Month: January 2016

In my last blog I went on a rant about how canola could start dripping if the canola is more than 4 C warmer than the air.  And this would be the case when the fan is initially turned on, but after a few minutes — after a few bin air exchanges; there are mitigating factors.

1. The roof, although it is a good conductor of heat, it might warm a bit.  It isn’t necessarily the same temperature as the outside air. It could warm up a deg or so.
2. The canola will be dried down just a bit, but not much, and this would also give us a small advantage in getting a bigger spread than 4 C.  But we know that it dries slowly, so in a few minutes, we can’t count on much, but in several hours we would get some benefit.
3. The canola will be cooled a bit, and this will also help as it will lower the EMC absolute humidity. This would be more important in the long run rather than in the short term of the first hour.
4. When the fan is off the RH of the air in the bin will be that of what the EMC equations dictate for canola at that temp and MC.  I call it the RHemc. However when we start the fan, we don’t have equalized air we have air that is coming into the bin with a much smaller absolute humidity.  It takes time for the air and moisture to equalize, and the air passing through the grain is only in contact with the grain for a minute or so.  We can’t expect the air to reach the value of absolute humidity or RHthres.  This is a non linear process, as the absolute humidity approaches the EMC absolute humidity asymtoticly.  Typically we describe these processes with something called a time constant.  One time constant is the time it takes to reach 63% of the EMC absolute humidity.  I am thinking that in our case it is probably one time constant.  I am devising a means to measure just what this is from our data and that will be a discussion for another blog.  In any event the absolute humidity of the air will be less than the EMC absolute humidity and this will have significant mitigating factor in dripping and increase the temp difference to perhaps 6 or 7 deg C for our canola example.

What if we have the situation in which we have a bin of canola at 11% MC and 30 C and we decide to turn the fan on when the outside temp is 20 C.  You wouldn’t think this would be a problem as that cooler dry air hits the grain it will instantly warm to 30 C, resulting in a lowering of the RH and it will dry the canola.  The problem starts when the air leaves the grain at 30 C, and hits the colder roof of 20 C.  As it cools it will saturate and condensation, dripping and literally raining will occur in the bin.  This water will run down the roof and collect in the canola on the upper side walls, and will also collect directly on the top layer of canola which at the least will form a crust and at the worst will form a hot spot of spoilage that could spread to the whole bin.

Using the grain drying calculator I plugged in 11% MC and found that condensation would occur with as little difference as 4 C.  If the grain temperature was more than 4 degrees C warmer than the outside air, we would get condensation.   I tried it for  grain temp / outside temp for 30 / 26, 20 / 16 and 10 / 6.  Only four degrees difference, and we were on the verge of condensation.

Let’s walk through this with a little more detail.  We have canola at 11% MC and 30 C.  When we calculate the EMC RH for this we get an RH of 80.1% or 83.8%, depending on whose EMC equation we use.  In terms of absolute humidity this is 24 or 26  gr/m^3.  In other words canola at 11% and 30 C wants air around it, or wants to equalize to having air that has about 24-26 grams of water in a cubic meter.  And that’s fine until this relatively warm moist air hits the much cooler roof that is the same temperature as the outside air, 20 C.  When it hits the inside of the roof, it cools to 20 C — and this is where the problem starts — air at 20 C can, at most hold 17 grams of water per cubic meter.  That’s at most, at saturation.  For every cubic meter of air that hits and is cooled by the roof;  25 – 17 = 8 grams of water will be squeezed out as drops of condensed water.  That may not sound like much, but let’s consider a typical flow of 3000 CFM or 180,000 CF per hour or in terms of cubic meters:   180,000/35 = 5142 cubic meters per hour.  Multiply this by our 8 gr per cubic meter and we get over 41 kg or 90 pounds of water dripping onto and into our canola every hour.   Do you think this might be a problem?

To prevent condensation we can’t allow the grain to be more than 4 deg warmer. This is why you should turn your fans on immediately, even while you are filling your bins.  Waiting, into the evening when the temperature is cooler is yes good for drying, but not so good for condensation.  You can see though why there is an argument for keeping the fan on continuously so that the grain temp is always chasing the outside temp and is keeping it to within 4 deg of the outside temp.

From experience we know that grain spoils more readily when it is hot and wet, and that it is the most secure, the safest, when it is dry and cool. But can we quantify spoilage?  There have been attempts at observing mold and fungus, but this was just too subjective.  In 1981 Fraser and Muir suggested that the reduction in germination capacity was related to spoilage and they arbitrarily declared that when the germination capacity was reduced to 95%, it constituted the first step or first stage of spoiling.  The number of days it took to have the grain deteriorate to this point was declared the number of SAFE days and it was empirically found to be:

Safe Days = 10^(6.234 -0.2118 MC – 0.0527 T)   wheat and cereals

= 10^(6.224 – 0.302 MC – 0.069 T)     canola and oilseeds

The objective for safe storage, is to maximize the number of safe days by lowering the temperature T, and the moisture content, MC. Look at how the number of safe days varies with temperature for ‘dry’ wheat:

14.5% @ 30⁰C =  38 safe days

14.5%   @ 20⁰C = 128 safe days

14.5%   @ 0⁰C = 1458 safe days

New Definition:   Spoilage Index = Ʃ ( 1/ safe days)   x 100

The value for spoilage index is equivalent to the amount of spoilage.  Anything less than 100 is very good, and essentially constitutes very little spoilage.  A value of 100 suggests that we have achieved our first level or stage of spoilage, and if the value is several hundred we are getting into some serious spoilage.

Example, if the calculation above  yields the number of safe days to be six, then after 3 days we will have an accumulated deterioration of:     (1/6 + 1/6 + 1/6) x 100 = 50%,

after 6 days : (6 x 1/6) x 100 = 100% of the way to 95% germination capacity

With our system we read the sensors for temperature and can calculate the moisture content, MC, and temperature, T,  every hour so we can refine the spoilage index to accumulate on an hourly basis to catch the dynamic changes in the bin, especially for the first few days. So when accumulating every hour the Spoilage Index would be:

Spoilage Index = Ʃ ( 1/ safe days)/24   x 100

Again if the spoilage index approaches 100; it means that we have reached the first stage of spoilage, the germination capacity has been reduced to 95%.

Spoilage index is an important tool in being able to measure spoilage such that different control strategies can be compared for the efficacy of safe grain storage.

In my last blog, I listed all the different fan control strategies.  From the very simple ,(on at night — off during the day), to the ultimate controller that required moisture cables.  But how much different are they?  Is the extra complexity and moisture cables worth it?

I think we all can agree that the water balance (water in/out) is the most accurate means of measuring whether of not we are drying or wetting, and by how much.  And sure it is great for turning the fan off, but it totally lacks in figuring out when to turn the fan on.

OK let’s compare control methods.  If we only want to control for ‘drying’ i.e. turn the fan on only when we have drying conditions, then we can easily use the water balance as a basis.  But let’s have another look at this; with water balance we are calculating the absolute humidity of the water going into the bin and the absolute humidity of the water coming out.  How different is that from our EMC method in which we input the MC and grain T to get a RH that can easily be converted to an absolute humidity.  In other words grain sitting in a bin will try to produce a specific RH with a given MC and T.  And with T and RH we can easily calculate the absolute humidity.  Now if the outside air has an absolute humidity that is less than the air in the bin’s absolute humidity we will get drying.

Let’s go through this again.  The fan is off and has been off for some time, at least an hour.  Therefore we can use EMC — everything must be in equilibrium. So we plug the grain T and MC into the appropriate EMC equation and get an RH.  From this RH and grain T we calculate the absolute humidity — the number of grams of water in one cubic meter.  Or perhaps we have a moisture cable and have the RH of the air directly.  Either way we end up with the absolute humidity.  And if this absolute humidity is greater than the absolute humidity of the outside air then we have a drying condition and a means to turn the fan on. Now this works out really well, because to use this the fan must be off, and for some time for things to equalize.  Once the fan is on, we start using the water balance criteria of more water coming out than going in to make a decision to turn the fan off.  This works out really neat:  the EMC technique does not work in flowing air, but is great for no flow; whereas the water balance does not work in no flow conditions but is great for flowing.  Therefore use EMC to turn the fan on, and water balance to turn it off.  The two techniques complement each other and are actually using the same thing as a decision criteria — the absolute humidity of the air inside and outside the bin.  There is no point in suggesting that one is better than the other, they in essence are the same.

The method in which we use temp difference is another story.  With temp difference, we only turn the fan on if the grain temp > outside air temp.  We know that the MC of the grain and the RH of the outside air are not used in this.  I did a correlation with the temp diff and water balance on some data we did where the fan ran continuously.  The R^2 value was  0.64 but what does that really mean?   How much better would this temp diff technique be if we did include the RH in some fashion.  Surely the RH does have some effect?  If it is raining outside with an RH of 100%, one in inclined to think we will not be drying?  I don’t care if the temp of the air is less than that of the grain.??  Can we look into this a bit more?

Here is what I did.  I took some old data from master_2015B file, sheet 08 09W which means it was from 2008, bin 9, wheat.  The run was for 288 hours with the fan running continuously.   I did the water balance calculation for each hour and determined how much drying occurred for each hour.   I then applied the temp difference calculation ( assume drying ~ grain Temp – air Temp)  As stated previously I did a correlation with what I consider to be the correct drying, the water balance, and got 0.64.  I also saw that if I used Tdiff as the control strategy there would be 12 hours out of the 288 in which Tdiff would wet the grain.  And if you qualify it with only turning the fan on when RH < 80%, then we are reduced to 6 hours of wetting.  If we further qualify Tdiff with an offset of 2.  Tg-Tair>2 or Tair + 2 < Tgrain, then we only have one hour of slight wetting (this includes RH<80%).

To summarize we got wetting:

1/288   offset of 2   RHair < 80%

6/288  offset 0    RHair < 80%

12/288  offset 0  RHair < 100%

If we used the Tdiff control with offset of 2 and RH < 80% we essentially won’t get any wetting, BUT we will miss out on some drying opportunities.  There are 29 hours in which Tdiff would have turned the fans off when indeed there was some drying and possibly a little warming of the grain.

We are starting to nit-pick here a bit.  The Tdiff is so simple, easy to understand, easy to implement with no heavy duty calculations, and no need for MC or moisture cables. Tdiff will take a little bit longer to get the grain dry, but it will just as cold and safe.  For simplicity we are giving up some drying opportunities.  What do we want?  Perfection or Simplicity?

I personally would like to go with the moisture cables and have the whole process automated with the ultimate controller; but I can certainly understand and appreciate those that would lean toward the simpler system — they have temperature cables and don’t want to purchase moisture cables — no problem, we still have something for you that works well.

• Continuous: The absolute worst situation is doing nothing with hot tough grain. This requires no attention, no sensors, and has a very high risk for spoilage. The number of safe days is small, and the grain is deteriorating quickly.   It still may work; it may dry the grain, but is very risky with the potential for the most spoilage. The next worst thing is to leave the hot tough grain for a week or so before turning it on only during the day. Or, leave it for a week or two, and then turn it on continuously, and finally turn it off after a hot day of running, leaving the grain hot, but somewhat dried. It is better to run the fan continuously right from the start, and quitting after a cold night, leaving the grain cold.
• On at Night: One requires no sensors, and only a little attention to turn the fan on at night and off by 9 the next morning. The fan should be turned on immediately, even while the bin is being filled. When the average grain moisture is dry, the process s should stop after a cold night of running. The grain will warm up slowly by maybe a degree a week, and once a month the grain should be cooled by running the fan on a very cold dry night. This technique does not check for dripping or condensation conditions. It could be automated with a simple timer controlling the fan’s actuator.
• Water Balance: This requires T and RH sensors on the air entering and leaving the bins. It also involves some calculations to determine the net amount of water leaving the bin. However the exhaust T and RH sensor are only effective when the air is moving or when the fan is on. When there is no air flow, the fan is off, the sensor are in stagnant air and do not represent the true readings of the exhaust air. So this is a great mechanism to determine when to turn the fan off ( no or little drying is occurring) but it is useless when in determining when to turn it on.
• Temp Difference: Tgrain > Tair This requires only two simple temperature sensors, one in the grain and the other in the outside air. No math or calculations are required, and the comparison can easily be done manually or with very simple electronics. This would be a simple technique that could be used for automatic control. It should be started as soon as the bin is filled, and end when the average of the grain is dry, after a cold night. It is lacking in that it does not take into account either the MC of the grain or the RH of the outside air. It is not perfect, but it is simple.
• EMC Calculator: The calculator requires the grain’s T ,MC, and type as well as the outside air’s T and RH. It could easily be configured to turn the fan on and off automatically, but it also could be used to control the fan manually.   For those that have temperature sensors and are not keen on changing to moisture sensors, this may be the way to go. The disadvantage with this method is that calculations must be made and that we are subject to the inaccuracies of the EMC equations. Also if we have an obscure grain, it may not be listed on the calculator. The fan should not be turned on if dripping or condensation will occur: RHthres > 100
• Moisture Cable on, Water Balance Off: This is the ultimate controller, but it requires a moisture cable strung from the center roof collar, with the highest RH/T sensor being in the air to get the T and RH of the exhaust air. The lower T/RH sensors would be in the grain.   For each of these sensor nodes, one gets the T and RH of the grain, and from the Saturation Equation one can calculate the absolute humidity of the air in the bin. Also using the Saturation Equation, one can easily calculate the absolute humidity of the Outside Air by using its T and RH. If the absolute humidity of the outside air is less than the grain air, then we have drying conditions and the fan should be turned on. Once the fan is on, we do a water balance calculation , using the RH and T of the outside air, and the exhaust air. When the calculation indicates that there is no longer a net flow of water out of the bin, the fan is turned off and in an hour we return to comparing the absolute humidity of outside ait, and grain air.   But there probably are more than one T/RH sensor in the grain and some sort of averaging must be done to determine just when to turn the fan on. Although this strategy depends on the installation of a more expensive moisture cable, it avoids the inaccuracies of the EMC equations , or even the knowledge of the grain type. However we might want to use the EMC equations to determine what the MC of the grain and to terminate the process when the average MC of the grain from top to bottom is dry. The grain should still be monitored, and maybe once a month the grain could be cooled to keep it as cold as possible.   An even more reliable system would have two moisture cables in the bin to detect faulty sensors. The fan should not be turned on if dripping or condensation will occur: the saturation absolute humidity of the outside air > absolute humidity of the grain air.