Background art Gaseous effluents are currently treated by means of various devices based on different operating principles. Therefore, there are centrifugal and filter- type mechanical dust separators, wet dust separators, and electrostatic filtration units.

Known centrifugal mechanical dust separators are also termed cyclones.

They are constituted by an axially symmetrical main body, in which the gas to be treated is introduced through appropriately shaped tangential ducts.

The gas accordingly assumes a vortical motion inside the main body. The rotary motion generates the centrifugal force required to propel the pollutant particles against the walls of the cyclone. In the classic configuration, the axially symmetrical main body of the cyclone is constituted by a first cylindrical section and by a second frustum-shaped section, which allows to trap gradually finer particles. The particles thus separated fall by gravity along the walls and are collected by gravity in a suitable hopper. The treated gas instead rises inside the cyclone with a substantially axial flow and exits through an appropriate duct.

Centrifugal mechanical dust separators are notoriously affected by drawbacks. Once the shape of the cylindrical and frustum-shaped main body is designed, the operating characteristics are set and it is not possible to vary them over the life of the cyclone. Moreover, there is the problem of limited dust separation efficiency for fine dust. Cyclones in practice allow to achieve a satisfactory standard only in the treatment of gases that contain particulates

of considerable size (> 20. 30 m).

Disclosure of the Invention The aim of the present invention is to overcome the drawbacks that affect the background art.

Within this aim, an object of the invention is to provide a gas fractionator-purifier whose operating characteristics can be changed and adjusted according to the operating requirements.

Another object of the invention is to allow to provide a gas fractionator- purifier that is easy to manufacture and therefore competitive even merely from an economical standpoint.

This aim and these and other objects which will become better apparent hereinafter are achieved by the gas fractionator-purifier according to claim 1.

Brief description of the drawings Further characteristics and advantages of the invention will become better apparent from the following detailed description of a particular embodiment thereof, illustrated only by way of non-limitative example in the accompanying drawings, wherein: Figure 1 is a schematic sectional side elevation view of the gas fractionator-purifier according to the invention, in a first embodiment; Figure 2 is a schematic plan view of the gas fractionator-purifier in a first embodiment; Figure 3 is a schematic sectional plan view of the gas fractionator-purifier in a first embodiment; Figure 4 is a schematic sectional side elevation view of the gas fractionator-purifier according to the invention, in a second embodiment.

Ways of can-vine : out the invention With reference to the figures, the gas fractionator-purifier, having a preferably vertical axis, is generally designated by the reference numeral 1.

It is constituted by a main chamber 2, which is fixed to a supporting frame that comprises an outer cylindrical enclosure, which in turn rests on a dust

collection hopper 16, and by an inner cylindrical frame 21, which supports the fixed inner parts.

Above the main chamber 2 there is a spiral duct 3 that is fixed to the chamber in a manner that is not shown. The spiral and slightly descending path of the spiral duct 3 connects the main chamber 2 to an inflow duct 4, which branches out from the chamber and is orientated tangentially to the main chamber 2. A rotary drum 5 is arranged inside the main chamber 2 and coaxially thereto. The drum is constituted by a solid disk 6 and laterally by a perforated cylindrical wall 7. The holes provided in the cylindrical wall 7 have typical diameters of 0. 50. 1. 00 mm and typically cover a surface equal to 20. 40%, depending on the mechanical characteristics of the material of which said wall is made. The rotary drum 5 is connected, by means of a shaft 8, to a per se known variable-speed motor assembly 9. A stator 10 is accommodated inside the rotary drum 5 and is constituted by a plurality of vanes 11. The stator 10 (flow regulator/straightener) and its vanes 11 are rigidly mounted on the inner cylindrical frame 21, which supports the main outflow duct 13, so that the rotary drum 5 can rotate freely about them. The shape of the vanes 11 of the stator 10 produces a free cavity 12, from which the main outflow duct 13 branches out. In the embodiment shown in Figure 1, between the free cavity 12 and the main outflow duct 13 there is an interposed connector 14, which allows to redirect the flow through 90°. In any case, it is evident that if the installation conditions were different, the path of the main outflow duct 14 also. might be different. For example, a connector with a different angle might be necessary, or no connector at all might be needed.

On the outside of the rotary drum 5, the walls of the main chamber 2 have an inclination that forms a tapering annular compartment 15, whose cross- section decreases downward. The tapering annular compartment 15 is connected to a hopper 16 by way of an additional compartment 15a. A retention cavity 20 lies above the hopper 16 and can be separated from it by

an optional perforated partition 17. An additional secondary outflow duct 18 branches out from the retention cavity 20.

Figure 4 illustrates another embodiment of the apparatus described so far.

It, too, is constituted by a main chamber 2 that is substantially symmetrical with respect to a vertical axis. A spiral duct 3 is arranged around the main chamber 2. The spiral and slightly descending shape of the spiral duct 3 connects the main chamber 2 to an inflow duct 4, which is orientated tangentially to the main chamber 2. Two rotary drums 5a and 5b are arranged inside the main chamber 2 coaxially thereto. The drums, like the one described earlier, are constituted by a solid disk 6 and by a perforated cylindrical wall 7. The rotary drums Sa and 5b are furthermore connected by means of a gearbox 19 to a per se known variable-speed motor assembly 9.

Two stators 10a and 10b are accommodated inside the drums. The stators 10 are rigidly mounted on the structure, so that the drums can rotate freely around them. The shape of the vanes 11 la and lib of the stators 10a and 10b produces two free cavities 12a and 12b, from which the main outflow ducts 13a and 13b branch out. In the embodiment shown in Figure 4, between the free cavity 12b and the main outflow duct 13b there is an interposed connector 14 that allows to redirect the flow through 90°, while no connector is interposed between the free cavity 12a and the main outflow duct 13a.

Externally with respect to the rotary drums Sa and Sb, the walls of the main chamber 2 have an inclination that forms a tapering annular compartment 15, whose cross-section decreases downward. The tapering annular compartment 15, by means of the compartment 15a comprised between the outer cylindrical enclosure and the inner cylindrical frame 21, leads to the hopper 16. A retention cavity 20 lies above the hopper 16, from which it is separated by a perforated partition 17. An additional secondary outflow duct 18 branches out from the retention cavity 20.

The gas to be treated is introduced through the inflow duct 4 with a certain pressure P4, whose energy corresponds to a certain velocity. More

particularly, the shape of the spiral duct 3 imparts to the gas and to the particles suspended therein a certain angular velocity. In this manner, a first fraction of particulate, the larger one, is propelled outward as in a cyclone of the known type, and strikes the walls of the main chamber 2. This first fraction of particles then falls along the outer walls of the main chamber 2 and reaches the hopper 16 by passing through the tapered annular compartment 15.

The remaining fraction of particulate is entrained in the vortical motion of the gas that rotates about the rotary drum 5. The drum 5 is in turn rotated by the motor assembly 9 by way of the shaft 8. In this step of the treatment, since the drum 5 and the gas to be treated have different angular velocities, a certain relative velocity persists between them. The fluid threads that move in contact with the wall 7 are aspirated by the holes with which said wall is extensively provided. This sudden change from the circular path forces an acceleration peak, which is applied to the gas but cannot be applied to the heavier parts suspended in the gas. This phenomenon prevents the particulate from entering, together with the gas, through the holes of the cylindrical wall 7. In this manner, the particulate falls along the cylindrical wall 7 and is conveyed into the hopper 16. The gas stream that has passed through the perforated cylindrical wall 7 with a radial motion encounters the vanes 11 of the stator 10. This generates a stream of gas that flows axially from the free cavity 12 toward the main outflow duct 13, where a pressure P13 stabilizes.

In the secondary outflow duct 18 (which as mentioned might also be shifted further down, in the connector with the hopper), where a pressure Pig is established, a secondary flow of gas is established which exits from the retention chamber 20. Said secondary flow assists the dust separation process, since the gas, in order to reach the retention chamber 20, must cross in succession the tapering annular compartment 15, the compartment 15a and the hopper 16. The flow of gas entrains the particulate in this path and

thus facilitates its removal. The presence of the perforated partition 17 and the large cross-section of the retention chamber 20 (with respect to the cross- sections of the compartment 15a and of the secondary outflow duct 18) allow to reduce considerably the residual turbulence in the gas stream, so as to limit the quantity of particulate that remains suspended. The stream of gas bled through the secondary outflow duct 18 must then be returned to the fractionator-purifier upstream of the spiral duct 3.

By adjusting the ratios among the three different pressures in the three ducts, it is possible to adjust the percentage of gas bled with respect to the total and therefore vary the efficiency of the purification process.

A large quantity of bled gas in fact corresponds to a highly efficient entrainment and removal of powders and therefore to a high efficiency of the entire process, but of course entails a reduction in the quantity of gas treated in the unit time. Vice versa, bleeding a small quantity of gas from the secondary duct 18 produces a scarcely efficient removal of dust and therefore a reduced efficiency of the entire treatment, but treats a large quantity of gas in the unit time.

In practice, for example, if P4 is set by an external system, P13 can be left to settle with respect to the value of the atmospheric pressure.

In this manner, by varying Pig the adjustment of the efficiency of operation is affected considerably. It is also possible to produce a negative pressure also in the main outflow duct 13, although this is not necessary for the operation of the apparatus.

Another adjustment of the operating characteristics of the apparatus is achieved by varying the rotation rate of the rotary drum 5. The higher the rotation rate, the lighter the fraction of gas that is aspirated through the holes of the cylindrical wall 7 of the rotary drum 5.

In the embodiment of the apparatus shown in Figure 4, the operation is qualitatively similar as regards dust separation. A considerable difference is instead the fact that the gas can pass through two different rotary drums Sa

and 5b, to which different rotation rates can be imparted. In this manner, by introducing a mixture of gas, it is furthermore possible to separate a first fraction that is rich in light components, which is aspirated through the faster drum, and a second fraction that is rich in heavier components, which is aspirated by the slower drum.

In this embodiment, of course, the pressures that regulate the operating characteristics of the fractionator-purifier are four: P4 in the inflow duct, P) g in the secondary outflow duct, and Pl3a and Pl3b in the two main outflow ducts.

The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims; all the details may be replaced with other technically equivalent elements. In practice, the materials used, as well as the dimensions, may be any according to requirements.

The disclosures in Italian Patent Application No. TV2002A000020 from which this application claims priority are incorporated herein by reference.