Running the Fan Continuously: What’s wrong with that?

Running the aeration fan continuously has been the conventional wisdom since aeration fans came into existence.  The truth is that no one really knew what was going on, and that eventually the grain would come down in moisture content. But there also was this lingering feeling that there must be times when drying was occurring and times when it was not.

I have also heard, many time, that one must keep their fans running because if you stop it, the moisture layer will collapse and a crust layer will form.  So, maybe it is safer to just leave it on.  And since we don’t really know when the conditions for drying are, maybe it is best to just leave it on.  Let’s address this one first.

In all the data we have collected at Indian Head, since 2007, we have never seen a distinct drying layer, or moisture band.  We have certainly see the bottom dry first, and in many cases at the end of the trial run, the top of the bin is still tough, while the bottom is over-dry.  But there is no distinct layer, or even pockets of moisture.  The change in moisture from one part of the bin to another, is a slowly changing continuum.  And when the fan is shut off, the temperature and moisture of the grain more or less remain constant, or at the very least changing ever so slowly.   Turning the fans off, is not the culprit reason for a crust forming.

Now for the other reason we leave the fans on: we don’t know when the conditions are right for drying.  Well, now we do.  We know the typical diurnal drying cycle and with the grain calculator one can exactly tell when there are drying conditions, and ever for conditions of condensation.  So not knowing when we have a drying condition, no longer is a valid excuse for running the fans continuously.

So what’s the problem in running your fans continuously?  In short more spoilage. Grain starts to deteriorate as soon as it comes off the combine. Storing grain can only slow down the spoilage process and there are two things that contribute to spoilage: higher grain temperatures and higher grain moisture. If you run the fan continuously during the day, there is a very good chance that you will be heating and wetting the grain — the exact things that contribute to spoilage.

To summarize, running the fan continuously will:

  1. Produce more spoilage
  2. Use more energy
  3.  The Figure below is an example of a run done in 2009. In the first few hours of operation the amount of water being removed from the bin is quite high. It was not unusual to have one percent moisture removed from the grain on the first day.  But at hour 21, we see something strange take place, the amount of water leaving the bin becomes negative; we are adding water to the bin. We are now into a well established 24 hour cycle of water being removed from the bin and then water added to the bin — almost equal amounts.  After the first 21 hours, there is essentially no drying taking place.  We are literally spinning our wheels, taking water out of the bin, and then putting it back in. And to make things worse we are adding the moisture during the day, as we heat the grain.  Warming the grain, and wetting it at the same time are just the conditions necessary to promote spoilage.
  4. Hourly Water Removed From Bin
    Hourly Water Removed From Bin


BINcast critique

When should the fan be turned out?  There is another calculator — other than mine that tells you when the fan should be operated.  But it is based on assumptions that are not realizable — judge for yourself.


Hi Ron
Have you seen the BinCast calculator (
Does it work on the same principles as your methods?


On Thu, Sep 22, 2016 at 3:34 PM, Ron Palmer <> wrote:

I have not seen BINcast as a webpage, but have followed the theory of operation.  It does not work; because they do not take into consideration the grain temperature. Here is the reasoning behind it, and at first glance, it appears to be a rational theory:
If one takes the outside air temp and relative humidity, and inserts these into an EMC equation for — let’s say barley — you will get an Equilibrium Moisture Content. Let’s assume this is 16%.  If I blow this air at my barley, it will eventually reach a moisture level of 16%.  And that is true, if this air is blown at the grain long enough, eventually the barley and air would reach equilibrium.  Reaching equilibrium means that the barley must be the same temperature as the air.  One can not assume that the barley is the same temp as the outside air, in fact it never is.  It takes hours and hours of blowing air (probably a thousand air exchanges) to change the barley’s temperature.  But the outside air is not a constant it is changing hour by hour, and the barley is always playing catch-up.  The outside air and the barley are NEVER in equilibrium.  Sorry but this is not a good calculator.  I can come up with example after example, where this calculator will give the wrong answer.
I use EMC equations, but for temperature I use the temperature of the grain, (because the air becomes the temp of the grain) and I use the EMC equations differently.  You will notice I ask for the moisture content of the grain; using the moisture content and the temperature of the grain, I get the equilibrium relative humidity. That is where the grain wants to put the air’s RH.
If you are really interested, I can give you concrete examples of how the BinCast is totally different than my calculator, and it makes the huge false assumption that the air and the grain are at the same temperature.
Ron Palmer

Convection Current Flow Rates

We have all seen those little diagrams of curved pointed lines that indicate convection currents in a grain bin.  How in the winter the air flows up through the middle and down the sides, and then in the summer where it flows down the center of the bin and up along the warm side walls.  We can also get convection currents right through the bin with air flowing through the fan, vents, and center opening.  But just how fast is the air moving with convection currents? Let’s see.

First of all, the only reason we get convection currents is because of the air being at two different temperatures.  Air is heavier if it is colder.  A column of cold air has more weight than a column of warm air, and consequently will push down with a pressure equal to the difference in weight.

According to , the weight of air is:

ºC         ºF         lbs/ft^3

10        50          7.786

21.1     70          7.492

32.2      90         7.219

If the outside air and the grain are the same temp, the air in the bin and air outside will have the same weight and there will be no pressure difference and therefore no convection current through the bin.   But let’s say that the grain is at room temperature of 21.1ºC , 70ºF, and the outside air is at 10º C.  That is quite a difference in temperature and also a difference in weight, 7.786 – 7.492 = 0.294 pounds per cubic feet.  But the grain in the bin is 20 feet high, so 20 x 0.294 = 5.88.  A column of air, 20 feet tall,  inside the bin weighs 5.88 lbs more than a corresponding 20 foot column of air outside. But 60% of this column is occupied by grain, and only 40% is air; so this column of air is only 2.35 lbs heavier.  But that is the pressure exerted over one square foot, psi or pounds per square inch is a more common unit of measure for pressure. 2.35/144 = .016 psi  This isn’t very much pressure, when you think of our car tires have 30 psi.   Another common pressure unit is inches of water.  How many inches of water would this pressure support? One psi would support a column of water 27.68 inches.  0.016 psi is equivalent to 0.452 “H20.

To determine the flow, one must have some appreciation for the grain resistance. From our past trials we saw that 3000 CFM required a pressure that would support 6 inches of water. Now if we said that pressure and flow were in a linear relationship, (it is not –it is closer to a squared),  the convection current would be about one thirteenth or 226 CFM.  This is a reasonable flow.  The bin we had was about 200 sq feet. so a flow of 226 CFM would be a vertical speed of 1 ft per second or about 1 kmph; but then again one must remember that this is only for a very significant temperature spread; as the temperature of the grain and the outside temperature become less, so too would there be a reduction in the convection current.

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 <> wrote:
> Jamie, this is exactly what my grain drying calculator was made for.  You can find it at  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:

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.

Notes to self about ideas for Future Blogs

  1.  Grain Drying 101    Drying Conditions if the absolute humidity of the air inside the bin is greater than the absolute humidity of the outside air.  Explain what absolute humidity is:  amount of water in one cubic meter, explain how it is different from relative humidity,  and that absolute humidity is calculated from the saturation curve and pro rated with the relative humidity.  Diurnal cycle and the night drying and the yardlight rule.
  2. Motor/Generator Set to make a portable system.  Especially good for remote sites that have no power.  Make use of the wasted heat from the motor.
  3. Plenum design, show how we waste energy (pressure) in going across the screen.  Better to use the electricity with a heater in front of a small fan. The pipe or plenum should be open on the bottom for the least resistance in getting the air into the grain.
  4. Time Constant Tau.  The time constant is the amount of time required for the outside air absolute humidity to get to 60% of the distance of the difference. A CFM/bu of 0.35 would be about 3 time contants and get us to over 90% of the way there.  This blog would show that 0.35 CFM/bu is kind of the sweet spot for using as much of the grains energy as possible, while still drying in a relatively short time.
  5. The argument for small fans:  less capital cost, less power infrastructure required, more uniform drying, use more of the inherent grains heat to dry, more efficient  — the downside is that it is a bit slower — but what is the hurry? How slow can you go.  The ideal fan size. CFM/bu
  6. More on condensation, and how to use the grain drying calculator to detect conditions for condensation.
  7. More specifics on how to use the grain calculator.
  8. Something for everybody, from the simplest strategy (yard light rule)  to the most advanced (absolute humidity) and the pros and cons of each. Depending on what you have.
  9. Long term storage, use whatever fan strategy until the end of January, in which you will get the grain frozen and cold as possible, and from January til the end of July keep the bin as sealed as possible.
  10. Roof vents don’t do much, and can actually result in condensation on the top layer of grain in the spring when the grain is cold and it is swiped by warm moist air circulating through the vents.
  11. Vertical in grain tubes.
  12. top dries first, compression causes heat
  13. safe days
  14. comparison of control strategies, continuous, nite, temp differential, absolute humidity.
  15. Flow from natural convection
  16. Energy in the grain to dry
  17. Grain Resistance
  18. Motor HP as we drop the flow in half, top down drying, HP 5 to 1
  19. First 24 Hours is critical
Ron Palmer
Ron Palmer