OK I am back, and yes it is that simple:
Turn the fan on if: grain temperature > outside air temperature
I have gone through all the control strategies, and as for a controller, this would be my choice because of its simplicity and ease of use. It does not guarantee the fan will only be on when you have drying conditions, but it does guarantee that you will have the safest , most secure storage with the least spoilage. It keeps your grain cold. The control strategy for only running the fan when you have drying conditions would be what I call the Absolute Humidity Controller.
It calculates the actual water content of the air inside the bin and the air outside the bin. It turns the fan on when the outside air contains less water than the air in the bin. It is much more involved and gives even a micro a hard time with Psychrometric equations, and EMC equations. It also requires the user to input the moisture content and type of grain. It is the ultimate in a control strategy that only runs the fan when there are drying conditions. But it is way more complex, and not as convenient for the user and therefore not as reliable and more expensive.
The strategy of:
Turn the fan on if: grain temperature > outside air temperature also dries the grain except for when the grain temperature is greater than, but only slightly greater than, the temperature of the grain AND when the outside relative humidity is close to 100%. But once the difference in temperature of the outside air and the grain becomes more than a few degrees, then even with the RH being 100%, you will still get drying. What is the chance of having an RH of 100% and only a slight difference in air/grain temp? Very very small. We could condition our strategy above by saying that we would only turn the fan on if:
grain temperature > outside air temperature, AND the RH < 85% — but is it worth it? This would require a humidistat etc. I don’t think it is worth it because of the probability of having a small differential temp and an RH > 85%
We can achieve almost the same thing by putting in an offset:only turn the fan on if:
grain temperature > (outside air temperature + 2 degrees)
So, what do we need to automate your fans — Are you ready for this?? All you need is a thermostat that is used to control baseboard heaters. You can find these at Lowes or Home Depot. It has a rotary knob that is set to a specific temperature. It is all mechanical ( I believe a bi-metalic strip), that closes a contact (than can handle a pile of current at 110 or 220 volts) and the contact closes when the air temperature is less than the indicated knob temperature. In our case we will set the knob temperature to that of the grain temperature and connect the contacts to the actuator’s (relay) coil. Most aeration fans are wired to have a latching actuator coil. This will need to be modified. Assuming we will have 220 volts with L1 and L2 power leads. The following would be wired in series, ignoring the Start and Stop switches. L1 – actuator or relay Coil – Thermostat – L2 The relay’s coil will only be activated when the contacts on the thermostat are closed, and that is when the temperature of the air is less than the temperature of the grain that is indicated on the knob. Your fan will only run when the air temp < grain temp.
Your grain temp will not change that quickly and maybe once a week you will need to adjust the temperature on the knob down to match the grain temp.
A more automated approach would adjust the temperature of the grain automatically. Thermistors are used to measure the temperature of the grain and the outside air. I used 10 K ohm thermistors. These are run into a comparator such as LM311, through a pulse integrated circuit and then into a solid state, high voltage relays to simulate the Start and Stop pulses on the fan. I have built these circuits and have wired up a couple of aeration fans with it. I called the system “Cool It” I still have some of these boards around, but I can not sell or even give these boards away as they would have to be certified by CSA or ULC and to go through all that hassle is not worth it. I suppose I would be willing to share the schematic, but I still would be a bit nervous about the liability.
I hope this gives you a start, with a cost effective, simple solution to automating your fans.