Achieving efficient, consistent production

Aggregate Industries' Oxford CMI parallel-flow asphalt plant in Englewood, CO served the company's production needs well at this location for over 20 years, but with the increasing challenge of working with new mix designs (Superpave, Stone Matrix Asphalt, modified polymers, etc.) and the use of RAP in many of those mixes, an upgrade was in order prior to the 2004 construction season.

Keeping only the cold feed bins, asphalt silo and controls from its old plant, Aggregate selected a Maxam Solo Drum Mixer equipped with a Raptor Recycle System that allows producers to heat their RAP material directly rather than indirectly (superheating the virgin aggregate material before adding the RAP).

The Maxam Solo Drum system is also equipped with a Maxamizer Heat Recovery System that separates the control of the mix temperature from control of the exhaust gas temperature by providing a separate burner and separate controller for each. The separate exhaust temperature control allows a producer to maintain a constant temperature of exhaust gas entering the baghouse, which supports consistent production rates even when varying mix designs.

The technology specifics

The Maxam Solo technology produces exhaust gas exit temperature of only 200 degrees through advanced, high-efficiency flighting, which allows up to 50 percent RAP content without superheating virgin aggregate because the RAP is radiantly heated by the burner flame to 300 degrees (hot enough to remove moisture but not hot enough to reach smoke point), and the baghouse outlet temperature is automatically controlled at 225 degrees with the Maxamizer Exhaust Heat Recovery Unit.

The heat recovery system captures exhaust temperatures exiting the drum at 180 to 200 degrees and then heats the gases to 225 degrees with a small burner before they enter the baghouse. This is accomplished with a flighting system that basically overflights the drying drum to transfer more heat to aggregate. This technology reduces the CFM and BTU requirements of the system, which translates into increased production of up to 20 percent and reduced fuel costs, as much as 5 to 10 percent per ton.

The advance counter-flow technology delivers greater fuel savings, higher production, increased RAP usage, safer baghouse operation and longer equipment life.

A significant improvement

Entering the second full year of production with the new plant, Bill Doe, plant manager, says the upgrade has been like night and day for increasing production, utilizing more RAP and achieving greater fuel efficiency in the operation.

Supporting three to four paving crews, as well as supplying a dozen or so private accounts out of the Oxford facility, Aggregate further enhanced the plant's efficiency by converting the burner from natural gas to waste oil. It required some creative thinking in making the modifications to the Hauck burner, especially with the high altitude environment of the plant's location, but a full year of operation has produced significant improvements; and switching the burner to waste oil is saving about $1 per ton in production costs.

"We were getting more requests for polymer mix designs and to use more RAP in some mixes we were producing, but our old drum really limited what we could do," Doe says. "The counter-flow system is much cleaner, we're able to produce the polymer designs without any problems, and we've been able to increase our RAP content without worrying about any blue smoke issues."

Aggregate Industries currently produces 400,000 to 500,000 tons per year out of the Oxford facility. And while the tons-per-hour production capacity is about the same as the old CMI parallel-flow drum, overall production has increased due to the efficient way the Maxam Solo counter-flow drum heats the mixes produced. Since virgin aggregate no longer has to be superheated to accommodate polymer additives or RAP, production time has been cut and the exhaust of blue smoke eliminated.

"Our maintenance time and costs are much lower as well," Doe says. "Now, part of that is due to the fact that we have a new plant, but we're also seeing less wear and tear on the drum since we no longer have to superheat the virgin aggregate. The money we're saving by burning waste oil to heat our mixes, along with the money we're saving by not having to superheat our virgin aggregate, should allow us to recoup our investment within a couple of years."

Ease of operation

For Doe, who's been with Aggregate for 20 years and manager of the Oxford facility for the past 14, a major advantage of the upgrade is the ease of operation from start-up to switching mix designs.

"When I fire up the plant in the morning, we can start producing within 15 minutes," he says. "And we're also able to switch mix designs a lot easier with the counter-flow system. The separate temperature controls allow us to maintain a constant temperature throughout the system at all times. It's really been a marriage of new and old technology that has really enhanced our overall production capabilities and efficiencies."

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