Adding supplemental heat when drying wheat with natural air isn’t usually necessary, even in September. But it may be needed this year if wet conditions continue, says Ken Hellevang, North Dakota State University Extension ag engineer.

Adding heat reduces the air’s relative humidity as well as the final grain moisture content. It should be used when outdoor air relative humidity is too wet to dry wheat to the desired moisture content.

However, adding too much heat causes wheat in the bottom of the bin to dry to lower than the desired moisture content, Hellevang says.

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 about 6 inches. Warming air by 5 degrees reduces the relative humidity about 10 percentage points. Warming air that is at 60 degrees with a 70% relative humidity by 5 degrees reduces the relative humidity to about 60%. This air will dry wheat to about 13.5% moisture content with just the heat from the fan.

A supplemental heater is not needed if the average outdoor air relative humidity is less than 70%. To determine the average, add the daytime and nighttime humidity levels together and divide by 2. If it exceeds 70%, then a little supplemental heat is needed. Even if the average relative humidity is 75%, the air will need to be heated only 2 or 3 degrees. For example, if 60-degree 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. The approximate heater size needed for wheat is about 1 kilowatt per fan horsepower, or 3,413 British thermal units per fan horsepower.

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 about 35 days using an airflow rate of 0.75 cubic feet per minute (cfm) per bushel, with the fan operating 24 hours per day. It is 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 — and is the preferred option.

Turn off the fans if the weather is foggy or rain is falling, Hellevang advises. 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.

Airflow, drying rates proportional

The drying rate is directly proportional to the airflow rate, Hellevang explains. 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 0.5 cfm per bushel. The airflow rate must be increased to raise the drying speed. Adding heat slightly increases drying speed, but primarily reduces the final grain moisture content. The maximum recommended moisture content for natural-air-drying wheat is 18%, which requires a minimum airflow rate of 1 cubic feet per minute per bushel; 17% requires 0.75 cfm per bushel; and 16% requires at least an airflow rate of 0.5 cfm per bushel to complete drying before significant deterioration occurs. However, the drying time is extremely long when using an airflow rate of 0.5 cfm per bushel.

The recommended minimum airflow rate is 0.75 cfm per bushel, and the maximum recommended initial moisture content is 17%. The maximum recommended wheat depth for drying is about 18 feet. A very rough rule of thumb is that 1 horsepower of fan is needed for each 1,000 bushels of wheat to obtain an airflow rate of 0.75 cfm per bushel. A centrifugal fan will be required. The NDSU grain drying and storage website, ag.ndsu.edu/graindrying, has a link to a fan selection program. With the program, you can determine the fan size needed to obtain the desired airflow or estimate the airflow provided by an existing fan.

Be careful with high-temperature drying

High-temperature drying is recommended if the wheat moisture content exceeds 17%, Hellevang says. However, use caution, because high temperatures affect the chemical structure and milling quality of the grain. High temperatures can damage baking quality severely, even though the grain kernels appear undamaged. The dryer temperatures that are allowed will vary with dryer type and design. Refer to the manufacturer’s recommendation for guidance.

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 wheat with 16% moisture content, and 130 degrees for wheat with 20% moisture content. 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.

Source: NDSU, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.