In my last blog, I showed how over-drying occurred from the increased temperature at the bottom of the bin caused by compression. Maybe we cannot eliminate it; but can we at least reduce it.
In our previous example we used a 2200 bushel-bin of barley at 20⁰ C and MC of 15%. A 5 HP aeration fan produced 3000 CFM with 6” H20 pressure. This resulted in the bottom being 4.2⁰ C warmer and dried to 11.6% MC all while the top remained at 15%.
Browsing around the internet, looking at fan curves, I found out that the air resistance is not linear like electrical resistance. Static pressure is proportional to the square of CFM (flow). And required horsepower, HP, is proportional to the cube of CFM. We can find these proportionality constants:
Pres = a CFM 2 6” = a 3 2 (3 is for 3,000 or CFM is in thousands) a = 6/9 = 0.666
HP = b CFM 3 5 = b 3 3 b = 5/27
Let’s see what happens if we reduce the CFM from 3 thousand to 2 thousand.
Pres = 0.66 (2 2 ) = 0.66 x 4 = 8/3 = 2.66 “ H20, less than half of what it was, 6.
And the required HP = 5/27 ( 2 3 ) = 40/27 = 1.48 or call it 1.5 HP
Isn’t that incredible, we only decreased the flow by two thirds, and in return we get a pressure that is less than half, and a required horsepower that is much less.
This reduced pressure will also decrease the temperature rise from compression.
2.66/406.8 = x/293, where x must be 1.9; that is we get a 1.9 ⁰C rise from compression. Let’s say we also get an increase in temperature from motor inefficiency, for a combined total of 2⁰ C or an absolute temperature of 22 ⁰C.
So we had at the top of the bin barley at 20 C and 15% MC. 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, so the air at the bottom has the same absolute humidity, or 12 grams per cubic meter. At the bottom of the bin, the temperature is 22 C, and the saturation humidity is 19.5 gr, giving an RH of 12/19.5 = 61.5%. Now using the grain drying calculator, find the MC for barley by trial and error with a temp of 22 and RH 61.5% and I get a MC of 13.7%. This is a spread of 1.3%. This is better, remember with a higher pressure of 6, we got the bottom to be 11.6%, and a spread of 3.4%.
The conclusion to this story is that a relatively small decrease in flow, will greatly reduce the pressure, which has a profound decrease in over drying the bottom. Reducing the flow will also increase the transit or contact air-to-grain time, giving the grain more time to pass water into the air.