US 3568920 A
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United States Patent  Inventor Torben Boss Nielsen  References Cited 1 N ggfi 'g Denmark UNITED STATES PATENTS Q f Dec24 '1968 3,187,997 6/1965 Gooch 233/7  Patented 9 1971 3,279,687 10/1966 Amero 233/7  Assignee separamr AIS 3,428,246 2/1969 Finkelston 233/7 Soborg, Denmark Primary Examiner-Robert W. Jenkins  Priority Jan. 10, 1968 Att0rney-Stevens, Davis, Miller & Mosher  Denmark  74/68  SCREW CENTRIFUGE ABSTRACT: A screw centrifuge comprising inner and outer centrifuge rotors rotating at differential rotational speeds with a screw secured to the outer surface of the inner rotor for conveying separated solid matter. Means for supplying raw material to the separating chamber between the rotors, comprises an axially extending supply duct communicating with radial bushings secured in the inner rotor via a dismountable insert having corresponding, radially orientated discharge apertures.
SCREW CENUGE This invention relates to a screw centrifuge having a separating chamber defined between two rotors rotating at different numbers of revolutions, a conveyor screw located in the separating chamber an secured to the internal rotor and a supply duct in the internal rotor for supplying raw material to the separating chamber between the two rotors through one or more apertures in the wall of said internal rotor.
Such centrifuges, which are also called decanters, are employed, inter alia, for a rough or preliminary separation of sludge such a sewer sludge and refuse from destruction plants, into a partially dried phase and a partially cleansed liquid phase, which, if desired, can both be treated further in other kinds of apparatus.
In the use of the centrifuge considerable wear may occur on the edges of the discharge aperture or apertures in the internal rotor, since the solid matter frequently contains highly abrasive constituents, especially sand. This does not apply to sewer sludge only, but also to dead animals, fish etc. With known centrifuges it is difficult to effect repairs on the internal rotor, which repairs normally comprise building up, such as by welding, fresh material to replace the worn off material, because the apertures situated at the bottom of the screws helix are only accessible with difficulty. Consequently, the operation is troublesome and time-consuming and, as a rule, it is impossible to carry it out in situ, because after having built up the worn parts by welding, it is often necessary to carry out a balancing of the rotor because of its rather high rotational speed during operation of the centrifuge. L
It is a primary object of the invention to provide a centrifuge of the kind referred to in which the above mentionedrepair of the internal centrifuge rotor can be carried out more readily than in the previously known centrifuge and without requiring skilled labor and specialized equipment normally not available to the user of the centrifuge.
According to the present invention there is provided a screw centrifuge comprising a hollow internal rotor and an external rotor surrounding said internal rotor and defining therewith a separating chamber, means for rotatably supporting said rotors and means for rotating each of said rotors at mutually different numbers of revolutions per minute, a conveyor screw secured to the outer side of, said internal rotor, means for supplying a raw material to be separated into said separating chamber, said supply means comprising a supply duct extending axially within said internal rotor and having spaced inlet and outlet ends, said outlet end being formed by an insert detachably secured within said internal rotor and having an axially facing inlet aperture and at least one radially facing discharge aperture, at least one bushing detachably secured in the wall of the internal rotor and having an inner end face located opposite a surface surrounding said discharge aperture of said insert, and means for separately discharging constituents of said raw material from said separating chamber after separation thereof.
Hereby the repair of a worn rotor is considerably simplified, as it is only necessary to replace the worn bushing or bushings and possibly the central insert with new parts which can be completely finished in advance including the balancing and can be supplied as spare parts togetherwith the centrifuge. It is therefore readily possible to effect repairs in situ, the centrifuge thus being out of operation after a short time only. It is, furthermore, an advantage that the insert and the bushings can, without unduly high expenses being incurred, be manufactured from particularly wear-resistant material, e.g. steel that is suitable for through-hardening, so that the service life is already increased by this means as compared to the known centrifuges.
The bushing may be provided with a collar which abuts against an internal contact surface of the internal rotor, which collar provides for a particularly simple assembly and securing of the bushing that can be inserted from the interior of the internal rotor and subsequently be pressed against the contact surface.
To facilitate the mounting and removal of the insert which normally will be situated approximately in the longitudinal center portion of the internal rotor, the insert may be secured to an end flange on a shaft part which is secured detachably to the internal rotor.
The insert may be cruciform or stellate and comprise a plurality of radial discharge ducts which, at their inner ends are connected to an inlet duct extending axially in the insert and each discharge duct may, moreover, be connected with the inlet duct via a duct extending obliquely within the insert. The oblique ducts mentioned counteract the accumulation of sludge in the raw material in front of a transverse wall which, in the insert, delimits the radial discharge ducts, so that more quick discharge of the material to the separating chamber is obtained.
In the following the invention is described in greater detail while referring to the accompanying drawings, in which:
FIG. 1 shows a longitudinal section through a part of a screw centrifuge embodying the present invention;
FIG. 2 is a longitudinal section on a larger scale through the insert shown in FIG. 1 in the centrifuges internal rotor, which section is taken along lines II-II in FIG. 3;
FIG. 3 is a section along lines III-III in FIG. 2; and
FIG. 4 is a section along lines IV-lVin FIG. 2 also showing the four discharge bushings and an adjoining part of the internal rotor without helix winding or convolution.
The screw centrifuge illustrated in FIG. 1 comprises an outer or external rotor l which, in a manner not shown in detail, is journaled rotatably in the centrifuges frame and is coupled to a driving mechanism. An :internal rotor 2 coaxial with rotor 1 is journaled rotatably in relation to the outer rotor 1 which, in FIG. 1 is illustrated by two diagrammatically shown bearings 3' and 4. The driving mechanism, not shown, is arranged to cause rotors l and 2 to rotate at slightly differing numbers of revolutions per minute, whereby a screw 5 fixed to the outside of the internal rotor 2 acts as conveyor of solid matter which, in the annular separating chamber 6 delimited between rotors 1 and 2, is separated from a raw material supplied.
The raw material is supplied through a pipe 7 which, at its one end is fixed in the internal rotor 2 coaxial with same and which opens into an insert 8 fixed in rotor 2, whereby the insert acts as distributor supplying four radially mounted insert bushings 9 that have discharge ducts 10 opening into the separating chamber 6.
As a consequence of the rotation of the rotors l and 2, a centrifuging of the supplied raw material takes place, which thus is separated into a solid and a liquid phase. The rpm. and the direction of rotation of the tworotors 1 and 2 are chosen in such a manner that screw 5 transports the separated solid matter towards the right-hand side in FIG. 1 to an outlet 11 at the right-hand end of rotor l. The liquid phase runs off through an overflow, not shown, at the left-hand end of rotor 1.
The insert 8, as shown in greater detail 181 FIGS. 2-4, has an axially extending hub 12,- the machined inner side of which encloses the end of pipe 7, in the periphery of which there is provided an O-ring l3, vide FIG. I, for sealing the connection. Internally of the hub 12, the insert 8 has a transverse wall 14 with a central aperture 15 in line with the bore in the hub 12. The aperture 15 and the bore of hub 12 cooperate to define an axial inlet duct in the insert. At a distance from the transverse wall 14 the insert ends in a closed, terminal wall 16. From the space between the two walls 14 and 16 issue four radial discharge ducts 17, which are in line each with one of the ducts 10 in the four bushings 9 referred to above, there are, moreover, in the wall 14, four smaller obliquely extending ducts 18, which each connect one of the radial ducts 17 with the space between the transverse wall 14 and the hub 12 where, as shown in FIG. 2, pockets 30 are formed in which solid matter from the raw material might splash back from the wall 14 and settle, if the ducts 17 were :not provided.
As is best seen from FIGS. 2 and 3, the main part of insert 8 is cross-shaped with four arms 19, each of which contains a radial duct 17. The radially outer terminal surface 20 of each arm is flat and lies, when the insert is mounted in the centrifuge, with a small clearance opposite the radially inner end face on a bushing 9, vide FIGS. 1 and 4. Each bushing 9 is provided with a collar 21 abutting on an internal shoulder surrounding the bore in the rotor 2, wherein the bushing is secured, preferably with a firm fit.
The insert 8 is secured to a flange 23 by means of four bolts 22 which flange, in turn, is welded to a tubular shaft 24, vide FIG. 1. The flange 23 has a recess for receiving and centering a machined projection 24 on the terminal wall 16 of the insert. As is seen from FIG. I, the shaft 24 serves partly for'the journaling of the internal rotor 2, in that the bearing 3 is mounted on the extreme end of the shaft. The tubular shaft with the insert 8 secured thereto is fixed to the rotor 2 by means of eight bolts 26, the heads of which abut against a flange 27 welded to the shaft 24, and the bolts are screwed into tapped holes in the rotor 2. As is seen from FIG. 4, the four bolts 22 lie in the corners between the four arms 19 of the insert end their heads abut against a ring 28, vide FIG. 1 which, in turn, abuts against machined surfaces 29 on the four arms of the insert, vide FIGS. 2 and 3.
For replacing the insert and/or one or more of the bushings 9, bolts 26 are first loosened, after which the shaft 24 together with the insert 8 can be removed axially from the internal rotor 2. The four collar bushings 9 are free after that and can be removed through the interior of the rotor 2. After the dismantling of the tubular shaft and the insert, these can be separated from each other by loosening of the screws 22. The assembly of the parts is effected in reversed sequence. The bushings 9 normally do not require to have extremely firm or close fit in rotor 2, since the centrifugal force occurring at the centrifuge s operation will assist in keeping them in place.
l. A screw centrifuge comprising a hollow internal rotor and an external rotor surrounding said internal rotor and defining therewith a separating chamber, means for rotatably supporting said rotors and means for rotating each of said rotors at mutually different numbers of revolutions per minute, a conveyor screw secured to the outer side of said internal rotor, means for supplying a raw material to be separated into said separating chamber, said supply means comprising a supply duct extending axially within said internal rotor and having spaced inlet and outlet ends, said outlet end being formed by an insert detachably secured within said internal rotor and having an axially facing inlet aperture and at least one radially facing discharge aperture, at least one bushing detachably secured in the wall of the internal rotor and having an inner end face located opposite a surface surrounding said discharge aperture of said insert, and means for separately discharging constituents of said raw material from said separating chamber after separation thereof.
2. A centrifuge as claimed in claim 1, wherein the bushing is provided with a collar abutting on an internal surface of said internal rotor when said bushing is mounted.
3. A centrifuge as claimed in claim 1 and further comprising a shaft detachably secured to said internal rotor and provided with an end flange, the insert being secured to said end flange.
4. A centrifuge as claimed in claim 1, wherein the insert is cruciform or stellate and comprises a plurality of radially extending discharge ducts, the inner end of each such ducts communicating with an inlet duct extending axially of said insert, and a plurality of obliquely extending ducts each connecting a discharge duct with the inlet duct.