What are the two types of drum mixers used in hot mix asphalt production?

• A. Parallel and counter flow mixers.
• B. Horizontal and vertical mixers.
• C. Batch and continuous mixers.
• D. Twin-shaft mixers.

Answer: (A)

Two drum-mixer types are defined by how the material and the hot gases move inside the drum: parallel-flow and counter-flow. In a parallel-flow drum, the aggregate, binder, and the hot exhaust gases travel in the same direction. They are heated as they move toward the discharge, but the binder can be exposed to hot gases for a longer period, so temperature control is essential to prevent aging or scorching of the asphalt.

In a counter-flow drum, the gases flow opposite to the aggregate. This setup creates separate zones: a drying zone where the aggregate is heated by the hot gases, and a mixing zone farther along where the asphalt is introduced and mixed. The asphalt is heated more gently and the hotter gases don’t contact the finished mix as directly, which improves heat transfer efficiency, reduces binder aging, and generally offers better temperature control and product quality.

Hence, the two types used in hot mix asphalt production are parallel-flow and counter-flow drum mixers.