Experts are still divided when it comes to night-only natural air drying.
Dr. Ron Palmer, project engineer with the Indian Head Agricultural Research Foundation, made waves in 2015, when he suggested that grain would dry better at night when air temperature outside was less than grain temperature. The idea flew in the face of conventional practice, which has fans running continuously.
Palmer’s argument is based on moisture and temperature data drawn from dozens of bin trials as far back as 2010.
The researcher compared moisture content of air entering the bin with the air leaving the bin on an hourly basis to determine if grain was drying or wetting throughout the day. Absolute humidity, or grams of water per cubic metre of air, was calculated for both outside air and bin exhaust using a function of relative humidity and temperature.
Results showed a regular wetting and drying cycle every 24 hours. In an average of 19 trials, Palmer found that grain dried best around 2 a.m. and that drying removed over 1.5 grams of water per cubic metre of air flowing through the bin roughly between 10 p.m. and 5:30 a.m. By just after 9:30 a.m., the cycle had changed from drying to wetting and did not switch back until almost 6 p.m. The worst time, according to Palmer’s data, was 2 p.m., when aeration added half a gram of water for every cubic metre of air pumped. The effect was worse on hot days, he added.
“We plotted this out and, to our surprise, yes, the drying actually takes place at night,” Palmer said.
Palmer makes an exception for filling bins, however. Fans should be left on continuously while bins are filled and left on until 9 a.m. the following morning, he said.
His findings led him to recommend farmers without sensors to turn fans off during daylight and to get grain as cold as possible, even if grain is dry.
He argues that cooling grain by 15 C will decrease moisture content by about one per cent while heating grain by 30° will increase moisture content by one per cent.
The other side
Joy Agnew, Agricultural Research Services project manager with the Prairie Agriculture Machinery Institute, disagrees.
Research has shown heat and moisture to be the two main factors in grain spoilage and, while Agnew says aerating at night will cool grain, little actual drying will occur and farmers needing to remove more than one or two per cent moisture should be cautious.
“What’s happening with night-only fan operation is you are passing cool air through warm grain, initially, and what happens is that cool air warms up as soon as it hits the grain,” she said. “It warms up without having a lot of moisture added to it initially, so now it has a huge capacity to remove moisture, so cool air passing through warm air does dry… but it’s short term because that relies on the grain being warm. As soon as the grain cools down, which usually happens after one night of operation, no longer do you have that effect of increasing the air’s capacity to dry.”
If grain must be dried, not just cooled, it is better to do it with warmer air that can hold more moisture and therefore has a greater drying capacity, she said. Grain should be cooled once grain approaches target dryness, something Agnew said will finish the drying process and avoid spoilage due to heat.
“With the later fall conditions that we’re seeing right now, you’re likely not going to get warm air, so that’s why if you need to remove more than two or three per cent moisture to safe dry, then you’re considering heated air drying options,” she said.
In an aeration fact sheet put out by PAMI, the organization acknowledged that changing temperature and relative humidity means that aeration will not dry grain during certain times of the day.
“This information can also be used to help even out the moisture content profile in a bin,” the fact sheet reads. “With most air distribution systems, the air flows from the bottom up. So to dry the grain at the top of the bin, the grain at the bottom becomes overdried. Air can be used to re-wet the overdried grain and result in an even moisture content profile.”
PAMI advises producers to turn aeration fans on as soon as ducts are covered and leave them on, “until the average temperature of the grain is at a safe store temperature.”
Fans can be turned off during rainstorms, the document adds.
Agnew is not the only one to weigh in on the topic. In 2013, two researchers from OPI Systems Inc. in Calgary and Digvir Jayas of the University of Manitoba advised against night-only aeration for drying.
While the experts agreed that fans should be turned on only when moisture was being removed, Jayas argued that aeration alone and only at night may not be enough to dry grain.
He added that continuous aeration would result in more uniform moisture levels in the bin and that it would be better to opt for continuous aeration if a farmer did not have sensors to calculate best drying conditions.
Palmer’s initial calls for night-only aeration did not account for relative humidity; something he now says was a gap. He has since added relative humidity into his findings and has developed an online calculator using grain temperature, air temperature and grain moisture to determine the highest relative humidity that a given crop can dry at.
The new data has not changed his views on night drying, however.
“We’ve got the data to show it — the data and the theory,” he said. “We’ve gone through something like 37 trial runs and you can clearly see that diurnal difference in drying, that drying occurs at night.”