Likewise, traditional “moisture migration” theory states that when grain is warmer than the outside air, the interstitial air near the wall sinks toward the bottom of the silo, circulates in the grain mass along the floor, warms up and rises through the center portion of the silo. As the air approaches the headspace it releases moisture into the grain mass near the center of the silo, then continues to circulate below the grain surface back to the wall. This theory has been used for many decades to explain significant moisture accumulation and the subsequent crusting in the upper grain layers of steel silos with and without plenums. However, our past research investigated heat and mass transfer in a stored grain mass by modeling the grain surfaces in the plenum and headspace of a grain storage silo as permeable boundary layers. The results predicted by our research group’s computer simulation model have shown that this traditional “moisture migration” theory does not explain surface crusting when aerated grain is stored at an appropriate temperature and moisture content considered safe for long-term storage.
Natural Air Convection Currents
Aeration involves forced convection of ambient air using fans generating airflow rates of 0.05 to 0.3 cfm/bu (~0.05 to 0.3 m3/min/tonne) for cooling grain, equilibrating moisture content, or exhausting fumigant at the end of a fumigation. The advantage of forced convection is that higher airflow rates result in faster cooling times. A limitation is that certain weather conditions are not suitable to achieve a desired cooling effect, especially in the tropics and subtropics. In such situations, chilled aeration may be needed. When aeration fans are not operating natural air convection currents move through the grain mass at rather slow speeds.
Figure 1 shows a circular natural air convection current in the middle of the graph between the radius of approximately 14 ft (4.3 m) and 24 ft (7.3 m) and between the vertical locations of 10 ft (3.1 m) and 50 ft (15.2 m) during summer storage in the Midwestern United States. The axis labeled z is the centerline of the silo and the r-axis is in the radial direction. The computer model predicted slow moving convection currents (6 to 8.5 ft/day; 1.8 to 2.6 m/day) that did not rapidly warm up the bulk of the silo after it had been cooled during winter storage, or change the moisture content any appreciable amount. Instead, by modeling the plenum and headspace as permeable boundary layers, the natural air convection currents always enter and originate from the plenum and headspace. The air in these regions, depending on the time of year and time of day, can either be at a lower or higher equilibrium relative humidity (ERH) than the grain mass. If air leaves the headspace at a higher ERH than the grain mass and enters the grain near the centerline, the grain absorbs moisture to equilibrate with the natural air convection currents. If air leaves the headspace at a lower ERH, the surface grain decreases in moisture content.
This moisture content increase (or decrease) is due to the natural air convection currents originating from the headspace, and not due to the traditional theory of moisture “migrating” directly from one area of the grain mass to another. As the natural air convection currents continue to travel downward through the grain mass toward the plenum, the temperature begins to increase and the ERH of the air decreases. As a result, the moisture content of the grain equilibrates with the natural air convection currents and decreases slightly in moisture content. The natural air convection currents that originate from the plenum and rise near the wall do not have a significant impact on the average moisture content of the grain. The grain near the plenum increases slightly in moisture when the plenum air has a higher ERH compared to the grain mass (or decreases if the ERH is lower). As the air approaches the headspace along the wall, the temperature increases and the grain decreases slightly in moisture content.
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Surface Crusting Cause
Top crusting also may occur when warm air is exhausted from the grain mass during cooling in cold weather, and during storage after grains were initially dried to moistures as low as 9% to 11%. This is a typical condition when corn (or rice) is transferred warm from a dryer into the storage structure. Condensation on the grain surface and underside of the roof usually also occurs in temperate climates early in the spring when weather conditions fluctuate between cold nights and warm days. Forced air headspace ventilation should be utilized to manage these condensation periods. When significant condensation occurs on silo walls and roofs, extended fan operation is needed to minimize excessive dripping. Exhaust airflow rates from extractor fans should be sized 125% to 150% greater than aeration airflows rates.
Peaked grain has a larger exposed surface area for moisture to condense on, and thus is more prone to spoilage than a leveled grain surface. Greater amounts of moisture also are caused by condensation from downspouts, and leaking roofs and hatch covers that allow rain and snow to enter the headspace. The development of “hot spots” in a stored grain mass is a typical indicator of grain spoilage due to excessive moisture. The grain under the loading spout is more densely packed with fine material and thus more prone to self-heating. Past research indicates that fine material concentration may reduce airflow through the core so much that it may take 2 to 3 times longer to cool a peaked versus leveled grain mass. This results in greater moisture shrink loss and higher electric energy costs due to excessive fan run times.
By properly ventilating the headspace air, the ERH of the air may be controlled. This will limit the moisture accumulation in the upper grain layers due to the natural air convection currents and condensation. In some structures passive venting may not suffice to effectively manage the headspace air-grain interface. In those cases, power exhausters are recommended that can be operated independently of the aeration fans, preferably using an automatic controller. Operating power exhausters for a limited time around sundown when steel roofs of silos cool quickly usually mitigates the potential for condensation.
Keeping Cold Grain Cold During Summer
The results of our past research support this management practice. Figure 2 shows the predicted grain temperature and moisture content distribution on July 28 under Midwestern U.S. climatic conditions in a 60-ft (18.3 m) diameter by 60-ft (18.3 m) eave height silo holding about 136,000 bushels (3,500 tonnes) of corn that was cored and leveled. The silo was cooled to about 32°F (0°C) given typical Indianapolis, Indiana, U.S., fall and winter weather conditions. After cooling, the fans were sealed and the silo was not aerated through the spring and summer storage period. Grain is a good insulator, which kept the center portions of the silo cold. By late summer, this silo still had about 60% of the total corn mass at a temperature of 60°F (15°) or less, which is below the threshold temperature for insect and mold development. Very little moisture movement was predicted to occur in the grain mass. Figure 3 shows the temperature and moisture content distribution in the same non-aerated silo but in the 12th month of storage (Oct. 1). The grain bulk had remained cool throughout the summer. Approximately 50% of the grain mass had remained at a temperature of 60°F (15°C) or less. The only regions that had warmed were along the plenum, wall, and headspace. The model predicted minimal maize weevil development until the end of the summer storage period. Concern over insect development in those regions when holding grain through the summer may be managed by treating interior silo surfaces before filling and the last amount of grain constituting the upper layer during filling the silo with an approved grain protectant.
The moisture content remained nearly uniform during storage. A small region near the center of the silo close to the grain surface absorbed some moisture (less than 0.1 percentage points) due to the natural air convection currents that originated from the headspace air and entered the grain mass. A small region near the plenum lost moisture due to the natural air convection currents that entered the plenum near the centerline. However, the overall change in average moisture content in the large silo resulted essentially in zero shrink loss during storage.
These findings have been incorporated in our continuing education and training materials for more than 10 years now and are supported by what good grain storage managers have practiced successfully even longer. Cold grain can be safely kept cold during summer storage while preserving grain quality, minimizing moisture shrink, and preventing insect development and spoilage.