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Additional Grain Drying Fans Not Normally Appropriate
To double the quantity of airflow for grain drying requires about five times the horsepower. To achieve an airflow rate of 0.5 cfm/bu. requires a 2.5 hp. fan, an airflow rate of 1.0 cfm/bu. requires a 10 hp. fan, and an airflow rate of 2.0 cfm/bu. requires a 45 hp. fan through a 21 foot diameter bin filled 17.5 feet deep with wheat.
A diminishing return for adding fans is quickly reached. A single 7.5 hp. fan provides an airflow rate of 0.86 cfm/bu, two fans provide an airflow rate of 1.20 cfm/bu (a 40% increase), three fans provide an airflow rate of 1.45 cfm/bu. (a 21% increase), four fans provide an airflow rate of 1.57 cfm/bu. (a 8% increase), and five fans provide an airflow rate of 1.62 cfm/bu. (a 3% increase).
The cost for drying wheat from 17% to 13% with $0.06/Kw-hr electricity is about $0.065/bu. with one fan, $0.094/bu. with two fans, $0.117/bu. with three fans, $0.143/bu. with four fans, and $0.174/bu. with five fans. The cost increases because the drying time is only reduced from 27 days to 14 days, while the horsepower increases from 7.5 to 37.5 hp. as additional fans are added.
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