DE29724707U1 - Wind sifter for separation of different grain sizes - has cross=sectional faces of rotor passages designed so that ratio of forces acting upon particles over part of passages is constant up to maximum deviation of plus or minus 10 per cent - Google Patents

Abstract

The wind sifter has at least one sifter rotor (4). The cross-sectional faces of the rotor passages (4.2) through which the air passes are designed so that the ratio of the forces acting upon the particles over a substantial part of the passages is constant up to a maximum deviation of plus or minus 10 per cent. The rotor blades are conical in shape and their apexes point inwards. The sides of the conical blades 4.1 of the sifter rotor may have two different pulling radii.

Description

PA Dr. A. Binder, Neue Bahnhofstraße 16, 89335 Ichenhausen ' 97106GBM - Schmidt & Co. GmbH & Co.KG - isodynamic notch:.

Wind sifter with sifting rotor

The invention relates to an air classifier with a classifying rotor according to the preamble of claim 1.

Similar air classifiers are basically known from the patent specification US 5,377,843 and the published patent application DE 38 38 871 A1.

The generic term used in this document is the published patent application DE 44 16 034 A1. This document proposes an air classifier with a rapidly rotating classifying rotor, the channels of which are designed in such a way that the radial flow speed of the incoming classifying air decreases from the outside to the inside. The aim of this is to achieve better classification. However, it was not disclosed to the expert how large the change in the radial flow speed should be in order to achieve optimal separation.

In addition, the patent specification GB 515,717 shows a similar air classifier with a horizontally arranged classifying rotor axis, whereby the classifying rotor has a central air outlet.

However, with this design, uncontrolled turbulence occurs in the classifying rotor due to unfavorable cross-sectional conditions between the channels of the classifying rotor and the air outlet channel, which can lead to the formation of streaks in the material being classified and, in the worst case, to the ejection of fine material already in the interior of the rotor, which impairs the classifying result.

The invention is based on the object of providing an air classifier with a classifying rotor which, due to its design, produces fines which, compared to the prior art, have a reduced proportion of oversized particles in the fines.

These objects are solved by the features of the first device claim.

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PADr. A. Binder, Neue Bahnhofstrasse 16, 89335 Ichenhausen .· .· . : : .: 97106GBM-Schmidt & Co. GmbH & Co.KG-isodynamischer.torb»·«» ·'· *«*« '«

Regarding the mechanisms and theory of air separation, reference is made to the publication Ulimann's Encyclopedia of Industrial Chemistry, VCH-Verlagsgesellschaft mbH, Vol. B2, pages 17-Iff.

The inventor has recognized that in order to achieve an accurate separation with necessary

is to create a viewing zone through which the material to be viewed passes, whereby in this viewing zone there should be a constant force ratio between the outward-directed centrifugal force F _z and the opposing air resistance force F _L over as large a distance as possible.
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The centrifugal force acting on a rotating particle is:

F _z = m*ü) ² *R = constant * &rgr; * d ³ * &ohgr; ² *R

or

V _T ² , V _T ²

F _z = m* —- = const. * &rgr; * d ³ * —-- ^Z R ^K R

with:

d diameter of the particle,

&rgr; density of the particle,

R distance of the particle from the center of rotation,

ω Angular velocity of the particle as it passes through the

sight rotor,

V _T Tangential component of the particle velocity.

Furthermore, the following applies to the air resistance of a particle flowing against it:

p _L d ²

F _L = c _w *--^* &pgr; *---* V _R = constant *d ² *V _R

with:
d diameter of the particle,

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PA Dr. A. Binder, Neue Bahnhofstrasse 16, 89335 Ichenhausen ' .! * , 97106GBM - Schmidt & Co. GmbH & Co.KG - isodynamic notch»' .

p _L density of the surrounding air,

c _w drag coefficient of the particle,

V _R radial velocity component of the surrounding air relative to the

particles.
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Thus, simplifying the constant parts of the equations and a certain particle diameter d results in a force ratio:

F _z ω ² * RV ₁ ²

— = const. *d* = const. * d *—-----

F _L V _R V _R * R

If the geometric design of the classifying rotor is designed in such a way that this force condition remains as constant as possible when the particles pass through the air channels of the classifying rotor, a relatively long-lasting field of tension is created for the particles passing through the channels, which leads to an improved separation of the oversized particles in the fine material. It is assumed here that there is essentially a turbulence-free, laminar flow and that the wall effects do not cause any significant change in the classifying process, or due to the lower air speed near a wall, only leads to the particles affected by them being separated from the classifying air flow.

Another possibility is to increase the ratio of the forces F _Z /F _L on the way through the channel of the classifying rotor, but this leads to a strong material loading of the classifying air flow, since particles that have already been sucked in are carried away again, i.e. migrate back in the channel and possibly lead to undefined conditions.

To implement the inventive concept, either the radial component of the classifying air, the angular velocity of the particle or both components simultaneously can be influenced as it passes through the channels of the classifying rotor depending on the distance to the center of rotation of the particle. This can be achieved, for example, by widening the radially arranged rotor channels (from the outside to the inside).

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PA Dr. A. Binder, Neue Bahnhofstrasse 16, 89335 Ichenhausen ' . ^: * , 97106GBM - Schmidt & Co. GmbH & Co.KG - isodynamic KotB.

by means of conically shaped rotor blades or by means of upper and lower discs of the classifying rotor that diverge conically from the outside to the inside, i.e. the height of the channels increases towards the inside, or a combination of both variants. In this case, the change in the passage areas should be inversely proportional to the radius. 5

Another embodiment of the rotor according to the invention can be to design the channel course with a constant passage area in such a way that the flow direction of the classifying air is deflected into the direction of rotation of the classifying wheel with a decreasing radius, so that the angular velocity of the particle increases with decreasing distance from the center of rotation and the product of G) ²⁺ R remains constant through the entire channel or at least a significant part of it.

Another possible variant can be any combination of the variants mentioned above. For example, the rotor can be designed in such a way that the upper and/or lower disks move away from each other towards smaller radii, while at the same time using blades that become thinner towards the center, which also create channels whose flow direction creates an angular velocity of the particles passing through that increases towards the inside, which on the one hand maintains a maximum entry area of the channels while at the same time maintaining the equilibrium condition described above for the forces acting on a particle of a certain diameter d.

A further design of the classifying wheel, which continues the inventive idea, consists in designing the interior of the classifying rotor in such a way that after the classifying air has passed through the channels of the classifying rotor, the air resistance forces F _L increase sharply compared to the centrifugal forces F _z and thus lead to a rapid removal of the solid particles that have penetrated inwards and prevent undesirable oversaturation of the classifying air. Possible designs could, for example, be radially mounted guide plates inside the classifying basket, which results in a constant angular velocity ω with increasing air velocity due to the narrowing air passage area. Another design, or one that can be carried out at the same time, could be a cone mounted inside the classifying wheel.

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PA Dr. A. Binder, Neue Bahnhofstraße 16, 89335 Ichenhausen * .j 97106GBM - Schmidt & Co. GmbH & Co.KG - isodynamic basket ¹ "'"

whose tip points in the direction of the outflowing classifying air and whose wall inclination is designed in such a way that a constant to increasing air velocity (without taking the tangential component into account) results for the classifying air inside.
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A further advantageous embodiment of the air classifier is to set at least two classifying rotors according to the invention coaxially into one another and, if necessary, to drive them separately. It can also be advantageous if an additional discharge space is provided between the classifying rotors so that the material separated here can be discharged from the classifier in a separate material flow.

Another advantageous design of the air classifier can be to connect a basket, which is designed in terms of its channel geometry according to the criteria explained in this application, directly to a channel leading out of the classifying chamber, which in this case must be rotatable, so that a closed basket unit is created that has no labyrinths in the area of the classifying chamber, so that the only path for the classifying air into the interior of the basket must be through the channels of the classifying rotor formed by the blades. In this way, it is avoided that incorrect grain can enter the interior of the classifying rotors, in which only the classified fine material should be located, through secondary air flows from the classifying chamber. This ensures that every grain that reaches the interior of the classifying rotors from the classifying chamber is exposed to the sorting process of counter-rotating centrifugal force and air resistance force described above to a well-defined extent.

All embodiments of the classifying rotor according to the invention can be used in any classifier type, such as compact classifiers with integrated fan, classifying chamber housing and separation chamber, cyclone air classifiers, cross-flow classifiers, air flow classifiers or other known classifier types.

The invention is described in more detail with reference to the following figures. They also show the following:

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PADr. A. Binder, Neue Bahnhofstrasse 16, 89335 Ichenhausen *.·*.·. *· *· , : i* , 97106GBM- Schmidt & Co. GmbH & Co.KG -isodynamicfiKertf.» .»'. ..'■ \.'*l-

Figure 1: Section through a compact sifter;

Figure 2: Section through the separating chamber housing of a cyclone air separator;

Figure 3: Section through the separating chamber housing of a cross-flow separator;

Figure 4: Classifying rotor according to the invention with parallel upper and lower disks;

Figure 5: Classification rotor with inclined/conical air channels;

Figure 6: Classification rotor with inclined air channels and parallel cover disks;

Figure 7: Classification rotor with curved/conical air channels;

Figure 8: Classifying rotor with radially arranged blades and conical cover disks;

Figure 9: Classifier with classifying rotor, without labyrinths.

Figures 1 to 3 show known types of air separators which are particularly suitable for use with the separating rotors according to the invention.

Figure 1 shows a longitudinal section through a compact sifter. The compact sifter consists of the rotating area with the drive 1, the circulating air fan 2, the sifting rotor 4 and the multi-stage spreading plate system 5. It also consists of the static area with the inner housing 6, which is made up of the sifting chamber 9, the guide vane system 8, the coarse material collection cone underneath and the outer housing 7, which acts as a fine material separator.

The inner housing 6 and the outer housing 7 form a cylindrical/conical double housing, whereby the two housings are connected via the circulating air fan 2 located at the top and the guide vane system 8 located at the bottom. The circulating air fan 2 generates a spiral-shaped ascending air flow in the classifying chamber. The feed material to be classified is introduced into the classifying chamber via an entry device (for example a central inlet chute, a tubular screw or an air conveyor trough), fed into the rotating spreading disc system and distributed into the air flow. Due to the effect of centrifugal, gravitational and air resistance forces, a separation into coarse and fine material takes place in the classifying chamber, particularly when the classifying air enters the classifying rotor 4. The fine material carried in the air flow flows through the classifying rotor 4, the circulating air fan 2 and is separated in the fine material separation chamber 10 by

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The coarse material falls into the coarse material collection cone 11 and is also discharged. The circulating air, which has been cleaned of fine material, returns to the classifying chamber via the ring-shaped guide vane system 8. By influencing the amount of circulating air by changing the speed of the circulating air fan and/or changing the rotational component of the circulating air by changing the speed of the classifying rotor 4, the size of the separated grain can be influenced and a finer or coarser classification can be achieved.

Figure 2 shows a longitudinal section through a classifying chamber housing of an exemplary cyclone air classifier. Not shown here is the separate circulating air fan and the separation system for separating the fines from the classifying air, such as a cyclone or a filter. The part of the cyclone air classifier shown has a classifier housing 12 with a classifying material feed device 3, a guide vane system 8, a classifying rotor 4 with the drive 1 and a fines blow-out head 13. The difference to the compact classifier from Figure 1 consists essentially in the separate arrangement of its individual components: classifier housing, circulating air fan and fines separator.

The separate circulating air fan (not shown here) generates the necessary classifying air, which is fed via connecting pipes to the classifying housing 12 in the area of the two-stage guide vane system 8. The incoming classifying air is given a rotational component by the guide vane system 8 and rises in a spiral shape into the classifying chamber 9. As it rises, the feed material to be classified, which is distributed in the classifying chamber via an input device 3 and a rotating spreading plate, is flooded with the classifying air and dispersed. The effect of the forces described above on the individual grains of the classifying material causes a separation into coarse and fine material, particularly when the classifying air enters the classifying rotor 4. The fine material carried in the air flow flows through the classifying rotor if it is sufficiently fine and is fed via the fine material blow-out head 13 and subsequent pipes (not shown here) to the fine material separator (for example a cyclone or filter) and separated there. The coarse material falls into the coarse material collection cone 11 and is discharged from there. The circulating air cleaned of the fine material returns via

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PA Dr. A. Binder, Neue Bahnhofstraße 16, 89335 Ichenhausen * . 97106GBM - Schmidt & Co. GmbH & Co.KG - isodynamicei*Keri>

Clean gas pipes, the recirculation fan and the guide vane system return to the classifying chamber and the cycle begins again. The separation grain size can be influenced by influencing the amount of recirculated air by changing the resistance on a swirl regulator of the recirculation fan or by changing the speed of the fan and/or by changing the rotational component of the recirculated air by changing the speed of the classifying rotor.

Figure 3 shows a longitudinal section through a cross-flow sifter, whereby, as in Figure 2, the circulating air fan and the fines separator of the system are not shown. The cross-flow sifter consists of the sifter housing 1, the input device for the sifted material 3, which here consists of a simple chute, and the spreading plate system 5 underneath, the sifting rotor 4 with the fines blow-out head and the guide vane system 8 arranged concentrically around the sifting rotor 4. A significant difference between the sifters shown in Figures 1 and 2 is that the blades of the guide vane system through which the sifting air is introduced into the sifting chamber and the blades of the sifting rotor are essentially at the same height. As with the cyclone circulating air sifter, a separate circulating air fan is used to generate the necessary sifting air. This is fed to the separator housing 12 via a connecting pipe and enters the separating chamber 9 through the guide vane system 8 as a horizontally running air spiral.

The feed material to be sifted is fed to the spreading disc system 5 via an input device 3, from which it is evenly scattered into the sifting chamber 9, which is located between the guide vane system and the sifting rotor 4. As it falls through the sifting chamber, the sifting material is ventilated by the spiral-shaped sifting air at right angles to the direction of fall. The counteracting air resistance and centrifugal forces separate the material into fine and coarse particles. The centrifugal force leads to the separation of the coarse particles, so that these coarse particles fall into the sifting cone. The fine particles are fed with the air flow through the sifting rotor 4 to the fine particles blow-out head 13 and via further pipes to the separating device (for example a cyclone or a filter). Here the fine particles are separated from the circulating air, and the cleaned circulating air returns to the sifting chamber via a clean gas line, the circulating air fan and the guide vane system. The separation grain size of the sifting process

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PADr. A. Binder, Neue Bahnhofstrasse 16, 89335 Ichenhausen ".·*,·. *: *| ,: :*,* 97106GBM- Schmidt & Co. GmbH & Co.KG - isodynamic-fcorl»»« >'. «?» %«* »5-«

can be brought about by a change in the amount of circulating air and/or a change in the rotational component of the circulating air by a change in the speed of the classifying rotor 4 or also by a change in the angle of attack of the guide vanes of the guide vane system 8.
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The following figures 4 to 8 show different classifying rotors, the special feature of which lies essentially in the design of the channels that lead the classifying air from the classifying chamber into the interior of the classifying rotor.

Figure 4 shows a cross section A-A and a longitudinal section B-B through a classifying rotor 4.

Section AA shows the classifying rotor with 12 rotor blades 4.1, which are arranged concentrically between an upper cover disk 4.5 and a lower cover disk 4.6. The blades have the shape of a circular sector and with their straight side surfaces 4.3 and 4.4 form channels 4.2 which widen inwards. The circular sectors of the blades 4.1 are aligned with their tip or their angle bisector towards the centre of the classifying rotor and their tip touches a common inner circle radius R ₁ and their outside touches an outer radius R _A. The channels 4.2, which are each formed by two rotor blades 4.1, have an inlet cross-section in the region of the outer radius F _A , which results from the product of the height of the channel, i.e. the distance between the upper and lower cover disks at the outer radius, and the width of the channel, i.e. the distance between the two adjacent rotor blades 4.1 in the region of the outer radius. The same applies to the inner edge of the channel. The following therefore applies:

F _A = H _A *B _A and F ₁ = H ₁ ⁺ B ₁ .

with:

F _A = external cross-sectional area of the channel 4.2

F ₁ = inner cross-sectional area of the channel 4.2

H _A = Height of the channel outside

&EEgr;, = Height of the channel inside

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PADr. A. Binder, Neue Bahnhofstrasse 16, 89335 Ichenhausen *. ^:# . ^: . &Iacgr;^# ! .: :*, 97106GBM - Schmidt & Co. GmbH & Co.KG - isodynamicei>fear*>»« >'« »*» V· V:«

B _A = Width of the channel outside B ₁ = Width of the channel inside

If the air density within the duct does not change, the radial component V _R of the air flowing through it is:

Q = F^V _8x = F ₁ ⁺ Vr ₁

Q = H _A · B^V _8x = H ₁ * B ₁ ^V ₁₁₁ .

with:

Q = air volume flow through the channel VRA ~ radial velocity component of the particle at the outer radius R _A Vr ₁ = radial velocity component of the particle at the inner radius R ₁

In order to obtain an optimal viewing result, it is now - as mentioned at the beginning - necessary to keep the ratio of centrifugal force and air resistance within the air channel as constant as possible. This means that the ratio of ω ² * R to V _R must remain constant within the rotor channel 4.2. Assuming that the angular velocity of a particle running through the channel 4.2 is equal to the angular velocity of the rotating viewing rotor 4, this results in:

R _A R ₁

Vra VR ₁

and thus the relationship

R _A * F _A = R ₁ ⁺ F ₁ .

Since in the given case the upper and lower cover plates 4.5 and 4.6 run parallel to each other, the following relationship results for the widths of the channels at the inlet and outlet of the channel B _A and B ₁ :

Ra

R ₁ B _A .

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If the channels 4.2 in the schematically shown classifying rotor are designed according to the above formula, it can be assumed that the particles passing through the channel are exposed to the same constant force ratio between air resistance and centrifugal force over the entire channel section, assuming a laminar flow with an incompressible gas.

This greatly reduces the probability that a particle of an incorrect size reaches the interior of the classifying rotor.

In section B-B it is shown that the rotor blades 4.1 are firmly connected to a closed upper cover disk 4.5, which is driven by a shaft 4.9. The lower cover disk is designed as a ring, which in turn is firmly connected to the rotor blades. A labyrinth 4.8 is attached to the lower side of the lower cover disk 4.6, which is intended to create as airtight a connection as possible to a fixed air duct through which the classifying air from the classifier is led to a separator. It is obvious that a key factor in ensuring that the classified fine material is free of imperfections is the quality of the labyrinth 4.8, i.e. that as few imperfections as possible are allowed to pass from the classifying chamber into the fine material flow that are not exposed to the actual separation process.

In order to avoid this risk of non-screened incorrect material flows, it is possible to firmly connect the classifying rotor to the air duct for the fine material flow leading out of the classifier, thus avoiding any labyrinths and thus incorrect flows between the classifying chamber and the fine material flow, and to only allow a sliding connection between the rotating classifying rotor and the permanently connected rotating air duct to a fixed onward line outside the classifying chamber. An example of such a design is described in Figure 9.

A similar embodiment of a classifying rotor is shown in Figure 5 in sections A-A and B-B. In this embodiment, the classifying rotor blades 4.1, whose shape corresponds to the blades in Figure 4, are set with their tips against the direction of rotation 4.10 of the rotor. This results in a particle that

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through the channel 4.2 from the outside of the classifying rotor 4 into the inside of the classifying rotor, a reduction in the angular velocity occurs, since the particle has a velocity component when passing through the channel that is opposite to the rotation. Furthermore, an additional change in the entry surface at the channel entry occurs because the upper cover plate 4.5 and the lower cover plate 4.6 are conical to one another, at least in the area of the classifying rotor blades 4.1, so that the entry height H _A is lower than the exit height H ₁ from the air channel 4.2. If the different angular velocity of a particle penetrating the channel 4.2 is taken into account, the following relationship results, which according to the invention should be fulfilled for an optimal classifying rotor:

<*> _A ² R _A * H _A * _BA ω, ² R, &bull;&EEgr;, &bull;&Bgr;, Vra 0 _Vr1 O <*>a ² R _a O ₁ ² R ₁

Figure 6 shows another variant of a classifying rotor according to the invention with channels 4.2 that also point backwards. In this case, the rotor blades 4.1 have an outer edge whose radius is congruent with the outer radius of the classifying rotor itself. The upper and lower cover disks 4.5 and 4.6 run parallel in this case.

Figure 7 shows a further embodiment of a classifying rotor 4. In this embodiment, the rotor blades and thus also the channels created thereby are curved and the upper and lower cover disks have a conical design according to Figure 5.

Figure 8 shows another embodiment of a classifying rotor with conical upper and lower cover disks 4.5 and 4.6, whereby the individual rotor blades 4.1 are designed as simple flat irons. This type of design makes it necessary to have a strong conical expansion between the upper and

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PA Dr. A. Binder, Neue Bahnhofstrasse 16, 89335 Ichenhausen

97106GBM - Schmidt & Co. GmbH & Co.KG - isodynamic fcoitf»« *»* *«■ '** ' ··

lower cover disk 4.5 and 4.6 in order to enlarge the channels 4.2, which are formed between the radially positioned rotor blades 4.1, from the outside to the inside in their cross section despite the radial inward adjustment of the blades and to be able to achieve the effect according to the invention.
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Figure 9 shows a further version of the sifter. The sifter has a cylindrical/conical outer housing 12 into which an input device, here an input screw 3, introduces the feed material A. The feed material A is fed onto a centrally arranged rotating spreading plate 5 and distributed in the sifter's sifting chamber 9. In the conical area of the sifting chamber housing, the air supply 14 with the guide vane system 8 is attached, through which the clean sifting air flows into the sifting chamber 9. Above the spreading plate there is a sifting rotor 4 with channels 4.2 designed according to the invention. The special thing about this sifter is that the channel 4.7 leading out of the sifter is firmly connected to the sifting rotor 4, which is closed except for the channels 4.2, and thus has no labyrinths within the sifting chamber 9 that could create an incorrect flow between the sifting chamber and the interior of the sifting rotor. The rotating duct 4.7 is suspended in a bearing 15 and seals the viewing space from the outside air with a labyrinth 16. At the transition between the rotating duct 4.7 and the fixed part of the exhaust air duct, another labyrinth 17 is arranged, which is intended to seal the viewing air flow from the outside air.

With all the embodiments shown, it is possible to achieve significantly better results in terms of a reduced oversize particle proportion in the fine material compared to the state of the art.

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List of reference symbols drive 1 Recirculation fan 2 Entry device 3 Sight rotor 4 Rotor blades 4.1 channel 4.2 1. Blade surface 4.3 2. Blade surface 4.4 Cover plate above 4.5 Cover plate below 4.6 Air duct 4.7 labyrinth 4.8 drive shaft 4.9 Rotation direction 4.10 Spreader disc system 5 Inner casing 6 Outer casing 7 Guide vane system 8th Viewing space 9 Fines separation room 10 Coarse material collection cone 11 Sifter housing 12 Fine material blow-out head 13 Injection channel 14 storage 15 labyrinth 16 labyrinth 17 fixed air line 18

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Claims (24)

1. Air classifier with at least one classifying rotor ( 4 ) for separating grain classes or particles of different densities with the aid of a rotating air flow, the classifying rotor ( 4 ) having a plurality of boundary surfaces ( 4.3 , 4.4 , 4.5 , 4.6 ) which form a plurality of channels ( 4.2 ) through which the classifying air flows from the outside to the inside, characterized in that the cross-sectional areas of the channels ( 4.2 ) are designed such that the ratio of the forces F _z /F _r acting on a particle is constant at least over a substantial part of the channels up to a maximum deviation of +/-10%. 2. Air sifter according to claim 1, characterized in that the deviation is a maximum of 5%. 3. Air classifier according to claim 1, characterized in that the deviation is a maximum of 1%. 4. Air sifter according to claim 1, characterized in that the force ratio is constant. 5. Air classifier according to one of claims 1-4, characterized in that the blades ( 4 , 1 ) of the classifying rotor are conical and their tip points inwards. 6. Air classifier according to claim 5, characterized in that the bisectors of the conical blades of the classifying rotor point to a common draft circle radius. 7. Air classifier according to claim 5, characterized in that the angle bisectors of the conical blades ( 4.1 ) of the classifying rotor point to a common center. 8. Air classifier according to claim 5, characterized in that the sides of the conical blades ( 4.1 ) of the classifying rotor have two different draft circle radii. 9. Air classifier according to one of claims 1-8, characterized in that classifying rotor blades are arranged at an angle > 0° to the radius, wherein the channels are directed with respect to their flow direction against the direction of rotation ( 4.10 ) of the rotor ( 4 ). 10. Air classifier according to one of claims 1-9, characterized in that at least one side (4.3, 4.4) of the classifying rotor blades has a curvature when viewed in cross section. 11. Air sifter according to claim 10, characterized in that the at least one curvature is circular. 12. Air sifter according to one of claims 10-11, characterized in that the at least one curvature is parabolic. 13. Air classifier according to one of claims 1-12, characterized in that the lateral surfaces ( 4.5 , 4.6 ) of the classifying rotor are at least partially conical in the region of the blades ( 4.1 ). 14. Air classifier according to one of claims 1-13, characterized in that the classifying rotor has a diameter/height ratio between 2:1 and 1:1. 15. Air classifier according to one of claims 1-14, characterized in that the outer surface of the classifying rotor through which classifying air flows and which rests on the inside of the blades is 0.75 to 1.25 times the open circular area of the classifying rotor at the air outlet. 16. Air classifier according to one of claims 1-15, characterized in that the outer surface of the classifying rotor through which classifying air flows and which rests on the inside of the blades is 0.9 to 1.0 times the open circular area of the classifying rotor at the air outlet. 17. Air classifier according to one of claims 1-14, characterized in that the open air passage area at the inner radius of the blades of the classifying rotor is equal to the open circular area of the outlet from the classifying rotor. 18. Air sifter according to the preamble of claim 1, characterized in that the channels have air passage areas that remain constant over their depth (from the outer radius to the inner radius). 19. Air sifter according to claim 18, characterized in that the walls ( 4.3 , 4.4 ) of the channels ( 4.2 ) run parallel. 20. Air classifier according to one of claims 1-19, characterized in that the classifying rotor and the adjoining air line for discharging the classifying air are firmly connected to the classifying rotor. 21. Air classifier with at least two classifying rotors arranged coaxially one inside the other, characterized in that at least one of the classifying rotors has the characterizing features of one of claims 1-20. 22. Air classifier according to claim 21, characterized in that the coaxially arranged classifying rotors are driven at different speeds. 23. Air classifier according to claim 22, characterized in that the inner classifying rotor has a higher speed than the outer one. 24. Air classifier according to one of claims 21-23, characterized in that a discharge space is provided between the coaxially arranged classifying rotors.