April 01, 1997
by Teresa Acklin
Understanding aeration systems and practices can prevent expensive losses in grain quality.
Aeration is a method of controlling the microclimate in a mass of stored grain. Its purpose is to preserve the original condition of the grain by providing an unfavorable environment for insects and microorganisms to grow. Losses in stored grain are related to the effects of biotic factors, such as molds and insects, and abiotic factors, such as poor storage design and management.
To prevent biotic factors from predominating, introducing air with selected properties through the grain mass in a timely and effective manner can provide conditions favorable to maintenance of grain quality and unfavorable to biotic factors. Aeration can be defined as the movement of selected ambient or suitably conditioned air through a grain mass to improve its storability.Purposes of Aeration
Aeration accomplishes five general goals. The first is to equalize the temperature of stored grains.
The main objective of equalizing the temperature of the grain mass is to prevent moisture migration. Warm or hot air rises and has greater moisture carrying capacity than cool air. If a mass of grain has hot spots caused by insects or microorganisms or diurnal changes in temperature, natural convection will occur in the grain mass, slowly causing the migration of moisture from the warm spots to the cooler grain.
Aeration's second purpose is to create low and uniform temperature in the grain mass. Cooling grain to temperatures below those favorable for microorganisms and insects to grow is one important purpose of aeration. However, cooling grain is more practical and achievable in temperate regions than in tropical areas.
Aeration also prevents heating in damp grains. The use of aeration with high airflow rates to dissipate heat in damp grain is common in large drying installations, especially during the peak of harvest.
Newly harvested grain with moisture above 18% generates significant heat due to microbial and grain respiration. Excessive heating leads to dry matter loss, mold growth and visible heat damage which usually results in a reduction of grain quality.
Aeration alters the gas composition in grain voids. Fumigation through an aeration system is often more convenient and practical in large silos than other methods, such as moving the grain or introducing fumigant tablets with the use of probes.
However, when using an aeration system for fumigation, care must be taken to prevent human and animal exposure to any fumigant that may escape. Fan operation should be stopped when the gas starts discharging into the atmosphere.
Finally, aeration can reduce grain drying operations. In some temperate areas, high airflow rates can help in bringing the grain mass to a lower moisture level.Heating Factors
Biological activity is a major influence on heat levels in stored grain. The combined respiration of grain, microorganisms and insects accounts for most of the heating reported in grain storage facilities.
Respiration in grains is a chemical reaction where grain's carbohydrate portion combines with the oxygen in the air to produce water, carbon dioxide and heat; the respiration process is accelerated by high grain moisture and temperatures. Respiration in microorganisms and insects also produces water and heat, which is a more significant factor than grain respiration under identical conditions.
Climatic conditions are another influence. Conditions of high ambient relative humidities and temperatures will cause moisture absorption in stored grain, thus creating favorable conditions for molds and insects to develop.
The thermal and biological properties of the grain itself are yet another factor, as these characteristics influence heating. Two of these properties are thermal conductivity and germinability; thermal conductivity affects heat transfer, while germination makes grain unfit for food use.
The type and method of storage also affects heating. Grains stored in metal silos tend to heat and cool down more rapidly than grains stored in concrete silos.Dissipating Heat
Several methods are available to dissipate heat in grain. Heat generated by biological activities in stored grains can be dissipated into the surrounding atmosphere by natural convection.
However, natural convection is a slow process and is effective only if grain is spread in a thin layer and if ambient conditions are favorable. Therefore, grains stored in bulk cannot be ventilated effectively by natural means.
Forced convection uses fans or blowers to force air movement in the grain mass to enhance the heat transmission process. With selected properties of the surrounding air coupled with a knowledge of the grain condition, cooling time can be predicted with an acceptable degree of accuracy using heat balance equations.
Moving grain from one storage silo to another introduces natural and forced convection or air around the grain. Additional cooling takes place as the stream of grain is moved by the belt conveyor from one point to another. In situations where an aeration system is not working or non-existent, moving the grain from one silo to another may be the only solution to a grain heating problem.Aeration Systems
Grain has four important properties that greatly influence the design and management of an aeration system. The first is hygro-scopity.
Hygroscopic materials such as grain adsorb or desorb moisture depending on the prevailing environmental conditions. Desorption occurs when the water vapor pressure in the grain is greater than that of the partial pressure of the surrounding air. On the other hand, when the vapor pressure of the grain is less than that of the surrounding air, adsorption will occur; the moisture will move from the air to the grain.
Desorption is commonly referred to as drying while adsorption is commonly known as a dehumidification or rewetting process. The moisture level reached by the grain either through desorption or adsorption is known as the equilibrium moisture content (E.M.C.) of the grain under that condition.
The specific heat of a grain also influences aeration systems. Specific heat is the amount of heat required to change the temperature of a unit weight of a substance by one degree. Available data on the specific heats of grains allow us to calculate the required air and time for an aeration operation to cool stored grains.
Thermal conductivity of grain is a measure of its capacity to transmit heat by conduction. The thermal conductivity of steel and concrete are about 450 times and five times that of wheat, respectively.
Available data indicate that the thermal conductivity of grains approaches that of water as its moisture content increases. The low thermal conductivity of grains, as compared with that of the silo wall, explains why the heat grain generates cannot be dissipated fast enough by conduction to prevent heat damage and dry matter loss.
Resistance to airflow is another property to consider. The different textures and shapes of grain influence their packing characteristics. These characteristics determine the percentage of void spaces in a mass of grain, which directly affects the degree of resistance to airflow by the grain mass.
Grain resistance to airflow is usually expressed in centimeters of water as measured by a manometer actually an expression of pressure. There is a relationship between grain resistance to airflow and required fan horsepower.
Clean grains stored in bulk have a fairly uniform resistance to airflow, and sufficient information is now available on the resistance of cereal grains to different airflow rates and grain depth. These data are used to design practical and effective aeration systems both in horizontal and vertical bulk storage structures.
On-farm aeration systems usually include perforated floors. The system typically consists of a silo with a perforated floor elevated from the ground and a blower or fan attached below the floor.
Vertical ducts are often used in small metal silos. The duct has perforations and a suction fan attached to it at the upper end.
Normally, one duct is placed at the center of a small silo, and several units can be used in horizontal bulk stores with the aim of providing as uniform an airflow as possible. Two vertical ducts (one inlet, one outlet) have been used in tall silos to provide high airflow rates and more uniform air distribution.
On-floor ducts permit the installation of an aeration system in horizontal bulk storage facilities. The duct can be permanent (below the floor) or portable (above the floor), depending on the use of the building and construction costs.
Permanent ducts are more expensive, but maintenance costs are much less. Portable ducts are less expensive to build, but have higher maintenance and labor costs. Both systems require a main duct and some distribution or lateral ducts, depending on the size of the building.
Normally, grain stored in bags does not require forced ventilation since it is exposed to the ambient air, and natural convection inside the warehouse helps dissipate heat generated by normal respiration. When bagged aeration is done, it requires proper planning well before the grain is stored. Aeration in bags can be accomplished by providing air channels on the floor and stacking the bags over them such that the air will not escape through the void spaces between individual bags.
This article is adapted from presentations made in Malaysia by Dr. Ulysses Acasio, Kansas State University, Manhattan, Kansas, U.S., under a program sponsored by the U.S. Feed Grains Council.