Proper temperature levels for stored grain
Figure 5. Average monthly temperatures at Windom and Hallock, Minn.
The purpose of aeration is to maintain a relationship between storage and outside temperatures that will minimize moisture migration and keep grain temperatures low to minimize deterioration rates (consistent with ambient temperatures).
Figure 5 shows the average monthly temperatures at Windom and Hallock, Minn. It might be argued that the ideal grain storage temperature would be the same as the average outside temperature. However, it is not necessary to cool the grain to the minimum average monthly temperature occurring in January, nor is it necessary to warm the grain to the highest average monthly temperature occurring in July.
Figure 6 shows the temperature levels of stored grain that provide a reasonable schedule that can be maintained with good aeration management. There are four calendar periods on figure 6:
- Cool-down period from September to December,
- Winter holding period from December to March,
- Spring warm-up period from March to June, and
- Summer holding period from June to September.
Figure 6. Grain storage aeration periods
The aeration management schedule is discussed for each of these periods. Proper management requires the use of these instruments:
- Thermometer to measure ambient and bin exhaust temperatures (One that shows maximum and minimum temperatures will provide additional desirable information.);
- Thermometer probe for smaller bins, or temperature cables in large bins (20,000 to 25,000 bu and up);
- Moisture tester that has been checked and calibrated with a known moisture tester (elevator tester, Brown Duvet tester, or oven drying); and
- Deep-bin probe for taking moisture samples from the bin.
Fan operation during cool-down periodsDuring the cool-down period, the objective is to reduce the temperature of the grain below 35° F but not lower than 20° F. In southern Minnesota a range of 25 to 35° F would be reasonable. In northern Minnesota the range could be 20 to 30° F. The fan should be operating when the average weather is capable of cooling. Since the stored grain has a large heat storage capacity, it serves to average the outside temperatures during fan operation. If the 24-hour average outside temperature is 10° F or more below the exhaust temperature, adequate cooling is being accomplished. A good guideline to follow is to let the fan run continuously night and day when the exhaust temperature from the bin is at or above the maximum daily temperature. The normal day-night temperature variation is about 20° F. When the exhaust temperature from the bin is the same as the maximum daily temperature (usually afternoon) the average 24-hour temperature will be about 10° F lower and adequate cooling is being accomplished. A question often raised is the effect of air relative humidity on fan operation during aeration. Table 4 shows the equilibrium moisture content of grain at several relative humidities for a 60° F temperature.
|*Oilseed sunflowers–estimated based on comparative storability.|
If the average relative humidity of the ambient air during fan operation is at or below the equilibrium moisture content of the grain, no moisture can be added to the grain. The average 24-hour relative humidity during reasonably fair weather is never high enough to cause a problem. A day or two of fan operation during rainy weather or other high relative humidity periods will do no harm if the fan operates for a day or two of fair weather following these periods. The only thing to avoid is extensive fan operation during rainy, wet weather after the fan has run enough to cool the grain to within 5 to 10° F of the average outside air temperature.
Aeration should be started as soon as grain is delivered to the bin. Grain out of a high-temperature dryer or grain combined during warm, sunny days will always be cooled by immediate fan operation. Cooling progress should be checked and the fan run enough to cool the grain to within 5 to 10° F of the average outside air temperature. Extensive fan operation beyond this (particularly in bad weather) should be avoided. The temperature of the grain can be reduced to the desired level in several steps if necessary.
The bin should be cooled to 20 to 35° F where it will be held during the winter holding period. Be sure all parts of the bin are cooled. The top center of the bin will be the last to cool with upward airflow and the bottom center will be the last to cool with downward airflow.
Fan operation during the winter holding period
When the fans are off during the winter holding period, they should be covered (with canvas or plywood) to prevent the grain near the ducts from getting too cold during severe winter weather. Large temperature differences result in condensation in the cold grain. Spoiled grain over the aeration ducts or perforated floor is a common problem caused by not covering the fan during extended off periods.
During the winter holding period, the grain should be checked weekly and the fan should be run periodically for a day or two during good weather when the outside temperature is near the temperature of the grain.
Fan operation during the spring warm-up period
Many operators do not warm the stored grain in the spring. If the grain is to be moved out by July and has not been cooled below 30° F, it does not need to be warmed. However, if there is a possibility that the grain will be held longer or is colder, it is desirable to warm it to 50 to 60° F. This is best done in several stages by starting early (end of March, first of April) and running the fans in fair weather (night and day) when the average 24-hour temperature is 10° F warmer than the grain. This means the fan is operating continuously when the minimum daily temperature is about the same as the bin exhaust temperature. When the exhaust temperature increases to the new level, wait until the weather warms up another 10° F and bring the temperature up another stage. Be sure to bring the entire bin up during each stage. If the fan is shut off before the entire bin has warmed up, there may be some condensation in the area between the cooler and warmer parts of the bin. This causes spoilage if left more than several days. The condensation is more severe with larger temperature differences.
Fan operation during the summer holding period
Be sure all the grain is warmed to 50 to 60° F by the middle of June. Again, be sure to cover the fan when it is off during the holding period. If the grain is between 50 and 60° F and the duct or plenum chamber is open, there may be condensation in this cooler grain during warm, high humidity periods during summer. This results in spoilage next to the ducts.
Check the grain periodically, and run the fan during cool, fair weather when the outside temperature is close to the grain temperature.
The top layer of grain warms up due to high temperatures between the grain and bin roof. It is better to move the air upward to carry this warm air out of the bin rather than draw it downward through the rest of the grain.
Direction of air movement
Aeration can be accomplished by moving the air up or down through the grain. The air delivery system will move the same amount of air either way. However, there are advantages and disadvantages to each. If the operator understands them, either direction can be used to do a good job.
The main advantage of moving the air down and exhausting it at the bottom is to minimize roof condensation when aerating warm grain during cold weather.
The main disadvantage of downward air is the uncertainty of knowing when aeration is complete. The grain at the bottom is the most difficult to check. A thermometer in the exhaust air, preferably on the suction side of the fan, helps check the progress of aeration; however, it will read the average exhaust temperature and may not reflect the higher temperature of the air leaving the bottom center. The operator must check to see if aeration is complete by probing with a thermometer or checking the temperature of some grain unloaded through the center hopper.
The main disadvantage of upward airflow is that moisture may condense when warm air hits a cold bin roof. This would happen when warm grain is aerated during cold weather. The problem can be minimized by starting aeration early and reducing the grain temperature over a longer period of time. The best way to operate fans—regardless of airflow direction—is to change the grain temperatures gradually, in several steps.
Screening, spreading, unloading
Accumulation of fine particles, weed seeds, and other foreign material interferes with airflow. Such accumulations are prime locations for increased mold and insect activity, which result in localized heating and grain deterioration. Normally, these accumulations are in or near the center of the bin due to separation as the grain flows toward the walls.
Several good management practices can reduce the storage risks incurred through accumulation of foreign material. Screening the grain reduces the amount of foreign material and greatly improves the storability. Screening is a must for long-term storage unless the grain is delivered in good, clean condition.
Spreaders are used to more uniformly distribute the foreign material throughout the storage. This helps provide more uniform airflow during aeration. However, there is research evidence to show that distribution of fine particles throughout the bin increases the resistance to airflow, which reduces airflow.
Another good management practice in bins equipped with center unloading hoppers is to unload some grain from the center to remove some accumulated material. Fill the bin so it is peaked and unload some of the grain (300 to 1,000 bu, depending on bin size). This removes some of the accumulation and increases airflow in the center if enough grain is unloaded to allow the center core to fill with clean grain. A better way to accomplish this, if possible, is to unload some grain periodically as the bin is filled.
It is impossible to provide a single answer to questions related to screening, spreading, and unloading the center. All of these practices improve the ability to manage stored grain. However, the degree of improvement depends on the amount of foreign material, bin size, grain temperature and moisture content, spreader performance, and unloading procedures. Each operator, based on experience and examination of the stored grain, can make reasonable decisions as to the risks involved in each case. Based on these considerations, the operator can decide which bins are the greatest risks and can arrange the unloading schedules to best meet the situation.
Some general guidelines are:
- Higher moisture grain should be unloaded first.
- Grain that has been dried in high-temperature bin dryers and stored in the same bin should be moved first.
- Grain delivered to bins for long-term storage (more than 10 months to one year) should be screened.
- If spreaders are not used in storage bins, the bins should be peaked and the centers unloaded immediately following filling.
- Even though spreaders are used, if the grain being delivered to storage contains large amounts of fine particles, weeds seeds, or other foreign material and the grain is not screened, the bin should be filled to the peak and the centers unloaded.
- If the grain is fed, the operator should unload some grain from all bins before completely unloading one.
- After all bins are filled, the operator should properly check the condition of each and arrange the unloading schedule to best manage the situation.