Miller Magazine Issue: 120 December 2019

82 ARTICLE MILLER / DECEMBER 2019 by Centaur (Centaur Analytics, 2018) can predict such airflow patterns and the corresponding temperature and moisture content profiles. Equalizing grain bulk temper- ature by aeration, such moisture migration is prevented. PREVENTION OF HEAD-SPACE AND DOWN SPOUT CONDENSATION Under-roof condensation is a different natural process than moisture migration within the grain bulk. Condensate that drips on the grain involves moisture in humid air, which accumulates in the head-space above the grain bulk and condenses on the under-surface of the bin roof. Bins with sufficient roof vent and open eave gaps (spacing of 1/2 to 1 inch) between sidewall and roof, generally have enough natural ventilation to avoid under-roof condensate. Con- densate is especially problematic in bins with eave gaps that are permanently sealed to prevent fumigant gas losses and easy grain access for insects. PREVENTION OF BIOLOGICAL HEATING OF DRY GRAIN In grain bulks where infestation is localized, insect pop- ulations develop in small pockets of grain. The lesser grain borer and the three primary weevil species found in grains in the United States — the rice weevil, the maize weevil, and the granary weevil are characteristic species that develop localized infestations in bulk grains, creating hot spots. Temperatures of heavily infested grain undergoing wide- spread heating are typically about 38° to 43°C. When heavy infestations are discovered, the grain should be fumigated immediately to stop insect activity. Then aeration should be used to cool the grain bulk. PREVENTION OF SPONTANEOUS HEATING OF MOIST GRAIN – In warm moist grain (equilibrium relative humidity greater than 70%), respiration can become very intensive due to mold devel- opment. High levels of respiration produce a phenomenon called “spontaneous heat- ing.” Heating of the grain bulk is detrimental to grain quality. In spontaneous heating, hot spot temperatures can easily reach 57° to 60°C creating steep temperature gradients between heated and surrounding cool grain. In bulks containing oil rich seeds such as cot- tonseeds, soybeans, and sunflower seeds at sufficiently high moisture conditions, very high temperatures are generated and “spontane- ous combustion” can occur, starting a fire. Do not operate aeration fans if fire is detected (by the smell of smoke or burning grain in the exhaust air stream) in a grain bulk. Aeration promotes fire by supplying more oxygen to the heating spot. LIMITED GRAIN DRYING BY AERATION A small, but significant drying effect (from 1/4% to 1/2% moisture loss per aeration cooling cycle) is typical- ly experienced, and during long-term aeration (multiple cooling cycles) up to 2% moisture reduction may occur while cooling large grain bulks. Because of the very low flow rates during aeration, the drying front moves slowly, and this small drying effect is usually limited to the grain near the entrance of the aeration air. This grain moisture loss is reflected in a corresponding shrinkage or market weight loss in the grain bulk. This must be considered in grain management as a cost for keeping grain safe for marketing. Aeration moisture shrinkage as well as “invis- ible” handling loss will affect facility records significantly and should be considered when grain receipt and deliv- ery records from storage facilities or sites do not tally. REMOVAL OF FUMIGANT RESIDUES AND ODORS The release of or desorption of fumigants at the end of a fumigation can be achieved with relatively low air flow rates. The aeration system can be operated intermittently (in pulses) to flush gas vapors from the grain bulk and stor- Fig. 2 – Temperature (oC) and relative humidity (%) readings recorded using a Centaur remote sensor inserted in a silo capacity of 800 tons containing paddy rice under aeration in Arkansas, U.S.A.