What Is an Air Classifier?

Dry air classification is a technical method of classifying dry air as a medium. The device that realizes this operation is called a dry air classifier.

Dry air classifier

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Dry air classification is a technical method of classifying dry air as a medium. The device that realizes this operation is called a dry air classifier.

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Dry air classifier

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Dry air

Dry air classification is a technical method of classifying dry air as a medium. The device that realizes this operation is called a dry air classifier. In the microscopic field, the forces acting on particles are usually: resistance, gravity, centrifugal force, Coriolis force, inertial force, electrostatic force, magnetic force, friction force, collision force, adhesion force and so on. Because the powder particles have different equivalent diameters or specific gravity, the magnitude of these forces is also different. The classifier is made by using the different effects of one or more of these forces. In order to facilitate selection and reference Design, according to the different forces and main structures, we make the following divisions:

1. Gravity air classifier principle: The particles are classified using different sedimentation speeds and movement trajectories in gravity and air medium resistance. It has a simple structure, low pressure drop and large processing capacity, but has poor classification accuracy.

1.1. Vertical flow type gravity classifier

Features: The airflow in the grading chamber moves upwards, and the particles whose terminal sedimentation speed is less than the airflow speed are carried out by the airflow; otherwise, the coarse powder collector that settles to the bottom.

1.2. Horizontal flow type gravity classifier

Features: The horizontal movement of the airflow in the grading room. According to different trajectories, fine powder is carried out by the airflow, and coarse powder is settled into the collector. The separation of particles is a two-dimensional process, which is not only related to its own sedimentation speed, but also to the length and height of the classification chamber.

1.3. Special flow type gravity classifier

Features: The typical model is the Zi type gravity classifier from German Alpine company. Each time the two-phase flow turns, it collides with the wall of the device to re-disperse the particles. The coarse powder loses the kinetic energy of upward movement due to the impact. The fine powder continues to rise with the air flow to achieve classification.

2. Inertial air classifier

Principle: Inertia is an inherent property of matter. It is determined by mass. When a particle is subjected to a force that changes its direction of movement during the movement, different movement trajectories will be formed according to the different inertia, thereby achieving classification. Its simple structure, no moving parts inside, high classification accuracy, but low output.

2.1. Jet Inertial Classifier

Features: The material is ejected outwards through the inner and outer discs at a very high peripheral speed. At the same time, different ejection curves are formed under the action of air spray to achieve the purpose of classification.

2.2. Jet-type inertial classifier

Features: A classification device that integrates the principles of inertial classification, rapid classification and Coanda effect. The spray sprays the powder, and at the same time, it is affected by the air flow and the Coanda effect in different directions, resulting in different trajectories and directions of particles of different sizes to achieve classification.

2.3. Other types of inertial classifier

Features and performance of common models:

Type Handling Capacity kg / h Separation Particle Size Origin Main Features

Jet type lower 20 German inertial, centrifugal force classification

Jet type 10-2000 0.5-30 Germany Sets inertial classification, rapid classification, wall attachment effect, can obtain multi-level products

Type VI 0.45-1800 0.3-10 Germany uses the combination of collision and inertia to achieve better dispersion effect and rapid classification.

This type of powder separator also includes: German company's V type (with a dispersing device); K type (simple structure, but the classification accuracy is not high); CI type (stable classification interface, in addition to the advantages of type VI, it can also be used For determining the density of spherical particles)

3 Centrifugal Force Air Classifier

Principle: The classification is achieved under the combined effect of air resistance, gravity and centrifugal force. Its classification accuracy is high and the processing capacity is large. According to different methods of generating centrifugal force, it can be divided into free vortex type, quasi free vortex type and forced vortex type centrifugal classifier.

3.1. Free eddy current centrifugal classifier

Features: The high-speed rotating air flow formed by the tangential volute or the guide blade enters the classification zone. The particles generate a strong centrifugal force under the action of rotating airflow. The centrifugal force of the coarse powder is greater than the air resistance and is thrown to the inner wall of the separator and rotated down to the lower discharge port. The centrifugal force of the fine powder is less than the air resistance and concentrated to the center of the separator. The air flow exits from the upper outlet. Commonly used such classifiers are cyclones and coarse powder separators.

3.2. Quasi-free vortex centrifugal classifier

Features: The basic features are similar to the free vortex type, and there are no moving parts inside. The main difference is that the latter is designed with a vortex-enhanced structure or a quasi-free vortex due to the irregular shape of the volute.

3.3. Forced vortex centrifugal classifier

Features: The introduction of specific mechanical motion devices to increase the centrifugal force of the particles in the classifier and generate forced vortexes to improve the classification accuracy and efficiency. It is still mainly completed under the combined action of air resistance, gravity and centrifugation. Because of its good dispersibility, stable strong flow field and obvious classification interface, it has become the most widely used classification device.

3.3.1. Traditional wind blade forced vortex centrifugal classifier

There are usually no guide blades in the interior, and the classification is performed only by the unstable centrifugal force field formed by the rotation of the large and small blades. The classification effect is not good.

3.3.2. Cage rotor type forced vortex centrifugal classifier

The casing is equipped with guide vanes, which are often accompanied by tangential air intake. A stable strong flow field and a clear classification interface are formed by the cage rotor, and the finished products with different particle sizes can be obtained by adjusting the rotation speed. The traditional air leaf type centrifugal classifier has improved significantly.

3.3.3. Rotary wall type forced vortex centrifugal classifier

The basic principle is the same as the other forced vortex centrifugal classifiers mentioned above, except that the rotary wall is used to generate the forced vortex, which is suitable for occasions with high concentration of materials and the classification is more precise.

3.3.4. Other types of forced eddy current classifier

Under different production conditions, in order to meet certain design requirements, the devices that generate forced eddy currents are not limited to traditional wind blades, cage rotors and rotating walls. The common types are as follows:

Disc-shaped rotor: classify the powder with the strong centrifugal force generated by the high-speed rotation of the disc. The representative model is a Japanese type classifier, which combines pulverization and classification into one, forming a closed circuit, compact structure and high efficiency, but the separated particles can not be easily discharged in time. The domestic PS type is a variant of it.

Needle wheel rotor, the air blade of the traditional air blade type centrifugal classifier is replaced by a rotor consisting of radial stacked needle seedlings, which can impact the material in multiple layers, and the gap between the needle seedlings can also pass through the wind at the same time. Sorting, the resulting centrifugal force field is also uniform and stable.

Special-shaped rotor: The rotor with an irregular shape is used, such as the classifier of a Japanese company, which introduces circulating airflow and performs repeated classification, which can achieve higher classification accuracy. Another prominent feature is that it is easy to control by microcomputer. Realize automation.

4 Combined classifier

Principle: It is the combination design of the previous models, which can often combine multiple advantages into one. It is the main method of designing a classifier before the current breakthrough in classification theory. For example, the model combination classifier of Mitsubishi Heavy Industries in Japan has the characteristics of coarse powder separator and type classifier; the type classifier of Danish FLS company is equivalent to coarse powder The combination of a separator and a cage separator; domestic.