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and baghouses) allow the plant operator to control the amount of fine aggregates that are returned to the completed asphalt mix.

Caltrans allows the return of fine aggregates to batch mixing facilities under the following conditions:

  • 1)

    Fine aggregates collected in “Cyclones” or “Knock-Out Boxes, may be uniformly returned at a point in advance of the aggregate sampling device normally at the hot elevator.

  • 2)

    Fine aggregates collected in a “Baghouse” shall be stored, kept dry, and proportioned by mass on a separate scale or cumulatively with the aggregate.

Currently, three types of systems are used for dust and emission control. Following are brief descriptions and operating procedures for each type of system.

Dry Collector - Consist of centrifugal collectors, commonly referred to as "cyclone" and expansion chambers,

which are

typically referred

to as

"knock-out"

or "drop-out

boxes".

These systems, are collectors after the drum. The material

usually, the first dust leaves the collected in these

types of systems is the fines collected

much coarser

than

in a baghouse

and

does not have a

profound effect

on

the

asphalt demand

when returned

to

the

the aggregate

stream, usually

at the

bottom of the

batch plant hot

(stone)

of to

mix.

Fines

from

these

types

collection systems can be returned

elevator.

Types of Deficiencies that May Be Encountered in Producing Asphalt Concrete

POSSIBLE CAUSES Excessive Aggregate M oistures Inadequate Separation at Bins Aggregate Feed Gates not Properly Set Over Rated Dryer Capacity Dryer Angle to Steep Im proper Dryer Operation Temperature Indicator Not Adjusted Properly Aggregate Tem perature too High Screens are W orn Out Faulty Screen Operation Bin Overflow not Functioning Bins Leaking Segregation of Aggregate in B Carryover in Bins due to Overlo Aggregate Scales out of Adjust Im proper W eighing Procedures Feed of M ineral Filler not Uniform Insufficient Aggregate in Hot Bins Im proper W eighing Sequence Insufficient Asphalt Too M uch Asphalt Faulty Distribution of Asphalt to Agg Asphalt Scales out of Adjustm ent Asphalt M eter out of Adjustm ent Undersized or Oversized Batch M ixing Tim e Incorrect Im properly Set or W orn Pugm ill Paddle Faulty Dum p Gate Asphalt & Aggregate Feed not Synchronized Occasional Dust Shakedown in Bins Irregular Plant Operation Faulty Sam pling

ed Screen Deck t

ate

A A A

A = Applies to Batch and Continuous Plants

PROBLEMS Asphalt Content does not Match Mix Design Aggregate Gradation does not Match Mix Design Excessive Fine Aggregate in AC Mixture Uniform Mix Temperature Difficult to Maintain Truck Weights do not Check with Batch Weights Excess Asphalt on Mix in Hauling Vehicle Excess Dust on Mix in Hauling Vehicle Large Aggregates are Uncoated Mixture in Truck is not Uniform Mixture in Truck Rich (Too Much Asphalt) on One Side Mixture Flattens in Truck Mixture Burned Mixture too Brown or Grey (Not Fully Coated) Mixture too Rich (Too Much Asphalt) Mixture Smokes in Back of Truck Mixture Steams in Back of Truck Mixture Appears Dull in Back of Truck

A

A

A

A

A AA AA

A

B

A AC A

BB

CA

AAAAA

AB AABAABBBAA AABAABBBAA

ABCBBA C A ABC BABCAA AAAA A AABAABBBAAB B A BBA AABCB A A A A A A A C A AAA AAAA BCB AABCB C C AABA A BA AB AA A AAA AAAA AAA BA A C = Applies to Continuous Plants AA B = Applies to Batch Plants

B

BB BB

B

AAABCBBA

C

A

B AABCB

C

A

B

Wet Collector - Also referred to as a wet scrubber, removes exhaust gases and fines from the drier. The fine particles mixed with exhaust gases are sprayed with water nozzles. The wetted fines, which are relatively heavy, are removed by centrifugal force and fall to the bottom of the collector. The water and fines flow out of the collector and into a settling pond while the hot gases and steam are exhausted into the air through the stack. This system does not return any of the collected fine aggregates back to the plant.

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