When should I turn my fan on? turn it off? — The ultimate controller!

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.

What is EMC (Equilibrium Moisture Content)?

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.

First Day is Critical

Grain starts to spoil the moment it is harvested.  The deterioration can be slowed down to almost nothing by cooling and drying the grain.  Grain can be cooled in a matter of hours with natural aeration, especially with cool night air.  Drying may take a little bit longer, but our data has shown that if grain is cooled by 15 deg C on the first day, that it will lower the moisture content (MC) by one percentage point.  So we see that cooling is drying if done with aeration.  Leaving the grain cool by itself in the bin does not result in any drying.  Even if your grain is dry, it is important to get it cooled as soon as possible, to put a halt to the deterioration process.  The fan should be run as soon as the grain covers the screen and left on until 9:00 the next morning.

At one time it was suggested that grain should be left hot to sweat out the moisture.  This is not a good idea; the grain is starting its deterioration process, it is spoiling and losing quality.  Get it cooled down with aeration as soon as possible.  Once it is cool,  it is safe from spoilage.  Then getting it dry is secondary.  You can take your time and use the calculator to figure out when to run the fan.

The take home is this:   If you have aeration fans, use them, especially the first day the freshly harvested grain is stored; don’t wait.

What happens when cold air hits warm grain?

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.

What about Supplemental Heat?

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.

Is dry grain really dry?

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.

Planning Future Blogs

I hope to have future blogs — in no particular order and I am putting this out there to perhaps get an idea of what I should do first:

  • Aeration — a useful tool to get ahead of the game, and lower risk.  Combine the grain tough, you can better manage the grain in the bin as opposed to in the field.
  • Tips on storing your grain, including the above, start the fan right away, keep the fan on for the first day, keep the grain as cold as possible, take out a load to get an inverted cone, put the toughest grain at the bottom by starting each new day, when the grain is tough with a new bin, and of course fan control strategies.
  • Using the new calculator http://planetcalc.com/4959/
  • Comparing the new calculator results with those of actual bin drying.
  • The first day, why it is so important  ie get the grain cold to be safe.
  • The ultimate control,  use the OPI moisture sensor, when the fan is off, calculate the moisture in the air around the grain, and when it is greater than the moisture in the outside air, then turn the fan on.  And to turn the fan off, check the moisture in the discharge air, as opposed to the outside air.
  • A list of control strategies depending on what you have:  no sensors,  temp sensor, temp and relative humidity sensor.
  • Diurnal Cycle
  • How fast does grain warm?
  • How fast does grain cool?
  • Cooling is drying?
  • Safety, is a factor of dry grain and cool grain, and the formula that provides  a measure of safety quantitatively.
  • Why the old way of using EMC of the air does not work, along with examples of actual data to show why.
  • Why using mean daily temperatures is misleading.
  • Why does grain dry in the bottom of the bin first.
  • Correlation of actual drying, with fan on only when grain temp > air temp.
  • Correlation of actual drying with calculator threshold RH results
  • How much should grain cool, when energy is used to evaporate the water in it.
  • Condensation on the roof, why does it occur, when does it occur, how to prevent it. Using the calculator to detect conditions for condensation. http://planetcalc.com/4959/
  • This year’s runs on all six bins.
  • Organizing all these topics and items into a more cohesive organized document.

So, I have a lot to do, and as the colder weather sets in, I will have more time to get at this.  Of course your comments and questions are a good source of inspiration and motivation.



Grain Drying Calculator

Natural air can be used for drying grain if the conditions are right. This calculator will determine if the ambient outside air is suitable for drying. The inputs are grain moisture content, %, grain temperature, C, and the outside air temperature, C. The output is the threshold relative humidity for several different types of grain. If the outside air’s relative humidity is less than this threshold drying will occur. A larger difference is indicative of better drying conditions.

The calculator grain drying calculator is found at planetcalc.com/4959/

When grain at a specific moisture content is allowed to equalize with the surrounding air, it will approach a relative humidity as determined by its moisture content. Equations relating the relative humidity, the temperature and the Moisture Content at Equilibrium have been developed (EMC) and can be found: ASAE D245.5 ‘Moisture Relationships of Plant-based Agricultrual Products’. These equations will give the relative humidity of a specific grain, that is at a specific moisture content and temperature. If we blow air into the grain that is at the grain temperature; but has a relative humidity below this EMC relative humidity, then drying will occur.

However the outside air temperature is not the same as the grain temperature as it swings up and down in its daily cycle and the grain temperature reluctantly chases. We must find this threshold relative humidity for the air that is at ambient temperature, not grain temperature. We will use pyschrometric saturation charts and equations to do this.

The EMC equations gave us the threshold relative humidity for air at the temperature of the grain. So, first calculate the maximum amount of water (saturation) that water could hold at this temperature (grams of water per cubic meter of air). Multiply this by the EMC threshold relative humidity as determined by the EMC equation, and this then will be the amount of water that is in the air for the grain at that temperature.

When the outside air hits the grain in question it will become the same temperature as the grain because it is much, much denser. It will have changed temperature, but will contain the same amount of water (absolute humidity). And we just calculated this absolute humidity for the air at EMC. Now calculate the saturation absolute humidity for air that is at ambient outside temperature. The ratio of the EMC absolute humidity over the saturation absolute humidity is the Threshold Relative Humidity for drying for air at ambient temperature.

If the outside relative humidity is same as this threshold relative humidity, then no drying or wetting will occur. If the outside relative humidity is greater, than wetting will occur. And if the outside relative humidity is less than this calculated threshold relative humidity, then drying will occur.

This calculator can also be used to determine when condensation will occur on the interior of the walls and roof. This is the case if the calculated threshold relative humidity is greater than 100. This happens when the air temperature is much less than the grain temperature. The cold outside air goes through the grain, is warmed and moisture is added but when it hits the cold exterior walls of the bin that are the same temperature as the outside air, this discharge air becomes over-saturated and water is expelled in the form of condensation. This condensation can run down and form pockets of grain. It is not recommended to run the fans if conditions have a threshold humidity > 100.

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.