The thermometry system is the main tool used to show and record the temperature of the mass of grains stored in bulk. In addition to allowing monitoring of the temperature, it allows the diagnosis of possible causes of problems observed in the mass of the product.
The thermometry consists of sensors that vary their electrical resistance according to the temperature of the grain mass and converters of units installed in the back, transform ohms (unit of resistance measurement) into degrees Celsius. It is evident that this relationship is fragile, especially considering that there are electrical contacts, humidity, dust, corrosion and so many other elements that can "deceive" the converter and thus show values diametrically opposed to reality. Thermometry is an ally, but one who you should always be suspicious of. Thus the probable cause for the fact presented above was the conversion error indicating false values. Therefore, it is recommended to review/general maintenance of the system once a year.
Artificial cooling equipment is accounted for as fixed assets and as such usually need the approval of the senior management of companies, which ends up requiring detailed storage managers to justify their purchase. Of course, in this context, the costs of this equipment must be addressed. If we look from the perspective of absolute values it may seem like a high investment, but if we look at the rate of return and its benefits in the quality and safety of storage, it is observed that it is technically and economically viable. For example, the investment required to apply the technology to a Storage Unit is usually on the order of 4 to 6% of the total invested and an amortization time between 12 to 24 months (without considering the financial costs), all depending on the product to be cooled, the geographical location and the season. As can be seen, the relative value results low and a short amortization time. Thus, the technology has been consolidated in the last decade, through the massive deployment, in all producing countries of South and Central America, whether cold or tropical climate, allowing the cooling of millions of tons of grains and seeds, regardless of climatic conditions and free of dangerous insecticides toxic to human and animal health. It's the clearest evidence of its viability.
These dryers are usually manufactured to operate with up to 3% impurities, for better fluidity of wet product, besides avoiding spaces occupied with impurities in place of grains. However, the lack or deficiency of pre-cleaning machines before the dryer, and the lack of maintenance causes the concentration of impurities and foreign matter inside the dryer, and these, when not removed, reach very low humidity and can go into self-combustion at temperatures equal to or greater than 60 °C.On the other hand, the drag of shards into the grain mass that occurs by the suction of furnace air, can also represent the spark needed for the start of a large fire. Thus, the ovens must have, after the combustion chamber, another chamber intended for the decanting of shards, which is done by reducing the speed of the air. This chamber should be cleaned at regular intervals of approximately 15 days. In the case of the use of solid fuel, the combustion chamber should be cleaned no later than two days in order to allow the good mixing of air with solid fuel, considering that poor quality firewood can produce up to 4% ash, which may obstruct the combustion air supply making a poor-quality burn. However, it can be stated, without a doubt, that lack of maintenance is the most common cause of fire in dryers and cleaning these dryers is not a simple task and carries risks, so it is usually postponed or avoided by operators.
The hot and humid air, retained between the top layer of the grains and the silo or bulk carrier cover, when coming into contact with the sheet metal (which is usually colder at night), reaches the dew point condensing the water vapor contained in the air which causes drops on the surface of the grain mass. As metals are good heat conductors, this phenomenon is more easily observed in the cover of silos where it has a greater intensity. Moisture comes from the very ambient air that occupies (through the gaps and openings) the space between the grain mass and the silo cover, or by inadequate aeration, or fungal activity within the grain mass, or by the drag of water from semi-wet grains via the aeration system, or the combination of these factors. The renewal of deficient air, in the cover of the silo or bulk carrier, worsens the problem causing qualitative and quantitative losses of the product, and with great losses to the storer. It is important to note that only the application of isolated layers on the outside of the warehouse cover may not be a definitive solution, because the reduction of 3 to 4 °C in the internal temperature of the air pocket may (depending on the temperature and initial humidity) cause damping on the surface of the grain mass. Currently there are already very efficient solutions to this problem on the market; like static wind hoods, which exhaust hot and humid air, through the action of the wind that when passing through its fins create the negative pressure necessary for this purpose. They do not use electricity and have no moving parts, being robust and durable.
Pre-cleaning machines are manufactured, in most cases, taking as reference soybean, with maximum water content of 18% (b.u.), maximum impurities index of 7%, with reduction capacity to 3%. If the machine is fed with another plant species and with a water content greater than the reference, its cleaning and production capacity will be reduced in some cases to less than 50%.
Here are some practical recommendations:
(a) level the machine and adjust for the uniform distribution of grains on the sieves;
b) Adjust the air flow, observing the grain drag, or the deposit of light impurities;
c) Regulate the grain flow in order to occupy up to approximately 50% of the top sieve;
d) Do not neglect manual cleaning of the sieves, as self-cleaning systems by means of rubber balls or scraping brushes may not have satisfactory results;
e) Observe the intensity of vibration of the sieves. Higher vibration intensity will allow the rapid passage of the grains over the sieve and the low intensity will allow the grains to on the sieves stay too long. Both conditions result in low machine yield and efficiency;
The probable reason was the concentration of impurities that were deposited in the center of the silo during its loading. Impurities hinder or prevent the passage of air (whether artificially cooled or environmentally), reducing or eliminating the possibilities 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 conditions for a temperature explosion, due to fungal infection, which must have caused the loss of the product in this sector of grain mass. Artificial cooling provides cold and dry air and therefore cannot cause this type of problem, but at the same time also, it does not solve problems related to operational or design failures.
For safe storage, a clean, dry, and cold product is recommended. The use of spreaders should be avoided and remove the products from the center of the silo to avoid the concentration of impurities in these points.
Here there are also controversies, because some researchers claim that it is a physical-chemical phenomenon, internal to the grain, which occurs by oxidation of carbohydrates, especially sugars, whose chemical reaction is exothermic (commonly called breathing) where;
Sugar + Oxygen (air) --------) Carbon dioxide + water vapor + heat.
Other researchers claim that it is a physical-chemical phenomenon generated by the action of fungi or insects, and consequently a phenomenon external to grains, 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 from the intergranular air) exert a decisive function to trigger 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 the consequences that everyone knows.
Dry, clean, and cold products are essential conditions to avoid these problems.
It is recommended that the monitoring of the temperature of the grain mass be carried out during the morning periods, between 7 and 9 hours, when the ambient temperatures are mild and a higher relative humidity, facilitating the transmission of electrical signals. At times of higher room temperature, the relative humidity is usually lower, and can affect the readings, because they change the electrical resistances of conductors and electrical contacts, thus modifying the ratio of electrical resistances "read" by the controllers at a certain distance.
The thermometry system shall be measured and calibrated periodically at least twice a year.
The use of insufflation or air suction through the grain mass during the aeration process is a matter of controversy and several pro and con questions for the different applications. Some researchers concluded that the decision to be made in favor of insufflation or suction should be made based on previous analyses of product conditions, ambient air, and the characteristics of the installation, because clearly, any of them can be used and bring the expected benefits, respecting the technical considerations observed for its use. The most frequently encountered situations in subtropical regions are: .
a) Hot air reduces the risk of grain damping and insufflation aeration may perform well even using air with relative humidity slightly higher ;
b) Suction may perform well when it is desired to cool grains in cold seasons, with ambient air, squealing the air from the upper layer of the mass ;
With the use of modern artificial aeration techniques, these differences end, because the air is artificially manipulated and injected into the grain mass through insufflation, allowing its total and uniform cooling, regardless of climatic conditions.
For the purposes of classification and commercialization, the maximum index of impurities allowed in the grain mass is 3%, established in the classification ordinances. However, the ideal storage condition is the total exemption of impurities, as they constitute an appropriate habitat for the development of insect pests, mites, bacteria, and fungi.
During loading, they accumulate in the center and walls of the silos or in the back of the "V" bulk carriers. A recommended practice for the partial solution of this problem is the removal of part of the load, from the bottom to the top of the grain mass, with the removal of a layer of approximately 5 mm thickness, removing all concentrated impurity and distributing it, superficially, in the product mass. In the case of vertical silos, the center ("tube of impurities ") should be removed.
The concentration of impurities at specific points of the mass impairs the distribution of air during aeration, which usually translates into increased temperature, and a degradation of quality, or a total loss of the product.
The most suitable solution for reducing the content of impurities is the installation of a cleaning machine after the drying system, allowing the operation called post-cleaning.
Scientific studies have made evident the mechanisms of resistance that pest insects develop to defend themselves from the active ingredients of some commercial insecticides, which constitutes a major problem for their control through the use of chemicals. According to some researchers, the main mechanisms of resistance are the reduction of the penetration of the insecticide by the cuticle of the insect, the metabolization of the insecticide by enzymes, and the reduction of sensitivity to the insecticide by the nervous system. The poor application or inadequate concentration of insecticides (low dosage) have also contributed to the increase of the resistances of these pests. Therefore, to overcome these problems, the adoption of good storage practices is recommended, integrated pest management, adequate use of insecticides, combined with alternative technologies such as artificial cooling and diatomaceous earth.
The main aspects to be observed are:
a) alignment and stretching of the conveyor belt;
b) verification of the operation of the mobile loading and unloading cables (tripper);
c) angular speed of the support rollers of the upper and lower branches. In case of locking, the roller should be replaced in order to avoid the possibility of frictional heating between them and the tape, and the consequent possibility of fires or explosions;
d) cleaning and lubrication of the bearings of the motor pulleys;
e) level and the period of oil change of the box reducing strength and speed;
f) cleaning impurities on the surface of the electric motor in order to ensure good heat dissipation. In case the tape is installed in closed galleries, it should be cleaned weekly.
The cooling of bulk grains can be carried out in silos out of metal, concrete, masonry, or any other building material, as well as in bulk warehouses. There are small, medium, and large chilling equipment on the market which can serve any size of storage unit. The aeration system should be good sized to ensure uniform distribution of air through the grain mass. A silo of 16,000 t, for example, can be cooled in 420 hours, at a temperature between 15 to 18 ºC within the grain mass and remain for several months without the need for a new application of cold. All grain has low thermal conductivity, which favors its thermal stability for a long period.
Energy consumption can range from 2 to 4 kwh per ton of grains, depending on plant species, water content, initial and final product temperatures, impurity index, air distribution systems, and average ambient temperature.
Generally, the operator cares about the temperature of the drying air while his concern should be with the temperature of the grains. This is because, for the same drying air temperature, depending on the dryer model, the grains may reach higher or lower temperature. In most models of dryers, especially tower models, mixed flows, most used in Brazil and other countries in South America, the drying air temperature should be maintained between the range of 80 to 100 °C, which leads to the temperature of the grains between 45 and 60 °C. Higher temperatures can cause irreversible damage to the product.