EP0442788B1 - Wind sieve with centrifugal action - Google Patents

Abstract

A pneumatic centrifugal separator comprises guide vanes disposed along the generatrices of a fictitious cylinder having a vertical axis, the guide vanes being adapted to impart to a gas stream entering the fictitious cylinder a rotary motion about the vertical cylinder axis, and a rotor coaxially positioned in the interior of the fictitious cylinder, the rotor being equipped with a first set of vertical blades distributed uniformly along the periphery of the fictitious cylinder and a second set of blades disposed between the blades of the first set and the cylinder axis. A gas stream and particulate material to be sorted is introduced between the guide vanes and the rotor, and the gas stream charged with particles of dimensions smaller than predetermined dimensions and sorted out of the particulate material is drawn out of a central outlet. The second set of blades is arranged to guide the streams of gas coming through channels between adjacent vertical blades of the first set to the central outlet.

Description

The subject of the present invention is a selector intended for separating from a stream of solid particles suspended in a gas stream the particles whose size is greater than a predetermined dimension and comprising guide vanes arranged along the generatrices of a fictitious cylinder , with a vertical axis, and able to communicate to the current of gas entering said imaginary cylinder, a rotational movement around the axis of the cylinder, a rotor placed inside said imaginary cylinder, the axis of which coincides with that of the cylinder and which is provided with vertical blades regularly distributed over its periphery, and a central outlet orifice placed above or below the rotor and through which is drawn the stream of gas charged with particles whose dimensions are smaller than said dimension predetermined.

In selectors of this type, the particles suspended in the gas stream are subjected to two opposing forces: a centrifugal force resulting from the rotational movement and a drag force due to the centripetal flow of the gas stream towards the outlet orifice central. Large particles are separated at the outer cylindrical surface of the rotor. If the distribution of the gas stream over the entire height of the turbine is uniform, there is only one critical particle diameter or cut-off diameter corresponding to one particle in equilibrium on the outer surface of the rotor. Particles with a diameter greater than the critical diameter are rejected against the guide vanes by centrifugal force and fall by gravity into a collecting hopper placed under the vanes; while the particles of diameter smaller than the critical diameter are entrained by the gas current through the rotor towards the central exit orifice.

In known devices, the rotor is equipped with blades of small width arranged on its periphery and, in operation, a vortex is formed in the center of the rotor in which a significant part of the kinetic energy of the gas stream is dissipated.

The object of the present invention is to improve the performance and to reduce the energy consumption of a selector of this type by means of making it possible to overcome the turbulence of the flow between the guide vanes and the rotor and to avoid the formation of a vortex in the rotor.

The selector object of the present invention is characterized in that the rotor comprises a second set of blades, arranged between the peripheral blades and the axis, and serving to guide the gas streams to the central outlet of the rotor charged with fine particles leaving the peripheral blades. The blades of this second set extend over the entire height of the rotor and can be arranged in radial planes or be inclined relative to these planes. They can be flat or have a certain curvature, and can be formed by an extension towards the axis of the peripheral blades. The central part of the end wall of the rotor opposite the outlet orifice may have a profiled shape, for example frustoconical, favoring the flow of gas towards the outlet orifice.

Thanks to this second set of blades, a large part of the kinetic energy of the gas stream is used to rotate the rotor, which makes it possible to reduce the power of the drive motor. Under certain conditions of use, it is even possible to eliminate this motor, the speed of the rotor, on which the cut-off diameter depends, then being adjusted by adjusting the orientation of the guide vanes.

To increase the cutting precision, it is advantageous to give the channels delimited by the peripheral blades of the rotor a section which grows from the outside towards the inside of the rotor, so that the centrifugal and drag forces acting on the grains whose diameter is equal to the cut-off diameter balance practically over the entire length of said channels.

As in all devices of this type, the guide vanes and the rotor are enclosed in an envelope which delimits, around the guide vanes, an annular chamber in which the gas stream and possibly the materials to be sorted are admitted. The gas stream can be admitted into this chamber tangentially or parallel to the axis of the device, from below. The raw materials can be suspended in the gas stream before it enters said chamber or introduced separately, from above, into the space between the rotor and the guide vanes; these two feeding modes can also be used simultaneously.

According to a preferred embodiment of the invention, a hopper in the form of an inverted cone is placed under the rotor and the guide vanes, in order to collect the particles whose dimensions are greater than the cut-off diameter, the envelope is of revolution, concentric to the rotor and also surrounds said hopper by providing around it a passage with annular section, and a vertical duct is connected to the bottom of said casing, under said hopper and coaxially with it, to bring the gas stream charged with particles to sort in said chamber, through said passage; in the plane where said conduit opens into said envelope, the diameter of the latter is significantly greater than that of said conduit so that the gases loaded with particles are subjected, when entering said envelope, to an expansion promoting the fall of heavy particles at the bottom of the envelope. Said conduit may extend upwards above the bottom of the envelope and delimit therewith an annular volume in which the large particles separated from the air stream will be collected in the expansion zone thus created, the bottom of said envelope being preferably inclined and provided at its lowest point with an orifice for discharging said particles. One or more deflectors constituted by flat or frustoconical rings may be fixed on the outside of said hopper to deflect the gas stream and promote the separation of large particles.

• La figure 1 est une coupe verticale d'un sélecteur réalisé conformément à l'invention;
• La figure 2 est une vue en coupe par un plan horizontal de l'appareil de la figure 1, et
• La figure 3 est une section droite de deux pales du rotor de l'appareil.

Other characteristics of the invention will appear on reading the description which follows and refers to the accompanying drawings which show, by way of non-limiting example, an embodiment of the invention and in which:

• Figure 1 is a vertical section of a selector made according to the invention;
• FIG. 2 is a sectional view through a horizontal plane of the apparatus of FIG. 1, and
• Figure 3 is a cross section of two blades of the rotor of the device.

The selector shown in the drawings comprises a casing 10 constituting the body of the apparatus and formed of a cylindrical upper part, of an intermediate part in the form of an inverted truncated cone, of a cylindrical lower part connecting to the small base of the truncated cone and of an inclined bottom comprising, at its lowest point, an evacuation orifice 12. An inlet pipe for the gases laden with particles to be sorted 14 crosses the bottom of the envelope and extends upwards approximately to the junction plane of the intermediate parts and lower. The conduit 14 is arranged coaxially with the envelope and its end is flared.

At its upper part, the envelope is closed by a cover 16 comprising a central opening at the edge of which is connected a gas evacuation duct 18.

A rotor 20 is placed in the upper part of the envelope, coaxial with it. It is fixed to the lower end of a vertical shaft 22 mounted, by means of rolling bearings, in a tubular support 24 fixed on the cover 16. The shaft is coupled to a variable speed control group 26 allowing the rotor to rotate at the desired speed.

The rotor 20 has a large number of vertical blades 28 regularly spaced around its periphery. The lower and upper ends of the blades are fixed, respectively, on a bottom 30 formed by a flat ring and a central truncated cone integral with the shaft 22, and on a ring 32. A baffle joint 34, integral of the cover 16, ensures sealing between the latter and the rotor.

et

$\text{Fc2 = Ft2}$

The blades 28 admit as plane of symmetry a plane containing the axis of the rotor and, as can be seen in FIG. 3, the channels formed between the blades have a width which increases from the outside towards the inside of the rotor (L1 <L2) so that the centrifugal force and the drag force acting on a particle of critical diameter (cut-off diameter) are balanced almost over the entire length of the canals. By calling Fc1, Ft1 the centrifugal and drag forces at the entrance of a channel and Fc2, Ft2 these same forces at the exit of the channel, this operating condition can be translated by the relations:

$\text{Fc1 = Ft1}$

and

$\text{Fc2 = Ft2}$

The profile of the blades can be easily determined from these mathematical formulas translating the equality of the centrifugal and drag forces acting on a particle of given density and diameter, with a given speed of the rotor. The equilibrium conditions can be satisfied, with a given blade profile, for different cutoff diameters, by adopting different rotational speeds for the rotor.

Instead of being arranged radially, the blades 28 could form an angle with the radial planes, the width of the channels delimited by the blades always increasing gradually from the outside to the inside.

The rotor further comprises a second set of blades 35, arranged between the blades 28 and the axis of the rotor. In the example shown, the blades 35 are constituted by flat sheets, located in vertical planes containing the axis of the rotor, and fixed on the frustoconical central part of the bottom 30 and on the upper ring 32. These blades have for aim of avoiding the formation of a vortex inside the rotor and make it possible to recover a significant part of the energy of the gas current passing through the rotor. The blades 35 could be inclined and / or form an angle with the planes containing the axis of the rotor, they could also be profiled in the manner of the blades of a turbine. The rotor thus formed can be compared to the rotor of a centrifugal compressor which would operate as a receiving turbomachine taking energy from a continuous flow of fluid to transform it into mechanical energy.

This construction of the rotor makes it possible to eliminate the vortex, which would form inside the rotor if the latter were devoid of the blades 35, and consequently, to recover the energy which would otherwise be lost in the vortex and, by reduction of gas speed, decrease abrasion wear and pressure drop.

The rotor is surrounded by a circular row of vertical guide vanes 36 regularly spaced around the rotor. These blades are provided at their ends with pivots 38 housed in holes of an upper ring 40 fixed on the upper end of the casing and of a lower ring 42 mounted on the upper edge of a frustoconical hopper 44 placed under the rotor, in the frustoconical part of the envelope, and supported by feet 46 fixed on the envelope.

The upper pivots are provided with levers 48 connected together by a hoop so that, whatever their orientation, all the blades form the same angle with the respective radial plane. An actuator acting on the hoop allows to adjust the orientation of the blades remotely.

The operation of the selector described is as follows:
The stream of gas charged with the particles to be sorted flows from bottom to top in the pipe 14. When it reaches the upper end of the pipe, it is subjected to a sudden expansion due to the large difference in the diameters of the pipe and of the envelope that surrounds it at this level. This results in a decrease in the speed of the gas which allows the largest particles to fall to the bottom of the envelope, in the annular space formed between the end of the duct and the envelope, and to be evacuated by the orifice 12. One or more deflectors 50 can be fixed on the hopper 44, above the duct 14, to improve this separation.

The gas stream then rises up to the upper part of the casing 10, maintaining an almost constant speed, then flows between the vanes 36, which impart a circular motion thereto, and enters the rotor through the channels formed between the blades 28. The particles whose dimensions are smaller than the cut-off diameter are entrained in the rotor by the gas stream and evacuated therewith by the conduit 18 which is connected to the suction opening of a fan through a dust collector to separate particles from the gas stream. Particles larger than the cut-off diameter are kept outside the rotor by centrifugal force and fall by gravity into the hopper 44, through an annular slot formed between the rotor and the ring 42. If one of these large particles accidentally enters one of the channels of the rotor, it will be rejected towards the outside since the profile of these channels is designed to that the centrifugal force exerted on such a particle exceeds the drag over the entire length of the channel. The particles collected in the hopper 44 are discharged through line 45.

As indicated above, at least part of the particles to be sorted could be introduced by one or more inlets 17 disposed above the ring 32 of the rotor and projected by centrifugal force against a skirt surround the ring 32 to then fall into the space between the vanes 36 and the rotor and be suspended in the gas stream flowing transversely.

The cut-off diameter depends, for a given gas flow, on the speed of rotation of the rotor. This is maintained at the value chosen by regulating the speed of the motor 26. Since, thanks to the provisions of the invention, the power transmitted to the rotor by the gas current which passes through it can be greater than that which is necessary to rotate it at the set speed, the motor 26 must be able to operate as a brake with speed regulation. The orientation of the blades 36 is adjusted, as a function of the speed of the rotor, so that the tangential component of the speed of the gas and of the particles at the periphery of the rotor is approximately equal to the peripheral speed of the rotor; this setting can be made manually or automatically. This measurement makes it possible to avoid impacts of the particles on the blades of the rotor and to obtain a homogeneous fluid speed over the entire width of the channels between blades of the rotor.

For certain uses, it is possible, thanks to the invention, to eliminate the motor 26, the rotor then being mounted idle. In this case, the rotor speed is maintained at the set value corresponding to the chosen cut-off diameter, by adjusting the orientation of the blades 36.

This possibility leads to appreciable savings, because it makes it possible not only to remove the drive motor from the rotor but also to use a lighter support structure for the rotor.

Instead of being admitted axially from below, as in the devices described, the gas stream could be admitted tangentially into the envelope, at the level of the vanes 36.

In the embodiment shown in the drawings, the increase in cross section, from the inlet to the outlet, of the channels formed between the blades of the rotor is achieved exclusively by increasing their width. We could also consider increasing their height by replacing the peripheral, flat part of the disc 30 and the flat ring 32 by frustoconical rings facing each other by their large base.

It is understood that these modifications and all those resulting from the substitution of equivalent technical means are within the scope of the invention.

Claims (9)

1. Centrifugal action air classifier comprising guide blades arranged along the generatrices of a fictitious vertical-axis cylinder, and capable of imparting a rotational motion around the axis of the said cylinder to a gas stream entering the said fictitious cylinder, a rotor placed coaxially inside the said fictitious cylinder and provided with vertical regularly-spaced blades on its periphery, means to introduce the particles to be sorted out between the blades and the rotor and a central outlet through which the gas stream laden with those particles the dimensions of which are smaller than a predetermined dimension is sucked, characterized in that the rotor comprises a second set of blades (35) which are arranged between the peripheral blades (28) and the axis, and guide the streams of gas which come out of the peripheral blades (28) to the central outlet of the rotor.
2. Air classifier according to claim 1, characterized in that the said blades (35) extend over the whole height of the rotor and are arranged in radial planes or inclined with respect to these planes.
3. Classifier according to claim 1 or 2, characterized in that the rotor end wall (30) opposite the central outlet is so streamlined as to favour the flow of the gas toward the said outlet.
4. Classifier according to claim 1, 2 or 3, characterized in that the rotor is mounted free and means are provided to adjust the direction of the guide blades (36) so as to maintain the speed of the rotor at a set value.
5. Classifier according to anyone of the above claims, characterized in that the rotor blades (28) are streamlined in such a way that the channels formed between the blades widen from the outside toward the inside of the rotor.
6. Classifier according to anyone of the above claims, characterized in that it includes an inverted truncated-cone shaped hopper (44) placed under the guide blades (36) and the rotor to collect those particles separated from the gas stream the dimensions of which exceed the said predetermined size, a casing of revolution (10) surrounding the guide blades and the hopper and a vertical duct for conveying the stream of gas laden with the particles to be sorted out (14) which is connected to the lower part of the said casing , the duct, the hopper and the casing being coaxial, and in that, in that plane in which the said duct opens into the casing, the diameter of the latter is markedly larger than that of the said duct, so that the gas laden with particles is subjected, on its entering the said casing, to a pressure reduction which favours the fall of the heavy particles down to the bottom of the casing which is provided with means (12) for discharging the said particles.
7. Classifier according to claim 6, characterized in that the said duct (14) extends upwardly above the bottom of the casing (10) and delimits with the latter a ring-shaped volume in which the heavy particles are collected.
8. Classifier according to claim 6 or 7, characterized by one or several ring-shaped deflectors (50) fixed on the outside of the said hopper (44), at a certain distance above the top end of the said duct (14).
9. Classifier according to anyone of claims 1 to 5, characterized in that it comprises a casing (10) which surrounds the said guide blades (36) and delimits with the same a ring-shaped inlet chamber for the gas stream, means for introducing at least a part of the particles to be sorted out from the top between the rotor and the guide blades (36) and means (14) for allowing the gas stream to pass into the said chamber, either tangentially or from the bottom.

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