Airflow resistance charts for grain storage management

Estimate the airflow rate in an existing bin

To determine the airflow (cfm) from an existing fan, install a static pressure gauge in the aeration ducting near the fan. With the bin full (or at the target grain level), turn on the fan and take the static pressure reading on the pressure gauge. The static pressure will change based on grain depth and grain type. Use the fan manufacture’s fan performance tables or curves (examples included below for Grain Guard’s in-line centrifugal fans), to estimate the airflow rate based on the static pressure reading.

Example: The static pressure reading is 4.6 in. H2O for a 5 hp in-line centrifugal grain guard fan (GGI-80511).

On the table at 4 in. H2O, the airflow is 4790 cfm and at 6 in. H2O it is 4050 cfm. From that it is estimated the airflow is likely around 4600 cfm.

The other method for estimating airflow rate uses the manufacturer supplied airflow-static pressure chart below. The point where a line, drawn vertically from 4.6 (in. H2O) on the horizontal axis, intersects the GGI-80511 (5 hp) fan curve, is the corresponding airflow rate for that fan and static pressure. Draw a line horizontally from the intersection point to the vertical axis to estimate the airflow rate (cfm), which for this example is approximately 4600 cfm (this is the same value that was estimated using the tables).

Determine the desired airflow rate for a new bin

Airflow resistance charts and fan performance tables/charts can be used to estimate the size of fan needed to achieve the desired airflow per volume (0.1 to 0.25 cfm/bu for aeration and 0.5 to 1 cfm/bu for natural air drying).

Example: 5000 bu of canola resulting in a grain depth of 20 ft. The desired airflow is at least 0.5 cfm/bu.

Calculate the airflow (cfm) required to achieve 0.5 cfm/bu (0.5 cfm/bu x 5000 bu = 2500 cfm). Estimate the airflow resistance using the chart shown below for canola and the same chart estimation method as in the previous example. The static pressure estimation at 20 ft of grain depth (horizontal axis) on the 0.5 cfm/bu curve is about 8 in. H2O. Use the fan tables or charts (above) to select the appropriate fan. At 8 in. H2O, the 5 hp in-line centrifugal fan (GGI-80511) is dead heading and only producing 1600 cfm, which is less than desired. The 7 hp in-line centrifugal fan (GGI-80711) however, is estimated to produce 4340 cfm which achieves the desired result with airflow of about 0.85 cfm/bu.

The above example illustrates how fan size selection may require a few iterations through the selection process. Theoretically, the 7 hp in-line centrifugal fan (GGI 80711) will generate 4340 cfm or 0.85 cfm/bu with 20 ft of canola. But the original assumption was for 0.5 cfm/bu which was used to estimate the airflow resistance. If the fan does indeed generate 4340 cfm or 0.85 cfm/bu, the resistance to airflow for 20 ft of canola now increases to approximately 15 in. H2O (which is off the chart for all sizes of the in-line centrifugal fan). But if you use the charts backwards and say you want 2500 cfm, the 7 hp GGI 80711 will need to operate at approximately 9 in. H2O. On the airflow resistance chart at 9 in H2O and 20 ft of canola, the resulting airflow rate will be slightly above 0.5 cfm/bu. So the 7 hp in-line centrifugal fan should be sufficient for a target airflow rate of at least 0.5 cfm/bu.

Individual fan models and types have unique airflow ratings, providing the ability to achieve a variety of airflow ranges for different depths of grain. For example, the chart below shows the relationship between static pressure (vertical axis) and airflow rate (horizontal axis) for four different types of fans (all 5 hp). At low static pressures (low grain depths and/or large seed sizes), the axial fan provides the highest airflow. However, at higher static pressures (above 7 in. H2O), only the in-line and high speed centrifugal fans provide any airflow. Fan type and size must be selected carefully based on expected static pressure, so make sure you are using the static pressure charts specifically for the type and size of fan you are investigating.

The airflow resistance charts for canola, wheat, barley, peas, lentils, flax and corn are provided on the next pages and can be used to estimate the expected static pressure based on grain depth, grain type, and target airflow rate (cfm/bu). Shedd’s curve and various equations were used to create the charts. These charts account for the airflow resistance of the grain only and do not account for added resistance due to chaff, fines, ductwork, or filling method (e.g.: grain spreader).