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.

Why horsepower alone isn’t enough

Horsepower describes the motor’s rated power, not the airflow delivered through your grain. Use the fan manufacturer’s performance curve at the operating static pressure. Free-air CFM is the flow at little or no external resistance; a loaded bin is a different operating condition.

02 / TRY THE NUMBERS

How much air per bushel?

Enter total airflow delivered to the bin and the grain quantity. The result is a ratio—not a fan-selection or drying-time estimate.

Total delivered airflow (CFM)Grain quantity (bushels)

Use measured or properly estimated airflow under load. Do not substitute a free-air rating or simply add fan nameplate ratings.

DELIVERED AIRFLOW ÷ BUSHELS

0.30 CFM/bu

6,000 CFM ÷ 20,000 bu = 0.30 CFM/bu

Current grain quantity

Half as many bushels, same assumed CFM

This comparison holds delivered CFM constant to explain division. In practice, changing grain depth can change resistance and airflow too.

What airflow should I aim for?

These four comparison bands help you read your result. UMN’s general recommendations are 0.05–0.5 CFM/bu for aeration and 0.75–1.5 CFM/bu for natural-air drying of grains and oilseeds. The bands below organize those recommendations; they are not four separate university-prescribed targets.

Your CFM/bu	What it means	How to use it
Below 0.05	Very low airflow	Below the general aeration range in this guide. Check delivered airflow and the storage plan. Zero means no airflow.
0.05 to below 0.75	Aeration and the transition toward drying airflow	0.05–0.5: the published aeration range. Above 0.5 but below 0.75: between the guide’s aeration and drying ranges; do not assume drying capacity from the ratio alone.
0.75–1.5	General natural-air drying range	The guide’s drying range for grains and oilseeds. Match the rate to crop, starting moisture, weather, and available drying time.
Above 1.5	Higher-airflow drying designs	Above the general range, but appropriate for some crop-specific plans. Northern-region wet-corn examples extend to 3.0 CFM/bu under specified conditions. Higher airflow is not automatically better.

Cooling: UMN’s separate on-farm aeration guidance recommends at least 0.1 CFM/bu. Conditioning: use cooling guidance for temperature maintenance. Intentional moisture adjustment needs a crop- and site-specific plan; aeration airflow alone does not establish a rehydration target.

Sources: UMN: Selecting fans and determining airflow · On-farm aeration · Crop-specific corn drying.

03 / THE FAN MEETS THE BIN

Same fan. More resistance. Less airflow.

A fan can move plenty of air with nothing in its way. Put grain in front of it, and the fan must push against resistance. In this example, increasing that resistance reduces the air delivered—even though the fan stays the same.

How hard is it to push air through the bin? Medium

Move right for more resistance. Watch the green dot move left toward less airflow.

Blue: what the fan can deliverGold: pressure the bin needsGreen dot: what this fan delivers through this bin

So what is the “operating point”?

It is the green dot where the two lines meet. At that airflow, the pressure the fan can provide matches the pressure needed to push air through the bin.

Read down from the dot: how much air moves. Read across to the left: the pressure needed. On a real manufacturer’s chart, those readings would be CFM and inches of water column.

It describes the fan’s actual performance in that setup—not a setting you choose or a recommended airflow target.

Conceptual curves with relative units, not manufacturer data or a bin calculation. The slider does not correspond to a specific crop, grain depth, or measured pressure.

Grain depth

A longer path through grain adds resistance. Measuring the actual depth is more useful than relying on bin capacity alone.

Crop & fines

Kernel characteristics, packing, and fines affect the air paths. Concentrated fines can make distribution uneven.

The whole air path

Include floors, ducts, transitions, and exhaust openings when evaluating the system—not just the fan.

Axial, centrifugal, or multiple fans?

Fan types have different performance curves. Centrifugal designs are often considered for higher-pressure duties, while axial designs are common at lower pressures. Choose by the actual curve, airflow requirement, and installation—not the type name alone.

Multiple fans can be useful, but more units do not automatically give more uniform air or twice the airflow. Their arrangement and the shared system resistance determine the combined performance. Review the layout with the fan supplier or system designer.

04 / MATCH THE JOB

Cooling and drying are different jobs.

Cooling: move the temperature front

The main objective is to lower grain temperature. Choose airflow and operating periods to move cooling air through the grain, while watching moisture effects and grain condition.

How much airflow is enough?

Choose airflow for the crop, starting moisture, depth, weather, and available time. The University of Minnesota’s northern-region natural-air corn guidance spans 1–3 CFM/bu for different starting moistures under its specified fall-filling conditions. That is not a universal target for every crop or goal.

Read the full natural-air corn drying guidance and assumptions.

Airflow carries the air. EMC guides its use.

Fan Buddy helps you explore weather-based EMC and plan fan time. GrainSight combines temperature and EMC readings with automated fan control. Match that guidance and automation with airflow suited to your storage job.

Explore EMC Open Fan Buddy Explore GrainSight components

05 / 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.