Airflow resistance charts for grain storage management
The information below was provided by the Prairie Agricultural Machinery Institute (PAMI) and was last updated on March 11, 2021.
Airflow resistance in a grain bin is an important measure as it dictates how much airflow (cfm or cubic feet of air per minute) your fan is pushing into the grain. The target airflow for aeration (cooling) is 0.1 to 0.25 cfm per bushel while the target airflow for natural air drying is 0.5 to 1 cfm per bushel. Note that airflow rates higher than 0.25 cfm per bushel will result in faster cooling or conditioning, but it may also result in some moisture loss or removal (depending on air and grain conditions). Similarly, you may achieve some moisture removal with an airflow rate lower than 0.5 cfm per bushel, but the moisture removal will be very slow.
Airflow resistance is measured in static pressure (inches of H2O) and is dependent on grain type and depth. As grain depth increases, airflow resistance increases (and airflow rate decreases).
This page describes two methods for using airflow resistance to know or predict airflow rate:
- For existing fans, use static pressure and fan curves to estimate the airflow rate in the bin
- For new bins, use the resistance-to-airflow charts to determine the fan type and size that will achieve the desired airflow rate
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.
In-line centrifugal | Static pressure (inches H2O) | ||||||
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Model | HP | 2 | 4 | 6 | 8 | 10 | 12 |
GGI-80311 | 3 | 3690 | 3020 | 2130 | 0 | 0 | 0 |
GGI-80511 | 5 | 5430 | 4790 | 4050 | 1600 | 0 | 0 |
GGI-80711 | 7 | 6550 | 5950 | 5220 | 4340 | 1560 | 0 |
GGI-81011 | 10 | 7750 | 7220 | 6550 | 5850 | 4960 | 3640 |
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).
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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.
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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.
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Relationship between static pressure and airflow rate for different types of fans Static pressure (inches of water) Axial Low speed centrifugal Static pressure (inches of water) High speed centrifugal In-line centrifugal 0 12300 9095 0 4505 6130 1 11130 8504 2 4225 5660 2 9510 7752 4 3890 5100 3 7490 6838 6 3480 4350 4 5290 5763 8 2976 0 5 3320 4526 10 2360 ‑ 6 2130 0 12 0 ‑
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).
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Airflow resistance canloa Target airflow rate (cfm/bu) Bushels Grain depth (ft) 0.1 cfm/bu 0.5 cfm/bu 1 cfm/bu 1.5 cfm/bu 1022 5 0.09 0.46 0.96 1.5 2045 10 0.36 1.92 4.17 6.71 3067 15 0.81 4.51 10.07 16.59 4090 20 1.46 8.34 19.08 31.99 5112 25 2.3 13.51 31.6 53.75 6134 30 3.35 20.14 47.99 82.64 7157 35 4.59 28.33 68.61 119.39 8179 40 6.05 38.17 93.82 164.74
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Airflow resistance wheat barley Target airflow rate (cfm/bu) Bushels Grain depth (ft) 0.1 cfm/bu 0.5 cfm/bu 1.0 cfm/bu 1.5 cfm/bu 1022 5 0.06 0.3 0.63 0.98 2045 10 0.24 1.26 2.7 4.33 3067 15 0.53 2.93 6.49 10.63 4090 20 0.96 5.4 12.25 20.4 5112 25 1.51 8.73 20.21 34.15 6134 30 2.19 12.99 30.61 52.34 7157 35 3 18.22 43.65 75.43 8179 40 3.95 24.5 59.55 103.84
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Airflow resistance peas Target airflow rate (cfm/bu) Bushels Grain depth (ft) 0.1 cfm/bu 0.5 cfm/bu 0.7 cfm/bu 0.9 cfm/bu 1.0 cfm/bu 1.2 cfm/bu 1.5 cfm/bu 2.0 cfm/bu 2.2 cfm/bu 2.5 cfm/bu 2.6 cfm/bu 1022 5 0 0.03 0.042801 0.058395 0.07 0.084338 0.11 0.167502 0.191297 0.229143 0.242322 2045 10 0.02 0.13 0.206535 0.289761 0.34 0.432489 0.6 0.910246 1.050482 1.275716 1.354689 3067 15 0.05 0.34 0.537762 0.767339 0.89 1.167196 1.63 2.534472 2.940746 3.596305 3.826916 4090 20 0.09 0.67 1.078049 1.556261 1.82 2.397357 3.38 5.311963 6.184586 7.596713 8.09447 5112 25 0.14 1.15 1.865979 2.716978 3.19 4.22412 6 9.495891 11.08263 13.65557 14.56379 6134 30 0.21 1.79 2.938078 4.306753 5.07 6.743411 9.62 15.32701 17.92089 22.13341 23.62196 7157 35 0.3 2.61 4.329339 6.380485 7.53 10.04729 14.39 23.0369 26.97466 33.37739 35.64181 8179 40 0.4 3.64 6.073541 8.991215 10.63 14.22481 20.44 32.85 38.51077 47.72425 50.98498
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Airflow resistance lentils Target airflow rate (cfm/bu) Bushels Grain depth (ft) 0.1 cfm/bu 0.5 cfm/bu 0.7 cfm/bu 0.9 cfm/bu 1.0 cfm/bu 1.2 cfm/bu 1.5 cfm/bu 2.0 cfm/bu 2.2 cfm/bu 2.5 cfm/bu 2.6 cfm/bu 1022 5 0.02 0.13 0.189881 0.258841 0.3 0.373454 0.5 0.740299 0.845167 1.011897 1.069942 2045 10 0.09 0.59 0.914032 1.281147 1.48 1.910157 2.63 4.012984 4.629778 5.620129 5.96731 3067 15 0.21 1.5 2.376289 3.387614 3.94 5.147639 7.19 11.15928 12.94446 15.82432 16.83722 4090 20 0.39 2.96 4.7587 6.863519 8.03 10.56286 14.88 23.36935 27.20143 33.40138 35.58646 5112 25 0.64 5.06 8.230257 11.97362 14.05 18.59879 26.38 41.75172 48.71689 60.00894 63.99435 6134 30 0.95 7.89 12.95094 18.96871 22.33 29.67549 42.31 67.36037 78.74299 97.22518 103.7553 7157 35 1.33 11.53 19.074 28.08919 33.13 44.19607 63.27 101.2091 118.4846 146.5696 156.5011 8179 40 1.79 16.06 26.74738 39.56732 46.75 62.55037 89.84 144.2802 169.1098 209.516 223.8145
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Airflow resistance flax Target airflow rate (cfm/bu) Bushels Grain depth (ft) 0.1 cfm/bu 0.5 cfm/bu 1.0 cfm/bu 1.5 cfm/bu 2.0 cfm/bu 2.5 cfm/bu 1022 5 0.104344 0.54735626 1.157123 1.826275 2.552331 3.333213 2045 10 0.422602 2.31424681 5.104663 8.334294 11.97645 16.01074 3067 15 0.962515 5.47882634 12.50144 20.91851 30.63139 41.56869 4090 20 1.731707 10.209326 23.95291 40.84185 60.63564 83.16642 5112 25 2.737686 16.6660644 40.02686 69.28115 103.958 143.7369 6134 30 3.987856 25.0028814 61.26278 107.3513 162.4623 226.0566 7157 35 5.489517 35.368196 88.17788 156.1198 237.9329 332.7851 8179 40 7.249876 47.9058115 121.2713 216.6164 332.0916 466.4895
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Airflow resistance corn Target airflow rate (cfm/bu) Bushels Grain depth (ft) 0.1 cfm/bu 0.5 cfm/bu 1.0 cfm/bu 1.5 cfm/bu 2.0 cfm/bu 2.5 cfm/bu 1022 5 0.013164 0.07329481 0.164111 0.270933 0.392711 0.528662 2045 10 0.054195 0.328222 0.785422 1.356347 2.031961 2.806093 3067 15 0.12534 0.81279993 2.03452 3.61054 5.511249 7.717163 4090 20 0.228771 1.57084405 4.063922 7.348332 11.35699 16.04662 5112 25 0.366595 2.64330805 7.015232 12.86194 20.05827 28.52447 6134 30 0.540862 4.06903956 11.0225 20.42825 32.07914 45.8441 7157 35 0.753571 5.88525238 16.2141 30.3128 47.86508 68.67185 8179 40 1.006672 8.12784402 22.71397 42.77219 67.84709 97.65294
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