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 particlesa
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.
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.