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State of California, Department of Transportation - page 19 / 52

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An example of poor cold feed adjustment, would be excessive amounts of coarse aggregate (½” x ¾”) appearing at the reject chute and the fine aggregate bin continually running empty. The plant operator can decrease the coarse aggregate feed and increase the fine aggregate feed percentages correcting the problem.

Controlling the cold feed system is the key to all subsequent operations! Problems at the cold feed contribute to a non-uniform mix, which in turn causes problems with the asphalt concrete mixture on the roadway.

Aggregate Drying And Heating Aggregates are delivered by a series of belts (collection, slinger) to the drum for heating and drying. The batch plant dryer is a revolving cylinder up to 12 feet in diameter and up to

50 feet in length.

Fuel used to

heat

the

aggregate

may

include

LPG,

propane,

natural

gas,

diesel,

heavy

fuel

oils

or

reclaimed oil.

A blower fan is

used to supply air for combustion of the fuel, and an exhaust fan is used to create a

draft

through

the

drum.

The

inside of the drum is equipped with longitudinal troughs or channels, called flights, that lift the aggregate and drop it in veils through the hot gases. The slope of the dryer drum, rotation speed, diameter, length and the flight configuration determine the length of time

the aggregate will spend in the dryer.

The inspector must be sure the burner fuel is achieving complete combustion, there should be no signs of black smoke in the air or soot in the settling pond. The exhaust fan creates the draft of air that carries the heat through the dryer drum and removes the moisture. Imbalance between draft air and blower air velocities can cause back pressure inside the drum. This creates a "PUFF-BACK" of exhaust gases at the burner end of the drum. Corrections to the draft air or blower air will correct this problem.

If the aggregate exiting the drier drum exceeds 1% moisture the inspector should look at incomplete fuel combustion, draft air vs. blower air imbalances (puff-back), drier drum slope exceeding ¾” per foot of drum length, worn or missing flights, or aggregate feed rates (TPH) exceeding the capacity of the burner to heat and dry the aggregate. Course aggregates with visible dark spots exiting the drier (use a shovel to sample drier discharge) and steam rising out of hot mix being loaded into trucks are tell tale sign indicating that the aggregate has not been dried enough. If the moisture content of the

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