Discharge GateLittle time is lost in discharging acompleted batch through the hydraulicallyactuated discharge gate because there isno segregation of materials. The gateopens across the entire length of thepaddle shafts and across the width(distance between the paddle shaftcenters) of the pugmill. Being relativelylarge, the gate opening permits accessfrom the bottom for maintenance andreplacement of parts.Plant AutomationThe batch asphalt plant is almostcompletely automated. After mixproportions and timers are set and theplant is started, the plant repeats theweighing and mixing cycles until stoppedby the operator or until a shortage ofmaterial or some unexpected malfunctioncauses the plant to shut down itself.The plant operator’s manual providedby the manufacturer, provides details onthe setup and adjustment of the automaticequipment. You must check the accuracy andadjustment of this equipment, particularlythe aggregate scales, asphalt scales ormeters, batching controls, and recordingequipment (if used). This should result introuble-free production; moreover, theentire plant should be checkedperiodically to ensure the finishedproduct meets specifications.CONTINUOUS-FLOW ASPHALT PLANTThe continuous-flow asphalt plant isshown in figure 8-2. Continuous-flowasphalt plants are equipped with positivedisplacement asphalt pumps. One type isregulated by changing the drive sprocketsor gears that are mechanically interlockedwith the aggregate feeders (fig. 8-16).The other is controlled by a calibratedremote control handwheel on the mixeroperator platform. When using the formertype, you must use the manufacturer’stables as a basis for determining theproper pump and sprocket combinations tofix the amount of asphalt discharge. Bydoing so, you can control the feeder gatesand asphalt pump while ensuring no changein setting can be made without theknowledge of the asphalt plant supervisor.The temperature of the asphalt goingthrough the positive displacement asphaltpump must be known at all times tomaintain constant asphalt proportioning.You should take frequent readings of thethermometer installed in the circulatingline just ahead of the pump. This allowsyou to make any necessary adjustments toFigure 8-16.-Spray-bar operation with asphaltdistribution mechanically interlock withaggregate feeders.compensate for volume changes in theasphalt when substantial temperaturechanges occur.Pugmill MixerThe function of a continuous-flowplant pugmill is almost identical to thepugmill in the batch mix plant. Theprimary difference is that the mixingprinciple is different. In a batch mixer,the materials are confined in the mixingchamber. In a continuous-flow plantpugmill mixer, the materials are propelledtoward the discharge. The mixing pressurevaries with the height or weight ofmaterial in the pugmill that can becontrolled by adjusting the dam gate atthe discharge. The height of material inthe pugmill mixer should not be allowed torise above the paddle tips, with theexception of the last set of paddles.To improve the mixing efficiency ofthe pugmill, you should make the followingadjustments:1. Raise the dam on the dischargeend of the mixer to hold thematerial in the mixing unit foralonger period of time at a depththat will further intensify themixing action.2. Adjust or reverse the pitch ofthe paddles to retard movement ofthe material through the pugmilland increase the degree of mixingaction within the unit.Nonuniform mixing can occur if themixer is overfilled. At maximum operatingefficiency, the paddle tips should bebarely visible in the material at the topof the arc during mixing. The batch ratingof the pugmill for the continuous-flowplant is the same as for the batch plant.8-15
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