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Aeration System Design for Cone-Bottom Round Bins

Before attempting to select, design or manage an aeration system, you should study the following publication: BAE-1101 “Aeration and Cooling of Stored Grain.”

 

Fact Sheet BAE-1101  discusses the importance of choosing the right airflow rate to obtain the desired aeration system capabilities. Power requirements, fan selection, control systems, and management suggestions are also explained.

 

This fact sheet presents information for the design or selection of aeration system components for cone-bottom round grain bins.

 

Cone-bottom bins have the advantage of complete and easy unloading. The cone also provides additional storage capacity in the bin.

 

Below-grade cone-shaped foundations can only be used in areas of low water tables. If the water table rises to the level of the cone, water may enter the cone and cause grain spoilage.

 

When only dry grain will enter the bin, a cone slope of 37° or 3 feet fall in a 4 foot run will allow complete unloading. When wet grain will be handled through the bin, the cone slope should be 45° or 1 foot fall in a run of 1 foot.

 

Aeration Systems

A typical aeration system for a cone-bottom round grain bin is shown in Figure 1. Round metal ducts are used to dis­tribute air in cone-bottom bins. The upper section of the duct is non-perforated with lower sections perforated. The bottom of the duct is usually open.

 

If a single duct does not have sufficient surface area to avoid excessive operating pressures, a second fan and duct are added as shown in Figure 1, with the air volume divided between the two fans. In large bins, three or more fans and ducts are sometimes required to deliver high airflow rates. In general, centrifugal fans with backward-inclined blades are required to deliver efficient air volumes when operating against the higher static pressures encountered in cone-bottom bins.

 

 Aeration system for a cone-bottom round grain bin.

 

Figure 1. Aeration system for a cone-bottom round grain bin.

 

 

Description of Terms

The following terms are used in the design procedure:

 

fpm = feet per minute, air velocity; 

CFM = cubic feet of air per minute, air volume; 

CFM/bu = cubic feet of air per minute per bushel, airflow rate

 

Static pressure is the pressure against which the fan must operate, expressed as inches of water.

 

Design Procedure

The design procedure for aeration systems involves:

 

  1. Determining bin capacity, selecting air-flow rate, and determining total air volume to be delivered; 
  2. Selecting ducts on the basis of surface area; 
  3. Determining operating static pressure; and 
  4. Selecting fans to deliver the required air volume when operating against the expected static pressure.

 

Determining Bin Capacity and Air-Flow Volume

Bin capacity is the number of bushels which can be stored in the bin plus the number which can be stored in the cone. The capacity of the bin with and without the cone can be determined from Table 1.  Bin capacity does not include storage in the roof section.

 

Example: Consider a 30 feet diameter bin with 19 foot sidewalls and a 45° concrete cone shaped foundation. The bin capacity with and without the cone can be found using Table 1. In addition to individual capacities, Table 1 gives the total capacity, bin plus cone. From Table 1, bin capacity without cone is 10,775 bu. The capacity of a 30-foot diameter, 45° cone is 2,820 bu. Therefore total capacity, bin plus cone, is 13,595 bu.  Table 1 can be used for bins with concrete cone shaped foundations and for steel hopper bottom bins.

 

It is best to aerate the grain with 1/5 cfm/bu (0.2 cfm/bu).  The total airflow requirement would be 0.2 cfm/bu multiplied by the total bushels in the bin.

 

0.2 cfm/bu x 13,595 bu = 2719 cfm 

Table 1.  Bin Capacity with/without cone.

Diameter (ft.) Height (ft.)  Bin Capacity (bu) Capacity due to Cone (bu)
45o cones
Capacity due to Cone (bu)
37o cones
Total Capacity (Bin+Cone),bu
45o cones
Total Capacity (Bin+Cone),bu
37o cones
15 8 1125 363 270 1488 1395
  13 1850 363 270 2213 2120
  16 2275 363 270 2638 2545
18 11 2250 610 450 2860 2700
  13 2650 610 450 3260 3100
  16 3275 610 450 3885 3725
  21 4300 610 450 4910 4750
21 13 3625 960 720 4585 4345
  16 4450 960 720 5410 5170
  24 6675 960 720 7635 7395
24 16 5825 1440 1080 7265 6905
  19 6900 1440 1080 8340 7980
  24 8725 1440 1080 10165 9805
  32 11625 1440 1080 13065 12705
27 19 8750 2060 1540 10810 10290
  24 11025 2060 1540 13085 12565
  32 14725 2060 1540 16785 16265
30 19 10775 2820 2120 13595 12895
  24 13625 2820 2120 16445 15745
  32 18175 2820 2120 20995 20295
33 24 16475 3850 2890 20325 19365
  27 18550 3850 2890 22400 21440
  32 21975 3850 2890 25825 24865
36 24 19625 4880 3660 24505 23285
  27 22075 4880 3660 26955 25735
  32 26150 4880 3660 31030 29810
  40 32700 4880 3660 37580 36360
42 27 30050 6315 4735 36365 34785
  32 35600 6315 4735 41915 40335
  40 44500 6315 4735 50815 49235
  48 53425 6315 4735 59740 58160
48 27 39250 7750 5810 47000 45060
  32 46500 7750 5810 54250 52310
  40 58150 7750 5810 65900 63960
  48 69775 7750 5810 77525 75585

 

Table 2 gives the maximum length of duct which may be placed down one side of the cone foundation for various bin diameters and the two cone slopes.

 

Table 2. Duct Lengths for Cone-Shaped Foundations or Bottoms.

Bin Diameter
(ft.)
45 o cones
1 in 1 slope
Max. length of duct (ft.)
37 o cones
3 in 4 slope
Max. length of duct (ft.)
14 9 8
15 9.75 9
18 12 10
21 14 12
24 16 14
27 18 16
30 20 18
36 24 21
42 28 25

Example:  For a 30 ft. diameter bin with 45° cone shaped bottom, the maximum length of a duct is 20 feet (using Table 2).

 

As shown in Figure 1, the air makes a turn when it leaves the fan and transition and enters the duct. This air turbulence is accompanied by an increase in static pressure in the initial section of the duct. For this reason, the initial section of the duct should be non-perforated for a distance equal to twice the duct diameter.

 

Max. length of duct (Table 2) – (2 x the duct diameter) = max. length of perforated duct

 

Example:  If a 12 inch diameter duct is to be used in our example bin (30 ft. diameter bin), the maximum length of perforated duct is 18 ft.

 

12 inches = 1 ft.

20 ft. (from Table 2) – (2 x 1 ft.) (1 ft. is the duct diameter) = 18 ft

 

If an 18 inch (1.5 ft.) diameter duct is to be used, the maximum length of perforated duct is 17 ft.:

 

20 ft. – (2 x 1.5 ft.) = 17 ft.

 

Determining Air Velocity through the Grain

Table 3 and Table 4 are used to determine total air volume and the air velocity through the grain when the desired airflow rate is known.  Table 3 is for 45 degree cones and Table 4 is for 37 degree cones. The choice of airflow rate is an important deci­sion. Higher aeration airflow rates give greater management flexibility and may allow the storage of grain with higher moisture content because the grain can be cooled and dried with natural air quicker. However, higher aeration airflow rates also require larger ducts, involve higher static pressures, and have greater power requirements. For a complete discussion of airflow rates, see BAE-1101.

 

Table 3 (Part A : Total Air Volume (cfm) ). Total air volume and air velocity through the grain for grain bins with 45 degree cones.

                 
      Total Air Volume (cfm)
      Air Flow Rate (cfm/bu)
Diameter (ft.) Height (ft.) Total Capacity
(Bin+Cone), bu
1/2 1/3 1/4 1/5 1/10 1/20
15 8 1488 744 496 372 298 149 74
  13 2213 1107 738 553 443 221 111
  16 2638 1319 879 660 528 264 132
18 11 2860 1430 953 715 572 286 143
  13 3260 1630 1087 815 652 326 163
  16 3885 1943 1295 971 777 389 194
  21 4910 2455 1637 1228 982 491 246
21 13 4585 2293 1528 1146 917 459 229
  16 5410 2705 1803 1353 1082 541 271
  24 7635 3818 2545 1909 1527 764 382
24 16 7265 3633 2422 1816 1453 727 363
  19 8340 4170 2780 2085 1668 834 417
  24 10165 5083 3388 2541 2033 1017 508
  32 13065 6533 4355 3266 2613 1307 653
27 19 10810 5405 3603 2703 2162 1081 541
  24 13085 6543 4362 3271 2617 1309 654
  32 16785 8393 5595 4196 3357 1679 839
30 19 13595 6798 4532 3399 2719 1360 680
  24 16445 8223 5482 4111 3289 1645 822
  32 20995 10498 6998 5249 4199 2100 1050
33 24 20325 10163 6775 5081 4065 2033 1016
  27 22400 11200 7467 5600 4480 2240 1120
  32 25825 12913 8608 6456 5165 2583 1291
36 24 24505 12253 8168 6126 4901 2451 1225
  27 26955 13478 8985 6739 5391 2696 1348
  32 31030 15515 10343 7758 6206 3103 1552
  40 37580 18790 12527 9395 7516 3758 1879
42 27 36365 18183 12122 9091 7273 3637 1818
  32 41915 20958 13972 10479 8383 4192 2096
  40 50815 25408 16938 12704 10163 5082 2541
  48 59740 29870 19913 14935 11948 5974 2987
48 27 47000 23500 15667 11750 9400 4700 2350
  32 54250 27125 18083 13563 10850 5425 2713
  40 65900 32950 21967 16475 13180 6590 3295
  48 77525 38763 25842 19381 15505 7753 3876

Table 3 (Part B : Air Velocity (fpm) ). Total air volume and air velocity through the grain for grain bins with 45 degree cones.

                 
      Air Velocity (fpm)
      Air Flow Rate (cfm/bu)
Diameter (ft.) Height (ft.) Total Capacity
(Bin+Cone), bu
1/2 1/3 1/4 1/5 1/10 1/20
15 8 1488 4.21 2.81 2.11 1.68 0.84 0.42
  13 2213 6.26 4.18 3.13 2.51 1.25 0.63
  16 2638 7.47 4.98 3.73 2.99 1.49 0.75
18 11 2860 5.62 3.75 2.81 2.25 1.12 0.56
  13 3260 6.41 4.27 3.2 2.56 1.28 0.64
  16 3885 7.64 5.09 3.82 3.05 1.53 0.76
  21 4910 9.65 6.43 4.83 3.86 1.93 0.97
21 13 4585 6.62 4.41 3.31 2.65 1.32 0.66
  16 5410 7.81 5.21 3.91 3.13 1.56 0.78
  24 7635 11.03 7.35 5.51 4.41 2.21 1.1
24 16 7265 8.03 5.36 4.02 3.21 1.61 0.8
  19 8340 9.22 6.15 4.61 3.69 1.84 0.92
  24 10165 11.24 7.49 5.62 4.5 2.25 1.12
  32 13065 14.45 9.63 7.22 5.78 2.89 1.44
27 19 10810 9.44 6.3 4.72 3.78 1.89 0.94
  24 13085 11.43 7.62 5.72 4.57 2.29 1.14
  32 16785 14.67 9.78 7.33 5.87 2.93 1.47
30 19 13595 9.62 6.41 4.81 3.85 1.92 0.96
  24 16445 11.64 7.76 5.82 4.66 2.33 1.16
  32 20995 14.86 9.91 7.43 5.94 2.97 1.49
33 24 20325 11.89 7.93 5.94 4.76 2.38 1.19
  27 22400 13.1 8.73 6.55 5.24 2.62 1.31
  32 25825 15.1 10.07 7.55 6.04 3.02 1.51
36 24 24505 12.04 8.03 6.02 4.82 2.41 1.2
  27 26955 13.25 8.83 6.62 5.3 2.65 1.32
  32 31030 15.25 10.17 7.63 6.1 3.05 1.53
  40 37580 18.47 12.31 9.23 7.39 3.69 1.85
42 27 36365 13.13 8.75 6.57 5.25 2.63 1.31
  32 41915 15.13 10.09 7.57 6.05 3.03 1.51
  40 50815 18.35 12.23 9.17 7.34 3.67 1.83
  48 59740 21.57 14.38 10.79 8.63 4.31 2.16
48 27 47000 12.99 8.66 6.5 5.2 2.6 1.3
  32 54250 15 10 7.5 6 3 1.5
  40 65900 18.22 12.15 9.11 7.29 3.64 1.82
  48 77525 21.43 14.29 10.72 8.57 4.29 2.14

 

 

Table 4 (Part A : Total Air Volume (cfm) ). Total air volume and air velocity through the grain for grain bins with 37 degree cones. 

                 
      Total Air Volume (cfm)
      Air Flow Rate (cfm/bu)
Diameter (ft.) Height (ft.) Total Capacity
 (Bin+ Cone), bu
1/2 1/3 1/4 1/5 1/10 1/20
15 8 1395 698 465 349 279 140 70
  13 2120 1060 707 530 424 212 106
  16 2545 1273 848 636 509 255 127
18 11 2700 1350 900 675 540 270 135
  13 3100 1550 1033 775 620 310 155
  16 3725 1863 1242 931 745 373 186
  21 4750 2375 1583 1188 950 475 238
21 13 4345 2173 1448 1086 869 435 217
  16 5170 2585 1723 1293 1034 517 259
  24 7395 3698 2465 1849 1479 740 370
24 16 6905 3453 2302 1726 1381 691 345
  19 7980 3990 2660 1995 1596 798 399
  24 9805 4903 3268 2451 1961 981 490
  32 12705 6353 4235 3176 2541 1271 635
27 19 10290 5145 3430 2573 2058 1029 515
  24 12565 6283 4188 3141 2513 1257 628
  32 16265 8133 5422 4066 3253 1627 813
30 19 12895 6448 4298 3224 2579 1290 645
  24 15745 7873 5248 3936 3149 1575 787
  32 20295 10148 6765 5074 4059 2030 1015
33 24 19365 9683 6455 4841 3873 1937 968
  27 21440 10720 7147 5360 4288 2144 1072
  32 24865 12433 8288 6216 4973 2487 1243
36 24 23285 11643 7762 5821 4657 2329 1164
  27 25735 12868 8578 6434 5147 2574 1287
  32 29810 14905 9937 7453 5962 2981 1491
  40 36360 18180 12120 9090 7272 3636 1818
42 27 34785 17393 11595 8696 6957 3479 1739
  32 40335 20168 13445 10084 8067 4034 2017
  40 49235 24618 16412 12309 9847 4924 2462
  48 58160 29080 19387 14540 11632 5816 2908
48 27 45060 22530 15020 11265 9012 4506 2253
  32 52310 26155 17437 13078 10462 5231 2616
  40 63960 31980 21320 15990 12792 6396 3198
  48 75585 37793 25195 18896 15117 7559 3779

 

 

 

Table 4( Part B : Air Velocity (fpm) ). Total air volume and air velocity through the grain for grain bins with 37 degree cones. (cont'd)

                 
      Air Velocity (fpm)
      Air Flow Rate (cfm/bu)
Diameter (ft.) Height (ft.) Total Capacity
(Bin+Cone), bu
2-Jan 3-Jan 4-Jan 5-Jan 10-Jan 20-Jan
15 8 1395 3.95 2.63 1.97 1.58 0.79 0.39
  13 2120 6 4 3 2.4 1.2 0.6
  16 2545 7.2 4.8 3.6 2.88 1.44 0.72
18 11 2700 5.31 3.54 2.65 2.12 1.06 0.53
  13 3100 6.09 4.06 3.05 2.44 1.22 0.61
  16 3725 7.32 4.88 3.66 2.93 1.46 0.73
  21 4750 9.34 6.23 4.67 3.74 1.87 0.93
21 13 4345 6.28 4.18 3.14 2.51 1.26 0.63
  16 5170 7.47 4.98 3.73 2.99 1.49 0.75
  24 7395 10.68 7.12 5.34 4.27 2.14 1.07
24 16 6905 7.64 5.09 3.82 3.05 1.53 0.76
  19 7980 8.82 5.88 4.41 3.53 1.76 0.88
  24 9805 10.84 7.23 5.42 4.34 2.17 1.08
  32 12705 14.05 9.37 7.02 5.62 2.81 1.4
27 19 10290 8.99 5.99 4.5 3.6 1.8 0.9
  24 12565 10.98 7.32 5.49 4.39 2.2 1.1
  32 16265 14.21 9.47 7.11 5.68 2.84 1.42
30 19 12895 9.13 6.08 4.56 3.65 1.83 0.91
  24 15745 11.14 7.43 5.57 4.46 2.23 1.11
  32 20295 14.36 9.58 7.18 5.75 2.87 1.44
33 24 19365 11.33 7.55 5.66 4.53 2.27 1.13
  27 21440 12.54 8.36 6.27 5.02 2.51 1.25
  32 24865 14.54 9.7 7.27 5.82 2.91 1.45
36 24 23285 11.44 7.63 5.72 4.58 2.29 1.14
  27 25735 12.65 8.43 6.32 5.06 2.53 1.26
  32 29810 14.65 9.77 7.33 5.86 2.93 1.47
  40 36360 17.87 11.91 8.93 7.15 3.57 1.79
42 27 34785 12.56 8.37 6.28 5.02 2.51 1.26
  32 40335 14.56 9.71 7.28 5.83 2.91 1.46
  40 49235 17.78 11.85 8.89 7.11 3.56 1.78
  48 58160 21 14 10.5 8.4 4.2 2.1
48 27 45060 12.46 8.3 6.23 4.98 2.49 1.25
  32 52310 14.46 9.64 7.23 5.78 2.89 1.45
  40 63960 17.68 11.79 8.84 7.07 3.54 1.77
  48 75585 20.9 13.93 10.45 8.36 4.18 2.09

 

Example: Using our sample bin (30 ft. diameter and 19 ft. height), suppose we wish to provide an airflow rate of 1/5 (0.2) CFM/bu. Table 3 gives a total air volume of about 2719 CFM and an air velocity through the grain of 3.85 fpm.  If the cone bottom has a 37° cone, use Table 4.

 

Determining Operating Static Pressure Due to Grain Depth

Tables 5, 6, 7, 8, and 9 can be used to determine the static pressure due to grain depth when the grain depth or bin sidewall height and the air flow rate through the grain are known. It is assumed the bin will be filled to the eaves and the top surface will be leveled, making grain depth and bin sidewall height equal. Table 5 is used when shelled corn is the grain to be aerated, Table 6 is used for barley and oats, Table 7 for soybeans and confectionary sunflowers, Table 8 for oil-type sunflowers and Table 9 for wheat and sorghum. The static pressure of canola is two to three times the static pressure of wheat. Therefore, if an existing aeration system designed for wheat is used for canola, check the velocity and pressure ratings of the system to ensure adequate airflow. Fact Sheet BAE-1110, “Storing Oklahoma Winter Canola,” covers the methods for storing Oklahoma winter canola. When the system will be used for more than one grain, design for the grain that gives the highest expected static pressure.

 

Table 5. Expected Static Pressure for shelled corn. 

Values in the table have been multiplied by 1.5 to account for fines and packing in the bin. (If corn is stirred, which tends to decrease airflow resistance, divide table values by 1.5.)

Expected static pressure (inches of water)

Grain depth (ft) 0.05 0.1 0.25 0.5 0.75 1 1.25 1.5 2
2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
4 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.2
6 0.1 0.1 0.1 0.1 0.2 0.3 0.3 0.4 0.6
8 0.1 0.1 0.1 0.2 0.3 0.5 0.6 0.8 1.2
10 0.1 0.1 0.2 0.3 0.5 0.8 1.1 1.4 2
12 0.1 0.1 0.2 0.5 0.8 1.2 1.6 2.1 3.2
14 0.1 0.1 0.3 0.7 1.2 1.7 2.3 3 4.6
16 0.1 0.1 0.4 0.9 1.6 2.4 3.2 4.2 6.4
18 0.1 0.2 0.5 1.2 2.1 3.1 4.3 5.6 8.7
20 0.1 0.2 0.7 1.6 2.7 4 5.6 7.3 11.3
25 0.2 0.4 1.1 2.6 4.6 7 9.7 12.8 19.9
30 0.3 0.5 1.6 4.1 7.2 11 15.3 20.3 31.9
40 0.5 1 3.1 8.1 14.6 22.6 31.9 42.5 *
50 0.7 1.6 5.3 14 25.6 39.9 * * *

* Static pressure is excessive--greater than 50 in. water.

 

 

Table 6. Expected Static Pressure for barley and oats. 

Values in the table have been multiplied by 1.5 to account for fines and packing in the bin.

Expected static pressure (inches of water)

Grain depth (ft) 0.05 0.1 0.25 0.5 0.75 1 1.25 1.5 2
     Airflow (cfm/bu)                    
2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
4 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.5
6 0.1 0.1 0.1 0.2 0.4 0.5 0.7 0.8 1.1
8 0.1 0.1 0.2 0.4 0.7 0.9 1.2 1.5 2.1
10 0.1 0.1 0.3 0.7 1.1 1.5 2 2.5 3.6
12 0.1 0.2 0.5 1 1.6 2.3 3 3.7 5.4
14 0.1 0.3 0.7 1.4 2.2 3.2 4.2 5.3 7.8
16 0.2 0.3 0.9 1.9 3 4.3 5.7 7.2 10.6
18 0.2 0.4 1.1 2.4 3.9 5.6 7.5 9.5 14.1
20 0.3 0.5 1.4 3 4.9 7.1 9.5 12.2 18.1
15 0.4 0.8 2.2 4.9 8.2 11.9 16.1 20.7 31.1
30 0.6 1.2 3.2 7.4 12.4 18.3 24.8 32.1 48.7
40 1 2.1 6 14.2 24.4 36.2 49.8 * *
50 1.6 3.4 9.9 23.8 41.4 * * * *

* Static pressure is excessive--greater than 50 in. water.

 

 

 

Table 7. Expected Static Pressure for soybeans and confectionary sunflowers. 

Values in the table have been multiplied by 1.5 to account for fines and packing in the bin.

Expected static pressure (inches of water)

Grain depth (ft) 0.05 0.1 0.25 0.5 0.75 1 1.25 1.5 2
  Airflow (cfm/bu)      
2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
4 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2
6 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.5
8 0.1 0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.9
10 0.1 0.1 0.1 0.3 0.4 0.6 0.8 1 1.5
12 0.1 0.1 0.2 0.4 0.7 0.9 1.2 1.6 2.3
14 0.1 0.1 0.3 0.6 0.9 1.3 1.7 2.2 3.3
16 0.1 0.1 0.3 0.8 1.2 1.8 2.4 3 4.5
18 0.1 0.2 0.4 1 1.6 2.3 3.1 4 6
20 0.1 0.2 0.6 1.2 2 3 4 5.1 7.7
25 0.2 0.3 0.9 2 3.4 5 6.8 8.8 13.4
30 0.2 0.5 1.3 3.1 5.2 7.7 10.6 13.7 21
40 0.4 0.9 2.5 5.9 10.3 15.4 21.4 28 43.4
50 0.6 1.4 4.1 10 17.6 26.7 37.2 49.1 *

* Static pressure is excessive--greater than 50 in. water.

 

 

Table 8. Expected Static Pressure for oil-type sunflowers. 

Values in the table have been multiplied by 1.5 to account for fines and packing in the bin.

Expected static pressure (inches of water)

Grain depth (ft) 0.05 0.1 0.25 0.5 0.75 1 1.25 1.5 2
     Airflow (cfm/bu)     
2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
4 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.3
6 0.1 0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.9
8 0.1 0.1 0.1 0.3 0.5 0.7 0.9 1.1 1.7
10 0.1 0.1 0.2 0.5 0.8 1.1 1.5 1.9 2.8
12 0.1 0.1 0.3 0.7 1.2 1.7 2.3 2.9 4.4
14 0.1 0.2 0.5 1 1.7 2.4 3.3 4.2 6.4
16 0.1 0.2 0.6 1.4 2.3 3.3 4.5 5.8 8.8
18 0.1 0.3 0.8 1.8 3 4.4 6 7.8 11.8
20 0.2 0.3 1 2.3 3.8 5.6 7.7 10 15.3
25 0.3 0.6 1.6 3.7 6.5 9.7 13.3 17.4 26.9
30 0.4 0.8 2.4 5.7 10 15.1 20.9 27.5 42.7
40 0.7 1.5 4.5 11.3 20.1 30.7 43 * *
50 1.1 2.4 7.5 19.3 34.8 * * * *

* Static pressure is excessive--greater than 50 in. water.

 

 

 

Table 9. Expected Static Pressure for wheat and sorghum.

Values in the table have been multiplied by 1.3 for wheat and 1.5 for sorghum to account for fines and packing in the bin.

Expected static pressure (inches of water)

Grain depth (ft) 0.05 0.1 0.25 0.5 0.75 1 1.25 1.5 2
     Airflow (cfm/bu)             
2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2
4 0.1 0.1 0.1 0.2 0.3 0.3 0.4 0.5 0.7
6 0.1 0.1 0.2 0.4 0.6 0.8 1 1.2 1.7
8 0.1 0.1 0.3 0.7 1.1 1.5 1.9 2.3 3.2
10 0.1 0.2 0.5 1.1 1.7 2.3 3 3.7 5.3
12 0.1 0.3 0.8 1.6 2.5 3.4 4.5 5.6 7.9
14 0.2 0.4 1 2.2 3.4 4.8 6.3 7.8 11.3
16 0.3 0.5 1.4 2.9 4.6 6.4 8.4 10.6 15.3
18 0.3 0.7 1.7 3.7 5.9 8.3 11 13.8 20
20 0.4 0.8 2.2 4.7 7.5 10.5 13.9 17.6 25.6
25 0.6 1.3 3.4 7.5 12.2 17.4 23.1 29.4 43.3
30 0.9 1.9 5.1 11.2 18.3 26.3 35.3 45 *
40 1.7 3.4 9.3 21.1 35.1 * * * *
50 2.6 5.4 15 34.8 * * * * *

* Static pressure is excessive--greater than 50 in. water.

 

 

Example: To design the system for our sample bin (19 ft. height) to store either corn or soybeans, consider both Table 5 and Table 7. Since the tables do not give data for 0.2 cfm/bu airflow rates, use the next higher airflow given. In this case, we would use 0.25 cfm/bu. The tables do not give data for 19 ft. height either, so we would calculate the static pressure that would be halfway between the pressures shown for 18 ft and 20 ft heights.

 

For corn (Table 5):  at 18 ft grain depth, pressure = 0.5 in. and at 20 ft., pressure = 0.7 in. Therefore, at 19 ft. grain depth, the pressure will be 0.6 in. or halfway between 0.5 and 0.7 in.

 

For soybeans (Table 7):  at 18 ft grain depth, pressure = 0.4 in. and at 20 ft., pressure = 0.6 in. Therefore, at 19 ft. grain depth, the pressure will be 0.5 in.

 

Since we would like to design our system to handle either crop, we must design for the greater pressure requirements.  In this case, it would be corn and we would use 0.6 in. for our pressure requirement due to grain depth.

 

Determining Static Pressure Due to Duct and Cone

Pressure requirements are also increased due to air traveling through the air ducts.  To find these pressure requirements, the surface area of the ductwork must be known.  Table 10 gives the duct surface area given the duct diameter and the duct length. Tables 11 and 12 use the surface area from Table 10 and the air-flow volume calculated earlier to determine the expected static pressure for the duct system and for the airflow through the cone part of the bin.  This pressure added to the pressure for the grain depth is the design pressure used to select the aeration fans.

 

Table 10. Duct Surface Area.

Duct Diameter
(inches)
Duct Length
(ft)
Duct Surface Area
(Sq. ft.)
6 10 13
  15 19
  20 25
  25 31
  30 38
  35 44
10 10 21
  15 31
  20 42
  25 52
  30 63
  35 73
12 10 25
  15 38
  18 45
  20 50
  25 63
  30 75
  35 88
15 10 31
  20 63
  25 79
  30 94
  35 110
18 10 38
  15 57
  20 75
  25 94
  30 113
  35 132
20 10 42
  15 63
  20 84
  25 105
  30 126
  35 147
24 10 50
  15 75
  20 100
  25 126
  30 151
  35 176
30 10 63
  15 94
  20 126
  25 157
  30 188
  35 220
36 10 75
  15 113
  20 151
  25 188
  30 226
  35 264
40 10 84
  15 126
  20 167
  25 209
  30 251
  35 293

Table 11. Expected static pressure due to duct and cone for corn or soybeans. 

Expected Static Pressure (inches of water)

 Total Air Volume (CFM)
Duct Surface Area (sq. ft.) 100 500 1000 2000 3000  4000  5000  6000  7000 8000 10000 20000 30000
10 0.26 2.5 7.5 * *  *  *  *  * * * * *
15 0.15 1.4 3.9 10 * * * * * * * * *
20 0.1 0.91 2.5 6 * * * * * * * * *
30 ** 0.54 1.4 3.7 6.5 * * * * * * * *
40 ** 0.35 0.92 2.5 4.4 6.4 8.5 * * * * * *
50 ** 0.26 0.7 1.75 3.2 5 6.8 8.5 * * * * *
60 ** 0.21 0.55 1.45 2.45 3.8 5.1 6.8 8 * * * *
80 ** 0.14 0.27 0.94 1.7 2.5 3.35 4.45 5.4 6.5 8.6 * *
100 ** 0.1 0.26 0.69 1.3 1.8 2.5 3.3 3.9 4.7 6.3 * *
150 ** ** 0.16 0.39 0.7 1.1 1.45 1.9 2.3 2.7 3.6 * *
200 ** ** 0.12 0.27 0.46  0.65  0.75  1.3  1.5 1.8 2.6 6.5 *
300 ** ** ** 0.15 0.27  0.39  0.5  0.7  0.85 1.05 1.49 3.75 6.8

** Static pressure is less than 0.1 in. water.
*Static pressure is excessive--greater than 10 in. water.

 

  

Table 12. Expected static pressure due to duct and cone for wheat, grain sorghum, oats, barley, or rye.

Duct Surface Area (sq.  ft.) 100 500 1000 2000 3000 4000 5000 6000 7000 8000 10000 20000 30000
10 0.8 5.5 13 * * * * * * * * * *
15 0.49 3.4 7.9 * * * * * * * * * *
20 0.35 2.4 5.5 12 * * * * * * * * *
30 ** 1.45 3.4 7.8 12.8 * * * * * * * *
40 ** 1 2.4 5.5 9 12.5 * * * * * * *
50 ** 0.8 1.85 4.2 6.8 10 13 * * * * * *
60 ** 0.65 1.5 3.45 5.4 7.9 10.1 13 15 * * * *
80 ** 0.45 0.8 2.4 3.8 5.5 7 9 10.8 12.5 * * *
100 ** 0.35 0.79 1.8 2.9 4.3 5.5 7 8.1 9.5 12.2 * *
150 ** ** 0.5 1.1 1.8 2.55 3.4 4.4 4.9 5.8 7.5 * *
200 ** ** 0.37 0.8 1.3 1.8 2.4 3 3.5 4.2 5.6 12.9 *
300 ** ** ** 0.5 0.84 0.12 1.5 1.85 2.2 2.6 3.45 7.9 13

 ** Static pressure is less than 0.3 in. water.
*Static pressure is excessive--greater than 15 in. water. 

 

Example:

  1. For the sample bin using 12 in. duct diameter and 18 ft. duct length, the duct surface area is 45 sq. ft. (Table 10). 
  2. Using Table 11 for corn or soybeans, 45 sq. ft. duct surface area, and 2719 cfm (rounded up to 3000 cfm) air volume, the static pressure due to the duct system and the cone is 3.8 in. (use pressure half way between 40 sq. ft. and 50 sq. ft. since there is not data for 45 sq. ft) 
  3. For the total pressure requirement, add the pressure due to grain depth and the pressure through the cone and duct system:  0.5 in. + 3.8 in. = 4.3 in.

When the bin will be used to store more than one grain, the grain which produces the highest static pressure should be used for design purposes. The static pressure of canola is two to three times that of wheat.

 

The static pressure due to duct and cone can be reduced by choosing a larger diameter duct. Table 13 compares the re­sults obtained for a 12-inch diameter duct with those obtained for 18- and 21-inch diameter ducts.

 

Selecting Fans

Fans are selected from the manufacturer’s rating curves or tables to deliver the required air volume when operating against the expected static pressure. Axial fans (propeller-type) are commonly used for aeration since they produce high air volumes at low static pressures. However, air volumes delivered by axial fans fall off rapidly as static pressures in­crease through the 3.5 to 5.0 inch range. Above this range, centrifugal fans with backward-inclined blades must be used.  In special designs, centrifugal fans will operate efficiently at static pressures of 20 inches or more.

 

Centrifugal fans operate with less noise than axial fans and should be used whenever fan noise may be a nuisance to neighbors. Centrifugal fans of 3 Hp or less cost two to three times as much as axial fans of the same Hp rating. Above 5 Hp, centrifugal fans cost 1.5 to 2 times as much as axial fans of the same Hp rating.

 

The lowest priced fan which will deliver the required air volume when operating at the expected static pressure is, of course, the most economical fan to buy. However, the most economical fan to operate is the fan with the lowest power consumption, measured in watts, while delivering the required air volume at the expected static pressure. Nominal horse­power rating is not a good measure of power consumption.

 

While final fan selection must be made from manu­facturer’s data, an estimate of the power requirement may be helpful for planning purposes. Equation 1 is used to estimate the power requirement, assuming a fan efficiency of 50 percent.

 

  hp = (cfm x Ps) ÷ (63.46 x efficiency)                               equation 1

  where:

  cfm = airflow in cfm

  Ps = static pressure in inches of water

  efficiency = fan efficiency (%)

 

Example: Our example calls for an air volume of 2719 CFM. If the operating pressure is 4.3 inches, the power requirement is about 3.68 Hp (select a 5 HP motor). Table 13 also compares the power requirements resulting from the use of a 12-inch diameter duct with those resulting from the use of 18- and 24-inch diameter ducts.

 

When selecting fans, consult the data from several manu­facturers. Tables 14 and 15 present typical performance data for axial and centrifugal fans, respectively. One manufacturer’s 5 Hp fan may be well matched to your needs while another’s 5 Hp fan may not. Fan performance data should be certified in accordance with standard test codes adopted by Air Moving and Conditioning Association, Incorporated and bear the AMCA seal.

 

Table 13. Static Pressures and Power Requirements resulting from three duct diameters in the example.

 Duct Diameter (inches)
  12 18 24
Maximum length of Perforated Duct (ft) 18 17 16
Static Pressure due to Duct and Cone (inches of water) For Corn or Soybeans 3.8 2 1.5
Total Static Pressure (inches of water)
For Corn or Soybeans
4.3 2.6 2.1
Power Required assuming 50% efficiency (Hp) For Corn or Soybeans 3.7 2.2 1.8

 

Table 14. Typical Performance Data for Axial Fans*.

 Static Pressure (Inches of water) 
CFM 
HP RPM 0.5 1 1.5 2 3 4  
1 3450 2880 2635 2360 1935 810 455  
3 3450 7000 6400 5700 5200 3700 2200  
5 3450 9700 9100 8600 8000 6500 4600  
7.5 3450 12800 12300 11600 11000 9800 7400  

 *This table is abbreviated. Intermediate static pressures and a much larger range of CFM values are normally shown

 

Table 15A. Typical Performance Data for Centrifugal Fan*. 

Static Pressure (inches of water)

CFM 2 RPM 2 HP 4 RPM 3 HP 6 RPM 6 HP 8 RPM 8 HP 10 RPM 10 HP 12 RPM 12 HP 14 RPM 14 HP
1520 1364 1.15 1753 1.96 2064 2.77 2332 3.6 2574 4.47 2794 5.37 3000 6.3
2026 1527 1.81 1894 2.89 2190 3.94 2446 4.99 2679 6.05 2891 7.14 3090 8.25
2532 1708 2.72 2050 4.07 2334 5.4 2584 6.71 2805 8.01 3010 9.3 3204 10.6
3039 1906 3.93 2221 5.5 2490 7.15 2730 8.75 2946 10.3 3145 11.8 3333 13.4

 *This table is abbreviated. Intermediate static pressures and a much larger range of CFM values are normally shown. Various speeds may be obtained using variable frequency drives or belt and sheave systems between a roteor and fan. Typical motor operating speeds are 1,760 rpm and 3,500 rpm (nominal).

 

Further Examples

 

Example 1:  An aeration system is desired for a 24 feet diameter bin with 16 feet sidewalls and 37˚ cone shaped foundation which will be used to store wheat. An airflow rate of 1/4 (0.25) CFM/bu is desired.

 

  1. Bin capacity is 6905 bu (Table 1).  Airflow volume:  6905 bu x 0.25 CFM/bu = 1726 CFM (also in Table 4).
  2. From Table 2, maximum duct length is 14 feet.
  3. Air velocity through the grain is 3.82 fpm (Table 4). The static pressure due to the 16 feet grain depth is 1.4 inches (Table 9).
  4. Maximum perforated duct if an 18 in. dia. duct is selected:  14 ft – (2 x 1.5 ft) = 11 ft.
  5. Duct surface area: The duct surface area of 10ft and 15ft duct is known (Table 10).  Estimate for 11ft duct length using interpolation:|
    38+((11-10)*(57-38)/(15-10)) = 38+ 3.8= 42 sq. ft.  (Table 10).
  6. Static Pressure due to cone and duct, as static pressure for 1726 cfm and 42 sq ft area is not given directly in Table 12 therefore we calculate the static pressure for  2000 cfm and 42 sq ft by interpolation:
       5.5 + ((42-40)*(4.2-5.5)/ ((50-40)) = 5.24 in. of water (Table 12).
  7.  Total static pressure:  5.24 + 1.4 in. = 6.64. 
  8.  Approx. HP:  hp = 
    (1726 * 6.64) / (63.46 * 50) = 3.61 HP         (equation 1) 
  9.  Choose a 5 HP fan.

 

Example 2:  Suppose a producer wishes to provide 1/2 (0.5) CFM/bu. for quick cooling of damp corn during harvest. The bin is 27 feet in diameter, has 24 feet sidewalls and a 45˚ cone shaped foundation.

 

  1. Bin capacity is 13085 bu (Table 1).  Airflow volume:  13085 bu x 0.50 CFM/bu = 6543 CFM (also in Table 3).
  2. From Table 2, maximum duct length is 18 feet.
  3. Air velocity through the grain is 11.43 fpm (Table 3). The static pressure due to the 24 feet grain depth is 2.4 inches (Table 5).
  4. Maximum perforated duct:  If 30 in duct is selected, round up to 3 feet, 18 ft – (2 x 3 ft) = 13 ft.
  5. Duct surface area, using interpolation and Table 10:
    63 + ((13-10) x (94-63) / (15-10)) = 63+18.6 =
    81.6 Sq. ft.= 82 Sq. ft.  (Table 10).
  6.  Pressure due to cone and duct, as static pressure for 6543 cfm and 82 sq ft area is not given directly in Table 11 therefore we calculate the static pressure for  7000 cfm and 75.4 sq ft by interpolation:
       5.4 + ((82-80) x (3.9-5.4) / (100-80)) = 5.25  (Table 11)
  7. Total static pressure:  5.25 + 2.4 in. = 7.65.
  8. Approx. HP :hp= (6543 x 7.65) / (63.46 x 50)=15.8 HP                          (equation 1).
  9. Choose a 20 HP fan

 

Other Considerations

Aeration systems for cone-bottom bins must operate as pressure systems-blowing air upward through the grain. For more information about pressure and suction systems, see BAE-1101.

 

There must be sufficient roof openings to allow the air to escape. The required air escape area, in square feet, is determined by dividing the total air volume by 1,500 fpm. If the bin roof is mounted off the sidewall, the slot under the eaves serves as air escape area. When additional area is required, roof vents should be installed until the air escape area requirement is met.

 

When aeration systems are operating, the unloading auger tube should be sealed to prevent the escape of air.

 

Smooth transitions should be used to connect fan outlets with duct inlets. Sudden reductions or increases in duct diameters should be avoided.

 

Carol Jones

Extension Agricultural Engineer

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