In my last blog, I was preaching about the dangers of over heating the grain because it could cause condensation. In fact I recommended that the grain should not be more than 5 deg C warmer than the outside air. But how long would it take to raise the grain 5 C? What would it cost? How many cubic meters of natural gas would be required? How much water would the combustion of that much natural gas be?
We will use Jim S. situation in which he has 3500 bu of wheat in a hopper bottom bin at 5 C and wants to heat it to 10 C with his 50,000 btu furnace.
Wheat is 60 lbs/bu. So in kg 3500 bushels weighs 60 x 3500/2.204 = 95,281 kg.
Specific Heat of Wheat is 1.36 kJ/kg C
For 1 C rise in 1.36 kJ/kg C x 95,281 kg = 129,582 kJ/C
For 5 rise (5 t0 10) 5 x 129,582 = 647,910 (assume that all energy went into wheat)
Our furnace puts out 50,000 btu and there is 1.055 kJ/btu so furnace puts out 52,750 kJ/hr. But we need 647,910 kJ, so it will take us 647,910/52,570 = 12.3 hrs to raise the temp of the wheat by 5 C.
Cost 1 GJ = $4.30 so .647 GJ = $2.78 (Sask Energy price)
Estimated Water Removal .2 % (MC 17% to 16.8%) – Assume that we will pull the wheat back to 0 C. This would be 190 kg of water removed. We would have to do this everyday for two weeks to get wheat from 17 to 14.4% and this would cost about $36 in natural gas.
1 cubic meter of natural gas produces 0.688 lbs of water in its combustion. There are 26.94 cubic meters required to produce a GJ of energy. But we only need 0.647 GJ to raise the temp 5 C, or 0.647 x 26.94 = 17.43 m^3. which would produce 17.43 x 0.688 = 12 lbs (5.44 kg.) of water. If we would use a tiger torch instead of the furnace, we would have taken 190 kg of water out of the wheat, but would have added 5.44 kg of water back into the wheat from the combustion of the natural gas. I think it is a good idea to prevent the flue or the smoke from getting into the grain. People are throwing out their conventional low efficiency furnaces, you could pick one out of the junk for almost nothing. All you need is the burner and the heat exchange. Even if the heat exchange is cracked, it would not be a problem.
I was doing some number crunching, concerning a call I got from Jim S from Wainwright Alberta. He was going to use a standard residential natural gas furnace to try to get his grain dry. I discovered something very interesting. IF the temperature of the grain is raised by more than 5 C above the ambient temperature, there will be serious condensation that will form on the inside of the bin, under the roof and on the walls. When the air goes through the grain, it warms up and takes on moisture, and that is fine; it is drying the grain, but when this warm moist air hits the cold roof, it can not hold all that water, so it dumps it as condensation and rains down on the grain, which then can form a crust and be a source for spoilage. This can happen even if the grain is dry. The rule of thumb is that there will be conditions for condensation if the grain is 5 C above that of the outside air. We can get a slightly better spread if the grain is dry as is shown below (numbers from my calculator planetcalc.com/4959/).
So, I think this is a serious problem with those attempting to use supplemental heat with their aeration fans: heating the grain by more than 5C can result in serious condensation problems at the top of your bin.
How much water will be condensing out as rain. Let’s say that Jim’s wheat is heated to 15 C and the outside air is 5 C; the wheat is 10 degrees higher in temperature than the outside air, so we suspect there will be condensation; but how much. Using the grain calculator (EMC equations) we discover that wheat @ 17% moisture at 15 C will produce an RH of 79.1% and an absolute humidity of 10.2 grams of water per cubic meter. When this air hits the cooler roof that is the same temp as the outside air, 5 C, it will cool to 5 C. The most water that air can hold at 5 C is 6.86 grams of water per cubic meter. The air when it was warmer at 15 C was holding 10.2 grams. The difference is the amount of water that will condense as liquid water on the roof, or fall onto the top layer of wheat as rain. For every cubic meter of air that is blown into the bin, 10.2 – 6.86 = 3.34 grams of water will rain down on the wheat. In one hour, it was determined earlier, that 5100 cubic meters of air flows through the bin. In one hour 5100 x 3.34 = 17,034 grams, or 17.034 kg, or 37.54 lbs of water is deposited onto the wheat. Can you imagine filling up the better part of a 5 gallon pail with water and throwing it onto your wheat — every hour?? And why do we get spoilage and a crust on the top??
In my last blog on night drying does not work, I explained how Jim S. from Wainwright was considering adding a natural gas furnace to his aeration fan to provide supplemental heat. He was getting no where with night drying, in fact the weather conditions were so bad that water was being added. Off the top of my head, I thought adding a furnace would be a good idea, but I would run some numbers to check it out.
Here is the situation: 3500 bushels of tough wheat at 17% Moisture Content. Let’s say his grain is near the freezing point, 0 C, and his fan can produce 3000 CFM or not quite the 1 CFM/bu recommendation. The immediate weather forecast is for cool temperatures and high humidity.
3500 bushels weighs 60 lbs/bu 2.204 lbs/kg 3500 x 60/2.204 = 95,281 kg
If we want to remove 1% MC we would need to remove 953 kg of water.
To evaporate 1 kg of water it requires 2257 k joules of energy, so to remove 953 kg of water, 2257 x 953 = 2,150,921 or about 2 GJ of energy. Checking with SaskEnergy, a GJ of natural gas would cost about $6.50. So, at least $30 of natural gas is required to lower the MC by 1% and $75 to bring Jim’s wheat down from 17 to 14.5%. This of course assumes that all of the energy we add will go into drying.
How big should the furnace be? When shopping for furnaces, you will see the spec as to the heat they give off is usually in BTUs or British Thermal Unit. And 1 btu = 1.055 kJ What they really mean in the spec is btu/hr; the per hour is implied. I found a ceiling furnace at Princess Auto with an output of 50,000 btu/hr. or 52.75 MJ/hr. We calculated above that we need 2,000 MJ of energy to get 1% moisture out. To get this energy into the wheat, it would take 2000/52.75 = 38 hours of running the furnace. Perhaps 100,000 btu –> 19 hours or even a 200,000 btu would take 9.5 hours.
OK so we have the furnace sized, and we have an idea of how much this is going to cost in natural gas; what’s next? How much should we heat the grain? Should we cycle the heating and cooling? If we are going to cycle — how long should the heat/cooling cycle be? I am going to assume here that the wheat is at 0 C and the outside air is also at 0 C, 76.2% RH. I ran some numbers with the grain drying calculator with the grain heated to various temps.
Looking at the first line, we heat the grain from 0 to 20 C. Because the wheat is 17% MC at 20 C, EMC equations tell us that it wants the air to be at an RH of 79.9%, which gives an absolute humidity of 23 gr per cubic meter. The air entering the bin had an absolute humidity of 3.7, so for every cubic meter of air flowing through the bin 23 – 3.7 = 19.3 gr. In one hour 5100 cubic meters of air flow through the bin so 19.3 x 5100 = 98.430 kg of water are removed in one hour. To remove one percent MC we need to remove 953 kg of water, and this would take about 10 hours.
This all looks really good, but we have one huge problem — CONDENSATION ! We see that the RHthres for the outside air is 302%. Anything over 100, and we will get condensation on the roof and walls of the bin. Here is what is happening: the air goes through the grain and becomes 20 C and RH 79.9. It is carrying 23 grams of water per cubic meter. This air hits the cold roof and is cooled to 0 C. But air can’t hold much water at 0 C, only 5 gr/m^3. The remaining 18 grams condenses and comes raining and running back into the wheat. If we want to avoid condensation we should only have the grain about 5 degrees warmer than the outside air. If we only raise the temp of the grain by 5 C, it will take a long time to remove 1% MC — 953/5.92 = 161 hours or about a week. It would take over 2 weeks to get Jim’s wheat dry. Trying to go faster with higher grain temps would be a dangerous exercise in creating condensation. Maybe we should reconsider this supplemental heat strategy and go with cherry picking good drying conditions. The wheat is safe, being as cold as it is, so what’s the hurry?
If you want to use supplemental heat, you better be careful.
I would tell Jim to use the smaller furnace, of 50,000 btu, which would give a temp rise of 8 C. The fan should only be run when the temp of the day is the highest, maybe at 6 hours a day. The heat should then be shut off and the fan left running into the coldest part of the night. I would do some cherry picking of good conditions using the drying calculator, and certainly not running the fan when Condensation conditions exist.
I got a call a couple of days ago from Jim S from Wainwright, Alberta. He said this night drying thing wasn’t working for him, and that he was considering buying a natural gas furnace to add some supplemental heat to the process. Would that work? I told him I thought so, but I would have to work out the numbers to see; but I’ll save that for another blog. First let’s have a look at why this night drying isn’t working out.
Jim has tough wheat, 17%, that was harvested cold and put in a 3500 bushel hopper bin. Attempts at night drying brought the temperature down, but because of the nasty wet weather they have been having, the moisture has remained close to 17. Jim thought that water may have been added. I told Jim that once you got the temperature of the grain down, you have taken the energy out of it that could have been used for drying. So it is quite possible that now that the grain is cold, that drying will cease.
Here is what I did to get a handle on what was happening in Jim’s case. I went to the website for BINcast and got the hourly temperature and relative humidity for Wainwright from Sunday Oct 14 til Thurs Oct 18. The temperature was bouncing around the freezing point, from +4 to -6 C. The relative humidity was high throughout this time, 90% – 95%. We didn’t know the temperature of the wheat, but we knew it was cold, so I assumed it was 4 C. I modeled the temperature of grain, so that it would chase the outside temp in a similar fashion as I saw how the grain temperature changed in our trials at Indian Head. At the end of this simulation, the grain temp was close to -1.5 C.
I now had all the information I needed to calculate the amount of hourly drying. To make a long story short, I used the principle of absolute humidity, and EMC equations to calculate the amount of drying for each hour. For the first few hours we did take out a bit of moisture until the temp of the grain came down, and then we started adding small amounts of water. We did get some drying when the temp went down to -5, but this was short lived. In the end we did indeed add water, 7.52 kg. This actually isn’t that much, it would raise the MC from 17% to 17.001% however we can definitely say that night drying did not work in these conditions. Maybe we should consider supplemental heat? Maybe Jim should buy his furnace. Using supplemental heat brings forth a whole host of other questions. What fuel should I use? How much will this cost? At what time of day do we apply the heat? How hot should we get the grain? Should we have a cooling cycle, or should we just apply the heat continuously. How long should the cooling cycle be?
In my previous blogs, I showed that a 15 deg C cooling of the grain resulted in a 1 percent decrease in moisture. So let’s say we want to raise the temp of the wheat by 15 C. The specific heat of wheat varies, but it is about 1.36 kJ/kg C. I will spare you the details but to heat 3500 bushels, 15 C, requires 2 GJoules.
I checked with SaskEnergy and used the internet to check out the cost of the different fuels for 1 GJ.
Natural Gas $6.48 /GJ
Gasoline $16 /GJ
Fuel Oil (Diesel) $21.65 /GJ
Propane $14.66 /GJ
Electricity $32.81/ GJ
So, we can see there is no decision, if you have natural gas, it is by far the cheapest supplemental fuel. Also 1 GJ , at least, is the required energy to remove 0.5% MC. Maybe we can get Jim’s wheat dry for $40 — if we do everything just right? As our benchmark, and challenge, we will use the same nasty weather conditions above, with the supplemental heat and see what happens?
Stay tuned for the next blog on playing with supplemental heat.
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:
Produce more spoilage
Use more energy
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.
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.
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.
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 http://www.engineeringtoolbox.com/air-density-specific-weight-d_600.html , 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.
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%
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.
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.
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.
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.
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.
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.
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.
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.
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?
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.
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 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:
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.
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:
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.