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ResearchReportKondinin Group

AUGUST 2019 No. 115 www.farmingahead.com.au

GRAIN AERATION SYSTEMSBE A COOL OPERATOR

Price: $95

Aeration systems on grain storages can take a number of forms, largely depending on the storage type. Aeration on an elevated

cone-base silo can be as simple as bolting a fan onto the bottom of the silo and pushing air through a plenum inside the silo to more evenly distribute the flow. Some silo manufacturers incorporate the plenum as a structural feature or a plenum can be installed with the fan, meaning aeration can be added at any stage.

For large, flat-bottom silos, aeration needs to be considered early in the planning stage prior to construction. Aeration channels at the base of the silo and the adapting ducting between the fan and the silo, known as the transition, will need to be set into the foundations. Alternatives include elevated full-floor aeration but in most cases, this too, needs to be known at the time of initial construction.

Aeration is an important tool for anyone storing grain. The benefits include providing

a uniform moisture and temperature profile through the grain stack. Where air-flow is sufficient, aeration can be used to dry grain and with careful air selection, aeration has the ability to drop grain temperatures to control insect reproduction.

It is important to note that under Australian conditions, aeration cooling will not kill insects, but rather control reproduction. This control temperature differs according to the insect species, a topic explored further into this report.

Grain aeration: chilling outAeration can be a powerful tool in the arsenal for growers storing grain on farm and an important option for maintaining the quality of grain in storage. The storage of products like oilseeds and pulses can benefit significantly from the use of aeration and for longer term storage, aeration of these commodities is essential. But uptake and knowledge of the methods and benefits of aeration around Australia varies. In this Research Report, Kondinin Group engineer and GRDC grain storage specialist, Ben White, explains the benefits and considerations of stored grain aeration.

2 Research Report August 2019 No. 115 www.farmingahead.com.au

RESEARCH REPORT GRAIN AERATION

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EARCH REPORT• KONDININ GROUP

Cone-base mounted fan. Full floor aeration. Channel aeration.

Low blow: Aeration is a tool used extensively in northern New South Wales and Queensland, but

other states have some catching up to do.

© Kondinin Group – Reproduction in whole or part is not permitted without permission. Freecall 1800 677 761

DaveyDavey

GRAIN AERATION COOLING AND DRYINGAeration is most commonly used to cool grain. To achieve this requires air at an appropriately low ambient air temperature at an airflow rate of two to three litres per second per tonne of grain.

As an example, a silo holding 100 tonnes of grain requires an airflow rate of between 200L and 300L per second to adequately draw heat out of the grain.

The flow rate is important because lower airflow rates may not push the cooling front through the grain stack, leaving a temperature gradient in the silo, potentially contributing to sweating and mould propagation.

One of the common misconceptions with aeration is that any aeration fans will dry grain down. But very high airflow rates need to be used to adequately dry grain. Grain storage experts recommend between 15L and 25L/sec/t; many times that of a typical aeration cooling fan.

Fan specification, capacity and therefore price can be very high relative to aeration cooling fans.

Because drying uses large volumes of air, the grain depth uniformity is important

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BENEFITSThe primary aim of an aeration cooling system is to utilise appropriate ambient air conditions to reduce the temperature of grain in storage.

A reduction in grain temperature brings with it a number of benefits.

Most importantly, cooler grain slows or stops grain storage insect pest reproduction rates. If the temperature of the grain is low enough, insect reproduction stops altogether.

The impact of lower grain temperatures on insect reproduction and reproductive halt differs between insect species:

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5www.farmingahead.com.au No. 115 August 2019 Research Report


Lesser grain borer• Female can lay up

to 300-400 eggs in a life cycle

• 35°C: Life cycle completed in four weeks

• 22°C: Life cycle completed in seven weeks

• Breeding stops below 18°C

Rust red flour beetle• Female can lay up

to 1000 eggs in a life cycle

• 35°C: Life cycle completed in three to four weeks

• 22°C: Life cycle completed in 11 weeks


Rice Weevil• Female can lay up





Flat grain beetle• Female can lay up




• Breeding stops below 17.5°C

Saw-tooth beetle• Female can lay up


• 35°C: Life cycle completed in three weeks


• Breeding stops below 17.5°C

Source: Kondinin Group

Figure 1. Insect life cycles with temperature


OTHER BENEFITSIn addition to the benefits of being able to manage insects, aeration also assists in maintaining grain quality in storage.

Examples of maintained quality include preserving colour in some pulses, for example lentils or oil quality in canola.

Aeration also evens out any moisture variations in the grain stack. This is particularly useful if there are a few green grains in the sample which may result in mould spots.

OILSEEDSOilseeds are particularly sensitive to moisture content in storage and therefore prone to damage. Considering oil content,

for example, can be over 50 per cent in canola, the remaining 50 per cent of the seed mass has to carry all the grain moisture. As a result, the true moisture content of the canola hull or meal can be double the indicative moisture content.

High moisture levels in canola can lead to respiration, creating hotspots and overheating. Hotspots in canola have been measured to exceed 60°C and in some cases, fires have started inside silos filled with canola, destroying the grain and silo and endangering lives trying to extinguish the fire.

High temperatures in stored canola can severely impact oil quality, by significantly increasing the free fatty acids (FFA) in the oil.

GERMINATIONIf storing seed grain, aeration cooling can help to reduce in-storage temperatures and maintain higher levels of germination. Two primary factors influence germination; moisture content and temperature. Higher levels of either temperature or moisture correlate with lower levels of seed viability.

Aeration cooling not only reduces the temperature of the grain, preserving viability (see figure 2), it also ensures a more uniform seed grain moisture profile.

An even seed grain moisture profile minimises the risk of higher moisture pockets of grain in the seed storage which are likely to contain a higher fraction of non-germinating seed. See figure 3.

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The canola oil on the left is within FFA specification, while the oil on the right was produced from canola which was poorly stored. Without aeration, grain temperatures in the silo were not uniform and large hot spots developed pushing the oil outside FFA specifications meaning it had to be discarded.

Figure 2. Influence of temperature on wheat germination stored at 12 per cent moisture content

Figure 3. Influence of moisture content (m.c.) on germination of wheat stored at 30°C

Source: CSIRO

Source: CSIRO


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Automation systems for aeration are well established having been developed over more than three decades. All of the systems have

a switching mechanism, ambient temperature thermometer and relative humidity sensor.

An aeration controller looks to harness the best ambient air conditions to efficiently

cool grain, switching fans on only when the temperature and humidity combined are conducive to cooling grain, and never above 85 per cent relative humidity.

Aeration control ensures effective operation

Autopilot: Aeration control systems like this time

proportioning controller take away the headache

of manually switching fans on and off, but need to

be understood to operate effectively.

The timing of aeration is critical in terms of the efficacy of aeration with different strategies required at different stages along the storage timeline. While manual switching of fans can work, it becomes tiresome and errors can happen. Automation is easily applied to the task, but there are a number of ways to approach automatic fan switching.


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HOW DO AERATION CONTROLLERS WORK?Aeration controllers switch aeration fans on and off in accordance with ambient air temperature and humidity conditions. These ambient condition boundaries can change depending on the grain’s storage term stage.

For example, freshly harvested grain put into a silo will have a range of moisture contents, not just between loads, but also between individual grains.

Respiration occurs after silo in-loading, raising the temperature of the grain. For example, grain put in the silo at 30°C could rise to 40°C depending on grain moisture content, putting the grain at risk of rapid insect reproduction, mould and quality loss.

At harvest and silo-fill, fans should be run continuously for a week to purge harvest heat and create a uniform moisture profile. The only circumstances under which a

controller should stop fans running during the purging phase is if the relative humidity levels rise above 85 per cent.

After the initial purging phase, the controller can be switched across to a cooling phase which selects optimal air to cool the grain. Under the right conditions and with the recommended air-flow rate of 2-3L/s/t, cooling can be surprisingly rapid. See figure 4.

Once cooled, the grain can be maintained at low temperatures in what is referred to by some controller manufacturers as a maintenance phase.

Some controllers have an automatic mode, automating the three step cooling process when from time of silo-fill.

TYPES OF AERATION CONTROLLERS There are three main aeration control methods; Set-Point Controllers (SPCs),

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Set-Point Controller (SPC).

Time-Proportioning Controller (TPC).

Adaptive Discounting Controller (ADC).


Time-Proportioning Controllers (TPCs) and Adaptive Discounting Controllers (ADCs).

The most simple and inexpensive of these is the SPC which uses a basic temperature and humidity sensor to measure ambient conditions and switch fans on and off when conditions move outside set parameters. Precision is relatively low compared to other controllers and one SPC can only control a limited number of fans, meaning they become less cost effective for medium to larger silo complexes. Because fan run time is not limited, SPC controllers are not as efficient as other options and cooling capacity may be limited.

TPCs are the most commonly used automated aeration controller in on-farm grain storage facilities. They use an algorithm to select the coolest air to run the fans for an average total of 24 hours per week. TPC units measure relative humidity and dry bulb temperature of the ambient conditions and use these to calculate a wet bulb temperature. The wet-bulb temperature trigger point is continually adjusted based on the measured ambient condition trends measured.

ADCs require the operator to enter data around the grain moisture and temperature for each load into the storage, facility volume and fan performance along with a target for moisture content and temperature. The system then applies a complex model to these input parameters along with measured relative humidity and dry-bulb temperature to calculate the best times to run fans. The system continually adapts the model to estimate the effect it has had on grain moisture and temperature and gets more selective about when fans are run.

HUMIDITY LIMITS BALANCE OUTMost aeration controllers specify an upper relative humidity limit of 80-85 per cent. This may seem high given the equilibrium moisture content of wheat at 25°C and 60 per cent relative humidity is 11.4 per cent moisture. Looking at the entire storage period, the average relative humidity over a storage period with an 85 per cent relative humidity upper limit imposed will typically be around 60 per cent. Exceptions to this rule of thumb are coastal areas which may see higher relative humidity figures.

WHAT IS EQUILIBRIUM MOISTURE CONTENT?Equilibrium moisture content is the grain moisture content at which the moisture content of the grain is equal to the relative humidity of the air around it. At the equilibrium, no moisture transfer occurs.

Introducing ambient air at relative humidity levels higher than the equilibrium moisture content will introduce moisture to the grain, which, if not managed with

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Table 1. Equilibrium moisture content: ASW Wheat15°C 20°C 25°C 30°C 35°C

Rela

tive

Hu

mid

ity (%

) 30 9.8 9.4 9 8.8 8.2 Grain moisture

content (%)

40 11 10.5 10 9.8 9.550 12.3 11.8 11.2 11 10.860 13.4 13 12.5 12.2 1270 14.5 14.2 14 13.5 13

Table 2. Equilibrium moisture content: Barley15°C 20°C 25°C 30°C 35°C

Rela

tive

Hu

mid

ity (%

) 30 9.3 8.9 8.6 8.4 8.1 Grain moisture

content (%)

40 10.7 10.4 10.2 9.9 9.650 12.2 11.9 11.6 11.4 11.160 13.8 13.5 13.2 13 12.770 15.5 15.3 15 14.8 14.5

Table 3. Equilibrium moisture content: Canola15°C 20°C 25°C 30°C 35°C

Rela

tive

Hu

mid

ity (%

) 30 5.2 5 4.9 4.8 4.6 Grain moisture

content (%)

40 5.9 5.8 5.6 5.5 5.350 6.8 6.6 6.5 6.3 6.160 7.9 7.7 7.5 7.3 7.170 9.5 9.2 9 8.7 8.5

Table 4. Equilibrium moisture content: Corn15°C 20°C 25°C 30°C 35°C

Rela

tive

Hu

mid

ity (%

) 30 9.1 8.8 8.5 8.2 7.9 Grain moisture

content (%)

40 10.8 10.4 10 9.7 9.450 12.3 11.9 11.5 11.1 10.860 14 13.5 13 12.6 12.270 15.7 15.2 14.7 14.2 13.8

Table 5. Equilibrium moisture content: Sorghum15°C 20°C 25°C 30°C 35°C

Rela

tive

Hu

mid

ity (%

) 30 11.2 10.8 10.4 10 9.8 Grain moisture

content (%)

40 12.8 12.5 12.3 11.8 11.350 14.2 14 13.6 13.3 1360 15.7 15.5 15.3 15 14.770 18.1 18 17.8 17.5 17.2

Table 6. Equilibrium moisture content: Oats15°C 20°C 25°C 30°C 35°C

Rela

tive

Hu

mid

ity (%

) 30 8.1 7.8 7.5 7.2 7 Grain moisture

content (%)

40 9.5 9.1 8.8 8.5 8.250 10.9 10.5 10.1 9.8 9.560 12.3 11.9 11.4 11.1 10.770 13.9 13.4 12.9 12.5 12.1

ample airflow can cause issues with mould and condensation issues, particularly near the aeration ducting. Conversely, too much dry air can pull moisture out of the grain,

causing shrinkage and reducing the stored grain mass.

Equilibrium moisture contents vary for different grains: see tables 1-6.


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There are a number of key variables that influence aeration airflow. Aeration fan style, capacity and performance relative to

backpressure from the grain are the dominant variables in how much air can be pushed through the grain.

The grain itself in terms of type of grain and depth of the stack are the other major variables. Smaller grain sizes and deeper grain stacks (taller silos) both increase backpressure and thereby reduce aeration fan performance. Any ducting, piping, bends or constrictions also increase

backpressure and thereby reduce air flow. Note that transition design is essential – a smoother transition with less sharp corners results in less backpressure. An abrupt transition between the fan and the silo can add up to 150Pa of static backpressure at a flow rate of 1000L/s.

Aeration fans the heart of the systemMatching an aeration fan to a grain silo requires a good deal of thought about influencing variables and what the grain grower wants the aeration system to do with the system. Getting this wrong can waste money on the purchase of an under-specification fan or in excessive power costs and grain shrinkage with an over-specified fan.

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Matched: Aeration fans need to be matched with the silo

and the products stored.


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Booklets and fact sheetson all things grain storage

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READING A FAN PERFORMANCE CURVEJust like pump performance, aeration fans have a performance curve which should be considered when specifying a fan for a grain storage.

Always ask the manufacturer for an aeration fan performance curve and use it along with the backpressure tables in this report to ensure adequate flow through the grain stack will be achieved.

Like an air compressor which specifies free air delivery (FAD), in most cases, aeration fan performance is specified as a zero static backpressure flow rate. In reality, there is always backpressure in operation so the specified flow rate is never achieved. Refer to the fan curve for performance under the expected operating conditions.

To read a fan performance curve, use the estimated static backpressure as specified for the specific grain in the tables and apply it with a small safety buffer of 10 per cent to account for transitional losses between the fan and the grain.

As an example: an F650 fan curve is shown to the right, fitted to a 128-tonne Dennys silo, the F650 would be pushing against around 8m depth of wheat.

The F650 delivers around 800L/s FAD (representing 6.25L/s/t) but under eight metres of wheat at an initial reduced flow estimate of 3L/s/t, the backpressure referenced in Figure 6 is likely to be around 850Pa which, referring to the fan performance curve, pulls the airflow rate back to about 450L/s FAD or 3.5L/s/t.

This allows for a 10 per cent reduction in airflow due to transition losses but means the F650 would be well suited to aeration cooling the 128t silo full of wheat.

If the silo was to be filled with canola however, the mass of grain would be reduced to 110t and while the airflow rate would drop to about half that under wheat

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0

200

400

600

800

1000

1200

0 100 200 300 400 500 600 700 800 900

Back

pre

ssur

e (P

a)

Fan output (l/s)

Figure 5. F650 example fan performance curve


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(say 1.7L/s/t) static backpressure would be 1000Pa (double that of wheat under the reduced airflow rate). This means that a single F650 fan would not be able to deliver sufficient air for this specific 128t silo if it was filled with canola.

A second or different pairing of fans could instead be considered if canola was to

be stored or the silo could be part filled to ensure sufficient air flow for cooling.

BACK PRESSURE CHARTSUnfortunately, there are no specific backpressure figures for Australian grain. This is an area of research requiring investment. But imperial tabular data from

Kansas State University has been converted to produce estimated backpressure charts (See figures 6 and 7).

These graphs are only for wheat but as a general rule for canola, the same centrifugal backward facing blade fan will deliver around half the airflow due to a doubling of the backpressure.

0

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3000

3500

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10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Back

-pre

ssur

e in

whe

at (

Pa)

Depth of grain (m)

1.2 l/s/t

1.8 l/s/t

2.4 l/s/t

3.0 l/s/t

3.6 l/s/t

Source: Adapted from K-State research and extension

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5 6 7 8 9 10

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e in

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at (

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Depth of grain (m)

1.2 l/s/t

1.8 l/s/t

2.4 l/s/t

3.0 l/s/t

3.6 l/s/t


Figure 7. Expected back-pressure for aeration cooling in wheat for a range of airflow rates: 10-25m (Excludes back-pressure due to ducting, ventilation constrictions and transitions)

Figure 6. Expected back-pressure for aeration cooling in wheat for a range of airflow rates: 5-10m (Excludes back-pressure due to ducting, ventilation constrictions and transitions)




VENTING CONSIDERATIONSEnsuring there are no restrictions for air once it has passed through the grain is important as it can introduce unnecessary static backpressure. Vents should also prevent rain and moisture blowing in even when in the open position.

For smaller silos, the lid can be opened slightly to provide roof ventilation, but if possible, lock it in place to prevent wind lifting the lid further and allowing rain to blow in.

For larger silos, there should be 0.1m2 of roof venting area per 500L/s of airflow as a rule of thumb. On a 1000t silo, with 2.5L/s/t blowing through it for example, around six roof vents each 300mm square would be required.

STYLES OF FANSTwo main fan styles are used in Australia and the most common is the backward, curved blade centrifugal fan. Centrifugal fans efficiently deliver airflow against higher static backpressure at lower power input levels. Single centrifugal fan outputs are limited to a maximum of around 3000L/s FAD so are regularly used in multiples for larger silos.

Axial fans are also used for high volume requirements but have a lower specific fan output (are not as efficient) at higher backpressures in comparison to centrifugal fans.

Centrifugal and axial fans are available manufactured in Australia and imported.

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Venting: As a rule of thumb, allow 0.1m2 of roof venting area per 500L/s of airflow.


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MULTIPLE FANSIt should be remembered that where a silo has more than one fan operating, the output of a single fan cannot be simply added or multiplied. Used in combination, the fans will each introduce an additional backpressure into the silo and fan output will subsequently reduce.

THE IMPORTANCE OF SEALINGIn a gas tight, sealable silo meeting AS2628 for sealing and fumigation, the aeration fan must also provide a seal to meet the standard.

This is usually in the form of a seal plate on the aeration inlet. This may need to be specified when ordering fans.

FAN MOTOR TECHNOLOGYFan motor technology has taken an efficiency leap in recent years with the introduction of Electronically Commutated (EC) fan motor technology. Kondinin Group testing has previously revealed EC-drive aeration fans from Ziehl Abegg found they deliver higher air flows at lower power input. Kotzur also supplies a number of its aeration fans running a Ziehl Abegg EC motor.

In addition to the improved efficiency of operation, other benefits include the absence of inrush current on start-up. Instead the fans slowly wind up to operating speed. Speed selection is also possible allowing fan output to be reduced to gain energy savings. For example, once a rapid cooling process at the

top-end output of 3L/s/t has been achieved, the fan could be cut back to deliver 2L/s/t for maintenance with a reduction in energy costs of about 50 per cent.

From an energy efficiency perspective, ask the manufacturer to provide fan curves with power consumption curves overlaid so the efficiency of the whole fan package including the motor, scroll and housing can be directly compared.

MEASURING PERFORMANCESometimes the only way of knowing how much air is going through a silo is to test the fan in-situ.

Farmers can use a wind-speed measuring device, for example a Kestrel wind meter,

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Multiple fans: One plus one does not always equal two.

Gas tight: A seal plate on the aeration fan inlet maintains sealing of the silo for fumigation.

Electronically commutated (EC) fan motor technology has come to grain aeration fans delivering more control and improved efficiency.


and a length of smooth internal surface pipe. Large PVC with a diameter suitably wide enough to encompass the air intake on the fan works well.

The cut length of the pipe should be around 8-10 times the diameter to ensure laminar flow of air through the pipe.

A small slot can be cut in the pipe to allow the insertion of the wind meter attached to a stick, of ample length to take measurements across the entire diameter.

After starting the fan up under normal operating conditions with grain in the silo, measurements can be taken across the cross section of the pipe and averaged.

Some wind-speed meters have a five-second averaging function allowing the operator to start the test and average the wind speed across the pipe transect over five seconds. Once the average speed is determined, this can be multiplied by the cross sectional area of the pipe to derive the fan output.

POWER REQUIREMENTS AND ACCESSPower requirements for aeration systems should be carefully considered. Where three-phase power is available, it can offer a lower cost of operation and higher output fan options. Very large storage facilities often opt for a three-phase powered aeration system.

Talk to local electricians and energy companies about the costs involved in

getting power to the site and consider alternatives depending on specific requirements.

Aeration controllers can start generators with a starting signal wire functionality.

Solar power is getting cheaper and while solar-powered systems have been demonstrated, they are yet to financially stack up for aeration. Until battery storage prices reduce, a generator is still a more economical option.

Acknowledgments: Philip Burrill, DAF Qld, Chris Warrick GRDC Stored Grain extension team.

More information: www.storedgrain.com.au

For grain storage advice or to arrange a grain storage workshop with the GRDC extension team:Phone 1800 WEEVIL (1800 933 845)

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Measure to manage: Intake velocity can be measured in-situ to understand how much air the fan is delivering.

BYO power: Generators can be the most economical option when sourcing power for a new grain storage

facility. Some aeration controllers can be used to start and stop the generator when needed.


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