August 31, 2016

31/08/2016: Aerating stored grain

by Peter Botta, PCB Consulting

Grain aeration is a popular grain storage tool used in Australia by farmers, offering harvest flexibility, increased marketing opportunities and better control of grain quality. As the range of chemical control options is reduced, grain aeration provides a powerful non-chemical stored grain insect management option.

Through manipulating grain temperature and moisture, aeration cools the grain stack and achieves a more uniform bulk, delivering an optimal storage environment. Not only does this inhibit insect activity, but also maintains grain quality. Aeration of stored grain has four main purposes - preventing mould, inhibiting insect development, maintaining seed viability and reducing grain moisture.

Without aeration grain is an effective insulator and will maintain its warm harvest temperature for a long time. Like housing insulation, grain holds many tiny pockets of air within a stack - for example 100 tonnes of barley requires a silo with a volume of about 130 cubic metres, 80m3 is taken up by the grain and the remaining 50m3 (38 per cent) is air space around each grain.

Without circulation, the air surrounding the grain will reach a moisture (relative humidity) and temperature equilibrium within a few days. These conditions provide an ideal environment for insects and mould to thrive and without aeration the grain is likely to maintain that temperature and moisture for months.

Air movement within the grain stack
Grain at the top of the stack is the hottest, as heat rises through the grain. The sun heats the silo roof and internal head space, resulting in the surface grain at the top of the silo heating up. 
Figure 1: Air movement within an aerated silo.
When grain is stored at moisture contents above 12 per cent, 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. This makes the top of the grain stack the most vulnerable to insect and mould activity and is unfortunately the last place aeration will get to. 
(See Fig. 1) 

From the aeration fan outlet, air will take the easiest route to the top of the grain stack - the path of least resistance. Poor aeration ducting can result in pockets of grain not being aerated. The peak of grain in a silo is a common place that aeration bypasses. The path of least resistance is to the side, below the peak of the stack as it is a shorter distance from the aeration ducting. Considering silo size – height and width, grain types stored, ducting type and configuration and fan size output needed are all important factors. The system must be fit for purpose to ensure successful results.

Cooling or drying
Grain aeration systems are generally designed to carry out either a drying or cooling function - not both. Aeration cooling can be achieved with airflow rates of 2–3 litres per second per tonne of grain delivered from fans driven by a 0.37 kilowatt (0.5 horsepower) electric motor.

Figure 2: Aeration for grain drying and cooling
Aeration drying can be achieved with fans delivering 15–25L/s/t, typically powered by 7kW (10hp) electric motors. Low-capacity fans cannot push this drying front through the grain fast enough to dry grain in the top section of a stack before it turns mouldy. (See Figure 2)

Management for cooling or drying
Managing the aeration system is different for cooling or drying, with fan run times required at different times of day and at different intervals.

An automatic aeration controller increases the efficiency of an aeration system by negating the need for manual fan control, but it’s vital to set the controller to operate the aeration fans for their designed purpose - either cooling or drying.

Read the full article HERE.

The Global Miller
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which is published by Perendale Publishers Limited.

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