New insights into natural air grain drying

Posted on 06.02.2017 | Last Modified 07.05.2019
Lead Researcher (PI): Ron Palmer
Institution: Indian Head Agricultural Research Foundation
Total WGRF Funding: $275,921
Co-Funders: None
Start Date: 2015
Project Length: 4 Years

To develop a fan control strategy using natural air that results in the safe storage of grain, that is efficient and results in less fan running time, and that results in more uniform drying of grain.

Project Summary:

This research project started over a decade ago in 2007.  At the time there was no formative practice as to how an aeration fan should be controlled; but, there was a vague notion that there must be a better, more effective, more efficient method to control the fans rather than just running them continuously and hoping for the best.  Eventually the grain would dry; but at what cost?  What was happening in that grain bin when the fan was running continuously? What ambient conditions yielded the best drying? Were there times when drying was not occurring? This research project had many questions to answer; but, the overall objective was to find a control strategy that would efficiently dry the grain. It would then be safe for storage and ready to sell.

A method was devised to measure the amount of drying by measuring the absolute humidity of the air entering and leaving the bin.  The absolute humidity is the amount of water in the air and it is a function of temperature and relative humidity. The difference in absolute humidity of the air flowing in and out of the bin in one hour renders the amount of water removed from the grain. This method of measuring drying, was instrumental in the subsequent research to measure and observe drying in real time.

A daily pattern of drying was discovered called the diurnal drying cycle and it clearly showed that the most drying occurred at night and the least during the day. This was controversial; yet undeniably the case.  The cold dry night air is warmed by the grain as it flows through the bin absorbing moisture from the grain.

The hourly drying graphs demonstrated a strong correlation between drying and cooling of the grain. It was observed that: “Cooling is Drying” or “Drying results in Cooling”.  This became the basis for a fan control strategy.   If cooling the grain resulted in drying; then it would be logical to only run the fan when cooling occurs. To cool the grain, one requires air that is colder than the grain. Consequently, the following control strategy was established: Only run the fan if the outside air temperature is less than the grain temperature. It is called Differential Temperature Control and it is the best control for keeping the grain cold.  This controller was built and tested. The duty cycle of the run-time of the fan started at approximately 50% but, quickly dropped to 20% or less as the grain cooled.

During the course of this research, the original objective of just getting the grain dry, was questioned.  It was believed that if you only got the grain dry, everything would be fine; the grain would have no spoilage. However, it was learned that the grain was deteriorating from the moment it was harvested.  Fraser and Muir (1981) [7,14] were able to measure the amount of spoilage and revealed that spoilage was a function of grain temperature and moisture content.  The best grain storage, that resulted in the least spoilage, would have the grain dry and cold.  Even if grain is dry, it can still be spoiling. To essentially stop spoilage, one must have the grain both dry and cool. The original objective of: drying the grain, needed to be reconsidered. To condition the grain for the best storage, with the least spoilage, one would have to include, cooling the grain.   The revised objective, for the best grain storage, with the least spoilage must include the grain temperature: keep the grain as cold as possible.

When the grain is cold, it possesses little heat energy to expel and evaporate more of its moisture. Even if grain is loaded into the bin at a high temperature, one should expect only modest drying with moisture content reduction of a couple of percentage points. The grain will need to be re-energized to expel more moisture.  Night drying only worked while the grain had some heat in it; once cold, there was no more drying.   Being cold the grain was safe from spoilage, even though it might be tough.

If the paramount objective of the farmer was to dry the grain, then the fan should run whenever a drying condition exists.  Drying conditions exist when the absolute humidity of the air leaving the bin is greater than the absolute humidity of the air entering the bin through the fan. By measuring the absolute humidity of the air entering and leaving the bin; it would be possible to determine drying conditions and when to turn the fan on.  It is called Absolute Humidity Control and it is the best control strategy for optimum drying: only run the fan if the absolute humidity of the air leaving the bin is greater than the absolute humidity of the air entering the bin.  This controller was built and tested. The duty cycle of the run-time of the fan was initially about 50% but it quickly dropped to 20% or less as the grain cooled and dried.

The calculations to determine the absolute humidity are rather onerous.  To determine drying conditions by hand with a calculator is complicated and tedious.  A calculator was designed and installed on to do the difficult calculations.  One simply inputs the grain temperature, its moisture content, and the temperature of the outside air. The calculator returns the threshold Relative Humidity, RHthres.  If the outside relative humidity is below this threshold, one has drying conditions. This calculator can be ported to a cell phone and used anywhere there is cell service.

The calculator has another purpose; it can be used to determine if condensation would form under the roof of the bin. Conditions for condensation exist if the calculated threshold relative humidity is greater than 100%.  It would not be advisable to run the fan under these conditions as condensed moisture would literally be raining down onto the grain.

It has been noticed by many that the bottom of the bin dries first and often over-dries by the time the top dries.  Why?  The air at the bottom of the bin is slightly warmer from compression. The compression was enough to support 5 or 6 inches of water, which resulted in warming the air by a few degrees.  The warmer air has more capacity to hold water, and thus dries the bottom more.  If one uses a bigger fan, one will get more compression, more of a temperature difference from top to bottom and more over drying the bottom.  Using a bigger fan with more compression can be counter productive in over-drying the bottom. The best flow was found to be about 0.4 CFM/bu.

Overall the project has been successful. Two control strategies have been developed, one for best storage and one for best drying. And a better understanding of what is happening in the bin has been found and made available to farmers.  The control strategies are being used by farmers. One farmer is using the absolute humidity controller for aeration on his 48,000 bushel bin. Another farmer is using the control strategy to keep bagged grain from spoiling in a sea-cargo-container as it is being shipped to its destination.

Much of the understanding of what is happening with Natural Aeration Drying (NAD) has been incremental and has been a collaborative effort.   A blog was started to record this journey and now this analysis and understanding has been recorded in detail.  Much of it has resulted from the analysis of the data, but a significant amount has been the result of farmers asking probing questions, which when worked out revealed the theory and gave a better understanding of the dynamics of aeration.

There has been one common parameter throughout all aspects of this research, and that is grain temperature.  Grain temperature is the dominant parameter in determining absolute humidity as well as securing the grain from spoilage.  On the one hand, hot grain is good because heat energy is required to push moisture out of the grain, but on the other hand, it is hot grain that spoils.   We want to get that heat energy into the grain for drying; but once hot, we want to cool it immediately to mitigate the spoilage.  It is a paradigm shift – we always thought that dry grain was paramount, and perhaps this would be the case if there was no control over temperature.  With NAD aeration, the temperature of the grain can be controlled; the grain can be cooled in a matter of hours and thus cool grain becomes more of an important ideal than dry grain.

This research project, has more than met its objectives.  Controllers have been designed and tested, the theory has corroborated the observation, and many bonus findings have been discovered, such as the cause of condensation on the roof, the calculator, and the reason the bottom dries first.