US3527401A - Drum centrifuge with rotating worm conveyor - Google Patents

Description

J. s. BYE-JQRGENSEN Erm.

DRUM CENTRIFUGE WITH ROTATING WORM CONVEYOR [/////////////////////////////{y//II United States Patent O U.S. Cl. 233--7 4 Claims ABSTRACT F THE DISCLOSURE This invention relates to a drum centrifuge for wastewater-sludge comprising a drum having two sections therein. Worm conveyor means mounted for free relative rotatable movement in one of said drum sections conveys moist solids which have been separated from the liquid. The other drum section has an oblique guiding surface along which the separated solids are guided and passed to the worm conveyor means. The worm conveyor means are disposed entirely within said one section and said other section is substantially free of obstructions to provide free axial and radial movement of the solids and liquids therethrough.

The present invention relates to a drum centrifuge, particularly for wastewater sludge, comprising a rotating confveyor worm for scraping out solids separated in the interior of the centrifuge; at least one feed conduit for the suspension to be separated, said feed conduit debouching in a part of the centrifuge provided in its interior with oblique, preferably substantially conical guiding surfaces, to which part of the centrifuge the conveyor worm does not extend; an exit opening for the separated solids; and at least one discharge for liquid totally or partly puried from solids.

Drum centrifuges are known in which a worm conveyor is placed inside the drum and rotate during operation at a speed differing from the speed of rotation of the drum itself. In other words, the worm conveyor has a rotation relative to the drum, and thereby the worm conveyor conveys the solids separated on the inner wall of the drum onto a discharge opening or place. A known drum centrifuge of this type is adapted so as to introduce the sludge-containing suspension at a point between the ends of the space in the drum, whereas the separated solids are discharged in one end and the centrifuged and thereby partly purified liquid is discharged in the other end 0f the drum. By this arrangement the two components to be separated, solids and liquid, move in mutual countercurrent and this counter-movement gives rise to so high a turbulence that the separation becomes unsatisfactory, because solids will have a tendency to return from the conveyor worm to the liquid. In use for instance for centrifuging wastewater sludge from biological water purification plants, the centrifuged liquid after such countercurrent conveying may still contain about 50% of solids, frequently in so iinely emulsified or dispersed form that a removal of said solids in a subsequent centrifugation step will cause difficulties.

It has been proposed to alter the described known drum centrifuge in such a manner that the feed conduit for the sludgecontaining suspension debouches in one end of the drum and the exit opening for the solids in the opposite end of the drum, whereas outlets for the discharge of purified liquid are situated in a zone between the ends of the drum. Hereby the sludge-containing suspension introduced in the centrifuge drum will be flowing in the same direction as the separated solids, which results in a considerably reduced turbulence and hence improved separation. By a drum centrifuge thus adapted, it is possible to achieve rather satisfactory separation, even of difiicultly separable suspensions such as organic sludge-containing liquids from biological wastewater-purification plants. i

In the above-mentioned known centrifutges containing worm-conveyors it is known to have the drum and the worm conveyor frusto-conical or substantially frusto-conical at least over part of their length. In such cases the exit opening for the solids is situated in the narrow end of the truncated cone and in a zone within the cylindrical surface of the liquid formed during operation of the centrifuge. It is hereby possible without diiiiculty to remove the solids in a condition freed from water to a considerable degree; for instance when the centrifuge is used for treating sludge from biological water-purification, it is possible to remove the solids from the centrifuge in a condition so that the solids can be shoveled and placed in heaps having a slope of for instance In the known drum centrifuges of this type the worm conveyor is situated at the inner wall of the drum along the full or approximately full length of the latter.

It has now been found that it is possible to achieve a still better separation effect or centrifugation-effect than is possible by the apparatuses discussed hereinbefore. This is possible if a drum centrifuge of the above-described main type is so constructed that the inlet for sludge-containing suspension and the discharge or outlet for water are placed in a part of the centrifuge drum where there is no worm conveyor, or rather to which the worm conveyor does not extend, and if furthermore this part of the centrifuge is provided with oblique, preferably substantially conical (frusto-conical) guiding surfaces along which the separated solids are led to the worm conveyor under the influence of the centrifugal forces. From the specification to the U.S. Pat. No. 3,322,336 a centrifuge is known constructed according to this principle, but in contradistinction to the above-mentioned centrifuges, it is not able to give a substantially turbulence-free treatment of the sludge-containing liquid, especially wastewatersludge, in the said part of the centrifuge to which the conveyor worm does not extend. This is due to the fact that the centrifuge described in the said U.S. Patent speciiication No. 3,322,336 in the said part 4'of the centrifuge is provided with a number of axial and radial partitions so as to subdivide the centrifuge part in question into several separating chambers. The radial partitions of these chambers by narrow slots or annular gaps, serving as discharges, are in distance from the inner surface of the drum wall and furthermore there are small openings in the partitions so as to allow liquid to pass. The openings and annular gaps or slots during the operation of the centrifuge cause turbulence in the sludge-containing liquid in the said part of the drum which is subdivided into said chambers. The partitions will hamper the movement of water and sludge in the direction of movement, especially axially relative to the longitudinal axis of the drum, and the part of the drum containing the conveyor worm is preferably substantially frusto-conical, the exit opening for the solids being situated in the narrow end of the truncated cone. If this is the case, the centrifuge drum may expediently have such a shape that the inner guiding surfaces, along which the solids slide onto the intake end of the conveyor worm are constituted by the inner side of the wall of a substantially frusto-conical part of the drum, the largest base-perimeter of the last mentioned truncated cone being joined to the largest base-perimeter of the first mentioned drum part, i.e. the one containing the conveyor, the spaces in the two parts of the drum being in open connection with each other in the axial direction. Thereby the drum substantially has the shape of a double truncated cone with the greatest thickness near the middle. The sludge-containing suspension is fed into the end of the drum into which the conveyor does not extend, and the inlet in fact debouches quite near the narrow end of that part of the drum. The solids are discharged in the opposite narrow end of the drum.

In a more preferred embodiment of the drum centrifuge according to the invention, the part of the drum containing the worm conveyor is also substantially frustoconical with the exit for the solids in the narrow end of this drum part. In this more preferred embodiment, however, the oblique guiding surfaces, along which the solids are led to the worm conveyor, are consituted by a substantially frusto-conical ldrum space or chamber the largest base-perimeter of which is situated near the inner end of the widest end of the worm conveyor, whereas the smallest base-perimeter of the said space is situated in a plane between the end planes of the worm conveyor. In this preferred case the two truncated or approximately truncated cones are uni-directional, i.e. have their narrowest end turned in the same direction. In its entirety the drum in this case has substantially the outer shape of a single truncated cone. This embodiment has the particular advantage that it may be constructed with smaller length compared to the capacity of the aforementioned embodiment, because of the total length of the drum will be little more than the length of the worm conveyor. The shorter length of the drum involves a shorter distance between the bearings, which may be of importance in large centrifuges and at the very high rotation-velocities which may come into question.

The discharge or discharges for liquid preferably start from an area in the centrifuge being situated near the circle of intersection between the cylindrical surface of the liquid during operation, and the largest base of the conical guiding surfaces. Hereby it is obtained that the purified liquid is discharged from an area in the centrifuge which is as distant as possible from the area in which the actual separation takes place, and this ensures the best possible purity of the discharged liquid.

A feed line for chemicals with which to treat the suspension may debouch in the centrifuge near the debouching of the feed line for sludge-containing suspension. In some cases it may be desirable to treat for instance wastewater sludge with various substances in order to cause various chemical reactions, and in the manner stated there is obtained the possibility of dosing the addition of the chemicals in a very varied manner; furthermore, the chemicals will come into contact with the suspension at once upon its introduction.

The drum centrifuge according to the invention will be described more in detail with reference to the drawing. In the drawing, FIGS. 1 and 2 schematically show longitudinal sections of two different embodiments of the drum centrifuge according to the invention.

In FIG. 1 a double frusto-conical dum 11 is fixedly mounted on a shaft 12 which in a manner not shown is rotatably mounted in bearings 13 on supports 14. The shaft 12 is driven in conventional manner, for instance by the aid of V-belts and a pulley 15. In one end the shaft 12 is enclosed in a coaxial tubular shaft 16, rotatably mounted on the shaft 12 by the aid of bearing .17 (not shown in detail). The shaft 16 is adapted to rotate with a velocity differing from that of the shaft 12 and may for instance be driven by V-belts and a pulley 18. It is possible to drive the two shafts from one source of power, suitable transmissions ensuring the speed of rotation of the two shafts being different.

In a suitable manner, for instance as in the embodiment shown, i.e. by the aid of spokes 19, the shaft 16 supports a Worm conveyor 20 adapted to convey solids onto an exit opening 21 in the one end of the drum. As the worm conveyor 20 is fxedly connected to the shaft 16, and as the drum 11 by the aid of the spokes 22 or in another suitable manner is fixedly connected to the 4 shaft 12, and as these two shafts rotate at different velocities, the worm conveyor 20 in operation will rotate relative to the drum 11, and this relative rotation causes the transport of the solids by the conveyor.

The worm conveyor does not extend along the entire length of the drum 11, but only along one conical part thereof, as in FIG. 1, the left frusto-conical end 23 of drum 11. The right-hand end 24 of drum 11 is likewise frusto-conical and the two frusto-cones are joined at the largest, common base perimeter. Frusto-conical part 24 of drum 11 accordingly is not provided with the conveyor work, but its inner wall constitutes a fnlsto-conical guiding surface for sludge-containing liquid suspension introduced into the drum in order to be separated by the centrifugal effect.

In the embodiment shown in FIG. 1, the suspension is introduced via a conduit 25 and a channel 26 in the shaft 12. This channel 26 debouches in the drum space through one or more openings 27 situated adjacent to the narrowest end of drum part 24. By the aid of suitable members (not shown) the conduit 25 is watertightly connected with the channel 26 in shaft .12 in a manner so as not to disturb the rotation. Near the widest plane of the drum 11 there is a number (for instance six) of orifices 28 for pipes 29 which extend through the wall of drum 11 and debouch into a discharge channel 30 for purified liquid; this discharge channel may for instance be limited by a cover 31 slidably connected to the drum 11 at the widest part of the latter.

In the shaft there may be provided, but need not be provided, a further channel 32 to be fed for instance with substances for treatment of the sludge. Channel 32 may be fed from conduit 33 and debouches in the drum space via one or more openings 34 adjacent to open ings 27.

Adjacent to the exit opening 21 there is a suitable discharge channel 35 through which the separated solids are discharged.

The drum centrifuge thus described functions in the following manner. When the suspension is supplied via conduit 25, channel 26 and openings 26, it flows into the space of the drum 11 in its right-hand frusto-conical part 24, wherein no part of the worm conveyor is present. The rotation of the drum will cause separation of liquid and solids, and this separation will take place only in the part 24 of the drum. Solely under the influence of the centrifugal force, the solids will slide along the inner conical wall of the drum part 24, this Wall acting as a guiding surface and conducting the solids onto the commencing end or intake end of the worm conveyor 20, situated near the widest part of drum 11. Worm conveyor 20 is also kept in rotation and at such a speed that it has a rotation relative to the drum, the direction of the relative rotation being so as to conduct the solids towards exit opening 2.1. Through this opening the solids leave the drum and are discharged via channel 35 from which they can be removed. Like the discharge channel 30, the channel 35 may pass a base plate (not shown) on which supports 14 are mounted.

During operation of the centrifuge, the surface of liquid adjusts itself along a cylindrical level or surface intimated in the drawing by broken lines 37. Adjacent to this surface, between it and the wall of the drum, the orifices 28 are situated. Through these orifices the water purified from solids enters pipes 29 and passes to the discharge channel 30. The pipes 29 may be led through a further channel in shaft 32. The orifices 28, however, should preferably be situated as shown in FIG. 1, i.e. close to the largest base of the truncated cone formed by drum part 24, because hereby the place where the liquid actually leaves the drum space is farthest possible from the place at which the suspension to be separated enters the drum space.

The level surface 3'7 has a longer distance from the drum and Worm conveyor axes than the periphery of the solids exit opening 21, and for that reason the solids leaving that opening have to a considerable degree been freed from liquid.

Simultaneously with the introduction of suspension from conduit 25, it is possible, if desired, to add liquid treatment substances from conduit 33 via channel 32 and openings 34. Such a liquid for addition to the suspension will thereby at once be intermixed with the suspension entering the drum space via openings 27, a good intermixing being thereby ensured.

The spokes 22 support the drum 23, 24 in such a man ner that the drum part 23 forms a projecting end. The spokes 22 may be replaced by a bearing inside the channel 35 and adapted in such a manner that the sludge from opening 21 can pass around such bearing, for instance through suitable opening in the prolonged drum part 23.

Since there are no substantial turbulence-creating hindrances to the free movement of the liquid in the drum part 24 containing no part of the worm conveyor, the centrifugation operation will essentially be free from turbulence and hence ensure an effective separation of liquid and solids.

The particularly preferred embodiment of the drum centrifuge shown in FIG. 2 comprises a substantially frusto-conical drum 41 iixedly mounted on a shaft 44 rotatably mounted on bearings 13 on supports 14. The mounting of drum 41 on shaft 44 may for instance be carried out by the aid of an end plate 43. In FIG. 2, end plate 43 is shown as a flat plate, but need not be flat. It may for instance be arched or conical (concave or preferably convex), or may otherwise deviate from the flat form. The end plate 43 is preferably convex, domeshaped.

Thc drum 41 is driven in a suitable manner, for instance -by the aid of belts and al pulley 15. In one end the shaft 44 is enclosed in a coaxial hollow shaft 45 which is rotatable relative to the shaft 44 in the same manner as shaft 16 (FIG. l) relative to shaft 12; shaft 45 may for instance be driven by belts over a pulley 18, and on shaft 45 is mounted a worm conveyor 46 by the aid of spokes 42. The worm conveyor is adapted to and driven in such a manner relative to the drum 41, that conveyor 46 transports solids centrifuged olf from the suspension in the direction from the widest to the narrowest end of said conveyor. At the narrowest end of the conveyor, the solids leave it and are discharged via the exit opening 21 and a discharge channel 35 in the same manner as the embodiment shown in FIG. I.

The worm conveyor 46 extends along substantially the entire length of the inner conical wall of drum 41. On the shaft 44 is mounted a substantially frusto-conical guiding means 47, made for instance of steel or some other metal. The orientation of guiding means 47 is the same as the orientation of the frusto-conical drum 41, i.e. the narrowest end of both is turned in the same direction; the narrowest end of the guiding means 47 is situated in a plane between the widest end of the very same guiding means 47 and the narrowest end of the drum. The larg est perimeter of the frusto-conical guiding means 47 is situated near the inner edge of the widest end of the conveyor Worm 46. Guiding means 47 together with end plate 43 define a substantially conical part or compartment 48 in the drum, and in this compartment the actual separation takes place. If end plate 43 is not at, the base of the truncated cone constituting said compartment will not be flat, but this has no influence on the principle of the compartment 48, which is that the inner (right-hand in FIG. 2) side of guiding means 47 serves as substantially conical guiding surface along which the solids slide to the intake end of the worm conveyor under the iniiuence of the centrifugal forces.

In the shaft 44 there is a channel 49 in connection with a feed conduit 50 for sludge-containing liquid or suspension to be separated in the drum centrifuge. The channel 49 debouches in compartment 48 by openings 51 situated adjacent to the narrowest end of guiding means 47. Guiding means 47 as mentioned act as guiding surface along which the solids separated from the suspension are led to the beginning end of worm conveyor 46. The embodiment accordingto FIG. 2 may also have a further channel 32 in shaft 44 and in connection with a supply conduit 33 for a treatment liquid such as chemicals with which to treat the suspension; such channel 32, if present, debouches in compartment 48 via openings 34 in the neighborhood of openings '51.

In end plate 43 there is a suitable number of holes or small pipes 5,2 through which liquid totally or partly purified from solids may leave the drum 41, to wit leave the drum from drum compartment 58. Via these holes or pipes the purified liquid enters a discharge channel 54 for purified liquid; this channel 54 may for instance be delimited by a screen 53.

The embodiment shown in FIG. 2 is shorter than that shown in FIG. l with regard to capacity, but essentially it functions in the same manner. When the suspension flows into the drum 46 via openings 51 and the drum rotates, a separation will be caused. Under the influence of the centrifugal forces, solids centrifuged off slide along the inner wall of guiding means 47 delimiting the compartment 48, and thereby the solids arrive to the largest perimeter of said guiding means. Here the solids are caught rby the worm conveyor 46 and conveyed onto the exit opening 21. The liquid level or surface adjusts itself as a cylindrical surface 37, and adjacent to this surface, yet between it and the outer wall or mantle of the drum, the openings or pipes 52 are situated; the purified liquid leaves the drum via these. In the embodiment according to FIG. 2, as well as in that according to FIG. 1, the actual spot of liquid discharge, i.e. the openings or pipes 52, are situated as far as possible from the area where the actual separation occurs and thereby one ensures the greatest possible purity of the discharged liquid; no countercurrents will be formed and no turbulence created.

The suspension introduced in the drum will in none of the embodiments come into flowing contact with the solids centrifuged off and thereafter being present in the turns of the worm conveyor; consequently the suspension cannot give rise to turbulence which would whirl up the solids and subdivide them. There furthermore are no hindrances to the flow in the compartment 48 and therefore no turbulence can be created there. The separation of solids and liquid accordingly becomes very efiicient.

In the present specication and appended claims, by the expressions oblique guiding surface, substantially conical or substantially frusto-conical and similar expressions, we mean rotational-symmetrical surfaces the generatrix of which is inclined or oblique to the axis, i.e. one end of the generatrix has a larger radial distance from the axis than the other end of the generatrix. From the radially nearest to the radially farthest end, the generatrix consistently is gaining larger distance from the axis but it need not be a smooth or continuous curve; the generatrix may have angles or bends. The generatrix may be a straight line, in which case the guiding surface is a true truncated cone. The generatrix may also be more or less curved, possibly sinuous, and convex or concave.

We claim:

1. A drum centrifuge for wastewater-sludge, comprising a drum having two sections therein, worm conveyor means mounted for relative rotatable movement within one of said drum sections to convey moist solids which have been separated from the liquid, said other drum section having an oblique guiding surface along which separated solids are disposed and guided by the action of centrifugal force, said guiding surface having its larger end disposed adjacent the inlet of the worm conveyor means whereby separated solids are passed from said guiding surface to said worm conveyor means inlet, feed inlet means conducting Wastewater-sludge into said other drum section, and puried water discharge means leading from said other drum section, said worm conveyor means being disposed entirely within said one drum section, said other section being constructed and arranged to provide an undivided and uncompartmentalized space substantially free of obstructions to facilitate free axial and radial movement of the solids and liquids therethrough, thereby to reduce turbulence to a minimum.

2. A drum centrifuge according to claim 1 wherein said one drum section and said worm conveyor means each have a substantially conical configuration, said worm conveyor means having a discharge at the smaller end of said one drum section, said oblique guiding surface being formed by the inner wall of a substantially truncated conical compartment, the largest end of which is disposed adjacent the largest end of said one drum section.

3. A drum centrifuge according to claim 2 wherein said adjacent larger ends of said one and said other compartments are disposed between the smaller ends of said one and said other compartments considered along a longitudinal axis.

4. A drum centrifuge according to claim 3 wherein said smaller end of said other Icompartment is disposed between the smaller and larger ends of said one compartment considered along a longitudinal axis.

References Cited UNITED STATES PATENTS 2,795,635 6/1957 McBride 233-7 3,322,336 5/1967 LOhse 233-7 FOREIGN PATENTS 16,5 32 Germany. 1,021,297 12/1957 Germany.

ROBERT W. JENKINS, Primary Examiner

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