October 27, 2008

High-speed batch or continuous-flow dryers have the highest bushel capacity per hour of any of the on-farm systems. Temperature, grain bed depth, and airflow rates are vastly different in high-speed, high-capacity dryers compared to deep-bed, in-bin drying systems. Air temperatures of 120°F to 140°F are typical in high-capacity dryers. Column widths of grain being dried are measured in inches (10 to 20 inches) in batch or continuous-flow dryers as opposed to feet (4 to 20 feet) for in-bin drying systems. Airflow rates of 50 to 100 cfm/bu are common in high-speed dryers as opposed to 1.0 to 2.5 cfm/bu for deep bed in-bin systems.

There are two limiting factors that affect the efficiency of high capacity systems. The first limiting factor is the rate moisture can migrate from the interior of the kernels to the surface where it can evaporate into the air stream. The second limiting factor is the short contact time the air stream has with the grain. Very high volumes of very hot and dry air moving through shallow beds of grain can dry the grain quickly but the air leaving the grain mass is much less saturated with moisture compared to deep-bed, in-bin drying systems. This results in higher energy cost per point of moisture removed per bushel as compared to in-bin systems. Some high-capacity dryers recover some energy by channeling the air used to cool the grain back into the drying chamber air stream or by re-circulating a high percentage of the previously heated air back through the grain mass.

High temperatures and uneven moisture content within the kernel result in a much higher incidence of stress cracks as compared to in-bin drying. Stress cracks created in the dryer result in a much higher percentage of broken kernels upon subsequent grain handling.

Dryeration

A variation using high-capacity dryers is known as dryeration. Dryeration is the name given to a system where hot grain is removed from the high-speed dryer one or two points of moisture above desired storage moisture, then transferred to a bin where it is allowed to temper for four to six hours before starting the fan for final cooling. The final one or two points of moisture are easily removed in the process of cooling the grain because the moisture deep inside the kernels has had time to redistribute during the tempering period. This method of grain drying increases the throughput capacity of the high-speed dryer and results in higher quality grain with fewer stress cracks than drying followed by rapid cooling.

Combination Drying

Another intermediate system using both the high-temperature dryer and in-bin aeration is called combination drying. With combination drying, producers "take the edge off" high moisture corn by drying the grain down to 20-22% moisture with the high-temp, high-speed dryer and then move the grain while still hot to a bin equipped with an aeration fan able to push at least two cfm/bu of unheated air through the grain mass to complete the process. This cuts some of the reliance on heat compared to completely drying the grain in the high-speed dryer and decreases the load on the high-temperature dryer even more than dryeration. It also cuts the energy cost if the heating fuel is the higher cost energy source.

If you have been completely drying and initially cooling your corn in the high-speed dryer but have bins equipped with mesh floors and high capacity aeration fans, either dryeration or combination drying can result in faster throughput, lower energy costs and higher quality grain.

Tom Dorn
UNL Extension Educator