- 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.
- 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.
- 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.
- 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.
- 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
- More on condensation, and how to use the grain drying calculator to detect conditions for condensation.
- More specifics on how to use the grain calculator.
- 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.
- 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.
- 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.
- Vertical in grain tubes.
- top dries first, compression causes heat
- safe days
- comparison of control strategies, continuous, nite, temp differential, absolute humidity.
- Flow from natural convection
- Energy in the grain to dry
- Grain Resistance
- Motor HP as we drop the flow in half, top down drying, HP 5 to 1
- First 24 Hours is critical
I don’t have a really short answer to question, but even if you didn’t know anything else, this would be a good rule to follow — even if your grain is dry.
1. As soon as your bin is full (even before it is full) turn the fan on and run it until 9:00 AM the next morning. The exception to this rule would be if it is raining or very high humidity.
2. Thereafter, run the fan every night until your grain is cooled down and dry. This might be for a few nights, or it might be for several weeks if you have tough grain. The cold clear nights will be the most effective for drying. Again if it is raining, don’t run the fans.
OK that’s the simplest; if you want to be more precise about drying conditions, you would only want to run the fan when there are conditions for drying and that is when the absolute humidity of the air outside is less than the absolute humidity of the air in the bin. Absolute humidity is the actual amount of water in the air. It is not the same as relative humidity; but relative humidity can be used to calculate the absolute humidity. And they are kind of ugly equations that are tough to do even with a calculator. So, I made an internet calculator that does the calculations for you. It can be found at planetcalc.com/4959/ You can load it onto your cell phone and use it like an app. You simply enter the moisture content of your grain, the temperature of your grain, and the temperature of the outside air. It calculates, what I call, the threshold relative humidity. If the relative humidity of the outside air is less than this you will have drying conditions and the fan should be turned on. If the outside air’s relative humidity is greater than this, you will not have drying conditions and obviously you should keep the fan turned off. If you really want to get up to speed on grain drying, read my blog at grain-aeration.com It explains everything, even why you get condensation in a bin and why the bottom always dries first etc.
Another rule of thumb that mostly works (98% accurate): You will have drying conditions when the outside air is at a lower temperature than the grain temperature and the relative humidity of the air < 85% This is not as accurate as the calculator because it does not take into account the moisture content of the grain; however it does not require any calculations or calculators
I hope this helps, let me know if you have any other questions
Here is a question I got from Quebec:
“Mr. Palmer, Last fall we discussed a lot about grain ventilation. I remember how you where generous on this subject. I want to share with you what we do. This year we will do other observations. The producer where we will make observations will get a 4 noodles cable with Stormax. (20’ x 20’ silo). However, control will be executed by water balance at least for the stop time. Re-start will be done manually or by timer followed automatic running. I will try to share you these informations including stormax data if possible. Do you think this approach is good ? Do you have suggestions ?
The producer is supposed to fill it with 2-3 crops (first : rye…temporary storage, second : Oats…temporary storage, third : soya). In the litterature I did not find EMC equation for rye. Is it similar to wheat ? Do you think Opi systems have it ?”
I gave the following reply:
Good to hear from you. I will try to answer your questions. I am not sure what a 4 noodle cable is, I am assuming it is one with 4 senors, that is 4 sensor temp and 4 senors for relative humidity. I am assuming that you are doing a water balance to stop by calculating the absolute humidity of the discharge air and comparing it to the absolute humidity of the inlet air. And you only stop the fan if the absolute humidity of the inlet air is greater than the absolute humidity of the discharge air? Great. To turn the fan on we can’t use the discharge air, but we do want to know what the absolute humidity of the air in the silo is? There are two ways to do this. 1. this is the best way and it can be done if you have a temp and RH sensor in the core of your grain or crop. Use the T and RH as you did before to determine the absolute humidity. 2. If you don’t have the RH sensor in the body of grain, and only the temp of the grain, you can use EMC equations. By plugging in the moisture content and temp of your grain, you can determine the RH to which it will find equilibrium. Use this RH and T to determine the absolute humidity. If this absolute humidity is greater than the absolute humidity of the outside air, then you have a drying condition and the fan should be turned on. You can use the calculator that I made to do this for you. Just plug in the temp and moisture content of your grain, and the outside temp, and the calculator gives you an RH thrreshold, if the outside RH is below this then you will be drying, anything above this, and it will be wetting. Actually the only thing I am doing with the calculator is the math for the EMC equations. And no I don’t know the EMC equation for rye, but with a little digging I am sure I could find it. I think it would be OK to use wheat — it would be close.
My calculator can be found at planetcalc.com Grain Drying Calculator
Grain Drying all comes down to something pretty simple. If the outside air has an absolute humidity less than the absolute humidity of the air inside your bin or silo, then you have a drying condition. The only thing you need to calculate the absolute humidity is the temperature and the relative humidity. When the fan is on, the relative humidity is read at the point of discharge and when the fan is off you get the relative humidity by reading an RH sensor directly in the body of grain, or if you don’t have this RH sensor then you must resort to using an EMC equation (or my calculator) to determine the RH.
Still thank you Mr Palmer, Yes it is supposed to be an Opi Cable with 4 points reading of T, RH and EMC. When you talk about a T&RH sensor in the grain, what position do you suggest : at the top, at the surface or in the grain and if so how deep ?
The sensor in the grain, the RH & T sensor, should represent as much as possible the entire grain bulk. We know that the bottom has more variation than the top. So I am thinking in the center of the bin or silo, about two thirds to three quarters the way up. You don’t want it too high because shrinkage may lower the grain and leave the sensor exposed to the air, which can be greatly influenced by the open discharge port. We also considered averaging several sensors to get an average T and RH, but one can get into trouble really quickly because the pyschrometric equations to calculate absolute humidity are non-linear, so I think using just a single sensor might be better.
Also the fan must be off for some time to use this sensor, as one must wait for the air in the grain to ‘equalize’ with the grain. I am thinking at least a half an hour or maybe an hour..
Integris of Opi Systems has the capability to start and stop automatically the ventilator by comparison of grain EMC with air intake. To stop ventilator, how this decision is good considering air is not equalized with the grain. To be correct and to take the good decision, ventilator should it be off for a while and then make the calculation after 0,5-1 h ? Do you think so ? Another explanation : Would they correct the reading, considering it is not in a stabilized situation ?
I also ask myself if the decision to stop or to start is depending of one, a combination or the average of all sensors in the bin.
I hope all these questions are not bothering you. Your informations are very usefull to understand.
EMC is equilibrium moisture content, notice the word equilibrium! You must wait some time for equilibrium to occur. Our data shows that you can not correctly do an EMC while the fan is running. You get the wrong value. The correct way to control the fan is with the absolute humidity, and if you have RH sensors, there is no need to even do an EMC.
I have been away for awhile as I was on holidays and then I got caught up in domestic chores. I have been meaning to do some blogging for awhile now, but that task has been degraded to a lower priority. I have been thinking about expanding my blogs to more than just grain drying, although I will continue with that as well. I want to turn my attention to my old work of agricultural mechanization. Here are some of things I am considering blogging.
- I have been in agricultural innovation for most of my career, and I would like to put some ideas that could be used to “think outside the box”. We get so used to doing things a certain way, that we don’t even consider new ways. If I were teaching the growing of crops to students, I would give them a real project. I would supply a bag of seeds, of which each student would select ten. At the end of say a hundred days, we would grade the students ability to grow the best plants by measuring the yield originating from those ten seeds. Then looking at the ones with the best results — study exactly what was done in the growing process. Now it is just a matter of joining the dots in coming up with the machinery to do this on a much larger scale.
- Getting rid of herbicides: If we grew plants in rows, or better yet grids in which the plants are equally spaced in the row, we could actually use the plants as guides or beacons and track and lock onto the position using simple video imaging. Anything green that is not in this grid, would by default be considered a weed and be removed with the surgical guidance of a water knife.
- Small is more efficient. I would go through all the reasons that small equipment, like maybe 8 feet wide, is much more efficient than the large wide equipment we have now.
- Direct injection spraying. Why aren’t we going there??
- How mechanized do we want to get? Are we building a house of cards. Are we really helping the world by taking that very small farmer from subsistence.
- Do we really want the highest yield no matter what? The cost? The environment?
In a previous blog, I explained how condensation could be a problem in the fall with warm grain and cold outside air. The rough rule of thumb was that there was a potential for condensation on the roof if the grain was ten degrees warmer than the outside; but a more precise determination could be made with the grain drying calculator. The recommendation was not to turn your fan on if the RH from the calculator was above 100%.
There is another time, in the spring that condensation could be a problem. When the grain is very cold and the outside air temp is much warmer and holding a fair amount of water. This air enters the bin through the fan entrance, the top or vents; and it hits the cold grain — it will cool past its dew point and condensation may well occur. This will happen mostly on the poor old top layer, that may well have had water dumped on it from fall condensation, and it is the last layer in the bin to dry. No wonder the crust always forms on the very most top layer.
To prevent condensation in the spring, the best thing to do would be to not let the warm air in. The bin should be sealed. The top lid closed tight, the fan covered and all vents covered. Indeed, this is a time when vents will actually be the cause for grain damage. Sealing the bin in mid winter, after the grain has been cooled to its lowest possible temperature, will also keep the grain as cold as possible for as long as possible. Yes the grain will still warm up from conduction through the walls; but it will be a warming without adding water. At the end of the summer, when the grain has warmed up and the nights again are getting cold, the bin can be opened up and the grain again cooled. And this process could be done for years of very safe grain storage.
With this blog, we would like to know what it might cost to use supplemental electrical energy to remove one percent moisture content, MC, from one bushel of wheat.
- All the energy goes into evaporation.
- wheat 60 lb/bu
- Cost of electrical energy is 0.12/kW hr.
- Latent heat of evaporation, energy to evaporate 2257 kJ/kg
One bushel is 60 lbs, so 1% MC would have a weight of 0.6 lbs or 0.2722 kg
2257/.2722 = 614.4 kJ to evaporate 1% MC from one bushel of wheat and to put this into hours divide by 3600 = 0.17 kW hr. and in terms of dollars, 0.17 x $ 0.12 = $ 0.02 to evaporate enough water out of a bushel of wheat to lower the MC by 1%. That doesn’t sound like much but for 3500 bushels, it would cost $71.68, and to drop by 2% MC it would be $143.36. It must also be remember that this is the ideal efficiency. In the real world, we may have to double our cost again.
Could we use the free heat in the air to dry grain? — I think so; but it will take careful management and control of the aeration fans.
We all know that it takes energy to dry grain and grain does contain energy. This energy is given up when the grain cools, but what if all the energy in the grain was used, as it cools for drying?
To answer this question we will use one cubic meter of wheat that will be cooled with aeration from 30⁰ to 5⁰ C. It has a MC of 15%. If all the energy in cooling could be used to dry the wheat, what could we get the MC down to – 14%? 13%? Let’s do the math and find out.
We know that it takes energy to evaporate water, the latent heat of evaporation; it is 2257 kJ/kg. Let’s assume that all the energy in the wheat, in going from 30⁰ C to 5⁰ C. will be used in evaporating water from the wheat. How much water (in %MC) can be removed?
1 bu = 1.2446 cubic feet 35.31 ft3 = 1 cubic meter 2.204 lbs/kg
One cubic meter of wheat has a weight of : 35.31 ft3/1.2446 = 28.57 bu. x 60 lb/bu. = 1714 lbs, 777 kg
One percent moisture, 1% MC would then be 7.77 kg. If wheat was 15% MC, then 116 kg would be water. (Remember we are dealing with one cubic meter or 28.57 bushels)
The wheat in going from 30⁰ C to 5⁰ C has how much energy to give. What is the specific heat? It can vary, and I found that it increases as the MC increases but for a MC of 15% it is about 2 kJ/kgC
The wheat has got energy:
2 x 777 kg x 25 C = 38,850 kJ and divide by how much energy is needed to evaporate
38850 /2257 = 17.2 kg of water can be evaporated with the energy from the wheat.
But 1% MC is 7.77 kg , 17.2 kg/7.77 = 2.21% reduction in MC. If the wheat was 15% and all the energy went into evaporating water in going from 30 to 5⁰ C, then the MC would be reduced by 2.2% and be dried from 15% MC to 12.8%.
Our data showed just about the same thing. Cooling the grain with aeration, we found that reducing the temperature by 15 C would remove 1% moisture. In some trials with a high MC, we found this to be: 10C/%. In this case, for wheat, the amount of cooling to decrease the MC by one is: 11⁰/%MC. This demonstrates that aeration (at least with our experimental trials) is efficient in using the energy of the grain. I thought we might be losing more of the energy to conduction, but it appears that almost all the energy in the grain goes into drying. I thought that by increasing the contact time, (decreasing CFM) that we could get more efficient, but it appears that we are already doing a pretty good job in using almost all the energy in the grain for drying. It also tells me that we can’t expect much more than 2% decrease in MC in going from 30⁰C to 5⁰C. If we must lower the MC by more than two percent, we might have to use supplemental heat, or carefully allow the outside air to warm the grain (Hopefully without adding water) and then cooling it to remove more water.
For this exercise we will use one cubic meter of barley at 30 C. And we know that it takes energy to evaporate water; this is called latent heat of evaporation, which is 2257 kJ/kg. Let’s assume that all the energy in the barley in going from 30 C to 5 C is used in evaporating water from the barley. How much water (in %MC) can be removed?
1 bu = 1.2446 cubic feet 35.31 cubic feet = 1 cubic meter 2.204 lbs/kg
One cubic meter of barley weighs 35.31 ft3/1.2446 = 28.57 bu x 48 lb/bu = 1361 lbs, 618 kg
One percent moisture, 1% MC would then be 6.18 kg. If barley was 15% MC then 92.6 kg would be water.
The barley in going from 30 C to 5 C has how much energy to give. What is the specific heat? It can vary, but I found one source: 1.36 kJ/kgC
1.36 x 618 kg x 25 C = 21012 kJ and divide by how much energy is needed to evaporate
21012 kJ /2257 kJ/kg = 9.3 kg of water can be evaporated with the energy in the barley.
But 1%MC was 6.18 kg/%MC , 9.3 kg/6.18 = 1.5% reduction in MC. If the barley was 15% and all the energy went into evaporating water in going from 30 to 5 C, then the MC would be reduced by 1.5% and be dried from 15% MC to 13.5%.
Our data showed just about the same thing. Cooling the grain with aeration, we found that reducing the temperature by 15 C would remove 1% moisture. In the case above, 1.5% resulted from a 25 C. A 1% reduction would require 16.66 deg reduction. This demonstrates that aeration (at least with our experimental trials) is very efficient at using the energy in the grain. I thought we might be losing more of the energy to conduction, but it appears that almost all the energy in the grain goes into drying.
Let’s work out an example of how long it will take to cool down a 3500 bushel bin of barley from 30 ⁰C to 20 ⁰C with air that is 15 C and flowing into the bin at 3000 CFM. We will make a wild assumption here and assume that the air will leave the grain at 30 C, making a 15 C increase. For this exercise we will use conservation of energy and we will ignore any drying or wetting that will result from latency heat.
First task is to find the energy that would be lost if the barley is cooled from 30 to 20 C. We must know the specific heat, Cp, of barley and I looked in a couple of places, but it was about 1.36 kJ/kg C. The weight of the barley is 3500 bu x 48 lbs/ bu x 1/2.204 kg/lb = 76,225 kg times 1.36 = 103,666 kJ for every degree C and we are changing ten degrees so the energy to be removed from the barley will be 1,036,660 kJ.
3000 CFM is 50 cubic feet per second. Air is 0.0807 lbs per cubic foot. 4.035 lbs/s or 1.83 kg/sec
The specific heat for air is close to 1 kJ/kg C so for every degree C we could acquire 1.83 kJ/s. For 15 C we could remove 27.45 kJ/s. How many seconds to remove 1,036,660 kJ? 1,036,660/27.45 =37765 sec, 629 min, 10.5 hours.
This shows that it is possible to cool the grain over a one night period (12 hours, 9 PM to 9 AM).
Now, if we used a smaller fan and produced only 2000 CFM, we would expect the cooling time to increase accordingly to 16 hours. Even for a much larger bin, of 10,000 bushels, we could cool it down in 48 hours (four 12-hour nights). One could argue that this would be for air that is consistently 15 C and it assumed ideal energy transfer. On the other hand there will be cooling affects resulting from drying; but that’s for another blog.
In my last blog, I showed how over-drying occurred from the increased temperature at the bottom of the bin caused by compression. Maybe we cannot eliminate it; but can we at least reduce it.
In our previous example we used a 2200 bushel-bin of barley at 20⁰ C and MC of 15%. A 5 HP aeration fan produced 3000 CFM with 6” H20 pressure. This resulted in the bottom being 4.2⁰ C warmer and dried to 11.6% MC all while the top remained at 15%.
Browsing around the internet, looking at fan curves, I found out that the air resistance is not linear like electrical resistance. Static pressure is proportional to the square of CFM (flow). And required horsepower, HP, is proportional to the cube of CFM. We can find these proportionality constants:
Pres = a CFM 2 6” = a 3 2 (3 is for 3,000 or CFM is in thousands) a = 6/9 = 0.666
HP = b CFM 3 5 = b 3 3 b = 5/27
Let’s see what happens if we reduce the CFM from 3 thousand to 2 thousand.
Pres = 0.66 (2 2 ) = 0.66 x 4 = 8/3 = 2.66 “ H20, less than half of what it was, 6.
And the required HP = 5/27 ( 2 3 ) = 40/27 = 1.48 or call it 1.5 HP
Isn’t that incredible, we only decreased the flow by two thirds, and in return we get a pressure that is less than half, and a required horsepower that is much less.
This reduced pressure will also decrease the temperature rise from compression.
2.66/406.8 = x/293, where x must be 1.9; that is we get a 1.9 ⁰C rise from compression. Let’s say we also get an increase in temperature from motor inefficiency, for a combined total of 2⁰ C or an absolute temperature of 22 ⁰C.
So we had at the top of the bin barley at 20 C and 15% MC. 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, so the air at the bottom has the same absolute humidity, or 12 grams per cubic meter. At the bottom of the bin, the temperature is 22 C, and the saturation humidity is 19.5 gr, giving an RH of 12/19.5 = 61.5%. Now using the grain drying calculator, find the MC for barley by trial and error with a temp of 22 and RH 61.5% and I get a MC of 13.7%. This is a spread of 1.3%. This is better, remember with a higher pressure of 6, we got the bottom to be 11.6%, and a spread of 3.4%.
The conclusion to this story is that a relatively small decrease in flow, will greatly reduce the pressure, which has a profound decrease in over drying the bottom. Reducing the flow will also increase the transit or contact air-to-grain time, giving the grain more time to pass water into the air.