We have all seen it, the bottom of the bin having a moisture content three or four percentage points below that of the top. Why?
To explain this phenomenon, we will use our setup with a 2200 bushel bin, a 5 HP aeration fan and we will look at two examples one with barley @ 20 ⁰C and MC 15%, and another with canola @ 20 ⁰C and MC 11%. In both cases the fan produced a pressure that supported a six inch column of water. This was measured with a home- made manometer which consisted of a plastic tube shaped into a U with coloured water in it. The air flow was 3000 CFM.
Even though the outside temperature is 20 ⁰C, the temperature of the air behind the fan is warmer because of heat given off by the motor and because of compression. Let’s first look at the heat from the motor. What air temperature rise can we expect from it?
I am guessing here, but let’s say that the 5 HP motor is 90% efficient, which means that 10% of the energy from the 5 HP motor will be going into heat and this is caused by wire resistance, bearing friction, and even air friction on the fan blades. 1 HP = 0.7475 kW so 5 HP = 3.737 kW and ten percent of that would be 0.3737 kW or 0.3737 kilojoules/sec will go into heating the air. The air is flowing at 3000 CFM or 50 ft3//sec and the weight of one cubic foot of air is 0.0807 lbs or 0.0366 kg, so 50 cubic feet would be 1.83 kg of air going by per second. The specific heat of air is close to 1 kJ/kg.K⁰ So the temperature rise of the air would be: ( 0.3737kJ/s / 1.83kg/s ) = 0.2 ⁰C. The heat from the motor would increase the temperature of the air from 20 to 20.2 ⁰C.
But the big increase in temperature is not from the motor but that of the compression. We know that the pressure behind the fan is enough to support a column of water six inches high. We know this because we measured it with our home-made manometer. How much of temperature rise will we get from this pressure or compression? There is a thermodynamic formula that relates pressure to temperature: PV = nRT. Pressure and Temperature are proportional. A typical pressure of 1 atmosphere will support a column of water 406.8 inches. And a typical temperature is 273 ⁰Kelvin. So an increase in pressure will produce a corresponding increase in temperature: 6”/4068” = x / 273 and this gives an x of 4 ⁰K or 4⁰ C. The air and grain at the bottom of the bin would be 24.2 ⁰C, and as the air flows to the top the compression would get less and less until at the top there would be none, and the temperature of the grain and air would be back to 20 ⁰C.
Does this increase in temperature affect the MC? Yes it does, and we will look at the barley at 15% MC and the outside air is 20 C and so is the barley at the top of the bin. Now we will use the grain drying calculator and plug 15 in for MC and 20 for both the outside air and grain temperature, this gives an RHthres of 68.6% and now we can use the relative humidity to absolute humidity calculator to see that the absolute humidity is 12 grams per cubic meter. We have assumed that the air at the top of the bin has reached equilibrium with the grain; the relative humidity of the air is 68.6% and the absolute humidity is 12 grams per cubic meter. We are at equilibrium – no drying or wetting is taking place at the top of the bin. However at the bottom of the bin the temperature is 4.2 ⁰ C warmer at 24.2. We are assuming that no drying or wetting is taking place so the absolute humidity at the bottom will be 12 grams per cubic meter; the only thing that has changed is the temperature. Using the humidity calculator again by setting the RH to 100 and the temp to 24 gives a saturation humidity of 21.8 gr, and since our absolute humidity is 12, the RH must be 12/21.8 = 49.6%. By using the grain drying calculator with an air and grain temp of 24.2 entered, and by trial and error entering MC until the RHthres is close to 49.6. I found that if I entered 11.6% for the MC, I got 49.4% for RHthres – close enough. This means that the barley at the bottom of the bin will be in equilibrium with the air around it at a MC of 11.6 and at the same time we have barley at the top that is 15% MC in equilibrium with the cooler air. This is a spread in MC of 3.4%. And yes we have seen this type of spread in our trial runs. The top to bottom spread in MC is quite commonly three or four percentage points different. And now we know why.
Does the same thing apply to oil seeds? It might even be worse, because the pressure might be higher with a grain that has more resistance; but let’s see what happens for a pressure of six inches. Let’s look at tough canola at 11% MC at the top of the bin and again at 20 ⁰C. The outside temperature is also 20, but after it gets heated and compressed by the fan it is now 24.2 C. Using the drying calculator, we plug in 11 for MC and 20 for both the grain and air temp, and it gives us 78.1% for RHthres. And then we use the humidity calculator to calculate the absolute humidity, 13.5 gr/m3. Since there is no drying taking place, the absolute humidity will be the same at the bottom of the bin: 13.5. The saturation humidity for 24.2 C is 21.8 and this then gives us an RH of 13.5/21.8 = 55.8%. Again with trial and error by plugging in different MC we find that a MC of 7.1% is the MC of the canola at which equilibrium is reached at 24.2 C and RH of 55.8%. The top of the bin is at 11% MC and the bottom is at 7.1 % MC – a whopping 3.9% difference.
I have seen fact sheets and guidelines for drying that suggest this difference in MC from top to bottom is actually some sort of front, and that to get the drying front right through to the top, one must use a bigger fan with lots of air flow, lots of pressure. But in understanding that an increased pressure will only result in more pressure, more compression; the spread in MC from top to bottom will only be worse. To mitigate this MC spread, I think there are a couple of things we can do. First, use smaller fans with less pressure, and secondly don’t run the fans continuously; give the moisture a chance to equalize. We also don’t want to have a pressure drop across the screen or perforated pipe. I would certainly be an advocate for open bottom pipes or louvers.
There is another complicating factor that aggravates this situation. I assumed in this analysis that no drying was taking place. We were at that point in time when the drying was done, and equilibrium was achieved at the top and bottom. However when we first start the fan, the grain at the bottom was just as tough as the grain at the top and the bottom would dry first. And when it dries, it must give up energy and heat to vaporize the water. The drying will be another cooling agent, cooling the air as it goes to the top. This will keep the top cool, the bottom warm – the top wet, the bottom dry. It also means that water will be added to the air, and that the absolute humidity of the air at the top of the bin will not be the same as at the bottom; it will be higher. Now we have air at the top that is even colder and wetter than it would be if no drying was taking place at the bottom. It is no wonder that the top grain has no chance of drying until the bottom is finished drying and much over dried.