Avoid Condensation During Cool Down

Hi Ron it Jamie @ Victoor Seed Farm Inc. We had talked last fall about grain drying. I had a question for you. We are having a wet fall so far in AB. Relative Humidity Very High but if air temp around 8’C at night and grain @ 24’C will I put much moisture into Bin as I want to Just Cool Grain Down. We only have Temp Cables in Bin not moisture & Temp Cables. Grain from 13% Moisture with 2000 Bus of a 10,000 Bus Bin Testing 16%

On Sep 7, 2016, at 2:18 PM, Ron Palmer <Ron.Palmer@uregina.ca> wrote:
> Jamie, this is exactly what my grain drying calculator was made for.  You can find it at planetcalc.com/4959/  you just punch in the moisture content, 16, and the grain temp, 24, and the outside air temp of 8 and it comes back and tells you the outside relative humidity, below which, drying will occur.  If you scroll down to hard spring wheat it will give you 214.4%.  This means that as long as you have an outside relative humidity of less than 214 you will be drying your grain.  So, let’s say the RH is 85% outside;  85 is much less than 214, so yes you will be drying, and because there is such a huge difference, you will be drying a lot.  However you have another problem–a big problem.  The moisture coming off your grain will condense when it hits the cold roof walls and roof, and the moisture that you just got out of your grain is going to be raining back down onto your wheat.  You should have cooled and dried your grain as soon as you put it in the bin, before it got so cold.  But what to do now?  Turn your fan on when it is warmer so that condensation will not occur.       I used the calculator and plugged in 20 for an outside temp, and it came back with a threshold RH of 97.4   In fact any number below 100 and you won’t get condensation on the inside of your bin.  So, let’s say you turn your fan on when it is 20 and RH is 70.  Will you be drying? Yes quite a bit.  Will you get condensation? No   The temp of the grain will come down fairly quickly, in just a few hours of running your fan the grain temp will maybe go to 22, and then we can use a lower outside temp. and still be below the 100.   And sure enough these numbers give a calculated threshold RH of 97.5
> So, here is what you do.  Wait for a slightly warmer day of 19 to 20 degrees.  Turn your fan on and as the temperature of the day goes down, the temp of the grain will also go down.  I am thinking that you should be able to chase the grain temp down fast enough so that you won’t be getting condensation.  But you can use the calculator to make sure you are not in a situation where the threshold RH is larger than 100.    I have loaded the grain drying calculator onto my Iphone and use it like an app.
> Another thing,  we found that cooling your grain down by 15 C will typically remove 1% moisture.  You cooling the grain from 24 to say 9 C should get your moisture close to 15%

FAQ: Why has it taken so long to discover these new grain drying techniques.

I was doing some reading from a comprehensive text on grain drying: “Drying and Storage of Grains and Oilseeds” by Brooker.  It occurred to me that there were some preconceived notions about grain drying that have held in abeyance some of findings that I have published in my blog.  Sometimes researchers must make assumptions to fill in for gaps in the unknown.  But that doesn’t mean that the assumptions should be challenged when more information comes to light.  I believe that is what is happening here, now that we have hourly data of what is happening for grain drying on farm sized bins in a typical prairie environment. I think it would be interesting to discuss the status quo assumptions.

  1. For the ambient drying conditions, the mean or average temperature is used. But there is a large difference in the high and low temperatures during the day.  Our data was collected hourly and a compilation of many years of experimental data has led to the discovery of the diurnal drying cycle, that drying takes place at night and quite commonly wetting occurs during the day.
  2. In the literature,  there is mention of the outside air (at the mean temp) comes into equilibrium with the grain.  And the implication is that this equilibrium comes about instantly.  It implies that the grain becomes the same temperature as the air.  But what actually happens is that the air becomes the temperature as the grain.  Grain is a thousand times more dense than air, and holds way more heat.  Sure after many many air exchanges the grain will start to move to the temperature of the ambient air.  But the ambient air is not at a constant temperature (assumed again to be the mean), it is changing all the time, hour by hour. As such the ambient air and grain are never in equilibrium.
  3. Farmers and researchers both know that the top of the bin is the last to dry.  Some of the literature even talk about a drying front or drying zone moving upwards.  Our data showed that the bottom grain tended to be a few degrees warmer than the top.  This indeed would cause the bottom to dry more.  But what is causing the increased heat at the bottom.  I believe it is compression.  When a gas, like air, is compressed it immediately heats up and as the air works its way to the top it decompresses and subsequently cools.  It is quite typical for a fan to produce a pressure of 5 or 6 inches of water, and if one works through the math for this pressure increase with the equation, PV=nRT, one will see that indeed the temperature will increase with the compression.  This is important to understand because the way to mitigate this top/bottom drying difference is to use smaller fans with less pressure.  I have seen recommendations that would suggest the opposite.  This problem can be solved by using bigger fans with more air flow, which can only be achieved by having more pressure, more compression.
  4. It is suggested that aeration fans can be used for drying with a higher airflow of 1 CFM/bu  or it can be used for cooling at a lower air flow of  0.1 CFM/bu.   Our data shows that drying can also occur at lower air flows and that drying and cooling are synonymous.   Our data shows that cooling the grain with an aeration fan will dry it.  Heating the grain typically wets it.  Pyschrometric equations provide the rationale for this occurrence.
  5. The latent heat to dry the grain must come from external sources or from the ambient air.  The inherent heat in the grain itself does not seem to be considered.
  6. There is no scientific reasoning to determine what the air flow rate should be.  I have seen recommendations like: “Get to know from experience”  or the popular belief is 1 CFM/bu for drying,  0.1 for cooling.  But I have not found any basis in science to back this.
  7. No control strategy.  It is assumed that the fans will run continuously, 24/7, and the number of drying hours are based on the mean temperature.  It turns out that there are periods of time with wetting.
  8. The way to prevent spoilage is to get your grain dry, and to do it as quickly as possible.  Indeed having your grain dry is an important factor in preventing spoilage but having your grain cool or cold is even more important.  We can get the grain cooled quickly, but it might take days or weeks to get it dry.  We have been kind of brain washed into thinking that the only thing that is important is to get your grain dry.   What is of utmost importance is to get your grain into a safe condition, one with the least spoilage.  We have to change our mindset into thinking:  “How can I get my grain into a safe condition, with the least spoilage, as quickly as possible?”  We can take our time at drying, what’s the hurry?
  9. A humidistat can be used to determine when there are drying conditions.  I read yesterday that one researcher felt that setting the humidistat at 55% was the threshold humidity.  What’s wrong with this?  First a humidistat measures relative humidity not humidity.  And relative humidity and absolute humidity are not the same.  If you give me the temperature and the relative humidity, I can calculate the absolute humidity, but by just giving the relative humidity it means nothing.  It seems to me that the implication is that one should be using the mean temperature again.   I am not sure — but I will say this, using just a humidistat will not be a good control strategy for your fan.  I know this from experience.  In the 1970s we had a grain dryer that we tried to use a humidistat for control — it did not work at all.  And logically now, I can see why.  We now know that if the air in the bin has more water in it (absolute humidity is high) than the ambient outside air; we will have drying.  Let’s say the air inside the bin is 20ºC @ 70% RH, using the absolute humidity table –> 12 gr/m^3 .  Now let’s assume the humidistat is reading an RH of 55%, will there be drying?  Yes if the outside air temp is 10ºC @ 55%  gives an absolute humidity of 5 grams, which is less than the absolute humidity of the air inside, 12 gr so we will have drying.  For every cubic meter of air flowing through the bin there will be 7 grams of water removed.  However let’s see what happens if it is not 10ºC, but rather much warmer at 25º C.  Then the absolute humidity for air 25ºC @ 55% RH is 13 gr.  At this temperature, for every cubic meter of air that flows through the bin we will be adding 1 gr of water.  We will be wetting the grain down.  In conclusion, we see that relative humidity means nothing, unless it is qualified with a temperature.
  10. You have to have heat to dry and drying can only take place on hot days. And yes there is some truth that it does take energy or heat to evaporate the water from the grain.  But the heat does not necessarily have to come from the air. There is a significant amount of heat in the grain itself especially if the grain is at a higher temperature.  The trick is to use as much of that inherent latent heat in the grain for drying.

What are Drying Conditions? Grain Drying 101?

What are the outside air conditions necessary for the drying of grain?   It really is the ultimate question for grain drying.

To determine the threshold relative humidity for drying, we need to know a few more things: the moisture content of the grain, the temperature of the grain, and the temperature of the outside air.  With these we can determine the threshold relative humidity; if the relative humidity is greater than this, we will not get drying in fact we will get wetting and if the relative humidity is below this we will get drying.   The more it is below this threshold, the more it will dry.

But before we get into this, we need to understand the basics of grain drying.  Air  carries the water from the grain.  If the air entering the grain bin acquires more water as it flows through the grain, drying will occur.  If the air being expelled from the bin has more water in it than the air entering the bin through the fan — there will be drying.

The amount of water that is in the air is called the absolute humidity and typically has the units of grams per cubic meter.   A cubic meter of air, one meter by one meter by one meter, will be carrying a certain amount of water, W, and it can be precisely determined from its temperature, T,  and its relative humidity, RH, using the following pyschrometric equation:

W = WS x RH/100

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

Where W (grams/m3) is the amount water in one cubic meter of air, Ws (grams/m3) is the maximum amount of water that saturated air can hold at a specific temperature (T), expressed in 0C, and relative humidity (RH)  %.

To avoid the math, a graph can be used:

Water in the Air
Water in the Air  can be determined by finding the temperature of the air on the horizontal axis and then going up to the relative humidity. The absolute humidity is horizontally to the left.  For example, if the air is 25º C with a 50% relative humidity, it will be carrying 12 grams of water per cubic meter.

A table could also be used and again a temperature of 25 ºC with a relative humidity of 50% will have air carrying 12 grams of water.  Sat Table



There is also a calculator online that can calculate the absolute humidity:  http://planetcalc.com/2167/

So, there are a number of ways to determine the absolute humidity of the air. If one knows the temperature and relative humidity of the air; the absolute humidity can be found by doing the math with the equation, or by using the graph or the table, or by going online and using the on-line calculator.

Now drying will occur if the absolute humidity of the outside air entering the bin through the fan, is less than the absolute humidity of the air being expelled from the bin.  For example, let’s say that the air outside entering the bin is 15°C @ 55% RH. The absolute humidity of the outside air is:  12.7 gr.  X  0.55 =  7 gr/m3.  The air being expelled is 25°C @ 45% RH with an absolute humidity: 23.7 gr x 0.45 = 10.67 gr/m3.  So for every cubic meter of air that flows through the bin of grain there is 10.67 – 7 = 3.67 grams of water being removed. Drying is occurring.

It should be noted that even though the relative humidity (RH) of the air entering, 55%, is greater than the RH of the air being expelled, 45%; the absolute humidity of the expelled air is higher than the outside air.  Relative humidity, by itself, means nothing; but if one knows both the RH and the temperature, then RH is very useful and can be used to easily calculate the absolute humidity.

If one knows the air flow through the bin, one can calculate the amount of drying.  In the above example 3.67 grams of water was removed for every cubic meter of air that flowed through the bin.  If the airflow was 3000 cubic feet per minute, CFM, then:

  • Are we drying? Yes 10.67 – 7 = 3.67 gr/m3
  • How much? 3000 CFM = 180,000 ft3/hr.

180000/35.41 x 3.67 = 18.6 kg/hr.  water  is removed every hour

The problem is that the air temperature and relative humidity continuously change during the day.  The temperature during the day can be more than 10 ºC higher than at night.

The above technique was used to measure the amount of grain drying done on an hourly basis with farm sized grain bins.  19 experimental drying trials were done with the fan running continuously, and the drying data was compiled to determine the amount of drying that was done in terms of the time of day.  A diurnal drying cycle was determined:

Diurnal Cycle

It can be seen that the greatest degree of drying occurred at night at about 2:00 AM, wetting occurred during the day, 14:00 or 2:00 PM and the transition from drying to wetting occurred at about 9:00 AM.

If drying occurs at night, and wetting during the day; wouldn’t it make sense to run the fans when we typically have the best drying conditions?  This was the basis for the recommendation that the fans should not be run continuously but rather only at night — the yard light rule:

On at night, you are bright; on  during the day, you will pay!

Finding the absolute humidity of the air inside the bin  involves the use of the temperature and relative humidity, but the bin is probably not equipped with relative humidity sensors. The relative humidity can be determined indirectly by  the use of EMC (Equilibrium Moisture Content) equations  —  a topic for another blog.

Grain Dries Much More at Bottom than Top! Why?

We have all seen it, the bottom of the bin having a moisture content three or four percentage points below that of the top. Why?

To explain this phenomenon, we will use our setup with a 2200 bushel bin, a 5 HP aeration fan and we will look at two examples one with barley @ 20 ⁰C and MC 15%, and another with canola @ 20 ⁰C and MC 11%. In both cases the fan produced a pressure that supported a six inch column of water. This was measured with a home- made manometer which consisted of a plastic tube shaped into a U with coloured water in it. The air flow was 3000 CFM.

Even though the outside temperature is 20 ⁰C, the temperature of the air behind the fan is warmer because of heat given off by the motor and because of compression. Let’s first look at the heat from the motor. What air temperature rise can we expect from it?

I am guessing here, but let’s say that the 5 HP motor is 90% efficient, which means that 10% of the energy from the 5 HP motor will be going into heat and this is caused by wire resistance, bearing friction, and even air friction on the fan blades.   1 HP = 0.7475 kW so 5 HP = 3.737 kW and ten percent of that would be 0.3737 kW or 0.3737 kilojoules/sec will go into heating the air. The air is flowing at 3000 CFM or 50 ft3//sec and the weight of one cubic foot of air is 0.0807 lbs or 0.0366 kg, so 50 cubic feet would be 1.83 kg of air going by per second. The specific heat of air is close to 1 kJ/kg.K⁰ So the temperature rise of the air would be: ( 0.3737kJ/s / 1.83kg/s ) = 0.2 ⁰C. The heat from the motor would increase the temperature of the air from 20 to 20.2 ⁰C.

But the big increase in temperature is not from the motor but that of the compression. We know that the pressure behind the fan is enough to support a column of water six inches high. We know this because we measured it with our home-made manometer.   How much of temperature rise will we get from this pressure or compression? There is a thermodynamic formula that relates pressure to temperature: PV = nRT.   Pressure and Temperature are proportional. A typical pressure of 1 atmosphere will support a column of water 406.8 inches. And a typical temperature is 273 ⁰Kelvin. So an increase in pressure will produce a corresponding increase in temperature:   6”/4068” = x / 273 and this gives an x of 4 ⁰K or 4⁰ C. The air and grain at the bottom of the bin would be 24.2 ⁰C, and as the air flows to the top the compression would get less and less until at the top there would be none, and the temperature of the grain and air would be back to 20 ⁰C.

Does this increase in temperature affect the MC? Yes it does, and we will look at the barley at 15% MC and the outside air is 20 C and so is the barley at the top of the bin. Now we will use the grain drying calculator and plug 15 in for MC and 20 for both the outside air and grain temperature, this gives an RHthres of 68.6% and now we can use the relative humidity to absolute humidity calculator to see that the absolute humidity is 12 grams per cubic meter. We have assumed that the air at the top of the bin has reached equilibrium with the grain; the relative humidity of the air is 68.6% and the absolute humidity is 12 grams per cubic meter. We are at equilibrium – no drying or wetting is taking place at the top of the bin. However at the bottom of the bin the temperature is 4.2 ⁰ C warmer at 24.2. We are assuming that no drying or wetting is taking place so the absolute humidity at the bottom will be 12 grams per cubic meter; the only thing that has changed is the temperature. Using the humidity calculator again by setting the RH to 100 and the temp to 24 gives a saturation humidity of 21.8 gr, and since our absolute humidity is 12, the RH must be 12/21.8 = 49.6%. By using the grain drying calculator with an air and grain temp of 24.2 entered, and by trial and error entering MC until the RHthres is close to 49.6. I found that if I entered 11.6% for the MC, I got 49.4% for RHthres – close enough. This means that the barley at the bottom of the bin will be in equilibrium with the air around it at a MC of 11.6 and at the same time we have barley at the top that is 15% MC in equilibrium with the cooler air. This is a spread in MC of 3.4%.   And yes we have seen this type of spread in our trial runs. The top to bottom spread in MC is quite commonly three or four percentage points different. And now we know why.

Does the same thing apply to oil seeds? It might even be worse, because the pressure might be higher with a grain that has more resistance; but let’s see what happens for a pressure of six inches. Let’s look at tough canola at 11% MC at the top of the bin and again at 20 ⁰C. The outside temperature is also 20, but after it gets heated and compressed by the fan it is now 24.2 C. Using the drying calculator, we plug in 11 for MC and 20 for both the grain and air temp, and it gives us 78.1% for RHthres. And then we use the humidity calculator to calculate the absolute humidity, 13.5 gr/m3. Since there is no drying taking place, the absolute humidity will be the same at the bottom of the bin: 13.5. The saturation humidity for 24.2 C is 21.8 and this then gives us an RH of 13.5/21.8 = 55.8%. Again with trial and error by plugging in different MC we find that a MC of 7.1% is the MC of the canola at which equilibrium is reached at 24.2 C and RH of 55.8%.   The top of the bin is at 11% MC and the bottom is at 7.1 % MC – a whopping 3.9% difference.

I have seen fact sheets and guidelines for drying that suggest this difference in MC from top to bottom is actually some sort of front, and that to get the drying front right through to the top, one must use a bigger fan with lots of air flow, lots of pressure.   But in understanding that an increased pressure will only result in more pressure, more compression; the spread in MC from top to bottom will only be worse. To mitigate this MC spread, I think there are a couple of things we can do. First, use smaller fans with less pressure, and secondly don’t run the fans continuously; give the moisture a chance to equalize. We also don’t want to have a pressure drop across the screen or perforated pipe. I would certainly be an advocate for open bottom pipes or louvers.

There is another complicating factor that aggravates this situation.   I assumed in this analysis that no drying was taking place. We were at that point in time when the drying was done, and equilibrium was achieved at the top and bottom. However when we first start the fan, the grain at the bottom was just as tough as the grain at the top and the bottom would dry first. And when it dries, it must give up energy and heat to vaporize the water. The drying will be another cooling agent, cooling the air as it goes to the top. This will keep the top cool, the bottom warm – the top wet, the bottom dry. It also means that water will be added to the air, and that the absolute humidity of the air at the top of the bin will not be the same as at the bottom; it will be higher. Now we have air at the top that is even colder and wetter than it would be if no drying was taking place at the bottom. It is no wonder that the top grain has no chance of drying until the bottom is finished drying and much over dried.


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.

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.

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