October 24, 2018
You still might be able to use a natural air system to dry soybeans to about 12% moisture for long-term storage, says Ken Hellevang, North Dakota State University Extension ag engineer. But time is working against you.
In late October and early November, there often aren’t enough warm days left to get soybeans dry just by blowing outside air through them. With an airflow rate of 1 cubic foot per minute per bushel (cfm/bu.), it will take about 60 days to dry 18% moisture beans down to about 12%. With an airflow rate of 1.5 cfm/bu, the drying time is reduced to about 40 days.
If the beans come off the field at a little drier —16% instead of 18% moisture — the drying time with an airflow rate of 1 cfm/bu will be about 50 days, Hellevang says.
The size of the fan required to achieve the higher airflow rates becomes excessive unless the grain depth is very shallow, Hellevang says.
If the depth of soybeans in a bin is 22 feet, each 1,00 bushels of soybeans will require about 1.0 hp of fan.
Achieving an airflow rate of 1.25 cfm/bu requires about 1.6 hp per 1,000 bushels
For an airflow rate of 1.5 cfm/bu, you will a about 2.5 hp per 1,000 bushels.
The fan type greatly affects the airflow provided per horsepower. Use the fan selection software program (https://bbefans.cfans.umn.edu) developed by the University of Minnesota, Hellevang suggests.
Adding supplemental heat to raise the air temperature by 3 to 5 degrees F will get the soybeans to about 11% moisture in 40 to 45 days.
Uneconomical at 35 degrees
However, the colder the air gets, the less moisture it will hold. When the average outside air temperatures gets down to about 35 degrees, natural air drying becomes inefficient and uneconomical. Adding heat at this point causes the bottom of the bin to be dried to a lower moisture content and it increases drying speed only slightly.
If the beans aren’t dry when the average outside temperature drops below 35 degrees, cool the soybeans to 20-30 degrees for overwinter storage. In the spring, when the air temperature rises, you can finish drying them, Hellevang says.
High temperature drying
High temperature dryers can be used to dry soybeans in a wet fall, but be careful, Hellevang says. Limit the dryer temperature to minimize damage to the beans. Refer to the manufacturer’s recommendations for maximum drying temperature.
Typically, the maximum drying temperature for nonfood grade soybeans is about 130 degrees. Even at that temperature, some bean skins and bean themselves will be cracked. One study found that with a dryer temperature of 130 degrees, 50-90% of the skins were cracked and 20-70% of the beans were cracked.
There is a risk of fires when drying soybeans, Hellevang says. Soybean pods and other trash can accumulate in the dryer and become combustible. Monitor the dryer continuously when drying soybeans. Make sure trash doesn’t accumulate and that soybeans continue to flow in all sections of the dryer.
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