Review: Aerating Stored Grain — Australian Guide

I was going through my files, and I came across an aeration guide that Guy Lafond had given me in 2011, along with his annotations.  It can be found at http://www.graintec.com.au/media/34545/Aerating%20stored%20grain%20-%20A%20Grains%20Industry%20Guide.pdf

It is called Aerating Stored Grain, Cooling or Drying for Quality Control, A grains Industry Guide by GRDC (Grains Research & Development Corporation) in Australia.

Guy passed away a couple of years ago, and in going through the report and reading his notes, it is almost as if he is talking to me.  I must have skimmed the report, but it is only now that I have slowly read the report and have appreciated the annotations.  Many of the findings in the report are in agreement with what we have found, and then there are guidelines that do not. I will now go through the report and perhaps in a somewhat disjointed way touch on the issues of agreement and disagreement.  You be the judge, and try to discern where the truth lies. Quotes from the booklet will be in italics, followed by my comment and Guy’s note.

This booklet explains the key differences and processes involved in aeration cooling and aeration drying.  We discovered right from the start of our analysis that cooling the grain also dries it. The two are not separate process, cooling is drying.  In fact this relationship can be approximately be quantified: cooling the grain by 15 ºC reduces the moisture content by one percent.

38 per cent is air space around each grain.  You may have noticed that we used 40% in the calculations done in my blogs.

Without circulation, the air surrounding the grain will reach a moisture (relative humidity) and temperature equilibrium within a few days.  In other words, you must leave your grain in a sealed bin for three days before you will get an accurate reading on your moisture cables.  Moisture cables use Equilibrium Moisture Content to calculate moisture; the air and the grain must be in equilibrium. The fans must be off for a long time to achieve equilibrium.  In other literature, I have seen times like 3 hours to reach equilibrium.  In actual fact, equilibrium is never reached, it is said to be asymptotic. It gets closer and closer but never reaches it.  That is why in my blogs I introduced the notion of a time constant, which is the time it takes to get about two thirds the way there.  Anyway the huge lesson here is that moisture cables can only give an accurate reading of Moisture Content if the fan is off for a considerable time.

The air in the head space, heats and cools each day creating ideal conditions for condensation to form, wetting the grain at the top of the stack.  The good news is that this guide does recognize a problem with condensation; the bad news is that they don’t give the specifics of the conditions that create condensation.  Using my calculator, http://planetcalc.com/4959/  we do have the specifics of when condensation occurs: it is whenever the RHthres is above 100%.  The more it is over 100, the more the condensation. It turns out that the rule of thumb is this: If the grain in the bin is more than 5º C warmer than the outside air, there are conditions for condensation.  The more the temp. difference, the more the condensation.

Grain aeration systems are generally designed to carry our either a drying or cooling function — not both.  This is what Guy had in his note on this: “Not so, drying occurs as grain is cooling”. Guy and I learned this by analyzing the data collected from bins with aeration since 2007.

Aeration cooling can be achieved with airflow rates of 2-3 litres per second per tonne delivered from fans driven by a 0.37 kilowatt (0.5 horsepower) electric motor.  I agree that low flows can achieve cooling within a matter of hours, and look at the size of the motor to do this , half a HP.  What I don’t agree with is that only cooling will take place with low flows; drying also takes place. Remember the rule of thumb that was developed from analyzing our data: Cooling the grain by 15 ºC removed 1% moisture content — regardless of the air flow rate.  In fact, I propose that more water will be removed with lower fan rates in terms of cooling.  We can make better use of the energy in the grain, to evaporate water if we do it a little slower, with lower air flow rates.  See my blog on air flows.

Aeration drying can be achieved with fans delivering 15-25L/s/t, typically powered by 7kW (10hp) electric motor. Again the message here is that only large air flows can dry grain; I disagree. Lower flows are actually more effective; not in terms of time, but in terms of actual moisture taken out.  Once we have taken the energy (heat) out of the grain, drying will no longer occur — with high or low flows. The trick is to use as much energy as we can for drying.  I think we can use this energy more efficiently to remove the water, if we go a little slower with lower flows.

Grain that is dry enough to meet specifications for sale (12.5% for wheat) can be cooled without drying.  I beg to differ, cooling is drying.  Guy’s comment was “really?”

After drying to the required moisture content, cool the grain to maintain quality. Guy’s comment to this: “Cooling and drying occur together”

Aeration cooling moves the air pockets around the grain, which evens out any hot or moist areas, creating a uniform stack.  Notice here they are talking about aeration cooling that use low air flows.  Therefore low air flows even things out, they do not create pockets or fronts of moisture or temperature.  I agree.

One way of measuring change in grain quality over time is seed germination. Exactly, this is what I have done by calculating the number of safe days, and the spoilage index.  It is sometimes uncanny how the same conclusion can be reached independently.

Stored grain deteriorates with time under any conditions, but poor storage conditions (high temperature and moisture) accelerate the deterioration process markedly.  I agree, you can’t stop spoilage, you can only slow it down. Grain in storage can never improve in quality, it can only get worse.

If aeration cooling is being used to hold moderately high-moisture grain temporarily until drying equipment is available, run fans continually while the ambient relative humidity is below 85 percent.  Guy notes: “Interesting Concept, Not sure if I agree.”  Again we have the assumption that low flows do not dry — but they do.  And where did the magic 85% come from?  What is it based on? I can show you tons of examples where drying takes place with RH above 85, and tons of examples where wetting takes place with the RH below 85.  We are missing some important parameters here like grain temp, and air temp.

However, do not operate the aeration fans on continuous mode for more than a few hours, if the ambient relative humidity is higher than 85 per cent.  Again, where does this magic 85% come from? And again, there are important parameters missing.  Guy notes: “Depends on Tº “.

After aeration fans have been running continuously to flush out the warm, humid air for 2-3 days, reduce run time to 9-12 hours per day for the next 3-5 days.  The difficulty is selecting the coolest air to run the fans and being on site to turn the fans on and off.  This is interesting and very close to my recommendation that the fan should be turned on, even as the bin is being loaded, and run until 9 the next day after which you only run the fan during the night, or the coldest hours.  Not during the day, with the hottest hours?  We learned that cooling is drying, and you can only cool if the air is colder than the grain.  The Aussies already knew this long before we did.

During this final phase they continually monitor ambient conditions and run fans on average during the coolest 100 hours per month.  Every time we pull the temperature of the grain down, we will be doing some drying, and by keeping it as cold as possible, the grain will be as safe as possible with the least spoilage. It is interesting that monitoring the ambient conditions is required, but exactly what are these ambient conditions in which one should run the fan??  I am suggesting that the fans be on if, and only if the air temp is less than the grain temp. and the RH < 80%.  We have found that if you follow this rule, that the fan duty cycle will be < 20% which is about 144 hours per month– amazingly close to 100 hours.  Again,  we have arrived at the same conclusion — independently.

Growers and bulk handlers need to  have an understanding of the effects of relative humidity and temperature when aerating grain.  I could not agree more, but the problem is, the report, is not giving us any understanding of the effects. Specifically, what temp and RH should I run the fan?  My calculator provides exactly what the temp and RH of the grain and ambient air should be to provide the conditions for drying AND for the conditions of condensation.

However, drying depends completely on the airflow through the grain.  I do not agree.  Grain will dry quite nicely, perhaps even more efficiently with low flows.

Operating in drying mode, aeration controllers select for air with low relative humidity.  Guy notes “Why only RH and not T as well ?”  They seem to keep missing the point that the amount of water in the air is determined not only by its RH but also, and maybe even more so, by its temperature.  They are missing the pyschrometric equation that relates RH and T to absolute humidity.

In rare situations aeration cooling fans can reduce grain moisture slightly, but they cannot reliably reduce grain moisture to a safe level.  I disagree.  The amount of drying depends on how efficiently we can use the energy in the grain to push the water out and evaporate it into the air.  Low air flows will do this as well as high air flows.  I would argue that low air flows actually do it better.

Much higher airflow rates are required for aeration drying in order to push a drying front through the grain bulk.  I disagree; in fact it is the larger airflow with the increased compression on the bottom of the bin that leads to over-drying the bottom and under-drying the top.  And there really is no drying front, it is more like a continuum, with the bottom being dried first and the top last.  And this is done more evenly if we have less compression on the bottom with a smaller fan and less flow.

Wheat at 16.5% MC at a temperature of 28 ºC was put into a silo with no aeration. Within hours the grain temperature reached 39 ºC and within two days it reached 46 ºC providing ideal conditions for mould growth and grain damage.  Guy notes: “I am not sure I agree with this.”

By monitoring the temperature and moisture content of the grain in storage, and reading the equilibrium tables for wheat or sorghum at the back of the booklet, a suitable relative humidity trigger point can be set.  Guy notes: “How to consider equilibrium moisture content?”  This suggests that I can take the temperature of the grain (let’s say 25) and the MC (let’s say 15%) and apply this to the EMC table to get a trigger RH of 82%.  What could be simpler?  Well, we missed a very major point:  The ambient air is NOT the same temperature as the wheat, therefore one can not consider the system to be in equilibrium and therefore the EMC table does not apply.  The trigger RH of 82 is for the air inside the bin, at equilibrium and at the same temperature as the wheat.   We can and do calculate the corresponding RH for the air outside with my calculator — but I would say that this important point is almost always overlooked or perhaps just not understood.

Firstly, warm air can transfer moisture from the grain more efficiently than cold air.  Guy notes: “Depends on RH?”  This has been a hang up with people for some time, and at first glance it seems obvious, of course warm air can dry better than cold air!!  But, wait a minute let’s examine this more carefully.  Drying occurs when the vapour pressure of the grain is larger than the vapour pressure of the air.  Vapour pressure depends on the moisture content and more importantly the temperature of the grain or the air.  When the grain temp is higher than the air temp, it most likely has a higher vapour pressure and drying occurs.  There are exceptions and one can conjure up an example where the air is just slightly cooler than the grain, at 100% RH and then wetting will occur.  But, as has been said in this report, with air,  RH < 85%, and a temp less than the grain, you are assured that you have drying conditions.    Sure warm air can hold more water than cold air, but the point that has been missed is that the warm air becomes the same temp as the grain as soon as it hits it.  If the grain is cold, the air will also become cold, and the air will not be able to hold the water it has, exceed 100% RH and dump the water onto the grain.  The warm air, that we thought was capable of carrying more water, and it could have if it stayed warm — but it doesn’t, it becomes almost the same temperature of the grain.  Sure eventually the grain will become the same temperature of the outside air, after hours and hours, but guess what by then the outside temperature has changed.  The outside ambient air temperature and the grain temperature are NEVER the same.  Yet people assume they are.  Big mistake.

Leave a Reply

Your email address will not be published. Required fields are marked *