EP0332031B1 - Zigzag sifter - Google Patents

Description

The invention relates to a zigzag sifter with a multiplicity of fluidically connected, zigzag-shaped, compressed air-operated sifting air channels which are connected to a collecting channel under compressed air.

The basic principle of a zigzag sifter for rising pipe wind sifting of granular goods is described in US-A-1 861 248.

This classifier is a simple, vertical, smooth tube with a rectangular cross-section that is alternately inclined to the right and to the left at the same angle to the vertical. The coarse portions of the feed slide down on the bottom wall of the channel. In doing so, they have to cross the visual air flow coming from below at the kinks. With each crossing there is a separation of fine parts, which in itself does not lead to a sharp classification. A multiple repetition can ultimately result in a very sharp separation of the fractions due to a multiplication effect.

A multi-channel zigzag sifter with sifting air ducts connected in parallel in terms of flow, which are connected to a gas distributor, is described in EP-A-163 836. The construction principle of such a sifter package with many small channel cross-sections allows significantly lower overall heights with the same number of kinks. In addition, the number of kinks for one and the same desired separation result can also be reduced because the thickness of the material layer to be sifted is so small that a significantly better viewing process takes place at the individual kinks.

Furthermore, a sighting system is described in CH-A-419 810, which has a group of sight tubes connected in parallel. This is a sifter cascade, in which the material to be sifted is passed one after the other under the feet of the individual viewing channels and is introduced into the individual viewing channels from below using visual air in the form of one or more sharp beams. The visible material on a conveyor belt is whirled up by the sharp line of sight air and blown into the sight pipes or sight channels.

For the dedusting of granular material at high throughputs, an airbed classifier was also developed, which e.g. is described in DE 1 507 686. This classifier combines a strongly fluidized fluidized bed with a system of parallel zigzag channels arranged above it. The feed bed moves through the fluidized bed in such a way that coarse material emerges from the bed at the end. The fine material is checked for its grain size in the zigzag channels and, if the size falls below a predetermined limit, is transferred from the classifying air to a separator. The coarse grain, on the other hand, falls back into bed.

In addition, multi-channel zigzag sifters are on the market, in which the material to be sifted is fed evenly to the channels via a distributor screw. The visible air flows in undosed to the parallel channels.

In order to avoid instabilities, such classifiers may only be operated with a low solids load.

The invention has for its object to take precautions in a multi-channel zigzag sifter of low height and high selectivity, which ensure that all channels are supplied evenly and constantly with sifting air regardless of the respective loading with solid matter.

This object is achieved in that the sifting air ducts each have an air distribution pipe for distributing the sifting air over the duct cross-sections and that in the individual air feeds branching off the collecting duct, sound restrictors with the same cross section are arranged, through which the sifting air flows at the speed of sound, so that in Each air distribution pipe receives an amount of air that is only determined by the cross section and the pressure in front of the sound throttle.

"Sound restrictors" are perforated screens through which the visible air flows at sound speeds in their working area. A sound choke is therefore assigned to each viewing duct, so that all ducts have the same air throughput with the same cross section of the perforated screens. Since the parallel viewing channels are all fed from the same collecting channel, the pressure in the collecting channel can be used to vary the viewing air flow rate of all channels evenly.

The visible air is blown into the ducts from below. For their uniform distribution over the channel cross section, an air distribution tube perforated on the upper side is advantageously provided, which is inclined by α = 5 ° to 60 ° to the horizontal. In this way it is achieved that the flow velocity in the channel increases above the feed source. For the particles to be discharged, this has the consequence that their discharge is not hindered by a blowing which decreases in the direction of the fall.

In practical operation of the new zigzag sifter, different numbers of solid particles can be supplied to the individual channels. Different solids loads in the classifier channels result in different pressure distributions, so that air flows back from more heavily loaded channels and then makes its way through less heavily loaded neighboring channels. This would lead to an unstable behavior of the classifier process as a whole and thus to a significant reduction in the selectivity. In order to make such backflows more difficult, the cross sections of the ducts are advantageously narrowed below the feed point for the classifying air in such a way that the coarse grain of the desired size can just fall freely out of the classifying duct. Due to the particles falling out, the narrowing of the cross-section is fluidically increased the more particles pass the constriction. However, this leads to the desired stabilization of the flow distribution.

According to a special embodiment of the proposal according to the invention, the constriction is given the shape of a zigzag channel. As is well known, zigzag channels have a particularly high resistance coefficient, which even exceeds that of labyrinth seals.

At the lower end, where all parallel sifter channels open into a common space, a control device is provided in the zigzag sifter according to the invention which maintains such pressure in this space that no sifting air flows out of the channels downwards, but on the other hand also no air from the surroundings is sucked in by the ducts. This is achieved by a corresponding regulation for the extraction of the classifier exhaust air.

Fig. 1

eine Aufrißdarstellung des Mehrkanal-Zick-Zack-Sichters,

Fig. 2

eine Seitenansicht und

Fig. 3

die Luftzuführung zu den Sichterkanälen.

An exemplary embodiment of the invention is explained in more detail below with reference to drawings. Show it:

Fig. 1

an elevation of the multi-channel zigzag sifter,

Fig. 2

a side view and

Fig. 3

the air supply to the classifier channels.

The zigzag sifter shown schematically in FIG. 1 consists of a housing 1 with a collecting space 2 for the discharged material with the desired grain size, the zigzag sifter package and the collecting duct 4 for supplying the sifting air. The zigzag sifter package consists of a plurality of vertically arranged, fluidically parallel zigzag channels 5 with attached return shafts 6. Through the return shafts 6, the undersize may be thrown back into a material bed. They can have smooth or zigzag channel walls.

For this purpose, an opening 7 is provided at the upper end of the return shaft 6 (see FIG. 2). The material to be sifted is fed through the entry opening 8 at the upper end of the sifter channels 5. At their lower end, the channels 5, as will be described in more detail below, are connected to the collecting channel 4 for supplying the classifying air. As can be seen from FIG. 2, each viewing channel 5 is equipped with an air distribution pipe 9 which, for example, is inclined at an angle of α = 45 ° to the horizontal is and has perforations 10 on its surface for a uniform air outlet. The classifying air is fed to the air distribution pipe via a diaphragm or throttle (sound throttle) 11 through which the speed of sound flows (see also description of FIG. 3). Below the air distribution pipes 9 are arranged zigzag-shaped narrow points (labyrinths) 12, the diameter of which is only slightly larger than the diameter of the material to be discharged (desired grain diameter). Due to the labyrinth 12, the coarse grain can just fall out of the viewing channel 5 unhindered. The labyrinth 12 ultimately serves to improve the selectivity if a different solids loading is to be expected in the classifier channels 5 and different pressure distributions result from this. The constrictions 12 counteract flow instabilities which result from such different pressure distributions.

The air supply to the air distribution pipes 9 can be seen from FIG. 3. The sifting air flows for the channels 5 are branched off in parallel from the collecting channel 4. Due to the arranged in the individual feeds to the air distribution pipes 9 throttles 11 ensures that in each air distribution pipe 9, regardless of the pressure behind the sound throttle, an amount of air flows that is determined only by the cross section and the pressure in front of the sound throttle. With the same cross section of the sound restrictors, the air throughput for all classifier channels is also the same.

In order to vary the visible air throughput of all channels 5 uniformly, only the pressure in the collecting channel 4 needs to be changed accordingly.

To ensure that neither air flows into or out of the channels through the collection space 2 below the cross-sectional constriction 12, the suction for the classifier exhaust air is provided with a control device which ensures that ambient pressure prevails in the collection space 2. The goods viewed are discharged from the collecting space 2 through a rotary valve.

Claims (5)

1. A zigzag sifter with a plurality of zigzag-shaped sifting air ducts (5), which are connected in parallel according to flow technology, are acted upon by compressed air and are connected to collecting ducts (4) similarly acted upon by compressed air, characterised in that the sifting air ducts (5) each comprise an air distribution pipe (9) for distributing the sifting air over the duct cross sections, and arranged in the individual air supply lines branching off from the collecting duct (4) are sound throttles (11), which have the same cross section and through which the sifting air flows at sonic speed, so that a quantity of air flows into each air distributing pipe (9) which is merely determined by the cross section and the pressure upstream of the sound throttle (11).
2. A zigzag sifter according to claim 1, characterised in that the air distributing pipes (9) are perforated (10) on their upper side and are inclined relative to the horizontal through α = 5° to 60° in the direction of flow of the pipe.
3. A zigzag sifter according to claims 1 to 2, characterised in that the ducts (5) are constricted (12) at their lower end in the region of the coarse grain outlet.
4. A zigzag sifter according to claim 3, characterised in that the constriction (12) comprises zigzag-shaped duct walls.
5. A zigzag sifter according to claims 1 to 4, characterised in that the exhaust for the exhaust sifting air is provided with a control device, which maintains such a pressure at the lower end of the ducts or constrictions that neither sifting air flows out nor air is drawn in from the environment at this point.

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