To prevent the onset of fermentation and degenerative phenomena that can generate from the moisture naturally present in the cereal, it’s necessary to treat the crops in order to remove the excess moisture.
Thanks to drying, the correct shelf life of the grain is guaranteed, and the cereal can be prepared for subsequent processing. At the time of harvesting, the humidity inside the cereal can typically vary from 17% to a maximum of 40%, while for selling the safety limit is typically between 13 and 14%. This level guarantees the higher shelf life by preventing crop degeneration mechanisms.
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Reaching the lowest value of tolerated moisture allows the cereal to be protected from attacks by harmful microorganisms during the storage period and to minimize losses both in terms of product quality and in terms of usable or saleable quantity.
Given the importance that this process plays for the protection of the crop of many farmers, Mecmar has collected in this article all the important information on how a grain dryer works.
Dynamics of drying
The drying process is based on the diffusion of moisture from the inside of the grain to the outside. The hot and dry air that touches the grain leads to a heating of the humidity inside the grain which tends to evaporate to saturate the hot air.
To further simplify, 3 main phases can be identified in the complex drying dynamics: in the first phase, the water present inside the kernel is extracted and pushed towards the outer wall; in the second phase, the evaporation of water by the air surrounding the kernels takes place; in the third phase, the air rich in humidity is expelled externally.
When the cereal is inserted inside the grain dryer, it has a uniform degree of humidity inside it, but when the humidity outside the kernels begins to evaporate (thanks to the hot air blown by the dryer), the water found in the internal begins to advance towards the outside, to re-establish a new balance of humidity. Therefore, by exploiting a current of hot air and the porosity of the product, all the water present on the surface is removed, followed by the water present in the core of the seed.
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Air heating
Hot air grain dryers exploit this principle of moisture diffusion from inside to outside and the continuous current of air that laps the grain allows to continuously remove the moisture that is released from the grain.
A correct balance of temperatures, air flows, pressures, and quantities of product to be treated allows to optimize the performance of the dryers.
Grain dryers typically consist of:
- a heat generator with an aeration system for generating the flow of hot air
- a desiccant body that contains the cereal to dry, inside which the excess water evaporates
- a control panel with the control, regulation, and safety devices of the entire system
- any complementary equipment designed for accumulating, cleaning, weighing, and handling the product.
Depending on how the air is heated, it is distinguished in direct and indirect heating systems. Direct heating systems mix the combustion gases with the ambient air and introduce them directly into the mass of cereals to dry. For this type of systems, it is essential to set the burners at the right temperature to burn the fuel perfectly. The advantage of this type of heating is in the high performance, as all the heat produced by combustion is used for drying.
In indirect heating systems, the external air passes around the combustion chamber and the bundle of pipes that keep the combustion fumes separate from the air used for drying. In this type of heat generators, the combustion products are expelled into the atmosphere and the efficiency (and the fuel savings) will be greater the greater the exchange surfaces.
The last phase of the drying process is cooling, a fundamental phase for proper product storage. Depending on the type of dryer, the methods in which the grain is cooled also change. In discontinuous dryers, cooling occurs at the end of drying and using the same fan previously used to blow hot air; in continuous dryers, cooling takes place in a dedicated area of the column or in a different cell. This phase is essential to prevent the formation of condensation and the formation of fungi or the onset of fermentation processes.
Types of dryer
The main distinction in the operation of a dryer, in addition to the type of heating, concerns drying. Drying can be in batches (with discontinuous or batch dryers, as Mecmar mobile grain dryers) or in continuous (with continuous cycle dryers, as Mecmar tower dryers). Discontinuous dryers are defined in this way because they perform a complete drying of an entire batch – equal to the capacity of the machine – after which the batch of cereal is emptied, and the machine is set up for drying the next batch. These dryers can be mobile and easily positioned as needed.
Generally, this type of dryer is the best choice for:
- those who have a few tons of product
- those who need to dry heads and tails of continuous dryers
- those who consider drying for third parties
- those who want to take care of the drying of their crop, limiting the fixed costs of the plant compared to fixed dryers
- those who need to dry different types of cereal.
Continuous dryers are defined in this way because the drying process of the grain proceeds uninterrupted, with the introduction of the wet product from one end of the dryer and the release of the dry and cooled product from the other. Usually, the machine is stationary and works continuously, even for several days or weeks.
These features make continuous dryers the ideal choice for:
- large drying plants (which typically require drying from 240 tons per day and up)
- farms with the need to dry large daily tons of grain
- those who need to dry large quantities of the same type of cereal
- those who have available grain transport and storage facilities.
Conclusion
Mecmar’s long experience in drying cereals has allowed it to thoroughly study the best technologies to create increasingly innovative and performing machines, but which respect the organoleptic qualities of the cereals during drying.
If you want to find out more about how grain dryers work and would like to find out which dryer could be right for you, do not hesitate to contact us: the Mecmar team is ready to answer all your doubts and curiosities.
