Supplemental Heat: Act V Using the Grain Drying Calculator, A Balancing Act

I have blogged about the grain drying calculator, http://planetcalc.com/4959/

I explained how to use it without supplemental heat, but not how to use it when supplemental heat is used.  The short answer is that you use it in the same fashion.  You still enter the moisture content of the grain, the temperature of the grain, and the ambient outside temperature of the air (before it is heated). The resulting RHthres will indicate whether or not drying will occur.

One might conclude, that you will get the same answer for RHthres whether you are applying heat or not; and this is quite true — at first, but if heat is applied the temp of the grain will increase.  My Act III blog on Supplemental Heat indicated that it would take 12.3 hours to heat the grain 5 ºC with a 50,000 btu heater.  However I failed to mention the two implied assumptions: 1) that the grain body heated up uniformly and 2) the ambient outside temp remained the same.  Neither of these assumptions are true.

The grain will heat at the bottom of the bin first, and the so called “warming-front” will slowly work its way to the top.  If you have a cable with multiple temperature sensors, you will see this first hand.

The ambient air temperature also varies by 10-15 ºC.  The air going through the heater will get a boost in temperature, above the ambient, by maybe 20-30 ºC. So, let’s say the outside air temperature has a low of 5, and a high of 15; and the temperature rise through the heater is 30 C.  Let’s say we turn the heater on when it is cold outside, 5 C and so is the grain, 5 C.  After one hour, the bottom of the bin might increase to 10 C, while the top stays at 5.  After two hours the grain will be even warmer at the bottom, maybe 15, and as time goes on the bottom will get warmer and warmer with a wave of heat slowly creeping up the bin.  We have a string of temperature sensors, so we can actually watch this wave.  Eventually the top of the grain will also be getting warmer.

Now comes the tricky part, when should the heat be turned off in order to curtail condensation?  Remember we never want the top of the grain temperature to exceed the outside ambient temperature by more than 5 C.   Let’s say that we are approaching the highest temp of the day, 15.  Maybe the top of the grain is 10 C, and the bottom 30 C.  Everything is fine — no condensation — yet.  As the day cools off, to 5 C, the wave of heat will continue to heat the grain at the top, even if we turn the heat off. In a few hours the top of the grain may become 15 C and as we approach the low of the day, 5C — we will have conditions for condensation — the grain is 10 C higher than the ambient temp.

What would be ideal would be to apply heat such that the top layer of grain is always 5 ºC warmer than the outside air.  There are two difficulties with this. First there is a huge delay from the time we apply heat at the bottom until it reaches the top layer.  It would be hours, and it depends on so many things, such as air flow rate, heater size, bin size, etc. And to anticipate this delay is really difficult.  The other complicating factor is that the ambient temperature changes, and sure we can get a forecast of the temperature, it is not entirely accurate. And then to monitor and control this whole mess.

Let’s take on the heat wave delay problem.  The outside air goes up and down in temperature in a somewhat predictable fashion.  Somewhat of a sinusoid in shape, with the warmest part of the day taking place a couple of hours after noon, and the coldest part of the day a couple hours after midnight. Typically we see a difference in the high to low temperature of 10 – 15 ºC.   We ideally would like to apply supplemental heat such that the top layer of grain is always 5 to 6 degrees warmer than the outside temp.  The problem is there is a delay in applying the heat to the bottom of the bin to when it gets to the top.  What is this delay?  To determine the delay we could model the system, but this is a trickly onerous task — and there are so many variables.  The other way to find out what this delay is, would be to review the data of our runs, and see what it is.  I did just that, and I examined two trial runs, 9 10P and 09 09W.  Clearly the delay was close to 6 hours.  The flow was close to 1.2 CFM/bu and if we lowered the flow to 0.3 CFM/bu we would increase the delay accordingly to 24 hours.  The temperature of the top layer of grain will be synchronized with the daily cycle of temperature change of the outside air. We can use the rate of flow of air to adjust the delay and set it where we get a 24 hour delay.

Now the other problem, we want to add supplemental heat to keep the top layer of grain  5 to 6 ºC above ambient, we want a 5-6 ºC rise, throughout the whole day.  We would keep the fan running continuously, and the heater running continuously to provide a 5 C rise.  What size heater do we need for this?

In my earlier blog: “Big Heater for supplemental heat Example” I had a farmer with a 1.4 million btu/hr heater at 5,000 CFM. It produced a 30 C rise in temp.  Supposing we have a 3500 bushel bin, we would need 1000 CFM to get the synchronizing daily delay of 0.3 CFM/bu.   The rise we would get with a 50,000 btu furnace would be 50,000/1,400,000 x 5000/1000 x 30 = 5.35 ºC — Perfect.  A 50,000 btu furnace with 1000 CFM should do the job to give a 5 C rise with a 24 hour delay.  We might have to use some trial and error to fine tune this.