Selecting fans and determining airflow for grain bins

Total airflow provided by a fan is usually expressed as cubic feet of air per minute (cfm).

Recommendations for drying or aerating a particular crop are given as airflow per unit of crop being served by the fan. For example, cfm per bushel (cfm/bu) is used for drying or aerating grains and oilseeds.

We list typical airflow recommendations in Table 1. Select fans that deliver airflow within the ranges given in the table, which are shown as cubic feet of air per minute per bushel (cfm/bu).

Note that greater airflows require larger fans and lead to greater costs, while lower airflows could result in unacceptable crop quality.

Airflow resistance and fan pressure are usually expressed in inches of water column (inches of water, or inches H2O). This term comes from gauges called u-tube manometers that are sometimes used to measure pressure (Figure 1).

You can make a u-tube manometer by fastening a clear plastic tube and a ruler to a board. Then pour some water, or water plus a small amount of antifreeze, into the tube.

Since manometers measure pressure relative to atmospheric pressure, leave one end of the tube open to the atmosphere. Attach the other end to the duct or plenum where you want to measure pressure.

Understanding the measurement

When a fan generates pressure, it forces water in the tube to move in the direction of lower pressure. The height difference of the water levels on the two sides of the tube, measured in inches, is the fan static pressure, inches of water (in. water).

In negative pressure or suction systems, pressure between the crop and the fan is less than atmospheric pressure and water in the manometer tube moves toward the fan. In positive pressure systems, water moves away from the fan.

You can buy dial-type pressure gauges that operate on a different principle but are calibrated to give readings in inches of water.

The crops’ airflow resistance and the fan pressure required to overcome it depend on how fast the air moves and how long and narrow the paths are. For grains and oilseeds, these factors are a function of the particular crop (size and shape of seeds), crop depth and airflow rate (cfm/bu) you're trying to provide.

Below, you’ll find the expected static pressure for barley and oats (Table 2), shelled corn (Table 3), soybeans and confectionary sunflowers (Table 4), oil-type sunflowers (Table 5) and wheat and sorghum (Table 6).

As you can see, at a given airflow rate, crop depth has a large effect on static pressure. Static pressure, in turn, greatly affects fan power requirements.

Short, large-diameter bins are recommended for natural-air grain drying because static pressure and required fan size are smaller than they would be in tall, narrow bins. Even though short bins cost more to install than tall ones with the same grain capacity, total drying costs are less because smaller fans use less electricity.

The full perforated floors used in grain bins generally have negligible resistance to airflow.

Airflow resistance of bin floors isn't significant unless open area is less than about 7 percent. Most commercially available floors have more than 10 percent open area.

Air supply ducts, tunnels and perforated air distribution ducts offer greater resistance to airflow than full perforated floors. This resistance can be quite large if ducts are too small or too long.

Use ducts that are large enough that air velocity is less than about 1,500 feet per minute. To calculate velocity, divide duct airflow in cfm by duct cross sectional area in square feet.

Also, try to keep duct length less than 100 feet. Unless you have better information, use 0.5 inches of water as an estimate of airflow resistance for duct systems.

Be aware that corrugated plastic ducts designed for air distribution have only 1 to 3 percent open area, and ordinary plastic tile designed for field drainage has less than 1 percent open area. Because plastic ducts have so little area for air exit, its airflow resistance can exceed 0.5 inches of water.

When using outdoor air to ventilate a bin or building, you need to provide adequately sized openings for air to move in and out of the structure. If openings are too small, they restrict airflow and increase fan pressure requirements.

Provide at least one square foot of inlet area per 1,000 cfm and an equal exhaust area, and make sure these vents or doors are open anytime the fan is operating.

Because of the way fan impellers (blades or rotors) are designed, the amount of air they can move decreases as the pressure they are working against increases. The airflow vs. pressure information for a particular fan is called the fan performance data.

Performance depends on the size, shape and speed of the impeller, and the size of the motor driving it. Performance widely differs among brands and models, even for fans with a motor of the same size.

 Access to fan performance data is essential for selecting fans and determining airflow provided by existing fans. Most manufacturers sell fans that have been tested using procedures specified by the Air Movement and Control Association International, Inc. (AMCA).

Manufacturers can provide you with performance data in the form of tables or graphs. Avoid fans that do not have AMCA data available.

Tables 7 and 8 exemplify the type of data you need. Figure 2 is a graphical presentation of the data for two fans from Table 7 and 8 that have motors of the same size. Note how much performance differs between the two fans.

Axial-flow fan performance

Centrifugal fan performance

Most fans can be categorized as either axial-flow or centrifugal (see Figure 3).

Axial-flow fans are sometimes called propeller fans, although that's actually just one type of axial-flow fan. Air moves in a straight line through axial-flow fans parallel to the axis or impeller shaft. The impeller has a number of blades attached to a central hub.

Centrifugal fans are sometimes called blowers or squirrel cage fans. The impeller is a wheel that consists of two rings with a number of blades attaching them.

Air enters one or both ends of the impeller parallel to the shaft and exits one side perpendicular to the shaft. The blades can be straight, slanted in the direction of airflow (forward-curved) or slanted opposite the airflow direction (backward-curved or backward-inclined).

These axial-flow type fans have between two and about seven long blades attached to a small hub. Fan diameter is usually large relative to the fan's length or thickness.

Some propeller fans are called panel fans and are designed for mounting in a wall or plenum divider. Some are belt-driven and some have the impeller hub directly attached to the motor shaft (direct-driven).

Propeller fans normally can't generate more than about 2 inches of water pressure.

They are most commonly used for potato ventilation, forced-air produce cooling, hay drying, exhausting air from attics or overhead spaces or general air circulation. They are seldom used for grain drying or aeration.

These axial-flow fans have a barrel-shaped housing and an impeller that has a large hub with a number of short blades attached to it.

They are generally direct-driven and the motor is cooled by the airstream. In positive pressure systems, the airstream captures the waste heat given off by the motor.

Vane-axial fans have guide vanes inside the fan housing to help reduce air turbulence.

Tube-axial and vane-axial fans are the most common types used for grain drying and aeration.

They are relatively inexpensive and fairly efficient when static pressure is less than about 4 inches of water. The main disadvantage is that these fans are very noisy.

The centrifugal fans used for crop drying and storage generally have backward-curved or backward-inclined blades. They are expensive, quiet and usually the most efficient fan type when static pressure exceeds about 4 inches of water.

The motor on centrifugal fans is normally outside the airstream. You need to install a special housing around the motor if you want to capture the heat it gives off.

Forced-air heating and ventilating systems often use centrifugal fans that have forward-curved blades.

If you operate these fans outside certain pressure ranges, the motors can overload and burnout. This makes them unsuitable for many crop drying and storage applications.

These fans have axial airflow, but use a centrifugal-type impeller. Price and operating characteristics are between those of backward-inclined centrifugal and tube-axial fans.

It’s sometimes necessary or desirable to install more than one fan to provide air to a common plenum or supply manifold for a duct system. Fans can be arranged in parallel or series (Figure 4).

Reasons for using multiple fans include:

• Total airflow, pressure or power requirements exceed the capabilities of the largest fan available from your dealer.
• The starting current for a single large fan motor is greater than the electrical system can handle. The maximum starting current is lower if several small fans are started one at a time.
• You want the ability to turn off some fans and operate with a lower airflow when conditions allow.
• Air distribution is sometimes more uniform when you use several small fans instead of one large one.

Parallel arrangement means fans are installed side-by-side or at several points along a manifold or plenum.

The most common applications have large total airflow requirements, but moderate pressure. When installing fans in parallel, they all face the same pressure.

Estimate the total airflow by adding the airflow provided by each fan at the expected pressure.

Series arrangement, in which fans are fastened in line or end-to-end, is not used very often. When used, it generally involves tube-axial or vane-axial fans in situations where pressure is relatively high, such as in deep grain bins.

Series arrangement is seldom used with centrifugal fans, and rarely connect more than two axial-flow fans in series. When arranging fans in series, each fan handles the same airflow.

Estimate total pressure by adding the pressure developed by each fan at the expected airflow.

1. Measure static pressure in the duct or plenum between the fan and the crop (Figure 1).
2. Drill a small hole (1/8 inch should be adequate) in the wall of the duct or plenum.
3. Press a tube from one side of a pressure gauge or u-tube manometer against the hole.
4. Take the pressure reading and use its absolute value (this means assume the reading is positive even if it's a negative pressure system) to determine airflow.

Use the AMCA performance data for that model fan at that pressure. To get airflow rate (cfm/bu, for example), divide the airflow from the performance table or graph by the amount of crop served by the fan.

For example, suppose fan #4 from Table 7 (see the fan performance data section above) is drying 10 tons of hay and the static pressure reading in the attached duct is 1 inch of water. The fan performance data in Table 7 shows that fan #4 provides 2,775 cubic feet of air per minute (cfm) against a pressure of 1 inch. Airflow per ton is 2,775 cfm / 10 tons = about 278 cfm per ton. This value is within the recommended range for hay drying given in Table 1 (see airflow recommendations by fan purpose section above).

Because airflow resistance and static pressure vary with the type of crop, crop depth, amount of fines present and the way the crop is piled, you need to repeat the above procedure and determine a new airflow anytime conditions change.

The first step in selecting a fan is to determine the total airflow it must provide.

Use the airflow rates in Table 1 as a guide (see the airflow recommendations by fan purpose section above). Choose an airflow rate, estimate the total quantity of crop to be served by the fan and then multiply the airflow rate by crop quantity to get total airflow requirement.

For example, if you want to supply 1 cubic feet of air per minute per bushel (cfm/bu) to natural-air dry corn in a 27-foot-diameter by 16-foot-deep bin with a full perforated floor, calculate airflow as follows:

Bin capacity = (pi) / 4 x (diameter)² x depth x 0.8 bu/cubic ft.
= 0.785 x 27 ft. x 27 ft. x 16 ft. x 0.8 bu/cubic ft.
= 7,325 bushels
Total airflow = 1 cfm/bu x 7,325 bushels = 7,325 cfm

The next step in selecting a fan is to estimate the pressure the fan will be operating against. For grains and oilseeds, use the desired airflow rate and expected crop depth and read the appropriate pressure value from Tables 2 through 6.

Remember to add 0.5 inches to the value from the table if you distribute air through a duct system. For hay, potatoes or other produce, use 1 inches of water as a pressure estimate unless a better number is available.

Continuing our example, Table 3 (see the shelled corn section above) indicates that the expected pressure for 16 feet of corn and an airflow rate of 1 cfm/bu is 2.4 inches of water.

Fans are usually described by the horsepower (hp) rating of the motor used to drive the impeller. It's helpful when selecting fans to first estimate the power requirement so you know where to start looking in the manufacturer's catalog.

Fan motor size depends on the total airflow being delivered, the pressure developed and the impeller's efficiency. Impeller efficiencies generally range from 40 to 65 percent. If we assume an average value of 60 percent, we can use the following formula to estimate the fan power requirement.

Fan power (hp) = airflow (cfm) x static pressure (in. water) / 3,814

In our example, fan power = 7,325 cfm x 2.4 in. water / 3,814 = 4.6 hp

Purchase cost and noise during operation can be important factors when selecting a fan, but the most critical factor is whether the fan can provide enough airflow at the expected operating pressure.

Start by looking at performance data for a fan with a motor rated just under the power value you calculated. If this fan provides more than enough airflow, look at the next size smaller. If your first pick is too small, try the next size larger.

If we use the fans listed in Table 7 (see the fan performance data section above) to select a fan for our example problem, we see that fan #7 (a 5.0-hp axial flow fan) comes closest to meeting our needs. Fans #6 and #10 wouldn't provide enough airflow at 2.4 inches of water and fans #8 and #11 would provide much more airflow than needed.

Sometimes fans produced by one manufacturer won't meet your needs and you'll have to look at another manufacturer's fans. Or, if you’re having trouble finding a fan that’s big enough, you might consider using several smaller fans (See the above section on multiple fans).