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State of California, Department of Transportation - page 12 / 52





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measured in an adjustable calibrated tank, for example, liters, gallons, cubic feet, or cubic meters could be used as the units of measure. This method is not used very much.

Cyclones - Usually the first component of a two-part emission control system. Hot exhaust gases, from the drier drum, containing fine particle of aggregate are carried through the cyclone. The movement of the hot gases and aggregate resembles a cyclone or tornado, hence the name. The larger aggregates fall to the bottom of the cyclone and are returned back to the aggregate steam by an auger or other means.

Wet Scrubber - Typically the second component in a two-part emission control system.

After the hot exhaust




gases that

have point

passed water

through the cyclone, they is sprayed from multiple

are forced through a nozzles to wet the

remaining fine aggregates and dust. motion, where the wetted fines fall fines then travel out into a settling atmosphere.

The wetted fines and gas are then sent into a circular out of the airflow due to the centrifugal force. The pond while the exhaust gases exit the stack into the

Knock Out Box - A rectangular shaped collector, used in combination with a baghouse. The system works on a principle of decreasing airflow, allowing coarser dust particles to fall out of the flow. The heaviest fines fall to the bottom of the knockout box where it can be returned to either the batch plant hot elevator or the drier drum mixer. The main purpose of the knock out box is to improve the efficiency of the secondary dust collector by reducing the quantity of fine material being collected in the baghouse.

Baghouse - The second portion of a dust collection system, comprised of a large metal structure containing hundreds of metal cages covered with cloth (Nomex, Kevlar, etc.) bags. At one end of the baghouse, a large exhaust fan pulls dirty air through the metal ducting connecting the baghouse and the drier drum. This process is similar to the way a vacuum cleaner operates, dirty air passing through a filter bag trapping the dirt and expelling clean air. The baghouse is divided into rows or sections, while most of the system is vacuuming a small portion of the baghouse is cleaning the filter bags. This cleaning process may include a system that shakes the bags, reverses the airflow backflushing the filter bags, or a combination of both. The fine particles fall to the bottom of the baghouse where they can be returned to the plant or rejected. Caltrans requires 100 percent returned or 100 percent rejected fines, less than 100 percent must be metered/weighed back into the plant.

Controller - The electronic hardware and circuitry that is adjusted to specific target values by the plant operator, then monitors and adjusts percentages of aggregate, asphalt binder, liquid anti-strip as specified by set tolerances.

Safety Inspection This safety inspection is performed during the CT 109 and follows the CAL-OSHA regulations, Business and Professions Code, and Caltrans Specifications. If you notice any safety concerns, please contact your District Weights and Measures Coordinator.

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