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The calculator below was created by the user's request. Request pretty good summarizes the idea, so I quote it here entirely.
Here it is:
"This calculator is used to set the airflow rate through a grain bin equipped with an aeration fan. It assumes that the airflow can be controlled with baffles, spill gates, or variable fan speed. It requires an airflow meter or anemometer. This calculator determines the airspeed needed at the top collar of a bin to achieve the desired airflow rate (CFM/bu Cubic Feet per Minute per bushel). Enter the values for the desired flow rate (CFM/bu), bin capacity (bu), and top collar diameter (ft). The output is the corresponding airspeed (ft/min, km/hr & m/sec) required at the bin's top collar. With the bin loaded with grain, measure the airspeed at the bin's top collar by holding the airflow meter in the cross-sectional plane of the top collar. Adjust the flow until the required airspeed is achieved."
So, the idea looks quite simple to me, although I have no connection with agriculture:
There is a desired airflow, cubic feet per minute per bushel, needed to dry grain. You can determine it from various tables which can be found on the internet, for example, here. These can include the location (i.e., state), date when the crop was harvested, and percent of moisture content.
After you have determined the desired airflow, say, 1.5 cfm/bu, you enter bin load, i.e., how many bushels are in there. Let's say you have a 27-ft diameter grain bin filled up to 11 feet - so you have about 5000 bushels loaded.
Having the two above numbers, you can determine the desired total air volume in cubic feet needed to go through the bin per minute. For our example, it is 7500 cfm
- Considering that there is no air loss, all this volume has to go out through the top collar. Knowing top collar diameter, you can determine airflow speed, which is needed to exhaust all volume.
So, if your measured exhaust airflow speed more or equals to a calculated one, you have your desired airflow. Will your fan be able to provide it? Well, it depends on the size and load of your grain bin - the more grain you have, the more its depth thus, the more the static pressure it creates, which prevents the airflow, hence, the less cfm you get from your fan. The method for calculating the maximum capabilities of a fan can also be found at the provided link above.