In the United States, natural air is commonly used to dry corn in a bin. The way that corn dries in that bin is dependent upon various factors, including harvest moisture content, grain depth, fan cfm, weather conditions and geography.

Typically, the harvest moisture content of corn being loaded into the bin ranges from 18% to 22%, and bins are generally equipped with design airflow of at least 1 cfm/bu. The typical grain depths (mostly up to eave height) used for natural air drying of corn range between 18' and 25'. Above this grain depth (25'), corn may still successfully dry some years, depending on the weather. However, during wet years, a bin of corn over 25' in depth may find itself in trouble. This also depends on the location of the bin. A certain grain depth and airflow may work just fine for one area, but may not yield the same positive outcomes in another area because of the difference in weather conditions specific to that area.

In-bin natural air drying is a deep-bed drying process that forces air through the grain mass, most commonly from bottom to top. The image below illustrates the progression of drying air through the grain mass, which causes the drying zone to move from bottom to top.

An air delivery system such as full-floor plenum or duct can be used - the full-floor system is most widely used. The pressure build-up at the plenum creates frictional heat due to the compression of air at constant volume. This frictional heat raises the temperature of air in the plenum. As a rule of thumb, 1" of static pressure creates 1°F temperature rise at the plenum.

To heat or not to heat? That is the question:

In order to speed up the natural air drying process, a heater needs to be attached to the fan. The heater can be fueled by either propane or natural gas (electric heaters can also be used, but are not common). The heat is not used to heat the grain mass, but rather reduce the relative humidity of the air and allow more productive air to be pushed the grain. Just a 10-15°F temperature rise due to added heat will reduce the relative humidity by 20-30%, based on ambient temperature. During periods of high humidity (mostly at night) a heater can be kicked on until an optimum drying relative humidity is reached. If the static pressure developed at the plenum is more than 7", then a heater is not required or necessary. In order to get 7" of static pressure, greater grain depth or greater airflow is needed. And that means more energy is required. Higher energy leads to higher electricity cost, which should be factored into the equation.

A disadvantage of creating higher static pressure at the plenum is it will over-dry the bottom layer of grain if drying weather prevails. And as a result, you could face financial loss due to shrinkage (moisture loss) of the grain as well as higher energy costs.

There are a lot of factors to account for in the use of heat during natural air drying. It will depend on drying time goals and deadlines, energy costs and the ability to manage and mitigate loss due to over-dried grain. Case studies below illustrate the differences in timing and costs when heat was applied to the natural air drying process.

Case Study 1: Natural Air Drying of corn at Waterloo, IA with 48' diameter and 25' grain depth. There is 1 cfm/bu of airflow using two 30hp low-speed CF fans. The corn was harvested at 20% MC on October 1 and target moisture content was 15%. The drying cost, drying end dates and final average moisture content (MC) with and without heater are given in the table below.

You can see in the top scenario when the heater was used, drying was done in 40-45 days. This can be a huge relief if you are facing deadlines. However, in using heat, a 1% average moisture content loss occurred. In the lower scenario when the heat was not used, fan run time increased and the end dates in 2010 and 2014 extended into late winter/early spring, which may cause some to worry about mold. However, the drying cost per ton was less than with heat.

Case Study 2: A similar natural air drying study was conducted at Greenville, SC, with the same bin dimensions, grain depth and cfm. The drying cost, drying end dates and achieved final average moisture content (MC) with and without heater are given in the table below.

Because Greenville, SC has an adequate weather pattern for drying, the differences between the heat and no heat scenarios were not as drastic. If a heater was used, it took 33-45 days to dry to goal. During drying with heat, there was 1% moisture content loss. If heater was not used for drying, it took 45-55 days. This is just 10-20 days longer than if heat was used. This slight increase in drying days will not put the grain under storage risk for Greenville, SC, in the same way that it would in the Waterloo, IA example.

Conclusions
Natural air drying of corn will always be affected by geographic location of the bin. Natural air drying with heater works well for Waterloo, IA; but may not needed for the typically warmer climate of Greenville, SC (if bin size and fan capacity are constant). Natural air drying with heater does carry the risk of over-drying the bottom layer of the grain - in both case studies, a 1% moisture content loss was seen in the scenarios with applied heat.

Want to see if your bin could use a heater to help you reach your natural air drying goals? Contact an AGI SureTrack Grain Specialist to discuss this and the BinManager technology that can successfully manage and condition your grain to targets.

BinManager, the industry-leading grain storage management solution, precisely measures exact temperature and moisture of your stored grain, helping you to monitor your grain and ensure it is being stored safely. BinManager also uses real-time data from the grain and outside air to automate when the fans and heaters run, which in turn can dry and cool the grain to safe storage conditions for long-term storage (depending on your storage goals).