Figure 8-8.Dust collector.

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 anti-air-pollution requirements. 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 8-7