The question is: how effective is the thermostat control in which the fan is on only when the outside air temp < grain temp. Let’s compare the thermostat control to running the fan continuously with essentially no control.
In August 2009 a 2200 bushel hopper bottom steel grain bin, was run for 117 hours, continuously; the amount of water removed each hour was recorded and shown in Figure 1. The temperature of the peas at hour one was 27 deg. C and the moisture content was 16%.
The cost to run the 3 HP fan for 117 hours @ $0.15 kWhr, where 1 HP is 0.7457 kW; 117 x 3 x 0.15 x 0.7457 = $ 39.26
At the end the temperature of the peas was 18.8 deg C. the moisture content was 14.65% and the Safe Days Index was 3.6%.
Now let’s apply the thermostat controller, such that the fan will only run while the outside air temperature is less than the pea temperature. Using this control strategy the fan will only run for the first 17 hours to reduce the temperature of peas to 12.7 deg C and reduces the moisture content to 14.33%. The safe days index under this control turns out to be 1.9%. If the Safe Days Index is 100% the grain would have deteriorated to have a reduced germination rate of 95%. The power to run the fan for 17 hours would be: $5.70
Clearly the thermostat control is better in terms of cost ( $39.26 to $5.70); the peas would be cooler ( 18.8 to 12.71 deg); more moisture is taken out so the peas are drier (14.65% –> 14.33%) and consequently the peas are safer with less spoilage with the spoilage index (3.6% –> 1.9%). Clearly the thermostat control is superior on all accounts compared to running the fan continuously for 117 hours.
The reason that the thermostat control is more effective is that it only runs when water is removed and does not pump water into the bin, back and forth needlessly.