Natural air grain drying cuts costs
Robert C. Hansen
  
Conserving energy and reducing costs continues to be a critical challenge for Ohio agriculture. Conventional drying of corn requires about 3 million BTU of fuel and electrical energy per bushel. With higher costs of electrical energy and propane this fall, farmers should consider natural air drying for shelled corn. This method also works for soybeans and wheat.

Natural air drying is an in-bin system with the following typical characteristics. The bin is equipped with f full perforated floor, one or more high capacity fans, a grain distributor and stairs. Fan size is selected based on 1 to 2 cfm/bu and air is unheated. Cleaning equipment is used remove broken kernels and fines. Initial shelled corn moisture content is generally 22 to 25% and drying time 4-8 weeks.

Energy can be saved
Going to natural air fro drying corn from 25 to 15% M.C. can save as much as 40-50% on drying costs. Natural air drying replaces fuel energy, usually propane, by using the natural drying potential of air over extended periods of time. However, more electrical energy is used to move the air through the bin. Total energy savings and cost will depend on fuel and electrical energy prices and drying time which varies with location in the state. In addition to energy savings, well-managed natural air systems can produce a superior quality product.

If propane costs $1.60/gal and electricity is $0.12/kwh, the total energy cost for high speed, high temperature drying (both drying and cooling) would be abut $33 to dry a hundred bushels of corn from 25 to 15%. A similar system that cools in the storage bin would cost about $29. Natural air drying reduces the cost to about $17. The savings in this example is 12 to 16 cents per bushel.

Airflow is key to success
The natural air drying rate is proportional to airflow rate. This means doubling airflow doubles drying rate and cuts drying time in half. Successful natural air drying requires enough air be provided to complete drying before unacceptable levels of deterioration occur. As corn moisture increases, higher airflow is required to achieve drying before spoilage. For corn at 20 to 22% moisture, the recommended airflow would be 1.0 cfm/bushel of bin capacity. At 22-24%, airflow must increase to 1.0 to 2.0, and for 24-26%, 2.0 to 3.0 cfm/bushel is required.

Upward airflow is recommended. With upward airflow, the grain at the top of the bins is most critical since it is last to dry. Upward airflow allows the most critical grain to be visually checked by the operator. Airflow must be sufficient to move the drying front (boundary between dry grain and grain that is not yet dry) to the top of the grain before spoilage occurs.

Generally, corn harvesting for natural-air drying should be delayed until October 15 in Ohio for two reason: 1) to take advantage of the free natural-air drying that occurs in the field, and 2) to reduce the probability of the occurrence of 60^oF days after the corn is harvested, stored and not yet dry. As air temperature increases, the time available for drying decreases faster than the drying capacity of the air increases. The only alternative is to increase airflow rates. Airflow rates for natural-air drying are primarily controlled by fan size, grain type, bin diameter, and grain depth (Hansen et al., 1990). Practical ranges for fan size and energy requirements limit the airflow rate and also vary according to initial moisture content.

Bin Diameter and Grain Depth
For trouble-free, natural-air shelled corn and soybean drying, a grain depth of 12 to 16 feet is preferred, with a maximum of 20 feet. Ideal bin diameters for natural-air drying are 24, 27 and 30 ft. At 18 ft grain depth, a 30 ft bin holds 10,200 bushels. For bin diameters larger than 30 ft, fan size and power requirements are often prohibitive and mechanical spreading and hand leveling become unreasonably difficult. Since standard grain bins are built using corrugated sheets with an effective depth of 32 inches, 9 ring of sheets is an ideal maximum height because it provides a half-ring for the air plenum, 7-1/2 rings for grain storage (20 ft deep), and one ring for easy access to the top of the grain for inspection and hand leveling. Unless the initial moisture content of the shelled corn is 20% or less, avoid the temptation to fill the 9-ring bin to the eaves during the single-fill drying.

Fan Selection
Fan size (in terms of horsepower) is based on grain type, bin diameter, grain depth, bin capacity in bushels, and desired airflow rate. Airflow requirements can be met with one, two or more fans on the same bin. Advantages and disadvantages of vane axial versus centrifugal fans need to be compared. Fan performance varies from on manufacturer to another. Working with a qualified dealer is required to make appropriate decisions. 

OSU Extension Bulletin 805 has more detailed information on the selection and operation of natural air grain drying systems in Ohio. Contact your county Extension office or me for a copy.

Robert C. Hansen, FABE Adjunct Assistant Professor and Research Scientist, can be reached at 330-263-3860, or hansen.2@osu.edu

 

This column is provided by the OSU Department of Food, Agricultural, and Biological Engineering.