US3078651A

US3078651A - Apparatus for classifying airdust mixture

Info

Publication number: US3078651A
Application number: US779985A
Authority: US
Inventors: Delfs Hans Jurgen
Original assignee: Babcock and Wilcox Ltd
Current assignee: Babcock International Ltd
Priority date: 1958-12-12
Filing date: 1958-12-12
Publication date: 1963-02-26
Anticipated expiration: 1980-02-26

Description

Feb. 26, 1963 H. J. DELFS APPARATUS FOR CLASSIFYING AIR-DUST MIXTURE Filed Dec. 12, 1958 Hans (f Deli-f5 IN V EN TOR. A

ATTORNEY llnited States Patent 3,078,651 APPARATUS FUR CLASSIFYHNG Am- DUST MKXTURE Hans Jurgen Delis, Hambnrg-Sasel, Germany, assignor to Eabcock 8: Wilcox Limited, London, England, a comparty of Great Britain Filed Dec. 12, 1958, Ser. No. 779,985 3 Claims. (Cl. 55-422) This invention relates to a classifier for classifying a dust-air mixture preferably from a fine grinding pulverizer with air swept discharge.

It is known to classify the dust-air mixture of direct firing pulverizers with the air of centrifugal forces. The dust-air mixture to be classified is conducted through a ring of tangentially directed guide vanes so that the dustair mixture entering the classifier is set in rotation and centrifugal forces act on each dust particle. Since the centrifugal force is proportional to the mass and velocity of a particle, the larger particles are subjected to a greater centrifugal force than the smaller particles when the composite mixture is giving a circular motion. The known centrifugal classifiers are based on this principle where the oversized particles which are, for example, not uitable for suspension burning, are returned to the grinding process.

It has been found in practice that with the increased boiler outputs and the necessary correspondingly large pulverizers, classification of the dust-air mixture with the known centrifugal classifiers can be effected only incompletely or with difficulty. With such large pulverizers, the centrifugal classifiers would have to have a very large volume and, as a result, could be accommodated within the pulverizer housing only with difficulty or not at all. This is particularly true in the case of a direct firing pulverizer because the available space within or closely adjacent the pulverizer is ordinarily insuflicient. The discharge of the dust-air mixture from the pulverizer, which is usually efiected through a single line also leads to diificulties particularly with regard to the distribution of the dust-air mixture to the points of use, as, for example, to individual burners.

The invention avoids the above drawbacks met with in practice in that the dust-air mixture is first reversed in the classifier and thereafter slowed down so that classification is carried out substantially without utilizing c ntrifugal force.

In the classifier in accordance with the invention, the separation capacity, contrary to known centrifugal classifiers, is independent of the size of the classifier.

In a preferred form of the invention, the dust-air mixture is subdivided into individual separate streams after completion of the classifying process so that the distribution difficulties mentioned above do not occur. In ac cordance with the invention, the classifier for carrying out the process is characterized in that the classifier cone is provided with a special guide cone having a reversal ring disposed above it to receive the dust-air mixture. A stepped grid is provided adjacent the guide cone and so spaced therefrom that the sum of its free cross-sectional area is greater than the free cross-sectional area of the annular gap between the stepped grid and the inner guide cone wall. In moving between these two parts, the dustair stream is abruptly diverted by the reversal ring while retaining a relatively high velocity and is then divide-d into a plurality of small streams in the stepped grid. As a result of the cross-sectional ratios indicated, a considerable reduction in velocity occurs so that the grits or oversized dust particles are separated out of the mixture and are returned to the raw material inlet, while air and the fine particles are removed from the space above the stepped grid.

In a preferred form of the invention, the classifier is provided with a stepped grid so designed as to form a cone substantially concentric with the guide cone.

The stepped grid can be advantageous formed by substantially concentric superposed rings, the diameter of which increases upwardly, the diameter of the lower ring being approximately that of the raw material chute entering the pulverizer. The rings of the stepped grid of the classifier may consist of rings formed by strips or bars of metal.

'Several discharge tubes or outlet ducts for the dustair mixture leaving the classifier may be provided so that a uniform distribution of the mixture to several points of use is possible.

The stepped grid may be suspended with the reversal ring and arranged so as to be adjustable in height. Thus, the free cross-section of the annular gap between the stepped grid and the inner guide cone wall can be varied. Thus, the velocity of the dust-air mixture passing through the gap can be varied to produce different finenesses of the finished material leaving the classifier. The spacings between the rings forming the stepped grid can also be adjustable, if desired.

The classifier of the present invention has yet further advantages, as compared with the known centrifugal classifier. The raw material chute passing through the classifier into the pulversizer may be vertical or arranged diagonally through the space between the classifier cover and the stepped grid. This results in a very considerab e adaptability of the arrangement of the classifier of the invention so as to accommodate the various spacing requirements with regard to pulverize feeding and discharge. In addition, the classifier of the present invention has a lower air flow resistance than a centrifugal classifier and is simpler and cheaper to manufacture.

Further details of the invention are found in the following description of a preferred example of the invention as shown diagrammatically on the drawings, where:

*FIG. 1 is a vertical section of a classifier constructed in accordance with the present invention as arranged on a pulverizer housing; and

FIG. 2 is a plan view of the classifier shown in FIG. 1 with the cover of the classifier partly removed.

The classifier arranged on mill housing 1 has a conical shell 2 and is closed at the top by cover 3. Immersion tubes 4 are passed through cover 3 for the removal of the dust-air mixture from space 5, while the raw material is passed by a chute 6 into the grinding space of a mill, which is only shown in outline. In this example of execution the chute 6 passes vertically through space 5. It can, however, also be passed diagonally through space 5 to meet prevailing conditions. In the conical section 2 of the classifier a guide cone 7 is provided, the outer wall 8 of which is spaced from the classifier outer wall 2, thus forming an annular gap 9. Above the guide cone 7 a reversal ring 10 together with the stepped grid 11 is suspended in the upper part of the classifier. The stepped grid 11 is formed by rings 12 made of flats arranged concentrically to chute 6, with a diameter increasing from below upwardly. The spacing of the stepped grid 11 from the inner wall 13 of guide cone 7 can be varied by adjusting means 14, so that an annular gap 15 with varying cross section can be created which simultaneously results in an alteration in the degree of fineness of the finished material.

The air stream laden with milled material coming for example from a direct firing mill is conveyed through annular gap 9 in the direction arrowed, reversed by reversing ring it and reaches annular gap 15, and is then split up by stepped grid 11 while being reversed into many part streams. Since the sum of the free cross sections in stepped grid 11 is greater than the free cross section of the annular gap 15, this results in a slowing down of the stream again diverted by the stepped grid so that separation of the grits results, which are led to the raw material inlet, while air andfines are conveyed from space 5 via one or several immersion tubes 4 to their point of use, the distribution of the air stream with finished material being effected in the classifier after classifications has been completed.

The classifier in accordance with the invention is not restricted in application to direct firing mills for fuels, but can with equal advantage be used in all cases, where classification of a dust-air mixture is involved, for example, when grinding mineral such as limestone, artificial fertilizers, and the like.

The invention is moreover not restricted to the use of air as carrier for the dust-air mixture, but can, of course, also be applied when instead of air some other gas is used as carrier medium.

What is claimed is:

1. Apparatus for classifying a dust-air mixture for removal of the coarse dust from the mixture which comprises an inverted frusto-conical casing having a closed base with a dust-air outlet therefrom, an element having inner and outer inverted frusto-conical surfaces coaxially positioned within said casing, said outer surface cooperating with said casing to define an upwardly and outwardly extending annular passageway therebetween, means for passing a dust-air mixture upwardly through said annular passageway, a reversal ring positioned at the upper end of said annular passageway to direct the flow of all of said mixture inwardly and downwardly along the inner surface of said element toward a centrally positioned coarse material discharge opening, and a stepped grid formed by a plurality of radially spaced ring members and forming an upward flow passageway between the ring members having a total flow area greater than the flow area of said annular passageway, the lower surface of said grid being substantially equally spaced throughout its extent from the inner surface of said element to define a second annular mixture flow passageway of substantially uniform width therebetween for distributed upward flow of air entrained fine dust through said grid.

2. Apparatus for classifying a dust-air mixture for removal of the coarse dust from the mixture which comprises an inverted frusto-conical casing having a closed base with a dust-air outlet therefrom, an element having inner and outer inverted frusto-conical surfaces coaxially positioned within said casing, said outer surface cooperating with said casing to define an upwardly and outwardly extending annular passageway therebetween, means for passing a dust-air mixture upwardly through said annular passageway, a reversal ring positioned at the upper end of said annular passageway to direct the flow of all of said mixture inwardly and downwardly along the inner surface of said element toward a centrally positioned coarse material discharge opening, a stepped grid formed by a plurality of radially spaced ring members, the lower surface of said grid being substantially equally spaced throughout its entire extent from the inner surface of said element to define a second annular mixture flow passageway or" substantially uniform width therebetween, for distribution of dust-air mixture flow upwardly through said grid toward said outlet and to thereby cause separation of coarse dust from said mixture, and means for adjusting the width between the inner surface of said element and said grid to control the size of coarse dust removed from said dust-air mixture.

3. Apparatus for classifying a dust-air mixture for removal of the coarse dust from the mixture which comprises an inverted frusto-conical casing having a closed base with a dust-air outlet therefrom, an element having inner and outer inverted frusto-conical surfaces coaxially positioned within said casing, said outer surface cooperating with said casing to define an upwardly and outwardly extending annular passageway therebetween, means for passing a dust-air mixture upwardly through said annular passageway, a reversal ring positioned at the upper end of said annular passageway to direct the flow of said mixture inwardly and downwardly along the inner surface of said element toward a centrally positioned coarse material discharge opening, a stepped grid formed by a plurality of radially spaced ring members the spaces defined by said ring members having a flow area greater than the flow area of said annular passageway, the lower surface of said grid being substantially equally spaced throughout its entire extent from the inner surface of said element to define a second annular flow passageway of substantially uniform width therebetween, and means for adjusting said grid to vary the substantially uniform width between said inner surface and the grid to control the size of coarse dust removed from said dust-air mixture.

References Cited in the file of this patent UNITED STATES PATENTS 83,506 Justi Oct. 27, 1868 137,438 Grassler Apr. 1, 1873 154,412 Nesmith Aug. 25, 1874 193,242 Grace July 17, 1877 378,507 Henderson et al. Feb. 28, 1888 759,553 Rood May 10, 1904 919,249 Ruddiman Apr. 20, 1909 1,598,261 Swem Aug. 31, 1926 1,818,905 McGee Aug. 11, 1931 2,039,692 Van Tongeren May 5, 1936 2,205,336 Beach June 18, 1940 2,580,317 Pekar et al. Dec. 25, 1951 2,641,335 Berg June 9, 1953 2,804,171 Yellott et a1 Aug. 27, 1957 FOREIGN PATENTS 160,100 Great Britain Mar. 17, 1921 Great Britain Jan. 28, 1932

Claims

1. APPARATUS FOR CLASSIFYING A DUST-AIR MIXTURE FOR REMOVAL OF THE COARSE DUST FROM THE MIXTURE WHICH COMPRISES AN INVERTED FRUSTO-CONICAL CASING HAVING A CLOSED BASE WITH A DUST-AIR OUTLET THEREFROM, AN ELEMENT HAVING INNER AND OUTER INVERTED FRUSTO-CONICAL SURFACES COAXIALLY POSITIONED WITHIN SAID CASING, SAID OUTER SURFACE COOPERATING WITH SAID CASING TO DEFINE AN UPWARDLY AND OUTWARDLY EXTENDING ANNULAR PASSAGEWAY THEREBETWEEN, MEANS FOR PASSING A DUST-AIR MIXTURE UPWARDLY THROUGH SAID ANNULAR PASSAGEWAY, A REVERSAL RING POSITIONED AT THE UPPER END OF SAID ANNULAR PASSAGEWAY TO DIRECT THE FLOW OF ALL OF SAID MIXTURE INWARDLY AND DOWNWARDLY ALONG THE INNER SURFACE OF THE ELEMENT TOWARD A CENTRALLY POSITIONED COARCE MATERIAL DISCHARGE OPENING, AND A STEPPED GRID FORMED BY A PLURALITY OF RADIALLY SPACED RING MEMBERS AND FORMING AN UPWARD FLOW PASSAGEWAY BETWEEN THE RING MEMBERS HAVING A TOTAL FLOW AREA GREATER THAN THE FLOW AREA OF SAID ANNULAR PASSAGEWAY, THE LOWER SURFACE OF SAID GRID BEING SUBSTANTIALLY EQUALLY SPACED THROUGHOUT ITS EXTENT FROM THE INNER SURFACE OF SAID ELEMENT TO DEFINE A SECOND ANNULAR MIXTURE FLOW PASSAGEWAY OF SUBSTANTIALLY UNIFORM WIDTH THEREBETWEEN FOR DISTRIBUTED UPWARD FLOW OF AIR ENTRAINED FINE DUST THROUGH SAID GRID.