Figure 8-8.Dust collector.
A major air pollution concern at an asphalt plant
centers around the combustion unit. Dirty, clogged
burners and improper air-fuel mixtures result in
excessive smoke and other undesirable combustion
products; therefore, close attention to the cleanliness
and adjustment of the burners and accessory equipment
is very important.
Another source of air pollution is aggregate dust.
The greatest dust emissions from the plant come from
the rotary dryer. Dust collectors are installed at this
location to reduce dust emissions to a level that meets
Most dust collectors (fig. 8-8) are centrifugal
(cyclone) units, either horizontal or vertical with single
or multiple shells. Dust particles enter the top of the dust
collector in the current of draft air from the dryer, drawn
by the fan(s) that pull(s) the flame and the hot gases
through the dryer. In the collector, the dust-laden air is
forced into a whirling motion.
Heavier dust particles in the exhaust gas stream are
separated by centrifugal force against the collector shell
and are carried to the lower outlet. If the collector works
efficiently, the finer dust that remains in suspension is
carried out the exhaust stack with the air. The fines
collected at the bottom of the cyclone are normally
picked up by a dust-return auger and returned to the
plant or wasted.
When required by specifications, a baghouse or wet
wash system is added to the dust-collecting system.
Several types of wet systems are used. They usually
consist of a short tower, with or without baffles, or
multiple horizontal tubes with spirals. The washer swirls
the high-velocity exhaust coming from the dust collector
through a fog and a fine spray to wash the gas. The
dampened fines are thrown to the sides by centrifugal
force. The material washes down the sides and
discharges, with the water, out the bottom of the washer.
The wastewater containing the dust must be properly
handled to prevent it from becoming another source of
pollution. Use of a wet wash system requires a large
source of water. Also, the output of the fan in the dust
collector must be increased by 10 to 20 percent because
of pressure loss in the tower.
The baghouse is a large metal housing, containing
hundreds of synthetic, heat-resistant fabric bags. The
bags are usually silicone-treated to increase their ability
to collect and retain very fine particles of dust. The
function of the baghouse is similar to the function of a
vacuum cleaner. A large vacuum fan creates a suction
within the housing that draws in dirty air and filters it
through the fabric of the bags. A typical unit may contain
as many as 800 bags to handle the huge volume of
exhaust gases from the aggregate dryer. Eventually, they
accumulate into what is called a dust cake that must
be removed before it reduces or stops the flow of dirty
air through the filter. Several methods for cleaning the
bags in the baghouse are used; however, the most
commonly used methods are as follows: flex the bags,
back flush the bags with clean air, or flex and back flush
the bags. The Jet-Pulse system is another method which
is similar to the back flush in that it produces a pulse of
positive pressure within the bag to dislodge the dust
cake. Dust removed from the bags drops into an auger
at the bottom of the baghouse and is normally