A Closer Look at Counterflow Rotary Coolers
June 1, 2008
An alternative to the co-flow and crossflow coolers generally being used in ethanol applications is the counterflow rotary cooler. In the counterflow rotary cooler, the flow of cool air and material work in opposite directions. As the warm dried distillers grain with solubles (DDGS) enters the inlet of the cooler spiral-feed-flights, it advances the DDGS into the lifting zone. In the lifting zone, specially designed lifters (or flights) lift and cascade the DDGS in the cooler air stream. The cooler air and entrained solids are filtered in a cyclone and recycled back to the combustion chamber.
Combined evaporative and convection cooling of the DDGS will provide a DDGS discharge temperature within 10 to 15°F (5.5 to 8.3°C) of the inlet air temperature. With special design considerations, DDGS discharge temperatures below 90°F (32°C) can be achieved year round in most locations.
This method has inherent advantages when compared to cross-flow (fluid-bed) and co-current flow (negative lift) coolers. The DDGS entering the cooler is exposed to the warmest air available within the cooler, minimizing the temperature shock and maximizing the evaporative cooling. The DDGS existing in the cooler is continuously in contact with the coolest air available.
In addition to improved cooling, a 2 percent to 3 percent reduction in DDGS moisture content due to evaporative cooling can be expected. This evaporative cooling means the dryer discharge moisture content can be raised by a corresponding amount. The result is an annual savings of approximately $100,000 for a typical 55 MGPY ethanol plant that uses natural gas as its fuel source.