When to run aeration fans: will they dry your grain or wet it?

It is the moisture grain ends up at if it sits in the same air long enough. Air at 50 F and 60% humidity brings corn to about 13.3 to 13.9%, by the two published corn equations this page uses. If that is under the moisture you want, running fans on that air dries the grain. If it is over, the fans put water back in.

It depends on the job. For natural-air drying of wet grain, NDSU Extension says to run the fans around the clock until the drying front comes out the top of the bin, and to shut them off only for steady rain, fog or snow; that advice was written for wheat. For cooling, run them when outside air averages 10 to 15 F under the grain temperature, which is also NDSU's rule. For dry grain in storage, run them on hours when the air's EMC is not over the moisture the grain is stored at. For winter, NDSU says to cool grain to 20 to 30 F; Minnesota says 20 to 30 F in the north of the state and 25 to 35 F in the south; Iowa State says 30 to 40 F and warns that going much under 30 F adds little and can form ice in the grain.

It depends on the temperature and on which published corn equation you trust. Corn held at 15.5% stays there in air of about 65 to 71% humidity at 40 F, 69 to 74% at 50 F and 72 to 77% at 60 F; wetter air than that pushes it over 15.5%. To dry corn toward a 15.5% target with a safety margin, keep the outside humidity under about 64% at 40 F, 68% at 50 F and 71% at 60 F.

Not for natural-air drying. NDSU Extension says to keep the fans running, and that running them only in the warmest, driest hours overdries the bottom of the bin and stretches the drying time. The exceptions it gives are rain that lasts more than a few hours, fog, and long snow. When average humidity runs over 70% for days, NDSU (writing about wheat) says to add a little heat rather than shut off.

Some. Fans warm the air typically 2 to 5 F, by Cornell Cooperative Extension's figure, and NDSU puts it at 4 to 5 F at about 6 inches of static pressure. Warmer air with the same water in it has a lower relative humidity: NDSU says 5 F of warming lowers humidity about 10 points. The math is RH at the fan equals RH outside times the saturation vapor pressure at the outside temperature, divided by the saturation vapor pressure at the fan temperature.

Aeration fans at 0.1 to 0.2 cfm per bushel cool grain but do not dry it safely; the top can spoil before a drying front gets there. Drying corn with natural air takes about 1 cfm per bushel or more. For cooling, NDSU, Iowa State and Nebraska Extension give about 15 divided by the cfm per bushel as the fan hours for one cooling front: at 0.1 cfm per bushel that is about 150 hours, within the 100 to 200 hours the guides give.

Published tables for the same air do not agree, so the page shows corn as a range from two published fits. For corn at 60 F and 70% humidity the page gives 14.2 to 15.1%, Minnesota Extension's table says 14.5% and Arkansas Extension's says 15.7%. An hour counts as drying to target only when the lower fit is a full point under the target and the higher fit is under it too, and any wetting call uses the higher fit.

I could not read the soybean constants in the ASABE standard at a source I trust, and corn numbers do not fit beans. Published soybean tables also disagree: Arkansas lists 13.7% at 60 F and 70% humidity, where NDSU lists 13.0%. Soybeans go on when I can check their equation.

The constants are the ASAE wheat set, and they match the University of Minnesota Extension wheat table to within a tenth of a point. Soft red winter wheat runs drier: Arkansas Extension lists 13.9% at 60 F and 70% humidity, where this equation says 15.2%. If you grow soft red, the page will call some hours wetting that would actually dry your wheat a little.