Issue link: http://read.dmtmag.com/i/266311
HOT-MIX MAGAZINE 18 VOLUME 19 NUMBER 1 I t's difficult for most asphalt plants to run the wide range of mixes demanded by the industry these days. High-RAP (reclaimed asphalt pavement) mixtures tend to overheat the bags in baghouses, which are the air pollution control systems on all asphalt plants. That is because the standard way to heat RAP is to mix it with super- heated virgin aggregate. When you run 50 percent RAP in a mix, you have less virgin aggregate to heat the RAP. The veil of shower- ing virgin aggregate in the drum is less dense in the drying zone. So you get a hole in the veil and the hot exhaust gases pass through the drum into the baghouse. Flights, or metal pocket-like devices, are attached to the inside of the rotat- ing dryer drum to lift and shower the virgin aggregate through the heated gases. You can add more flights inside the drum to increase the density of the veil. That will reduce the stack gas temperature to the point that you can run high-RAP mixes. But a problem arises when you try to take the same drum, with the added flights, and run a 100-percent virgin aggregate mix. Now the veil is much too heavy for virgin mixes and not enough heat will escape to keep your baghouse up to tempera- ture. And a baghouse that becomes too cold will cause mud deposits to build up on the bags because of moisture condensation. What's more, mud may build up in the feed end of a counterflow dryer or within the exhaust ductwork itself. With mud amassing throughout the system, the plant's operation may slog to a halt. WILLETS POINT ASPHALT CORP At Willets Point Asphalt Corp., Flushing, New York, owner Ken Tully uses his Astec Double Barrel ® plant with a warm mix system to run both virgin mixes and mixes with 40 to 45 percent RAP—or more—in them. Until recently, he would get high stack gas tempera- tures with high-RAP mixes, and low stack temperatures with virgin mixes. "We had some problems in the baghouse with mudding of the bags," Tully said. To resolve the problem, Astec installed its patent-pending V-Pack ™ Stack Temperature Control System and stainless steel combustion flights. Those ele- ments, plus some changes to the binder injection location, make up the Double Barrel ® Enhanced RAP. Tully's plant has been retrofitted with the system, which takes RAP capability above 60 percent. V-FLIGHTS ARE KEY A key element of the patent-pend- ing V-Pack TM Stack Temperature Control System consists of "V-flights," which are patent- pending drum flights with a deep V-shape. They produce a uniform veil of virgin aggregate across the drum, regardless of how full the drum is, the plant's production rate, or the RAP percentage used. Moreover, Astec's V-Pack Stack Temperature Control System moni- tors the exhaust gas temperatures at the baghouse inlet as the pri- The Astec V-Pack TM Stack Temperature Control System controls exhaust gas temp across a range of mix types and operating conditions. THE KEY TO HIGH-RAP MIX PRODUCTION THE KEY TO HIGH-RAP MIX PRODUCTION Astec's patent-pending V-Pack™ Stack Temperature Control System yields success 13609_Astec_HotMixV19N1.indd 18 2/18/14 11:24 AM