Adding supplemental heat when drying wheat generally is not recommended for most of North Dakota, even when cooler temperatures are associated with a late harvest, according to Ken Hellevang, NDSU Extension agricultural engineer and grain-drying expert.
"Adding heat primarily will change the final moisture content but only slightly increase the drying speed," he says. "Adding heat frequently causes wheat in the bottom of the bin to dry to a moisture content lower than desired. In addition, 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 when the fan is operating at a static pressure of 6 to 7 inches. Wheat will dry to about 13% moisture during average statewide September weather conditions of 58 degrees and 65% relative humidity due to the heat added by the fan. The moisture content of the wheat would be 14.5% without fan heat.
The fan warming the air just 4 degrees, from 58 to 62 degrees, reduces the relative humidity from 65% to 56%. Therefore, even with the cooler and damper air, running the fan 24 hours a day is best. Supplemental heat generally is not needed even for the cooler, damper conditions.
Only running the fan during the warmer and drier portion of the day will overdry the wheat and lengthen the drying time, Hellevang says. The estimated drying time is 35 days to dry wheat from 18% to 13% using an airflow rate of 0.75 cubic feet per minute (cfm) per bushel with the September air temperature at 62 degrees and relative humidity at 56% after being heated 4 degrees by the fan.
Drying the wheat will take 62 days 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% moisture. The air temperature will be about 64 degrees with 55% relative humidity during the warmer 12 hours of the day, and about 68 degrees 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 31 days.
Even for conditions that may occur in the northern part of the state in late September to early October, the air needs to be warmed only about 7 degrees to reduce the relative humidity from 70% to the desired 57% 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 kilowatt (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, Hellevang says. Drying wheat from 17% to 12.2% moisture will take about 27 days with an average August air temperature of 69 degrees and an airflow rate of 0.75 cfm per bushel. Drying wheat from 17% to 13% will take about 32 days with an average September temperature of 58 degrees and the same airflow rate.
The drying rate is directly proportional to the airflow rate. If drying 16% 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. Adding heat to a bin will cause the wheat to be dried to a lower moisture content, but it will decrease the drying time very little.
Hellevang recommends shutting off the fans during foggy or rainy weather if it lasts for more than a few hours. Wheat up 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 the drying cost will be almost the same when drying 17% moisture wheat or 15% moisture wheat using natural-air drying because drying time decreases only slightly for a lower initial moisture content. The time to dry wheat from 17% to 13% moisture using an airflow rate of 0.75 cfm per bushel is 31 days. The drying time is 28 days for wheat starting at 16% moisture and 27 days when starting at 15%.
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% moisture wheat will pick up 4 points of moisture, (from 17% to 13%), while air going through 15% moisture wheat only picks up 2 points, (from 15% to 13%). Therefore, waiting for 17% wheat to dry to 15% in the field produces no advantage.
For flour milling, avoiding temperatures above about 150 F is important because of the effect of high temperatures on the chemical structure of the grain. A common practice for 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.
Dryer temperature will vary with dryer type, but a general recommended maximum drying air temperature for drying milling wheat in a continuous-flow dryer is 150 F for 16% moisture content and 130 F for 20% moisture content wheat.
For more information, contact Hellevang at (701) 231-7243 or firstname.lastname@example.org or check out the NDSU Extension Service's publications on grain drying and storage at www.ag.ndsu.edu/pubs/grainsto.html.
Source: NDSU Agriculture Communication