Duct airflow calculator
Aeration Fan Calculator
OSU Biosystems Engineering

This calculator provides an estimate of static pressure of air as it flows through a grain mass in a grain aeration or closed-loop fumigation system.    For more information on grain aeration consult Oklahoma Cooperative Extension Fact Sheets: BAE1100, BAE1101, BAE1102 and BAE1103.

Parameters specific to the grain storage environment are used by this calculator to determine the power required to move air through a grain mass.  These parameters are discussed below in the order that they appear on the calculator web page:

 

Enter the height of the grain in the bin in feet.  Power increases linearly with grain depth.   This model limits the height between 2 and 200 feet.

Grainbin

Enter the bin diameter in feet.  This model assumes a round bin.  If the bin is not round, enter a diameter that gives the same area as the non-round bin.  (For example a rectangular flat storage structure 60 feet by 40 ft has an equivalent diameter of 57.1 feet)  This model limits diameter between 2 and 200 feet.

Select the type of grain from the drop down list.  The type of grain affects the resistance to airflow.  Generally coarser commodities have lower resistance to airflow.

Enter aeration rate in CFM/bushel.  This value depends on the commodity, local climate and what the objective of the aeration is.  For aeration of wheat the desired airflow rate is usually between 0.2 to 0.3 CFM per bushel in southern states. In cooler climates 0.1 CFM/bu is the recommended rate. For drying, higher rates up to 2 CFM/bu may be recommended.  For Closed-Loop Fumigation (CLF) systems, typical aeration rates are 0.002 to 0.010 CFM/bu.  The model in ASABE standard D272.3 is based on empirical data.  If entered aeration rate is outside of the limits supported by the standard, an error message will appear under the aeration rate textbox.  If the desired aeration rate is outside of the supported limits, the results should be treated with caution.

Enter the static pressure of the duct system.  This parameter can be estimated using the duct loss calculator webpage link at the top of this page.   For grain bins with perforated floors and where the aeration fan is directly connected to the bin plenum, the static pressure of the short duct can often be assumed to be negligible.  For long ducts and/or bins with perforated ducts instead of perforated floors, the duct static pressure should be calculated based on desired airflow and then entered here.  Closed-loop fumigation system static pressures in ducts are typically significantly greater than the static pressure through the grain and must be included for proper fan selection.

Fan efficiency depends on the type of fan and is generally published by the fan manufacturer.  If no fan specific information is available, 60% is a good starting estimate.

Grain packing factor. Resistance to airflow through grain depends on several factors.  ASABE standard D272.3 suggests the following:

1. For a loose fill (not packed) of clean, relatively dry grain, use 1.0 as a grain packing factor.

2. For a loose fill of clean grain having high moisture content (In equilibrium with relative humidity exceeding 85%), use only 0.8 as a grain packing factor

3. Packing of the grain in a bin due to settling or disturbance will cause higher resistance to air flow and suggests a packing factor up to 1.5. 

4. For airflow through bulk grain in the horizontal direction the grain packing factor may be as low as 0.6 for some commodities.  This calculator assumes vertical airflow through the grain mass.

5.  Foreign material mixed with grain can have a significant effect on airflow resistance.  Generally resistance to air flow is increased if the foreign material is finer than the grain, and resistance to air flow is decreased if the foreign material is coarser than the grain. 

The default value in the calculator is 1.25.  See ASABE standard D272.3 for more information.

CALCULATE button uses the information entered above to calculate the following estimates:

-Grain volume = height * diameter^2 * pi / 4

-Airflow rate = Aeration rate * Grain volume

-Time for air to go through grain bulk = Grain volume / (Airflow rate * Grain porosity)

-Static pressure across grain bulk = Grain height * A * Airflow rate^2 / ln(1 + B * Airflow rate)   A and B are specific to each commodity type.

-Fan power estimate = Pressure across grain bulk * Airflow rate with appropriate unit conversions.  You should use the Fan Power Estimate as a guide to the amount of power needed to move air through the grain bin.  To more accurately select a fan, use the "Airflow rate" and "Static pressure across grain bulk" to select a suitable fan using the fan manufacturer's specification.