There are two types of drum-mix plants: parallel-flow drum-mix plants and counter-flow drum-mix plants (conventional and double barrel). In drum-mix plants, cool aggregate is delivered from cold-feed bins to a dryer-mixer and then into a silo through a slat conveyor. The mixture gradation control is exercised
through establishing suitable proportions of individual aggregate fractions in cold-feed bins and the rate of aggregate supplied by the feeder belt. The conveyor is equipped with a weight and speed-control system that enables control over the coating plant’s throughput in tons per hour. The general categories of drum-mix plants depend on the flow direction of the aggregate relative to the hot air movement from the burner. In parallel-flow drum-mix plants, the aggregate and hot air move in the same direction, while in counter-flow drum-mix plants, they move in opposite directions.
In classic parallel-flow drum-mix plants, the asphalt binder is delivered to a dryer- mixer and injected on the aggregate tumbling inside, which poses a risk of direct contact between the binder and exhaust gases from the dryer’s burner. That is why various solutions have been adopted that place the binder batching point away from the burner and the aggregate drying zone.
In counter-flow drum-mix plants, the aggregate moves in the direction opposite to the movement of exhaust gases. Many precautionary design measures have been built in and are supposed to provide a significant reduction in emissions, a reduction in the exhaust gas temperature, and protection of the binder against overheating. A lengthened part of the drum where mixing takes place, an extra coater, and an embedded burner are a few examples of these solutions. In counter-flow drum-mix plants of the double-barrel type, the aggregate is dried in an inner drum, then discharged into the surrounding outer drum, where it is mixed with binder while being protected from the burner’s high temperature. The final asphalt mix is transported to a storage silo with a conveying device.
The SMA aggregate mix contains more than 70% m/m coarse particles, which means that a substantially higher amount of energy is needed to dry and heat them than when manufacturing asphaltic concrete. Furthermore, it may be necessary to reduce the output of the coating plant and to extend the veil of aggregate flowing through the dryer-mixer per time unit to obtain the proper aggregate temperature.
Adding a loose fiber stabilizer represents a severe handicap from a production control standpoint; the location of batching should be chosen carefully so that the loose stabilizer cannot be captured by hot exhaust gases. However, the application of granulated fibers does not present major problems, provided that the point of their addition into the mix is properly determined. Adding granulate to the drum behind the burner (parallel-flow or conventional counter-flow) or to the outer drum before adding binder (counter-flow double-barrel system) is the rule.
