Parallel-flow drum-mix plants

The dryer/heater of aggregates differs from the one used in batch plants in the following aspects (a) the burner is placed at the top end of the drum, (b) the configuration of the flights inside the drum is more complex and (c) the drum is longer. 

The mixing zone is protected from the radiant heat and the burner flames by a special flame shield or another appropriate device. The hot bitumen is usually added under pressure from the discharge end. The filler and the reclaimed asphalt, if used, are also added in the mixing zone. 

One of the advantages of the drum mixer is the fact that exhaust gases contain a very low percentage of dust. The reduction is due to the fact that the dust produced in the drying/ heating zone is eliminated by the bitumen added in the mixer. However, environmental legislation imposes the use of dust collectors. The small dust (filler) quantity collected is usually re-supplied to the drum. 

In general, a drum-mix plant is relatively simple and smaller in size than a batch plant. Regarding the hourly output, it may reach up to 500-550 tonnes per hour. Thus, this type of plant is considered as the most appropriate in cases of high demand. 

The other advantages of the parallel-flow drum-mix plant are as follows lower capital cost in comparison to batch plant, lower space requirement, higher outputs can be achieved, 

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Figure 8.2 Typical layout of parallel-flow drum-mix plant for hot mix asphalt. (From EAPA and NAPA, The Asphalt Paving Industry A Global Prospective, 2nd Edition, GL 101. Brussels EAPA, 2011 Courtesy of National Asphalt Pavement Association, http //www.asphaltpavement.org.)...

All the other production stages are exactly the same as those in the parallel-flow drum-mix plant. 

The basic advantages of a counterflow plant over a parallel-flow drum-mix plant are that it is more environment friendly, fuel efficiency increases and higher percentages of reclaimed asphalt can be used. 

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