Figure 8-7.-Dryer.when calibrating gates through which fine aggregatesare flowing.For uniform flow, gates that feed coarse aggregateshould not be set at a height less than 2 1/2 to 3 timesthe largest aggregate size; for example, if a gate isfeeding aggregate that has a maximum size of 1 inch,the gate should not be set at less than 2 1/2 or 3 inches.Sometimes it may be necessary to restrict the openingwidth to provide the necessary opening height.Before you set the cold-feed gates, the productionvolume of the plant in normal operation must bedetermined. this can be estimated from the plant size(dryer, screening, and mixing capacities) and mixingcycle time. Then, using the gate calibration charts, eachgate is set to deliver its share of the desired volume ofaggregate.Grading of the individual cold aggregate isdetermined by sieve analysis. The percentage of eachsize of aggregate to be used is calculated by trial anderror.The proportions required on the basis of thesepercentages will determine the gate settings. Thesesettings should be checked by the same method used incalibrating the gate originally.The setting should be considered tentative becausethe cold aggregate may vary in grading and moisturewith the weather and other conditions that will affect itsbulking and flow.The hot bins should be watched carefully and thecold aggregate feeders regulated to see that they do notrun out of material or overflow.DryerFrom the aggregate cold-feed system, aggregatesare delivered to the dryer. The dryer (fig. 8-7) is arevolving cylinder in which the aggregate is dried andheated by an oil or gas burner. The cylinders used rangefrom 3 to 10 feet in diameter and from 15 to 40 feet inlength. A cylinder is usually equipped with longitudinalcups or channels (called lifting flights) that lift theaggregate and drop it in veils through the burner flameand hot gases. The slope of the cylinder, its speed,diameter, length, and the arrangement and number offlights control the length of time required for theaggregate to pass through the dryer.The dryer performs two functions: (1) it removesmoisture from the aggregate and (2) it heats theaggregate to mixing temperature.The dryer includes an oil or gas burner with a blowerfan to provide the primary air for combustion of the fueland an exhaust fan to create a draft through the dryer.For efficient dryer operation, the air that is combinedwith the fuel for combustion must be in balance with the8-5
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