amount of fuel oil being fed into the burner. The exhaust fan creates the draft of air that carries the heat through the dryer and removes the moisture. Imbalance among these three elements causes serious problems. With fuel oil, lack of sufficient air or excess flow of fuel oil can lead to incomplete combustion of fuel. The unburned fuel leaves an oily coating on the aggregate particles–a coating  that  can  adversely  affect  the  finished  mixture. Lack of balance between the blower air and draft air velocities can create hack pressure within the dryer drum, causing puff back at the burner end. Puff back indicates that the draft is not sufficient to accommodate the air pressure being introduced by the burner blower. The solution is to increase the draft or to reduce the pressure of the blower air. Dryer burners using natural gas or liquid petroleum gas  rarely  develop  combustion  problems;  however,  an imbalance  between  gas  pressure,  combustion  air,  and draft  can occur. Make sure the gas burner you use is the correctly type for the pressure of the gas available. The temperature of the aggregate, not the asphalt, controls  the  temperature  of  the  mix.  Overheating  the aggregate  can  harden  the  asphalt  during  mixing. Underheating the aggregate is difficult to coat with asphalt  and  the  resulting  mix  is  difficult  to  place; therefore, a pyrometer, which is a reliable and accurate temperature-measuring  device,  should  be  installed  in the dryer discharge in full view of your burner operator. The  pyrometers  are  sensitive  instruments,  designed to measure the very small electrical current induced by the heat of the aggregate passing over the sensing element.  The  pyrometer  must  be  completely  shielded from the heat and plant vibrations. The head of the device is usually located several feet away from the dryer 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 and it must be recalibrated. Two types of pyrometers are used. They are as follows: (1) the indicating pyrometer, which is usually located at the discharge chute of the dryer, and (2) the recording  pyrometer.  The  recording  head  of  this instrument is usually located in the plant control room. The   major   difference   between   the   indicating pyrometer and the recording pyrometer is that the indicating pyrometer gives a dial or digital reading, and the recording pyrometer records aggregate temperatures on 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, a shielded thermocouple that protrudes into the main hot-aggregate stream in the discharge chute of the dryer. Pyrometers should be cleaned periodically. Dust accumulating  on  them  may  cause  a  time  lag  in temperature  measurement.They  should  also  be  checked frequently for accuracy. A simple way to do this is to put the  sensing  element  of  the  pyrometer,  together  with  an accurate thermometer, in an oil or asphalt bath. Being cautious of the flash point for the bath, slowly heat the oil or asphalt and compare readings from the pyrometer and  thermometer.  These  readings  should  be  taken  at temperatures  below,  through,  and  above  the  expected operating  temperature  range. Another  means  to  check  the  accuracy  of  a temperature-indicating device is to take two shovel loads  of  hot  aggregate  from  the  dryer  discharge  chute and dump them in a pile on top of each other. The top shovel  load  of  hot  aggregate  keeps  the  bottom  shovel load  of  aggregate  hot  while  the  temperature  is  taken. Inserting  the  entire  stem  of  an  armored  thermometer  into the hot aggregate pile will give a temperature reading that can be compared to the reading on the pyrometer. Several thermometer readings may be necessary to get accurate temperature data. A  moisture  check  of  the  hot  aggregate  can  be performed at the same time a temperature indicator check  is  performed.  From  the  two  shovel  loads  of aggregate, observe the aggregate for escaping steam or damp spots. These are signs of incomplete drying or porous aggregate releasing internal moisture which may or may not be a problem, Another procedure used to check the moisture content is to take adry, clean mirror, shiny spatula, or other reflective item and pass it over the aggregate slowly and at a steady height. Observe the amount of moisture that condenses on the reflective surface. With experience, you will be able to detect excessive  moisture  consistently.  These  quick-moisture checks are useful in determining whether a more precise laboratory  moisture  test  should  be  preformed. Dust Collector Manufacturers have designed asphalt plants to have equipment that restrict the escape of pollutants from the plant. Even so, during the operation of an asphalt plant, some gaseous and particulate pollutants may escape into the air. These pollutants must be controlled and limited to  meet  established  clean  air  regulations.  As  the supervisor, you must be fully aware and familiar with the  local  laws  concerning  air  pollution. 8-6