Grain dryers configured so that different numbers of ducts in a grain column are selectable for cooling

A grain dryer, comprising:

• first and second grain columns, each configured to receive grain to be dried and each having respective first wall and a respective second wall;
• a plenum between the first and second grain columns, the plenum comprising an inner channel containing a burner and a respective outer channel, of a pair of outer channels, between the inner channel and the respective first wall of each of the first and second grain columns;
• a respective air cavity, of a pair of air cavities, between the respective second wall of each of first and second grain columns and a respective heat shield of a pair of heat shields;
• a blower below the burner in an interior of the grain dryer between the respective first walls of the first and second grain columns;
• a plurality of first ducts in each of the first and second grain columns and extending from the respective first wall of each of the first and second grain columns to the respective second wall of each of the first and second grain columns, wherein the first ducts have first inlet/outlets that open into the respective outer channels, of the pair of outer channels, through the respective first walls, wherein ends of the first ducts opposite to the first inlet/outlets are closed by the respective second walls;
• a plurality of second ducts in each of the first and second grain columns and extending from the respective first wall of each of the first and second grain columns to the respective second wall of each of the first and second grain columns, wherein the second ducts have second inlet/outlets that open into the respective air cavities through the respective second walls, wherein ends of the second ducts opposite to the second inlet/outlets are closed by the respective first walls;
• a plurality of dampers, the dampers of the plurality of dampers respectively configured to selectively close each of the respective outer channels, of the pair of outer channels, at different locations along a length of the respective outer channels;
• wherein when a particular damper of the plurality of dampers selectively closes each of the respective outer channels at a particular location along the length of each the respective outer channels:
• cooling air, in response to operation of the blower, flows from the respective air cavities into second ducts, of the plurality of second ducts, below the particular dampers through the second inlet/outlets of the second ducts below the particular dampers and into the respective first and second grain columns from the second ducts below the particular dampers and from the respective first and second grain columns into first ducts, of the plurality of first ducts, below the particular dampers and into regions in the respective outer channels below the particular dampers through the first inlet/outlets of the first ducts below the particular dampers;
• and
• heating air, in response to operation of the blower, flows from regions in the respective outer channels above the particular dampers into first ducts, of the plurality of first ducts, above the particular dampers through the first inlet/outlets of the first ducts above the particular dampers and into the respective first and second grain columns from the first ducts above the particular dampers and from the respective first and second grain columns into second ducts, of the plurality of second ducts, above the particular dampers and into the respective air cavities through the second inlet/outlets of the second ducts above the particular dampers.