Engineering Notes Index

Minnesota/Wisconsin Engineering Notes
Fall 1997

Plan to Aerate Grain in Late November

Bill Wilcke, Minnesota Extension Engineer

The best strategy for preventing mold growth in stored grain is to keep the grain cool. And the best way to cool grain in the upper midwest, is to use aeration fans to blow outdoor air through it. The target temperature for winter storage of grain in the upper midwest is 20° to 30°F and long term weather records indicate that average outdoor temperatures will fall to this range in late November. So mark your calendars now to remind yourself to start aeration fans about that time of the year, but also watch the thermometer and weather forecasts to make sure that air temperatures really are in the desired 20° to 30°F range.

Once you start an aeration cycle, it is important to 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/bu (cubic feet of air per minute per bushel of grain), you might be able to cool the grain in as little as 15 hours. But if your bin was designed purely as a storage bin and is equipped with a duct or pad system and a relatively small fan, you might be surprised how long the fan needs to run to complete cooling. Many grain storage systems are designed to provide just 0.1 cfm/bu and in these bins, cooling can take 150 hours or more (six days!). Cooling times, in hours, are estimated by dividing the number 15 by the airflow in cfm/bu; this is just an estimate, however, and you actually need to measure grain temperatures to verify that cooling is complete.

For high airflow aeration systems that have short cooling times, you can watch short-term weather forecasts for the desired weather and then run the fan continuously until the grain is cooled. For low airflow systems, however, the weather is likely to fluctuate quite a bit during the week-long cooling cycle, and you need to make some decisions about how to control the fan. Here are some options to consider:

• The simplest option, and one that actually works fairly well, is to watch the long-term weather forecast for the desired weather and then just let the fan run continuously until the grain is adequately cooled. The cooling process might be slowed by if you run into a day or so of warmer weather during the cooling cycle, but just continue to run the fan until the re-warmed grain is re-cooled. For the most part, you don't need to worry about humidity during cooling cycles because binned grain rewets pretty slowly&endash;especially at low airflow.
• With a little more active management, you can watch the daily weather forecasts, and manually turn the fan off during warm or humid weather. In most cases, this kind of intermittent fan operation shouldn't hurt the grain. But remember, for every hour that you turn the fan off during the middle of an aeration cycle, you'll have to run another hour at the end of the cycle to complete cooling.
• You could also install some kind of control system that automatically switches the fan on and off as the weather changes. The simplest control is a thermostat that stops the fan when the outdoor temperature gets too high. Next, you could add another thermostat that stops the fan at low temperatures. Then, you could add a humidistat that stops the fan at high humidities. Or you could buy a sophisticated programmable controller that monitors air temperature, grain temperature, and relative humidity, and only runs the fan when air conditions would keep the grain in a specified moisture range. With any type of automatic control, it is important to make sure the fan runs long enough to complete cooling. Also, no control system is completely fail-safe, so you need to understand how the system is supposed to work and then keep checking to make sure that it is working properly.

After grain is cooled to 20° to 30°F in late fall, it might not be necessary to aerate again before it is fed or sold. But keep monitoring grain temperatures during winter and if you start to see differences of more than about twenty degrees from one part of the bin to another, it would be good to run another aeration cycle to re-cool grain to the desired storage temperature of 20° to 30°F.

If you have questions about grain storage, feel free to contact Bill Wilcke at wwilcke@mes.umn.edu or (612) 625-8205.

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