4. Mix Production

4.7.1 Basics of Dust Collection

The amount of airborne dust is a function of the material being dried, the velocity of the air in the dryer, and the location of the binder injection point that captures the smaller particles.

The emission-control system on most asphalt plant facilities generally consists of primary and secondary dust collectors. The dust collectors are situated downstream of the dryer and filter the air that exits the drum.

4.7.2 Primary Collectors

The purpose of the primary collector is to collect and remove the larger dust particles contained in the exhaust gas stream. The typical primary collectors are called the knockout box or the cyclone collector.

The knockout box is the simplest type of primary collector (see Figure 48). The exhaust gas flows through an expanded chamber, causing the airspeed to decrease. The chamber also contains plates to cause a change in the direction of airflow. The speed reduction and direction changes cause the larger dust particles to drop out of the airstream to the bottom of the box, where they are reintroduced in the dryer.

Source: Gencor Industries
Figure 48. Knockout Box Returning Coarse Fines to the Dryer

Cyclone collectors are more efficient than knockout boxes and operate on the principle of centrifugal separation. The exhaust stream circulates inside the collector (shown in Figure 49), and particles hit the outside wall and drop to the bottom of the cyclone. Speed and directional changes also assist as the exhaust is discharged through the top of the collector. The fines collected at the bottom of the cyclone are picked up by a dust-return auger and may be returned to the plant or removed.

Source: Asphalt Institute
Figure 49. Cyclone Collector Returning Coarse Fines to the Dryer

4.7.3 Secondary Collectors

The purpose of the secondary collector is to filter out the finest dust particles. The most common type of secondary collector is referred to as a baghouse. A baghouse (see Figure 50) is a large metal housing containing hundreds of heat-resistant fabric filter bags. A typical unit may contain as many as 800 bags. It operates on a principle similar to a vacuum cleaner. The dust-laden exhaust gases are pulled through the filter bags supported on long wire cages to prevent collapse. The dust is trapped on the outside of the bag as the air passes through the filter cloth to the clean air side, effectively cleaning all the dust from the exhaust stream. When properly operated, baghouses can be very efficient, removing over 99 percent of the dust from the dryer exhaust.

Source: CMI Roadbuilding, Inc.
Figure 50. Baghouse Dust Collector

Dust continuously collects on the outside of the fabric filter while the dryer is in operation. Periodically, a pulse of air is passed through the bag in the opposite direction, causing the bag to flex in reverse. The collected dust drops off the bag and falls to the bottom hopper of the baghouse. This collected dust can either be returned to the mix production process or removed. When dust is returned to the plant process, certain operational practices are important to keep the dust return uniform and consistent. Non-uniform introduction of baghouse fines back to the plant can cause erratic volumetric test results. Refer to Section 4.3.3.