AIRFLOW & STORAGE

Right air.
Enough airflow.

EMC helps explain what the air can do. Airflow helps that air move through the grain. Understand the fan, the grain load, and the job before comparing systems.

01 / THE BASICS

Three numbers. One connected picture.

VOLUME

CFM

Cubic feet per minute: how much air the fan delivers at a particular operating pressure. A CFM rating only makes sense with its test conditions.

AIR PER GRAIN LOAD

CFM/bu

Delivered CFM divided by bushels in the bin. This puts airflow in context: the same air volume serves a smaller grain load differently.

RESISTANCE

Static pressure

The pressure the fan works against, commonly expressed in inches of water column (in. H₂O). Grain, floors, ducts, and the rest of the air path contribute resistance.

02 / START WITH YOUR FAN & BIN

Put the airflow in perspective.

Choose a fan you recognize and set the grain depth. First, we’ll divide its free-air rating by the bushels. Then we’ll see how the grain changes that starting number.

Choose your fan

ManufacturerHorsepower

Fan model / published configuration

How closely does it match?

Check the fan label for model, wheel size and RPM. Similar horsepower alone does not mean similar airflow.

Fans running into the same bin

Multiple-fan examples assume identical fans operating in parallel into a shared plenum.

Set the grain load

Crop

Crop data source →Bin diameter (feet)Level grain depth 35 ft

Diameter and grain depth share the same scale. The 40-ft reference wall and illustrative roof are not your bin’s measured height. Air enters below the grain.

Estimated grain quantity—Level-fill volume × 0.8 bu/ft³. Depth is measured above the air floor; peaked grain is not included.

FREE-AIR CFM—

BUSHELS—

STARTING CFM/bu—

This is the free-air comparison, before accounting for resistance. It is not the airflow through a filled bin. Keep going to find the estimate under load.

03 / PUT GRAIN IN THE WAY

The air needs pressure to get through.

Air travels through the spaces between kernels. Crop type and the length of that path change the pressure needed to move it. Here, compare the pressure required for the same CFM per bushel at the same depth.

Try another cropGrain depth 35 ft

How much pressure does each crop need?

Same grain depth. Moving right asks for more air per bushel; moving up means more pressure is needed. The chart shows up to 50 inches of water; higher requirements continue beyond the frame. Shared source curves appear once in the legend. Packing allowances differ by source, so comparisons are illustrative.

Where does Shedd’s relationship fit?

Shedd’s grain-resistance work connects airflow through a grain bed with pressure drop and grain depth. Airflow per floor area matters too: the same total CFM moves differently through a wide bin and a narrow one.

This example uses published crop-and-depth pressure tables from UMN, Oklahoma State and the Canadian Grain Commission / PAMI, interpolating between listed values. It does not apply a single generic Shedd coefficient to every crop. The next step combines that resistance with your selected fan’s actual performance table.

Supported airflow varies by crop: 0.05–2.0 CFM/bu for the UMN and Oklahoma State tables; 0.1–1.5 for canola; 0.1–2.6 for peas and lentils; and 0.1–2.5 for flax. This walkthrough uses depths of 8–40 ft. Gaps mean the source has no value or the result lies outside the supported range. Fines, packing, uneven grain and obstructions can change actual resistance.

Read the crop resistance tables →

04 / MATCH THE FAN TO THAT RESISTANCE

Here’s what moves through the bin.

The blue line shows what your selected fan can deliver. The gold line shows the pressure needed to push that much air through this grain load. Where they meet is the estimated airflow through the bin.

Compare another fanAir distribution

The duct allowance is a simplified estimate. Floors, transitions and vents must suit the installation. Comparisons use the same fan count and bin settings.

Selected fan(s)Bin resistanceAirflow through this bin

READ ACROSS

Pressure

The effort needed to push the air through this grain load, in inches of water.

READ DOWN

Delivered CFM

The total air moving through the bin each minute. This is the CFM we use next.

I have a static-pressure reading

Use a pressure reading from the plenum between the fan and grain, taken while the selected fan(s) are running. Enter the positive magnitude for a suction system.

Use my measured pressure for the final resultMeasured static pressure (inches of water)

This reads CFM directly from the selected fan curve. The crop/depth charts remain modeled comparisons; the final result is labeled separately.

View the selected fan’s published data

Manufacturer’s original table →

Single-fan performance; values between rows are interpolated.
Static pressure (in. water)	CFM

05 / BRING IT BACK TO EACH BUSHEL

The number that matters under load.

Use the delivered airflow from the previous step and divide it by the same grain quantity. Now the CFM/bu reflects the fan working against pressure.

ESTIMATED CFM/bu UNDER LOAD

—

Starting free-air comparison —

Delivered airflow —

Operating pressure —

Change the depth and watch both charts 35 ft

As pressure rises…

Selected fan(s), fixed current bushels. This isolates the effect of pressure on CFM/bu.

As the bin fills…

Same fan(s) and diameter. Depth changes both resistance and bushels. Lines stop wherever published data cannot support an estimate.

Compare crops at your current depth, with the same fan(s) and diameter. All values below use estimated resistance. Sources have different packing assumptions; shared curves give identical estimates, not independently tested crop differences.
Crop	Pressure	Delivered CFM	CFM/bu

What airflow should I aim for?

Start with delivered CFM/bu, then match the crop and job. These four bands summarize UMN’s broad guidance; NDSU’s crop-specific recommendations below add the moisture and seasonal context. The bands are a reading aid, not four universal design targets.

Your CFM/bu	What it means	How to use it
Below 0.05	Very low airflow	Below UMN’s general aeration range. Check delivered airflow and the storage plan.
0.05 to below 0.75	Aeration and some lower-airflow drying cases	UMN’s broad aeration range is 0.05–0.5. Drying at lower airflow needs a crop-specific plan; NDSU includes a 0.5 CFM/bu wheat example below.
0.75–1.5	General natural-air drying range	UMN’s broad range. NDSU’s spring examples call for at least 1.0 for corn and soybeans; 0.75 applies to the wheat, barley and oil-sunflower cases below.
Above 1.5	Higher-airflow drying designs	Use crop-specific guidance. UMN’s northern-region fall corn recommendations extend to 3.0 for specified starting moistures and filling conditions.

Broad ranges: UMN fan selection · Higher-airflow examples: UMN natural-air corn drying.

Match airflow to starting moisture

These are NDSU’s spring natural-air drying recommendations. The starting-moisture limits belong with the airflow values; they are not final storage-moisture targets.

Crop	Airflow (CFM/bu)	Starting moisture limit
Corn	At least 1.0	21%
Soybeans	At least 1.0	15–16%
Wheat	At least 0.75	17%
Barley	At least 0.75	16%
Oil sunflowers	0.75	15%

For corn, soybeans and oil sunflowers, NDSU starts spring drying around 40°F average outdoor temperature; for wheat, around 50°F. Grain condition and storage time already used also matter. NDSU: Proper Spring Grain Drying and Storage Critical (2020).

Wheat at lower moisture: NDSU also lists 0.5 CFM/bu for wheat up to 16%, 0.75 for 17%, and 1.0 for 18% in its wheat-drying guidance. Its general recommendation there is 0.75 CFM/bu with an initial limit of 17%; slower airflow takes longer. NDSU wheat-drying guidance (2019).

Cooling / aeration: NDSU’s durum guide, revised March 2026, describes 0.2 CFM/bu as typical for farm bins and estimates about 75 fan hours for a cooling pass at that rate. This is an example, not a universal minimum. NDSU durum production guide. UMN separately recommends at least 0.1 CFM/bu for on-farm aeration. UMN aeration guidance.

Conditioning: Temperature maintenance and intentional moisture addition are different jobs. Cooling airflow does not establish a universal reconditioning target; match moisture adjustment to the crop, EMC, temperature, available time and grain condition.

Airflow carries the air. EMC guides its use.

Use the airflow estimate alongside crop moisture, temperature, weather and available time. Fan Buddy helps plan useful fan hours; GrainSight combines temperature and EMC readings with automatic fan control.

Explore EMC Open Fan Buddy Explore GrainSight components

Calculation method & source limits

Level-fill bushels = π × diameter² ÷ 4 × grain depth × 0.8. Crop pressure is interpolated linearly across the published CFM/bu and depth tables. Fan CFM is interpolated linearly between published pressure readings. The estimate solves for the pressure where fan delivery and grain resistance agree.

For identical parallel fans, flows add at the same pressure before solving against the shared grain load. A second fan does not automatically double the delivered airflow. The duct option adds a constant 0.5-inch allowance as a simplified educational estimate.

These are uniform-grain, level-fill examples using published test data. They do not model uneven airflow, fines pockets, fan wear, air-density corrections or every installation loss. An exact fan selection still gives an estimate of bin performance. Values outside either source’s range are not extrapolated.

Crop sources are used as published, without an added packing multiplier. UMN supplies corn, barley/oats, soybeans/confection sunflowers and oil sunflowers. Oklahoma State’s shared wheat/sorghum table includes its stated packing allowances. Canadian Grain Commission / PAMI supplies clean-grain tables for canola, peas, lentils and flax; these exclude added fines and packing. This is not a standardized, same-condition trial across every crop. Use measured pressure with the matching fan curve to refine an existing-bin estimate.

Canadian Grain Commission / PAMI crop tables · Oklahoma State Table 9: wheat and sorghum. Only the grain-pressure table is used from Oklahoma State; its separate cone and duct calculations are not applied here.

Sources reviewed September 2026: Sukup Fans and Heaters · Brock centrifugal ratings · Brock axial ratings · UMN crop resistance and fan selection. Match catalog configurations and RPM to your equipment; inclusion is not a sales ranking.

06 / PRACTICAL CHECKS

Start with the grain and the air path.

Before filling

Identify the fan model and obtain its performance curve.
Check floors, ducts, transitions, and exhaust openings.
Plan for the crop, actual grain depth, and intended storage job.

After filling

Manage grain peaks and concentrated fines using appropriate equipment and procedures.
Record the grain quantity, moisture, and depth.
Determine airflow at the loaded operating condition.

During operation

Track temperature, moisture samples, fan time, and changing air conditions.
Follow progress through the grain rather than assuming the whole bin changes at once.
Adjust the plan as conditions and storage goals change.

When comparing upgrades

Compare delivered airflow at the required pressure, not just horsepower.
Evaluate grain depth and air distribution alongside fan capacity.
Check electrical requirements and system compatibility with your supplier.

Use safe procedures for inspections and grain handling; do not enter a bin to perform these learning exercises.

Common assumptions worth checking

“The same fan works for every bin.” Crop, depth, and the air path change its operating point.

“Low CFM/bu means air cannot reach the top.” Low airflow can slow progress; distribution also depends on resistance and the air path. The ratio alone does not describe every part of the bin.

“Warm air is always best.” Match temperature and EMC to the grain and goal. Drying potential and storage conditions need to be considered together.

“More airflow is always better.” Compare useful performance, energy use, and the job you need to complete.