and the flight configuration determine the length of time the aggregate will spend in the dryer.
The inspector must be sure that the burner fuel is achieving complete combustion, there shouldn’t be any signs of black smoke in the air or soot in the settling pond. The exhaust fan creates the draft of air that carries the heat through the dryer drum and removes the moisture. Imbalances between draft air and blower air velocities can cause back pressure inside the drum. This creates a "PUFF-BACK" of exhaust gases at the burner end of the drum. Corrections to the draft air or blower air will correct this problem.
A good rule of thumb for determining the heat transfer between the burner and aggregate is to compare the exhaust gas temperature at the drum exit and the completed AC mix temperature. If both temperatures are within 20 F the drum mixer is operating efficiently and the fuel usage is optimal. If the temperature difference exceeds 20 F the heat transfer is not as efficient, primarily due to the lack of a uniform veil of aggregate falling through the heated air. Regular maintenance of the internal components, such as drying flights, and retainer rings will typically correct the problem as well as reduce emissions.
If the aggregate exiting the dryer drum exceeds 1% moisture, the Plant Inspector should look at incomplete fuel combustion, draft air vs. blower air imbalances (puff-back), drier drum slope exceeding ¾” per foot of drum length, worn or missing flights, or reducing the aggregate feed rates (TPH).
Dust Collection Systems All continuous mix plants have a small amount of fine aggregates carried out of the drier drum with the exhaust gases. In order to meet Federal, State and Local air quality regulations, dust collection systems must be used to capture dust and exhaust gases that would otherwise be released into the atmosphere. In some areas of California, strict air quality standards have required Contractor’s to be pretty innovative. Not only are the dust collection systems in use, but the entire AC plant has been enclosed within a building. These types of systems utilize multiple filtering systems to clean the exhaust gas discharged into the atmosphere. In addition, some dust control systems allow the plant operator to control the amount of fine aggregates returned to the completed asphalt mix.
Fine aggregate collected in dust control systems may be returned to the aggregate production stream without any further proportioning if returned at a rate that is commensurate with overall plant production, and if returned at or before the mixer.
Fine aggregate returned at a rate less than 100 percent of the collected rate, shall be
proportioned by a method that uniformly feeds the material with an accuracy of percent. Supplemental fine aggregate shall be discharged directly into the mixer.
Currently, three types of systems are used for dust and emission control.