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Dries
aggregate and coats it with asphalt cement to produce HMA. Major
components are a drum, burner, exhaust fan, aggregate bins, aggregate
feed system, and dust collection system. All are mounted on a single
trailer frame. A tractor with a fifth wheel attachment can transport
the unit.
The drum functions as a combination
dryer and mixer. Steel spokes attach the drum to two steel tires. The
tires ride on four motor-driven trunnions that rotate the drum. The
burner is mounted on the elevated end of the drum and produces hot
gases to dry the aggregate. The exhaust fan at the other end of the
drum pulls the hot gases through the drum.
The aggregate system has two aggregate bins, two feeder belts, and a
slinger conveyor. Each bin has a divider and two adjustable gates that
control passage of the material onto the feeder belt. Up to four
different aggregates can be used simultaneously, each with its own feed
rate. The feeder belts empty onto the slinger conveyor, which
discharges the aggregate into one end of the drum just below the
burner. The aggregate travels in the same direction as the burner gases
Flights feed the aggregate through the rotating drum and shower it
through the hot gas stream. As the hot dried aggregate reaches the
other end of the drum, liquid asphalt is injected into the aggregate.
Mixing flights ensure that the asphalt uniformly coats the aggregate.
The finished mix exits the drum through a chute near its end.
The dust collection system is mounted at the discharge end of the drum.
It limits the amount of dust that can escape into the atmosphere. Major
components of the system are a motor driven fan, an oversized knock-out
box, a water spray system, a cyclonic separator and an exhaust stack.
The dust collection system is designed to avoid discharging water
droplets from the exhaust stack. This is a problem inherent in dust
systems of competitive units because they lack a cyclonic separator.
Our cyclonic separator not only removes water from the dust wetting
process, but also removes water created when the exhaust fan compresses
and condenses the gas stream.
The fan pulls the gas stream, which includes dust produced by drying
the aggregate, through the drum and into the knock-out box. The box
slows down the gas stream causing larger dust particles to fall out of
the stream. A perforated baffle in the box significantly increases the
efficiency of dust removal by redirecting the gas stream. The larger
dust particles collect in the bottom of the box where a dust screw
conveys it back into the hot mix.
As the gas stream continues its journey through the knock-out box it
passes through a water spray compartment in one end of the box.
Particles of dust remaining in the gas stream are wetted by the water
and trapped in the water. The dirty water can be drained into a holding
pond.
The exhaust gases and fine dust particles not captured in the knock-out
box pass through the fan and into the cyclonic separator. The gases and
dust laden water droplets swirl around inside the cyclone so that
centrifugal force separates the water and dust particles from the gases
before they exit the stack. The water droplets and fine dust particles
collect in the cone at the bottom of the cyclone where they can be
discharged.
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