Dakota Farmer

A little supplemental heat is good; too much is bad.

September 17, 2014

3 Min Read

Adding supplemental heat isn't usually necessary when drying wheat, but may be needed this year in the Northern Plains if wet conditions continue, says Ken Hellevang, North Dakota State University Extension Service agricultural engineer and grain-drying expert.

But you have to be careful not to add too much heat.

Air will be warmed 4 to 5 degrees F as it passes through the fan on a bin of wheat when the fan is operating at a static pressure of 6 to 7 inches, he says.

bring_heat_dry_wheat_1_635465578124500000.jpg

Warming air by 5 degrees F reduces the relative humidity about 10 percentage points. Warming air that is at 60 degrees F with a 70% relative humidity by 5 degrees F reduces the relative humidity to about 60%. This air will dry wheat to about 13.5% moisture content with just fan heat. If the average relative humidity exceeds 70%, then a little supplemental heat is needed. Even if the average relative humidity is 75%, the air only will need to the heated 2 or 3 degrees F.

For example, if 60 degree F air has a relative humidity of 75%, warming the air 3 degrees in addition to the 4 degrees from the fan reduces the relative humidity to 59% and permits drying wheat to about 13.5% moisture.

"Only running the fan during the warmer and drier portion of the day lengthens the drying time," Hellevang says. "The estimated drying time during September is 35 days using an airflow rate of 0.75 cubic feet per minute (cfm) per bushel with the fan operating 24-hours per day and 62 days when the fan is operated just during the warmer 12-hour portion of the day. Running the fan 24 hours a day, and adding supplemental heat if necessary, permits drying to the desired moisture content faster than only operating the fan 12 hours per day.

Turn off the fans if it is foggy or raining. Wheat up to 16% moisture can be without airflow for a few days, but wheat at 18% moisture should not be without airflow for more than a day or two due to the potential for heating and spoilage.

Drying rate
The drying rate is directly proportional to the airflow rate, Hellevang says.

 If drying 16% moisture content wheat using an airflow rate of 1 cfm per bushel takes 21 days, it will take 28 days with an airflow rate of 0.75 cfm per bushel and 42 days at 0.50 cfm per bushel. The airflow rate must be increased to increase the drying speed.

The maximum recommended moisture content for natural-air drying wheat is 18% with an airflow rate of 1 cubic feet per minute per bushel (cfm/bu), 17% for 0.75 cfm/bu, and 16% for 0.5 cfm/bu to complete drying before significant deterioration occurs. Generally an airflow rate of 0.75 cfm/bu and limiting the initial moisture content to 17% is recommended. The maximum recommended wheat depth for drying is 18 to 20 feet.

A fan selection program from the NDSU can help you determine the fan size needed to obtain the desired airflow or the airflow provided by an existing fan.

High temp drying
High temperature drying is recommended if the wheat moisture content exceeds 17%, Hellevang says.

However, caution is required because high temperatures affect the chemical structure and milling quality of the grain. A common practice of some millers is to test a sample of the grain for milling properties before purchasing. High temperatures can damage baking quality severely even though the grain kernels appear undamaged.

Allowable dryer temperature will vary with dryer type and design, but a general recommended maximum drying air temperature for milling wheat in a cross-flow dryer where some of the wheat approaches the drying air temperature is 150 degrees for 16% moisture content and 130 degrees for 20% moisture content wheat.

Frequently a plenum air temperature about 30 degrees warmer is used in dryers where the kernel temperature remains below plenum temperature and the wheat kernel is not damaged.

Drying wheat will be slower than corn drying due to the reduced air temperature and airflow rate.

Source: NDSU Extension

Subscribe to receive top agriculture news
Be informed daily with these free e-newsletters

You May Also Like