A centrifugal separator apparatus is known which comprises a tube into which the fluid is introduced and which is driven into rotation around its own axis. A drawback of such apparatus is that the centrifuge tube must be rotated at very high speeds, 60,000 r. p. m. or greater, in order to achieve a satisfactory degree of separation of particles from the fluid in which they are suspended or entrained.

We have now devised a centrifugal separator apparatus which is able to provide a satisfactory degree of separation when driven at substantially lower rotary speeds than prior art centrifugal separators.

In accordance with the present invention, there is provided a centrifugal separator apparatus which comprises a centrifuge tube which is closed at one end thereof, and mounted for rotation around its axis, drive means for rotating said centrifuge tube around its said axis, an inlet tube extending into said centrifuge tube from the second, opposite end thereof and to a point adjacent said closed end of said centrifuge tube, and a manifold coupled to said second end of the centrifuge tube and formed with a first outlet for dirty fluid and a second outlet for clean fluid.

In use, particle-laden fluid is fed longitudinally through the inlet tube and into the centrifuge tube, the fluid accordingly entering the centrifuge tube adjacent the closed end of the latter. The fluid now reverses its direction of flow, to pass longitudinally through the annular space between the inlet tube and the centrifuge tube towards the manifold. Owing to the rotation of the centrifuge tube, the fluid separates into two streams, a radially outer stream which is

relatively heavily laden with particles and a radially inner stream which is relatively lightly laden with particles.

We have found that a satisfactory separation performance can be achieved at relatively low rotational speed (e. g. 30,000 r. p. m.) of the centrifuge tube.

Preferably the manifold is formed with a longitudinal duct which forms a continuation of the centrifuge tube.

Preferably the outlets for dirty fluid and clean fluid comprise respective first and second frusto-conical passages which are formed in the manifold, communicating at their narrow ends with the longitudinal duct and at their wider ends with respective outlet ports.

In one embodiment, the longitudinal duct in the manifold is of uniform diameter along its length and preferably of substantially the same diameter as the inner diameter of the centrifuge tube. In another embodiment, the longitudinal duct in the manifold is of larger diameter in the section between the end of the centrifuge tube and the first frusto-conical outlet passage, than in the section between the first and second frusto-conical outlet passages.

The inlet tube may be stationary. Alternatively, at least a part of the inlet tube, which is disposed within the centrifuge tube, may be arranged to rotate. It is expected that this modification should improve the separation performance of the apparatus.

Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which: FIGURE 1 is a longitudinal sectional view of a first embodiment of centrifugal separator apparatus in accordance with the present invention; and FIGURE 2 is a longitudinal sectional view of a second embodiment of centrifugal apparatus in accordance with the present invention.

Referring to Figure 1 of the drawings, there is shown a centrifugal separator apparatus which comprises a base 10, a cylindrical casing 12 mounted on the base 10 and extending vertically upwards from the base, and a manifold 20 mounted on the top of the cylindrical casing 12. A centrifuge tube 14 is positioned within the casing 10, the axes of the centrifuge tube 14 and casing 10 being coincident. The centrifuge tube 14 is mounted on bearings, e. g. foil bearing assemblies one of which is shown at 16, for rotation around its axis at high speed. The cylindrical casing 12 is split into upper and lower sections 12a, 12b: the stator part 18 of an air turbine is mounted between the two casing sections 12a, 12b and the rotor part 19 of the air turbine is fixed to the centrifuge tube 14.

The bottom end of the tube 14 is closed: optionally, and as shown, a cruciform 15 is fitted into the tube 14 at its bottom end.

The manifold 20 comprises a body having an axial duct 22 having a reduced-diameter section 22a at the upper end of the manifold. An inlet tube 24 is fitted into the reduced- diameter duct section 22a of the manifold and extends axially along the duct 22 and the centrifuge tube 14 to terminate a short distance from the cruciform 15 at the bottom end of the centrifuge tube.

The manifold 20 comprises three sections 20a, 20b, 20c fitted one on top of another. The lower two manifold sections 20a, 20b co-operate to define a first frusto-conical passage 26 extending radially outwardly from the duct 22 to an annular passage 27, from which an outlet tube (not shown) for dirty gas projects tangentially. The upper two manifold sections 20b, 20c co-operate to define a second frusto-conical passage 28 extending radially outwardly from the duct 22 to an annular passage 29, from which an outlet tube (not shown) for clean gas projects tangentially. It will be noted that each of the frusto-conical passages 26,28 has its narrow end directed

towards the centrifuge tube 14. Also, the passage 28 for clean gas is substantially wider than the passage 26 for dirty gas.

In use, gas is fed under pressure into the top, projecting end of the inlet tube 24, this gas having particles entrained in it. The inlet gas passes along the tube 24 to exit from the opposite, lower end thereof, following which the direction of gas flow is reversed and the gas passes upwards in the annular space between the outer surface of the inlet tube 24 and the inner surface of the centrifuge tube 14. Owing to the rotation of the centrifuge tube 14, the flow of gas forms into two streams, a radially outer stream which is relatively heavily laden with particles, and a radially inner stream which is only relatively lightly laden with particles.

The first stream, of dirty gas, passes through the first frusto-conical passage 26 and out of the manifold 20 via the dirty gas outlet. The second stream, of. clean gas, passes through the second frusto-conical passage 28 and out of the manifold 20 via the clean gas outlet.

We have built a centrifugal separator apparatus as shown in Figure 1, the centrifuge tube being 310 mm long. We tested the apparatus using 0.3 micron particles entrained in air, the centrifuge tube being driven at 30,000 r. p. m. Under these test conditions, only 5 to 6% of the air passed out of the dirty gas outlet, the remaining 95 to 94% passing out of the clean gas outlet. Typically 60% of the particles were separated into the dirty gas outlet. By increasing the rotational speed of the centrifuge tube to 60,000 r. p. m., an improvement in the separation performance was achieved, 70 to 80% of the 0.3 micron particles being separated into the dirty gas outlet.

Figure 2 shows a centrifugal separator apparatus which differs from the apparatus of Figure 1 as regards the internal design of the manifold 20. In particular, in the first section 20a of the manifold, the duct 22 is larger in diameter than the

centrifuge tube 14 and is also larger in diameter than in the second section 20b of the manifold. We have built and tested an apparatus as shown in Figure 2. Using 0.3 micron particles entrained in air and driving the centrifuge tube at 30,000 r. p. m., we found approximately 15% of the air was separated into the dirty gas outlet stream. However, approximately 70% of the particles were carried in the dirty gas stream. When the centrifuge tube speed was increased to 60,000 r. p. m., 90% of the particles were carried in the dirty gas stream.

We have also built and tested the apparatus of Figure 2 with a shorter centrifuge tube, 150mm in length instead of 310mm. At 30,000 r. p. m., still 15% of the air was separated into the dirty gas stream. 60% of the 0.3 micron particles were separated into the dirty gas stream. When the rotational speed of the centrifuge tube was increased to 60,000 r. p. m., approximately 80% of particles were separated into the dirty gas stream.

It will be appreciated from the above that a satisfactory separation performance can be achieved at the relatively low rotational speed of 30,000 r. p. m. and even with a relatively short centrifuge tube of 150mm. The separation performance can however be improved by increasing the rotational speed.

Whilst in the above-described embodiments the inlet tube 24 is stationary, in a modification of either embodiment, at least part of the inlet tube may be arranged to rotate: in particular, the part of the inlet tube which lies within the centrifuge tube 14 may be mounted to rotate with the centrifuge tube, a seal being provided between this part of the inlet tube and the remaining part, which is disposed within the manifold 20.