By Bruce MacKellar, Michigan State University Extension
Using bin aeration to dry corn and soybeans with natural air is a common practice in many areas of the country. However, the process does require more management than using fans to adjust grain temperature. The Purdue Extension publication AED-20, “Managing Dry Grain in Storage,” reports that drying front movement is around 50 times slower than the temperature front movement in a normal grain mass. To successfully use natural air to dry grain in storage bins, it takes significantly more cubic feet per minute (cfm) in fan capacity compared to what it takes to manage temperature. This is important because we often do not have long periods of time with low relative humidity natural air with which we can dry the crop in early spring in Michigan.
The relative humidity of the ambient air is important. If the humidity is too high, moisture can be added to the grain in the bin. To avoid re-wetting, we can refer to the grain equilibrium moisture content at various temperatures and relative humidity levels of the natural air.
Soybean equilibrium moisture content. The combination of temperature and relative humidity levels in the yellow cells with an asterisk (*) are good for drying beans. It is generally hard to find many days that have low humidity levels at temperatures below 40 F during Michigan spring weather to dry soybeans.
Source: Kenneth Hellevang, NDSU Extension Service, from ASAE D245.4, Modified Henderson Equation.
In order to dry soybeans in a grain bin with natural air early in the season, you must be able to find sufficient periods of time in the forecast with advantageous temperature and relative humidity values to be able to move the drying front through the grain mass. We generally want to avoid temperatures in the 30s as the relative humidity levels are usually too high for effective drying to occur. As the temperatures warm into the 40s and 50s, average relative humidity levels begin to drop, allowing for more effective drying opportunities.Source : msu.edu
How long it will take grain to dry depends on the cubic feet per minute of air flow you can get from of your bin aeration system. Usually, 1.0 to 1.5 cfm per bushel is the minimal airflow needed for drying grain. More airflow will dry grain faster. Grain drying will begin at the bottom of the bin and move up towards the top. The mechanism of moisture removal is evaporation. If fans cannot be run long enough for the drying front to reach the top of the bin and dissipate, a higher moisture zone may appear above the drying front. Short pauses for overnight increases in relative humidity may or may not be beneficial, but pauses for cold snaps, periods of fog or during heavy rainfall can reduce rewetting. Movement of the drying front to the top of the grain mass and out of the bin is important for longer term grain storage. Incomplete removal of moisture can result in crusting of the beans or layers of spoilage at the drying front.
It is important to understand that the air-drying process will bring grain temperature up to the ambient air temperature. Because it is easier to cool grain more quickly than move the drying front, temperatures can be managed with night aeration, especially on cool, low humidity nights, without adding too much moisture to the lower levels of the grain. However, the ability to do this depends on the temperatures this spring. Remember, when grain is placed in the bin at harvest at normal moisture levels for longer term storage, it is a common practice for us to “refrigerate” the grain and seal the fans off. Bin drying with aeration this spring will leave grain at the air temperatures that occur at the time of aeration for drying. Warmer grain, particularly with elevated moisture levels, should be monitored closely for signs of spoilage or increased insect activity.
You may want to consider upgrading fan capacity on your grain bin if you need to air dry wet soybeans this spring. Be sure to also evaluate the venting capacity of your bin. Adding larger fans without adequate exhaust capacity may reduce the efficiency of your new fan substantially.
Kenneth Hellevang, North Dakota State University stored grain specialist, has an excellent set of resources addressing requirements for air drying grains, including a page established at the University of Minnesota that helps you to determine fan size for bins of various depths and diameters to achieve your desired cfm/bushel.