What are the outside air conditions necessary for the drying of grain? It really is the ultimate question for grain drying.
To determine the threshold relative humidity for drying, we need to know a few more things: the moisture content of the grain, the temperature of the grain, and the temperature of the outside air. With these we can determine the threshold relative humidity; if the relative humidity is greater than this, we will not get drying in fact we will get wetting and if the relative humidity is below this we will get drying. The more it is below this threshold, the more it will dry.
But before we get into this, we need to understand the basics of grain drying. Air carries the water from the grain. If the air entering the grain bin acquires more water as it flows through the grain, drying will occur. If the air being expelled from the bin has more water in it than the air entering the bin through the fan — there will be drying.
The amount of water that is in the air is called the absolute humidity and typically has the units of grams per cubic meter. A cubic meter of air, one meter by one meter by one meter, will be carrying a certain amount of water, W, and it can be precisely determined from its temperature, T, and its relative humidity, RH, using the following pyschrometric equation:
W = WS x RH/100
Ws = 0.000289 T3 + 0.010873 T2 + 0.311043 T + 4.617135
Where W (grams/m3) is the amount water in one cubic meter of air, Ws (grams/m3) is the maximum amount of water that saturated air can hold at a specific temperature (T), expressed in 0C, and relative humidity (RH) %.
To avoid the math, a graph can be used:
A table could also be used and again a temperature of 25 ºC with a relative humidity of 50% will have air carrying 12 grams of water. 
There is also a calculator online that can calculate the absolute humidity: http://planetcalc.com/2167/
So, there are a number of ways to determine the absolute humidity of the air. If one knows the temperature and relative humidity of the air; the absolute humidity can be found by doing the math with the equation, or by using the graph or the table, or by going online and using the on-line calculator.
Now drying will occur if the absolute humidity of the outside air entering the bin through the fan, is less than the absolute humidity of the air being expelled from the bin. For example, let’s say that the air outside entering the bin is 15°C @ 55% RH. The absolute humidity of the outside air is: 12.7 gr. X 0.55 = 7 gr/m3. The air being expelled is 25°C @ 45% RH with an absolute humidity: 23.7 gr x 0.45 = 10.67 gr/m3. So for every cubic meter of air that flows through the bin of grain there is 10.67 – 7 = 3.67 grams of water being removed. Drying is occurring.
It should be noted that even though the relative humidity (RH) of the air entering, 55%, is greater than the RH of the air being expelled, 45%; the absolute humidity of the expelled air is higher than the outside air. Relative humidity, by itself, means nothing; but if one knows both the RH and the temperature, then RH is very useful and can be used to easily calculate the absolute humidity.
If one knows the air flow through the bin, one can calculate the amount of drying. In the above example 3.67 grams of water was removed for every cubic meter of air that flowed through the bin. If the airflow was 3000 cubic feet per minute, CFM, then:
- Are we drying? Yes 10.67 – 7 = 3.67 gr/m3
- How much? 3000 CFM = 180,000 ft3/hr.
180000/35.41 x 3.67 = 18.6 kg/hr. water is removed every hour
The problem is that the air temperature and relative humidity continuously change during the day. The temperature during the day can be more than 10 ºC higher than at night.
The above technique was used to measure the amount of grain drying done on an hourly basis with farm sized grain bins. 19 experimental drying trials were done with the fan running continuously, and the drying data was compiled to determine the amount of drying that was done in terms of the time of day. A diurnal drying cycle was determined:
It can be seen that the greatest degree of drying occurred at night at about 2:00 AM, wetting occurred during the day, 14:00 or 2:00 PM and the transition from drying to wetting occurred at about 9:00 AM.
If drying occurs at night, and wetting during the day; wouldn’t it make sense to run the fans when we typically have the best drying conditions? This was the basis for the recommendation that the fans should not be run continuously but rather only at night — the yard light rule:
On at night, you are bright; on during the day, you will pay!
Finding the absolute humidity of the air inside the bin involves the use of the temperature and relative humidity, but the bin is probably not equipped with relative humidity sensors. The relative humidity can be determined indirectly by the use of EMC (Equilibrium Moisture Content) equations — a topic for another blog.