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Feb 21


The Differences Between Wet and Dry Type Air Classifiers

Summary: A comparative study of the two main types of high-efficiency classifiers.

The Differences Between Wet and Dry Type Air Classifiers

An air classifier is a machine used in industries to separate coarse particles from fine particulate matter and heavier particles from lighter ones. In high-efficiency classifiers, the materials are neatly divided, therefore reducing over-grinding.

There are two streams of products resulting from any classifier: A fraction containing fine particles and the one containing coarse particles.

High-efficiency classifiers or separators have had the greatest impact on the product quality and in reducing consumption of electricity. They are easy to maintain and are designed in an optimum manner for convenient use. You don’t have to think of paying huge power bills to operate these high-efficiency classifiers.

The principle of operation of a classifier is based on the differences in specific gravity, densities of materials and terminal falling velocities of particles in liquid and air medium.

Air classifiers are used for a broad range of materials and obtain different fractions of powders. They are employed in the production of ceramics, building materials, abrasives, powder metallurgy and in many other industries.

The separation of particles is performed by the speed with which the particles fall through air or water medium. The rate of particles in a fluid medium is not only dependent on the size, but it's also dependent on the specific gravity and shape of the particles.

The different rates of movement of particles of various densities and sizes, when suspended in a fluid medium, is made use of by classifiers by making appropriate arrangements to collect the particulate fractions as they move in the various regions. It also uses the different effects forces such as the centrifugal force and gravity have on the particles. Based on the separation principles of the air classifiers, they are classified into two types - wet classifiers and dry classifiers.

Wet Classifiers

Wet classifiers use the principle of liquid fluidization to separate coarse particles from fine particles. The fundamental principle of wet classification is that fine particles move slower than coarse particles at equal density and low-density particles move slower than high-density particles at the similar size.

Also, the motion of the particles in the fluid can be either free or hindered movement. If a particle has interference from other particles, it moves slower than a particle not surrounded by other particles. This is due to the increase in density and viscosity of the slurry. They are called hindered movement and free flow respectively and is valid both for centrifugal and gravity classification.

Industrial classification of materials using wet classifiers can be carried out in various types of classifiers, and they are either (i) cyclones, (ii) hydraulic classifiers, and (iii) mechanical classifiers.

They all work following the principle that when the particles are suspended in water, which carries out a slight movement upwards about the particles, the particles that are below a particular density and size are transported away by the flow of water. However, the coarse and denser particles will settle.

The different wet classifiers are cone classifier, hydro clones classifier, gravity settling tank, spiral classifier, and rake classifier.

Dry Classifiers

Dry classifiers use the principle of air fluidization to separate coarse particles from fine particles. Dry air classifiers use the aerodynamic characteristic of the materials in the process of separating them into their categories. A particular material's aerodynamic component depends on the geometry, size and density of the particle.

The classification process involves the interaction of material particles, a moving stream of air, and the gravitational force, all within a confined volume. In the interaction, the drag force and the gravitational force are exerted in different directions upon the materials.

The outcome is that materials that have a high drag-to-weight ratio are suspended in the air stream. However, materials with a small drag-weight ratio tend to settle. Conventionally, the fraction that paid is termed ‘air-classified substantial fraction’ while the fraction that is suspended is referred to as the ‘air classified light portion’. The enclosed volume in which the separation takes place is called an ‘air classifier’.

The density of material is not the only characteristic of a particle that affects the separation process in air classifiers. Likewise, moisture has an effect on the separation. This is as a result of its influence on the density of a material. There are some designs that an air classifiers may come in, but the three principal groups of models are vertical, horizontal and inclined. They all require proper dust collection, blower, separator, and control facilities.