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Beware fake news about natural air drying this spring

Ken Hellevang, NDSU Extension ag engineer, addresses the inaccuracies of the night fan theory.

February 7, 2020

4 Min Read
row of steel grain bins
DAY DRYING: Grain in bins await drying in spring. Be leery of advice telling you to only run fans at night to dry your grain this spring. Lon Tonneson

There’s information being circulated that natural air drying of grain occurs at night and not during the day, and therefore fans should be operated at night and not during the day.

“This theory has several inaccuracies,” says Ken Hellevang, North Dakota State University Extension ag engineer.

At airflow rates used for natural-air drying, the grain temperature rapidly changes and is fluctuating during a 24-hour period. The time required to change the grain temperature can be estimated by dividing 15 by the airflow rate. At 1 cubic foot per minute per bushel, changing the temperature of the grain in the bin will take only about 15 hours.

During the daytime, the grain is being warmed. Then as outdoor air temperatures cool during the evening, the grain at the bottom of the bin gradually cools and the air is warmed by the grain. As the air is warmed, the moisture-holding capacity is increased.

During the forenoon, the grain at the bottom of the bin gradually is warmed by the air and the air is cooled. The cooling of the air limits the moisture-holding capacity, but the warming of the grain creates drying potential later.

Moisture is removed by evaporation, which requires that the grain and air going through it be warm enough to evaporate the moisture until the moisture equilibrium is reached, based on the equilibrium moisture content (EMC) of the grain.

If the grain was not warmed during the day, the grain would remain at the night temperature, so running the fan the next night would bring cold, damp air in on cold grain. Little, if any drying would occur.


For example, a producer is attempting to dry 16% moisture soybeans in April when the average temperature is 42 degrees F and 70% relative humidity. The soybeans would be expected to dry to about 13.5% moisture if the fans run 24 hours per day.

Typically, the temperature varies by 20 degrees to 25 degrees during a 24-hour period, and the relative humidity changes as the air temperature changes unless moisture is added or removed from the air. If the fans were operated just at night, the grain would be about 32 degrees, and the air condition would be about 32 degrees and 100% relative humidity.

Based on the soybean EMC, operating the fan just at night would add moisture to the soybeans.

“Unfortunately, there are no new ways to dry grain,” Hellevang says. “The laws of nature continue to apply.”


Hellevang recommends starting to natural-air dry corn and soybeans this spring when the average air temperature is about 40 degrees. This year, the maximum initial corn moisture for natural air drying using an airflow rate of at least 1 cubic feet per minute per bushel is 20% due to the increased potential for deterioration because of damaged or immature kernels. The expected drying time for corn is about 45 to 50 days using an airflow rate of 1 cfm/bu.

Drying time is proportional to the airflow rate, so at an airflow rate of 1.25 cubic feet per minute per bushel, the drying time is about 35 to 40 days. Adding heat will change the final corn moisture but will change the drying speed only slightly.

Ensure that the fan’s airflow rate is adequate by checking fan charts or estimate airflow by using the fan selection program available at the NDSU grain drying and storage website.

Shut off fans

If temperatures cool to an average of about 30 degrees, the fans can be stopped, Hellevang advises. Wait until the temperature again averages at least about 40 degrees before starting the fans. Cool the grain by operating the fans at night or other cool periods before shutting off the fans to extend the storage life of the grain.

Some producers are concerned about shutting fans off because that leaves a drying front in the grain. The drying front is the area in the grain mass where the drying is occurring. The dry air comes in contact with wet grain at the bottom of the drying zone, picks up moisture until it comes into equilibrium with the grain in the drying zone and then carries that moisture through the wet grain above the drying zone and out of the bin.

“There is nothing magical about the drying front or zone,” Hellevang says. “The grain and the drying zone will be in the same condition several days later when the fan is started again.”

Fans should be shut off during rainy days and during fog.

Source: NDSU, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.

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