Centrifugal separator with spiral-formed passages The present invention relates to a centrifugal separator for separating a fluid containing heavy particles into a relatively light fraction and a relatively heavy fraction containing the particles, comprising a drum-shaped rotor with two opposite gable walls, and at least one spiral-shaped partition wall, which is arranged between the gable walls such that at least one spiral-shaped separation passage with a central inlet end and a peripheral outlet end are formed in the interior of the drum-shaped rotor. The centrifugal separator further comprises at least one inlet for supplying the fluid to be separated centrally into the rotor to the inlet end of the separation passage, a heavy fraction outlet for discharging the heavy fraction from the outlet end of the separation passage, a light fraction outlet for discharging the light fraction from the interior of the drum-shaped rotor, and an operation device adapted to rotate the rotor at a speed such that fluid flowing through the separation passage separates in the latter into light and heavy fractions exclusively as a result of the influence of centrifugal forces.

Such centrifugal separators are known from for example patent specifications SE 9470, DE 141959, GB 157974 and US 4161275.

SE 9470 shows a centrifugal separator in which the light fraction flows in the direction inwardly toward the centre of the rotor, i. e. in the opposite flow direction of the heavy fraction.

DE 141959 shows a centrifugal separator functioning in the similar manner as as that of the separator according to SE 9470 and having a through hole for heavy fraction in the partition wall between the spiral turns.

GB 157974 shows a centrifugal separator having three spiral- shaped separation passages horizontally oriented. An additional, perforated partition wall extends along each separation passage. The light fraction flows radially

inwardly through the perforated partition wall and further downwardly through holes in the lower gable wall to the light fraction outlet.

US 4161275 shows a centrifugal separator with the light fraction outlet situated centrally in the rotor and with pockets formed in the spiral-shaped partition wall for receiving and accumulating polluting particles (heavy fraction).

The object of the present invention is to provide a new simple and efficient centrifugal separator of the kind discussed above, which can replace sedimentation basins, be used as a sludge dewaterer or pre-dewaterer of sludge containing liquid to be dewated by a decanter, and be used for separating environmentally harmful particles from gases, for example smoke gases.

This object is obtained by a centrifugal separator of the kind initially described characterised in that the partition wall comprises a tapering portion tapering towards the outlet end of the separation passage and having at least one elongate free edge spaced from the gable walls, the free edge being formed as an overflow for light fraction which in addition works as a passage obstacle for heavy fraction. In principle, the new centrifugal separator works differently than any of the known centrifugal separators described above, namely by the fact that the light and heavy fractions have the same flow direction in the separation passage. As a result the new centrifugal separator will be easy to operate and have a relatively high capacity.

The new centrifugal separator is well suited for separating a fluid in the form of liquid and may advantageously replace conventional sedimentation basins. In this case the centrifugal separator is operated at a relatively low rotational speed. The capability of the sedimentation basin of separating a fraction having a high share of particles

from for example sewage depends on the sedimentation capability of the particles and the volume and shape of the basin, which from economical and practical reasons limits the separation efficiency of the sedimentation basin. A great advantage of the sedimentation basin however is that it has low operational costs.

Thus, the new centrifugal separator is able to clean sewage, which mainly takes place in the following manner. During a first phase the sewage is de-aerated when it flows through the initial spiral turns of the separation passage, in order to avoid flotation of the particles. During a second phase clean water separates (light fraction) from sludge containing particles (heavy fraction) when the sewage flows through the remaining spiral turns of the separation passage. During a last third phase the upper layer of the sewage flows in the form of clean water over the overflow to the light fraction outlet, while the sludge discharges through the heavy fraction outlet.

The new centrifugal separator may also be operated at a relatively high rotational speed. In this case it may advantageously in many cases replace decanters intended for dewatering sewage sludge. This is so because a decanter functions badly for large water flows having a low content of suspended particles. Therefore the sludge fraction from a sedimentation basin usually must be thickened before it can be supplied to the decanter. Another problem of the decanter is that flocks usually prevailing in sewage sludge are beaten into smaller particles by the rotating decanter drum when the sludge enters the decanter. These smaller particles are difficult to separate in the strong liquid flow through the decanter. A further problem of the decanter is that the sludge is pressed together on the innerwall of the decanter drum, which makes it difficult to dewater the sludge, since particles in the sludge tend to form a cover that prevents passage of surplus water. The compression of the sludge also makes it difficult to axially transport the particles without

causing stirring, which results in re-wetting, so that the possibility of obtaining a high dryness of the resulted dewatered sludge is reduced. With a centrifugal separator according to the invention the above mentioned problem thus can be substantially eliminated.

Research of mechanical sludge dewatering aiming at obtaining as high dryness as possible has proved that it is realistic to obtaine about 50 % dryness provided that sludge to be dewatered is conditioned in a suitable manner. Also, it has been proved that a reduction of the thickness by 50 % of a thin sludge layer may multiple the separation rates by four.

These research results suggest that the centrifugal separator according to the invention is beneficial to use for dewatering sewage sludge, since the sludge layer in the spiral-shaped separation passage can be made thin.

According to the invention the new centrifugal separator may also advantageously be used as a pre-dewaterer of sludge containing liquid to be dewatered further by a decanter.

In case the new centrifugal separator is operated at a relatively high rotational speed it works in the same manner as described above for a low rotational speed in connection with separation of liquid, with the additional fact that the stronger centrifugal forces acting on the liquid result in that water in the particles are released from the latter and separate into the light fraction. The light water fraction can successively be discharged via the overflow so that substantially all free water in the separation passage is removed, which means that the heavy particles containing fraction can be discharged with a very high dryness.

Centrifugal separator according to the invention may also advantageously be used for separating a fluid in the form of a gas, for example separation of environmentally harmful particles from smoke gas.

Deflecting means may be arranged in the separation passage for deflecting flowing light fraction towards the elongate free edge, so that the separation of the light gas or liquid fraction is facilitated.

The tapering portion of the partition wall may taper linearly or progressively, or step by step towards the outlet end of the separation passage, and suitably has at least two elongate free edges spaced from the gable walls, wherein each one of the free edges is formed as a light fraction overflow also acting as a passage obstacle for heavy fraction.

The tapering portion extends along at least one, preferably several turns of the spiral-shaped partition wall. Suitably, the tapering portion may extend along at least one fourth of the length of the spiral-shaped partition wall. From a physical point of view the separation passage should extend in as many spiral-shaped turns as possible, in order to achieve a long staying time of the fluid in the separation passage, which results in a good separation result. A suitable diameter of the spiral-shaped partition wall may be about 1,2 meter and the number of turns formed by the partition wall may be about 100 turns. For applications that require relatively low rotational speeds of the rotor the width of the partition wall may be about one meter, whereas the width the partition wall can be substantially smaller for applications that require relatively high rotational speeds.

With advantage, the partition wall may be formed with projections or indentations to create local turbulence in the passing fluid, which speeds up the separation. Suitable, the projections or indentations extend diagonally or along curves towards the middle of the partition wall.

The heavy fraction outlet may be connected to a control valve for controlling the outgoing flow of the heavy fraction. Also the light fraction outlet may be connected to a control valve for controlling the outgoing flow of the light fraction.

The invention is described in more detail in the following with reference to the accompanying drawings, in which Figure 1 shows an axial cross-section through a rotor of a centrifugal separator according to an embodiment of the invention, Figure 2 schematically shows a side-view of a spiral- shaped partition wall in the rotor according to the figure 1, Figure 3 shows a partition wall in an outspread shape, Figure 4 shows a modification of the embodiment according to figure 1, and Figure 5 shows a decanter provided with a centrifugal separator according to the invention.

Figure 1 shows a drum-shaped rotor 1 of a centrifugal separator according the invention with two gable walls 2 and 3, and a tubular shaft 4, to which the gable walls 2,3 are attached. A spiral-shaped partition wall 5 is arranged in the interior of the rotor 1 between the gable walls 2,3 and forms a spiral-shaped separation passage 6 with a central inlet end 7 and a peripheral outlet end 8, see figure 2. The interior 9 of the tubular shaft 4 and an elongated hole 10 in the wall of the shaft 4 form an inlet for supplying a fluid to be separated to the separation passage 6 at the inlet end 7 thereof. Possibly, two different fluids may be mixed in the interior of the shaft 4 by supplying the fluids from opposite ends of the shaft 4, so that a fluid composed of said two different fluids is supplied to the hole 10.

This mixing procedure may be beneficial to do when a chemical substance facilitating separation is to be added.

An operation device, not shown, of a conventional kind is adapted to rotate the rotor 1 at a speed which results in

that the fluid flowing through the separation passage 6 separates in this into light and heavy fractions exclusively as a result of the influence of centrifugal forces. Thus, the separation result is not affected by gravity.

The partition wall 5 consist of a band-shaped plate having a linear tapering portion 11 with two side edges 12 and 13, see figure 3. The side edges 12,13 are folded upwardly so that they form two overflows at mutual sides of the tapering portion 11 of the partition wall 5. In this embodiment the partition wall 5 is in total 250 meter long and one meter broad up to the tapering portion 11, which extends along more than half the length of the partition wall 5. With the partition wall 5 in a rolled up state and mounted in the rotor 1 the portion 11 thus forms a tapering chute that guides particles in the heavy fraction towards the apex end of the partition wall 5, where the heavy fraction is discharged from the centrifugal separator via a heavy fraction outlet 14, whereas the light fraction passes across the side edges 12, 13 and is discharged from the centrifugal separator via a light fraction outlet 15. Of course, several spiral-shaped partition walls maybe arranged in the rotor 1, so that several separate separation passages are formed.

Figure 4 shows a modified rotor 16, in which a spiral-shaped partition wall 17 similar to the partition wall 5 is arranged, but having a differently shaped tapering portion 18. Thus, the portion 18 tapers progressively along one single curved side edge 19 towards one short end of the partition wall 17. A control valve 20 is adapted to control the outgoing flow of the heavy fraction and a control valve 21'is adapted to control the flow of the outgoing light fraction.

Figure 5 shows a decanter 22, provided with a centrifugal separator 23 according to the invention. The centrifugal separator 23 is arranged at the inlet end of the decanter 22 with the rotor oriented coaxial with a decanter screw 24 of

the decanter 22. Sewage sludge to be dewatered by a decanter 22 first enters into the centrifugal separator 23 where the sludge is dewatered to a consistency that optimises the function of the decanter 22.