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.