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Adding supplemental heat when natural-air drying wheat generally is not needed for most of North Dakota, but may be needed this year if wet conditions continue, according to Ken Hellevang, North Dakota State University Extension Service agricultural engineer and grain-drying expert.
“Adding heat reduces the air relative humidity and the final grain moisture content, so it should be used when outdoor air relative humidity is too wet to dry wheat to the desired moisture content,” he says. “Adding too much heat frequently causes wheat in the bottom of the bin to dry to a lower than desired moisture content.”
Air will be warmed 4 to 5 degrees as it passes through the fan on a bin of wheat when the fan is operating at a static pressure of 6 to 7 inches. Warming air by 5 degrees reduces the relative humidity about 10 percentage points. Warming air that is at 60 degrees with a 70 percent relative humidity by 5 degrees reduces the relative humidity to about 60 percent. This air will dry wheat to about 13.5 percent moisture content with just fan heat. A supplemental heater is not needed if the average relative humidity is less than 70 percent.
If the average relative humidity exceeds 70 percent, then a little supplemental heat is needed. Even if the average relative humidity is 75 percent, the air only will need to the heated 2 or 3 degrees.
For example, if 60 degree air has a relative humidity of 75 percent, warming the air 3 degrees in addition to the 4 degrees from the fan reduces the relative humidity to 59 percent and permits drying wheat to about 13.5 percent moisture.
“Only running the fan during the warmer and drier portion of the day lengthens the drying time,” Hellevang says. “The estimated drying time during September is 35 days using an airflow rate of 0.75 cubic feet per minute (cfm) per bushel with the fan operating 24-hours per day and 62 days when the fan is operated just during the warmer 12-hour portion of the day. Running the fan 24 hours a day, and adding supplemental heat if necessary, permits drying to the desired moisture content faster than only operating the fan 12 hours per day.
Turn off the fans if it is foggy or raining. Wheat up 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 rate is directly proportional to the airflow rate. If drying 16 percent moisture content wheat using an airflow rate of 1 cfm per bushel takes 21 days, 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.
The maximum recommended moisture content for natural-air drying wheat is 18 percent with an airflow rate of 1 cubic feet per minute per bushel (cfm/bu), 17 percent for 0.75 cfm/bu, and 16 percent for 0.5 cfm/bu to complete drying before significant deterioration occurs. Generally an airflow rate of 0.75 cfm/bu and limiting the initial moisture content to 17 percent is recommended. The maximum recommended wheat depth for drying is 18 to 20 feet.
A fan selection program is linked from the NDSU Grain Drying and Storage website (http://www.ag.ndsu.edu/graindrying). With the program you can determine the fan size needed to obtain the desired airflow or the airflow provided by an existing fan.
High temperature drying is recommended if the wheat moisture content exceeds 17 percent. However, caution is required because high temperatures affect the chemical structure and milling quality of the grain. A common practice of some millers is to test a sample of the grain for milling properties before purchasing. High temperatures can damage baking quality severely even though the grain kernels appear undamaged.
Allowable dryer temperature will vary with dryer type and design, but a general recommended maximum drying air temperature for milling wheat in a cross-flow dryer where some of the wheat approaches the drying air temperature is 150 degrees for 16 percent moisture content and 130 degrees for 20 percent moisture content wheat. Frequently a plenum air temperature about 30 degrees warmer is used in dryers where the kernel temperature remains below plenum temperature and the wheat kernel is not damaged. Drying wheat will be slower than corn drying due to the reduced air temperature and airflow rate.
Source: North Dakota State University
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