US3371782A

US3371782A - Centrifugal air classifiers

Info

Publication number: US3371782A
Application number: US512308A
Authority: US
Inventors: Meyer Paul; Humphreys Geoffrey Daniel
Original assignee: Meyer Paul; Humphreys Geoffrey Daniel
Current assignee: GEOFFREY DANIEL HUMPHREYS
Priority date: 1964-12-11
Filing date: 1965-12-08
Publication date: 1968-03-05
Anticipated expiration: 1985-03-05
Also published as: GB1114314A; DE1507683B2; DE1782775A1; US3371783A; DE1507683A1; AT281566B; DE1782775B2; DE1782775C3

Description

March 5, 1968 p, EYE ETAL 3,371,782

' CENTRIFUGAL AIR CLASSIFIERS Filed Dec. 8, 1965 2 Sheets-Sheet 1 31 5: x a; .51 K 57 March 5, 1968 Filed Dec. 8, 1965 P. MEYER ETAL CENTRIFUGAL AIR CLASSIFIERS 2 Sheets-Sheet '2 United States Patent Office 3,371,782 Patented Mar. 5, 1968 3,371,782 CENTRIFUGAL AIR 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. 512,308 Claims priority, application Great Britain, Dec. 11, 1964, 50,647/64; Sept. 8, 1965, 38,374/65 8 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 pe riphery 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 carrier-fluid. 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 flow path 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 cut 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 efficient 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 effective 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.

It is the object of this invention to reduce or to eliminate the above drawbacks for constant ratio type air classifiers of the powder fed type.

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 this invention in a rotary air classifier comprising a casing and a rotor mounted to revolve therein and wherein the rotor has a top plate and a bottom shroud plate and radial blades extending between the two plates, the shroud plate is sloped downwardly and outwardly to permit gravity to assist in rejection of the particles at or near cut size and not normally rejected by centrifugal force exceeding the drag exerted by the carrier fluid and means is provided for introducing material to be classified into the rotor in a direction such that it is thrown by centrifugal force across the air stream through the classifying zone.

Material to be classified may be introduced to the rotor by means of a feed duct having a central point of discharge whereby the particles are subjected to the action of centrifugal force before entering the classifying zone.

Preferably the rotor has a downwardly inclined top plate and the bottom shroud plate is in two sections which are spaced apart to provide an entrant slot for introduction of material to be classified.

The invention is illustrated in the accompanying drawings in which:

FIGURE 1 is a sectional elevation of a constant ratio type powder fed classifier in accordance with the invention.

FIGURE 2 is a sectional elevation of a classifier having design of casing to enable the raw material to be introduced by means of an air slide and provided with a rotor designed to accommodate the increased air flow.

Referring to the drawings, the classifier comprises a casing indicated generally at 22, mounted to rotate within which is a rotor 2, the rotor 2 being fast on a central shaft 3 which is journalled in bearings in a housing 23 extending upwardly through the conical rejects section of the casing 22.

The rotor consists of a top plate 52 and a bottom plate, the latterbeing in two sections 534, of which the outer section 4 is curved outwardly and downwardly, being formed integrally with a horizontal plate 54', by which the rotor is secured to the shaft 3. The inner section 53, whose lower perimeter is spaced from the upper perimeter of section 4 to form an annular opening 58 is integral with the lower end of the duct 32. In place of the single opening 58 access may be through a series of slots.

The plates 52 and 4-53 which make up the rotor are joined by a series of radial blades 5. The top plate 52 is curved outwardly and downwardly and it will be seen that the cross section of the rotor blade is such that the chamber increases in depth from the perimeter to the centre. Particles of material, when they have entered the rotor are submitted to the action of centrifugal force operating in opposition to the incoming air stream, the magnitude of which force is directly proportional to the distance of the particles from the centre of the rotor.

Particles to be classified enter the rotor chamber through a hopper 33 whose lower end opens into the axially extending duct 32. Discharge of the rejects is through a discharge orifice 15 at the bottom of the rejects section 22 while the separated fines and air collect in the central space of the rotor chamber and then pass upwardly and out at the top into a volute chamber 35, having a discharge pipe 36.

Air is admitted through a feed pipe 31 which opens into an annular chamber 34 above the cover plate 21 of casing 22 which is joined by conical section 51 to the underside of volute chamber 35. The inner wall of chamber 34 is constructed in the form of a volute so that the air pressure is evenly balanced. Opening out of the bottom of chamber 34 is a cylindrical passage 57 provided by a flange plate 57 whose lower end terminates immediately opposite the inlet to the rotor. Radial Vanes 52' spaced equidistantly around the top plate 52 reduce leakage of air over the top of the rotor.

It is a feature of this invention that by extending the feed duct 32 for the material to be classified to the bottom of the rotor chamber, it enables the articles to be introduced in an area which coincides with a central section of the classifying zone.

In the embodiment illustrated, where the duct 32 connects with the inverted conical section plate 53'; the plate 53' which is spaced from the bottom plate 54 of the rotor, provides the boundary wall of a radially extending flow passage, through which the incoming material passes finally entering the interior of the rotor through opening 58 at its perimeter.

By introducing the particles at a central point it secures the advantage that they are thrown by centrifugal force across the air stream from below and at an acute angle with the result that the spreading of the dust is more effective. Cut size particles may rebound from the top plate and under gravity pass down the sloping plate to leave the periphery of the rotor with the rejects. Such impact also causes some of the ultra fines adhering to the rejects to splinter off, which is important in rinsing the rejects.

Referring now to FIGURE 2, there is shown an arrangement similar to that of FIGURE 1 but having a piped connection 60 for a fluidized feed leading directly into the duct 32.

The rotor comprises a bottom plate 54 which is modified in that it extends radially outwards in a horizontal plane for a substantial distance and beyond the entry slot 58, the perimeter 4 being at an angle instead of being curved as in FIGURE 1.

It will be seen therefore that inwardly of the entry slot, the cross-sectional area of the rotor has a stepwise or sudden increase in depth, the purpose being to accommodate the additional air flow caused by the fluidised feed, the air flow resulting from which constitutes a substantial air leak into the rotor.

This stepwise increase is brought about by the inner section 53 being out of line with the outer section 54.

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 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 for admission of carrier fluid from the casing, means for introducing material to be classified into the rotor in a direction such that it is thrown by centrifugal force across the air stream in the classifying zone, the fluid and fines discharging through a central orifice in the rotor and the shroud plate being sloped downwardly and outwardly 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, 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 material into the classifying zone and so arranged that the material on entering the rotor is thrown by centrifugal force across the carrier fluid stream passing through the classifying zone, said shroud plate being sloped down- 4 wardly and outwardly to permit gravity to assist in the rejection of particles near cut size and not rejected by centrifugal force exceeding the drag exerted by the carrier fluid, means for supplying carrier fluid to the casing and means for imparting rotation to the rotor.

3. 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 introduction of 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, means including a duct connecting the feed inlet for material to the rotor and so arranged that on entering the retor the material is thrown by centrifugal force across the stream of carrier fluid passing through the classifying zone, said bottom shroud plate sloping downwardly and outwardly 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, means connecting said inlet to a source of carrier fluid and means for imparting drive to said shaft.

4. A rotary air classifier as claimed in claim 3 comprising a rotor wherein the bottom shroud plate includes inner and outer sections spaced apart to provide an entrant slot to the classifying zone and wherein the inlet duct for material unites at its lower end with the inner section so that incoming material enters the rotor beneath the inner section of the shroud plate.

5. A rotary air classifier as claimed in claim 4 when used for treatment of the material in a fluidized condition characterised in that the inner and outer sections of the shroud are axially spaced so that the flow section of the rotor inwardly of the entrant slot for raw material has a stepwise increase to accommodate the additional air flow from the fluidized bed.

6. A rotary air classifier as claimed in claim 3 comprising a rotor having a downwardly inclined top plate.

7. A rotary air classifier as claimed in claim 3 in which the inlet for material to be classified comprises a feed hopper on the casing, an axial duct open at one end to receive material from the hopper and at its other end discharge into the rotor and in which the bottom shroud plate is in two sections, the inner section being attached to the said duct and the outer section being fast on the motor shaft and spaced from the inner section to provide an entry slot for material to be classified.

8. A rotary air classifier as claimed in claim 7 in which the outer section of the shroud plate comprises a plate that lies radially to the rotor shaft and combines with the inner section to provide a passage leading from said duct to the entry slot of the rotor.

References Cited UNITED STATES PATENTS 2,199,015 4/1940 Toenfeldt 209144 X 2,276,761 3/1942 Carey 209144 2,338,779 1/1944 Mutch 209-144 FRANK W. LUTTER, Primary Examiner.

TIM R. MILES, Examiner.

Claims

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 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 FOR ADMISSION OF CARRIER FLUID FROM THE CASING, MEANS FOR INTRODUCING MATERIAL TO BE CLASSIFIED INTO THE ROTOR IN A DIRECTION SUCH THAT IT IS THROWN BY CENTRIFUGAL FORCE ACROSS THE AIR STREAM IN THE CLASSIFYING ZONE, THE FLUID AND FINES DISCHARGING THROUGH A CENTRAL ORIFICE IN THE ROTOR AND THE SHROUD PLATE BEING SLOPED DOWNWARDLY AND OUTWARDLY 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, MEANS FOR SUPPLYING CARRIER FLUID TO THE CASING AND MEANS FOR IMPARTING ROTATION TO THE ROTOR.