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.