The thermometry system is the main tool used to display and register the temperature of the grain mass stored in bulk. Besides allowing monitoring of the temperature, it allows the diagnosis of possible causes of problems observed on the product’s mass. The thermometry consists in sensors that vary their electrical resistance as a function of the grain mass temperature and converter units installed in rear, that transform ohms (resistance measurement unit) to degrees Celsius. It is clear that this relation is fragile, especially considering that there are electrical contacts, humidity, dust, corrosion and many other elements that can trick the converter and show values diametrically opposed to real. Thermometry is an ally, but one must always be doubtful. Thus, the probable cause for the presented fact was the conversion error indicating incorrect values. Therefore, a general system review/maintenance is recommended once a year.
The artificial cooling equipment is accounted as fixed assets and as such, usually requires the approval of companies’ CEOs and CFOs, which ends up requiring to the storage manager detailed justifications and reasons for the purchase. Naturally, in this context equipment costs cannot be ignored. If we analyze only absolute values, it may seem like a high investment, but if we look at the rate of return and the benefits in quality and safety of storage, it is technically and economically viable.
As an example, the necessary investment to apply technology in a Storage Unit is usually around 4 to 6% of the total invested and the repayment time from 12 to 24 months (without considering financial costs), all this, depending on the cooling product, the geographic location and the season. As you can see, the relative value is low and the amortization time is short. That is the main reason why this technology has consolidated, in the last decade, through massive implantation in all producing countries from South and Central America. Whether they are tropical or cold weather countries, millions of tons of grains and seeds are being cooled despite climate conditions and free of dangerous insecticides, toxic to human and animal health. This is the clearest evidence of its viability.
These dryers are generally manufactured to operate with up to 3% impurities, for better flow of the wet product and to avoid impurities taking grain space. However, the lack or deficiency of precleaning machines before the Dryer, and the absence of maintenance cause impurities concentration inside the dryer. When not removed, these impurities reach very low humidity and may start self-combustion at temperatures of 60°C or more.
On the other hand, the drag of sparks into the grain mass that occurs through the furnace air suction can also represent the spark necessary for the beginning of a great fire. Consequently, the furnaces must have, after the combustion chamber, another chamber for the settling of sparks, which is done by reducing the air velocity. This chamber should be cleaned with regular intermissions of approximately 15 days. In the case of the use of solid fuel, the combustion chamber should be cleaned at least once in every two days in order to allow air to be mixed with the solid fuel, considering that poor quality wood can produce up to 4% ash, which could obstruct the combustion air supply by performing a poor quality burn.
However, it can be affirmed without a doubt that the lack of maintenance is the most common cause of fires in dryers and cleaning these dryers is difficult and risky, and is why it is often delayed or avoided by operators.
The hot and humid air trapped between the top layer of grains and the silo cover, upon contact with the metal sheet (which is generally colder during the night), reaches the dew point by condensing the water vapor contained in the air, which causes drips on the surface of the grain mass. As the metals are good heat conductors, this phenomenon is more easily observed in the coverage of the silos with greater intensity.
Humidity comes from the ambient air that occupies (through cracks and openings) the space between the grain mass and the silo cover, by inadequate aeration, fungal activity within the grain mass or by grain water trawling, through the aeration system, or the combination of these factors. The renovation of deficient air, in the coverage of the silo, worsens the problem causing qualitative and quantitative product losses, and with great damages to the storage unit. It is important to emphasize that only the application of isolation layers on the outside of the warehouse cover may not be a definitive solution, since the reduction of 3 to 4°C in the internal air temperature may (depending on the temperature and initial humidity) cause moisture on the surface of the grain mass. There are already very efficient solutions to this problem on the market today; such as Eolic Static Exhaustion Fans, which exhaust hot and humid air through the action of the wind, which, while passing through its paddles, creates the negative pressure necessary for this purpose. They do not use electrical energy and have no moving parts, being robust and durable.
Pre-cleaning machines are manufactured, in most cases, taking soybean as the grain of reference, with a maximum water content of 18% (b.u.), maximum impurities index of 7%, with a reduction capacity of 3%. If the machine is fed with another vegetable species with higher water content than the reference, its cleaning and production capacity will be reduced, in some cases, to less than 50%.
Some practical recommendations:
a) Level the machine and adjust for uniform grain distribution on the sieves;
b) Adjust the air flow, observing the drag of grains or the deposit of light impurities;
c) Adjust the grain flow to occupy up to approximately 50% of the top sieve;
d) Do not neglect the manual cleaning of sieves, since self-cleaning systems using rubber balls or scraper brushes may not have satisfactory results;
e) Observe the sieves vibration intensity. Higher vibration intensity will allow rapid passage of grains onto the sieve and the low intensity will allow the grains to remain too long over the sieves. Both conditions result in poor performance and efficiency of the machine.
The probable reason was the concentration of impurities that were deposited in the center of the silo during its load. Impurities obstruct the air flow (either artificially cooled or ambient), reducing or eliminating the possibility of lowering the temperature of the grain mass at these points. The combination of temperature, humidity and impurities (which are usually infected with fungal spores), offers ideal condition for a temperature explosion due to fungal infection, which may have caused product loss in this sector of the grain mass. Artificial cooling provides cool, dry air and consequently cannot cause such problems, but at the same time, it does not solve problems related to operational or design failures.
For safe storage, clean, dry and cold products are recommended. Avoid the use of spreaders and remove the products from the center of the silo to avoid the concentration of impurities at these points.
There are also controversies here, some researchers say that it is a physical-chemical phenomenon, internal to the grain, that occurs by the oxidation of carbohydrates, especially sugars, whose chemical reaction is exothermic (commonly called breathing) where: Sugar + Oxygen (air) --------) Carbonic gas + water vapor + heat. Other researchers say that this is a physical-chemical phenomenon generated by the action of fungi or insects, and consequently external to the grain, or the combination of both factors: intrinsic and extrinsic.
Either way, the effects are devastating and millions of tons of grain can be lost in a few days or weeks. The temperature and humidity of the grain (consequently intergranular air) play a decisive role in activating the phenomenon. The fungi themselves, through their enzymes, create the ideal environment for their development, raising the temperature and humidity of the intergranular air to dangerous levels, causing a true "temperature explosion", with consequences known by everyone. Dry, clean and cold products are essential to avoid problems.
It is recommended to perform grain mass temperature monitoring during the morning periods, between 7 and 9 o'clock, when ambient temperatures are mild and the relative humidity is higher, facilitating the transmission of electrical signals. In times of higher ambient temperature, the relative humidity is generally lower and can significantly affect the readings, as they alter the electrical resistances of the conductors and electrical contacts, thus modifying the ratio of electrical resistances viewed by the controllers from a certain distance. The thermometry system should be assessed and calibrated periodically, at least twice a year.
The use of insufflation or suction through the mass of grains during the aeration process is a cause of controversy and several pros and cons questions for different applications.
Some researchers have concluded that the decision making in favor of insufflation or suction should be made on the basis of prior analysis of the product's conditions, ambient air and the characteristics of the installation, since clearly any of them can be used and bring the expected benefits, respecting the technical considerations recommended for its use. The most frequent situations found in subtropical regions are:
a) The hot air reduces the risk of the wetting of grains and the aeration by insufflation can perform well even using air with a slightly higher relative humidity;
b) The suction can perform well when it is chosen to cool grains in the cold seasons, with ambient air, sucking air from the mass’s upper layer. With the use of modern artificial aeration techniques, these differences end because the air is artificially manipulated and injected into the grain mass by insufflation, allowing its total and uniform cooling, regardless climatic conditions.
For classification and commercialization purposes, the maximum impurities allowed in the grain mass are 3%, established in the regulations. However, the ideal storage condition is the total exemption of impurities, since they constitute a suitable habitat for the development of pest insects, mites, bacteria and fungi.
During loading, they accumulate in the center and on the silos’ walls or in the bottoms of the "V" type warehouses. A recommended practice for partial problem solution is to remove part of the load from the bottom to the top of the grain mass by removing a layer approximately 5 mm thick, removing all the concentrated impurity and distributing it superficially in the product mass. In the case of vertical silos, the center ("impurity tube") must be removed.
Impurities concentration, at specific points in the mass, prejudices air distribution during aeration, which usually results in elevated temperatures and quality degradation or total product loss. The most suitable solution for the reduction of the impurities is the installation of a cleaning machine after the drying system, allowing the post-cleaning operation.
Scientific studies have made evident the resistance mechanisms that insects develop to defend themselves against the active principles of some commercial insecticides, which is a major problem for their control through the use of chemicals. According to some researchers, the main mechanisms of resistance are the reduction of insecticide penetration by the insect cuticle, the metabolism of the insecticide by enzymes and the reduction of sensitivity to the insecticide by the nervous system. Poor application or inadequate concentration of insecticides (low dosage) have also contributed to the increase in the resistance of these pests. Therefore, to overcome these problems good storage practices, integrated plague management, proper use of insecticides combined with alternative technologies such as artificial cooling and diatomaceous earth are recommended.
The main aspects to be observed are:
a) Alignment and stretching of the conveyor belt;
b) Verification of the tripper cables operation;
c) Angular velocity of the supporting rollers of the upper and lower sections. In case of locking, the roller must be replaced in order to avoid the possibility of frictional heating between the roller and the belt, and the consequent possibility of fires or explosions;
d) Cleaning and lubrication of the sheaves of the motor pulleys;
e) Level and periodical oil change of the force reduction gearbox;
f) Cleaning of impurities on the surface of the electric motor in order to guarantee its regular dissipation of heat. In case the conveyor belt is settled in closed galleries, its cleaning must be performed weekly.
Cooling of grains in bulk can be done in metal, concrete, masonry or any other building material, as well as warehouses. There are small, medium and large coolers on the market, which allow you to fit any size of storage unit. The aeration system should be well dimensioned to ensure uniform distribution of air through the mass of grains.
A silo of 16,000 tons, for example, can be cooled in 420 hours, at a temperature from 15 to 18°C within the grain mass and remain for several months without new application of cooling. Grains present low thermal conductivity, which favors its thermal stability for a long period. The energy consumption may diverge from 2 to 4 Kw/h per ton of grain, depending on the plant species, water content, initial and final product temperatures, impurities index, air distribution system and average ambient temperature.
Usually the operator worries about the drying air temperature, while his concern should be with the grain temperature. This is because, for the same drying air temperature, depending on the dryer model, the grains can reach higher or lower temperatures.
In most dryer models, especially the tower ones, of mixed flows (most used in Brazil and other South American countries), the temperature of the drying air must be maintained between the range of 80 and 100°C, which takes the grain temperature between 45 and 60°C. Higher temperatures can cause irreversible damage to the product.