Supplemental Heat: Act IV Using a Gen Set for Heat

In my last blog it was established that the grain should not be heated by more than 5 C and that a 50,000 btu furnace could do this in 12 hours. What if we used the heat from a small gas driven generator set?  The electric power generated could be used to power the aeration fan,  and this set up would be ideal for remote bins where power from the grid is not available.  OK let’s consider this.  But how big should the gen set be?  What will it cost for the gen set, and the fuel to dry Jim S’s 3500 bushels of wheat that is now 17%?

Jim is using a 5 HP fan.  1 HP is  .7457 kW, so the gen set must be able of producing at least  5 x .7475 = 3.7 kW.  At Princess Auto, I found a 7500 Watt, Westinghouse, gas Generator that had a 6.6 gallon (25 liter) fuel tank that would last 11 hours at  half load.  It cost about \$1000.  We would be burning 25/11 or 2.27 liters per hour.

The specific energy of gasoline is 46.4 MJ/kg and 1 liter of gas weighs 1.64 lbs/2.2 = 0.743 kg.  One liter of gas will produce 46.4 x .743 = 34.47 MJ or 34,470 kJ.  But we burn 2.2 liters per hour, 2.2 x 34,470 = 75,845 KJ expended as heat and electricity in one hour.  We said we need 3.7 kW to power the fan, but a Watt is in terms of per second and we want it as per hour, so multiply by 3600.  3.7 x 3600 = 13,320 kJ per hour.  The energy produced as heat will be the total energy produced by the burning gas, minus the electrical energy:  75,845 kJ – 13,320 kJ = 62,525 kJ per hour.

In the previous blog, we saw that our 50,000 btu furnace put out 52,750 kJ/hr and we saw that this would be the energy necessary to raise the temperature of the wheat by 5 C.  That is at 100% efficiency.  However we will be losing some of the heat; not all the heat energy will be going into raising the temp of the grain.  Our gen set produces a little more heat than the furnace, and doing 11 hour runs would be enough.  A 5 C rise in temp, is like pulling the temperature of the grain down by 5C.  Using our rule of thumb that 15 C decrease reduces the MC by 1%, then 5C would reduce it by 0.3 percentage points.  To go from 17 to 14.4 would take 2.6/.3 = 8.6 eleven hour runs.  Let’s say 9 runs. Each run uses 25 liters, and let’s say a liter costs us \$1.  The cost for the fuel would be 9 x 25 = \$225.  One must bear in mind that this is for both the electricity to drive the fan but also for the supplemental heat.

One way to make this better would be to convert the gas gen set to natural gas.  With an inexpensive conversion kit, this can easily be done.  The cost of gas is about 2.5 times that of natural gas, so we could easily get our price of fuel down to \$100.

Another idea, would be to use a car, truck or tractor that runs on gas, and simply park it right up against the fan so that the radiator is right next to the input of the fan.  Shroud the whole thing with a tarp, so that the fan sucks in all the heat from the idling vehicle.  If it is burning about 2 liters per hour, it will be giving off about the same amount of heat as our gen set, and will raise the temperature of the wheat by 5 C.  We would avoid the cost of the gen set, but we would be paying over \$200 in gas to get the grain dried.

In all of this estimating, it was assumed that the daily temp was constant.  But we all know that the temp changes by 5, 10 or even 15 deg over a day.  Can we use this variation in temp to save some money.  Also on any given day we will have better and worse drying conditions.  The humidity can be high or it can be low. Maybe we could do some careful cherry-picking to only do our runs when we will maximize our drying and reduce our overall cost?  Should we be cycling heating and cooling?  Just more to consider for future blogs.

Supplemental Heat: Act III How Long to Heat Grain 5 C

In my last blog, I was preaching about the dangers of over heating the grain because it could cause condensation.  In fact I recommended that the grain should not be more than 5 deg C warmer than the outside air.  But how long would it take to raise the grain 5 C?  What would it cost?  How many cubic meters of natural gas would be required?  How much water would the combustion of that much natural gas be?

We will use Jim S. situation in which he has 3500 bu of wheat in a hopper bottom bin at 5 C and wants to heat it to 10 C with his 50,000 btu furnace.

Wheat is 60 lbs/bu.  So in kg 3500 bushels weighs 60 x 3500/2.204 = 95,281 kg.

Specific Heat of Wheat is 1.36 kJ/kg C

For 1 C rise in   1.36 kJ/kg C  x 95,281 kg = 129,582 kJ/C

For 5 rise (5 t0 10)  5 x 129,582 = 647,910 (assume that all energy went into wheat)

Our furnace puts out 50,000 btu and there is 1.055 kJ/btu so furnace puts out 52,750 kJ/hr.  But we need 647,910 kJ, so it will take us 647,910/52,570 = 12.3 hrs to raise the temp of the wheat by 5 C.

Cost  1 GJ = \$4.30   so .647 GJ = \$2.78  (Sask Energy price)

Estimated Water Removal   .2 %  (MC 17% to 16.8%) – Assume that we will pull the wheat back to 0 C.  This would be 190 kg of water removed.  We would have to do this everyday for two weeks to get wheat from 17 to 14.4% and this would cost about \$36 in natural gas.

1 cubic meter of natural gas produces 0.688 lbs of water in its combustion.  There are 26.94 cubic meters required to produce a GJ of energy.  But we only need 0.647 GJ to raise the temp 5 C, or  0.647 x 26.94 = 17.43 m^3.  which would produce  17.43 x 0.688 = 12 lbs  (5.44 kg.) of water.   If we would use a tiger torch instead of the furnace, we would have taken 190 kg of water out of the wheat, but would have added 5.44 kg of water back into the wheat from the combustion of the natural gas.  I think it is a good idea to prevent the flue or the smoke from getting into the grain.  People are throwing out their conventional low efficiency furnaces,  you could pick one out of the junk for almost nothing.  All you need is the burner and the heat exchange. Even if the heat exchange is cracked, it would not be a problem.

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??

Supplemental Heat: Be Careful

In my last blog on night drying does not work, I explained how Jim S. from Wainwright was considering adding a natural gas furnace to his aeration fan to provide supplemental heat.  He was getting no where with night drying, in fact the weather conditions were so bad that water was being added.  Off the top of my head, I thought adding a furnace would be a good idea, but I would run some numbers to check it out.

Here is the situation: 3500 bushels of tough wheat at 17% Moisture Content. Let’s say his grain is near the freezing point, 0 C, and his fan can produce 3000 CFM or not quite the 1 CFM/bu recommendation.  The immediate weather forecast is for cool temperatures and high humidity.

3500 bushels weighs  60 lbs/bu   2.204 lbs/kg     3500 x 60/2.204 = 95,281 kg

If we want to remove 1% MC we would need to remove 953 kg of water.

To evaporate 1 kg of water it requires 2257 k joules of energy, so to remove 953 kg of water,  2257 x 953     =  2,150,921 or about 2 GJ of energy.  Checking with SaskEnergy, a GJ of natural gas would cost about \$6.50.  So, at least \$30 of natural gas is required to lower the MC by 1% and \$75 to bring Jim’s wheat down from 17 to 14.5%. This of course assumes that all of the energy we add will go into drying.

How big should the furnace be?  When shopping for furnaces, you will see the spec as to the heat they give off is usually in BTUs or British Thermal Unit.  And 1 btu = 1.055 kJ  What they really mean in the spec is btu/hr; the per hour is implied.  I found a ceiling furnace at Princess Auto with an output of 50,000 btu/hr.   or 52.75 MJ/hr.  We calculated above that we need 2,000 MJ of energy to get 1% moisture out.  To get this energy into the wheat, it would take 2000/52.75 = 38 hours of running the furnace.  Perhaps 100,000 btu –> 19 hours or even a 200,000 btu would take 9.5 hours.

OK so we have the furnace sized, and we have an idea of how much this is going to cost in natural gas; what’s next?  How much should we heat the grain?  Should we cycle the heating and cooling?  If we are going to cycle — how long should the heat/cooling cycle be?   I am going to assume here that the wheat is at 0 C and the outside air is also at 0 C, 76.2% RH.  I ran some numbers with the grain drying calculator with the grain heated to various temps.

Grain-Temp  RH-bin-air abs-hum-bin-air RHthres-outside-air  abshum dif  5100m

20                     79.9             23 gr H2O           302%                              19.3               98.43

15                     79.1%          10.2                     217.6%                           6.5           33.15 kg

10                    78.2%           7.4                       154.2%                           3.7           18.87 kg

5                      77.2               5.3                        108.4                              1.6            5.92 kg

0                     76.2                3.7                         76.2%                                0               0 kg

Looking at the first line, we heat the grain from 0 to 20 C.  Because the wheat is 17% MC at 20 C, EMC equations tell us that it wants the air to be at an RH of 79.9%, which gives an absolute humidity of 23 gr per cubic meter. The air entering the bin had an absolute humidity of 3.7, so for every cubic meter of air flowing through the bin 23 – 3.7 = 19.3 gr.  In one hour 5100 cubic meters of air flow through the bin so 19.3 x 5100 = 98.430 kg of water are removed in one hour. To remove one percent MC we need to remove 953 kg of water, and this would take about 10 hours.

This all looks really good, but we have one huge problem — CONDENSATION !  We see that the RHthres for the outside air is 302%.  Anything over 100, and we will get condensation on the roof and walls of the bin.  Here is what is happening: the air goes through the grain and becomes 20 C and RH 79.9. It is carrying 23 grams of water per cubic meter.  This air hits the cold roof and is cooled to 0 C.  But air can’t hold much water at 0 C, only 5 gr/m^3. The remaining 18 grams condenses and comes raining and running back into the wheat.  If we want to avoid condensation we should only have the grain about 5 degrees warmer than the outside air.  If we only raise the temp of the grain by 5 C, it will take a long time to remove 1% MC —  953/5.92 = 161 hours or about a week.  It would take over 2 weeks to get Jim’s wheat dry.  Trying to go faster with higher grain temps would be a dangerous exercise in creating condensation.   Maybe we should reconsider this supplemental heat strategy and go with cherry picking good drying conditions.  The wheat is safe, being as cold as it is, so what’s the hurry?

If you want to use supplemental heat, you better be careful.

I would tell Jim to use the smaller furnace, of 50,000 btu, which would give a temp rise of 8 C.  The fan should only be run when the temp of the day is the highest, maybe at 6 hours a day. The heat should then be shut off and the fan left running into the coldest part of the night.  I would do some cherry picking of good conditions using the drying calculator, and certainly not running the fan when Condensation conditions exist.

Night Drying Doesn’t Work?

I got a call a couple of days ago from Jim S from Wainwright, Alberta.  He said this night drying thing wasn’t working for him, and that he was considering buying a natural gas furnace to add some supplemental heat to the process. Would that work?  I told him I thought so, but I would have to work out the numbers to see; but I’ll save that for another blog. First let’s have a look at why this night drying isn’t working out.

Jim has tough wheat, 17%, that was harvested cold and put in a 3500 bushel hopper bin. Attempts at night drying brought the temperature down, but because of the nasty wet weather they have been having, the moisture has remained close to 17.  Jim thought that water may have been added.  I told Jim that once you got the temperature of the grain down, you have taken the energy out of it that could have been used for drying.  So it is quite possible that now that the grain is cold, that drying will cease.

Here is what I did to get a handle on what was happening in Jim’s case.  I went to the website for BINcast and got the hourly temperature and relative humidity for Wainwright from Sunday Oct 14 til Thurs Oct 18.  The temperature was bouncing around the freezing point, from +4 to -6 C.  The relative humidity was high throughout this time, 90% –  95%.  We didn’t know the temperature of the wheat, but we knew it was cold, so I assumed it was 4 C.   I modeled the temperature of grain, so that it would chase the outside temp in a similar fashion as I saw how the grain temperature changed in our trials at Indian Head. At the end of this simulation, the grain temp was close to -1.5 C.

I now had all the information I needed to calculate the amount of hourly drying. To make a long story short, I used the principle of absolute humidity, and EMC equations to calculate the amount of drying for each hour. For the first few hours we did take out a bit of moisture until the temp of the grain came down, and then we started adding small amounts of water. We did get some drying when the temp went down to -5, but this was short lived.  In the end we did indeed add water, 7.52 kg.  This actually isn’t that much, it would raise the MC from 17% to 17.001% however we can definitely say that night drying did not work in these conditions.   Maybe we should consider supplemental heat?  Maybe Jim should buy his furnace.  Using supplemental heat brings forth a whole host of other questions.  What fuel should I use? How much will this cost?  At what time of day do we apply the heat?  How hot should we get the grain?  Should we have a cooling cycle, or should we just apply the heat continuously. How long should the cooling cycle be?

In my previous blogs, I showed that a 15 deg C cooling of the grain resulted in a 1 percent decrease in moisture.  So let’s say we want to raise the temp of the wheat by 15 C. The specific heat of wheat varies, but it is about 1.36 kJ/kg C. I will spare you the details but to heat 3500 bushels, 15 C, requires 2 GJoules.

I checked with SaskEnergy and used the internet to check out the cost of the different fuels for 1 GJ.

Natural Gas   \$6.48 /GJ

Gasoline          \$16 /GJ

Fuel Oil (Diesel)  \$21.65 /GJ

Propane            \$14.66 /GJ

Electricity           \$32.81/ GJ

So, we can see there is no decision, if you have natural gas, it is by far the cheapest supplemental fuel.  Also 1 GJ , at least, is the required energy to remove 0.5% MC.  Maybe we can get Jim’s wheat dry for \$40 — if we do everything just right? As our benchmark,  and challenge, we will use the same nasty weather conditions above, with the supplemental heat and see what happens?

Stay tuned for the next blog on playing with supplemental heat.