US3799353A

US3799353A - Continuous centrifuge

Info

Publication number: US3799353A
Application number: US00297440A
Authority: US
Inventors: K Pause
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-10-15
Filing date: 1972-10-13
Publication date: 1974-03-26
Anticipated expiration: 1991-03-26
Also published as: NL7213714A; IT968920B; DE2151476B1; JPS4847659A; DE2151476C2; DD98841A5; CS161809B2; CH563811A5; FR2156059B1; FR2156059A1; GB1412006A

Abstract

A continuous centrifuge for separating liquid and solid phases of a suspension has a centrifuge basket rotatable about an upright axis and having an upper open end, and a perforate liner in the basket conically diverging towards the upper open end. An annular baffle is mounted adjacent the upper open end and defines therewith a radially extending annular outlet gap through which retained solid phase can pass in radially outward direction under centrifugal force. An annular flow-regulating member is mounted on and rotatable with the basket and has a cylindrical surface juxtaposed with spacing from the outer end of the annular gap so that centrifugally ejected solid phase will contact this surface. The flow regulating member is mounted on the basket with slight freedom of axial movement relative thereto.

Description

United States Patent [1 1 Pause CONTINUOUS CENTRIFUGE [76] Inventor: Kurt Pause, Zedernstr. l3,

Grevenbroich, Germany [22] Filed: Oct. 13, 1972 [21] Appl. No.: 297,440

[30] Foreign Application Priority Data Oct. 15, 1971 Germany 1. 2151476 [52] US. Cl. 210/369, 210/391 [51] Int. Cl. Bold 33/02 [58] Field of Search 2l0/369-376, 210/391 56] References Cited UNITED STATES PATENTS 3,302,794 2/1967 Laven 210/369 R26,844 3/1970 Cuza 210/369 X 3,225,934 12/1965 Van Riel 210/370 [451 Mar. 26, 1974 Primary ExaminerGranville Y. Custer, Jr. Assistant Examiner-DeWalden W. Jones Attorney, Agent, or FirmMichael S. Striker [57] ABSTRACT A continuous centrifuge for separating liquid and solid phases of a suspension has a centrifuge basket rotatable about an upright axis and having an upper open end, and a perforate liner in the basket conically diverging towards the upper open end. An annular baffle is mounted adjacent the upper open end and de- 1 fines therewith a radially extending annular outlet gap through which retained solid phase can pass in radially outward direction under centrifugal force. An annular flow-regulating member is mounted on and rotatable with the basket and has a cylindrical surface juxtaposed with spacing from the outer end of the annular gap so that centrifugally ejected solid phase will contact this surface. The flow regulating member is mounted on the basket with slight freedom of axial movement relative thereto.

14 Claims, 8 Drawing Figures PAIENIEBIARZSIBM I 3.799353 SHLU 1 [1F 5 Angle of epose ""AIENTEDMAREBIW SHEET 2 m 5 3793353 FlG.3b

PAIENIED was m 3.799353 SHEU 30F 5 msmanmzsmm 3799353 sum s of, 5

. 1' CONTINUOUS CENTRIFUGE BACKGROUND OF THE INVENTION fuge.

Continuous centrifuges of this type are well known, and are'particularly frequently employed in the sugar production. It is known to utilize such centrifuges as so-called thin-layer centrifuges which usually have an internal screen. It is also known to provide protection for the screen by mounting at the outlet end of the centrifuge basketan annular plate whichv rotates with the basket and which extends in parallelism with the upper edge of the basket at the outlet end, being spaced therefrom so that an annular channel exists through whichthe material to be ejected, that is the retained solid phase of the suspension, such as sugar, can pass. This space is made large enough so that even larger lumps of mattercan pass through itjThe purpose of this arrangement is to provide a protection against rebounding, because lumps of solid matter which could rebound would cause damage or-destruction of the thin screen liner. The arrangement neither intends to nor is capable of producing a thick layer in the interior ofthe centrifuge.

However, thick-layer centrifuges are also known, with the distributor compartment for the incoming suspension being of substantially bell-shaped configuration and serving to .back up the flow of incoming suspension to.some extent, to assure that the suspension which behaves like Newtons liquid, that is like a liquid without flow limit, will always move to the centrifuge screen as a stiffened theologically inherently viscous mass, where it can form a thick layer. A constant 'supply of the suspension to the screen is assured with this arrangement, and fluctuations in different suspensions are eliminated in the Newtonian flowrange.

Another thick-layer centrifuge determines the thickness of the layer on the conical screen of the basket by a filler plate, and by varying the degree of angular inclination of the basket wall in the region where the solid phase is ejected, the ejection of the solid phase is'to be capable of regulation. Because the inclination of the circumferential wall of the basket cannot'be adjusted, it is not possible to vary the amounts of solid phase which are ejected per unit of time in this construction. Moreover, this particular centrifuge does not make it possible to produce a suspension layer of a particular thickness on the conical basket because the incoming suspension is supplied onto the screen of the basket as a Newtonian'liquid.

Further advances in the art of continuous centrifuges, particularly those for processing of sugar suspensions, are highly desirable and are very much soughflafter in the art. To properly put into perspective the considerations according to the present invention with respect to what is' known, it is desirable to consider firstly the incoming sugar suspension from a rheological point of view. It will be seen that supersaturated sugar solutions for white-sugar suspensions are'usually so boiled and subsequently so crystallized in stirring devices at slow cooling to 73-72C. that ap- 2 proximately percent by weight of crystal mass (specific weight 1.58 kgldm and 50 percent by weight of syrup (specific weight, temperature dependent, 1.4

'kg/dm are obtained. Related to the weight unit kilogram, 50 percent of sugar crystals require a volume of 0.317 dm This means that the specific weight of the suspension is 1.49 kg/dm.

The pouring weight of the sugar crystals is 0.9 kg/dm, and the sugar mass thus requires only about 57 percent of a volume of space, whereas 43 percent are hollow areas which accommodate syrup. However, only 67 percent of the total syrup weight is contained in these hollows so that 33 percent of the syrup weight is free or unbound. Since in the stirring devices the suspension is mechanically agitaged, the sugar crystals are freely floating in the syrup without any significant relative contact with one another. Such a suspension acts, from a rheological 'point of view, as Newtonian liquid, that is a liquid without flow limit. The flow speed gradient, serving as a measure of the speed differentials from layer to layer in the suspension, it is directly proportional to the product of shear stress and fluidity of the suspension, and inwardly proportional to the product of shear stress and the reciprocal value of the fluid ity, that is the coefficient of viscosity. The sum of all flow speed gradients, related to a particular flow cross section, yields the coefficient of flow, that is the median flow speed of a medium is dependent upon the cross section through which it flows.

When the sugar suspension is to be separated into its liquid and solid phases the sugar crystals are retained on a screen through which syrup can pass. This means that as i the syrup moves away the sugar crystals will Finally obtain contact withone another, support one another and no longer reduce their total volume. Given the aforementioned weight and volume data, it will be seen that this occurs when the unbound or free syrup has disappeared through the screen. The specific weight of the residual suspension is now 1.5 kg/dm, of which 60 percent by weight is sugar and 40 percent by weight is syrup. Beginning at-this particular weight distribution, the stiffened suspension has an intrinsic viscosity, from a rehological point of view, that is the speed gradient no longer increases linearly with the shear stress, but instead increases according to an exponential function with an exponent which is greater than I. In this relationship, the coefficient of viscosity and the exponent vary at constant shear stress, but in the relationship of solid to liquid phase.

' During increasing removal of liquid from the suspension the whole range of relationships comes into play, and at the transfer from the fluidity of Newtonian liquid to flowing according to intrinsic viscosity no flow limit is yet observable. Such a limit becomes increasingly observable, however, the more the suspension approaches complete removal of liquid phase. At zero speed the flow limit in effect represents an angle, the so-called angle of slide, the tangent to which is a measure for the flow limit. The angle of slide is zero in the Newtonian. liquid, that is the surface of the liquid at rest orients itself at right angles to the action of an energy gradient, that is in acceleration. Without externally acting forces a liquid is always subject to gravitational acceleration and forms a horizontal surface. In a centrifuge the acceleration acting upon the liquid can reach a substantial multiple of gravitational acceleration without changing the angle of flight. The surface of the liquid will always extend at right angle to the resulting acceleration. In the flow range of inherent velocity the flow behavior at zero speed, related to the angle of slide, is also independent of acceleration, which is understandable because the thrust, the shear stress, is directly proportional to the normal force, that is the speed gradient.

The boundary of inherently viscous flow behavior of sugar suspensions is at 98 percent by weight of sugar crystals and 2 percent by weight of syrup. If the weight of syrup drops below this point, sugar crystals act as poured material, under the influence of mass force.

A further reduction of the syrup proportion decreases the angle of slide further, until when the sugar is dry and the proportion of syrup has dropped to zero, a boundary value is reached which depending on the grain distribution is within a scatter range.

These considerations, based on rheological behavior, make it clear that in the prior art centrifuges it is impossible for a thick layer to develop which can be chosen at a desired thickness and which will flow at a desired speed. Although the baskets of some of these continuous centrifuges have a varied angle of inclination on their inner surface, the admitted suspension will always enter the permeable basket wall as a Newtonian liquid and will assume a flow speed which corresponds to its fluidity and the energy gradient resulting from the angle ofinclination of the drum wall. Because the torrential of the energy gradient is constant, the flow speed is accelerated and thus increases and leads to a thinning of the suspension layer. An additional thinning results from the increase in the angle of the drum wall. During the separation of the suspension, syrup is ejected, and the suspension is subject to an increasing flow limit, that is an increasing angle of slide, with the fluidity decreasing and the acceleration similarly decreasing.

The angle of inclination of the peripheral wall of known continuous centrifuge baskets or drums is so chosen that it is smaller than the largest angle of slide in the inherently viscous range of sugar suspensions. This should mean that the sugar layer from which the syrup has been removed cannot leave the basket, because beginning at the condition where the angle of slide ofthe suspension is greater than the angle of inclination of the basket wall, there is a retardation of the flow movement which would rapidly lead to a zero flow speed, if there would not remain a thrust component in axial direction from the beginning of the sugar layer. At the beginning in the range of purely Newtonian flow-behavior this thrust component is substantial, but decreases as the flow limit increases until it reaches zero and than becomes converted into a braking component.

If it is attempted to increase the layer thickness by adding more suspension into the basket, then it will be observed that not only the boundary of the shift to inherently viscous flow will move, but also the energy gradient will increase. Moreover, there is the danger that the following increments of the sugar layer, having a lower flow speed which may even be zero and having a lower angle of slide, will slip onto the slower-flowing previous or downstream increments of the layer, and will flow over it and ahead ofit due to the larger energy gradient. These conditions are known to those skilled in the art, and it is therefore conventional practice in the centrifuging of sugar for the sugar-centrifuge operator to so regulate a thin-layer continuous centrifuge that optimum flow conditions of the flow behavior of the total layer on the conical basket will obtain, and that only sugar will be ejected.

It is also known in the art that when other suspensions are separated, for instance dextrose suspensions,

the different angle of slide range requires a different angle of inclination of the basket wall, in order to be able to obtain the optimum flow conditions mentioned above, for the particular material.

It is further known in the art that thin-layer continu ous centrifuge baskets cannot be emptiedby continuing to operate them and terminating the supply of suspension. If this is done, the necessary thrust component will disappear and the sugar layer will simply remain stationary on the conical inner surface of the basket wall.

Given the above considerations and relationships of inherently viscous flowing on the inclined surface of a centrifuge drum, it will be understood that the throughput and degree of removal of liquid of a sugar suspension in such a centrifuge will be self-determining in the conical basket of a thin-layer centrifuge. The reason is the condition of the suspension depending upon the crystal distribution, the proportionality of solid phase and sugar solution and the temperature involved. It is therefore common practice to supply water which can be regulated as to the amount, to influence the total flow behavior as related to the constant angle of inclination of the basket wall.

In many instances the imperfect conditions in known thin-layer centrifuges are acceptable because brown sugars and other secondary and tertiary sugar products need not meet high requirements as to residual moisture and purity. in the case of white sugar, however, there are requirements made with respect to purity, ash content, the color and crystal quality, which cannot be met when known thin-layer centrifuges or known thick-layer centrifuges are employed.

A substantial difference exists between white sugar suspensions and tertiary-sugar or sugar by-product suspensions from a rheological point of view. The syrup of white sugar suspensions has a substantially lesser viscosity at a temperature of 7273C. than the molasses of the sugar by-products at a temperature of only 3540C. The greater purity of the syrup with respect to the molasses increases the fluidity of the syrup, even beyond the difference resulting from the temperature differential. This means that the change from Newtonian flow behavior to inherently viscous flow behavior occurs much more rapidly in white sugar suspensions than in suspensions of the other types, and that the change in the flow speed gradient per unit of time is greater in white sugar suspensions.

It is also to be pointed out that a centrifuge is always supplied with so-called one-boil sugar, that is sugar which has been boiled once. It is a peculiarity of this type of sugar that the crystal distribution always varies in almost identical size ranges which can be determined by screen analysis and which, as experience has shown, have a coefficient of variation between 25 and 35. However, the relationship of crystal proportion to syrup is not uniform in white sugar suspensions, and it will be understood that a shift of this relationship strongly influences the fluidity in the Newtonian flow range.

A crystal distribution, related to a median crystal size, can vary with reference to another one, the pouring rate of a one-boil sugar is almost constant. This is all the more true as in a centrifuge a very uniform packing of the crystals can be obtained and the pouring rate mentioned earlier (without the so-called free syrup) represents the boundary between Newtonian flow behavior and inherently viscous flow behavior. In the region of the pouring rate behavior, in the case of one-boil sugar, the amount of size differential in the crystal mix becomes noticeable when the sugar has pouring characteristic, that is the characteristic which a constant energy gradient, will just be capable of freely flowing through this diameter of the iris diaphram. In the course of these investigations there were examined, inter alia, two types of one-boil sugar, type A and type B. The nozzle value of type A was 5 mm. and that of type B was 3 mm. The median crystal size for type A was at 0.9 mm. mesh, and for type B at 0.55 mm. mesh. The maximum angle of slide, in inherently viscous state, equalled 34 in the case of type A and 32 in type B. V

Another comparative evaluation was drawn between these two types of sugars, the so-called area value. This particular evaluation serves for orientation purposes only; it is not a factual value, because starting from the values of the mesh width of the median crystal size it refers to the relationship of the surface of a parallelepiped to its volume. This volume of. a parallelepiped with a largest edge length equalling the median mesh width M M X 0.5M X 0.5M or a total of 0.25 M. The wetted surfa e. is 2.5M}. The relationship of the wetted surface to thevolume of the parallelepiped (sugar crystals=octahedrons) is 2.5M /O.'25M lO/M. This is an'orientationvalue which facilitates a comparison of the amount of residual liquid adhereing to the surface of the crystals under known and equal conditions, for instance under the influence of the centrifuga'l effect in the centrifuge. The area value is 1 1.1 for the sugar type A, and 18.2 for the sugar type B, of 1:1.64 at a dimension MM".

Residual moisture of liquid adhereing to the crystal surfaces, is' related to the unit weight; this' means that the sugar type B is drier than the sugar type A given residual moisture which is identical in terms of the weight.

The surface relationship of A and Binfluences the inherently viscous flow behavior in the presence of sugar syrup.

SUMMARY OF THE INVENTION It is a general object of the present invention to provide an improved continuous centrifuge.

More particularly it is an object of the present invention to provide an improved continuous centrifuge for separating the liquid and solid phases of a suspension, particularly for separating sugar from the liquid phase of a suspension. w

An additional object of the invention is to provide such a continuous centrifuge in which the dwell time of the solid phase in the centrifuge basket can be regulated at will, independent of the thickness of the layer on the interior of the basket.

In keeping with these objects, and of others which will become apparent hereafter, one feature of the invention resides, in a continuous centrifuge for separating the liquid and solid phases of a suspension, particularly in a sugar centrifuge, in a combination which comprises a centrifuge basket mounted for rotation about an upright axis and having an inner circumferential surface which conically diverges in upward direction to an upper open inlet end of the basket. A perforate liner is provided in the basket, overlaying the inner surface thereof. Admitting means admits a suspension into the basket for centrifugal separation of the liquid and solid phases of the suspension, and for retention of the latter phase as a flowable mass. An annular baffle is mounted adjacent the open outlet end and defines therewith a radially extending annular outlet gap communicating with the interior and having an outer end communicating with the exterior of the basket, for centrifugal ejection of the flowable mass from the basket through this gap. An annular flow-regulating member is rotatable with the basket for regulating the outflow of the mass through the outlet gap and has a surface which concentrically surrounds the baffle at a predetermined fixed radial distance from the outer end of the outlet gap. This surface extends axially beyond the outer end by a distance greater than the slope angle capable of being formed by the mass when centrifugally ejected throught the outlet gap and onto the surface of the member. Mounting means mounts the member on the basket for rotation therewith and for limited axial movement relative thereto and to the baffle.

Advantageously, the width of the annular gap is approximately 10 percent greater than the greatest nozzle value of the flowable mass, and the spacing between the outer end of the gap and the surface concentrically surrounding it should also be slightly greater than this greatest nozzle value.

A centrifuge so constructed assures that the flow regulating member will'cause, depending upon the contents of solid phase inthe suspension and the pouring angle, a correspondingly thick layer of solid phase on the inner surface of the perforate liner. The precisely defined annular gap permits a predetermined quantity of solid mass to pass through, which mass than contacts the surface concentrically surrounding the gap and forms a pouring angle on this surface. An axial movement of the flow-regulating member assures that,

depending upon the extent of axial movement and the frequency per unit of time, a desired regulatable quantity of solid mass, that is solid phase of the suspension, willleave the centrifuge basket.

Because the thickness of the layer can be regulated at will, the invention achieves the advantage that a precisely metered ejection of flowable or solid mass takes place, independently of the crystal size of the solid mass, for instance the sugar, related statistically per unit of time. This has not been possible with the continuous centrifuges known heretofore.

To assure a proper flow of the flowable mass into the annular gap, and to avoid dead flow zones, the invention further provides for the inner portion of the annular baffle to extend conically into the interior of the centrifuge basket.

Furthermore, the inner surface of the flow-regulating member is advantageously conically configurated, and the angle of inclination of this surface is smaller than the pouring or slide angle of all solid materials to be processed in the centrifuge. The cone angle may widen in the direction in which the material is ejected, and at the end or side facing away from this direction the flow-regulating member may be delimited by an annular transverse wall portion. Such a construction facilitates the ejection of the dry centrifuged solids to one side from a frictional point of view.

To make the shear resistance during ejection of the solids in the direction of ejection smaller than in the opposite direction the invention further suggests that the annular baffle have a larger diameter than the outlet end of the centrifuge basket itself. Furthermore, the circumferential wall portion of the flow-regulating member having the surface concentrically surrounding the outer end of the outlet gap, is advantageously provided as an annular magnetizable core and contacts an annular bead of the annular baffle against which it is pressed by at least three circumferentially distributed spring units. An electromagnet is located above the baffle and the flow-regulating member at one circumferential location, being mounted at a stationary component of the centrifuge, and the movement of the flow-regulating member in upward axial direction away from the basket is delimited by an annular abutment provided on the baffle itself. This assures that when the electromagnet is energized, it lifts the flowregulating member in opposition to the biasing force of the springs until it contacts the aforementioned abutment. Because the electromagnet is stationary and the flow-regulating member turns with reference to it, the flow-regulating member will perform relative to the basket an axial tumbling movement so that during each rotation of the basket each circumferential incremental portion of the flow-regulating member will perform the same axial movement. In other words, the flowregulating member is never moved axially in it entirety, but only increments are moved axially as they pass into and out of the range of the electromagnet.

The inclination and axial length of the centrifuge basket influences the development of a thick layer of solid phase. To make it possible to reduce the length of the centrifuge basket it is further contemplated that the basket itself and/or the perforate liner be provided with inner surfaces having different angles of inclination, which are greater than the angle of slide of the solid phase which obtains at the particular region of the basket or perforate liner, that is which is assoicated with that degree of liquid removal which has been reached by the solid at the time it moves to a particular surface portion of the basket or liner.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a graph showing the angle of slide of two sugar layers of different crystal distribution at various degrees of moistness;

FIG. 2 is another graph showing the angle of slide of the sugar layers of FIG. 1, as related to their contact with a perforate support through which moisture can escape;

FIG. 3a is a diagram showing the sugar outflow through an opening with an intercepting member beneath the opening in one position;

FIG. 3b is similar to FIG. 3a but shows the intercepting member in a different position;

FIG. 4 is an axial section illustrating the novel centrifuge according to the present invention;

FIG. 5 is a graph showing one operational principle of the centrifuge illustrated in FIG. 4; and

FIGS. 6a and 6b are diagrammatic detail views, illustrating specific details of the centrifuge, FIG. 6b being a section on line A-B of FIG. 60.

DESCRIPTION OF THE PREFERRED EMBODIMENTS I Discussing the drawing now in detail, and referring first to FIG. 4 showing an exemplary structural embodiment of the novel centrifuge, it will be seen that reference numeral 1 designates the conically configurated centrifuge basket which has a bottom wall 2. Secured to the bottom wall 2 is a drive shaft 3 which can rotate about its longitudinal axis in the diagramatically illustrated bearings 4 (one shown).

Substantially mid-way between its bottom wall 2 and its upper outlet end where it is provided with a radial flange 6, the basket 1 has a plurality of outlet openings 5 through which the so-called green syrup" is ejected. In the region of the flange 6 there is another plurality of outlet openings 7 for ejection of the so-called high green syrup. A conical baffle 8 is provided which prevents contact of the high green syrup with the centrifuged dry sugar crystals.

The drum may be provided, in a manner and at distances known per se, with concentric undercut annular grooves 5a in which the syrup is collected which flows behind the perforate liner insert 9, and'from which it passes through the outlet openings 5 to the exterior of the drum or basket 1.

The liner insert 9 is a self-supporting basket-shaped element having a bottom wall 10, a conical circumferential wall 11 and a flange 12 provided on the latter. Screen openings 13 are provided in the bottom wall 10 and communicate with one or more radial channels 14 which in turn communicate with the space IS'defined between the circumferential wall of the basket 1 and the circumferential wall of the insert 9. Screen openings 16 are provided in the circumferential wall 11 and the latter has in the lower region a lesser angle of inclination than in the upper region as shown in FIG. 4. Mounted within the liner 9, above the wall 10, is a retarding bell 17 which can be raised and lowered and which is pushed via a sleeve 18 and webs 19 onto a pin or bolt 20 of the wall 10 or the wall 2, like the german Pat. No. 1,272,229. A conduit 21 communicates with the interior of the bell 17 and supplies the suspension into the latter. A nozzle 22 communicates with a conduit 23 through which wash water can be sprayed into the interior of the liner 9, and a known device 24 is provided by means of which the thickness of the layer of sugar 42, that can form on the inner circumferential surface of the wall 11, can be regulated.

The liner 9 could also be composed of a supporting screen and a working screen, one overlaying the other and both being mounted and supported on the inner surface of the basket 1.

In any case, the two angles of inclination of the two portions of the wall 11 are so selected that they are greaterthan the greatest angle of slide of the solid phase that is to be separated in the centrifuge, that is in this case the greatest angle of slide of the white sugar types in their inherent viscosity flow range. This angle of inclination can be accommodated to the changing angle of slide of the, sugar, that is the angle as it changes with progressing removal of syrup. The angle of inclination at every point isalways greater than the greatest angle of slide of the sugar in the liquidremoval phase inwhich the sugar will be at the time it overlies the particular portion of the wall.

Due to the greater angle of inclination of the wall with respect to the greatest possible angle of slide of the sugar there is obtained an energy gradient for the flow of the sugar layer on the wall 11. Because of the fact that the outlet gap is greater than the maximum nozzle value (which term was explained before), a flow speed for the outflowing sugar 42 would be obtained, which in every case would be much to great for the desired treatment time of the sugar in the centrifuge. In other words, if the sugar were allowed to flow at this flow speed, 'it would not dwell or remain in the centrifuge for'the time necessary to impart to it the desired degree of dryness.

Before discussing further structural details of the centrifuge in FIG. 4, the following considerations should be taken into account especially with respect to the angle of slide, and for better explanation these comments will be related to the earlier-described sugar types A and B.

Y The finer-crystal sugar B is more ready to slide than the sugar A, when it is ,dry and has a small proportion of syrup admixed with it. Evidently, a small proportion of syrup in the sugar type B will mean, due to the large only a small change in the friction of the crystals with reference to one another. The same amount of proportion of syrup (by weight) in the sugar type A will cause a'thicker film on the surfaces of the sugar crystals and will reduce the friction (and thus the angle of slide) more than in the case of the sugar type B. The boundary to Newtonian flow, given an angle of slide of 0 is close in both types of sugar, but the fine-crystal type B requires somewhat more syrup .because the larger number of crystals per unit of time will have more frictional points of contact than the smaller number of crystals of the coarse crystal type A.

A further characteristc of syrup-containing sugar in the inherentviscou's flow range must be mentioned. The angle of slide refers to the degree of friction between sugar crystals. If the. sugar is on a metal surface,

the friction is less. If the sugar is maintained against sliding on an inclined surface, withthe inclination of the surface being greater than the sliding angle of sugar on sugar, than the sugar layer will not move and can be increased to a desired thickness. The angle of inclination of the sugar layer will remain constant, because if additional sugar is poured on, it will slide off and will contact the base support, forming a mound with the constant angle of slide.

.surface area of the total number of crystals involved, 1

Given these comments, reference is now made to FIG. 2 where a perforate metal support is shown, which is inclined as illustrated and on which two sugar types A and B are shown to be heaped. A dam or base support provides them from freely sliding off and they are shown in movement which corresponds to the outlet at the lower end of the dam. This movement is not free because the speed at which it takes place is not associated with the existing energy gradient, but is rather significantly less.

It will be evident from this that the layer as a whole will slide, because the individual layers will all be retained in the plane of the angle of slide, so that only the lower layers can slide on the previous more strongly inclined surface with respect to which they further haveless friction than sugar on sugar. Because the inclined surface is pervious, syrup can pass through it and as a result the angle of slide will vary. Beginning with an angle of slide of zero (where the inherent viscous flow begins) up to the vrnaximum angle ofslide of the sugar at which it is ejected from the centrifuge, the two boundaries of the sugar types A and B have been illustrated, according to the change in the angle of. slide depending upon the proportionality of the two components (sugar and syrup); this is illustrated in FIG. 1. The movement here in question takes place at uniform speed, not at accelerated speed as would be the result of the energy gradient.

For purposes of the following explanations, relating in particular to FIGS. 3a and 3b, it is also to be considered that the outlet opening at the lower end of the dam retaining the layers in FIG. 2 from freely flowing, is just large enough for the sugar to pass through in accordance with its nozzle value. This means of course that sugar type A require a larger outlet gap than sugar type B. The outflow speed corresponds to the energy gradient, reduced by the friction according to the angle of slide.

The two layers have been shown to be of approximately identical thickness, so that the speed of sliding of the layer A on the inclined surface would be greater than that of the slide B. In other words, the amount of sugar A flowing through the gap per unit of time is greater than that of the sugar B. An influencing of the speed is possible only by increasing the gap or making the layer thicker. The lower boundary of the quantity of outflow per unit of time thus determines the sugar characteristic and indifferent sugar types this lower boundary varies substantially, as can be determined from the nozzle value. It must be kept in mind in this connection that in the centrifuging of sugar, quantities of sugar must be passed out of the basket which are much too large to subject them to the necessary time required for separation and washing.

In order to obtain, despite the constant and much too large outlet gap of the centrifuge, a desired regulatable quantity of sugar outflow per unit of time, considerations were undertaken which will now be explained with reference to FIGS. 3a and 3b. FIG. 3a shows a container-C which has at its lower end an opening 0 through which sugar can flow at the largest nozzle value that is to be encountered. Located beneath the container C, extending transversely to the direction of outflow, is a slide S the bottom BW of which is inclined in one direction at a lesser angle than the smallest angle of slide of dry sugar. At one side the slide S is provided with a vertical side wall SW which extends upwardly beyond the plane of the outlet opening 0. The latter is vertically bounded to different extents, that is it will be seen that the right-hand vertical wall of the container C extends downwardly below the lower edge of the left-hand wall which is opposed to it. Sugar passing through the opening will fall onto the bottom wall BW and will form on both sides of the opening its specific pouring angle, as shown in FIG. 3a. Because the slide S is so long that the pouring angle will not make the sugar reach the end of the slide, the friction in the sugar prevents further outflow of sugar once this pouring angle has been reached, that is no further sugar will flow out through the opening 0.

If, when this condition is reached, the slide S is moved toward the right from the position of FIG. 3a to the position of FIG. 3b, than the sugar which is already on the slide S will not follow this movement due to the fact that the uneven delimitation of the opening 0 (one wall of the container C extends downwardly below the lower edge of the other) produces in ths sugar layer a shear surface which is inclined towards the displacement of the slide S and which has a substantially greater friction than if the shear surface were purely horizontal. Also, the friction of the sugar with respect to the-bottom wall BW is much less than that of sugar on sugar, aside from which the inclination of the bottom wall BW in the manner illustrated in FIGS. 3a and 31;, further reduces the friction. The receding slide S would thus yield a corresponding volume at the right which is not filled with sugar. However, this does not in fact take place because as the volume becomes available it is immediately filled by sugar which flow out through the opening 0 so that the sugar in fact never loses contact with the bounding surfaces of the slide S.

On the other hand, the existing friction assures that the left-hand portion P of the sugar must remain in position, that is that it cannot follow the slide along. Because of this it loses its support by the bottom wall BW and must slide off.

A movement of the slide S towards the left back to the position of FIG. 3a causes the sugar on the slide to move along with the same without changing its volume. The shear surface of the sugar at the outlet 0 is now inclined in the direction of the return movement of the slide S and affords only a small shear resistance. However, the friction of sugar with respect to the bottom wall BW is increased over what existed before.

Depending upon the extent of the displacement of the slide W, the frequency of displacement per unit of time, and the pouring angle of the sugar it will be understood that, in a statistical sense and related to a unit of time, a predetermined quantity of sugar will thus fall off the slide S, that is the quantity corresponding to the portion P in FIG. 3b. This means that with such an arrangement quantities of sugar can be passed and regulated which, related to the cross section of the opening 0 and the energy gradient, amount only to a small fraction of the quantity that would pass if the outlet opening 0 were not so obstructed. The quantity of sugar passing through can be continuously regulated between zero and a maximum value.

This principle as illustrated with respect to FIGS. 3a and 3b has been used for regulating the outflow of material in the embodiment of the novel centrifuge which has been shown in FIG. 4.

FIG. 4 shows that the insert 9 has a flange 12 with which there is connected an annular baffle 25 in such a manner as to define a precisely dimensioned annular outlet gap 26 between them. The outer diameter of the baffle 25 is greater, in accordance with the invention, than the greatest diameter of the flange 12 to which it is connected by means 43 FIG. 6 of several profiled connecting components which are so constructed that they permit a proper outflow of the solids through the gap without interfering therewith. If the insert 9 is replaced with the conventional dual supporting and working screens mentioned earlier, than the flange 12 can be omitted and in this case the baffle 25 is directly connected with the flange 6 and forms therewith the gap 26.

The thickness of the gap is somewhat greater, for instance approximately lO percent, than the maximum nozzle value of any solids which are to be separated in the centrifuge from a suspension. To avoid dead flow zones during outflow of the solids, the baffle 25 is provided with a conical annular portion 27 and it is further provided in the region of its inner diameter with an annular collar 28 having in its outer edge an abutment 29. In the region of the juncture of the collar 28 with the remainder of the baffle 25 there is provided an annular abutment 30.

The invention further provides for an annular flowregulating member 31 an inner cylindrical surface 32 of which concentrically surrounds the outer end of the gap 26 with radial spacing thereform. The surface 32 conically diverges in the direction of discharge, that is downwardly in FIG. 4, and at an angle which is smaller than the pouring angle of all solids which are to be centrifuged in the apparatus. At the upper end of the surface 32, that is the end which faces away from the discharge side, there is provided an annular transverse wall portion 33 which is located upwardly of the baffle 25 and whose inner periphery rests on the annular bead 30. The axial length of the surface 32 is so selected that a pouring angle 34 of the centrifugally ejected solid material (sugar) can form when the material issues from the gap 26; this is illustrated in FIG. 4. It will be seen that the flowregulating member 31 in the embodiment of FIG. 4 thus performs the same function as the slide S in FIGS. 30 and 3b.

The differential location of the lower edges of the side walls of the container C in FIGS. 3a and 3b is reproduced in the embodiment of FIG. 4 by the fact that the one edge of the baffle 24 extends radially beyond the flange l2. Particularly the outer annular portion of the flow-regulating member 31, that is the portion 33,

constitutes a magnetizable core of appropriately magnetizable material, and the inner region of the member 31 is connected with the baffle 25 by means of at least three biasing arrangements 35. These biasing arrangements are equi-angularly distributed about the axes of rotation of the centrifuge and each utilize a bolt 36, a stack of dished springs 38 and nuts 37, so that the springs press the member 31 with a precisely predetermined force against the shoulder or abutment 30. Above the member 31 there is mounted an electromagnet 39 of well known construction, which is located at one circumferential location of the centrifuge basket and is fixedly mounted, for instance on the centrifuge frame 40. An air gap 41 exists between the upper surface of the portion 33 of the member 31 and the underside of the electromagnet 39; this air gap can lifted at that portion which is located beneath the electromagnet 39 until it abuts the abutment 29. When the drum 1 with the member 31 rotates. the highest point of the member 31, that is the point where it is raised closest to the electromagnet 39, will shift circumferentially. with respect to the drum or basket 1' so that, during each complete 360 degree rotation of the basket, each point of the member'3l has performed an axial movement withrespect to the basket 1. The axial movement corresponds to an axial movement of the portion 31 between the highest and lowest points, determined by the abutment 29 and 3.0, respectively.

If the wall 11 of the insert 9 carries a sugar layer 42, than sugar passes through the gap 26 at a quantity which is determined by pouring angle and friction in the' absence of theoperation of the flow-regulating member 31. If, however, the member 31 has imparted to it the tumbling or. 'wobbling movement just described with the aid of the electromagnet 39, than sugar will be ejected through the gap 26 in accordance with the principle described above with respect to FIGS. 3a and 3b. Because the tumbling movement is delimited between two fixed abutments, the extent of axial displacement is the same during each rotation of the basket and, giving a uniform number of rotations, a constant quantity of sugar will be expelled per unit of time.

It will be appreciated that with this arrangement the dwell time of the sugar, that is the time during which the sugar will remain in the basket, is determined by the thickness of the layer and is variable by varying this thickness. The thicker the'layer, given the ejection of aconstant quantity of sugar through the gap 26 per unit of time, the lesser will be the How movement of the sugar on the surface of the wall 11 as compared to a thinner layer. 7

This movement of the member 31 with respect to the basket 1 between an upper and a lower limit can also be produced by' a stream of liquid under pressure, a

stream of air under pressure or mechanically for instance with the aid' of a force-transmitting roller, instead of with the electromagnet 39 If the amount of solid (sugar) to be ejected per unit of time is to be variable, the abutment 29. must be located high enough so that it permits a lifting of the member 31 in axial direction to an extent which is greater than the possible or' expected maximum ejection of solid per unit of time.

The electromagnet 39 can be supplied with electrical energy by an adjustable transformer (not illustrated because it is well known in the art).

The characteristic of the spring arrangemnt 38 is so selected that thecharacteristic line of the springs intersects the characteristic lines of different voltages applied to the electromagnet overthe entire control range for the electromagnet andover the entire air gap variation. FIG. shows this graphically, with the magnetic holding force via the air gap 41 between magnet 39 and member 31 acting as illustrated, given the voltage applied as the parameter. Because the magnetic force does not directly appear, but acts only indirectly as the force of a tilting movement, it is the movement and not the force which has been designated on the ordinate of the graph in F 1G. 5. The arrangements 35 produce a counter moment when themember 31 is tilted by the magnet 39. Because the characteristic of each unit 35 corresponds to that of each other unit, and because the characteristic is linear, the sum of all spring forces, that is of the counter moments produced by the springs during each phase of rotation, is equal and constant. Therefore, FIG. 5 shows the sum of all of these spring forces.

it is clear from FIG. 5 that as the voltage applied to the electromagnet is increased, the exteritto which the member 31-will be axially raised extends upwardly of the intersection between spring moment and magnetic moment of the respective voltage value, and the quantity of solid which can pass through the gap 26 can be continuously varied during operation between zero value-and the maximum value determined by the location of the abutment 29.

The thickness of the layer 42 is regulated by means of the device 24 which is well known (in german Pat. application P 27 57 475.0-23) to those skilled in the art; The swell time is shortened if this is done, but if the dwell time isto be maintained unchanged than the device 24 must be adjusted so as to provide for a lesser regulation of the thickness so that the thickness can be greater. This also can be effected without interrupting the continuous operation of the centrifuge.

Generally speaking it should be pointed out that as far as the requirement of regulation for sugar and analogous suspensions is concerned, there is a contradiction in so far as crystal mixes with large surface values require a relatively thin layer and a long dwell time, whereas crystal mixes with small surface values can be operated in a thicker layer and at shorter dwell time.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above.

While the invention has been illustrated and described as embodied in a continuous centrifuge, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of .prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within'the meaning and range of equivalence of the following claims.

- What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. In a continuous centrifuge for separating the liquid and solid phases of a suspension, particularly in a sugar centrifuge, a combination comprising a centrifuge basket mounted for rotation about an upright axis and having an inner circumferential surface which conically diverges in upward direction to an upper open outlet end of the basket;

a perforate liner in said basket and overlying said inner surface;

admitting means for admitting a suspension into said basket for centrifugal separation of the liquid and solid phases of such suspension, and retention of the latter phase as a flowable mass;

an annular baffle mounted adjacent said open outlet end and defining therewith a radially extending annular outlet gap communicating with the interior and having an outer end communicating with the exterior of said basket, for centrifugal ejection of the flowable mass from the basket;

an annular flow-regulating member rotatable with said basket for regulating the outflow of said mass through said outlet gap, said member having a surface concentrically surrounding said baffle at a predetermined fixed radial distance from said outer end of said outlet gap and extending axially beyond said outer end by a distance greater than the slope angle capable of being formed by said mass when centrifugally ejected through said outlet gap and onto said surface of said member; and

mounting means mounting said member on said basket for rotation therewith and for limited axial movement relative thereto and to said baffle.

2. A combination as defined in claim 1, wherein the width of said gap and the distance of said member from said outer end of said gap, slightly exceed the diameter from an opening through which said mass is just capable of freely flowing.

3. A combination as defined in claim 1, wherein the width of said gap exceeds by substantially 10 percent the diameter of an opening through which said mass is just capable of freely flowing, and wherein the distance of said member from said outer end of said gap slightly exceeds said diameter.

4. A combination as defined in claim 1, said baffle having a radially inner portion of conical taper which extends through said open outlet end into said basket.

5. A combination as defined in claim 1, wherein said surface of said member diverges conically in axial direction of said basket at an angle which is smaller than the angle of repose of said flowable mass.

6. A combination as defined in claim 5, wherein said surface diverges conically in the direction of ejection of said flowable mass.

7. A combination as defined in claim 6, said member including an annular wall portion provided with said surface and having two axial ends one of which is open for ejection of said flowable mass, and a transverse wall portion provided on said annular wall portion in the region of the other axial end thereof and extending transversely of the axis of rotation of said basket.

8. A combination as defined in claim 7, said upper open outlet end of said basket having a first outer diameter, and wherein said annular baffle has a second outer diameter which is greater than said first outer diameter.

9. A combination as defined in claim 7, said annular baffle having an end face facing axially away from said basket and being provided with a raised abutment, said transverse wall portion overlying said end face at least in part and normally contacting said abutment, and at least said annular wall portion being of magnetizable material; and further comprising electromagnet means mounted adjacent to said transverse wall portion at one circumferential locus of said basket slightly spaced from said transverse wall portion along said axis.

10. A combination as defined in claim 9, wherein said mounting means comprises at least three mounting units distributed circumferentially about said upright axis and each comprising spring means permanently tending to bias said transverse wall portion towards said abutment.

11. A combination as defined in claim 10, and further comprising limiting means intermediate said transverse wall portion and said electromagnet means so as to limit axial displacement of the former in direction toward the latter.

12. A combination as defined in claim 1, further comprising a plurality of equi-angularly spaced profiled portions mounting said annular baffle on at least one of said basket and liner.

13. A combination as defined in claim 1, said perforate liner having an interior circumferential surface facing inwardly of said liner and away from said inner surface; and wherein at least one of said circumferential surfaces diverges from the region of the lower end of said basket to substantially midway between said ends at a smaller 'first angle, and from substantially midway of said basket to said upper end at a larger second angle.

14. A combination as defined in claim 13, wherein said first and second angles are both greater than the angle of slide of said flowable mass.

Claims

1. In a continuous centrifuge for separating the liquid and solid phases of a suspension, particularly in a sugar centrifuge, a combination comprising a centrifuge basket mounted for rotation about an upright axis and having an inner circumferential surface which conically diverges in upward direction to an upper open outlet end of the basket; a perforate liner in said basket and overlying said inner surface; admitting means for admitting a suspension into said basket for centrifugal separation of the liquid and solid phases of such suspension, and retention of the latter phase as a flowable mass; an annular baffle mounted adjacent said open outlet end and defining therewith a radially extending annular outlet gap communicating with the interior and having an outer end communicating with the exterior of said basket, for centrifugal ejection of the flowable mass from the basket; an annular flow-regulating member rotatable with said basket for regulating the outflow of said mass through said outlet gap, said member having a surface concentrically surrounding said baffle at a predetermined fixed radial distance from said outer end of said outlet gap and extending axially beyond said outer end by a distance greater than the slope angle capable of being formed by said mass when centrifugally ejected through said outlet gap and onto said surface of said member; and mounting means mounting said member on said basket for rotation therewith and for limited axial movement relative thereto and to said baffle.

13. A combination as defined in claim 1, said perforate liner having an interior circumferential surface facing inwardly of said liner and away from said inner surface; and wherein at least one of said circumferential surfaces diverges from the region of the lower end of said basket to substantially midway between said ends at a smaller first angle, and from substantially midway of said basket to said upper end at a larger second angle.