Grain Ops: Minimizing shrink loss during grain storage

by Dirk Maier and Ben Plumier
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Minimizing shrink loss during grain storage
Figure 1: Predicted velocity profile of the natural air convection currents in the grain mass a non-aerated 136,000-bushel (3,500-tonne) silo of corn at the end of July given typical Midwestern U.S. weather conditions in Indianapolis, Indiana, U.S.
 
According to traditional “moisture migration” theory, when grain is colder than the outside air, the air in the grain mass (interstitial air) close to the silo wall rises straight up into the upper layers of the grain, moves toward the center of the grain mass and sinks down near the centerline of the silo. As the natural air convection currents approach the plenum, the air releases moisture into the grain and moves back out toward the silo wall. The region near the plenum in the center of the silo presumably continues to accumulate moisture.

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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