Under each bin is a belt feeder upon which the aggregateis proportioned.The plant should be equipped with a means to obtainsamples of the full flow of aggregates from each coldfeed and the total cold feed. These samples are requiredto perform a sieve analysis of the dried aggregate.Cold feed controls consist of the following:1. Sieve analysis of aggregate in each bin.2. Calibrate feeders–both belt speed and gateopening.3. Established bin proportions.4. Set belt drive speeds and gate openings.Once the gates are calibrated, they should bechecked regularly to ensure they remain properly set.All settings should be considered tentative because thecold aggregate used in the mix normally varies ingrading and moisture content; therefore, adjustment ofthe gates is required to maintain a uniform flow.Drum-mix plants require the use of a continuousweighing system on the cold-feed conveyer belts.In-line belt weighers, known as weigh bridges, arebelt-weighing devices used to weigh the combinedaggregate passing over the conveyer belt. A readoutindicates the weight of the flow over the scales at anygiven instant. No material should ever be diverted fromthe conveyer belt after it passes the belt weigher.The in-line belt weigher is usually located betweenthe head and tail pulley of the cold-feed belt conveyer,This location tends to lessen variations in readingscaused by impact loading, rollback of aggregate, orchanges in belt tension.In drum-mix plants the aggregate is weighed beforedrying. Undried aggregates may contain an appreciableamount of moisture that can influence the weight;therefore, an accurate measurement of the moisturecontent is important. From the weight measurement,adjustments can be made to the automatic asphaltmetering system to ensure that the amount of asphaltdelivered to the drum is correct for the amount ofaggregate minus its moisture content.The moisture content of the cold-feed aggregatesshould be monitored at the beginning of each day andabout the middle of the day. When conditions make themoisture content vary, it should be checked morefrequently.Figure 8-18.-Drum mixer zonesAsphalt MeteringThe drum-mixer is normally equipped with a systemto add asphalt to the aggregate inside the drum mixer.Called the asphalt metering and delivery system, it is acontinuous mechanical proportioning systeminterlocking with the aggregate weigh system to ensurethe exact asphalt content of the mix. The weight of theaggregate delivered into the mixer, as measured by theweigh belt, is the basis for determining the quantity ofasphalt delivered into the drum.Asphalt proportioning is accomplished byestablishing the necessary rate of asphalt delivery ingallons per minute to match the aggregate delivery intons of dry aggregate per hour. The asphalt delivery rateis adjusted to correspond to the weight measurement ofthe aggregate prosing over the belt scale.Drum-Mix OperationThe mixer is the heart of a drum-mix plant.Compared to a conventional batch plant rotary dryer, themixer is similar in design and construction except thatthe drum-mixer can be divided into two sections: (1) aprimary or radiation zone and (2) a secondary orconvention/coating zone (fig. 8-18).Aggregates enter the primary zone where heat fromthe burner dries and heats it. Then the aggregate movesto the secondary zone where asphalt is added, andaggregates and asphalt are thoroughly blended.Continued drying also occurs in the secondary zone. Themixture of hot asphalt and moisture released from theaggregate produces a foaming mass that traps the finematerial (dust) and aids in the coating of the largerparticles.Drum-mixers are equipped with flights to direct theaggregate flow and spread the aggregates into a veil8-17
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