October 4, 2012
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 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.
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.
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.
Holes from abrasion and normal wear and tear typically appear in the upper outside casing, inside drum, and upper and lower tailings cones. Patch it with steel, and you’re back in business. In extreme conditions, parts or wear liner replacement becomes necessary.
5 Surface moisture content.
Under normal operation, this must be 1.5 percent or less. If the moisture content is any higher, the finer particles will cling to larger ones, making it difficult to get a good quality classification. If the moisture content, whether it comes from rainfall or dust suppression spraying, is very high, the water is centrifuged out, resulting in equipment clogging. Natural inherent or internal moisture content usually has no effect on air classifier performance.
Always check for vibration. Excessive vibration could cause the classifier’s structural integrity to fail prematurely. Vibration will also put undue stress on the gear unit bearings and seals.
7 Proper lubrication.
It’s important to maintain the proper oil level and drip setting for the lower bearing and proper interval greasing of the upper bearings. The gear unit oil should also be drained and replaced after the recommended hours of operation. Consult your owner’s manual for recommended lubrication practices.
8 Belt tension.
The belt tension should be checked periodically to maintain belt life and power transmission. Over-tightened belts could be the cause of vibration. Always follow your manufacturer recommended intervals when it comes to checking belts.
9 Gear unit backlash in 90-degree gear drives.
Gear backlash is the amount of clearance between the ring gear and pinion gear. Maintaining the proper amount of backlash is important. If it is too tight, you’ll wear the gears out, which will not only reduce the lifespan of the gears, but it also can result in top bearing problems in the gear unit. An annual check of the backlash in the gears is imperative to maximize the life of the gears. Again, consult your owner’s manual for proper backlash amounts and procedures.
10 Bearing temperature.
High bearing temperatures and wear are generally not an issue because classifiers are relatively slow-speed machines and don’t handle the aggressive work of pulverizers. However, a high bearing temperature can be an indication of an impending bearing failure and could save you from having a major failure. Bearings have been known to fail simply because they were not properly monitored.
As part of an expanded preventive maintenance program, a vibration analysis expert can take a vibration reading as a baseline and then compare it to annual readings. They can identify and isolate any suspect bearings.
Remember, wear and tear is based on the characteristics of your specific material. For instance, silica sand applications may have to be inspected every 30 to 60 days to troubleshoot any wear problems, whereas soft limestone-like materials will give you a much more generous maintenance schedule. Each deposit from each territory is different depending on the chemical property of the material, so always be aware of the silica or quartz content.
Keep on top of your maintenance schedule and your air classifier will earn its keep, doing what it was designed to do — turning common stone into common currency.
William Chapman is lab manager and William MacNeil is lab service technician, both with Sturtevant Inc.