How to Control Dust During Production
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The father-son team of Lloyd and Jason Vivant ran the plant for McNamara Contracting in Rosemount, Minnesota, 25 years ago. Lloyd wanted to control the amount of dust going back to his mix evenly and reject the difference. He set up a system to do just that. Today, the system design still works well for Jay. The size of the bin has increased to take on unwashed material. The savings from not washing material is threefold: 1. Cost of washing is abated; 2. Plant production rates are increased with drier material; and 3. Energy use decreases with drier material. Because the minus 200 dust product may vary excessively at times, this can be an ideal opportunity to reject the excess. Let’s look at how you might do this at your facility. The aggregate drying process inherently separates the fines from the on-spec material. When air velocity changes, whether that’s due to production, moisture or temperature variations, through the dryer, so does the size and amount of fines separated. Consequently, the loading of the baghouse changes and so does the amount of dust returned. Complicating the issue further is the baghouse discharging the dust in sags and surges, which can be as high as plus or minus 21% over a three-minute period. Adding insult to injury, the hot stops allow the bags to relax when the fan is turned off causing most of the dust to fall and fill the auger below. When the plant is started again, the dryer initially sees the full auger surge (+59%). Because the bags had released most of their dust, the augers run empty until the bags build up the dust cake again. This situation then causes the dust to go from the surge to a sag (-59%) until it slowly builds up the dust cake over a five-minute period. There are some basic steps the plant operator must execute to ensure he isn’t chasing ghosts and is getting the correct fines metered back to the drum. Let’s look at those steps.
46 // September 2021
BY CLARENCE RICHARD
1. Practice correct operating procedures, which involve slowly changing from one production rate—or temperature—to another, maintaining consistent aggregate moistures, and slowly changing exhaust damper or fan speed settings. 2. Invest in the right equipment, such as a surge bin with flow measuring and control. You want the baghouse dust to be continuously fed to the surge bin (indicated by the number 1 in the photo). In order to use all the dust while smoothing all the sags and surges, the operator will set his vane feeder discharge speed so the level fluctuates between the low and middle-level bindicators during normal operation and between the lower and high-level bindicators during a
hot start. Notice the change in bin-level tolerance is significantly different between steady production and startups; startups can take the system five minutes to recover. It’s that five minutes of dust that forces us to use the extra volume available in the storage bin. The surge bin should be sized to absorb these fluctuations. When rejecting dust, the operator can set the dust flow controller to the percentage wanted. The flow scale (indicated by the number 2 in the photo), reports to the controller and increases or decreases the vane feeder depending on the plant rate. The excess fills the surge bin and is rejected out the second surge bin auger discharge port.
