Asphalt -

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 with a wet scrubber, for example, a baghouse can return the dust for use rather than wasting it and no local settling pond is needed.

Typical applications have a range of particle sizes, ranging from smoke well below one micron to coarse.  Other asphalt applications can have very fine dust, mostly submicron. Aramid (Nomex) filter bags are the standard used in the industry.  With a typical dust, smoke is not generally collected, but overall emissions are .01-.02 gr/ft³, well below the US standard of .04 gr/ft³.  The air-to-cloth ratio ranges between 4/1 and 6/1, depending on the fineness of the particulates.

It’s important to preheat the baghouse prior to start-up to prevent condensation from forming.  The preferred inlet temperature is about 250°, however the preheat temperature should be about 100° higher.  Under no circumstances should the temperature inside the baghouse exceed 400°, the maximum operating temperature of Nomex.

Prior to 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.  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.