Supplemental Heat — Cycling Works the Best

I was just talking to a farmer, who farms west of North Battleford.  He had the same difficulties this fall with harvest and had plenty of tough grain to deal with.  He purchased a diesel heater, that put the equivalent of 75,000 btu/hr. into a 5000 bushel bin with a 3 HP aeration fan.   He conducted a little experiment — when did he get the best drying?  When the supplemental heat was left on continuously, or when it was cycled  heating the grain up and cooling it down.  He told me that clearly the cycling resulted in much better drying, and he was curious as to why.   I have received this same question from others.  Here is what I think is happening.

It comes down to vapour pressure.  The air has a vapour pressure that is trying to push water into the grain.  The two factors that effect vapour pressure for air are the amount of water in the air and the temperature.  The dominant factor for the vapour pressure for air is definitely temperature — the higher the temperature the more the pressure.  The grain also exerts a vapour pressure and again the dominant factor is temperature.  So, if the grain is warmer than the surrounding air, the grain most likely will have a greater vapour pressure and water will be pushed out of the grain and into the air.  The opposite will occur when the surrounding air is at a higher temperature than the grain — it is most likely that the air will have a higher pressure and water will move from the air into the grain.  Wetting will occur.

When supplemental heat is applied continuously, the grain becomes more or less the same temperature as the air.  As such the vapour pressures of each are more or less the same, and little drying occurs.  On the other hand if the grain is warmed, and then cooled with cool or even cold air with very little vapour pressure; there will be lots of water being pushed out of the grain and into the air. Lot’s of drying.   Back to our old adage “Cooling is drying” and our data indicates our rule of thumb:   “For every 15 C that the grain is cooled, 1% moisture will be removed.

Here is what our farmer did to get good results with cycling.  He had temperature sensors on a cable spaced 4 feet apart.

He would run the heat (75,000 btu/hr) into his bin until he got the first 3 or 4 sensors on the bottom of the bin to 25 C. This may take a day or two.  He would then cool the grain, using cold night air (5 C), and get the grain down to maybe 10 C.   Each cycle removed about 1% moisture.   After a couple of cycles, he moved the entire contents of the bin to another bin.  He called it tipping?  And if the grain was still not dry he would go through another cycle.  He always ended up with grain being brought to as low a temperature as possible.

And this brings me to another note, as I think of it.  I was at a conference in which Sarah Foster gave a presentation on germination rates, etc.  She was from a seed lab in Winnipeg.  I asked her if you could ever get seeds too cold,  No, definitely not was her reply.  However can you get grain cold enough to kill off the fusarium?  Yes was her reply, but she wasn’t sure at exactly what temperature.  She is looking into it.

Back to my farmer;   he is so pleased with the drying technique that he has come up with that he now plans to start harvest when his grain is a couple of points higher than dry.  He said he might be able to get a two day jump on things. And later in the fall, he will be able to start earlier in the morning.  Yes, he might be paying a bit for supplemental heat (diesel fuel) but that is offset by getting his grain safely into the bin.  And now that he knows how to get his grain dry in a matter of days, he can still sell his crop in short order.

 

 

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