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Is It Appropriate to Add Heat to Natural Air Drying?
Adding heat to a natural air drying system will increase the speed of drying slightly, but may cause over-drying and will increase the drying cost. An airflow rate of 0.75 cfm/bu will dry 17% moisture wheat to 13.3% in about 32 days with 70 degree, 60% relative humidity air. Warming the air ten degrees will reduce the drying time to about 25 days, but the wheat will over-dry to 10.7%. This is an over-drying cost of 16 cents/bu; 12 shrink + 4 heat.
The drying time is reduced to about 24 days by increasing the airflow rate to 1.0 cfm/bu. This requires the fan horsepower be increased from 5 to 10 hp, on a 21-foot diameter bin filled 17.5 feet deep with wheat. The drying cost increases by 3 cents/bu., from 5 to 8 cents/bu.
Adding supplemental heat is appropriate if it is cool and wet. For example, on average it is 62 degrees and 67% relative humidity during mid-August to mid-September in Northeast North Dakota. With these air conditions, wheat will only dry to about 14.6% moisture from 17% in about 34 days with an airflow rate of 0.75 cfm/bu. Warming the air by 5 degrees results in the wheat being dried to about 12.9% moisture in about 31 days. The additional cost for the heat is about 3 cents/bu at an electric rate of 3 cents/KWH.
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