Drying ’Flowers in November?
by Ken Hellevang
Based on average North Dakota temperatures and relative humidity, the equilibrium moisture content (EMC) of oil-type sunflower is about 10% for November, 11% for December through February, 10% in March, 9% in April and 8% in May. For confection seeds, it is about 14% for November, 15% for December through February, 14% in March, 11.5% in April and 10%in May. Sunflower seeds above these moisture contents will gradually dry to the EMC, based on temperature and relative humidity.
The estimated natural-air drying time for 17% moisture oil sunflower —given October conditions 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. The drying time in November is about 47 days due to the reduced moisture-carrying capacity of the air at 27 degrees.
Adding heat to warm the air 5 degrees reduces the final moisture content to about 8.5% and the drying time to about 32 days. Warming the air by 10 degrees reduces the final sunflower moisture content to about 7% and drying time to 36 days. Some supplemental heat will reduce the drying time, but more supplemental heat appears to actually lengthen the drying time and over-dry the sunflower.
The estimated allowable storage time chart for cereal grains can also be used for estimates on oil sunflower. Oil sunflower at 15% moisture is equivalent to cereal grains at 20%. The allowable storage time for 17% oil sunflower at 40 degrees is about 60 days. Airflow through the sunflower seeds is required to keep them cool. The allowable storage time for 17% sunflower at 60 degrees is only 15 days.
Moisture meters generally will not give accurate readings for seed temperatures below about 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. It’s also important to remember to adjust the temperature meter reading for seed temperatures above 40 degrees unless the meter does it automatically. The adjustment may be three (3) percentage points for sunflower near 40 degrees. Read and follow the operator’s manual for accurate readings.
Sunflower seeds coming out of a high-temperature dryer will have a moisture gradient that will generally cause the moisture meter to give a reading lower than the correct value. This causes what is commonly referred to as a “moisture rebound,” where the sunflower seeds appear to increase in moisture after being placed in storage. To determine the amount of error, measure the moisture content of the sunflower coming from the dryer, place the sample in a sealed container for at least 12 hours, and then recheck the moisture level.
The expense involved in high-temperature drying consists primarily of the propane and the capital or fixed cost. The propane cost can be estimated on a “per-point of moisture removed per-hundredweight basis” by multiplying the propane price per gallon by 0.037.
For a cross-flow column dryer, the expected propane cost, is about $0.074 per point of moisture per hundredweight using $2.00 propane. The estimated propane cost to dry sunflower from 20% down to 10%, using $2.00 propane, is 74 cents/cwt. The capital and fixed cost might be about 47 cents/cwt., resulting in a total cost of $1.21/cwt.
To estimate the propane cost on a per-point of moisture per-bushel basis, multiply the propane price by 0.012. For a cross-flow column dryer, the expected propane cost is about $0.024 per point of moisture per bushel using $2.00 propane. The estimated propane cost to dry sunflower from 20% down to 10%, using $2.00 propane, would be 24 cents/cwt. The capital and fixed cost might be about 15 cents/cwt., for a total cost of 39 cents/cwt.
Any dryer using an open flame to heat the air poses a constant fire hazard. Fine fibers from sunflower seeds or other plant materials may be ignited by the burner and carried to the seeds, causing them to ignite. This fire hazard can be reduced by turning portable dryers into the wind so airborne fibers are blown away from the dryer intake, or by pointing permanent dryers into the prevailing wind.
Clean the dryer, air ducts and area around the dryer at least daily. Frequently remove the collection of sunflower lint on the dryer column and in the plenum chamber, because the material becomes extremely dry and can be ignited during dryer operation.
A major concern is that some sunflower seeds will hang up in the dryer or be stopped by an accumulation of material and become overdried. Make sure the dryer is completely cleaned after each batch, keep sunflower seeds moving, and check a continuous-flow dryer regularly (hourly) to see that the seeds are moving.
High-speed dryers are like a forge when a fire gets going. However, fires can be controlled if they are noticed immediately — which makes constant monitoring necessary. Many fires can be extinguished by shutting off the fan to cut off the oxygen. A little water applied directly to the fire at an early stage may extinguish it if shutting off the fan fails to do so.
A fire extinguisher for oil-type fires should be used for oil sunflower fires. Many dryers are designed so that sunflower seeds can be unloaded rapidly in case of a fire and before the dryer is damaged. In some dryers, only the part of the dryer affected by the fire needs to be unloaded.
Ken Hellevang is professor and extension agricultural engineer with North Dakota State University.
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