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