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State of California, Department of Transportation - page 36 / 52

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Dry Collectors When a baghouse is in use, expansion chambers, usually referred to as "knock-out" or "drop-out” boxes are the first collector after the dust leaves the dryer drum. The material captured in these types of systems is much coarser than the fine dust collected in a baghouse and does not have a profound effect on asphalt demand when returned to the drum. The fine aggregate normally associated with these types of systems must be returned to the plant at a rate of 100 percent or 100 percent rejected. Caltrans specifies if the return of rate is other than 100% of the collection rate it shall be metered.

Wet Collectors - typically referred to as a "wet scrubber", this system removes exhaust gases and very fine dust from the dryer drum. These fine dust particles are mixed with exhaust gases as they exit the drier drum. As the dirty air enters the wet

scrubber, nozzles emitting a saturate the dust laden air. fines, which are relatively removed by centrifugal force

fine spray The wetted heavy, are and fall to

fines flow out

of the

collector

and into a

settling pond

while

the hot

gases and

the

bottom

of

the

collector.

The

water

and

steam are the stack.

exhausted into the air through This system does not return any

of

the

collected

fine

aggregates

back

to

the

plant. When this type of system is in use, the mix design created for mixing operation is typically in error. A portion of the fine aggregate mix design is now in the wash pond, decreasing the surface area. With a area the asphalt demand should also be less.

this continuous used during the reduced surface

Baghouse

  • -

    removes

dust

passing

the

#200 sieve, utilizing a principle similar to a giant vacuum cleaner. Exhaust fans create the suction that pulls the fine dust from the drier drum. After the dust-laden air enters the baghouse, the exhaust fans pull the dirty air through cloth bags to trap the fine dust against the exterior of the bag. A baghouse is typically divided into sections, while some sections are collecting dust, others are using reverse air, a shaker system or a pulse cleaning system to release dust from the outside of

the bags.

Dust then falls to the floor of the baghouse and travels by auger or conveyor to

a central point outside of the system.

From this point, all of the collected fines may be

rejected or returned at a rate commensurate with the overall plant production.

Caltrans

specifies

if

the

return

rate

is

other

than

100

percent

of

the

collection

rate,

it

shall

be

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