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.

 

 

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: 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.

Running the Fan Continuously: What’s wrong with that?

Running the aeration fan continuously has been the conventional wisdom since aeration fans came into existence.  The truth is that no one really knew what was going on, and that eventually the grain would come down in moisture content. But there also was this lingering feeling that there must be times when drying was occurring and times when it was not.

I have also heard, many time, that one must keep their fans running because if you stop it, the moisture layer will collapse and a crust layer will form.  So, maybe it is safer to just leave it on.  And since we don’t really know when the conditions for drying are, maybe it is best to just leave it on.  Let’s address this one first.

In all the data we have collected at Indian Head, since 2007, we have never seen a distinct drying layer, or moisture band.  We have certainly see the bottom dry first, and in many cases at the end of the trial run, the top of the bin is still tough, while the bottom is over-dry.  But there is no distinct layer, or even pockets of moisture.  The change in moisture from one part of the bin to another, is a slowly changing continuum.  And when the fan is shut off, the temperature and moisture of the grain more or less remain constant, or at the very least changing ever so slowly.   Turning the fans off, is not the culprit reason for a crust forming.

Now for the other reason we leave the fans on: we don’t know when the conditions are right for drying.  Well, now we do.  We know the typical diurnal drying cycle and with the grain calculator one can exactly tell when there are drying conditions, and ever for conditions of condensation.  So not knowing when we have a drying condition, no longer is a valid excuse for running the fans continuously.

So what’s the problem in running your fans continuously?  In short more spoilage. Grain starts to deteriorate as soon as it comes off the combine. Storing grain can only slow down the spoilage process and there are two things that contribute to spoilage: higher grain temperatures and higher grain moisture. If you run the fan continuously during the day, there is a very good chance that you will be heating and wetting the grain — the exact things that contribute to spoilage.

To summarize, running the fan continuously will:

  1. Produce more spoilage
  2. Use more energy
  3.  The Figure below is an example of a run done in 2009. In the first few hours of operation the amount of water being removed from the bin is quite high. It was not unusual to have one percent moisture removed from the grain on the first day.  But at hour 21, we see something strange take place, the amount of water leaving the bin becomes negative; we are adding water to the bin. We are now into a well established 24 hour cycle of water being removed from the bin and then water added to the bin — almost equal amounts.  After the first 21 hours, there is essentially no drying taking place.  We are literally spinning our wheels, taking water out of the bin, and then putting it back in. And to make things worse we are adding the moisture during the day, as we heat the grain.  Warming the grain, and wetting it at the same time are just the conditions necessary to promote spoilage.
  4. Hourly Water Removed From Bin
    Hourly Water Removed From Bin

     

Notes to self about ideas for Future Blogs

  1.  Grain Drying 101    Drying Conditions if the absolute humidity of the air inside the bin is greater than the absolute humidity of the outside air.  Explain what absolute humidity is:  amount of water in one cubic meter, explain how it is different from relative humidity,  and that absolute humidity is calculated from the saturation curve and pro rated with the relative humidity.  Diurnal cycle and the night drying and the yardlight rule.
  2. Motor/Generator Set to make a portable system.  Especially good for remote sites that have no power.  Make use of the wasted heat from the motor.
  3. Plenum design, show how we waste energy (pressure) in going across the screen.  Better to use the electricity with a heater in front of a small fan. The pipe or plenum should be open on the bottom for the least resistance in getting the air into the grain.
  4. Time Constant Tau.  The time constant is the amount of time required for the outside air absolute humidity to get to 60% of the distance of the difference. A CFM/bu of 0.35 would be about 3 time contants and get us to over 90% of the way there.  This blog would show that 0.35 CFM/bu is kind of the sweet spot for using as much of the grains energy as possible, while still drying in a relatively short time.
  5. The argument for small fans:  less capital cost, less power infrastructure required, more uniform drying, use more of the inherent grains heat to dry, more efficient  — the downside is that it is a bit slower — but what is the hurry? How slow can you go.  The ideal fan size. CFM/bu
  6. More on condensation, and how to use the grain drying calculator to detect conditions for condensation.
  7. More specifics on how to use the grain calculator.
  8. Something for everybody, from the simplest strategy (yard light rule)  to the most advanced (absolute humidity) and the pros and cons of each. Depending on what you have.
  9. Long term storage, use whatever fan strategy until the end of January, in which you will get the grain frozen and cold as possible, and from January til the end of July keep the bin as sealed as possible.
  10. Roof vents don’t do much, and can actually result in condensation on the top layer of grain in the spring when the grain is cold and it is swiped by warm moist air circulating through the vents.
  11. Vertical in grain tubes.
  12. top dries first, compression causes heat
  13. safe days
  14. comparison of control strategies, continuous, nite, temp differential, absolute humidity.
  15. Flow from natural convection
  16. Energy in the grain to dry
  17. Grain Resistance
  18. Motor HP as we drop the flow in half, top down drying, HP 5 to 1
  19. First 24 Hours is critical
Ron Palmer
Ron Palmer

Condensation a problem in fall and spring.

In a previous blog, I explained how condensation could be a problem in the fall with warm grain and cold outside air.  The rough rule of thumb was that there was a potential for condensation on the roof if the grain was ten degrees warmer than the outside; but a more precise determination could be made with the grain drying calculator.  The recommendation was not to turn your fan on if the RH from the calculator was above 100%.

There is another time, in the spring that condensation could be a problem. When the grain is very cold and the outside air temp is much warmer and holding a fair amount of water. This air enters the bin through the fan entrance, the top or vents; and it hits the cold grain — it will cool past its dew point and condensation may well occur.  This will happen mostly on the poor old top layer, that may well have had water dumped on it from fall condensation, and it is the last layer in the bin to dry.  No wonder the crust always forms on the very most top layer.

To prevent condensation in the spring, the best thing to do would be to not let the warm air in.   The bin should be sealed.  The top lid closed tight, the fan covered and all vents covered.  Indeed, this is a time when vents will actually be the cause for grain damage.  Sealing the bin in mid winter, after the grain has been cooled to its lowest possible temperature, will also keep the grain as cold as possible for as long as possible.  Yes the grain will still warm up from conduction through the walls; but it will be a warming without adding water.  At the end of the summer, when the grain has warmed up and the nights again are getting cold, the bin can be opened up and the grain again cooled.  And this process could be done for years of very safe grain storage.

Energy to Lower MC by 1% for one bushel of wheat

With this blog, we would like to know what it might cost to use supplemental electrical energy to remove one percent moisture content, MC, from one bushel of wheat.

Assumptions:

  1. All the energy goes into evaporation.
  2. wheat  60 lb/bu
  3. Cost of electrical energy  is 0.12/kW hr.
  4. Latent heat of evaporation,  energy to evaporate  2257 kJ/kg

One bushel is 60 lbs, so 1% MC would have a weight of  0.6 lbs or 0.2722 kg

2257/.2722 = 614.4 kJ to evaporate 1% MC from one bushel of wheat  and to put this into hours divide by 3600  =  0.17 kW hr.   and in terms of dollars,  0.17 x $ 0.12 = $ 0.02 to evaporate enough water out of a bushel of wheat to lower the MC by 1%.    That doesn’t sound like much but for 3500 bushels, it would cost $71.68, and to drop by 2% MC it would be $143.36.  It must also be remember that this is the ideal efficiency.  In the real world, we may have to double our cost again.

Could we use the free heat in the air to dry grain?  —  I think so; but it will take careful management and control of the aeration fans.

Wheat Energy Dries Itself

We all know that it takes energy to dry grain and grain does contain energy. This energy is given up when the grain cools, but what if all the energy in the grain was used, as it cools for drying?

To answer this question we will use one cubic meter of wheat that will be cooled with aeration from 30⁰ to 5⁰ C. It has a MC of 15%. If all the energy in cooling could be used to dry the wheat, what could we get the MC down to – 14%? 13%? Let’s do the math and find out.

We know that it takes energy to evaporate water, the latent heat of evaporation; it is 2257 kJ/kg. Let’s assume that all the energy in the wheat, in going from 30⁰ C to 5⁰ C. will be used in evaporating water from the wheat. How much water (in %MC) can be removed?

1 bu = 1.2446 cubic feet       35.31 ft3 = 1 cubic meter     2.204 lbs/kg

One cubic meter of wheat has a weight of : 35.31 ft3/1.2446 = 28.57 bu.    x 60 lb/bu. = 1714 lbs, 777 kg

One percent moisture, 1% MC would then be 7.77 kg.   If wheat was 15% MC, then 116 kg would be water. (Remember we are dealing with one cubic meter or 28.57 bushels)

The wheat in going from 30⁰ C to 5⁰ C has how much energy to give. What is the specific heat? It can vary, and I found that it increases as the MC increases but for a MC of 15% it is about 2 kJ/kgC

The wheat has got energy:

2 x 777 kg x 25 C = 38,850 kJ and divide by how much energy is needed to evaporate

38850 /2257  = 17.2 kg of water can be evaporated with the energy from the wheat.

But 1% MC is 7.77 kg ,   17.2 kg/7.77 = 2.21% reduction in MC. If the wheat was 15% and all the energy went into evaporating water in going from 30 to 5⁰ C, then the MC would be reduced by 2.2% and be dried from 15% MC to 12.8%.

Our data showed just about the same thing. Cooling the grain with aeration, we found that reducing the temperature by 15 C would remove 1% moisture. In some trials with a high MC, we found this to be: 10C/%. In this case, for wheat, the amount of cooling to decrease the MC by one is: 11⁰/%MC.  This demonstrates that aeration (at least with our experimental trials) is efficient in using the energy of the grain. I thought we might be losing more of the energy to conduction, but it appears that almost all the energy in the grain goes into drying. I thought that by increasing the contact time, (decreasing CFM) that we could get more efficient, but it appears that we are already doing a pretty good job in using almost all the energy in the grain for drying. It also tells me that we can’t expect much more than 2% decrease in MC in going from 30⁰C to 5⁰C. If we must lower the MC by more than two percent, we might have to use supplemental heat, or carefully allow the outside air to warm the grain (Hopefully without adding water) and then cooling it to remove more water.

Seed Energy: How much moisture can the energy in the grain remove?

For this exercise we will use one cubic meter of barley at 30 C. And we know that it takes energy to evaporate water; this is called latent heat of evaporation, which is 2257 kJ/kg. Let’s assume that all the energy in the barley in going from 30 C to 5 C is used in evaporating water from the barley. How much water (in %MC) can be removed?

1 bu = 1.2446 cubic feet       35.31 cubic feet = 1 cubic meter     2.204 lbs/kg

One cubic meter of barley weighs 35.31 ft3/1.2446 = 28.57 bu     x 48 lb/bu = 1361 lbs, 618 kg

One percent moisture, 1% MC would then be 6.18 kg.   If barley was 15% MC then 92.6 kg would be water.

The barley in going from 30 C to 5 C has how much energy to give. What is the specific heat? It can vary, but I found one source: 1.36 kJ/kgC

1.36 x 618 kg x 25 C = 21012 kJ and divide by how much energy is needed to evaporate

21012 kJ /2257 kJ/kg = 9.3 kg of water can be evaporated with the energy in the barley.

But 1%MC was 6.18 kg/%MC ,   9.3 kg/6.18 = 1.5% reduction in MC. If the barley was 15% and all the energy went into evaporating water in going from 30 to 5 C, then the MC would be reduced by 1.5% and be dried from 15% MC to 13.5%.

Our data showed just about the same thing. Cooling the grain with aeration, we found that reducing the temperature by 15 C would remove 1% moisture. In the case above, 1.5% resulted from a 25 C. A 1% reduction would require 16.66 deg reduction. This demonstrates that aeration (at least with our experimental trials) is very efficient at using the energy in the grain. I thought we might be losing more of the energy to conduction, but it appears that almost all the energy in the grain goes into drying.

 

How much Time is Required to Cool Grain

Let’s work out an example of how long it will take to cool down a 3500 bushel bin of barley from 30 ⁰C to 20 ⁰C with air that is 15 C and flowing into the bin at 3000 CFM. We will make a wild assumption here and assume that the air will leave the grain at 30 C, making a 15 C increase. For this exercise we will use conservation of energy and we will ignore any drying or wetting that will result from latency heat.

First task is to find the energy that would be lost if the barley is cooled from 30 to 20 C. We must know the specific heat, Cp, of barley and I looked in a couple of places, but it was about 1.36 kJ/kg C. The weight of the barley is 3500 bu x 48 lbs/ bu x 1/2.204 kg/lb = 76,225 kg   times 1.36 = 103,666 kJ for every degree C and we are changing ten degrees so the energy to be removed from the barley will be 1,036,660 kJ.

3000 CFM is 50 cubic feet per second. Air is 0.0807 lbs per cubic foot.   4.035 lbs/s or 1.83 kg/sec

The specific heat for air is close to 1 kJ/kg C so for every degree C we could acquire 1.83 kJ/s. For 15 C we could remove 27.45 kJ/s. How many seconds to remove 1,036,660 kJ?   1,036,660/27.45 =37765 sec, 629 min, 10.5 hours.

This shows that it is possible to cool the grain over a one night period (12 hours, 9 PM to 9 AM).

Now, if we used a smaller fan and produced only 2000 CFM, we would expect the cooling time to increase accordingly to 16 hours. Even for a much larger bin, of 10,000 bushels, we could cool it down in 48 hours (four 12-hour nights). One could argue that this would be for air that is consistently 15 C and it assumed ideal energy transfer. On the other hand there will be cooling affects resulting from drying; but that’s for another blog.