Cereal: Drying and Storing Wheat

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Pub 811: Agronomy Guide > Cereal > Drying and Storing Wheat

Excerpt from Agronomy Guide for Field Crops
Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

• Drying and Storing Wheat
  • Table 4-21. Recommended Airflow for Natural-Air and Low-Temperature Wheat Drying
• Natural-Air and Low-Temperature Drying
  • Table 4-22. Maximum Recommended Air Temperatures for Drying Milling and Seed Wheat
• High-Temperature Drying
• Determining Airflow
  • Table 4-23. Equilibrium Moisture Content for Soft Winter Wheat Exposed to Air
• Equilibrium Moisture Content
• When to Run the Fan

Drying and Storing Wheat

Winter wheat is sometimes harvested at higher moisture contents because of impending wet weather or to reduce harvest losses. Wheat is considered dry at 14.5% moisture.

Winter wheat must be dried to 13%-14% moisture content for safe, long-term storage.
Drying Systems

Three different systems can be used to dry wheat:

• natural-air drying bin
• low-temperature dryers (less than 40°C)
• high-temperature or high-speed dryers (temperatures greater than 40°C)

Adapted from Wilcke, William F., Hellevang, Kenneth J. Wheat and Barley Drying. FS-5949-GO, 1992. University of Minnesota, Extension Service.
¹ CFM = cubic feet per minute.

Natural-Air and Low-Temperature Drying

Natural air drying of wheat will only occur when the relative humidity of the outside air is below the equilibrium moisture content of the grain. The effectiveness of natural air drying systems is greatly reduced during rainy periods and at night when temperatures are cool and relative humidity levels are normally high. When air temperatures fall below 10°C, forced ambient air will not pick up as much moisture, and supplemental heat may be required. Extended periods of humid weather may also require additional heat to affect drying. Raising the temperature of the incoming air by 5°C will dry the air but should not overdry the grain at the bottom of the bin. See Table 4-21, Recommended Airflow for Natural-Air and Low-Temperature Wheat Drying, for recommended airflow rates for natural-air and low-temperature wheat drying.

Minimum requirements for natural-air drying:

• full aeration floor in the bin
• level grain surface across the entire bin
• minimum airflow of 6.5 L/sec/m3 (0.5 CFM/bu), preferably 9.7 L/sec/m3 (0.75 CFM/bu) or more
• clean wheat with no weed seeds or fines
• accurate moisture determination of the wheat in the bin
• accurate outside air temperature and relative humidity measurement
• an understanding of wheat equilibrium moisture content
• on/off switch for the fan

A full aeration floor is essential to move air uniformly through the entire bin contents. With a partial aeration floor or air duct system, dead areas will exist, leading to potential spoilage problems. Weed seeds, green trash and fines accumulations in the bin will restrict or divert airflow. Air moving through the wheat mass will take the path of least resistance.

Copright: Farm Drying of Wheat, Canadian Grain Commission, Sept 1992.
Wilcke, William F., Hellevang, Kenneth J. Wheat and Barley Drying. FS-5949-GO, 1992. University of Minnesota, Extension.

High-Temperature Drying

With high-temperature drying, large volumes of heated air, 40°C or higher, are used to accomplish drying in a few hours or days. Corn dryers could be used but it may be necessary to reduce the drying temperature to avoid loss of starch quality and germination. It is important not to exceed the recommended maximum air temperatures for drying milling wheat Table 4-22, Maximum Recommended Air Temperatures for Drying Milling and Seed Wheat, which are dependent on the type of dryer used and the end use of the wheat.

For safe drying, the temperature of grain kernels should never exceed 60ºC. Check the contract to determine if heated air drying is allowed to condition seed wheat.

The baking quality of wheat is reduced if the temperature of the grain reaches 60°C for any significant length of time. When heated air dryers are used, it is a worthwhile precaution to have samples evaluated to ensure the dried grain meets market standards.

Tough wheat can be dried with natural air under good drying conditions. Natural-air drying of wheat requires careful management by the operator since wheat gives up and takes on moisture easily. Only run the fan when outside conditions will result in drying progress.

Do not run the fan continuously, night and day, as the wheat will re-wet at night. The progress you make during the day will be undone during the night.

Determining Airflow

Sufficient airflow is needed to move drying air through the entire wheat mass. To remove moisture, the minimum airflow required is 6.5 L/sec/m3 (0.5 CFM/bu); anything less will only change the temperature but not the moisture content of the wheat. Higher airflow rates of 9.75 L/sec/m3 (0.75 CFM/bu) or greater help speed up the drying process. These higher airflow rates may be difficult to achieve, requiring much higher fan horsepowers. The small kernel size of wheat causes the spaces between the kernels to be small. Moving large amounts of air through deep beds of wheat will take a large fan with high static pressure capability. If this bin and fan combination is capable of supplying 26 L/sec/m3 (2 CFM/bu) when filled with corn, only fill it one-half to one-third that depth with wheat. With axial flow fans, filling the bin with wheat to one-third the depth of corn is a good starting point.

To determine the CFM/bu value for a bin, determine the number of bushels in the bin and the static pressure that the fan is operating against. A simple manometer connected to the air plenum below the perforated floor will show the static pressure (inches of water displaced in the column). See Figure 11-1, Home-Built Manometer, on page 183, for a diagram of a homemade manometer. Determine the fan output at the measured static pressure using the fan performance curve.

To calculate CFM/bu airflow, divide the CFM output of the fan at the measured static pressure by the number of bushels in the bin (1 CFM/bu = 13 L/sec/m3).

If adequate airflow cannot be achieved, one strategy is to partially fill the bin. In this way, the fan will be operating at less static pressure and will deliver higher airflow rates per bushel.

Equilibrium Moisture Content

Researchers have developed equilibrium moisture content tables Table 4-23, Equilibrium Moisture Content for Soft Winter Wheat Exposed to Air, that allow the prediction of the final moisture content of winter wheat when exposed to air with a certain temperature and relative humidity.

For example, you can find the equilibrium moisture content of wheat exposed to outside air at 25°C and 80% relative humidity. In Table 4-23, for example, find the point at which the 25°C row and the 80% relative humidity column intersect. This point will be the equilibrium moisture content for wheat at the outside air conditions stated. Given enough time, the wheat will dry down to 14.4% moisture content.

When to Run the Fan

Air temperature and relative humidity levels should determine fan operation, not the time of day. On some days, drying can be accomplished from 9 AM until midnight, while on others it may only be from 9 AM to 6 PM. Check the temperature and relative humidity of the air frequently throughout the day. As the wheat loses moisture, drier outside air is needed to continue to make drying progress. If the equilibrium moisture content on a given day is less than the moisture content of the wettest wheat, drying is possible and the fan should be on. Install a humidistat that will activate the fan at preset humidity levels. The operator can adjust the relative humidity level at which the fan is activated.

The wheat at the top of the bin will be the last to dry. Each day of fan operation will push a drying front up through the bin. This drying front may not reach the top of the bin that same day. Be sure to take moisture samples at the same depth each time to know how the moisture content is changing at that depth. Bins with stirrators will have fairly uniform moisture levels throughout the entire bin as a result of the mixing that has been done.