By Ed Misajet
Fred Prill doesn’t pull any punches when the subject of new batch plant facilities is brought up in conversation. “Very few contractors with the exception of some in the extreme northeast are interested in batch plants anymore—at least not new ones,” states Prill, an equipment consultant with decades of experience working with leading asphalt plant manufacturers. “Batch plants, in their time, were the state of the art, very functional, and met the needs of most asphalt producers.”
Batch plants, used at the turn of the century, were simple in design. A one-ton batch plant (60 tons per hour [tph]) was the typical size. The quality of the mix generated by these plants met the specifications of the day. However, the quality of the mix produced was manually controlled by the skills of the plant operator. The primary requirement in the early days was for the aggregate to be completely coated with liquid asphalt. These needs were reflected in the plant’s design. A typical batch plant was equipped with a hopper or bin to introduce the aggregate, a small dryer to heat the stone, hot screens to separate the aggregate into several sizes, weigh hoppers to control proportioning and a pugmill to blend the asphalt cement (AC) and aggregate.
As time went on, the needs and requirements changed and batch plant design grew more complex. During the 1950s, interstate construction required hot mix asphalt (HMA) contractors to increase production capability. Typical batch plant size increased to six tons to meet production demands. Larger screens were added to accommodate the production of multiple mixes. Otherwise, batch plants changed little until about 30 or 35 years ago.
The major changes that followed were the onset of automatic computer batching controls, more efficient dust collectors and the emergence of surge silo systems. Plants got a lot bigger and cleaner. They added silos to minimize truck turnaround time. But plants still produced one batch at a time. Expensive pugmill mixers and multideck screens were still required. They resulted in high initial costs plus high maintenance costs, which were the batch plant’s ultimate downfall.
Wave of the Future—Continuous Mix Asphalt Facilities
Continuous mix type plants emerged in the early 1970s as a viable option to batch plants. A typical batch facility produced 150 to 300 tph whereas continuous mix plants produced 300 to 500 tph. The first drum plants capable of producing high tonnages were parallel flow design systems, where aggregate flow was parallel to the direction of the heat source.
Early parallel flow drum plants were designed to be portable because of market demand. Acceptance of this design was also due in part to the involvement of the Boeing Corporation, one of the first manufacturers of the parallel flow system. Even though the design had some inherent technical flaws, the plant still sold.
One drawback was the lack of air pollution controls. Parallel flow systems retained a high percentage of fines and moisture during the heating and mixing process, leading to relatively dirty stack emissions.
Another drawback to the parallel design was the problem of controlling mix temperature. A parallel flow design is a low temperature plant by nature. This is important because most systems such as these are not very practical for urban applications where there are longer haul times, job waiting times and a higher percentage of handwork requiring higher mix temperatures. Low temperatures plus high moisture retention in the mix also equated to compaction difficulties, thus greatly reducing the operating ranges of paving crews.
Additionally, there was an increasing use of reclaimed asphalt pavement (RAP). Successful inclusion of RAP is inherently a high temperature process to transfer heat to the virgin aggregate before RAP is introduced into the mix. This became the final death knell for the parallel flow asphalt plant design because it caused increasingly excessive emissions. Heavy hydrocarbons and relatively high particulate emissions ultimately could not be corrected during the drying and mixing process. These issues could be moderated but not eliminated.
This situation, in turn, created problems in the baghouse. Moisture and hydrocarbons from heavy fuels, AC vapors and heated RAP could plug the collector bags, compromising the baghouse efficiency and could, on occasion, create a baghouse fire hazard. In order to solve these problems, plant manufacturers needed to refine the design.
Counter Flow Designs
Plant engineers began to work on issues such as moisture, excessive emissions and superheating of the baghouse. They believed that a counter flow design would improve plant performance.
The counter flow drum plant is designed for aggregate to flow counter to the heat source allowing for high aggregate temperatures and low stack temperatures. This design coupled with innovations such as flighting systems, conveying systems, and burner design advances solved the problems of earlier drum plant designs.
Modern counter flow designs include an all-in-one drum mixer where aggregate is heated, liquid asphalt is added and RAP and other additives are incorporated in a single chamber. Other designs include expanded recycling collars, separate post-dryer mixing systems and batch plant retrofits. All of these designs are centered on the use of high RAP contents and eliminating blue smoke in the exhaust stack.
Learning to Use RAP
Today’s requirements are vastly different from when drum mix plants first came into use. “In the early days, using RAP was innovative, but problematic,” says Prill. The RAP was already coated with asphalt and contained moisture. When the RAP was introduced into the mix process, problems such as steam explosions, cooking the asphalt or severe temperature drops could occur.
As experience was gained using RAP, manufacturers were able to develop plant designs which minimized the problems and provided HMA producers the ability to handle RAP successfully.
Superpave and Modified Binders
A significant development for HMA producers in the last few years is the emergence of Superpave and modified asphalt binders. These materials created some new challenges for plant designers. Pumping and storing some modified binders required plant modifications. In some cases, additional storage tanks were needed to hold the new Performance Grades or the additional asphalts used in mixes containing RAP. Some modifications of the dryer’s flights were needed to thoroughly dry the coarser Superpave aggregate blends. After doing a few jobs, HMA producers have, for the most part, learned to manage the new materials.
“All of the drum plant technology in the world means nothing if not for the utilization of the silo system. They go hand in hand. Some modern drum plants have production rates in excess of 600 tph. If you don’t have a viable silo system, you are still limited to how fast your truck turnaround is,” according to Prill. All drum plants utilize silo systems, some in clusters of as many as eight silos.
These silos are filled in several ways—by a dragslat, a transfer conveyor or a bucket elevator. A silo system enables the contractor to store high tonnages of different mixes from a few hours to a few days. By filling multiple silos during low demand periods, an HMA producer can supply a variety of mixes to secondary customers while the plant produces mix for the primary need. This flexibility enables the producers to furnish several jobs at the same time as demand dictates.
“Drum plant innovations have added versatility for the contractor,” says Prill. With modern drum mix plants, there is little reason to consider a new batch plant.”
Ed Misajet is principal of Infinite Images. He can be contacted at (502) 292-2125.