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Natural-Air/Low-Temp Drying of Sunflower
Monday, September 14, 2015
filed under: Harvest/Storage
While drying sunflower is “old hat” for many Upper Midwest growers, that’s not the case for everyone. I receive calls about sunflower every year, with the quantity, not surprisingly, depending on the type of autumn we’re experiencing. In recent years, the majority of inquiries have centered on air drying sunflower in a bin, i.e., the fundamentals of natural air and low temperature drying. With that in mind, here are a few reminders that hopefully will be useful to numerous producers as they harvest their 2015 sunflower crop and dry it down for storage.
Moisture Measurement
Even the more-expensive farm meters work off an electrical measurement, and that measurement is affected by where the moisture exists within the seed. Since the reading is influenced more by moisture near the seed surface, the hull may indicate the sunflower seed is actually dry — whereas the true moisture is higher.
That, of course, leads to what we’ve long called “moisture rebound.” To get a more-accurate measurement, first check the seed moisture with your meter; then place the sample in a sealed container; and then recheck it six to 12 hours later (having warmed the sample to around 70°F). By then the moisture will have equalized throughout the seeds, and your reading will be accurate.
The easiest way to calibrate your on-farm meter is to take a sample, check it, take the same sample to your elevator, check it there — and then compare.
Natural Air Basics
Natural-air and low-temperature bin drying is energy efficient when designed properly, and it also facilitates a rapid harvest since bins can be filled at the same rate as the seeds are being harvested and transported to the bin site. Drying typically takes between three to six weeks, depending on the initial seed moisture content, airflow rate and outdoor temperature.
The table below shows the required airflow rates and drying time for drying oil sunflower at various moisture contents, using air at 47°F and 65% relative humidity (based on average North Dakota conditions in October). Drying times will be about twice as long at the (North Dakota) November average of 27° due to the reduced moisture-holding capacity of the air at colder temperatures.
| Drying Time |
Moisture
Content | Airflow
(cfm/bu) | Fan Time
(Hours/Days) |
17% | 1.00 | 648 / 27 | 15% | 1.00 | 480 / 20 | 15% | 0.75 | 720 / 30 | 15% | 0.50 | 960 / 40 | 13% | 1.00 | 336 / 14 | 13% | 0.75 | 504 / 21 | 13% | 0.50 | 672 / 28 |
Add enough heat when the air humidity is too high to dry the sunflower to the safe moisture content. Generally, that which will warm the air about five (5) degrees is the maximum amount required; anything more than that can cause over-drying. As a rule of thumb, about two (2) kilowatts (kW) of heater will be required per fan motor horsepower.
The equation for calculating the heat requirement in Btu is as follows: Btu/hr = cfm x 1.1 x temperature increase. You can convert Btu to kW by dividing by 3,413 Btu/kW.
A perforated floor is recommended. Since air does the drying, it’s imperative for air to reach all of the sunflower in the bin. That’s accomplished easier with a perforated floor system rather than with ducts — although it can work with ducts if they are spaced no more than one-half the grain depth apart and the distance from duct to bin wall does not exceed one-fourth the grain depth.
When It’s Not a Good Fit
The downside of relying strictly on a natural-air/low-temp drying system in the Northern Plains comes in a late, wet fall. We can experience weather patterns where the sunflower is drying down in the field and everything looks great as of early October — and then we get some rain (maybe even snow), and sunflower that was at 12 or 13% moisture ends up around 19 or 20%.
With a natural air system, sunflower at 15% is at a comfortable moisture level. Get up around 17% or higher, though, and you need a lot of airflow. For oil-type sunflower at 17% moisture, we need a good 1.0 cfm per bushel. That translates, under “typical” North Dakota conditions, into about 27 days of fan time for proper drydown. It the ’flowers go in at 15% moisture, we’d be looking at about 0.75 cfm per bushel and 30 days of drying time.
It may get too cold (near or below 32°) to efficiently dry in November, so then the drying would need to be completed in the spring. What works well in October does not in November. So the earlier we can get that drying process started, the more efficient we’ll be.
It takes about double the fan horsepower to go from 0.75 cfm up to 1.0 — which is why I’m not very enthused about natural air drying for sunflower that’s above 15% moisture. Also, the potential for sunflower deterioration is much greater for 17% moisture ’flowers.
Remember to leave the fill or access door open when operating fans near or below freezing temperature. These openings serve as a pressure relief valve in case the bin vents become covered and sealed with frost and ice. The pressure from a fan can damage the roof if the vents become sealed.
When in Storage
Sunflower can be stored for short periods in the fall at 12% moisture with adequate airflow to keep the seeds cool. Long term, however, the recommendation for oil sunflower remains at not more than 10% during the winter and 8% in summer. (For nonoils, it’s 11% during winter and 10% in summer.)
Aeration to control seed temperature is essential. If aeration is not available, sunflower should be rotated between bins during the storage period.
Cooling the sunflower also reduces the potential for seed deterioration from insects and mold. Sunflower should be cooled to 40°F or below before or soon after it is placed in the bin, and then to about 25° for winter storage.
Bins should be checked initially every two weeks for moisture condensation on the roof, and for crusting and changes in temperature within the pile. After the seeds have been cooled sufficiently for winter storage, the sunflower should be checked on at least monthly.
For More Information: www.ag.ndsu.edu/graindrying