US2998137A - Centrifugal screen - Google Patents

Description

Aug. 29, 1961 z, v 2,998,137

CENTRIFUGAL SCREEN Filed Feb. 13, 1959 4 Sheets-Sheet 1 5 H63 L /////1,,//// ,//L/- Aug. 29, 1961 z, VANE CENTRIFUGAL SCREEN 4 Sheets-Sheet 2 Filed Feb. 13, 1959 F/GS MFA/70K jaw [41%.

Aug. 29, 1961 2. VANE CENTRIF'UGAL SCREEN 4 Sheets-Sheet 3 Filed Feb. 13, 1959 Aug. 29, 1961 z. VANE CENTRIFUGAL SCREEN Filed Feb. 13, 1959 United States Patent Canada Filed Feb. 13, 1959, Ser. No. 793,057 6 Claims. (Cl. 209-262) This invention relates to centrifugal screens and to a method of separations performed therein. This application is a continuation-in-part of my co-pending application Serial No. 712,000, filed January 29, 1958, now Patent No. 2,963,154.

Ordinary screens with flat or curved active surfaces, depending on gravity as the main acting force, usually require some shaking or rotative movement to enhance the screening action. A considerable wear of the surfaces in contact and a further comminution of the handled materials are inevitable accompanying effects. The possibilities of varying the grain size of materials passing therethrough are either none or very reduced; to obtain a change in the grain size, one screening surface is usually replaced by another. Gravity being a fixed value, the output of such screens, per unit of surface, is limited and cannot be increased when the screened material is not propelled forcibly to traverse the screen faster. Choking and stopping of the slots is frequent, thus involving olfperiods for cleaning. This can be only partly avoided by propelling the materials through the screens by the use of a fluid carrying medium and by reducing the angle of approach at which the fine solids are presented to the screening surface. The use of a carrier makes it necessary to separate such carrier from the screened solids in a subsequent step. The resulting number of fractions is limited to two, undersize and oversize, even in the screens using centrifugal force.

This invention improves the above prior art by a new method of screening which consists in helically spinning a fluid carrying a mixture of fine solids in a vortex chamber and in separating, in a radial direction, the solid particles through a series of screening members fixed in a position adjacent the periphery of the vortex. The method is carried out in a new apparatus wherein a screening surface is circular in shape, preferably as an inverted truncated cone, and divided along the axis into sections, each of them having an individual collecting space provided 'with variable valves to regulate the throughflow of fluid therein. With this invention, any desired number of fractions can be obtained from one screening operation. The method further provides for an inexpensive way of separating a surplus of the carrier fluid from the screened product.

It is an object of this invention to provide a compact apparatus and a method for classification of fine solids according to grain size by centrifugal force wherein fluidsuspended solids are separated into a number of fractions differing in size, and wherein the size may be varied during the process without any change of screening surfaces.

It is another object of the invention to provide a device for separating the surplus of the carrier fluid, liquid or gaseous, from said solids fractions in a subsequent separation step on the discharge side of the screening members, using the energy of the primary screening.

Other objects will be apparent from the following description and accompanying drawings in which:

FIGURE 1 is an elevation partly in section of a classifier according to the invention,

FIGURE 2 shows a plan view section thereof along the line 22 in FIGURE -1,

FIGURE 3 is an elevation of the first screening member of the apparatus,

"ice

FIGURE 4 shows a plan view of another screening member,

FIGURE 5 represents a plan view of the classifier sectioned along the line 5-5 in FIGURE 1,

FIGURE 6 shows a variation of a screening member in plan view,

FIGURE 7 is an elevation partly in section of a variation of the apparatus in FIGURE 1,

FIGURE 8 represents an elevation in section of the main part of a separating attachment for threshe-rs according to the invention,

FIGURE 9 is a plan view of the structure part of FIGURE 8, and

FIGURE 10 shows an elevation of a conical core completing said attachment in FIGURE 8.

FIGURES 1 and 2 represent an apparatus for wet screening of line solids according to grain size wherein comminuted solid materials are classified in preparation for a subsequent density separation as is current practice in mining and similar industries. A liquid carrier is used and the device is designed to classify the handled materials into seven fractions, namely the undersize in five groups plus one of oversize solids which are all sink components, while the float components, if any, are separated in one group without screening along with the remainder of the carrier. This structure is assembled with five independent ring sections inserted in a housing, the screening surface being divided into shorter sleeves.

The numeral 1 1 designates a shell supporting the structure shaped as an inverted truncated cone, closed at the bottom by a base plate 12. A flange 13 welded to the shell provides a tight connection of the two members by bolts or rivets. Adjacent the base plate 12, a cylindrical inlet chamber 14 is defined outwardly by a massive ring :15 of some wear resistant material, such as hard rubber, fixed within the shell 11. The liquid carrier is fed in this chamber 14 at a high velocity by a tube 16 provided with a tangential inlet, whereby the carrier is imparted a spin, while fine solids enter the same space by a feeding pipe 17; they are introduced in the form of a slurry. An inner conical vessel 18 is mounted within said inlet chamber, co-axially therewith, extending through a major part of the length of the shell 11 to supply an additional liquid into the circuit; it is closed at its top, provided with four series of openings 19, 20, 211 and 22, and screwed to the base plate 12 by screws preferably with sunk-in heads. This vessel 18 receives the additional liquid carrier by a tube 23 fixed in the bottom plate 12. All three of the infed flows may be subjected to flow rate control by suitable valves 16, 17' and 23 of any well known type. By its outside surface, the vessel 18 defines the inner wall of a vortex chamber 24 which is bounded radially outwardly by an inverted conical screen made up of five ring sections 25, 26, 27, 28 and 29. These ring sections extend from the inlet chamber 14 to a top cover 30 closing the shell 11 tightly at its larger upper end. The upper edge of the shell 11 is flanged and the removable cover 30 is bolted thereto, as shown in FIGURE 1. In its center, the cover 30 supports a separating sleeve 311 fixed to the cover by a flange 32. In a top covering tightly said sleeve 31, another sleeve 33 is mounted concentrically therewith by means of another flange 34. The sleeve 33 is tightly closed on its top. These three member 30, 31 and 33 may be welded together to be removable along with the cover 30, and to provide a tight closing of the vortex chamber 24 at its upper end. The sleeves 31 and 33 are provided with tangential outlets 35 and 36 respectively to evacuate the oversize and the flat components with a remainder of the carrying liquid and may be subject to flow rate controls by any suitable valves.

The ring sections 25 to 29 of the screening system are separate members, inserted into the shell 11 and so designed that they all define by their inner curved surfaces the vortex chamber 24, common for all of them, while the inner surface of the shell 11 bounds their spaces outwardly. In the axial direction, the space of each of the sections 25 to 2.9 is limited by supporting plates numerals 25' to 29' shaped as annular spacers, their outer edges being flanged as shown to fit into the inner space of the tapered shell 11 at the point thereof for Which each section is designed. Due tothe conical shape of the shell 11, the only means required to secure these supporting plates in their position is the pressure exerted on the complete system of sections by the bolts fixing the cover 30 to the shell 11. Said pressure acts in the direction of the shell axis and drives the plates into the narrowing space of the shell as a wedge. The plates should be strong enough to stand that pressure, they are at right angles to the axis of the apparatus and axially aligned therewith. The central hole of each supporting plate has a diameter corresponding to that of the chamber 24 at the point for which each plate is designed. In each space between two adjacent supporting plates 25' to 29', there is fixed the acting part of a section, a screening member, numerals 25 to 29", in a position adjacent the separating chamber 24. These members may not extend over the whole distance between the two supporting plates, so the vacant space is filled up with a solid wall of some wear resistant material, such as solid rubber. Numerals 37 designate such fillers which will, in some cases, be divided horizontally into two parts to secure the screening member in a most suitable position within each section. The inside periphery of each filler 37 is shaped to conform exactly, along with all screening members and supporting plates, to the tapered form of the vortex chamber 24 which is defined outwardly by them. All the fillers 37, the supporting plates 25 to 29' and the screening members 25" to 29" form a compact column, and that part of the plates which projects outwardly of said column defines, along with the wall of the shell 11, and with the outer periphery of fillers and screening members a suitable space s for compound separations on the discharge side of each screening member 25" to 29". In radial direction, said fillers 37 are so dimensioned as to insure a stability of the column of ring sections compressed by the bolts of the cover 30 within the shell 11. The screening members are constructed with the same requirement in mind. The acting part of each screening member is set up in this apparatus with a series of partitions 38, placed in a vertical position in a circle and circumferentially equally spaced to form radial passages, slots 39. Their shape and thickness vary from section to section: in the screening member 25 shown in FIGURE 3, they consist of plates forming the laminae 38 with the slots 39 therebetween. In the following ring sections 2 6 to 29, the slots are decreasing in number and with the increasing diameter of each screen section the partitions 38 are growing in thickness so that they finally take a form of small pads. The slots are wider in order to separate coarser particles, and at the same time they are less numerous to insure a suflicient velocity to the throughflowing fluid carrier. A certain minimum velocity is necessary for imparting a spin to the fluid carrier on the discharge side of the member and to insure a suflicient margin for the flow rate control. Thus, the sum of the slots cross-section areas in each screening member is determined by the volume of fluid carrier compelled through slots at the required velocity. All partitions should be fixed between two circular washers 40 so that each screening member forms one rigid piece. In FIGURE 4, a screening member is shown in which the slots 39 are curved to better impart aspin to the flow passing 'therethrough. These slots when viewed in the chamber 24 in direction of flow shown by an 'arrow, are slanting backwards, the

4 included angle A being acute, and the spin they impart to the flow on the discharge side of the screening member, is the reverse of that in chamber 24. Except for the screen slots, the assembly of all sections presents a smooth inside taper of a funnel-like arrangement which involves a reduced friction angle for a helical flow in an upward direction. The number and dimensions of the slots is to be calculated from the entire volume of carrier liquid passing therethrough at a required velocity and coming in a radial direction from the openings 19 to 22 of conical vessel 18. These openings should be able to supply to each section the same volume of liquid as is evacuated by the same, moreover, they should be placed in front of or slightly ahead of any screening member to which they belong. They may be given a helical direction to enhance the spinning motion in the main flow by the added liquid.

All ring sections 25 to 29 are so constructed that a circular space s is provided for secondary separations on the discharge side of each screening member. In a vertical direction, each said space s, called hereinafter a secondary chamber, is defined by two supporting plates, except for section 25 bounded at the bottom by the wall 15, and section 29, defined upward by the cover 30. The slots in all screen members include an acute angle A with a tangent tg drawn to the circumference of the vortex chamber 24, as in FIGURES 2 and 4. Due to this approximately tangential direction, the liquid passing through said slots is imparted a spin when it enters said circular secondary chambers and flows along the curved wall of the shell 11. All these fine flows are thus united in the secondary chamber and, due'to said spin, are under efiect of the centrifugal force. The angular velocity of this spin will depend on the linear velocity of the flow which will be inversely proportional to the cross-section area of the secondary chamber s. A suitable cross-sectional dimension of the chamber s can be obtained when both the radial and the axial dimensions of each respective screening member and of the filler 37 have been taken into account along with the equation ruling the throughflow in vessels having a varying cross-sectional area:

Velocity As stated above, this area of each secondary chamber s, shown in FIGURE 1, is bounded vertically by the Wall of the shell 11 on one side, and by the outside periphery of both the filler 37 and of the respective screening member on the other side. The vertical height of the chamber s is that of the two last named members together. This area may be developed by varying the radial depth of both the filler and the screening member, within the limits imposed by reasons of stability of the whole column, and by varying the vertical height of the filler 37. The main purpose of the filler is to increase the spacing between two adjacent screening members so that the flow through one of them does not disturb the flow in the other. This is desirable when the radial flow through the ring sections is subject to considerable variations. Where these are negligible, the fillers 37 may be vertically reduced or omitted.

Three d-iiferent arrangements of the secondary chamber s are shown in FIGURE 1 to perform the compound separations tending to the thickening of the suspension discharged by each screening member into its'respective secondary chamber. In the sections 25 and 26, the screening members are placed adjacent to the upper limit of the section, and in the section 25 the unique filler 37 is narrowed at its base to enlarge, in theradial direction, the secondary chamber s at its bottom. This enlarged bottom space is then divided by an intercepting sleeve 41 placed vertically and welded to the supporting plate 25 which serves as bottom'to the secondaiy chamber. Its upper edge may be beveled to reduce there- 'sistance to the spinning flow. Its vertical height is chosen so that said upper edge be placed slightly below the lower edge of the screening member 25". Two compartments; concentric and adjacent in a horizontal plane, are thus created at the lower part of the secondary chamber 25, the inside one, marked i and the outside one, marked 0. Each of these compartments is drained by a series of outlet passages, numeral 42 for the inside and numeral 43 for the outside compartments. The outlet pas: sages should be equally spaced on the circumference of the shell 11 and the flow therefrom so controlled that each point of each of said compartments will be drained evenly. The function of the screen member 25 and the separating effect of the sleeve 41 would be impaired if one half of the intercepting sleeve is drained more than the other, since the fluid, cutting short its way toward the single outlets 42, 43, would not be shared uniformly by all slots 39 of the same ring section. A multiplicity of outlets 42 and 43 one section would require collecting troughs outside the shell 11, not shown in the drawings. Such necessity may be avoided and the number of the outlets reduced when the draining is regulated by a device shown in FIGURE 5. Both compartments are closed upwardly by circular covers 46, 47, in the form of annular spaces, provided with holes 48, or indented on their edges. The purpose of these openings is to increase the resistance to the inflow of separated partial flows into said compartments i and 0 at those points which are adjacent to an outlet passage, and to facilitate said inflow on points distant from said passages; by varied cross-sectional areas of said holes 48, their absorbing capacity is made substantially equal on the whole circumference of the covers 46 and 47, and the unique outlets 42 and 43 are thus enabled to drain both compartments evenly, on each point thereof.- Said covers are made of sheet-metal or a similar material, the holes or indentures are graduated from small to large so that the smallest openings are those close to the single drain, while the largest are those most remote therefrom. The covers may be sectioned to make the assembly easier. The flow discharged by the screening member 25" into the secondary chamber s being in rotation, thus under influence of centrifugal force, sub jects the separable components in suspension to a density separation. The heavy components tend to reach the inner wall of the shell 11 where they are intercepted by the compartment 0, while the clean liquid tends toward the compartment i. The proportion of the two flows depends only on the flow rate controls in the respective outlets 42 and 43. The sum of the liquid discharged by the screening member 25 depends also upon the pressure of the fluid carrier and the cross-sectional area of the openings 19 in the conical vessel 18, which liquid discharged radially onto the spinning solids in the main flow performs an efiicacious scrubbing action able to rid the coarse solids of the fine mud clinging thereto. This deslirning, as the first step of the classification according to grain size, is thus facilitated along with the secondary action in the secondary chamber s, by the fact that the radius of the spin in both actions is relatively small,

thus the angular velocity and the centrifugal pressure are relatively much higher than in the remaining screen sections. This advantage derives from the tapered form of the vortex chamber 24.

The following section 26 shows a similar arrangement of parts except for the filler 37 which is not caved. Section 27 is arranged differently: The screening member 27" is placed in the center of the vertical height of the section, the filler 37 being divided in two rings as shown, and the screening member 27" is inserted between them so that it is located in the center of the height of the section. The separated solids in this section are coarser than in the preceding two sections, they therefore tend less to be entrained upwardly by the flow since they obey more the pull of gravity and sink to the bottom of G the secondary chamber 27-s. In this section, the ce1i-' trifugal tension is less active due to the larger radius of the same, said tension may also be lessened by reducing slightly the volume of the added liquid from the vessel 18 by damping the outlets draining the secondary chamber s. The outlets 42 for the clean carrier liquid are placed at the top of the secondary chamber s while those evacuating the separated material at 43 are placed close to the bottom. To prevent solids leaving the slots 39 in the screening member 27" from being flung upward toward the outlets 42, a deflecting partition 49 is mounted at the upper edge of said screening member; it is solid with the upper washer thereof. They may be produced in one piece. The flow coming radially out from the slots 39 and rotated due to the acute included angle A, FIGURE 2, is deflected downward by said partition 49, and only when therein suspended solids take a downward direction, the spinning liquid is allowed to rise up through a gap between the partition 49 and the inner wall of the shell 11. This separation is thus performed by a combined elfect of the centrifugal force and gravity, the coarse particles being more able to overcome the viscosity of the carrier than the fine ones in section 25. When only one outlet 42 is used for the clean carrier, an extended partition 49 can be used to regulate the draining; it may either close the gap between the screening member 27" and the shell 11 completely and be provided with holes or indentures of various sizes on its whole perimeter as in the case of covers 46, 47, or, as shown in FIGURE 1, it may be trimmed so that there is left a clear narrow space between the shell 11 and the partition 49 on the whole circumference of the same, but of uneven width, thus leaving a very narrow passage for the liquid at the point adjacent said unique outlet 42, while gradually increasing in width at the more remote points from said outlet 42; the absorbing capacity of the outlets is thus extended on the whole circumference of the secondary chamber s by equalizing the resistance to the flow, and said chamber is thus drained evenly. The outlet 43 evacuating the fine solids is smaller in size and due to its distance from the screening member 2 may either work without any protection, or it may be provided with a partition 50 of a similar character as that at 49, but covering only a part of the circumference of the secondary chamber s. Such partition 50 should be fixed, preferably on the inside wall of the shell 11, thus obliging the flow coming from the nearest slots 39 of the screening member 27" to make around about way, as shown by an arrow. Section 29 is constructed without any partition in the secondary chamber s, since said chamber is more voluminous, it is handling more coarse solids which show no tendency to rise upward, since the flow is slow due to the large volume of the chamber, and the centrifugal tension is lower therein because of the slower flow and of the larger radius of rotation. The conical vessel 18 may' be without holes adjacent to the section 29, but extends to the level of the cover 30 to fill up the central space of the chamber 24 as a core.

In operation, the carrying liquid is introduced tangentially under pressure by the tube 16 into the inlet chamber 14 and subjected to a spinning motion. The fine solids, which have already been subject to some preliminary screening as in a grinder, are pressed into the same chamber 14 by the feeding pipe 17 as a slurry in such a quantity that the suspended solids just cover in a fine layer the inside surface of the screen 25". The contact therewith need not be long to obtain the expected result when the draft through the slots 39 is strong enough. The additional liquid is supplied under pressure into the vessel 18 by the tube 23. The fine mud is separated first in the section 25- through the slots 39 by acombined effect ofthe centrifugal'tension and the lateral flow in the radial direction coming from the openings 19 in the vessel 18. The suspended mixture of the remaining solids then continues its helical travel along the vortex chamber 24 to meet successively the screening members 26" to 29", the slots 39 of which are set up by a correct flow rate control of all participating inlets 16, 17 and 23, by variable valves 16, 17" and 23', and of all outlets to receive particles, the size of which increases from section to section. The particles are kept in contact with the screening surface by the centrifugal force combined with said lateral flow. Some incidental float particles, lighter than the carrying liquid, are driven to the wall of the vessel 18. The separating sleeve 31 will intercept the oversize of the sink components, while the sleeve 33 evacuates the float particles and the surplus of the carrying liquid. Due to the relatively low angular velocity of the flow and to the pressure at which the process is maintained by a proper volume control of all inlet and outlet openings, and to the presence of the vessel 18, no vacuum is formed in the central layers of the chamber 24.

In this separation, the volume control of the intake and of the outgoing partial flows are the main adjustments for controlling the grain size of solids passing through the screening members, and any change in the flow passing therethrough will involve a variation in the size of solids carried through the slots. The size of the slots need not vary from one section to the other, yet, when the other control means have been applied properly, the screened solids will show a different size from section to section. The fluid in the vertical flow is driven toward the outer wall of the chamber 24, and fine peripheral layers thereof are removed by the multiple slots 39. The depth of the layers removed in each ring section depends upon the volume control in the outlets 42 and 43, wherein suitable valves regulate the draft. The removed layers contain the fine solids and the readiness of each particle to enter a slot will depend upon its mass, thus mainly on its size. Said valves thereby perform a function of variable screen slots, although the size of said slots is constant. Consequently, the grain size of the product may be adjusted during the operation, or even changed, which may be an appreciable advantage. On the other hand, the screening effect of the screen is not affected by the varying composition of the handled mixture, since a solid particle cannot enter a screen slot, unless the absorbing capacity of said slot is set up to receive it. Due to inertia, the higher the speed of a suspension flowing past the screen slots, the more difficult will it be for a solid particle to change its direction and to pass from the whirling fluid column into a slot. The cross-section area of the vortex chamber 24 increases in the direction of flow due to its tapered form, and the linear velocity of the flow therein, being inversely proportional to said area, is slowing down progressively. This slowing down is desirable for separating coarser and heavier components. The vessel 18 supplies only that volume of fluid which is removed in the separated partial flows. The small particles passing from the spinning fluid into a slot will change their direction easier than the large ones because of the smaller momentum they possess. The screens according to this invention perform satisfactorily if the respective densities of the constituents in the treated mixtures are not sharply apart from each other. When they do differ considerably, either an allowance should be made for the grain size within the classes, or it would be advisable to operate the screening in two or more stages, thus avoiding too high density differences of the components in any one operation.

Besides the size of the slots 39, the included angle under which a solid particle must change its direction to enter a slot becomes an element of impedance that may be made variable, thus an important means of control of the grain size. In any one of the sections 25 to 29 of FIGURE 1, the variable angle control may be applied. When this variability is desired during the process, its elements, placed insidea. secondarychamber s, are operated by a shaft turning, or by a lever swiveling in a bearing fixed on the shell 11. Such a movement is very short and the link or joint can be made impermeable by a flat piece of rubber fixed to the two parts, i.e. to the shell 11 and to the lever. In the FIGURE 6, a-series-of directing vanes 51 suspended by journals 52 on two washers 5-3 and 54, which are fixed in a ring section, can be swiveled by a narrow ring 55, made from apiece of sheet-metal and provided with holes 56 in which another series of journals 57, fixed on the vanes 51, can pivot. The holes 56 are oval to allow the journals 57 a short spiralling movement. A lever 58 suspended in the shell 11, by a bearing 59, controls a short rotative movement of the ring 55 to which it is linked at 60, thus swiveling the whole row of the vanes 51 into a new position marked by dotted lines, the vanes thus providing a changed'angle B with the tangent tg drawn to the helical path. The rubber packing 61 is fixed both to the shell 11 and to the lever 58 respectively, by a washer 62 and by a metal strip 63.

When the directing vanes 51 pivot in the slots, the shape and the cross-sectional area of the slots are changed along with the angle A, thus modifying the velocity and the volume of fluid flowing therethrough. The crosssectional area of the slots may be varied without altering the included angle A, when the screening member is formed by two complementary belts of sheet metal or a similar material, punched to produce corresponding spaced apertures. The material punched is bent to form the desired tangential angle with the remainder of the belt. One of the two belts is fixed in position, while the other is maintained in a sliding contact with the first one and movable. The aperture size in the whole screening member may be varied by moving the sliding belt. The limits of movement of the movable belt are set by the projecting vanes of the two belts. Projecting vanes may be provided on one of the two belts only, namely on the discharging side of the screen, while the other, on the receiving side, is punched with plain holes. Finally, both belts may have plain holes without any projecting vanes.

FIGURE 4 represents a classifier differing in some details of construction from that in FIGURE 1. The apparatus is constructed without any continuous outer shell, the five screening members are all one piece of sheetmetal or plate, rolled up and welded in one truncatedcone, screen 64. Holes are punched at five different 10- cations to form five screening members. If the punched material cannot be bent outwardly to form short outlet passages, a belt can be added wherein the punched material is bent so as to have the desired included angle A with the tangent drawn to the helical path of fluid in the vortex chamber. The "vessel 18 is a conical hollow standard supporting the cover 30, bolted thereto. The cover 30 may be reinforced by a beam in the form of a cross or by similar method. Said cover is bolted on its perimeter to a flanged edge of the screen 64, which is made of material strong enough to support individually constructed secondary chambers 65. Each secondary chamber is built separately and may be slipped onto the tapered screen 64 when the lower edge thereof, flanged outwardly, is narrow enough, as shown in FIGURE 7. A flange 66 fixing it to the base plate is removable, made up in sections. Each secondary chamber s may be extended in a radial direction, thus making possible a number of modifications. One of them is shown by way of example.

The screening member 64", produced by punching the screen 64 in a series of radial passages, is enveloped on the outside by a housing 67, fitting on the tapered screen 64. A flange 68 encircling the screen 64 as a hoop is fastened to the screen by screws, or rivets or spot welding; it may be sectioned, and not covering the whole circumference of the screen. The housing 67 is. screwed to this flange 68 by screws pulling indirection of the axis of the apparatus, thus producing-a tight conacce e? 9, nection due to the taper of the screen. A fine strip of rubber may be put between the two surfaces. Outwardly, a secondary chamber s is preferably shaped as a truncated cone, and provided at its bottom with outlets 69 evacuating the separated solids, and outlets 70 for the clean carrier liquid. A horizontal partition 71, shaped as an annular space, and extending radially from the screen 64, is fixed thereto by screws just under the screening member 64". The slots may be ground in the screen 64 at a desired angle, or a belt with directing vanes may be placed, supported by said partition 71. The slots of the screening member 64" being cut in an acute angle, they impart the liquid passing therethrough a spinning motion in a reverse direction to that in the main separating chamber. Such angle protects the screen against stopping. If the screened material is such that it does not choke the slots, this angle may be obtuse and the spin in the secondary chamber has the same direction as the spin in the chamber 24. At a given velocity, the increased angle increases the readiness of particles to enter the slot. The partition 71 directs the spinning carrier towards the outside periphery of the chamber s, thus preventing it from cutting short its way toward the outlet 70. Due to the centrifugal tension produced by said spin, the fine solids are driven to the periphery of the chamber s until they are evacuated by the outlets 69, while the clean carrier is obliged to reach in the radial inward direction the outlets 70, located adjacent to the screen. The centrifugal action keeps the solids away from said outlets 70. Suitable flow rate control valves placed in the outlets 69 and 70 determine the proportion of the two partial flows. In FIGURE 7, only one outlet of each kind is shown at each screen section. The secondary chamber s may be constructed large enough to avoid a too great irregularity in draining. If desired, the draining may be controlled by increasing the number of outlets 69 and 70 for each section, or by using regulating covers as shown in FIGURE 5, except for their position and shape, since they should be placed vertically as sleeves in front of the outlets 69 and 70. In this variation, one intercepting sleeve 32 with its outlet 34 is used, the other details being the same as in FIGURE 1.

In the FIGURES 8, 9 and 10, another appliaction of the invention is shown, using a gaseous carrier. It is a separating attachment for a thresher. Its purpose is to finish the threshing operation by separating all remaining kernels and all kernels bearing ears from the coarse straw. In a modern thresher, the concave grating within which the cylinder rotates extracts nearly 90 percent of all kernels from the straw; the operation is then followed by a separation on flat screens which are in a continuous back-and-forth movement accompanied by a current of air. In said concave, the grain is moved by the cylinder at a speed of about 50 to 75 feet per second on a semi-circular path and loses most of its kernels. On the flat screens, it moves about 2 or 3 feet per second to finish the separation. Because of this slowing down,v

the screens handle a thick layer of straw, while in the thresher concave the grain passes in a very dispersed order. This dispersion is the reason why the centrifgual force is able to separate so quickly and easily most of the kernels.

This invention tends to maintain very high the velocity imparted to the grain in the concave so that the mixture produced by the cylinder is conveyed in a dispersed order over a grate similar to the concave, and subjected simultaneously to centrifugal force. The effect of the centrifugal action is increased by an artificial aid draft through the screen slots. The screens in a standard thresher are often required to be exchanged, since they handle materials of difierent density and qualities. In this invention, this exchange may be dispensed with in most cases because the variable air draft provides a suflicient compensation. This advantage of adjusting a screening effect by varying an air blast will be especially appreciated in the threshing operations. Plugging of the screens in a standard thresher is frequent due to the low velocity of mate; rials passing through them; the herein disclosed screen-, y.

FIGURE 8 shows an elevation in section along a vertical axis of the centrifugal screen for threshers; FIGURE 9 is a plan view of the same, FIGURE 10 is an elevation of a core defining a helical path in said screen. The attachment is assembled with two separate parts: a funnel 72 and the core 84. The funnel 72 defines a separat ing chamber 73; it is made of sheet metal and two screening members 74 and 75 are inserted in its tapered wall.

They define one continuous helix. On the discharge side of each member 74, 75, a secondary separating chamber is fixed, numeral 76 and 77 respectively. They are sealed 011' from each other by -a wall 92 shown in dotted lines in FIGURE 9. The funnel 72 is provided with a conduit 78 for evacuating the coarse straw; the chamber 77 has a conduit 79 for evacuating the kernel bearing ears, and the chamber 76 has a conduit 80 for the fine dirt and an outlet 81 for the clean kernels. provided with air flow rate controls by suitable dampers of any well known type. The screening member 74 and 75 form one belt along a helix in the Wall of the funnel 72, each of said members being constituted by a series of partitions equally spaced and producing suitable slots, the center line of which includes an acute angle with a tangent drawn to the curve of said helical path. If viewed in the direction of the flow, said partitions slant backwards as the slots in FIGURE 4. Each partition in the member 75 may be produced by cutting the material through on three sides of a rectangle and bending said material along the fourth side. The device thus produced is similar to a Venetian blind, the slots being about one inch and one half wide. The member 74 contains finer slots, about one inch wide. It can be made when a continuous helical slot is cut out in the wall of the funnel 72, and in this slot suitable strips of metal fixed by riveting or welding. These strips would be bent and/or twisted on their ends so that the slots thus produced include the desired angle with the periphery of the main separated chamber 73. On the discharge side of the member 74, within the housing for the secondary chamber 76, a deflecting partition 82 is;

provided to prevent the kernels from entering the inlet-of the conduit 80 evacuating the air with some fine dirt. The kernels are collected in the outer corner of the sloped bottom of the chamber 76 to be evacuated by the outlet 81. Finally, a tangential inlet 83 for the material to be screened is attached to the funnel 72, for directing said material toward the first partitions of the member 74.-

The helical path, limited outwardly by the belt of the two screening members 74, 75, is further defined by the removable core 84, shown in FIGURE 10. It is a hollow body shaped as an inverted cone closed on its base by a cover and bearing outside on its curved wall two helical vanes 85 and 86, defining in the separating chamber 73 said helical path-in the direction of the radius, while inwardly said path is defined by the curved wall of the core- 84. The vane 86 is straightened at 87 to suit into the tangential inlet 83. The two vanes 85 and 86 may be joined by a belt closing tightly the idle space between them, thus forming one thread of a conical screw, unless said space is used for a second helical path, parallel with the first one, and equipped in a similar way as the members 74 and 75. If left idle, said space is closed upwardly by the cover of the core 84. The width of the active spacing between the vanes 85 and 86 should be the length of the laminae in the two screening members 74, 75, and

the apex angle of the cone should be approximately that of the funnel 72. To vary the air blast through the slots of the member 75, the curved wall of the cone is provided with fine holes 87 in the area adjacent said member.

All these conduits are 75' and the cover of the cone with an additional flow rate controlled air pipe 88. To find quickly the right position of the core in the funnel 72, a pin 89-is fixed in the core 84 and a slot 99 is provided in the funnel 72. Some clamps 91, of any suitable well known kind, insure the respective position of the two parts against the air pressure. The three separating chambers with their respective conduits have, of course, no direct communication with the outside air.

In the operation, the material to be screened is propclled by the pressure of a thresher tan, not shown, through the'inlet 83. A combined action of the centrifugal tension produced by the spin on the helical path and of the air blast through the slots due to a correct flow rate control of all outlets, separates the finer particles on the finer slots of the first screening member 74. The passage of air through this first screen is regulated by the volume control placed in the conduit 80' for the fine dirt, and in the outlet 81 for the kernels. In the second stage, the kernels bearing ears are separated on the member 75 from the coarse straw, and evacuated by the conduit 79 from the secondary chamber 77 for rethreshing. The blast is regulated here by the flow rate control in said conduit 79, and, if desired, by -a volume control placed in the air pipe 88, said air producing a lateral pressure in the helical path, so that whatever the material handled may be, every valuable particle may be intercepted by the member 75; Due to the constant loss of air by draft through slots in the member 74, the linear velocity of the flow is decreasing by the time the suspended material reaches the member 75; this would result in a loss of pressure by centrifugal action which, however, is partly compensated for by the increased angular velocity due to the decreasing radius of the spiral path. A further compensation is thus achieved by the added air through fine holes 37. It will be noted that this added air need not penetrate through slots in member 75 into the chamber 77, but, on the contrary, it will flow toward the outlet 78 along the spiral path and enhance the flow velocity there. The direction it takes depends mainly on the flow rate controls in the outlets 78 and 79. The velocity of the screened'material along the spiral path must be higher than the velocity of the draft through slots if choking of these is to be prevented. The air entering from holes 87 presses onto the suspended material and increases the friction against the partitions whereby the finer part of the mixture, the kernels bearing ears, are checked in their speed to be collected by the slots. The cars diifer far less in size and density from the coarse straw than the freekernels and need to be helped in separation by a correct proportioning between the two flows: that along the helical path and that through the slots of member 75. On the other hand, the free kernels are very fine and heavy, they are pressed much more toward the member 74 whereby they lose quickly most of their speed by friction and are, therefore, ready to enter the slots. The draft through slots of the member 74 aims at separating also the fine light dirt and-chaff into chamber 76 so that said fine particles are prevented from being collected by the member 75 with cars which must go back to the thresher. The remaining coarse straw is evacuated by the conduit 78. This is facilitated by the fact that in the modern threshers the straw is cut in short pieces before threshing, and the combines in the open fields handle, at the threshing operations, ears with short straw only. In the secondary chamber 76, the mixture of fine dirt with the kernels, entering by the slots of the member 74 is projected by a turning motion toward the outside wall, and the heavy kernels are collected on the sloped bottom thereof outwardly, while the fine dirt is evacuated beneath the partition 82 in an inward radial direction by the conduitsil. The kernels leave the chamber 76 by the outlet 31. to be subjected, alongtwith those separated on the concaveof the thresher, to adensityseparation, which isthe object-of acoepending applicationforpatent. The

core 84 being removable facilitates the inspection of the screen.

I claim:

1. An apparatus for centrifugal. screening, comprising in combination: a series of ring sections fixed in a position axially adjacent and axially aligned to each other and defining a symmetrical continuous m-ainvortex chamber common :for all said sections, said vortex chamber having a fixed inlet chamber and an outlet portion axially opposite to each other in said vortex chamber, said inlet chamber having attached thereto a feed tube provided with a tangential inlet for a carrier enabling said carrier to be imparted a spin, and a feed pipe for a separable material, flow rate control means in said feed tube and in said teed pipe; said main vortex chamber having. a diame-ter increasing progressively from said inlet chamber to said chamber outlet portion, each of said ring sections,

having an inner wall bounding said main vortex chamber provided with a screening member, each screening member in said series of sections havinga receivingside radially, adjacent to said main vortex chamber, each ring section being further provided on the outer periphery thereof with a closed secondary chamber, each of said secondary chambers being in communication with a discharge side of only one screening member but sealed ofl from adjacent secondary chambers; means in said screening members to discharge primary separated fractions of said carrier into said secondary chambers, means to evacuate said fractions outwardly and means to-control the flow rate of each of them, and means in said vortex chamber outlet portion to discharge the surplus of said' carrier and oversize components of said separable material.

2. The apparatus of claim 1 wherein said screening members are provided with slot-s the centerline of which includes an acute angle with the tangent drawn to the periphery of the main vortex chamber so that partitions defining said slots are slanting backwards when viewed in direction of the flow, said angle being a measure of'protection against blocking said slots by materials treated.

3. The apparatus of claim 1 wherein said screening members are provided with means to discharge said primary separated fractions into said secondary chambers in a substantially tangential direction thus impartingthem a spin, means in said secondary chambers to sub-separate centrifugally said primary fractions into secondary fractions, and means to evacuate said secondary fractions outwardly while controlling the flow rate of each of them.

4. The apparatus of claim 1 wherein separate collecting spaces are provided 'for said secondary fractions, drained by a reduced number of outlets, said collecting spaces being provided with drain regulating covers evacuating said secondary 'fractions evenly along the circumference of each collecting space.

5. The apparatus of claim 1, provided further with an inner vessel-positioned centrally in said main vortex chamher and provided with openings substantially opposite to said screening members to supply an additional fluid carrier in a radial outward direction, means for supplying.

said additional carrier into said vessel and means to control the flow rate of said additional carrier.

6. A separating attachment, for threshers including; a funnel shaped housing for a main separation chamber; a helical path in said funnel with a screening member therein to screen coarse materials discharged by a thresher, said member being divided lonigtudinally in two parts, the first part separating kernels from the coarse straw, the second part separating kernels bearing ears,

each said part of said screening, member communicating with one of two secondary separation chambers fixed on the discharge side of said tunnel to collect and to further separate such separated material; one evacuating conduit attached to said tunnel for said coarse straw; one conduit attached to thesecondof said secondarv chambers 13 for evacuation of said ears; two conduits attached to the first of said secondary chambers for evacuating respectively said kernels and a surplus of air with some incidental fine di-rt; a partition dividing said latter secondary chamber; a hollow core with helical vanes attached thereto to define said helical path Within said funnel, said core being provided with a tight cover and with a duet therein for an additional air supply to enhance the separation in the second part of said screening member dealing with said ears; a series of fine openings on said core at that portion thereof which faces said second part of said screening member; a feeding channel connecting said thresher to said funnel to supply said material to be 14 screened in suspension with an air flow; a flow rate control means in each of said outlets and inlets to regulate the volume and the velocity of the air flow inall parts of the attachment, thus controlling the quality of separated products.

References Cited in the file of this patent UNITED STATES PATENTS 2,855,099 De Koning Oct. 7, 1958 FOREIGN PATENTS 800,745 Netherlands Sept. 3, 1958

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