Aeration, cooling of stored grain
March 11, 2016
Stored grain problems such as mold, insects, fungi and spoilage start with or become worse due to improper grain moisture and temperature.
Temperature differentials in the grain cause moisture to migrate to the top and center of a stored grain mass, causing molds to develop and insects to feed and reproduce, according to an Oklahoma Cooperative Extension Service report written by Carol Jones, professor, Oklahoma State University.
Aeration keeps the grain cool and maintains uniform temperatures and moisture in the grain mass. In aeration, a fan is used to pump outside air through the grain in the storage facility. Eventually, the grain attains the temperature of the air traveling through void spaces in the mass, Jones said.
Each type of grain has different storage requirements for temperature and moisture, requiring a specific aeration system. The time of harvest can have an impact, given the varying outside temperatures. Oilseeds in general require cooler and dryer storage conditions than wheat or corn (maize).
Designing an aeration system involves first determining the airflow rate and direction needed to achieve specific aeration goals, Jones said. After the rate is determined, the air distribution system can be designed and fans selected to deliver the required air volume.
Airflow is typically expressed as cubic feet of air per minute, per bushel of stored grain (CFM/bushel). Rates start at 0.05 CFM/bushel and increase as shown in Table 1 (page 78). Light aeration systems (0.1 CFM/bushel or less) require long operation times to lower grain temperatures and are suited to cooler climates. Medium and fast systems (0.2 to 0.8 CFM/bushel) are used periodically to lower and equalize temperatures in stored grain.
For high speed grain cooling, 1 CFM/bushel or greater is required, Jones said. Grain can be cooled in one or two nights of fan operation.
Don’t confuse grain cooling with natural air drying, which requires high speed airflows (greater than 1 CFM/bushel). In natural air grain drying, damp grain is placed in a bin and the fan is operated continuously for several weeks until all the grain has dried, she said. Grain cooling includes an initial period of fan operation to cool grain followed by periodic operation to maintain even temperatures throughout.
The airflow rate should be chosen according to how the system will be used. In general, if grain must be cooled quickly, a faster airflow is ideal. For example, if aeration will be used to prevent moisture migration, only a light system is needed. If moisture content during storage will be 1% or 2% above safe levels, a faster aeration system is desirable, Jones said.
The time required for grain cooling will depend on the airflow rate, foreign material in the grain and the amount of evaporative cooling that takes place. Doubling the airflow rate will cut cooling time in half.
Airflow direction. Pressure (upward) airflow is preferred by many grain managers over suction (downward) airflow, Jones said, for several reasons:
- Aeration fans are designed to deliver maximum airflow against pressure; fans push more mass of cool dense air than warm light air;
- Pressure fans develop more uniform airflow through the grain than suction systems;
- Condensation on top of the grain is visible and usually dries by the end of the aeration cycle. Bottom condensation damage caused by suction fans can’t be seen until the bin is unloaded; and
- Pressure fans eliminate winter bin roof collapse as roof vents may freeze over during suction cooling.
Aeration fans may be either axial (propeller) type or centrifugal (squirrel-cage) type as shown in Figure 1 (page 76). Axial type fans are less expensive and are normally used when static pressure will not exceed 4 or 5 inches of water, Jones said.
Centrifugal type fans with backward-inclined blades give more consistent air delivery over a range of pressures than most axial type fans and, in special designs, can operate at 20 or more inches of static pressure.
The power needed for aeration increases as airflow rate and grain depth increase. A doubling of airflow rate or grain depth causes about a four-fold increase in power required. In addition, different grain types have different resistance to airflow. For these reasons, aeration fans vary greatly in horsepower.
With light aeration systems, fractional horsepower fans are often used. As airflow rates increase to 0.5 CFM/bushel or more and grain depths of 20 feet or more, the power needs may exceed 1 horsepower for every 1,000 bushel of bin capacity. In large bins, multiple fans of 20 or 25 horsepower may be required. Fans should not be selected by type or by horsepower, but from the manufacturer’s rating tables or curves to deliver the required air volume at the expected static pressure, Jones said.
Grain bins with fully perforated drying floors are ideal for aeration, but cleaning under the floor can be difficult and erection costs can be higher. Removable floor ducts are an alternative to drying floors and can be obtained in large square, cross, “Y,” or “U” patterns.
Cross-flow aeration systems can be used to reduce the resistance to airflow encountered in aeration of tall grain bins. The lower power requirements reduce operating expense and capital costs.
Bins need adequate roof exhaust vents to minimize condensation and maximize airflow, Jones said. There should be a minimum of one square foot of roof vent opening for every 1,500 CFM of airflow. An alternative to installing roof openings during aeration fan operation is to open roof fill caps and access doors to allow air to escape.
Roof vents that are designed to open under pressure or vacuum can remain closed between aerations to help reduce insect infiltration.
It is possible for aeration fans to suck rain and snow into a grain bin. Short storms (three to six hours in duration) usually do not create a moisture problem. But during longer storms or continuous foggy weather, it is advisable to stop aeration until the weather improves, Jones said.
The fans may be controlled by manual, semiautomatic or automatic controls. Manual controls require a single on-off switch. Aeration fan controls include time clock or electric eye for night operation, high- or low-limit thermostats and high- or low-limit humidistats. Multiple commercial control packages are available ranging from simple temperature and humidity controls for a single bin to more elaborate controls for multiple fan operation.
With semiautomatic or automatic controls, an elapsed-time clock should be placed in the circuit. If the aeration system is not operating long enough to achieve temperature equalization within the stored grain, then the control settings should be changed to allow longer or more frequent operation.
During cooling seasons, aeration systems must be operated several times, each time long enough to equalize temperatures throughout the entire grain mass.
Small motors can be started and run by temperature and humidity control switches if the contacts are rated for these loads. Magnetic motor starters should be used for all motors of 0.5 horsepower or larger. Automatic aeration temperature controllers using time delay to minimize inrush current and peak power loads can control two or more larger fan motor starters in sequence.
Thermostats and humidistats should be adjusted and maintained according to the manufacturer’s recommendations. Controls should be cleaned at least once a year, and more often if operated in dusty surroundings, Jones said.
A level grain surface is an important part of controlling grain temperature and insect activity. Temperatures and insects in grain peaks are difficult to control. Level grain does not vary as much in temperature as peaked grain, which follows outside air temperatures because air blows against grain slopes through roof eave openings.
Clean grain and/or use grain spreaders to distribute foreign material and level the grain surface, thereby reducing storage risks from insects and mold.
Level surfaces improve temperature uniformity and make it easy to enter the bin for inspection. Foreign matter tends to accumulate in a column under the bin bill point. This restricts airflow and increases the chances of insect infestation and mold development, resulting in grain heating and spoilage.
Coring is a method of leveling a peaked bin and removing part of the center column of trash and foreign material, Jones said. Remove 10 to 20 bushels from each 3- to 4-foot layer of grain during bin filling (see Figure 2, page 76). The peak is then hand leveled at the end of filling.
Although aeration is not intended as a drying system, minor changes to moisture content can occur. The air surrounding stored grain has an equilibrium moisture content which is the point at which grain and air don’t exchange moisture. This equilibrium changes with temperature and moisture content of the grain. If grain is aerated with air having a relative humidity above the equilibrium, the grain will slowly gain moisture by absorbing water from the air. Conversely, if the air’s relative humidity is below the equilibrium, the grain will dry by transferring water to the air.
The amount of drying or rewetting that takes place during aeration depends on how dry the grain is when placed in storage, the airflow rate, how long the aeration system is operated, and, of course, the air’s humidity, Jones said.
At low airflow, drying is a slow process. Light aeration systems (less than 0.1 CFM/bushel) will rarely reduce average moisture content more than 1%, even when operated for long periods. Fast aeration systems can cause greater moisture loss if operated continuously. This moisture loss might be desirable if grain moisture content is above safe levels. It can also be undesirable if the grain dries below the market standard and the owner suffers additional shrinkage.
If some drying is desired, fast aeration systems can be operated continuously while periodically sampling grain moisture. If drying is not desired, fast aeration systems can be operated at night only as needed for cooling and temperature equalization.
Rewetting of grain by aeration with humid air is a very slow process. Operation of light or medium-speed aeration systems in humid weather will have little effect on moisture content, Jones said. However, if airflow rates above 0.5 CFM/bushel are used for aeration, fans should not be operated for more than a few hours when humidity is extremely high.
Because the higher airflow will cool grain rapidly, it is best to wait for lower humidity before cooling. Normal aeration rates are much too low for drying grain and can’t keep wet grain safe in warm weather. If harvested grain moisture is too high (13% for wheat), solve the moisture problem before putting it in storage.
The cost of installing an aeration system varies widely and depends on the specific installation. Availability of electricity, existing bin equipment such as a drying floor and roof vents, level of automation desired, and bin configuration all affect the total capital cost.
But aeration generally costs less than grain fumigants or protectants, or losses from mold and insect damage, and maintains grain quality, Jones said.