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.

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.

Roof Drips


Hi Ron,

The situation is this:

–        His flax was harvested at 8 % moisture roughly a week ago

–        It was put in the bin right away, with no drying, or fans

–        Two days ago he decided to put the fan on because the flax was at 27 C. He ran it for a day and night.

–        When he checked back, there was moisture on the inside roof of the bin, the grain has not changed temperatures


I suggested he start running the fan 9 pm to 9 am, he could also open the hatch on good weather days overnight.. And could consider augering out half the flax and then back into the bin to force air through it.

Did he actually add moisture to the grain by running the fan during the day?

Any thoughts, comments are appreciated.


Good question:
I don’t have all the facts here, but I will run with what you have given me.  The flax is 27 C with a moisture content of 8%.  It’s been standing like this for a while so it will have reached equilibrium with the air in the bin  at 27 C and a relative humidity of 65% ( I used the Henderson EMC equations for canola, I didn’t have the product coefficients for flax, but they should be similar to canola as they are both oil-seeds).  Now when this air hits the cold bin roof, it will cool to the point at which it can no longer hold water. This can all be calculated using the pyschrometric saturation graph and in equation form:

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

where T is in deg C, and Ws is the most water that air can hold at that temperature (saturated) and is in units of gr/m^3

The relative humidity tells us what percentage of this saturated amount is in the air.  I have attached.   In our case with air at 27C and 65% relative humidity, the air is holding 17.3 gr/m^3.  But air at 19.5 C can only hold , at most 17.3 gr/m^3  –if the air gets any colder it will start dropping it as liquid water.  So if the bin roof is any colder than 20 C, you will find that water is condensing and running down the sides.  Even though the flax is dry!

We certainly don’t want water raining down on the flax.  Normally your advice of running the fan from 9 PM to 9AM is good advice, but not so much here, because at this time of year, you can pretty much guarantee that the roof will be colder than 20 C and we will get more condensation.

I think the best thing to do is wait for a sunny day (sun heats roof) above or at least close to 20 C, and start the fan and cool the flax down, and as the flax cools, the air surrounding it will contain less and less water.  For example once we get the flax down to 20 C (still at 8% MC) the temperature of the roof can be lower before condensation occurs; that is 12 C.  And once you get the flax to 15 C, the roof will have to be at or below 7 C before condensation will occur.

How do I know all this?  I have made a grain drying calculator, that I now run in Excel, but I hope to get an app made for an iphone.  It does all the math, the only thing you input is the moisture content of the grain,  the temperature of the grain, and the outside air temperature.  It gives you what I am calling the threshold relative humidity.  If the outside relative humidity is less than this threshold humidity, your grain will dry; and if the outside relative humidity is greater than this threshold relative humidity your grain will get hydrated.   And you can go through exercises like we just did above to know when your bin is going to start raining inside.  It is really neat.  I am not releasing it yet, we will be validating it on our past years of drying data.  And I don’t have the coefficients for all the grains yet.  I don’t have them for flax, rye, peas, and oats.  I would like to release the calculator with all the common grains.  Stay tuned!  And I would be more than willing to answer any other questions.
Just another thought, if the farmer’s flax is rising in temperature (heating) then you may not be able to wait for the roof to heat up — get the fan turned on, you have to get that flax cooled right now, even if there is some condensation.

Ron Palmer

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.