AGI’s engineers design storage solutions with the purpose of securing the value of the stored commodity inside. Aeration is one of the most common and safest practice available to preserve the stored grains quality in both industrial and farm facilities without the use of chemicals. Its proper implementation has a significant role in the prevention of kernel deterioration, grain losses due to insect and mold infestations. Along with these, aeration also provides an environmentally friendly and cost-effective solution that limits the risk of pests developing resistance to chemical agents and finally, can potentially improve business performances.
Senior Director of Product Management,
Storage & Structures
There is an increasing global need of energy consumption and footprint reduction in our industry and moreover, the FAO (Food and Agricultural Organization of United Nations) claims that over one third of food production is wasted every year before consumed. Thus, the importance of preserving your stored grain is critical to meet the growing global food demands. Other more specific assessments on grain storage proves that grain losses during storage is about 1 to 5% in temperate climates and about 10% in warmer climates.
Population growth increases food demand and changing climate conditions are affecting the agricultural productivity worldwide. The cereal production has always been higher if compared to the utilization rate, apart from the last few years where the trend has changed, becoming equal or lower. The consequence of this is a flattening of stock level trends (See Fig. 1), with a main concern that cereal bulk prices have the potential to continue to increase in the future. Finally, under investigation is the correlation between allergenic and storage conditions and treatments.
Understanding that grain losses can be reduced through proper grain conditioning and that the efficiencies of the aeration systems can be improved by provisions and best practices already available, it is easy to recognize the importance that education and training around the benefits of sustainable storage and aeration can have on global food challenges and future generations.
It is important to note that every grain system comes with unique challenges and components that must be considered. There are many factors that must be considered to deliver the right aeration system for the project including airflow requirements and size of the system.
THE NATURE OF THE AERATION SYSTEM
There is always the need of storing a certain commodity, for an amount of time in a particular kind of environment. The response to this need comes from literature and agricultural science, which provides safe storage charts. With the chart shown, we can compare the commodity (in this case wheat - Fig. 6) and the safe storage time, with its temperature and the moisture contained. Therefore, we know there is a need for a certain temperature and moisture surrounding our seed during storage.
Also, aeration avoids natural migrations which can cause the moisture to concentrate or condense in some regions of the bin and it is interesting to see how this behavior is mirrored.
In warmer regions the moisture usually migrates to the bottom center and the central well gets stuck, (Fig. 3) while in cold regions the moisture tends to migrate to the top pile making crusts and germinations.
The hygroscopic nature of grains
A certain temperature and a certain moisture for our seed can be successfully obtained by knowing that the grain is hygroscopic. Hygroscopic means that the kernel can take up or release moisture from or to the atmosphere in which it is placed. Wood, fertilizers and even sugar are examples of hygroscopic commodities. Therefore, the technology and the knowledge today allows us not only to control the grain temperature, but also the grain moisture, thanks to the development of EMC (equilibrium moisture content) charts (Fig. 4 is an example). This can be developed specifically for your commodity at any of our AGI Labs.
First, we need to know there is an inner environment in (Fig. 2) our storages which involves: the grain itself, air and water vapors, heat and moisture, and insects and mites. These factors are all interconnected to each other.
To have safe storage conditions we need to keep the product under certain temperature and moisture ranges as prescribed by the already mentioned safe storage charts.
There are a combination of factors to consider, because an insects life is influenced mainly by temperature while mold activity is influenced by moisture.
As the grain is hygroscopic, we can ensure the proper temperature and moisture content requirements by playing with the temperature and the relative humidity of the air surrounding our seed, taking advantage of the mentioned EMC chart (Fig. 4).
As an example, in order to keep the wheat under the 12% moisture content, the action is to provide air with such conditions of the white regions. If air is provided with conditions on the red field, the moisture will increase.
At this point the needed storage condition are known, and also the quality of the air that needs to be provided to the grains in order to guarantee the prescribed conditions.
The air might already be available in the atmosphere quite frequently, and this is the case of temperate climates where the aeration system might be designed for an airflow rate of 6 to 8 m3/h/ton of storage as a general rule.
This is different for warmer regions, where fresh hours are limited and the airflow rate has to be doubled in order 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 empirical 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 compared to a channel system, not only in terms of airflow uniformity 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 insignificant element, but if not properly designed it can be a critical 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 important task to be accomplished is understanding the allowable 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 variables: geographical area, ambient conditions, yearly variations 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:
1) Basic rule: aerate when ambient temperature is at least 10 °C lower than grain temperature.
2) Aerate in the daily hours where the outside temperature and relative humidity are close to the average values (See fig. 8).
3) Asses a target moisture content for the grain preservation and take advantage of the EMC (equilibrium moisture content) charts to understand when the outside conditions are suitable for aeration.
4) Following the seasonal variations of the ambient temperature in temperate climates, keeping in mind the low thermal conductivity of the grain, it is possible to plan the storage operations in order to limit (even kill or stop) the insect activity. This is a typical method used also along with refrigerators. In the case that bad management of the grain has taken place, these are some signals that can be detected during storage:
1) High temperature readings (so called “Hotspots”) in some localized sensor only within the grain bulk usually a key signifier of insect infestation.
2) Strong weight difference (reduction) between the incoming quantity stored and the delivered grain is signal of insect’s activity.
3) Product sticking on the sidewall sheets after the unload is usually sign of moisture migration within the grain pile, humidity condensation on the roof or excessive moisture of the grain.
FUTURE VISION & RECENT MARKET TRENDS
Roof natural ventilation
There is always a top to bottom temperature gradient inside the bin roof, as the hot and moist air tend to concentrate naturally to the top. By having a gap on the roof peak as well as on the eave perimeter it is possible to allow the hot air to be released to the atmosphere and replaced with fresh air entering from the eave. This natural ventilation prevents the roof from sweating. There is also no need for a roof exhauster, therefore removing energy consumption and management.
When the aeration system is operational the natural airflow stops and allow the air in excess to be released from both peak and eave gaps, and it will restart as soon as the temperature gradient is in place again.
Basic systems only monitor the temperature of the grain bulk and struggle to provide the best distribution and quantity of sensors needed to detect “hot spots” accurately. “Hot spots” are where insects are actively spoiling the product.
Recent advancements in grain bin monitoring systems provide temperature cables as well as a humidity and CO2 cable. These are able to detect insect activity with more accuracy by measuring the CO2 concentration and its relative variations.
Implementing a weather station and additional computer technologies allow for a complete automated system, with the capability to start the fan when the outer air conditions are suitable for the business needs without the need of human supervision. With this, operational savings are also possible.
AGI offers a range of safety and temperature sensing systems that are available with cloud-based functionality for remote controlling and alarm.
Other systems and practices
• The uses of grain ladders and/or spreaders strongly reduce the ratio of kernel breakage and improve the granulometric distribution within the gran pile. A more uniform granulometric distribution results in a more uniform airflow.
• Double skinned bins or painted bins, both technologies are capable of improving the thermal insulation of the sidewalls and then become very effective if combined with refrigeration units in extremely warm regions.
• Bin coring is another important operation that consists of unloading the center core of grains located over the central discharge gate. Just 10-15% of the product should be unloaded and reloaded to the top. The benefits include an improved aeration on the bottom layers and a reduced risk of central gate unloading issues.
• Circulation of grain in some cases can reduce the overall grain temperature of about 1-2°C and stop localized insect infestations by spreading them around.
A SIMPLE EXERCISE…AN INTERESTING RESULT
Considering just one single bin with a capacity 5,500 tons of wheat and the following assumptions:
7000 m3 Volumetric capacity
5500 MTon Capacity based on wheat
200 €/Ton Current wheat price (assumption)
The overall investment cost is about 150-200.000 Euro including civils and assembly costs. This cost is sustained once for the time of the investment. With a wheat price assumption of 200 Euro/ton, every year an amount equivalent to 1.1 million Euro is stored in the bin.
Let’s now assume additional money is invested in systems and technologies with the aim of reducing the product losses of an average of 1% per year.
1% reduction product losses / year (assumption)
1.1 M€ * 1% = 11.000 €
Assuming a 25 year investment the NPV ≈ 150.000 € ≈ Net present value of the overall investment
This provides for an additional cash flow of about 11,000 Euro. Assuming a 25 year investment, the calculated net present value is estimated 150,000 Euro, which is almost equal to the overall investment amount.
To learn more about the benefits of aeration and conditioning and to find out what method is best for your storage facility contact AGI, EMEA@aggrowth.com or visit www.aggrowth.com