By Sam McNeill
Canola and wheat crops are maturing rapidly in western Kentucky and point to an earlier than normal harvest. With favorable weather predicted in the next few days across much of the state, harvest progress will likely be well ahead of the 5-year average despite intermittent/spotty rain showers.
Limits of Field Drying
The extent and rate of field drying is largely dependent on rainfall and relative humidity, then temperature and the amount of wind and sunshine. Canola and wheat give up and re-absorb moisture more quickly than corn and have different equilibrium moisture properties. Whether drying in the field or in a bin, the limits of drying are dictated by the average ambient conditions, as shown in Table 1.
Clemson University provides a free tool to quickly calculate the equilibrium moisture content of SRW wheat (as well as corn and soybean) for specific locations (by zip code) based on a five-day weather forecast. After selecting the type of grain from a drop-down list, the user chooses between three prediction equations or a composite which averages their values. Designed to help grain managers make decisions on when to harvest and/or operate drying or aeration fans, this tool was used to predict the trend in moisture changes from June 7-13 for Mayfield, Princeton, Elizabethtown and Lexington with results shown in Figure 1a-1d. The values shown for wheat were provided by the model, while the values for canola were calculated and inserted by the author. Note that a drying trend of 3-6 points is predicted for these areas and will approach moisture levels suitable for natural air or low temperature drying systems (10% for canola and 15% for wheat) in all regions. Keep in mind that most weather models predict temperatures more accurately than relative humidity or rainfall, especially scattered showers (so local conditions will vary). Still, this is a useful tool for predicting short-term changes in moisture.
Implications for Fan Operation
The recommended minimum airflow rate for in-bin drying and aeration systems is 1.0 and 0.1 cfm per bushel of grain. The University of Minnesota Biosystems Engineering Department provides a free, useful decision tool for evaluating existing systems to determine the amount of air that can be delivered for a given bin-fan combination and manage the depth of grain to achieve these limits. For example, the performance of a 24-inch, 10 hp axial fan (3950 cfm at 5 inches of static pressure) on a 30-ft diameter bin is shown for both crops in Table 2. Note that the minimum drying rate for canola and wheat can be achieved at a depth of 9 feet and 12 feet, respectively, and that the minimum rate for aeration is exceeded by 5 at respective depths of 12 and 18 feet. Considering the rule of thumb that the time needed to move a cooling cycle through grain is estimated by dividing 15 by the amount of air per bushel, this can be achieved in 25 hours for canola and 30 hours for wheat (at depths of 12-ft and 18-ft, respectively).
Source : uky.edu