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Sunflower Drying & Storage: A Late-Season Memo

Monday, November 1, 2010
filed under: Harvest/Storage

As any North Dakota or northern Minnesota farmer can readily attest, natural-air drying of crops is a lot tougher in November than in October — whether you’re talking corn, beans or sunflower.

“In our area, we’re looking at a big change in temperature as we go from October to November,” affirms Ken Hellevang, North Dakota State University professor and extension agricultural engineer. “Our average October temperature is nearly 50 degrees; the average for November is under 30.”

To put that into a sunflower drying perspective, “the estimated natural-air drying time for 17% moisture oil sunflower — given an October condition of 47 degrees and 65% relative humidity — is about 27 days when using an airflow rate of 1.0 cubic foot of air per minute per bushel,” Hellevang notes. A similar air flow rate in November — with an ambient temperature of 27 degrees — would require 47 days of drying simply because of the reduced moisture-carrying capacity of that cooler air.

Heating the air even slightly has a significant effect. Adding enough heat to warm the air just five degrees typically can bring those seeds’ moisture content down to about 8.5% within 32 days, according to Hellevang.

The NDSU drying and storage specialist says he’s not getting nearly as many calls on drying sunflower as he once did. With North Dakota’s corn acreage having increased dramatically in recent years, most inquiries focus on that commodity. And with much of the state’s corn crop not being harvested until latter October and into November, the conversation often centers on high-temperature drying.

“With corn, if we don’t start drying until the end of October, it’s pretty difficult to do any drying with natural air,” Hellevang points out. Many corn producers want to be able to market their crop during the winter, he adds. “And you can’t do that if you’re sitting there with wet corn.

“So corn has really pushed a lot of high-temperature dryers onto North Dakota farms. And once farmers have one, they’ll tend to use it for their other crops (sunflower, soybeans) as well.”

Now more than ever, energy efficiency is a major consideration for owners of high-temperature dryers. Newer models often boost efficiency by reclaiming heat from the cooling portion of the dryer. “Some of the dryers have gone to ‘vacuum cool’ or ‘heat reclaim’ where they’re taking the air through the corn (or sunflower) and then into the fan and burner,” Hellevang notes. “So they’re capturing all that warm air.”

While not a new concept, it can have big economic implications. Traditionally, “we’ve always said it will save 20-25% in energy costs during the drying process,” the NDSU ag engineer relates. “But it can be even higher, depending on outside temperatures. I know of a number of growers who say they’ve [achieved] over 30% savings. Because if you compare bringing in outside air at 10 degrees versus 40 degrees, there’s a big difference in the amount of energy required to heat that air.”

Hellevang developed a formula that growers can use to easily calculate the cost of energy required for drying sunflower seeds: simply multiply the per-gallon cost of propane by 0.037. That will equal the dollars per hundredweight of seed per point of moisture removed. (For corn and wheat drying, he uses a factor of 0.022.)

Will lowering the heat result in greater energy efficiency? No, says Hellevang. Actually, the opposite is true. As shown in the accompanying figure (right), the amount of energy required to dry corn in a conventional crossflow dryer declines as temperature increases. The same is true with sunflower, Hellevang says. “Run the dryer at the maximum recommended temperature,” he advises. But just as importantly, don’t overdry beyond what you’ll need for good storage. That’s just going to cost you more money — and increase the threat of fires.

Hellevang says the main issue in high-temperature drying of sunflower seeds remains that of fire hazard due to “fines” and other trash. “Any dryer using an open flame to heat the air poses a constant fire hazard,” he points out. With sunflower, seed fines or other plant material may be ignited by the burner and carried to the high-oil seeds, causing them to ignite.

“Clean the dryer, air ducts and area around the dryer at least daily,” Hellevang suggests. “Frequently remove the collection of sunflower lint on the dryer column and in the plenum chamber, because that material can become extremely dry and be ignited during dryer operation.”

Sunflower seeds that become “hung up” in the dryer or stopped from flowing by an accumulation of materials will get overdried — and thus turn into a real fire threat. “Make sure the dryer is completely cleaned after each batch, keep the sunflower seeds moving, and check a continuous-flow dryer regularly (hourly) to see that the seeds are moving,” Hellevang advises.

“High-speed dryers are like a forge when a fire gets going,” he emphasizes. “However, fires can be controlled if they are noticed immediately — which underscores the need for constant monitoring.”

When it comes to properly storing sunflower seeds for an extended period, the rules haven’t changed much through the years. Here’s a summary of Hellevang’s recommendations:

• Sunflower should be stored at 10% or less moisture if the intent is to market the crop within six months following harvest. Seeds held through the spring or summer should be at 8% or less for oil-type sunflower and 10% or less for nonoils.

• An aeration fan is meant to cool the seeds; it is not an adequate tool for drying or moisture removal. Sunflower should be cooled to 40 degrees or below before (or soon after) it is put into the bin, and to about 25 degrees for winter storage. Insects will not cause damage or multiply if seed temperature is below 40 degrees.

• Fans should be operated when the outdoor air temperature is 10 to 20 degrees lower than that of the seeds. Fans can be run even during periods of intermittent high humidity, but should be turned off during rainy or damp weather. Cover the fan once it is turned off.

• Once in the bin, sunflower should be sampled weekly until seed temperatures are cooled to the desired winter storage temperature. Then sample the seeds every three to four weeks during the winter months. Sampling should take place again bi-weekly if the seeds are being held into the warm spring and summer months.

• Check the seeds, not the bin. When sampling, probe the sunflower seed pile and be observant for temperature, moisture, insect, fungi and odor differences from the previous inspection. (Writing down your observations each time for future reference is a good practice.) If the probe is hot, take immediate action.

• Should a problem be detected, try to stabilize it with aeration. Should that fail, remove the seeds from the bin immediately, as the problems will only increase.

Meters & High-Moisture Seeds

Are today’s portable moisture meters more accurate than their older counterparts when it comes to measuring “high-moisture” sunflower seeds?

Not really, says NDSU’s Ken Hellevang.

“And that’s true with any of our grains," he observes. “Most meters are calibrated to be most accurate on dry grain. Whenever we get into high moisture — whether it’s sunflower, corn, soybeans or wheat — the meters will have more variability.”

Changes in seed or kernel structure also tend to affect measurement accuracy, he adds. “If we have a year with low test weights or higher percentages of immature seeds, such things will affect the meter reading. We can’t necessarily say it will be off on the high end or on the low end; we just know it will be affected.

“We’re using electrical characteristics in farm moisture meters to measure moisture content. But those characteristics are changed by these other types of parameters.

“So the best way to get a truly representative reading is to take several samples.”

Remember, too, that moisture meters commonly will not give accurate readings when seed temperatures are below 40 degrees Fahrenheit. “To get an accurate reading, place the sunflower sample in a sealed container and allow it to warm to room temperature before taking the measurement,” Hellevang advises.

“Also, adjust the temperature meter reading for seed temperatures above 40 degrees, unless your meter does so automatically. That adjustment may be as much as three percentage points. Read and follow your operator’s manual in this regard.”

Finally, as any veteran sunflower producer knows, it’s always important to keep “moisture rebound” in mind for seeds coming out of a high-temperature dryer. The term refers to a moisture gradient that will generally cause the moisture meter to give a reading that’s lower than the correct value.

It all revolves around where within the seed the moisture is being measured. “The reading is much more influenced by the moisture near the surface,” Hellevang points out. “So the hull can indicate that the seed is actually dry, whereas the true moisture is higher. That’s where the ‘rebound’ is coming from: the electrical measurement tends to measure the hull and not the kernel.”

To determine the amount of error, measure the moisture content of the sunflower coming out of the dryer, place the sample in a sealed container for at least 12 hours, and then recheck the moisture level. — Don Lilleboe

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