Miller Magazine Issue: 139 July 2021

55 COVER STORY MILLER / july 2021 to take the maximum advantage of the limited cooling time. Or it can even be unavailable, as in cases in regions close to the equator, where the artificial refrigerated air should be considered. Aeration system overview The most common technology to service the condition of the grain is the aeration system. There is a fan, blowing air into a transition which is connected to a floor capable to release said air which travels all along the grain bulk and then it is released again into the atmosphere by the means of roof vents or fans. We then use a monitoring system to record the conditions and the effects of the aeration onto the grain bulk. The design process is mirrored and it starts from the gran bulk. Depending on the amount of air needed (reference the previous chapter) and the type of commodity, by empir- ical formulation it is possible to evaluate the static pressure required to guarantee the proper airflow needed to travel through the bulk. This static pressure is also affected by the flooring technology because it is a source of load losses. It is obvious that a full-floor system is more efficient if com- pared to a channel system, not only in terms of airflow uni- formity but also in terms of load loss reduction. Because the load losses are a function of the velocity, and in the full-floor they are surely lower than in a trench system. Measures can be taken to improve the trench system efficiency by having wider or deeper main distribution channels. The same applies to transition, it looks like an insignifi- cant element, but if not properly designed it can be a crit- ical bottleneck which makes the whole system inefficient. The static pressure requirements for the bulk and the load losses inherent to the distribution system are all the data needed to size the blowing fan. The importance of having a proper design of the ventilation system is vital to achieve favorable grain quality results. Allowed storage period When proper ventilation is frequently performed and when the internal and external ambient conditions in term of temperature and humidity are well known, the first im- portant task to be accomplished is understanding the al- lowable storage period. The safe storage charts are usually expressed in days that relate the storage condition in terms of temperature and moisture content of the grains with a safe storage period. “Safe” means the conditions are suitable for the good conservation of the specific commodity. Exceeding this period, the possibility of developing insects and molds and likelihood of spoilage and germination become much greater. As an example, if we have the grain with 16% moisture and 20° Celsius, therefore the safe storage time is between 40 and 120 days, but if the temperature and moisture are both reduced to 15° C and 14% moisture the safe storage time is more than doubled (See fig. 7). When to aerate? Aeration is probably one of the most complicated tasks within the storage management field. The reason is that there is not an exact correct or wrong method, but the method must be set according to a large number of vari- ables: geographical area, ambient conditions, yearly varia- tions of ambient conditions, type of commodity, conditions of commodity, type and capacity of the aeration system, storage period expected, etc. Given the above, it is suggested to consult a specialist in the specific field to provide the best method for the given case of grain aeration needs. However, listed below are some of the most popular methods adopted in the metallic silos market: Figure 7