A grain drying system and process includes a controller that is electronically coupled to a variable speed fan and to a heater or a heat pump to supply air through a plenum and the grain. Also coupled to the controller are ambient temperature and humidity sensors, internal plenum temperature and a humidity sensor. The controller adjusts a fan speed in combination with operating the heater or heat pump to deliver a target equilibrium moisture temperature, during a first period when the ambient air is outside the equilibrium moisture target. The controller operates the fan at a minimum speed, during a second period when the ambient air is outside the equilibrium moisture target, and the controller is unable to obtain equilibrium moisture target air in the plenum in view of operational limits of the fan and the heater or heat pump.
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