Natural Air Grain Drying and Storage

By Dunling Wang, PhD, PAg, Provincial Specialist, Alternative Cropping Systems, Regina

Warm grain with a high moisture content is at great risk of spoiling, often resulting in downgrading and financial losses for producers. To prevent grain spoilage, both temperature and moisture must be properly managed in stored grain.

In recent years, drying grain by forcing natural air through the storage bin has gained popularity because of its relative effectiveness and low cost. Depending on the selected airflow rate and whether a supplement heat is provided, blowing natural air through stored grain can serve different purposes:

• Aeration – to cool and create an even temperature grain profile in the storage bin. The airflow rate is often set in the range of 0.1 to 0.25 cubic feet per minute per bushel (cf/m∙bu).
• Natural air drying – to blow ambient air (5 to 25 C) through grain to reduce grain moisture content when the ambient air has the capacity to dry. The airflow rates can be set in the range of 0.5 to 1.0 cf/m∙bu.
• Natural air drying with supplemental heat to improve drying efficiency – by adding heat to a NAD fan to increase the temperature of the air going into the storage bin, resulting in increased air-drying capacity and efficiency.

For natural air drying, airflow rate is important as it dictates the air volume that the fan is pushing into the grain. Dr. Ron Palmer from the University of Regina conducted tests using natural air and found that a flow rate of 0.5 to 1.0 cf/m∙bu is adequate for drying without a heat supplement. Turning on the fan immediately after the harvested grain is loaded into the bin can remove a large amount of moisture from the grain and reduce the grain temperature.

The drying capacity of natural air depends on its temperature and relative humidity (RH); but the efficiency of drying is also influenced by the grain type. The equilibrium moisture content (EMC), the moisture level that the grain will equilibrate to the air condition, is often used to predict if the ambient air can effectively remove moisture from grain. The ambient air has the capacity to dry when its EMC is less than the moisture of the grain.

For example, if the ambient air has a temperature of 10 C with RH of 60 per cent, the EMC of the air for hard red spring wheat is 13.6 per cent (see table below). That means when the air temperature stays constant at 10 C and RH at 60 per cent, wheat would eventually equilibrate to 13.6 per cent moisture content (MC) at 10 C. However, air conditions never stay constant, so this is a very complicated process.

When the goal is to dry grain, the fans should be on when the EMC is less than the desired grain MC. To dry wheat to a moisture level of 14.4 per cent, the most effective time to run the fan would be when the EMC of the air is much less than 14.4 per cent as in the range of the area highlighted in red in Table 1 below.

The EMC tables for different crops can be found on PAMI's website. Producers must monitor the grain MC until it reaches an average safe level in the bin. If necessary, producers should turn the grain to ensure it is uniformly dried.

Drying grain using natural air without supplemental heat works well when daytime temperatures are above 5 C. However, when both the grain and the ambient air temperatures drop below 5 C, supplemental heat may become necessary to enhance the drying potential of the natural air.

For every 10 C increase in temperature, the air moisture holding capacity doubles. Producers might want to use a higher temperature intake air (greater than 10 C) in late fall for increased drying efficiency. However, some cautions must be taken as this can cause the grain temperature to increase inside the bin, causing grain spoilage due to reduced moisture removal if airflow is low (e.g., ≤0.5 cf/m∙bu).

Research from the Prairie Agricultural Machinery Institute (PAMI) suggested that airflow rates should be maintained between 0.75 to 1.0 cf/m∙bu for any natural air drying with supplemental heat.

When dealing with high-moisture grain, producers can also use a grain dryer which provides extra drying capacity with higher temperatures and high throughput air. It is tempting to dry grain fast with high temperatures, but proceeding slowly with a moderate temperature increase, such as 15 C, is much safer and a better choice. Fast drying can cause damage to the grain, crack seed coats, and increase the risk of a fire. In addition, running supplement heat continuously is not recommended as it raises grain temperature and decreases drying efficiency. It becomes more effective to switch off the heater when the bottom half of the bin reaches 25 C and then run cold air to cool off the grain to about 10 C. Repeat this warm and cool air cycle.

Once the grain is dried, it should be cooled using aeration to within 5 C of the ambient air temperature to prevent condensation. If the grain at the center is more than 5 C warmer than the grain at the bin wall, moisture can start accumulating near the bin lid. Therefore, ensuring adequate ventilation in the headspace in the stored grain is important to prevent condensation. A minimum of one square foot of vent space for every 1000 cubic feet per min of air flow is required. You can also consider using active ventilation in the headspace to expel moist air more effectively. Prolonged storage requires continual monitoring of grain temperatures – at least once a month. Using aeration at a low airflow rate (e.g., 0.1 cf/m∙bu) as necessary and keeping the grain as cool as possible, should be the goal of grain storage, particularly in the spring and summer.

For details of drying grain with natural air, please refer to the Natural Air Grain Drying factsheets.