15 May 2001
(1) Batch plant.
(a) General. A batch plant is illustrated in figure 2-1. Cold feed hoppers have individual
feeders for each of the aggregates to be used in the mixture. These feeders must be set so that the
desired percentage of each aggregate is fed into the plant. The rate of feed may be controlled by the
gate opening, belt speed, or other methods depending on the type of cold feed. If the aggregate feeders
are improperly set, a combination of the following problems may occur:
-- One of the aggregate hot bins will overflow with material while another hot bin runs low on
-- The gradation of the aggregate in the mix being produced will not meet the design gradation.
-- The amount of natural sand may vary from design proportion and may exceed the amount
allowed in the specifications.
(b) Cold feed bin calibration. Before the start of a project the cold feed bins should be
calibrated so that each bin will feed the desired rate of material. The cold feed calibration involves
feeding one aggregate at a time onto a belt that is common to all aggregates. The speed of this belt
should be determined prior to calibration of the feeders. One way to do this is to divide the belt length by
the time required for one revolution. After the material is fed onto the belt, the material over a given
length (for example; 2 meters (6 feet)) should be completely removed and weighed. The following
relationship can be used to convert the weight of the sample taken to kilograms per hour (pounds per
hour) and later to metric tons (tons) per hour:
R = rate of feed, kilograms per hour (pounds per hour)
W = weight of sample, kilograms (pounds)
S = speed of belt, meters per second (feet per second)
L = length of belt sampled, meters (feet)
Each aggregate should be fed at four to five different feeder settings and the rate of feed determined; a
plot of this data showing the relationship between rate of feed (kilograms or metric tons (pounds or tons)
per hour) and feeder setting (gate opening, feeder belt speed, or other method for setting aggregate
feeder) should be used for each aggregate. These plots can be used to set each cold feed bin to feed at
the desired rate.
(c) Dryer. After the aggregate cold feed bins have been properly set, the aggregate is
carried up the cold elevator and through the dryer. The dryer removes the moisture from the aggregate
and heats the aggregate to the desired temperature.
(d) Dust collector. A dust collector collects the dust created in the dryer and other plant
components and adds all or any portion of it back to the mix at the hot elevator. The plant should have
the capability to remove any desired portion of the collected dust from the mixture.