November 3, 2009

The CropWatch article, Estimating Drying Times for High Moisture Corn and Natural Air (October 29, 2009), estimated drying times for corn between 16% and 21% moisture  using natural (unheated) air with a range of temperatures and relative humidity levels typical of November.

In much of the state, however, sunny days and dry conditions have been rare and corn is being harvested at much higher moisture contents. In these instances. the high moisture corn likely can't be dried to the recommended 15% moisture before ambient air temperatures became so cold that continuous fan operation isn't worthwhile.

This leaves two alternative management strategies:

1. Dry with natural air for as long as weather conditions are conducive to removing significant moisture from the corn, then convert to a holding strategy, monitoring the grain for any sign of heating. This strategy requires running the aeration fans intermittently to detect heating or moldy smells while keeping the grain temperature uniform throughout the bin and between 25°F and 30°F. For more information on holding higher moisture grain during the cold, winter months see How to Reduce On-Farm Grain Drying Energy Costs on the CropWatch Surviving High Input Costs Web site.

Scroll down to the Intermittent Fan Operation heading. This section includes information on how to hold the grain to minimize deterioration and a table on equilibrium moisture content. This table shows the moisture content achievable using air with the stated temperature and relative humidity properties. It can help you decide when natural aeration is no longer feasible.

2. Add heat to the air being pushed through the corn. Contrary to what many people believe, heating the air does not change the quantity of moisture in the ambient air. When you heat the air it can hold more total water vapor, so the relative humidity of the air changes. Air with a relative humidity of 50% contains half of the total amount of water that it can hold. If you heat air at 50 degrees and 50% relative humdity to 70°F, the relative humidity will drop to 25% and the air's water-holding capacity will increase. Heating the air allows it to carry away more moisture from the grain bin per cubic foot of airflow.

A rough rule of thumb is that adding 20°F to air reduces its relative humidity by half. The next 20°F increase drops the new value by half, the third 20° F rise drops that value by half. Table 2 in  How to Reduce On-Farm Grain Drying Energy Costs shows how changing the temperature of an air mass affects its relative humidity.

Table 1 (below) was generated by a computer model. Various air properties are listed in the far left column. The next column shows the air temperature and relative humidity after the air is heated 20 degrees above the ambient temperature. The remaining cells indicate the days to dry from the stated moisture content to 15% moisture using the heated air.

Table 2 is organized like Table 1 but assumes the ambient air has been heated 35 degrees above ambient air temperature.

Tom Dorn
Extension Educator, Lancaster County