Aeration of Stored Wheat in Humid Night Air
A 300 t bulk of wheat is stored in a flat-store with a perforated floor and a 3,000 m³/h aeration fan. Grain moisture is 13.6% wet basis and temperature is 30 °C after harvest. The fan delivers about 0.17 m³/min per tonne, which is within the recommended range for wheat. Night ambient air is 18 °C and 80% relative humidity; daytime air is 35 °C and 45% RH. The store has a roof but no ventilation at the ridge, and the top grain surface shows damp patches near the walls. The farmer wants to run the fan continuously for three days to cool the grain to 20 °C before covering it. Electricity is available only at night. Equilibrium moisture content of wheat at 18 °C and 80% RH is about 15.4%, and at 35 °C and 45% RH about 11.8%.
1. Using the equilibrium moisture content data, assess whether continuous night aeration will cool the grain or rewet it.
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The grain is at 13.6% moisture. Night air at 18 °C and 80% RH has an equilibrium moisture content of about 15.4% for wheat. Because 15.4% is above 13.6%, the grain will tend to absorb moisture from the air rather than dry. The air is cooler than the grain, so it will provide some cooling, but the simultaneous moisture uptake can raise the surface-layer moisture and create a rewetting front.
Day air at 35 °C and 45% RH has an EMC of about 11.8%, which is drying, but it is hotter than the grain and cannot cool the bulk. Running the fan continuously at night is therefore not a safe cooling strategy. The fan should run only when the ambient EMC is below the target grain moisture, or the grain should first be dried to a lower moisture so that the humid night air is less damaging.
2. Recommend an aeration schedule and an alternative if suitable cooling air is not available during the storage period.
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The fan should be controlled by an aeration controller or by manual psychrometric checks, not by a simple timer. Run the fan only when the ambient EMC for wheat is below about 13.0–13.5%, which may occur on clearer nights with RH below roughly 60% at 18 °C. If such conditions occur, aerate in several short passes rather than three continuous days, and monitor the top layer moisture.
If suitable cool, dry air does not occur, the alternative is to use a batch or continuous dryer to reduce grain moisture to about 12.5% before attempting cooling, or to use refrigerated/dehumidified aeration if the store is high-value. Do not cover the grain while it is at 30 °C and 13.6% moisture with damp patches present; that traps moisture and encourages mould and insects. Improve ridge ventilation and keep the grain surface level to reduce uneven airflow.
3. Explain the damp patches near the walls and the condensation risk, and state what immediate actions should be taken.
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The damp patches are consistent with moisture migration: warm grain in the centre drives moisture vapour toward cooler wall and roof surfaces, where it condenses. The store has no ridge ventilation, so humid air accumulates at the top and near the walls. Running the fan with 80% RH night air would add more moisture and worsen the problem.
Immediate actions:
- Stop continuous night aeration until the ambient EMC is below the grain moisture.
- Open or improve ridge ventilation to let humid air escape, while keeping rain out.
- Level the grain surface and check for roof leaks or wall cracks.
- Probe the damp patches and test moisture. If local moisture exceeds about 14.5%, remove or dry those patches separately; do not blend them into the bulk.
- Monitor grain temperature and moisture every few days. If heating or musty odours develop, dry or market the grain promptly rather than attempting further aeration with humid air.