Principles of aerating grain for cooling and drying
John Cameron and Peter Hughes explain the capabilities of various storage systems to reach certain objectives
Aeration can be used to cool or dry grain, depending on the system capability and how it is managed.
The system being used, the temperature and relative humidity of the air that is available and the grower"s or system"s ability to select the right air for the targeted grain temperature and moisture content will determine the result.
Changing grain storage temperature is a quick process compared to changing grain moisture. Cool grain is far less prone to quality loss. To maintain grain quality and avoid the build-up of hot spots of mould or insects, regular air movement and air change is needed, once grain temperature has been stabilised. Low flow-rate aeration cooling fans should be turned on at appropriate times to move fresh, cool air into and around the grain. Exhaust vents must be open when aerating. In many areas the best cooling conditions usually occur late at night and early in the morning.
To cool grain, relatively low flow-rates of air at around two litres per second per tonne (L/s/t) are pushed up through the grain stack. The air takes heat from (or hot air can add heat to) the grain and is then vented to the atmosphere.
Ambient air can also be used to dry grain. Here, high flow-rates of air at a temperature and humidity that will remove water from the grain is pumped through the grain. Providing the air will dry and not re-wet the grain, the grain will dry from the bottom of the silo, with a drying front moving upwards through the grain stack.
Aeration drying is a much slower process than aeration cooling or hot-air drying. The time it takes and the moisture content of grain after a drying front has reached the top of the grain stack depend on the quality of the air used. Several drying fronts may be needed to dry grain to receival standards. If aeration is to be used for drying, check that the fan and ducting have sufficient flow-rate and pressure to force a moisture-change front through the grain quickly enough to prevent mould development. It is also critical to ensure that flow fields are even and grain depth is not too deep.
Air with greatest capacity to dry occurs most during the day when temperatures are high and relative humidity low, but this is not always the case. Very hot dry air can over-dry and crack grain. It is the average quality of the inlet air that determines the final grain moisture content.
A flow field describes the way air moves in a grain stack. Air, like water or electricity ,follows the path of least resistance. Different depths of grain provide different back-pressures or levels of resistance. Air takes the easiest route to the surface and if the grain depth varies, or if poor duct design leads to uneven air distribution, pockets of grain can remain warm or fail to dry, and hot spots for mould and insects can develop. Grain spreaders are often used in the top of flat-bottomed drying bins. Where grain is deep, especially with dense-packing small grain like canola, more fan pressure is needed to maintain flow-rate and the drying front will take longer to reach the top.
Air temperature, humidity and grain moisture content determine the grain temperature resulting from aeration. For this reason, most aeration cooling with low flow rate fans is usually done during the colder night hours. Due to evaporative cooling, aeration of high-moisture grain results in greater cooling than in low-moisture grain. For example; aeration of 14 per cent moisture grain results in temperatures more than 4cC cooler than aeration of 10 percent moisture grain with the same air.
Some growers elect to manually switch on their cooling fans or use timers, while others prefer an automated controller. The main type of controller used for cooling is the time-proportioning controller.
Time-proportioning controllers are fitted with sensors to measure air temperature. Some also measure humidity. When protecting grain quality during long-term storage, time-proportioning controllers are programmed to try to select the best one-seventh of hours per month (about 24 hours per week) in which to aerate and cool grain. Fans are switched on and off automatically. They do this by selecting air with the greatest capacity to cool when it is available. Controllers with relative humidity sensors can prevent fans turning on when relative humidity is too high,such as in a fog.
Grain left in air of a certain temperature and humidity for long enough will eventually reach a moisture content in equilibrium with the air. This is called the Equilibrium Grain Moisture Content (EGMC). Table 1 shows EGMC for wheat at different air temperatures and relative humidities. To reach equilibrium, moisture must diffuse from the grain to the air. This is a much slower process than cooling. Drying grain in a stack will take weeks.
For the same relative humidity, more drying will be achieved with warmer air. A good supply of air of moderate to low relative humidity is needed to dry grain to Australian receival standards. As a rough guide, air around an average humidity of 50 to 60 per cent will get close to receival standards of 12 per cent for cereals.
In coastal environments, or areas where the air supply can be humid for extended periods, it is more difficult to dry grain. However, even in difficult drying environments, air can still often be found to dry grain from very high moisture contents down to around 15 to 16 per cent. The difficulty is usually in drying from 15 or 16 per cent down to receival standards of 12 to 13 per cent.
In these environments, automatic controllers would help select suitable air to dry grain. Also, additional heating can be fitted to artificially increase the drying potential of the air by heating it by 4°C to 10°C. Heating the air also reduces relative humidity (RH). The combined effect of lower RH% and higher temperature provides air with greatly increased drying capability - although increasing the temperature too much can result in over drying and damaged grain.
(Assumes a flow-rate of one to 4L/s/t. Use caution if controlling systems with airflow outside this range or in partly filled bins.) Fans can be started as soon as grain covers the ducts. For grain that is dry or of moderate moisture:
A controller that automatically selects the most suitable air for cooling is the best way to manage cooling. Where fan capacity exceeds the needs of cooling/maintenance, and when grain is cool and at the low moisture levels needed for safe long-term storage, reduce fan run-hours or disconnect unneeded fans. This minimises the risk of potentially re-wetting the grain, which could happen if large volumes of over-moist air were pumped in.
If air conditions are suitable for drying, fans should be turned on and left on. When conditions are such that grain is no longer drying, turn the fans off and turn them back on when air conditions improve. The moisture content that grain will dry to is determined by the average condition of the air used.
To calculate if air of a certain quality will or will not dry grain, training in calculating Equilibrium Grain Moisture Content is needed. It should be noted that different types of grain have slightly different equilibrium grain moistures. Automated drying controllers simplify the process of selecting suitable drying air. The recently released Adaptive Discount Controller (ADC, see page 17) can improve drying efficiency by only selecting air appropriate to the targeted grain moisture. If suitable air is not available, supplementary heating can be used to raise the air inlet temperature, thus reducing humidity.
If supplementary heating is unavailable and the available air will not dry the grain, a short-term holding measure maybe to change strategy and cool the grain, to maintain its quality until better drying air is available.
Aeration bins must have regular inspection, particularly if high-moisture grain is involved. The smell of the air leaving the bin is one of the most reliable indicators if the system is working or not.
Check the smell of air leaving the silo every day during the first two to three weeks, then at least once a week. A sweet, fresh smell is good, while off, musty/mouldy smells are sign of a problem.
As the moisture-change/drying front moves up the grain stack, probing the top of the bin for moisture will tell you when the front has reached the top. The moisture content through the bin will be determined by the average air quality that has been pumped through the stack over time.
It is important to ensure that drying fronts do not become stalled or stuck for any length of time. This zone is warm and wet and must be kept moving.
Hygiene: When selecting a storage system, keep in mind the need to keep it clean of all residues that can harbour insects.
Ducts: A range of duct types are available. Some are only suited for use with certain grains and will block up if other grains (for example canola) are stored.
Vents: All aeration silos need adequate venting - the more the better. This does not mean that aerated silos cannot also be sealed. Several manufacturers produce aeration-drying and cooling silos that can be sealed to enable effective fumigation when needed.
GRDC Research Code DAQ00028
For more information:QDPI&F website: www.dpi.qld.gov.au/home/, search for "grain storage"; stored grain research laboratory website: sgrl.csiro.au; Agriculture WA website: www.agric.wa.gov.au; Peter Hughes (Qld) ,07 4688 1564; Peter Botta (Vic), 03 57611647; Peter Fulwood (SA), 08 8568 6422; Chris Newman (WA), 08 9366 2309; John Cameron (NSW), 02 9482 4930
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