March 11, 2011
Spring Grain Drying Recommendations for Large Bins
Managing large grain bins — those with diameters of 36-48 ft and heights of 28-32 ft — may require adding drying capacity and increasing monitoring efforts with warmer spring conditions.
In recent years there has been a trend toward larger bins than were used 20 years ago. Most of these bins are equipped with a single 30 hp centrifugal aeration fan, which likely doesn’t provide the drying capacity necessary for a full bin of grain.
With spring warm-up underway, you’ll want to quickly assess stored grain quality and begin drying efforts as necessary. This is particularly true for large bins where drying and aeration will take longer, depending on grain volume and fan capacity.
Fan Capacities and Drying Potential
A 30 hp fan will only produce about 0.25 cubic feet of airflow per minute per bushel of grain in a bin that's 48 ft in diameter and filled to a depth of 31 ft. Airflow values of 0.2 cfm/bu are adequate for pushing temperature fronts through the grain, but inadequate for drying grain.
Adding a second 30 hp fan on a separate transition duct nearly doubles the airflow to 0.47 cfm/bu. Adding a third duct and fan brings the airflow up to 0.67 cfm/bu.
A single 40 hp centrifugal fan will push 0.32 cfm/bu through the bin. Adding a second 40 hp fan and transition duct nearly doubles the airflow to 0.61 cfm and a third 40 hp fan and duct results in 0.85 cfm/bu airflow.
Most experts recommend using at least 1.0 cfm/bu airflow for grain drying. This recommendation is based on the probability of being able to dry the grain to a safe moisture content for long-term storage in a reasonable time.
Drying time is proportional to airflow. Under constant temperature and relative humidity, it will take four times as long to dry grain with 0.25 cfm/bu airflow as it would at 1.0 cfm/bu airflow. Over this extend period, some grain will dry much sooner than other grain in the bin, opening the door to increased losses due to fungal growth and deterioration.
For example, when drying with natural air and assuming an average air temperature of 50°F and average relative humidity of 50%, it would take about 8.3 days of continuous fan operation to dry a bin of corn with a starting moisture content of 17% (assuming 1.0 cfm/bu airflow). If the airflow is only 0.25 cfm/bu, it would take 33.2 days of continuous operation.
Grain drying in a bin occurs in a fairly narrow zone which moves upward through the grain assuming the air is pushed through from the bottom of the bin. The grain above the drying front remains unchanged or can actually be further wetted somewhat by the saturated air leaving the drying zone. The concern with drying grain using very low rates of airflow is the time the grain in the upper part of the bin remains at a high moisture content. This grain will be subject to attack by fungal organisms before the drying front reaches it.
Monitor Bins Now for Signs of Deterioration
Soon average air temperatures will climb into the mid to upper 50s. Grain held in the bin above safe storage moisture content (15% for corn) is in danger of beginning to spoil when warmer temperatures return.
Monitor stored grain twice a month for signs of heating. Air takes the path of least resistance through grain. There could be pockets of wetter grain buried in the grain mass. This is usually the result of an accumulation of fines or perhaps a frost dam which can impede airflow through a portion of the grain mass during winter.
The first symptoms of heating in the grain will likely appear next to the bin wall, especially on the sunny side of the bin where the grain will be warmer.
Also look for heating at the top center of the bin. When the grain is colder than the ambient outside temperature, the grain near the side walls is warmed by the radiant heating through the bin wall. This warmer air rises through the grain mass next to the wall and exits at the top surface of the grain. The air in the colder grain in the center of the bin sinks down, replacing the air rising next to the wall. Air in the head space of the bin is drawn into the grain in the center of the bin where it contacts the colder grain. Moisture can condense out of the air onto the cold grain mass. Wetting and warming the grain creates ideal conditions for fungal growth which, in turn causes additional heating in the grain.
If you have a grain thermometer, take the grain temperature near the bin wall about every 20 feet around the outside of the bin and a couple of places near the middle of the bin. If you are using a mercury thermometer, let the probe stay in place at least five minutes before taking each reading. If there is more than a 10 degree difference in temperature between the highest and lowest readings in the bin, run the aeration fan long enough to push a temperature front through the entire grain mass. The estimated time (hours) to push a temperature front through a bin is 15 divided by the airflow rate cfm/bu. If the airflow is 1.0 cfm/bu, it would take 15 hours. If the airflow is 0.25 cfm/bu, the estimated time is 15 /0.25 = 60 hours
If you don’t have a grain thermometer, turn on the aeration fan and lean into the access hatch or climb into the bin. Does the air hitting your face feel warmer than expected? Do you detect a musty odor? Does condensation form on the inside surface of the bin roof on a cold day? If you detect any of these symptoms, continue running the fan long enough to push a temperature front through the bin. If the bin is equipped with a stirring system, run two or three rounds to break up hot spots and to equalize the moisture throughout in the grain mass.
If the warning signs are present and the bin is not equipped with a stirring system, pull some grain out of the bin and monitor the condition of the grain coming out of the auger. If you detect heating, run the aeration fans to cool the grain and to dry the grain if air properties allow. Level the grain surface if the remaining grain will be left in place.
How to Finish Drying Cold Grain in Spring
Cold grain (less than 40°F) should be warmed in stages. Run a warming front through the bin when the outside air temperature is 10 to 12 degrees higher than the grain temperature. Trying to warm grain in larger steps can result in moisture condensation onto the grain mass. This occurs when the grain temperature is below the dew point temperature of the ambient air, depending on the relative humidity. This is especially problematic if the grain temperature is below freezing as the condensation could be in the form of frost and a frost dam can block airflow.
Once you start aeration to finish drying the grain and the grain temperature is above 40°F, run the fan continuously until the drying front passes completely through the bin and the top of the bin is down to 15% moisture. Ideally, you would like your corn to be 15% moisture and 40°F if held into May. This is dry enough to nearly stop mold growth and cool enough to stop insect activity. If you plan to hold corn into summer it should be 14% and 60°F.
UNL Extension Educator, Lancaster County