Fans and Pedestals
Pedestals are a comprehensive solution to the challenge of cooling grain stores. The crop pedestals are suitable for cooling other crops too including oilseeds. Pedestals are currently used in farm and commercial stores from 2m - 9.3m depth.
Principles of aerating grain for cooling and drying
Aeration can be used to cool or dry grain, depending on the system's capability and how it is managed.
The system is 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 the 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 the 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 a 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 for grain cooling
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 per cent 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 from turning on when relative humidity is too high, such as in a fog.
Air for grain drying
Grain left in the 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.