Grain Aeration

First we must agree on what the objective of the controller would be?  I assume that we only want to run the fan when conditions are right for drying; and secondly that we want the grain to be as safe (least spoilage or deterioration) as possible.  And finally, that we want to do this in the most economical way.

What do we need?  We need to know the temperature (T) and relative humidity (RH) of the outside air near the intake of the fan.  We also need at least one OPI moisture cable with T and RH sensors every 4 ft.  This string could be hung pretty much down the middle, with the highest sensor bud, being just under the center ring, but above the grain.  This sensor would be used to detect the T and RH of the air being discharged from the bin.  Also a controller than can turn the fan on and off based on pyschrometric equations. (I did this in a previous blog- How much water is in the air?)

We mentioned before the importance of getting the fan turned on ASAP.  The fan would be turned on while the bin is being filled with the freshly harvested grain, whether it is dry or not.  Then every hour the amount of water in the air would be calculated for the air entering and leaving the bin, using the T and RH sensors for the outside air, and the OPI  T and RH sensor, at the top of the bin sampling the discharge air.  These psychrometric equations can calculate the water in a specific volume of air,  by only knowing the T and RH of the air.  If there is more water in the discharge air as compared to the input air, the fans would remain on.  When an hourly decision time has a calculation that shows that there is more water going into the bin than out, the fan would be shut off.  Then we wait for an hour and then will decide whether or not the fan will be turned on?  But we can’t use our water in/out balance technique that we used to turn the fan off because now that the fan is off, the air around the top sensor is not necessarily representative of the discharge air.  We need to make the decision to turn the fan on based on something else.

To make the decision to turn the fan on, we will base it on the principle of the grain drying calculator .   We can easily calculate the amount of water in the outside air using the T and RH plugged into the psychrometric equations. But the outside air becomes the grain temperature (or very close to) when it hits it, and since we know how much water is in that air, we can calculate its RH for its new temperature, again using the psychrometric equations at a temperature of the grain as given by the OPI sensor.  If this newly calculated RH is less than the RH of the OPI sensor at that sensor bud, then we have a drying condition, for at least that bud.  But the sensor buds are every 4 feet along the OPI string, so a calculation must be done for each bud that is in the grain.  When a majority of the buds have a RH higher than the calculated RH, then we have a majority of the buds indicating that a drying condition does exist, and therefore the fans should be turned on.  Then we wait an hour and use the water balance technique to turn the fan off.

The OPI also can give us the moisture content of the grain by utilizing EMC equations.  However the fans do not have to be operated until all the grain is dry, but only until the average is dry.  When the grain is eventually unloaded, the over dry and under dry grain can be blended to give an overall grain that is just dry.  The above technique also cools the grain, and even if there is some slightly tough grain (probably at the top), it will not spoil because it will be cool or cold.

And that’s it, the ultimate controller.  The really neat thing is that it does not depend on the type of grain, the accuracy of EMC equations, or the moisture content of the grain (other than when we are deciding to terminate the operation when the average of the grain is dry).  It will keep the grain cool and safe, with the minimum amount of fan time.  It is the ultimate.

For years farmers have been advised to use EMC as a guide to determine when conditions are right for running their fans in order to dry their grain.  But, how does this work?  What is EMC anyway?  Why aren’t more people using it as a guide?  Well, let’s see if we can’t answer these questions.

First let’s define EMC.  If one takes a grain, or any biological material for that matter, at a certain moisture content (MC) and put it into a sealed container, and left it for sometime such that the temperature of the grain and that of the air  inside become the same. They have reached equal temperature (T) and are thus in equilibrium.  Depending on the moisture content of the grain and the grain type, the air will reach a certain relative humidity (RH).

This experiment of putting a specific grain, with a specific MC and T into a sealed container; waiting for it to reach equilibrium, and then recording the RH was done thousands of times.  All these points were fitted into an equation to obtain  EMC equations  in which the MC is a function of the T and RH.  Or we can get RH as a function of the T and MC.  This has been done by many researchers such as Henderson,  Chung, Pfost, and Hasley; each with a slightly different nasty looking equation.  They involve natural logarithms and exponents and I will spare you the details.  Each grain has a different set of coefficients.  The American Society of Ag Engineers (ASAE) have published these equations as “Moisture Relationships of Plant-Based Agricultural Products” ASAE D245.5 Oct95. To avoid the ugly equations, EMC is usually presented as a table, but the tables were generated from these equations.

How can this be used by a farmer?  If you have a bin of wheat, in which the fan has been off for some time — at least an hour — and for all intense purposes there is no air entering or leaving the bin, (consider it sealed).  We can take the T and RH, plug it into the appropriate EMC equation with coefficients for that particular grain and we can get the MC of that grain.  The accuracy of the MC is sometimes in question, but usually it’s within one percentage point of the MC. There are many things that erode the accuracy. RH sensors have error, the density of the grain varies, and the variety of the grain will affect the accuracy as well as the amount and kind of dockage and foreign material.

What is EMC of the air?  The recommendations for using EMC to determine if one has good drying conditions goes something like this:

If ambient air has a T of 10 C and RH of 60%, we plug this into the EMC equation for wheat and get 14.2% MC. The EMC of the air is 14.2%. That means if the air conditions stay constant at 10 C and 60% RH, wheat would eventually equilibrate to 14.2%.  Whether the wheat started with a moisture content lower or higher than 14.2% — it would eventually end up with a MC of 14.2%.  If the EMC of the air is less than the MC of the grain, then drying conditions exist.

The above statements are true, but we have some assumptions that are ridiculous to realize.  The outside T and RH are not constants, they are changing hourly.  And the temperature of the wheat is assumed to be the temperature of the air when doing the EMC calculation, and in fact the temperature of the grain is never the same as that of the air.  The grain temperature is always following the air T.  Maybe in a lab you can produce a constant T and RH for hours and hours and eventually the wheat will become the same temperature, but one has no control over the T and RH of the outside air.  And what happens in the mean time — when the grain is at a much different temperature than the air?

Let’s use the above example, with the wheat being 14.4% and at 5 C.  The example says that the EMC of the air is 14.2%, and since this is less than the MC of the grain, we would think that drying would occur.  But it does not, in fact wetting or hydration occurs.  Why?  When the outside air, at 10 C, hits the grain at 5 C, it instantly becomes the same temperature as the grain, 5C (because the specific heat of grain is so much greater than the air).  The amount of water in the air, or absolute humidity, remains the same, and therefore a reduction in temperature will result in a higher RH.  The RH of the air will increase , as the T decreases to 5 C.  In using the grain drying calculator we see that the outside RH must be less than 45.1% to have drying.  Since the outside RH is well above 45; indeed by 15%, we will get some pretty serious wetting.  The calculator takes into account the grain temperature, and the fact that the outside air will become the same as the grain temperature as soon as it hits it.  It predicts the drying conditions right now, not what eventually will happen.  The calculator does not assume a constant air T and RH.  Outside conditions change, and sometimes rapidly.  The calculation should be done every hour to determine the current drying conditions.   EMC of the air is an absurd concept; the air is not in equilibrium with the grain, as the name EMC would suggest.  EMC equations can be used to predict drying conditions, but they must be used correctly and not rely on unrealistic assumptions.

Suppose we have one cubic foot of wheat at a temperature of 30 C, and we pass one cubic air through it at 10 C.  We would expect the temperature of the wheat to go down, and the temperature of the air will increase — but by how much?  We will use something called the specific heat; it is the amount of energy that a substance holds due to its temperature.  I looked up the specific heat for wheat and it is 1.67 kJ/kg C, which means for every degree C, one kilogram of wheat holds 1.67 kilo joules of energy or heat.  Likewise the specific heat of air is 0.716 kJ/kg C

We want to get everything in terms of a cubic foot, so we will need some conversion factors.  Air weighs .0807 lbs per cubic foot or .0366 kg per cubic foot. One bushel is 1.2446 cubic feet, and we will assume that wheat is 60 lbs per bushel.

When the air comes into contact with the grain, the grain will lose the same amount of energy as the air gains.  Using the conversion factors we see that one cubic foot of wheat is 48.2 lbs or 21.88 kg. So the specific heat of wheat in terms of a cubic foot would be:    1.67 x 21.88 = 36.53 kJ/ft^3  C

Air weighs  0.0807 lbs per cubic foot   or .0366 kg per cubic foot.  The specific heat in terms of a cubic foot is  0.716 kJ/kg C  x  0.0366 kg/ft^3 = 0.0262 kJ/ft^3 C

So the wheat has way more energy (36.53) than the air (0.0262); in fact it has 36.53/0.0262 =  1,394 times more,  and with difference in temperature, 30 -10 = 20. The wheat will go down in temp, but only 1/1394 x 20 = 0.0143 deg C below 30, 29.98 C and the air will increase 1,393/1,394 x 20 = 19.98  above its 10 to 29.98 C

So to conclude, we see that the air becomes almost the same temperature as the wheat, and because the surface area of the wheat is so large, it is clear that this heat exchange would happen without delay.

Another way to look at this is that one air exchange would move the temperature of the wheat approx  .02/20, one thousandth the difference in temperature, but after many heat exchanges, the temperature of the grain will go down, and the difference becomes less.  So it might take a thousand air exchanges to get the temperature of the wheat from 30 to 20.  If our fans have a flow of 1 CFM/bu. and since one bu is close to one cubic foot, we would get an air exchange every minute.  A thousand air exchanges would take over 16 hours.  The data that we have collected over the years shows that cooling the grain is much faster than this, it can be cooled to near the air temp in a matter of hours with air flow close to 1 CFM/bu.  There is more at play here and there must be something else, the grain is being cooled by water evaporating from the grain into the air.  This is called the latent heat of evaporation; and this will be a topic for another day.

So, you want to use supplemental heat because your grain is a few percentage points above dry but too cold for natural drying.  Let’s say it is 5 C, and moisture content is 15% where dry is 12%. Are you looking at getting it dry quickly so that you can sell it, or is it that you just want to get it dry for long term storage?  No matter what, if you have it cooled down to 5 C, it won’t spoil; even if it is a few points on the tough side.
Sure adding supplemental heat will speed the drying process, but there is a down side — it costs money.  But OK we will consider spending some money to get it dried quicker, then you must decide what you will use for a heat source, and when should you apply it.  First,  I would apply the heat when the outside air is the driest and that is at night, especially a clear night when the relative humidity is low (less than 70%).  I would apply the heat for a few hours, let’s say from 9 PM until 2 AM and then you must immediately cool it down — say 2 AM until 6 AM.  The drying does not really take place with the heating, it takes place when the grain is cooled.  Several short sessions of heating and cooling are more effective and efficient than trying to do it all in one session.
But what should I use for a heat source?  You could use a petroleum product, like gas, diesel, propane or natural gas; of these I would go with natural gas if you have it, because it is the cheapest, and burns the cleanest. However, these petroleum products have some down sides.  First when they burn, they give off water, and that’s kind of counter productive.  They also will leave a residual petroleum odor on the crop — this may or may not be a problem?  And you are indeed playing with fire. For example if you have a flame-out, you could fill your whole bin with a very explosive mixture.  Or you could easily start a fire if the fan quits,  etc.     Another source of heat that could be used is a vehicle.  Pull a half ton, or tractor right up to the fan intake, and throw a tarp around the whole thing so that the fan sucks in all the heat from the vehicle. Leave the vehicle idle while the fan picks up all the heat from the burning gas or diesel.  This has a couple of advantages, first you are not putting the water, byproduct of the burning, into the grain; it is going out the tailpipe.  And secondly it is much safer, you are not playing with fire.  For every gallon of gas that you burn, you will produce about 123,000 kilo joules (kJ) which is equivalent to 34.16 kWh  At $0.11 a kWh we could easily make a case for using an electric heater in front of the fan.  Maybe get one of those 220 volt, one or two kW industrial heaters?
How much energy do I need to get the grain dry? In other words, what is this going to cost me?   I did a bit of number crunching for wheat to give you an idea.   I  considered one kilogram of wheat, that is at 14% MC, 20 C, and I want to lower the MC by 1%.  I need to remove 10 grams of water that is a liquid inside the wheat and have it evaporate into the air.  The energy to do this is called ‘heat of vaporization’ and for wheat it is 2714 kJ/kg.  To evaporate 10 grams of liquid water from the wheat it would take 27.14 kJ.   We said earlier that one gallon of gas has about 123,000 kJ, which in turn is about 34.16 kWh which would cost us about $4.  And 123,000/2.714 = 44,890 grams of water which is about 100 lbs of water.  So in a perfect world where all the supplemental heat energy goes into evaporating water out of the wheat,  it would cost us four dollars for every 100 pounds of water we removed from the crop. In reality it may be two or three times this, so it will be more like $10 per 100 lbs.  and I haven’t included the cost of running the fan which is about 40 cents an hour.  Now, that seems reasonable, but let’s consider drying down, a 5000 bushel bin of wheat by only 1%. 5000 bushels of wheat weighs 5000 x 60 = 300,000 with 1% being 3,000 lbs of water to be removed.   At $10 per hundred pounds, this would cost at least $300 for the heat alone. And that’s for just a one percent reduction in MC.

OK, but who says you have to use supplemental heat.  You can still dry your grain, even when it is cold and the outside air is cold  when the conditions are right.
I have developed a calculator that tells you when the conditions are right.  It is at http://planetcalc.com/4959/  and you simply enter the MC of your grain,  say 15% ?, the grain temp, say 5 C ? , and the outside air temp, let’s say it is 3 C, Then press the calculate button, and it gives you the threshold RH to which drying will occur with any relative humidity below this.  The more the spread the better.  If your grain is not on the table, use a grain that is close. For sunflower with the above numbers, you should get a threshold RH of 94.4%.   And when I look at my remote weather station, I see that the current relative humidity is 89%.  This is below the calculated threshold RH of 94.4, and it would dry your sunflower seeds, but since there isn’t much spread it wouldn’t be much.  I would wait for better conditions with a bigger spread before I would run the fan.  For example if everything was like it was above, except that today’s RH was 60%, then we would have really good drying conditions, and I would certainly run the fan.  Running the fans only when the calculator determines good drying conditions will dry your grain; it might take a while, but it will dry your grain will keep your grain safe from spoilage and won’t cost you much.