Recommendations for the design of perforated floor for grain aeration

Perforated metal floor provides negligible resistance to airflow if the area of openings in the metal is greater than 20% of the total area of metal. For dryeration systems where the airflow is substantially higher than for maintenance aeration (0,5 m³min^-1t^-1) or for in-bin drying systems (1 m³min^-1t^-1) it is convenient that the area of openings should be greater, with a maximum of 40%.
The hole size to be used in the perforated floor depends on the size of the kernel being cooled. For example, flooring for canola (2 mm diameter) must have holes smaller than is necessary for wheat (about 4 mm) or corn (about 8 mm). The smallest holes required to prevent canola from passing through the floor cause higher resistance to the airflow than do the larger holes in the flooring suitable for cereal grains, so flooring with small diameter perforations should not be specified for larger grains.
The plenum of the bin (the empty space below the perforated flooring) should be free of obstructions to allow the air to freely flow to all the directions. This allow for an uniform distribution of the airflow.
The transitions connect the fan with the airflow distribution system. The shape and size of the fan outlet is usually not the same as that of the entrance to the air plenum or duct in a bin. Transition ducts must transfer air from the fan outlet to the bin without leakage and excessive friction losses. Transitions should also reduce turbulence and velocity of the air leaving the fan. The entrance of the transition duct must match the size and shape of the fan outlet, usually circular for axial flow fan and circular or rectangular for centrifugal fans. A sharp expansion or contraction at the fan outlet causes energy losses. The shape of the transition should change gradually from that of the fan outlet to the shape of the air duct in the bin or the opening through the bin wall into the under-floor plenum. The taper of the duct should be less than a 20° angle from the centerline of the transition to reduce turbulence as the cross-section area and shape change. The cross-section area of the transition outlet should give an outlet air velocity equal to or less than 7.5 m/s.

References
Navarro y Noyes. 2002. The mechanics and physics of modern grain aeration Management. CRC Press. ISBN 0-8493-1355-4. Chapter 5: Aeration system design pp 195-247