Asphalt in Dust Collection Systems
In the hot asphalt industry, baghouses have become the air cleaning system of choice for particulate removal, being more economical and simpler to operate than other options. Compared to a wet scrubber, for example, a baghouse can return the dust for reuse rather than wasting it, and no local settling pond is needed.
Particle Size Range
Typical applications have a range of particle sizes:
- Smoke well below one micron to coarse particles
- Other applications have very fine dust, mostly submicron
- Aramid (Nomex) filter bags are the industry standard
Performance Specifications
- Overall emissions: .01-.02 gr/ft³ (well below US standard of .04 gr/ft³)
- Air-to-cloth ratio: 4/1 to 6/1 (depending on particulate fineness)
- Maximum operating temperature: 400°F (Nomex limit)
Preheating Requirements
It's important to preheat the baghouse before start-up to prevent condensation from forming. The preferred inlet temperature is about 250°F; however, the preheat temperature should be about 100°F higher. Under no circumstances should the temperature inside the baghouse exceed 400°F, the maximum operating temperature of Nomex.
Cyclone Pre-Filtering
Before the baghouse, it is usual to filter out the larger particulates with a cyclone. Without this pre-filtering, dust exiting the dryer could overload the baghouse, and an excess of coarse particles could form too porous of a cake on the bags to filter out the fines. Additionally, large particles can abrade the bags, shortening their life.
Design Consideration: The cyclone should only be designed for moderate efficiency (not over 90%), since some medium-to-large particles are necessary to form a foundation on the bags so the fines may be collected. A cyclone adds some pressure drop to the system (on the order of 5″ W.G.), so the exhaust fan must be sized accordingly.