Aeration system management
April 1, 2008
The purpose of aeration is to maintain a consistent relationship between grain temperature and outside air temperature that will minimize moisture migration and keep grain temperatures low to minimize deterioration rates.
Insects will cause more problems when the temperature is above 56 degrees F/13 degrees C and mold is always a problem, especially at temperatures above 48 degrees F/ 9 degrees C.
The aeration system allows the operator to change the temperature of stored grain using outside or ambient air that is a different temperature than the grain. Most of the time the grain will be cooled but there may be situations where you want to raise the grain’s temperature.
The goal of aeration management is to change the temperature of the stored product economically and incrementally to a temperature sufficient for safe storage based on the storage time requirements. The majority of the time this means cooling maize (corn) and soybeans to below 48 degrees F/8 degrees C and cooling wheat below 56 degrees F/13 degrees C as fast and safely as possible.
The type of climate a grain storage facility is located in plays an important role in determining aeration strategies. For instance, 25% moisture maize arriving at an elevator in a hot and humid climate poses different challenges than 25% moisture maize arriving at a storage facility in a dry and cool climate.
The operator needs to determine whether the aeration system will be used for holding wet grain, cooling dry grain, long- or short-term storage maintenance, or long-term storage during the spring and summer.
The following guidelines will serve the operator well for all of these conditions:
Keep stored grain as uniform as possible in terms of temperature, quality and moisture, as the odds of successful grain storage will increase with a uniform grain temperature, moisture content, cleanliness, etc.
Try to run the aeration fans when the outside ambient temperature is within 15 degrees F/9 degrees C of the grain temperature. Condensation potential inside the bin (silo) increases exponentially when the grain ambient air temperature differential exceeds 15 degrees F/9 degrees C. This may require some planning and good management, but it will prevent multiple storage problems.
When you start an aeration front, make sure you finish it. Moving air causes an aeration front to be started from the bottom up with a positive aeration system and from the top down in a negative system. If the fans are stopped and then restarted, the aeration process starts over as well. Uneven temperatures in a bin significantly increase the potential for grain spoilage, moisture migration and crusting. Significant temperature differential fronts may even cause crusting in the middle of the grain mass that will prohibit future aeration from moving through the crusted area.
HOLDING WET GRAIN
Many grain storage facilities do not aerate wet grain because it is stored for a relatively short period of time before drying. It can be argued that wet grain has the greatest chance of spoilage and therefore deserves the best aeration system in the facility. Positive aeration systems with roof exhaust fans are recommended for wet grain. The potential for water to be removed from higher moisture grain increases during aeration based on the ease of water removal from the higher moisture grain (see chart, page 65). Aeration rates of 1/5 (0.20) cubic feet per minute per bushel (cfm/bu) or more are recommended, but even a minimum amount of air is better than no aeration. An aeration rate of 1/5 (0.20) cfm/bu will maintain maize at 16.5% to 17% moisture in a normal year in most parts of the Midwestern U.S.
Many facilities do not aerate wet grain because their wet holding bins do not have aeration, or the aeration system in the bin storing high-moisture maize is a positive system without roof exhaust fans. Aeration of high-moisture maize with a positive system without roof fans will cause condensation under the bin roof, which in turn will cause crusting and spoilage on the surface of the grain.
Positive air passing through the grain will pick up moisture and release it when the now wetter air contacts the roof. The ability of the exhaust air to release moisture increases as the temperature differential between the exhaust air temperature and the roof temperature rises. Condensation will almost always occur if the roof temperature is 10 to 15 degrees F/6 to 9 degrees C or more different than the exhaust air temperature. Negative systems are not recommended for wet grain since moisture and fines are pulled down and tend to plug and blind the aeration ducts. Assuming the aeration system was designed correctly, low amperage readings on negative aeration fan motors may indicate a fan that is not moving air due to blinded or plugged aeration ducts or plugged roof vents.
COOLING DRY GRAIN
When cooling dry grain, aeration should start as soon as grain covers the aeration duct in the bin. Temperature detection cables will monitor the cooling process and should be checked at least on a daily basis and more frequently if the aeration front appears to be moving slower than expected. Cooling progress should be checked and the fans should run long enough to cool the grain to within 5 to 15 degrees F/3 to 9 degrees C of the average outside air temperature.
Grain from the fall harvest should be cooled to about 35 to 40 degrees F/2 to 4 degrees C when it will be stored during the winter months or into the spring or summer. The entire grain mass should be as uniform in temperature as possible. The top center of the bin will be the last to cool with positive aeration and bottom center will be the last to cool with negative aeration. It is in these two areas that fines tend to accumulate, increasing the chance of spoilage if the grain temperature is not uniform. Coring the bin will help redistribute the fines to lessen their concentration.
LONG, SHORT-TERM WINTER STORAGE
During the winter, monitor the grain temperature. If it is stable, no further aeration is required or recommended.
The cost of covering the positive fan inlets using inlet dampers can be justified. Large temperature differentials between the grain and the outside air combined with the chimney effect of the aeration tunnels connected to open inlet fans may cause condensation near the ducts inside the bin. Negative fans should always have discharge dampers covering the fan opening for safety.
SPRING, SUMMER STORAGE
Many grain storage facility operators do not aerate in the spring if the grain will be carried into next fall. If the decision is made to “warm up” the grain from 35 degrees F to 48 degrees F/2 degrees C to 9 degrees C, it should be done when expected fair weather periods provide sufficient fan operation time to warm the grain. A 1/10 (0.10) CFM system will require about 80 hours of continuous fan operation in the summer to change the grain temperature 15 to 20 degrees F/9 to 12 degrees C and a 1/5 (0.20) CFM system will require about 40 hours of continuous fan operation for the same temperature change.
Warming the grain above 48 degrees F/9 degrees C will increase potential mold growth opportunities. Moving grain with a greater than 15 degrees F/9 degrees C air/grain temperature differential increases condensation or sweat potential. Choosing the lesser of two evils is a management decision. Monitoring the grain temperature will tell the operator if the grain is stable or not.
Exhaust fans may be run without main fans on hot summer days to prevent condensation caused by temperature differentials of over 15 degrees F/9 degrees C between the ambient temperature and the dead air space inside the silo. Condensation is completely avoided if the dead air temperature and the roof temperature are the same. Wheat arriving at the facility during the summer harvest seems to benefit from aeration even when cooling is not taking place, as insects do not like an environment with moving air.
EFFECTS OF RAIN AND HUMIDITY
Questions about operating aeration fans while it’s raining are often asked. The answer is it depends on how long the rain is expected to continue. Keep on aerating with a positive system and do not start with a negative system if the rain is expected to last less than 12 to 24 hours. If negative air is being used when the rain starts, the aeration should be stopped if the rain is expected to last more than 6 to 12 hours. The negative aeration fan system should be restarted as soon as possible after the rain ends.
The cause for concern is much lower with positive aeration systems, since a positive aeration fan will add a 2 to 5 degree F/1 to 3 degree C heat rise to the air entering the bin. The air molecules are compressed, creating friction (thus heat) between the air molecules as a factor of the tunnel velocity of the air. The temperature increase will cut the relative humidity of the air entering the bin under pressure. The warmer air will thus cause a slight drying effect.
A general rule is that humidity is cut in half with every 14 degrees F/9 degrees C temperature increase, so it is recommended to continue aeration in a positive system even if it starts to rain if the ambient temperature is expected to remain stable. Negative aeration fans do not add any heat to the air pulled into the bin, and it really becomes a management decision to continue or stop aeration if extended rain is in the forecast based on the expected duration of high humidity. The flexibility of positive aeration systems with roof exhaust fans in rain is just another reason to justify the added expense of a roof exhaust fan system.
Scott Chant is president of Safe-Grain/Maxi-Tronic Inc. He can be reached at email@example.com.