addition may be conducted in either a screw conveyor or a pugmill. Besides the direction of purge gas flow relative to soil feed direction, there is one major difference in configuration between countercurrent and cocurrent rotary dryers. The purge gas from a countercurrent rotary dryer is typically only 350 °F to 500 °F and does not require cooling before entering the baghouse where fine particles are trapped. A disadvantage is that these particles may not have been decontaminated and are typically recycled to the dryer. Countercurrent dryers have several advantages over cocurrent systems. They are more efficient in transferring heat from purge gas to contaminated soil, and the volume and temperature of exit gas are lower, allowing the gas to go directly to a baghouse without needing to be cooled. The cooler exit gas temperature and smaller volume eliminates the need for a cooling unit, which allows downstream processing equipment to be smaller. Countercurrent systems are effective on petroleum products with molecular weights lower than No.2 fuel oil. In cocurrent systems, the purge gas is 50 °F to 100 °F hotter than the soil discharge temperature. The result is that the purge gas exit temperature may range from 400 °F to 1,000 °F and cannot go directly to the baghouse. Purge gas first enters an afterburner to decontaminate the fine particles, then goes into a cooling unit prior to introduction into the baghouse. Because of the higher temperature and volume of the purge gas, the baghouse and all other downstream processing equipment must be larger than in a countercurrent system. Cocurrent systems do have several advantages over countercurrent systems: The afterburner is located upstream of the baghouse ensuring that fine particles are decontaminated; and because the heated purge gas is introduced at the same end of the drum as the feed soil, the soil is heated faster, resulting in a longer residence time. Higher temperatures and longer residence time mean that cocurrent systems can be used to treat soils contaminated with heavier petroleum products. Cocurrent systems are effective for light and heavy petroleum products including No. 6 fuel oil, crude oil, motor oil, and lubricating oil. Asphalt plant aggregate dryer Hot-mix asphalt plants use aggregate that has been processed in a dryer before it is mixed with liquid asphalt. The use of petroleum contaminated soils for aggregate material is widespread. Aggregate dryers may either be stationary or mobile. Soil treatment