Engineer says running fans continuously is not a smart move

Ron Palmer says moisture levels often rise during the day, even when the fans are running, and it makes sense to only operate them intermittently

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Conventional wisdom says you should run fans 24/7 for six weeks or so after harvest to dry down grain.

But conventional wisdom is wrong and will not only needlessly run up your power bill, but could also lower grain quality, says a University of Regina professor of electronic systems engineering.

Instead, Ron Palmer recommends running the fan for the first 24 hours, and then only turning it on at night or when it’s cold.

Studies at the Indian Head Research Centre that continuously measured grain moisture levels found they fell for the first 24 hours of aeration. But after that, moisture levels would fall and rise in a more or less daily cycle. In other words, the grain was drying out and then absorbing moisture, often in a single day. Even stranger, the lowest moisture readings were in early morning and the highest at the end of the day — no matter how warm and dry that day was.

It’s the opposite of what you’d expect — we all know wet clothes dry faster when it’s warm — but it’s not that simple, Palmer said at the Farming Smarter conference last month.

“We talk about humidity when we really mean relative humidity,” said Palmer. “But that’s not a measure of the wetness of the air, or how much water is in the air. Relative humidity is not a lot of use when we want to know if a kernel of grain will give up its moisture to the air or absorb moisture from the air.”

The Indian Head moisture data intrigued Palmer, who is well known for his pioneering work in the development of auto steer, but also has had a lifelong interest in aeration.

He decided to look at a whole bin of grain, rather than wondering about different parts of it, and just consider the amount of moisture going in and coming out of the bin. He measured the amount of air, its relative humidity, and air temperature entering and exiting the bin, and then calculated the amounts of moisture.

He found that during the first 24 hours after grain was loaded into the bin, aeration lowered grain temperature and removed 1,198 pounds of moisture from a 2,000-bushel bin, or 1.8 percentage points.

Adding and subtracting

After that, aeration benefits were smaller and more erratic. Some days, the grain lost over 300 pounds of water, while on others, it absorbed similar amounts. During a cool week, the grain lost 617 pounds of moisture.

“After the first 24 hours, the grain consistently dried overnight and at 9 o’clock in the morning, it flipped into gaining moisture,” said Palmer.

“The air you’re pushing into the bin is wettest at 1 p.m. and driest at 6:30 a.m. That’s when dew forms, so we think of it as wetting, but all the moisture has dropped out of the air then. Dry, cool air dries and cools grain. In the afternoon, the warm, moist air hits cool grain and can’t hold its moisture (much like the air outside at 6 a.m.) and it gives up its moisture to the grain.

“When it’s warm, aeration wets the grain. It’s not just a waste of energy, it’s damaging the grain, heating it and wetting it.”

The only times Palmer recorded grain drying during the day was on cold days.

“If the aeration is cooling the grain, it’s drying the grain,” he said. “Except for that critical first 24 hours, run aeration only when the outside air temperature is the same or lower than that of the grain. Cooling is virtually the same as drying grain.”

Palmer modifies this advice slightly to allow for warming of the air from compression by the fan and heat from the motor. In one of his bins, compression raised temperatures, but not moisture levels, near the bottom of the bin by 2 C compared to the top.

To avoid creating a drying gradient (there is no drying front, he said), Palmer advises measuring the temperature difference between high and low in the bin and adding this “offset” to the outside air temperature when deciding whether to run the aeration fan.

“If the air temperature plus the offset is less than the grain temperature, run the aeration,” he said. “Or, you could use a smaller fan motor.”

Palmer tested his strategy by filling two 2,000-bushel bins with barley at 25 per cent moisture, one with continuous aeration and the other aerated according to his strategy (see box).

Conventional aeration did remove more moisture (5,114 kilograms versus 3,690), but Palmer’s method cut the number of hours the fans ran by nearly 80 per cent. So instead of a $533 electrical bill, a farmer using Palmer’s method would only pay $93 for the cost of running the fan. The grain was harvested on Aug. 29, so there were many nights when the air temperature plus the offset were above the grain temperature, which meant the fan didn’t cut in or only ran for a few hours.

The barley in the second bin obviously needed more aeration, but was in good condition, he said.

“We kept the grain as cool as possible, and we never put any moisture back into the grain,” said Palmer. “Drying the grain with cool air is the safest and cheapest way to aerate grain. And for less than $20 in electronics you can automate the system.”

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