Night-Only Drying Too Simplistic for safe grain dry down, Research Suggests

by recently ran a story featuring work out of the Indian Head Agriculture Research Foundation that suggests farmers could save on drying costs by only turning aeration fans on at night, when the air temperature sunk below the temperature of the gain in the bin. Once we posted the story, more than a few farmers and scientists raised some concerns over this method. Dr. Digvir Jayas, VP of Research at the University of Manitoba and a grain-drying ecosystem expert, contacted us to refute the claim that night-only drying is safe and effective. He’s not saying night-only drying won’t work, he’s saying it’s too simplistic, that the fan should operate in an optimal window for drying (a function of more than just temperature) instead of day or night on/off control only. Recent research he’s published, in conjunction with OPI Systems Inc., out of Calgary, points to a much more complex answer on how to most efficiently dry grain.

Jayas stresses that “appropriate and automated control strategies are required to optimize the drying performance with uniform drying, energy efficiency, minimum under/over drying, and to reach the target moisture level without significant dry matter and quality loss prior to winter storage. In natural air drying and aeration, fan ON/OFF by guesswork may result in significant spoilage and/or shrink loss.”

The work done at IHARF only allowed a single cooling cycle, which doesn’t fully capture the grain drying ecosystem. Now, bear with us here, as it’s going to get technical, but safe storage of grain is very important, so we’re going to get wordy but for good reason.

Jayas explains that “for grain drying, it is important to understand Equilibrium Moisture Content (EMC). Natural air drying is governed by the principle of EMC, which is the moisture content that grain will attain if exposed to air at a specific relative humidity (RH) and temperature (T) for a long enough duration of time. If the EMC of the air delivered to the grain (plenum EMC), including fan and heater warming, is lower than the grain moisture content, moisture will be released from the grain to surrounding air (drying). If the plenum EMC is greater than the grain moisture content, the grain will absorb moisture at a slower rate compared to drying to reach equilibrium with delivered air.”

It is not always true that warm air holds more water.

Jayas elaborates from here: “Air is the mixture of gases that contain water in it as vapor. The combination of temperature and relative humidity determines how much water is present in air, how much maximum water air can hold and its drying potential. Water content of air (measured in grams of water/kilograms of dry air) at a specific T/RH can be read from a psychometric chart. It is possible that cold air with high RH has the same water content as warm air with low RH. For example, air at 10 degrees C with 80% RH and air at 25 degrees C with 30% RH hold the same amount of water: 6 g/kg of dry air. Therefore, it is not always true that warm air holds more water. However, warm air has potential to hold more water vapor if it is cooled and humidified (cold grain and warm air). Cold air will hit dew point temperature at 100% RH quicker than warm air and cause condensation sooner. This is the reason why cold air has less potential for grain drying.”

The fan should operate in a desired EMC window instead of day or night on/off control only

Jayas has completed research with Chandra Singh and Ronald Larson of OPI Systems Inc., where they calculated average day and night ambient temperatures and RH for Indian Head, Sask., for two weeks, during the 4th week of September and 1st week of October 2010, using Weather Canada data. Average night and day time T/RH were 8.10C/80.4% and 16.30C/53.9%, respectively, and the air had 5.39 g of water at night and 6.22 g of water per kg of dry air at daytime. The daytime air has slightly higher moisture than nighttime air; however, the corresponding EMC at day and night was 13.1% and 18.8%, respectively, as calculated using American Society of Agricultural and Biological Engineers (ASABE) standard curve. This explains how the grain would tend to equilibrate to 13.10% moisture during the day and significant drying would occur. Grain is likely to rehydrate during the night when EMC is high. In natural air drying, the bottom layer grain is typically over dried by the time the drying front reaches the top of the grain bin. Running the fan sometimes with higher EMC evens out the bottom layer of moisture by rehydrating the bottom layer, and simultaneously dries the high moisture top layers. This reduces over drying cost. Therefore, the fan should operate in a desired EMC window instead of day or night on/off control only.

The concept of cool air night drying would work only if the grain is always warmer than the ambient air, Jayas says. However, in practice, when air of nearly 1 cfm/bu is blown through the grain, it will quickly bring down the grain temperature close to the ambient air. The IHARF work showed that wheat was chilled to 4 degrees C from 20 degrees C and peas were cooled to 13 degrees C from 25 degrees C in less than 20 hours. Further drying would not occur unless grain is heated again. Due to heating, however, significant dry matter loss and grain spoilage would occur before the drying front reaches the top of the grain bin. Moreover, when cool air passes through the warm grain, it may condense near the head space and around exhaust vents as warm air cools down again, due to cold ambient conditions. Cold temperature operation also creates the potential to freeze kernels together, leading to crusting and potential airflow blockage and subsequent grain damage and/or blockage of exhaust vents, resulting in roof damage.

Jayas and his colleagues have looked at several, high-tech grain drying combinations and used modeling software to compare night-only aeration to more selective fan use. “We used OPI Integris’ Pro modeling software to run simulations using historical hourly weather data (2006-2010) from Regina, Sask., for drying wheat from 17.0% to 14.0% with starting date of September 15th. We found that the fan ON During Night only control strategy gave very poor drying results in comparison to other control strategies, especially the inability to complete the drying cycle in the time required. A self-adapting variable heat (SAVH) strategy gave the best drying results and grain reached the target moisture between October 22nd and November 27th with 437-578 h of fan run time, negligible heater hours and without significant dry matter loss. The SAVH strategy consumed less fan hours than natural air drying and dried the grain with the lowest moisture spread (maximum uniformity).”

So, there you have it. While night-only drying may work, it’s not necessarily the most efficient or safest way to dry grain based on broader scope research done through the University of Manitoba. That said, Jayas is not saying to turn on fans and leave them on either — there are times when night only drying would be the best thing to do, and times when the fans should run all day or may require supplemental heat.


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