US 3371783 A
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March 5, 1968 Filed Dec. 8, 1965 P. MEYER ETAL CENTRIFUGALAIR CLASSIFIERS 4 Sheets-Sheet 1 V 17 3 I 8 l 2 I? 37 I I. 7, I u
March 5, 1968 MEYER ETAL 3,371,783
CENTRIFUGAL AIR CLASSIFIERS Filed Dec. 8, 1965 4 Sheets-Sheet 2 i mwlgob 2 .ar mama/7M March 5, 1968 P. MEYER ETAL 3,371,783
CENTRIFUGAL AIR CLASSIFIERS 4 Sheets-Sheet 5 Filed Dec. 8, 1965 4 Sheets-Sheet 4 Filed Dec. 8, 1965 United States Patent 3,371,783 CENTRIFUGAL AFR CLASSIFIERS Paul Meyer, 19 Paget Road, London, N. 16, England, and Geoffrey Daniel Humphreys, Woodbridge, Ash Road, Hartley, near Dartford, Kent, England Filed Dec. 8, 1965, Ser. No. 512,309 Claims priority, application Great Britain, Dec. 11, 1964, 56,647/64 18 Claims. (Cl. 209-144) This invention relates to centrifugal air classifiers for ground or other solid particles of varying size and/or density.
Rotary air classifiers are known in which a stream of air or other carrier fluid is induced to flow from the periphery of a rotor spirally inwards to emerge at the eye thereof. Material to be classified is either introduced into the rotor at some position between the periphery and the eye or enters the rotor at its periphery together with the carrienfluid. In the rotor, the suspension is subjected to an interplay of centrifugal and drag-forces such that, as many as pOsSible of the particles below a required cut size, the fines, emerge from the eye of the rotor while as many as possible of the oversize or rejects are ejected at the periphery of the rotor.
To further this action, rotors are fashioned with shrouds such that the depth of the path of the air increases towards the centre, i.e., with decreasing radius, centrifugal force decreases to such a degree that, for particles of a given size, the radial drag-force component and the centrifugal force are in the same ratio over the whole flowpath between periphery and eye of the rotor. While for the fines, this ratio exceeds 1, and for the rejects it is less than one, the ratio is approximately unity for the cut size. For this reason there is a tendency for out size particles to collect in the rotor, particularly if the rotor is mounted on a substantially vertical axis and is given a flat or inward and downward sloping bottom shroud.
Furthermore, while the above constant ratio type of rotor makes a high degree of separation possible in principle, this in practice is limited by the means applied to impose the rotation on the air-stream with its dust entrained to a greater or smaller degree, as well as by means employed particularly in rinsing the rejects, to eliminate agglomerated fines therefrom.
Where the particles are introduced as a powder-flow into a constant ratio type rotor at some position between the periphery and the eye of the rotor (powder fed type), the minimum air quantity, to make efilcient separation possible will be pumped through the rotor to keep down the power input, both on pumping and on turning the rotor. With the constant ratio type rotor, which makes efiective separation possible at high dust burdens, this leads to rotors which are of low depth at their periphery, and hence are compact and easy to manufacture.
Where the particles enter the rotor periphery together with the air (air-swept type), usually coming directly from some grinding machine, the dust burden in the air is usually much less than a constant ratio rotor could handle. In order not to impose too high a pressure loss on the air-stream, the radial inward speed of the air must be kept within limits so that rotors of very considerable depth at the eye result, which are somewhat clumsy and expensive. At best, a compromise between sharpness of cut, pressure-loss of the air, and power input into the rotor, at some values less than optimum is called for.
It is the object of this invention to reduce or to eliminate the above drawbacks for constant ratio type air clas- 3,371,783 Patented Mar. 5, 1968 sifiers, respectively of the powder fed and of the airswept types.
It is a further object of this invention to provide a classifier adapted to be fed directly from air transport systems with very high powder to air ratios, such as air-slide and similar systems.
According to the present invention in a rotary air classifier having a rotor counted to revolve about a vertical axis within the casing, the rotor comprising a top plate and a bottom shroud plate and the classifying zone defined between said plates being subdivided by radial blades, the shroud plate is sloped downwardly towards the centre to permit gravity to assist in rejection of particles near cut size and not rejected by centrifugal force exceeding the drag exerted by the carrier fluid, and an opening or openings is provided at the bottom of the shroud plate for discharge of the cut-size particles.
In carrying out the invention as applied to a rotary air classifier, which is swept by the carrier fluid, the classifier comprises a cylindrical casing including an upper portion and a lower portion; a rotor mounted to revolve within said lower portion; an inner shell within the casing and having a tangentially disposed inlet for dust-laden-fluid in order to set up a cyclone efiect within the shell, means connecting said shell and rotor by which material is introduced to the rotor as it revolves; the upper portion of the casing having a central duct for discharge of carrier fluid and fines from the rotor and the lower portion having a bottom opening for discharge of classified particles; and said rotor comprising a top plate and a bottom shroud plate, the classifying zone defined between said plates being subdivided by radial blades, said zone being open at its perimeter to the lower end of the admission passage for the dust-ladernair; the shroud plate being sloped downwardly towards the centre to permit gravity to assist in rejection of particles near cut size and not rejected by centrifugal force exceeding the drag exerted by the carrier fluid and means for imparting rotation to the rotor.
This invention will now be described with reference tot he accompanying drawing in which:
FIGURE 1 is a sectional elevation of a constant ratio type powder-fed classifier according to this invention, and
FIGURE 2 is a plan view of its rotor.
FIGURE 3 is a sectional elevation of a second em bodiment of powder-fed classifier.
FIGURES 4 and 5 are a sectional elevation and a plan of air-swept classifier.
FIGURES 6 and 7 are a sectional elevation and a plan of a further embodiment of air-swept classifier.
FIGURES 8 and 9 are a sectional elevation and plan of a further embodiment of an air-swept classifier.
Referring first to FIGURES 1 and 2, the classifier comprises a rotor 2 fast on a central shaft 3, mounted to revolve inside a stationary casing.
The rotor consists of a bottom plate or table having radial blades 5, surmounted by a top plate or cowl so shaped that the cross section of the rotor chamber increases in depth from the perimeter to the centre.
Particles of material to be classified, when they have entered the rotor, are subjected to the action of centrifugal force, the magnitude of which force is directly proportional to the distance of the particles from the centre of the rotor. In existing classifiers, it has been the practice to build the rotor to Careys design (see British Patent No. 490,393) to ensure that the inward radial velocity of the fluid progressively diminishes towards the centre of the rotor table in such a manner that at all points it just balances the centrifugal force acting upon a particle of the size at which a cut is desired. In consequence the particles of the size at which a separation is required, describe circles and move neither outwardly nor inwardly. Finer particles move inwardly in spirals and are drawn away with the fluid to discharge through an orifice at the top of the casing. Coarser particles move outwardly in spirals and fall into the bottom of the casing and there withdrawn.
Slits 14 in the blades enable the powder to pass from a compartment between one pair of blades to the next where it is against subjected to the action of the air stream in the adjacent compartment.
We have now found that by providing an inwardly and downwardly directed slope in the bottom plate of the rotor that it enables the cut size particles to be separately extracted and furthermore that the classifier can be operated more satisfactorily at lower rotor speeds.
In the embodiment of FIGURES 1 and 2, the classifier comprises an annular casing having a conical lower section 22, terminating at its lower end in a discharge orifice 15. Extending into the lower section 22, is a cylindrical housing 23, within which is rotatably mounted the shaft 3 which carries the rotor.
The rotor, according to this invention, comprises a flat top plate or cowl 12 and a bottom shroud 24, which is curved downwardly towards the central axis so as to provide a central bottom opening, which is partly closed by a circular plate 26, plate 26 being spaced apart from the bottom opening so as to provide an annular discharge slot 27 for extraction of the cut size particles as hereinafter described. Instead of the slot 27, the plate 26 may be joined to the bottom of the shroud and a series of holes provided for the same purpose.
Connecting the top plate 12 to the shroud 24 are the series of radial blades 5, these blades, as in previous constructions, being formed with vertical slits so as to permit the material to be classified, to move from the compartment on one side of the blade to that on the other. Immediately above the top plate 12 of the rotor is an annular plate 21 which at its inner end connects with a duct 19, thereby enclosing the lower section 22 of the casing within which the rotor operates and separating it from the upper section 20. The duct 19 is open at its lower end to the eye of the rotor and provides a discharge for the carrier fluid and fines, its upper end opening into a volute chamber 35, from which leads an outlet pipe 39 for the air and fines that have been separated.
Extending downwardly through the chamber and duct 19 is a tube 32 which at its upper end connects with a hopper 33 for introduction of material to be classified, the lower end of the tube opening into the centre space of the rotor. Certain of the blades 5 are cut away so as to provide radially projecting entry passages 36 for feeding the powdered material into the classifying zone.
The tube 32 rotates with the rotor and a bearing 17 together with a seal 37 is provided at the top part of the casing within which the tube revolves, the hopper 33 being attached to the bearing housing.
Carrier fluid, i.e., air is admitted through a pipe 31 which opens into an annular passage 34 in the upper section of the casing above the cover plate 21. The inner wall 37 of passage 34 is constructed in the form of a volute, the in-coming air passing downwardly through a bottom opening 38 so that the air enters the rotor at its perimeter where the distance separating the top plate 12 and shroud 24 is a minimum.
By constructing the chamber wall 37 as a volute with the point of entry at the beginning of the spiral, it has the advantage that the air pressure within chamber 34 can be maintained at an even balance over the entire perimeter. Likewise the chamber 35 communicating with the discharge pipe 39 is in the form of a volute.
As can be seen from FIGURE 2, the passages 36 for entry of powder leave open a sufficient area between each passage so that air carrying the fines, which passes 4 into the eye 28 of the rotor, has an unhindered egress to the chamber 35.
To prevent any direct passage of air by-passing the outside edge of the rotor, a skirt device in the form of upper and lower, i.e., axially spaced skirt elements are provided on the shroud plate 24 of the rotor. One skirt 40 is fixed to the outer perimeter of the plate 24 and the other skirt 40' is fixed to its lower end. By arranging for the clearance between each skirt element and the casing 22 to be a minimum air-leakage past the rotor is, in practice, virtually eliminated.
In the construction according to this invention, those rejects at or near cut size which are thrown against the lower shroud and not ejected under an excess centrifugal force, move down the shroud under gravity and are ejected through the lower slot or opening 27 by centrifugal force. This has the advantage that classification can take place at lower centrifugal fields and correspondingly lower radially inward air-speeds. This permits savings in classifier drive power and on air-power which are substantial on big units with large throughputs.
Referring now to FIGURE 3, there is shown a further embodiment wherein the rotor is suspended from a hollow shaft 43 carried in bearings in the upper section of the casing, the shaft 43 supporting the feed hopper 33. The rotor, as in the FIGURE 1 arrangement, comprises a top plate 12 and an inwardly curved shroud 24, extending between which are blades 5, the eye of the rotor in this case, facing downwardly and discharging directly into a pipe 41 which curves sidewardly out of the rejects cone 22 or lower section of the casing. It will be seen that a gap is provided between the bottom opening of the rotor and the outlet pipe 41. This gap, which is similar to the discharge opening 2'7, forms a secondary outlet for particles which have been pushed against the shroud 24 by centrifugal force and/ or by gravity and are running down it due to gravity.
The provision of the skirts 40 and 40' together with the vanes 29, limits the volume of air which can bypass the rotor: Furthermore, the provision of extension blades 42, on the underside of the skirt plate 40, which serve to maintain the rotation of any such air look at rotor speed, together with the existence of the reject slot 27' makes any kind of seal unnecessary at this position.
Material to be classified enters the hollow shaft 43 through the hopper 33 and passes directly into a conical space 48' provided between a coverplate 48 and the top plate 12 and then is thrown outwardly when it falls through openings 47 in the top plate to enter the rotor, these openings being in a position about half way along the classifying zone.
The hollow shaft 43 is fixed into a conical section 48 of the coverplate 48 which, in its turn, is fixed to the top plate 12 of the rotor just outside the feed slots 47.
Vanes 49 are fixed to the top plate of the rotor beyond the perimeter of the cover plate 48, and the main casing 22 has a cylindrical rim 50 which overlies the outside edge of the vanes 49. With the casing 22 having a fairly small clearance to the cover plate 48 and the vanes 49 sweeping the space between the two restrictions, an effective seal is formed keeping down air-leakages over the top of the rotor, without involving really tight, and hence expensive, clearances anywhere.
In operation, powder to be classified is introduced by means of the hopper 33 and feed shaft 43 and thence distributed on the rotating plate 12, passing into the rotor through slots 47. With air under pressure or suction being distributed through volute 34, classification takes place in the rotor as described above, with most rejects emerging from the periphery of the rotor and falling down the rejects cone 22, through the clearances between shirt 40 and the plate 40' and the cone.
Air and fines emerge through pipe 41 while what rejects that pass down the inclined shroud will be ejected through a secondary rejects exit slot 27. No fines will find their way out through the slot because of a leakage flow of clean air past the underside of the rotor, entering exit pipe 41 through slot 27'. This leakage flow, conversely, will have insufficient speed to carry any rejects with it into pipe 41, the rejects moving outwards under centrifugal force.
To avoid air leakage between the feed hopper and the classifying zone of the rotor which, if excessive, would upset classification, the powder feed may be introduced through a rotary valve or equivalent device-not shown on the drawing.
Referring now to FIGURES 4-9, there is shown the application of this invention to an air-swept classifier. In the arrangement shown in FIGURES 4 and 5, the outer casing consists of an upper cylindrical section 75 surmounting a lower conical section 76 which is closed at its lower end by a base plate 77. The rotor comprises a flat top plate 62, radial blades 63, a curved bottom plate or shroud 67 and a bottom plate 65, the whole being mounted for rotation on a shaft 3 similar to the embodiment of the invention illustrated in FIGURES 1 and 2.
Material to be classified enters the rotor at its perimeter and the rejects pass out through the perimeter opening and through a gap 69 between the plate 65 and the shroud 67 whence they are discharged through a rejects outlet 78. Removal of air and fines is upwardly through a pipe 71 and into a volute 73.
Dust-laden air enters through an inlet pipe 79 (FIG- URE 5) which opens into a cylindrical shell 80 which operates as a cyclone and is open at the top to allow air from the centre to pass first upwardly and then downwardly through an annular passage 80" between shell 80 and the casing 75 when it enters the perimeter of the rotor as denoted by the arrows. The bottom of the shell Sil is bounded by an annular plate 68, which terminates short of the inner surface of the shell so as to provide a peripheral slot 86 for dust falling down the iner wall surface of the shell into the preclassifying zone, some air being introduced with it. It should be noted that the lower end of the shell 84 extends beneath plate 68 so as to direct the down fiow of air through slot 36 across the rotor where it encounters the main stream that descends through passage 80: this air stream causes the dust particles to accelerate radially inwards of the rotor, the dust particles, like the air already having a substantial circumferential velocity.
Sealing skirts 82 and 83 and a number of vanes 67 are provided as in the FIGURE 3 embodiment.
In operation the majority of particles are swept into the centre of the rotor although some of the heavier ones will fall down and be directed by the skirt 82 directly into the reject cone 76 and pass into the outlet 78.
In practice, it has been found that a deep rotor with a short classifying section gives the right order of airspeed to keep pressure losses low. The rejects are rinsed by the action of the air-stream on the curtain of material in the zone outside the outer circumference of the constant ratio rotor.
FIGURES 6 and 7 show a modification of the airswept classifier of FIGURES 4 and 5 in which the shell 80, which functions as a cyclone, incorporates a lower conical section 87 so as to direct the dust particles through an opening 86 and thence directly into the classification zone of the rotor through an annular opening 86 in the top plate 63 of the rotor. Drive to the rotor shaft may be by conventional means. In the embodiment shown in FIGURE 6 drive is directly from a motor M through a bevel pinion train of gears B.
It will be seen that the plate 63' of the rotor, as a result, has slightly greater radial depth and consequently the heavier rejects, together with all the dust which passes directly via 86-86 into the rotor, are rinsed in the rotor itself. It also follows that the casing as well as the inner shell can be of reduced diameter.
The air classifier shown in FIGURES 8 and 9 operates in a manner similar to that of FIGURES 6 and 7 but with the difference that the discharge pipe 71 for the air and fines is provided with valve plates or similar means in its wall providing a by-pass from the entry of the cyclone chamber 80 to the discharge pipe. In the embodiment illustrated, two Valve plates 9b are fitted to the outer perimeter of the pipe so as to permit of hinging movements about axes extending parallel to the axis and the tube so that by varying the opening of the valve plates, a greater or smaller amount of air can be bypassed directly into the discharge pipe 71. It will be noted that position of the by-pass valves W is in the upper portion of the chamber so that by-pass air will only contain superfines, the passage of which through the constant ratio rotor would only involve waste of power both on the air and on the rotor drive.
This enables the design of the constant ratio rotor itself to approximate much more to the .flat shape of the powder-fed designs. This amounts to a considerable manufacturing simplification and saving of space compared to the deep rotors of air-swept designs feeding all the air through their rotors.
If desired the valve means 90 is regulated automatically through some regulating device indicated diagrammatically at 91, for instance, so as to maintain a constant air flow through the rotor with varying pressure drops across the rotor.
What is claimed is:
1. A rotary air classifier including a casing provided with an inlet for material to be classified and a discharge outlet for classified particles, and a rotor mounted to revolve about a vertival axis within the casing, said rotor comprising a top plate and a bottom shroud plate, the classifying zone defined between said plates being subdivided by radial blad-es, said rotor having a central orifice for discharge of fluid and fines, said zone being open at its perimeter for admission of carrier fluid from the casing, said shroud plate being sloped downwardly and towards the centre to permit gravity to assist in rejection of particles of approximately cut size and not ejected by centrifugal force exceeding the drag exerted by the carrier fluid, said shroud plate having at least one central opening for ejection of the rejected particles, means for supplying carrier fluid to the casing and means for imparting rotation to the rotor.
2. A rotary air classifier including a casing provided with an inlet for material to be classified, a discharge outlet at the bottom for classified particles and a second discharge outlet for the carrier fluid and fines and a rotor mounted to revolve about a vertical axis within the casing, said rotor comprising a top plate and a bottom shroud plate, the classifying zone defined between said plates being subdivided by radial blades, said zone being open at its perimeter to admission of carrier fluid from the casing, means connecting said inlet to the rotor for feeding raw material into the classifying zone, said shroud plate being curved inwardly and downwardly towards the centre to permit gravity to assist in the ejection of particles of approximately cut size and not rejected by centrifugal force exceeding the drag exerted by the carrier fluid, said shroud plate having at least one central opening for ejection of the rejected particles, means for supplying carrier fluid to the casing and means for imparting rotation to the rotor.
3. A rotary air classifier as claimed in claim 2 comprising a rotor having a central opening, said opening being closed at its lower end by a bottom plate spaced from the bottom shroud plate so as to provide an annular slit or slits and means including a central duct open at one end to the second discharge outlet and connecting at its other end with an opening in the top plate of the rotor, through which the carrier fluid and fines are discharged and tube means extending through said duct for conveying material to be classified from the inlet in the casing to the rotor.
4 A rotary air classifier as claimed in claim 3 in which the tube means is provided at its lower end with branch passages opening in to the classifying zone.
5. A rotary air classifier as claimed in claims 2 wherein the rotor is provided with a circumferentially extending skirt device, said device having minimum clearance with the wall of the casing.
6. A rotary air classifier as claimed in claim 5 in which the skirt device comprises axially spaced skirt elements carried by the shroud plate.
7. A rotary air classifier as claimed in claim 6 in which radial vanes are provided on the outside of the shroud plate.
8. A rotary air classifier as claimed in claim 6 in which the top plate of the rotor is provided with radial blades to minimize air leakage.
9. A rotary air classifier including a cylindrical casing having a conical rejects section in which is a discharge outlet for classified particles, said casing being provided with an inlet for material to be classified and an inlet for carrier fluid, said classifier comprising a drive shaft mounted to revolve about a vertical axis concentric with that of the casing, a rotor fast on said shaft and including a top plate and a bottom shroud plate, the classifying zone defined between said plates being subdivided by radial blades, said classifying zone being open at its perimeter for inflow of carrier fluid from the casing, said bottom shroud plate sloping downwardly to the centre to permit gravity to assist in the ejection of particles of approximately cut size and not rejected by centrifugal force exceeding the drag exerted by the carrier fiuid, said shroud plate having at least one central opening for ejection of the rejected particles into said rejects section, means connecting said inlet to a source of carrier fluid and means for imparting drive to said shaft.
it). A rotary air classifier as claimed in claim 9 in which the rotor comprises a bottom shroud plate having a central opening and the casing includes an outlet pipe for fluid and fines, said pipe extending through the rejects section and registering at its inner end with the opening in the shroud plate, said end being spaced from said opening to provide an annular slot for discharge of rejects.
11. A rotary air classifier, which is swept by the car rier fluid, said classifier comprising a cylindrical casing including an upper portion and a lower portion; a rotor mounted to revolve within said lower portion; an inner shell within the casing and having a tangentially disposed inlet for dust-laden-fiuid in order to set up a cyclone effect within the shell, passage means connecting said shell and rotor by which material is introduced to the rotor as it revolves, the upper portion of the casing having a central duct for discharge of carrier fluid and fines from the rotor and the lower portion having a bottom opening for discharge of classified particles; said rotor comprising a top plate and a bottom shroud plate, the classifying zone defined between said plates being subdivided by radial blades, said zone being open at its perimeter to the lower end of said passage means for the dust-laden-fiuid; said shroud plate being sloped downwardly towards the centre to permit gravity to assist in the ejection of particles of approximately cut size and not rejected by centrifugal force exceeding the drag exerted by the carrier fluid, said shoud plate having at least one central opening for ejection of the rejected particles, and means for imparting r0- tation to the rotor.
12. A rotary air classifier as claimed in claim 11 Wherein the rotor is provided with a circumferentially extending skirt device, said device having minimum clearance with the wall of the casing.
13. A rotary air classifier as claimed in claim 11 in which the shell is spaced from the casing to provice an annular flow passage whose lower end is situated opposite to the preclassification zone of the rotor.
14. A rotary air classifier as claimed in claim 13 in which said inner shell has a bottom plate in which are openings for ejection of the rejected particles providing direct access to the preclassification zone of the rotor.
15. A rotary air classifier as claimed in claim 11 in which the inner shell has a bottom plate in which is an annular opening registering with an annular opening in the top plate of the rotor, by which dust-laden-air is introduced to the classification zone of the rotor.
16. A rotary air classifier as claimed in claim 15 in which the bottom plate includes a conical portion to direct dust particles through the opening giving access to the rotor casing.
17. A rotary air classifier as claimed in claim 11 comprising a discharge duct having valve means providing a by-pass between the cyclone chamber within said shell and the discharge duct.
18. A rotary air classifier as claimed in claim 17 in which the valve means consists of an opening in the wall of the duct, a valve plate by which the opening normally remains closed, and means for adjusting the position of said valve plate to vary the size of the opening.
References (Iited UNITED STATES PATENTS 2,199,015 4/1940 To-ensteldt 209144 X 2,276,761 3/1942 Carey 209-l45 X 2,338,779 1/1944 Mutch 209144 3,989,595 4/1963 Kaiser 2G9--144 FOREIGN PATENTS 222,989 8/1962 Austria.
FRANK W. LUTTER, Primary Examiner.
TIM R. MILES, Examiner.