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Drying Wheat During a Cool & Late Harvest
Adding supplemental heat generally is not recommended for most of the state, even with cooler temperatures. Adding heat will primarily change the final moisture content of the grain and only "slightly" increase the drying speed. Also, shutting fans off at night is not recommended.
Air will be warmed by 4 to 5 degrees as it passes through the fan on a bin of wheat operating at a static pressure of 6 to 7 inches of water gage. During an "average" year, this heat added from the fan will likely contribute to over- drying the wheat. Wheat will dry to about 13.3% with air at 69 degrees F and 60% relative humidity, which is the average for August across the state of North Dakota. When this air is heated 4 degrees by the fan, the wheat will dry to about 12.2%. After passing through the fan, the average August air entering the grain will be 73 degrees F and 52% relative humidity. During an average year, it is very important to run the fan during the night when the relative humidity is higher to reduce the amount of over-drying.
Wheat will dry to about 13.0% moisture during average statewide September weather conditions of 58 degrees F and 65% relative humidity. The moisture content of the wheat would be 14.5% without the fan heat. However, the fan warming the air just 4 degrees, from 58 to 62 degrees F, reduces the relative humidity from 65% to 56%. Therefore, even with the cooler and damper air, it is best to run the fan 24-hours per day. Supplemental heat is not generally needed even for the cooler damper conditions.
Running the fan just during the warmer and drier portion of the day will cause the wheat to be over-dried and lengthen the drying time. The estimated drying time is 26 days to dry wheat from 18% to 13% using an airflow rate of 1.0 cfm/bu with September conditions of 58 degrees F and 65% relative humidity. The air is 62 degrees F and 56% relative humidity after being heated 4 degrees by the fan. It will take 47 days to dry the wheat if the fan is operated during the warmer 12 hours each day. In addition, the wheat will dry to about 11.5% moisture. The air will be about 64 degrees F and 55% relative humidity during the warmer 12 hours of the day, and about 68 degrees F and 48% relative humidity after being heated by the fan. If these conditions existed 24 hours per day, the drying time would be reduced to about 23.5 days. However, since the fan is only operated half time, it takes 47 days to complete the drying and the wheat is over-dried.
Even for conditions that may occur in the northern part of the state in late September to early October, the air only needs to be warmed about 70 degrees F to reduce the relative humidity from 70% at 50 degrees F to the desired 57% at 57 degrees F to dry the wheat to 13.5% moisture. Since the air is warmed about 4 degrees by the fan, only an additional 3 degrees needs to be provided by a supplemental heater. A rule-of-thumb on wheat is that 1 Kw of heater per horsepower of fan motor will warm the air about 5 degrees. Therefore, only about a 3 Kw heater is needed for a 5 hp fan to provide the desired amount of heat.
The drying time will be longer at cooler temperatures, because the cooler air cannot hold as much moisture. It will take about 27 days to dry wheat from 17% to 12.2% with an average August air temperature of 69 degrees F and an airflow rate of 0.75 cfm/bu. It will take about 32 days to dry wheat from 17 to 13.0% with an average September temperature of 58 degrees F and the same airflow rate.
The drying rate is directly proportional to the airflow rate. If it takes 21 days to dry 16% moisture wheat using an airflow rate of 1.0 cfm/bu., it will take 28 days with an airflow rate of 0.75 cfm/bu., and 42 days at 0.50 cfm/bu. The airflow rate must be increased to increase the drying speed. Adding heat to a bin will cause the wheat to be dried to a lower moisture content and increase the drying speed only a very little.
Shut off the fans during foggy or rainy weather if it lasts for more than a few hours. Wheat at 15 to 16% moisture can be without airflow for a few days, but wheat at 18% moisture should not be without airflow for more than a day or two due to the potential for heating and spoilage.
The drying time and therefore the drying cost will be almost the same drying 17% moisture wheat and 15% using natural-air drying. This is because drying time decreases only slightly for lower initial moisture contents. The time to dry wheat to 13% moisture using an airflow rate of 0.75 cfm/bu. starting at 17% is 31 days, at 16% is 28 days, and at 15% is 27 days. This occurs because the air going through the wetter wheat removes more moisture than the same air going through drier wheat. Air going through 17% wheat will pick up 4 points of moisture, 17-13, while air going through 15% wheat only picks up 2 points, 15-13. Therefore, there is no advantage in waiting for 17% wheat to dry to 15% moisture in the field.
Kenneth Hellevang, Ph.D., P.E.
Extension Engineer, Professor
Agricultural & Biosystems Engineering Department
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