The drying rate depends on the difference in moisture content between the drying air and the grain kernel. The rate of moisture movement from high moisture grain to low relative humidity air is rapid. However, the moisture movement from wet grain to moist air may be very small or nonexistent. At high relative humidities, dry grain may pick up moisture from the air. The airflow rate also affects drying rate.
Air carries moisture away from the grain, and higher airflow rates give higher drying rates.
Airflow is determined by fan design and speed, fan motor size and the resistance of the grain to airflow. Deeper grain depths and higher airflow rates cause higher static pressures against the fan. Higher static pressures decrease fan output. As air enters the grain, it picks up some moisture, which cools the air slightly. As air moves through a deep grain mass, the air temperature is gradually lowered and relative humidity increased until the air approaches equilibrium with the grain.