October 27, 2008

Tom Dorn, UNL Extension Educator

With energy prices increasing dramatically in recent years, grain producers are asking how to reduce the cost of drying grain on the farm. This paper discusses management strategies to minimize related energy costs.

Field Dry

It goes without saying, the least costly method of drying corn is to let it dry naturally in the field for as long as possible. Given good drying conditions (low humidity, light winds, and warm temperatures), corn can lose one-third to one-half point of moisture per day. At this drying rate, the corn would dry naturally in the field in about the same amount of time as if the corn were harvested and dried in a bin using natural (unheated) air drying, moving about 1 cubic foot of air per minute per bushel (cfm/bu) through the grain.

Grain Drying 101

All mechanical grain drying systems use a fan to push or pull air through the grain mass. The time required to dry grain is a function of the initial and final moisture content of the grain, the rate of airflow through the grain (cubic feet per minute per bushel, cfm/bu) and the air properties, temperature and initial humidity level.

Management Tips for In-bin Grain Drying

In deep-bed drying systems (in-bin drying), air is normally pushed through the grain from the bottom of the bin and it is exhausted out the top of the bin. As the air moves through the grain, moisture evaporates from the grain into the passing air. Eventually, the moisture content of the grain nearest the fan comes into equilibrium with the incoming air and no further drying takes place in that zone. The area within the grain mass where moisture is being removed from the grain is known as the drying zone. The top of the drying zone is the point at which the relative humidity of the air is in equilibrium with the moisture content of the grain and no more moisture from the grain can evaporate into the air. The drying zone moves through the grain in the direction of airflow.

Natural Air Drying

Natural air drying uses unheated air to dry grain. It can take several days to several weeks to dry a bin of corn using natural air. Never the less, under favorable drying conditions, natural air drying can be the least expensive drying method and usually results in the highest grain quality of any mechanical drying method. The minimum recommended airflow rate in Nebraska for in-bin natural air drying is 1.0 cfm/bu up to 18% moisture corn, 1.25 cfm/bu up to 20% moisture, and 1.5 cfm/bu up to 22% moisture. If the airflow rate of the fan is too low to meet the recommendation above, the bin could be partially filled.

A shallower grain depth results in less static pressure for the fan to overcome, which results in more airflow output (cfm) from the fan. Since partially filling the bin results in fewer bushels in the bin, more air is pushed through fewer bushels, significantly increasing cfm/bu. Many times reducing grain depth a foot or two will result in airflows which meet the minimums stated above. For information on reducing grain depth to speed drying, see the September 8, 2006 CropWatch article, Reduce Grain Depth To Save Time/Energy When Drying Grain.

Intermittent Fan Operation

Natural air drying can take a long time, especially later in the season when the air temperature is cooler. Continuous aeration is recommended, rain or shine, when the moisture content of the corn is above 18% and grain temperature is above 50°F. Once the moisture content of the corn has dropped below 18% and the temperature of the grain is 40°F or less, a producer may consider intermittent fan operation. When the air temperature and relative humidity is not conducive to drying corn, fan operation can be discontinued and run only when conditions are favorable to dry.

This recommendation is contingent on the producer's ability and willingness to closely monitor the corn to detect the first sign of heating in the bin. After November 15, the fan can be turned off when corn moisture at the top of the bin is less than 17% and corn temperature is less than 35°F. If the weather forecast calls for a period of warm weather, resume aerating when the equilibrium moisture content in Table 1 indicates that some drying can be accomplished. For example, if the air temperature is above 40°F and the relative humidity is below 60%, the fans could be restarted on corn above 14.5% moisture. Once started, continue fan operation until average temperatures drop below 35°F again. The eventual goal is to get corn to 15.5% moisture and 30°F if sold by December and 15% moisture and 30°F if held into the new year. (The fans will have to be run again in the spring for temperature management.)

Stirring System Management When Drying With Natural Air

Research has found stirring grain being dried with natural air actually increases the time required to dry the grain because it disrupts the drying zone, resulting in exhaust air leaving the grain mass less saturated. Considering the long drying times associated with natural air drying, continuous stirring can cause significant grain damage and results in costly wear to the stirring device.

If a stirring device is installed in a bin being dried by natural (unheated) air, the stirring device should be run during the filling period to reduce the pack factor of the grain from the filling operation, to redistribute fines and to level the grain. Stirring should then be discontinued to allow a drying zone to develop and move through the grain. Since the bottom of the bin will be somewhat over dried by the time the drying zone approaches the top of the bin, a final stirring just before the drying zone is pushed completely through the bin will help to equalize the moisture content of the grain in the bin.

If the corn has not completely dried by December 1 and the producer decides to hold the corn until better drying weather returns, the stirring device should be run a couple of rounds before discontinuing continuous fan operation to equalize the moisture content of the grain mass.

Heated Air In-Bin Drying

Heating the air increases its ability to carry away more water vapor as heating lowers the relative humidity of the air. When adding supplemental heat, the relationship between temperature rise and relative humidity is not linear. Table 2 presents the effect on the relative humidity of air when adding supplemental heat.

A rough rule of thumb is the relative humidity drops by one-half for each 20°F rise in temperature. Natural air at 60°F and 50% relative humidity will have a relative humidity of 25% if heated to 80°F. Adding another 20°F to raise the temperature from 80°F to 100°F cuts the relative humidity by about half again and results in a drop to 13.5%. The third 20°F rise to 120°F lowers the relative humidity by about half again to 7.6%. The notable point is the second 20°F increment of added heat results in half as much reduction in relative humidity (half of half) and the third increment results in only one-eighth as much reduction (half of half of half). To minimize energy cost for drying grain, keep the temperature rise to a moderate level. The biggest savings in drying time versus energy input for in-bin drying systems is achieved with the first 20°F rise in air temperature.

Stirring System Management When Drying With Heated Air

Management of stirring devices is different for heated air drying than natural air drying, especially for high temperature drying (over 40°F temperature rise). The relative humidity of the incoming air is so low with heated air drying, the grain on the bottom of the bin becomes over-dried by several percentage points by the time the drying front is pushed through the full depth of the grain. Stirring devices, if installed, should be run continuously with high-temperature heated in-bin drying systems to help equalize the moisture content of the grain mass and avoid over-drying at the bottom of the bin.

In-Bin Layer Drying

If a producer has several bins equipped with drying fans and is able to switch over from filling one bin to another in a reasonably short time, filling and drying several bins in layers could reduce drying time and energy consumption by 22%-36% as compared to completely filling each bin in turn before beginning to fill the next bin.

For more information on layer drying see Fill Drying Bins in Layers to Reduce Drying Time and Energy Cost.