Many farmers are building larger grain bins than were common 30 years ago. A typical on-farm grain bin traditionally was 27 to 36 feet in diameter and would store grain to a depth of 18 to 22 feet. Many new grain bins are 42 to 48 feet in diameter and store grain to a depth of 28 to 32 feet — almost three times the volume of the earlier bins.
These large bins work well for storing dry grain when equipped with aeration fans capable of pushing 0.3 cubic feet of air per minute per bushel (0.3 cfm/bu) through the bin, says Tom Dorn, Nebraska Extension educator in Lancaster County.
Many fan manufacturers produce a 10-hp axial-flow fan capable of producing 0.3 cfm/bu. in a 48-foot diameter bin with corn 30 feet deep. In cases where there is low static pressure — the pressure required to push air through the grain bin — axial flow fans will produce more airflow than a centrifugal fan with the same horsepower.
Using a 48-foot diameter bin for drying corn is a much different situation. The minimum airflow recommended for drying corn in Nebraska is 1 cfm/bu. A 48-foot diameter bin with grain 30 feet deep would require three 40-hp centrifugal fans on separate transition ducts to produce 1 cfm/bu. airflow.
Static pressure is affected by two parameters: grain depth and airflow, according to Dorn. At a given grain depth and airflow, the diameter of the bin does not affect the static pressure.
There are several management changes you could make to reduce the initial cost of the grain bin and associated equipment. These changes will also reduce operating costs for years to come. Reducing grain depth can reduce horsepower requirements significantly. In a 48-foot diameter bin
if grain were loaded into the bin to a depth of 30 feet, three 40-hp centrifugal fans could produce the needed 1.0 cfm/bu.,
if grain were loaded into the bin to a depth of 25 feet, two 40-hp centrifugal fans could produce 1.0 cfm/bu., or
if grain were loaded to a depth of 18 feet, one 40-hp centrifugal fan could produce the required 1.0 cfm/bu.
Since the airflow remains the same in all three scenarios, the time required to dry the grain would be the same.
Two fans can dry 83% as much grain per batch as three fans. One fan can dry 60% as much grain per batch as three fans.
When buying a new bin, if you decide that you're willing to dry smaller batches of grain at a time, one-third or even or two-thirds of the expense for fans, transition ducts, control boxes, and wiring can be saved. Also, for each fan that's eliminated, the electrical use for fan operation drops 33%. Reducing grain depth from 30 feet to 25 feet eliminates one fan and associated equipment and reduces electricity use by 33%. Likewise, reducing grain depth from 30 feet to 18 feet reduces electricity use by 67%.
For more information, see EC710, Management of In-bin Natural Air Grain Drying Systems to Minimize Energy Cost. Also see the Lancaster County Extension web page Grain Storage Management.