Cooling Front & Run the Fans Just Long Enough

Run them long enough for one full cooling front to pass through the grain. The hours needed are about 15 ÷ (cfm per bushel): at 0.1 cfm/bu a front takes ~150 hours, at 0.2 cfm/bu ~75 hours, and at 0.3 cfm/bu ~50 hours. The tool computes this from your fan delivery and total bushels. Stop only after the front has exited the top of the bin.

It uses the published rule that a cooling front needs roughly 15 hours of fan run-time per unit of specific airflow (cfm per bushel). So hours per front = 15 ÷ cfm-per-bushel, where cfm-per-bushel is your fan's delivery divided by the bushels in the bin. The literature places the constant between 12 and 20 hours; 15 is the standard design figure from MWPS-13.

Switch them off once a complete cooling front has moved out the top of the grain — not before. If you stop mid-front you leave a warm, moisture-laden zone at the far end of the bin that can spoil. Confirm the front has exited by checking a temperature cable or feeling the exhaust air: when the exhaust matches the new cool air temperature, the front is through.

Cooling aeration typically uses 0.1 to 0.2 cfm per bushel — far less than the 1–2 cfm/bu used for drying. Below ~0.05 cfm/bu the front moves impractically slowly; above ~0.5 cfm/bu you are in drying-grade airflow. The calculator flags whether your setup is below, inside, or above the recommended cooling band.

Static pressure is the resistance the grain column puts up against the airflow, measured in inches of water (in-H₂O). It rises steeply with airflow rate and grain depth and is higher for fine grains like wheat or sorghum. If the required pressure exceeds what your fan can develop (a typical farm fan tops out near 4 in-H₂O), the fan can't deliver the airflow you assumed — so the cooling will be slower than planned.

It uses Shedd's airflow-resistance curves (ASAE D272), fitted as a power law: pressure per foot of grain ≈ a × (cfm/ft²)^1.5, where the coefficient a depends on the grain. Multiplying by your grain depth gives the total static pressure. The fan-adequacy bar compares that to a typical 4 in-H₂O farm fan.

Wheat kernels are smaller and pack more tightly, so they leave smaller air passages and resist airflow more than shelled corn. At the same airflow velocity wheat develops roughly 1.6× the static pressure of corn. Grain sorghum and paddy rice are higher still. That means for the same fan you get less airflow — and a slower cooling front — in fine grains.

Yes — 0.15 cfm/bu sits squarely in the recommended 0.1–0.2 cfm/bu cooling band. At that airflow one cooling front takes about 100 hours (15 ÷ 0.15). It is efficient on electricity while still cooling the whole bin within a few days of fan operation, which is the goal of aeration as opposed to drying.

Typically a fall cool-down (often two fronts to step the temperature down), one final winter front to hold grain at 30–40 °F, a gentle spring warm-up to avoid condensation, and one or two summer fronts on cool nights. The tool multiplies the hours-per-front by the cycles for your selected season so you can budget total fan hours and electricity.

Aeration moves a cooling front, not primarily a drying front, so moisture change is small if you aerate with air at a sensible humidity. The real risk is the opposite: stopping mid-front, or pulling warm humid air through cool grain, can deposit moisture and cause crusting. Aim to aerate when outside air is cooler than the grain and not saturated.

Cool grain stores longer because mold growth and insect activity slow sharply below about 50 °F and nearly stop near freezing. Cooling also evens out temperature differences that drive moisture migration and hot spots. A few hundred hours of fan time per season is cheap insurance against spoilage in a full bin.

Yes — the math depends only on total bushels, fan delivery, the depth the air travels through, and the floor (or inlet) area. Enter the depth and area for your structure. The cooling-front hours come from cfm/bushel; the static pressure comes from the depth and airflow velocity through your floor area, whatever the shape.