Fan Control Strategies from the Worst to the Ultimate

For years farmers have wondered what to do — when should they turn the fan on, when should they turn the fan off.  Intuitively they knew that there were ambient conditions that were better for drying and there were times when there were terrible conditions for drying; like when it was raining.  The experts for years have not really known either, so the recommendation for years has been to just leave the fan on continuously until you thought the grain was dry.  For years this has been working, but it does beg the question:  Is there a better way.   I have now been looking at this for years now, and I have been analyzing the data that we have been collecting for eight years now, and the following is what I think should be done to control your fan.  It goes from worst practice to what I think is the ultimate or optimum controller.  I will have more details on each of these in subsequent blogs.

  • The worst thing to do is nothing.  Let that hot grain coming in off the field sit for a while and sweat.  This is not good, your crop is starting the process of spoiling. Even if the grain is coming in dry, you still need to cool it down to stop the spoilage process.  There are two factors in keeping your grain safe from spoilage, and that is cooling and drying.  You can get the grain cool quickly, so get it cooled down quickly.  Turn the fan on as soon as the floor or screen is covered — don’t wait.
  • Turn the fan on only on hot days.
  • Turn the fan on, only during the day.
  • Turn the fan on continuously.
  • Turn the fan on only at nights.
  • Turn the fan on, on cold nights only.
  • Turn the fan on if the grain temperature is greater than the outside air temp, and the relative humidity is less than 80%.
  • Turn the fan on when the outside relative humidity is less than the threshold relative humidity given by the grain dryer calculator  and turn it off when the outside RH > threshold RH from the grain drying calculator. I have done correlations with this strategy comparing it to when the drying actually occurs and it has R correlation values > .75.
  • The ultimate would be to have the fan turned on when the conditions are right, and headed for good drying with the grain drying calculator, and then shutting the fan off when there is no drying.  This is calculated by watching how much water is going into the bin, and how much is coming out.  As long as there is a net amount of water coming out of the bin, we will keep the fan on, but as soon as we see more water going in than coming out.  This requires temperature and RH sensors on both the air going in and being discharged.  The temperature and relative humidity are plugged into the pyschrometric equation for saturated air and pro rated with the relative humidity to get the absolute humidity.  We need a few more sensors with this, but it is the ultimate, the Cadilac, of controllers.

Suck or Blow?

I have often been asked whether to blow air into the bin, or suck it out.  The short answer is to blow it in.  There are several reasons to blow:  First we will utilize the heat given off by the motor to aid in the drying process.  If we suck, that heat is just wasted.    Also when we blow, there is compression of the air that occurs, which manifests itself in the form of heat.  We have measured this compression to be 5 or 6 inches of water (supports a column of water in a manometer) and this results in an increase temperature of a couple of degrees.  This also helps in the drying process.  And the third reason is somewhat more practical.  Blowing will keep all the little holes in the screen open, whereas sucking will plug the holes with fines.

How Much Water is in the Air?

Obviously, the amount of water that is in the air affects its drying ability. Less water results in a lower vapour pressure and a better chance that the grain vapour will overcome it. But how does one determine the amount of water in the air. It turns out that it isn’t that onerous. When air is holding the absolute most it can hold, it is said to be in saturation. The saturated amount of water that air can hold is determined almost exclusively by its temperature and it can be obtained from something called a psychrometric chart. The formula for this is:

Ws = 0.000289 *T3 + 0.010873 * T2 + 0.311043 * T + 4.617135

Where Ws is the saturated amount of water (grams) per cubic meter at a given temperature, T (degrees Celsius, ⁰C).

If air is cold, it cannot hold much water.  At 0 ⁰C the most it can hold is 4.6 gr/m3   whereas when the air is hot at 35 ⁰C it can hold almost ten times that at 41.2 gr/m3.

Vapour Wars

Grain drying is a battle between two forces of vapour pressure, that of the air and that of the grain. The water in the air has a force or pressure trying to push its water into everything it comes in contact with. Likewise grain has a vapour pressure in which water is being pushed out of the seed. When air vapour pressure is greater than grain vapour pressure, water is pushed into the grain, and the grain is wetted down. However when the vapour pressure of the grain exceeds that of the air; water is transferred from the grain to the air and we have drying. The vapour pressure (Vp) of the air is determined by its temperature and by the amount of water it is carrying. At any given temperature the greatest vapour pressure occurs when the air is holding its maximum capacity and this is called the saturated vapour pressure (Vps). Compared to atmospheric pressure (14.7 psia), vapour pressure is typically much smaller (1psia).

Grain vapour pressure is determined by the temperature of the grain, the moisture content (MC) and the type of grain. It is measured indirectly by leaving it equalize in vapour pressure with its surrounding air in a sealed environment. This has resulted in the creation of Equilibrium Moisture Content equations which will be discussed later.

Learning the Process of Grain Aeration

Throughout the upcoming blogs, I intend to go through the basic science of how grain aeration dries or wets grain.  I will take you on the journey of how this project started, what we learned along the way, and in the end I will give you the specifics that you need to know to control your fan.  The conditions that will result in drying — turn the fan on; and conditions in which there will be wetting — turn your fan off.  There are very few people who know about this, so you will be on top of very recent research.  I am excited, not only in telling you about the new control strategy but also in educating you as to why it is the ultimate way to control your aeration fan.   Comments of course will be welcomed.