The primary purpose of aeration is to maintain the condition of the crop stored in a bin. Hot spots cause spoilage and provide an environment in which bug infestations can develop. An aeration system moves a low volume of air through the crop, dissipating heat and preventing hot spots.
Aeration offers only minimal drying, so a system designed for aeration should not be used to dry crop. Aeration does not offer the advantage of an earlier crop harvest as offered by natural air drying and pressure curing.
As a rule of thumb aeration requires an airflow of 0.1 - 0.15 CFM per bushel.
Natural Air Drying
Natural air drying is a process which removes moisture from stored crops. With a natural air drying system you can harvest a crop with a 16-20% moisture content and remove moisture down to an optimal level. Once drying is complete the natural air drying system can provide aeration for long term crop storage.
A natural air drying system dries a crop by evaporation. Air flows past kernels, absorbing moisture and carrying it out of a bin. A drying front starts at the bottom of the stored crop and as the bottom kernels dry, the drying front moves upwards towards the top of the bin.
The drying process may take several weeks, depending upon the crop condition, airflow, outside humidity and temperature. Natural air drying requires that a crop is dried within the allowable storage time before spoilage has a chance to occur, so sufficient airflow is essential. Airflow of 0.5-1 CFM is normal for a natural air drying system.
In certain conditions of high relative humidity (a factor of cooler temperatures) air cannot absorb moisture so drying cannot occur. For this condition, low temperature supplemental heaters should be used. The purpose of the heaters is not so much to raise the temperature, but to lower the relative humidity of the air by slightly increasing its temperature (for more information on heaters, click on the Heaters tab on the left.)
Natural air drying offers a number of advantages:
A crop dried in storage is crop not dried in the field. Crops can easily loose grade waiting to dry 14% in the field.
Department of Agriculture tests show that wheat kernel damage is at a minimum when combined at 20% moisture. Having the option to dry in the bin allows the grower to combine at a higher moisture content.
Optimum moisture content
Field drying can result in a crop dropping below 14% moisture. A natural air drying system allows drying to be stopped at 14%, keeping the weight in the kernel and putting cash in your pocket.
Insects will remain dormant when a stored crop is below 50ºF (or 10ºC). Using airflow when the outside air temperature is cool enough can keep a crop below the critical temperature.
Crops harvested at a higher moisture content are often 2-6 pounds per bushel greater than crops harvested with lower moisture content. This happens because the kernel retains more nutrients. When a crop is left in the field some of the nutrients are absorbed by the stalk.
Kernel quality is often much higher for crops conditioned with natural air compared to crops subjected to heated drying. Natural air leads to less shell cracking and better color, test weight and germination percentage.
Low drying cost compared to heat dryers
Natural air drying systems have low capital costs and operating costs compared to batch drying systems which use heat to dry.
Peace of mind
Natural air drying offers opportunities for crop management and reduces the risks imposed by the weather.