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.
Month: February 2020
Natural Aeration or Natural Drying??
I wanted to ask about using the terms “aeration” and “natural air-drying.” I’m trying to avoid confusion – I found in a grain aeration spreadsheet by PAMI the following definition:Aeration = grain conditioning/cooling low airflow rate (0.1-0.2 cfm/bu) Natural air Drying = removing moisture from grain high airflow rate (1-2 cfm/bu)
Natural aeration or Natural Air Drying would be using the natural ambient air (no supplemental heat) to condition the grain. I think it is generally accepted that the fan is pumping air into a steel bin. Our research has shown that Cooling is Drying and whether you have an air flow of 1 CFM/bu or 0.1 CFM/bu, you will be cooling and drying the grain. I am not sure where the distinction between airflows for cooling and drying came about, but in fact they are related. Even at very low air flows, you will still be drying — albeit somewhat slower, but; there are advantages to drying slower. 1. Because the higher flows make for more pressure on the bottom of the bin, they also will create more of a difference in top to bottom of the bin drying. Slower flows have a more even distribution of top/bottom drying. 2. You will use more of the heat energy in the grain to push the water out of the grain. 3. You will use much less electrical energy with a smaller fan — even if it takes a bit longer. We found the sweet spot for flow to be about 0.4 CFM/bu. You get the advantages, as mentioned above, and you still get the grain dried in a reasonable time. But back to answering your question: Natural Air Drying and Natural Aeration are really the same thing.
Do you agree with these definitions? Natural air-drying seems to be both a somewhat general term for the two methods and also a specific term for the high airflow rate method – thus my confusion.
Also, would you have any figures on the costs to run a grain aeration fan? You said it would be pennies on the dollar – do you know where I can find more specific numbers?
Here is the math that you can use to calculate the cost. In Sask the cost of a KiloWatt Hour is about $ 0.13 1 HP requires 0.74 KW So to run a 1 HP fan for 1 hour cost 9.62 cents — lets round it off to 10 cents. A 5 HP fan would cost 50 cents per hour or 12 dollars a day. A 10 HP fan would cost $24/day. You can take it from there.
I was looking at your presentation and the diurnal drying cycle graph – it shows drying starting at 6:00 pm and wetting starting at 9:30 am (with best drying happening at 2 am). So, after that initial 24 hour drying period during/after harvest, are those the times farmers should be following for natural aeration?
After the initial 24 hours, the fan should only be run if the outside air temperature is less than the grain temperature. This will probably be at night. And as the temperature of the grain goes down, there will be warm nights that the fan should not be run at all.