September 8, 2006

The time required to dry grain in a bin is a function of the initial moisture content of the grain, the desired final moisture content of the grain, the temperature and relative humidity of the air passing through the grain and the rate of airflow through the grain (cubic feet per minute per bushel: cfm/bu).

Table 1 shows airflow resistance for shelled corn. An increase in static pressure is required to push a given rate of airflow (cfm/bu) through grain as the depth of grain increases. Static pressure also must increase to push increasing rates of airflow (cfm/bu) through a given depth of grain.

Since drying time is directly related to the rate of airflow, we want airflow rates to be as high as practical when drying grain. The variable we can manipulate to our advantage when trying to reduce energy cost for fan operation is to reduce grain depth and thereby lower the static pressure the fan must overcome.

If you were building new bins, you could build larger diameter and shorter bins to keep static pressure low while not sacrificing the number of bushels dried per batch. Consider the differences between when a 27-foot diameter bin and a 30-foot diameter bin are used to dry 8,000 bushels of corn at one time. Grain depth in the 27-foot bin would be 17.5 feet while grain depth in the 30-foot bin would be only 14.2 feet.

Using the University of Minnesota FANS computer program to compare these scenarios provides some interesting results. It would take 10.55 horsepower (hp) to push 1.25 cfm/bu through 8,000 bushels of shelled corn in a 27-foot diameter bin. To push the same1.25 cfm/bu through 8,000 bushels of shelled corn in a 30-foot diameter bin would only take 6.32 hp; a savings of 4.23 hp.

Assuming electricity cost is $0.08/kWh, and if one were drying shelled corn using natural air in mid to late October (assuming 20 days drying time), the drying cost in the 30-foot diameter bin could easily be $0.02 per bushel less than in the 27-foot diameter bin.

A management alternative for the larger diameter bin would be to select a fan which requires the same 10.55 hp but delivers more airflow. Once again, the easiest way to analyze this is to use the FANS program because it can calculate the interaction of the system curve and the fan curve. By using the program you can see that it takes the same horsepower to push 1.54 cfm/bu through the 30-foot bin as needed for 1.25 cfm/bushel in the 27-foot diameter bin. For deep-bed in-bin drying, drying time is directly proportional to airflow. The airflow in the 30-foot bin is 23% higher for the same horsepower and therefore drying time would be 23% less in the larger diameter bin.

If you're working with existing grain storage facilities, the same principles can apply. Save time and energy required for fan operation by partially filling a drying bin instead of filling it to the full depth. Since most producers move grain through drying bins into larger storage bins anyway, it only takes a little extra labor to dry grain in smaller batches.

Tom Dorn
Extension Educator, Lancaster County