It'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 superheated virgin aggregate. When you run 50 percent RAP in a mix, you have less virgin aggregate to heat the RAP. The veil of showering 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 rotating 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 temperature. 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 temperatures 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 elements, 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-pending V-PackTM 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 monitors the exhaust gas temperatures at the baghouse inlet as the primary reference for control. As the exhaust gas temperature rises, the control system checks it against a set point. When the temperature exceeds the set point, the control system speeds up the drum's rotational speed and controls temperature to the set point. The veil thickens, which brings down the stack gas temperature. The drum speed can be varied from a minimum of about 7 rpm to a maximum of about 12 rpm. (Normal for Astec drums without the system is 8 rpm.)
Because the V-flights shower material uniformly without regard to how much material is in them, they never leave a hole in the veil. More revolutions per minute place more aggregate in the air.
"When you manually adjust your flighting with a virgin mix, it's very hard to maintain (stack gas) temperature because you have a lot of material in the drum," said Tully. "And then to go to a high percentage RAP mix, you have just 60 percent of the virgin aggregate that would be in there at 100 percent virgin. With this V-Pack, it enables you to run the target temperature that you're looking for.
"We set the stack temperature at 250 degrees F (121 degrees C)," said Tully. "Previously, we would be running like 350 degrees F (176 degrees C) on a high-RAP mixture. And we would struggle to maintain 220 or 230 degrees F (104 or 110 degrees C) on a virgin mix. The V-Pack system improves our production for high-RAP mixes, and even for virgin mixes, because it allows you to maintain that temperature and you can run it at higher speeds. Normally, we would run like 300 TPH (272 MTPH) on a virgin mix, and now we can run 375 TPH (340 MTPH) with the V-Pack."
Previously, Tully was having some problems with heat deformation of his flights in the combustion zone. The answer to that, he said, was to install stainless steel flights in the combustion zone; those are not V-shaped flights. Stainless steel can withstand more heat than mild steel.
Running Warm Mix
Tully also runs warm mix with RAP in his Astec Double Barrel® plant. To make warm mix, he typically activates the foaming attachment on the plant that injects a tiny amount of water into the liquid binder—and yet Tully heats the asphalt up to 325 degrees F (163 degrees C). He can achieve the same 375 TPH (340 MTPH) as with a virgin mix.
Warm mix enables Willets Point to maintain workability of the asphalt mixture to a lower temperature than with conventional hot mix. "Conventional mix will be difficult to use after the temperature drops to 250 degrees F (121 degrees C) or so," said Tully. "But the warm mix will allow another 20 to 30 degrees of workability. If a regular mix has a problem at 250 degrees F (121 degrees C), warm mix will be workable up to 230 or 220 degrees F (110 or 104 degrees C). You'll still be able to shovel it, rake it, and get your compaction. If we ship out the warm mix at 325 degrees F (163 degrees C), that will give them another hour and a half or two hours in the truck to allow them to use it."
C.R. Jackson Another asphalt producer to use the patent-pending V-Pack Stack Temperature Control System is C.R. Jackson Inc., Columbia, South Carolina. The company runs a wide variety of mixes including both traditional hot mix, and warm mix, with and without RAP. The company tries to run as much RAP as possible. They also run an open-graded friction course mixture (OGFC).
The V-Pack System did produce the desired advancement on an Astec Double Barrel® for C.R. Jackson, said Malcolm Swanson, president of Astec, Inc. "We first saw this when making OGFC for an Interstate highway project," Swanson recalled. "Whereas the plant previously had been unable to make more than 150 TPH (136 MTPH) of this particular mix, after the V-Pack was installed, the plant easily produced 350 TPH (317 MTPH) of the same mix."
Flights, or metal pocket-like devices, are attached to the inside of the rotating dryer drum to lift and shower the virgin aggregate through the heated gases.
"V-flights" shower aggregate uniformly across the drum, no matter how much material is in the drum.