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When Natural Air Drying Does, Or Doesn't, Work

Tuesday, August 1, 1995
filed under: Harvest/Storage

Many Northern Plains sunflower producers can attest to the efficiency of natural air drying systems. Others, however, can relate experiences which are not so satisfactory.

The difference often boils down to the time of year when the harvest and drying process was taking place. For Dakota and Minnesota sunflower producers, natural air drying works very well under typical late September and October conditions. It is not, however, an efficient way to dry wet seeds in November or early December — particularly if that sunflower’s moisture is in the mid-’teens or higher.

“If we have 15-percent moisture sunflower and we’re using an air flow rate of one cubic foot per minute (cfm) per bushel, our sunflower will dry to around 8.5 percent in about 20 days, under October conditions (table, next page),” says Ken Hellevang. “So it is working very well.”

Shift into November, however, and the scenario isn’t as nearly as positive, notes the North Dakota State University exten-sion agricultural engineer. “By November, we’re looking at cooler and higher-humidity conditions,” he points out. “The equili-brium moisture content is still pretty good; but our drying time has doubled because of the cooler temperatures (table).

“That’s been the scenario we’ve often run into during the past few years: We end up harvesting in early November, start trying to dry — and run out of time.” The picture is even less satisfactory in Decem-ber (table). “By then we’re to the point of getting the seeds down to only 11-percent moisture — and yet it’s taking us 50 days to do so,” Hellevang remarks.

“Once we’re into mid-November, our ability to dry sunflower with a natural air drying systems is basically gone,” the North Dakota drying/storage specialist concludes.

So what are some options for the Northern Plains producer who finds himself locked into a late harvest with wet seeds — yet may not have a high-temperature dryer on his premises?

One option would be to add a small amount of supplemental heat — five degrees or so — through the natural air system. The heat will help bring down the seeds’ moisture content; but also will, of course, add some expense. “Typically, we’d use a heater kilowatt rating that’s about the same — or slightly larger — than the fan horsepower,” Hellevang explains. “If we’re talking a five-horsepower fan, we’re probably looking at between a five- to 10-kilowatt heater. So the cost will be perhaps two times or more than that of a strictly natural air system.”

Another option — and a less expensive one — would be to simply cool the seeds to 25-30 degrees and store them wet over the winter. Then, in early spring, once the outside air is warmer and drier, turn on the fans and lower the moisture content to around eight to nine percent. As long as the

etting seed moisture content down to the proper level is the focus of every sunflower producer who babysits a high-temperature dryer or turns on the fans of a natural air system. But that’s not the entire story. It’s just as important to make certain one’s moisture meter is providing an accurate measurement of the amount of water in those seeds. An inaccurate meter can negatively affect both the quality of the grain and the number of dollars ending up in the producer’s pocketbook.

While many commercial elevators have gone to infrared-based moisture meters, some elevators and most producers are still utilizing electronic systems. Most such meters have a temperature adjustment chart for sunflower — and Ken Hellevang says he cannot stress enough the importance of using that chart.

“If a person ignores the adjustment chart when measuring sunflower moisture, he’s really going to have significant error,” the NDSU drying/storage specialist remarks. “And even with those charts, it’s important to remember that many meters become inaccurate when temperatures fall below roughly 40 degrees. So the producer needs to review those adjustment charts to see whether they can adjust for sunflower at 28 degrees or whatever the cooler temperature may be.”

As he has for years, Hellevang reminds sunflower growers to let cold seed samples warm up to room temperature before taking a moisture reading. The samples must first be placed inside a sealed container before being brought indoors to warm up, he cautions.

If taking samples off a high-tempera-ture dryer, of course, one must keep the “moisture rebound” phenomenon in mind. The disparity between the sunflower meat and the more-porous hull means that a reading of 10 percent right out of the dryer might translate into 12 or 13 percent once the moisture has reached equilibrium throughout the entire seed. “The meter would have been fooled,” Hellevang notes. “Typically, the higher the dryer temperature and the wetter the sunflower going into the dryer, the greater the ‘rebound.’ ”

There are two other points the NDSU ag engineer emphasizes regarding moisture testing of harvested sunflower.

First, it’s important to take samples that are representative of the entire field or storage. Moisture content can vary from area to area in a field — and perhaps even spot to spot within a single sunflower plant head. So as with any sampling procedure, the more samples the better.

Second, when filling the moisture meter with seeds, it’s important to be as exact as possible on the amount placed in the meter. “If you’re sloppy with how accurately you’re measuring that weight, the corresponding moisture content reading is going to be even sloppier,” Hellevang advises. For example, if the sample is supposed to be 250 grams on a 10-percent moisture reading, a weight deviation of just one percent (2.5 grams) would result in a moisture deviation of 0.2 percent, for a reading of 9.8 percent.

“By and large, most moisture meters do as good a job as we can expect of them,” Hellevang concludes. “But sometimes we forget there are a number of factors involved — and all these factors can affect the accuracy of that moisture measure-ment.”
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