Airflow and equipment
Since the purpose of aeration is temperature control, the quantity of air required depends on the desired rate of temperature change. An airflow rate of one-tenth cubic foot of air per minute per bushel of grain (0.1 cfm/bu) will change the temperature of a bin in 100 to 200 hours of fan operation. The temperature change occurs as a cooling or warming front moves through the grain and is not complete until the front is completely through the bin.
The fan time required is proportional to the airflow rate. An airflow of 0.05 cfm/bu takes twice as long as 0.1 cfm/bu to change the temperature of a bin. An airflow of 0.5 cfm/bu requires only 0.2 the time for a temperature change as 0.1 cfm/bu.
Although stored grain can and is being aerated with airflow rates as low as 0.03 to 0.02 cfm/bu in commercial storages, a minimum of 0.1 cfm/bu is recommended for on-farm grain storages since this level of airflow can be easily attained. Higher airflows allow the operator to get the job done faster and therefore do not require as much attention. In fact, there is an increase in the use of higher airflow aeration (0.2 to 0.5 cfm/bu) to help manage stored grain, particularly shelled corn, at higher moisture contents (15.5 to 18 percent) for feeding and/or blending. It should be remembered that increasing the moisture content of stored grain is accompanied by an increase in storage risk. However, well-designed aeration systems and good management allow the producer to gain the advantage of storing at higher moisture contents.
The operator must gain experience in the time it takes to change the temperature in the bin by monitoring the temperatures during periods of fan operation. Nonuniformity in airflow and uncertainty of airflow rates makes this necessary. Inadequate fan time is probably the major source of problems when aerating stored grain.
Fan and equipment selection for aeration
Figure 2 shows a variety of aeration systems. The round bins are all shown with the surface of the duct flush with the floor. This facilitates ease of unloading with underfloor and sweep augers. Properly designed above-floor ducts perform as well as flush ducts and are commonly used in storages originally constructed without aeration.
Figure 2. Typical grain storage aeration systems
Figure 3 shows some possible duct arrangements for flat storages, where proper duct arrangement is generally more of a problem. Figure 4 gives a guideline for duct spacing which provides good air distribution in flat storages.
Figure 3. Aeration duct arrangements for flat storages
Figure 4. Guideline for duct spacing in flat storages
Table 3 contains the static pressures that can be used to select fans for aerating wheat and shelled corn with duct systems. For complete details and a fuller understanding of equipment selection, see Selecting Fans and Determining Airflow for Crop Drying, Cooling, and Storage, FO-5716.
Table 3. Static pressures* (inches of water) for fan selection
|Grain depth||Shelled corn||Wheat|
|0.2 cfm/bu||0.1 cfm/bu||0.2 cfm/bu||0.1 cfm/bu|
|Up to 20 feet||1.0 inches||1.0 inches||2.0 inches||1.5 inches|
|20 to 30||1.5||1.0||4.5||2.5|
|30 to 40||2.5||1.5||7.5||4.0|
|40 to 50||4.0||2.0||–||5.5||*Static pressures are listed in increments of one-half inch with a minimum on one inch.|
Proper sizing of the duct flow area and the perforated surface area is necessary for adequate performance. Enough perforated surface should be installed to provide one square foot for each 25 cfm. For example, if a fan is selected to move 1,500 cfm, 60 square feet of perforated surface area should be installed (1,500 ÷ 25 = 60). The ducts carrying the air should have enough cross-sectional area to provide one square foot for each 1,500 to 2,000 cfm. For example, the same fan moving 1,500 cfm would require a duct with one square foot cross section (1,500 ÷ 1,500 = 1).