August 22, 2008

Also see How to Reduce Energy Cost for High Capacity Grain Drying in this issue of CropWatch.

With energy prices up dramatically in recent years, grain producers are asking how to reduce the cost of drying grain on the farm. This article will discuss in-bin grain drying.

It goes without saying, the least cost method of drying corn is to let the grain dry naturally in the field for as long as possible. Given good drying conditions (low humidity, wind, and warm temperatures), corn can lose one-third to one-half point of moisture each day. At this drying rate, the corn would dry naturally in the field from 18% to 15% moisture in about the same time as if the corn were harvested and dried in the bin using natural (unheated) air using about 1 cubic foot per minute per bushel (cfm/bu) airflow. Producers with grain drying facilities usually hedge their bets by starting harvest early and mechanically drying part or all of their grain.

Grain Drying 101

The biggest savings in drying time versus energy input for in-bin drying systems is achieved with the first 20°F to 40°F rise in air temperature.

All mechanical grain drying systems use a fan to push 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.

In deep-bed drying systems (in-bin drying), air is normally pushed through the grain from the bottom of the bin and 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. The area within the grain mass where moisture is evaporating into is known as the drying zone. The top of the drying zone is the point at which the relative humidity of the air approaches 100% and no more drying can take place. The moisture content of the grain above the drying zone remains unchanged or may be slightly wetted by the saturated air leaving the drying zone. The drying zone moves through the grain in the direction of airflow as the air continues to remove moisture.

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. Under favorable drying conditions, natural air drying can be the least expensive drying method and usually results in the highest quality grain of any mechanical drying method. The minimum recommended airflow rate in Nebraska for in-bin natural air drying of corn is 1.0 cfm/bu for corn up to 18% moisture, 1.25 cfm/bu for corn up to 20% moisture, and 1.5 cfm/bu up to 22% moisture. If the airflow rate is too small to meet the recommendation above, the bin could be partially filled. The shallower grain depth results in less static pressure for the fan to overcome, which translates into more airflow output (cfm) from the fan. Since partially filling the bin results in fewer bushels in the bin, you are pushing more cfm through fewer bushels, thus significantly increasing cfm/bu. 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.

Stirring System Management When Drying With Natural Air

Research has found that stirring when using natural air to dry grain actually prolongs 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 damage to the grain and results in costly wear to the stirring device.

If a stirring device is installed in a bin of grain being dried by natural (unheated) air, the stirring device should be run during the filling period to reduce the pack factor from the filling operation, to redistribute fines and to level the grain. Stirring should then be discontinued to allow a drying zone to develop in 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.

Heated Air Drying

Table 1. Effect raising the air temperature has on relative humidity. Air Temperature Relative Humidity 50 72 60 50 70 35 80 25 90 18 100 13.5 110 10 120 7.6 130 6 140 4 Assumptions: Elevation 1,000 feet. Dew point 41.4oF.

Heating the air increases its ability to carry away more water vapor, (heating lowers the relative humidity of the air). When adding supplemental heat, the relationship between temperature rise and relative humidity is not linear. Table 1 presents the effect on the relative humidity when adding supplemental heat. All values shown in the table assume the dew point temperature (a measure of the absolute water vapor content of the air) is a constant 41.4°F.

A rough rule of thumb is the relative humidity drops by one-half for each 20°F rise in temperature. For example, natural air at 60oF 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 to 40°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 20-35% as compared to completely filling each bin in turn before beginning to fill the next bin.

In-bin drying is the most economical way to dry grain. Adopting these management techniques can preserve grain qualilty while minimizing energy cost.

Tom Dorn
Extension Educator, Lancaster County