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August 26, 2004 Click here to download an Audio clip to accompany this story. (318KB wav file) Click here to download an Audio clip to accompany this story. (501KB wav file) Click here to download an Audio clip to accompany this story. (392KB wav file)
Drying Wheat During a Cool and Late Harvest Adding supplemental heat when drying wheat is generally not recommended for most of the state, even with cooler temperatures, according to a North Dakota State University Extension Service engineer and professor. “Adding heat will primarily change the final moisture content, but only slightly increase the drying speed,” says Kenneth Hellevang, NDSU Agriculture and Biosystems Engineering Department. “Also, shutting fans off at night is not recommended.” Air will be warmed by 4 to 5 degrees as it passes through the fan in a bin of wheat operating at a static pressure of 6 to 7 inches of water gauge. During an average year, the heat added from the fan will likely contribute to overdrying. Wheat will dry to about 13.3 percent with an air temperature of 69 degrees and 60 percent relative humidity, which is the average for August across the state. When the air is heated 4 degrees by the fan, the wheat will dry to about 12.2 percent. After passing through the fan, the average August air entering the grain will be 73 degrees with 52 percent relative humidity. During an average year, it is important to run the fan during the night, when the relative humidity is higher, to reduce the amount of overdrying. Wheat will dry to about 13 percent moisture during average statewide September weather conditions of 58 degrees and 65 percent relative humidity. The moisture content of the wheat would be 14.5 percent without fan heat. However, the fan warming the air just 4 degrees, from 58 to 62 degrees, reduces the relative humidity from 65 percent to 56 percent. Therefore, even with the cooler and damper air, it is best to run the fan 24 hours a day. Supplemental heat is not generally needed even for the cooler, damper conditions. Only running the fan during the warmer and drier portion of the day will cause the wheat to be overdried and lengthen the drying time. The estimated drying time is 26 days to dry wheat from 18 percent to 13 percent using an airflow rate of 1 cfm per bushel with September conditions of 58 degrees and 65 percent relative humidity. The air temperature is at 62 degrees and 56 percent relative humidity after being heated 4 degrees by the fan. It will take 47 days to dry the wheat if the fan is operated just during the warmer 12 hour period of the day and the wheat will be dried to about 11.5 percent moisture. The air temperature will be about 64 degrees with 55 percent relative humidity during the warmer 12 hours of the day, and about 68 degrees and 48 percent 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 for 12 hours, it takes 47 days to complete the drying and the wheat is overdried. “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 7 degrees to reduce the relative humidity from 70 percent at 50 degrees to the desired 57 percent at 57 degrees to dry the wheat to 13.5 percent moisture,” Hellevang says. “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 heat per horsepower of fan motor will warm the air about 5 degrees. Therefore, only about a 3 kw heater is needed for a 5 horsepower 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 percent to 12.2 percent with an average August air temperature of 69 degrees and an airflow rate of 0.75 cfm per bushel. It will take about 32 days to dry wheat from 17 percent to 13 percent with an average September temperature of 58 degrees and the same airflow rate. “The drying rate is directly proportional to the airflow rate,” Hellevang says. “If it takes 21 days to dry to 16 percent moisture using an airflow rate of 1 cfm per bushel, it will take 28 days with an airflow rate of 0.75 cfm per bushel and 42 days at 0.50 cfm per bushel. 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, but increase the drying time very little.” Shut off the fans during foggy or rainy weather if it lasts for more than a few hours. Wheat at 15 percent to 16 percent moisture can be without airflow for a few days, but wheat at 18 percent 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 the drying cost will be almost the same when drying 17 percent moisture wheat and 15 percent using natural-air drying because drying time decreases only slightly for a lower initial moisture content. The time to dry wheat to 13 percent moisture using an airflow rate of 0.75 cfm per bushel, starting at 17 percent, is 31 days. The drying time is 28 days at 16 percent moisture and 27 days at 15 percent. “This occurs because the air going through the wetter wheat removes more moisture than the same air going through drier wheat,” Hellevang says. “Air going through 17 percent moisture wheat will pick up 4 points of moisture, 17-13, while air going through 15 percent moisture wheat only picks up 2 points, 15-13. Therefore, there is no advantage in waiting for 17 percent wheat to dry to 15 percent in the field.” ### Source:
Kenneth Hellevang, (701) 231-7243, kjh-eng@ndsuext.nodak.edu |
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