amount of fuel oil being fed into the burner. The exhaustfan creates the draft of air that carries the heat throughthe dryer and removes the moisture. Imbalance amongthese three elements causes serious problems. With fueloil, lack of sufficient air or excess flow of fuel oil canlead to incomplete combustion of fuel. The unburnedfuel leaves an oily coating on the aggregate particles–acoating that can adversely affect the finished mixture.Lack of balance between the blower air and draft airvelocities can create hack pressure within the dryerdrum, causing puff back at the burner end. Puff backindicates that the draft is not sufficient to accommodatethe air pressure being introduced by the burner blower.The solution is to increase the draft or to reduce thepressure of the blower air.Dryer burners using natural gas or liquid petroleumgas rarely develop combustion problems; however, animbalance between gas pressure, combustion air, anddraft can occur. Make sure the gas burner you use is thecorrectly type for the pressure of the gas available.The temperature of the aggregate, not the asphalt,controls the temperature of the mix. Overheating theaggregate can harden the asphalt during mixing.Underheating the aggregate is difficult to coat withasphalt and the resulting mix is difficult to place;therefore, a pyrometer, which is a reliable and accuratetemperature-measuring device, should be installed inthe dryer discharge in full view of your burner operator.The pyrometers are sensitive instruments, designedto measure the very small electrical current induced bythe heat of the aggregate passing over the sensingelement. The pyrometer must be completely shieldedfrom the heat and plant vibrations. The head of thedevice is usually located several feet away from thedryer and is connected to its sensing elements by wires.Any change in the connecting wire length, size, splice,or coupling will affect the calibration of the device andit must be recalibrated.Two types of pyrometers are used. They are asfollows: (1) the indicating pyrometer, which is usuallylocated at the discharge chute of the dryer, and (2) therecording pyrometer. The recording head of thisinstrument is usually located in the plant control room.The major difference between the indicatingpyrometer and the recording pyrometer is that theindicating pyrometer gives a dial or digital reading, andthe recording pyrometer records aggregate temperatureson paper in graph form providing a permanent record.Both types of pyrometers are quite similar in operation.Both pyrometers have a sensing element; that is, ashielded thermocouple that protrudes into the mainhot-aggregate stream in the discharge chute of the dryer.Pyrometers should be cleaned periodically. Dustaccumulating on them may cause a time lag intemperature measurement.They should also be checkedfrequently for accuracy. A simple way to do this is to putthe sensing element of the pyrometer, together with anaccurate thermometer, in an oil or asphalt bath. Beingcautious of the flash point for the bath, slowly heat theoil or asphalt and compare readings from the pyrometerand thermometer. These readings should be taken attemperatures below, through, and above the expectedoperating temperature range.Another means to check the accuracy of atemperature-indicating device is to take two shovelloads of hot aggregate from the dryer discharge chuteand dump them in a pile on top of each other. The topshovel load of hot aggregate keeps the bottom shovelload of aggregate hot while the temperature is taken.Inserting the entire stem of an armored thermometer intothe hot aggregate pile will give a temperature readingthat can be compared to the reading on the pyrometer.Several thermometer readings may be necessary to getaccurate temperature data.A moisture check of the hot aggregate can beperformed at the same time a temperature indicatorcheck is performed. From the two shovel loads ofaggregate, observe the aggregate for escaping steam ordamp spots. These are signs of incomplete drying orporous aggregate releasing internal moisture which mayor may not be a problem, Another procedure used tocheck the moisture content is to take adry, clean mirror,shiny spatula, or other reflective item and pass it overthe aggregate slowly and at a steady height. Observe theamount of moisture that condenses on the reflectivesurface. With experience, you will be able to detectexcessive moisture consistently. These quick-moisturechecks are useful in determining whether a more preciselaboratory moisture test should be preformed.Dust CollectorManufacturers have designed asphalt plants to haveequipment that restrict the escape of pollutants from theplant. Even so, during the operation of an asphalt plant,some gaseous and particulate pollutants may escape intothe air. These pollutants must be controlled and limitedto meet established clean air regulations. As thesupervisor, you must be fully aware and familiar withthe local laws concerning air pollution.8-6
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