Cooling and Drying 18% Corn With an Undersized Fan
A 30-ft-diameter, 24-ft-eave bin holds 12,000 bu of 18% corn harvested at 80 °F. The only aeration is a 15-hp centrifugal fan rated 6,000 cfm at 4 in. w.g., ducted to a full perforated floor. With 24 ft of corn over the floor the system static pressure is about 6.5 in. w.g., and the fan curve gives roughly 3,500 cfm at that point. The owner wants the corn held until March and sold at 15%. The October forecast is 45 °F nights and 65 °F days at about 70% RH, and the fan will run continuously unless you direct otherwise. Power costs 11 ¢/kWh.
1. Is this fan capable of natural-air drying the bin to 15%? Support your answer with airflow per bushel, static-pressure and equilibrium-moisture reasoning.
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Airflow. 3,500 cfm ÷ 12,000 bu = 0.29 cfm/bu. Natural-air drying of corn at 18% in cool fall weather needs roughly 1.0–1.5 cfm/bu. The system is short by a factor of 3–5, and the cause is depth: 24 ft of 18% corn with fines generates 6.5 in. w.g., which walks the fan far down its curve from the 6,000-cfm rating.
Equilibrium moisture. At 45 °F and 70% RH the EMC of corn is roughly 16% - noticeably higher than the ~14.7% you would get from the same air at 77 °F. Air at those conditions physically cannot pull the kernel below about 16%, no matter how long the fan runs.
Conclusion. No. Two independent limits bind: not enough air, and air that is too cool/wet to reach 15%. Realistic options, in rough order of cost:
- Move the grain to a shallower depth or a drying floor to get near 1 cfm/bu and add 5–10 °F of supplemental heat to drop the EMC below 15%.
- Batch-dry a portion and blend.
- Sell early at a moisture discount rather than risk a spoiled bin.
Airflow alone will not get this corn to a spring-marketable 15%.
2. The owner intends to run the fan around the clock. What grain temperature can actually be reached, and how should the fan be scheduled?
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Fan heat is the ceiling. A 15-hp fan puts essentially all of its shaft power into the air:
15 hp × 2,545 BTU/hp·h = 38,200 BTU/h
Air mass at the actual operating point: 3,500 cfm × 0.075 lb/ft³ × 60 = 15,750 lb/h.
Temperature rise through the fan: 38,200 ÷ (15,750 × 0.24) = 10.1 °F.
So 45 °F night air enters the grain at about 55 °F. Because the fan heats the air before it ever touches the corn, the grain asymptotically approaches ~55–57 °F, not the ambient 45 °F. Running at 65 °F in the afternoon puts 75 °F air into the bin - warmer than the grain - and you would rewarm the mass and feed mold.
Cooling load and schedule. Sensible heat to remove from 80 °F to 55 °F: 12,000 bu × 56 lb/bu × 0.4 BTU/lb·°F × 25 °F ≈ 6.7 million BTU. At an average grain-to-air ΔT near the front of ~20 °F, the air carries off about 75,000 BTU/h, so expect roughly 100–200 hours of fan time - several days of continuous running, longer in practice because the last zone of the bin cools slowly.
Recommendation. Run only at night and early morning, when ambient is lowest, and stop during warm afternoons. Accept that this achieves cooling, not drying - 18% corn held at ~55 °F will still degrade over the five months to March, so the real fix remains mechanical drying or early sale.
3. The owner proposes blowing 65 °F afternoon air because it "feels drier" than the damp night air at 45 °F. Evaluate that reasoning.
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The reasoning is half right and the conclusion is wrong. At 65 °F/70% RH the air does carry less absolute moisture per pound than 45 °F/70% RH, so its EMC is slightly lower and its drying potential per pound is real. But three effects swamp that:
- Cooling is the first priority at 18%. The bin's immediate risk is mold growth, and mold rate is dominated by grain temperature. Cooling from 80 °F toward 55 °F buys far more safe storage time than a marginal moisture gain.
- The fan adds ~10 °F. Afternoon air enters the grain at ~75 °F, which is above the current grain temperature in the upper zone. Blowing warm air rewarms grain that was previously cooled and can move a condensation front through the mass.
- The airflow is already inadequate. At 0.29 cfm/bu the air is not moving through the grain fast enough to dry it front-to-back; adding 5% more drying potential per pound changes nothing.
Correct answer: schedule on temperature, not on perceived dryness - night and early morning only - and treat drying as a separate problem requiring more airflow or heat. If the owner wants a defensible rule, use a target: run when ambient is at least 10–15 °F below the current grain temperature, and never when ambient plus the 10 °F fan rise exceeds the grain temperature.