Cooling Front & Run the Fans Just Long Enough
Cools shelled corn
A cooling front needs roughly 15 hours of fan run-time per cfm-per-bushel to travel through the whole grain mass. Enter your fan, bushels, depth and floor area to get the hours per front, total run-time, static pressure and fan adequacy — and know exactly when to switch the fans off.
Set the aeration job
Needs 58.3" H₂O; a typical farm fan delivers ≈ 4". Short by 54.3".
Next: the grain column needs 58.3" of static pressure — more than a typical 4" fan. Use a higher-pressure fan, lower the grain depth, or accept reduced airflow.
Cooling-front hours ≈ 15 ÷ (cfm/bushel); static pressure from Shedd's airflow-resistance curves × grain depth. Front speed scales with airflow; never stop a cycle mid-front or you leave a warm, mold-prone zone at the far end of the bin.
Grain aeration — key facts
- Hours per front
- ≈ 15 ÷ (cfm per bushel)
- Cooling airflow band
- 0.1 – 0.2 cfm/bu
- Drying airflow
- 1 – 2 cfm/bu (much higher)
- Static pressure
- a × (cfm/ft²)^1.5 × depth (Shedd)
- Typical farm fan
- ≈ 4 in-H₂O maximum
- 0.15 cfm/bu front
- ≈ 100 hours
- Stop the fans when
- the front has exited the top
- Winter hold target
- 30 – 40 °F (insects & mold stall)
- Source
- MWPS-13, ASAE D272 (Shedd's data)
- Privacy
- Runs in your browser; nothing uploaded
Grain airflow resistance & cooling-front hours
Static pressure per foot of grain rises with airflow velocity and is higher for fine grains. Hours per cooling front at the standard cooling airflows follow 15 ÷ cfm-per-bushel.
| Grain | Test weight (lb/bu) | Resistance coeff. a | dP/ft @ 5 cfm/ft² (in-H₂O) | Airflow note |
|---|---|---|---|---|
| Shelled corn | 56 | 0.0573 | 0.64 | Medium resistance; fines raise it sharply. |
| Wheat | 60 | 0.0950 | 1.06 | Higher resistance than corn; small kernels pack. |
| Soybeans | 60 | 0.0420 | 0.47 | Low resistance; large smooth seed. |
| Grain sorghum | 56 | 0.1100 | 1.23 | High resistance; very fine grain. |
| Oats | 32 | 0.0610 | 0.68 | Light, hulled; moderate resistance. |
| Barley | 48 | 0.0780 | 0.87 | Awned kernels; moderate-high resistance. |
| Sunflower | 28 | 0.0330 | 0.37 | Very low resistance; bulky oil-seed. |
| Rice (paddy) | 45 | 0.0890 | 1.00 | High resistance; rough hull, fines. |
Seasonal cooling targets
| Season | Target grain temp | Cooling cycles | Goal |
|---|---|---|---|
| Fall cool-down | 50 °F | 2 | First cooling after harvest — knock the field heat down to ~50 °F. |
| Winter holding | 35 °F | 1 | Final winter front — hold grain at 30–40 °F to stall insects & mold. |
| Spring warm-up | 50 °F | 1 | Gentle warm-up to avoid spring condensation as outside air heats. |
| Summer holding | 60 °F | 2 | Keep summer grain below 60 °F; cool on cool nights only. |
What a cooling front is — and why timing the fans matters
When you start an aeration fan, the air entering the bin reaches the grain's temperature very quickly, but the cooling front — the boundary where cool air has actually changed the grain temperature — moves through the mass slowly, like a slow tide. It advances in the direction of airflow at a rate set almost entirely by the specific airflow. The widely used rule of thumb is about 15 hours of run-time per cfm-per-bushel for one full front. Run less than that and the front never reaches the far end, leaving warm grain that can grow mold and breed insects.
This calculator turns your fan size and bin into the numbers that actually matter: the airflow in cfm per bushel, the hours per cooling front, the total run-time over the season's cycles, and the static pressure the grain develops at that airflow (from Shedd's resistance curves) checked against what a typical fan can deliver. Most hub airflow tools stop at cfm/bushel; this one tells you how long the fans must run and the moment you can switch them off.
Know when to stop
Run the full front, then switch off — no wasted electricity, no warm zone.
Real Shedd pressure
Static pressure from published airflow-resistance curves, by grain.
Catch an undersized fan
The adequacy bar warns when the depth exceeds your fan's pressure.
Plan the season
Total fan hours across fall, winter, spring and summer cooling cycles.
Frequently Asked Questions
How many hours should I run aeration fans to cool a bin?+
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.
How is the cooling-front time calculated?+
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.
When can I switch the aeration fans off?+
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.
What airflow (cfm per bushel) do I need for cooling?+
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.
What is static pressure and why does it matter?+
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.
How does the tool work out static pressure?+
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.
Why is wheat harder to aerate than corn?+
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.
Is 0.15 cfm per bushel good for cooling grain?+
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.
How many cooling cycles do I run in a season?+
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.
Does running fans add moisture to the grain?+
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.
Why cool stored grain at all?+
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.
Can I use this for a flat store or a bag, not a round 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.