Aeration

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Ambient Aeration

Aeration is a process of forcing air through grain to reduce its temperature. It is a very useful storage management tool which can preserve grain from deterioration, especially where the moisture content of the grain is above its safe level. Aeration can be used as effectively in sealed stores as in unsealed ones - sealed stores merely requiring the provision of an air-exhaust ventilator which can be sealed whenever fumigation is to be carried out.

In terms of storage design, the requirements for ambient aeration are simply (a) to provide some form of perforated ducting on the floor through which air can be blown into the grain, and (b) venting above the grain for air exhaust. (With downwards aeration the floor ducting is used for exhausting the air and the roof opening is the air inlet.) Floor ducting can be in the form of corrugated circular or semi-circular ducts on top of the floor surface, or troughed ducts flush with the floor surface. The latter are more costly, but allow for easier removal of grain from horizontal floors.

Good design of aeration systems is essential for efficient cooling. For instance, ducts must be of adequate size for the required air-flow, and should be located to ensure good distribution of cooling air throughout the grain mass. In general, duct sizes should be such as to limit air velocities to no more than 10 m/s, and duct surface area should be sufficient to limit air velocity at the duct/grain interface to around 0.2 m/s. Detailed discussion of duct design is beyond the scope of this bulletin. However, besides the many papers on the subject, there are now several computer software packages which can be used for their design.

Reducing grain temperatures by aeration offers numerous benefits. It reduces the rate of insect population growth; it reduces the rate of microbial (or mould) development; it preserves germination viability and it prolongs the effectiveness of insecticide chemicals where these are used. If temperatures can be reduced low enough (to around 10C wet bulb), insect population growth rates can be stopped altogether. In all cases, it is the wetbulb temperature which governs the benefits that can be achieved from aeration cooling.

It is apparent that ambient aeration requires periods (e.g. at night time) of low wet bulb temperature to effect cooling of the grain. Such conditions are not always available in tropical climates, particularly during the wet season. The major requirement during such times is more often on drying rather than cooling, however where aeration cooling is required or warranted (for instance after drying, or to delay mould development prior to drying), some air cooling and/or dehumidification may be needed to achieve the requisite conditions (see below).

To be effective, aeration requires the use of a well defined strategy and a good control system for operation of the fans. Where these are not effective, aeration can result in high costs for little or no gain, and can even be counter-productive. The key to successful aeration, is to design the cooling rate to minimise grain spoilage. For instance in the case of high moisture grain, it may be beneficial to begin with high rates of aeration to achieve a modest temperature reduction so as to delay the onset of fungal activity. Subsequently a reduced rate of aeration can be used to achieve further temperature reductions at a slower rate. Selection of appropriate ambient air to achieve optimum temperature loss is the basis of aeration strategy.

When air is forced through a grain mass, it carries with it first a 'temperature front', and then a 'moisture front'. The temperature front moves quite rapidly, while the moisture front moves very slowly. Above the temperature front, the grain remains at its initial temperature and moisture level; below it the grain is at the wet-bulb temperature of the aeration air. Below the moisture front, the grain is at the same RH as the aeration air. With aeration, the aim is to ensure that the temperature front is a cooling front, and that heating fronts are largely avoided. Ideally, aeration air should also be selected to avoid the creation of wetting fronts, however at normal rates of aeration the speed of the moisture front is so slow that wetting problems seldom cause more than very localised damage.

The speed of the temperature front is governed largely by the rate of air-flow, and the temperature of the aeration air. Surprisingly, it is largely independent of the initial grain temperature. To design an aeration system, it is necessary to know (approximately) the climatic conditions so as to define the condition of the ambient air that is likely to be available for grain cooling. Also the initial temperature and moisture levels of the grain need to be known, so as to determine the maximum time that can be allowed for the cooling front to pass through the grain before deterioration begins. Given these factors it is possible to calculate fan and duct sizes, and to predict the performance of the system in operation. As already stated, several software packages are now available which can be used to perform the calculations and produce an optimum design for fans, ducts, etc.

Numerous control systems are available for controlling fan operation, some being relatively simple, and others more complex. Most are based on micro-processing technology, and they differ principally in the degree of sophistication in the monitoring systems that they use. One of the simplest (yet effective) systems is the 'set-point' controller which monitors only dry-bulb ambient air temperature. It is designed to provide a predetermined amount of aeration (in terms of the number of hours per week that the fans will operate) by automatically adjusting the minimum 'set-point' air temperature at which the fans will start. By continuously monitoring air temperatures and fan operating hours, the controller calculates the optimum periods for aeration to achieve maximum temperature reductions, whilst ensuring a predetermined number of fan hours per day. The number of fan-hours of aeration can be adjusted manually; by reducing the fan-hours, the controller is able to select colder air to achieve lower grain temperatures, however the time taken to cool the grain will be increased. The strategy for grain stored at high temperature is thus to begin with 'rapid' aeration during which the controller select the coldest air whilst achieving fan operation for (say) 50% of the time until the cooling front has passed through and the entire grain mass has been initially cooled. This may take a week, depending in the air-flow rate. The controller is then adjusted so that the fans to operate only during the coldest 15% of the time. This will cause a further reduction in temperature (of maybe only 2 or 3 degrees), however this cooling front may take a month to pass through the grain.

More sophisticated (and more expensive) controllers may measure not only dry and wetbulb air temperature, but grain temperatures as well. Such systems can ensure that aeration will only occur when ambient air is sufficiently cool to maintain a cooling front - i.e. by avoiding all air that is warmer than the grain. Such a strategy will provide no aeration at all at times when air temperatures remain above grain temperature. This may not be a problem if the grain is dry and insect-free, but it could result in major problems if localised heating goes undetected.

 

Refrigerated Aeration

Aeration with refrigerated air achieves much lower temperatures when ambient conditions are warm. It is an expensive method of disinfestation compared to fumigation, but can be justified for storage of grains such as malting barley and seed grains in hot conditions, where maintenance of germination viability is important. Technically, the requirements are the same as for ambient aeration, except that no fan control is required since the system will operate 100% of the time until the temperature front has passed through the grain mass. An evaporative cooling system is used to reduce air temperature and to remove moisture. It is useful to place the fan between the cooling unit and the store, so that heat from the fan can be used to raise the air temperature by a few degrees, thus reducing its relative humidity and minimising risk of grain wetting.

By recirculating the cooling air, it is possible to maintain a sealed storage system. In this way the grain may first be fumigated to render it insect-free, and then cooled to preserve quality, with the fumigant still present.


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