Fan Control Strategies == Worst to Best

  • Continuous: The absolute worst situation is doing nothing with hot tough grain. This requires no attention, no sensors, and has a very high risk for spoilage. The number of safe days is small, and the grain is deteriorating quickly.   It still may work; it may dry the grain, but is very risky with the potential for the most spoilage. The next worst thing is to leave the hot tough grain for a week or so before turning it on only during the day. Or, leave it for a week or two, and then turn it on continuously, and finally turn it off after a hot day of running, leaving the grain hot, but somewhat dried. It is better to run the fan continuously right from the start, and quitting after a cold night, leaving the grain cold.
  • On at Night: One requires no sensors, and only a little attention to turn the fan on at night and off by 9 the next morning. The fan should be turned on immediately, even while the bin is being filled. When the average grain moisture is dry, the process s should stop after a cold night of running. The grain will warm up slowly by maybe a degree a week, and once a month the grain should be cooled by running the fan on a very cold dry night. This technique does not check for dripping or condensation conditions. It could be automated with a simple timer controlling the fan’s actuator.
  • Water Balance: This requires T and RH sensors on the air entering and leaving the bins. It also involves some calculations to determine the net amount of water leaving the bin. However the exhaust T and RH sensor are only effective when the air is moving or when the fan is on. When there is no air flow, the fan is off, the sensor are in stagnant air and do not represent the true readings of the exhaust air. So this is a great mechanism to determine when to turn the fan off ( no or little drying is occurring) but it is useless when in determining when to turn it on.
  • Temp Difference: Tgrain > Tair This requires only two simple temperature sensors, one in the grain and the other in the outside air. No math or calculations are required, and the comparison can easily be done manually or with very simple electronics. This would be a simple technique that could be used for automatic control. It should be started as soon as the bin is filled, and end when the average of the grain is dry, after a cold night. It is lacking in that it does not take into account either the MC of the grain or the RH of the outside air. It is not perfect, but it is simple.
  • EMC Calculator: The calculator requires the grain’s T ,MC, and type as well as the outside air’s T and RH. It could easily be configured to turn the fan on and off automatically, but it also could be used to control the fan manually.   For those that have temperature sensors and are not keen on changing to moisture sensors, this may be the way to go. The disadvantage with this method is that calculations must be made and that we are subject to the inaccuracies of the EMC equations. Also if we have an obscure grain, it may not be listed on the calculator. The fan should not be turned on if dripping or condensation will occur: RHthres > 100
  • Moisture Cable on, Water Balance Off: This is the ultimate controller, but it requires a moisture cable strung from the center roof collar, with the highest RH/T sensor being in the air to get the T and RH of the exhaust air. The lower T/RH sensors would be in the grain.   For each of these sensor nodes, one gets the T and RH of the grain, and from the Saturation Equation one can calculate the absolute humidity of the air in the bin. Also using the Saturation Equation, one can easily calculate the absolute humidity of the Outside Air by using its T and RH. If the absolute humidity of the outside air is less than the grain air, then we have drying conditions and the fan should be turned on. Once the fan is on, we do a water balance calculation , using the RH and T of the outside air, and the exhaust air. When the calculation indicates that there is no longer a net flow of water out of the bin, the fan is turned off and in an hour we return to comparing the absolute humidity of outside ait, and grain air.   But there probably are more than one T/RH sensor in the grain and some sort of averaging must be done to determine just when to turn the fan on. Although this strategy depends on the installation of a more expensive moisture cable, it avoids the inaccuracies of the EMC equations , or even the knowledge of the grain type. However we might want to use the EMC equations to determine what the MC of the grain and to terminate the process when the average MC of the grain from top to bottom is dry. The grain should still be monitored, and maybe once a month the grain could be cooled to keep it as cold as possible.   An even more reliable system would have two moisture cables in the bin to detect faulty sensors. The fan should not be turned on if dripping or condensation will occur: the saturation absolute humidity of the outside air > absolute humidity of the grain air.

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