Last year we measured the pressure drop across the perforated pipe in the bin when the aeration fan was one. We typically saw a one inch (of water) drop across this perforated pipe, (used a manometer). My thoughts were that an open bottom pipe or plenum would preclude this drop. I sketched out what such a thing could be and I ended up with design that had an open bottom pipe leading to a Christmas Tree looking rocket in the middle of the bin. louvers placed on the outside at 45 degrees provided direct grain exposure to the air from the fan.
The center tree structure provides more air in the middle to accommodate the peak of grain that occurs at the top of the bin, and would result in a more even distribution of airflow.
Because there is no pressure drop across the bottom, there would be an increase in flow and the lower compression would yield a lower temperature with less discrepancy to top/bottom drying. Overall the drying would be more effective and efficient.
The 18 ” and 6″ pipes could also be open bottom to provide more direct exposure. Being open bottom would preclude any fine buildup which plugs holes and also is a wonderful breeding ground for pathogens.
Open bottom pipes, would add more exposure for a total of 35 sq feet. If the material used for the louvers are 10% perforated, this will increase the exposure to 43 sq. feet. Remember that a 10% perforated pipe (18″) provides only 10.3 sq. feet. With this much more exposure, 4 X, it is obvious that this would be a better air delivery system.