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Natural air drying simpler, cheaper

Three grain storage bins used for natural air drying study at the IHARF research farm at Indian Head, Sask. A diesel generator, used to power the fans, is in the foreground. Photo by Ron Palmer, IHARF.

An option for natural air drying other than continuous fan operation is being put forward by Ron Palmer, an electrical systems engineer with the Indian Head Agricultural Research Foundation (IHARF), a 1200-acre, non-profit producer-directed applied research organization in Saskatchewan.

It isn’t fancy, but it is simple and cheap, as Palmer describes it. And if you’re skeptical, it won’t be difficult to test.

The IHARF study of natural air drying began in 2007, and is being funded through the 2017 growing season by the Western Grains Research Foundation. Other sponsors include Agriculture and Agri-Food Canada, Great West Controls, and Advancing Canada’s Agriculture and Agri-Food Saskatchewan.

According to Palmer, the purpose of the study is to develop a fan control strategy for natural, unheated air that results in safe storage of grain, requires less fan running time and dries grain quickly for early sales. “Safety” of the storage reflects the number of days grain can stay in storage before the germination rate (quality) falls to 95 per cent of whatever rate it had when it went into storage. The faster it reaches a stable cool and dry condition, the better the quality will be and the longer it can be stored safely.

To the end of 2014, Palmer worked with spring wheat, barley and field peas in typical farm-size bins with 33 trial runs. Two 2250-bushel bins and four 3500-bushel bins were paired for the trials – each filled at the same time with the same lot of grain. The typical continuous operation strategy was compared to experimental options, with 3-hp and 5-hp fans.

All bin runs from 2007 to 2013 with continuous fan operation were examined to determine the average rate of drying on an hourly basis. It was observed that there was consistently a significant amount of drying occurring in the first 24 hours of all continuous runs. “Thus, we suggest that it is important to have the fan on immediately as the grain comes in from the field,” Palmer says.

After the first 24-hours, his analysis of the drying curves became very interesting. “There was a daily cycle of drying and wetting appearing to repeat every 24 hours… in general, drying occurred at night and occasionally during cool days,” he notes.

Palmer’s research showed a direct relationship between grain temperature and air temperature. Drying was occurring whenever the grain temperature was decreasing. Drying was not occurring when the grain temperature was rising. In fact, grain in storage was being re-wetted by warmer outside air – moisture from the warm air was condensing on the cooler grain, and was being gradually absorbed into the grain.

“There are some producers who are intuitively following a control practice of only running the fans on hot days,” Palmer notes. “This does result in drying the grain, but it also keeps the grain hot which in turn reduces the number of safe days of storage, which could lead to mould development and spoilage.

“The common practice of running the fans continuously ‘works,’ but it needlessly cycles the grain through hot wet conditions which increases grain moisture and encourages spoilage,” he adds. “There are many days that the fan is running and is actually damaging the grain, by warming it up and adding moisture to the grain.”

The better option, he continues, is “cool fan operation.” Ideally, operate fans only at night when the air is cooler than the grain – resulting in much less fan time and cooler, safer grain.

Palmer found out that the first day was extremely critical. After that, continuous fan operation was a waste of fan operation and energy, and a waste of money.

“We would remove one per cent of the grain moisture content within that first 24 hours. After that, we fell into the cycle of drying at night and wetting in daytime. Leaving the air on continuously took out more water than we put in, eventually, but we could run the thing for a whole week without getting anywhere. It was just cycling back and forth, water in, water out. We were spinning our wheels, doing nothing.”

Thus, continuous natural air drying (airflow 1-2 cfm/bu) of the grain resulted in bins of warmer grain with higher moisture.

On the basis of this new information, Palmer suggests, the better focus for grain in storage is to “drive the temperature down” as far as you can.

His two-stage advice for best control of natural air drying is: 1. Turn on the fan immediately when filling a bin with warm grain; and 2. Leave fan on until 9 a.m. next day.

After that, get the grain as cold as possible by leaving the fan on when the outside temperature is less than grain temperature.

Palmer notes that one can adjust the drying time and the fan time by including an offset of one or two degrees to alter the threshold temperature. An offset of only one degree may lower the duty cycle of the fan by about five per cent, he says. The grain will be cooler and safer, but the drying time will increase. Work is being done to determine how the offset affects this balance. A sophisticated controller could include this offset.

Source: Ron Palmer, IHARF.

Safe days
As Palmer studied research data from instruments on the IHARF bins over several years, he realized that maintaining the grain quality was as important as getting it dry economically.

“Really, we want the grain safe. We don’t want any spoilage. Grain starts to spoil the minute it comes off your combine,” he says. “The question is, how can I store that grain with the least amount of spoilage to keep the quality as high as possible?”

That led him to studies from the 1980s that led to a spoilage formula. The Fraser and Muir formula determines the safe storage time for cereal grains based on grain moisture and storage temperature. Safe storage life is 38 days at 30 degrees and 14.5 per cent moisture; at the same moisture and 20 degrees, it has 128 safe days; at zero or colder, the safe days are almost unlimited.

“Two things go into secure, safe storage. We’ve been ignoring one of them. The one is dry. The other is cool or cold,” Palmer says. “How your grain is stored determines the number of safe days. If you want to keep your grain safe, keep it dry and cool.”

Going back to his data from hundreds of cycles as grain in storage warmed and cooled, Palmer saw that for every 10 to 15 degrees that the grain is cooled, about one per cent moisture was removed - simply because cold air holds less moisture than warm air.

“Cooling your grain is drying your grain. The two are one. You can actually build a controller now that would only be drying your grain if the outside temperature was less than your grain temperature. If it’s warmer outside, turn off the fan. If it’s less (than the grain temperature), turn on the fan. I’ve built the controllers and they work,” he says.

A company in Regina has started developing a controller for this purpose, to be controlled from a smartphone. It will monitor the temperature of grain in storage and outside air. At a threshold the farmer can set, it will activate or turn off the fans.

“We’re going to try that product this fall,” Palmer says. “We’re going to play with that offset, to see how it influences the on/off time.”  

More to do
There’s more to do, Palmer admits. For instance, there’s discussion about what happens inside the bulk of grain in a bin. To this point, he’s treated it as a “black box” where those dynamics are ignored. He’s been measuring amounts of moisture going into the bin and amounts coming out.

“We’re actually loading these bins this year with sensors for moisture and relative humidity to find out what is really going on, and how the drying is taking place, inside the bin. With the temperature and relative humidity I will be able to calculate the moisture content of the grain, at points throughout the bin. That will be interesting to see with real data, not assumptions. Predictions and assumptions could be wrong if you miss something.”

There’s an “art” to drying grain, Palmer adds. “We’re looking at the possibility of using smaller fans, producing less than one cfm/bushel. They may take a longer time to dry but you’ll get more consistent, more uniform drying from top to bottom – maybe,” he says.

In the remaining project years, he also may try reversing fans, using bins larger than 10,000 bushels, results with natural air drying for oilseeds and tests to clarify the “drying front” concept as moisture changes while grain is in storage.

Finally, good science will produce consistent results. He’s hopeful that other work will confirm his findings or reveal issues that he has missed.


September 4, 2015  By John Dietz


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