There is a lot of grain in storage this fall and some of it is probably in places where grain has never ventured in the past. Grain that goes out of condition during storage is lost income. The best strategy for preventing mold growth and insect damage in stored grain is to keep the grain cool. And the most effective way to cool grain is to use aeration fans. The target temperature for winter storage of grain is 20 to 30 degrees. Typically, late November is the time to begin the aeration process if normal weather patterns prevail. What happens if grain isn’t aerated? If grain is not aerated in the fall, air near the bin wall cools and migrates toward the bottom of the bin. This creates a convection current. The air then rises up through the warm grain and picks-up moisture in the form of water vapor. This air continues to move toward cooler grain near the top surface where the moisture condenses and causes spoilage (see figure). The most common location of wet or spoiled grain is at the top-center of the bin or on the cold north wall.
How long will it take? After starting the aeration cycle, run the fan long enough to move a cooling front all the way through the bin. How long this takes depends on the airflow per bushel delivered by the fan. If you are cooling grain in a bin that was designed for drying (full perforated floor and large fan) and provides 1 cfm per bushel (cubic feet of air per minute per bushel of grain), the grain may cool in as little as 15 hours. However, a bin designed primarily for storage equipped with a simple duct and a relatively small fan will take significantly longer. These systems are often designed to provide just 0.1 cfm per bushel. If this is the case, cooling can take up to six days. Cooling times, in hours, are estimated by dividing the number 15 by the airflow in cfm per bushel. Some of the temporary grain storage facilities that are being used in 1999 have aeration systems that deliver less than 0.1 cfm per bushel, which means aeration cycles could take much longer than 150 hours. Many temporary storage facilities have non-uniform airflow, where some parts of the grain pile have a much lower airflow per bushel than other parts. In situations where airflow is not uniform, it is important to check grain temperatures at many different points and to run the fans until all parts of the pile are cooled. Bill Wilcke, University of Minnesota Ag Engineer, suggests that if initial grain temperature is much higher than 50 degrees, it is probably best to cool the grain in 15 to 20 degree stages. For example, if the grain is 65 degrees, wait for 40 to 50 degree weather and run the fans until the grain is cooled to that range. Then, stop the fans until outdoor temperatures drop another 15 to 20 degrees, and resume fan operation until all grain is cooled to 20 to 30 degrees. Positive vs. negative pressure Either positive pressure (air pushed upward) or negative pressure (air drawn downward) can be used to aerate grain. However, positive pressure aeration systems do tend to provide more uniform air distribution in large grain piles that have long aeration ducts. One caution with positive pressure systems is condensation under the roof when you aerate warm grain during cool weather. You can reduce condensation problems by opening doors and hatches to provide plenty of air movement over the top surface of the grain during aeration and by cooling grain in stages so that the difference between outdoor temperature and grain temperature is less than 20 degrees. Checking grain temperature It is extremely important to complete each cooling cycle before shutting the fans off completely. If this is not done, a moisture front will remain within the grain mass. A thermometer can be used to check the grain temperature at the bottom and top of the bin to indicate if a cooling cycle is complete. Once grain is cooled to 20 to 30 degrees, you might not have to aerate it again during the winter. Check the grain every two to four weeks for signs of mold, insects, crusting, or heating. If you find problems, use the aeration fans to cool and dry the problem area. If aeration doesn't rectify the problem, remove the problem grain to prevent additional spoilage.
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