Managing Large Grain Bins for Potential Mycotoxin Contamination

Managing Large Grain Bins for Potential Mycotoxin Contamination

If mycotoxins are suspected in corn, grain depths in large bins may need to be adjusted to facilitate quick drying.

September 14, 2012

Table 1. With one 40 hp centrifugal fan on a 48-foot diameter bin, grain depth may need to be limited to 8 feet or less to achieve an airflow over the recommended minimum of 1.25 cfm/bu (see data in red).

Grain depth, ft Airflow cfm/bu
30 0.33
25 0.40
20 0.51
15 0.68
10 1.02
8 1.27
7 1.46
6 1.70
 
Table 2. With two 40 hp centrifugal fan on a 48-foot diameter bin grain depth may need to be limited to 15 feet or less to achieve an airflow over the recommended minimum of 1.25 cfm/bu (see data in red).
Grain depth, ft Airflow cfm/bu
30 0.63
25 0.77
20 0.97
15 1.32
10 2.01

Many farmers are building much larger grain bins than was commonly done 30 years ago. A typical on-farm grain bin traditionally was 27 to 36 feet in diameter and would hold 18 to 22 feet of grain depth. Now it is common on many farms to build 42- to 48-foot diameter bins that can hold 28 to 32 feet of grain depth.

These large bins work well for storing dry grain and regulating grain temperature. The low airflow rates that are good for controlling grain temperature require relatively low static pressure to move the air through the grain mass. Generally, fans intended for temperature control are sized to produce 0.2 to 0.3 cubic feet of air per minute per bushel (cfm/bu) through the grain. Many fan manufacturers produce a 10 horsepower axial-flow fan capable of producing 0.3 cfm/bu in a 48-foot diameter bin with 30 feet of corn depth.

Using a 48-foot diameter bin for drying corn is a much different situation. The minimum airflow recommended for drying corn in Nebraska is 1.0 cfm/bushel. An even higher airflow is required for grain that has, or even might have storage molds that could continue to grow and produce mycotoxins in storage. The higher airflow will dry the grain more quickly. Drying time is proportional to the airflow, cfm/bu, and the airflow a fan can produce is affected by the grain depth. Therefore, I recommend reducing grain depth to provide higher airflow (1.25 cfm/bu or higher) and shorter drying times. 

Remember, while a minimum of 1.25 cfm/bu is recommended, providing more airflow is better. Drying time is inversely proportional to the airflow (cfm/bu). An airflow of 2 cfm/bu will dry corn in half the time as an airflow of 1 cfm/bu.

See Tables 1 and 2 for examples of how airflow at various grain depths changes depending on the available fan power.  To achieve the recommended 1.25 cfm/bu in a 48-foot diameter bin using only one 40 hph fan, grain depth would need to be limited to 8 feet or less.  If two 40 hp fans are used, grain depth needs to be 15 feet or less to achieve the recommended airflow.

Resources

For more information on grain drying , see

Tom Dorn
Extension Educator, Lancaster County