Supplemental Heat: Act II Serious Problem: Condensation

 

  I was doing some number crunching, concerning a call I got from Jim S from Wainwright Alberta.  He was going to use a standard residential natural gas furnace to try to get his grain dry.  I discovered something very interesting.   IF the temperature of the grain is raised by more than 5 C above the ambient temperature, there will be serious condensation that will form on the inside of the bin, under the roof and on the walls.  When the air goes through the grain, it warms up and takes on moisture, and that is fine; it is drying the grain, but when this warm moist air hits the cold roof, it can not hold all that water, so it dumps it as condensation and rains down on the grain, which then can form a crust and be a source for spoilage.  This can happen even if the grain is dry.  The rule of thumb is that there will be conditions for condensation if the grain is 5 C above that of the outside air.  We can get a slightly better spread if the grain is dry as is shown below  (numbers from my calculator  planetcalc.com/4959/).

Grain    Moisture Content   Grain Temp   Outside Air Temp  RHthres  Temp Spread
Wheat            17                            4                           0                        101%               4
Wheat            17                            24                        20                       102                4
Wheat            15                            25.5                     20                       100.1             5.5
Wheat            15                            6                           0                        101               6
Wheat            14                            7                          0                          98.9%          7
Canola           10                            5.5                       0                        100.1             5.5
Canola           12                            4.5                       0                        105                 4.5
Canola           12                            24                        20                        103.9            4
Flax                 10                         5.5                         0                         100.1             5.5

Flax                12                           4.5                        0                          100.4            4.5
Flax               12                            24                         20                         102.6             4

So, I think this is a serious problem with those attempting to use supplemental heat with their aeration fans:  heating the grain by more than 5C can result in serious condensation problems at the top of your bin.

How much water will be condensing out as rain.  Let’s say that Jim’s wheat is heated to 15 C and the outside air is 5 C; the wheat is 10 degrees higher in temperature than the outside air, so we suspect there will be condensation; but how much. Using the grain calculator (EMC equations) we discover that wheat @ 17% moisture at 15 C will produce an RH of 79.1% and an absolute humidity of 10.2 grams of water per cubic meter.  When this air hits the cooler roof that is the same temp as the outside air, 5 C, it will cool to 5 C.  The most water that air can hold at 5 C is 6.86 grams of water per cubic meter.  The air when it was warmer at 15 C was holding 10.2 grams.  The difference is the amount of water that will condense as liquid water on the roof, or fall onto the top layer of wheat as rain. For every cubic meter of air that is blown into the bin, 10.2 – 6.86 = 3.34 grams of water will rain down on the wheat.  In one hour, it was determined earlier, that 5100 cubic meters of air flows through the bin.  In one hour 5100 x 3.34 = 17,034 grams, or 17.034 kg, or 37.54 lbs of water is deposited onto the wheat.  Can you imagine filling up the better part of a 5 gallon pail with water and throwing it onto your wheat — every hour??  And why do we get spoilage and a crust on the top??

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