ration, as well as the amount of wash liquid which can be employed with a given size rotor capable of operation at a given speed of rotation, is limited. Efforts to crowd the prior machines beyond these limits by increasing the feed rate and amount of wash water result in definite impairment of the separating efliciency -of particular machines, According to my observations, the

amount of fresh wash liquid which can be used during operation of the machine is a measure for the capacity which can be obtained without sacrifice in separating efficiency. Limitations as to capacity and the amount of wash water which can be employed serve to limit the range of usefulness of such return type centrifuge machines, and in many instances their application is made impractical for economic reasons.

Accordingly, a further primary object of the present invention resides in the provision of new centrifuges of the continuous return type which overcome the foregoing limitations and disadvantages of prior return machines.

It is another object of my invention to provide a new centrifuge of the continuous return type which for -a given diameter of rotor and speed of rotation will afford a relatively high capacity in comparison with capacities heretofore obtained with prior machines.

A further object resides in providing an improved centrifuge of the continuous type which permits use of relatively greater amounts of wash liquid in the underflow return circuit with high separatory efiiciency and improved regulation of the amounts of feed and wash liquids passing either way through the machine with improved soluble blocking.

A still further object of my invention is provision of new return type centrifuges and methods of utilizing and controlling the return of underflow material, whereby much higher capacities can be obtained in machines of given size without sacrifice in separating efliciency and whereby plugging or clogging of the underflow nozzles is alleviated.

Another object of the invention is to provide a centrifuge of the continuous type having a return circuit for underflow material which does not require the use of an excessive number of underflow discharge nozzles, and which is consequently relatively efficient with respect to power consumption.

Still another object of the invention is. to provide a novel centrifuge method characterized by the novel manner in which optimum separating conditions are maintained in the separating chamber, and by the manner in which the returned underflow material is utilized to minimize or prevent the build up of tetrahedrons of solid material between the underflow nozzles.

Additional objects and features of the invention will appear from the following description in which the preferred embodiments have been set forth in detail in conjunction with the accompanying drawings, and within the scope of the present invention as defined by the appended claims.

The foregoing objects I accomplish in general by the use of properly proportioned and arranged underflow return tubes which distribute the returned underflow and wash liquid between the bowl nozzles adjacent the bowl periphery in a manner and at velocities that prevent accumulations of solids between the nozzles, minimize or eliminate clogging of the nozzles, and, for soluble reoovery, displace original feed liquid with wash liquid causing the original feed liquid to pass out in the overflow, all with a substantially greater degree of control and efficiency than has heretofore been available in return type machines.

In accomplishing these new improved and highly desirable results I have discovered that the following factors are interrelated and of varying importance: (a) the num- 'ber and position of discharge of the return tubes with respect to the bowl nozzles, (b) tube size, the differential head created by relative bowl entry diameters and overflow discharge diameter, (d) the speed, size and shape of rotor bowl, (e) the nature of the separation desired, (1) the viscosity and other characteristics of the process liquids, (g) the percentage and physical characteristics of the process solids, and (h) the number and size of. underflow nozzles vs. tube size and number.

The correct position for tube discharge is important,

and should be substantially midway between adjacent bowl nozzles for optimum results. Tubes slightly leading midposition are not too harmful. In a series of tests made with a liquid containing refined starch solids slurried in salt water which gives generally typical results, and with the return tube discharges leading and lagging the exact mid-nozzle location, the overflow invariably contained considerably less solids with the tubes discharging at mid-position between nozzles than when shifted from mid-position. Lagging mid-location was very detrimental and slightly leading mid-location was somewhat worse than mid-location, but not too detrimental for use. As mid-location is approached the separator results improve. Tubes discharging parallel to the periphery of the bowl are not especially beneficial over tubes discharging radially.

These tests show that the inside pipe size or cross sectional area of the tubes is important in relation to the number of tubes used, volume of fluid passing through the tubes, the differential head pumping the fluid through the tubes and the nature of the fluid itself. The velocity of the return tube jet discharge must be great enough to keep the bowl, nozzles and the tubes themselves cleared. Tubes too 'large in area relative to area of underflow nozzles plugged up with solids. The tube length may be varied without adversely affecting clarification of the overflow to any substantial extent so long as the clearance between the end of the tube and bowl is sufficient to avoid clogging of the tube, and so long as velocity of the jet discharge is sufiicient. The minimum clearance of the tube ends should be about equivalent to one-fourth of the tube outlet diameter.

.Now referring to the drawings:

Figure 1 is a side elevational view in section illustrating one specific embodiment of my invention.

Figure 2 is a diagrammatic plan view on a reduced scale, illustrating particularly a portion of a return circuit for returning underflow material.

Figure 3 is an enlarged cross-sectional detail of a portion of the rotor and particularly illustrating a part of the underflow return circuit.

Figure 4 is a quarter plan section of the rotor, illustrating the underflow discharge nozzles and the positioning of the underflow return tubes.

Figure 5 is an enlarged cross-sectional development taken along the line 5-5 of Figure 3.

Figure 6 is a cross-sectional detail like Figure 3 but illustrating another embodiment of the invention.

Figure 7 is a cross-sectional detail like Figure 3 but illustrating another embodiment of the invention.

Figure 8 is a cross-sectional detail taken along the line 8-8 of Figure 7.

Figure 9 is a cross-sectional detail like Figure 3 but illustrating a further embodiment of the invention.

Figure 10 is a cross-sectional detail taken along the line 10-10 of Figure 9.

. That embodiment of the invention illustrated in Figures 1 to 5 inclusive consists of a rotor 10 which is disposed in the housing 11. The rotor is carried by the vertical shaft 12' which in turn is connected by coupling 13 to the vertical drive shaft 14.

For convenience the rotor 10 is made of a number of separable annular parts. Thus, the upper and lower frustro-

conical bowl parts

16 and 17 are fitted together and held in assembled relationship by the threaded lock ring 18.

Parts

16 and 17 are formed to provide the inner separating chamber 19 having upper and lower conical outer wall sections converging outwardly from the axis of rotation to the bowl discharge nozzles hereinafter described. The fit between these parts is sealed by suitable means such as the resilient O-ring 20. The lower part 17 is providedwith an inner frustro-conical hub 21 which is fitted upon the lower end of the shaft 12 and forms one wall of the feed passages for the separating chamber. The lower 7 extremity of the shaft 12 is threaded to receive the nut 22 and the threaded sleeve 23. Nut 22 serves to hold the bowl assembly and an impeller assembly hereinafter described to shaft 12. Sleeve 23 is flanged to provide a disc like lower face 24, for a purpose to be presently explained. To provide means for the discharge of centrifugally separated overflow, the upper section 16 is provided with a cylindrical extension 26, which terminates with the annular over flow lip 27. A conical sleeve-like member 28 is positioned within rotor section 16 and is provided with a lower flared portion 29 overlying the inner portion 21 of the bowl section 17 Member 28 is provided with inwardly extending circumferentially spaced ribs 31, and the spaces between these ribs form outwardly discharging passages 32 for introduction of feed material. Feed material introduced through passages 32 enters the separating chamber 19 at relatively high velocity through the annular orifice 33 adjacent the lower end of the upwardly inclined frustro-conical lower outer wall of chamber 19.

The exterior ribs 34 of member 28 serve to maintain spaced relationship with the upper rotor section 16, thus permitting flow of overflow material to the lip 27. A stack of spaced conical-shaped separating discs 36 located by ribs 34 have outer horizontal discharge lips having a diameter approximately equal to that of the feed annular orifice 33. This provides a settling or separating region or zone surrounding the feed orifice and the outer edges of the discs for effective separation of the solids from the feed and for separation of the feed from the return in a manner that will more fully hereinafter appear. During operations of the rotating chamber 19 the feed material entering through inlet 33 will resolve itself into an annular zone whose inner boundary is located approximately at the outer edges of the discs 36 and surrounds the discharge or the inlet orifice 33 thus assuring that the feed inlet is wholly within the settling or separating zone. As separation of the solids from the feed material commences, the

outer boundary of the separating zone resolves to a position radially inward of the peripheral margin of the separating chamber and outer zone of separated solids occupies the annular space which surrounds the outer boundary of the separating zone. This separation of the feed material into three concentric zones with the lighter materials or solubles occupying the inner zone, the feed material occopying a mid zone and the heavier materials or solids occupying the outer zone is well known to those familiar with continuous rotor centrifuges.

The centrifugally separated underflow material is discharged from the rotor through the nozzles 41, which are mounted at circurnferentially spaced points (Figure 4) on the outer peripheral margin 42 of the rotor. The details of construction of these nozzles may vary, but to reduce power loss it is desirable that they discharge material backwardly with respect to the direction of rotation. In the nozzle construction illustrated in Figure 4 the rotor is provided with the radially extending openings 43, which are threaded to receive the collars 44. A bushing 46 is fitted within and seated upon a collar 44, and forms a mounting for the body of a nozzle 41. The flow passage through this nozzle includes the portion 48 which extends in a radial direction, and the orifice portion 49 which is directed backwardly with respect to the direction of rotation.

In that form of the invention illustrated in Figures 1 to 5 inclusive, the inlet ends of the nozzles 41 all communicate with an annular recess 51, a so-called racing chamber, which in turn communicates on its inner side with the main separating chamber 19. While this racing chamber is not necessary in the practice of this invention, it has been found helpful to soluble blocking although it is not particularly helpful to overflow clarification or separation of solids. As shown particularly in Figure 3 the annular recess 51 is defined by the spaced upper and lower

horizontal surfaces

52 and 53, and the peripheral surface margin 54.

The outer stationary housing 11 is preferably made from a number of separable annular sections including particularly the parts 56, 57 and 58 (Figure 1). The upper section 56 forms an annular volute or chamber 59 for receiving overflow material discharging over the lip 27, and carries the

concentric conduits

61 and 62, which provide a passage 63 for introducing feed material into the feed well formed by the member 28. Housing sections 57 and 58 are formed to provide annular chamber or volute 64, which receives underflow material discharged from the nozzles 41. Filler rings 66 and 67, carried by the housing sections 57 and 58, may be provided to form between them a relatively restricted annular throat passage 68 through which jetting material enters volute 64. By maintaining a liquid level in the restricted throat passage 68 kinetic energy of the discharging underflow material is substantially converted to static pressure head in the volute chamber 64.

In the embodiment of Figures 1 to 5 inclusive the underflow return circuit comprises lower housing 77 (Figure l) forming inlet passage 78 to nozzle 79. Inlet 78 is in communication with the discharge end of conduit 81 (Figure 2), the inlet end of which connects tangentially with the underflow volute chamber 64. Valve controlled pipe 82 is provided for controlled introduction of wash liquid into conduit 81 and final drawoff of underflow material is eflected through valve controlled pipe '83.

A needle valve member 84 (Figure 1) carried by the threaded operating stem 86, which extends to the exterior of the housing and is provided with the operating hand wheel 87, is preferably provided to control the velocity of the jet of return under-flow and wash liquids. By turning the hand wheel 87 the effective flow area in the orifice 88 of nozzle 79 may be varied as may be desirable to keep the velocity of the jet at the proper point.

The centrifuge rotor provided with an impeller assembly receives the material discharged upwardly through nozzle 79. This assembly comprises a conical-shaped impeller part 89 attached to the rotor part 17, provided with a downwardly faced axial opening 91 aligned with and directly above the nozzle 79. Circumferentially spaced radial vanes 92 impart rotary velocity to returned fluid material which is impelled upwardly along the conical inner surface of bowl section 89.

A plurality of return tubes 93 (Figures 1 and 3 to 5) are located in the lower part of centrifuge chamber 19 overlying the inner conical surface of the bowl section 17. The lower inner ends of the tubes 93 communicate with the impeller chamber and in the construction shown are secured to the sleeves 94, which in turn are removably seated within the openings 96 formed in the bowl section 21. Suitable sealing means such as a resilient O-ring 97 serve to prevent leakage past each sleeve. The outer ends 98 of the tubes 93 are substantially unobstructed, and discharge adjacent the outer periphery of separating chamber 19. As illustrated in this embodiment the tubes are bent to discharge horizontally and at right angles to the surface 54 into racing chamber 51 midway between nozzle assemblies 41 as illustrated in Figure 4, and the number of tubes equals the number of nozzles. The diameter of tubes 93 and their location is an important feature of my present invention. As hereinbefore pointed out their diameter must be such as to maintain adequate velocity to prevent clogging within the tubes and also prevent accumulation of solids between rotor nozzles at the inside periphery of the rotor. Optimum results are secured when they discharge midway between nozzles.

It will be seen that from the structure so far described that complete separation between the fed material and the returned underflow is maintained with the feed entering the main separating chamber 19 radially well within the discharge ends of tubes 93.

With twelve nozzles and tubes as illustrated in this embodiment, the distance between the discharge end of each tube 93 and the inlet ends of the adjacent nozzles used for particular applications as hereinbefore set forth,

it is generally desirable to maintain a relationship be- :tween the chord distance from the discharge end of tube to adjacent nozzle and the radial distance from axis of rotation to discharge end of tube of unity or less.

A shroud or barrier 161 preferably frustro-conical in shape, between the recess 51 and the main part of the separating chamber aids in soluble blocking. With this barrier or shroud, communication between recess 51 and the main part of separating chamber 19 is through the annular space 164.

A drain pipe 1% (Figure 1) connected to the lower end of housing 11 serves to remove spilled material. Guide rods Ill 7 urged downwardly by compression spring 108, facilitate assembly of the machine.

Operation In operation of the machine illustrated in Figures 1 to inclusive a suitable feed material as for example mill starch liquor containing finely divided starch particles, gluten particles and solubles, is supplied to feed passage 63 while the machine is in operation. Within the separating chamber centrifugal forces act upon the material to separate the starch from the lighter gluten. The feed liquid containing solubles and gluten is carried off continuously in an overflow established over the lip 27. An underflow of separated starch and carrying liquid is established through the discharge nozzles 4-1, fills the volute chamber 64 and establishes a pressure head in the return circuit 81, whereby a controlled large portion of the underflow material is continuously returned to the centrifuge rotor. The returned material jets upwardly from the nozzle orifice 88 into the impeller section 89' and is then discharged outwardly through the tubes 93, to be finally delivered into the confined annular recess or racing chamber 51. In a typical starch and gluten separation 85% or more of the discharged underflow material is continuously returned into the centrifuge rotor in this fashion and underflow material redischarges through the nozzles 41, together with freshly separated underflow solids.

The use of a Wash water or wash liquid introduced by way of pipe 82 is generally desirable to promote efliciency of separation, particularly where the feed material contains solubles which it is desired to block or to keep out of. the underflow. Wash liquid introduced through pipe 82 serves to displace feed liquid containing solubles from the underflow material at the inner periphery of the rotor bowl and forces it into the overflow to the extent of the displacement thus blocking the solubles from passing into the underflow. The wash liquid which displaces the feed liquid passes out into the underflow. When less wash liquid is employed than the liquid removed in the underflow, a condition of so-called downflow exists within the rotor; and when more wash liquid is used than the amount of liquid removed in the underflow an upflow condition exists. During downflow part of the feed liquid passes out with the underflow, and during upflow part of the wash liquid moves countercurrently through the separating chamber for discharge with the overflow.

The relative amount of wash liquid used in practice is dependent upon the type of operation. For maximum displacement of solubles into the overflow, it is the practice to employ wash liquid in sufiicient amounts to maintain an upflow condition. Where the soluble separating requirements are not exacting, the amount of wash water employed may be reduced to provide a downflow condition. In instances where two or more centrifuges are used in cascade, the first centrifuge may operate with downflow and the second with upflow.

While it is impossible to make direct observations as to 8. what takes place within the 'centrifugechamber 19 according to my present observations and understanding, the feed fluid passes down and outward through section 28,

. passages 32 and orifice 33 adjacent the bottom of chamher 19 and is forced upward by'the inner conical surface of section 10 of the bowl into the zone of chamber 19 surrounding the discs 36 where heavier feed overflow components are separated and pass out through orifice 104 (Figure 3) into racing chamber 51, and the overflow passes upward through the disc stack 36 over lip 27 into volute 59. In the stack further separation of heavier components occurs from the liquids passing upward through the stack.

From chamber 51 the underflow discharges through nozzles 41 .into volute 64. Underflow in the proportion desired is divided and part is drawn off through pipe 83 (Figure 2) while the remainder is returned through conduit 81, orifice 79 (Figure l) and the impeller assembly t hrough tubes 93 with the desired amount of wash liquid added through pipe 82 (Figure 2).

The mixture of underflow and wash liquid discharges through the tubes 93 with relatively high radial velocity and with a jet action from the outer ends of these tubes. Since the material is confined within the tubes until the instant of its discharge from the outer end thereof, rotary velocity is imparted to it equal to about the pcripheral velocity of the outer ends of the tubes. Since discs 36 terminate short of the outer ends of the pipes 93, returned material as it enters recess or chamber 51 has a relatively high apparent specific gravity compared to the material in the main separating zone of chamber 19 surrounding discs 36 and this contributes to the jet action from tubes 93. The underflow material discharging through tubes 93 apparently largely fills the peripheral bowl space or recess 51 between the nozzles and flows circumferentially in a peripheral layer of live slurry discharging through underflow nozzles 41. While flowing through recess 51 or along the outer bowl periphery when recess 51 is not used, a continual exchange of solids and liquid occurs between the returned live slurry layer and the material in the adjacent separating zone. Thus, centrifugally separated solids from the feed move outwardly from the main separating chamber through the space 104 into the recess 51. Assuming that the amount of wash water or liquid is suflicient to provide upflow conditions, wash liquid flows from the recess 51 through space 104 in Figure 3 into the main separating chamber, to thereby efiiectively displace feed liquid and solubles from the centrifugally separated underflow solids. It will be noted that this continuous exchange between material in the outer live slurry layer and material in the main part of the separating chamber or adjacent separating zone is relatively evenly distributed completely about the rotor, whereas the returned material discharging from the outer ends of the tubes 93' is sharply localized in regions located midway between the discharge nozzles 41. Therefore, the exchange takes place while the bulk of the material in recess 51 or the live slurry layer is flowing circumferentially toward nozzles '41.

The returned peripheral live slurry layer is apparently maintained in a continual state of agitation due to eddy currents imparted to the material within the tubes 93, and to further turbulence and eddy currents caused by material jetting from the outer ends of these tubes against the bowl periphery between the nozzles. Such turbulence or eddy currents are largely confined to the live slurry layer and do not spread into the main separating chamber to interfere with eflicient separating action. However, the formation of tetrahedrons present in the prior art machines due to collection of centrifugally separated solids between the nozzles is either prevented in entirety or maintained within such size limits as to avoid interference with the operation of the machine. I attribute this highly important effect to the sweep action coupled with the high apparent specific gravity of the returned live slurry layer flowing circumferentially through the recess 51.

An important characteristic of my invention is attainment of higher separating efliciencies with considerably higher capacities and/or use of considerably larger amounts of wash water than are possible in prior machines. With my invention, the rate of feed can accordingly be greatly increased over the prior art machines, and the amount of wash liquid can be greatly increased to secure etficient separation with respect to both solids and solubles. I attribute this to the manner in which liquid from the returned material is transferred to the main separating chamber, and also to the substantial or complete elimination of tetrahedrons of accumulated solids between nozzles which interfere with capacity and separating efliciency. As previously stated the relative amount of wash liquid or water employed can vary 1 according to the separating requirements and the capacity desired. When operating with upflow conditions a relatively high capacity may be maintained while at the same timethe rotor may be supplied with sufiicient wash water to provide an upflow with a substantially more efiicient displacement of solubles into the overflow as et er of 0.064 inch. Twelve tubes 93 were employed,

each having an internal diameter of 0.31 inch. The outer ends of these tubes were located midway between adjacent nozzles. The return inlet opening 91 had a diameter of 1.5 inches and the internal diameter of the overflow lip was 3.8 inches. The machine was operated on 14.5 Baum starch feed containing 25.8% solids. This liquor was supplied to the machine at a feed rate of 34,000 cc. per minute with discharge of the overflow at the rate of 28,000 cc. per minute, and the underflow drawoff at 26,700 cc. per minute. The underflow was maintained at 18.2 Baum and contained 32% dry solids. Fresh wash water was added at the rate of 20,700 cc. per minute. The material discharged from the underflow nozzles at the rate of approximately 226,700 cc. per minute, and underflow was continuously returned to the rotor at the rate of about 200,000 cc. per minute. The separating capacity in terms of dry starch per twenty-four hours was 31,000 pounds, and this was attained with highly eflicient separation (minimum starch solids in overflow and minimum solubles in underflow drawoff). This contrasted with a maximum capacity of about 10,000

pounds per twenty-four hours for a machine of comparable size constructed substantially as illustrated in Patent 1,945,786, and operated with comparable separating efficiency. During operation of the machine there was complete freedom from clogging of the underflow nozzles and tubes 93, and there was no substantial evidence of .accumulated tetrahedrons when the machine was inspected after test runs such as are present in the prior machines.

The machine referred to above by way of example was not provided with a barrier or shroud member 101.

,cient displacementaction occurred to displace such sol- -ubles into'the overflow.

After extended periods of operation dismantling of the machine (made according to the foregoing sample) re vealed that no accumulation of solid material, such as formed in prior machines, occur-red in the centifuge chamber or in the annular recess 51. According to my observations and present understanding, the absence of such accumulated solids is largely responsible for the absence of plugging or clogging of the underflow discharge nozzles, and also contributes to attaining the remarkably high separating capacity. In addition it is well known that accumulations of starch or like solid material in the centrifuge machines used for processing food products is objectionable because such deposits offer a possible source of contamination by bacteria or other micro-organisms, or may require expensive periods of shutdown for cleaning the machine.

Previous reference has been made to the relatively high velocity flow through the tubes 93. High velocity flow through the tubes is desirable not only because it causes the desired circumferential peripheral live slurry flow between the nozzles, but in addition it prevents clogging of the passages through the tubes. Relatively high flow velocity through the tubes 93 is due in part to the relatively high differential head applied to the returned material. This can be better understood by reference to Figure 1. Thus as in the case of the dimensions specified by way of example, the inlet 91 of impeller section 89 is comparatively small relative to the diameter of the overflow lip 27. At rot-ative speeds such as used to obtain good separation, the relatively large difference in diameter at these points results in a substantial difference in hydraulic head between the underflow material being returned into the recess 51 at the outer ends of the tubes 93, and the head on the material moving outwardly through the separating chamber at the outer ends of tubes 93. For the particular machine specified in the foregoing example, the differential head at the outer ends 98 of the tubes 93 calculates theoretically to be of the order of about 180 feet of water for a rotational speed of 7,000 r.p.m. In practice I prefer that the differential head he of the order of from 50 to 100 feet of water or more.

My invention can be modified in various ways within the scope of the appended claims. Thus castings or other structures can be used in place of the tubes 93, and which provide comparable passages for discharging the returned underflow material into localized regions intermediate the underflow discharge nozzles, in substantially the same manner as the tubes.

The return circuit need not be a direct closed return passage, as illustrated in Figure 2. Thus it is possible to include in the return circuit a vent box or flotation cell as disclosed in Patent 2,039,605.

In the embodiment of the invention illustrated in Figure 6, the recess 111, corresponding to recess or racing chamber 51 of Figures 1 to 5 inclusive, is defined by an outer curved wall 112, and the upper and

lower surfaces

113 and 114. The tubes 116, corresponding to tubes 93, have their outer ends 117 terminating within the recess .111. The tubes are provided with pads 113 for seating upon the adjacent surface of the bowl. It will be noted that in this embodiment the shroud or barrier member 101 has been omitted.

In the embodiment illustrated in Figures 7 and 8 the construction of the bowl is further simplified. Tubes 121 deliver the returned material into the outer portion of the separation chamber and intermediate the discharge nozzles. A shroud 122 overlies the tubes 121 and another shroud or barrier 123 extends between the tubes 121, .and downwardly from the outer periphery of the shroud 122. This construction forms in effect an annular space 124 which serves in place of the annular bowl recess 51 of Figure 1.

A further simplified embodiment of the invention is illustrated. in Figures 9 .and 10. In this instance the bowl'is constructed the same'as in FigureT and the tubes 126 discharge underflow material into the outer peripheral portion 127 of the separating chamber. The .outer ends of the tubes 126 are located midway between the underflow discharge nozzles. This embodiment makes possible higher capacity than conventional centrifuges, although it is not as effective as the embodiments .previously described in making possible high capacity together with eflicient displacement of solubles by wash.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the .appended claims rather than by the foregoing description, and all changes which come withinthe meaning and range of equivalency of the claims, are therefore intended to be embraced therein.

I claim:

1. In a centrifuge, a rotor having an annular separating chamber provided with a passage for introducing a fluid feed material into the chamber, and also having provision for discharge of a centrifugally separated overflow; underflow discharge nozzles mounted at spaced points on the peripheral margin of the rotor; and means for continuously returning underflow material back into the centrifuge rotor separately from the feed material including substantially unobstructed tubes arranged to discharge streams of returned underflow material into the periphery of said chamber at localized regions substantially midway between adjacent ones of said nozzles.

2. In a centrifuge, a rotor having an annular separating chamber with an inlet for fluid feed material and constructed to discharge centrifugally separated overflow radially inward of said inlet and centrifugally separated underflow radially outward of said inlet; underflow discharge nozzles mounted at spaced points on the peripheral margin of the rotor; and means for continuously returning underflow material separately from said feed material back into said rotor to maintain a circumferentially flowing zone of material of underflow discharge consistency surrounding the zone of separationincluding substantially unobstructed tubes having their inlet ends radially inward of said feed inlet and discharging streams of underflow material under a comparatively high differential hydraulic head into localized regions adjacent the periphery of said chamber and substantially midway between adjacent ones of said nozzles.

3. In a centrifuge, a rotor having an annular separating chamber convergent in a direction outwardly from the axis of rotation with an unobstructed separating zone, and a passage for introducing a fluid feed material intosaid chamber adjacent the inner periphery of said zone; a discharge for the centrifugally separated overflow radially inward of said zone; an annular confined recess formed in said rotor having an inner open side communicating with the outer periphery of said separating zone; spaced underflow discharge nozzles having their inlet ends communicating with said recess; and means for continuously returning underflow material through said separating zone into said recess including an im peller carried by said rotor and tubes having their inlet ends isolated from the feed material connecting with the impeller extending outwardly to discharge returned underflow material into said recess at circum-ferentially :spaced localized regions located substantially midway between the inlet ends of adjacent ones of the nozzles at a suflicient velocity to maintain a live annular zone of material of discharge consistency in said recess.

4. In a centrifuge, a rotor having an annular separating chamber convergent in a direction outwardly from the axis of rotation; passages for introducing fluid feed material into said chamber and removing centrifugally separated overflow therefrom; a confined annular recess having its inner open side communicating-with .the .outer portion of said separating chamber; underflow discharge nozzles mounted .at spaced points on the peripheral margin of said recess; and means for continuously returning underflow material 'back into said rotor comprising an impeller carried by the rotor, circumferentially spaced tubes extending outwardly. from said impeller having their inner ends communicating with the impeller to receive fluid material therefrom, separated from said feed material, and having their outer open ends terminating within said confined recess at regions located substantially midway between the inlet ends of adjacent ones of said nozzles; whereby the underflow material continuously returned into said rotor by way of said impeller and said tubes maintains a circumferential-1y flowing zone of material of discharge consistency throughout the length of said recess.

5. -In a centrifuge, a rotor having an annular separating chamber and provided with a passage for introducing fluid feed material into the chamber and provision for the discharge of a centrifugally separated overflow, the cross-sectional contour of the separating chamber being convergent in a direction outwardly from theaxis of rotation, the peripheral margin of the bowl being formed to provide a confined annular recess having its inner open side freely communicating with the outer portion of the separating chamber; a plurality of underflow discharge nozzles mounted at regularly spaced intervals on the peripheral margin of the bowl andhaving their inlet ends communicating with said recess at spaced points, and means for introducing a supplementalfluid material into said rotor comprising an impeller carried by the rotor, tubes connected to the impeller and extending outwardly from the same having their outerends discharging into said recess at circumferentially spaced regions located substantially midway between the inlet ends of adjacent ones of said nozzles, the maximum distance from the discharge end .of each tube and the inlet end of an adjacent nozzle being a minor fraction of the radial distance from the axis of the rotor :to the discharge ends of the tubes.

6. In a centrifuge, a rotor having an annular separating chamber convergent outwardly from the axis of rotation; a feed passage for introducing a fluid feed material into said chamber; a passage for the discharge of a centr-ifugally separated overflow from said chamber; a stack of separating discs in said chamber surrounded by an unobstructed separating zone; an annular recess surrounding said zone; spaced underflow discharge nozzles having their inlet ends communicating with said confined recess; and means for continuously returning heavier underflow material back into said recess comprising tubcs havingtheirinlet ends isolated from said feed passage, and extending outwardly through said separating zone into said recess outward of saidseparating discs and substantially midway between the inlet ends of adjacent ones of said nozzles, providing for continuous return of heavier underflow material into'said recess for redischarge through said nozzles, whereby a circumferentially flowing zone of material of substantially dis charge consistency is maintained'in said recess.

7. In a centrifuge, a rotor having an annular separating chamber provided with passages for-introdue1ng a fluid feed material and -for the discharge of acentrifugal- 1y separated overflow,thecross-sectional contourof the separating chamber beingconvergent outwardly from the vertical axis of rotation; an annular recess surrounding and communicating with said chamber; a stackof separating discs centrally disposed in saidchamber surrounded by an unobstructed separating zone c0mmun1cating with said recess; spaced underflow discharge nozzles having their inlet ends communicating withsaid recess; andmeans for continuously returning heavier underflow material back into the centrifuge chamber, said means comprising an impeller carried by said roto r, tubes having their inner ends connecting with saidumpeller and extending, outwardly through said separating zone 'and discharging into said recess, the outer ends of the tubes being located intermediate the end planes of said stack and substantially midway between the inlet ends of adjacent ones of said nozzles, said tubes serving to continuously return heavier underflow material into said recess for redischarge through said nozzles; whereby a zone of material of substantially discharge consistency is maintained in portions of said recess extending be tween the outer ends of the tubes and the inlet ends of the nozzles.

8. The centrifuge as set forth in claim 7 in which the maximum distance from the discharge end of each tube and the inlet end of an adjacent nozzle is a minor fraction of the radial distance from the axis of the rotor to the discharge ends of the tubes.

9. In a centrifuge, a rotatable chamber provided with a radially unobstructed substantially annular separating zone merged into an annular collecting region having a continual peripheral margin defined by the inner surface of the rotatable chamber; a feed inlet for said separating zone adjacent its radially inner boundary; an inner annular zone adjacent the inner boundary of said separating zone; an overflow outlet for said inner zone radially inward of said inner boundary; spaced underflow discharge nozzles in the chamber wall having inlets in said peripheral margin; and spaced tubes extending into said chamber and beyond said separating zone with their inlet ends isolated from said feed inlet and closer to the chamber axis than said inner boundary of the separating zone and their outlet ends freely spaced from said peripheral margin a distance at least equal to one-fourth the effective tube diameter and located to discharge fluid fed therethrough into said collection region at localized areas substantially midway between adjacent ones of said spaced nozzles.

10. In a centrifuge, a rotatable chamber provided with a radially unobstructed annular separating zone merged into an annular collecting region having a continual peripheral margin defined by the inner surface of the rotatable chamber; a feed inlet for said separating zone; an overflow outlet for said separating zone radially inward of the inner boundary of said separating zone; spaced underflow discharge nozzles in the chamber wall and having inlets in said peripheral margin; spaced tubes extending into said chamber and beyond said separating zone with their inlet ends isolated from said feed inlet and closer to the chamber axis than the inner boundary of the separating zone and their outlet ends located to discharge fluid fed therethrough into said collecting region substantially midway between adjacent ones of said spaced nozzles.

11. In a centrifuge, a rotatable chamber provided with a radially unobstructed annular separating zone merged into an annular collecting region having a continual peripheral margin defined by the inner surface of the rotatable chamber; a feed inlet to said separating zone for a primary fluid; an overflow outlet for said separating zone radially inward of the inner boundary of said separating zone; spaced underflow discharge nozzles in the chamber wall and having inlets in said peripheral margin; spaced tubes extending into said chamber and beyond said separating zone with their inlet ends isolated from said feed inlet and closer to the chamber axis than the inner boundary of the separating zone and their outlet ends located to discharge fluid fed therethrough into said collecting region substantially midway between adjacent ones of said spaced nozzles; and means for feeding a supplementary fluid through said tubes into said separating zone.

12. In a centrifuge, a rotatable chamber provided with a radially unobstructed annular separating zone merged with an annular collecting region having a continual peripheral margin defined by the interior peripheral surface of the rotatable chamber; a feed inlet for said separating zone adjacent its radially inner boundary; an overflow outlet for said zone radially inward of said inner boundary; spaced underflow discharge nozzles in the chamber wall having inlets in said peripheral margin; and spaced tubes extending into said chamber and beyond said separating zone with their inlet ends isolated from said feed inlet and closer to the chamber axis than the inner boundary of the separating zone and the cross-sectional area of each of said tubes being of a size and their outlet ends being located to discharge fluid fed therethrough into said collecting region at concentrated portions of said continual peripheral margin substantially midway between adjacent ones of said spaced nozzles.

13. In a centrifuge, a rotatable chamber provided with a separating zone; a feed inlet for said chamber adjacent the bottom of said separating zone, an overflow outlet for said separating zone disposed radially closer to the rotating axis than said inlet and axially separated from said inlet; underflow discharge nozzles for said chamber radially outward of said separating zone; and substantially unobstructed tubes extending into said chamber and beyond said separating zone with inlet ends radially inward of said feed inlet and overflow outlet disposed to freely discharge material passing therethrough into localized regions substantially midway between adjacent ones of said underflow nozzles.

References Cited in the file of this patent UNITED STATES PATENTS 1,945,786 Peltzer Feb. 6, 1934 2,525,629 Zimmerman Oct. 10, 1950