A service technician should check these 10 items to help identify and fix problems with air classifiers.

by William Chapman and William MacNeil

Air classifiers are normally employed when dry material particle size for separation is too fine for screening. The beauty of an air classification system is that it defeats the blinding and breaking issues associated with screening. The most significant advantage is its dry process for de-dusting aggregates. This can, ultimately, eliminate the need for water or settling ponds; saving money, land, and the environment.

Air classifiers are rugged by design, but they do require a degree of maintenance to keep them running at peak performance.

Air classifiers work by combining the principles of centrifugal and drag force, collision, and gravity to generate a balanced high-precision method of classifying particles according to size and density. While density does play a role in air classifier separation, the internal air currents are mostly affected by the overall mass and weight of the particles in the feed. Lighter and smaller particles are removed by the airflow, while heavier and larger particles are not entrained in the airflow. If the lower-density material also has a finer particle size, then air classifiers can be very effective. However, large particles with low density can have a similar mass and weight as some small particles with high density. This can reduce the effectiveness of an air classifier’s density separation.

Even though air classifiers are rugged by design, they do require a degree of maintenance to keep them running at peak performance.

Here are 10 items to look at that will enable a technician to better identify any problems being experienced or allow for an in-house fix.

1 Proper venting.

Is the “fines” side of the classifier properly vented? Plenty of dust to contend with results from improper venting. The fines side of the classifier has positive air pressure, so you want to be able to vent or remove it to prevent dusting. In quarry applications, the venting can be eliminated, because dusting can be controlled with fine water mist.

2  Selector blade adjustment.

Maintain a proper gap between the top of the selector blades and the underside of the drum cover; the general rule of thumb is 1/4 inch or as close as you can get without contact.

Is the clearance between the selector blades and the inside drum cover correct to specifications in the owner’s manual? It is one of the most common problems encountered by service technicians. It is very important to maintain a proper gap between the top of the selector blades and the underside of the drum cover; the general rule of thumb is 1/4 inch or as close as you can get without contact. An excessive gap will result in unwanted oversize particles in the fines fraction.

3  Feed rate.

Are you over feeding or under feeding the classifier? Whether you employ inclined belts, screw feeders, or air slides, it is important to maintain relative consistency in the feed rate. Material-to-air-flow ratio can have dramatic effects on classifier efficiency.

4  Excessive wear.

An annual check of the backlash in the gears is imperative to maximize the life of the gears.

Check the internal areas of the classifier for wear. If you discover any holes, then your material could start to migrate between the fines and coarse side of the classifier by sheer gravity. This will obviously throw your product out of spec.