Category: Basics

There is a false sense of security when we think that dry grain is actually dry.  Let’s take for example wheat, at 20 deg C, with a moisture content (MC) of 14%. Wheat is typically considered dry if it has a MC < 14.5%; but just how much water is in the grain as opposed to in the air?  Let’s see!  Consider a volume or space of one cubic meter.  This is 27.5 bushels and at 60 lbs per bushed, this would have a weight of 1650 lbs.  We said the MC was 14%, so 1650 x 0.14 = 231 lbs or 104.78 kilograms.  That is, one cubic meter of wheat at an MC of 14% has 104,780 grams of water in it.  Now air at 20 deg C that is saturated, 100% relative humidity (RH) contains 17.5 grams of water.  But we shouldn’t be looking at saturated air in the bin, it is more likely to be around 75% RH and one cubic meter of air at 20 C, 75% RH would have 13.2 grams.  However in our one cubic meter of grain, the grain occupies 60% of the space, and 40% is air.  Therefore the water in the air is 0.40 x 13.2 = 5.28 grams.

So, this so called dry grain is actually caring 104,780 grams of water per cubic meter, and the air surrounding it is holding a tiny fraction of that, 5.28 grams. Even wet air at 100% RH contains thousands of times less water than the so called dry grain it surrounds.  So, it is not so dry after all.  And it is truly amazing that a seed can retain such a large amount of water.

What we have learned here is important to understanding grain drying. Let’s suppose that we were blowing air through the wheat and that it went from 50% RH to 75% by picking up some water from the grain.  How many air exchanges would it take to lower the MC of the grain by one percent?  14% was 104.780 grams of water,  so 1% would be about 7,484 grams of water to be removed.   One air exchange going from 50 to 75% would take out 1.75 grams.  Therefore we would need over 4,000 air exchanges to lower the MC by one percent.  Now one bushel is 1.2446 cubic feet, but the air in between the wheat would only be about a half a cubic foot.  At an air flow rate of  1 CFM/bu, an air exchange would occur every half a minute and we need 4000 air exchanges, 4000/ (60 x 2) ,which would take 33 hours.   It would take over a day of absolutely ideal conditions to knock down the MC by one percent.  If we are using the grain dryer calculator this means we would need the outside RH to be 25% less than the threshold RH for 33 hours. This example contains gross approximations and assumptions like the grain and air remaining at 20 C (it won’t);   but it does suggest how an estimate of the drying time could be determined.

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.

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.

I have given many presentations in which I use a power point presentation.  This has been converted to  .pdf and be found at:

http://www.farmingsmarter.com/wp-content/uploads/2013/12/Palmer-A-practical-guide-to-natural-aeration-grain-drying.pdf

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.

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.