What Size Fan Do I Need for My Grain Bin?
The right grain-bin fan size is not determined by bin diameter or horsepower alone. You need to know how much air you want to move, usually expressed as CFM per bushel, and then select a fan capable of delivering that airflow against the static pressure created by the grain and aeration system.
In plain English:
Figure out how much air the bin needs first. Then find a fan that can actually deliver that air once the bin is full.
That's the part that gets missed.
A fan capable of moving plenty of air with nothing in front of it can move considerably less air once it's pushing through a full bin of grain.
Here's how to size it properly.
Step 1: Decide What You Want the Grain Bin Fan to Do
Before talking horsepower, ask why you're running the fan.
Are you:
- cooling stored grain;
- maintaining more uniform grain temperatures;
- aerating grain;
- natural-air drying; or
- trying to accomplish more than one of those jobs?
The answer matters because different jobs require different amounts of airflow.
The University of Minnesota Extension gives a broad recommendation of 0.05 to 0.5 CFM per bushel for aeration of stored grains and oilseeds, compared with 0.75 to 1.5 CFM per bushel for natural-air drying. [extension.umn.edu]
Those are ranges, not a promise that a particular number is correct for every bin.
Crop, moisture, grain depth, weather, storage goals and the rest of the system all matter.
Benny's tip: If somebody asks how many horsepower you need before asking what you're trying to accomplish, you're starting at the wrong end of the problem.
Step 2: Determine How Many CFM You Need
Once you have an airflow target, the basic math is simple:
Bushels × CFM per bushel = required total CFM
For example, suppose you have 20,000 bushels of grain and the system is being designed around 0.2 CFM/bu.
20,000 × 0.2 = 4,000 CFM
Your airflow target would therefore be 4,000 CFM.
But this is where an important distinction comes in.
You don't just need a fan advertised as capable of moving 4,000 CFM.
You need one capable of delivering your required airflow at the static pressure your system will create.
That's a much different question.
Step 3: Account for Grain Depth
A bushel isn't the only thing the fan has to overcome.
How those bushels are stacked matters.
Air has to travel through small spaces between kernels. The friction created as the air moves through that grain produces resistance. The fan has to develop enough pressure to overcome that resistance. [extension.umn.edu]
Generally, deeper grain creates a tougher job for the fan.
This is why you can't reliably say:
“A 30-foot bin needs a 5 HP fan.”
Two 30-foot-diameter bins could have:
- different grain depths;
- different crops;
- different airflow targets;
- different aeration floors;
- different transitions or duct arrangements; and
- very different static-pressure requirements.
They may therefore need different fans.
Step 4: Understand Static Pressure
Static pressure sounds complicated, but the basic idea isn't.
Static pressure is the resistance your fan is pushing against.
If resistance goes up, a fan generally moves less air.
That's why fan manufacturers publish performance curves or tables showing how much airflow an individual fan delivers at different pressures.
This is also one reason grain depth is so important.
The University of Minnesota Fan Selection for Grain Bins tool demonstrates the relationship directly by estimating static pressure from variables including crop, bin diameter, floor type, grain depth and desired CFM/bu, then comparing that requirement with actual fan data. [bbefans.cf...ns.umn.edu]
That's much more useful than selecting from horsepower alone.
Step 5: Look at the Fan Curve
This is where we go from estimating airflow to actually picking equipment.
Suppose your calculations say your bin needs:
4,000 CFM
Now suppose your expected static pressure is:
3 inches of water
The question isn't:
Which fan says 4,000 CFM on the box?
It's:
Which fan delivers at least the required airflow at 3 inches of static pressure?
That's where the manufacturer's fan-performance data comes in.
The operating point matters because airflow changes with resistance. Your full-bin operating condition is what matters, not simply how much air a fan can move under little or no resistance.
So, How Many Horsepower Do I Need?
Horsepower comes after airflow and static pressure, not before them.
That's worth repeating.
Don't size a grain-bin fan from horsepower alone.
A larger motor can provide the power necessary to do more work, but horsepower alone does not tell you exactly how much air a specific fan will move through your grain.
Fan design matters too.
An axial fan and centrifugal fan with similar motor horsepower can have different performance characteristics.
That's why our lesson on axial versus centrifugal grain-bin fans belongs right next to this one in Benny's Schoolhouse.
First determine the job.
Then airflow.
Then static pressure.
Then look for the fan whose performance meets those requirements.
That's when horsepower becomes meaningful.
Does Bin Diameter Matter?
Yes, but not by itself.
A wider bin generally holds more grain at a given depth, which affects the total amount of airflow needed.
But diameter alone still doesn't tell us what fan should go on it.
Consider two farmers with the same diameter bin.
One might use it primarily for aerating already-dry grain.
The other might expect much greater airflow for natural-air drying.
Even though their bins have the same diameter, their required CFM can be substantially different because they're asking their fans to do different jobs. Broad University of Minnesota guidance illustrates the difference in airflow requirements between aeration and natural-air drying. [extension.umn.edu]
Does the Type of Grain Matter?
Yes.
Different crops create different resistance to airflow, so crop is one of the inputs used when estimating static pressure and selecting a fan. [extension.umn.edu], [bbefans.cf...ns.umn.edu]
Fines and grain condition can also affect the resistance encountered in an actual bin.
So if you're calling A&S about a fan, don't just say:
“I've got a 30-foot bin.”
Tell us what's going in it.
That gives us useful information.
What About Axial vs. Centrifugal Fans?
Once you understand the airflow and pressure requirement, you can start narrowing down fan type.
As a general rule:
Axial fans are commonly used where high airflow is needed at relatively lower resistance.
Centrifugal fans become increasingly useful when you're dealing with higher static-pressure requirements.
But don't use that rule of thumb as a substitute for checking actual fan performance.
A&S also carries inline centrifugal options. Current A&S inline centrifugal offerings include models intended for grain aeration and natural-air drying where higher static pressure is needed. [asagsales.com]
For larger and deeper-bin applications, A&S also currently lists low-speed centrifugal fans from Caldwell and AGI in 5 to 25 HP configurations. [asagsales.com]
The right answer still comes back to the performance curve.
What About Single-Phase vs. Three-Phase Power?
Don't wait until after selecting the fan to check your electrical service.
A&S's current grain-storage lineup includes fan configurations using both single-phase and three-phase power. [asagsales.com]
Available service can limit your equipment choices, particularly as horsepower increases.
Before ordering, know what power is actually available at the bin.
Electrical installation and modifications should be performed according to applicable requirements and manufacturer instructions by qualified personnel.
What Information Should I Have When I Call A&S?
This is the easy part.
If you want help narrowing down a fan, gather:
1. Bin diameter
How wide is the bin?
2. Grain depth
How deep will the grain actually be?
3. Crop
Corn, soybeans, wheat, barley, or something else?
4. Bushels
What's the approximate stored capacity or operating volume?
5. What you're trying to accomplish
Are we aerating, cooling, conditioning, natural-air drying, or doing something else?
6. Aeration system
Full floor, ducts, or another configuration?
7. Electrical service
Single phase or three phase, plus the available voltage/service information.
8. Existing equipment
If you're replacing a fan, get the manufacturer and model number. Photos of the fan, transition and nameplate can also help identify what you currently have.
That information gets us much closer to answering:
“What fan do I actually need?”
instead of guessing from horsepower.
A Quick Fan-Sizing Example
Let's put the process together.
Suppose we're discussing a bin holding 20,000 bushels.
For illustration, we'll use a desired airflow of 0.2 CFM/bu.
Calculate required airflow
20,000 bushels × 0.2 CFM/bu = 4,000 CFM
So we need approximately 4,000 CFM for that selected planning target.
Then ask about resistance
Now we need:
- crop;
- grain depth;
- floor/aeration system; and
- other factors affecting static pressure.
Then check actual fans
Once we have an estimated static pressure, we compare candidate fans at that pressure.
Maybe one fan delivers enough air.
Maybe another doesn't.
Maybe moving to a different fan design makes more sense.
That's fan sizing.
The answer did not come from saying, “20,000 bushels equals X horsepower.”
And that's the main lesson.
The Bottom Line
If you're asking,“What size fan do I need for my grain bin?”, don't start with horsepower.
Start with:
1. What is the fan supposed to accomplish?
2. How many bushels are we serving?
3. How many CFM per bushel does the application require?
4. What static pressure will the fan operate against?
5. Which fan delivers the required CFM at that pressure?
2. How many bushels are we serving?
3. How many CFM per bushel does the application require?
4. What static pressure will the fan operate against?
5. Which fan delivers the required CFM at that pressure?
Then check horsepower, fan type, electrical service and physical compatibility.
That's how you move from guessing at fan size to selecting a fan that actually fits the job.
Need Help Sizing a Grain Bin Fan?
A&S works with grain-bin fans and aeration equipment for operations across Minnesota, North Dakota and South Dakota. A&S Ag Sales currently carries fans along with transitions, electric motors and other grain-storage equipment. [asagsales.com]
Send A&S your bin diameter, grain depth, crop, intended use and electrical-service information. We'll help narrow down the fan options instead of making you guess from horsepower.
Frequently Asked Questions
How many CFM does my grain bin fan need?
Start with the number of bushels and the desired airflow in CFM/bu. Multiply them together to determine total required airflow. The correct CFM/bu depends on what you're trying to accomplish. University of Minnesota Extension lists broad ranges of 0.05 to 0.5 CFM/bu for aeration and 0.75 to 1.5 CFM/bu for natural-air drying of grains and oilseeds. [extension.umn.edu]
What horsepower fan do I need for my grain bin?
You can't reliably determine fan size from horsepower alone. Establish your required airflow and expected static pressure first, then check manufacturer performance data to see which fan actually delivers the required CFM under those conditions.
Does a deeper grain bin need a bigger fan?
Deeper grain generally increases airflow resistance, so the fan must be evaluated at the resulting static pressure. Depth alone doesn't determine the fan because crop, desired airflow, bin configuration and other factors also affect the system. [extension.umn.edu]
Can a grain-bin fan be too big?
More airflow isn't automatically better. University of Minnesota Extension notes that greater airflow requires larger fans and increases costs, while insufficient airflow can create crop-quality problems. The goal is to size the system for its intended job. [extension.umn.edu]
Do I need an axial or centrifugal fan?
It depends primarily on required airflow and static pressure. Once those are known, compare appropriate axial, inline centrifugal and centrifugal options using their actual performance data.
Is free-air CFM the number I should use?
No. For a grain bin, the important number is the airflow the fan delivers while operating against the system's static pressure, because the grain and aeration system resist airflow. [extension.umn.edu]