Aeration strategies

by Harmon Towne
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My first experience out of college in 1963 was working with farmers and country elevators on drying and storage of grain. One of the main topics that I discussed at meetings was aeration. The U.S. Department of Agriculture (USDA) had done a lot of work in the 1940s on maintaining the quality of grain during storage. Although much of the initial research had to do with flat storage, by the early 1960s this information was being used for concrete silos as well as steel grain bins.

Research had shown that as the temperatures changed with the seasons, convection currents were active in the bin because of temperature differences in the grain mass. Since grain is a good insulator, the central core temperatures remained fairly constant while grain next to the wall and on the surface changed with the ambient temperatures. With cool air settling along the wall of the bin and warm air rising in the center, minute amounts of moisture were transferred to the surface, and condensation took place. This eventually led to spoilage and if the core temperatures remained above about 50 degrees F, insect activity also occurred.

By using small amounts of air movement through the grain, the grain mass temperatures could be equalized, eliminating moisture migration and insect activity. Air flows of 1/20th CFM (cubic feet per minute) per bushel were recommended because this was thought to simulate the natural air flow.

Tests showed that at these low rates it took about 300 hours of continuous fan operation to equalize the temperature of the grain to the ambient temperature. The recommendations were also to use a suction system to keep condensation from occurring on the top of the grain pile.

As the size of storage structures increased and the desire to cool the grain faster became more prevalent, airflows were increased to 1/10th CFM per bushel. This reduced the time to cool the grain in half. As electricity supplied to the farms improved, larger fans became more practical. The trend of higher airflow has continued for today’s operations. It is now quite common for both farm and commercial setups to use up to 1/5th CFM per bushel. Also, the trend with the higher airflows has been to use a push system rather than a suction system. With the higher airflows a suction system caused the perforated floors to become plugged, creating higher static pressures and thus lower airflow through the grain. Keep in mind, however, that airflows higher than this can create a drying effect on the grain mass.

In order to make sure all of the grain in storage gets the benefit of some airflow, the distribution system is critical. Air flows through grain, like water, flows downhill. It follows the path of least resistance. The higher the velocity, the more straight line effect you see. It is therefore imperative to slow the air speed down enough to get it to all of the grain mass. The two major areas are the distribution tunnels and the surface area of these tunnels.

The ideal velocity for the distribution tunnel is 1,000 feet per minute (1,000 CFM per square foot of cross section). With this slow speed, the air will flow evenly to all areas of the tunnel. However, with the higher airflows and the height of the bins, it is not economically feasible for this size of tunnel. The velocity of the air exiting the fan will normally be about 4,000 feet per minute (fpm) or 4,000 CFM for every square foot of cross section. The transition going into the tunnel should then slow the air down to no more than 2,000 to 2,500 fpm. Airflow in the tunnel of 4,000 fpm will create one inch of static pressure and may not move through the perforated area as uniform as preferred. This also reduces the amount of air that the fan can provide for aeration.

With these lower velocities, the pressure in the tunnel will allow for fairly uniform airflow through the perforated surface of the tunnel. In order to get the air to flow throughout the entire grain mass, the surface velocity should not be over 40 fpm. It is better if you can have 30 fpm when the bin is full so that as the bin is emptied the flow doesn’t exceed the 40 fpm figure.

It is also necessary to make sure that the tunnel spacing is adequate to ensure that all grain gets the benefit of some air. There should be at least two to three feet of solid cover on the tunnels at the sidewalls of the structure. Recalling that air follows the path of least resistance, the concern is that most of the air will flow up the sidewall. However, keeping in mind that velocity and quantity of air will cause change in direction due to higher resistance, air will move throughout the grain mass with proper spacing.

The vertical and horizontal distance should never be greater than 1.5 times the diagonal distance. Following these guidelines with proper tunnel size and surface area will ensure that air can get to all of the grain mass. There are, however, some additional factors that can inhibit the flow of air.

When the bin is filled without the use of a spreader, the course material flows to the outside of the bin and the fines and trash accumulate in the center of the bin. There will also be a peak of grain in the center of the grain mass. These two factors create greater resistance to airflow and in some cases may totally block any air from aerating this area. It is important to eliminate this core blockage. After filling the bin, the center of the bin should be cored to eliminate this fine buildup. Failing to do so will create an area for spoilage and insect activity that can cause the grain to form chunks or crusts that may block the discharge sump when starting to unload the bin.

Venting

Proper venting of the roof area is also very important. I prefer to see at least one square foot of vent space for every 1,000 CFM of air. You should never exceed 2,000 CFM per square feet of vent space. The location of the gravity vents is a topic that will give almost as many answers as there are individuals questioned.

In most steel bins there is almost always an opening between the sidewall and roof. With that in mind, I prefer the gravity vents to be about one-third of the way up the roof as this puts about one-half of the grain on each side of the vents. It is also important to have some venting near the peak of the roof as that it where the highest heat buildup will occur during the summer.

When power vents are used, the location of the gravity vents can be low in the roof. The power vents will draw air through these vents and wipe the roof to reduce possible condensation. If the power vents cannot be located near the peak of the roof, there should also be gravity vents at this location. The goal is to keep from having heat buildup in the peak. The amount of air provided by the power vents should be equal to or greater than the amount of air provided by the aeration fans. Once the grain is cooled, the power vents can be used to keep the heat from building in the over space.

Storage Temps

During the fall and winter aeration time, the grain should be cooled to 38 to 40 degrees F. Cooling the grain any lower only uses up unnecessary energy. Once you start the fans they should not be shut off until the entire grain mass has reached a uniform temperature.

Remember, at 1/10th CFM per bushel it will require 150 hours of run time to cool the grain.

As the average temps continue to fall, it may require you to run the fans for several weeks to get the temperature to the proper level. Once you have started the process, do not shut the fans off because of rain or snow. While it is true that a small amount of re-wetting may occur, far more damage can be caused by stopping the fans and letting heat buildup in the grain mass. Any moisture added to the grain will quickly be removed once the weather improves.

Once the grain has been properly aerated to the desired temperature and the fans turned off, the fans should be blocked with a cover over the entrance. Air blowing against the fans will enter the system, changing temperatures in the grain which may cause uneven temperatures and create migration. The same advice applies during the spring and summer.

Should the grain be warmed up during the spring? The early philosophy was that warming the grain should occur in the spring to eliminate the reverse moisture migration concept. It is now more accepted to only warm the grain to about 50 degrees F. Since grain is such a good insulator, you can restrict insect activity by keeping the core temperature at this temperature. There is also little evidence that moisture migration occurs during this storage timeframe.

One additional benefit of the pressure aeration system is to check the seal at the base of the bin. With the aeration system running, you should check the base angle seal completely around the bin.

One of the greatest areas of grain spoilage is located at the base when not sealed properly. If air is leaking out at any place, water will certainly leak in when the system is off causing grain spoilage.

Following proper design criteria and operating procedures of your aeration system will go a long way in maintaining the quality of your grain during storage, but remember that you cannot improve the quality of the grain, only maintain it
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